The University of Michigan – Visiting Prof. HKU p. 1 S. W. Pang
ELEC 7364 Lecture Notes Summer 2008
Ion Implantation
by STELLA W. PANG
from The University of Michigan, Ann Arbor, MI, USA
Visiting Professor at The University of Hong Kong
The University of Michigan – Visiting Prof. HKU p. 2 S. W. Pang
Channel Doping Requirements
Shallow Junction down to 10 nm High Doping Concentration up to 1020 cm-3 Need to have Precise Placement of Dopants at high Dose Minimize Thermal Diffusion During Annealing
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Advantages of Ion Implantation - Precise Dose and Depth Control - Less Lateral Distribution for Short Channels - Wide Range of Dopants Can Be Selected - Complex Doping Profile Can Be Made - Room Temperature Mask
Disadvantages of Ion Implantation - Damage by High Energy Ions: Annealing
Needed - Damage Related Enhanced Diffusion, Junction
Leakage, and Dopant Channeling - Equipment Costly and Large; Lower Throughput - Safety Hazards - Toxic Gases (e.g. AsH3, PH3,
B2H6, …), High Voltage, Radiation
Diffusion vs. Ion Implantation
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Ion Source: Convert gases to ions by electron bombardment
- Electrons generated by heating filament or cathode (lower current, <1 mA)
- Electrons generated by magnetically confined rf plasma (High Current, I~25 mA, usually for shallow junctions with low ion energy)
Mass Spectrometer: Separate ions with different mass by bending them with B field
High Voltage Accelerator: Change ion energy Eion from few KeV (Typical) to MeV (High Voltage)
x, y deflection system: Beam scanning to cover wafers. Typical beam size ~5x20 cm2
Target Chamber: Wafers loading with rotation, tilt, and dose integration
Typical Ion Implanter
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Ion Implantation System
Ion Source to Supply Ions to be Extracted to Mass Spectrometer Ions Selected by Electromagnetic Field (Mass of Ions) and
Accelerated to Desired Energy Scanners to Deflect Ion Beam to Cover Area on Wafer Ion Dose Monitored by Dose Integrator
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Scanning and rotation improve uniformity and minimize wafer heating. Heating can be significant at high dose or high I
Particles could be generated due to erosion of beam line components or microdischarging. Need to have new design of ion source and beam extraction components, use of hard materials, and use of low ion density and large area beam to reduce the static force that carry particles to wafers
Rapid Thermal Annealing (RTA) after implantation for shallow junctions. Anneal with fast ramp rate (>100°C/s for RTA compared to 15°C/min for furnace), better time, temperature, and ambient control
Special Considerations for Ion Implanter
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Typical Implanters – Medium I, Medium E; few µA to 1 mA; 20-200 KeV; up to 1015 ions/cm2; ion implantation time ~10 s/wafer
High I, Low E Implanters; For shallow junctions, high throughput, high dose; Up to 30 mA; 0.2-80 KeV; up to 1016 ions/cm2; ion implantation time ~10 s/wafer
- Ion source consists of rf plasma and magnetron - Needs cooling and scanning to avoid heating - Needs new design of beam line components to avoid
particles High I, High E Implanters; For deep junctions with low thermal
budget, up to 3 MeV and 5 µm deep - Used to form deep well for substrate isolation to reduce
capacitance, increase packing density, lower power consumption, and reduce latch up
- Thick photoresist mask needed (>5 µm). The ions also break down the polymer, causing outgassing. The emitted species can ionize or neutralize incoming ions and cause dose error. The photoresist is hardened which makes it different to remove
Ion Implanter Types
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Ion Stopping - Ions loss energy through collisions with target,
stop when energy is dropped to 0 - A. Nuclear Stopping - Elastic collisions with
nuclei with stopping power:
M1 and M2 are atomic mass of ions and target Z1 and Z2 are atomic number of ions and target Nuclear energy loss dominates at low E
Ion Distribution - Nuclear Stopping
€
Sn =2.8x10−15M1Z1Z2
(M1+M2 ) (Z123 + Z2
23 )(eV − cm2 )
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- B. Electronic Stopping - Inelastic collisions with electrons with stopping power :
Electronic energy loss dominates at high E
Rate of energy loss:
Total distance ion can travel before stopping:
where Eo is the initial ion energy
Ion Distribution - Electronic Stopping
€
Se = K E(eV − cm2 )
€
dEdx
= Sn + Se
€
R =dE
(dE /dR)0
E0
∫
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Rp = projected range, average distance ions travel before stopping; increases with ion energy E and decreases with ion mass mi
B in Si: Rp=0.07 µm at 20KeV and 0.3 µm at 100KeV 100 KeV B: SiO2: Rp=0.3 µm; AZ: Rp=1 µm; W: Rp=0.08 µm
ΔRp=standard deviation of Rp; Cp=Peak concentration at x=Rp Total Dose:
Typical Dose: 1012 ion/cm2 for VT adjustment 1015 ion/cm2 for Junction formation
Gaussian Ion Distribution Profile
€
C(x) = Cpe−(x−Rp )
2
2ΔRp2
(ions / cm3 )
€
Q = C(x)dx0
∞
∫ = 2πCpΔRp( ions / cm2 )
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Projected Range and Straggle
Ion Collision and Stopping are based on Statistics Range (Depth of Implanted Peak) and Straggle (Spread of Profile)
Increase with Ion Energy Heavier Ions have Shorter Range and Straggle (e.g. BF2
+ is used to Reduce Depth)
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At x = Rp ± ΔRp, C(x) = Cpe-0.5 = 0.6 Cp Annealing after Ion Implantation – Limited Source Diffusion
Gaussian Solution for Limited Source Diffusion
Find Qo: x shifted to (x - Rp) Diffusion from both sides (factor of 2) Increase spreading from 2Dt to 2Dt + ΔRp
2
Dopant Profile and Thermal Anneal
€
C(x,t) =Qo
πDte−x 2
4Dt
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Dopant Profile Evolution
Peak Concentration Decreases by 1/sqrt(t) At x = 2*sqrt(Dt), Dopant Concentration Decreases by 1/e At Longer Time, Peak Concentration Decreases and Dopants
Spread Further Out
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Thick mask for selective ion implantation
Usually with mask thickness = Rp+4ΔRp
SiO2 and Si3N4 about 0.5 µm thick and photoresist
about 1 µm thick
Often, ions are implanted through a layer of SiO2 to
place Cp at Si Surface (also keeps the surface from
contamination)
Ion Implantation Mask
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Ion Distribution Under Mask
Mask Needs to be Thick Enough to Block Ions (Typically, N(xo) <0.1 NB), High Temperature NOT Needed
Doping Profile Spreads Vertically and Laterally An Oxide Layer Can be Used to Place Dopants with Maximum
Concentration Right on Si Surface
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LSS Model for implantation to amorphous materials
For single crystal Si, open channels in between atoms reduce nuclei stopping - with mostly electronic stopping, ions can travel much deeper in substrate
Ion penetration range is not predictable due to channeling and dechanneling
To reduce channeling and get predictable Rp: - ion implantation at 7° off(100) - pre-amorphizing surface by another implant - implant through oxide - use heavier ions (e.g. BF2 rather than B)
Ion Channeling - Deeper Junction
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Ion Channeling - Deeper Ion Penetration
Ion Range Modeled Based on Amorphous Substrate Channeling Causes Much Deeper Penetration with Only Electron
Stopping Channeling Reduced by Implanting 7° off (100) Orientation
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Remove lattice damage due to high energy ions
600-1000 °C
1-30 min
Thermal energy to move Si to lattice sites and dopants to substitutional sites
Low thermal budget needed for shallow xj
Annealing After Ion Implantation
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Rapid Thermal Annealing - Shallow Junction
High Intensity Lamps to Rapidly Heat Wafers with Fast Ramp Rate of >100 °C/s to High Temperature of 950 - 1050 °C
Limit Thermal Diffusion (Small ʻDtʼ) while Removing Ion Induced Damage
Can Also be Used to Grow Thin Oxide or Nitride
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L = 0.25 µm, xj = 100 nm; L = 0.18 µm, xj = 75nm; L = 0.1 µm, xj = 40 nm; L = 0.05 µm, xj = 20 nm Low ion energy (1 KeV), high dose, high ion
mass Rapid Thermal Annealing – Temperature, time,
gas ambient, ramp rate (e.g. 1050 °C, 1 s) Pre-amorphization and 7° off (100) to avoid
channeling Abrupt junction – Halo Implantation with a ring
of well dopant self-aligned to gate
Shallow Junctions
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To adjust VT positively: Acceptors (e.g. B)
To adjust VT negatively: Donors (e.g. As, P)
where Qi = Ion dose (ions/cm2) Oxide Capacitance = (F/cm2)
VT Adjustment
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ΔVT =qQi
Cox
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Cox =εoxxox
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Change VT from +1V (enhancement mode) to -1V (depletion mode) for n-channel MOSFET with xox=30 nm
ΔV= -2 V, negative shift, need donors (As or P) ion implantation to get ΔVT = 2V
Qi = 1.44x1012 ions/cm2 Qi includes only dopants in Si, not the ones in SiO2
Example of VT Adjustment
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2 =1.6x10−19C(Qi )
3.9x8.85x10−14F / cm / 30x10−7cm
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Molecular Ions: Higher mass for acceleration and transport of ions at sub-keV, more stable space charge, easier to extract large current. (e.g. Use B10H14 molecules instead of 11B+)
Plasma Doping: Reduce time needed for scanning in conventional implanter; Extract dopant ions directly from plasma for implantation of entire wafer
Laser Doping: Laser melting a shallow depth when exposed to dopant vapor, follow by recrystallization when cooled without annealing
Alternative Doping Techniques