The twin well process allows for separate optimization of n-type and p-type transistors. It involves depositing a lightly doped epitaxial layer on an n+ or p+ substrate, then forming n-wells and p-wells in this layer through independent doping steps. This allows the dopant concentrations to be carefully tuned to produce desired device characteristics for both transistor types. The key steps are tub formation through n-well and p-well implantation and diffusion, polysilicon gate formation, and contact definition and metallization to connect the transistors. The main advantage is obtaining balanced performance from n-type and p-type transistors through separate well optimization.

1. Applied Electronics –PT
Coimbatore - india

2. Twin well Process
Provide separate optimization of the n-type and p-type transistors
This means that transistor parameters such as threshold voltage, body
effect and the channel transconductance of both types of transistors can be
tuned independently
n+ or p+ substrate, with a lightly doped epitaxial layer on top, forms the
starting material for this technology.
The n-well and pwell are formed on this epitaxial layer which forms the
actual substrate.
The dopant concentrations can be carefully optimized to produce the
desired device characteristics because two independent doping steps are
performed to create the well regions.

3. The starting material is either an n+ or p+ substrate with a lightly doped
epitaxial layer, which is used for protection against latch up
This process provides separately optimized wells, balanced performance n-
transistors and p-transistors may be constructed
Tub formation
 Thin-oxide construction
 Source and drain implantations
 Contact cut definition
 Metallization
Twin well Process Continued…,

4. Twin well Process Steps
Step1:
N or p type substrate is taken Initially
Step2:
Epitaxial Layer Deposition, Lightly Doped Epitaxial Layer is
Deposited above n+ or p+ Substrate.
Electrical Properties of Layer is Fixed by Dopant and its
Concentration.
The Aim of This Step is to Deposite High-Purity Silicon Layer.
Concentration of Dopant Distributed Throught the Layer
Step3:
Tub Formation
n -well-Formation
Protect certain region in this by using an oxide nitride mask
Phosphorus implantation
Form n-well
Entire substrate to an oxidation process
The oxide is going to be formed only over the n-well
The rest of the portions are protected by the oxide nitride mask
This n-well will also be driven deeper

5. Step4:
p -well-Formation
Protect certain region in this by using an oxide nitride mask
Boron implantation
Form p-well
Entire substrate to an oxidation process
Implant The p-well
Step 5:
Polysilicon Layer is Formed The Overall Surface
Step 6:
Polysilicon gates Are Formed for n-well and p-well by Using Photo-Etching
Process
Step 7:
Using Blanket Implantation(Auto Aligning The Wells)
Step 8:
n+ Diffusion is Formed in n-well
p+ Diffusion is Formed in p-well
These Are Used For VDD and VSS Contacts(Substrate Formation)

6. Step9:
Contact Cuts Are Defined In Both The Wells
Step10:
Metalization Process (Metal Contacts Are Created)

7. p-
p-epitaxial
p well N well
p+n+
gate oxide
Al (Cu)
tungsten
SiO2
SiO2
TiSi2
Dual-Well Trench-Isolated CMOS
field oxide

8. Advantages of Twin Tub Method:
Separate Optimized Wells Are Available
Balanced Performance of N and P Transistors is Obtained(Speciality)

9. Advance Twin Tub Process