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

The University of Michigan – Visiting Prof. HKU p. 3 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 4 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 5 S. W. Pang

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

The University of Michigan – Visiting Prof. HKU p. 6 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 7 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 8 S. W. Pang

  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 )

The University of Michigan – Visiting Prof. HKU p. 9 S. W. Pang

-  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

∫

The University of Michigan – Visiting Prof. HKU p. 10 S. W. Pang

  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 )

The University of Michigan – Visiting Prof. HKU p. 11 S. W. Pang

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)

The University of Michigan – Visiting Prof. HKU p. 12 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 13 S. W. Pang

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

The University of Michigan – Visiting Prof. HKU p. 14 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 15 S. W. Pang

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

The University of Michigan – Visiting Prof. HKU p. 16 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 17 S. W. Pang

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

The University of Michigan – Visiting Prof. HKU p. 18 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 19 S. W. Pang

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

The University of Michigan – Visiting Prof. HKU p. 20 S. W. Pang

  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

The University of Michigan – Visiting Prof. HKU p. 21 S. W. Pang

  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

€

ΔVT =qQi

Cox

€

Cox =εoxxox

The University of Michigan – Visiting Prof. HKU p. 22 S. W. Pang

  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

€

2 =1.6x10−19C(Qi )

3.9x8.85x10−14F / cm / 30x10−7cm

The University of Michigan – Visiting Prof. HKU p. 23 S. W. Pang

  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