MEMS: Fabrication

Lecture 5: Micromachining

Prasanna S. GandhiAssistant Professor,Department of Mechanical Engineering,Indian Institute of Technology, Bombay,

Recap: Last Class

E-beam lithographyX-ray lithographyIon beam lithographyOxidation

Today’s Class

Si wafer preparationClean room fundamentalsChemical etching processAnisotropic EtchingSilicon micromachining

Surface micromachiningBulk micromachiningHow to produce devices

Si-wafer preparation

Czochralski process

Cutting, CMP, cleaning

Clean Room Fundamentals

NeedClass of a clean environmentClass X clean room not more than X particles (of size 0.5µm or larger) per cubic foot of airHow this cleanliness is produced and maintained??

Clean Room Fundamentals

Air conditioning plantHEPA filters air recirculation through these filters

Chemical Etching

Isotropic etchingEtchant: HNA mixture.HNA can dissolve 550µm thick silicon wafer in about 20 min.HNA mixture removes silicon equally in all directions.SiO2 etch: 10-30nm/min

Without agitation (5)

With agitation (20)

Chemical Etching

Isotropic etchingUndercutEtch bias

Materials & etchants*Lift off process

Without agitation (5)

Chemical EtchingMechanisms

AnodicDopant as etch stopPrecautions

Etch diagrams

Chemical Etching

Choice of etchant:Ease of handlingToxicityEtch rateTopology of the surface to be etchedEtch selectivity of mask material and other materials

Chemical Etching

Anisotropic bulk etchingEtchant: KOH, EDP

(ethylendiamine, pyrocatechol and water), TMAH<111> direction has lower etching rates than <100>Can produce grooves, slanted/vertical walls

<110> surface wafer

<100>

<010>

<001><111>

<100> surface wafer

Chemical Etching

Silicon crystal geometry*

Examples of use of the crystal geometry in etchingFundes regarding etch shapes under different conditions*

<100>

<010>

<001><111>

Anisotropic Etching

KOH, EDP and TMAHEDP etches oxide 100 times slower than KOH, KOH, TMAH dangerous to eyeKOH less dangerous than EDP & TMAHEtch curves*: 5hrs to etch 300µm thick waferH2 bubbles during KOH etching of SiEDP ages quickly in contact with oxygen producing red brown color, vapor is harmfulHF dip is necessary for EDP: native oxide problem

<100>

<010>

<001><111>

Table 5.1 : Negative and Positive Resisits

Lithography Name Type Sensitivity γOptical Kodak 747 Negative 9 mJ/cm2 1.9

AZ-1350J Positive 90 mJ/cm2 1 .4PR102 Positive 140 mJ/cm2 1.9

e-beam COP Negative 0.3 µC/cm2 0.45GeSe Negative 80 µC/cm2 3.5PBS Positive 1 µC/cm2 0.35PMMA Positive 50 µC/cm2 1.0

X-ray COP Negative 175 mJ/cm2 0.45DCOPA Negative 10 mJ/cm2 0.65PBS Positive 95 mJ/cm2 0.5PMMA Positive 1000 mJ/cm2 1.0

Secondary flat

Primary flat

Secondary flat

Primary flat

Primary flat

Primary flat

Secondary flat

Primary and Secondary wafer flats are used to identify orientation and type.

(100) n-type (100) p-type

(111) n-type(111) p-type

A’

A

Top View

[110]

54.7o

Cross Section A-A’

A square <110>-oriented mask feature results in a pyramidal pit.

A

A’A

A’A

A’

Convex corners are rapidly undercut

54.7o

[100] Silicon

[110][100] Masking Layer

Cross Section A-A’

[100]

Cross Section A-A’

Convex corners where {111} planes meet are not stable. They are rapidly undercut. This permits creation of suspended structures.

A’

ABoundary of rectangular pit[110]

Undercut regions

[100] Masking Layer

Cross Section A-A’

Any mask-layer feature, if etched long enough, will result in a rectangular V-groove pit beneath a rectangular that is tangent to the mask features, with edges oriented along <110> directions.

5o misalignment

Boundary of rectangular pit[110]

The effect of misalignment is to enlarge the etched region. Thisfigure shows the effect of a 5o misalignment for a rectangular feature.

Re-entrant resist profile

Wafer with

photoresist

Directional evaporation (e-beam)

Strip resist and

lift off metal

Illustrating the lift-off method for patterning evaporated metals.

Wafer with double-layer

undercut masking

Directional evaporation gradually

closes opening

At closure, a sharp tip is formed

The use of a modified lift-off process to create sharp tips.

After lift-off

Conclusions

Chemical etchingIsotropic Anisotropic

Liftoff processBulk micromachining

Next class

Plasma based processesPlasma etchingRIE

SputteringPE CVD

Wafer Cleaning Process

NeedCMP- Chemical mechanical polishing

RCA cleaning

Chemical Vapor Deposition (CVD)

Chemical reaction in vacuum chamberHigh temperatures (>300oC)Polysilicon, SiO2, Si3N4, tungston, titanium, copper etc. can be depositedLow pressure CVD (LPCVD)Plasma Enhanced CVD: low temperaturesPressure, temp, gas flow

Wafer

Gases

Temperature > 300oC