The document details the process and components of photolithography, including the definition, requirements such as substrate and photoresist, and the advantages of positive over negative photoresist. It describes the construction of a UV exposure system and the steps involved in photolithography, from surface preparation to pattern transfer and metal deposition. Future goals include creating a Schottky diode and conducting I-V characterization.

1. PORTABLE UV-EXPOSURE SYSTEM FOR
OPTICAL MICROLITHOGRAPHY

2. CONTENTS
Defining Photolithography
Definition
Basic requirements in photolithography
Substrate
Photoresist
Composition of a photoresist
Positive and Negative photoresist
Advantage of Positive Resist over Negative resist
Photomask
UV exposure system
Construction and Application
Process of Photolithography
Futuristic goals

4. What is Photolithography ?
Photolithography is a familiar process used in microfabrication for
transferring the geometrical pattern made on a mask to a substrate
by means of radiation (X-ray, UV, DUV, e-beam).
It comes from a Greek word, which mean light-stone -writing.
The basic requirements for photolithography are:-
 Substrate
 Photoresist
 UV exposure system
 Photomask
 Developer
and Removal solution

5. Substrate
Substrate is the material where the geometrical pattern has to
be encrypted.
We have used n-type Si-wafer
as a substrate in our project
work.
The Si-wafer is double side
polished having resistivity of
10 ohm cm and dimension of
2 inch.

6. Photoresist (PR)
PR are photosensitive chemical compounds.
Composition of PR are :-
 Polymer - Change solubility when exposed to light
 Solvent - provide liquid medium to the polymer
 Sensitizer - enable chemical change in polymer
 Additives - various chemicals to obtain desired process results
We will use S1818 and S1813 photoresist.
The composition of S1818 are mixed cresol novolak resin (polymer), electronic
grade propylene glycol (solvent), monomethyl ether acetate (solvent), diazo
photoactive compound cresol (sensitizer), non-ionic Surfactant (additive).
When exposed to light, either PR get harder or become softer depending upon its
composition. So, the PR divided into two types, viz- positive photoresist (+PR) and
negative photoresist (- PR).

7. + PR and - PR

8. Advantage of + PR over - PR
The unexposed portion do not swell much because of that it has a high
step coverage.
Response to 300-400 nm spectral range.
Have higher resolution.
Lesser pin holes are created.
Exposure speed is about 10-60 sec
whereas for – PR the speed is of several minutes.
Have higher exposure sensitivity,
etching resistance and contrast.
Have lesser thermal flow.
Few disadvantages are :-
 Costly
 Lesser adhesion capability

9. Photomask
A photomask is a thin plate having opaque and transparent
parts arranged in a definite pattern.
The opaque part is generally made up of chrome metal film.
This film is planted on a fused silica.

10. UV Exposure System
The UV exposure system is an essential part for optical microlithography. It works
as a radiation source.
The system consists of
 An external circuit- the circuit consist of a mercury lamp, a ballast and
parallely connected capacitor.
 An cuboidal shape tin box- it has two sliders S1 and S2 .

11. External Circuit
The external circuit is necessary for glowing the Mercury bulb.
The circuit is shown in the figure 1.
The copper ballast is used for ignition and current limitation during
discharge and the capacitor is connected parallely for power factor
correction.

12. Design Of The Box
The box is of cuboidal shape
made up of tin having dimension
7 X 7 X 14.5 inch cube.
The box has two slider S1 and S2.
S1 works as a ON/OFF switch for
exposure system.
S2 is for focusing radiation on the
substrate.

13. Process of Photolithography
Surface Preparation
Coating
Soft baking
Alignment
Exposure
Resist Development
Hard Baking
Processing Using the Photoresist as a
masking Film (Etching)
Stripping using Removal solution
Cleaning
photoresist
silicon substrate
oxide
Ultraviolet Light

14. Future Goals
Transferring the pattern from mask to n-type silicon substrate.
Deposition of metal on the patterned silicon substrate by vapor
deposition method to create Schottky diode.
I-V (current-voltage) characterization of Schottky diode.

15. References
Mark D. Huntington;Teri W. Odom, “A portable, benchtop photolithography
system based on a solid-state light source”, SMALL 7(22):3144-
7,NOVEMBER 2011
Murat Kaya Yapici and Ilyas Farhat, “UV-LED exposure system for low cost
photolithography”, proc. Of SPIE Vol. 9052 90521T, 2014
NPTEL Lecture series at VLSI Technology by Dr. Nandita Dasgupta, IIT
MADRAS, 2009
Mod-01, Lec-15, “Schottky junction and Ohmic contacts”, NPTEL lecture by
Dr. M.R. Shenoy, IIT Delhi
Material safety data sheet, MICROPOSIT S1818 photoresist ; mod-41340 6.00
US US MSDS_US