Wet%20Bulk%20Micromachining%20

1
Wet Bulk Micromachining

• a STIMESI II tutorial23.02.2011

Per Ohlckers, Vestfold University College
www.hive.no Per.Ohlckers_at_hive.no
Daniel Lapadatu, SensoNor Technologies
www.multimems.com daniel.lapadatu_at_sensonor.no
2
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

3
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

4
Manufacturing Processes

• Serial (e.g. Focused Ion Milling - FIB) vs. batch
  (e.g. bulk Si micromachining) vs. continuous
  (e.g. doctors blade).
• Additive (e.g. evaporation) or subtractive (e.g.
  dry etching).
• Projection (almost all lithography techniques)
  vs. truly 3D.
• Mold vs. final product.

There are many different techniques that are
being used. However, in general, batch
processing is the most powerful technique and
used mostly.
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.

6
Classification of Bulk Silicon Etching

• Bulk Micromachining is a process that produces
  structures inside the substrate by selective
  etching.

Wet Etching
Dry Etching
Crystal orientation dependent
Process dependent
Isotropic
Anisotropic
Isotropic
Anisotropic
AcidicEtchants
AlkalineEtchants
BrF3 XeF2
F-basedplasmas
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.

8
Bulk Silicon Etching Examples
Deep cavity by wet, anisotropic etching
Recess etch by RIE
Release etch by RIE
9
Wet Silicon Etching Examples
Isotropic etching with HNA (HF Nitric Acid
Acetic Acid)
Anisotropic etching with KOH
(110)
(100)
10
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

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.
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.
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).

14
Crystallographic Planes and Directions

• (abc) denotes a plane.
• abc denotes a family of equivalent planes.
• abc denotes the direction perpendicular on
  (abc) plane.
• ltabcgt denotes a family of equivalent directions.
• 100, 110 and 111 are the most important
  families of crystal planes for the silicon
  crystal.

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.

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.

17
Wafers Used in MultiMEMS

• P-type, 150 mm Si wafer.
• (100) 0.5º Surface.
• 110 0.5º Primary Flat.

18
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

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.

20
Isotropic Etching of Silicon Etchants
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 HNO3HF ratio,- Etch limited by oxide
  removal.
• Low HNO3HF ratio- Etch limited by oxide
  formation.
• Dilute with water or acetic acid
• CH3COOH is preferred because it prevents HNO3
  dissociation.

Iso-Etch Curve (from Robbins et al.)
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.

23
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

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.

25
Etching Setup
Laboratory setup for wet chemical etching of
silicon.
The principles for industrial manufacturing
equipment are the same.
26
Anisotropic Etching of Silicon Etchants
27
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.

Si 2OH ? Si(OH)22 4e 4H2O 4e ? 4OH
2H2 (gas) Si(OH)22 4OH ? SiO2(OH)22 2H2O
Overall Si 2OH 2H2O ? ?
SiO2(OH)22 2H2 (gas)
28
Anisotropic Etching of (100)-Si

• 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.

29
Cavity Geometry for (100)-Si
Anisotropically etched cavity in (100) silicon
with a square masking film opening oriented
parallel to the lt110gt directions.
30
Mask Undercutting

• (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.

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.

32
Cavity Geometry for (110)-Si
33
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

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.

35
Time-Stopped Etching

• Example of etching stopped at an arbitrary depth,
  exhibiting a flat floor.

36
Etching Stopped by 111 Walls

• Example of etching stopped by the intersecting
  111 walls, exhibiting a pyramidal groove.

37
Boron Etch-Stop Technique

• Chemical selective etching
• Etch rate depends on boron concentration.
• Etching stops if boron concentration exceeds
  51018 cm3.
• 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.)
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.

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.

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.

41
Wafer Holder for ECES
A practical way to make a wafer holder to be used
for electro-chemical selective etching.
The principles for industrial manufacturing
equipment are the same.
42
Electrochemical Etch-Stop Mechanism

• Etch-stop achieved by reverse biasing the pn
  junctions.
• More in the Bulk Silicon Etching tutorial...

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

44
Etch-Stop on Multi-Level Junctions
45
Outline

• 1. Background and Motivation
• 2. The Silicon Crystal
• 3. Isotropic Wet Etching
• 4. Anisotropic Wet Etching
• 5. Selective Etching
• 6. Convex Corners

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.

47
Compensation for Convex Corners
Etching without corner compensation structure
Etching with corner compensation structure
Corner Compensation in Silicon (from Gupta et al.)
Corner compensation of mask is difficult to
establish as a repeatable process highly
dependant on etching parameters.
48
Corner Compensation Structures
Compensation Structure
Desired Result
A simple approach to convex corner compensation
(from Wei Fan et al.)
49
Designing with Undercut Corners
Fabrication of MEMS - MEMS Technology Seminar
(from Burhanuddin Yeop Majlis)
50
(No Transcript)