View
199
Download
3
Tags:
Embed Size (px)
DESCRIPTION
MICRO ELECTRO MECHANICAL SYSTEMS
Citation preview
Department of Instrumentation & Control Engineering, MIT, Manipal
Lecture #08
MOSFET Fabrication
1
Department of Instrumentation & Control Engineering, MIT, Manipal
Contents
1. CMOS inverter cross-section
2. Fabrication procedure
3. Inverter cross-section with well and substrate taps
4. Flow diagram
5. Fabrication procedure – Basic steps
6. Mask set
7. Fabrication steps
8. Testing
9. Die cut and assembly
2
Inverter Cross-section
3
• Typically use p-type substrate for nMOS transistors
• Requires n-well for body of pMOS transistors
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n+
p substrate
p+
n well
A
YGND V
DD
n+ p+
SiO2
n+ diffusion
p+ diffusion
polysilicon
metal1
nMOS transistor pMOS transistor
Fabrication Procedure
4
• Well
– Requires to build both pMOS and nMOS on single wafer.
– To accommodate both pMOS and nMOS devices, special
regions must be created in which the semiconductor type
is opposite of the substrate type.
– Also Known as Tubs.
– Twin-tubs
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Inverter Cross-section with Well and Substrate taps
5
• Typically use p-type substrate for nMOS transistors
• Requires n-well for body of pMOS transistors
• Substrate must be tied to GND and n-well to VDD
• Metal to lightly-doped semiconductor forms poorconnection
• Use heavily doped well and substrate contacts / taps
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n+
p substrate
p+
n well
A
YGND V
DD
n+p+
substrate tap well tap
n+ p+
VDD
A=0 Y=1
GND
OFF
ON
Flow Diagram
6S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Create n-Well regions and Channel Stops region
Grow Field Oxide and Gate Oxide
Deposit and pattern Polysilicon Layer
Implant sources, drain regions and substrate contacts
Create contact Windows, deposit and pattern metal layer
Fabrication Procedure – Basic Steps
7
• Masks: Each Processing steps in the fabrication procedure requiresto define certain area on the chip. This is known as Masks.
• Chips are specified with set of masks
• Minimum dimensions of masks determine transistor size (and hencespeed, cost, and power)
• Feature size f = distance between source and drain
– Set by minimum width of polysilicon
• Feature size improves 30% every 3 years or so
• Normalize for feature size when describing design rules
• The ICs are viewed as a set of pattern layers of doped Silicon,Polysilicon, Metal and Insulating Silicon Dioxide.
• A layer must be Patterned before the next layer of material isapplied on the chip.
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Inverter Mask Set
8
• Transistors and wires are defined by masks
• Cross-section taken along dashed line
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
GND VDD
Y
A
substrate tap well tap
nMOS transistor pMOS transistor
Detailed Mask Views
9
• Six masks
– n-well
– Polysilicon
– n+ diffusion
– p+ diffusion
– Contact
– Metal
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Metal
Polysilicon
Contact
n+ Diffusion
p+ Diffusion
n well
Fabrication Steps
10
• Start with blank wafer
• Build inverter from the bottom up
• First step will be to form the n-well
– Cover wafer with protective layer of SiO2 (oxide)
– Remove layer where n-well should be built
– Implant or diffuse n dopants into exposed wafer
– Strip off SiO2
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
Oxidation
11
• Grow SiO2 on top of Si wafer
– 900 – 1200 C with H2O or O2 in oxidation furnace
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
SiO2
Photoresist
12
• Used for lithography.
• Lithography is a process used to transfer a pattern to layer onthe chip. Similar to printing process.
• Spin on photoresist (about 1 mm thickness)
– Photoresist is a light-sensitive organic polymer
– Positive Photoresist: Softens where exposed to light
– Negative Photoresist: Harden where exposed to light, Notused in practice generally.
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
SiO2
Photoresist
Lithography
13
• Expose photoresist through n-well mask
• Strip off exposed photoresist
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
SiO2
Photoresist
Etch
14
• Etch oxide with hydrofluoric acid (HF)
– Seeps through skin and eats bone; nasty stuff!!!
• Only attacks oxide where resist has been exposed
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
SiO2
Photoresist
Strip Photoresist
15
• Strip off remaining photoresist
– Use mixture of acids called piranah etch
• Necessary so resist doesn’t melt in next step
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
SiO2
n Well
16
• n-well is formed with diffusion or ion implantation
• Diffusion
– Place wafer in furnace with arsenic gas
– Heat until As atoms diffuse into exposed Si
• Ion Implantation
– Blast wafer with beam of As ions
– Ions blocked by SiO2, only enter exposed Si
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n well
SiO2
Strip Oxide
17
• Strip off the remaining oxide using HF
• Back to bare wafer with n-well
• Subsequent steps involve similar series of steps
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substrate
n well
Polysilicon
18
• Deposit very thin layer of gate oxide
– < 20 Å (6-7 atomic layers)
• Chemical Vapor Deposition (CVD) of silicon layer
– Place wafer in furnace with Silane gas (SiH4)
– Forms many small crystals called polysilicon
– Heavily doped to be good conductor
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Thin gate oxide
Polysilicon
p substraten well
Polysilicon Patterning
19
• Use same lithography process to pattern polysilicon
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Polysilicon
p substrate
Thin gate oxide
Polysilicon
n well
n Diffusion
20
• Use oxide and masking to expose where n+ dopants should be diffused or implanted
• N-diffusion forms nMOS source, drain, and n-well contact
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p substraten well
n Diffusion
21
• Pattern oxide and form n+ regions
• Self-aligned process where gate blocks diffusion
• Polysilicon is better than metal for self-aligned gates because it doesn’t melt during later processing
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n+ Diffusion
p substraten well
n Diffusion
22
• Historically dopants were diffused
• Usually ion implantation today
• But regions are still called diffusion
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n wellp substrate
n+n+ n+
n Diffusion
23
• Strip off oxide to complete patterning step
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
n wellp substrate
n+n+ n+
p Diffusion
24
• Similar set of steps form p+ diffusion regions for pMOSsource and drain and substrate contact
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
p+ Diffusion
p substraten well
n+n+ n+p+p+p+
Contacts
25
• Now we need to wire together the devices
• Cover chip with thick field oxide
• Etch oxide where contact cuts are needed
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Contact
p substrate
Thick field oxide
n well
n+n+ n+p+p+p+
Metalization
26
• Sputter on aluminium over whole wafer
• Pattern to remove excess metal, leaving wires
S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
M etal
p substrate
Metal
Thick field oxide
n well
n+n+ n+p+p+p+
Testing
27S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Defective IC
Individual integrated circuits are tested to distinguish good die
from bad ones.
Die cut and assembly
28S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Good chips are attached to a lead frame package.
Die Attach and Wire Bonding
29S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
lead frame gold wire
bonding pad
connecting pin
Final Test
30S.Meenatchisundaram, Department of Instrumentation & Control Engineering, MIT, Manipal
Chips are electrically tested under varying environmental conditions.