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Photolithography Overviewhttp://www.intel.com/research/silicon/mooreslaw.htm
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Photolithography OverviewMoore’s law only holds due to photolithography
advancements in reducing linewidths
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Photolithography OverviewAll processing to create electric components and circuits rely on
photolithography
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Photolithography OverviewTypical MOS transistor NMOS = n-type carrier across
gate
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Photolithography OverviewTransistor fabrication
N-MOS P-MOS
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Photolithography OverviewInterconnect
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Photolithography Overview• Chapter 1 sections 1-7 :
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Photolithography Overview
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Photolithography Overview• Basic process flow
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Photolithography Overview
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Photolithography Overview• Wafer clean: removal of Organics and metalics
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Photolithography OverviewHMDS Hexamethyldisilazane
Prime: Replaces surface adsorbed H2O and gives off Ammonia. This material produces a bond with the wafer surface creating a polar surface ( electrostatic).
•No surface wetting by Photoresist occurs on an un-treated SiO2 surface with these bonded hydroxyl groups. Basically the Photoresist is hydrophobic and will not adhere to a hydrophilic surface. The HMDS is a “ hydroxyl getter” and creates a hydrophobic surface, which the Photoresist had good adhesion.
•BOTTOMLINE: Priming adjusts the surface energy of the wafer so that it is comparable to the surface energy of the Photoresist.
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Photolithography OverviewPhotoresist
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Photolithography OverviewSpin Coat
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Photolithography OverviewSpin Coat: RPMs: Spread or cast, Ramp,and terminal
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Photolithography OverviewSpin Coat
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Photolithography OverviewSpin Coat
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Photolithography OverviewSoftbake
• Removes solvent from film and stablizingcoating: typical:
• 90C to 120C I-line DNQ• 100C to 130C: DUV CAR
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Photolithography OverviewAlignment
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Photolithography OverviewPhotoresist Exposure
DNQ photoresist
actinic radiation
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Photolithography OverviewDUV: Photoresist Exposure wavelengths below 200nm
All use excimer lasers Note 248nm = KrF laser
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Photolithography OverviewPhotoresist Exposure
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Photolithography OverviewExposure and feature type
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Photolithography OverviewPositive and Negative Tone Photoresists
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Photolithography OverviewContact/Proximity/Projection printing
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Photolithography OverviewProjection printing: Typical stepper
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Photolithography OverviewProjection printing: Numerical Aperture
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Photolithography OverviewProjection printing: High NA lenshttp://www.research.ibm.com/journal/rd/411/singh.html
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Photolithography OverviewProjection printing: Resolution
http://www.research.ibm.com/journal/rd/411/singh.html
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Photolithography OverviewProjection printing: Depth of Focushttp://www.research.ibm.com/journal/rd/411/singh.html
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Photolithography OverviewProjection printing: Depth of Focushttp://www.research.ibm.com/journal/rd/411/singh.html
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Photolithography OverviewOptical lithography Performance: Resolution
NA and wavelength coherent systems
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Photolithography OverviewPhotoresist Standing waves:
reflection/interference
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Photolithography OverviewPhotoresist Post Exposure Bake
• Purposes: key idea• DNQ/Novolak positive tone: diffusion bake : Diffusion of
PAC to improve CD contact by removing standing waves.• PAG/Novolak negative tone: (acid hardened resist: AHR)
Diffusion of H+ ion to react with polymer causing polymer to become insoluble.( PAG: Triazine)
• DUV PAG/Blocking group/PHS: Diffusion of H+ ion to react with blocking group causing PHS to become soluble
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Photolithography OverviewPhotoresist exposure and dissolution
• Key ideas:• DNQ and Novolak Resin I-line 365nm system• 1. DNQ or Photo Active Compound PAC is an Inhibitor: It inhibits
Development rate when present! There is very little dissolution in an OH solution.
• 2. Photolytic conversion of DNQ to ICA in by exposure to Near UV radiation ( Hg lamp) increases development rate
• PAG and Poly Hydroxystyrene PHS DUV CAR 248nm system• 1. PAG Photo-Acid-Generator creates an Acid (H+) upon exposure to
248nm radiation. There is very little dissolution in an OH solution.• 2. The addition of thermal energy using a PEB bake causes the H+ to
diffuse and react with the blocking group, causing the exposed area to become soluble.
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Photolithography OverviewPhotoresist Development Threshold dose
Develop exposed photoresist in TMAH (2.38%) basic solution
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Photolithography OverviewPhotoresist CDs Dose Vs linewidth
Polysilicon Linewidth DUV photoresist0 Focus offset.
y = -0.0267x + 1.0649R2 = 0.9727
0.58
0.6
0.62
0.64
0.66
0.68
0.7
13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18 18.5 19Exposure Dose
Phot
ores
ist l
inew
idth
CD
um
ACENALLALRAULAURZCENZLLZLRZULZURLinear (AUL)
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Photolithography OverviewPhotoresist CDs Dose Vs Spacewidth
Contact CD Vs Exposure Dose Mj/cm2 IX405 i-line Photoresist Nominal 0.80u
y = 0.0013x + 0.5853R2 = 0.927
0.65
0.70
0.75
0.80
0.85
0.90
0.95
100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250
ASML 5500/100C Exposure dose mj/cm2
Con
tact
Pho
tore
sist
C
D m
icro
ns
201CEN DILinear (201CEN DI)
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Photolithography OverviewI-line Positive tone Photoresist 365nm
http://www.jsrusa.com/resists.htm
Line dense
isolated line
isolated space
contact
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Photolithography OverviewI-line negative tone Photoresist 365nm
http://www.jsrusa.com/resists.htm
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Photolithography OverviewDUV Photoresist KrF 248nm
http://www.jsrusa.com/krfr11je.htm
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Photolithography OverviewDUV Photoresist OH contamination issue
http://www.iemw.tuwien.ac.at/publication/workshop0600/Hudek.html
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Photolithography OverviewPhotoresist Post-develop bake Hardbake
• Improve adhesion of photoresist for subsequent wet processing:
• Wet etches: BHF, Acetic acid, H2O2
• Plateup: Au or Cu• Increasing hardbake temperature will cause
photoresist patterns to flow.
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Photolithography OverviewPhotoresist Etch
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Photolithography OverviewPhotoresist Etch: RIE
http://www.iemw.tuwien.ac.at/publication/workshop0600/Hudek.html
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Photolithography OverviewPhotoresist removal: Strip Post etch