1. Prof. Jang-Ung Park (박장웅)
Chapter 8.
Ion Implantation
Prof. Jang-Ung Park (박장웅)

2. Doping Process using Thermal Diffusion
[ Doping by diffusion process ]
: Dopants(vapor type) can be introduced by diffusion from gas ambients in quartz-tube furnaces.
[ diffusion coefficients (diffusivity) of impurities in Si ]
Gas-type dopant precursors:
B2H6 (diborane), PH3 (Phosphine), AsH3 (Arsine)
carrier gas
(N2 or Ar)
: For this control in the diffusion doping, the surface must be clean and exposed to a uniform flow of dopants in the gas ambient; that is, the localized patches of contaminants or native oxide layers must be impeded.
: From a production-line technology standpoint, this requisite control of surfaces and gas flow is difficult to achieve reproducibly.

3. Ion Implantation
: Uniform and controlled introduction of dopants is possible.
: In ion implantation, a beam of dopant ions of fixed energy ( keV) is swept across the surface of the semiconductor.
: The ions have a sufficiently high velocity (~ 107 cm/s) to penetrate through the surface and come to rest within the semiconductor at depths of 10~100 nm below the surface.
Masking material :
SiO2, metal layers, Organic films, etc
: The penetration depths are determined by the energy of the incident ion and are not blocked by surface contamination or native oxide layers.

4. Ion Implantation [ Control of doping concentration ]
: In ion implantation, a current is measured directly with a current meter connected btw the sample and electrical ground.
QI : total number of implanted dopants/cm2
FI : flux of incident ions/cm2s
tI : the implantation time
qI : positive charge per ion (1.610-19C for singly ionized ions)
A : area of the implanted surface
Ion current (I) =
Total integrated charge (in coulombs) :
[ Advantages of Ion implantation ]
: Doping concentration can be controlled over a wide range ( dopants/cm3) by adjusting the ion beam current and implantation time.
: Selected areas can be implanted by use of masks.
- mask thickness (~ 200 nm) » ion-penetrated depth (~ 100 nm)
- After ion implantation, all the mask material is removed for next processing steps.
[ Disadvantage ]
: The penetration of energetic ions into the semiconductor damages the crystal structure, because the ions collide with semiconductor atoms and displace them off lattice site (amorphous layer can be formed).
 High-temperature annealing step is required after ion implantation.

5. Rapid Thermal Anneal (RTA) to restrict the diffusion
Ion Implantation
[ High-temperature annealing (~1000oC) after ion implantation ]
Conventional thermal Anneal causes drive-in diffusion.
(diffusion length is determined by annealing time.)
Rapid Thermal Anneal (RTA) to restrict the diffusion
(laser anneal, spike anneal, impulse anneal using flash lamps, etc)
Temperature (T)
Reaction rate
Crystal reordering : high activation energy (dominant at high Temp.)
RTA
Conventional
Tanneal
diffusion: low activation energy
(dominant at low Temp.)
Time
1 / Temperature (T-1)

6. Ion Implantation Systems
: First, dopant atoms (vapor) are introduced into the ion-source container.
: The dopant atoms in the source are ionized by energetic electrons emitted from a hot filament.
- By the electron-electron collision, the dopant atom is left to in an ionized state with a positive charge.
: The positive dopant ions enter the acceleration tube which is an insulating column with a vacuum inside.
- the dopant ions can be accelerated without suffering impact collisions with residual gas atoms in the column.
MI : mass of dopant atom
vI : velocity (~ 105 m/s)
VI : acceleration voltage (~100 kV)

7. Ion Range Distributions
: When an energetic ion penetrates a semiconductor, it undergoes a series of collisions with the atoms and electrons in the target.
: In these collisions, the incident particle losses energy at a rate dE/dx of a few to 100eV per nanometer.
 Penetration depth is related to the energy loss.
(x)
Range distribution:
E0 : incident energy of the ion
RP: penetration depth
dE/dx : energy loss rate
In the absence of crystal orientation effects,
Dopant concentration:
average concentration of implanted ions:

8. Ion Range Distributions
: The lighter ions such as B, with their higher velocities, have greater penetration in Si than the heavier ions such as As. ( As velocity is higher, the duration to transfer their energies decreases.)
- Mass effect RP
ΔRP
: The ranges of the ions increase nearly linearly with energy of the incident ions.
Rp  2.35ΔRp = Δxp
: SiO2 is often used as a masking material.
Here density of SiO2 is close to density of Si.
Rp(SiO2)  Rp(Si)
- Density of substrate

9. Ion Range Distributions

10. Ion Range Distributions
[ Comparison of dopant profiles ]
[ Transverse straggle in ion implantation ]
(x)
ΔRt : transverse straggle
: Transverse straggle is important in defining the penetration at the edge of a mask (ΔRt = ΔRp).

11. Ion Range Distributions
[ Channeling effect ]
: With single-crystal substrates of Si or GaAs, the orientation of the ion beam with respect to the crystallographic axes of the substrate can have a pronounced effect on the range distribution.
: When an ion trajectory is aligned along atomic rows, the positive atomic potentials of the line of atoms steer the positively charged ion within the open space or channels btw the atomic rows.
: These channeled ions do not make close-impact collisions with the lattice atoms and have a greater range than those of non-channeled ions.

12. Ion Range Distributions
[ How to avoid the Channeling effects ]
: The depth distribution of channeled ions is difficult to characterize under routine implantation conditions.
The substrate holders are often tapered so that the wafers are mounted 7o off normal: this minimizes channeling effects.
: Some ions originally incident angles greater than the critical angle can be scattered into a channeling direction. It is difficult to avoid channeling effects completely.
(ii) A thin amorphous layer is grown on top, before the ion implantation process.

13. Energy-Loss Processes
[ Two different mechanism of Energy loss (dE/dx) ]
Nuclear collision : energy is lost in displacements of the substrate atoms.
Electronic collision: the incident ion excites or ejects electrons from atomic orbitals.
A. Nuclear collision
can involve large discrete energy losses and significant angular deflection of the trajectory of the ion.
induces lattice disorder by the displacement of substrate atoms from their lattice position.
B. Electronic collision
involve much smaller energy loss per collision, and negligible deflection of the ion trajectory.
induces negligible lattice disorder.

14. Energy-Loss Processes
[ Rate of energy loss (dE/dx) versus (energy)1/2, showing nuclear and electronic loss contributions ]
- Nuclear collision : dominant at low energies (E < E2).
- Electronic collision: dominant at high energies (E > E2).
Ion
E1 (Si)
E1 (GaAs)
E2 (Si)
E2 (GaAs) B
3 keV 7 17 13 P 29
140 As 73
103
800 Sb
180
230
2000 Bi
530
600
6000
For light ions in Si (100-keV boron), electronic collisions are dominant.
For 100-keV As, nuclear energy loss can be significant.
Formation of the amorphous layer is less significant, if doses are too low or ions are too light.

15. Energy-Loss Processes
[ Rate of energy loss versus energy of As, P, and B ions in Si ]
- The rate of electronic energy loss is proportional to the ion velocity (is not dependent on Z and M).
- Coefficient ke does not depend strongly on the ion species.
(ke  1 (eV)1/2 per nm)
Subscripts 1 and 2 refer to ion and substrate.
Z and M are the atomic number and mass, respectively.
In the case where nuclear stopping predominates, the projected range Rp can be estimated by

16. Implantation Damage
As an energetic ion slows down and comes to rest in a substrate, it makes a number of collisions with the lattice atoms.
In these collisions, sufficient energy T may be transferred from the ion to displace an atom from its lattice site.
The displaced atoms can in turn displace other atoms, and so on – thus creating a cascade of atomic collisions.
Light ions (B): loses energy primarily in electronic collisions, and has only occasional large-energy transfer collisions along its path.
Heavy ions (Sb): loses energy primarily in nuclear collisions, and hence produces a dense cascade of collisions and hence disorder along the track (forming amorphous regions).
Formation of the amorphous layer is less significant if doses are too low or ions are too light. (H+ is preferred for RBS.)

17. [ Energy transfers in collisions btw two isolated particles ]
Implantation Damage
[ Energy transfers in collisions btw two isolated particles ]
For incident particle, mass : M1
velocity : v
energy : E0.
The energy T transferred in the collision equals E2 and the maximum energy transfer Tmax occurs for a head-on collision with  = 0.
at M 1 = M 2, Tmax = E0.
at M 1 « M 2, Tmax « E0.

18. Implantation Damage
The minimum energy Ed required to displace a Si atom from a lattice site is about 15 eV. (If a lattice atom receives less energy than Ed, it will not be displaced.)
If the energy transfer is greater than 2Ed, the displaced atom can displace another lattice atom.
The number of atoms ND displaced by an incident ion :
(for heavy ions)
: High-dose-rate implantation: the flux of energy ions is sufficient to cause an increase in the substrate temperature (~several hundred degrees).
 Thermal annealing of the disorder occurs (amorphous  crystals).

19. Epitaxial Growth of Implanted Amorphous Silicon
[ Solid Phase Epitaxy ]
By use of self-ion implantation, Si ions implanted into < > oriented Si.
: One forms an amorphous layer several 100 nm thick on a single-crystal substrate.
- When the sample is annealed in a furnace at ~ 550oC, the amorphous layer reorders on the underlying single-crystal substrate.
The regrowth rate is also strongly dependent on the orientation of the Si substrate.
( Samples with the < > direction perpendicular to the surface have the slowest growth rate, while < > samples have the fastest : related to the packing density.)
[ Ion Implantation for Gettering ]
Ar ions are implanted into the back side of the wafers.
- During high-temperature annealing, impurities that are already present in the wafer can diffuse and be captured (or gettered) at the grain boundaries.

20. Energy-Loss Processes
[ Rate of energy loss (dE/dx) versus (energy)1/2, showing nuclear and electronic loss contributions ]
- Nuclear collision : dominant at low energies (E < E2).
- Electronic collision: dominant at high energies (E > E2).
- Nuclear energy loss: dependent on probability of collision cross-section.
T : amount of energy transfer during collision
d : probability of collision (related to the cross-section and contact time)
: As E > E1, d decreases and thus (dE/dx)n also decreases.