FABRICATION TECHNIQUES
BULK MICROMACHINING: H2&3
Bulk micromachining: MEMS fabrication technique where material is
removed from the bulk volume of a Si-wafer to create 3D structures.
Structures are formed by etching into substrate itself.
Thickness: tens to hundreds of µm
DEPOSITION
Lithography: coating with photoresist (with spin coater) > add
photomask with pattern (positive/negative) > UV > development: wafer in
solution > removal of uncured material > 2D pattern of photoresist on
wafer > photoresist is now etch mask (> etching > remove mask)
Only determines where to etch (not how deep, …)
Deposition techniques:
+ PVD
1
,Physical vapor deposition (PVD): thin-film deposition technique for
adding material to the wafer PHYSICALLY
Material is physically removed from a solid source > converted into
vapor in a vacuum > condensed onto a wafer to form a film
3 options:
o Thermal evaporation
o E-beam evaporation
o Sputtering: Ar into vacuum chamber > plasma by ionizing the
gas > Ar+ accelerated toward target > ions knocks atoms out
of target > atoms condense on wafer forming film
PLASMA: Ar-gas
into vacuum chamber (low pressure) > RF voltage applied with frequency
13.56 MHz > oscillating E-field > free electrons are accelerated >
collision with Ar-atoms > Ar+ + 2e- > plasma with free electrons, Ar+, free
radicals, …
Chemical vapor deposition (CVD): deposition of material onto wafer
CHEMICALLY (conformal)
Reactive gas to heated wafer > chemical reaction at surface of wafer
> film grows onto wafer
Growth rate:
o For monocrystalline: at high T & small growth rate
Crystals have enough time to form & grow
o For polycrystalline: at lower T & high growth rate
o To grow on metals: plasma enhanced CVD, but lower quality
Low pressure CVD: more directionality (because of increased
mean free path)
2
, Laser beam CVD: laser beam focused on substrate, defines where
deposition needs to happen (locally heats up substrate or breaks
chemical bonds at low T)
Thermal oxidation USES Si (SiO2 is grown out of the Si wafer), CVD
doesn’
Step coverage: deposited film is not
equally thick everywhere, especially on
sidewalls & sharp corners causes
electrical discontinuities (we need
conform deposition)
3
, WET ETCHING (after lithography)
Wet etching: patterned wafer (after lithography) in etchant solution >
chemical reaction on uncured material > it solves in solution & rest is kept
Isotropic: etches in all directions (vertical & horizontal) at same
rate & does not depend on orientation of crystal planes
o Round pits BUT risk of
undercutting!!!
o Etchant = HNA
HNO3 + HF + CH3COOH
Anisotropic: etches only in vertical direction
o V-pits
o ONLY WITH MONOCRYSTALLINE Si & does depend on
orientation of Si
100: fast
110: medium
111: very slow, Si atoms
are more bound (more neighbors)
Angle between 100 & 111: 54.74°
o Etchant = KOH(/TMAH)
100 wafer:
- Oxidation: of Si with OH- >
Si(OH)22+
o Si(OH)22+ again reacts
with OH- > Si(OH)62-
= SOLUBLE IN WATER
4
BULK MICROMACHINING: H2&3
Bulk micromachining: MEMS fabrication technique where material is
removed from the bulk volume of a Si-wafer to create 3D structures.
Structures are formed by etching into substrate itself.
Thickness: tens to hundreds of µm
DEPOSITION
Lithography: coating with photoresist (with spin coater) > add
photomask with pattern (positive/negative) > UV > development: wafer in
solution > removal of uncured material > 2D pattern of photoresist on
wafer > photoresist is now etch mask (> etching > remove mask)
Only determines where to etch (not how deep, …)
Deposition techniques:
+ PVD
1
,Physical vapor deposition (PVD): thin-film deposition technique for
adding material to the wafer PHYSICALLY
Material is physically removed from a solid source > converted into
vapor in a vacuum > condensed onto a wafer to form a film
3 options:
o Thermal evaporation
o E-beam evaporation
o Sputtering: Ar into vacuum chamber > plasma by ionizing the
gas > Ar+ accelerated toward target > ions knocks atoms out
of target > atoms condense on wafer forming film
PLASMA: Ar-gas
into vacuum chamber (low pressure) > RF voltage applied with frequency
13.56 MHz > oscillating E-field > free electrons are accelerated >
collision with Ar-atoms > Ar+ + 2e- > plasma with free electrons, Ar+, free
radicals, …
Chemical vapor deposition (CVD): deposition of material onto wafer
CHEMICALLY (conformal)
Reactive gas to heated wafer > chemical reaction at surface of wafer
> film grows onto wafer
Growth rate:
o For monocrystalline: at high T & small growth rate
Crystals have enough time to form & grow
o For polycrystalline: at lower T & high growth rate
o To grow on metals: plasma enhanced CVD, but lower quality
Low pressure CVD: more directionality (because of increased
mean free path)
2
, Laser beam CVD: laser beam focused on substrate, defines where
deposition needs to happen (locally heats up substrate or breaks
chemical bonds at low T)
Thermal oxidation USES Si (SiO2 is grown out of the Si wafer), CVD
doesn’
Step coverage: deposited film is not
equally thick everywhere, especially on
sidewalls & sharp corners causes
electrical discontinuities (we need
conform deposition)
3
, WET ETCHING (after lithography)
Wet etching: patterned wafer (after lithography) in etchant solution >
chemical reaction on uncured material > it solves in solution & rest is kept
Isotropic: etches in all directions (vertical & horizontal) at same
rate & does not depend on orientation of crystal planes
o Round pits BUT risk of
undercutting!!!
o Etchant = HNA
HNO3 + HF + CH3COOH
Anisotropic: etches only in vertical direction
o V-pits
o ONLY WITH MONOCRYSTALLINE Si & does depend on
orientation of Si
100: fast
110: medium
111: very slow, Si atoms
are more bound (more neighbors)
Angle between 100 & 111: 54.74°
o Etchant = KOH(/TMAH)
100 wafer:
- Oxidation: of Si with OH- >
Si(OH)22+
o Si(OH)22+ again reacts
with OH- > Si(OH)62-
= SOLUBLE IN WATER
4
