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ECE 6466 “ IC Engineering ” Dr. Wanda Wosik

: Silicon VLSI Technology Fundamentals, Practice and Modeling by J. D. Plummer, M. D. Deal, and P. B. Griffin. Chapter 3 Crystal Growth, Wafer Fabrication and Basic Properties of Silicon Wafers. ECE 6466 “ IC Engineering ” Dr. Wanda Wosik. UH; F2014. Basic Crystal Lattice.

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ECE 6466 “ IC Engineering ” Dr. Wanda Wosik

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  1. :Silicon VLSI TechnologyFundamentals, Practice and Modelingby J. D. Plummer, M. D. Deal, and P. B. Griffin Chapter 3 Crystal Growth, Wafer Fabrication and Basic Properties of Silicon Wafers ECE 6466 “IC Engineering” Dr. Wanda Wosik UH; F2014

  2. Basic Crystal Lattice Silicon in microelecronics: Single Crystal Polycrystalline Amorphous periodic small crystals no long range arrangements order between of atoms atoms Crystal lattice is described by a unit cell with a base vector (distance between atoms) Types of unit cells Body Centered Cube Face Centered Cube

  3. Directions and Planes in Crystals Directions (vector components: a single direction is expressed as [a set of 3 integers], equivalent directions (family) are expressed as < a set of 3 integers > Planes: a single plane is expressed as (a set of 3 integers h k l = Miler indices) and equivalent planesare expressed as {a set of 3 integers} Miler indices: take a,b,c (multiple of basic vectors ex. x=4a, y=3a, z=2a) reciprocals (1/4, 1/3, 1/2)-> common denominator (3/12, 4/12, 6/12) -> the smallest numerators (3 4 6) lattice constant

  4. Wikipedia

  5. Silicon Crystal Structure Diamond lattice (Si, Ge, GaAs) Two interpenetrating FCC structures shifted by a/4 in all three directions All atoms in both FCCs covalent bonding Diamond Atomsinside one FCC come from the second lattice (100) Si for devices (111) Si not used - oxide charges

  6. http://www.iue.tuwien.ac.at/phd/hoessinger/node26.html http://conceptualphysics.in http://www.tomshardware.co.uk/behind-the-closed-doors-of-amd,review-15637-7.html

  7. http://www.molecularsciences.org/book/export/html/125

  8. • Silicon has the basic diamond crystal structure - two merged FCC cells offset by a/4 in x, y and z. See 3D models • Various types of defects can exist in crystal (or can be created by processing steps. In general these are detrimental to device performance. http://oes.mans.edu.eg/courses/SemiCond/applets/education/solid/unitCell/home.html http://cst-www.nrl.navy.mil/lattice/struk.jmol/a4.html

  9. Defects in Crystals Point defects, line defects, volume defects

  10. Dislocation Formation by Point Defects Agglomeration Intrinsic point defects in a crystal Nv and NI increase with T Agglomeration of Interstitials collapse After Shimura

  11. Stacking Faults and Grain Boundary Perfect stacking OISF Induced by oxidation Missing (111) plane SFs bound by dislocations After Campbell After Shimura

  12. Types of Dislocations Dislocation line Screw Dislocations Start the contour here Dislocation line | | Burger vector - screw Burger vector - edge b Edge Disclocation inserted plane b In Si 60° dislocations are formed in processing After Shimura

  13. Silicon Crystal w/o Dislocations Process induced dislocations deteriorate devices Grown Si crystal is dislocation-free Dangling bonds of a half plane affect physical and electrical properties (here 60°) Pure edge dislocation After Shimura

  14. Propagation of Dislocations by Climb Motion Shift After Shimura

  15. Motion of Dislocations by Glide • Stress induced by • T gradient T • Mismatch of thermal expansion coeff., layers, precipitates Easy motion of dislocations T After Wolf&Tauber

  16. Raw Material and Purification After Wolf&Tauber

  17. Purification and Preparation of Electronic Grade Semiconductor MGS EGS Si Crystal 2SiHCl3+H2→2Si+6HCL gas solid MSG Refined quartzite (SiO2) ppb purity of EGS for CZ or FZ 300 °C MGS+HCl →SiHCl3 Grind MGS to powder 98% pure Liquid at RT After Wolf&Tauber

  18. Crystal Growth • Si used for crystal growth is purified from SiO2 (sand) through refining, fractional distillation and CVD. • The raw material contains < 1 ppb impurities. Pulled crystals contain O (≈ 1018 cm-3) and C (≈ 1016 cm-3), plus any added dopants placed in the melt. • Essentially all Si wafers used for ICs today come from Czochralski grown crystals. • Polysilicon material is melted, held at close to 1417 ˚C, and a single crystal seed is used to start the growth. • Pull rate, melt temperature and rotation rate are all important control parameters. → Introduces SiO2 in CZ; Oi≈1017-1018cm-3 → C ≈ 1015-1016 cm-3 Ar ambient

  19. Czochralski Growth •Load EGS+Impurities P, B, As •pump-out, •seed down, •pull fast, •pull slow EGS 100 kg Neck confines dislocations • Crystal solidifies • increases - pull rate decreases seed (More information on crystal growth at http://www.memc.com/co-as-description-crystal-growth.asp Also, see animations of http://www.memc.com/co-as-process-animation.asp) (Photo courtesy of Ruth Carranza.))

  20. Details of Czochralski Growth Rotation of crucible and crystal in opposite Directions improves growth and doping uniformity Ar Oxygen incorporation - important for intrinsic gettering 1017-1018 cm-3 Carbon contributes to native defects 1015-1016 cm-3 After Shimura

  21. Oxygen Concentrations in CZ Silicon Si melts - C-Si growth Role of temperature After Campbell

  22. Requirement for Larger Crystals 450 mm ~2016? 12 inches After Wolf&Tauber

  23. Wafer Preparation and Specification Mark wafer earlier (laser process) to track their process flow • Grind crystal to a diameter (200mm750µm) … 850µm thick • Grind flats (the primary and secondary) • Saw of the boule into wafers • Lapping, etching (batch process in acids etching Si) 20 µm, polishing (chemical-mechanical) 25µm removes damage and improves flatness ±2µm SiO2 10nm in NaOH/DI Suspension Al2O3 CMP 3Si +4HNO3+18HF 3H2SiF6+4NO+8H2O Thickness increases with diameter: 300 mm – 775mm vs. 450 mm – 925 mm

  24. Orientation of ICs on Silicon Wafers Si cleaves along {111} For (100) the {111} planes are along <110> Primary and Secondary Flats plane Notches instead of flats – or Notchless wafers? After Shimura

  25. Float Zone Method for Crystal Growth • An alternative process is the float zone process which can be used for refining or single crystal growth. No crucible - no impurities High resistivity Si Add Dopants (gas) PH3 B2H6 EGS ESG

  26. Crystal Growth and Wafers Fabrication • After crystal pulling, the boule is shaped and cut into wafers which are then polished on one side. http://www.youtube.com/watch?v=AMgQ1-HdElM http://sunedisonsemi.com/index.php?view=100-notch&l1=27&l2=55&l3=62

  27. After Wolf&Tauber

  28. Modeling CZCrystal Growth • We wish to find a relationship between pull rate and crystal diameter. • Freezing occurs between isotherms X1 and X2. • Heat balance (A B C): latent heat of crystallization + heat conducted from melt to crystal = heat conducted away. C=Radiation B=conduction in solid A=Heat of crystallization Freezing interface Liquid to solid → HEAT  (1) For crystal uniformity T uniformity is important Vpmax ~1√r Large crystal requires slow pull rates

  29. • The rate of growth of the crystal is (2) where vP is the pull rate and N is the density. (3) • Neglecting the middle term in Eqn. (1) we have: • In order to replace dT/dx2, we need to consider the heat transfer processes. • Heat radiation from the crystal (C) is given by the Stefan-Boltzmann law (4) • Heat conduction up the crystal is given by (5)

  30. • Differentiating (5), we have (6) (7) • Substituting (6) into (4), we have • kS varies roughly as 1/T, so if kM is the thermal conductivity at the melting point, (8) (9) • Solving this differential equation, evaluating it at x = 0 and substituting the result into (3), we obtain (see text): (10) • This gives a max pull rate of ≈ 24 cm hr-1 for a 6” crystal (see text). Actual values are ≈ 2X less than this.

  31. Dopant Segregation During Crystal Growth “k” affects doping uniformity 1 After Wolf&Tauber

  32. Modeling Dopant Behavior During Crystal Growth • Dopants are added to the melt to provide a controlled N or P doping level in the wafers. • However, the dopant incorporation process is complicated by dopant segregation. Segregation Coefficients of Various Impurities in Silicon • Most k0 values are <1 which means the impurity prefers to stay in the liquid. • Thus as the crystal is pulled, NS will increase.

  33. Solid Solubility After Campbell

  34. Dopant Incorporation During CZ Growth • If during growth, an additional volume dV freezes, the impurities incorporated into dV are given by solidified (12) Removed → from the melt (13) (14) Initial in melt During the growth • We are really interested in the impurity level in the crystal (CS), so when incremental volume freezes (15) (16) where f is the fraction of the melt frozen (Vs/V0).

  35. Uniformity of Crystal Doping tail seed ko=0.8 • Plot of Eq. (16). • Note the relatively flat profile produced by boron with a kS close to 1. • Dopants with kS << 1 produce much more variation in doping concentration along the crystal. k0=0.35 where f is the fraction of the melt frozen. k0=0.023

  36. After Wolf&Tauber

  37. Radial Doping Nonuniformity After Shimura

  38. Zone Refining and FZ Growth Segregation of Impurities Between Solidus and Liquidus (in FZ Growth) RF -> melt=zone moving Poly-Si Crystal C0 original concentration in the rod I - the number of impurities in the liquid dI=(C0-k0CL)dx • In the float zone process, dopants and other impurities tend to stay in the liquid and therefore refining can be accomplished, especially with multiple passes • See the text for models of this process. Distribution of a dopant along the crystal

  39. Float Zone Growth; Removal of Impurities For one pass only → k0 small gives better refining Better crystal purity

  40. Modeling Point Defects in Silicon • Point defects (V and I) will turn out to play fundamental roles in many process technologies. • The total free energy of the crystal is minimized when finite concentrations of these defects exist. (17) Sf entropy & Hf enthalpy of defect formation • In general and both are strong functions of temperature. • Kinetics may determine the concentration in a wafer rather than thermodynamics. • In equilibrium, values for these concentrations are given by: (18) (19) Smaller concentrations than those of dopants/carriers(hard to measure) @1000°C CI0≈1012cm-3 and CV0≈5x1013 cm-3

  41. Point Defects • V and I also exist in charged states with discrete energies in the Si bandgap. • In N type Si, V= and V- will dominate; in P type, V+ and V++ will dominate. • Shockley and Last (1957) first described these charged defect concentrations (see text). Note: • The defect concentrations are always << ni. ( doping EF point defect concentrations) • As doping changes, the neutral point defect concentrations are constant. • However, the charged defect concentrations change with doping. \ the total point defect concentrations change with doping. (20) (21)

  42. Point Defects I and V increase with T & are very mobile and affect many various processing Very difficult to measure • Concentrations of I and V are different (surface generation & recombination= sinks, defects) • Equilibrium concentrations change instantaneously with T • Non-equilibrium concentrations possible (after implantation, CZ growth - freeze leads to swirls, oxidation) These concentrations can be measured (ex. RBS) @1000°C 1012 cm-3 5x1013 cm-3 ni≈7.14x1018cm-3 Point Defects are very fast: Diffusivities of V and I >> dopants’ diffusivities

  43. Fermi-Dirac distribution not sharp at high T Neutral – do not depend on F-level HEAT IT to 1000°C Low doped vacancies interstitials Highly doped Still extrinsic semiconductor so F-level in the upper half. “p” region “n” region

  44. Continue the Example • At 1000 ˚C, the P region will be intrinsic, the N region is extrinsic. Note: • ni relative to doping in the two regions. • V0 is the same in the two regions. • Different charge states dominate in the different regions.

  45. Oxygen and Carbon in Silicon 10-20 ppm (5x1017-1018cm-3) • Interstitial oxygen improves crystal strength (≈25%) • Oxygen (thermal 450°C) donors TD (1016cm-3) affect  resistivity; TD dissolve T>500°C • Precipitations weakens strength, getter impurities, • contribute for dislocation and SF formation volume expansion (stress) alleviated by V and I grow Si-Si → Si-O-Si Growing precipitates consume V release I Solubility of oxygen 1.2x1018cm-3 in melt 30 Å embryos grow @700 °C • precipitation at T • heterogenous or • homogeniuos Co*1000°C=1017cm-3 CarbonCC ≈ 1016 cm-3is usually substitutional in Si, affects SiO2 precipitates and high T thermal donors (650-1000°C), interacts with point defects and some dopants Processes that generate V cause SiO2 precipitates’ growth I cause smaller growth compensate stress

  46. Measurements of the Grown Crystal Resistivity xj<<t r >>s Average resistivity Sheet resistance

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