Ion Implantation

Two-step diffusion

2NSpDt Q = N → solid solubility limit → huge. p⇡ s

To get a low dose requires small Dt, which is difficult to control.

Ion implantation • small, controllable doses • profiles can be tailored, not restricted to error function or gaussian profiles • much faster → one wafer completed in seconds • low-temperature (sort of), so more flexibility in processing

Expensive !!

EE 432/532 ion implantation – 1 V

ion mass separator B ion acceleration x,y sample chamber 25 kV - 1 MV scanning V

plasma source

gas inlet

Ion implantation system • Ions (B+, P+, As+) are created in a plasma. (We’ll study it later.) • Desired ions are filtered out using mass separation. • Dopant ions are accelerated to high energies (25 keV – 1 MeV). • Ion beam is deflected and rastered across the wafer surface.

EE 432/532 ion implantation – 2 Example of a “smallish” ion implanter.

EE 432/532 ion implantation – 3 ion stopping

nuclear – ion cores interact • incoming ion is strongly deflected • lattice atoms are knocked out of place

+

ion electronic – ion interacts interacts with electron cloud of the lattice. • energy is lost as the ion drags through cloud • similar to viscous friction lattice

EE 432/532 ion implantation – 4 • In stopping the ions, most of the energy is lost through electronic interactions. • Nuclear interactions still have a strong effect – randomized motion and crystal damage. • Detailed theories for nuclear stopping in solids have existed for several decades. Linhardt, Scharff, and Schiott (c. 1963) provided the first unified treatment that could be applied to semiconductors. Known as LSS theory. The treatment is beyond our purposes here, but you are welcome to read further on your own. The theory provides a statistical description of the dopant profile. Range → average depth straggle → deviation or variance

• Higher energies → deeper range and more straggle. • Lighter ions → deeper range and more straggle. EE 432/532 ion implantation – 5 Range and straggle for implants into silicon

boron B 1 phosphorus 0.1 antimony P arsenic B As As P Sb Sb

0.1 0.01 range range (µm) straggle straggle (µm)

boron phosphorus antimony arsenic

0.01 0.001 10 100 1000 10 100 1000 implant energy (keV) implant energy (keV)

Rp and ∆Rp are determined by ion energy.

EE 432/532 ion implantation – 6 Implant profile

simplest description 1019 N 2 P Q (x Rp) 18 ∆R N (x)= exp 2 10 P p2⇡Rp " 2(Rp) #

1017 Q → dose ) –3

16 Rp → range (average depth of ion travel) N (cm 10

∆Rp → straggle (variance in ion depth) 1015

R Q is fixed by the implant time 1014 P 0 0.1 0.2 0.3 0.4 0.5 0.6 1 t Q = I (t0) dt0 x (µm) q beam Z0 Measure the beam current. Stop peak concentration after the required amount of charge Q (in the form of dopant atoms) has N = P p arrived. 2⇡RP

EE 432/532 ion implantation – 7 Junction depth same as diffusion – look for where doping profiles cross

1019 N P

N (xj)=NB 18 ∆R 10 P

NB 1017 ) (x R )2 –3 N = N exp j p B P 2 16 " 2(Rp) # N (cm 10

1 15 N 2 10 p P xj = Rp 2(RP ) ln xj2 ± NB xj1 R  ✓ ◆ 1014 P 0 0.1 0.2 0.3 0.4 0.5 0.6 Note that there might be two x (µm) junctions. Usually not desirable, but a possibility.

EE 432/532 ion implantation – 8 Damage and annealing

The top surface of the semiconductor crystal (RP plus few ∆RP) will be heavily damaged. Probably becomes completely amorphous.

Damage can be healed by annealing. At high temperatures, the dislodged atoms can “diffuse” back into the correct lattice positions.

Annealing at 1000°C for 30 minutes is typically enough to restore crystallinity.

EE 432/532 ion implantation – 9 Dopant diffusion during annealing Of course, during the anneal, dopant will diffuse.

Before anneal After anneal

1E+19 1E+19

1E+18 1E+18

1E+17 1E+17

1E+16 1E+16

1E+15 1E+15

1E+14 1E+14 0 0.1 0.2 0.3 0.4 0 0.1 0.2 0.3 0.4

We lose some of the advantages of the ion implant.

EE 432/532 ion implantation – 10 Accounting for diffusion

1E+19 implant 2 Q (x RP ) 1E+18 N (x)= exp 2 p2⇡RP " 2(RP ) #

1E+17

1E+16 diffusion 2 Q (x RP ) 1E+15 N (x)= exp 2p⇡Dt " 4Dt #

1E+14 0 0.1 0.2 0.3 0.4

2 (RP ) Treat the implant like it was a diffusion with (Dt) = imp 2 Add the Dt’s, as we’ve discussed with diffusions

Q (x R )2 N (x)= exp P (R )2 (R )2 0 P 1 P 4 2 + Dt 2 ⇡ 2 + Dt EE 432/532 @ A ion implantation – 11 r h i h i