1C 245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 1EE C245 – ME C218Introduction to MEMS DesignFall 2007Prof. Clark T.-C. NguyenDept. of Electrical Engineering & Computer SciencesUniversity of California at BerkeleyBerkeley, CA 94720Lecture 8: Surface MicromachiningC 245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 2Lecture Outline• Reading: Senturia Chpt. 3, Jaeger Chpt. 11, Handout: “Surface Micromachining for MicroelectromechanicalSystems”• Lecture Topics:ª Finish diffusionª Polysilicon surface micromachiningª Stictionª Residual stressª Topography issuesª Nickel metal surface micromachiningª 3D “pop-up” MEMSª Foundry MEMS: the “MUMPS” processª The Sandia SUMMIT process2EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 3Metallurgical Junction Depth, xjxj= point at which diffused impurity profile intersects the background concentration, NBLog[N(x)]NONBx = distance f/ surfacexje.g., p-type Gaussiane.g., n-typeLog[N(x)-NB]NO-NBNBx = distance f/ surfacexjNet impurity conc.n-type regionp-type regionEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 4Expressions for xj• Assuming a Gaussian dopant profile: (the most common case)• For a complementary error function profile:()BjojNDtxNtxN =⎥⎥⎦⎤⎢⎢⎣⎡⎟⎟⎠⎞⎜⎜⎝⎛−=22exp,⎟⎟⎠⎞⎜⎜⎝⎛=BojNNDtx ln2()BjojNDtxNtxN =⎟⎟⎠⎞⎜⎜⎝⎛=2erfc,⎟⎟⎠⎞⎜⎜⎝⎛=−oBjNNDtx1erfc2→→3EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 5Sheet Resistance• Sheet resistance provides a simple way to determine the resistance of a given conductive trace by merely counting the number of effective squares• Definition:• What if the trace is non-uniform? (e.g., a corner, contains a contact, etc.)EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 6# Squares From Non-Uniform Traces4EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 7Sheet Resistance of a Diffused Junction• For diffused layers:• This expression neglects depletion of carriers near the junction, xj→ thus, this gives a slightly lower value of resistance than actual• Above expression was evaluated by Irvin and is plotted in “Irvin’s curves” on next few slidesª Illuminates the dependence of Rson xj, No(the surface concentration), and NB(the substrate background conc.)() ()11 −−⎥⎦⎤⎢⎣⎡=⎥⎦⎤⎢⎣⎡==∫∫jjxoxojsdxxNqdxxxRμσρSheet resistanceEffective resistivityMajority carrier mobilityNet impurity concentration[extrinsic material]EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 8Irvin’s Curves (for n-type diffusion)Example.Given:NB= 3x1016cm-3No= 1.1x1018cm-3(n-type Gaussian)xj= 2.77 μmCan determine these given known predep. and drive conditionsDetermine the Rs.p-type5EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 9Irvin’s Curves (for p-type diffusion)Example.Given:NB= 3x1016cm-3No= 1.1x1018cm-3(p-type Gaussian)xj= 2.77 μmCan determine these given known predep. and drive conditionsDetermine the Rs.n-typeEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 10Surface Micromachining6EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 11Silicon SubstratePolysilicon Surface-Micromachining• Uses IC fabrication instrumentation exclusively•Variations: sacrificial layer thickness, fine- vs. large-grained polysilicon, in situvs. POCL3-dopingSilicon SubstrateFree-StandingPolysiliconBeamHydrofluoricAcidReleaseEtchantWafer300 kHz Folded-Beam Micromechanical Resonator NitrideInterconnectPolysiliconSacrificialOxideStructuralPolysilconIsolationOxideEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 12Polysilicon7EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 13Why Polysilicon?• Compatible with IC fabrication processesª Process parameters for gate polysilicon well knownª Only slight alterations needed to control stress for MEMS applications• Stronger than stainless steel: fracture strength of polySi ~ 2-3 GPa, steel ~ 0.2GPa-1GPa • Young’s Modulus ~ 140-190 GPa• Extremely flexible: maximum strain before fracture ~ 0.5%• Does not fatigue readily• Several variations of polysilicon used for MEMSª LPCVD polysilicon deposited undoped, then doped via ion implantation, PSG source, POCl3,or B-source dopingª In situ-doped LPCVD polysiliconª Attempts made to use PECVD silicon, but quality not very good (yet) → etches too fast in HF, so release is difficultEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 14Polysilicon Surface-Micromachining Process Flow8EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 15Layout and Masking Layers• At Left:Layout for a folded-beam capacitive comb-driven micromechanical resonator• Masking Layers:1stPolysilicon2ndPolysiliconAnchor OpeningAA′Capacitive comb-drive for linear actuationFolded-beam support structure for stress reliefEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 16Surface-Micromachining Process Flow• Deposit isolation LTO (or PSG):ª Target = 2μmª 1 hr. 40 min. LPCVD @450oC• Densify the LTO (or PSG)ª Anneal @950oC for 30 min.• Deposit nitride:ª Target = 100nmª 22 min. LPCVD @800oC• Deposit interconnect polySi:ª Target = 300nmª In-situ Phosphorous-dopedª 1 hr. 30 min. LPCVD @650oC• Lithography to define poly1 interconnects• RIE polysilicon interconnects:ª CCl4/He/O2@300W,280mTorr• Remove photoresist in PRS2000Silicon SubstrateNitrideInterconnectPolysiliconIsolationOxideSilicon SubstrateSilicon SubstratePhotoresistCross-sections through A-A′9EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 17Surface-Micromachining Process Flow• Deposit sacrificial PSG:ª Target = 2μmª 1 hr. 40 min. LPCVD @450oC• Densify the PSGª Anneal @950oC for 30 min.• Lithography to define anchors• Etch anchorsª RIE using CHF3/CF4/He @350W,2.8Torrª Remove PR in PRS2000ª Quick wet dip in 10:1 HF to remove native oxide• Deposit structural polySiª Target = 2μmª In-situ Phosphorous-dopedª 11 hrs. LPCVD @650oCSilicon SubstrateSacrificialOxideSilicon SubstratePhotoresistSilicon SubstrateStructural PolysilconEE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 18Surface-Micromachining Process Flow• Deposit oxide hard mask (PSG)ª Target = 500nmª 25 min. LPCVD @450oC• Stress Annealª 1 hr. @ 1050oCª Or RTA for 1 min. @ 1100oC in 50 sccm N2•
View Full Document
Unlocking...