Wet Bulk Micromachining –

a STIMESI II tutorial23.02.2011

Per Ohlckers, Vestfold University College www.hive.no Per.Ohlckers@hive.no

Daniel Lapadatu, SensoNor Technologies www.multimems.com daniel.lapadatu@sensonor.no

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 !