Lithography, originating from the Greek words meaning 'stone' and 'to write', is a technique for transferring geometric shapes onto smooth surfaces, with its modern applications critical in IC manufacturing. It includes various methods such as photolithography, electron beam lithography, and x-ray lithography, each having specific processes and advantages for creating intricate patterns on semiconductor wafers. The process involves several steps including coating with photoresist, exposure, development, and etching, ensuring precise patterning essential for microfabrication.

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.