Lithography is the process of transferring patterns of geometric shapes in a mask to a radiation sensitive material called resist,which cover the surface of semiconductor wafer.

1. PRESENTED BY
ANJANI S
PONDICHERRY UNIVERSITY
S1 M-TECH ECE
20304001

2.  The term Lithography comes from the Greek word, lithos,
means "stone“ and graphein, means "to write”.
 Lithography is the transfer of geometric shapes on mask to
smooth surface.
 First introduced by German author Alois Senefelder at 1771-
1834.
 Important lithographic company of 19th century was Currier
& Ives, 1852.
 In the 20th and 21st century, it becomes an important
technique with unique dramatic capabilities in the Art field.

3.  Lithography is the most complicated, expensive and critical
process of modern IC manufacturing.
 Lithography transforms complex circuit diagrams in to pattern
which are define on the wafer in a succession of exposure and
processing steps .
 Typically 8-25 lithography steps and several hundred processing
steps between exposure are required to fabricate a packed IC.

4.  PHOTOLITHOGRAPHY
 ELECTRONE BEAM LITHOGRAPHY
 X-RAY LITHOGRAPHY
 INTERFERENCE LITHOGRAPHY
 SCANNING PROBE LITHOGRAPHY
 CHARGED PARTICLE LITHOGRAPHY
 NANOIMPRINT LITHOGRAPHY

5.  Photolithography, also called optical lithography or UV
lithography.
 Process used in micro fabrication to pattern parts on a thin
film or the bulk of a substrate (also called a wafer).
 Uses light to transfer a geometric pattern from a photo mask
to a photosensitive chemical photoresist on the substrate.
 A series of chemical treatments, etches the exposure pattern
into the material or enables deposition of a new material in
the desired pattern upon the material underneath the
photoresist.
 40 to 50% total wafer process time .

6.  A CMOS wafer may go through the photolithographic cycle
as many as 50 times.
fig: Photolithographic process

7.  Surface cleaning
 Spin coating with photoresist
 Soft baking
 Mask alignment
 Exposure
 Development
 Post baking
 Plasma Etch-Or Add Layer
 Post process cleaning
 Final Inspection

8.  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)

9.  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.

10.  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/100

12.  To ensure reproducible processing by removal of excess
solvent from the resist.
 The silicon wafer coated with photoresist is heated at 75-
85°C for 45 sec.
 Improve adhesion by reducing stress.
 The thickness of the resist is usually decreased by 25%.

13. Fig :SOFT BAKING

14.  The coated wafer is placed in an apparatus called mask aligner
in very close proximity to a photo mask.
 Photo mask has photographic emulsion on one side.
 The pattern has clear and opaque areas.
 A highly collimated UV light is turned on.
 The areas of wafer that are not covered by the opaque areas of
the photo mask are exposed to UV.

15.  Master patterns which are transferred to wafers .
 Types:
Photographic emulsion, Fe2O3 , Cr on glass
Cr on quartz glass
 Dimensions:
4”x4”x0.060” for 3 inch wafers
5”x5”x0.060” for 4 inch wafers

16. Fig: mask alignment and exposure

17.  The exposed wafer is then put in a developer solution.
 The developer solution will remove the exposed or
unexposed regions of photoresist.
 Removal of regions depend on the type of photoresist used
[positive or negative resist].

18. Negative Photoresist
• Becomes insoluble
after exposure
• When developed, the
unexposed parts
dissolved.
• Cheaper
positive Photoresist
• Becomes soluble after
exposure
• When developed, the
exposed parts
dissolved.
• Better resolution

19. Fig: development

20.  Post bake in an oven at a temperature about 150℃ for 30 to
60 minutes.
 Used to stabilise and harden the developed photoresist.
 Post bake removes any remaining traces of the coating
solvent or developer .
 Introduces some stress into the photoresist .
 Some shrinkage.
Fig: post bake

21.  To remove unwanted regions, where are not protected by
resist.
 Two main Etching Methods :
Wet Chemical Etching
– Difficult to control
– Cheapest
Dry Etching
– Ion Beam
– Plasma

22.  For etching wafers are immersed in or sprayed with
hydrofluoric acid(HF) solution.
 HF will etch the sio2 layer.
 HF will not attack the underlying silicon.
 Etch rate = The rate at which etching process occurs.
 Etching is a isotropic process.

23. Fig: Etching process

24.  Plasma etching with O2 (ashing) .
 Simple solvents are generally sufficient for non-post baked
photoresists:
 The remaining photoresist is finally removed or stripped off
with a mixture of sio2 and hydrogen peroxide.
 A step of washing and drying complete the required window
in the oxide layer. Overall photolithographic process

25. Overall photolithographic process

26.  Easy to produce pattern of smaller dimensions.
 High resolution compared to proximity lithography.
 Low production cost.
 Improved production

27.  Specialized technique for creating extremely fine
pattern(∽50nm).
 The preferred method for producing the stamps used
for Nano-imprint lithography.
 Utilizes an accelerated electron beam focusing on an
electron-sensitive resist.
 Basic process design is same as photolithography.

28. Fig: electron beam lithography

29.  ADVANTAGES OF EBL
 Print complex pattern directly on wafers.
 Eliminates the diffraction problem.
 High resolution.
 Flexible technique.
 DISADVANTAGES OF EBL
 Slower than optical lithography.
 Expensive and complicated
 Forward scattering
 Backward scattering

30.  X-ray lithography (XRL) is an advanced version of optical
lithography in which shorter wavelengths are used.
 Process used in electronic industry to selectively remove parts
of a thin film.
 It uses X-rays to transfer a geometric pattern from a mask to
a light-sensitive chemical photoresist.
 X-ray resists.
 X-ray mask.

31. Fig:x-ray lithography

32.  ADVANTAGES OF XRL
 Less diffraction effect.
 High resolution.
 Large area(large depth of focus).
 Excellent resist profiles
 DISADVANTAGES OF XRL
 Distortion in absorber.
 Cannot be focused through lens.