Lithography

Question 1

What are the main stages of a thin film patterning process?

Answer 1

Photolithography - photoresist deposition, alignment exposure, development

Question 2

What is a lithography process? How do we classify the lithography processes? Please compare them and comment on their applicability

Answer 2

Lithography - pattern transfer

Classified by radiation used ie photo, X-ray etc

Question 3

Explain the photo-resist processing stages. Outline their purpose and comment on the factors that can affect the photo-resist quality at each stage

Answer 3

clean - remove impurities and dust
pre-bake and primer coating - promotes adhesion of PR
Photoresist spin coating - improve quality of coating
soft bake - evaporating most of solvents that absorb radiation and affect adhesion
alignment and exposure - determines feature size
development - soluble areas dissolved to form pattern
hard bake -evaporating all solvents, improving etch and implantation resistance
inspection - cannot rework after etch so much check

Question 4

Please draw a simple diagram and outline the work principle of industrial equipment for photoresist processing (eg applying, baking, developing of photoresist)

Answer 4

see notes

Question 5

Why do we need resist materials?

Answer 5

Restricts etchant or prevents it from attacking the underlying material where it remains in place after development

Question 6

What are the main requirements for the resist materials

Answer 6

High resolution - thinner PR has higher resolution, lower etching and ion implantation resistance
High etch resistance
Good adhesion
Wider process latitude - higher tolerance to process condition change

Question 7

What type of resist materials do you know? Please list the PR components and outline their purpose

Answer 7

Polymer - solid organic material, transfers designed pattern to wafer surface, changes solubility due to photochemical reaction when exposed to radiation
Solvent - dissolves polymers into liquid -> thinning PR, allow application of thin PR layers by spinning
Sensitisers - additives that control or modify photochemical reaction of resist during exposure determines exposure time and intensity
Additives - various added chemical to achieve desired process results, such as dyes to reduce reflection

Question 8

Please compare the positive and negative photoresists

Answer 8

Negative - becomes insoluble after exposure, when developed unexposed parts dissolve, mostly polyisoprene type, exposed becomes cross-linked polymer with higher chemical etch resistance, unexposed dissolves in development solution
Positive - soluble after exposure, when developed the exposed parts dissolved, better resolution, eg novolac resin polymer, acetate type solvents, sensitisers cross-linked within the resin, energy from the light dissociates the sensitiser and breaks down the cross-links, exposed part dissolve in developer solution, image the same that on the mask, higher resolution, commonly used in IC fabrications

Question 9

Which are the main performance metrics of a patterning process?

Answer 9

Resolution - minimum feature dimension hat can be transferred with high fidelity to a resist film
Registration - how accurately patterns on successive mask can be aligned with respect to previously defined patterns
Throughput - number of wafers that can be exposed/unit time for a given mask level

Question 10

Explain the effect of the light diffraction, numerical aperture, and depth of focus on the image resolution during a patterning process

Answer 10

Light diffraction - optical lens collect diffracted light and enhance image
Numerical aperture - ability of lens to collect diffracted light
Depth of focus - range that light is in focus and can achieve good resolution of projected image

Question 11

Please draw a simple diagram and outline the work principle of the equipment for photoresist alignment processing

Answer 11

Scanner-stepper

Question 12

Please outline the purpose of a phase-shifting mask

Answer 12

Phase-shifting mask - destructive interference

Question 13

Please list the exposure light sources, which are used in photolithography

Answer 13

Mercury lamp
Excimer laser
Fluorine laser

Question 14

What is the next generation lithography process?

Answer 14

Extreme UV
E-beam
I-line
DUV
ArF

Question 15

Explain the principle of the extreme UV, X-ray e-beam and ion-beam lithography processes outline their advantages and disadvantages

Answer 15

Extreme UV -
Xray - gold mask, 15nm resolution, high cost, photomask device
ebeam - below 100nm resolution, high cost, main use research
ion-beam - 1 um resolution, high cost, research

Question 16

Draw a simple diagram outline the principle of work of a machine for e-beam lithography

Answer 16

SEM setup

Question 17

Please compare the NGL processes which you know

Answer 17

table