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UNIT PROCESS
DIFFUSION - PART 2
Dr. S. K. BhatnagarProfessor and Head,
R & D Center for Engg. And Science
J. E. C. Kukas, Jaipur
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Pre-deposition and drive-in
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Diffusion Systems
Tube
Open tube
Sealed tube
Diffusion source
Solid
Liquid
Gaseous
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S. K. Bhatnagar 4
Patent No.2291/CAL/73 (1973)
S. K. Bhatnagar and O. P. Wadhawan
"Improvements In Or Relating To An
Apparatus For Diffusion Of Impurities InSemiconductor Using Liquid Dopants
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S. K. Bhatnagar 5
h123
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S. K. Bhatnagar 6h154
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g207
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q251a
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Atomic model of diffusion
Purpose - to understand diffusion process at high conc.
mechanism for diffusion
Significance of atomic model crystal is isotropic
Diffusion coeff. is a function of
Doping conc.
Depth level
lattice sites are minima of potential well
dopant atom may occupy substitutional or
interstitial lattice site.
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Kinds of exchange
Direct exchange
Vacancy exchange
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Fairs Vacancy Mechanism
Used to describe diffusion at
Low and moderate conc.
Temperatures < 10000 C
According to this mechanism each atom inSi forms a covalent bond with its fournearest neighbors in order to fill its
vacancy
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Diffusion in SiO2
Simple class of diffusion
Dopant distribution at Si SiO2 interfaceis assumed to be in equilibrium
Dopant conc. At the interface is describedby segregation coeff.
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Techniques for measurement ofdiffusivity
SIMS Technique Secondary Ion MassSpectroscopy
High energy ions are directed towards the
surface of the specimen so as to sputter thematerial
Emitted ions are detected and measured
Diffusivity of the dopant is estimated fromthis
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Techniques for measurement ofdiffusivity - RBS
RBS Technique Rutherford BackScattering Technique
He ions bombard the target at high energy
The ions penetrate significant distance beforebeing scattered.
Back scattered ions are detected.
Diffusivity is estimated from this data
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Techniques for measurement of diffusivity BST
BST test Bias Temperature Stress Test
MOS structure is fabricated
Electric bias and temperature are applied
C V characteristics are plotted
Diffusivity is estimated from this data
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Mechanism for diffusion in SiO2
Exchange mechanism
Interstitial mechanism
Substitutional
mechanism Interstitially mechanism
Relaxation mechanism
Grain Boundary Diffusionmechanism
Ring mechanism
Vacancy mechanism
Crowdion mechanism
Divacancy mechanism
Dislocation Pipemechanism Diffusion
Surface Diffusionmechanism
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Interstitial Vacancy and Substitutional
mechanisms are of importance
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Kinds of Diffusion
D is a function of dopant conc. andTemperature
Intrinsic Diffusion N
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What is lateral diffusion? State itssignificance in IC Technology.
Raj Univ. 2002, 2003Diffusion is an isotropic process. In a
masked diffusion, the dopant diffuses in
all the directions. Near the mask edge sideways diffusion is substantial. This is calledLateral Diffusion. To take this into account
three dimensional diffusion is to beconsidered. In IC technology this results inloss of critical dimensions.
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Lateral diffusion may be as large as 0.8 x
vertical junction depth. However ICtechnology has used this phenomenongainfully also by clever calculation andaccurate control over diffusion process.This is clear from the accompanyingdiagrams. Post implantation thermalprocess is carefully designed to take
advantage of the lateral diffusion.
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As the depth of diffusion depends on
crystallographic orientation, lateraldiffusion can, in principle, be controlled byproper choice of crystal orientation anddiffusing element.
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Important Parameters
Surface Conc. the conc. Of the dopantsat the surface. This controls the dopingprofile and junction depth of the next
diffusion. Dose 0 : Total quantity of the dopant
present per unit volume of the wafer
0 = 0N(x,t)dx
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Explain the electric field effect on diffusion.What is maximum value of electric field
enhancement factor? Raj. Univ. 2004
When the dopants are ionized at the
diffusion temp. a local electric field is setup between the ionized impurity atomsand the electrons or holes. The conc.Gradient of these ionized impurity atomsalso produces an internal electric fieldwhich enhance the diffusivity of ionizedimpurity atoms.
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Garima page 2.21
This internal electric field is related toelectrical potential V(x) as:
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Important Parameters
Junction Depth Depth of the junctionbelow the wafer surface.
Physical location where the conc. Of the
diffusing species is equal to the conc. Of thebackground species is called metallurgical
junction.
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Measurement of junction depth
By angle lapping andstainning
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Measurements
Junction Depth
Sheet Resistance
Effective line width
Concentration - profile
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Junction depth measurement
Angle Lapping
Lapping at 1 to 5 degree angle
Cleaning
Staining: few drops of Nitric acid in 100 c.c.
HFExpose in strong light (1000 watt bulb) for 2
minutes surface of p type material willbecome dark due to change in reflectivity
Measurement
Calculation -
Grooving
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q30
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h157
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Cooperative Diffusion Effects q222
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g223
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Thank You
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Diffusion Profiles
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g174
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g176a
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g185
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g187
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g189
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g192
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g220
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S. K. Bhatnagar 48h179c
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h153b
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h122
h125
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h125
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Diffusion Systems
Diffusion Profiles
Measurements
Rapid Thermal Annealing (RTP)
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h124
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h155b
Difference between theory and practice
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Difference between theory and practice
Spread in values of surface concentrationof the dopant
Theoretical values are not achieved
Impurity profile varies from theoreticalprediction
Effect of physical location of the wafer in
the furnace
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q239
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RS
Distance from gas input end of the diffusion tube
Solution
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Solution
Based on Henrys law research wasundertaken
Patent No.2291/CAL/73 (1973)
S. K. Bhatnagar and O. P. Wadhawan
"Improvements In Or Relating To AnApparatus For Diffusion Of Impurities In
Semiconductor Using Liquid Dopants
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Henrys Law: Surface concentration of thedopant is proportional to the partialpressure in gas phase
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Distance from gas input end of the diffusion tube
RS
Center of theprocess tube
Normalequipment
Patented
equipment
Mechanism of diffusion
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Mechanism ofdiffusion g151b