Doping: Depositing impurities into Si in a controlled manner
Doping: Depositing impurities into Si in a controlled manner Overview Diffusion vs Implantation Mechanism,Models Steps Equipment Goal: Controlled Junction Depth Controlled dopant concentration and profile P+ P+
Source N well Wafer (Substrate): P Type Drain Preferred location of maximum concentration need not be the surface Ion Implantation Diffusion & Ion
Implanatation Bombardment of ions SOURCE Electric Field OXIDE Ions Wafer (Substrate)` BLOCK Junction is where N
=P Can also be used when doping N in N Diffusion & Ion Implantation Diffusion Solid-in-solid high temperatures (1000 C) Distances covered are in um or nm Diffusion OXIDE Wafer (Substrate)`
BLOCK Mechanism , Models Substitutional (10-12 cm2/s) Interstitial replacement (10-6 cm2/s) Interstitial movement Substitutional preferred (better control) Au, Cu diffuse by interstitial mechanism B, P etc by substitutional mechanism Two ideal cases Constant source, limited source Using Ficks First & second law J = Flux D - Diffusivity of A in B N- Concentration x - distance
J D N x N 2 N D 2 t x Models Constant Source Concentration at x=0 is No N (0, t ) N o x N (, t ) 0
N ( x, t ) N o erfc ( 2 Dt N ( x, 0) 0 Complementary Error Function Q N ( x, t )dx 2 N 0 0 ) N x
0 x 0,t N ( x, t ) dx Q 0 N (, t ) 0 N ( x, 0) 0 Total Dose Q Limited source Dose Q = constant Approx by Delta Fn
Dt Q N ( x, t ) e Dt x2 4 Dt Models Constant Source Concentration at x=0 is No Important Parameter : Dt
Impurity Concentration N0 species, temp and time 1 2 3 Distance from Surface Models Limited Source
Dose Q Important Parameter : Dt Impurity Concentration N0 Area under the curve is constant 1 3 2 Distance from Surface If you normalize, erfc drops faster than Gaussian
Diffusivity E kT D D0 e Diffusivity Follows Arrhenius behavior Wafer goes through heating cycles many times in the process Effective Diffusivity * time = sum (Diffusivity * time) Concept of thermal budget D
Dt total Diti i 1000 T Diffusion Max absorption (at a given temp) Usually quite high Good for emitter and collector, but not for base Not all dopant can contribute to electron/hole near solubility limit Solubility limit in the range of 10 20/cm3 at 1000o C Diffusion into silicon Faster on grain boundaries 10 times in poly silicon
Diffusivity in SiO2 usually very low (Segregation occurs) Junction Formation N Impurity Conc Carrier Conc Jn P Distance from surface
Diffusion: Drive In: Dopant re distribution Deposited dopant must be pushed into Si Re-distribution of dopant Oxidation of exposed Si to protect OXIDATION Dopant Diffusion *Dopant profile changes due to diffusion * Also due to preference for Oxide/Silicon: N-type piles up in Si, P-type depletes in Si Diffusion: Steps OXIDE
Dep Diffusion BLOCK 1.Pre Clean To remove particles Thin oxide grows 2.HF Etch To remove oxide Not too much! 3.Deposit (pre dep) Deposit enough to be higher than the solubility limit 4.Drive In High temp to enable diffusion
inside Si Also forms SiO2 (with high 5.Deglaze (HF Etch) dopant concentration) Oxide may act as dopant source in future 2-STEP diffusion (usual) steps Removing highly doped oxide may be problem (for dry etch) Diffusion: Dep: schematic Wafers are Horizontal Gas Flow
Better Uniformity Less wafers per batch Vertical Poor Uniformity More wafers per batch (or can have smaller chamber) Gas Flow Dummy wafers placed in the beginning & end Doping: Gas phase Dopant can be in Gas/Liquid/Solid state, but is typically carried using N2 in gaseous form
Chamber Carrier Gas (N2) + Source Reaction gas *Carrier gas may be bubbled through liquid source *Carrier gas may pass over heated solid source * inert gas can provide volume to maintain laminar flow Doping: Gas phase
Reaction/Diffusion Limited Phosphorus oxy chloride 4 POCl3 3 O2 2 P2O5 6 Cl2 Phosphine 2 PH 3 4 O2 P2O5 3 H 2O 2 P2O5 5 Si 4 P 5 SiO2 Arsenic Oxide 2 As2O3 3 Si 3SiO2 4 As Diborane 300o C
B2 H 6 3 O2 B2O3 3 H 2O B2 H 6 6 CO2 B2O3 3 H 2O 6CO Boron Tribromide 4 BBr3 3O2 2 B2O3 6 Br2 2 B O 3 Si 4 B 3 SiO 2 3 2 Solid phase Solid Source Slugs between wafers Lower through put Cleaning is issue (slugs can break) Safer to handle(no toxic vapor at room temp)
Spin coating (with solvents) Similar to photo resist coating Cost of extra spin/bake steps thickness variations Doping: Solid phase Phosphorous pentoxide 2 P2O5 5 Si 4 P 5 SiO2 Arsenic Oxide 2 As O 3 Si 3SiO 4 As 2 3 2 Antimony Tri Oxide 2 Sb2O3 3Si 3SiO2 4 Sb
Boron Trioxide 2 B2O3 3 Si 4 B 3 SiO2 Tri Methyl Borate (TMB) 900o C 2(CH 3O )3 B 9O2 B2O3 6 CO2 9 H 2O Side diffusion Issues Increases with temperature/time Limits the space between devices Maximum dopant concentration is near surface
==> majority of current near surface (Surface tends to have max defects) ==> less control Dislocation generation (thermal drive in) Surface contamination (dep) Low dopant concentration and thin junction (small junction depth) are difficult At 0.18 um , junction depth is ~ 40 nm At 0.09 um, junction depth may be 20 nm Issues: Side diffusion Side diffusion (Lateral Diffusion) BLOCK Wafer (Substrate)`
Diffusion OXIDE BLOCK Example of Real systems : *Hitachi-Vertron V *1m x 3.5m x 3.3m *200 mm wafer *150 wafers at a time * higher thermal budget, * good control, uniformity * high throughput *Hitachi-Zestone VII *2m x 3m x 3m
*300 mm wafer *one wafer at a time * lower thermal budget, * better control, uniformity * low throughput Example of Real systems : Protemp Gettering To remove unwanted impurities Try to get them to the back of wafer Defects Ar implant Dep SiN/SiO2 (stress) Oxygen during crystal growth (intrinsic)
High Conc P on back of wafer Measurement Sheet Resistance (average) Four point probe, VDP (Van der Pauw) Bevel Interference Dye SIMS Diffusion: Summary Diffusion Temp, Time, Thermal budget Doping (more important for older nodes) Relevant for all nodes 2 step (constant source, limited source) Solid/Liq/Gas
Ion Implantation Somewhat similar to Sputtering Dopant goes inside the silicon sputtering deposits on the surface Used for controlled doping concentration profile (depth) Equipment Mechanism Issues Summary Equipment N e u tra l B e a m T rap and Beam
G a te 900 A n a ly z in g M agnet Focus A c c e le r a tio n Tube B ea m T ra p a n d G a te P la te Y - A x is Scanner
w a fe r in w a fe r P ro cess cham ber Ion S o u rce Peter van Zant 1. Ion Source Gas or solid source (no liquid source) Solid heated to obtain vapor (P2O5) effectively gas source Mass flow meters (to control the flow better) Gas usually Fluorine based
AsF5 , BF3 , SbF3 , PF3 , PF5 Ionization chamber low pressure (milli/ micro torr) to ionize and minimize contamination heated filament (thermionic emission) positively charged ions created 2. Analyzing Selection, analyzing, mass analyzing, ion separation Similar to Mass Spectroscope Usually the second stage (before acceleration) Magnetic field to control the path Charge to Mass Ratio Some of the species from BF3 source B , BF , BF2
Selection of B+ B BF BF2 3. Acceleration Acceleration needed for implantation Positive ions accelerated with ring anodes Energy range: 5 keV for low, 2 MeV for high High energy ==> high throughput few seconds per wafer Beam Current
Medium current : 1 mA High current: 10 mA Current ~ Dose Beam Focus (magnetic/electric) SOI High Current Oxygen 100 mA 10 mA Low Energy 1 mA High
Current Low Current keV High Energy MeV 4. Scanning Beam size ~ 1 sqr cm Wafer size 200 mm or 300 mm Issues: neutral atoms need to be removed because... dose calculated by current integrator Electrical (beam) scanning & Mechanical (wafer) scanning Beam Scan:(medium current)
beam moves outside the wafer for turn controlling XY plates may be destroyed by discharge Rotate wafer for uniformity Wafer scan: (high current) Beam shuttering: (electrical/mechanical) turn beam off when not on wafer 5. Target chamber End chamber low particle, high vacuum Wafer held on clamp (more particles) OR ESC (less particles) Anti-static devices on the chamber Integrate the current to measure dose For 2+ ions, divide by 2 and so on... Wafer charging: minimize by connecting wafer to ground (with a charge
counter) dielectrics may get damaged use flood gun to provide electron (and count it in measurement) Mechanism Electrons attract the +vely charged ions Nuclei repel the +vely charged ions Inelastic collision: Electron (ionization) Nuclear (nuclear reactions) Elastic collision Electron Nuclear (atom substitution) At low energy Nuclear collisions predominant
At high energy electronic collisions predominant Variation in stopping cross section Gaussian profile expected (projected range Rp) Implantation Mask with Photoresist or oxide resist for medium and low energy, moderate dose high energy/high dose: increase in temp Resist re-flow Cross link (for organics) less soluble (stripping an issue) Faraday Cage Retain secondary electron from wafer Otherwise, wafer under dosed -Ve Bias e-
Issue: Transverse Straggle implant OXIDE Gaussian BLOCK Transverse Straggle (Diffraction) Even in implantation, dopants present in lateral direction Channeling Some ions will move through channels
without experiencing nuclear or electron collision for a long time ==> No Gaussian Profile Channeling 1. Hold the wafer at an angle (~ 8 degree) BLOCK ==> increase transverse straggle(called undercut) Also causes
shadow ==> Too much angle is also a problem Shadow Undercut Channeling 2. Dep amorphous material on the top implant OXIDE BLOCK
It has to be very thin and not stop ions 3. Damage top of wafer and make it amorphous (eg high energy silicon implant) Channeling 4. Increase temperature ==> reduce channel cross section Channeling critical angle ~ (Z/E) 1/2 ==> Low energy implants more likely to channel TED Transient Enhanced Diffusion Damage during implantation
==> point defects (vacancies) interstitial silicon atoms reduced during anneal Channel dopant diffuse to surface ==> VT modification Solid State Technology RTA Anneal to heal the damage Diffusion during anneal an issue High temp repair is faster than anneal Repair energy barrier 5 eV, diffusion barrier 3 or 4 eV 1. Adiabatic (laser, heats surface , < micro sec) profile control difficult (not used) 2. Thermal flux ( micro to 1 sec) laser, ebeam, flash lamp
surface+bulk heating rapid cooling ==> point defects 3. Iso thermal (W-Halogen lamp) 30 sec (1100 C) Diffusion vs Ion Implantation Dep+Diffusion: depends on chemical nature and solubility Implantation: on energy of ion beam Expensive Better Control of junction depth, dose, profile Less transverse straggle
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