A Primer on CMOS Technology

Objectives:

1.To Introduce about CMOS technology

Basic CMOS Fabrication Flow

Fabrication Steps Lithography

Key Characteristics:

• Bulk Process• Twin Wells• Shallow Trench Isolation• Poly gate material• Lightly doped Source/Drain extensions• Salicided gate,drain source areas• Subtractive metarialzation

The Fabrication Steps

FEOL BEOL

Front-End-Of-Line :

1.Initial Wafer

• Either p-type or n-type wafer

• Epitaxial layer of gentle doping

2.n-well Formation

• A mask is used to define the geometry of n-well

• Donor atoms are implanted into indicated region

• This will be p-channel MOSFET

3.p-well Formation:

• Do the complementary job of n-well formation

4.Active area definition:

• MOSFETs & diffusion areas are defined by subsequent mask

• Thin buffer oxide layer SiN layer is used to cover the entire surface

5.Shallow trench isolation:

• Trenches are cut into the bulk materials• Trenches filled up with oxide materials• Remove the materials by using Chemical

mechanical polishing• CMP is used to get most planer surface which is

essential for photolithography process

6.Gate dielectric Formation:

• The Wafer is layered by extremely thin (2nm-5nm) oxide layer

7.Polysilicon Deposition and patterning:

• The entire surface is covered with poly silicon film(200nm)

8.n-channel source/drain extension:

• N-type material is imposed to extend the source & drain

9.p-channel source/drain extension

• Do the complementary job of n-channel

10.Side-wall or oxide spacers:

• An oxide layer is deposited and etched away to leave an insulating wall

11.n-type doping:

• A heavier and deeper implant is used to make drain and sources of the n- channel MOSFETs

• Side wall spacers prevent penetration of doping atoms

12.p-type doping:

• Complementary job of n-type doping is done here

13.Salicidatioin :

• Wafer is covered by thin Highly conductive silicide film

• That serve lower electrical resistivity to source drain and gate

Back-End-Of-Line:

1.First interlevel dielectric :

o Layer of Silicon dioxide is deposited

o Another CMP planarization step is done to make layer the first interevel dielectric

2.Contact plug formation:

o Mask CONTACT defines the region where metal shell connect to adiffusion or polysilicon region

o Tungsten is deposited to form the plug

3.Deposition of patterning of first metal layer:

o Metal layer is deposited entire the layer surface

o Metal layer removed selectively to leave behind those parts that are defined metal1 mask

4.Second inter level dielectric:

o The second interlevel dielectric deposited and planarized

5.Via plug formation:

o Mask via1 defines those location where a first metal structure shall connect

6.Deposition of patterning of second metal layer:

o The Second layer of metal is obtained by way of subtractive metalization

7.Third interlevel dielectric followed by via plug formation:

o Dielectric deposition , planarization , plug formation , metal deposition and metal patterning is done in this step

o Two mask is required per metal layer

8.Deposition of patterning of third metal layer :

o Topmost metal layer

9.Overglass and bond pad openings :

o The Wafer surface is covered by a Layer of Silica

Process Monitoring:

• Capcitance-Voltage Characteristics are obtained from MOSCAP

• The Resistivity of all Conductive layer is determined Van der Pauw structure .

• Fault Wafers are detected and sorted out

Photolithography

Photolithography has Four major Parts:

1.An Illumination Source

2.A mask

3.An exposer subsystem

4.Photoresist Materials

A Practical Photolithographic machine:

Traditional optical Lithography:

o 546 nm E-line

o 436 nm G-line

o 405 nm H-line

o 364 nm I-line

Deep UV Lithography:

o 248 nm (KrF6)

o 193 nm (ArF6)

o 157 nm (F2)

Resolution Enhancement Techniques:

1.Phase Shift Masks:

o Improving line Separation by cancelling out light waves using destructive interference

o Illumination must be from a partially coherent source.

2.Off-axis Illumination:

o Like as Phase shift masking

o But there is no extra phase shifter

o Slightly tilted illumination axis

3.Optical Proximity Correction:

o Applies inverse distortion to the mask to precompensate the inperfection

o Overaccutuated corners such as serifs and hammer heads are typical for OPC

4.Computerized resolution enhancement :

PSM & OPC are used in computerized way in high volume high repetitive layout ( like RAM)

Post Optical Lithography:

1.Electron beam direct write lithography:

o Layout pattern get written in photoresist layer by an electron beam

o It is too slow for mass production

2.Nano imprint lithography:

o Here Photoresist is not used

o Patterns are printed on polymer coating by pressing a stencil on the wafer surface

Some Change Has been made in Fabrication Industry:

o Copper Has Replaced Aluminum as interconnect material:

First Fabrication process with Cu as Interconnect material :

o Low-permittivity interlevel dielectrics are replacing Silicon-di-oxide:

Low permittivity dielectric is using for

1.Good mechanical strength

2.Process Compatibility

3.Thermal Stability

4.Low moisture absorption

Intel’s 65 nm processed material where Carbon doped oxide CDO (€r=2.9)is used as interlevel dielectric instead of Silicon-di-Oxide(€r=3.9)

THE END