Introduction to MEMS

Transcription

1 Introduction to MEMS Lecture Outline 1. Photolithography 2. Thermal Oxidation 3. Wet Etching 4. Dry Etching 5. Unconventional Microfabrication

2 Photolithography

3 Conventional Photoresists Typically consist of 3 components: -resin or base material a binder that provides mechanical properties (adhesion, chemical resistance, etc) -photoactive compound (PAC) -solvent control the mechanical properties, such as the viscosity of the base, keeping it in liquid state.

4 Positive Photoresist (I) Two-component DQN resists: Currently the most popular positive resists are referred to as DQN, corresponding to the photo-active compound, diazoquinone (DQ) and resin, novolac (N), respectively. Dominant for G-line and I- line exposure, however, these resists cannot be used for very-short-wavelength exposures. Novolac (N): - a polymer whose monomer is an aromatic ring with two methyl groups and an OH group. - it dissolves in an aqueous solution easily. - solvent added to adjust viscosity, however, most solvent is evaporated from the PR before exposure and so plays little part in photochemistry Diazoquinone(DQ) % weight - photosensitive UV - DQ Carboxylic acid (dissolution enhancer) R

5 Positive Photoresist (I) -Photoactive compound (DQ) is insolvable in base solution. -Carboxylic acid readily reacts with base to form a water soluble salt -resin/carboxylic acid mixture will rapidly takes up water (the nitrogen released in the reaction also foams the resist, further assisting the dissolution) -The chemical reaction during the dissolution is the breakdown of the carboxylic acid into water-soluble amines such aniline and slat of K (or Na depending on the developer). -Typical developer KOH or NaOH diluted with water Advantages of DQN photoresists: the unexposed areas are essentially unchanged by the presence of the developer. Thus, line width and shape of a pattern is precisely retained. novolac is a long-chain aromatic ring polymer that is fairly resistant chemical attack. The PR therefore is a good mask for the subsequent plasma etching.

6 PMMA (Ploymethyl methacrylate) Positive Photoresist (II) short-wavelength lithography: deep UV, extreme UV, electron-beam lithography resin itself is photosensitive (Slow) (pro s) high resolution (con s) Plasma etch tolerance of the resist is very low. it needs to have thick PMMA to protect the thin film, otherwise the PMMA will disappear before the thin film does t PR t film Plasma etching Si dissociation of PMMA changes the chemistry of the plasma etch and often leads to polymeric deposits on the surface of the substrate. Low sensitivity it needs to add PACs or to elevate exposure temperature to increase the speed ( the elevation of temperature can also increase the contrast). PR thin file oxide resist feature with aspect ratio higher than 4 is not considered to mechanical stable.

7 Negative Photoresist (I) Based on azide-sensitized rubber such as cyclized polyisoprene Advantages Negative photoresists have very high photospeeds Adhere to substrate without pretreatment Disadvantages Swelling of photoresists during the development. - an after develop bake will make the lies to return to their original dimension, but this swelling and shrinking process can cause the lines to be distorted. The minimum feature size of negative PR is limited to 2 μm Dirt on mask causes pinhole Developer is usually organic solvent - less ecological

8 Spin on Photoresist Deposition Methods Thickness of PR t : = ν K S 2 ω R t = thickness K = constant S= fraction of solids υ= viscosity ω= angular velocity R = radius Spray on Plate on

9 Pattern Transfer (I) The remaining image after pattern transfer can be used as a mask for subsequent process such as etching, ion implantation, and deposition

10 Pattern Transfer (II) Liftoff Process the film thickness must be smaller than that of the resist the potions of the film on the resist are removed by selectively dissolving the resist layer in an appropriate liquid enchant used extensively for high-power MESFETs used for patterning the materials lacking a highly selective etchant.

11 Thermal Oxidation

12 Thermal Oxidation A method for growing a film of SiO 2 from a single-crystal silicon (SCS) wafer or a polysilicon thin film - high temperature process ( C) - used extensively in commercial ICs and MEMS - thermal oxidation by far is the most important method for growing a SiO 2 thin film in contrast several other methods : PECVD and electrochemical process. - one of the major reasons for the popularity of silicon ICs is the case with which silicon forms an excellent oxide, SiO 2 Why is it done: -Masking materials (Lab #2_WG, Pre-Lab #2_FC) -Electrical isolation (Lab #4_FC) -Surface modification (eg. refractive index in Lab #5_WG) -Biocompability -Thermal isolation -Sacrificial layer

13 SiO2 fro IC and Surface Micromachining MOSFET Thin film beam structure fabricated By Surface Micromachining SiO 2 Si Poly Si SiO2 used as gate oxide, field oxide,.. SiO2 used as a sacrificial material

14 Desired Properties Electrical - high breakdown strength - low amount of undesirable charges interface trapped charge, mobile ion charges Mechanical - no pin holes - uniform (thickness and density) Selected Physical Constants of Thermal Silicon Oxide

15 How Does Silicon Oxidize? Dry Oxidization : Si (solid) + O 2 (gas) SiO 2 (solid) Wet Oxidization: Si (solid) + 2H 2 O (gas) SiO 2 (solid) + 2H 2 (gas) Silicon is consumed in the process Oxidization occurs at the Si-SiO 2 interface, NOT on top of the oxide The interface produced by thermal oxidization is not exposed to atmosphere, minimizing the impurities 0.56 X ox 0.44 X ox Thickness of Si Thickness of SiO 2 = Molar volume (Si) Molar volume (SiO 2 ) = Molecular weight (Si) Density (Si) Molecular weight (SiO 2 ) Density (SiO 2 ) 28.9 g/mol 2.33 g/cm 3 = = g/mol 2.21 g/cm 3

16 Structures of SiO 2 (Silica) 2.65 g/cm 3 Tetrahedral structure 2.21 g/cm 3 (a) Basic structural unit of silicon dioxide. (b) Two-dimensional representation of a crystalline structure of silicon dioxide (quartz crystal lattice). (c) Two-dimensional representation of the amorphous structure of silicon dioxide.

17 Kinetics of Thermal Oxidization Since oxidization occurs at the Si-SiO 2 interface : -O 2 or H 2 O must diffuse through the previous grown oxide film - Oxidization (growth) rate will fall with time and oxide thickness C 0 : concentration of the oxidizing species (oxygen or water vapor) at the air-sio 2 interface, molecules/cm 3 C s : concentration of the oxidizing species at the SiO 2 -Si interface, molecules/cm 3 F 1 : oxygen (or water vapor) flux through the oxide layer F 2 : reaction flux F=F 1 =F 2

18 Kinetics of Thermal Oxidization (Cont.) Fick s first law of diffusion dc D( C0 Cs ) F1 = D dx x D: diffusion coefficient of the oxidizing species X: the thickness of the oxide layer already present Reaction of the oxidizing species with Si F = κc s 2 κ: the surface reaction rate constant for oxidization Growth rate of the oxide layer thickness dx dt F DC0 / C1 = = C1 x + ( D / κ ) C 1 : the number of molecules of the oxidizing species in a unit volume of silicon oxide. for O 2 : C 1 =2.2x10 22 /cm 3 H 2 O: C 1 =4.4x10 22 /cm 3 Steady state F = F 1 = F 2 Initial condition: x(t=0) = d 0 2 2D 2DC0 x + x = ( t + τ ) κ C d 0 : initial oxide thickness τ ( d + DC 2 0 2Dd0 / κ ) C1 / 2 1 F = DC0 x + ( D / κ ) τ: time coordinate shift to account for the initial oxide layer d 0 0

19 Model of Thermal Oxidization (Cont.) ) ( τ κ + = + t C DC x D x General relationship for the oxidization of Si The oxide thickness after an oxidizing time t + + = 1 ) ( DC t C D x τ κ κ for small value of t for large value of t ) ( 1 0 τ κ + t C C x ) ( τ + t C DC x ) ( 2 +τ = + t B Ax x Compact form of the oxidization of SI where A=2D/κ, B=2DC 0 /C = 1 ) ( ) ( 2 τ t A B A t x κ D A = C C D B = / ) / 2 ( DC C Dd d κ τ +

20 Growth Rate Regimes 1 4 B x( t) = A 1+ ( t + τ ) A Short Times with (t+τ)<< A 2 /4B: B x= ( t+ τ) A Linear rate constant Long Times with (t+τ)>> A 2 /4B, t>>τ: x 2 = B( t +τ ) Parabolic rate constant

21 Linear and Parabolic Rate Constant v.s. Temperature (linear: reaction limit) (parabolic : diffusion limit) Arrhenius Relationship B = B0 e E a kt B A = B A 0 e E a kt Rate constant (B/A) varies as exp(-e a /kt) Ea (~2 ev) agrees with the energy to break Si-Si bond (1.83 ev) Rate constant depends on orientation Rate constant (B) varies as exp(-e a /kt) Ea (1.24 ev for dry and 0.71 for wet oxidization ) agrees with the activation energy of diffusion ( 1.18 ev for O 2 and 0.79 for H 2 O).

22 Oxidization Graph Used for quick look-up or confirming the calculations How long does it take to grow 0.2 um oxide using Dry Oxidation at 1200 C? How long does it take to grow 1 um oxide with 0.2 um initial oxide using Wet oxidation at 1050 C

23 Oxidization Furnace 1 L/min

24 Oxidize Thickness Characterization Profilometry: Oxide etched away over part of the wafer and a mechanical stylus is dragged over the resulting step. Ellipsometry: Polarized laser light is incident on the oxide covered wafer. The polarization of the reflected light, which depends on the thickness and index of refraction (known) of the oxide layer, is determined and used to calculate the oxide thickness. Color (P.55 a reference color chart for thermally grown oxide) Light reflected from the surface of an oxidized silicon wafer will experience constructive interference when the path length in the oxide is equal to an integer multiple of the wavelength of the light. 2 x 0 = kλ / n x 0 : oxide thickness k: 1,2,3, λ : wavelength of the incident light n: refractive index of SiO 2, 1.46

25 Wet Etching

26 Pattern Transfer (I) The remaining image after pattern transfer can be used as a mask for subsequent process such as etching, ion implantation, and deposition

27 Etch Rate: Etch Parameters - rate of material removal (μm/min) - function of concentration, agitation, temperature, density and porosity of the thin film or substrate, Etch Selectivity: - relative (ratio) of the etch rate of the thin film to the mask, substrate, or another film Etch Geometry: - sidewall slope (degree of anisotropy)

28 Types of Etching Processes Anisotropic Isotropic Anisotropic: - best for making small gaps and vertical sidewalls - typically more costly Isotropic : -quick, easy, cheap -best to use with large geometries, when sidewall slope does not matter, undercut/release -rounding of sharp anisotropic corners to avoid stress concentration

29 Wet Etching Mixtures of acids, bases, and water -HF, H 3 PO 4, H 2 SO 4, KOH, H 2 O 2, HCl,.. Can be used to etch many materials - Si, SiO 2, Si 3 N 4, PR, Al, Au, Cu, Etch Rate: - wide range Etch Selectivity - typically quite high - sensitive to contamination Etch Geometry: - typically isotropic, some special cases are anistrpoic

30 Hydrofluoric Acid (for SiO2) Selective (room temperature) - etches SiO 2 and not Si - will also attack Al, Si 3 N 4,.. Rate depends strongly on concentration - maximum: 49% HF ( concentrated) ~ >2 μm/min - controlled: 5 to 50:1 ( timed ) ~ <0.1 μm/min Dangerous! - not a strong acid - deceptive (looks just like water) - penetrate skin (adsorption) and attacks slowly - will target bones Etch Geometry - completely isotropic (used to undercut/release) Reactions: SiO 2 + 6HF H 2 SiF 6 (aq) + 2H 2 O

31 Buffered HF (for SiO2) Buffered HF (BHF), also called Buffered oxide etch (BOE) addition of NH 4 F to HF solution - control the ph value - replenish the depletion of the fluoride ions to maintain stable etching performance SiO 2 + 4HF + 2NH 4 F (NH 4 )2SiF 6 + 2H 2 O SiO 2 + 3HF 2- + H + SiF H 2 O

32 Phosphoric Acid (for Si x N y ) Selectively (high-temperature) - etches SixNy and not Si or SiO2 - etches Al and other metals much faster Rate: - Slow! R ~ > μm/min for H 3 PO 4 at C Tough Masking Materials Needed - PR will not survive - Oxide is typically used Etch Geometry - completely isotropic

33 HNA (for Silicon) Iso-etch Curve (From Robbins. et al) -mixture of nitric (HNO 3 ), hydrofluoric (HF) and acetic (CH 3 COOH) acids -HNO 3 oxides Si, HF removes SiO 2, repeat Si + 4HNO 3 SiO 2 + 2H 2 O + 4NO 2 SIO 2 + 6HF H 2 SiF 6 + 2H 2 O -high HNO 3 :HF ratio (etch limited by oxide removal) -low HNO 3 :HF ratio (etch limited by oxide formation) -dilute with water or acetic acid (CH 3 COOH) -acetic acid is preferred because it prevents HNO 3 dissociation

34 Orientation-Dependent Etching KOH -Si +OH - + 2H 2 O SiO 2 (OH) H 2(g) - Etch rate : {110} > {100} >> {111} - Used at the elevated temperature (~80 0 C) - Resist will not survive, oxide is attacked slowly - Nitride is not attacked (best masking material) Neuron well Optical DNA Sensor

35 Miller Indices Miller Indices of a plane is determined by - Take the intercepts of the plane along the crystallographic axes eg. 2, 1, 3 - The reciprocal of the three integers are taken and are multiplied by the smallest common denominator eg. 1/2, 1/1, 1/3 (multiple by 6) (3, 6, 2) - If a plane is parallel to an axis, its intercept is at infinity and its Miller index is zero - Important plans in Si lattice - {1,0,0} : (1,0,0), (01,0), (0, 0,1), (1,0,0), (0,1, 0),.. - {1,1,0} - {1,1,1}

36 Etch rate of Si in KOH Depends on Crystallographic Plane

37 Etch Rate of Oxide in KOH

38 Anisotropic Etching : Si in KOH {100} cross-section SiO 2 on top of Si wafer {110} W b W b = W l cot = W 2 0 l

39 Bulk Micromachining

40 Bulk Micromachining

41 Dry Etching

42 Wet Etching vs Dry Etching In wet etching, the etch reactants come form a liquid source In dry etching, the etch reactants come form a gas or vapor phase source and are typically ionized -Atoms or ions from the gas are the reactive species that etch the exposed film Selectivity : In general, dry etching has less selectivity than wet etching Anisotropy: In general, dry etching has higher degree of anisotropy than wet etching Etch Rate: In general, dry etch has lower etch than wet etching Etch Control: Dry etching is much easier to start and stop than wet etching F - H 2 O HF H 2 O F - HF H + HF H + H 2 O SF6 CF 4 CF 2 2+ CF 4 CF 3 + F CF 5 - Wet Etch Dry Etch

43 Etch Mechanism Generation of etching species -Without generating the etching species etching will not proceed Diffusion to surfaces -etching species must get to the surface to react with the thin or substrate molecules -the mechanics of getting to the surface can limit aspect ratio, undercuttig, uniformity Adsorption -can also effect aspect ratio Reaction: -strong function of temperature (Arrhenius relationship) -obviously effect the etch rate Desorption -can stop etch if the reacted species is not volatile Diffusion to bulk gas -can lead to non-uniform etching due to dilution of un-reacted etching species The slowest dominates!!!

44 Gas Phase Etch Reactive Gas - Gas adsorbs on the surface, diassociates, and then reacts Xenon Difluoride Etching XeF 2 Adsorb Xe + 2 F Particular features of XeF 2 etching -extremely selective -fast etch rate -isotropic

45 RF-Plasma-Based Dry Etching A plasma is fully or partially ionized gas composed of equal numbers of positive and negative charges and a different number of unionized molecules. A plasma is produced when an electric field of sufficient magnitude is applied to a gas, causing the gas to break down and become ionized.

46 Dry Etch Chemistries Materials Etch Gases Etch Products Si, SiO2, Si3N4 CF4, SF6, NF3 SiF4 Si Cl2, CCl2F2 SiCl2, SiCl4 Al BCl3, CCL4, SiCl4, Cl2 AlCl3, Al2Cl6 Organics O2, O2 + CF4 CO, CO2, H2O, HF other: (W, Ta, Mo..) CF4 WF6,..

47 Methods of Dry Etching Physical etching: (e.g. sputtering etch) -mechanical/physical interaction -positive ions are accelerated and strike substrate with high kinetic energy, some energy is then transferred to surface atoms, which leads to material removal -negative ions cannot reach the wafer and therefore plays no role in the etching -Highly anisotropic and Low Selectivity Chemical etching: -neutral or/and ionized species interact with the material s surface to form volatile products -High product volatility is important so that the reaction product would not coat the surface and prevent further etching -Isotropic & High Selectivity Combinations of Chemical and Physical Etching -Anisotropic profile, reasonably good selectivity, and moderate bombardment-induced damage. (physical) (chemical) (physical & chemical)

48 Degree of Anisotropy Degree of Anisotropy l R t R 1 Af 1 = 1 1 = hf Rvt 1 Rv For isotropic etching: R l = R v and A f = 0 For anisotropic etching: R l = 0 and A f = 1

49 Unconventional Microfabrication

50 Soft Lithography Developed by Whitesides, et. al A set of techniques for microfabrication that based on the use of soft substrate materials, printing of SAMs, and molding of polymers Micromolding Microtransfer molding Replica molding Microcontact Printing

51 Micromolding

52 Rigid Materials vs. Soft (Elastomeric) Materials Rigid materials Crystalline silicon, amorphous silicon, glass, quartz, metals Advantages: Photolithography process is mature and well developed (eg. PR against etching) Bulk-etching for forming two- and three-dimensional shapes Batch process compatible with IC process Silicon dioxide: good quality, stable chemically and thermally Disadvantages: Expensive Brittle Opaque (for silicon) in the UV/Vis regions Low dielectric strength (Si) Surface chemistry is difficult to manipulate Packaging/Bonding: Anodic bonding Fusion bonding Polymer bonding Good and Bad!!

53 Rigid Materials vs. Soft (Elastomeric) Materials Soft materials PDMS, PMMA, Polyimide, Hydrogel, etc.. Advantages: Quick Inexpensive Flexible Transparent to visible/uv Durable and chemical insert Surface property easily modified Improved biocompatibility and bioactivity Disadvantages: Low thermal stability Low thermal and electrical conductivity Techniques for microfabrication not as well developed Packaging/Bonding: Through surface modification easy but not robust

54 Rapid Prototyping Procedure for Soft-lithography

55 Different Types of Micromolding of Polymers (Silicon, Su8, etc.) Replica Molding Microtransfer molding

56 Microcontact Printing Printing on a planar substrate with a rolling stamp Printing on a planar substrate with a planar PDMS stamp Printing on a curved substrate with a planar stamp

57 Problems in Soft Lithography Pairing Occurs at high aspect ratio Caused by weight and surface tension Occurs at low aspect ratio Caused by compression force while inking Sagging Shrinking during curing Swelling by nonpolar solvents Shrinking / swelling

58 PDMS (Polydimethylsiloxane) Deforms reversibly Can be molded with high fidelity Optically transparent down to ~300nm Durable and chemically inert Non-toxic Inexpensive Upon treatment in oxygen plasma, PDMS seals to itself, glass, silicon, silicon nitride, and some plastic materials

59 Micro/nano Imprinting Dot pattern imprinted into PMMA ( polymethymethacrylate ). The dots have a 25 nm diameter and 120 nm period (Renstrom) Ti/Au dot pattern on a silicon substrate fabricated using nano-imprinting and a lift-off process. The dots have a 25 nm diameter and 120 nm period

60 Hydrogel Based Microfabrication Hydrogel Fabrication Photosensitive (polarity like negative PR) Liquid-phase photo-polymerization Laminar flow-aided patterning Functional (stimuli-responsive) and non-functional materials Fabrication of fluidic channels, actuators, valves, pumps Typical polymerization time: 5-40 sec (UV light) Minimal Total fabrication time of a system ( <10 min) A hydrogel jacket valve in a T channel (D. Beebe)

61 Fabrication of a valve in a Hydrogel Microchannel 2-D and 3-D micro fluidic network

62 Geometry Control during Fabrication by Using Laminar Flows

63 Other Soft Material (Polymer) Technique Injection molding Features > 500 nm Useful for plastics, metals, ceramics, etc Laser Direct Writing / Stereolithography (3-D Fabrication) Laser-assisted deposition Laser-assisted polymerization X-ray lithography LIGA (x-ray lithography, electroplating, molding)