Crystals and wafer prep - University of Texas at Austin

Crystals and wafer prep - University of Texas at Austin

Bulk crystal growth melting points silicon: 1420 C GaAs: 1238 C quartz: 1732 C starting material: metallurgical-grade silicon by mixing with carbon, SiO2 reduced in arc furnace T > 1780C: SiC + SiO2 Si + SiO + CO common impurities Al: 1600 ppm (1 ppm = 5 x 1016 cm-3) B: 40 ppm Fe: 2000 ppm P: 30 ppm used mostly as an additive in steel Dean P. Neikirk 1999, last update January 31, 2020 1 Dept. of ECE, Univ. of Texas at Austin Preparation of electronic-grade silicon gas phase purification used to produce high purity silicon ~ 600C crud + Si + HCl

SiCl4 (silicon tetrachloride) SiCl3H (trichlorosilane) SiCl2H2 (dichlorosilane) chlorides of impurities trichlorosilane (liquid at rm temp), further purification via fractional distillation now reverse reaction 2SiHCl3 + 2H2 (heat) 2Si + 6HCl after purification get Al: below detection B: < 1 ppb (1 ppb = 5 x 1013 cm-3) Fe: 4 ppm P: < 2 ppb Sb: 1 ppb Au: 0.1 ppb Dean P. Neikirk 1999, last update January 31, 2020 2 Dept. of ECE, Univ. of Texas at Austin Czochralski crystal growth silicon expands upon freezing (just like water) if solidify in a container will induce large stress CZ growth is container-less images from Mitsubishi Materials Silicon http://www.egg.or.jp/MSIL/english/ msilhist0-e.html Dean P. Neikirk 1999, last update January 31, 2020 3 Dept. of ECE, Univ. of Texas at Austin Diameter control during CZ growth seed pull direction rotation images from Mitsubishi Materials Silicon http://www.egg.or.jp/MSIL/english/msilhist0-e.html critical factor is heat flow from liquid to solid interface between liquid and solid is an isotherm temperature fluctuations cause problems! already grown crystal is the heat sink balance latent heat of fusion, solidification rate, pull rate, diameter, temperature gradient, heat flow

diameter inversely proportional to pull rate (typically ~ mm/min) Dean P. Neikirk 1999, last update January 31, 2020 4 Dept. of ECE, Univ. of Texas at Austin temperature Diameter control during CZ growth increment solidified liquid solid dT d x1 dT d x2 latent heat of fusion heat flux (power) released is dm Ad x L L L A v pull dt dt dT

d x3 dx = vpull dt ; dm = A dx critical factor is heat flow from liquid to solid heat flux (power) balance heat released + as solidifies thermal diffusion in liquid from hot liquid towards solidification interface thermal diffusion in solid from solidification interface = towards cooler sides/end of boule dT dT liquid A solid A d x1 d x2 interface between liquid and solid should be an isotherm dT temperature fluctuations cause problems! 0 d x1 L A v pull Dean P. Neikirk 1999, last update January 31, 2020

5 Dept. of ECE, Univ. of Texas at Austin Diameter control during CZ growth temperature increment solidified liquid solid dT d x2 dT 0 d x1 heat flow balance becomes L A v pull dT d x3 or dT solid A dx 2 thermal current dT

L v pull d x2 solid so critical factor is relation between temperature gradient in boule and boule size thermal current proportional to cross sectional area A and vpull if the only heat sink were at the end of the boule: thermal resistance inversely proportional to A, directly proportional to length of boule l temperature change (voltage) = Ithermal Rthermal l L T L A v pull v pull l A solid solid dT T L v pull

dx l solid net effect: just what we got above! dT/dx independent of diameter diameter doesnt appear!!! Dean P. Neikirk 1999, last update January 31, 2020 6 Dept. of ECE, Univ. of Texas at Austin Diameter control during CZ growth temperature increment solidified liquid solid dT d x2 dT 0 d x1 v pull dT d x3 solid d T

L d x2 but most of the heat is lost via radiation from the SIDES of the boule! thermal current still proportional to cross sectional area A (diameter)2 and vpull if the heat sink is from sides of boule: thermal resistance inversely proportional to perimeter diameter, temperature change (voltage) = Ithermal Rthermal constant 2 T constant diam v pull diam v pull diam diam v pull T 1 net effect: diameter is inversely proportional to pull rate Dean P. Neikirk 1999, last update January 31, 2020 7 Dept. of ECE, Univ. of Texas at Austin Impurities in Czochralski Grown Silicon choice of crucible material is crucial:

must be stable at high temperatures (~1500 C) carbon: saturates solution and causes poly growth refractories: too much metal in materials quartz: in exclusive use for silicon growth dissolution of quartz crucibles into melt is major concern: function of relative velocity between melt & crucible almost all oxygen present in silicon melt is due to the dissolution of the SiO2 crucible most of this oxygen evaporates in the form of SiO Dean P. Neikirk 1999, last update January 31, 2020 8 Dept. of ECE, Univ. of Texas at Austin Doping and segregation effects during crystal growth when two dissimilar materials / phases are in contact, the concentration of an impurity across the interface is NOT NECESSARILY CONTINUOUS segregation (distribution) coefficient CL element Csolid/Cliquid

Al 0.002 As 0.3 when a volume of liquid freezes, if k < 1, what is concentration in solid? must be less than in liquid what happens to extra impurities? rejected into melt increased [impurity] in melt if Co is initial melt concentration, and X is fraction solidified k 1 CS kCo1 X Dean P. Neikirk 1999, last update January 31, 2020 9 B 0.8 O 1.25 P 0.35 Sb 0.023 k = 0.5, Cliquid = 1e17 1.0E+18 Concentration

(#/cm3) C k S 1.0E+17 1.0E+16 0 1 Percent Solidified Dept. of ECE, Univ. of Texas at Austin Doping and segregation effects during crystal growth segregation effects can be used intentionally to purify semiconductor material zone refining consists of repeated passes through the solid by a liquid zone Cn 1 x Cn x 1 1 k e k x L Zone Refining k = 0.5 float zone silicon used for high resistivity Concentration (#/cm3)

1E+18 1E+17 1E+16 images from Mitsubishi Materials Silicon http://www.egg.or.jp/MSIL/english/msilhist0-e.html Dean P. Neikirk 1999, last update January 31, 2020 10 0 position x (L=0.1) 1 Dept. of ECE, Univ. of Texas at Austin Oxygen in CZ Silicon concentrations typically in 1016 - 1018 cm-3 range segregation coefficient k ~ 1.25 more in solid than liquid contact area between crucible and melt decreases as growth procedes oxygen content decreases from seed to tang end effects of oxygen in silicon ~ 95% interstitial; increases yield strength of silicon via "solution hardening" effect as-grown crystal is usually supersaturated (occurs above about 6 x 1017) Dean P. Neikirk 1999, last update January 31, 2020

11 Dept. of ECE, Univ. of Texas at Austin Oxygen complexes in silicon usually donor-like two classes of complexes: "old thermal donors" very small silicon-oxygen atom clusters very rapid formation rates in 400-500 C range ( 1010/cm3sec) "new thermal donors" slow formation rate above 500 C slow dissolution rate at high temperature ~1013 cm-3 @ 2 hours, 900 C ~1011 cm-3 @ 2 hours, 1150 C donor behavior possibly due to surface states of large SiOx complexes Dean P. Neikirk 1999, last update January 31, 2020 12 Dept. of ECE, Univ. of Texas at Austin Gettering in Silicon Wafers devices fabricated only in the top five or ten microns of the wafer: use gettering to provide a sink for unwanted defects in the bulk of the wafer gettering sites provide sinks for impurities generated during the processing mobile impurities device region

bulk wafer bulk faults back side damage backside damage: (pre-gettering) mechanical damage produces high strain regions impurities nucleate on dislocations; if wafer stresses are kept small during subsequent processing dislocations will remain localized on back Dean P. Neikirk 1999, last update January 31, 2020 13 Dept. of ECE, Univ. of Texas at Austin Intrinsic Gettering and Oxygen Precipitates wafer starting material: initial oxygen content between ~3.5 and ~8 x 1017 cm-3 denuded zone formation: high temperature step (1050 C) reduces interstitial oxygen content via diffusion of O to surface formation of internal gettering sites: low temp step (500-600 C) creates large reserve of small, stable oxygen precipitates higher temperature step (700-900 C) causes growth of larger SiOx complexes subsequent thermal processing creates dislocation loops associated with SiOx complexes

actual starting material oxygen concentration and process determined by trial device fab and performance evaluation. Dean P. Neikirk 1999, last update January 31, 2020 14 Dept. of ECE, Univ. of Texas at Austin Denuded zone preferential (decorating) etch used to reveal stacking faults and precipitates OSF: oxidation induced stacking faults from: Sze, VLSI Technology, 2nd edition, p. 46. Dean P. Neikirk 1999, last update January 31, 2020 15 Dept. of ECE, Univ. of Texas at Austin Wafer preparation boule forming, orientation measurement old standard: flatperpendicular to <110> direction; on large diameter notch used instead

inner diameter wafer saw wafer slicing <100> typically within 0.5 <111>, 2 - 5 off axis images from Mitsubishi Materials Silicon http://www.egg.or.jp/MSIL/english/msilhist0-e.html Dean P. Neikirk 1999, last update January 31, 2020 16 Dept. of ECE, Univ. of Texas at Austin Wafer prep (cont.) lapping grind both sides, flatness ~2-3 m ~20 m per side removed edge profiling etching chemical etch to remove surface damaged layer ~20 m per side removed polishing chemi-mechanical polish, SiO2 / NaOH slurry ~25 m per polished side removed gives wafers a mirror finish cleaning and inspection Dean P. Neikirk 1999, last update January 31, 2020

17 Dept. of ECE, Univ. of Texas at Austin Wafer specifications wafer diam. thickness thickness variation bow 150 mm 0.5mm 675m 25m 50m 60m 775m 25m = 10m warp

200 mm 300 mm 0.2mm = 100m warp: distance between highest and lowest points relative to reference plane bow: concave or convex deformation Dean P. Neikirk 1999, last update January 31, 2020 18 Dept. of ECE, Univ. of Texas at Austin Wafer diameter trends desire is to keep number of chips (die) per wafer high, even as die size increases 300 250 200 150 100 50 0 1970 1975 1980 1985 Year

1990 1995 challenge: thermal nonuniformities, convection currents become more significant as diameter grows Dean P. Neikirk 1999, last update January 31, 2020 19 Dept. of ECE, Univ. of Texas at Austin Wafer specifications Sematech From Sematech document: International 300 mm Initiative, Technology Transfer # 97113407A-ENG next generation: 300 mm wafer diameter 25x25 mm die size yields 89 complete die Dean P. Neikirk 1999, last update January 31, 2020 20 Dept. of ECE, Univ. of Texas at Austin Silicon wafer production 1999: 4.263 billion square inches, $5.883 billion

$1.38 per square inch, $0.21 per square cm 100mm, 150mm: 2.808 billion square inches (65.9% of total) 200mm: 1.441 billion square inches (33.8%) 300mm: 0.014 billion square inches of silicon (0.3%) 2000, expected: 4.692 billion square inches, $6.475 billion 2001, expected: 5.204 billion square inches 2003, expected: 200mm: 2.892 billion square inches 300mm: 0.112 billion square inches from EE Times, Advanced silicon substrates prices rise as wafer glut eases by J.Robert Lineback Semiconductor Business News (01/12/00, 2:04 p.m. EST) Dean P. Neikirk 1999, last update January 31, 2020 21 Dept. of ECE, Univ. of Texas at Austin Volume Silicon processing costs 2001 processing cost date reference: ICKnowledge, http://www.icknowledge.com/economics/wafer_costs.html advanced CMOS process, ~0.13 micron, 300mm wafers, ~25 mask levels: about $5 per cm2 reference: ICKnowledge, http://www.icknowledge.com model assumes a 30,000 300mm wafer per month fab running at 90% of capacity

thats about 21 million cm2 / month! about 40 wafer starts per hour 2001 world-wide wafer starts, 8 (200mm) equivalent: ~5 million wafers per month (~1.5billion sq. cm per month) from http://www.semichips.org/downloads/SICAS_Q4_01.pdf MOSIS (ref http://www.mosis.org/Orders/Prices/price-list-domestic.htm 1.5 micron cmos ~$200 per square mm, 5 to 20 parts per lot cost ~$4K- $1K per cm2 0.18 micron ~$1-2K per square mm for 40 parts cost > ~$2.5K per cm2 Dean P. Neikirk 1999, last update January 31, 2020 22 Dept. of ECE, Univ. of Texas at Austin Silicon Oxides: SiO2 Uses: diffusion masks surface passivation gate insulator (MOSFET) isolation, insulation

Formation: grown / native thermal: highest quality anodization deposited: C V D, evaporate, sputter vitreous silica: material is a GLASS under normal circumstances can also find crystal quartz in nature m.p. 1732 C; glass is unstable below 1710 C BUT devitrification rate (i.e. crystallization) below 1000 C negligible Dean P. Neikirk 1999, last update January 31, 2020 23 bridging oxygen non-bridging oxygen silicon network modifier network former hydroxyl group Dept. of ECE, Univ. of Texas at Austin

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