l6 Ion Implantation

8/3/2019 l6 Ion Implantation

1/19

ION-IMPLANTATION


2/19

BASICS

Ionized impurity atoms areaccelerated by anelectrostatic field and

made to strike the surfaceof wafer

By measuring ion currentand adjusting electrostaticfield, the penetrationdepth and dose can becontrolled


3/19

SYSTEM SET-UP

Consists of:

Ion source,Acceleration

tube, MassAnalyzer,End station


4/19

MASS ANALYZER The ion beam is passed through a magnetic sector that

selects a particular ionic species An ion of mass Mand charge qmoving at a velocity vin a

circular path will experience a force:

According to the mass, the trajectory will bedifferent a slit is used at the appropriateplace to extract desired species

The kinetic energy of the ion is given by its extractionvoltage:

rMv

qB B

M

q

V 1

2 extE qV Mv vqV

M

extext1

2

22


5/19

Ions are accelerated after mass separation.

Acceleration done in vacuum to avoid collisions.

Neutral species undesirable as they cannot be

deflected by electrostatic potential duringscanning will get implanted near center ofwafer.

Each species will have a different mass.

Neutral species are removed by deflecting theions (or bending the ion beam) so that the neutralswill not respond to the electrostatic field and willtravel straight and strike a beam stop.


6/19

BASIC ARRANGEMENT:


7/19

Vertical projected range The total distance traveled

by the ion before it stops iscalled range R.

Rp is the average vertical

distance for a uniformbeam= vertical projectedrange

Energy loss due to twomechanisms Interaction of incidentions with electrons of

target atom Interaction with lattice

ions


8/19

Implantation Profile

Total distance traversed by ion is called Range, R. The average penetration depth of the ions is called

projected range Rp. Due to a distribution, there is a statistical fluctuation along

the direction of the projected range, called straggle, Rp.

The statistical fluctuation perpendicular to incidentdirection is called lateral straggle, R.

RpR

Ion Beam

Rpx

z

y

Log (ionConcentration)


9/19

TARGET-ION INTERACTION:

Energy loss due to two mechanisms Electronic stopping interaction with

electron cloud around target atom Collisions with electrons around atoms

transfers momentum and results in localelectronic stopping

Nuclear stopping interaction withnucleus of target


10/19

Nuclear stopping

Collision with nuclei of the lattice atoms

Scattered significantly

Causes crystal structure damage.

Electronic stopping

Collision with electrons of the lattice atoms

Incident ion path is almost unchanged Energy transfer is very small

Crystal structure damage is negligible


11/19

THEORY OF STOPPINGMECHANISM:

Total rate of energy loss = sum of energy lossper unit length due to each mechanism

Total projected range is given by

[ dE/dx ]total = Sn(E) + Se(E)


12/19

Stopping profile:

Ec: Critical energy

dE/dx : Energy lost


13/19

Peak concentration:


14/19

Dopant concentration can be expressedas a function of depth:

Where is the dose.

2

2

2exp

2 p

p

pR

RxR

xN


15/19

Ion channeling: When the ion direction is such

that it orients itself along majorcrystallographic direction, ionstravel a great distance beforestopping

Results in Deep junctions For each species such as B, P,

etc, there is a critical anglewhen this begins

Avoided by implanting at smallangle ~ 7 tilt

By using a thin screening layer ofPR or SiO2


16/19


17/19

Implant damage:

Light ions and heavy ions cause different

type of damage profiles


18/19

Annealing: Removes implant damage Restores crystalline structure Activates dopants Done usually at 850 1000C in N2 ambient Long times (>30min) help remove defects

completely But cause significant dopant diffusion

Secondary defects start forming at

about 500-600C

Removed at 850 1000C

Amorphous layer recrystallizes when

annealed at 600C for 30min (called

solid phase epitaxy)


19/19

RTA

High temperature attained in short time.

Two methods

Array of lamps

Laser

High throughput

No change in implantation profile.

2

2

( )

2( 2 )2

( , )2 ( 2 )

x Rp

Rp DtN x tRp Dt

e

Documents

l6 Ion Implantation