chap2 CMOS Technology

Allen and Holberg - CMOS Analog Circuit Design
Page II.1-8
Photomask
UV Light Photomask
Photoresist
Polysilicon
Allen and Holberg - CMOS Analog Circuit Design
Chapter 8 Simple CMOS Opamps
Chapter 9 High Performance Opamps
CIRCUITS
Chapter 5 CMOS Subcircuits SIMPLE Chapter 6 CMOS Amplifiers
Chapter 2 CMOS Technology
Page II.1-7
Areas not exposed to UV light are soluble in the developer Steps: 1. Apply photoresist 2. Soft bake 3. Expose the photoresist to UV light through photomask 4. Develop (remove unwanted photoresist) 5. Hard bake 6. Etch the exposed layer 7. Remove photoresist
t3
Finite-source diffusion:
N0
Gaussian
t1<tBiblioteka <t3N(x)NB
t1
t2 Depth (x)
t3
Allen and Holberg - CMOS Analog Circuit Design Ion Implantation
Page II.1-4
Ion implantation is the process by which impurity ions are accelerated to a high velocity and physically lodged into the target.
0.5-0.8 mm 125-200 mm
n-type: 3-5 Ω -cm p-type: 14-16 Ω -cm
Allen and Holberg - CMOS Analog Circuit Design Oxidation
Page II.1-2
The process of growing a layer of silicon dioxide (SiO2)on the surface of a silicon wafer.
Allen and Holberg - CMOS Analog Circuit Design
Page II.0-1
II. CMOS TECHNOLOGY
Contents II.1 II.2 II.3 II.4 II.5 II.6 II.7 II.8 Basic Fabrication Processes CMOS Technology PN Junction MOS Transistor Passive Components Latchup Protection ESD Protection Geometrical Considerations
Original Si surface tox
SiO 2
0.44 tox
Si substrate
Uses: • Provide isolation between two layers • Protect underlying material from contamination • Very thin oxides (100 to 1000 Å) are grown using dry-oxidation techniques. Thicker oxides (>1000 Å) are grown using wet oxidation techniques.
Allen and Holberg - CMOS Analog Circuit Design Diffusion
Page II.1-3
Movement of impurity atoms at the surface of the silicon into the bulk of the silicon - from higher concentration to lower concentration.
Concentration peak N(x)
NB 0
Depth (x)
Allen and Holberg - CMOS Analog Circuit Design Deposition
Page II.1-5
Deposition is the means by which various materials are deposited on the silicon wafer. Examples: • Silicon nitride (Si3N4) • Silicon dioxide (SiO2) • Aluminum • Polysilicon There are various ways to deposit a meterial on a substrate: • Chemical-vapor deposition (CVD) • Low-pressure chemical-vapor deposition (LPCVD) • Plasma-assisted chemical-vapor deposition (PECVD) • Sputter deposition Materials deposited using these techniques cover the entire wafer.
Allen and Holberg - CMOS Analog Circuit Design Etching
Page II.1-6
Etching is the process of selectively removing a layer of material. When etching is performed, the etchant may remove portions or all of: • the desired material • the underlying layer • the masking layer Important considerations: • Anisotropy of the etch lateral etch rate A = 1 - vertical etch rate • Selectivity of the etch (film toomask, and film to substrate) film etch rate Sfilm-mask = mask etch rate Desire perfect anisotropy (A=1) and invinite selectivity. There are basically two types of etches: • Wet etch, uses chemicals • Dry etch, uses chemically active ionized gasses.
Page II.1-9
Polysilicon
Photoresist
Photoresist Polysilicon
Polysilicon
Positive Photoresist
Allen and Holberg - CMOS Analog Circuit Design II.2 - CMOS TECHNOLOGY TWIN-WELL CMOS TECHNOLOGY Features • • • •
Path of impurity atom
Fixed atoms
Impurity final resting place
• Anneal required to activate the impurity atoms and repair physical damage to the crystal lattice. This step is done at 500 to 800 °C. • Lower temperature process compared to diffusion. • Can implant through surface layers, thus it is useful for field-threshold adjustment. • Unique doping provile available with buried concentration peak.
Page II.2-1
Two layers of metal connections, both of them of high quality due to a planarization step. Optimal threshold voltages of both p-channel and n-channel transistors Lightly doped drain (LDD) transistors prevent hot-electron effects. Good latchup protection
Allen and Holberg - CMOS Analog Circuit Design Perspective
Chapter 10 D/A and A/D Converters
Page II.0-2
Chapter 11 Analog Systems
SYSTEMS
Chapter 7 CMOS Comparators COMPLEX
Allen and Holberg - CMOS Analog Circuit Design II.1 - BASIC FABRICATION PROCESSES