integrated circuits dr. esam yosry lec. #6. lithography introduction lithography photolithography...
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INTEGRATED CIRCUITSDr. Esam Yosry
Lec. #6
Lith
og
rap
hy
Introduction
Lithography
Photolithography
Photoresist Process
Types of Photoresist
Photomask
Clean Rooms
Intr
od
uct
ion
(Chip
Fabri
cati
on C
ycl
e)
Intr
od
uct
ion
(Pro
cess
es)
Oxidation
Diffusion
Ion Implantation
Deposition
Etching
Lithography
Deposition Removal Patterning Modification of
electrical properties
Lithography
• Lithography is the process of transferring patterns drawn on a mask to a thin layer of radiation sensitive material (resist) covering the surface of the material to be etched (oxide or metal).
• The main equipment is optical ultraviolet unit (UV wavelength 200 to 400 nm).
• In this case the mask is called photomask (PM), the resist is called photoresist (PR) and the process is called photolithography.
Lithography
• Patterning process consists of mask design, mask fabrication and wafer printing.
Lithography
• In case of X-ray (wavelength 0.2 to 5 nm) the resist is called X-ray resist and the process is called X-ray lithography.
• In electron-beam (EB) lithography (wavelength ≈ 0.1 nm) using EB resist.
• The shorter the radiation wavelength the smaller the feature size that can be produced.
• Since the EB radiates very small spots, no mask is needed and direct exposure of the EB resist is carried out (serial exposure of the resist or direct writing on the resist).
Lithography
• When masks are used (photo- and X-ray- lithography), we irradiate the whole resist surface using parallel exposure.
• EB lithography is used when the product volume is small because masks are very expensive to fabricate.
• Masks are themselves fabricated by EB lithography for shaping chromium metallic layer deposited on quartz transparent supports.
• Masks are only used to reproduce very large number of wafers exceeding 10000.
Lit
hog
rap
hy
• As processing equipment and technologies become more advanced, transistor size shrinks
• • In the 70’s, 8
• Now, 0.02
• This is due to advancement in lithography
• .• Each layer of dopant or
material has its own layout (15 – 20 mask).
Photolithography
Photolithography is the heart of integrated circuit processing. It is the method used to transfer the individual circuit design knowledge onto the silicon wafer.
Probably one half of the wafer fabrication costs go into obtaining proper photolithography.
The basic elements consist of:• An Align and Expose Tool• Masks containing design information• Photo-sensitive Resist
Basics of Photolithography
Silicon Wafer with Silicon Dioxide
Silicon Wafer
SiO2
To be etched
Basics of Photolithography
Apply photo resist
Silicon Wafer
SiO2
Photoresist
Basics of Photolithography
Apply photomask
Silicon Wafer
SiO2
Photomask
Basics of Photolithography
Apply Ultraviolet Light
Silicon Wafer
SiO2
Photomask
Decreasing feature size require the use of shorter λ
Basics of Photolithography
Develop photoresist
Silicon Wafer
SiO2
Basics of Photolithography
Etch Silicon Dioxide
Silicon Wafer
SiO2
Basics of Photolithography
Strip Photoresist
Silicon Wafer
SiO2
This flow chart shows the typical process used for VLSI Lithography.
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
Photoresist Process
1.Substrate Cleaning and Preparation• The surface is as clean as possible
immediately after a high temp operation.
• Acid cleaning may be necessary if wafers sit for a long time.
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
The goal of coating is to apply a uniform, defect free film of photoresist over The entire wafer. Spin Coating is used for VLSI• accelerate the wafer - fast• spin at constant speed to get uniform
thickness
2. Wafer
Coating
Before spin After spin
PR Wafer PR
Chuck
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
The Soft-Baked is used to:• Drive off solvent from resist. (reduced to ~5%)• Improve adhesion and anneal stresses in resist
Typical process Temp is 90-100°COvens
• Convection oven - Very stable over time. ~ 30 min
• IR oven - Most popular. 3-4 min.• Hot plate - very effective for single wafers.
3. Pre-Bake
(Soft-Bake)
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
To produce the desired photochemical effects in the shortest time possible.
Time is important because:• Wafers are individually processed• Align machines are expensive
However, short times usually mean lower resolution.
Exposure to UV light chemically changes the resist
4. Expose
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
Silicon Wafer
SiO2
Photomask
Development removes the unwanted photoresist.• Positive Resist - Area exposed is removed• Negative Resist - Area not exposed is
removed
5. Develop
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
Silicon Wafer
SiO2
• Hardens Resist Prior to Etch
• Temperature is critical
• Too high a Temp (>120°C) causes resist to flow
• Extra high Temp (>180°C) causes resist to lift.
6. Post-Bake
100-120°C
30 minutes
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
Photoresist Process
• The object of this inspection is to verify the
photoresist is acceptable prior to etch.
• A complete rework can now be performed if
the photoresist is not acceptable.
7. Inspection
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Photoresist Process
• Dry Plasma Etch is commonly used in
commercial manufacturing lines.
• We use Wet Etch with acids because of the
cost and flexibility of the process.
8. Etch
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
Silicon Wafer
SiO2
Photoresist Process
9. Strip
SUBSTRATE CLEANING SPIN COAT
DEVELOP
PRE-BAKE
EXPOSE
INSPECTION
POST-BAKE
ETCH STRIP
• It is a complete removal of the Photoresist.
• Dry Plasma Strip in O2 is commonly used in
commercial manufacturing lines.
• We use Wet Chemical Strip in special solvents
because of the cost and flexibility of the
process.
Silicon Wafer
SiO2
Photoresist ProcessPH
OTO
RES
IST A
RE
A
Types of Photoresist
Photomask
• The pattern to beetched on the wafersurface is drawn onthe pohtomask
• Photomasks are made from chromium
• Many masks are needed in recent CMOS technologies. The # of masks depends on the process complexity
Clean Rooms
Photolithography must be carried out in a clean room otherwise dust particles causing fabrication errors.
The total number of dust particle are controlled with temp and humidity.
Clean rooms standards:Class 100 < 100 pp ft3
(0.5µ)
Class 10 < 10 pp ft3 (0.5µ)
Class 1 < 1 pp ft3 (0.5µ)
Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.
Thanks