Crystallography Overview for MEMS Learning Module

Transcription

1 Southwest Center for Microsystems Education (SCME) University of New Mexico MEMS Fabrication Topic Crystallography Overview for MEMS Learning Module This booklet contains six (6) units Knowledge Probe (KP) Primary Knowledge (PK) Growing Crystals Hot Ice Activity The Miller Index Activity Breaking Wafers Activity An Origami Crystal Activity Final Assessment The purpose of this learning module is to introduce the science of crystallography and its importance to microtechnology. Activities provide for additional exploration into crystallography and its applications. Target audiences: High School, Community College, University Made possible through grants from the National Science Foundation Department of Undergraduate Education # , , and Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and creators, and do not necessarily reflect the views of the National Science Foundation. Southwest Center for Microsystems Education (SCME) NSF ATE Center 2010 Regents of the University of New Mexico Content is protected by the CC Attribution Non-Commercial Share Alike license. Website:

2 Crystallography Overview for MEMS Knowledge Probe (pre-test) Participant Guide Introduction The purpose of this learning module is to introduce the science of crystallography and its importance to microtechnology. Activities provide additional exploration into crystallography and its applications. This knowledge probe helps to determine your current understanding of crystallography, silicon crystal structure, and Miller Indices as it relates to microsystems technology and fabrication. This pre-quiz should be completed prior to starting the Crystallography Overview for MEMS Learning Module. There are 15 questions. Answer each question to the best of your knowledge. 1. Which of the following BEST defines Crystallography? The science of a. developing arrangements for atoms in solid matter. b. determining the arrangement of atoms in solid matter. c. studying the properties of atoms in solid crystal. d. developing new crystals using atomic structures. 2. Matter without a regular arrangement of atoms is called a. Amorphous b. Chaotic c. Polycrystalline d. Crystalline 3. In a crystal, the simplest repeating section of atoms is called the a. Single crystal b. Poly crystal c. Unit cell d. Crystal seed 4. Polycrystalline solids consist of small crystals called that are separated by. a. Unit cells, cell boundaries b. Amorphous solids, amorphous boundaries c. Single crystals, crystal boundaries d. Grains, grain boundaries Southwest Center for Microsystems Education (SCME) Page 1 of 4 Fab_Crystl_KP10_PG_mc_April2017.docx Knowledge Probe

3 5. What type of solid is peanut brittle? a. Amorphous b. Chaotic c. Polycrystalline d. Crystalline 6. Which of the following is NOT a characteristic of monocrystalline silicon? a. Longer range order compared to polycrystalline silicon b. Well-ordered silicon atoms arranged in a lattice structure c. Many single crystalline solids held together by ionic bonds 7. The material properties of a silicon wafer are determined by surface atoms and the of the silicon wafer. a. crystal orientation b. doping concentration c. long range order d. bandgap 8. What is the roadmap or compass called for identifying the crystal planes of single crystals? a. X-ray diffraction b. Crystallography c. Miller Index d. Cartography 9. Which of the following BEST describes why crystalline silicon is used for microsystems fabrication? a. Readily abundant and can be easily formed into polycrystalline ingots and cut into wafers b. Its relatively short range order, strength, and unique ability to be etched along grain boundaries c. Its semiconductor properties that allow it to act as an insulator or a conductor depending on design d. Its unique electrical and mechanical properties that make it possible to form specific well-defined structures Southwest Center for Microsystems Education (SCME) Page 2 of 4 Fab_Crystl_KP10_PG_mc_April2017.docx Knowledge Probe

4 10. There are different configurations for unit cells with each configuration having different number of atoms. A face centered cubic structure has how many atoms? a. 1 b. 4 c. 7 d A silicon atom has valence electrons that are shared with other silicon atoms when forming a crystal. a. 8,8 b. 6,2 c. 4,4 d. 2,6 12. What is the Miller Index notation for the yellow plane in this diagram? a. (100) b. (010) c. (001) Z d. (110) e. (111) Y X Southwest Center for Microsystems Education (SCME) Page 3 of 4 Fab_Crystl_KP10_PG_mc_April2017.docx Knowledge Probe

5 13. The following diagram represents different in the same crystal structure. a. Grains b. Unit cells c. Growth structures d. Crystal planes 14. In the manufacture of silicon wafers, the crystal orientation of the wafers is determined by the a. purity of the silicon b. orientation of the seed crystal c. type of seed crystal d. pull rate of the ingot 15. The bulk modulus of a microsystems structure is determined by the properties of the material in which they are constructed. a. dimensional b. electrical c. mechanical d. optical Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 4 of 4 Fab_Crystl_KP10_PG_mc_April2017.docx Knowledge Probe

6 Crystallography Overview for MEMS Primary Knowledge (PK) Participant Guide Description and Estimated Time to Complete The purpose of this learning module is to introduce the science of crystallography and its importance to microtechnology. Activities provide additional exploration into crystallography and its applications. This PK unit reviews the science of crystallography as it relates to the construction of microsystem (MEMS) components. You will study three types of solids (amorphous, polycrystalline, and crystalline) and will learn how to identify crystal orientation based on Miller indices. Estimated Time to Complete Allow about 40 minutes Introduction Crystallography is the science of determining the arrangement of atoms in solid matter. Solids with an irregular arrangement of atoms are amorphous or noncrystalline structures. Such solids include glass, soot, plastics, and gels. Solids composed of atoms arranged in a definite pattern with a repeating structure are crystalline structures. These structures include diamonds, ice, quartz, and an old favorite, rock candy. All solid matter is either amorphous or crystalline, or a type of crystalline matter called polycrystalline. One of the objects below is amorphous. The other is crystalline. Which is which? Pretty obvious, isn't it? Notice the cloudiness of the amorphous glass (bottom left) compared to the clarity of the crystalline diamond (top right). Southwest Center for Microsystems Education (SCME) Page 1 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

7 Because the atoms of crystalline structures "fit together" so well, a crystal is typically very strong. This characteristic is invaluable for the construction of micro and nanosized devices. The fabrication of microsystems requires a type of crystalline substrate in order to build microsized structures such as cantilevers, diaphragms, gears, comb drives, and electronic circuits. The image to the right is a MEMS popup mirror used to redirect optical data. MEMS popup mirror [Courtesy of Sandia National Laboratories, SUMMIT Technologies, This unit discusses three topics of crystallography: The types of solid matter (amorphous, polycrystalline and crystalline) Miller Indices (a method of describing planes and directions within a crystal) Growing crystals Objectives State at least one example for each type of solid matter (amorphous, polycrystalline and crystalline). Discuss the importance of crystal structures in MEMS fabrication. Identify the direction of a crystal plane using the Miller index notation. Key Terms (The key terms are defined in the glossary at this end of this unit.) Amorphous Crystalline Crystallography Grains Grain Boundaries Miller Indices Polycrystalline Unit cell Southwest Center for Microsystems Education (SCME) Page 2 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

8 Crystallography Crystallography is the science of determining the arrangement of atoms in solid matter. This science is important to the advancement of applied sciences and technologies, and material science. It provides information necessary for the development of metal and metal alloy structures, ceramics, glasses, and polymers. For micro and nanotechnologies, it provides information for the design and development of micro and nano-sized components. Microcantilever Array [Image courtesy of Dr. Christoph Gerber, Institute of Physics, University of Basel. This cantilever array was developed by the Cantilever Array Sensor Group at the Swiss Nanoscience Institute.] The scanning electron microscope (SEM) image above shows a microcantilever array that was etched from a crystalline silicon substrate. The crystalline structure of silicon allows for the fabrication of micro-sized sensors such as these cantilevers that are strong, ultrasensitive, and fastresponding. Such sensors can be used in a variety of applications in chemistry, physics, biochemistry and medicine. 5 Southwest Center for Microsystems Education (SCME) Page 3 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

9 Solid Arrangements Amorphous vs. Crystalline structures Matter without a regular arrangement of atoms is called amorphous or non-crystalline. Matter composed of atoms arranged in a definitive pattern with a repeating structure is called a crystal. (See illustrations above.) Crystals consist of a repeating structure called a unit cell. The Unit Cell Unit Cell and Unit Cell configuration The unit cell is the simplest repeating unit in a crystal. In a single crystal, all unit cells are identical and oriented the same way (fixed distance and fixed orientation). The opposite faces of a unit cell are parallel (see graphic of unit cell). The edge of the unit cell connects equivalent points. The resulting structure is a lattice. The figure above illustrates a unit cell for a crystalline structure. Southwest Center for Microsystems Education (SCME) Page 4 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

10 The Lattice The pattern of a crystal is like the repeating pattern on wallpaper. The motif is analogous to the unit cell and the arrangement of the motif over the surface is like the lattice. The lattice is a repetition of unit cells and when viewed from different angles or planes one would see different geometries or patterns. Check out this 3D crystal viewer. 1 ( This applet allows you to move a crystal around so you can see it from different angles. (Select Diamond from the Archives list.) Southwest Center for Microsystems Education (SCME) Page 5 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

11 All Unit Cells are Not Alike There are several different configurations for unit cells. The simplest being the three configurations below. The Simple Cubic Structure is a unit cell consisting of one atom. You are probably confused by that because you see eight atoms; however, remember that unit cells form a lattice and the edge of the unit cell connects to equivalent points. Therefore, each of the atoms you see in the simple cubic structure contributes ONLY 1/8 of itself to the unit cell. As the crystal structure forms, seven more unit cells bond with each of the eight atoms. To see this in action, watch an animation of how a body-centered cubic configuration forms a crystal. 2 ( Pay close attention to how each corner cell bonds to other unit cells. How many atoms are there in a "body centered cubic structure"? in a "face centered cubic structure"? Southwest Center for Microsystems Education (SCME) Page 6 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

12 How many atoms are there in a "body centered cubic structure"? If you said TWO, you are correct! in a "face centered cubic structure"? If you said FOUR, you are correct! The body centered cubic has ONE atom from the eight corners, then the stand-alone atom on the middle: TWO atoms The face centered cubic has ONE atom from the eight corners, then ONE-HALF an atom from each of the face centered atoms: 1 + ½ * 6 = 4 Carbon Unit Cell This is the unit cell for Silicon (Si), Germanium (Ge), and carbon (C). Identify the "face-centered atoms". This unit cell can combine with other unit cells in a variety of ways. To see variety of structures formed by the carbon unit cell, Google Carbon structures and view images. You should find structures such a carbon sheets, carbon nanotubes, bucky balls (also called fullerenes), and diamonds. Southwest Center for Microsystems Education (SCME) Page 7 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

13 What's What? Earlier, we talked about solids being a crystalline structure (e.g., diamond) or an amorphous structure (e.g., glass). However, not all crystal structures are alike. A true crystal or single crystal structure is one continuous crystal. Sometimes a crystal structure is made of many, single crystals. These are polycrystalline structures. Which of the following graphics (a, b, or c) illustrates crystalline, polycrystalline or amorphous? Correct answer: a. polycrystalline b. amorphous c. crystalline Let's take a closer look at the three arrangements. Amorphous (Noncrystalline) Question: What do you think of when you hear the word "amorphous"? When a solid's atoms are randomly "arranged" in a non-predictable order, the solid is referred to as amorphous. Which of the following are amorphous solids? Styrofoam Window glass Salt Wax and paraffin The pattern of a tiled floor Amorphous solid structure of Silica Glass Peanut brittle Southwest Center for Microsystems Education (SCME) Page 8 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

14 If you said all but salt and a tiled floor, you are correct. When you break a piece of peanut brittle, it does not break along a straight edge. Instead it shatters into pieces of different sizes and different shapes. It shatters because it is amorphous, having no definitive edges. Peanut Brittle Amorphous solids have the following characteristics: No long range order exists at the atomic level. No predictability in the position of atoms, even over a short distance (i.e. a few nanometers). An amorphous solid cannot be cut (cleaved) like a crystal. It shatters rather than breaks along a plane. Amorphous "arrangement" Polycrystalline Scanning electron microscope image of a polycrystalline carbon in a diamond structure. [Courtesy of Prof. Dean Aslam, Michigan State University] Crystalline structures are either single crystal or polycrystalline (poly being "many"). In both structures the atoms are arranged in a pattern consistent with the unit cell. Diamonds formed in nature are single crystal diamonds. However, polycrystalline diamonds (like the one shown above) are being fabricated for use in high temperature cutting tools, cell phones, and are being explored for use in MEMS, high-frequency, high temperature and radiation hard device applications. 4 Some metals and metal alloys are polycrystalline. As like diamonds (carbon), silicon can be either polycrystalline or crystalline. Southwest Center for Microsystems Education (SCME) Page 9 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

15 Grains Polycrystalline materials are made up of a myriad of small individual crystal grains. The grains randomly arrange to form the final structure. In the photo, the individual grains of this polycrystalline mineral sample are clearly visible. Each grain is a small crystal. Can you see how the grains connect to each other to form this polycrystalline structure? In amorphous materials, the unit cells are randomly arranged throughout the material random distances, random orientations. Grain crystals do not form. Remember the peanut brittle and how it shattered? How do you think polycrystalline material would break? If you said that it would break according to the individual grains, you were correct! However, in polycrystalline material the grains are not aligned predictably to each other. In a monocrystal, the entire solid is a single gigantic grain. In the figure of the polycrystalline material, note the "grain boundaries". Polycrystalline solids have the following characteristics: Long range order exists. Polycrystalline solids consist of crystal grains stuck together; Each crystal grain consists of billions and billions of atoms with predictable placement. Each grain is a mono-crystal. Polycrystalline solids do not shatter like amorphous solids. When broken, they tend to break along the grain boundaries (the boundaries form when individual grains are joined). Polycrystalline structure showing grains and grain boundaries Based on these characteristics, what are some other examples of polycrystalline solids? Southwest Center for Microsystems Education (SCME) Page 10 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

16 Crystals Crystals are defined by a regular, well-ordered atomic lattice structure. A lattice consists of stacked planes of atoms. Because the atoms of the crystal fit together repeatedly and are held together by strong electrical attractions between each other, a crystal is typically very strong. High quality diamonds found in jewelry consist of tight, dense carbon lattices as illustrated in this image of a diamond structure. The less compact the carbon lattices, the less valuable the diamond. Other crystal solids include gemstones, salt, sugar, some metals, pure silicon, and germanium. Crystals have the following characteristics: Extremely long range order and predictability exists with very few defects. If you could get inside a crystal, you could move from one end to the other and see no difference in the placement of the atoms. The environment is always the same throughout the crystal solid. Crystals can be cut along flat planes called cleavage faces. Cutting a crystal is essentially separating one lattice plane from its adjacent plane. This produces a near perfectly flat surface. A Closer Look at the Silicon Crystal Silicon crystal is widely used in micro and nanotechnologies. A silicon (Si) atom has four valence electrons that are shared with four other atoms to form four covalent bonds when forming a crystal. By sharing electrons this way, each atom s valence shell is complete. This results in solid matter that is electrically stable and a poor conductor of heat. In the graphic below, notice that the outer energy level has four electrons and space for four more. On the right, you should see that each silicon atom is bonded to four other silicon atoms. In other words, each "electron space" is filled by one electron from one other silicon atom. The figure on the right is a two-dimensional crystal lattice or sheet. Southwest Center for Microsystems Education (SCME) Page 11 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

17 The orientation of the silicon crystal denotes which crystal plane is exposed on the wafer surface. (Refer to the graphic below for the following discussion). The left most image is a silicon crystal. The middle images highlight two different planes within the silicon crystal. Think of looking at the same crystal from two different directions. The images on the right are what you would see looking at the face of each plane. Same crystal, same distance between unit cells, and same orientation of unit cells. However, looking at different planes, presents a different picture. Silicon Crystal Planes [Graphic courtesy of Khalil Najafi, University of Michigan] Southwest Center for Microsystems Education (SCME) Page 12 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

18 Silicon Properties The surface properties of a silicon mono-crystalline wafer vary depending on the orientation of the lattice relative to the wafer surface. This orientation affects the properties of the wafer, the number of atoms on the wafer surface, and the wafer's conductivity (electronic properties) and reaction potential. For example, the ability to etch silicon crystal in potassium hydroxide is dependent on this orientation (what arrangement is presented to the surface). Also, a silicon crystal has different bending (mechanical) properties depending on its orientation to applied stresses. Count the number of atoms in the two plane faces shown above right. Revisit 3D crystal viewer 1. It might help you understand this graphic better. Use the diamond as an example. Rotate the diamond unit cell to find each of the following planes. Southwest Center for Microsystems Education (SCME) Page 13 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

19 Crystal Planes Planes are the second level or organization in crystal structure. They describe the orientation of the crystal, which is dependent on the orientation of the individual unit cells within the crystal. Each type of plane is unique, differing in atom count and binding energies and therefore in chemical, electrical and physical properties. The Miller Index helps us to identify crystal planes. The Miller Index The Miller index is a roadmap or compass for identifying the crystal planes of crystals. Miller indices are three digit notations that indicate planes and direction within a crystal. These notations are based on the Cartesian coordinate system of x, y, and z. The Cartesian coordinate system is illustrated using the three vectors (axes) x, y, and z. Referring to the graphic Cartesian Coordinates, the x-axis vector is denoted [1,0,0] y-axis vector is denoted [0,1,0] z-axis vector is denoted [0,0,1] (Think of the "1" as being "1 unit" out from the origin or 0,0,0.) Alternate vectors are indicated with < >, such as <100>, <010>, or <001>. Cartesian Coordinates Southwest Center for Microsystems Education (SCME) Page 14 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

20 Identifying the Crystal Plane Crystal planes, each perpendicular to its respective vector (or axis: x,y,z) Crystal planes are perpendicular to their corresponding axis. For example, the plane perpendicular to the [1,0,0] axis or x-axis, is the (1,0,0) plane (shown in the figure). Notice that the (100) plane is perpendicular to the X vector and parallel to the plane formed by the Y and Z axes. In a crystal there are an infinite number of (100) planes. To visualize this, think of each card in a deck of cards standing on end, perpendicular to the table top and parallel with all of the other cards in the deck. Each plane in a crystal structure has a unique notation and the notation depends on the plane s orientation. (1,0,0) or (100) is perpendicular to the x-axis (0,1,0) or (010) is perpendicular to the y-axis (0,0,1) or (001) is perpendicular to the z-axis The above graphic illustrates a unit cell relative to the x-y-z axes and the yellow plane denoted (100). Alternate planes are denoted using {}, such as {100}, {010} and {111}. Southwest Center for Microsystems Education (SCME) Page 15 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

21 The (100) Plane Z Y X The (100) plane is perpendicular to the x-axis (x-vector), but parallel with the plane formed by the y and z axes. Can you see this in the graphic? If not, start with the point at which the (100) plane touches the x-axis. Is the plane perpendicular to the x-axis? Now move along the bottom edge of the plane toward the right. Is this edge parallel to the y-axis? You should now be able to see that the vertical edge of the (100) plane is parallel to the z-axis. Southwest Center for Microsystems Education (SCME) Page 16 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

22 What's What? What are the Miller indices for each of these planes? Southwest Center for Microsystems Education (SCME) Page 17 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

23 Why is Crystal Orientation Important? Microsystems consist of structures with defined edges, lengths, widths or thicknesses. They also require certain electrical (e.g. resistance), mechanical (e.g. bulk modulus), and optical (Index of Refraction) properties. Each of these properties can be different in different crystal orientations. The structures in the Scanning Electron Microscopy (SEM) images below show a mono-crystalline silicon wafer that has been coasted with silicon nitride on both the front and back surfaces. The silicon nitride acts as a membrane on one side of the wafer and a hard mask on the other side of the wafer. The left image shows the membrane. In the right image, the membrane is the brown color and the hard mask is the green color on the top of the wafer. The openings in the hard mask allow one to etch through the silicon wafer to the membrane on the opposite side. Note the sharp edges of the silicon. This was not accomplished by accident. A (100) silicon substrate and KOH (potassium hydroxide) etchant were used. The chemical reaction between the KOH and the silicon resulted in the anisotropic etch, the selective removal of material in one direction more than in another direction. The picture on the right shows the backside of a wafer. (Left) Diaphragm (membrane) for MEMS pressure sensor over an etched silicon substrate [SEM courtesy of University of Michigan] (Right) Backside view of the etched silicon crystal wafer. The green represents silicon nitride thin film (hard mask) on top of the silicon substrate while the gold (or brown) is a thin film of silicon nitride (membrane) on the opposite side of the wafer. [Courtesy of the MTTC / University of New Mexico] By choosing specific wafer crystal orientation and etchant, one can create a multitude of different shaped structures: V-grooves Micro fluidic channels Cantilevers and bridges Mesas or pyramid shaped structures Cavities and holes Southwest Center for Microsystems Education (SCME) Page 18 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

24 Determining the orientation To determine the orientation of a silicon crystal wafer, crystallographers use x-rays aimed at a tiny piece of the wafer containing trillions of identical atoms. The specific periodic arrangement of the atoms within the crystal diffracts the x-rays onto an electronic detector or film. The resulting diffraction pattern on the film or detector gives the crystallographer the information needed to determine the actual orientation of the tiny seed crystal and the spacing of the atoms. A computer reconstructs the orientation from the diffraction pattern. The images below show the resulting patterns of three planes of a silicon crystal. Indicate which image represents each of the following planes. (Think about the spacing of atoms and the number of atoms in different silicon planes.) a. (111) b. (100) c. (110) Crystal orientation of three different planes of a silicon crystal. X-ray was used to create these images. [Images printed with permission and from the personal collection of Christopher C. Jones 3 ] As you can see, there is quite a bit of information in the patterns: Spacing between dots Relative orientations Angle between patterns and different dots Such a pattern can be reconstructed into a 3-D image for a better view of the crystalline structure. This same process is used to determine the double helix structure of DNA. Technicians crystallize the DNA, then put an x-ray beam through it. Because they have to use a weak x-ray beam, exposure time is long; however, eventually a diffraction pattern appears. Southwest Center for Microsystems Education (SCME) Page 19 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

25 Answer: ( 100), (110) and (111), respectively (HINTS: The (100) pattern has fewer atoms and right angles are distinct in the pattern. The (111) pattern has the most atoms on the surface.) Another method to determine the crystal orientation of a silicon wafer is to break it. Remember that a crystal is a lattice structure; therefore, when a silicon wafer breaks it will break along a lattice plane. To see this for yourself, complete the SCME "Breaking Wafers Activity" which is part of this overall learning module. Southwest Center for Microsystems Education (SCME) Page 20 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

26 Making a silicon wafer The CZ (Czochralski) method of growing a silicon ingot How is a silicon crystal formed and the orientation of a silicon wafer substrate determined? 1. First we start with very pure silicon material ( % pure!) 2. Melt the pure silicon in a crucible. (This molten silicon is called the melt.) 3. Lower a seed crystal into the melt (top left image). Silicon atoms in the melt align to the same crystal orientation. As the seed crystal is slowly pulled out of the melt, a large crystal ingot or boule is formed. 4. To "grow" this silicon crystal or ingot, rotate the seed and the crucible with the melt in opposite directions while slowly pulling the seed crystal upward. 5. The slower the "pull", the larger the diameter of the crystal ingot that forms. (This process is the Czochralski (CZ) Method of growing silicon.) The seed crystal acts as a starting point for the alignment of the atoms in the molten silicon. The alignment of the seed crystal relative to the melt determines the orientation of the subsequently grown silicon crystal. The wafers cut from this crystal will maintain this orientation. Southwest Center for Microsystems Education (SCME) Page 21 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

27 The Ingot The resulting ingot is cylindrical in shape, 25.4 mm (~1 inch) to 450 mm (~18 inches) in diameter and several meters long. Once cooled, the ingot is ground to a perfect cylinder. The cylinder is sliced into thin wafers using diamond coated wires or saw blades. Each slice is polished to create silicon wafers, also referred to as substrates. Microsystems are constructed on or within these substrates depending upon the type of process used surface or bulk, respectively. Summary Solid matter is either amorphous, polycrystalline or crystalline. Silicon polycrystalline wafers are widely used as the substrate for microsystems. These wafers provide the electrical and mechanical properties needed to build the components for electromechanical systems. Crystal orientations (100) and (111) are commonly used. Food For Thought Explain why the quality of a diamond is determined by its crystalline structure? Why are only polycrystalline and crystalline materials used as substrates for microsystems components and devices? Food for Thought / Answers Explain why the quality of a diamond is determined by its crystalline structure? Why are only polycrystalline and crystalline materials used as substrates for microsystems components and devices? Southwest Center for Microsystems Education (SCME) Page 22 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

28 Glossary Amorphous: Without order. Lacking definite form. Band tailing: A characteristic that helps to define a material as being a conductor, insulator or semiconductor. Band tailing reduces band gap and increases conductivity. Crystalline: A uniform arrangement of atoms / molecules in all directions. Crystallography: The science of determining the arrangement of atoms in solid matter. Grains: Small crystals. Grains are comprised of several unit cells of a crystal. Grain Boundary: The edge formed by adjourning grains in a polycrystalline structure. Miller Index: A notation system in crystallography for planes and directions in crystal lattices. Miller indices: Three integers identifying a type of crystal plane. Polycrystalline: Solid matter that is made of many smaller crystallites or grains with varying orientation. The variation in direction can be random or directed, possibly due to growth and processing conditions. Unit cells: A unit of atoms arranged in a definite pattern with a repeating structure. A unit cell is a crystal. References and Resources "3D Crystal Viewer" Applet. Body-centered Cubic". Barbara L. Sauls and Frederick C. Sauls. King's College. Pennsylvania. Scientific Photographs by Christopher C. Jones and his study of Crystal Symmetry. Making a Practical Diamond Device. Evince Technology. Applied Diamond Electronics. Nanomechanical Cantilever Array Sensors. Lang, Hans Peter. Hegner, Martin. Gerber, Christoph. Springer Handbook of Nanotechnology. January Cantilever Array Sensor Group. Swiss Nanoscience Institute, Institute of Physics, University of Basel. Crystallography Presentation by Matthias Pleil, SCME Crystallography, MATEC Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 23 of 23 Fab_Crystl_PK10_PG_April2017.docx Crystallography PK

29 The Miller Index Activity Crystallography Overview Learning Module Participant Guide The purpose of this learning module is to introduce the science of crystallography and its importance to microtechnology. Activities provide additional exploration into crystallography and its applications. Description and Estimated Time to Complete In this activity you explore crystal planes by learning how to identify and notate them using The Miller Index also referred to as Miller Indices. By the end of this activity, you should be able to denote basic crystal planes using Miller indices notation as well as create a three-dimensional model of various planes. Estimated Time to Complete Allow at least 30 minutes to complete this activity. Introduction By definition a crystal or a crystalline solid is a solid material consisting of atoms or molecules arranged in a repeating pattern. This pattern forms a lattice structure of stacked planes that extend in all three spatial dimensions. The well-ordered, repeatable bonds between atoms or molecules are typically very strong. The repeatability and predictability of the bonds and atomic structure of a crystal make it an ideal substrate for MEMS fabrication. Knowing the atomic structure of a substrate enables the design and fabrication of many simple and complex microdevices. Typical crystalline substrates used in the fabrication of microdevices include silicon, polysilicon, and gallium arsenide 1. Southwest Center for Microsystems Education (SCME) Page 1 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

30 Choosing a crystalline substrate with a specific crystal orientation provides a desired structural effect on a micro-scale. Below are several micro-components that are formed along one or more planes of a crystal substrate. The first component is a microcantilever with a combdrive used to move the cantilever back and forth. The second image is a reference pressure chamber on the backside of a micro-pressure sensor. The third image is an out-of-plane accelerometer that shows an inertial mass and suspension beams fabricated within a crystal substrate. Components in all three of these images were designed and fabricated using specific planes of silicon crystal. [Cantilever image on the left courtesy of Sandia National Laboratories, Pressure sensor chamber courtesy of the MTTC / University of New Mexico, Accelerometer courtesy of the University of Michigan] How do we identify the many planes and vectors in a crystal structure? Miller index notation is a roadmap or directional compass for identifying the crystal planes and directions (vectors) within crystals. Miller indices are three digit notations that indicate planes and vectors within a crystal. These notations are based on the Cartesian coordinate system of x, y, and z. The Cartesian coordinate system is illustrated using the three vectors (axes) x, y, and z. Other coordinate systems are used for more complex crystal structures. Vector Notation Referring to the graphic Cartesian Coordinates, the x-axis vector direction is denoted [1,0,0] or [100] y-axis vector direction is denoted [0,1,0] or [010] z-axis vector direction is denoted [0,0,1] or [001] (Think of the "1" as being "1 unit" or 1 unit cell out from the origin or 0,0,0.) Here is another way to look at it. Let s look at the [100] vector. If you were to start at the origin (the intersection of the three axes) and take one step (1 unit) down the x-axis and no steps in the directions of the y and z axes, you would be on the [100] vector. Southwest Center for Microsystems Education (SCME) Page 2 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

31 Vectors parallel to the primary vectors are shown as <100>, <010>, or <001>. Notice in the graphic to the left, the primary vector is [010] and the vectors parallel to it are denoted <010>. Plane Notation Crystal planes are perpendicular to their corresponding axis or vector. For example, the plane perpendicular to the [100] vector or x-axis is the (100) plane (shown in red in the figure to the right). Each crystal plane has a unique notation. (1,0,0) or (100) is perpendicular to the x-axis (Red) (0,1,0) or (010) is perpendicular to the y-axis (Yellow) (0,0,1) or (001) is perpendicular to the z-axis (Green) Alternate planes or parallel planes are shown in {}, such as {100}, {010}, and {001}. The figure to the right shows the (100) plane and a set of planes {100} parallel to it. Southwest Center for Microsystems Education (SCME) Page 3 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

32 Miller index notation can also refer to a "negative" plane, or a parallel plane on the opposite side of the origin (0,0,0) or the opposite plane of the unit cell. For instance, the (010) plane would refer to the plane parallel to the (010) plane on the opposite side of the unit cell or one unit from the origin on the negative y (-y) axis. The graphic below illustrates this. As you can see, the reference plane in the unit cell is one unit from the origin in the positive direction (010). The plane opposite (010) in the unit cell is denoted (010) as are the additional planes in the y direction. To better understand Miller Index Notation, stop and take a few minutes (7:41 to be exact), to view this video: An Activity on Miller Index Notation. This video illustrates what we have been discussing about vector and plane identification and notation. [If the above link doesn t work, copy and paste this URL: ] Southwest Center for Microsystems Education (SCME) Page 4 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

33 The Unit Cell The unit cell is the simplest repeating unit in a crystal. In a single crystal, all unit cells are identical and oriented the same way (fixed distance and fixed orientation). The opposite faces of a unit cell are parallel (see graphic of unit cell below). The edge of the unit cell connects equivalent points. The resulting structure is a lattice. The figure below illustrates a unit cell for a crystalline structure. Miller Indices and the Unit Cell The (100), (010), (001), ( 00), (0 0) and (00 ) planes form the faces of the unit cell. Southwest Center for Microsystems Education (SCME) Page 5 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

34 The (101), (110), (011), (10 ), (1 0) and (01 ) planes form the sections through the diagonals of the unit cell and are denoted below. Study these planes and relative notations and make sure you understand them. Why is this important? Shown below on the left is a microscopic image of the backside of a (100) silicon wafer. The chemical reaction between a potassium hydroxide (KOH) etchant and the (100) silicon substrate, results in this desired anisotropic etch along the (111) crystal plane. Because of the crystalline structure and bonds of the crystal in the (100) silicon, material (silicon) is selectively removed (or etched) along the (100) and (111) planes, leaving the desired cavity. The etch occurs both vertically (down the (100) planes) and sideways (along the (111) planes) at an etch rate of 400:1, meaning that the vertical etch of the (100) plane is 400 times faster than the sideways etch of the (111) plane. This is because the lattice of the (111) plane is the denser or has more silicon atoms on its surface than the surface of the (100) planes. The angle of the (111) plane is always relatively to the (100) plane in a silicon crystal. The predictability of this chemical reaction on a monocrystalline silicon wafer allows for a micro-cavity that can be used for many purposes. Backside of etched silicon substrate Courtesy of the MTTC / University of New Mexico Southwest Center for Microsystems Education (SCME) Page 6 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

35 Activity Objectives and Outcomes Activity Objective To use the correct Miller index notations for different planes and vectors in crystal structures. Activity Outcome By the end of this activity you should be able to how to determine the Miller Index notation for a variety of planes and vectors within a Cartesian coordinate system and relative to a unit cell at the origin of the system. Resources SCME Crystallography Overview for MEMS PK Supplies / Equipment Supplies for each team (provided by instructor) 3 sticks or rulers (representing the x, y, z axes) Tape 1- piece of cardboard approximately 12"x12" in size (representing a plane ) 1 18 piece of string (representing a vector or direction) This activity should be performed in teams of 2 or 3 in order to promote discussion and a better understanding of the concept. Documentation Answers to the Post-Activity Questions An on-line tutorial Again, you can refer back to this tutorial while you are doing this activity. An Activity on Miller Index Notation [ Southwest Center for Microsystems Education (SCME) Fab_Crystl_AC00_PG_April2017.docx Page 7 of 12 The Miller Index Activity

36 Activity: Miller Indices Models Procedure Complete this activity with one or two other students. 1. Create a Cartesian coordinate system using the three rulers or sticks. Designate the x, y, or z-axis. For the purpose of this exercise, use the Cartesian orientation shown in the "Crystal Planes" image shown previously in this activity. Make sure that your team members know which ruler represents which axis (x, y, or z). 2. Position the piece of cardboard perpendicular to and in the middle of the "x" axis (1 unit length), and parallel to the y-z plane. The cardboard in this orientation represents the (100) crystal plane. Southwest Center for Microsystems Education (SCME) Page 8 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

37 3. Using the string, indicate the [100] vector. Remember that this vector is perpendicular to the (100) plane. 4. Position the piece of cardboard perpendicular to and in the middle of your "y" axis. The distance should represent 1 unit length. The cardboard in this orientation represents the (010) crystal plane. 5. Using the string, show the [010] vector, the vector perpendicular to the (010) plane and one unit from the origin. Southwest Center for Microsystems Education (SCME) Page 9 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

38 6. Position the cardboard to represent the (001) crystal plane. 7. Use the string and show the [001] vector. Make sure that everyone in your team agrees. 8. Using the cardboard and the string model the (110) plane and the [110] vector. Make sure that everyone in your team agrees. 9. Now position the cardboard in the (111) orientation and the string for the [111] vector. 10. Model the ( 00), (0 0) and (00 ) planes. 11. Model the (101), (011), (10 ), (1 0) and (01 ) planes. 12. Using the string, again model the following vectors. Create a drawing below that illustrates these vectors relative to a unit cell. [100], [010], [001], [011], [110], [111] Southwest Center for Microsystems Education (SCME) Page 10 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

39 Post-Activity Questions 1. What does it mean when a crystal plane is noted like this: (100)? 2. Name all of the faces of the "unit cell" using the Miller Index notations. 3. Draw a unit cell and show the (011) plane relative the x-y-z axes. 4. Draw the (101) plane relative the x-y-z axes. 5. Using Miller indices, name the following crystal plane, relative to the unit cell 6. Using Miller indices, name the following crystal plane, relative to the unit cell. 7. Using Miller indices, name the following crystal plane, relative to the unit cell. Southwest Center for Microsystems Education (SCME) Page 11 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

40 Summary Crystal orientation is a very important aspect of microsystems fabrication. Knowing the orientation of a crystal is imperative to being able to design and fabricate functional microstructures because the physical, chemical and electrical properties of each plane can be different. The Miller Index allows us to identify and notate specific crystalline planes relative to the Cartesian coordinate system and the unit cell. References 1. Gallium arsenide. Wikipedia Lattice Planes and Miller Indices. University of Cambridge. Disclaimer The information contained herein is considered to be true and accurate; however the Southwest Center for Microsystems Education (SCME) makes no guarantees concerning the authenticity of any statement. SCME accepts no liability for the content of this unit, or for the consequences of any actions taken on the basis of the information provided. Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 12 of 12 Fab_Crystl_AC00_PG_April2017.docx The Miller Index Activity

41 Growing Crystals: Hot Ice Activity Participant Guide Description and Estimated Time to Complete In this activity sodium acetate trihydrate is used to demonstrate how crystals grow from a seed crystal. Using a hot plate, you will dissolve polycrystalline sodium acetate trihydrate in a beaker, creating a supersaturated liquid solution. Once the solution cools, you will trigger crystal growth by placing a seed crystal in the supersaturated sodium acetate trihydrate solution. Estimated Time to Complete Allow at least one hour to complete this activity. Introduction MEMS (microelectromechanical systems) are fabricated using monocrystalline silicon wafers. The wafers are cut from a silicon ingot that is formed by melting chunks of polycrystalline solids in a large crucible. Once melted a seed crystal is placed in the liquid silicon to stimulate crystal growth for a specific crystal orientation. Over several hours a long ingot of pure monocrystalline silicon is slowly pulled from the melt. Below are the steps for growing this monocrystalline ingot. 1. First we start with very pure polycrystalline silicon material ( % pure!) 2. The pure silicon is melted in a crucible at 1425 C. (This molten silicon is called the melt.) 3. A seed crystal is precisely oriented and mounted on a rod then lowered into the melt (left image). Silicon atoms in the melt align to the same crystal orientation of the seed. 4. As the seed crystal is slowly pulled out of the melt, the seed and the crucible are rotated in opposite directions A large crystal ingot or boule is formed by controlling the temperature gradient of the melt, the speed of rotation, and the rate of the pull of the rod. The slower the "pull", the larger the diameter of the crystal ingot that forms. (This process can take several days to complete and is called the Czochralski (CZ) Method of growing silicon.) The seed crystal acts as a nucleation site for the alignment of the atoms in the molten silicon. The alignment of the seed crystal relative to the melt determines the orientation of the subsequently grown silicon crystal ingot. The wafers cut from this crystal maintain this orientation. Southwest Center for Microsystems Education (SCME) Page 1 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

42 Once the ingot is formed, it is removed from the equipment, then ground down to form a smooth cylinder with a consistent diameter from end to end. The ingot is now cut into silicon wafers using a diamond saw or diamond coated wires. The individual wafers are polished for a smooth, uniform surface. This process is called polishing and lapping. In this activity you will simulate the crystal growing process by melting sodium acetate trihydrate then stimulate the growth of a crystal by exposing the melt to a seed. Two types of reactions occur during this activity endothermic and exothermic. Endothermic and Exothermic Reactions The melting of the sodium acetate trihydrate to form the supersaturated solution and subsequent recrystallization demonstrates both endothermic and exothermic reactions. Endothermic - As the sodium acetate trihydrate solid is heated above its melting point of 58 C it absorbs heat energy (an endothermic reaction). Continued heating allows for all of the sodium acetate trihydrate to dissolve. This process results in an unstable supersaturated solution. Exothermic - When one initiates recrystallization of the supersaturated solution, the system crystallizes or freezes (forms a solid) between C, giving off heat energy (an exothermic reaction). Southwest Center for Microsystems Education (SCME) Page 2 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

43 Below is how this reversible reaction is notated. The left side of the equation is the liquid state (l) and the right side is the solid state (s). When reading right to left, heat is added to the solid state resulting in a liquid. Reading left to right, when the supersaturated liquid solution crystalizes, a solid forms giving off heat.!!!2!3!2 3!2!!!!!2!3!2 3!2!!+ 19.7!!/!!!! (heat) In this experiment the dissolved supersaturated solution of sodium acetate trihydrate (the liquid) cools below room temperature to about 15 C (59 F). When this temperature is reached a seed crystal is added causing the liquid solution to rapidly crystallize or freeze, releasing its heat (left to right in the preceding equation). Once crystallized, the heat can be felt through the beaker by holding the beaker in the palm of the hand. This exothermic reaction of the sodium acetate trihydrate gives off enough heat to reach its freezing point of 58 C (136 F) which is hot enough to cause burns; therefore caution is recommended. Activity Objective and Outcomes Activity Objective Demonstrate the growth of a crystal by heating sodium acetate trihydrate below its melting point to form a supersaturated liquid solution, and then initiating crystal growth by placing a seed crystal into the liquid. Activity Outcomes This activity has two possible outcomes. One is the formation of a solid crystal by placing a seed into the dissolved sodium acetate trihydrate. The other outcome is to form a crystal structure by pouring the liquid sodium acetate trihydrate into a flat container or watch glass containing a seed crystal. Resources NurdRage How To Make Hot Ice The Complete Guide Uploaded Oct. 17, Fisher Scientific Material Safety Data Sheet (MSDS) for Sodium Acetate Trihydrate Southwest Center for Microsystems Education (SCME) Page 3 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

44 Supplies (*Needed for each team. The other items can be shared) 50 grams of 100% Sodium Acetate Trihydrate* 200ml beaker* Watch glass* 3 ml plastic pipette* Beaker tongs Hot plate Weigh boat Small pitcher (for distilled water) Weighing scale (grams) Stainless steel chemistry scoop* Glass Casserole dish used for the Cooling Bath Distilled water for adding to the sodium acetate trihydrate Tap water for the cooling bath Thermometer Personal Protective Equipment (PPE) Gloves Safety Glasses/Goggles Activity: Growing Crystals Hot Ice Procedure: Procedural Notes: Read these procedural notes and the entire procedure before starting this experiment. a. The more sodium acetate trihydrate crystals you use the longer it takes to dissolve and cool. By using 50 grams, you can keep the time of this experiment to less than an hour. b. The addition of water is to help speed up the dissolution of the sodium acetate trihydrate crystals. The sodium acetate trihydrate crystals can dissolve without adding water because they have three water molecules attached; however, NOT adding water would slow down the dissolution, adding to the overall time to do this experiment. c. Once the sodium acetate trihydrate is in liquid form you may notice crystals forming on the stainless steel (SS) scoop that you use to stir the solution. This crystal formation indicates that the sodium acetate trihydrate is NOT completely dissolved. Once you see no crystal formation on the SS scoop when stirring, the sodium acetate trihydrate is completely dissolved. Southwest Center for Microsystems Education (SCME) Page 4 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

45 Procedure Steps 1. Locate the Fisher Scientific Material Safety Data Sheet (MSDS) for sodium acetate trihydrate. (Do an online search) Review the MSDS and answer the following questions before starting this experiment. a. What is the chemical formula (molecular formula) for sodium acetate trihydrate? b. What are the potential acute health hazards of working with sodium acetate trihydrate? c. At what temperature ( C and F) will sodium acetate trihydrate auto-ignite? d. What is the melting temperature for sodium acetate trihydrate? e. What are the handling requirements for sodium acetate trihydrate? f. What PPE (personal protective equipment) should one wear while doing this activity? g. What personal precaution should one take while doing this activity? 2. Turn on the hot plate to high. 3. Using the scoop and the weigh boat, weigh out 50 grams of sodium acetate trihydrate. Transfer to the 200 ml beaker. 4. Place the beaker with sodium acetate trihydrate on the hot plate. 5. Using the stainless steel chemistry scoop, stir the crystals frequently to distribute the heat evenly. The sodium acetate should start to dissolve. Sodium acetate trihydrate dissolves at 58 C. 6. As the sodium acetate trihydrate dissolves, crystals start to form on the sides of the beaker. Fill the pipette with 2 ml of distilled water and rinse off the crystals as they appear. At this point you can reduce the temperature of the hot plate to a medium high. 7. If the liquid starts to cloud or crystallize, add more liquid, 1 ml at a time, until the liquid clears. 8. While the sodium acetate trihydrate is dissolving, prepare the cooling bath by adding cold tap water to the glass casserole dish, about 2 inches deep in the dish. 9. Once the crystals have dissolved into a clear liquid, check the solution to ensure that it is completely dissolved by stirring the liquid with the SS scoop. If any crystals form on the scoop, then the sodium acetate trihydrate is NOT completely dissolved. Continue to heat. The sodium acetate trihydrate is completely dissolved when no crystals form on the SS scoop. 10. Once completely dissolved (no clouding or crystal formations), use the beaker tongs to remove the beaker from the hot plate and place in the cooling bath. Cover the beaker with a watch glass to prevent premature crystallization of the sodium acetate trihydrate. 11. Feel the sides of the beaker. When it feels cool to the touch and below room temperature, check the temperature with the thermometer. WARNING: Before checking the temperature, thoroughly clean the tip and surfaces of the thermometer to prevent premature crystallization. Any dirt or debris will become a nucleation site for crystal formation. 12. Once the solution reaches 15 C, extract a seed crystal (the size of a dust particle if possible) from the bag of sodium acetate trihydrate. Perform one of the following: a. Drop the dust size crystal of sodium acetate trihydrate into the beaker. This seed crystal initiates crystallization. You should see a crystal forming in all directions starting at the seed. (Suggestion: Take a video or pictures of the crystal formation.) Southwest Center for Microsystems Education (SCME) Page 5 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

46 b. Place the dust size seed crystal on the watch plate. Slowly pour out the dissolved sodium acetate trihydrate into the watch glass on top of the seed. As you pour, the sodium acetate trihydrate should crystallize into a stalagmite formation. (Suggestion: Take a video or pictures of the crystal formation.) Note: You can reuse crystallized sodium acetate trihydrate. Return the beaker to the hot plate and slowing reheat, adding distilled water if needed. You can continue to reuse the solution until it becomes contaminated. Post-Activity Questions 1. Which procedure did you perform (12a or 12b)? a. Explain exactly what you saw. b. In your own words, describe the crystallization process that you observed. 2. Why does sodium acetate trihydrate work for this experiment? 3. What are the melting point and the freezing point of sodium acetate trihydrate? 4. What is supercooling? 5. What is the purpose of the seed crystal? 6. What are two applications for sodium acetate? Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 6 of 6 Fab_Crystl_AC14_PG_April2017.docx Growing Crystals: Hot Ice AC

47 Breaking Wafers Activity The Crystallography Learning Module Participant Guide Description and Estimated Time to Complete The purpose of this learning module is to introduce the science of crystallography and its importance to microtechnology. Activities provide additional exploration into crystallography and its applications. In this activity you will further explore the crystal planes of silicon by breaking two silicon wafers. By the end of this activity, you should be able to tell from a piece of silicon the specific crystal orientation of the silicon crystal. Estimated Time to Complete Allow at least 15 minutes to complete this activity. Introduction MEMS (microelectromechanical systems) are fabricated using monocrystalline silicon wafers. The wafers are cut from a silicon ingot that is formed by melting chunks of polycrystalline solids in a large crucible. Once melted a seed crystal is placed in the liquid silicon to stimulate crystal growth for a specific crystal orientation. Over several hours a long ingot of pure monocrystalline silicon is slowly pulled from the melt. Below are the steps for growing this monocrystalline ingot. 1. First we start with very pure polycrystalline silicon material ( % pure!) 2. The pure silicon is melted in a crucible at 1425 C. (This molten silicon is called the melt.) 3. A seed crystal is precisely oriented and mounted on a rod then lowered into the melt (left image). Silicon atoms in the melt align to the same crystal orientation of the seed. 4. As the seed crystal is slowly pulled out of the melt, the seed and the crucible are rotated in opposite directions A large crystal ingot or boule is formed by controlling the temperature gradient of the melt, the speed of rotation, and the rate of the pull of the rod. Southwest Center for Microsystems Education (SCME) Page 1 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

48 The slower the "pull", the larger the diameter of the crystal ingot that forms. (This process can take several days to complete and is called the Czochralski (CZ) Method of growing silicon.) The seed crystal acts as a nucleation site for the alignment of the atoms in the molten silicon. The alignment of the seed crystal relative to the melt determines the orientation of the subsequently grown silicon crystal ingot. The wafers cut from this crystal maintain this orientation. The resulting ingot is cylindrical in shape, 25.4 mm (~1 inch) to 450 mm (~18 inches) in diameter and several meters long. Once cooled, the ingot is ground to a perfect cylinder. The cylinder is sliced into thin wafers using diamond coated wires or saw blades. Each slice is polished to create silicon wafers, also referred to as substrates. Microsystems are constructed on or within these substrates depending upon the type of process used surface or bulk, respectively. Southwest Center for Microsystems Education (SCME) Page 2 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

49 To determine the orientation of a silicon crystal wafer, crystallographers use x-rays aimed at a tiny piece of the wafer containing trillions of identical atoms. The specific periodic arrangement of the atoms within the crystal diffracts the x-rays onto an electronic detector or film. The resulting diffraction pattern on the film or detector gives the crystallographer the information needed to determine the actual orientation of the tiny seed crystal and the spacing of the atoms. A computer reconstructs the orientation from the diffraction pattern. The images below show the resulting patterns of three planes of a silicon crystal. Indicate which image represents each of the following planes. (Think about the spacing of atoms and the number of atoms in different silicon planes.) a. (111) b. (100) c. (110) [Images printed with permission and from the personal collection of Christopher C. Jones 1 ] What characteristics helped you to identify the correct orientation of these planes? An easier way to determine the crystal orientation of a silicon wafer is to just break it. So let's do that in this activity. Southwest Center for Microsystems Education (SCME) Page 3 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

50 Activity Objectives and Outcomes Activity Objective State the crystal orientation of a silicon wafer by breaking the wafer into smaller pieces and observing the resulting shape. Activity Outcome By the end of this activity you should be able to look at a piece of a silicon wafer and state its crystal orientation: (100) or (111). Resources SCME Crystallography Overview for MEMS PK Supplies / Equipment Supplies provided by instructor Safety glasses or goggles Ice Pick or pointed metal implement (e.g., a Philips screwdriver, a large nail) Hammer (For tapping the end of the metal implement) Two large sheets of paper or poster paper Supplies included in kit Two silicon wafers of (100) orientation Two silicon wafers of (111) orientation Wafer holders and packing 1 Crystallography Learning Module Instructor Guide 1 Crystallography Learning Module Participant Guide Documentation Answers to the Post-Activity Questions Southwest Center for Microsystems Education (SCME) Page 4 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

51 Activity: Breaking Wafers Procedure: 1. Place two pieces of paper or poster paper side-by-side on the table top. 2. Remove one wafer from each of the wafer holders and place them side-by-side on the two pieces of paper. 3. Place another piece of paper over one of the wafers. (This is to minimize wafer shards from flying off the table.) 4. Put on your safety glasses. 5. Place the tip of the ice pick or screw driver close to the center of one of the wafers. 6. With the hammer or your hand, gently, but firmly, tap the handle of the ice pick until you hear the wafer break (snap). 7. Repeat steps 4 and 5 with the second wafer. 8. You will see that that wafers break at either right (90 ) angles or at approximately 60 angles. a. What is the orientation of the wafer surface plane that breaks at 90 angles: (100) or (111)? b. What is the orientation of the wafer that breaks at 60 angles: (100) or (111)? 9. What would be the result of breaking one of the pieces of each wafer? 10. Test out your hypothesis. Break one of the pieces of each wafer. 11. Answer the Post-Activity Questions Post-Activity Questions 1. In this activity, you broke two silicon wafers. One wafer had a (100) crystal surface plane orientation and the other a (111) surface plane orientation. In the making of the original silicon ingot, what determined the crystal orientation of the silicon wafer? 2. At what approximate angle did the (111) wafer break? 3. At what approximate angle did the (100) wafer break? 4. Did each wafer continue to break at the same angle when you broke the smaller pieces? Why or why not? 5. Which orientation (100) or (111) has more silicon atoms exposed to the wafer's surface? 6. Why is crystal orientation important in the fabrication of microsystems? Southwest Center for Microsystems Education (SCME) Page 5 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

52 7. Identify the crystal orientation of each of the following pieces of silicon. Summary The most commonly used orientation for MEMS fabrication is the (100) and less frequently the (111). These crystal orientation determines the electrical and mechanical properties for components of electromechanical systems. An example of when crystal orientation is very important is in the anisotropic etching of crystalline silicon. For example, KOH (potassium hydroxide) is used to etch crystalline silicon; the (111) plane etches at about µm / minute while the (100) plane etches at 1.4µm/minute, about 500 times faster! The picture shows the surface of this wafer as the (100) plane and the results of a KOH backside etch that etched along the (111) plane. References 1. "Crystal Symmetry". Scientific Photographs by Christopher C. Jones Images of wafers, broken wafers and etched wafers courtesy of the Manufacturing Technology Training Center (MTTC) at the University of New Mexico. Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 6 of 6 Fab_Crystl_AC10_PG_April2017.docx Breaking Wafers Activity

53 An Origami Crystal Activity Participant Guide Description and Estimated Time to Complete In this activity you will use a template to construct a representation of a silicon crystal. The final structure will actually be a rhombicuboctahedron, one of 13 Archimedean solids or a convex polyhedral. Certain faces of the template are marked with specific plane notations from Miller Index. Once the polyhedral is constructed, the markings will illustrate the crystal plane of each face of the polyhedral. You will use the Japanese art of origami to construct this solid from a template. Estimated Time to Complete Allow at least one hour to complete this activity. Southwest Center for Microsystems Education (SCME) Page 1 of 5 Fab_Crystl_AC12_PG_April2017.docx Origami Crystal Activity

54 Introduction Crystals are defined by a regular, well-ordered atomic lattice structure. A lattice consists of stacked planes of atoms. The bonds between the atoms are typically very strong. Silicon crystal is widely used in micro and nanotechnologies. The orientation of the silicon crystal denotes which crystal plane is exposed on the wafer surface. (Refer to the graphic below in the following discussion). The left most image shows the silicon crystal structure, also known as face-centered cubic or diamond cubic. It is the same structure carbon forms in a diamond. The middle images highlight two different planes within this structure. Think of looking at the same crystal from two different directions. The images on the right are what you would see looking at the face of each plane. Same crystal, same distance between unit cells, and same orientation of unit cells. However, looking at different planes, presents a different picture. Silicon Crystal Planes [Graphic courtesy of Khalil Najafi, University of Michigan] The Miller index is a roadmap or directional compass for identifying the crystal planes and directions within crystals. Miller indices are three digit notations that indicate planes and direction within a crystal. These notations are based on the Cartesian coordinate system of x, y, and z. The Cartesian coordinate system is illustrated using the three vectors (axes) x, y, and z. Referring to the graphic Cartesian Coordinates, the x-axis vector direction is denoted [1,0,0] y-axis vector direction is denoted [0,1,0] z-axis vector direction is denoted [0,0,1] (Think of the "1" as being "1 unit" out from the origin or 0,0,0.) Cartesian Coordinates Southwest Center for Microsystems Education (SCME) Page 2 of 5 Fab_Crystl_AC12_PG_April2017.docx Origami Crystal Activity

55 Crystal planes, each perpendicular to its respective vector (or axis) Crystal planes are perpendicular to their corresponding axis. For example, the plane perpendicular to the [1,0,0] vector or x-axis is the (1,0,0) plane (shown in the figure). Each crystal plane has a unique notation. (1,0,0) or (100) is perpendicular to the x-axis (0,1,0) or (010) is perpendicular to the y-axis (0,0,1) or (001) is perpendicular to the z-axis Crystal orientations (100) and (111) are commonly used for microsystems fabrication. Activity Objectives and Outcomes Activity Objectives Using the Japanese art of origami, folding paper into objects, you will construct a rhombicuboctahedron that represents a silicon crystal. Activity Outcomes The final product must have clean edges and flat faces. When you work in a small startup company or even a large research lab, many tasks are performed by hand. Attention to detail as well as fine motor skills is a premium! Your outcome should show that you planned your strategy before tackling this task. Take your time and build a quality product! Try not to have excessive tape or glue showing on the finished work. Once constructed you will be asked to interpret the information on at least three of the origami faces. Southwest Center for Microsystems Education (SCME) Page 3 of 5 Fab_Crystl_AC12_PG_April2017.docx Origami Crystal Activity

56 Resources Template by Jack Judy, Associate Professor, Electrical Engineering, UCLA Supplies 12 origami templates printed on cardstock are contained in the SCME Crystallography kit. If you do not have the kit, you can go to for the template. A pair of thin scissors, Exacto knife, or razor blade Documentation The silicon crystal cube Post Activity Questions with answers Activity: An Origami Crystal NOTE: For simplification purposes, we will refer to the final outcome as a "cube" rather than a rhombicuboctahedron or polyhedral. The image to the right shows the constructed origami crystal. Procedure: 1. Watch the video "Origami Crystal animation". ( ) 2. Carefully cut out the template provided. Be sure to cut along the thin lines. 3. Fold the tabs and carefully construct the cube. You may have to try several times in order to get it to fit together correctly. 4. Use a small amount of glue or double sided tape to hold your cube together. 5. If you make a mistake and need a new template, be sure to print the template on heavy paper or cardstock. 6. Answer the Post-Activity Questions. Southwest Center for Microsystems Education (SCME) Page 4 of 5 Fab_Crystl_AC12_PG_April2017.docx Origami Crystal Activity

57 Post-Activity Questions 1. How do the faces of this cube relate to a silicon crystal? 2. What do the "folds" represent relative to a silicon crystal? 3. What is the difference between (010) and (010)? 4. Why is crystal orientation important in the fabrication of microdevices? 5. Look carefully at the final cube. What can you say about the importance of crystal orientation related to following: a. Etch rates on the exposed planes? b. Oxidation growth rates on the exposed planes? Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 5 of 5 Fab_Crystl_AC12_PG_April2017.docx Origami Crystal Activity

58 Crystallography Overview for MEMS Final Assessment Participant Guide Introduction The purpose of this assessment is to determine your understanding of crystallography, silicon crystal structure, and Miller Indices as it relates to microsystems technology and fabrication. There are 15 questions 1. Which of the following BEST defines Crystallography? The science of a. developing arrangements for atoms in solid matter. b. determining the arrangement of atoms in solid matter. c. studying the properties of atoms in solid crystal. d. developing new crystals using atomic structures. 2. Matter without a regular arrangement of atoms is called a. Amorphous b. Chaotic c. Polycrystalline d. Crystalline 3. In a crystal, the simplest repeating section of atoms is called the a. Single crystal b. Poly crystal c. Unit cell d. Crystal seed 4. Polycrystalline solids consist of small crystals called, that are separated by. a. Unit cells, cell boundaries b. Amorphous solids, amorphous boundaries c. Single crystals, crystal boundaries d. Grains, grain boundaries 5. What type of solid is peanut brittle? a. Amorphous b. Chaotic c. Polycrystalline d. Crystalline Southwest Center for Microsystems Education (SCME) Page 1 of 4 Fab_Crystl_FA10_PG_mc_April2017.docx Assessment

59 6. Which of the following is NOT a characteristic of monocrystalline silicon? a. Longer range order compared to polycrystalline silicon b. Well-ordered silicon atoms arranged in a lattice structure c. Many single crystalline solids held together by ionic bonds 7. The material properties of a silicon wafer are determined by surface atoms and the of the silicon wafer. a. crystal orientation b. doping concentration c. long range order d. bandgap 8. What is the roadmap or compass called for identifying the crystal planes of single crystals? a. X-ray diffraction b. Crystallography c. Miller Index d. Cartography 9. Which of the following BEST describes why crystalline silicon is used for microsystems fabrication? a. Readily abundant and can be easily formed into polycrystalline ingots and cut into wafers b. Its relatively short range order, strength, and unique ability to be etched along grain boundaries c. Its semiconductor properties that allow it to act as an insulator or a conductor depending on design d. Its unique electrical and mechanical properties that make it possible to form specific well-defined structures 10. There are different configurations for unit cells with each configuration having different number of atoms. A face centered cubic structure has how many atoms? a. 1 b. 4 c. 7 d. 15 Southwest Center for Microsystems Education (SCME) Page 2 of 4 Fab_Crystl_FA10_PG_mc_April2017.docx Assessment

60 11. A silicon atom has valence electrons that are shared with other silicon atoms when forming a crystal. a. 8, 8 b. 6, 2 c. 4, 4 d. 2, What is the Miller Index notation for the yellow plane in this diagram? a. (100) b. (010) Z c. (001) d. (110) e. (111) Y X 13. The following diagram represents different in the same crystal structure. a. Grains b. Unit cells c. Growth structures d. Crystal planes Southwest Center for Microsystems Education (SCME) Page 3 of 4 Fab_Crystl_FA10_PG_mc_April2017.docx Assessment

61 14. In the manufacture of silicon wafers, the crystal orientation of the wafers is determined by the a. purity of the silicon b. orientation of the seed crystal c. type of seed crystal d. pull rate of the ingot 15. The bulk modulus of a microsystems structure is determined by the properties of the material in which they are constructed. a. dimensional b. electrical c. mechanical d. optical Support for this work was provided by the National Science Foundation's Advanced Technological Education (ATE) Program through Grants. For more learning modules related to microtechnology, visit the SCME website ( Southwest Center for Microsystems Education (SCME) Page 4 of 4 Fab_Crystl_FA10_PG_mc_April2017.docx Assessment

62 Southwest Center for Microsystems Education (SCME) Learning Modules available for scme-nm.org MEMS Introductory Topics MEMS Fabrication MEMS History MEMS: Making Micro Machines DVD and LM (Kit) Units of Weights and Measures A Comparison of Scale: Macro, Micro, and Nano Introduction to Transducers Introduction to Sensors Introduction to Actuators Problem Solving A Systematic Approach Crystallography for Microsystems (Crystallography Kit) Deposition Overview Microsystems (Science of Thin Films Kit) Photolithography Overview for Microsystems Etch Overview for Microsystems (Bulk Micromachining An Etch Process Kit) MEMS Micromachining Overview LIGA Micromachining Simulation Activities (LIGA Micromachining Lithography & Electroplating Kit) Manufacturing Technology Training Center Pressure Sensor Process (Three Activity Kits) A Systematic Approach to Problem Solving Introduction to Statistical Process Control Learning Microsystems Through Problem Solving Activity and related kit MEMS Applications MEMS Applications Overview Microcantilevers (Microcantilever Model Kit) Atomic Force Microscope Overview Micro Pressure Sensors and The Wheatstone Bridge (Modeling A Micro Pressure Sensor Kit) Micropumps Overview BioMEMS BioMEMS Overview BioMEMS Applications Overview DNA Overview DNA to Protein Overview Cells The Building Blocks of Life Biomolecular Applications for biomems BioMEMS Therapeutics Overview BioMEMS Diagnostics Overview Clinical Laboratory Techniques and MEMS MEMS for Environmental and Bioterrorism Applications Regulations of biomems DNA Microarrays (DNA Microarray Model Kit available) Microtechnology of Pacemakers Revised January 2017 Nanotechnology Nanotechnology: The World Beyond Micro (Supports the film of the same name by Silicon Run Productions) Safety Hazardous Materials Material Safety Data Sheets Interpreting Chemical Labels / NFPA Chemical Lab Safety Personal Protective Equipment (PPE) Check our website regularly for the most recent versions of our Learning Modules. For more information about SCME and its Learning Modules and kits, visit our website scme-nm.org or contact Dr. Matthias Pleil at mpleil@unm.edu