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Principles of Ion Implant
- Generation of ions
- dopant gas containing desired species
- BF 3 , B 2 H 6 , PH 3 , AsH 3 , AsF 5
- plasma provides positive ions
- (B^11 ) +, BF 2 +^ , (P^31 ) +^ , (P^31 ) ++
- Ion Extraction
- Ions are extracted from the source due to a high electric field
- Ion Selection
- Magnetic field mass analyzer selects the appropriate ion (mass & charge)
- Ion Acceleration
- Further accelerate ions giving the ions their final kinetic energy.
- Beam Scan / Disk Scan
- Provides a uniform dose of ions over the wafer surface.
- In-situ Dose Monitoring
Implant Mechanics
Plasma source and ion extraction
Vext
Gas feed
To pump
Plasma Chamber
mT level pressure
variable extraction voltage (typically ~30KV )
Nielsen-type gaseous source
Ion selection ( Analyzing Magnet)
Resolving Aperture
R radius of curvature
Magnetic Field
V : ion velocity
F = q v xB = M v^2 / R
KE = qVext = ½M v^2
M /q = R 2 B 2 / (2 Vext)
Mass to charge ratio of the selected ions:
selected ions
Heavier ions
The ions are extracted from the source and analyzed in a magnetic field. The Lorentz force makes the ions take a curved path with a radius of curvature that depends on the mass of each ionic species. By adjusting the magnetic field strength, only the selected ions will enter the accelerating column.
Centripetal Force
B | v
v
B F
R-H-R
X
Ion Acceleration
Resolving Aperture
Ground
Vacc
E field
Column length = 3 ‘
Final Kinetic Energy of the Ion = q ( Vext + Vacc )
Example: Vext = 30 KV Vacc = 70 KV Energy of the Ion = 100 KeV
μΤ level pressure
Projected Range (Rp)
Rp (μm)
Ion Energy (KeV)
Rp of Boron, Phosphorous, Arsenic and Antimony in Silicon as a function of the ion energy
Rp depends on incident and target atomic masses (complex)
Beam Scanning
Electostatic scanning (low/medium beam current implanters (I < 1mA)
This type of implanter is suitable for low dose implants. The beam current is adjusted to result in t > 10 sec/wafer. With scan frequencies in the 100 Hz range, good implant uniformity is achieved with reasonable throughput.
V (^) y Vertical Scan
Horizontal Scan
Si wafer
Y
X
Scan Patterns
V (^) x
In-situ Dose Control
End Station
A
Vsuppression ( - 500 V )
VFaraday cup ( - 100 V )
ion beam
secondary electrons
wafer
back plate
Ground
I Ι^ dt =^ Q=A q^ φ
DOSE MONITORING
