Lithography Fabrication
1
Jadhav Avinash J
 2K13E11
 Savitri Bai Phule Pune University

What is Lithography?
2
 Lithography(Greek word) means printing is done
on stone.
 Photo-litho-graphy: light-silicon wafer-printing.
 Components in photolithography:
(1)Mask (2)Photoresist (3)UV exposure system

Overview of the Photolithography Process
3

Steps Used in Photolithography
 Surface cleaning
 Spin coating with photoresist
 Soft baking
 Mask alignment
 Exposure
 Development
 Hard baking
 Plasma Etch-Or Add Layer
 Post process cleaning
 Final Inspection
4

Surface Cleaning
5
 Typical contaminants that must be removed prior to
photoresist coating: dust from scribing or cleaving
(minimized by laser scribing)
 photoresist residue from previous photolithography
(minimized by performing oxygen plasma ashing)
 atmospheric dust (minimized by good clean room
practice)
 bacteria (minimized by good DI water system)

6
 films from other sources:
-solvent residue
-H2 O residue
-photoresist or developer residue
-silicone
 For particularly troublesome grease, oil, or wax
stains: Start with 2-5 min. soak in 1,1,1-
trichloroethane (TCA) or trichloroethylene (TCE)
with ultrasonic agitation prior to acetone

Spin coating with photoresist
7
 Wafer is held on a spinner chuck by vacuum and
resist is coated to uniform thickness by spin coating.
 Typically 3000 - 6000 rpm for 15-30 seconds.
 Resist thickness is set by: primarily resist viscosity
secondarily spinner rotational speed
 Most resist thicknesses are 1-2 μm for commercial Si
processes.

8
 Resist thickness is given by
t =square of( kp)/root of(w1),
where,
k = spinner constant, typically 80-100
p = resist solids content in percent
w = spinner rotational speed in rpm/1000

Stages of Resist Coating
9

Soft baking
10
 Used to evaporate the coating solvent.
 Typical thermal cycles:90-100°C for 20 min. in a
convection oven 75-85°C for 45 sec. on a hot plate
 Microwave heating or IR lamps are also used
 Optimizes light absorbance characteristics of
photoresist

11

Mask alignment & Exposure
12
Transfers the mask image to
the resist-coated wafer
Activates photo-sensitive
components of photoresist
Three types of masking
(1) Contact printing
(2) Proximity printing
(3) Projection printing

13

14
 Contact printing capable of high resolution but has
unacceptable defect densities. May be used in
Development but not manufacturing.
 Proximity printing cannot easily print features below
a few mm in line width. Used in nano-technolgy.
 Projection printing provides high resolution and low
defect densities and dominates today.They print » 50
wafers/hour.

Positive resist & Negative resist
15

Development
16
 Soluble areas of photoresist are dissolved by
developer chemical
 Visible patterns appear on wafer
 Quality measures:
 –line resolution
 –uniformity
 –particles & defects

Hard baking
17
 Used to stabilize and harden the developed
photoresist prior to processing steps that the resist
will mask.
 Postbake removes any remaining traces of the
coating solvent or developer.
 Higher temperature than soft bake (120-150 degree)

Plasma Etch-Or Add Layer
18

Post process cleaning
19
 Plasma etching with O2 (ashing)
 Simple solvents are generally sufficient for non-postbaked
photoresists:
 Positive photoresists:
aceton
trichloroethylene (TCE)
 Negative photoresists:
methyl ethyl ketone
methyl isobutyl ketone


Final Inspection
20
 Photoresist has been completely removed
 Pattern on wafer matches mask pattern (positive
resist)
 Quality issues:
–defects
–particles
–step height
–critical dimensions

Any Questions???
21