1C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 1EE C245 – ME C218Introduction to MEMS DesignFall 2007Prof. Clark T.-C. NguyenDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, CA 94720Lecture 6: Etching, I/IC 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 2Announcements• ME’s should have been moved from the waitlist to the class• EE’s should already be in good shape• Any other students with problems getting into the class?ª See me immediately after classª Or see Jo Bullock, 205 Cory, [email protected]• Discussion section room too smallª according to the class scheduler, 299 Cory can hold only 25 students (reality is actually different)• Discussion section room options:ª 550 Cory (Hughes Room): 9-10 a.m. ª 550 Cory (Hughes Room): 10-11 a.m. ª 293 Cory: 12-1 p.m. ª 293 Cory: 1-2 p.m. *ª 293 Cory: 4-5 p.m. *2C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 3Lecture Outline• Reading: Senturia, Chpt. 3; Jaeger, Chpt. 2, 4, 5• Lecture Topics:ª Etchingª Ion Implantationª DiffusionC 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 4Etching3C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 5Etching Basics• Removal of material over designated areas of the wafer• Two important metrics:1. Anisotropy2. Selectivity1. Anisotropy–a) Isotropic Etching (most wet etches)If 100% isotropic: df= d + 2hDefine: B = df–dIf B = 2h Ö isotopicPR PRhdfdC 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 6Etching Basics (cont.)b) Partially Isotropic: B < 2h(most dry etches, e.g., plasma etching)Degree of Anisotropy: (definition)PR PR021 =−=hBAfIf 100% isotropic10 ≤<fAanisotropic4C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 7Etching Basics (cont.)2. Selectivity -PRPoly-SiSiO2SiPRPoly-SiSiO2SiIdeal EtchActualEtchOnly poly-Si etched (no etching of PR or SiO2)Perfect selectivityPRPoly-SiSiO2PR partially etchedSiO2partially etched after some overetch of the polysiliconC 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 8Etching Basics (cont.)μ1Why overetch?mμ4.0PRdm=μ4.0Thicker spots due to topography!mddμ56.04.12 ==10nmGate oxide45°Poly-Si → conformal if deposited by LPCVDThus, must overetch at least 40%: 40% overetch → (0.4)(0.4) = 0.16 μm poly = ??? oxideDepends on the selectivity of poly-Si over the oxide5C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 9Etching Basics (cont.)Define selectivity of A over B:∴Selectivity of A over Be.g., wet poly etch (HNO3+ NH4+ H2O) =PRpolySVery high (but PR can still peel off after soaking for > 30 min., so beware)baabRERES....=Etch rate of AEtch rate of B1152=SiOpolyS(very good selectivity)e.g., polysilicon dry etch:1752−=SiOpolyS14=PRpolyS(but depends on type of etcher)Regular RIEECR: 30:1Bosch: 100:1 (or better)C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 10Etching Basics (cont.)oxide! of nm 20816.0=This will etch all poly over the thin oxide, etch thru the 10nm of oxide, then start etching into the silicon substrate →needless to say, this is bad!with better selectivity:e.g.,nm3.53016.0=40% overetch removes (better)(Can attain with high density Cl plasma ECR etch!)If 40% overetch removes 182=SiOpolyS1302=SiOpolyS6C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 11Wet Etching• Wet etching: dip wafer into liquid solution to etch the desired filmª Generally isotropic, thus, inadequate for defining features < 3μm-wide• General Mechanism -1. Diffusion of the reactant to the film surface2. Reaction: adsorption, reaction, desorption3. Diffusion of reaction products from the surfacewaferetchSolvent bathPR PRSiPR PRSiooFilm to be etchedo ReactantReactionproductsC 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 12Wet Etching (cont.)• There are many processes by which wet etching can occurª Could be as simple as dissolution of the film into the solvent solutionª 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 bath2. Usually fast etch rates (compared to many dry etch processes)3. Usually excellent selectivity to the film of interest7C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 13Wet Etching Limitations1. Isotropicª Limited to <3μm featuresª But this is also an advantage of wet etching, e.g., if used for undercutting for MEMS2. Higher cost of etchants & DI water compared w/ dry etch gas expenses (in general, but not true vs. deep etchers)3. Safetyª Chemical handling is a hazard4. Exhaust fumes and potential for explosionª Need to perform wet etches under hood5. Resist adhesion problemsª Need HMDS (but this isn’t so bad)C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 14Wet Etch Limitations (cont.)6. Incomplete wetting of the surface:ª 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 betterwaferBut this will lead to nonuniformetching across the wafer.Pockets where wetting hasn’t occurred, yet (eventually, it will occur).Wetted surfaceSolvent bath8C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 15Wet Etch Limitations (cont.)7. Bubble formation (as a reaction by-product)ª If bubbles cling to the surface → get nonuniform etchingNon-uniform etchingPRPRSi waferBubble (gaseous by-product)Film to be etchedSolution: Agitate wafers during reaction.C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 16Some Common Wet Etch ChemistriesWet Etching Silicon:Common: Si + HNO3+ 6HF H2SiF6+ HNO2+ H2+ H2O(isotropic)(nitric acid)(hydrofluoric acid)(1) forms a layer of SiO2(2) etches away the SiO2Different mixture combinations yield different etch rates.9C 245: Introduction to MEMS Design Lecture 6 C. Nguyen 9/13/07 17Silicon Crystal Orientation• Silicon has the basic diamond structureª Two merged FCC cells offset by (a/4) in x, y, and z axesª From right:# available bonds/cm2(111)# available bonds/cm2(110)# available bonds/cm2(100)xyza[100](100) planeIncreasing@ coordinate (1,0,0)(100) plane ⊥to this vectorxyza][110](110) planexyza][111]e(111) planecoordinate (1,1,0) defines vector(110) plane ⊥to this vector@ coordinate (1,1,1),
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