EE C245 ME C218 Introduction to MEMS Design Fall 2008 Prof. Clark T.-C. Nguyen Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, CA 94720 Lecture 6: Process Modules III: Lithography, h Etching EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 1
Lecture Outline Reading: Senturia, Chpt. 3; Jaeger, Chpt. 2, 3, 5, 6 Film Deposition Evaporation Sputter deposition Chemical vapor deposition (CVD) Plasma enhanced chemical vapor deposition (PECVD) Epitaxy Atomic layer deposition (ALD) Electroplating Lithography Etching Ion implantation ti Diffusion EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 2
Atomic Layer Deposition (ALD) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 3
Atomic Layer Deposition (ALD) Precursor A Precursor B Fundamental Components: Self-limiting limiting surface reactions of suitable precursor compounds A & B A & B then form the desired product S in a binary reaction cycle consisting of two sequential half-reactions EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 4
ALD Video Run the ALD video EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 5
ALD Capability Excellent conformality, even at the bottom of the trench! (aspect ratio ~60:1) Al 2 O 3 EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 6
ALD Versus CVD ALD CVD Highly reactive precursors Precursors react separately on the substrate Less reactive precursors Precursors react at the same time on the substrate Precursors must not decompose Precursors can decompose at at process temperature process temperature Uniformity i ensured by the Uniformity i requires uniform flux saturation mechanism of reactant and temperature Thickness control by counting the number of reaction cycles Thickness control by precise process control and monitoring Surplus precursor dosing Precursor dosing important acceptable EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 7
ALD Versus Other Deposition Methods Method ALD MBE CVD Sputter Evapor PLD Thickness Uniformity i Good Fair Good Good Fair Fair Film Density Good Good Good Good Poor Good Step Coverage Good Poor Varies Poor Poor Poor Inteface Quality Good Good Varies Poor Good Varies Number of Materials Fair Good Poor Good Fair Poor Low Temp. Deposition Good Good Varies Good Good Good Deposition Rate Fair Poor Good Good Good Good Industrial Apps. Good Fair Good Good Good Poor EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 8
Electroplating EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 9
Metal Electroplating Electroplating: the process using electrical current to coat an electrically conductive object with a thin layer of metal Useful when very thick (>1μm) metal films are needed d Evaporation and sputtering generally suffer from excessive stress when films get too thick get peeling Anode (often made of the metal to be plated) Cation Anion Cathode (part to be plated) Cu SO 4 2- Cu 2+ Cu 2+ SO 4 2- CuSO 4 (aq) 1. Switch on external supply of direct current 2. Metal at anode is oxidized to form cations with a (+) charge 3. Cations are attracted to the (-) charge on the cathode 4. Cations get reduced by e - s at the cathode, depositing the metal (in this case, Cu) Electrolyte EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 10
Wafer-Level Implementation Wafer Preparation: areas where plating is to occur must have electrical l access to the DC voltage source Often use a seed layer that accesses all plating locations Electrical Connector DC Voltage Source Wafer Wafer Holder Container Electrolyte l t Solution Counter Electrode Ti/Au Silicon Substrate Aluminum Photoresist Al layer insures electrical contact to plating areas, despite patterned Ti/Au Need not be the metal to be electroplated Often just a platinum electrode In this case, must replenish electrolytic l ti solution after time EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 11
Lithography EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 12
Lithography Lithography Method for massive patterning of features on a wafer pattern billions of devices in just a few steps Four Main Components (that affect resolution) I. Radiation Source Designated pattern (clear or dark field) II. Mask emulsion chrome Mask (glass/quartz) Generated Photoresist from layout III. Photoresist (~1μm-thick) Film to be patterned (e.g., poly-si) IV. Exposure System contact, step and repeat optics this is where the real art is! EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 13
Lithography (cont.) The basic Process (Positive Resist Example) Exposed converts light to another form after reaction with light (e.g., (+)-resist: polymer organic acid) Dip or spray wafer with developer if (+) resist, developer is often a base Etch protects film; open areas of film get etched Film Film Film Si Si Si Remove EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 14
Lithography (cont.) With each masking step usually comes a film deposition, implantation and/or etch. Thus, the complexity of a process is often measured by # masks required. NMOS: 4-6 masks Bipolar: 8-15 masks BICMOS: ~20 masks CMOS: 8-28 masks Multi-level metallization Comb-Drive Resonator: 3 masks GHz Disk: 4 masks Now, take a closer look at the 4 components: EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 15
I. Radiation Source I. Radiation Source Several types: optical, (visible, UV, deep UV light), e-beam, X-ray, ion beam The shorter the wavelength Better the resolution we have all of these in our μlab Today s prime choice due to cost and throughput. Can expose billions Optical Sources: of devices at once! Mercury arc lamp (mercury vapor discharge) 200 365 405 435 546 nm I-line G-line (we have both in our μlab) For deep UV, need Excimer laser (very expensive) Glass opaque, so must use quartz mask and lens EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 16
II. Mask II. Mask has become one of today s biggest bottlenecks! Electronic computer representation of layout (e.g., CIF, GDSII) A single file contains all layers tape mask generator Masks for each layer Mask Material: Fused silica (glass) inexpensive, i but larger thermal expansion coeff. Quartz expensive, but smaller thermal expansion coeff. EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 17
III. Photoresist (optical) Negative Positive Pictorial Description: Si develop Si develop Si Si Exposed Area: remains removed EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 18
III. Photoresist (optical) Mechanism: Negative photoactivation Positive photoactivation Polymerization (long, linked Carbon chains) Developer solvent removes unexposed Converts exposed to organic acid Alkaline developer (e.g.,koh) removes acid EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 19
III. Photoresist (optical) Issues: Negative Polymerized swells in solvent bridging problem Exposed and polymerized Positive Doesn t adhere well to SiO 2 Need primer: HMDS (hexamethyl disilazane) SiO 2 Poor adhesion HMDS SiO 2 Good adhesion at both HMDS interfaces EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 20
Typical Procedure for Lithography Clean Wafer Dry Wafer Deposit HMDS Spin-on Soft Bake Very important step 30 min. @ 120 C pre-bake (for oxide on wafer surface) 30-60 sec @ 1000-5000 rpm Topography very important: Thicker and unfocused overexpose underexpose 2 min @ 90 C Improve adhesion and remove solvent from Align & Expose Oxygen plasma (low power ~ 50W) Develop Descum Post Bake EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 21
IV. Exposure System/Optics Contact Printing Proximity Printing Photoresist Mask in contact with wafer Problem: mask pattern can become damaged with each exposure must make a new mask after x number of exposures Photoresist Mask in very close proximity but not touching 1X printing very useful for MEMS can expose surfaces with large topography (where reduction printers cannot) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 22
IV. Exposure System/Optics Projection Printing Dominates in IC transistor fabrication 5X or 10X reduction typical Mask minimum features can be larger than the actual printed features by the focused reduction factor less expensive mask costs Less susceptible to thermal variation (in the mask) than 1X printing Can use focusing tricks to improve yield: mask Photoresist Step & repeat wafer Dust particle will be out of focus better yield! EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 23
Etching Basics Removal of material over designated areas of the wafer Two important metrics: 1. Anisotropy 2. Selectivity 1. Anisotropy a) Isotopic Etching (most wet etches) d d f h If 100% isotropic: d f = d + 2h Define: B = d f d If B = 2h isotopic EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 24
Etching Basics (cont.) b) Partially Isotropic: B < 2h (most dry etches, e.g., plasma etching) Degree of Anisotropy: (definition) A f 1 B = 0 2h = if 100% isotropic 0 < A f 1 anisotropic EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 25
Etching Basics (cont.) 2. Selectivity - Poly-Si SiO 2 Ideal Etch Poly-Si SiO 2 Only poly-si etched (no etching of or SiO 2 ) 2 Si Actual Etch Si Perfect selectivity Poly-Si SiO 2 partially etched SiO 2 partially etched after some overetch of the polysilicon EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 26
Etching Basics (cont.) Why overetch? 2 d = 1.4 d = 0.56 μm Thicker spots due to topography! 45 1μ 0.4μm μ 0.4μm = d Poly-Si conformal if deposited by LPCVD 10nm Gate oxide Thus, must overetch at least 40%: 40% overetch (0.4)(0.4) = 0.16 μm poly =??? oxide Depends on the selectivity of poly-si over the oxide EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 27
Etching Basics (cont.) Define selectivity of A over B: S ab = E. R. E. R. a b Etch rate of A Etch rate of B Selectivity of A over B e.g., wet poly etch (HNO 3 + NH 4 + H 2 O) S poly 15 = 1 (very good selectivity) S = y g ( p poly SiO 2 = e.g., polysilicon dry etch: S poly SiO 2 Very high (but can still peel off after soaking for > 30 min., so beware) 5 7 = 1 4 = 1 Regular RIE (but depends on type of etcher) S ECR: 30:1 poly Bosch: 100:1 (or better) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 28
Etching Basics (cont.) 8 If S SiO = poly 2 40% overetch removes 1 0.16 = 20 nm of oxide! This will etch all poly 8 over the thin oxide, etch thru the 10nm of oxide, then start with better selectivity: etching into the 30 silicon substrate e.g., S SiO = poly 2 needless to say, this 1 is bad! (Can attain with high density Cl plasma ECR etch!) 0.16 40% overetch removes = 5.3nm (better) 30 EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 29
Wet Etching Wet etching: dip wafer into liquid solution to etch the desired film Generally isotropic, i thus, inadequate for defining features < 3μm-wide wafer etch Solvent bath General Mechanism - Si Film to be etched o Reactant Si o o Reaction products 1. Diffusion of the reactant to the film surface 2. Reaction: adsorption, reaction, desorption 3. Diffusion of reaction products from the surface EE C245: Introduction to to MEMS Design Lecture 63 C. Nguyen C. Nguyen 9/16/08 9/5/07 30 30
Wet Etching (cont.) There are many processes by which wet etching can occur Could be as simple as dissolution of the film into the solvent solution Usually, it involves one or more chemical reactions Oxidation-reduction (redox) is very common: (a) Form layer of oxide (b) Dissolve/react away the oxide Advantages: 1. High throughput process can etch many wafers in a single bath 2. Usually fast etch rates (compared to many dry etch processes) 3. Usually excellent selectivity to the film of interest EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 31
Wet Etching Limitations 1. Isotropic Limited to <3μm features But this is also an advantage of wet etching, e.g., if used for undercutting for MEMS 2. Higher cost of etchants & DI water compared w/ dry etch gas expenses (in general, but not true vs. deep etchers) 3. Safety Chemical handling is a hazard 4. Exhaust fumes and potential for explosion Need to perform wet etches under hood 5. Resist adhesion problems Need HMDS (but this isn t so bad) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 32
Wet Etch Limitations (cont.) 6. Incomplete wetting of the surface: Solvent bath Pockets where wetting hasn t occurred, yet (eventually, it will occur). Wetted surface wafer But this will lead to nonuniform etching across the wafer. For some etches (e.g., oxide etch using HF), the solution is to dip in DI water first, then into HF solution the DI water wets the surface better EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 33
Wet Etch Limitations (cont.) 7. Bubble formation (as a reaction by-product) If bubbles cling to the surface get nonuniform etching Film to be etched Si wafer Bubble (gaseous by-product) Non-uniform etching Solution: Agitate wafers during reaction. EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 34
Some Common Wet Etch Chemistries Wet Etching Silicon: Common: Si + HNO 3 + 6HF H 2 SiF 6 + HNO 2 + H 2 + H 2 O (isotropic) (nitric ic (hydrofluoric acid) acid) (1) forms a layer (2) etches away of SiO 2 the SiO 2 Different mixture combinations yield different etch rates. EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 35
Silicon Crystal Orientation z z z a (100) plane a (110) plane e a [111] y y y [110] ] @ coordinate x [100] (111) plane <110> plane ] x coordinate x to this vector (1,1,0) (1,0,0) @ coordinate <100> plane defines (1,1,1), <111> plane to this vector vector to resulting vector Silicon has the basic diamond structure Two merged FCC cells offset by y( (a/4) in x, y, and z axes From right: # available bonds/cm 2 <111> # available bonds/cm 2 <110> # available bonds/cm 2 <100> EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 36 Incre easing
Anisotropic Wet Etching Anisotropic etches also available for single crystal Si: Ointtin Orientation-dependent nd etching: <111>-plane more densely packed than <100>-plane Faster E.R. Slower E.R. in some solvents One such solvent: KOH + isopropyl alcohol l (e.g., 23.4 wt% KOH, 13.3 wt% isopropyl alcohol, 63 wt% H 2 O) E.R. <100> = 100 x E.R. <111> EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 37
Anisotropic Wet Etching (cont.) Can get the following: <111> <100> SiO 2 54.7 Si (on a <100> -wafer) <110> <111> SiO 2 (on a <110> - wafer) Si Quite anisotropic! EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 38
Wet Etching SiO 2 SiO 2 + 6HF H 2 + SiF 6 + 2H 2 O Generally used to clear out residual oxides from contacts bubble 300nm HF nt Problem: Contact hole is so thin that surface tensions don t allow the HF to get into the contact Generally the case for VLSI circuits oxide native oxide can get this just by exposing Si to air 1-2nm-thick Solution: add a surfactant (e.g., Triton X) to the BHF before the contact clear etch 1. Improves the ability of HF to wet the surface (hence, get into the contact) 2. Suppresses the formation of etch by-products, which otherwise can block further reaction if by-products get caught in the contact EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 39
More Wet Etch Chemistries Wet etching silicon nitride Use hot phosphoric acid: 85% phosphoric acid @ 180 o C Etch rate ~ 10 nm/min (quite slow) Problem: lifted during such etching Solution: use SiO 2 as an etch mask (E.R. ~2.5 nm/min) A A hassle dry etch processes more common than wet Wet etchining aluminum Typical etch solution composition: 80% phoshporic acid, 5% nitric acid, 5% acetic acid, 10% water (H 2 PO 4 ) (HNO 3 ) (CH 3 COOH) (H 2 O) (1) Forms Al 2 O 3 (aluminum oxide) (2) Dissolves the Al 2O 3 Problem: H 2 gas bubbles adhere firmlly to the surface delay the etch need a 10-50% overetch time Solution: mechanical agitation, periodic removal of wafers from etching solution EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 40
Wet Etch Rates (f/ K. Williams) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 41
For some popular films: Film Etch Chemistries EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 42
Physical sputtering Plasma etching Reactive ion etching ~ + + + + + ++++ Dry Etching All based upon plasma processes. RF (also, could be μwave) ) Develop (-) bias) (+) ions generated by inelastic collisions with energetic e -1 s Get avalanche effect because more e -1 s come out as each ion is generated. Plasma (partially ionized gas composed of ions, e - s, and highly reactive neutral species) E-field wafer Develops (+) charge to compensate for (+) ions will be accelerated to the wafer EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 43
Physical Sputtering (Ion Milling) Bombard substrate w/ energetic ions etching via physical momentum transfer Give ions energy and directionality using E-fields Highly directional very anisotropic ions plasma film Si Steep vertical wall EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 44
Problems With Ion Milling etched down to here Once through the film, the etch will start barreling through the Si film Si 1. or other masking material etched at almost the same rate as the film to be etched very poor selectivity! 2. Ejected species not inherently volatile get redeposition non-uniform etch grass! Because of these problems, ion milling is not used often (very rare) EE C245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/16/08 45