Lec' 13 Ion implantation

1
Lec. 13 Ion implantation

• Ion implantation is a materials engineering
  process by which ions of a material can be
  implanted into another solid, thereby changing
  the physical properties of the solid.
• Ion implantation is used in semiconductor device
  fabrication and in metal finishing, as well as
  various applications in materials science
  research.
• The ions introduce both a chemical change in the
  target, in that they can be a different element
  than the target, and a structural change, in that
  the crystal structure of the target can be
  damaged or even destroyed.

2
Ion implantation

• Ion implanter is a high voltage particle
  accelerator producing a high-velocity beam of
  impurity ions that can penetrate the surface of
  the silicon target wafer.

Neutral beam trap/beam gate
10 -175 kV
-
Focus
Resolving aperture
Beam trap
Integrator
Analyzing magnet
-
Acceleration tube
Q
X-axis y-axis Scanner Scanner
Ion source
-
25 kV
Wafer
3
Ion implantation Ion source

• The ions are generated in an ion source which
  consists of an oven where the particles are
  vaporized, and the arc chamber, where the
  particles are ionized mainly by the bombardment
  of the atoms or molecules with electrons but also
  by atom/atom and atom/molecule collisions. Due to
  a well designed magnetic field in the arc
  chamber, which increases the path length of the
  electrons, and by the external generation of
  electrons, the electron/atom collision
  probability is increased, enabling a larger ion
  density. The figure shows one example of an ion
  source, a schematic description of a Harwell
  Freeman ion source .

• There are various types of ion sources differing
  in the design of the arc chamber and in the
  electron generation method.
• Penning ion source - Bernas ion source
• Radio-frequency gas ion source.
• Duoplasmatron -Microwave ion source
• The ion beam which leaves the particle source is
  divergent. It is focused by an electric or
  magnetic field lens to avoid ion loss to the wall
  of the beam line and to ensure that the ion beam
  reaches the wafer in a reasonably well defined
  focus.

4
Ion implantation Mass analyzing

• After leaving the ion source the beam contains a
  lot of atom and molecule species in several
  charge states.
• The desired dopant is separated from the
  remaining elements by an analyzing magnet.
• Mass spectrometry (previously called mass
  spectroscopy or informally, "mass-spec" and MS)
  is an analytical technique used to measure the
  mass-to-charge ratio of ions. It is most
  generally used to find the composition of a
  physical sample by generating a mass spectrum
  representing the masses of sample components.
• .

• One way of mass analyzing, and perhaps the
  easiest to understand is Time-of-flight (TOF)
  analyzer. Where an electric field is used to
  accelerate the ions through the same potential,
  and then measures the time they take to reach the
  detector. If the particles all have the same
  charge, then their kinetic energies will be
  identical, and their velocities will depend only
  on their masses. Lighter ions will reach the
  detector first.

• The detector is the final element of the mass
  spectrometer. The detector records the charge
  induced or current produced when an ion passes by
  or hits a surface. In a scanning instrument the
  signal produced in the detector during the course
  of the scan versus where the instrument is in the
  scan (at what m/q) will produce a mass spectrum,
  a record of ions as a function of m/q.

5
Ion implantation Mass analyzer

• The ions are accelerated to a high speed by an
  electric field after which they are directed into
  a magnetic field.
• The magnetic field applies a force to each ion
  perpendicular to the plane defined by the
  particles' direction of travel and the magnetic
  field lines.
• This force deflects the ions (makes them curve
  instead of traveling in a straight line) to
  differing degrees depending on their
  mass-to-charge ratio.
• The lighter ions are deflected more than the
  heavier ions because according to Newton's second
  law of motion the acceleration of a particle is
  inversely proportional to its mass. Thus the
  magnetic field deflects the lighter ions more
  than the heavier ions.
• The detector measures the deflection of each
  resulting ion beam. From this measurement, the
  mass-to-charge ratios of all the ions produced in
  the source can be determined.
• From this information it is possible to
  determine the chemical composition of the
  original sample (i.e. that both sodium and
  chlorine are present in the sample) and the
  isotopic compositions of its constituents (i.e.
  whether the ratio of 35Cl to 37Cl has been
  changed by some process).

6
Ion implantation Mass analyzer

• The mass analyzing magnet in the implanter is
  used to filter out the undesired ions, hence the
  detector part is removed a slit is placed at the
  location where the desired ion would go through
  according to its mass-charge-ratio.
• When no magnetic field applied, a charged
  particle moving with a velocity v through a
  magnetic field B will experience a force F
• The equation of motion tells that if B is normal
  to v then there is no change in the velocity
  components along the movement direction.

7
Ion implantation Mass analyzer
x
Slit plane

• The radius r, and hence the position at the slit
  plane, changes with the magnetic field B, the
  particle mass-charge ratio and the initial
  velocity.
• The high voltage accelerator column adds energy
  to the beam (up to 5 MeV) and accelerate the ions
  to their final velocity. Hence, the initial
  velocity is fixed.
• In order to select a specimen with a particular
  mass-charge-ratio, the strength of the magnetic
  field is adjusted through the applied dc current.
  
• The scanning electrodes are used to scan the beam
  across the silicon wafer to produce uniform
  implantation and achieve the desired dose.
• Then the deflection is proportional to the
  strength of the electric field.

r
z
Ex -
8
Lect. 14 Ion implantation

• The Silicon wafer is maintained at a good contact
  with a metallic target holder so electrons can
  readily flow from or to the wafer to neutralize
  the implanted ions.
• The total dose is then measured from the total
  current flow
• where I is the current
  in Amperes, T is the total implantation time, A
  is the wafer area and m is 1 for singly ionized
  species and 2 for doubly ionized species.
• The target wafer is maintained at low temperature
  to prevent undesired spread of
• impurities by diffusion.
• In principle any element can be implanted as
  long as it can be ionized.
• The cost of the ion implantation system is very
  high but the advantages of the system outweigh
  the cost.

9
Ion implantation

• For an ion with initial energy E moved by dr in a
  solid material, then the totalenergy at the
  output is E-T, where T is the energy lost during
  the process.
• The loss is due to two mechanisms, nuclear
  collisions and electronicinteraction.
• Collision and electronic interaction is a
  probabilistic process, hence we define the
  concept of cross section.
• If particles are flowing through a unit area with
  a certain probability of colliding with a
  nuclear , hence the cross section is an area
  inside thatunit area where the probability of
  colliding is assumed 100 and outsidethe cross
  section the probability of colliding is 0.
• Then the energy lost due to the dr displacement
  of ions with concentration of N atoms per unit
  volume is
• The suffixes e and n refer to electronic
  interaction and nuclear collision respectively.
• The amount of energy loss, T, can be in any range
  between 0 and Tmax, hence
• S is the individual ion energy loss cross section

d?n
E
dr
E-T
10
Ion implanation

• Upon each interaction the ion losses part of its
  energy till it comes to rest at a certain
  location Rp.
• In solving this statistical type of problem, an
  easy way is to consider a one dimensional case
  and solve for the vertical ion distribution.
• Gaussian distribution for the ions can be
  proposed (Linhard, Scharff and Schiott, LSS
  theory).

N(x)
Np e-2Np

• Rp is called the projected range defined as the
  average distance an ion travels before it stops.
• The spread of the ion distribution, DR, is
  referred to as the straggle.
• The implanted dose can be calculated as
• If the implant is completely contained within the
  silicon.

x
Rp DRp DRp

• Typical dose rages from 1010/cm3 to 1018/cm3
  which is impossible to produce using diffusion.
• Hence, ion implantation often used to replace the
  predeposition step in two-step diffusion.

11
Ion implantation

• The ion range and struggle are calculated
• For the nuclear collision stopping, a classical
  mechanics model for moving particles can be used

12
Ion implantation
r
Increasing the velocity, vo, the interaction time
? is reduced and hence the integration value. At
very high speed the collision stopping is not
dominant.
13
Ion implantation

• The other process that causes ion stopping is
  the electron stopping power.
• When interacting with ions in the target
  material, the ion experiences a drag force that
  is proportional to ion velocity .
• Unlike the nuclear collision, electronic
  interaction does not change the direction of the
  ion, just reduces its energy.

Seke E1/2 Ke107 (eV)1/2/cm for silicon
14
Ion implantation
Projected range for Si and SiO2 are identical
15
Lect 15 Ion implantation
ke for silicon is 107 eV1/2/cm
16
Ion implantation
Impurities spreading from one point. (Monte Carlo
calculations)

• Selective implantation
• To selectively implant impurities in specific
  regions of the wafer, a barrier material is used
  Silicon-dioxide or silicon-nitride.
• Although blocked, the ions implant beneath the
  barrier layer.
• The spread of the impurities beneath the barrier,
  ??R- is referred to as transverse-struggle.

17
Ion implantation

• The ions penetrate the SiO2 and Si in very
  similar ways, hence, the thickness of the barrier
  layer should guarantee an Si-SiO2 surface
  concentration small enough not to affect the
  background concentration, NB.
• SiO2 and Si3N4 are common barrier materials
• Ion implantation is low temperature process.
  Hence, photo-resist can be used as a barrier.
• Compared to SiO2, Si3N4 requires 0.85 of the
  thickness of the dioxide barrier.
• Photo-resist, however, requires1.8 of that of
  SiO2.

18
Ion implantation

• The junction depth, xj,is calculated as
• When the peak of the implantation is positioned
  atthe surface of the Si, Irvins curves can be
  used directly to calculate the sheet resistance
  as it was the case of diffusion.
• If the profile is impeded underneath the surface

19
Ion implantation

• If the dose is high enough the ions knock atoms
  out of the silicon lattice resulting in amorphous
  implanted region.
• The dose that causes the damage is referred to as
  the critical does.
• The heavier the impurity the lower the dose
  required.
• The lattice damage can be fixed by an annealing
  step that follows the implantation.
• The annealing process heating the wafer for 30
  min at temperatures between 800 and 1000 oC.
• At these temperatures, silicon atoms move back to
  the lattice and the impurities become
  electrically active, except for concentrations
  above 1019cm-3.
• The annealing process causes the impurities to
  diffuse, and hence the impurities distribution
  spreads.

20
Ion implantation

• Along certain crystal directions, tubular holes
  exist allowing the ion to travel long distances
  without undergoing high angular collisions.
• The ion would eventually stop due to electronic
  stopping power. Hence the traveled distance is
  much higher than the range predicted from the LSS
  theory.
• Tunneling can be reduced by tilting the wafer,
  screening with a silicon dioxide layer, or
  damaging the first layer. The last two introduce
  an amorphous layer with random atoms orientation.
• Usually a combination of tilt(7o) and
  rotation(30o) minimizes the tunneling as the
  crystal appears less structured.

Tubular hole lt110gt