Photolithography, also called optical lithography or UV lithography, is a process used in microfabrication to pattern parts on a thin film or the bulk of a substrate (also called a wafer). It uses light to transfer a geometric pattern from a photomask (also called an optical mask) to a photosensitive (that is, light-sensitive) chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. In complex integrated circuits, a CMOS wafer may go through the photolithographic cycle as many as 50 times. Photolithography shares some fundamental principles with photography in that the pattern in the photoresist etching is created by exposing it to light, either directly (without using a mask) or with a projected image using a photomask. This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing. This method can create extremely small patterns, down to a few tens of nanometers in size. It provides precise control of the shape and size of the objects it creates and can create patterns over an entire surface cost-effectively. Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions. Photolithography is the standard method of printed circuit board (PCB) and microprocessor fabrication. Directed self-assembly is being evaluated as an alternative to photolithography

1. LITHOGRAPHY
KAROLINEKERSIN.E
Assistant Professor
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2. LITHOGRAPHY IN ART
• Lithography comes from the Greek word, lithos, means "stone“ and graphein, means
"to write”.
• Lithography is a method for printing using a stone (lithographic limestone) with a
completely smooth surface.
• This method was invented in 1796 by German author and actor Alois Senefelder as a
cheap method of publishing artwork on paper or to print text. Lithography works
because of the mutual repulsion of oil and water.
• The image is drawn on the surface of the print plate with a fat or oil-based medium
such as a wax crayon, which may be pigmented to make the drawing visible.
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3. LITHOGRAPHY IN IC
FABRICATION
• Lithography (or patterning) refers to the series of steps that establish the shapes,
dimensions, and location of the various components of the integrated circuit (IC).
• The current progress in IC design, with the decreased dimensions (miniaturization) of the
chip and increased density of transistors, is possible only if smaller areas on the wafer
surface can be patterned.
• This is primarily the function of lithography.
• Thus, the success of modern IC design is due largely to lithography.
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4. CONTD..
• Create a pattern with the dimensions established by the circuit design.
• Place the pattern correctly with respect to the crystal orientation and
other existing patterns.
• After the pattern is created, either the defined part of the wafer surface
is removed (trench creation) or left behind (island creation) or new
material is deposited.
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5. CONTD..
• Lithography is also used to expose certain parts of the wafer surface for doping (either with a
hard mark for thermal diffusion or with a soft mask for ion implantation).
• The correct placement of the circuit pattern involves alignment or registration of various
masks.
• An IC wafer fabrication process can require forty or more patterning steps.
• Alignment of these individual steps is critical to form a working IC.
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6. PHOTOLITHOGRAPHY
• Photolithography is the process of transferring geometric
shapes on a mask to the surface of a silicon wafer.
• Photolithography, also called optical lithography or UV
lithography, is a process used in microfabrication to pattern
parts on a thin film or the bulk of a substrate (also called a
wafer)
• Photolithography is a patterning process in which a
photosensitive polymer is selectively exposed to light
through a mask, leaving a latent image in the polymer that
can then be selectively dissolved to provide patterned
access to an underlying substrate.
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7. PHOTOLITHOGRAPHY STEPS
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8. PHOTOLITHOGRAPHY STEPS:
1 Wafer Cleaning:
In the first step, the wafers are chemically cleaned to remove organic,
ionic, and metallic impurities.
2 Barrier Layer Formation:
After cleaning, silicon dioxide, which serves as a barrier layer, is
deposited on the surface of the wafer.
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9. 3.Photoresist Application:
• Photoresist is applied to the surface of the wafer by high-speed
centrifugal spinning.
• This technique, known as "Spin Coating," produces a thin uniform
layer of photoresist on the wafer surface.
• In this process a liquid solution of photoresist is give out from the
wafer by rapid spin and produce uniform thin layer (0.5µm to 2.5µm).
• Spin coating/ spinner typically runs at 1200rpm to 4800rpm for 30sec
to 60sec
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10. COMPONENTS OF PHOTORESIST
• Polymer - this is a light sensitive polymer whose structure changes on exposure to light. The
desired property is usually change in solubility in a specific solvent.
• Solvent - The solvent is used to thin the resist so that is can be applied on the wafer by a
spin on process. The solvent is usually removed by heating to around 100 ◦C, called soft
bake process.
• Sensitizers - these are used to control the chemical reaction during exposure.
• Additives - various chemicals that are added to achieve specific process results, like dyes.
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11. Positive Photoresist:
• Positive photoresists is exposed to UV light, the underlying material
is to be removed.
• In these resists, exposure to the UV light changes the chemical
structure of the resist so that it becomes more soluble in the
developer.
• The exposed resist is then washed away by the developer solution.
In other words, "whatever shows, goes”.
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12. CONTD..
• Positive resists directly transfer the pattern from the mask onto the wafer.
• This is because the mask protects the portion of the resist below it from exposure to UV
radiation.
• The rest of the resist, that is exposed, becomes more soluble and can be easily removed.
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13. Negative Photoresist:
• Negative photoresists behave in just the opposite
manner. Exposure to the UV light causes the negative resist
to become polymerized, and more difficult to dissolve.
• Therefore, the negative resist remains on the surface
wherever it is exposed, and the developer solution removes
only the unexposed portions.
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14. CONTD..
• Negative resists, transfer the negative of the mask pattern to the wafer.
• This is similar to the negative process in film photography.
• For negative resists, the portion that is protected by the mask pattern is more soluble, since
it is not exposed to UV radiation, while the radiation hardens the rest of the resist
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15. 4.Prebaking
• Prebaking is the step during which almost all of the
solvents are removed from the photoresist.
• The photoresist become photosensitive after
prebaking.
• Photoresist is prebake at 90Co to 100Co for 5min to
30min.
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16. 5.Mask Alignment and Exposure
• A mask or "photo mask" is a square glass plate with a
patterned combination of metal film on one side pattern
transferred onto the wafer surface.
• The mask contains the hard copy of the pattern that has to be
transferred to the different wafers during lithography. For a
given integrated circuit, there are multiple masks, which have
to be aligned for proper device fabrication.
• Masks have alignment markers included with the pattern,
which can be used for this purpose
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17. MASK MAKING
• The alignment markers are usually much smaller than
the typical dimensions of the pattern.
• The mask material is made of borosilicate glass or
quartz with a sputter deposited chrome layer on top.
The chrome layer is 100 nm thick.
• There is also a photoresist layer deposited on top of
the chrome.
• A laser writer is used to ‘write’ the pattern on the mask.
Different laser wavelengths (365, 248 or 193 nm) and
lenses are used to write the pattern on the mask.
• The choice of the wavelength depends on the smallest
dimension on the pattern.
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18. • The laser writing process is sequential (line by line) and can take hours depending on the
complexity of the pattern.
• The mask pattern took approximately 7 hours to write, using a 365 nm laser wavelength.
After the pattern is written, a suitable developer is used to remove the unexposed
photoresist.
• After that, the exposed chrome layer is removed (using an acid bath etch) and then the
remaining photoresist is removed to leave behind the chrome desired pattern on glass.
• There are also cleaning and drying steps to remove any excess solvent and keep the mask
free of dust particles
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19. METHODS OF MASK ALIGNMENT
There are three primary exposure methods:
• contact
• proximity
• projection.
Contact Printing:
• In contact printing wafer is brought into physical contact with photo
mask. Because of the contact between the resist and mask, very high
resolution is possible.
• The problem with contact printing is that fragments trapped between
the resist and the mask, can damage the mask and cause defects in
the pattern.
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20. Projection Printing
• Projection printing avoids mask
damage entirely.
• An image of the patterns on the mask
is projected onto the wafer, which is
many centimeters away.
• To achieve high resolution, only a
small portion of the mask is imaged it
has about 1-micron
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21. Proximity Printing
• The proximity exposure method is similar to
contact printing except that a small gap, 10 to 25
microns wide is maintained between the wafer
and the mask.
• This gap minimizes (but may not
eliminate) mask damage.
• Approximately 2 to 4 micron resolution is
• possible with proximity printing
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22. 6.Development
• Development is a process in which exposed/non-
exposed area is dissolved by developer.
• Most commonly used developer is tetra methyl
ammonium hydroxide is used in concentrations of 0.2 -
0.26.
• Developer is important in controlling the development
uniformity.
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23. • Therefore two methods are mainly used i.e. spin development and spray
development.
• During spin development wafers are spun and developer is poured onto the rotating
wafer.
• In spray development, the developer is sprayed rather than poured, on the wafer by
using a nozzle that produces a fine spray over the wafer.
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24. 7.Hard-Baking
The hard bake is used to harden the final resist
image at the temperature (120°C - 150°C), so
that it will hold out the harsh environments of
etching.
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25. 8.Etching
• Etching is performed either using wet chemicals such as
acids, or more commonly in a dry etching (by exposing the
material to a bombardment of ions) .
• The photoresist will “resists” the etching and protects the
material covered by the resist.
• When the etching is complete, the resist is stripped leaving the
desired pattern.
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26. DRY AND WET ETCHING
The etching process that involves
using liquid chemicals or etchants to
take off the substrate material is
called wet etching.
In the plasma etching process, also
known as dry etching, plasmas or
etchant gases are used to remove the
substrate material.
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27. 9.Stripping
• After the imaged wafer has been etched the remaining
photoresist must be removed.
• There are two classes of stripping
techniques; wet stripping and dry stripping.
• A simple example of stripper is acetone.
• Acetone tends to leave residues on the wafer.
• Most commercial organic strippers are phenol-based.
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28. ADVANTAGES
Photolithography can etch a pattern into an
integrated circuit with a single beam of
ultraviolet light and does not require any
additional materials.
highly efficient
Controls the exact size and shape of the
entire substrate.
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29. DISADVANTAGES
Very expensive
Photolithography
requires extremely clean
conditions that are void
of all contaminants,
liquids, and
environmental hazards.
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30. APPLICATIONS OF
PHOTOLITHOGRAPHY
• IC designing process
• Fabrication of microcircuits
• Semiconductor industry
• Sensors
• Microprocessor
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31. THANK YOU
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