EE-527: MicroFabrication
Negative Photoresists
R. B. Darling / EE-527
A Little Structural Organic Chemistry - 1
alkanes: (no double bonds):
H2 H2 H2 H2
H3C C C C C
alkenes: (at least 1 double bond):
H H H2 H2
H3C C C C C
alkynes: (at least 1 triple bond):
H2 H2
H3C C C C C
[alkenes and alkynes are unsaturated hydrocarbons.]
R. B. Darling / EE-527
A Little Structural Organic Chemistry - 2
R OH
alcohol
R O R'
ether
R CHO
aldehyde
O
R C R'
ketone
O
R C OH
acid
O
R C O R'
ester
R O OH
hydroperoxide
R O O R'
peroxide
CH3OH
HCHO
HCOOH
methanol
formaldehyde
formic acid
R. B. Darling / EE-527
A Little Structural Organic Chemistry - 3
benzene
OH
phenol
CH3 toluene
CH3
(ortho-)
xylene
CH3
(meta-)
CH3 xylene
H3C
CH3
(para-)
xylene
H3C
orthometapara-
means straight out
means beyond
means opposite
R. B. Darling / EE-527
Common Monomers and Their Polymers - 1
MONOMER
POLYMER
H2C CH2
H2C CH2
ethylene
H2C CH
CH3
propylene
H2C CH
Cl
vinyl chloride
H2C CH
n
polyethylene (PE)
H2C CH
CH3
n
polypropylene (PP)
H2C CH
Cl
n
polyvinyl chloride (PVC)
H2C CH
n
styrene
polystyrene (PS)
R. B. Darling / EE-527
Common Monomers and Their Polymers - 2
MONOMER
H2C CH
H2C CH
CO
CO
O
O
CH3
methyl acrylate
CH3
H2C C
CH3
n
polymethyl acrylate (PMA)
CH3
H2C C
CO
CO
O
O
CH3
methyl methacrylate
All rubbers are
diene polymers
POLYMER
CH3 n
polymethyl methacrylate (PMMA)
H
H2C C C CH2
CH3
H
H2C C C CH2
CH3
isoprene
polyisoprene (PI)
(2-methyl-1,3-butadiene)
n
R. B. Darling / EE-527
Common Monomers and Their Polymers - 3
H OH
HO
HO
H OH
H
H
HO
HO
O
H
H
H
OH
HO
OH
O
H
D-glucose
H
HO
H
D-manose
H OH
O
HO
H
H
H
O
OH
HO
H OH
O
HO
OH
O
H
H
cellulose
H
HO
O
R. B. Darling / EE-527
Atomic Weights
hydrogen
1.0079
carbon
12.011
nitrogen
14.0067
oxygen
15.9994
silicon
28.0855
sulfur
32.06
chlorine
35.453
(distribution of isotopes gives rise to fractional atomic weights)
R. B. Darling / EE-527
Molecular Weights
ethylene
C2H4
2(12.011) + 4(1.0079) = 28.05 g/mole
propylene
C3H6
3(12.011) + 6(1.0079) = 42.08 g/mole
vinyl chloride
C2H3Cl
2(12.011) + 3(1.0079) + 35.453 = 62.50 g/mole
styrene
C8H9
8(12.011) + 9(1.0079) = 105.16 g/mole
methyl acrylate
C 4 H 6 O2
4(12.011) + 6(1.0079) + 2(15.9994) = 86.09 g/mole
methyl methacrylate
C 5 H 8 O2
5(12.011) + 8(1.0079) + 2(15.9994) = 100.12 g/mole
isoprene
C5H8
5(12.011) + 8(1.0079) = 68.12 g/mole
1 mole is Avogadro’s number of particles: NA = 6.023 x 1023
R. B. Darling / EE-527
Molecular Weight of a Polymer
• Mp = nMm
• n = number of units
• Mm = molecular weight of monomer
• Mp = molecular weight of polymer
• For use in a photoresist resin, need a molecular weight of
around 100,000 - 200,000 for proper viscosity, melting
point, softening point, and stiffness.
• Example:
– To get Mp = 100,000 using isoprene (Mm = 68.12 g/mole), need to
get chains of average length of n = 100,000/68.12 = 1468 units.
– This would lead to a molecule that is too long for proper
photolithographic resolution, so need to coil the chains to make the
lengths shorter and to increase the mechanical stiffness.
R. B. Darling / EE-527
Polyisoprene Rubber
• 2-methyl-1,3-butadiene (isoprene) spontaneously
polymerizes into natural latex rubber (polyisoprene).
• Polyisoprene becomes sticky and looses its shape at warm
temperatures.
• Natural latex rubber is the only known polymer which is
simultaneously:
•
•
•
•
•
elastic
air-tight
water-resistant
long wearing
adheres well to surfaces
CH3
H2
C C
H
C
H2
C
n
polyisoprene
C5H8
R. B. Darling / EE-527
Cyclicized Poly(cis-isoprene) - 1
• Poly(cis-isoprene) is the substrate material for nearly all
negative photoresists.
– cis- CH3 groups are on the same side of the chain
– trans- CH3 groups are on alternatingly opposite sides of the chain
– cis-isoprene is needed in order to curl the chains up into rings;
(trans-isoprene will not work; CH3 groups would hit each other).
• Two protons are added to cis-isoprene to further saturate
the polymer and induce curling into cyclicized versions.
H2 CH
3
C
C
C
H2C
C
H2
H2
C
C
CH3
CH2
n
monocyclic poly(cis-isoprene)
C10H16
Bicyclic and tricyclic forms are also possible.
(This is usually part of the proprietary part of
photoresist manufacture.
R. B. Darling / EE-527
Cyclicized Poly(cis-isoprene) - 2
• Cyclicized poly(cis-isoprene) allows greater solids content
in coating solutions and is less subject to thermal crosslinking.
Property
Uncyclicized
Cyclicized
Average Molecular Weight
~ 106
~ 104
Density
0.92 g/mL
0.99 g/mL
Softening Point
28 C
50-65 C
Intrinsic Viscosity
3-4
0.36-0.49
Unsaturation
14.7 mmole/g
4-8 mmole/g
R. B. Darling / EE-527
Vulcanization (Cross-Linking) of Rubber
• Vulcanization of rubber uses sulfur atoms to form bridging
bonds (cross-links) between polymer chains.
• Sulfur is thermally activated; it is not photosensitive.
CH3
H
C
C
H2
H
C CH C C
S
H
C
CH3
CH3
H
C
C
H2
C
CH3
H
CH
C
C
H2
H
C
H
C
C
H2
C
S
S
H
C
H2
C C
C CH
CH3
H2 H
C C
CH3
R. B. Darling / EE-527
Components of a Negative Photoresist
• 1. Non-photosensitive substrate material
– About 80 % of solids content
– Usually cyclicized poly(cis-isoprene)
• 2. Photosensitive cross-linking agent
– About 20 % of solids content
– Usually a bis-azide ABC compound
• 3. Coating solvent
– Fraction varies
– Usually a mixture of n-butyl acetate, n-hexyl acetate, and 2butanol
• Example: Kodak KTFR thin film resist:
– work horse of the semiconductor industry from 1957 to 1972.
R. B. Darling / EE-527
ABC Photosensitive Cross-Linking Agent
CHO
O
+2
CH3
4-methylcyclohexanone
N3
4-azidobenzaldehyde
O
H3N
H
C
H
C
NH3
CH3
2,6-bis(4-azidobenzal)-4-methylcyclohexanone
"ABC"
R. B. Darling / EE-527
Bis-Azide Cross-Linking Agents
• “bis” means oppositely oriented---needed to attach both
ends of the cross linker to two different substrate strands.
• It plays the same role as sulfur in vulcanization of rubber.
• The ABC bis-azide compound is photosensitive instead of
being thermally activated.
• Photosensitivity arises from explosophore groups on ends:
– N3 azide group
– NO2 nitro group
– Lead azide Pb(N3)2 is a primary explosive...
• Nitrenes are photoionized azide groups with a triplet
ground state and a singlet excited state which is extremely
reactive and capable of bonding to hydrocarbon chains.
R. B. Darling / EE-527
Bis-Azide Cross-Linking Chemistry - 1
R
N3
hν
R
N*
+
N2
photolysis of nitrene group: the only photoreaction
R
+
HC
+
HC
N*
1
R
H
N
C
+
C
2
R
N
3
R
NH
4
R
N
R
NH
+
O2
+
5
HC
R
NO2
6
R
R
H
N
C
NH2
+
C
R. B. Darling / EE-527
Bis-Azide Cross-Linking Chemistry - 2
– Photolysis of nitrene group by ultraviolet light is the only
photoreaction.
– Reaction 1 is the desired pathway which leads to one end of the
ABC compound being cross-linked to an isoprene strand.
– Reactions 2, 3, and 4 are an alternative pathway to the same result,
involving an intermediate ground state and a radical state of the
nitrene.
– The ground state nitrene can combine with O2. (Reaction 5)
• This competes with cross-linking.
• This can be used in an image reversal process.
– The radical state nitrene can steal an additional proton from an
isoprene strand and terminate the ABC compound without forming
a cross link. (Reaction 6)
• This competes with cross-linking, also.
R. B. Darling / EE-527
Cross-Linking Efficiency
• χ is the efficiency of thermal cross-linking of nitrene
groups to isoprene strands, set by the rates of reactions
1,2,3,4 versus 5,6.
• φis the quantum efficiency of photolysis of the azide
groups, set by the wavelength and absorption of the resist.
• Φ = φχ is the quantum yield of cross-link bond formation.
• For a bis-azide resist, two bonds are needed (one on each
end) to form a cross-link between isoprene strands; thus:
• Φ = φ2χ2
– This requires two photons per cross-link, and thus has very low
photographic speed.
– This allows great variety in the substrate polymer chains.
R. B. Darling / EE-527
The Gel Point
– All sites for cross-linking (chromophores) are equally likely; thus,
larger polymer chains are more likely to bind together than small
ones.
– A many-branched supermolecule results from increased exposure.
– This supermolecule permeates the irradiated area forming a lattice
which solvent atoms can penetrate, but not disperse.
– The polymer chains have at this point been rendered insoluble to
the solvent, and the exposure required to produce this is called the
Gel Point.
R. B. Darling / EE-527
The Gel Curve
percent of solids by weight in a gel
Gel Fraction, W
initial slope
1.0
0.5
Gel Point
0.0
~ 5 x 1016 photons/cm2, or
~ 8.3 x 10-8 Einsteins/cm2
Exposure, E
(linear scale)
Exposure is measured in Einsteins/cm2
(1 Einstein = 1 mole of photons)
R. B. Darling / EE-527
The Sensitometric Curve
percent of solids by weight in a gel
Gel Fraction, W
initial slope is the photographic contrast, γ
1.0
working point range
0.5
Gel Point
0.0
DG ~ 10-20 mJ/cm2
Exposure Dose, D
(logarithmic scale)
Exposure energy dose in mJ/cm2
R. B. Darling / EE-527
Exposure and Dose Calculations
NA = Avogadro’s number = 6.022 x 1023 particles/mole
h = Planck’s constant = 6.626 x 10-34 J-s
c = speed of light in vacuum = 2.998 x 108 m/s
NAhc = 0.1196 J-m/mole
E = exposure in Einsteins/cm2
D = exposure energy dose in mJ/cm2
Assume nearly monochromatic exposure illumination of wavelength λ
hc
λ
is the energy per photon
N A hc
λ
Therefore:
is the energy in a mole of photons (one Einstein)
EN A hc
D=
λ
R. B. Darling / EE-527
Optical Absorption by the Resist
A = fraction of light that is absorbed by the photoresist layer
dr = thickness of the photoresist layer
α r = optical absorption coefficient of the photoresist layer
ρr = density of the photoresist layer
R1 = reflectivity of the air - photoresist boundary
R2 = reflectivity of the photoresist - substrate boundary
I(z)
single pass:
resist
layer
A = (1 − R1 )(1 − e − α r d r )
double pass:
[
A = (1 − R1 )(1 − e − α r d r ) 1 + R2e − α r d r
I ( z ) = I 0 (1 − R1 ) e − α r z
]
z
R. B. Darling / EE-527
Cross-Link Fraction
E = exposure in Einsteins/cm2
A = absorbed fraction of light
Φ = quantum efficiency of cross-link formation
C = cross-link fraction
EAΦ = number of formed cross-links in moles/cm2
drρr = mass of resist in grams/cm2
drρr/2Mm = number of possible cross-links in moles/cm2
therefore, the fraction of possible cross-links is proportional to the exposure:
C=
EAΦ 2 M m
d r ρr
R. B. Darling / EE-527
Gel Point Exposure
• The gel point occurs when each polymer strand, on
average, has one cross-link. Thus,
C gel
Mm 1
=
=
Mp n
The gel point exposure is thus:
E gel
d r ρr
d r ρr
=
=
AΦ 2 M m n AΦ 2 M p
For Φ = 1, A = 1, dr = 1 µm, Mp = 105 g/mole, and λ= 365 nm, obtain that
Egel = 0.25 x 10-9 Einsteins/cm2 and Dgel = 0.1 mJ/cm2.
This is a benchmark for negative resist systems.
R. B. Darling / EE-527
The Flory Function - 1
W = gel fraction (fraction of solids)
C = cross-link fraction
n = degree of polymerization (an integer) = Mp/Mm
f(n) = distribution of polymerization, a log-normal distribution
β = dispersity, typically 0.6 - 2.2
n0 = average polymer chain length
The Flory function relates the cross-link fraction C (proportional to exposure) to
the resulting gel fraction W (solids content) as a function of the average chain
length and dispersity of the polymer.
∞
 (ln n − ln n0 ) 
f (n) = exp −

2
2
β


2
W = 1−
∑ n f (n) [1 − CW ]
n =1
n
∞
∑ n f (n)
n =1
R. B. Darling / EE-527
The Flory Function - 2
Gel Fraction, W
1.0
β=0
β=1
β = 1.5
β=2
0.5
Exposure, C/Cgel
0.0
0
1
2
3
4
5
6
7
8
9
10
11
12
R. B. Darling / EE-527
Dispersity and Contrast
• The slope of the sensitometric curve is the photographic
contrast of the resist:
2 − β2
 dW 
e
=


dE
E

gel point
gel
 dW 
− β2
− β2


=
=
=γ
2
ln(
10
)
e
4
.
606
e
 d (log E ) 
gel point

• Desire a minimally dispersed polymer to optimize the
sensitometric curve.
– Age increases the dispersity of the polymer.
– This is a key factor in limiting the shelf life of photoresist.
R. B. Darling / EE-527
Negative Photoresist Ingredients
•
•
•
•
1.
2.
3.
4.
–
–
–
–
–
–
–
Non-photosensitive substrate material
Photosensitive cross-linking agent
Coating solvent
Other additives: (usually proprietary)
antioxidants
radical scavengers
amines; to absorb O2 during exposure
wetting agents
adhesion promoters
coating aids
dyes
R. B. Darling / EE-527
Negative Photoresist Development - 1
• The unexposed (uncross-linked) areas of resist as well as
polymer chains that have not been cross-linked to the
overall network of the gel must be dissolved during
development.
• Negative photoresist developers are solvents which swell
the resist, allowing uncross-linked polymer chains to
untangle and be washed away.
• A sequence of solvents is often used to keep the swelling
reversible.
• The swelling of the resist during development is the largest
contributor to loss of features and linewidth limitations.
R. B. Darling / EE-527
Negative Photoresist Development - 2
volume expansion factor, V/Vo
10
chloroform
toluene
benzene
5
increasing
solubility
parameter δ
hexane
1
0
ethanol (base solvent)
0%
50%
100%
molecular percent
of solvent in ethanol
R. B. Darling / EE-527
Negative Photoresist Strippers
• Most commonly used are:
– Methyl ethyl ketone (MEK)
– Methyl isobutyl ketone (MIBK)
CH3
O H2
H3C C C CH3
methyl ethyl ketone (MEK)
H3 C
O
C
C
CH3
CH3
methyl isobutyl ketone (MIBK)
R. B. Darling / EE-527
Single Component Negative Photoresists
• Electron beam irradiation produces cross-linking.
• An anion A- is needed to complete the reaction.
CH3
H2
H2 H
C C C C
CH
CO
CO
O
O
CH2
CH2
CH
CH3
O
CH2
CH2A-
e-beam radiation
+
A-
O
CH2
epoxide group
H
C
R CH
CH2
O-
O
CH2A
CH2AR CH
O-
n
R
+
H
C
H2C
O
R CH
R
O
e-beam radiation
CH2
CH R
O-
glycidyl methacrylate and ethyl acrylate copolymer
R. B. Darling / EE-527