Solid State Sensors. Microfabrication 8/22/08 and 8/25/08

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1 Solid State Sensors Microfabrication 8/22/08 and 8/25/08

2 Purpose of This Material To introduce the student to microfabrication techniques as used to fabricate MEMS Sensors Understand concepts not specifics ELEC 5730/6730 Microelectronic Fabrication offered for more detail Basic concepts will be reinforced throughout the semester

3 MEMS Gear Drive Courtesy: Sandia

4 MEMS Characteristics Typically based around a silicon substrate Typically use batch fabrication processes borrowed from micro-electronics fabrication technology Sequential process: one device at a time Batch process: many devices at the same time Sometimes compatible with micro-electronics fabrication

5 Microfabrication Lab Safety Many dangerous chemicals and equipment used Safety is VERY important Chemical safety Acids severe burns Toxic liquids and gases Flammable liquids Explosive potential Equipment safety High voltage UV radiation High pressure High/low temperatures Glassware (sharp if broken) Proper clothing required Clean room suit Protective gloves Safety goggles

6 Wet Chemical Processing Vapors are pulled up and out of the lab through a ventilation hood Acids and solvents processed separately

7 MEMS Materials What are MEMS devices made out of?

8 Typical MEMS Materials Silicon MEMS Silicon substrate Polysilicon Silicon dioxide Metallization Non-metallic metallic layers Non-Silicon MEMS C compounds Ceramic Polymers PCB laminate Metals

9 Silicon Substrates Silicon is a hard, brittle semiconductor material Single crystal Si is grown in ingots, sawn into thin wafers and polished Most common substrate material for electronics Typical size used for MEMS: 100mm diameter and 500μm thick; but other sizes are available Diamond crystal structure: different crystalline planes (100), (110) and (111) have different properties

10 Silicon Characteristics Hard and Brittle Young s s modulus of ~160 GPA Stainless steel: 200 GPA Deforms elastically Does not deform plastically Good material for making micro springs and other structures

11 Si Wafers Can Also Be Injected with impurities to: Low or high resistivity N-type or P-typeP Process called Doping Etched (micromachined( micromachined) Chemically converted to glass (silicon dioxide) at its surface

12 Polysilicon Polysilicon is polycrystalline silicon Deposited on top of Si wafers, silicon dioxide layers, etc Typically deposited in a Low Pressure Chemical Vapor Deposition (LPCVD) chamber Thin films, typically 5µm Can be doped to make n-type n or p-typep Can also be micromachined LPCVD System

13 Silicon Dioxide Silicon Dioxide (SiO 2 ) easily grown on the silicon surface in a furnace using an oxygen torch and a hydrogen torch Oxidation A good electrical insulating material Typical thickness: 1μm Can be used as a insulator, structurally, as an etch mask and as an optical window Pure silicon dioxide substrates Oxidation and diffusion furnace

14 Silicon Based MEMS Materials: Metallization Thin metal layers deposited using vapor deposition techniques such as sputtering and electron beam deposition. Layers up a few μm m thick Variety of metals available, including aluminum, titanium, nickel and gold Thin or thick layers deposited by electro or electroless plating Layers exceeding 100μm m thick are possible Example metals that are available include copper, nickel and gold

15 Sputtering and Electron Beam High vacuum operation ~1x10-7 Torr 1 atm = 760 Torr Sputtering High speed Ar ions knock atoms off target Deposit on substrate E-beam evaporation Electron beam vaporizes target surface Atoms deposit on substrate Atoms chemically bond to substrate Deposition Typical cleanroom suit

16 A Typical Vapor Metallization Ti-Ni Ni-Au: E-beam E deposited onto a Si wafer with a thin silicon dioxide coating Ti: 500A, adhesion layer, forms good chemical bond with silicon dioxide Ni: 1000A, diffusion barrier layer and a working layer Prevents Ti from diffusing into the Au Au: 2000A, prevents oxidation of the Ni and can be wirebonded Ti-Ni Ni-Au deposited without breaking vacuum

17 Silicon Based MEMS Materials: Non-Metallic Layers Silicon Nitride: Si 3 N 4 A useful dielectric material and a structural material Deposited using CVD (LPCVD or plasma enhanced (PE) CVD) Polyimides Plastic thin layers that can be spun on and heat cured Useful as dielectric layers, passivation,, membranes and etch masks Epoxies Spun on and heat cured Useful as structures, plating molds and etch masks

18 Non-Silicon MEMS Materials: Carbon Compounds SiC: : Silicon Carbide Young s s modulus > 400GPA (Si( Si: : 160GPA) Useful for high temperature applications Diamond Young s s modulus > 900GPA Useful optical properties Carbon Nanotubes Young s s modulus ~ 1000 GPA Many interesting properties

19 Non-Silicon MEMS Materials: Ceramic Ceramics: Alumina (Al 2 O 3 ) hard, brittle, insulating, high-temperature applications Can be laser cut to realize MEMS devices Low Temperature Cofired Ceramics (LTCC) Easily machined in the green stage Stacked to realize 3-D 3 D structures Laminated with heat and pressure

20 MEMS Gas Flow Sensor in LTCC

21 Non-Silicon MEMS Materials: PCB Laminate and Polymers Laminate Materials: FR-4 4 laminates (E-glass and resin) Polyimides (Kapton,, etc.) Liquid Crystal Polymer (LCP) Individual layers can be micromachined (laser cutting or plasma etching) Layers can be laminated into multi-layer layer structures

22 Micromachined RF Switch Array Laminated micromachined polyimide layers Electrostatic actuators turn switches on and off

23 Non-Silicon MEMS Materials: MEMS devices can be made by plating metals into temporary molds Metals Courtesy Denso Corp: powered by remote magnetic field

24 Microfabrication Terminology Understanding the language

25 Cleanliness Very important in microfabrication 100µm m width human hair: 10µm m device feature Class X cleanroom: : less than X 0.5µm m particles per cubic foot Ex: Class 10,000: > 10, µm m particles per ft 3 AU microlab: Class 1000/2000 in open areas Class 100/200 in photolithography room

26 Wafer Cleaning Very important to begin a microfabrication process with clean wafers Remove organics (such as finger prints) Remove dust, etc from surface May need to chemically clean surface to remove unwanted layers, such as silicon dioxide

27 Additive and Subtractive Additive Process Processes The deposition of a layer or volume of a material Example Processes: CVD, sputtering, spin coating, plating, oxide growth Subtractive Process The removal of material Example Processes: wet etching, plasma etching, laser machining, polishing

28 Aspect Ratio The aspect ratio refers to the ratio of depth to width of an etched hole or a grown structure. It is often used as a metric for comparison of microfabrication processes

29 Aspect Ratio Illustration High Aspect Ratio High Aspect Ratio Low Aspect Ratio Low Aspect Ratio Holes Structures Si Substrate

30 Patterning and Etching The process of transferring a designed pattern into a physical layer called Patterning Photolithography results in a pattern being transferred to a (often temporary) layer of photoresist The patterned photoresist is often used to pattern a permanent layer (such as Si substrate) through the selective removal of the exposed material, i.e. Etching

31 Mask and Undercutting Mask A layer, usually temporary, that protects a material underneath during a process Often a patterned layer of photoresist Undercutting The result of a subtractive process where material is removed from up under a mask or other layer. Usually undesirable.

32 Photolithography The selective exposing and/or masking of the wafer/ device surface with a temporary layer of photoresist,, to limit the effects of a process on the layer underneath Process Spin on photoresist and cure in an oven Using a photolithography mask, selectively expose the photoresist to UV light Develop the photoresist Wash off the UV light affected photoresist (positive PR) or the unaffected photoresist (negative PR) Process the wafer accordingly Remove the remaining photoresist

33 Photograph of a Photolithography Mask Glass mask Dark features are chrome on the glass surface

34 Spinner for Applying Photoresist to a Wafer

35 Photograph of a MA/BA6 Mask Aligner

36 Removing Used Photoresist Can be removed chemically using solvents such as acetone Can be removed by Ashing An oxygen plasma treatment that burns organics off the surface Matrix Asher

37 Isotropic and Anisotropic Isotropic A process, such as etching, that has equal effect in all directions Anisotropic A process that does not have an equal effect in all directions

38 Isotropic and Anisotropic Illustrations Isotropic Etch Photoresist Etch Mask Anisotropic Etch Undercutting Si Substrate

39 Sacrificial Layer A temporary deposited layer whose purpose is to create a gap between two layers Process The sacrificial layer is deposited and patterned The upper layer is deposited and patterned The sacrificial layer is removed Release the process of removing the sacrificial layer that frees a structure to move or deform.

40 Sacrificial Layer Illustration (1) Photoresist applied to wafer and patterned Photoresist (2) Thin metal layer deposited on surface Photoresist Si Substrate Si Substrate (4) Sacrificial photoresist layer removed (3) Metal layer patterned Photoresist Si Substrate Si Substrate

41 Micromachined Micro-Bolometer Type of structure that can be fabricated using sacrificial layers Courtesy: LETI

42 Terms Associated with Etching Selectivity The properties of an etchant with regard to what it will and will not etch A highly selective etchant might only etch 1 material Wet etch A liquid chemical etchant that removes material from a solid by chemically converting it to a liquid or a gas, or by converting it to solid particles carried away by the liquid etchant. Example wet etchants: : acids and bases Dry Etch A gas or plasma etchant that removes material from a solid by chemically converting it to a gas. Example dry etchants: : SF 6, CF 4, Cl 2, O 2, XeF 2

43 Example Wet Etchants KOH Anisotropic etchant for silicon Extremely high selectivity Etch rate of the 100 plane ~ 400 times faster than the etch rate of the 111 plane Acetic/nitric/phosphoric acid mixture Used to etch aluminum Diluted nitric acid HF Used to etch copper Used to etch silicon dioxide

44 Example Dry Etchants XeF 2 gas Used to etch silicon O 2 plasma Used to etch organics such as photoresist SF 6 plasma Used to etch silicon Can be made highly anisotropic using inductively coupled plasma DRIE techniques Cl 2 plasma Used to etch aluminum CF 4 plasma Used to etch silicon dioxide Used to etch silicon

45 Reactive Ion Etching (RIE) Dry etching process Vacuum chamber High speed ions bombard substrate and sputter off material Chemical reaction converts sputtered material (solid) to a gas Bosch Process Iterative vertical RIE etch step Followed by passivation step to minimize lateral etching Deep RIE (DRIE) RIE etching deep into a substrate STS ASE Bosch Process Si Etcher

46 A Dry Etched Si Wall Bosch dry etching process results in horizontal micro trenches on vertical surfaces

47 Ion Milling Dry etching process Vacuum chamber High speed ions sputter material off of a masked target The target has a patterned mask to limit the milling to the exposed substrate areas Patterned photoresist mask Patterned thin layer of Ti used as a mask Process not used much anymore

48 Grayscale Lithography Special photolithography mask that blocks UV light over a grayscale or analog range Unexposed, partially exposed, fully exposed photoresist DRIE etches through partially exposed photoresist at a rate proportional to its percentage exposed DRIE etches the Si undereneath as the photoresist goes away Transfers the grayscale pattern into a 3D shape in the Si

49 Stiction When two micro-structures come into contact, they tend to stick together Due to several forces acting on small masses: Electrostatic forces between charged surfaces Capillary forces during a wet etch release can pull micro- structures together, that are then held together by Van der Waals forces Spot welding This is a major problem in MEMS devices

50 Critical Point Drying Process for removing a liquid from between 2 micromachined structures to prevent stiction from capillary forces Process performed in a CPD temp/pressure chamber Liquid is replaced with liquid carbon dioxide Carbon dioxide is frozen: dry ice Dry ice is sublimated (solid directly to gas)

51 Dicing Process by which wafers are cut into individual die Uses an automated diamond-bladed saw When? IC s: last step before packaging MEMS: often before releasing fragile structures

52 Automated Dicing Saw

53 Wafer Bonding The process of permanently attaching 2 wafers together Bonding processes: Gluing or using an adhesive between the 2 wafers High Temperature Bonding: : high temperatures (1000 o C) are used to make a chemical bond at a metal/glass interface Anodic Bonding: : A high voltage (~400V) is applied across a glass/silicon interface at around 350 o C to create a glass-silicon silicon chemical bond Silicon Fusion Bonding: : 2 silicon wafers are chemically treated to be very hydrophobic and are brought into contact at high temperatures es (300 o C to 800 o C) Useful in hermetically sealing MEMS devices Used to make complex 3-D 3 D devices

54 SOI Wafers SOI Silicon on Insulator A type of wafer sometimes used in making MEMS devices, and for making high speed microelectronics Consists of three layers A thick silicon base <Handle layer> - Bottom Layer A thin silicon dioxide layer <Box layer> - Middle Layer A thin silicon layer <Device layer> - Top Layer For MEMS: manufactured by wafer bonding 2 wafers together and grinding and polishing one of them back to the desired thickness

55 SOI Wafer Illustration Si Device Layer SiO 2 Box Layer 50μm 1μm 500μm Si Handle Layer

56 MEMS SOI Fabrication Process Pattern device layer and Bosch process DRIE down to Box layer Dice wafer into individual die Remove most of the Box layer with HF acid Timed HF Release Replace liquid HF acid with alcohol solution Critical point drying Metallization E-beam Ti-Ni Ni-Au

57 Illustration of a MEMS Device in an SOI Wafer Process Released, suspended structure Metallization Anchor structure 50μm 1μm 500μm

58 MEMS Gyroscope Made with the MEMS SOI Fabrication Process

59 Liftoff Process A process for patterning thin metal films being deposited onto a substrate. The process starts with a negative-image image photoresist (PR) pattern applied to the substrate, preferably with undercut side walls. A thin metal layer is deposited, nearly perpendicular to the substrate surface. The PR results in a discontinuity in the metal layer. The metal layer on top or the PR is removed with the PR.

60 Liftoff Process Illustration (1) Photoresist applied to wafer and negatively patterned (2) Thin metal layer deposited on surface Photoresist Si Substrate Si Substrate (3) Photoresist removed leaving patterned metal on the substrate Si Substrate

61 Doping Doping is the process of introducing impurities into silicon to make it p-type p or n-typen 2 Doping methods: diffusion and ion implantation Makes Si electrically conductive Required for making microelectronic devices (diodes, transistors, etc.) Used to make a wet etch stopping point by changing the etch rate. Used to make piezoresistors for strain detection

62 Wirebonding Attachment of tiny wires between the die and the package it has been put in Wires provide electrical connection to the device Au and Al are typical wirebond wire materials Diameters as small a 25µm m are typically used for low power applications

63 Automatic Thermosonic Wirebonder

64 Surface Micromachining Addition and subtraction of layers of materials on top of the substrate Example materials: metal films, polysilicon, polyimide, epoxies (SU-8) Silicon Substrate Polysilicon Cantilever Beam

65 Bulk Micromachining Removal of the substrate material Dry (Plasma) or wet (chemical) etching Silicon Substrate

66 Bulk Micromachined Vibration Isolation Platform Device Photograph Typical Frequency Response 40 Measured Filter Response vs. Frequency Magnitude, db Frequency, Hz Vibration sensitive MEMS device A MEMS spring-mass mass-damper mechanical low pass filter

67 Lamination Thin patterned layers stacked together and bonded through heat and pressure Printed circuit boards, ceramics 5 mil PCL-FR mils 0.7 mils 2.3 mils 0.7 mils 5 mils 0.7 mils 2.3 mils 0.7 mils 5 mils 22.4 mils 3.7 mil thick A11 no-flow Prepreg 0.5 oz. Cu Spring Soldered electrical contact pin

68 A High-G G Accelerometer Made in Printed Circuit Board Laminate Top Layer Removed Integrated with Electronics Spring Proof mass Capacitive position detection

69 LIGA Deep lithography (X-ray or UV) through thick photoresist to make plating molds on Si Electroplating into PR mold Silicon Substrate Silicon Substrate Silicon Substrate

70 LIGA Micro-Gears Courtesy: Karlsruhe Nuclear Research Center

71 Precision Laser Cutting Cutting into or through materials with a high- precision, computer controlled laser Dime

72 Screen Printing Metal screen mesh with a patterned emulsion is used to transfer patterns to a substrate Conductive and nonconductive pastes Thick film layers up to ~100µm m thick Substrate is dried at ~100 o C Substrate is fired at ~900 o C Multiple layers can be applied to a substrate

73 Sandia MEMS 6-Gear 6 Drive Train