Semiconductor Technology 5b Doping by Diffusion PDF

H.K. Trieu Welcome to the lecture Please ask questions if something is unclear. There are no stupid questions. Any question will improve your understanding !!! only the slides and the voice will be captured! for internal use within the Semiconductor Tech...

H.K. Trieu Welcome to the lecture Please ask questions if something is unclear. There are no stupid questions. Any question will improve your understanding !!! only the slides and the voice will be captured! for internal use within the Semiconductor Technology class only further dissemination is not allowed Disclaimer H.K. Trieu "The use of video recording is a service provided by the TUHH or the respective lecturer. No claims can be derived against the TUHH or the lecturer by its use or non-use. Without the prior written consent of the lecturer, neither the presentation nor the content made available through it (especially graphic images, audio and video sequences, HTML codes, buttons and text) may be copied, reprinted, published, sent, transmitted or in any way distributed. Explicitly permitted is the production of a single copy for exclusive personal, non-commercial use, but only provided that the content is not changed and all references to copyrights, patents, trademarks and other property rights are included on the copies or added in case of excerpts. NO LIABILITY is assumed: - for the content, in particular for the correctness, completeness and timeliness of the information - for uninterrupted, continuous use of the video recording, - for damage caused by incorrect, incomplete, omitted or delayed retrieval of the recording - for direct or indirect damage that is (could) be related to the use or non-use of the video recordings. The TUHH or the lecturer reserves the right to change, add to, or delete parts of the service or the entire service or to temporarily or permanently stop the publication of video recordings without prior notice." for internal use within the Semiconductor Technology class only further dissemination is not allowed Sequence of content H.K. Trieu  Ch 1 Introduction  Ch 2 Fundamentals of materials  Ch 3+4 Crystal production + Wafer production  Ch 8a+b Patterning processes (Lithography) + Resolution enhancement  Ch 7a Deposition processes  Ch 7c CVD (chemical vapor deposition)  Ch 7d PVD (physical vapor deposition)  Ch 9 Etching processes  Ch 6 Oxidation  Ch 7b Epitaxy  Ch 5a+b+c Doping + Diffusion + Implantation  Ch 10 Process Integration  Ch 11 Packaging and interconnect technologies for internal use within the Semiconductor Technology class only further dissemination is not allowed  5b - 1 H.K. Trieu Semiconductor Technology 5b. Doping by Diffusion Prof. Dr.-Ing. Hoc Khiem Trieu Hamburg University of Technology Institute of Microsystems Technology for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion 5b - 2 H.K. Trieu  Foreign atoms (dopants) diffuse through the semiconductor according to concentration gradients  It is a thermally activated process (exponentially dependent on T)  Doping profile depends on T-Time relation and on boundary conditions Diffusion paths in solid material:  Diffusion by neighbors changing places (exchange diffusion)  Diffusion through interstitial lattice sites (interstitial diffusion)  Diffusion by moving through free latties sites (substitution diffusion) Exchange diffusion Direct exchange of places by lattice neighbors In crystals, very unlikely Plays practically no role in monocrystals for internal use within the Semiconductor Technology class only further dissemination is not allowed Interstitial diffusion 5b - 3 H.K. Trieu Concept: atoms wedge themselves between the lattice atoms from one interstitial site to the next Principle of lattice diffusion Frequency of jumps is jump frequency J Is excited thermally though lattice vibration with frequency 0 (1013 -1014Hz) Activation energy with EA (0.6 up to 1.2 eV)  EA  „4“ due to J  4 o exp   coordination number at lattice temperature T:  kT  of diamond lattice At 300 K (room temperature) approx. 1 jump/min -> 4 Jump possibilites for internal use within the Semiconductor Technology class only further dissemination is not allowed Substitution diffusion 5b - 4 H.K. Trieu Concept: Dopant atom moves through lattice sites by swapping to neighboring site For that, neighboring site must be free (dependent on vacancy concentration) Activation energy to create a vacancy (Schottky defect) ES ( ~2 eV) Activation energy to break the bond for place exchange EB ( ~2 eV) at lattice temperature T: ES  EB  S  4 o exp    kT  Principle of substitution diffusion Due to ES+EB : 3 - 4 eV At 300 K: 1 jump in 1045 years Can be increased significantly with additional vacancies Exchange through vacancies for internal use within the Semiconductor Technology class only further dissemination is not allowed Interstitial and substitution diffusion 5b - 5 H.K. Trieu Frequently a combined diffusion process: combined movement with independent parallel particle flows – in interstitial with concentration NI (cm-3) – through site exchange with concentration NS (cm-3) The slower of the two effects limit the process Probabilities whether NI In interstitial sites Interstitial diffusion NI  NS or NS In lattice sites Substitution diffusion NI  NS S NS I NI Effective jump frequency eff eff   NI  NS NI  NS More frequent: dependent combined diffusion process Dissociation from lattice into interstitial site Migration through interstitial sites Re-incorporation into lattice for internal use within the Semiconductor Technology class only further dissemination is not allowed Influence of defect density 5b - 6 H.K. Trieu Diffusion is strongly dependent on vacancy density nV At higher defect density, interstitial diffusion limits the process Vacancy concentration is quickly set I NI Jump frequency is determined by interstitial eff  sites NI  NS At lower defect density, substitution diffusion limits the process Vacancy concentration nV must be established first S nV Jump frequency in substitution diffusion eff  determined by vacancies nV  N S for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion forces 5b - 7 H.K. Trieu  Without driving forces, diffusion progresses randomly in all directions  Driving forces are:  Diffusion gradients  Electrical fields  Very high concentrations and vacancy densities Diffusion with concentration gradients With locally differing concentrations (gradients), N diffusion occurs against the concentration gradients x a in diamond lattice: distance of atoms with lattice constant a 3 a a in each case two neighbors are equivalent Concentration N1 and N2 in regions 1 and 2 calculated from number of particles n1 and n2 3 n1 3 n2 N1  N2  aA aA To derive the diffusion coefficients for internal use within the Semiconductor Technology class only further dissemination is not allowed Derivation of diffusion coefficients 5b - 8 H.K. Trieu Particle flow I across the border: N N1  N 2 n n1  n2 eff a   eff A  N1  N 2  due to x a t 2 2 3 3 follows the particle flow: n a 2 N  A  I  JA eff t 6 x Diffusion coefficient D eff a 2 Diffusion coefficient is, through the D 6 jump frequency, exponentially N dependent on the temperature 1. Fick´s Law: J  D x  E  Interstitial diffusion DZ  Do exp  A   kT   E  EB  Substitution diffusion DS  Do exp  S   k T  4 0 a 2 Diffusion constant Do  6 for high diffusion rate, sufficiently high temperatures are necessary for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion with potential gradients 5b - 9 H.K. Trieu Field-supported diffusion interstitial diffusion without electric field At higher temperatures in semiconductors, the dopants and the lattice atoms are ionized Additional driving force, if electric field E is present   Drift velocity vd ( with mobility µ) vd   E   Particle flow due to field: J E   NE interstitial diffusion with electric field electric field 1. Fick´s Law with field term: N J g  J D  J E  D   NE x Formation of electric fields: distance  Voltage in substrate during diffusion (hardly feasible) Energy course in interstitial  Atomic core and charge carrier have different mobilities in lattice diffusion with and without field Electrons and holes diffuse faster than donors and acceptors Charge carriers „trail behind“ dopants for internal use within the Semiconductor Technology class only further dissemination is not allowed Doubling of D with field effect 5b - 10 H.K. Trieu  kT Einstein relation: D q Electrical field strength owing to gradient kT 1 dn Ex   with electron concentration n (in stationary state) q n dx Resulting particle flow  dn  N J   D 1   (Substitute field strength with gradients)  dN  x N  dn  J   Deff gilt: Deff  D 1   x  dN  N 2   N  With n doping, the electron concentration follows: n  ni      1  2 ni  2 ni         dn 1  1   Varies between 1/2 (light doping N ) and 1  1 dN 2   2 n  2  (high doping) -> max. Deff = 2D   i   1    N    Further strong increase of diffusion coefficients through interaction with charged defect sites. for internal use within the Semiconductor Technology class only further dissemination is not allowed Concentration-dependent diffusion 5b - 11 H.K. Trieu  At higher concentration, interstitial diffusion is dominant Larger diffusion coefficient, which means rapid diffusion  Equilibrium reaction of dissociation process NS N I + nV between Substitution sites NS interstitial states NI and vacancy sites nV  Rather on right side at higher concentration (interstitial and vacancy) at lower concentration on left (substitution) Rapid diffusion at higher concentration Slow diffusion at lower concentration Formation of a „box profile“ „Box profile“ of boron and arsenic doping in Si for internal use within the Semiconductor Technology class only further dissemination is not allowed Influence of dislocations on the diffusion 5b - 12 H.K. Trieu Dislocations create vacancies and interstitial atoms High dislocation density: High vacancy concentration nV Diffusion limited only by interstitial transport NZ Deff  DZ NZ  NS Low dislocation density Low number of vacancies Small change in the interstitial concentration NI Diffusion limited by vacancy transport nV n Deff  DV  DV V nV  N S NS for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion in silicon 5b - 13 H.K. Trieu Rapidly diffusing  Dopants (substitution diffusion): Donors (group V): Acceptors (group III) diffusion coefficient [cm²/s] P B As Al slope Sb Ga  Impurities: (interstitial diffusion) (groups I and VII) Li, Na, K, He, H2  Transition metals (interstital-substitution diffusion) Pd, Au, Fe, Cu: „Life span killer“ Slowly diffusing Temperature dependence of diffusion coefficients in silicon for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion in gallium arsenide 5b - 14 H.K. Trieu Diffusion mechanisms less distinctive Diffusion takes place in sublattices (Ga, As) Dopants diffusion coefficient [cm²/s] Acceptors (group II) Interstitial-substitution diffusion in Ga lattice: Zn Donors (group VI) Be Substitution diffusion in As lattice: Cd Hg S Mg Se Te Group IV: Substitution diffusion in Ga and As lattices Si (amphoteric) Temperature dependence of Ge, C, Sn diffusion coefficients in GaAs for internal use within the Semiconductor Technology class only further dissemination is not allowed Theoretical description of diffusion 5b - 15 H.K. Trieu Particle flow characterized by two differential equations: Fick`s first law: particle flow proportional to concentration gradients dN C ( x , t ) j( x , t )   D  D dx x It is possible that the flow might change from cross section to cross section. Dopants might accumulate (inflow > outflow) or the region between the cross sections might become depleted. This behavior finds expression in the continuity equation. Continuity equation (conservation law for particles) C ( x, t ) j ( x, t )  t x By differentiation of Fick‘s first law, Fick‘s second law results, which is frequenly called the diffusion equation. C ( x, t ) C 2 ( x, t ) Fick`s second law: D t x 2 for internal use within the Semiconductor Technology class only further dissemination is not allowed Theoretical description of diffusion 5b - 16 H.K. Trieu  Holds for concentration-independent diffusion coefficients limited validity at high concentrations high dislocation densities locally varying unequal doping and dislocation density  The diffusion equation, as a 2nd order partial differential equation, has many solutions, which are dependent on the beginning and boundary conditions Most important process conditions (boundary conditions):  Constant concentration on the surface: unlimited (infinite) source  Constant particle number (finite source) N0 = const. d.h. N(x=0) = const., t>0 N N0 , 0  x  h, t 0 N N0 N  x ,t   N0 N0 0, x  h, t 0 With that: N0  h  S , [ S ]  cm2 h x x for internal use within the Semiconductor Technology class only further dissemination is not allowed Constant surface concentration 5b - 17 H.K. Trieu „unlimited, inexhaustable, infinite source“ Diffusion from the gas phase C ( x ,0 )  0 Diffusion out of heavily doped glasses C ( 0, t )  C s Boundary and initial conditions for Fick‘s 2nd law: C( , t )  0  x  Solution: C ( x , t )  C s erfc   2 Dt  Concentration changes with erfc: complementary inexhaustable source (Gaussian) error function  2 x / 2 Dt  y2  C ( x, t )  C s 1   e dy    0  for internal use within the Semiconductor Technology class only further dissemination is not allowed Constant particle number 5b - 18 H.K. Trieu  „limited, finite source“ Constant particle number S   C ( x , t )dx „Preallocation“ and inward diffusion 0 Ion implantation C ( x ,0 )  0 Boundary and initial conditions for Fick`s 2nd law: C( , t )  0 S C ( 0,0 )  S Delta-Function of source S   x 2  S  x2  Gaussian Solution: C ( x , t )  exp      exp    Dt   2 Dt   Dt  4 Dt  distribution Boundary concentration decreases! S produced by preallocation during t1 from unlimited source D1t1 S  2C s  Concentration changes with finite source for internal use within the Semiconductor Technology class only further dissemination is not allowed Higher order effects 5b - 19 H.K. Trieu  The diffusion coefficient depends on diffusing material and temperature.  i.e. D is dependent on the concentration -> nonlinear ODE  No analytical solution of diffusion equation possible  Process simulators for internal use within the Semiconductor Technology class only further dissemination is not allowed Production of different dopants 5b - 20 H.K. Trieu Consecutive diffusion processes Overcompensation of previously introduced concentration Further diffusion of previously introduced concentration Fall of border concentration of previously introduced concentration Same doping type: Contact diffusion Setting of field patterns (e.g. backside drift field in solar cells) Doping profile for backside drift field unlimited limited source Doping profile for contact diffusion for internal use within the Semiconductor Technology class only further dissemination is not allowed Production of pn junctions 5b - 21 H.K. Trieu pn junction: Forms when NA=ND n type doping, erfc profile Use of limited source to reduce doping of p type doping, Gaussian profile the base. concentration of n doped wafer Doping concentration [1/cm³] emitter base collector Actual and theoretical doping profile of an npn transistor for internal use within the Semiconductor Technology class only further dissemination is not allowed Two-dimensional diffusion 5b - 22 H.K. Trieu Diffusion takes place through openings in masks diffusion mask unlimited source  Diffusion is isotropic (equal in all directions)  Two-dimensional solution of Fick‘s 2nd law For line source ( function in x direction, width w in y) SLw  r2  C( r , t )  exp    SL: particle number/length 2 Dr  4 Dt  diffusion mask limited source  Lateral infiltration of mask corresponding to the depth (75-85%), with concentration dependence of D (65- 70%) Sinking of concentration at edge of mask With unlimited source, constant doping in window With limited source, reduced concentration at edge Shifting of pn transition (short circuit with metal contacts possible) Increase of contact resistance (Schottky contact) Line doping profile for internal use within the Semiconductor Technology class only further dissemination is not allowed Anomalies in diffusion behavior 5b - 23 H.K. Trieu Locally increased diffusion coefficient through:  High vacancy concentration  A large amount of interstitial atoms Doping profile with  Electric fields higher concentration  Mechanical stresses (steeper transition than with efc or Repeated diffusion at high concentrations Gaussian profile) Emitter push effect Many interstitial atoms and vacancies due to high emitter concentrations, migrate into the base N increase base diffusion coefficient base profile prior to Electric field produced by emitter majority carriers leading the way emitter diffusion Base diffusion against concentration gradient at E-B junction E electric field due to emitter concentration Cause for the Doping profile with emitter-push effect emitter-push Flatter emitter effect Longer base N Higher base concentration than expected resulting base profile after emitter diffusion for internal use within the Semiconductor Technology class only further dissemination is not allowed  Diffusion under oxidating surfaces 5b - 24 H.K. Trieu Influence on diffusion coefficient With oxidation of Si to SiO2, high interstitial concentration of Si is produced. (the SiO2 molecule requires more space than a single Si atom) Also lowered vacancy concentration (thermodynamic equilibrium) Increased diffusion with interstitial-substitution diffusion (B, P, As) „oxide enhanced diffusion“ (OED) DOED  Di  D(T , t , pO2 , Orientieru orientation ng ) Does not hold for Sb (vacancy diffusion) diffuses more slowly Diffusion under oxidating surface Difference in the penetration depth enhanced diffusion decreased diffusion more point defects less point defects with diffusion under oxidating surface oxidation time [h] lowered oversaturation vacancy for internal use within the Semiconductor Technology class only concentration further dissemination is not allowed  Influence on doping concentration 5b - 25 H.K. Trieu  Moving edge condition: interface SiO2-Si shifts during oxidation in the crystal Edge concentration dependent on:  Diffusion out of surface i.e. With oxide, lower than with open surface  Diffusion coefficient in oxide  Solubility in oxide (segregation coefficient K) Further diffusion von B (cm-3) Equilirium concentration EquilibriumSiof dopant in Si during oxidation K at 1200°C boron concentration Equilibriu m concentrat Equilibrium ion of concentration dopantin of dopant inSiO SiO22 relative Konzentration e.g. boron with higher H- Oxid Silizium oxide silicon oxide Oxid silicon Silizium concentration (moist oxidation) e.g. boron concentration Konzentration Konzentration concentration Stronger diffusion out of surface due to rapid t=700 s) diffusion into SiO2 , relative K1, very fast at SiO2-Si interface a SiO2-Si interface K>1, langsame diffusion in SiO2 Diffusion in SiO2 (P) (Ga) for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion at masking edges 5b - 26 H.K. Trieu Masking (SiO2, Si3N4) leads to thermomechanical stresses (different thermal expansion coefficients of materials) Expansion of Si lattice Creation of vacancies Increase of diffusion coefficients for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion at grain boundaries 5b - 27 H.K. Trieu Increased defect density at grain boundaries 10- to 100-fold higher diffusion coefficient at grain boundary Segregation at grain boundary possible Diffusion from grain circumference into grain with unimpaired D Penetration depth depends on grain size and grain boundary properties polysilicon single crystal silicon Principle of grain boundary diffusion for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion processes for silicon 5b - 28 H.K. Trieu Processes: Requirements:  Gas phase diffusion In each case, thermal activation Gaseous source High temperatures (900 to 1200oC, +/-0,5°C) Liquid source For sufficiently brief diffusion times (10 s bis h) Solid source  Solid state diffusion and penetration depths (0.1 to 10 µm) Doped oxides as many wafers as possible (30 to >100) Polysilicon simultaneously (batch process) Gas phase diffusion process for silicon tube reactor carrier with wafers closing cap Theoretical and actual setup of diffusion reactors for internal use within the Semiconductor Technology class only further dissemination is not allowed Reactors for gas phase diffusion 5b - 29 H.K. Trieu Gaseous sources:  Especially simple supply  Simple uniform distribution  High toxicity (in ppb range) dilution with hydrogen  High edge concentration leads to rougher, non-etchable surface coating with dopant Addition of oxygen (3-10 vol%) promotes the formation of SiO2 and reduces the formation of difficult-to-etch skin made of silicides and other compounds of the doping material Doping from oxide wafers on carrier Boron: diborane B2H6 2 B + 3 H2 exhaust B2H6 + 3 O2 B203 + 3 H2O (300oC) valves and MFC Phosphorous: phosphine 2 PH3 2 P + 3H2  2 PH3 + 4 O2 P205 + 6 H2O exhaust Arsenic: arsine 2 AsH3 2As + 3H2  dopant gas bubbler 2 AsH3 + 3 O2 As203 + 3 H2O for internal use within the Semiconductor Technology class only further dissemination is not allowed Reactors for liquid phase diffusion 5b - 30 H.K. Trieu Liquid sources:  Relatively simple supply (thermostatted washing bottle („bubbler“))  Simple uniform distribution  Low toxicity  Few doping agents  High edge concentration leads to rough, non-etchable surface coating with dopant Addition of oxygen doping from oxide wafers on carrier exhaust Phosphorous: Phosphorous oxychloride 4 POCl3 + 3 O2 2 P205 + 6 Cl2 valves and MFC 4 BCl3 + 3 O2 2 B203 + 6 Cl2 4 BBr3 + 3 O2 2 B203 + 6 Br2 The oxidation of BBr3 runs markedly faster than with BCl3 (around 5s to 100s with liquid dopant source comparable process parameters) temperature-controlled bath for internal use within the Semiconductor Technology class only further dissemination is not allowed T and gas flow-time progression 5b - 31 H.K. Trieu Solid sources:  Partially relatively simple supply (e.g. ceramic wafers)  Simple uniform distribution  Low toxicity  Complex process control  High edge concentration leads to rough, non-etchable surface coating with dopant Addition of oxygen doping from oxide Dopants: Sb2O5 Sb P2O5 P As2O3 As + H2 B2O3 B BN B T H2/N2 Temperature and gas flow-time O2/N2 progression for doping from solid sources Time for internal use within the Semiconductor Technology class only further dissemination is not allowed Reactors for diffusion from solid sources 5b - 32 H.K. Trieu platinum crucible with wafers on source material carrier Open powder source exhaust valves and MFC closed box Powder source with increased source exhaust vapor pressure (box process) diffusion tube MFC boron nitride wafers exhaust valves silicon wafers Dopant oxides as ceramic flow controller wafers for internal use within the Semiconductor Technology class only further dissemination is not allowed Solid diffusion processes for Si 5b - 33 H.K. Trieu Dopant applied as layer in elemental form (evaporate, spin-coat) as oxide (doping glass): spin-coated, application from gas phase in oxidating atmosphere From doped polysilicon Doped oxide  Very simple process control Semiconductor  Simple uniform distribution  Low or no toxicity Doping with solid diffusion  Surface concentrations beneath the solubility limit possible in semiconductor  But critical reproducibility with low surface concentration Dopants: Sb2O5 as layer or in SiO2 Sb P2O5 P As2O3 As B2O3 B BN B e.g. Spin-cloated from silicate compounds in organic solution: Spin and dry at 300oC for internal use within the Semiconductor Technology class only further dissemination is not allowed Diffusion from doped polysilicon 5b - 34 H.K. Trieu Emitter diffusion: polysilicon gate electrode  Extremely high doping (degeneration) 1021 cm-3 phosphorus gate oxide deliver higher emitter efficiency single crystal silicon 1016 cm-3 boron channel region of  Heterojunction (interface layer of two different MOS transistor semiconductor materials) PolySi-Si poly Si mono Si  Critical in thin gate oxides (in FET with thin gate oxides, not only would the source and drain be doped but the channel as well) concentration As B Diffusion from polycrystalline silicon for internal use within the Semiconductor Technology class only depth further dissemination is not allowed Diffusion processes for gallium arsenide 5b - 35 H.K. Trieu Due to high vapor pressure of arsenic Encapsulation of substrate Melt down in ampule with As reservoir Dopant in Ga melt (Zn in Ga) or As powder mixture (Zn:As) Cover with doped SiO2 layers (from silicate emulsions) platinum crucible with wafers on Zn-Ga-As source material carrier ZnAs2 exhaust Zn-Ga valves and MFC H2 Zn-Ga Diffusion from solid sources in closed ampule Dependence of doping profile in GaAs on source type (T=850oC) for internal use within the Semiconductor Technology class only further dissemination is not allowed Reactor configurations 5b - 36 H.K. Trieu Multi-zone hot wall tube reactor for batch processes Long tube for > 100 wafers Three to five zone oven (400 - 1300oC) for even temperature profile (T <  1oC) Temperature control for time gradients („ramp“) to prevent dislocation formation Horizontal or vertical Tube diameter: 10 cm (3“) to 40cm (300 mm) Automatic intake mechanism Substrate carrier: self-supporting or slide „Flow-Box“ at entrance Tube cleaning using HCl-, Cl2- rinsing between processes (capturing of sodium)  Simple setup  Low space and cost requirements  High wafer throughput  Relatively long duration (10 min) to thermal stabilization  Not suitable for short time processes for internal use within the Semiconductor Technology class only further dissemination is not allowed Setup of multi-zone hot wall reactors 5b - 37 H.K. Trieu air filter and fan MFC loading station temperature sensor temperature control unit heater power supply Theoretical and actual setup of multi-zone hot wall reactors for internal use within the Semiconductor Technology class only further dissemination is not allowed RTP (Rapid Thermal Processing) reactors 5b - 38 H.K. Trieu  Halogen-lamp heated reactor  Single wafer process  Extremely high time-temperature gradients (~ 1000K/s)  Short process times (sec to min)  Temperatures to > 1300oC  Simple setup  Suitable for short time processes 10 sec  Low space requirement  Short duraction (s) to thermal stabilization  Casette-casette process TEMP °C tungsten halogene lamp  Realization of extremely small „thermal budgets“ (temperature-time product) arc lamp  Low slide throughput T-time profile of a  Critical temperature control RTP process time for internal use within the Semiconductor Technology class only further dissemination is not allowed Setup of RTP reactors 5b - 39 H.K. Trieu Schematic and real setup of plate and lamp- heated RTP-reactors for internal use within the Semiconductor Technology class only further dissemination is not allowed Local diffusion 5b - 40 H.K. Trieu Masking using high-temperature resistant patternable layers Suitable materials for Si and GaAs: SiO2: deposited or through thermal oxidation (Si) Si3N4: deposited Required layer thickness i.e. < 1µm, patterning takes place using photo-etching technologies mask thickness [µm] mask thickness [µm] boron diffusion phosphorus diffusion diffusion time [min] diffusion time [min] Required thickness of SiO2 for masking against boron and phosphorous for internal use within the Semiconductor Technology class only further dissemination is not allowed Problems with diffusion doping 5b - 41 H.K. Trieu Reproducibility in production conditions (temperature and gas flow)  Diffusion in two steps  Coating of surface with saturation concentration of dopant (predeposition). Since the solubility of the dopant in Si is limited => concentration on the surface of the wafer is not determined by the relatively-difficult-to-control dopant supply, but instead by the saturation concentration.  Infusion of the deposited amount of dopant (drive in). But: maximum doping in silicon limited by the solubility of the impurities  e.g. for arsenic, maximum 4% in silicon  not every desired ohmic resistance achievable for internal use within the Semiconductor Technology class only further dissemination is not allowed Current significance of diffusion processes 5b - 42 H.K. Trieu Application for example in the doping of polysilicon (POCl3, liquid donor) In all other process modules (e.g. drain/source for MOS) replaced by ion implantation  More exact quantity control  More complex profiles possible  Lower lateral (outward) diffusion through deep implantation for internal use within the Semiconductor Technology class only further dissemination is not allowed