520/580.495 photolithography (ii) andreas g. andreouandreou/495/archives/2002/lecturenotes/... ·...
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A.G. Andreou 2000 1
520/580.495Photolithography (II)
Andreas G. Andreou
Lecture notes from
Positive Photoresists and Photolithography by R. Darling
http://www.engr.washington.edu/~cam/PROCESSES
A.G. Andreou 2000 2
Lecture Summary
� Positive Photoresists� Photoresist Spinning� Mask Aligning and Exposure
R. B. Darling / EE-527
Advantages of Positive Photoresists
• Most commonly used in the IC industry.• Superior to negative photoresists because:
– They do not swell during development.– They are capable of finer resolution.– They are reasonably resistant to plasma processing operations.
R. B. Darling / EE-527
Phenolic Resins - 1
• Phenolic resins are condensation polymers of aromaticalcohols and formaldehyde.
• Bakelite was the first thermosetting plastic.• Phenolic resins are readily cross-linked by thermal
activation into rigid forms.• Most phenolic resins are readily dissolved by aqueous
alkaline solutions, e.g. NaOH, KOH, NH4OH.
R. B. Darling / EE-527
Phenolic Resins - 2OH
HC
H
O
phenol
formaldehyde
OH OH OHH2C
H2C
H2C
bakelite
HO
Hwater
+
+
R. B. Darling / EE-527
Phenolic Resins - 3OH
HC
H
O
formaldehyde
OH OH OHH2C
H2C
H2C
HO
Hwater
+
+
CH3
CH3 CH3 CH3
para-cresol
novolac
R. B. Darling / EE-527
Important Properties of the Base Phenolic Resin
• average molecular weight– typically in the range of 1000 to 3000 g/mole– (8 to 25 repeating units in the polymer chain)
• dispersity of the molecular weights• isomeric composition of the cresols
– ortho-cresol– meta-cresol– para-cresol
• relative position of the methylene linkages (--CH2--)
R. B. Darling / EE-527
Cresol IsomersOH
OHH2C
CH3
CH3
para-cresol
OHOH
CH3
CH3
ortho-cresol meta-cresol
OH
CH3
OH
CH3
H2C
H2C
ortho-cresol resin meta-cresol resin para-cresol resin
R. B. Darling / EE-527
Cresol Isomer Properties
• Single isomers and smaller molecular weights are desirable• Manufacture of positive photoresist relies heavily upon
obtaining only a single isomer of the resin, usually para-cresol.
• Each monomer is [C8H8O] (120.151 g/mole)
Isomer Methylene Link Molecular Weight Dissolution Rate Plastic Flow Temp.
ortho-cresol 3 2100 g/mole 2.7 A/sec 85 C
meta-cresol 1 15000 g/mole 0.7 A/sec 73 C
para-cresol 1 1600 g/mole 3.0 A/sec 119 C
R. B. Darling / EE-527
Photoreaction in a Positive Photoresist
O
N2
COOH
+
+
H2O
N2
hν
diazonaphthaquinone (DQ)
indene carboxylic acid (ICA)
R. B. Darling / EE-527
Dissolution of Phenolic Resins - 1
• Because of the OH groups, phenolic resins are hydrophylicand are readily dissolved by aqueous alkaline solutions.
• Diazonaphthaquinone (DQ) is a hydrophobic and non-ionizable compound.
• When phenolic resins are impregnated with DQ, theybecome hydrophobic and their dissolution is greatlyinhibited.
• After exposure, DQ is converted into indene carboxylicacid (ICA) which is hydrophylic and very ionizable.– This allows the developer to wet and penetrate the novolac resin.
• Phenolic resins which contain ICA instead of DQ arereadily dissolved by aqueous alkaline developers.
R. B. Darling / EE-527
Dissolution of Phenolic Resins - 2
0 10 20 30 40
0.1
1.0
10
100
1000
exposed novolac + DQ
unexposed novolac + DQ
DQ concentration, weight percent
Dis
solu
tion
rate
, nm
/sec
in 0
.15
M N
aOH
R. B. Darling / EE-527
DQ Primary PhotoreactionO
N2
O
N2+
+ H2O
CO COOH
diazonaphthaquinone (DQ) a carbene
a ketene indene carboxylic acid (ICA)
hν
ring contraction
hydration
photolysis
See Otto Suess, 1944 and 1947 papers in Annalen.
R. B. Darling / EE-527
Physical Requirements on the Photoactive Component
• Need an overlap of the absorption spectrum with theemission spectrum of the exposure source, e.g. a Hg lamp.
• Need bleachability at the exposure wavelength so that thephotoreaction is able to reach the resist-substrate interface.
• Need compatibility with the base resin (novolac) so thatthe two form a single, miscible phase.
• Need thermal stability so that the photoactive dissolutioninhibitor does not break down at prebake temperatures.
• Photoactive dissolution inhibitors are often modified toalter their spectral absorption, thermal stability, andmiscibility characteristics.
R. B. Darling / EE-527
Spectral Absorption of Novolac, DQ, and ICA
200 300 400 500 600
1.0
10
100
1000
10,000
Wavelength, λ, nm
313
365
405
437
Hg arc lamp linesnovolac
DQ (unexposed photoinhibitor)
ICA (exposed photoinhibitor)
R. B. Darling / EE-527
Primary Components of a Positive Photoresist
• Non-photosensitive base phenolic resin– usually novolac
• Photosensitive dissolution inhibitor– usually a DQ-derived compound
• Coating solvent– n-butyl acetate– xylene– 2-ethoxyethyl acetate
• very carcinogenic, TLV = 5 ppm• now removed from most positive photoresists
R. B. Darling / EE-527
Secondary Components of a Positive Photoresist
• Antioxidants• Radical scavengers• Amines to absorb O2 and ketenes• Wetting agents• Dyes to alter the spectral absorption characteristics• Adhesion promoters• Coating aids
R. B. Darling / EE-527
Sensitometric Curve for a Positive Photoresist
1 10 100 1000 10,000
0.0
0.5
1.0
Exposure Dose, D, mJ/cm2
Developed Resist Thicknessnormalized to 1.0
working pointfor the resist
R. B. Darling / EE-527
Novolac Dissolution - 1
• A minimum concentration of [OH-] is required to producea net forward rate:
H3C OH
CH2
+ OH-
H2O+H3C O-
CH2
The dissolution rate is R = kCn, where C is the base concentration.
For NaOH solutions, R = (1.3 x 105) [Na+]1 [OH-]3.7 Angstroms/second.
R. B. Darling / EE-527
Novolac Dissolution - 2
Dissolution Rate, Angstroms/secondSolutionUnexposed Exposed
0.15 M NaOH 20 14000.15 M KOH 10 860
0.15 M NaOH +0.1 M Na2SiO3
270 3400
0.15 M NaOH +0.1 M Na3PO4
350 2800
0.15 M NaOH +0.1 M Na2CO3
270 2400
Typical data for different developer solutions:
R. B. Darling / EE-527
Positive Photoresist Exposure Latitude
-0.4
0.0
+0.4
Exposure Dose, D, mJ/cm2
-0.2
+0.2
Critical Dimension Shift, µm
200180160 220150 170 190 210 230
Exposure Latitude
BLOAT
SHRINK
lines and islands
spaces and windows
Kodak Micro Positive 820
30 min. @ 95 C prebake in convection oven
30 sec. @ 71 C develop with agitation, 1:2 mix
working point: 185 mJ/cm2
R. B. Darling / EE-527
Positive Photoresist Prebake Latitude
-0.4
0.0
+0.4
Prebake Temperature, degrees C
-0.2
+0.2
Critical Dimension Shift, µm
Prebake Latitude
BLOAT
SHRINK
lines and islands
spaces and windows
Kodak Micro Positive 820
185 mJ/cm2 exposure
30 sec. @ 71 C develop with agitation, 1:2 mix
working point: 95 C
90 95858075 100 105 110 115
R. B. Darling / EE-527
Photoresist Spin Coating
• Wafer is held on a spinner chuck by vacuum and resist iscoated 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
• Resist thickness is given by t = kp2/w1/2, where– k = spinner constant, typically 80-100– p = resist solids content in percent– w = spinner rotational speed in rpm/1000
• Most resist thicknesses are 1-2 µm for commercial Siprocesses.
R. B. Darling / EE-527
Photoresist Spin Coating
vacuum chuck
resist dispenser
wafer to be coated
excess resist flies off during rotation
time
speed
slowcoat
level out
spindown
photoresist
R. B. Darling / EE-527
Spinning Artifacts
• Striations– ~ 30 nm variations in resist thickness due to nonuniform drying of
solvent during spin coating– ~ 80-100 µm periodicity, radially out from center of wafer
• Edge Bead– residual ridge in resist at edge of wafer– can be up to 20-30 times the nominal thickness of the resist– radius on wafer edge greatly reduces the edge bead height– non-circular wafers greatly increase the edge bead height– edge bead removers are solvents that are spun on after resist
coating and which partially dissolve away the edge bead
• Streaks– radial patterns caused by hard particles whose diameter are greater
than the resist thickness
R. B. Darling / EE-527
Prebake (Soft Bake) - 1
• Used to evaporate the coating solvent and to densify theresist after spin coating.
• Typical thermal cycles:– 90-100°C for 20 min. in a convection oven– 75-85°C for 45 sec. on a hot plate
• Commercially, microwave heating or IR lamps are alsoused in production lines.
• Hot plating the resist is usually faster, more controllable,and does not trap solvent like convection oven baking.
wafer
photoresist
hot plate chuck
coating solventis driven off
R. B. Darling / EE-527
Prebake (Soft Bake) - 2
• A narrow time-temperature window is needed to achieveproper linewidth control.
• The thickness of the resist is usually decreased by 25 %during prebake for both positive and negative resists.
• Less prebake increases the development rate:
50 60 70 80 90 100
0
100
200
300
400
temperature, °C
dissolution rate, nm/sec.
R. B. Darling / EE-527
Prebake (Soft Bake) - 3
• Convection ovens:– Solvent at surface of resist is evaporated first, which can cause
resist to develop impermeable skin, trapping the remaining solventinside
– Heating must go slow to avoid solvent burst effects
• Conduction (hot plate):– Need an extremely smooth surface for good thermal contact and
heating uniformity– Temperature rise starts at bottom of wafer and works upward,
more thoroughly evaporating the coating solvent– Generally much faster and more suitable for automation
R. B. Darling / EE-527
Overview of Align/Expose/Develop Steps
uniform UV exposure illumination
chrome on glass photomask
photoresist (PR)
substrate waferlatent image created inphotoresist after exposure
(x,y,θ) alignment ofmask to substrate
wet chemical development
NEGATIVE PHOTORESIST POSITIVE PHOTORESISTPhotoresist is photopolymerized whereexposed and rendered insoluble to thedeveloper solution.
Exposure decomposes a developmentinhibitor and developer solution onlydissolves photoresist in the exposed areas.
R. B. Darling / EE-527
Photomasks
• Master patterns which are transferred to wafers• Types:
– photographic emulsion on soda lime glass (cheapest)– Fe2O3 on soda lime glass– Cr on soda lime glass– Cr on quartz glass (most expensive, needed for deep UV litho)
• Dimensions:– 4” x 4” x 0.060” for 3-inch wafers– 5” x 5” x 0.060” for 4-inch wafers
• Polarity:– “light-field” = mostly clear, drawn feature = opaque– “dark-field” = mostly opaque, drawn feature = clear
R. B. Darling / EE-527
Mask to Wafer Alignment - 1
– 3 degrees of freedom between mask and wafer: (x,y,θ)– Use alignment marks on mask and wafer to register patterns prior
to exposure.– Modern process lines (steppers) use automatic pattern recognition
and alignment systems.• Usually takes 1-5 seconds to align and expose on a modern stepper.• Human operators usually take 30-45 seconds with well-designed
alignment marks.
alignment mark on wafer,created from prior processing step
alignment mark on mask,open window in chrome throughwhich mark on wafer can be seen
R. B. Darling / EE-527
Mask to Wafer Alignment - 2
• Normally requires at least two alignment mark sets onopposite sides of wafer or stepped region.
• Use a split-field microscope to make alignment easier:
L R
L R
R. B. Darling / EE-527
Mask to Wafer Alignment - 3
• Visual alignment:– Process of getting wafer coarsely centered under mask– All that is needed for the first mask of the set, since no patterns on
the wafer exist yet– Accomplished by special windows on a dark field mask
R. B. Darling / EE-527
Oriel Alignment Fixture
vacuum chuckmask holder
wafer stage1 2 3
mask
mask retainer wafer or substrate
height adjustment ring
(x,y,θ) adjustment micrometers
foam rubbermask gasket
main chambergasket
R. B. Darling / EE-527
Postbake (Hard Bake) - 1
• Used to stabilize and harden the developed photoresistprior to processing steps that the resist will mask.
• Main parameter is the plastic flow or glass transitiontemperature.
• Postbake removes any remaining traces of the coatingsolvent or developer.
• This eliminates the solvent burst effects in vacuumprocessing.
• Postbake introduces some stress into the photoresist.• Some shrinkage of the photoresist may occur.• Longer or hotter postbake makes resist removal much
more difficult.
R. B. Darling / EE-527
Postbake (Hard Bake) - 2
• Firm postbake is needed for acid etching, e.g. BOE.• Postbake is not needed for processes in which a soft resist
is desired, e.g. metal liftoff patterning.• Photoresist will undergo plastic flow with sufficient time
and/or temperature:– Resist reflow can be used for tailoring sidewall angles.
100°C 110°C 120°C 130°C 140°C
R. B. Darling / EE-527
Photoresist Removal (Stripping)
• Want to remove the photoresist and any of its residues.• Simple solvents are generally sufficient for non-postbaked
photoresists:– Positive photoresists:
• acetone• trichloroethylene (TCE)• phenol-based strippers (Indus-Ri-Chem J-100)
– Negative photoresists:• methyl ethyl ketone (MEK), CH3COC2H5
• methyl isobutyl ketone (MIBK), CH3COC4H9
• Plasma etching with O2 (ashing) is also effective forremoving organic polymer debris.– Also: Shipley 1165 stripper (contains n-methyl-2-pyrrolidone),
which is effective on hard, postbaked resist.