Lecture Outline EE C245 ME C28 Introduction to MEMS Design Fall 200 Prof. Clark T.-C. Nguyen Dept. of Electrical Engineering & Computer Sciences University of California at Berkeley Berkeley, CA 94720 Reading: Senturia, Chpt. 3; Jaeger, Chpt. 2, 4, 5 Lithography Etching Wet etching Dry etching Semiconductor Doping Ion implantation Diffusion Lecture Module 4: Lithography, Etching, & Doping EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 2 Lithography Lithography Method for massive patterning of features on a wafer pattern billions of devices in just a few steps Lithography EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 3 Four Main Components (that affect resolution) Designated pattern (clear or dark field) emulsion chrome Generated from layout I. Radiation Source II. Mask Mask (glass/quartz) Photoresist (~μm-thick) III. Photoresist Film to be patterned (e.g., poly-) IV. Exposure System contact, step and repeat optics this is where the real art is! EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 4
Lithography (cont.) 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 Film developer if (+) resist, developer is often a base Film Etch protects film; open areas of film get etched Film Remove 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: 8-5 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 LecM 4 C. Nguyen 8/20/09 5 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 6 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 Today s prime choice due to cost and throughput. Can expose billions Optical Sources: of devices at once! Mercury arc lamp (mercury vapor discharge) we have all of these in 200 365 405 435 546 nm our μlab 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 LecM 4 C. Nguyen 8/20/09 7 II. Mask II. Mask has become one of today s biggest bottlenecks! Electronic computer representation of layout (e.g., CIF, GDSII) Masks for each layer tape mask generator A single file contains all layers Mask Material: Fused silica (glass) inexpensive, but larger thermal expansion coeff. Quartz expensive, but smaller thermal expansion coeff. EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 8 2
III. Photoresist (optical) III. Photoresist (optical) Pictorial Description: Negative develop Positive develop Mechanism: Negative photoactivation Polymerization (long, linked Carbon chains) Developer solvent removes unexposed Positive photoactivation Converts exposed to organic acid Alkaline developer (e.g.,koh) removes acid Exposed Area: remains removed EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 9 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 0 III. Photoresist (optical) Typical Procedure for Lithography Issues: Negative Polymerized swells in solvent bridging problem Exposed and polymerized Positive Doesn t adhere well to O 2 Need primer: HMDS (hexamethyl disilazane) O 2 Poor adhesion HMDS Clean Wafer Dry Wafer Deposit HMDS Spin-on Soft Bake Align & Expose Very important step 30 min. @ 20 C Topography very important: pre-bake (for oxide on wafer surface) Thicker and unfocused 30-60 sec @ 000-5000 rpm overexpose underexpose 2 min @ 90 C Improve adhesion and remove solvent from Oxygen plasma (low power ~ 50W) O 2 Good adhesion at both HMDS interfaces EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 Develop Descum Post Bake EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 2 3
IV. Exposure System/Optics Contact 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 Proximity Printing Mask in very close proximity but not touching X printing very useful for MEMS can expose surfaces with large topography (where reduction printers cannot) EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 3 Projection Printing Photoresist IV. Exposure System/Optics Step & repeat Dominates in IC transistor fabrication 5X or 0X 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 X printing Can use focusing tricks to improve yield: wafer mask Dust particle will be out of focus better yield! EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 4 Etching Basics Removal of material over designated areas of the wafer Two important metrics:. Anisotropy 2. Selectivity Etching. Anisotropy a) Isotopic Etching (most wet etches) d d f h EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 5 If 00% isotropic: d f = d + 2h Define: B = d f d If B = 2h isotropic EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 6 4
Etching Basics (cont.) Etching Basics (cont.) b) Partially Isotropic: B < 2h (most dry etches, e.g., plasma etching) Degree of Anisotropy: (definition) B A f = = 0 if 00% isotropic 2h 0 < A f anisotropic 2. Selectivity - Poly- O 2 Ideal Etch Actual Etch Poly- O 2 Only poly- etched (no etching of or O 2 ) Perfect selectivity Poly- O 2 partially etched O 2 partially etched after some overetch of the polysilicon EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 7 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 8 Why overetch? 45 Etching Basics (cont.) 2 d =.4d = 0.56μm 0nm Gate oxide μ 0.4μm Thicker spots due to topography! 0.4μm = d Poly- conformal if deposited by LPCVD Thus, must overetch at least 40%: 40% overetch (0.4)(0.4) = 0.6 μm poly =??? oxide Depends on the selectivity of poly- over the oxide EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 9 Define selectivity of A over B: S ab Etching Basics (cont.) Selectivity of A over B e.g., wet poly etch (HNO 3 + NH 4 + H 2 O) 5 S poly O = (very good selectivity) 2 S poly E. R. = E. R. = a b Etch rate of A Etch rate of B Very high (but can still peel off after soaking for > 30 min., so beware) e.g., polysilicon dry etch: Regular RIE 5 7 S poly O = (but depends on type of etcher) 2 4 S poly = ECR: 30: Bosch: 00: (or better) EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 20 5
Etching Basics (cont.) 8 If S O = poly 2 40% overetch removes 0.6 = 20 nm of oxide! This will etch all poly 8 over the thin oxide, etch thru the 0nm of oxide, then start with better selectivity: etching into the 30 silicon substrate e.g., S O = poly 2 needless to say, this is bad! (Can attain with high density Cl plasma ECR etch!) Wet Etching 0.6 40% overetch removes = 5.3nm (better) 30 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 2 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 22 Wet Etching Wet Etching (cont.) Wet etching: dip wafer into liquid solution to etch the desired film Generally isotropic, thus, inadequate for defining features < 3μm-wide General Mechanism - Film to be etched o Reactant o o Reaction products wafer etch Solvent bath. 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 MEMS Design LecM 4 C. Nguyen 8/20/09 23 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:. 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 LecM 4 C. Nguyen 8/20/09 24 6
Wet Etching Limitations. 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) Wet Etch Limitations (cont.) 6. Incomplete wetting of the surface: Solvent bath wafer Pockets where wetting hasn t occurred, yet (eventually, it will occur). Wetted surface 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 LecM 4 C. Nguyen 8/20/09 25 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 26 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 wafer Bubble (gaseous by-product) Non-uniform etching Solution: Agitate wafers during reaction. Some Common Wet Etch Chemistries Wet Etching licon: Common: + HNO 3 + 6HF H 2 F 6 + HNO 2 + H 2 + H 2 O (isotropic) (nitric (hydrofluoric acid) acid) () forms a layer of O 2 (2) etches away the O 2 Different mixture combinations yield different etch rates. EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 27 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 28 7
z a (00) plane licon Crystal Orientation x [00] <0> plane ] @ coordinate to this vector (,0,0) <00> plane to this vector y licon has the basic diamond structure Two merged FCC cells offset by (a/4) in x, y, and z axes From right: # available bonds/cm 2 <> # available bonds/cm 2 <0> # available bonds/cm 2 <00> z a (0) plane EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 29 x Increasing y [0] coordinate (,,0) defines vector z e a [] ] () plane x @ coordinate (,,), <> plane to resulting vector y Anisotropic Wet Etching Anisotropic etches also available for single crystal : Orientation-dependent etching: <>-plane more densely packed than <00>-plane Slower E.R. Faster E.R. in some solvents One such solvent: KOH + isopropyl alcohol (e.g., 23.4 wt% KOH, 3.3 wt% isopropyl alcohol, 63 wt% H 2 O) E.R. <00> = 00 x E.R. <> EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 30 Anisotropic Wet Etching (cont.) Can get the following: <> <00> O 2 (on a <00> - wafer) 54.7 O 2 <0> <> (on a <0> - wafer) Quite anisotropic! EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 3 Wet Etching O 2 O 2 + 6HF H 2 + F 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 to air -2nm-thick Solution: add a surfactant (e.g., Triton X) to the BHF before the contact clear etch. 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 LecM 4 C. Nguyen 8/20/09 32 8
More Wet Etch Chemistries Wet Etch Rates (f/ K. Williams) Wet etching silicon nitride Use hot phosphoric acid: 85% phosphoric acid @ 80 o C Etch rate ~ 0 nm/min (quite slow) Problem: lifted during such etching Solution: use O 2 as an etch mask (E.R. ~2.5 nm/min) 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, 0% water (H 2 PO 4 ) (HNO 3 ) (CH 3 COOH) (H 2 O) () Forms Al 2 O 3 (aluminum oxide) (2) Dissolves the Al 2 O 3 Problem: H 2 gas bubbles adhere firmlly to the surface delay the etch need a 0-50% overetch time Solution: mechanical agitation, periodic removal of wafers from etching solution EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 33 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 34 For some popular films: Film Etch Chemistries Dry Etching EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 35 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 36 9
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 - s Get avalanche effect because more e - 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 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 Develops (+) charge to compensate for (+) ions will be accelerated to the wafer Steep vertical wall EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 37 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 38 Problems With Ion Milling etched down to here Once through the film, the etch will start barreling through the film. 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 LecM 4 C. Nguyen 8/20/09 39 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 40 0
Plasma Etching Plasma (gas glow discharge) creates reactive species that chemically react w/ the film in question Result: much better selectivity, but get an isotropic etch Plasma Etching Mechanism:. Reactive species generated in a plasma. 2. Reactive species diffuse to the surface of material to be etched. 3. Species adsorbed on the surface. 4. Chemical reaction. 5. By-product desorbed from surface. 6. Desorbed species diffuse into the bulk of the gas 3 4 Film to be etched MOST IMPORTANT STEP! (determines whether plasma etching is possible or not.) EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 4 2 plasma 5 6 Ex: Polysilicon Etching w/ CF 4 and O 2 CF 4 CF 4+ + CF 3+ + CF 2+ + CF + + F + + F 0 + CF 2+ + plasma Neutral radical (highly reactive!) CF 6, F 4 e - + CF 4 CF 3 + F + e - both volatile dry etching is possible. F is the dominant reactant but it can t be given a direction thus, get isotropic etch! isotropic component F 0 F 0 F 4 poly EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 42 Ex: Polysilicon Etching w/ CF 4 and O 2 isotropic component F 0 F 0 F 4 poly Problems:. Isotropic etching 2. Formation of polymer because of C in CF 4 Solution: add O 2 to remove the polymer (but note that this reduces the selectivity, S poly/ ) Solution: Use Reactive Ion Etching (RIE) Reactive Ion Etching (RIE) Use ion bombardment to aid and enhance reactive etching in a particular direction Result: directional, anisotropic etching! RIE is somewhat of a misnomer It s not ions that react rather, it s still the neutral species that dominate reaction Ions just enhance reaction of these neutral radicals in a specific direction Two principle postulated mechanisms behind RIE. Surface damage mechanism 2. Surface inhibitor mechanism EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 43 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 44
RIE: Surface Damage Mechanism RIE: Surface Inhibitor Mechanism reactive radical plasma + + + film Enhanced reaction over Relatively high energy impinging ions (>50 ev) produce lattice damage at surface Reaction at these damaged sites is enhanced compared to reactions at undamaged areas reactive radical film plasma + (+) ions breakup the polymer layer + + get reaction Non-volatile polymer layers are a product of reaction They are removed by high energy directional ions on the horizontal surface, but not removed from sidewalls no reaction Result: E.R. at surface >> E.R. on sidewalls Result: E.R. @ surface >> E.R. on sidewalls EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 45 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 46 Deep Reactive-Ion Etching (DRIE) DRIE Issues: Etch Rate Variance The Bosch process: Inductively-coupled plasma Etch Rate:.5-4 μm/min Two main cycles in the etch: Etch cycle (5-5 s): SF 6 (SF x+ ) etches Deposition cycle: (5-5 s): C 4 F 8 deposits fluorocarbon protective polymer (CF 2- ) n Etch mask selectivity: O 2 ~ 200: Photoresist ~ 00: Issue: finite sidewall roughness scalloping < 50 nm dewall angle: 90 o ±2 o Etch rate is diffusion-limited and drops for narrow trenches Adjust mask layout to eliminate large disparities Adjust process parameters (slow down the etch rate to that governed by the slowest feature) Etch Etch rate rate decreases with with trench trench width width EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 47 EE C245: Introduction to MEMS Design LecM 4 C. Nguyen 8/20/09 48 2