Download Principles of Ion Implant Implant Mechanics

Principles of Ion Implant
• Generation of ions
– dopant gas containing desired species
• BF3, B2H6, PH3, AsH3, AsF5
– plasma provides positive ions
• (B11)+, BF2+, (P31)+, (P31)++
• 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
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
1
Implant Mechanics
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
2
Plasma source and ion extraction
Gas feed
Nielsen-type gaseous source
Plasma
Chamber
+ ions
mT level pressure
To pump
Vext
variable extraction voltage
(typically ~30KV )
Microelectronic Engineering
K.D. Hirschman
Rochester Institute of Technology
Silicon Processes: Ion Implantation
3
Ion selection ( Analyzing Magnet)
V : ion velocity
KE = qVext = ½M v 2
B|v
R
X
radius of curvature
Resolving Aperture
R-H-R
B
v
F
selected ions
Heavier ions
Magnetic Field
F = q v xB = M v 2 / R
Mass to charge ratio of the selected ions:
Centripetal Force
Microelectronic Engineering
Rochester Institute of Technology
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.
M /q = R2 B2 / (2 Vext)
K.D. Hirschman
Silicon Processes: Ion Implantation
4
BF3 Gas Spectrum
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
5
PH3 Gas Spectrum
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
6
Ion Acceleration
µΤ level pressure
Resolving
Aperture
Final Kinetic Energy of the Ion =
q ( Vext + Vacc )
E field
Example: Vext = 30 KV
Vacc = 70 KV
Energy of the Ion = 100 KeV
Vacc
Ground
Column length = 3 ‘
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
7
Projected Range (Rp)
Rp of Boron, Phosphorous, Arsenic and Antimony in Silicon as a
function of the ion energy
Rp (µm)
Rp depends on incident
and target atomic masses
(complex)
Ion Energy (KeV)
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
8
Beam Scanning
Electostatic scanning (low/medium beam
current implanters (I < 1mA)
Vy
Scan Patterns
Si
wafer
Vertical Scan
Vx
Y
X
Horizontal Scan
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.
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
9
In-situ Dose Control
End Station
secondary electrons
wafer
ion beam
back plate
VFaraday
cup
( - 100 V )
Vsuppression
( - 500 V )
I
A
DOSE MONITORING
∫ Ι dt = Q=A q φ
Ground
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
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Mechanical Scanning
Stationary Ion Beam
Rotating Disc
( 1200 rpm )
Beam Area ~ 20 cm2
(diameter ~ 5cm)
Si
wafer
Excellent wafer cooling needed.
Substantial load/unload time.
15 - 25 wafers /disc.
Excellent throughput for high
dose implants.
ƒ High current (~20mA)
ƒ Scan speed adjustment to insure
uniform dose
ƒ
ƒ
ƒ
ƒ
Microelectronic Engineering
Rochester Institute of Technology
K.D. Hirschman
Silicon Processes: Ion Implantation
11