LJ07 Diffusion 3 Aug2010

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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

    S. K. Bhatnagar 2

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    S. K. Bhatnagar 3

    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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    S. K. Bhatnagar 7

    g207

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    S. K. Bhatnagar 8h126

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    S. K. Bhatnagar 9q201

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    S. K. Bhatnagar 10

    q251a

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    S. K. Bhatnagar 11q239

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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

    S. K. Bhatnagar (2009) 13

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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

    S. K. Bhatnagar (2009) 14

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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.

    S. K. Bhatnagar (2009) 15

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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

    S. K. Bhatnagar (2009) 16

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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

    S. K. Bhatnagar (2009) 17

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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

    S. K. Bhatnagar (2009) 18

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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

    S. K. Bhatnagar (2009) 19

    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.

    S. K. Bhatnagar (2009) 21

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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.

    S. K. Bhatnagar (2009) 22

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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.

    S. K. Bhatnagar (2009) 23

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    S. K. Bhatnagar 24h151b

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    S. K. Bhatnagar 25h152

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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

    S. K. Bhatnagar (2009) 26

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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.

    S. K. Bhatnagar (2009) 27

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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.

    S. K. Bhatnagar (2009) 29

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    Measurement of junction depth

    By angle lapping andstainning

    S. K. Bhatnagar 30

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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

    S. K. Bhatnagar (2009) 35

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    S. K. Bhatnagar 36

    g223

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    S. K. Bhatnagar 37

    Thank You

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    S. K. Bhatnagar 38

    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 46h179a

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    S. K. Bhatnagar 47h179b

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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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    S. K. Bhatnagar 52

    Diffusion Systems

    Diffusion Profiles

    Measurements

    Rapid Thermal Annealing (RTP)

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    S. K. Bhatnagar 53h153a

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    h124

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    h155b

    Difference between theory and practice

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    S. K. Bhatnagar 57

    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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    S. K. Bhatnagar 58

    q239

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    RS

    Distance from gas input end of the diffusion tube

    Solution

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    S. K. Bhatnagar 60

    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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    S. K. Bhatnagar 61

    Henrys Law: Surface concentration of thedopant is proportional to the partialpressure in gas phase

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    S. K. Bhatnagar 62

    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