this presentation highlights the 3 methods of coating of photo resis and highlights their advantages and disadvantages.

1. PHOTORESIST COATING ON
SILICON WAFERS
BY
KARTIKEY GROVER
LALIT AGGARWAL

2. WHAT IS MEMS
• It’s a technology of very small device.
• MEMS are made up of components between 1
to 100 micrometres in size and generally range
in size from 20 micrometres to a millimetre.
• The fabrication of MEMS evolved from the
process technology in semiconductor device
fabrication.

4. Patterning
• Patterning in MEMS is the transfer of a pattern
into a material.
• In patterning we use lithography.
• Lithography is the transfer of a pattern into a
photosensitive material by selective exposure
to a radiation source such as light.
• In order to do it we need photoresist coating.

5. CATEOGORIES OF PHOTORESIST
• A positive resist is a type of photoresist in which the
portion of the photoresist that is exposed to light
becomes soluble to the photoresist developer. The
portion of the photoresist that is unexposed remains
insoluble to the photoresist developer.
• A negative resist is a type of photoresist in which the
portion of the photoresist that is exposed to light
becomes insoluble to the photoresist developer. The
unexposed portion of the photoresist is dissolved by
the photoresist developer.

6. GENERAL CHEMICAL COMPOSITION
OF PHOTORESIST
• Different chemicals may be used for
permanently giving the material the desired
property variations:
• Poly(methyl methacrylate) (PMMA),
• Poly(methyl glutarimide) (PMGI)
• Phenol formaldehyde resin (DNQ/Novolac),
• SU-8.

7. THREE MAJOR PHOTORESIST COATING
TECHNIQUES
• SPIN COATING
• SPRAY COATING
• ELECTRODEPOSITION OF PHOTORESIST

8. SPIN COATING
• It is the standard coating
method for flat wafers in IC
technology
• In short, an excess amount of a
solution is placed on the
substrate, which is then rotated
at high speed in order to spread
the fluid by centrifugal force.

9. Spin coating : Stages

10. SPIN COATING
• Solution viscosity and the spinning speed are
the only parameters which influence the layer
forming process
• The thickness approximately decreases with
the (increasing) square-root of the spin speed
(in rpm)

11. ADVANTAGES OF SPIN COATING
• Spin coating is a mature technique and uses
commercially available equipment and resists.
• used at all stages of processing on all types of
substrate layers.
• The high resist film thickness homogeneity as
well as the short coating times makes spin-
coating the most-applied coating technique at
least in microelectronics.
• The process optimization focuses only on these
two parameters.

12. DISADVANTAGES OF SPIN COATING
• The deeply etched features cause a physical
obstruction to the solution flow, preventing
complete coverage.
• Depends on the position of cavity.
• Sizes and shapes of the cavities also have
influence on the resist uniformity
• The reproducibility of spin coating is very less.

13. 1. The lowest uniformity
variation can be observed at
H/V = 2/3, 1, 3/2, which
indicates a better resist
uniformity in the square or
large rectangle cavities.

14. SPRAY COATING

15. SPRAY COATING
• It uses an Ultrasonic atomization technology that has unique
advantage that is based on two features: its ability to generate
very small droplets of an extremely narrow range diameter
size, and its ability to apply these droplets gently on the
substrate with minimum “bounce back” from the target
surface.
• To get the proper droplet size distribution of photoresist, a
resist solution with lower viscosity is necessary

16. • By optimizing the coating
process a good uniform
resist layer has been
deposited on wafer with
375μm-deep cavities.
• The uniformity of resist
layer obtained by this spray
coating technique is better
than with the spin coating

17. ADVANTAGES OF SPRAY COATING
• This technique uses much less resist than spin
coating. The spray process requires no spin off
photoresist so that it can effect up to 70% less resist
consumption to spin process.
• The resist thickness is repeatable over all cavities
with the same size, regardless the position of cavities
on the wafer
• Spray coating has no thickness variation caused by
directional effect of spinning

19. (a) Spin Coating (b) Spray Coating (c) spray coating with angle control

20. DISADVANTAGES OF SPRAY COATING
• variations in resist thickness are observed if cavities with a
large difference in size are present on the same wafer. The
resist thickness at the bottom of a small cavity is thicker that
the one in a large cavity. If the difference in dimension of
cavities is large, it will lead to a large variation in resist
thickness between cavities.
• Photoresist spray coating has more variables than spin
coating.

21. ELECTRODEPOSITION OF
PHOTORESIST
• The process used in electroplating is called Electrodeposition.
• The process uses electrical current to reduce cations of a
desired material from a solution and coat a conductive object
with a thin layer of the material
• Wafer to be deposited is the cathode of the circuit.
• the anode is made of the metal to be plated on the part.

22. ADVANTAGES OF
ELCECTRODEPOSITION
• The main advantage of this ED coating is the
conformal resist layer independently of the
geometry of the nonplanar features

23. • For that reason, ED coating is the most suited
technique to pattern structures that run in and
across cavities or when a smaller line width is
required

24. DISADVANTAGE OF ED
• This technique always requires a conductive
(metal) surface.
• The set up and process handling are more
complicated than the other two coating
techniques.
• The coating bath should be checked and
maintained frequently in order to get a
reproducible process.

25. COMPARISON

26. CONCLUSION
• Spin coating is a simple method but it does
not give a good and reproducible result on
deep cavities of different sizes and randomly
distributed across the wafer
• Spray coating and ED deposition certainly
bring better results but they need special
equipment
• ED deposition is perfectly suited for patterning
metal layer

27. THE END
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