Photoresist

Chapter 8
Photoresists

Ch7 - Areal Image:
Pattern of radiation at the
surface of the wafer during
exposure.
To transfer a pattern, radiation
changes the properties of the
light sensitive material so that a
replica (or a negative of the
mask) of the mask is left behind.

Photoresists (PR) or resists
Three components:
Resin, base material
Photoactive compound (PAC)
Solvent
Measures of performance:
• Sensitivity: amount of light energy necessary to create the
chemical change in the PR
• Resolution: smallest feature that can be reproduced in PR
Types of PR:
• Positive: i Exposed regions dissolve more quickly during
development process (has better resolution than negative)
• Negative: Unexposed regions dissolve more quickly during
• Negative: Unexposed regions dissolve more quickly during
development process

Organic Materials and Polymers
Aromatic ring: 6 Carbon atoms arranged in a planar hexagonal
structure with a single H atom attached to each C atom.
Figure 8.1 Diagram of simple benzene aromatic ring. The delocalized pi-
bond electrons are in a ring that t surrounds the nuclei. The symbol
indicates the currently accepted ring notation.

Substitute:
Methyl group
Chlorine
Additional
Benzene ring
Figure 8.2 Some aromatic-based compounds based on (A) single-site substitution,
Figure 8.2 Some aromatic based compounds based on (A) single site substitution,
(B) double-site substitution, where the first term defines the position (ortho, meta, or
para) of the second substitution relative to the first, and (C) aromatic condensation.

Polymers are formed by linking together
many smaller repeating units called
monomers; many are C-based. One of
the simplest is: polyethylene
Monomer: Ethylene: C 2 H 4
Polymers can be linear or branched
Branching, cross-linking increases
strength and density
Positive Tone resist: if exposure leads to
breaking polymer chains, the polymer
dissolves easily in developer
Negative Tone resist: if exposure leads to
cross linking, polymer dissolves slowly in
developer
Figure 8.3 (A) Polyethylene, an
example of a simple polymer. (B)
Branched-chain polymers. (C)
Cross-linking.

Positive resist
In i-line and g-line exposures, a class of compounds, DQN
DQ: diazoquinone is the PAC and the resin material is
Novolac (benzene ring with substituted methyl and OH
groups)
-evolved from materials to make blueprints
Solvents added to adjust viscosity, much of this evaporates
before the exposure is done
Figure 8.4 Meta-Cresol novolac, a commonly used resin
material in g- and i-line applications. The basic ring
structure may be repeated from 5 to 200 times.
Figure 8.5 Diazo quinone (DQ), the
most commonly used photoactive
compound for g- and i-line
applications. The right-hand ring is not
an aromatic but has a double bond.

N 2 is weakly bonded and UV light will free N 2 from Carbon ring
leaving behind a reactive site, more stable to move 1 Carbon
outside the ring with oxygen covalently bonded to form Ketene
Presence of H 2 O, final
rearrangement results in
carboxylic acid
Figure 8.6 Photolysis and subsequent reactions of DQ upon UV exposure.

Light, water are the main drivers of this process
Starting material will not dissolve in the developer; carboxylic acid,
however, readily reacts with and dissolves in basic solutions (pH >
7); novolac resin dissolves easily in developer
Typical developer solutions are KOH or NAOH diluted with water
Dark areas of the photomask will protect the resist from light and
will “stay” after developing → positive image
Negative resists are suited to features > 2 µm due to swelling during
development; they are based on a resin of cyclized polyisoprene p and
bis-aryl diazide (PAC); these compounds lose N 2 on light exposure
and form free radicals that react with a site in the resin; this cross-
linking produces a highly insoluble film in the developer and
pattern is formed by washing away the unpolymerized resist →
negative image

Contrast (γ) curves: a way to characterize resist
Procedure for determining i contrast:
1) spin coat a layer of PR
2) Measure PR thickness
3) Expose for short period of time,
Exp. dose = Intensity * t exp Units: mJ/cm 2
4) Immerse wafer in developer for fixed time
5) Rinse and measure remaining resist
Assuming positive resist and short exposure time, very little of the
PAC has changed to be soluble in the developer so resist thickness
is nearly the same as original thickness.
Repeat experiment for increasingly larger doses and obtain a
contrast curve by plotting resist thickness versus log of incident
dose.

Contrast, γ , is the slope of the line at the steepest part of the curve,
D o is the lowest energy needed to begin the photochemical
reactions, D 100 is the energy for which all resist is removed
Low exposure region
Opposite behavior because resist
is insoluble in developer
High exposure region
Figure 8.7 Contrast curves for idealized resists: (A) postitive tone and (B) negative tone.

Can be thought of as a measure of the ability of a resist to distinguish
between light and dark portions of the mask
γ =
1
log10(
D100
/ D
Typical resists have contrast values around 2-5
Another measure of performance is the critical modulation transfer
function (CMTF) for the resist:
CMTF D100
− D0
= resist D + D
In terms of contrast…
CMTF resist
0
10
10
)
1/ γ
=
1/ γ
Typical values are ∼0.3; provides a single number to help evaluate resolution; If
MTF < CMTF, image will not be resolved.
**Resolution is a combination of ability for light intensity to transfer pattern
into resist and resist to distinguish that image.
−1
+ 1
100
0

Figure 8.9 Typical process flow
in a photolithography step.
1) Dehydration bake at 150-200 ºC in vacuum or
nitrogen atmosphere for removing surface moisture
on wafer to improve PR adhesion
2) Wafer is primed with Hexamethyldisilazane
(HMDS) - a commonly used adhesion promoter
3) PR is applied by spin coating: small amount of
resist dispensed on wafer and the wafer chuck
(platform) spins rapidly (2000-6000 rpm)
1
T R
∝
ω
4) Softbake (prebake) at ∼ 90-100 ºC will help to
evaporate solvents in the resist
5) Exposure to optical source
6) PEB may be done to stabilize resist
7) Spray coating, or immersion, of wafer in
developer promotes PR removal in certain areas
8) Hardbake increases adhesion of PR to underlying
film to stabilize resist against the etch step or ion
implantation steps

Effects of hardbake temperature;
resist profile tends to reflow at
high hardbake temperatures.
Figure 8.11 Resist profiles of a 1-µm line and
space and a large feature in SPR-2FX resist for
different hardbake temperatures (courtesy
Shipley).

Industrial systems are called “coat and develop tracks” where
wafers move from hotplates to PR dispensing stations, spin coating
platforms, etc.
Figure 8.12 Top view of a photoresist
processing system including cassette load
and unload, resist application, bake and
From:
develop stations, and a central robot; more
http://classoneequipment.com/equipdetails.aspEQID
t / i t il modern systems are in a controlled
=853
environment and integrated into an exposure
tool (courtesy silicon Valley Group).

Absorbance: a decrease in absorption of the photoresist after
it has been exposed is called “bleaching”
.
Bleaching can provide a
more uniform exposure
because as the top layers
of the resist are exposed,
they become partially
transparent, allowing a
fuller exposure of the
lower layers.
Figure 8.13 Total absorbance of a layer of SPR511-A resist
before and after exposure. The difference between the two
curves is the actinic absorbance (courtesy Shipley).

Exposing resist over topology causes resist thickness
variations; this has created a need for planarization of the
layers → chemical mechanical planarization (CMP)
Figure 8.14 Cross-sectional view of resist as it covers a vertical step.