伯克利课程MEMS设计IntroductiontoMEMSdesign3
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• Silicon also exhibit a strong piezoresistive effect
• These properties, combined with silicon’s exceptional mechanical characteristics, and well-developed manufacturing base, make silicon the ideal material for precision sensors
compressive oxide still showing
(lower right)
MEMS Design & Fab
ksjp, 7/01
(100)
(110) (111)
(110)
(111)
(110)
Clever KOH etching of (100)
Clockwise from above: Ternez; Rosengren; Keller
ksjp, 7/01
MEMS Design & Fab
Anisotropic Etching of Silicon
<111>
<100>
54.7 Silicon Substrate
• Anisotropic etches have direction dependent etch rates in crystals • Typically the etch rates are slower perpendicularly to the crystalline
MEMS Design & Fab
ksjp, 7/01
Surface micromachining material systems
• Structure/
sacrificial/
etchant
• Polysilicon/ Silicon dioxide/ HF
• Silicon dioxide/ polysilicon/ XeF2
ksjp, 7/01
Crystal Planes & Miller Indices • [abc] in a cubic crystal is just a direction vector • (abc) is any plane perpendicular to the [abc] vector • (…)/[…] indicate a specific plane/direction • {…}/<…> indicate equivalent planes/direction
e.g.: (100),(111) Cos1((1 0 0) /(1)( 3))
MEMS Design & Fab
ksjp, 7/01
Miller indices
[001]
[abc]
[100]
[010]
c a b
MEMS Design & Fab
ksjp, 7/01
[001]
1/c
(abc)
• Pressure sensors and accelerometers were the first to be developed
Silicon pressure sensor chip Packaged pressureMsEMeSnDseosrign & Fab
ksjp, 7/01
KOH etching: atomic view
MEMS Design & Fab
ksjp, 7/01
Residual stress gradients
ksjp, 7/01
A bad day at MCNC (1996).
MEMS Design & Fab
Hinges
Deposit first sacrificial Deposit and pattern first poly
(111)
(110)
MEMS Design & Fab
ksjp, 7/01
Surface Micromachining
Deposit sacrificial layer
Pattern contacts
Deposit/pattern structural layer
Etch sacrificial layer
MEMS Design & Fab
ksjp, 7/01
KOH Etching
• Etches PR and Aluminum instantly • Masks:
• SiO2 • compressive
• SixNy • tensile
• Parylene! • Au?
MEMS Design & Fab
ksjp, 7/01
Bulk, Surface, DRIE • Bulk micromachining involves removing material from
the silicon wafer itself
• Typically wet etched
• Traditional MEMS industry
MEMS-specific fabrication
• Bulk micromachining • Surface micromachining • Deep reactive ion etching (DRIE) • Other materials/processes
MEMS Design & Fab
Deposit and pattern second sacrificial
Pattern contacts Deposit and pattern 2nd poly
Etch sacrificial
ksjp, 7/01
• Isotropic silicon etchants
• HNA • HF, nitric, and acetic acids • Lots of neat features, tough to work with
• XeF2, BrF3 • gas phase, gentle • Xactix, STS selling research & production equipment
ksjp, 7/01
Typical 100 wafer
<100>
<111>
Cross-section in (110) plane
ksjp, 7/01
The wafer flat is oriented in the [110] direction
MEMS Design & Fab
<100>
<111>
(110)
(111)
(111)
(111)
MEMS Design & Fab
ksjp, 7/01
Rosette
“Amplified” etch rate Masking layer
Lateral undercut
ksjp, 7/01
Un-etched silicon
MEMS Design & Fab
MEMS Design & Fab
ksjp, 7/01
Bulk micromachined cavities
• Anisotropic KOH etch (Upperleft) • Isotropic plasma etch (upper right) • Isotropic BrF3 etch with
planes with the highest density • Commonly used anisotropic etches in silicon include Potasium
Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TmAH), and Ethylene Diamine Pyrochatecol (EDP)
Micromachining Ink Jet Nozzles
Microtechnology group, TU Berlin
MEMS Design & Fab
ksjp, 7/01
Bulk Micromachining
• Anisotropic etching allows very precise machining of silicon
• <111> etch rate is slowest, <100> and <110> fastest • Fastest:slowest can be more than 400:1 • KOH, EDP, TMAH most common anisotropic silicon etchants
• Aluminum/ photoresist/ oxygen plasma
• Photoresist/ aluminum/ Al etch
• Aluminum/ SCS
EDP, TMAH, XeF2
• Poly-SiGe
poly-SiGe
DI water
ksjp, 7/01
MEMS Design & Fab
but looks like surface micromachining!
MEMS Design & Fab
ksjp, 7/01
Bulk Micromachining • Many liquid etchants demonstrate dramatic etch rate
differences in different crystal directions
Angles between directions can be determined by scalar product: the angle between [abc] and [xyz] is given by ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(theta)
• Artistic design, inexpensive equipment
• Issues with IC compatibility
• Surface micromachining leaves the wafer untouched,
but adds/removes additional layers above the wafer surface, First widely used in 1990s
Residual stress gradients
More tensile on top
More compressive on top
Just right! The bottom line: anneal poly between oxides with similar phosphorous content. ~1000C for ~60 seconds is enough.
STM image of a (111) face with a ~10 atom step. From Weisendanger, et al., Scanning tunnelling microscopy study of Si(111)7*7 in the presence of multiple-step edges, Europhysics Letters, 12, 57 (1990).
c
[010]
1/b 1/a
a
[100]
[abc] b
MEMS Design & Fab
ksjp, 7/01
[001]
{100}
(001)
[010]
[100]
(100)
(010)
MEMS Design & Fab
ksjp, 7/01
[001]
[010]
[100]
(110)
(111)
MEMS Design & Fab
MEMS Design & Fab
ksjp, 7/01
Etch stops in anisotropic silicon etching
• Electrochemical etch stop • High boron doping (~1e20/cm)
MEMS Design & Fab
ksjp, 7/01
• Typically plasma etched
• IC-like design philosophy, relatively expensive equipment
• Different issues with IC compatibility
• Deep Reactive Ion Etch (DRIE) removes substrate
• These properties, combined with silicon’s exceptional mechanical characteristics, and well-developed manufacturing base, make silicon the ideal material for precision sensors
compressive oxide still showing
(lower right)
MEMS Design & Fab
ksjp, 7/01
(100)
(110) (111)
(110)
(111)
(110)
Clever KOH etching of (100)
Clockwise from above: Ternez; Rosengren; Keller
ksjp, 7/01
MEMS Design & Fab
Anisotropic Etching of Silicon
<111>
<100>
54.7 Silicon Substrate
• Anisotropic etches have direction dependent etch rates in crystals • Typically the etch rates are slower perpendicularly to the crystalline
MEMS Design & Fab
ksjp, 7/01
Surface micromachining material systems
• Structure/
sacrificial/
etchant
• Polysilicon/ Silicon dioxide/ HF
• Silicon dioxide/ polysilicon/ XeF2
ksjp, 7/01
Crystal Planes & Miller Indices • [abc] in a cubic crystal is just a direction vector • (abc) is any plane perpendicular to the [abc] vector • (…)/[…] indicate a specific plane/direction • {…}/<…> indicate equivalent planes/direction
e.g.: (100),(111) Cos1((1 0 0) /(1)( 3))
MEMS Design & Fab
ksjp, 7/01
Miller indices
[001]
[abc]
[100]
[010]
c a b
MEMS Design & Fab
ksjp, 7/01
[001]
1/c
(abc)
• Pressure sensors and accelerometers were the first to be developed
Silicon pressure sensor chip Packaged pressureMsEMeSnDseosrign & Fab
ksjp, 7/01
KOH etching: atomic view
MEMS Design & Fab
ksjp, 7/01
Residual stress gradients
ksjp, 7/01
A bad day at MCNC (1996).
MEMS Design & Fab
Hinges
Deposit first sacrificial Deposit and pattern first poly
(111)
(110)
MEMS Design & Fab
ksjp, 7/01
Surface Micromachining
Deposit sacrificial layer
Pattern contacts
Deposit/pattern structural layer
Etch sacrificial layer
MEMS Design & Fab
ksjp, 7/01
KOH Etching
• Etches PR and Aluminum instantly • Masks:
• SiO2 • compressive
• SixNy • tensile
• Parylene! • Au?
MEMS Design & Fab
ksjp, 7/01
Bulk, Surface, DRIE • Bulk micromachining involves removing material from
the silicon wafer itself
• Typically wet etched
• Traditional MEMS industry
MEMS-specific fabrication
• Bulk micromachining • Surface micromachining • Deep reactive ion etching (DRIE) • Other materials/processes
MEMS Design & Fab
Deposit and pattern second sacrificial
Pattern contacts Deposit and pattern 2nd poly
Etch sacrificial
ksjp, 7/01
• Isotropic silicon etchants
• HNA • HF, nitric, and acetic acids • Lots of neat features, tough to work with
• XeF2, BrF3 • gas phase, gentle • Xactix, STS selling research & production equipment
ksjp, 7/01
Typical 100 wafer
<100>
<111>
Cross-section in (110) plane
ksjp, 7/01
The wafer flat is oriented in the [110] direction
MEMS Design & Fab
<100>
<111>
(110)
(111)
(111)
(111)
MEMS Design & Fab
ksjp, 7/01
Rosette
“Amplified” etch rate Masking layer
Lateral undercut
ksjp, 7/01
Un-etched silicon
MEMS Design & Fab
MEMS Design & Fab
ksjp, 7/01
Bulk micromachined cavities
• Anisotropic KOH etch (Upperleft) • Isotropic plasma etch (upper right) • Isotropic BrF3 etch with
planes with the highest density • Commonly used anisotropic etches in silicon include Potasium
Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TmAH), and Ethylene Diamine Pyrochatecol (EDP)
Micromachining Ink Jet Nozzles
Microtechnology group, TU Berlin
MEMS Design & Fab
ksjp, 7/01
Bulk Micromachining
• Anisotropic etching allows very precise machining of silicon
• <111> etch rate is slowest, <100> and <110> fastest • Fastest:slowest can be more than 400:1 • KOH, EDP, TMAH most common anisotropic silicon etchants
• Aluminum/ photoresist/ oxygen plasma
• Photoresist/ aluminum/ Al etch
• Aluminum/ SCS
EDP, TMAH, XeF2
• Poly-SiGe
poly-SiGe
DI water
ksjp, 7/01
MEMS Design & Fab
but looks like surface micromachining!
MEMS Design & Fab
ksjp, 7/01
Bulk Micromachining • Many liquid etchants demonstrate dramatic etch rate
differences in different crystal directions
Angles between directions can be determined by scalar product: the angle between [abc] and [xyz] is given by ax+by+cz = |(a,b,c)|*|(x,y,z)|*cos(theta)
• Artistic design, inexpensive equipment
• Issues with IC compatibility
• Surface micromachining leaves the wafer untouched,
but adds/removes additional layers above the wafer surface, First widely used in 1990s
Residual stress gradients
More tensile on top
More compressive on top
Just right! The bottom line: anneal poly between oxides with similar phosphorous content. ~1000C for ~60 seconds is enough.
STM image of a (111) face with a ~10 atom step. From Weisendanger, et al., Scanning tunnelling microscopy study of Si(111)7*7 in the presence of multiple-step edges, Europhysics Letters, 12, 57 (1990).
c
[010]
1/b 1/a
a
[100]
[abc] b
MEMS Design & Fab
ksjp, 7/01
[001]
{100}
(001)
[010]
[100]
(100)
(010)
MEMS Design & Fab
ksjp, 7/01
[001]
[010]
[100]
(110)
(111)
MEMS Design & Fab
MEMS Design & Fab
ksjp, 7/01
Etch stops in anisotropic silicon etching
• Electrochemical etch stop • High boron doping (~1e20/cm)
MEMS Design & Fab
ksjp, 7/01
• Typically plasma etched
• IC-like design philosophy, relatively expensive equipment
• Different issues with IC compatibility
• Deep Reactive Ion Etch (DRIE) removes substrate