EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa08/lectures/Lec10p.Surface... · EE C245 – ME C218 Introduction to MEMS Design Fall 2007 Prof Clark TProf. Clark

EE C245 – ME C218Introduction to MEMS Design

Fall 2007Fall 2007

Prof Clark T C NguyenProf. Clark T.-C. Nguyen

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

Berkeley, CA 94720y

L t 10 S f Mi hi i III / M h i

EE C245: Introduction to MEMS Design Lecture 10 C. Nguyen 9/30/08 1

Lecture 10: Surface Micromachining III / Mechanics of Materials

Lecture Outline

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

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


“Foundry” MEMS:Th MUMPS PThe MUMPS Process


MUMPS: MMultiUUser MEMS PProcesSS

•Originally created by the Microelectronics Center of North Carolina (MCNC) → now owned by MEMSCAP in France

• Three-level polysilicon surface micromachining process for prototyping and “foundry” services

• D i d t i • Designed to service as many users as possible; basically an attempt to provide a

Micromotor fabricated via MUMPSan attempt to prov de a

universal MEMS process• 8 photomasks• $4,900 for 1 cm2 dies


MUMPS: MMultiUUser MEMS PProcesSSMicromotor Example


MUMPS Process Flow

• Deposit PSG on the starting n-type (100) wafers

• Anneal to heavily dope the wafersnn a to h a y op th waf rs• Remove the PSG• LPCVD 600 nm of low stress

nitride• LPCVD 500 nm of polysilicon• Lithography using the POLY0(cf)

mask and RIE etching to pattern g pthe poly0 ground plane layer

• LPCVD 2 μm of PSG as the 1st

sacrificial layer• Lithography using the DIMPLE(df)

mask (align to poly0)• RIE 750 nm deep to form dimple

ivias• Lithography using the ANCHOR1

(df) mask (align to poly0)• RIE h i d t th


• RIE anchor vias down to the nitride surface

Alignment: Choice of Aligning Layer


MUMPS Process Flow (cont.)

• LPCVD 2 μm undoped polysilicon• LPCVD 200 nm of PSG• A l f 1 h @ 1050oC• Anneal for 1 hr. @ 1050oC

This both dopes the polysilicon and reduces its residual stress

• Lithography using the POLY1(cf) g p y g ( )mask to define structures (align to anchor1)

• RIE the PSG to create a hard mask first, then …

• RIE the polysilicon

• LPCVD 750 nm of PSG• Lithography using the P1_P2_VIA

(df) k t d fi t t t


(df) mask to define contacts to the poly1 layer (align to poly1)

Utility of Hard Masks in Lithography



• Recoat with photoresist and do lithography using the ANCHOR2(df) mask to define openings where poly2 contacts nitride or poly0 (align to poly0)

• RIE the PSG at ANCHOR2 iopenings

• LPCVD 1.5 μm undoped polysiliconP D 00 P G h d • LPCVD 200 nm PSG as a hard

mask and doping source• Anneal for 1 hr @ 1050oC to

dope the polysilicon and reduce dope the polysilicon and reduce residual stress

• Lithography using the POLY2(cf) • Lithography using the POLY2(cf) mask (align to anchor2)

• RIE PSG hard mask• RIE poly2 film


• RIE poly2 film• Remove PR and hard mask

Need to Anneal After Every Thick Dep



• Lithography using the METAL (df) mask (align to poly2)

• Evaporate titanium (Ti) (as an Evaporate titanium (Ti) (as an adhesion layer for gold)

• Evaporate gold (Au)• Lift ff t PR d d fi • Liftoff to remove PR and define

metal interconnects• Coat wafers with protective PR• Dice wafers• Ship to customer

Poly1 Rotor Poly1 Stator

• Customer releases structures by dipping and agitating dies in a 48 8 wt % HF solution or via 48.8 wt. % HF solution or via vapor phase HF

• Anti-stiction dry, if neededFinal Structure: Micromotor


MUMPS: MMultiUUser MEMS PProcesSS

•Originally created by the Microelectronics Center of North Carolina (MCNC) → now owned by MEMSCAP in France

• Three-level polysilicon surface micromachining process for prototyping and “foundry” services

• D i d t i • Designed to service as many users as possible; basically an attempt to provide a

Micromotor fabricated via MUMPSan attempt to prov de a

universal MEMS process• 8 photomasks• $4,900 for 1 cm2 dies


polyMUMPS Minimum Feature Constraints

•Minimum feature sizeDetermined by MUMPS’ photolithographic resolution and alignment precisionalignment precisionViolations result in missing (unanchored), under/oversized, or fused featuresU i i f t l h b l t l Use minimum feature only when absolutely necessary

Nominal [μm] Min Feature [ ]

Min Spacing [ ]Nominal [μm] [μm] [μm]

POLY0, POLY1, POLY2 3 2 2POLY1 POLY2 VIA 3 2 2POLY1_POLY2_VIA 3 2 2

ANCHOR1, ANCHOR2 3 3 2DIMPLE 3 2 3DIMPLE 3 2 3METAL 3 3 3

HOLE1, HOLE2 4 3 3


,HOLEM 5 4 4

MUMPS Design Rules (cont.)

Cross Sections

Oxide1 Poly0

Mask LevelsPOLY0

Oxide1 Poly0


ANCHOR1


Poly1 Poly0GNH

y

Oxide1Poly0 H

OKR

Cross Sections

POLY0

Mask Levels

ANCHOR1

POLY1


ANCHOR1





The Sandia SUMMIT ProcessThe Sandia SUMMIT Process


Sandia’s SUMMiT V

• SUMMiT V: “Sandia Ultra-planar Multi-level MEMS Technology 5” fabrication process

Fi l l ili f i hi i Five-layer polysilicon surface micromachining processOne electrical interconnect layer & 4 mechanical layersUses chemical mechanical polishing (CMP) to maintain Uses chemical mechanical polishing (CMP) to maintain planarity as more structural layers are realized14 masks


SUMMiT V Layer Stack

• Uses chemical mechanical polishing (CMP) to maintain


• Uses chemical mechanical polishing (CMP) to maintain planarity as more structural layers are realized

Chemical Mechanical Polishing (CMP)• Used to planarize the top surface of a semiconductor wafer or other substrate

• U b i d i h i l l (i • Uses an abrasive and corrosive chemical slurry (i.e., a colloid) in conjunction with a polishing pad

Wafer and pad are pressed togetherf p p gPolishing head is rotated with different axes of rotation (i.e., non-concentric) to randomize the polishing

Top View

Side ViewDI Water

Carrier/Chuck

Slurry Platen

Slurry Pad Conditioner

PadC i

WaferBacking-Film

Pad


Pad ConditionerCarrier Chuck

CMP: Not the Same as Lapping

• Lapping is merely the removal of material to flatten a

Lapping Chemical Mechanical Polishing• CMP is selective to certain films and not selective to of material to flatten a

surface without selectivity• Everything is removed at

films, and not selective to others

y gapproximately the same rate

CMP

Stops at non-selective layerRemoves diff.

materials at Lapping

materials at same rate


Actual SUMMiT Cross-Section

•No CMP until after the first three polySi layers• 1 μm mmpoly1 and 1.5 μm mmpoly2 can be combined to form a 2.5 μm polysilicon film

• Refer to the SUMMiT V manual (one of your handouts) for more detailed information on masks and layout instructions


more detailed information on masks and layout instructions

New Topic: Mechanics of Materials

• Reading: Senturia, Chpt. 8g p• Lecture Topics:

Stress, strain, etc., for isotropic materialsThin films: thermal stress residual stress and stress Thin films: thermal stress, residual stress, and stress gradientsInternal dissipationpMEMS material properties and performance metrics


Vertical Stress Gradients

• Variation of residual stress in the direction of film growth• Can warp released structures in z-direction


ElasticityElasticity


Normal Stress (1D)

If the force acts normal to a

surface then the surface, then the stress is called a

normal stressnormal stress

z σz

σy

z

∆z

x

y

Diff ti l

σx∆x

∆y

∆z


Differential volume element

Strain (1D)


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EE C245 – ME C218 Introduction to MEMS Design Fall 2007ee245/fa08/lectures/Lec10p.Surface... · EE C245 – ME C218 Introduction to MEMS Design Fall 2007 Prof Clark TProf. Clark