Where were we?
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Simple Si solar Cell!
•Two Levels of Masks - photoresist, alignment
•Etch and oxidation to isolate – thermal oxide, deposited
oxide, wet etching, dry etching, isolation schemes
•Doping - diffusion/ion implantation
•Metallization - Materials deposition, PVD, CVD
Conventional Optical Lithography
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Photolithography is how patterns are transferred into
chips, and its the driving force behind Moore’s law.
Light source and condenser
Mask
Projection/reduction
optics
Photoresist
Substrate
Photoresist - Basic Process
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Light: h
Light: h
Mask
Photoresist
Wafer
Developing
Photoresist
Wafer
Positive Resist
Negative Resist
Operating wavelengths, defines Resists
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Traditionally, the light sources have been high pressure Hg arc
lamps (It’s what we use in this course). The aligner (why do we
need to align?) filters out specific emission lines from the lamp for
exposure of the photoresist
•g-line - = 436nm
•i- line - = 365nm (what we have)
The drive to smaller feature size also means shorter wavelength.
Why? 1) reduced absorption length 2) higher energy per photon 3)
less diffraction 4) less refraction
Generations after the above include (tend toward excimer laser
light sources):
•=248nm (KrF)
•=193nm (state-of-the-art, ArF - 30nm features with high index
immersion)
•=157nm (F2 in development)
Node (International Technology Roadmap for Semiconductors)
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
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10 m — 1971
3 m — 1975
1.5 m — 1982
1 m — 1985
800 nm (.80 m) — 1989
600 nm (.60 m) — 1994
350 nm (.35 m) — 1995
250 nm (.25 m) — 1998
180 nm (.18 m) — 1999
130 nm (.13 m) — 2000
90 nm — 2002
65 nm — 2006
45 nm — 2008
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32 nm — 2010
22 nm — 2011
16 nm — approx. 2013
11 nm — approx. 2015
Photoresists - (the film in the camera)
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
3 Component Material (relates to i-line and g-line)
Matrix or Resin
CH2
OH
• Determines mechanical/chemical properties
• Partially controls resistance to etching
• Novolac – Industry Standard for + resist
M
OH
PAC: Photo Active Compound
• Changes properties due to optical exposure
• Positive becomes soluble/Negative becomes insoluble
• Controls exposure time/resolution
Solvent
• Dissolve the matrix and PAC
• Determines Viscosity
• Controls processing conditions (spin speed, time, bakes, etc.)
M
CH2
Photoresist Processing
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Wafer clean, dehydration, priming
• Main Goal is Good Adhesion: Critical during developing and etching
• Clean – device integrity
• Dehydration – Organics are hydrophobic
• Priming – Particularly important for oxides
• HMDS: Hexa Methyl Di Silazane
• Mist Chamber: Absorb a monolayer
Organic Loving
R
Si
HO
Si
O
O
H
O
H
R
H
H
OH
O
H
H
Water Loving
Wafer Coating
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
#1: Dispensing Stage Not Critical
• Few hundred RPM
• Spread resist over wafer (65-85% solvent)
#2: Acceleration
Basically not
a Problem
• 2000-6000 RPM in a tenth of a second
• Key to unifomity (solvents begin evaporating)
#3: Spin Speed
• Controls Thickness
• Force Balance: Centrifugal vs. Viscous Drag
#4: Spin Duration
• Primary Goal is to Evaporate Solvent (30%)
Photoresist
Dependenttypically use
manufacturers
data sheets
Softbake and Exposure
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Purpose of Softbake Step (80 – 110 ºC)
• Drive-off solvent (5%)
• Improve Adhesion
• Anneal out stress in polymer film
•Control film dissolution rate
2 Methods:
• Hot plate: 30 – 60 seconds
• Oven: 25 - 30 minutes
• Considerations: Throughput, reproducibility
Exposure
• Depends on lamp intensity, spectrum
• Depends critically on everything else, both before and after
Post Exposure Bake?
• Diffusion of PAC to remove standing waves
•Chemically Amplified Resist, necessary to activate the photoacid generator
Development and Hard Bake
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
Developer (positive DQN)
• Aqueous NaOH/KOH Solution
• Temperature Sensitive: +/- 1ºC
• Bath or Spray
• Time again depends on everything else
Hardbake
• Improves (hardens) Resistance
• Detrimental to Profiles
• Hotplate or Oven
• Temperature Sensitive
Photolithography in General
• Complex multidimensional phase space
• Requires careful optimization and tight process control
Photoresist, the rest of the story
PHGN/CHEN/MLGN 435/535: Interdisciplinary Silicon Processing Laboratory
-Contrast curves, bleaching, planarization
-Positive vs. Negative. Which is better? 1) positive 2)
negative
-Deep UV photoresists (Novolac absorbs below 250nm, PAC
not capable of resolution below 250nm)
-Chemically Amplified Resists (CAR)
-Polymethyl methacrylate (PMMA)
-Resists for Next Generation Lithography - Don’t exist
-Environmental Issues?