======================SILICON_PHYSICS=========================== silicon Atomic number: 14 Atomic weight: 28.086 Density: 2329 [293 K];2525 [at m.p.] kg m-3 Molar volume: 12.06 cm3 Velocity of sound: 2200 m s-1 Hardness Mineral: 6.5 Melting point: 1683K Boiling point: 2628K Thermal conductivity: 148 [300 k] W m-1 K-1 Water Standard: 2.42W Specific Heat: 0.76joule/gm*C Fusion: 39.6 kJ mol-1 Vaporization: 383.3 kJ mol-1 Linear Hole Mobility SEMICONDUCTOR ( T =290K). Si Ge GaAs InSb Energygap,eV 1.106 0.67 1.351 0.17t Temp coef EG -4 -4.5 -5 -2.7 eV/*K*10e4 Melting point, C 1412 958 1238 523 Thermal conduct 1.421 0.521 0.4411 0.17 W/cm-*C Thermal coeff l 4.2 5.5 5.7 linear expans C^-1*10e9 Lattice constant_A 5.42 5.65 5.65 6.48 Dielectric constant 11.8 16.0 11.1 15.9 Elec mobility, 1350 3900 6800 80000k cm2/v-sc T dependence Hole mobility 480 1900 680 4000~ Melt Coeff Eg Electron Density Point Expan Gap Mobility Light Mas Heavy Mass Element gm/cm3 0C 10-6/C (eV) cm2/v-s cm2/v-sec B 2.34 2075 1.40 1 2 C diamon 3.51 3800 1.18 5.30 1800 1600 Si 2.33 1417 4.20 1.09 1500 1500 480 Ge 5.32 937 0.10 0.66 3900 14000 1860 Sn-alpha 5.75 231.90 0.08 144000 1600* As 5.73 814 3.86 1.20 Sb 6.68 630.50 10.88 0.11 S-aplha 2.07 112.80 64.10 2.60 Se 4.79 217 36.80 1.80 1 Se 4.82 2.30 0.005 0.15 Te 6.25 452 16.80 0.38 1100 10000 700 Breakdown Si 30v/u Glass 600V/u Dielectric Si 11.7 Dielectric glass 3.9 Therm expan Si 2.5E-6 Therm expan SiO2 5E-6 Specific heat 0.76joule/g*C 1.0 joule/g*C thermal conduct 8 4 Watts/meter*C Nitride Si/N ratio 0.75->1 density 2.5-.3E3 Kg/cube_meter dielectric 6-9 breakdown field 6E6 V/cm expansion 46E-6 delta_L/L per C Si 1.00 ->1.46 Watts/cm*C GaAS 0.44 Cu 4.05 Gold 3.09 Silver 4.14 Kovar 0.2 BeO 2.34 saphire 0.25 Al2O3 0.188 Temp dependence T^-2.7 T^-2.33 ni, 1/cm3 1.5e10 2.4e13 1.35X10'6 § ni(Temp) T*exp(-1.21/kT) T*exp(-0.785/kT) In equilibrium, p*n*= n_i^2 net product independent of doping densities n=N_D+p donor density. Current flow J=e*E, conductivity given by sigma = q*(mu_n*n +mu_p*p). mobilities, ave drift velocity per unit electric field, q electron charge. Diffusion current -q*D_p*dp/dx for electrons -qD0 dp/dx. for holes diffusion D related to mobilities by D/mu= k*T/q kT/q is about 26 millivolts. ------------------------------------------------------ Valence Number radius_Ang Ionization Gallium 3 31 1.26 0.065ev Boron 3 5 0.81 0.045ev Aluminum 3 13 1.26 0.057ev Indium 3 44 1.44 0.160ev Phosphours 5 15 1.13 0.044ev Arsenic 5 33 1.18 0.049ev Antimony 5 51 1.36 0.0395ev Silicon 4 14 1.17 ------------------------------------------------------- SEMICONDUCTOR ( T =290K). Si Ge GaAs InSb Energygap,eV 1.106 0.67 1.351 0.17t Temp coef EG -4 -4.5 -5 -2.7 eV/*K*10e4 Melting point, C 1412 958 1238 523 Thermal conduct 1.421 0.521 0.44110.17 W/cm-*C Thermal coeff l 4.2 5.5 5. linear expans C^-1*10e9 Lattice constant_A 5.42 5.65 5.65 6.48 Dielectric constantt 11.8 16.0 11.1 15.9 Elec mobility, 1350 3900 6800t 80000k cm2/v-sc T dependence Hole mobility 480 1900 680 4000~ Hole mobility Temp T^-2.7 T^-2.33 ni, 1/cm3 1.5e10 2.4e13 1.35X10'6 § ni(Temp) T*exp(-1.21/kT) T*exp(-0.785/kT) p*n*= n_i^2 net product independent of doping densities n=N_D+p donor density. Current flow J=e*E, conductivity given by sigma = q*(mu_n*n +mu_p*p). mobilities, ave drift per unit electric field, q electron charge. Diffusion current -q*D_p*dp/dx for electrons -qD0 dp/dx. for holes diffusion D related to mobilities by D/mu= k*T/q kT/q is about 26 millivolts. high doping levels mobility decreases additional scattering from impurity high electrifields mobility also decreases, carriers at limiting velocity almost independent of field. limiting velocity is of order of ~10e6cm/ sec. recombination rate for excess electrons in p material, dn/dt= -(n-n_0)/tau_n no is the equilibrium density and tau_n, is called the lifetime of electrons. excess decays as exp (-t/tau_n). Lifetime very sensitive puritles several microseconds down to few nanoseconds. Lifetime can be reduced for such applications by introducing impurity atoms (such as gold) which act as recombination centers. Density Point Expan Gap Mobility Light Mas Heavy Mass Element gm/cm3 0C 10-6/C (eV) cm2/v-s cm2/v-sec B 2.34 2075 1.40 1 2 C diamon 3.51 3800 1.18 5.30 1800 1600 Si 2.33 1417 4.20 1.09 1500 1500 480 Ge 5.32 937 0.10 0.66 3900 14000 1860 Sn-alpha 5.75 231.90 0.08 144000 1600* As 5.73 814 3.86 1.20 Sb 6.68 630.50 10.88 0.11 S-aplha 2.07 112.80 64.10 2.60 Se 4.79 217 36.80 1.80 1 Se 4.82 2.30 0.005 0.15 Te 6.25 452 16.80 0.38 1100 10000 700 Si SIO2 number 14 atomic weight 28.09 60.8 Water Standard: 2.42 Lattice_const_A 5.43 melt_C 1420 1700 atoms_per_cc 5.00E22 2.3e22 density_Kg/m^3 2329 2.27 liquid 2525 Molar volume:cm3 12.06 thermal_conduct 84W therm_concuct_W/cmK 1.5 0.014 specif_heat_J/Kg*C 760 1000 expan dL/L per C 2.6E-6 0.5E-6 dielectric 11.8 3.9 ionization_ev 1.1 8v breakdown_V/um 30 600 therm_deltaL/L*C 2.9E-6 0.5E-6 therm_cond_W/cm-C 1.5/ 0.014 liquid 4.3 Youngs_mod_KG/mm^2 10890 Young_modulus_GPa 113 Rigidity modu_GPa 39.7 Bulk modul_GPa n.a. Poisson's ratio: 0.42 HardnessMineral: 6.5 Fusion_kJ_mol-1 39.6 Vapor_kJ_mol-1 383.3 Vel_sound_m_per_s 2200 Therm_con_W_m-1_K-1 148 1.4 Melting point: 1683 Boiling point: 2628 Crit_temperature: 5159 Debye temperature: 647 Molar_volume_cm3 12.06 Reflectivity_% 28 Si melt 1420C Cu melts 1083 Gold 1030 Alum 660 Si storaage 300 50/50 solder melts 200 indium melts 156 Silcon operates 175 mil spec 155 -65 industrial 125 -25 commerical 85 0C rho_silver 1.63E-8_ohm_meters rho_cu 1.72E-8_ohm_meters rho_brass 7.00E-8_ohm_meters rho_alum 2.66E-8_ohm_meters ni 1.45E10/cubic_cm silicon at 300K about same if doped intrinsic mobility 1450cm^2/(V*sec ) electrons 480 holes Elect resistivity: 0.001 [273 K] &Ohm; m magnetic suscept -1.8 x 10-8 (s) kg-1 m3 NITRIDE Si/N ratio 0.75->1 density 2.5-.3E3 Kg/cube_meter dielectric 6-9 breakdown field 6E6 V/cm Resistance (ohms/square) versus doping 100 ................................................ | p . . . . . . | |n . . . . . . | | n . . . . . . | | . . . . . . | 10|....n..p........................................| | . . . . . . | | . p . . . . . | | .n . . . . . | | . . . . . . | 1|..........n...p.................................| | . . . . . . | | . . p. . . . | | . .n . . . . | | . . . . . . | .1| ................n.......p......................| | . . . . . . | | . . . n . . . | | . . . . p . . | | . . . n. . . | .01|...............................n..p.............| | . . . . . . | | . . . . .n . | | . . . . . p . | | . . . . . p | .001|.........................................n......| | . . . . . . n | | . . . . . . p n| | . . . . . . p | | . . . . . . p| .0001|________________________________________________| E14 E15 E16 E17 E18 E19 E20 E21 doping( /cm^3) Mobility (cm/sec)*(cm/V) versus doping ................................................. | . . . . . . | | . . . . . . | | . . . . . . | |e e . . . . . | 1000 |.............e..................................| | . . . . . . | | . . . . . . | | . . e . . . | | . . . . . . | |h...h...........................................| | . h . . . . . | | . . h . .e . . | | . . . . . . | | . . . . . . | 100 |.........................h.........e............| | . . . . . e e | | . . . . . . | | . . . . h . . | | . . . . . h h | |................................................| | . . . . . . | | . . . . . . | | . . . . . . | | . . . . . . | 10 |________________________________________________| E14 E15 E16 E17 E18 E19 E20 E21 doping( /cm^3) ----------------------work function------------------------- ............ ____ ................. ^ | | ^ | ~ ~ | Work_F_Metal ~ ~ Work_F_Silicon | | | _____V________| |______Ec ^ | | | | V_fb | |. . . . | _ V _ _ _ _ _ |_ _ |_ _ _ _ _ _ Eg V Al | |_____ Ev |SiO2| P type V_flatBand Work_Func_Metal - Work_Func_Silicon Barrier Energies referred to SiO2 Metal Photon MOS Vacuum_Work_Function AL 3.2 3.21 4.20 Si 4.35 5.15 Mg 2.5 2.4 3.7 Ni 3.7 3.6 4.74 Cu 3.8 3.8 4.52 Ag 4.15 4.2 4.31 Au 4.1 4.1 4.7 tau_p 1/(sigma_p*v_th*N_t) lifetime in low level injection N_t concention of centers of recombination v_th sqrt(3*k*T/m) = ~ 1e-7cm/sec thermal velocity sigma_p capture cross section n N_c*exp( (E_c - E_f)vt) p N_v*exp( (E_f - E_v)vt) Fermi Dirac 1/(1+exp( (E - E_v)vt) ) Fermi Dirac distribution n_i^2 N_c*N_v*exp ( -E_g/vt) intrinsic carriers Phi_Fn vt*ln(N_D/n_i) Phi_Fp vt*ln(N_A/n_i) Phi_T Phi_Fp + | Phi_Fn | built in pn junction V q*C_B*W^2/(2*K_s*e_0) W sqrt( 2*K_s*e_o*Phi_T/(q*N_A) ) depletion width I_R I_gen + I_sh + I_surface reverse leakage I_gen q*n_i*W*Area/tau_g generation leakage funct_of 1/tau_g funct_of Num_traps I_gen(V) I_gen_0*sqrt(V) from W I_gen(T) I_gen_0*exp(-E_g/(2*K*T)) from ni I_sh (q*n_i^2*Area/N_D)*sqrt( D_n/tau_r) shockley component funct_of sqrt(Num_traps) "diffusion current" I_sh(T) I_sh_0( -E_g/( K*T)) n_i ni_i_0*exp(-E_g/(2*K*T)) T_c when I_gen(T) = I_sh(T) at or below room D_n v_t/mu_n 34cm2 diffusion coefficent v_t K*T/q thermal voltage mu_n 1300cm^2/V-s @ room mobility D_p v_t/mu_p 13cm2 diffusion coefficent mu_p 500cm^2/V-s @ room mobility L_p sqrt(D_p*tau ) diffusion length tau 34u -> 2000u lifetime I_surface 4e9/cm2-V about surface state leakage ======================SILICON_DEVICES=========================== n n n n n n ____________ Ec ............ Ef N type _____________Ev p n n ____________ Ec ............ Ef P type _____________Ev p p p p p p Ef Fermi Potent where prob of electron is 50% f(E) 1/(1+exp( (E-EF)/KT )) Tau TF = Wb^2/(2*Diff_b) vt KT/q Thermal energy KT/q Diffion/mobitity Vfermi_total .4V + .3V = .7V ________________________________________ | | :++|--: --|++ | | | :++|--: --|++ | | | ^ :++|--: --|++ | | | | :++|--: --|++ | | | :++|--: --|++ Metal| ___ |Metal | N :++|--: P --|++ |__| | | | :++|--: --|++ | |___| | | :++|--: --|++ | |_______|_____:__|__:__________|_________| | 0 V .7V .7V ___ |_________________________________________| | |___| Vfermi_total .3V + .4V = .7V ________________________________________ | +|- :++|--: ---|+++ | | +|- :++|--: ---|+++ | | +|- :++|--: ---|+++ | | +|- :++|--: ^ ---|+++ | ___ | +|- :++|--: | ---|+++ Metal|__| | |Metal +|- N :++|--: P ---|+++ | |___| | +|- :++|--: ---|+++ | | +|- :++|--: ---|+++ | |_______|_____:__|__:__________|_________| | .1V .7V .8V ___ |_________________________________________| | |___| Vfermi_total .4V + .4V = .8V ________________________________________ | | :++|--: ---|+++ | | | :++|--: ---|+++ | | | ^ :++|--: ^ ---|+++ | | | | :++|--: | ---|+++ | ___ | | :++|--: ---|+++ Metal|__| | |Metal | N :++|--: P ---|+++ | |___| | | :++|--: ---|+++ | | | :++|--: ---|+++ | Zero V |_______|_____:__|__:__________|_________| | 0V .8V .8V ___ |_________________________________________| | |___| Vfermi_total .3V + .3V = .6V _______________________________________ | +|- :+|-: --|++ | | +|- :+|-: --|++ | | +|- :+|-: --|++ | | +|- :+|-: --|++ | | +|- :+|-: --|++ Metal| ___ |Metal +|- N :+|-: P --|++ |__| | | +|- :+|-: --|++ | |___| | +|- :+|-: --|++ | |_______|______:_|_:__________|_________| | .1V .6V .7V ___ |________________________________________| | |___| contact potential V (KT/q)*ln(A) 680mV = E+20 660mV= E+19 V_fermi_p (KT/q)*ln(N_accept/ni) V_fermi_n (-1)*(KT/q)*ln(N_donor/ni) Breakdown BVbco_V 95*(rho_epi_ohm_cm)^(.722) BVceo_V BVbco_V/( (Beta_max+1)^(.25) ) BVbco_thickLimited_V 36*(w_um)^(.861) together with other carbon _ / \ '| | \ ' | | ___ '___\ /'___) (___/ | ` ' -_ \ \ ' - \_| The four covalent bonds of carbon Silicon Crystal Diamond structure _ , _ ( ) . _ , (_) Œ _ (_) / (_) ` : (_)/ \/: : . : \/ ,(_) : . : (_)-_ :` _ : : _(/) -_ _(_) : : (_)/ (_): : : \/ : `:_ : : (_)_ (_)_(_): : / -_.` _-- . `: :/ . ` (_) (_) (_)` _ . . ` (_) . . ` . ^ z /|\ | [100] PLANE | | __|____ | |____|_____\ | / | / y |/______| /1 ^ |/_ |___ Crystal plane x [100] MOS has lowest surface states ^ z /|\ | [110] PLANE | _- |_- | _-| | | |____|_____\ | / _- 1 / y |/_- /1 |/_ x [111] Highest tensile strength Highest density of attoms ^ z /|\ | 1 [111] PLANE |\ /| \ | | \ | |___\______\ / / _- 1 / y |/_- /1 |/_ x ^ z [001] /|\ | _ [111] | /| | |__________\ / / y [010] / |/_ x [100] : : : Silicon Crystal Diamond structure _ , _ ( ) . _ , (_) ' _ (_) / (_) ` : (_)/ \/: : . : \/ ,(_)_ : . : (_)-_ : -_ : : _(/) -_:_(_) : : (_)/ (_) | : : \/ : \| : : (_)_ (_)_(_): : / -_.` _-- . \: :/ . ` (_) (_) (_)` _ . . ` (_) . . ` . Temp dependence T^-2.7 T^-2.33 ni, 1/cm3 1.5e10 2.4e13 1.35X10'6 § ni(Temp) T*exp(-1.21/kT) T*exp(-0.785/kT) In equilibrium, p*n*= n_i^2 net product independent of doping densities n=N_D+p donor density. Current flow J=e*E, conductivity given by sigma = q*(mu_n*n +mu_p*p). mobilities, ave drift velocity per unit electric field, q electron charge. Diffusion current -q*D_p*dp/dx for electrons -qD0 dp/dx. for holes diffusion D related to mobilities by D/mu= k*T/q kT/q is about 26 millivolts. high doping levels mobility decreases additional scattering from impurity high electrifields mobility also decreases, carriers at limiting velocity almost independent of field. limiting velocity is of order of ~10e6cm/ sec. recombination rate for excess electrons in p material, dn/dt= -(n-n_0)/tau_n no is the equilibrium density and tau_n, is called the lifetime of electrons. excess decays as exp (-t/tau_n). Lifetime very sensitive puritles several microseconds down to few nanoseconds. Lifetime can be reduced for such applications by introducing impurity atoms (such as gold) which act as recombination centers. silicon latin silex or silicis meaning flint. 25.7% of the earths crust. invent transistor Bardeen, Brattain, and Shockley in 1947. Germanium (Ge) original semiconductor material used to fabricate diodes and transistors. narrow bandgap of Ge (0.66 eV),relatively large leakage Germanium oxide (Ge02) could act as such a layer but it is difficult to form, is water soluble, and dissociates at 800'C. larger bandgap of silicon (1.1 eV) smaller leakage currents maximum operating temperature of 150'C. Si02 easy to form chemically very stable. electronic grade silicon is about one tenth as costly as germanium. ----------------------BREAKDOWN_V---------------------- %_of_Isat At given voltage given percent Isat forms holes/electrons I_leakage Isat + Isat*P% + Isat*P%^2+.. Isat/(1-P%) P% (V/Vbeakdown)^3 I_leakage Isat/(1- ( (V/Vbeakdown)^3 ) ) Pbeta% (Beta+1)*P% if beta get into the picture Vcbo Vcbo/((1+Beta)^sqrt(1/3)) Breakdown BVbco_V 95*(rho_epi_ohm_cm)^(.722) BVceo_V BVbco_V/( (Beta_max+1)^(.25) ) BVbco_TLV 36*(w_um)^(.861) BVbco_thickLimited_V ------------------------------------------------------- Deposit PSG vapox APCVD atmossheric pressure Deposit PSG vapox LPCVD low pressure Deposit PSG vapox vt drift act eng 1.2ev less than 1mV drift with delta 5v stress contact potential (kt/q)*ln(A ) 680 at E20 25C facter 10 less time to fail ------------------------------------------------------------- dopant and contaminant concentrations in silicon. atoms/cm', about 5e22 Si atoms/cm3 in single crystal silicon, impurity concentration of 5x1016 /cm3 equals 1 ppma SINGLE CRYSTAL SILICON polycrystalline material short minority carrier lifetimes, defects occurring at grain boundaries method obtaining such single crystal Si for VLSI fabrication, 1) Raw material (e.g. quartzite, a type of sand), refined by multi-stage process which produces electronic grade polysilicon (EGS), 2) polysilicon used to grow single crystal silicon by Czochra!ski (CZ) crystal growth or float zone (Fz) growth. Single crystal silicon commercially available in either(100) or (llI)-orientations (other orientations (110) obtained on special order ). Ct growth single crystal ingots are pulled from molten silicon contained in a crucible. Czochralski silicon preferred can withstand thermal stresses better than FZ materia1 is able to offer internal gettering mechanism Float zone crystals, because grown without making contact to any container or crucible, can attain higher purity land thereby higher resistivity) than CZ silicon. Devices and circuitsneed high purity starting material (e.g. high voltage, high power devices) are typically fabricated from FZ silicon. (EGS) Electronic Grade Polysilicon Single crystal silicon grown from melts of electronic grade polycrystalline to achieve controlled doping during single crystal growth, EGS in the parts per billion atoms (ppba) raw material which EGS is refined (quarzite), contains high levels of impurities (e.g. aluminum levels of -3x1020 lcm3 refining process reduce impurities by approximately eight orders of magnitude refinement procedure involves four major stagesS: Reduction of quartzite to metallurgical grade silicon (MGS) with a purity of approximately 98%, Si02 in form of quartzite reacted with carbon to yield silicon and carbon monoxide: Si02 + 2C => Si + 2C0 Quartzite is relatively pure form of Si02 presence of coal, coke, or wood chips) reduced to MGS. name MGS derived from fact this purity sufficiently refined as an alloy material in manufacture of aluminum or for producing silicone polymers. only a small fraction refined into EGS quartzite, coal (or coke), wood chips (for porosity) loaded into submerged elecuode arc furnace electric arcs exceed 2000'C, in those regions SiC formed. SiC reacts with Si02 to form Si, SiO, and CO. silicon is drawn while SiO and CO gases escape through the spaces created by the presence of the wood chips. Large quantities of electrical power are consumed in Conversion of MGS to trichlorosilane (SiHC13), trichlorosilane (SiHC13) formed by reaction of anhydrous hydrogen chloride and MGS. MGS is first ground to a fine powder. powdered Si is then treated with HCI to form SiHC13. reaction of solid Si and gaseous HC1 occurs at 300'C in presence of a catalyst. Both formation of SiC13 and chlorides of impurities (e.g. AIC13, BC13) takes place in this step. resultant SiHC13 is a liquid at room temperatures (boiling point 31.8'C), hence can be purified by distillation. Purification of SiHC13 by distillation, SiHC13 separated by fractional distillation. Upon conclusion SMC13 is highly refined SiHC13 must be deposited as semiconductor grade polysilicon, then converted to single crystal silicon, to determine the impurity levels. (CVD) Chemical vapor deposition Si from purified SiHC13, as EGS highly purified SiHC13 converted into polycrystalline silicon (EGS) by CVD in the presence of hydrogen. process takes place in a reactor (Fig. 5b) first proposed referred to as the Siemens process. 2SiHC13 (gas) + 2H2(gas) => 2Si (solid) + 6HCI (gas) (2) starting surface is a thin silicon rod (called a slim rod) a nucleation surface for depositing silicon. large rods (200 mm idiameter and several meters long) deposition takes several hundred hours. After deposition, EGS processed into three products: a) one-piece crucible charges; b) nuggets (random sized pieces); c) poly-rods (Fig. 6). first two used as charges in CZ growth rods used for float-zoning single crystal ingots. To evaluate the purity of EGS conversion to single crystal silicon by FZ technique, primary flat usually positioned relative to crystal direction. primary flat orientation found using x-rays, to produce a Laue photograph used for several Automated wafer handling equipment to obtain correct alignment, devices on wafer can be oriented to crystal direction with flat as reference. Smaller flats are called secondary flats, utilized to identify orientation and conductivity type of the wafer Since automated equipment relies on the flats flat dimensions precisely machined. sawing operatio ingot rigidly mounted Wafers of <100> orientation normally cut "on orientation" <111> wafers generally cut "off orientation for epitaxial processing applications damaged and contaminated layer is chemically etched away A relatively non-porous and clean wafer backside also produced by this step. wet etch procedure typically etchant solution of hydrofluoric, nitric, and acetic acids chemical-mechanical polishing" step used to produce reflective, scratch, damage free surface accomplished by mounting unpolished slices onto a carrier, then putting on polishing machine. a powered platen drives a polishing pad material across the wafer surface. colloidal silica sluny of sodium hydroxide and fine (-100A) Si02 particles, is dripped onto the table. frictional heat of sliding mounted wafers causes sodium hydroxide to oxidize silicon (i.e.the chemical part of the process). oxide is abraded away by silica particles (i.e. mechanical part). Following polishing cycle which 25 Lm removed) wafers subjected to series of chemical dips and rinses to remove polish slurry. cleaning process concludes this step. Another option being offered is a layer of polysilicon or mechanical damage on backside of wafers for extrinsic gettering purposes (RTP) rapid thermal processing a relatively new technique thermal steps utilized in many other processes that an Gettering term used to describe a variety of processing techniques that remove harmful defects or impurities from regions on wafer which devices fabricated. Various crystal defects in a simple cubic lattice a) interstitial impurity ion | | | | | | | -0--0--0--0--0--0--0- | | | |A | | | -0--0--0--0--0--0--0- | | | | | | | -0--0--0--0--0--0--0- | | | | | | | edge dislocation | | | | | | | -0--0--0-0-0--0--0- | | \ / | | -0--0--0--0--0--0--0- | | | | | | -0--0--0--0--0--0--0- | | | | | | self-interstitial | | | | | | -0--0--0--0--0--0--0 | | | |0 | | -0--0--0--0--0--0--0- | | | | | | | -0--0--0--0--0--0--0- | | | | | | | coherent precipitate of substitutional atoms | | | | | | | -0--0--A--A--A--0--0- | | | | | | | -0--0--A--A--A--0--0 | | | | | | | -0--0--0--0--0--0--0- | | | | | | | small dislocation loop formed by agglomeration of self-interstitials | | | | | | | -0--0--0-0-0--0--0- | | \ A / | | -0--0--0--0--0--0--0- | | | | | | -0--0--0--0--0--0--0- | | | | | | substitutional atom widening the lattice | | | | | | | -0--0--0--0--0--0--0- | | | | | | | -0--0---0-A-0---0--0- | | | | | | | -0--0--0--0--0--0--0- | | | | | | | vacancy | | | | | | | -0--0--0--0--0--0--0- | | | | | | -0--0--0 0--0--0- | | | | | | -0--0--0--0--0--0--0- | | | | | | | small dislocation loop formed by agglomeration of vacancies | | | | | | | -0--0--0-0-0--0--0- | | \ / | | -0--0--0--0--0--0--0- | | | | | | -0--0--0--0--0--0--0- | | / \ | | -0--0--0-0-0--0--0- | | | | | | | substitutional impurity atom compressing the latticelO | | | | | | -0--0--0--0--0--0--0- | | | | | | | -0--0-0---A---0-0--0- | | | | | | | -0--0--0--0--0--0--0- | | | | | | | Point defects 0 0 0 0 0| 0 0 0 0 0 0 0 | 0 0 0 0 0| 0 0 0 0 0 0 _ 0 0 0 _/ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Schottky defect ___ 0 0 0 0 0 0 0 0 0 0/ 0 0 0 0 0 0 0 0 0 0 0 |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 interstitial arriving from surface 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 --- > 0 0 0 0 / 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 c) Frenkel defect. Intrinsic point defects important in kinetics of diffusion. diffusion of many impurities depends on the vacancy concentration. low packing-density of the diamond lattice (34% versus 74% for a fee lattice) implies large spaces between atoms (interstices) which atoms of same size can be placed without shifting of neighboring atoms structural condition of the silicon lattice favors incorporation of interstitials, why Si lattice fewer vacancies than metal lattices extrinsic point defects, involving foreign atoms. non-Si atoms occupy lattice sites, defects referred to as substitutional impurities. bonafide point defects, atoms are larger or smaller crystalline regularity perturbed. Impurity also occupy non-lattice sites, called interstitial impurities. To be electrically active, atoms usually must be located on lattice sites. inherent solubility of impurity in silicon crystal. is a maximum specific concentration depends on the element and temperature solubility most impurities increases up to a temperature, then decreases as approaches its melting point. known as retrograde solubility. dislocations form by growth and multiplication of dislocation loops in the bulk, or from dislocations generated at the surface in response to stresses created in crystal. Stresses can arise i a) diff expansion due to temp variations in crystals''; b) intro substitutional impurities cause stresses between doped and undoped crystal regions called misfit dislocations.); c) compressive stresses from volume mismatches arise during some precipitation events d) coefficient of thermal expansion stresses caused by layers present on surface of crystal7 involves growth of thermal oxide and suppression of oxidation on other by nitride layers. Dislocation causing stresses result primarily from tensile stress silicon nitride, and volume expansion of oxide formed in walls.) stresses arise in a variety of ways seed crystals undergo high thermal stress when immersed into Si melt during single crystal ingot exterior of seed crystal is brought to melt temperature, interior much cooler. Stresses resulting from expansion at surface and interior of seed crystal induce dislocations lead to plastic deformation of crystal. (Plastic deformation is permanent deformation of material that remains after stress released. Elaitic deformation, lost upon release of stress. Plastic deformation occurs when elastic limit [or yield strength] is exceeded.) Dislocations in wafers induced by thermal stress during furnace operationss. Upon removing wafers from furnace, edges of wafers cool faster wafers are held vertically and spaced closely in wafer boats. edges radiate heat to cooler surroundings stress S from uneven cooling S = cr ff T cr= coeff thermal expansion silicon 4e6 cm /cm'K, ff= Youngs modulus ( 1.5 x 1012 dyn lcm2, T = temperature difference between edge and center (temperature gradients in cooling wafers can reach >150'C). If stress exceeds yield strength dislocations will form. stress from 150'C gradient 0.9 x 10' dyn lcm2, larger than yield strength (0.45e9 dyn /cm2 850'C, thus introduce dislocations. yield strength of CL Si wafers is impacted by the presence of impurities, as oxygen. Dislocations can climb and glide. Climb occurs when point defects are absorbed by the dislocation line. Thus, if a self-interstitial is captured, an edge dislocation moves as shown in Fig. 5a, while the absorption of vacancy causes line to climb in opposite direction. Dislocation loops also change size by climb-type events (absorption of point defects). Movement of the dislocation in the surface defined by its line and | _____| |/ | | | | | | | | | | | | . ._/ |. _- | - Typical characteristics of single-emitter test transistor (emitter area 3 x 8 m) with emitter collector short showing up in ICEO IVCE (---) ICE /VCB ( ). and IEB IVEB (...) are "hard"57 _________ p / ___/____ _________ n | / p \/_____ ___/____ _________ n | \/___ n \/_____ p ___/____ b) Schematic of enhanced emitter diffusion model explain collector-emitter pipe formation CRYSTAL DEFECTS ON DEVICE PROPERTIES T influenced by crystalline defects include: a) leakage currents in p-n junctions; b) collector-emitter leakage currents in bipolar c) minority carrier lifetimes; d) gate-oxide quality; e) threshold voltage uniformity in MOS devices; f) resistance to warpage wafers during thermal process Leakage Currents precipitates and dislocations increase pn junction leakage. transition metal precipitates in pn junction produces leakage due to mid-gap energy levels at low voltages, and a "soft" leakage component at higher voltages Dislocations and thereby extrinsic stacking faults that cross pn junctions, The formation of generation-recombination (g-r) centers defect sites junctions, and the decoration of dislocations Collector-to-emitter leakages have been correlated with dislocations from emitter to collector. If dislocation decorated with metallic impurities, permit significant current between collector and emitter dislocation role in enhancing diffusion along dislocation during emitter formation can lead to emitter-collector pipes, precipitates contributes to locally retarded dffjsion of dopant atoms in shallow double-diffused structures. dopant appears not to diffuse as rapidly in region of precipitate, forming a localized spike pointing upward toward the wafer surface2 precipitate apparently dissolves during first diffusion. second diffusion is thus effected by precipitate, locally narrow separation of emitter and collector at spike causes excessively large reverse currents. not expected to occur in ion-implanted devices. Minority Carrier Lifetimes mean time spent before they recombine Gate Oxide Defects in silicon subscrate MOS devices: 1) oxide leakage current; 2) oxide breakdown voltage. both correspond to stackig faults at silicon substrate generated by metallic contamination during oxidation. correlate with high defect density on wafer surface. SEMICONDUCTOR ( T =290K). Si Ge GaAs InSb Energygap,eV 1.106 0.67 1.351 0.17t Temp coef EG -4 -4.5 -5 -2.7 eV/*K*10e4 Melting point, C 1412 958 1238 523 Thermal conduct 1.421 0.521 0.44110.17 W/cm-*C Thermal coeff l 4.2 5.5 5. linear expans C^-1*10e9 Lattice constant_A 5.42 5.65 5.65 6.48 Dielectric constantt 11.8 16.0 11.1 15.9 Elec mobility, 1350 3900 6800t 80000k cm2/v-sc T dependence Hole mobility 480 1900 680 4000~ Hole mobility Temp T^-2.7 T^-2.33 ni, 1/cm3 1.5e10 2.4e13 1.35X10'6 § ni(Temp) T*exp(-1.21/kT) T*exp(-0.785/kT) p*n*= n_i^2 net product independent of doping densities n=N_D+p donor density. Current flow J=e*E, conductivity given by sigma = q*(mu_n*n +mu_p*p). mobilities, ave drift per unit electric field, q electron charge. Diffusion current -q*D_p*dp/dx for electrons -qD0 dp/dx. for holes diffusion D related to mobilities by D/mu= k*T/q kT/q is about 26 millivolts. high doping levels mobility decreases additional scattering from impurity high electrifields mobility also decreases, carriers at limiting velocity almost independent of field. limiting velocity is of order of ~10e6cm/ sec. recombination rate for excess electrons in p material, dn/dt= -(n-n_0)/tau_n no is the equilibrium density and tau_n, is called the lifetime of electrons. excess decays as exp (-t/tau_n). Lifetime very sensitive puritles several microseconds down to few nanoseconds. Lifetime can be reduced for such applications by introducing impurity atoms (such as gold) which act as recombination centers. Resistance (ohms/square) versus doping 100 ................................................ | p . . . . . . | |n . . . . . . | | n . . . . . . | | . . . . . . | 10|....n..p........................................| | . . . . . . | | . p . . . . . | | .n . . . . . | | . . . . . . | 1|..........n...p.................................| | . . . . . . | | . . p. . . . | | . .n . . . . | | . . . . . . | .1| ................n.......p......................| | . . . . . . | | . . . n . . . | | . . . . p . . | | . . . n. . . | .01|...............................n..p.............| | . . . . . . | | . . . . .n . | | . . . . . p . | | . . . . . p | .001|.........................................n......| | . . . . . . n | | . . . . . . p n| | . . . . . . p | | . . . . . . p| .0001|________________________________________________| E14 E15 E16 E17 E18 E19 E20 E21 doping( /cm^3) Mobility (cm/sec)*(cm/V) versus doping ................................................. | . . . . . . | | . . . . . . | | . . . . . . | |e e . . . . . | 1000 |.............e..................................| | . . . . . . | | . . . . . . | | . . e . . . | | . . . . . . | |h...h...........................................| | . h . . . . . | | . . h . .e . . | | . . . . . . | | . . . . . . | 100 |.........................h.........e............| | . . . . . e e | | . . . . . . | | . . . . h . . | | . . . . . h h | |................................................| | . . . . . . | | . . . . . . | | . . . . . . | | . . . . . . | 10 |________________________________________________| E14 E15 E16 E17 E18 E19 E20 E21 doping( /cm^3) Silicon Crystal Diamond structure _ , _ ( ) . _ , (_) ' _ (_) / (_) ` : (_)/ \/: : . : \/ ,(_)_ : . : (_)-_ : -_ : : _(/) -_:_(_) : : (_)/ (_) | : : \/ : \| : : (_)_ (_)_(_): : / -_.` _-- . \: :/ . ` (_) (_) (_)` _ . . ` (_) . . ` . ^ z /|\ | [100] PLANE | | __|____ | |____|_____\ | / | / y |/______| /1 ^ |/_ |___ Crystal plane [111] Highest tensile strength Highest density of attoms ^ z /|\ | 1 [111] PLANE |\ /| \ | | \ | |___\______\ / / _- 1 / y |/_- /1 |/_ x [110] Early Bipolar ? ^ z /|\ | [110] PLANE | _- |_- | _-| | | |____|_____\ | / _- 1 / y |/_- /1 |/_ x [001] Early Bipolar ? ^ z [001] /|\ | _ [111] | /| | |__________\ / / y [010] / |/_ x [100] : Miller indices important planes in a cubic crystal. ___________ / / |_____________ |____________ /| /| / ___/| /\ | / | [001] / | /| __/ | /| \ | / | / | / |___/ | /|| \_ | /___________/ | /__/ | / || \ | | | | | // | | / /| \ | | | |010| /| | [110] | /| | | \_ | | | | | | | | | | | \ | | |_______|___| | | | | / | [111] \ | | / | / | |___________|_ || |________ _\| | / | / | / __/ || / ___ | / [100] | / | / ___/ || / ___ |/ |/ | / ___/ |// ____ |___________| |/__/ |/___ |/ // MOS devices fabricated on (100)-wafers smallest surface state densities on such orientations. ___________ / / /| /| / | [001] / | / | / | /___________/ | | | | | | | |010| | | | | | |_______|___| | / | / | / | / | / [100] | / |/ |/ |___________| tensile strength silicon highest <111> highest density of atoms. oxidize more rapidly than (100)-planes, |____________ /\ | /| \ | /|| \_ | / || \ | / /| \ | /| | | \_ | | | | \ | | / | [111] \ | || |________ _\| || / ___ || / ___ |// ____ |/___ // Silicon Crystal Diamond structure _ , _ ( ) . _ , (_) ' _ (_) / (_) ` : (_)/ \/: : . : \/ ,(_)_ : . : (_)-_ : -_ : : _(/) -_ _(_) : : (_)/ (_) | : : \/ : \| : : (_)_ (_)_(_): : / -_.` _-- . \: :/ . ` (_) (_) (_)` _ . . ` (_) . . ` .