Thara Srinivasan

1
MEMS Fabrication Process Flows and Bulk Silicon
Etching

• Thara Srinivasan
• Lecture 2

Picture credit Alien Technology
2
Lecture Outline

• Reading
• Reader Kovacs, pp. 1536-43, Williams, pp.
  256-60.
• Senturia, Chapter 2.
• Todays Lecture
• Tools Needed for MEMS Fabrication
• Photolithography Review
• Crystal Structure of Silicon
• Silicon Etching Techniques

3
IC Processing
Cross-section
Masks
Cross-section
Masks
N-type metal oxide semiconductor (NMOS) process
flow
Jaeger
4
CMOS Processing

• Processing steps
• Oxidation
• Photolithography
• Etching
• Diffusion
• Evaporation and Sputtering
• Chemical Vapor Deposition
• Ion Implantation
• Epitaxy

Jaeger
Complementary Metal-Oxide-Semiconductor
5
MEMS Devices
Microoptomechanical switches, Lucent
6
MEMS Devices
7
MEMS Processing

• Unique to MEMS fabrication
• Sacrificial etching
• Thicker films and deep etching
• Mechanical properties critical
• Etching into substrate
• 3-D assembly
• Wafer-bonding
• Molding
• Unique to MEMS packaging and testing
• Delicate mechanical structures
• Packaging before or after dicing?
• Sealing in gas environments
• Interconnect - electrical, mechanical, fluidic
• Testing electrical, mechanical, fluidic

sacrificial layer
structural layer
Package Dice Release
8
Photolithography Masks and Photoresist

• Photolithography steps
• Photoresist spinnning, 1-10 µm spin coating
• Optical exposure through a photomask
• Developing to dissolve exposed resist
• Photomasks
• Layout generated from CAD file
• Chrome or emulsion on glass
• 1-3 k

dark-field
light-field
9
Photoresist Application

• Spin-casting photoresist
• Polymer resin, sensitizer, carrier solvent
• Positive and negative photoresist
• Thickness depends on
• Concentration
• Viscosity
• Spin speed
• Spin time

www.brewerscience.com
10
Photolithography Tools

• Contact or proximity
• Resolution Contact - 1-2 µm, Proximity - 5 µm
• Depth of focus

• Projection
• Resolution - 0.5 (?/NA) ? 1 µm
• Depth of focus Few µms

11
Materials for MEMS

• Substrates
• Silicon
• Glass
• Quartz
• Thin Films
• Polysilicon
• Silicon Dioxide, Silicon Nitride
• Metals
• Polymers

Silicon crystal structure l 5.43 Å
Wolf and Tauber
12
Silicon Crystallography
z
z
z
001
(110)
y
y
y
010
(100)
(110)
(111)
x
x
x
100

• Miller Indices (hkl)
• Normal to plane
• Reciprocal of plane intercepts with axes
• (unique), family
• Direction
• Move one endpoint to origin
• unique, ltfamilygt

111
13
Silicon Crystallography

• Angles between planes, ?
• ? between abc and xyz is given by axbycz
  (a,b,c)(x,y,z)cos(?)
• 100 and 110 45
• 100 and 111 54.74
• 110 and 111 35.26, 90 and 144.74

14
Silicon Crystal Origami
110 (101)
111
111
(111)
(111)
100 (100)
110 (101)
111
111
(111)
(111)
100 (001)

• Silicon fold-up cube
• Adapted from Profs. Kris Pister and Jack Judy
• Print onto transparency
• Assemble inside out
• Visualize crystal plane orientations,
  intersections, and directions

110 (011)
110 (011)
110 (101)
111
111
(111)
(111)
100 (100)
100 (010)
100 (010)
110 (110)
110 (110)
110 (110)
110 (110)
110 (011)
110 (011)
110 (101)
111
111
(111)
(111)
100 (001)
15
Silicon Wafers

• Location of primary and secondary flats shows
• Crystal orientation
• Doping, n- or p-type

Maluf
16
Properties of Silicon

• Crystalline silicon is a hard and brittle
  material that deforms elastically until it
  reaches its yield strength, at which point it
  breaks.
• Tensile yield strength 7 GPa (1500 lb
  suspended from 1 mm²)
• Youngs Modulus near that of stainless steel
• 100 130 GPa 110 169 GPa 111 188 GPa
  
• Mechanical properties uniform, no intrinsic
  stress
• Good thermal conductor
• Mechanical integrity up to 500C

17
Bulk Etching of Silicon

• Etching modes
• Isotropic vs. anisotropic
• Reaction-limited
• Etch rate dependent on temperature
• Diffusion-limited
• Etch rate dependent on mixing
• Also dependent on layout and geometry, loading
• Choosing a method
• Desired shapes
• Layout and uniformity
• Surface roughness
• Process compatibility
• Safety, cost, availability

Maluf
18
Wet Etch Variations

• Etch rate variation due to wet etch set-up
• Loss of reactive species
• Evaporation of liquids
• Poor mixing (etch product blocks diffusion of
  reactants)
• Contamination
• Applied potential
• Illumination

19
Anisotropic Etching of Silicon

• Etching of Si with KOH
• Si 2OH- ? Si(OH)2 2 4e-
• 4H2O 4e- ? 4(OH) - 2H2

• Crystal orientation relative etch rates
• 110100111 6004001
• 111 plane has three backbonds below the surface
• Energy explanation
• 111 may form protective oxide quickly

lt100gt
Maluf
20
KOH Etch Conditions

• 1 KOH 2 H2O (wt.), stirred bath _at_ 80C
• Si (100) ? 1.4 µm/min
• Etch masks
• Si3N4 ? 0
• SiO2 ? 1-10 nm/min
• Photoresist, Al fast
• Micromasking by H2 bubbles leads to roughness
• Stirring displaces bubbles
• Oxidizer, surfactant additives

Maluf
21
Undercutting

• Convex corners bounded by 111 planes are
  attacked

Maluf
Ristic
22
Undercutting

• Convex corners bounded by 111 planes are
  attacked

23
Corner Compensation

• Protect corners with compensation areas in
  layout, Buser et al. (1986)
• Mesa array for self-assembly test structures,
  Smith and coworkers (1995)

Alien Technology
Hadley
Chang
24
Corner Compensation

• Self-assembly microparts, Alien Technology

25
Other Anisotropic Etchants

• TMAH, Tetramethyl ammonium hydroxide, 10-40 wt.
  (90C)
• Al safe, IC compatible
• Etch rate (100) 0.5-1.5 µm/min
• Etch ratio (100)/(111) 10-35
• Etch masks SiO2 , Si3N4 0.05-0.25 nm/min
• Boron doped etch stop, up to 40? slower
• EDP (115C)
• Carcinogenic, corrosive
• Al may be etched
• Etch rate (100) 0.75 µm/min
• R(100) gt R(110) gt R(111)
• Etch ratio (100)/(111) 35
• Etch masks SiO2 0.2 nm/min, Si3N4 0.1
  nm/min
• Boron doped etch stop, 50? slower

26
Boron-Doped Etch Stop

• Control etch depth precisely with boron doping
  (p)
• B gt 1020 cm-3 reduces KOH etch rate by 20-100?
• Gaseous or solid boron diffusion
• At high dopant level, injected electrons
  recombine with holes in valence band and are
  unavailable for reactions to give OH-
• Results
• Beams, suspended films
• 1-20 µm layers possible
• p not compatible with CMOS
• Buried p compatible

27
Microneedles
Ken Wise group, University of Michigan
28
Microneedles
Wise group, University of Michigan
29
Microneedles
Ken Wise group, University of Michigan
30
Electrochemical Etch Stop

• Electrochemical etch stop
• n-type epitaxial layer grown on p-type wafer
  forms p-n diode
• p gt n ? electrical conduction
• p lt n ? reverse bias
• passivation potential potential at which thin
  SiO2 layer forms

• Set-up
• p-n diode in reverse bias
• p-substrate floating ? etched
• n-layer above passivation potential ? not etched

Maluf
31
Electrochemical Etch Stop

• Electrochemical etching on preprocessed CMOS
  wafers
• N-type Si well with circuits suspended from SiO2
  support beam
• Thermally and electrically isolated
• TMAH etchant, Al bond pads safe

Reay et al. (1994)
32
Pressure Sensors

• Bulk micromachined pressure sensors
• In response to pressure load on thin Si film,
  piezoresistive elements detect stress
• Piezoresistivity change in electrical
  resistance due to mechanical stress
• Membrane deflection lt 1 µm

p-type substrate frame
Maluf
Integrated Pressure Sensor, Bosch
33
Isotropic Etching of Silicon
pure HF reaction-limited

• HNA hydrofluoric acid (HF), nitric acid (HNO3)
  and acetic (CH3COOH) or water
• HNO3 oxidizes Si to SiO2
• HF converts SiO2 to soluble H2SiF6
• Acetic prevents dissociation of HNO3
• Etch masks
• SiO2 etched at 300-800 ?/min
• Nonetching Au or Si3N4

pure HNO3 diffusion-limited
Robbins
34
Isotropic Etching Examples
Tjerkstra, 1997

• 5 (49) HF 80 (69) HNO3 15 H2O (by
  volume)
• Half-circular channels for chromatography
• Etch rate 0.8-1 µm/min
• Surface roughness 3 nm

• Pro and Con
• Easy to mold from rounded channels
• Etch rate and profile are highly agitation
  sensitive

35
Dry Etching of Silicon

• Dry etching
• Plasma phase
• Vapor phase
• Plasma set-up and parameters
• RF power
• Pressure
• Nonvolatile etch species

• Plasma phase etching processes
• Plasma etching
• Reactive ion etching (RIE)
• Inductively-coupled plasma RIE

36
Plasma Etching of Silicon

• SF6
• Plasma phase
• Vapor phase

37
High-Aspect-Ratio Plasma Etching

• Deep reactive ion etching (DRIE)
• Inductively-coupled plasma
• Bosch method for anisotropic etching, 1.5 - 4
  µm/min
• Etch cycle (5-15 s)
• SF6 (SFx) etches Si
• Deposition cycle (5-12 s)
• C4F8 deposits fluorocarbon protective polymer
  (-CF2-)n
• Etch mask selectivity SiO2 2001, photoresist
  1001
• Sidewall roughness scalloping lt 50 nm
• Sidewall angle 90 2

Maluf
38
DRIE Issues

• Etch rate is diffusion-limited and drops for
  narrow trenches
• Adjust mask layout to eliminate large disparities
• Adjust process parameters (etch rate slows to lt 1
  µm/min)
• Etch depth precision
• Etch stop buried layer of SiO2
• Lateral undercut at Si/SiO2 interface
• footing

Fig 3.15 p.68 Maluf
Maluf
39
DRIE Examples
Keller, MEMSPI
40
Vapor Phase Etching of Silicon

• Vapor-phase etchant XeF2
• 2XeF2(v) Si(s) ? 2Xe(v) SiF2(v)
• Etch rates 1-3 µm/min (up to 40)
• Etch masks photoresist, SiO2, Si3N4, Al, metals
• Set-up
• Closed chamber, 1 torr
• Pulsed to control exothermic heat of reaction
• Issues
• Etched surfaces have granular structure, 10 µm
  roughness
• Hazard XeF2 reacts with H2O in air to form Xe
  and HF

Xactix
41
Etching with Xenon Difluoride

• Example

Pister group
42
Laser-Driven Etching

• Laser-Assisted Chemical Etching
• Mechanism
• Etch rate 100,000 µm3/s 3 min to etch
  500?500?125 µm3 trench
• Surface roughness 30 nm RMS
• Serial process patterned directly from CAD file 
  

.
Laser-assisted etching of A 500?500 µm2 terraced
silicon well. Each step is 6 µm deep.
Revise, Inc.