Créer une présentation
Télécharger la présentation

Download

Download Presentation

: Silicon VLSI Technology Fundamentals, Practice and Modeling by J. D. Plummer, M. D. Deal, and P. B. Griffin

686 Vues
Download Presentation

Télécharger la présentation
## : Silicon VLSI Technology Fundamentals, Practice and Modeling by J. D. Plummer, M. D. Deal, and P. B. Griffin

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**: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; F2013**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**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**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**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**• 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**Defects in Crystals**Point defects, line defects, volume defects**Dislocation Formation by Point Defects Agglomeration**Intrinsic point defects in a crystal Nv and NI increase with T Agglomeration of Interstitials collapse After Shimura**Stacking Faults and Grain Boundary**Perfect stacking OISF Induced by oxidation Missing (111) plane SFs bound by dislocations After Campbell After Shimura**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**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**Propagation of Dislocations by Climb Motion**Shift After Shimura**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**Raw Material and Purification**After Wolf&Tauber**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 98% pure Grind to powder Liquid ar RT After Wolf&Tauber**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**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.))**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**Oxygen Concentrations in CZ Silicon**Si melts - C-Si growth Role of temperature After Campbell**Requirement for Larger Crystals**12 inches After Wolf&Tauber**Wafer Preparation and Specification**Mark wafer earlier (laser process) to track their process flow • Grind crystal to a diameter (200mm750µm) … 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**Orientation of ICs on Silicon Wafers**Si cleaves along {111} For (100) the {111} planes are along <110> Primary and Secondary Flats plane After Shimura**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**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.memc.com/index.php?view=Process-Animations-**After Wolf&Tauber**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**• 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)**• 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.**Dopant Segregation During Crystal Growth**“k” affects doping uniformity 1 After Wolf&Tauber**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.**Solid Solubility**After Campbell**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).**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**Radial Doping Nonuniformity**After Shimura**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**Float Zone Growth; Removal of Impurities**For one pass only → k0 small gives better refining Better crystal purity**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**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)**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**Fermi-Dirac**distribution not sharp at high T Neutral – do not depend on F-level Low doped Highly doped Still extrinsic semiconductor so F-level in the upper half. interstitials vacancies**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.**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**Measurements of the Grown Crystal**Resistivity xj<<t r >>s Average resistivity Sheet resistance