MEMS Fabrication

Cristina Rusu Imego AB

2011-02-21

2011-02-21 – Cristina Rusu MEMS

• Semiconductors as mechanical materials • Bulk micromachining – Dry etching – Wet etching • Surface micromachining – MUMPs • Polymer MEMS • Wafer bonding

2011-02-21 – Cristina Rusu Technology: Micromachining

Micro El ect ro M ech ani cal S yst ems (MEMS) Micro System Technology (MST)

. Fabrication process similar to that used to make computer chips (Integrated Circuits) •Cappgable of High Precision • Can Operate at High Volumes • Produces Parts at Low Cost

. Silicon is… • Extremely pure • Compatible with electronics • Suitable for micro-scale production … and it has outstanding mechanical properties

2011-02-21 – Cristina Rusu MEMS vs CMOS

• CMOS compatible processes – No Au, no alkali metals (K, Na, ..) – Limited thermal budget (After doping)

2011-02-21 – Cristina Rusu Semiconductors as mechanical materials

• First paper: ”Silicon as a mechanical material” (Kurt Petersen, 1978) – Stiffness: Young’s modulus of Si (130 GPa) close to that of steel – No plastic deformation – (Almost) no fatigue

• Other semiconductor materials that are used as mechanical materials: – GaAs, InP , ...

2011-02-21 – Cristina Rusu Other MEMS materials

• Polymers – Direct patternable: • UV: SU-8, Polyimide, BCB SU-8 SU-8 • Synchrotron X-ray: PMMA – Etchable • Polyimide, BCB – Moldable: • COC, PDMS, PMMA, Parafin – Evaporable • Parylene • Ceramics – Glass: P yrex, B orofl oat , Q uart z – LTCC

PMMA

2011-02-21 – Cristina Rusu Aspect ratio

= ratio of the depth to the width of hole / structure

2011-02-21 – Cristina Rusu MEMS

• Semiconductors as mechanical materials • Bulk micromachining – Dryyg etching – Wet etching • Surface micromachining – MUMPs • Polymer MEMS • Wafer bonding

2011-02-21 – Cristina Rusu Bulk micromachining

• Dry etching – Deep reactive ion etching (DRIE) – Inductively coupled plasma (ICP): The Bosch process

• Wet etching – Isotropic (HNA) – Anisotropic (KOH, TMAH, ...)

2011-02-21 – Cristina Rusu DRY Etching - principle Reactant Products

Mask Bombardment Impulse transfer Physical etching

Film/substrate (a) Reactant Products

Mask Adsorption Desorption Chemical etching Reaction

(b) Reactant Products

Mask Adsorption Desorption Ion-enhanced reaction Synergetical

E.g. XeF2 or SF6

2011-02-21 – Cristina Rusu Physical: tapered etching

2011-02-21 – Cristina Rusu Synergetical: vertical etching

2011-02-21 – Cristina Rusu Typical etching

2011-02-21 – Cristina Rusu The Bosch process

2011-02-21 – Cristina Rusu Cryogenic DRIE

• Principle

– SF6/O2 plasma

– At cryogenic temperatures (T < -100 C), a passivating SiOxFy layer forms on top of the silicon surface – sputtered away from horizontal surfaces by directional ion bombardment.

– thickness of the passivation layer is mainly determined by the O2 flow rate (more O2, more passivation)

• Superior sidewall quality

2011-02-21 – Cristina Rusu http://www.clarycon .com/etch_mech_pic .html

2011-02-21 – Cristina Rusu Artifacts in dry etching

Notching (ion trajectory distortion RIE lag or ARDE & chemical etching) Aspect ratio dependent etching

Faceting, Ditching (Trenching) and Redeposition2011-02-21 – Cristina Rusu Advanced dry etching (1)

2011-02-21 – Cristina Rusu Advanced dry etching (2)

2011-02-21 – Cristina Rusu Typical RIE Gases

Typical etch rate Material Typical etchant Typical mask (µm/min)

SF6 ~ 3 - 8(DRIE)8 (DRIE) Si Photo resist, SiO2, Al BCl3 + Cl2 ~ 0.5

SiO2 CF4 ~ 0.02 Photo resist, Al

Si3N4 CHF3 ~ 0.1 - 0.2 Photo resist, Al

GaAs CCl2F2 + O2 ~ 0.2 Ni, Al, Cr

SiC SF6 ~ 0.2 - 0.5 Photo resist, Al

Al Cl2 ~ 0.3 Photo resist

Au CCl2F2 ~ 0.05 Photo resist

2011-02-21 – Cristina Rusu Wet etching

• Isotropic etching – Same etch rate in all directions – Lateral etch rate is about the same as vertical etch rate – Etch rate does not depend upon the orientation of the mask edge

• Anisotropic etching – Etch rate depends upon orientation to crystalline planes – Lateral etch rate can be much larger or smaller than vertical etch rate, depending upon orientation of mask edge to crystalline axes – Orientation of mask edge and the details of the mask pattern determine the final etched shape

• Can be very useful for making complex shapes

• Can be very surprising if not carefully thought out

• Only certain “standard” shapes are routinely used

• MhhMuch cheaper th thdan dry et thithiching techniques

• Higher safety risk for lab personnel: bases & acids instead of confined plasma 2011-02-21 – Cristina Rusu Crystal planes in silicon

• Silicon: Face Centered Cubic (FCC)

[100]

[111] [010]

[001]

2011-02-21 – Cristina Rusu Anisotropic wet etching - orientation dependent etching

2011-02-21 – Cristina Rusu <100>

2011-02-21 – Cristina Rusu Si

2011-02-21 – Cristina Rusu <011>

2011-02-21 – Cristina Rusu Anisotropic wet etching: AFM tips

resistors

Tip connection

2011-02-21 – Cristina300 μ Rusum KOH

• Comparatively safe and non-toxic • High crystal plane selectivity

• Limited SiO 2 selectivity • Not CMOS compatible: potassium (K) • Careful cleaning can allow KOH-etched wafers (Piranha cleaning) in not too picky CMOS facilities

2011-02-21 – Cristina Rusu Tetra-Methyyy()l Ammonium Hydroxide (TMAH)

• CMOS compatible

• Lower crystal plane selectivity: (111):(011):(100) 1:60:20

• High selectivity towards SiO2

• Pooso,coison, corros ive

2011-02-21 – Cristina Rusu Crystal alignment

• Identifying the correct crystal alignment

– Flat alignment: ±1º (standard)

– Test etch + alignment

– Alignment forks (Vangbo and Bäcklund): ±0.05º

2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching

Wafer flat

<011>

<100> <111>

2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching Wafer flat

2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching

Wafer flat

45o

2011-02-21 – Cristina Rusu Alignment forks (Vangbo & Bäcklund)

2011-02-21 – Cristina Rusu Corner compensation structures

2011-02-21 – Cristina Rusu Solution: corner compensation structures

2011-02-21 – Cristina Rusu Simulation software

• Cellular automata -based simulation – 3D continuous • Intellisuite AnisE •Fast • Does not simulate surface roughness

– Monte Carlo • CoventorWare: Etch3D • Advantage: precise • Slow, heavy on resources (memory, cpu)

2011-02-21 – Cristina Rusu AnisE

2011-02-21 – Cristina Rusu CoventorWare Etch3d

1406µm

575µm 700µm 575µm

140µm

500µm 575µm

290µm

2011-02-21 – Cristina Rusu MEMS

• Semiconductors as mechanical materials

• Bulk micromachining – Dry etching – Wet etching

• Surface micromachining – Stiction – Lithophraphy – MUMPs

• Polymer MEMS

• Wafer bonding

2011-02-21 – Cristina Rusu Evaporation Drying - Stiction

2011-02-21 – Cristina Rusu Stiction = Big problem in MEMS

Capillary force greater than structural stiffness

• The microstructures may remain stuck to substrate even after dry .

• Cause: solid bridging, van der Waals force, electrostatic force, hydrogen bonding, etc

2011-02-21 – Cristina Rusu Supercritical Drying Evaporation Drying

Material Tc (ºC) Pc (atm) Pc (psi) Water 374 218 3204 Methanol 240 80 1155 CO231731073 Sublimation Drying Vapour phase Etching

T-butyl alcohol – freezes at 26 ºC P-dichlorobenzene – freezes at 56 ºC

Anhydrous HF vapour avoiding liquid-gas transition 2011-02-21 – Cristina Rusu Stiction Reduction Strategies

Reduce Adhesion Area • dimples • surface roughening • low surface-energy coatings

Integrate supporting microstructures • increase tolerance of capillary forces

Examples: • microtethers •microfuses •sacrifici al supporti ng l ayers ( ex. ph o toresi s t) • coat devices with low surface-energy films

2011-02-21 – Cristina Rusu Lithography issues

• MEMS: often ”large” height differences – Spray coating

– Proximity exposure  Still lower resolution

2011-02-21 – Cristina Rusu Surface micromachining, e.g. polyMUMPs

• Cost per submission is $3,200/academic, $4,500/commercial – 1cm2 die area per submission – 15 identical dice returned (~$2/mm2)

• Dicing, bonding , HF release are all available for additional cost

• Parameterized and static design cells are free online

• Design services are available for additional cost

• 2-5 weeks time to evaluate/test chips and revise design for next scheduled run

2011-02-21 – Cristina Rusu polyMUMPs process flow

2011-02-21 – Cristina Rusu Example – IR microspectrometer

2011-02-21 – Cristina Rusu Different MUMPs processes

• PolyMUMPs – 8 lithography levels , 7 physical layers – 3 Poly layers – 1 Metal layer • SOIMUMPs – 10 or 20 µm structure layer – Double-sided pattern/etch – 2 Metal layers • MetalMUMPs – 10 lithography layers – Thick electroplated Ni (18-22 µm) Source: MEMSCAP

2011-02-21 – Cristina Rusu MEMS

• Semiconductors as mechanical materials • Bulk micromachining • Surface micromachining • Polymer MEMS • Wafer bonding

2011-02-21 – Cristina Rusu Polymer MEMS

• Fabrication methods

• Polymers – Parylene – PDMS – Paraffin – Polyimide, BCB – SU-8 – PMMA – ...

2011-02-21 – Cristina Rusu Polymer fabrication methods (1)

Injection moulding Hot embossing Casting

2011-02-21 – Cristina Rusu Polymer fabrication methods (2)

Stereolithography Ink jet printing

2011-02-21 – Cristina Rusu Parylene

• Poly-para-xylylene

• Vapor-phase deposition – Low-temperat ure process ( <100 ºC) – Very conformal (~100mbar)

• Advantages: – Low surface roughness – Stress free – Excellent dielectric breakdown properties <1µm – Pinhole free for film thicknesses >0. 5µm – Low autofluorescence – Biocompatible

• Adhesion:

– Silanization recommended for Si, Si3N4, SiO2 and Al surfaces –O2 plasma treatment recommended for polymer surfaces – No preparation required for Cr, Au, Ti

• Microfabrication 2011-02-21 – Cristina Rusu – Etched using O2 and CF4 atmosphere PDMS

• Polydimethylsiloxane • Silicone  Not allowed at MC2 • Biocompatible • Can be spun or poured on mold

• Easily bonded to PDMS, Si, SiO2 using O2 plasma activation • Low cost

2011-02-21 – Cristina Rusu Paraffin

• Properties – Low thermal conductivity – Low electrical conductivity – Low chemical reactivity – High thermal expansion during phase change – High boiling point

• Applications – Microactuators

2011-02-21 – Cristina Rusu Polyimide & Benzo cyclobutene (BCB)

• PI: Kapton

• Spin-on

• Patterning – Photopatternable (negative) – storage temperatures < -25ºC

– Etch abl e (O 2 pl)lasma)

• Advantages – Chemical & thermal stability – Low water uptake – Biocompatibility (PI) – Multilayer deposition

2011-02-21 – Cristina Rusu SU-8

• Negative photoresist (UV) • High aspect ratios (> 18) can be obtained • Higher aspect ratios (>60) with synchrotron X-ray • Can be used as mold or as structural material

2011-02-21 – Cristina Rusu PMMA • Polymethylmethacrylate (Plexiglass) • X-ray patternable • Aspect ratios: 50-500 (freestanding-supported) when applied in LIGA • 100-3000 µ m thick

(Also used as e-beam resist, but a slightly lower thicknesses)

LIGA

2011-02-21 – Cristina Rusu Other materials

• Hydrogels – Good for chemical sensing applications

• Biodegradable materials – Polyglycolic acide (PGA), polylactic acid (PLDA), ... – Fabrication methods: imprinting, hot embossing, stereolithography, laser micromachining

2011-02-21 – Cristina Rusu MEMS

• Semiconductors as mechanical materials • Bulk micromachining – Dryyg etching – Wet etching • Surface micromachining – MUMPs • Polymer MEMS • Wafer bonding

2011-02-21 – Cristina Rusu