EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa07/lectures/Lec8... · 2007-09-20 · 4 EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 7 Sheet Resistance

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C 245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 1

EE C245 – ME C218Introduction to MEMS Design

Fall 2007

Prof. Clark T.-C. Nguyen

Dept. of Electrical Engineering & Computer SciencesUniversity of California at Berkeley

Berkeley, CA 94720

Lecture 8: Surface Micromachining

C 245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 2

Lecture Outline

• Reading: Senturia Chpt. 3, Jaeger Chpt. 11, Handout: “Surface Micromachining for MicroelectromechanicalSystems”

• Lecture Topics:Finish diffusionPolysilicon surface micromachiningStictionResidual stressTopography issuesNickel metal surface micromachining3D “pop-up” MEMSFoundry MEMS: the “MUMPS” processThe Sandia SUMMIT process

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EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 3

Metallurgical Junction Depth, xj

xj = point at which diffused impurity profile intersects the background concentration, NB

Log[N(x)]

NO

NB

x = distance f/ surfacexj

e.g., p-type Gaussian

e.g., n-type

Log[N(x)-NB]

NO-NB

NB

x = distance f/ surfacexj

Net impurity conc.

n-type region

p-type region


Expressions for xj

• Assuming a Gaussian dopant profile: (the most common case)

• For a complementary error function profile:

( ) Bj

oj NDt

xNtxN =

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−=

2

2exp, ⎟⎟

⎠

⎞⎜⎜⎝

⎛=

B

oj N

NDtx ln2

( ) Bj

oj NDt

xNtxN =⎟⎟

⎠

⎞⎜⎜⎝

⎛=

2erfc, ⎟⎟

⎠

⎞⎜⎜⎝

⎛= −

o

Bj N

NDtx 1erfc2

→

→

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Sheet 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.)


# Squares From Non-Uniform Traces

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Sheet 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 slides

Illuminates the dependence of Rs on xj, No (the surface concentration), and NB (the substrate background conc.)

( ) ( )11 −−

⎥⎦⎤

⎢⎣⎡=⎥⎦

⎤⎢⎣⎡== ∫∫

jj x

o

x

oj

s dxxNqdxxx

R μσρ

Sheet resistance

Effective resistivity

Majority carrier mobility

Net impurity concentration

[extrinsic material]


Irvin’s Curves (for n-type diffusion)

Example.Given:NB = 3x1016 cm-3

No = 1.1x1018 cm-3

(n-type Gaussian)xj = 2.77 μmCan determine these given known predep. and drive conditions

Determine the Rs.

p-type

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Irvin’s Curves (for p-type diffusion)

Example.Given:NB = 3x1016 cm-3

No = 1.1x1018 cm-3

(p-type Gaussian)xj = 2.77 μmCan determine these given known predep. and drive conditions

Determine the Rs.

n-type


Surface Micromachining

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Silicon Substrate

Polysilicon Surface-Micromachining

• Uses IC fabrication instrumentation exclusively

• Variations: sacrificial layer thickness, fine- vs. large-grained polysilicon, in situvs. POCL3-dopingSilicon Substrate

Free-Standing

PolysiliconBeam

HydrofluoricAcid

ReleaseEtchant

Wafer

300 kHz Folded-Beam Micromechanical Resonator

NitrideInterconnectPolysilicon

SacrificialOxide Structural

PolysilconIsolationOxide


Polysilicon

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Why Polysilicon?

• Compatible with IC fabrication processesProcess parameters for gate polysilicon well knownOnly 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 MEMSLPCVD polysilicon deposited undoped, then doped via ion implantation, PSG source, POCl3, or B-source dopingIn situ-doped LPCVD polysiliconAttempts made to use PECVD silicon, but quality not very good (yet) → etches too fast in HF, so release is difficult


Polysilicon Surface-Micromachining Process Flow

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Layout and Masking Layers

• At Left: Layout for a folded-beam capacitive comb-driven micromechanical resonator

•Masking Layers:

1st Polysilicon

2nd Polysilicon

Anchor Opening

A A′ Capacitive comb-drive for linear actuation

Folded-beam support structure for stress relief


Surface-Micromachining Process Flow• Deposit isolation LTO (or PSG):

Target = 2μm1 hr. 40 min. LPCVD @450oC

• Densify the LTO (or PSG)Anneal @950oC for 30 min.

• Deposit nitride:Target = 100nm22 min. LPCVD @800oC

• Deposit interconnect polySi:Target = 300nmIn-situ Phosphorous-doped1 hr. 30 min. LPCVD @650oC

• Lithography to define poly1 interconnects

• RIE polysilicon interconnects:CCl4/He/O2 @300W,280mTorr

• Remove photoresist in PRS2000

Silicon Substrate

NitrideInterconnectPolysilicon

IsolationOxide

Silicon Substrate

Silicon Substrate

Photoresist

Cross-sections through A-A′

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Surface-Micromachining Process Flow

• Deposit sacrificial PSG:Target = 2μm1 hr. 40 min. LPCVD @450oC

• Densify the PSGAnneal @950oC for 30 min.

• Lithography to define anchors

• Etch anchorsRIE using CHF3/CF4/He @350W,2.8TorrRemove PR in PRS2000Quick wet dip in 10:1 HF to remove native oxide

• Deposit structural polySiTarget = 2μmIn-situ Phosphorous-doped11 hrs. LPCVD @650oC

Silicon Substrate

SacrificialOxide

Silicon Substrate

Photoresist

Silicon Substrate

Structural Polysilcon


Surface-Micromachining Process Flow• Deposit oxide hard mask (PSG)

Target = 500nm25 min. LPCVD @450oC

• Stress Anneal1 hr. @ 1050oCOr RTA for 1 min. @ 1100oC in 50 sccm N2

• Lithography to define poly2 structure (e.g., shuttle, springs, drive & sense electrodes)

Hard bake the PR longer to make it stronger

• Etch oxide mask firstRIE using CHF3/CF4/He @350W,2.8Torr

• Etch structural polysiliconRIE using CCl4/He/O2@300W,280mTorrUse 1 min. etch/1 min. rest increments to prevent excessive temperatureSilicon Substrate

Silicon Substrate

Silicon Substrate

Oxide Hard Mask

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Surface-Micromachining Process Flow

• Remove PR (more difficult)Ash in O2 plasmaSoak in PRS2000

• Release the structuresWet etch in HF for a calculated time that insures complete undercutting If 5:1 BHF, then ~ 30 min.If 48.8 wt. % HF, ~ 1 min.

• Keep structures submerged in DI water after the etch

• Transfer structures to methanol

• Supercritical CO2 dry release

Silicon Substrate

Silicon Substrate

Free-StandingPolysilicon Beam

HydrofluoricAcid

ReleaseEtchant

Wafer


Polysilicon Surface-Micromachined Examples

• Below: All surface-micromachined in polysilicon using variants of the described process flow

Free-Free Beam Resonator

ThreeThree--Resonator Micromechanical FilterResonator Micromechanical Filter

FoldedFolded--Beam CombBeam Comb--Driven ResonatorDriven Resonator

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Structural/Sacrifical Material Combinations

H2O2, hot H2OPoly-GePoly-SiGe

TMAH, XeF2SiAl

XeF2Poly-SiSiO2

O2 plasmaPhotoresistAl

HF, BHFSiO2, PSG, LTOPoly-Si

EtchantSacrificial MaterialStructural Material

•Must consider other layers, too, as release etchantsgenerally have a finite E.R. on any material

• Ex: concentrated HF (48.8 wt. %)Polysilicon E.R. ~ 0Silicon nitride E.R. ~ 1-14 nm/minWet thermal SiO2 ~ 1.8-2.3 μm/minAnnealed PSG ~ 3.6 μm/minAluminum (Si rich) ~ 4 nm/min (much faster in other Al)


Wet Etch Rates (f/ K. Williams)

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Film Etch Chemistries

• For some popular films:


Issues in Surface Micromachining

• Stiction: sticking of released devices to the substrate or to other on-chip structures

Difficult to tell if a structure is stuck to substrate by just looking through a microscope

• Residual Stress in Thin FilmsCauses bending or warping of microstructuresLimits the sizes (and sometimes geometries) of structures

• TopographyStringers can limit the number of structural levels

Substrate

BeamStiction

Stringer

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Microstructure Stiction


Microstructure Stiction

• Stiction: the unintended sticking of MEMS surfaces

• Release stiction:Occurs during drying after a wet release etchCapillary forces of droplets pull surfaces into contactVery strong sticking forces, e.g., like two microscope slides w/ a droplet between

• In-use stiction: when device surfaces adhere during use due to:

Capillary condensationElectrostatic forcesHydrogen bondingVan der Waals forces

Stiff Beam

Rinse Liquid

Wide Beam

Anchor

Substrate

Substrate

BeamStiction

Documents

EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa07/lectures/Lec8... · 2007-09-20 · 4 EE C245: Introduction to MEMS Design Lecture 8 C. Nguyen 9/20/07 7 Sheet Resistance