Course Outline

1. Introduction
2. Materials of Microsystems Engineering
3. Clean Rooms/Yield
4. Thin Films
5. Lithography
6. Surface Micromachining
7. Bulk Micromachining
8. LIGA
9. Packaging Technology

Microsystems Engineering Prof. Dr. Michael Rüb 1

6. Surface Micromachining Fachhochschule Jena
 6. Surface Micromachining

6.1. Differences Surface- and BulkMM

6.2. Sacrifical Layer Technology
6.3. Typical Process Challenges in SurfaceMM
6.4. Monolithic Integration
6.5. Foundries for SurfaceMM
6.6. Examples for Products Made in SurfaceMM

Microsystems Engineering Prof. Dr. Michael Rüb 2

6. Surface Micromachining Fachhochschule Jena
 Bulk micromachining Surface micromachining

LIGA technique

Microsystems Engineering Prof. Dr. Michael Rüb 3

6. Surface Micromachining Fachhochschule Jena
 BulkMM   Surface MM

Bulk MM
-structures are etched into the substrate
-single crystalline material

Surface MM
-structures are built up as additve layers
on a wafer; deposition of thin layers
-polycrystalline material

Microsystems Engineering Prof. Dr. Michael Rüb 4

6. Surface Micromachining Fachhochschule Jena
 Examples from Application

Microsystems Engineering Prof. Dr. Michael Rüb 5

6. Surface Micromachining Fachhochschule Jena
 6. Surface Micromachining

6.1. Differences Surface- and BulkMM

6.2. Sacrifical Layer Technology
6.3. Typical Process Challenges in SurfaceMM
6.4. Monolithic Integration
6.5. Foundries for SurfaceMM
6.6. Examples for Products Made in SurfaceMM

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6. Surface Micromachining Fachhochschule Jena
 Process Sequence of Sacrifical Layer Technology

1. Deposition of Sacrifical Layer

- process dep. on material
- e.g. thermal oxidation for SiO2

2. Patterning of Sacrifical Layer

- opening for anchor structure to
substrate
- patterning according to needs
isotropic (wet)
anisotropic (plasma etch)

3. Deposition of Functional Layer

- process dep. on material
(e.g. PVD, CVD)
- sacrifical layer is completely
covered

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6. Surface Micromachining Fachhochschule Jena
 Process Sequence of Sacrifical Layer Technology

4. Patterning of Functional Layer

- process dep. on material used
e.g. DRIE plasma etch
-etch stopp on sacrifical layer

5. Removal of Sacrifical Layer

- selective and isotropic etching
of sacrifical layer
- selective; no attack of structural
layer;
- isotropic; remove layer also
underneath of functional layer

Microsystems Engineering Prof. Dr. Michael Rüb 8

6. Surface Micromachining Fachhochschule Jena
 Process Sequence of Sacrifical Layer Technology

Properties of SurfaceMM
- additive technology
- functional layers in most cases
polycrystalline silicon
aluminium

Purpose of Sacrifical Layer

- sacrifical layers are oxides or polymers
- sacrifical layers define vertical distance
of functional layer to substrate

Criteria for Selecting a Sacrifical Layer

- istropic etch chemistry available
- high selectivity against functional layer
- process compatibility; e.g. CMOS
compatible.

Microsystems Engineering Prof. Dr. Michael Rüb 9

6. Surface Micromachining Fachhochschule Jena
 6. Surface Micromachining

6.1. Differences Surface- and BulkMM

6.2. Sacrifical Layer Technology
6.3. Typical Process Challenges in SurfaceMM
6.4. Monolithic Integration
6.5. Foundries for SurfaceMM
6.6. Examples for Products Made in SurfaceMM

Microsystems Engineering Prof. Dr. Michael Rüb 10

6. Surface Micromachining Fachhochschule Jena
 Challenges in SurfaceMM

Removal of Sacrifical Layer only possible on lateral distances (small areas)

- etch is isotropic i.e. material beneath a functional layer must be removed

by large undercutting

-result: long etch times

-high selectivity with respect to functional layer needed

Resulting Design Rules:

-use only structures with small lateral
dimensions
-large areas need to be perforated

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6. Surface Micromachining Fachhochschule Jena
 Challenges in SurfaceMM
Low Stiffness in z-direction
- root cause: functional layers are only
a few µm thick

- consequence: structures easily escape

in z-direction

Internal Stress
- frequently structures with high ratio
length/thickness are used

-structures must be extremly straight

(and free of stress) and parallel to
substrate in order to avoid
substrate contact

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6. Surface Micromachining Fachhochschule Jena
 Annealed Polycrystalline Silicon

at sufficient temperature (1050°C)

- compressive stress is converted into tensile stress
- further anneling leads to complete disappearance of stress

tensile

compressive

time

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 Challenges in SurfaceMM

Sticking

- if movable structures touch the substrate, they may remain glued to the
ground by adhesive forces
- processing problem with wet chemical etching of sacrifical layer
-> when removing the liquid medium, capillary forces always exit, which
pull the structures to the ground

Solution 1: Application of special drying processes

Solution 2: Spacers and bumpers

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6. Surface Micromachining Fachhochschule Jena
 Typical process sequence
- Wet chemical etching of sacrifical layer
- Rinsing with water or isopropanol (removes HF)
- Drying of substrate

Open areas dry first -> dops of liquid remain between structure and substrate

Liquid drops get smaller and smaller -> surface tension pulls microstructure
to substrate

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6. Surface Micromachining Fachhochschule Jena
 Vaporisation (Liquid)  Sublimation (Solid)

Liquid fills gap Liquid is solidified (solid material)

Liquid drop gets smaller in 3dim Isothermal sublimaton

 adhesion forces pull the Column is getting smaller only in 2dim
structure towards the substrate No suface tension  no pulling force

After vaporisation structure sticks After sublimation  free standing

to substrate structure, which does not stick

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6. Surface Micromachining Fachhochschule Jena
 Dichlorobenzene Process

Dichlorobenzene melts at 60°C and is soluble

in alcohol Pressure

Process sequence for removing HF-acid solid liquid

Rinsing with deionised water
Rinsing with 70°C Isopropanol
Rinsing with 70°C Dichlorobenzene triplepoint
Cooldown to room temperature
Dichlorobenzene is transfered to solid state
Isothermal sublimation of Dichlorobenzene
Temp
(evacuation)

Thin structures 1000µm in length can be

manufactured without sticking

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6. Surface Micromachining Fachhochschule Jena
 Thermodynamics – Critical Point
phase diagram of carbondioxide

solid
In thermodynamics the critical point
is defined as the point where the
supercritical fluid difference between the gaseous and
the liquid state seize to exist.

liquid
pressure

The critical point defines the upper

critical point end of the vapour pressure curve.

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 Supercritical Drying
Isothermal increase of pressure
Isobaric heating beyond the critical point Pressure critical
point
phase transition from liquid to
solid
supercritical phase
liquid
interface liquid/gaseous is avoided
no surface tension
triplepoint
Isothermal relaxation into gaseous state
phase transition from supercritical
phase to gaseous state
no interface Temp
no surface tension

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6. Surface Micromachining Fachhochschule Jena
 Example for „supercritical drying“

„CO2 supercritical drying“

critical point: T=31°C und p=72,8atm

• Rinsing of etched structures with deionised water

• Water is replaced by Methanol

• Move wafer into pressure chamber

• At 25°C and 80atm Methanol is replaced by liquid CO2

• At identical pressure (80atm) CO2 is heated and transfered to supercritical

phase

• Bringing the chamber to ambient pressure and temperature

 CO2 is transfered to gaseous state

Microsystems Engineering Prof. Dr. Michael Rüb 20

6. Surface Micromachining Fachhochschule Jena
 6. Surface Micromachining

6.1. Differences Surface- and BulkMM

6.2. Sacrifical Layer Technology
6.3. Typical Process Challenges in SurfaceMM
6.4. Monolithic Integration
6.5. Foundries for SurfaceMM
6.6. Examples for Products Made in SurfaceMM

Microsystems Engineering Prof. Dr. Michael Rüb 21

6. Surface Micromachining Fachhochschule Jena
 Monolithic Integration in Surface Micromachining

Monolithically integrated
acceleration sensor

Micromechanical acceleration
sensor and signal processing
electronics integrated in one
chip

Alternatively
„Hybrid integration“ was
possible

i.e. mechanics and

signal processing on two separate
chips that are connected by
wires

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6. Surface Micromachining Fachhochschule Jena
 Integration of Electronics before Mechanics

• First signal processing electronics in CMOS technology is produced

•Then, mechanical structures

 Electronics can be „buried“ beneath mechanics

Challenges DLP, TI
Only CMOS compatible process steps
can be applied
 temperature
 no KOH
 do not destroy metal and doping

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6. Surface Micromachining Fachhochschule Jena
 Integration of Mechanics at the same Time as Electronics

Process:

• Micromechanical elements are composed of the same layers as electronics

 usually restrictions on polysilicon thickness:
0,5µm poly thickness @ 0,8µm CMOS process

Challenges:

• „Free moving“ structures of thin layers are more susceptible to sticking

 structures have to be designed smaller
 restrictions in mechanical properties

Microsystems Engineering Prof. Dr. Michael Rüb 24

6. Surface Micromachining Fachhochschule Jena
 6. Surface Micromachining

6.1. Differences Surface- and BulkMM

6.2. Sacrifical Layer Technology
6.3. Typical Process Challenges in SurfaceMM
6.4. Monolithic Integration
6.5. Foundries for SurfaceMM
What are Foundries good for?
Bosch OMM Foundry
MUMPS Foundry
6.6. Examples for Products Made in SurfaceMM

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6. Surface Micromachining Fachhochschule Jena
 Why Do We Need Foundries?

• A foundry enables access to an established process line

(approx. 5000€/ 1- 100 Chips, depending on complexity)

• Foundries require strict design rules!!

When you are thirsty,

then normally you
wouldn´t buy a cow, but
you would buy milk from
a retailer

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6. Surface Micromachining Fachhochschule Jena
 Why Do We Need Foundries?
What types of „milk“ is available?

Surface Micromachining Foundry

BULK Micromachining Foundry

Microelectronics Foundries (Samsung, Grace-Semi, IMS-Stuttgart)

Mikroelektronik Foundries (Samsung, Grace Semi, IMS Stuttgart etc.

Massive advantage for product development

 engineering sample
 prototype
 mass product

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6. Surface Micromachining Fachhochschule Jena
 Foundry grants access to
 cheap prototypes
to high volume production

with identical process sequence

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6. Surface Micromachining Fachhochschule Jena