[PDF] - ME 645: MEMS: ME 645: MEMS: Design, Fabrication Design Fabrication PDF Document

SLIDE 1

1

ME 645: MEMS: ME 645: MEMS: Design Fabrication Design Fabrication

Lecture 4: Lithography 1

Design, Fabrication Design, Fabrication and Characterization and Characterization

P.S. Gandhi P.S. Gandhi Mechanical Engineering Mechanical Engineering IIT Bombay IIT Bombay

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Recap: Last Class Recap: Last Class

Materials for MEMS

VLSI

VLSI-

based fabrication processes:

based fabrication processes: preliminary concepts preliminary concepts

Lithography

Material removal

Material addition

Combination of all these to make devices:

Combination of all these to make devices: surface micromachining: Examples surface micromachining: Examples

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

SLIDE 2

2

Recap: Surface Recap: Surface Micromachining Micromachining

Combination of

Combination of lithography and etching lithography and etching

Defined with respect to

Defined with respect to deposited films instead deposited films instead

U-V RAYS

f Si substrate itself
f Si substrate itself

Wafer substrate

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Surface Micromachining Surface Micromachining Material Systems Material Systems

Structural Sacrificial Release Etch I solation

Polysilicon SiO2 Buffered HF Si3N4+ SiO2 Polyimide Aluminum PAN etch SiO2 LPCVD Si3N4 Polysilicon XeF2 SiO2 + Al Aluminum Photoresist Oxygen plasma SiO2

SLIDE 3

3

Recap: HW Example Recap: HW Example

How will you make a

How will you make a microchannels microchannels on

n
How will you make a

How will you make a microchannels microchannels on

n

surface of Si wafer using the processes surface of Si wafer using the processes learnt so far? What will be the masks? learnt so far? What will be the masks?

Can you control the shape of cross

Can you control the shape of cross section of section of microchannel microchannel? What way? ? What way?

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Microchannels Microchannels

Mask geometry: PPR Mask geometry: PPR

Exposure to U-V RAYS

Developing

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Etching Removal of photoresist

Substrate Wafer

SLIDE 4

4

Comments Comments

Exactly same process can be utilized to get more

Exactly same process can be utilized to get more complex complex microchannel microchannel structures structures

We have assumed that etchant affects only the

We have assumed that etchant affects only the top side however typically its on all sides and top side however typically its on all sides and hence protection of other sides necessary. Its hence protection of other sides necessary. Its usually done by oxidation process (oxide grows usually done by oxidation process (oxide grows everywhere) everywhere)

Q: How will you make fluid flow through these?

Q: How will you make fluid flow through these? How to close the channels? Any ideas!! How to close the channels? Any ideas!!

How can we control the cross section?

How can we control the cross section? We will now study these processes in depth We will now study these processes in depth

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Lithography: Types Lithography: Types

O ti l lith h O ti l lith h

Optical lithography

Soft Lithography

Electron beam lithography

X-

ray lithography

ray lithography

Ion beam lithography

Dip Pen lithography

SLIDE 5

5

Lithography Lithography

Positive Positive Photoresist Photoresist (PPR) (PPR) Positive Positive Photoresist Photoresist (PPR) (PPR)

MASK U-V RAYS

Wafer substrate Si, glass, etc.

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

Some fundamental Some fundamental questions questions

Q: what affects the minimum feature that can

Q: what affects the minimum feature that can Q: what affects the minimum feature that can Q: what affects the minimum feature that can be produced or transferred faithfully?? be produced or transferred faithfully??

What is the chemistry of positive and negative

What is the chemistry of positive and negative photoresists photoresists? Importance from resolution stand ? Importance from resolution stand point point

What are limits of this process?

What are limits of this process? What are limits of this process? What are limits of this process?

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in

SLIDE 6

6 Photoresist Photoresist

I mportant parameters

Optical Lithography Optical Lithography

Photoresist

Ways of exposure

Contact printing

Proximity printing

Projection printing

W l th f li ht W l th f li ht

Wavelength of light

Intensity of light

Width w of the feature size: Diffraction effects

Contact printing

Types

Optical Lithography Optical Lithography

Contact printing

Proximity printing

Projection printing

Projection printing What is the one we saw in animation? What is the one we saw in animation?

SLIDE 7

7 Mask pressed against resist (0 05 Mask pressed against resist (0 05 0 3atm press) 0 3atm press)

Process Adv/ Disadv

Contact Printing Contact Printing

Mask pressed against resist (0.05

Mask pressed against resist (0.05 – 0.3atm press) 0.3atm press)

Very high resolution (< 1

Very high resolution (< 1μm) possible m) possible

Flexible mask

Problems

Nonuniform

Nonuniform contact, resolution decreases contact, resolution decreases

Contact produces defects in mask and wafer (13

Contact produces defects in mask and wafer (13 def/cm2 after 5 exp def/cm2 after 5 exp 37 def/cm2 after 15) : pinholes, 37 def/cm2 after 15) : pinholes, scratches, fractures etc. scratches, fractures etc.

Reduced mask life

Need?? Motivation

Proximity Printing Proximity Printing

Need?? Motivation

Contact printing problems

Need to study some fundamentals of light to

Need to study some fundamentals of light to understand understand

Limitations on feature size that can be produced

Selection of process parameters to achieve min

Selection of process parameters to achieve min feature size feature size

SLIDE 8

8

Fundamentals Fundamentals

Proximity printing

Animation: practical Animation: practical difficulties??? difficulties???

Diffraction of light

Assume Fresnel diffraction

Diffraction Limits Diffraction Limits

Assume Fresnel diffraction

λ < < g < W

< < g < W2/λ

Q factor

Larger the Q more faithful

x w Mask Wafer

g W Q λ 2 =

g Q g Q is the image is the image

Smaller the gaps g and

Smaller the gaps g and shorter the wavelength shorter the wavelength λ better the resolution better the resolution

g

SLIDE 9

9 The effects of diffraction The effects of diffraction

Diffraction Limits Diffraction Limits

The effects of diffraction

The effects of diffraction are prominent at the are prominent at the corners corners

Solution: make

Solution: make modifications in the mask modifications in the mask to get appropriate to get appropriate geometrical features geometrical features g

Another effect: standing

Another effect: standing waves along thickness: waves along thickness: reflection from reflection from substrate substrate interference interference

Projection Printing Projection Printing

Projecting image of mask on the substrate

Expensive projection optics compatible to UV

Expensive projection optics compatible to UV light necessary light necessary

Higher resolution than proximity printing

Larger separation between the mask and the

Larger separation between the mask and the substrate substrate

Reduction while projecting so better for higher

resolution. Masks with lower resolution will do
resolution. Masks with lower resolution will do

for getting higher resolution. for getting higher resolution.

SLIDE 10

10 Usually made up of glass substrate with Cr Usually made up of glass substrate with Cr

Masks for Masks for Lithography Lithography

Usually made up of glass substrate with Cr

For exposure at low frequencies quartz or Al

For exposure at low frequencies quartz or Al2O3

3 as

as mask substrate have been used. mask substrate have been used.

Process of making masks???

E-

Beam direct write lithography

Beam direct write lithography

Thank You Thank You

PRASANNA S GANDHI gandhi@me.iitb.ac.in PRASANNA S GANDHI gandhi@me.iitb.ac.in