wet bulk micromachining –
DESCRIPTION
a STIMESI II tutorial 23.02.2011. Wet Bulk Micromachining – . Per Ohlckers, Vestfold University College www.hive.no [email protected]. Daniel Lapadatu, SensoNor Technologies www.multimems.com [email protected]. Outline. 1. Background and Motivation 2. The Silicon Crystal - PowerPoint PPT PresentationTRANSCRIPT
Wet Bulk Micromachining –
a STIMESI II tutorial23.02.2011
Per Ohlckers, Vestfold University College www.hive.no [email protected]
Daniel Lapadatu, SensoNor Technologies www.multimems.com [email protected]
Slide 2
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 3
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 4
Manufacturing Processes Serial (e.g. Focused Ion Milling - FIB) vs. batch (e.g. bulk Si
micromachining) vs. continuous (e.g. doctor’s blade). Additive (e.g. evaporation) or subtractive (e.g. dry etching). Projection (almost all lithography techniques) vs. truly 3D. Mold vs. final product.
FIB
There are many different techniques that are being used.
However, in general, batch processing is the most powerful technique and used mostly.
Slide 5
Micromachining Bulk Micromachining is a process that produces structures
inside the substrate by selective etching. Surface Micromachining is a process that creates structures
on top of the substrate by film deposition and selective etching.
Surface MicromachiningBulk Micromachining
Slide 6
Classification of Bulk Silicon Etching Bulk Micromachining is a process that produces structures
inside the substrate by selective etching.
Wet Etching Dry Etching
Isotropic Anisotropic Isotropic Anisotropic
AcidicEtchants
AlkalineEtchants
BrF3 XeF2
F-basedplasmas
Crystal orientationdependent
Processdependent
Slide 7
Etching Features Etch rate:
► Rate of removal of material/film.► Varies with concentration, agitation and temperature of etchant,
porosity and density of etched film.
Etch selectivity:► Relative etch rate of mask,
film and substrate.
Etch geometry:► Etching depth (R);► Mask undercut (U); ► Slope of lateral walls (S);► Bow of floor (B);► Anisotropy.
U
R
SB
Slide 8
Bulk Silicon Etching: Examples
Deep cavity by wet, anisotropic etching
Recess etchby RIE
Release etch by RIE
Slide 9
Wet Silicon Etching: ExamplesIsotropic etching with HNA
(HF : Nitric Acid : Acetic Acid) Anisotropic etching with KOH
(110)
(100)
Slide 10
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 11
Structure of Single Crystal Silicon Face-centred cubic (fcc) structure
(diamond structure) with two atoms associated with each lattice point of the unit cell.
One atom is located in position with xyz coordinates (0, 0, 0), the other in position (a/4, a/4, a/4), a being the basic unit cell length.
Lattice constant a = 5.43 Å.
The arrangement of the silicon atoms in a unit cell, with the numbers indicating the height of the atom above the base of the cube as a fraction of the cell dimension.
Slide 12
Miller Indices Miller indices are a notation system in crystallography for
planes and directions in crystal lattices. A lattice plane is determined by three integers h, k and l, the
Miller indices, written (hkl). The indices are reduced to the smallest possible integers with the same ratio.
Determining the Miller indices for planes by using the intercepts with the axes of the basic cell.
Slide 13
Determining Miller Indices Example:
► Take the intercepts of the plane along the crystallographic axes, e.g. 2, 1 and 3.
► The reciprocal of the three integers are taken: 1/2, 1/1 and 1/3.
► Multiply by the smallest common denominator (in this case 6): 3, 6 and 2.
► The Miller indices of the plane are: (362).
y
xO
z
1 2 3
1
2
3
4
1
Slide 14
Crystallographic Planes and Directions (abc) denotes a plane. {abc} denotes a family of
equivalent planes. [abc] denotes the direction
perpendicular on (abc) plane. <abc> denotes a family of
equivalent directions. {100}, {110} and {111} are
the most important families of crystal planes for the silicon crystal.
Slide 15
Single Crystal Silicon Wafers Primary and secondary flats
indicate the dopant type and surface orientation.
Wafer diameter in current fab standards: from 100 to 300 mm.
Wafer thickness in current fab standards: from 250 to 600 µm.
Surface orientation:► (100) for MOS and MEMS;► (110) for MEMS;► (111) for bipolar. Secondary flat
Primary flat
Slide 16
Standard 100 mm Wafers The position of the flat(s) indicates the surface orientation and
the type of doping. The primary flat on (100) and (110) wafers is along
the [110] direction.
Orientation of flats for 100 mm wafers
p-(111) n-(111) n-(100)p-(100)
Secondary flatPrimary flat
Slide 17
Wafers Used in MultiMEMS P-type, 150 mm Si wafer. (100) ± 0.5º Surface. [110] ± 0.5º Primary Flat.
Wafer's Primary Flat
<110> Directions
(100) Surface
x
y
z
O
{100}
[110]
Si Wafer
Slide 18
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 19
Isotropic Wet Etching of Silicon All crystallographic directions are
etched at the same rate. Features:
► Etchants are usually acids;► Etch temperature: 20... 50 °C;► Reaction is diffusion-limited;► Very high etch rate (e.g. up to
50 µm/min);► Significant mask undercutting.
Masking is very difficult:► Au/Cr or LPCVD Si3N4 is good. ► SiO2 may also be used for shallow
etching.
Si
Si
Mask
with stirring
without stirring
Si
Slide 20
Isotropic Etching of Silicon: Etchants
HF HNO3CH3COOH + H2OHF HNO3CH3COOH + H2OHF HNO3CH3COOH + H2O
10 ml30 ml80 ml25 ml50 ml25 ml 9 ml75 ml30 ml
Etchant(Diluent)
TypicalComposition
22 °C
22 °C
22 °C
Temp.
0.7... 3.0
40
7.0
Etch Rate[µm/min]
Mechanism
oxide removal:
HNO3 + H2O + HNO2 → 2HNO2 + 2OH– + 2h+
Si4+ + 4OH– → SiO2 + H2
SiO2 + 6HF → H2 SiF6
hole injection:oxidation:
Slide 21
Silicon Etching with HNA HNA: mixture of
► 49,23% HF, ► 69,51% HNO3 and ► acetic acid (CH3COOH) or
water (H2O) as diluent.
HNO3 oxidizes the silicon, HF removes the oxide:► High HNO3:HF ratio,
- Etch limited by oxide removal.► Low HNO3:HF ratio
- Etch limited by oxide formation.
Dilute with water or acetic acid:► CH3COOH is preferred because it
prevents HNO3 dissociation.
(µm/h)
Iso-Etch Curve (from Robbins et al.)
Slide 22
Isotropic Etching of Glass Single- or double-side etching of glass wafers is achieved either
by using HF-water solution or HNA. Typical etch rate for borosilicate glass in HNA: 1.9 µm/min. Applications:
► Etching cavities and through-holes;► Etching gas/fluid channels.
GLASS
Undercut
CavityThrough-hole Mask
Slide 23
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 24
Anisotropic Wet Etching of Silicon Crystallographic directions are
etched at different rates. Features:
► Etchants are usually alkaline;► Etch temperature: 85... 115 °C;► Reaction is rate-limited;► Low etch rate (ca. 1 µm/min);► Small mask undercutting.
Masking is very difficult:► LPCVD Si3N4 is good; ► SiO2 may also be used with some
etchants.
Si
Mask<111>
54.74°
<100>
<110>
Si
Slide 25
Etching Setup
The principles for industrial manufacturing equipment are the same.
Laboratory setup for wet chemical etching of silicon.
Slide 26
Anisotropic Etching of Silicon: Etchants
EtylenediaminePyrocathecolWater
750 ml120 gr100 ml
115 °C 0.75 35:1SiO2 Si3N4
metals
Etchant(Diluent)
TypicalComposition Temp. Etch Rate
[µm/min]Etch Ratio(100)/(111)
MaskingFilm
EtylenediaminePyrocathecolWater
750 ml120 gr240 ml
115 °C 1.25 35:1SiO2 Si3N4
metals
KOHWater + Isopropyl
44 gr100 ml 85 °C 1.4 400:1 SiO2
Si3N4
TMAHWater + Isopropyl
220 gr780 ml 90 °C 1.0 100:1
SiO2 Si3N4
Slide 27
Overall: Si + 2OH– + 2H2O → → SiO2(OH)2
2– + 2H2 (gas)
Chemistry of Anisotropic Etching Etching phases:
► Transport of reactants to the silicon surface;► Surface reaction;► Transport of reaction products away from the surface.
Key etch ingredients:► Oxidisers;► Oxide etchants;► Diluents and transport media.
Mechanism
Si + 2OH– → Si(OH)2
2+ + 4e–
4H2O + 4e– → 4OH– + 2H2 (gas)Si(OH)2
2+ + 4OH– → SiO2(OH)2
2– + 2H2O
Slide 28
Anisotropic Etching of (100)-Si
P-SUBSTRATE
N-LAYERS
BETCH
Truncated PyramidalCavity
OXIDE
{111} Planes
(100) Plane
V-GrooveCavity
Pyram idalCavity
<110> Directions{111} Planes
Cavity defined by:► {111} walls
– slow-etching planes;► {100} floor
– fast-etching plane.
Final shape of cavity depends on:► Mask geometry; ► Etching time.
Shape of cavity:► Truncated pyramid; ► V-groove;► Pyramid.
Slide 29
Cavity Geometry for (100)-Si
Wb = W0 – 2·l·cot(54.7°)
Anisotropically etched cavity in (100) silicon with a square masking film opening oriented parallel to the <110> directions.
Slide 30
Mask Undercutting
<110>
Si
Mask<111>
<100>
(a)
Si(b)
Si(c)
Si(d)
(e)
(a) is a pyramidal pit bounded by the {111} planes.
(b) is a type of pit expected from slow undercutting of convex corners.
(c) is a type of pit expected from fast undercutting of convex corners.
In (d), further etching of (c) produces a cantilever beam suspended over the pit.
(e) illustratates the general rule for undercutting assuming a sufficiently long etching time.
Slide 31
Anisotropic Etching of (110)-Si Cavity defined by:
► {111} walls– slow-etching planes;
► {110} floor– fastest-etching plane;
► {100} bottom side walls– fast-etching planes;
Final shape of cavity depends on:► Mask geometry; ► Etching time.
Cavity shape:► Rhombic prisms;► Hexahedric prisms.
Mask
A A'
Mask<111 >
<11 0>
a)
b)
{100 }
{11 0 }
<111 >
90 º
<100>
70 .5 º
<111>
45 º
Slide 32
Cavity Geometry for (110)-Si
Slide 33
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 34
Methods for Selective Etching Time etching methods:
► Calculate the needed etching time on the basis of the etching rate.– Easy, but inaccurate method, as etching rate varies with the chemical condition of
the etchant and geometrical factors limiting the agitation of the etch. Typical accuracy: ± 20 µm.
► Inspect the depth of the etched cavity in appropriate time intervals until desired depth is reached.
– Time consuming, but improved accuracy. Uneven etching depth from cavity to cavity due to chemical and geometrical factors is still a problem. Typical accuracy: ± 10 µm.
Chemical selective techniques:► The etching stops when a chemically resistive layer is reached.
– Typical accuracy: ± 3 µm.
Electrochemical selective techniques: ► The etching stops on reverse biased pn junctions.
– Typical accuracy: ± 1 µm.
Slide 35
Time-Stopped Etching Example of etching stopped at an arbitrary depth, exhibiting a flat
floor.
Slide 36
Etching Stopped by {111} Walls Example of etching stopped by the intersecting {111} walls,
exhibiting a pyramidal groove.
Slide 37
Boron Etch-Stop Technique Chemical selective etching:
► Etch rate depends on boron concentration.
► Etching stops if boron concentration exceeds 5·1018 cm–3.
Boron stop layer is manufactured:► By diffusion deposition,
implantation or both; ► On the opposite surface of the
wafer with respect to the etch cavity.
Boron-dependent etch rate of silicon (from Seidel et al.)
Slide 38
Boron Etch-Stop Mechanism Interstitial bonds require more energy to be broken. The electrons supplied by the etchant recombine with the holes
in the bulk, rather than participating in the chemical reaction.
Silicon atom Substitutionalboron atom
Interstitialboron atom
Slide 39
Boron Etch-Stop Shortcomings Electronics cannot be integrated in the boron stop layer.
► Solution: depositing an epitaxial layer atop the stop layer, with appropriate doping as substrate material for integrated devices.
– Controlling the autodoping of the epi-layer is challenging.
n-type epitaxial layer
p+ type boron stop layer
n-type substrate
Slide 40
Electrochemical Etch-Stop (ECES) Electrochemical selective etching:
► Etch rate depends on the applied potential.
► Etching stops if the applied potential exceeds a threshold value, called passivation potential.
► Low-doped material, both p- and n-type, can be passivated:
– To be used as substrate for integrated components such as piezoresistors.
High accuracy, typically ± 1 µm:► Achieved by using well-controlled
implantation and diffusion techniques.
KOH and TMAH can be used:– Both avoid the health dangers of EDP.
n p
Pt
V+ –
p
etchant
oxide
stir bar
Slide 41
Wafer Holder for ECES
The principles for industrial manufacturing equipment are the same.
A practical way to make a wafer holder to be used for electro-chemical selective etching.
Slide 42
Electrochemical Etch-Stop Mechanism
V
EtchRate
Vn
Silicon Etching
Vp
Anodic Passivation
Vpp
+
N-layersP-layers
Etch-stop achieved by reverse biasing the pn junctions. More in the Bulk Silicon Etching tutorial...
Slide 43
ECES for MultiMEMS Electrochemical etch-stop allows 3 different thicknesses:
► Full-wafer thickness (400 µm) For heavy seismic masses;
► Epi-layer thickness (3 µm) For thin membrane, springs;
► N-well thickness (23 µm) For thick membranes, masses, bosses…
Slide 44
Etch-Stop on Multi-Level Junctions
Thin Membrane
Very Thin Membrane
Thick Membrane (Mass/Well)
Slide 45
Outline
1. Background and Motivation
2. The Silicon Crystal
3. Isotropic Wet Etching
4. Anisotropic Wet Etching
5. Selective Etching
6. Convex Corners
Slide 46
Undercutting of Convex Corners High etch-rate of high-index planes:
► Severe undercutting of convex corners;► Truncated pyramids or V-grooves as final cavities.
undercut
{100}
{111}
{221}
Mask will be undercut until {111} planes are exposed.
Mask
{111}
Slide 47
Compensation for Convex Corners
Etching without corner compensation structure
Etching with corner compensation structure
Corner compensation of mask is difficult to establish as a repeatable process; highly dependant on etching parameters.
Corner Compensation in Silicon (from Gupta et al.)
Slide 48
Corner Compensation Structures
A simple approach to convex corner compensation (from Wei Fan et al.)
Desired Result
CompensationStructure
Slide 49
Designing with Undercut Corners
Fabrication of MEMS - MEMS Technology Seminar (from Burhanuddin Yeop Majlis)
Slide 50
Thank you for your attention !