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Chapter 8 Ion implantation
NE 343: Microfabrication and thin film technology
Instructor: Bo Cui, ECE, University of Waterloo; http://ece.uwaterloo.ca/~bcui/
Textbook: Silicon VLSI Technology by Plummer, Deal and Griffin
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Random tilt and rotation
Looking at Si at different orientations
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Channeling effect
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Critical Angle: defined as the maximum angle between the ion and the channel for a glancing collision to occur.
Where Z1 is the incident ion atomic number, Z2 is the target atom atomic number, E is the acceleration energy in keV (voltage), and d is the atomic spacing in the direction of the ion path in angstroms.
Note: channeling is more likely for heavy ions and lower energies.
Channeling effect: critical angle
But another source says the opposite:
The effect is particularly pronounced when implanting light atoms on axis into a heavy matrix sine the ion’s atomic radius is much less than the crystal spacing.
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Phosphorus impurity profiles for 40keV ion implantations to silicon at various angles from the <110> axis.
Impurity distribution due to channeling effect
Even (the complicated) Pearson profile fails in the case of crystalline silicon where ion channeling may occur.
The resultant profile can be described by a “Dual-Pearson” distribution.
long “tail”
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A thin screen oxide which is amorphous is often used, causing some randomization of incident beam before it enters the lattice.
Most IC implantation is done off axis. A typical tilt angle is 7o.
Methods to reduce channeling effect
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Destroy the lattice before implantation
Methods to reduce channeling effect
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Example (channeling)
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Chapter 8 Ion implantation
NE 343: Microfabrication and Thin Film Technology
Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca
Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin
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Ion – substrate interaction
The ions are stopped at random positions, mostly not in crystalline sites, so not active as dopant (need anneal to active them).
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Ion implantation energy loss mechanisms
Nuclear stopping, crystalline Si substrate damaged by collision.
Electronic stopping, electronic excitation creates heat.
LSS theory: in 1963, Lindhard, Scharff and Schiott proposed that the energy loss of incident ion can be divided into two independent process, namely nuclear stopping and electronic stopping. Total energy loss is the sum of the two processes.
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Nuclear and electronic stopping power
Energy loss rate
(N is target atom density)
Nuclear stopping power
(unit: eV cm2)
Electronic stopping power
Ion range
Light ions/at higher energy: more electronic stopping
Heavier ions/at lower energy: more nuclear stopping
H+: electronic stopping dominates
B+: electronic stopping dominates
As+: nuclear stopping dominates
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Nuclear stopping power: Coulomb scattering (assume elastic)
Incident ion interacts with nucleus of stationary ion. b = impact parameter
The angles depend on masses and on b.
Maximum energy loss: is when b = 0, φ = 0:
To first order approximation:
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Nuclear stopping power: Coulomb scattering (assume elastic)
At 100keV and for Z=15, ion velocity ≈ 106 m/s! 1000× faster than speed of sound in solids.
So ion is far past nucleus before nucleus can displace in response to Coulomb force.
As fast particle have less interaction time with the scattering nucleus, the nuclear energy loss is small at very high energies/velocity.
Thus the nuclear energy loss tends to dominant towards the end of the range when ion has lost much of its energy and where nuclear collisions produced most of the damage.
Light ion slowed down at the end of range
Heavy ion is slow (v ∝ M-1/2), more loss
There are also inelastic collisions that transfer energy.
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Electronic stopping power:
also Coulomb interactions, but inelastic
Non-local: ion experiences drag due to “free” or polarizable electrons.
Local: passing ion causes internal electronic transitions => energy and moment transfer
Because electrons can follow fields up to optical frequencies (velocities of 105 m/s - 100 times faster than phonons), electronic losses dominate at higher ion velocities.
ion velocity => charge separation, drag
incident ion attracts electron polarization
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Electronic stopping power
Charge and momentum exchange due to local electronic interaction, ion changes direction
Drag force exerted on a moving ion in a dielectric medium, ion moves in same direction
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Electronic and nuclear stopping power: summary
Electronic collisions dominate at high energies.
Nuclear collisions dominate at low energies.
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E2: energy where electronic stopping power is equal to nuclear stopping power.
Electronic and nuclear stopping power
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Electronic and nuclear stopping power
B+: light, electronic stopping dominates
As+: heavy, nuclear stopping dominates
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Chapter 8 Ion implantation
NE 343: Microfabrication and Thin Film Technology
Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca
Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin
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EOR damage
Damage at end of range (EOR)
The main disadvantage of ion implantation is the production of lattice damage (vacancies and interstitials, or V/I) which may evolve from simple point defects into complex dislocations or voids.
Eventually, implantation leads to an amorphous silicon structure (can be used for channeling reduction).
Most damage can be repaired by annealing. However, annealing cycles of 30 min at close to 1000oC can cause considerable spreading of the implant by diffusion.
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(Si)Si→SiI + SiV
Damage to the lattice: target atom displacement
Energetic incident ions collide with target atoms, leading to their displacement.
The result is an interstitial atom and a vacancy, V-I pair = Frankel defect.
The displaced atoms may have energy high enough to further displace other target atoms along its path.
I: interstitial; V: vacancy
Vacancy
Self interstitial
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Number of displaced target atoms
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Amorphization
Critical dose
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Damage distribution
More crystalline damage at end of range, Sn > Se
Less crystalline damage, Sn < Se
Most damage is done by nuclear interactions (nuclear stopping)
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Damage distribution
For light ion (lighter than target), small energy transfer to target atom for each collision, generate few displaced target atoms, and ion scatted at large angle. Low density non-overlapping damage, but over large area with a saw-tooth shape.
For heavy ion, large energy transfer for each collision, small scatter angle. The displaced atom can further displace other target atoms. Small range, large damage density over small volume.
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Damage density distribution
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Chapter 8 Ion implantation
NE 343: Microfabrication and Thin Film Technology
Instructor: Bo Cui, ECE, University of Waterloo, bcui@uwaterloo.ca
Textbook: Silicon VLSI Technology by Plummer, Deal, Griffin
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As ion
Si atom
Annealing repair damage and activate dopants
After implantation, we need an annealing step, usually under Ar, N2 or vacuum.
A typical ~900oC, 30min will:
Restore silicon lattice to its perfect crystalline state - silicon atoms can move back into lattice sites at these temperatures.
Put dopants into Si substitution sites for electrical activation - nearly all of the implanted dose becomes electrically active except for impurity concentrations exceeding 1019/cm3.
Restore the electron and hole mobility – now that the lattice becomes perfect again.
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Due to the high activation energies required to annihilate defects (~5eV), it is often easier to regrow the crystal from an amorphous layer via SPE (activation energy ~2.3eV in Silicon) than it is to anneal out defects. Thus, two schemes for implants are used:
Implant above the critical dose and use low temperature anneal to regrow material.
Implant below the critical dose and use high temperature anneal to get rid of defects.
Solid state epitaxy (SPE): when substrate has been rendered amorphous, the crystallinity is repaired by SPE, where crystal reforms using the underlying undamaged substrate as a template. Most of impurities are incorporated into the growing lattice .
Solid phase epitaxy
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Stable defects formation near a/c interface
If the substrate is amorphous, it can re-grow by solid state epitaxy (SPE).
But, the tail of the damage beyond the a/c (amorphous/crystalline) interface can nucleate stable, secondary defects (defects caused directly by implanted ion are primary defects), and cause transient enhanced diffusion (TED).
TED is the result of interstitial damage from the implant enhancing the dopant diffusion for a brief transient period.
It is anomalous diffusion, because profiles can diffuse more at low temperatures than at high temperatures for the same Dt.
TED is the biggest single problem with ion implantation because it leads to huge enhancements in dopant diffusivity and difficulty in achieving shallow junctions.
Physically based understanding of TED has led to the methods to control it (rapid thermal annealing, or RTA).
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Rapid thermal processing/annealing
Dopants can diffuse during high temperature anneal (activation energy ~3-4eV)
To minimize this unwanted diffusion, one can use Rapid Thermal Processing (RTP) or Rapid Thermal Anneal (RTA).
RTA is extremely important for shallow junction devices.
Applied Materials 300mm RTP System
Rapid heating source:
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Small desktop RTA system mostly intended for research (wafer size 100 mm)
Max temp 1000°C, heating rate 200°C/s, cooling rate 40°C/s at 1000°C
A commercial RTA system
This slide set is not covered in the course, and they are not refined/polished, may have some errors.