Anodic Bonding And The Crack Opening Method Experiment. Individual Study ECE 199 Spring Quarter Advisors: Dr. Richard D.

Transcription

1 Anodic Bonding And The Crack Opening Method Experiment Individual Study ECE 199 Spring Quarter 2000 Advisors: Dr. Richard D. Nelson Allen Kine

2 Research Students: Cop Van Le # Jun Kue Lee # Desired Units: 4.0 (No Design Units) Submitted: June 16, 2000

3 This is the fixture for the crack-opening method of the anodic bond between the silicon and the glass. As seen in this picture, the glass is much thicker than the silicon. The razor blade is used to cut through the anodic bond to measure the crack formed at the bond line. Using the equation relating the crack length to the bond strength, our project involved comparing the bond strength of several different types of bonded materials. Table of Contents Cover Page Table Of Contents List Of Illustrations

4 I. Introduction II. History On The Anodic Bonding III. Silicon Wafer a. Pure Silicon b. Silicon Crystal IV. Anodic Bonding a. How To Make The Bond b. How Silicon Wafer And Glass Get Bonded c. How to Measure Anodic Bonding Crack Length And Bond Strength V. How To Set Up The Fixture VI. Other Issues On The Crack Opening Method a. The Defect In Silicon-Glass Bonding b. The Mechanical Problem Of Stabilizing The Blocks To Hold The Blade And Bonded Sample Fixed c. The Blade Problem VII. About The Mask Design VIII. Conclusion Work Cited Appendix

5 List of Illustrations Figure 1: The Czochralski method for growing silicon crystal Figure 2: Silicon Wafer Figure 3: Schematic drawing of the typical apparatus used in the anodic bonding Figure 4: Schematic showing the joining of silicon wafer and Pyrex in anodic bonding Figure 5: Schematic of the crack technique Figure 6: Schematic of the first design of the crack experiment Figure 7: Schematic of the second design of the crack experiment Figure 8: In this fixture, the blade is on the right side, attached to a metal block, which is connected to a lead-screw device. The sample is placed on the block that moves horizontally, connected to another lead-screw device perpendicular to the other one Figure 9: There were spots of particles in the bond surface, surrounded by the rings, as discussed before. However, it was very difficult to take photos of these details. It is recommended to view the samples under the microscope in person Figure 10: As seen in this picture, the bonded sample is actually taped

6 to the block. We used scotch tapes to hold it in place, so we were not very happy with its stability Figure 11: Typical view of the razor blade, under the microscope. This picture was taken using a digital camera, focusing on the view from the eyepiece of the microscope Figure 12: As shown in this picture, there bonded sample has many long grooves across the bond. In this picture, it is hard to see any defects in it, however they are detected under the microscope...20 INTRODUCTION There are many different ways to bond silicon wafers with different materials. One of the popular ways to bond silicon wafers with other materials is anodic bonding. The anodic bonding process has a variety of commercial applications including pressure sensors, photo-voltaics, and micro-electronic device packaging. It has been used to bond many different materials such as ceramics with metal, ceramics with semiconductors, ceramics with ceramics, and semiconductors with semiconductor pairs. For this project, we are trying to understand the anodic bonding of silicon wafer with glass. We are using borosilicate glass or Pyrex to bond with silicon wafer. For this anodic bond we will try to measure its bonding force with the crack technique. HISTORY ON THE ANODIC BONDING Many people have thought about bonding of silicon wafer with other materials about a quarter of century a go. The procedures that are involved in the bonding have very high temperature and pressure. It was estimated that the temperature was 1225 degree Celcius, and the pressure was 2000 psi. These high in temperature and high in pressure are hard to achieve. Therefore, an alternative approach was discovered. Wallis and Pomerantz (1) introduced a technique for bonding of silicon wafer fused quartz. This idea was passed on to Frye et al. (2) who used it to investigate for the bonding of silicon-on-silicon (SOI) applications. This new technique is working on low temperature, and some electric field. The temperature is lowered in the range from 850 to 950 degree Celcius and an externally applied electric field that is in the range from 20 to 50 volts. The external electric field replaced the hydrostatic pressure which providing the pulling force needed to achieve intimate contact of the wafer surfaces. The result of the second process shows many advantages over the first techniques. The bonding with high temperature and an applied electric field, making the process incompatible with batch processing and prone to

7 contamination (4). This technique is called anodic bonding. SILICON WAFER Silicon wafers come from silicon-growth crystals. However, pure silicon does not exist in nature. It is believed that silicon (Si) is the second most abundant element in the earth s crust and a component in numerous compounds (4). Usually, silicon is found together with silica (impure SiO2) and the silicates (Si + O + another element). Since it is never found to be a single pure element in nature, pure silicon is a man-made material. Pure Silicon To obtain pure silicon from its compound, numerous processes are involved. First, there must be a silicon compound. This silicon compound is then put into a purifying and separation process. The silicon compound is placed into an electric furnace and is heated with carbon. At a certain point, when the temperature rises high enough, the silicon compound will melt. When the silicon melts and becomes liquid, the carbon will pull the oxygen away from the impure SiO2. This will reduce the level of SiO2 in the compound, which means that the silicon compound has more silicon element than O2, and it can be assumed that the compound is impure silicon. The impure silicon is then chlorinated to yield SiCl4 or SiHCl3. Now the SiCl4 or SiHCl3 is in a liquid state at room temperature. Then, this SiCl4 or SiHCl3 is chemically treated in a reaction that yields pure silicon. This chemical reaction occurs through the heating of SiCl4 or SiHCl3 in a hydrogen atmosphere. For example, for the SiCl4, the reaction process can be described as [ SiCl4 + 2H2 4HCl + Si ] (4). Figure 1. The Czochralski method for growing silicon crystal

8 Silicon Crystal To obtain a silicon crystal, pure silicon is cooked by using a method known as the Czochralski. Figure 1 shows the Czochralski method for growing silicon crystal. Through the Czochralski method, the pure silicon is placed in a quartz crucible and heated in an inert atmosphere to melt the silicon. A small silicon crystal has already been prepared to be used as a tool to pull out the melted silicon. The small silicon crystal is called a silicon seed crystal. It is clamped to a metal rod and then dipped into the melting silicon. Once the silicon seed crystal comes in contact with the melted silicon, its thermal temperature will increase. At the same time, the temperature of the melted silicon that is closed to the silicon seed crystal will decrease. Eventually, the two different temperatures at the contact will reach equilibrium. At this point the melted silicon begins to freeze and stick onto the silicon seed crystal. This freeze silicon can be viewed as part of the extension of the silicon seed crystal. Then the silicon seed crystal is allowed to rotate slowly. While it is rotating, it is also being pulling upward from the melted silicon. As the silicon seed crystal is pulling out, it allows more melt silicon to freeze from the bottom of the silicon seed crystal. The freeze silicon, which is an extension of the silicon seed crystal, is the silicon crystal that we want, and it is also known as an ingot. It is in a cylindrical shape and about 200mm in diameter and one to two meters in length. This ingot or silicon crystal is then cut into thin pieces, circle shapes, which are known as silicon wafers. Figure 2 shows example of silicon wafers. Figure 2. Silicon wafer

9 ANODIC BONDING One of the techniques that is used to hold two pieces of solid materials together is called anodic bonding. In anodic bonding, the process relies on charge migration to produce bonded wafers. This usually works well for element with high alkali metal content. Silicon wafer is one of the main materials that is used in the bonding process. Glass is another material that has high in alkali metal, which can be good to be used for the bonding process. Therefore, people often use anodic bonding process to bond silicon to glass. When these materials are bonded, they can be used to measure their bond strength. To measure bond strength of these bonded materials, another special technique is involved. The glass that is used for the anodic bonding is a special man made glass also known as Pyrex borosilicate glass. This type of glass has a sodium oxide (Na2O) content of 3.5 percent. The reason that the Pyrex borosilicate glass is picked over other substances to bond with silicon is that it has a thermal expansion coefficient closely matches that of silicon. Other advantages of glass to silicon bonding are the benefits of the electrical, thermal and optical properties of the glass. Many experiments show that the glass provides dielectric isolation properties that significantly reduce parasitic capacitance in capacitance transducers. In manufacturing, the optical transparency of glass is taken advantage to enable backside visual inspection of microstructures (5). How To Make The Bond To bond silicon wafer and glass in an anodic bonding process, it involves high temperature, and high apply voltage, and of course, some equipment. The temperature in the bonding is in the range from 200 to 500 degree Celsius and the voltage is in the range from 500 to 1000 volts. Figure 3 shows a schematic drawing of the typical apparatus that is used in the anodic bonding of silicon wafer and glass. The apparatus consists of a chamber and two electrodes. Figure 3. Schematic drawing of the typical apparatus used in the anodic bonding

10 The hot plate of the chamber serves as an anode. The aluminum block serves as a cathode. When the silicon wafer and glass are ready to be bonded, they should be placed between the two electrodes. They should be placed in such a way that the silicon wafer is electrically connected to the anode and the glass wafer to the cathode. Usually, the glass wafer is on the top and the silicon wafer is at the bottom. With many experimental results, they show that the critical process parameters for the anodic bonding are the magnitude and duration of the applied potential, the temperature of the wafers during the bonding cycle, and the area to be bonded. The voltage that is applied in the bonding process is depending on the thickness of the glass and the controlling temperature. Often, the operating temperatures are near the glass-softening point, but should be below its melting point. The bonding process is usually completed in five to ten minutes. How Silicon Wafer And Glass Get Bonded To understand how silicon wafer and glass stick together in an anodic bonding process, we must know the element that make up silicon and the glass that are used in the bonding. Through experiments were showed that the elements that make up the glass to be bonded (Pyrex borosilicate glass or Pyrex) has sodium oxide (Na2O). It was found that there is a content of 3.5 percent of sodium oxide in the Pyrex. When the silicon wafer and the Pyrex are put together and placed in the anodic bonding chamber, heat is then added. At a certain temperature the Pyrex is hot enough and becomes soften. Since the Pyrex is softened, an applied voltage produces an electric field between the silicon wafer and the Pyrex. The electric field exists because the applied voltage makes the presence of the mobile metal ions to exploit to the high negative voltage of the Pyrex. The high negative voltage pulls most of the positive metal sodium ions (Na+) to the top, attracting them, and neutralized them. As the result of the positive ions moved away to the cathode, they leave behind permanent negative ions. These permanent negative ions then form a depletion region between the silicon wafer and the Pyrex themselves. The depletion region is a space charge region between the silicon and the Pyrex. This depletion region gives rise to a large electric field, which is between the silicon wafer and the Pyrex. As the result of the electric field, the silicon wafer and the Pyrex are pulled into contact with one another. The strong electrostatic attraction between the silicon wafer and glass wafers, fixing them firmly in place. The mobility of these positive ions are then further enhanced by increasing the bonding temperatures up to 500 degree Celsius. In addition, the electric field makes oxygen from the glass to transport to the glass-silicon interface where it combines with silicon to form SiO2, creating the permanent bond that bond the silicon and glass together. Figure 4 shows the schematic joining of the anodic bonding of silicon wafer with Pyrex.

11 Figure 4. Schematic showing the joining of silicon wafer and Pyrex in anodic bonding How To Measure Anodic Bonding Crack Length And Bond Strength When the silicon wafer and the Pyrex are bonded, the sample can be used to measure its bonding strength. There are many techniques that have been used to measure anodic bonding strength. One of the techniques that is often used is known as the crack technique introduced by Maszara (6). In this crack technique, a blade is inserted between the sample to make the crack. This technique is very simple and easy to apply. The strength of an anodic bonding is given in term of the surface energy that keeps the two materials, silicon wafer and Pyrex, stick together. This surface energy is defined by Gilman as γ = (3/32)(E)(d^3)(y^2)/(L^4) [1] In Equation 1, γ is the surface energy that defined in term of the parameters E, d, y, and L. Where E is known as the Young s modulus number. The parameter d is the thickness of a wafer, and y is the thickness of a blade. A crack is generated and spreads until a crack length L is reached which reflects the balance between elastic and surface energies (4). Figure 5 shows a schematic of the crack technique generated by a blade for measuring the anodic bonding strength.

12 Figure 5. Schematic of the crack technique HOW TO SET UP THE FIXTURE To set up the fixture for testing the bonding strength, we come up with a sketch first. We draw the design that we want and try to improve it along the way. In our first design, the bonded sample is placed on top of a big block, with the glass side up. There are additional blocks attached around the sample to hold the sample in position. Then, we place the blade horizontally parallel to the bond line, and we use a hammer to hit the blade, to force it to enter the bond line. After several hits on the blade, we expect to see a crack developed as a result. The glass side would bend upward as a result of bond breakup, but the silicon side would stay because it is supported by a block from the below. The figure 6 shows how this first design looks like.

13 Figure 6. Schematic of the first design of the crack experiment However, we decided that using the hammer to hit the blade to force it in the bond length is not very reliable, because it would be impossible to determine how much force is exerted for each hammer hit, and the amount of length that razor enters the bond. Also, we needed to use a clamp to hold the bonded sample in position during the bond cutting. Figure 7. Schematic of the second design of the crack experiment In figure 7, we replaced the hammer with a screw attached to a clamp, and holding a blade on the other side. We used another clamp, holding the bonded sample and the block underneath together. The main advantage of this design is that we can determine the amount of length that the razor moves toward the bond line. This is done by measuring the length of the spacing between the threads of the screw. Then, divide that length (typically 1/100 of inch) from 1 inch, to get a ratio of 100 threads per inch. It is

14 more desirable to get threads with higher ratio of number of threads per inch. Later, we came up with some more modification of our design, mainly allowing the clamp to move horizontally, instead of being attached stationary to a block. This is done to allow some mobility for the blade, enabling it to move deeper into the bond length. We used a device called a lead-screw to realize this new feature, and the mechanical engineering department (the wind tunnel lab) has been very supportive in assisting us with the design. The figure 8 shows our final design for the fixture. Figure 8. In this fixture, the blade is on the right side, attached to a metal block, which is connected to a lead-screw device. The sample is placed on the block that moves horizontally, connected to another lead-screw device perpendicular to the other one.

15 OTHER ISSUES ON THE CRACK OPENING METHOD From the figures 6 and 7, as well as the discussion so far on the crack opening method, it is easy to assume that the crack opening method would be easy and with good results. However, we were faced with some new problems when we actually started the crack opening method. These problems ranged from the defect in silicon-glass bonding, mechanical problem of stabilizing the blocks to hold the blade and bonded sample fixed, as well as the fact that none of us had previous experience with this experiment outcome. (As it turns out, the anodic bonding was too strong to be cut simply with the razor blade.) The Defect in Silicon-Glass Bonding When the bonding is made in the clean room laboratory, one would expect (or hope) that all the bonding are made perfect, and there is no defect in the bonding. However, we learned that our bonded samples have some defects, mainly due to the foreign particles in the clean room that gets trapped in the bonding process. These particles get in the way of bonding, and they prevent the bonding from becoming complete. Around these particles, a ring of air space is created, and we can see these rings under the microscope. For our experiment, such rings are not considered to be extremely important, because there are not many of them. However, these rings would contribute to decrease in the overall bond strength of the sample. Figure 9. There were spots of particles in the bond surface, surrounded by the rings, as discussed before. However, it was very difficult to take photos of these details. It is recommended to view the samples under the microscope in person. The Mechanical Problem of Stabilizing the Blocks to Hold the Blade and Bonded Sample Fixed The blocks that hold the razor blade (or the attached clamp), and the bonded sample are attached to the main base block with screws. And, to hold the sample attached to a block, we even had to use

16 scotch tapes. (because drilling a hole on a sample to hold it to the block would not be a good idea) Thus, all the surrounding components were not fully stable and fixed to a position, often swaying sideways or along the bond line, during the bond cutting. When, these components move slightly, it prevents us from accurately measuring the amount of length that the razor blade enters the bond line. (our main goal) Figure 10. As seen in this picture, the bonded sample is actually taped to the block. We used scotch tapes to hold it in place, so we were not very happy with its stability. Worse, as the clamps were not fully fixed, the razor blade would often bend instead of cutting through the bond line. At other instances, the razor blade would shave off the part of silicon, but it would still not be able to cut through the bond line. We realized that the anodic bond that was created between the samples was too strong to be cut be cut simply with the razor blade. Also, after several unsuccessful attempts on the bond cutting, the sample would be ruined and we had to replace it with a new bonded sample. After several more failures, we felt frustrated, so we even resorted back to the hammer banging plan to hit the razor blade to force it in the bond line. However, even this attempt failed, proving that the anodic bonding is too strong for our experiment design. The Blade Problem Ideally, one would like to have a blade that is shaped like a perfect rectangle, which would enter the bond line uniformly, and make it easier to calculate the distance the razor blade travels into the bond line. However, the razor blade has a tip shaped like an isosceles triangle. So, the bond crack is created proportionally to the varying width of the thickness of the razor blade, at least initially. We were interested in the angle formed by the sides at the tip of the razor blade, so we examined it under the microscope. The figure 11 is a typical picture of the blade tip.

17 Figure 11. Typical view of the razor blade, under the microscope. This picture was taken using a digital camera, focusing on the view from the eyepiece of the microscope. In order to measure the angle at the tip of the blade, there are several steps involved. First, measure the thickness (which is the base of the isosceles triangle) of an actual blade. This is best done using a micrometer, preferably with an accuracy of inch or so. Second, view the razor blade tip under the microscope, take a photo of the view, then measure then calculate the ratio of isosceles triangle height to the base. Third, multiply this ratio by the thickness of an actual blade. This is the height of the actual blade, just looking at the triangle region. Finally, use the mathematical equation: tan (θ/2)=(w/2)/h, where w and h are the dimensions of an actual razor blade. The angle θ is defined between two sides of the isosceles triangle. The main reason why we would like to measure the angle is as follows: suppose we are able to push the razor blade deep enough into the bond line, then we know exactly what the width of the bond crack formed at the bond line. It is the width of the razor blade. However, mostly likely, we would not be able to push the razor blade deep enough. So, if the razor blade is only half way inside the bond line. Then, we would use the angle θ, and the length of the razor blade pushed inside the bond, to calculate the width of the bond crack formed at the bond line.

18 ABOUT THE MASK DESIGN In this project, before the bonding takes place, we have to design a mask. The reason that the mask is made because we don t want air traps inside the bond area between the silicon wafer and the glass. Silicon wafer and glass have two different surfaces. The silicon wafer surface is smoother than the glass surface. The glass surface seems smooth to people naked eyes. However, when one views the glass under a microscope, its surface is very rough. Figure 12. As shown in this picture, there bonded sample has many long grooves across the bond. In this picture, it is hard to see any defects in it, however they are detected under the microscope. When the silicon and glass are bonded without the mask, the roughness of the glass will trap air between the bonded areas. This will weaken the bond strength. Therefore, the mask is needed to let all the air out of the bonded areas and helps made the bond strength stronger by increasing more contact areas. CONCLUSION At the beginning of our research project, the anodic bonding and its process has been thought of as mysterious and complicated. From our little background in the study of silicon and its chemical properties, we believed that the anodic bonding would be no different than a typical bonding between two metals. Moreover, we initially thought that the method of measuring the bond strength would be simple and predictable. However, as we progressed in developing the crack-opening method fixture, designing the mask for the silicon, and finally attempting to crack the bond using the razor blade and the lead-screw device, we faced some new challenges that we have not planned, such as the defects in silicon bonding, mechanical instability in the fixture, and finally the sheer force of anodic bonding. In the end, we were not even able

19 to crack the bonding, instead the razor blade would bend and it would only shave off the silicon side of the bonding. Of course, we also had received many supports from the mechanical engineering department (the wind tunnel lab), as well as Travis and Mr. Allen Kine. We realize that our experiment was not much of success in measuring the bond strengths, but we hope that our work this quarter will enable future students to improve upon our experiment and aid in the future research development of anodic bonding. Work Cited 1. Wallis and Pomerantz D.I., Journal Of Applied Physics, v40,3946 (1969). 2. Frye, Griffith, and Wong Y.H., Journal Of The Electrochemical Society. v133, 1673 (1986). 3. Maszara W. P. Silicon-On-Insulator: A Review. Journal Of The Electrochemical Society. Jan 1991: Martini, Steinkirchner, and Gosele U., The Crack Opening Method In Silicon Wafer Bonding-How Useful Is It? Journal Of The Electrochemical Society. Jan 1997: Maszara, Goetz, Caviglia, and McKitterick J.B., Journal Of Applied Physics, v31, 4943 (1988). Appendix: One of the final tasks in our experiment was to measure the razor blade angle at its tip. As discussed in the paper, the tip end of the blade forms an isosceles triangle, thus we can determine the height of the blade tip. (as discussed in p.19 ) The following page shows an example of such method. We determined that the angle formed between two equal sides of an isosceles triangle is 2θ = = 19.76

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