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INTEGRATED CIRCUITS

INTEGRATED CIRCUITS. Dr. Esam Yosry. Lec . #3. Introduction Diffusion Process Diffusion Mechanisms Why Diffusion? Diffusion Technology Diffusion Equation Lateral Diffusion Measuring Dopant Concent . & Depth. Diffusion. Introduction ( Chip Fabrication Cycle).

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INTEGRATED CIRCUITS

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  1. INTEGRATED CIRCUITS Dr. EsamYosry Lec. #3

  2. Introduction • Diffusion Process • Diffusion Mechanisms • Why Diffusion? • Diffusion Technology • Diffusion Equation • Lateral Diffusion • Measuring Dopant Concent. & Depth Diffusion

  3. Introduction(Chip Fabrication Cycle)

  4. Introduction(Processes) • Oxidation • Diffusion • Ion Implantation • Deposition • Etching • Lithography • Deposition • Removal • Patterning • Modification of electrical properties

  5. Diffusion Process • Diffusion: is a process by which impurity atoms move through a crystal • Diffusion is the redistribution of atoms from high concentration regions to low concentration regions • Diffusion is becoming the old way of doping, but it is still used for deep doping

  6. Diffusion Process • Diffusion process is carried out at 1000 °C to give atoms thermal energy • There are more than one possible mechanism by which the heatedatomscan move through the heated crystal

  7. Diffusion Mechanisms • Substitutional : B, P, As • Interstitial : Au, Cu, Fe • Thermal activation E (eV) • Es > Ei

  8. Why Diffusion? • Placement of doped regions (Source/Drain) determine the short-channel c/cs of MOS device

  9. Diffusion Technology • The introduction of impurities by diffusion into wafer is usually carried out by exposing the wafer to an inert (nitrogen) gas containing the desired dopant in a diffusion furnace • The Two-Step diffusion is: • Predeposition then Drive-In

  10. Diffusion Technology • Predeposition: doping often proceeds by initial predep step to introduce the required dose of dopant into the substrate (source on Infinite source) • Drive-In: a subsequent drive-in anneal then redistributes the dopant giving the required junction depth and surface concentration (source off limited source)

  11. Diffusion Technology P-type dopant atoms SiO2 SiO2 P-doped region Masked part of the wafer n-type wafer Typically, a thermally grown oxide mask is used, and openings are made in the oxide over the regions where you want to dope.

  12. Diffusion Technology • Oxide can be relatively thin, since the diffusivity of dopants in SiO2 << in Si. Need thicker oxide for longer predeposition processes • Oxide grows on the Si wafer during diffusion and traps the dopant in the wafer so it does not diffuse out.

  13. Diffusion Technology • A junction is the separation between a region n-dopant and p-dopant region • Junction location is where the concentration of electrons = concentration of holes Metallurgical Junction

  14. Diffusion Technology Diffusion Sources

  15. Diffusion Technology Diffusion Furnace • Exactly the same as the oxidation furnace. • The nature of the ambient gas depends on the type of diffusion required. Tube Batch Boat

  16. Diffusion Equation • Fick’s 1st Law: • Continuity Equation: • Fick’s 2nd Law (Diffusion Equastion): D is the diffusion constant in cm2/s

  17. Diffusion Equation • Dopant redistribution is described by Fick’s 1st Law, how the flux of dopant depends on the doping gradient • -ve indicates that the flow is down the concentration gradient

  18. Diffusion Equation • Fick’s 2nd Law describes how the change in concentration in a volume is determined by the change in fluxes in and out of the volume ∆N

  19. Solution of the Diffusion Equation • Under constant surface concentration Ns (Infinite source): (Predeposition) • Under constant dose Q (Limited source): (Drive-In) surface concentration Shallow highly concentrated doped region near the surface diffusion length Ld Dd td >> Dptp

  20. Solution of the Diffusion Equation • Predeposition: (Diffusion from infinite source for short time at low temperature) log scale Ns NB x Xj Shallow Junction

  21. Solution of the Diffusion Equation • Drive-in: (Diffusion from a limited source for long time at high temperature) Deep Junction Xj

  22. Solution of the Diffusion Equation At any given time we can plot the concentration versus distance

  23. Error Function Solution of the Diffusion Equation

  24. Error Function Solution of the Diffusion Equation

  25. Error Function Solution of the Diffusion Equation

  26. Example 1 • Calculate the junction depth and the dose of dopant Q introduced into an n-type silicon substrate with a bulk background concentration of 1015 cm-3 after a diffusion from infinite source at 975 °C for 60 min with 3.5 x 1020 cm-3 surface concentration and diffusion constant is 1.5 x 10-14 cm2/s. • Solution • NB=1015 cm-3,T=975 °C, t=60 min, NS= 3.5x1020 cm-3, D=1.5x10-14 cm2/s. NB / NS = 2.9x10-6, 2√Dt =1.47x10-5 cm. From table erfc-1 (2.9x10-6 ) =3.3 Xj=3.3x1.47x10-5 =49 µm = 0.49 micron Q=NsLd /√Π = 1.47x10-5x 3.5x1020 /1.77 = 2.9x1015 cm-2

  27. Example 2 • Calculate the junction depth for the sample predep-diffused in example1 after a drive-in for 4.5 hours at 1100 ° C and diffusion constant at this temperature is 2.5 x 10-13 cm2/s. • Solution • T=1100 °C, t=4.5 hr, D=2.5x10-13cm2/s. • We must first make sure that the condition Ddtd >> Dptpis valid. • √(Dd td )= √ (2.5x10-13x 4.5x60x60) = 0.64 micron • √Dptp= √ 1.5x10-14x 60x60 = 0.074 micron • Thus the condition is valid and we can use where No=(2x3.5x1020/ Π) x 0.074/0.64= 2.5x 1019 cm-3. Xj =12.8x10-5 x√ln(2.5x1019/1015)= 4 micron

  28. Example 3 • Calculate the concentration of a boron diffusion into silicon (doped with Phos. to 1X1015/cm3) after a 5 hour diffusion at 1100 C. The boron Qois 5X1011/cm2 . Calculate the N(x) at 1, 2 and 3. • Solution • Obtain the diffusion coefficient at 1100 C,  D = 0.2 /hr1/2 from the graph on the previous page • T=1100 °C, t=5 hr.

  29. Example 3 Q/(Dt)1/2 = 6.0X1015 boron atoms / cm3 At x = 0, N(x) = 6.0X1015atoms / cm3 At x = 1, N(x) = 4.4X1015atoms / cm3 At x = 2, N(x) = 1.7X1015atoms / cm3 At x = 3, N(x) = 0.36X1015atoms / cm3 6.0X1015 N(x) 1.0X1015 x 0 1 2 3 Distance in the silicon in microns

  30. Lateral Diffusion Dopant diffuses laterally under the oxide diffusion mask Diffusion through an oxide window 70 - 80% Xj Xj

  31. Measuring Dopant Concentrations and Depth • There are several techniques: • Sheet Resistance • 2. Bevel and Etch • Mechanically groove the wafer surface. • 3. Capacitance-Voltage Measurement • 4. Spreading Resistance Profilometry • Can measure doping between 1013 – 1021 atoms/cm3 • 5. Secondary Ion Mass Spectroscopy • Takes a long time

  32. Sheet Resistance Measuring Dopant Concentrations and Depth • Sheet Resistance is defined by looking at a small piece (square) of silicon with dimension “L” on a side and xjdeep L

  33. Sheet Resistance Measuring Dopant Concentrations and Depth • The resistivity of a cube is given by: + V - L I L L

  34. Sheet Resistance Measuring Dopant Concentrations and Depth • The sheet resistance of a shallow junction is For uniformly doped material + V - I Sheet Resistance L sq L L

  35. Capacitance-Voltage Measurement Measuring Dopant Concentrations and Depth • Place a biased metal contact on the semiconductor surface to form a depletion region of width W and a capacitance of C

  36. Capacitance-Voltage Measurement Measuring Dopant Concentrations and Depth • Measure the capacitance of the depletion region as a function of bias voltage so the substrate doping is given by: • Dose not work for high doping (>1018 atoms/cm3)

  37. Thanks Many thanks to Prof. Hany Fikry and Prof WaelFikry for their useful materials that help me to prepare this presentation.

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