Post on 21-Nov-2014
Fabrication of CMOS Integrated Circuits
CHAPTER 4
John P. Uyemura
Wafer Preparation
Thin Films
Photo-lithography
Cleaning
Front-EndProcesses
EtchIonImplantation
Planarization
Test &Assembly
DesignWaferPreparation
• Polysilicon Refining
• Crystal Pulling
• Wafer Slicing & Polishing
• Epitaxial Silicon Deposition
Polysilicon Refining
Chemical Reactions Silicon Refining: SiO2 + 2 C Si + 2 CO Silicon Purification: Si + 3 HCl HSiCl3 + H2
Silicon Deposition: HSiCl3 + H2 Si + 3 HCl
Reactants H2 Silicon Intermediates H2SiCl2
HSiCl3
Crystal PullingQuartz Tube
Rotating Chuck
Seed Crystal
Growing Crystal (boule)
RF or ResistanceHeating Coils
Molten Silicon(Melt)
Crucible
Materials Polysilicon Nodules * Ar * H2
* High proportion of the total product use
Process Conditions Flow Rate: 20 to 50 liters/min Time: 18 to 24 hours Temperature: >1,300 degrees C Pressure: 20 Torr
3/15/98 PRAX01C.PPT Rev. 1.0
Wafer Slicing & Polishing
The silicon ingot is grown and individual wafers are sliced.
The silicon ingot is sliced into individual wafers, polished, and cleaned.
silicon wafer
p+ silicon substrate
OXIDATION PROCESS IN FABRICATION
WHAT IS OXIDATION?• It is a way to produce a layer of Silicon
dioxide on the surface of a wafer
• Normally referred to as “Thermal Oxidation”
• Process is done at very high temperatures
• Two types– Dry Oxidation– Wet Oxidation
WHY SiO2 ?
• Has excellent dielectric properties
• One among the best insulators
• Relatively easy to obtain when compared to other insulators
• Unwanted SiO2 can be removed selectively to Si without any problems
• It is very stable and chemically inert
• SiO2 is normally used
– As Gate oxides (thin layer insulating the gate and channel area)
– For electrical isolation between different layers on the chip
– For Masking– As Field oxides (thick layer insulating the
device from other devices on the chip)– For Passivation
OXIDATION PROCESS
DRY OXIDATION• Requires temperature of 8000C to 11000C
• Slow process (about 10 hours for 700nm oxide)
• Easily controllable
• Si + O2 SiO2
• Pure high quality oxide
• Used for Gate oxide
WET OXIDATION• Inlet gas is a mixture of water vapor and
oxygen
• Si + 2H2O SiO2 + 2H2
• Nearly 10 times faster than dry oxidation
• Temperatures of 7000C to 12000C
• Medium quality oxide
• Used for obtaining Field oxide
CVD• Chemical vapour deposition (CVD) is
used where Si cannot be directly oxidized • Wafer (substrate) is exposed to volatile
compounds (called precursors)• These react and decompose on the
substrate and produce the desired deposit• CVD can also be used for deposition of
– Polycrystalline Silicon– Silicon Nitride
Detailed Description of First Photolithographic Steps Only
• Top View
• Cross-Section View
A A’
B’B
C
C’
D
D’
A A’
B’B
A A’
B’B
Bulk CMOS Process Description
• N-well process
• Single Metal Only Depicted
• Double Poly
A A’
B’B
n-channel MOSFET
S
D
G
S
D
BG
A A’
B’B
S
D
BG
W L
A A’
B’B
n-channel MOSFET
Capacitor
p-channel MOSFET
Resistor
A A’
B’B
N-well Mask
A A’
B’B
N-well Mask
~
Blank Wafer
p-doped Substrate
ExposeDevelop
Photoresistn-well Mask
Implant
~
A A’
B’B
A-A’ Section
B-B’ Section
PhotoresistN-well MaskExposureDevelop
A-A’ Section
B-B’ Section
Implant
N-well Mask
A-A’ Section
B-B’ Sectionn-well
A A’
B’B
Active Mask
A A’
B’B
Active Mask
Active Mask
A-A’ Section
B-B’ Section
Field Oxide Field Oxide Field Oxide
Field Oxide
A A’
B’B
Poly1 Mask
A A’
B’B
Poly1 Mask
A A’
B’B
n-channel MOSFET
Capacitor
P-channel MOSFET
Resistor
Poly 1 Mask
A-A’ Section
B-B’ Section
Gate Oxide Gate Oxide
A A’
B’B
Poly 2 Mask
A A’
B’B
Poly 2 Mask
Poly 2 Mask
A-A’ Section
B-B’ Section
A A’
B’B
P-Select
A A’
B’B
P-Select
P-Select Mask – n-diffusion
A-A’ Section
B-B’ Section
n-diffusion
P-Select Mask – p-diffusion
A-A’ Section
B-B’ Section
p-diffusion
A A’
B’B
Contact Mask
A A’
B’B
Contact Mask
Contact Mask
A-A’ Section
B-B’ Section
A A’
B’B
Metal 1 Mask
A A’
B’B
Metal 1 Mask
Metal Mask
A-A’ Section
B-B’ Section
A A’
B’B
Contact Mask
A A’
B’B
Contact Mask
Contact Mask
A-A’ Section
B-B’ Section
A A’
B’B
Metal 1 Mask
A A’
B’B
Metal 1 Mask
Metal Mask
A-A’ Section
B-B’ Section