Si wafer - The Accelerator Laboratory at University of Helsinki

Strålningsskador vt 2014
Säteilyvauriot kl 2014
Radiation Damage Spring term 2014
10. Macroscopic/application
consequences of irradiation
10.5 Beneficial electronic properties
n Basic semiconductor physics teaches that to get any
beneficial electrical functionality out of Si, one needs to dope
it with impurity dopant (dopning / douppaus tai seostus)
atoms
n
Most common ones B as acceptor of
electrons, P and As as donor
n Basic idea is simple: introduce atoms
into Si lattice places
n But achieving this with a uniform
concentration is actually not so easy
n
[Pic: http://sv.wikipedia.org/wiki/
Dopning_%28fysik%29]
Diffusion was first used, but does not lead to an uniform
concentration
n Since the 1970’s, it has been done industrially by ion
implantation
Strålningsskador 2014 – Kai Nordlund
The Si implantation process physics, 1
n The Si dopant implantation process actually proceeds in
many steps and involves complex defect physics
n
Here we present the directly implantation-relevant parts,
neglecting all the other lithographic, deposition, etching, etc.
methods that are of course also crucial for processing
n Step 1: ion implantation of entire Si wafer (300 mm in
industry)
Accelerator
Energies ~ keV
[Ventra, ed.: Intro to nanoscale science & techn.]
Strålningsskador 2014 – Kai Nordlund
5 – 300 mm
The Si implantation process physics, 2
n Initially the implantation leads to defect buildup, measurable e.g.
by Rutherford backscattering/channeling (RBS-C) (Rutherfordbakåtspridning/kanalisering/ Rutherford-takaisinsironta/kanavointi)
n Measures fraction of He scattered backwards due to defect atoms
in crystal channels
n After a fluence of the
order of 1015 ions/cm2
the Si typically amorphizes
(= 100% displaced atoms in
RBS-C)
n
More accurate value: nuclear
energy deposition of 6x1023
eV/cm3 => amorphization
[Dennis and Hale, J. Appl. Phys.
49 (1978) 1119]
Strålningsskador 2014 – Kai Nordlund
[K. Nordlund et al, Phys. Rev. B 52, 15170 (1995)]
The amorphization mechanism of Si?
n Although any number of experiments has shown the amorphization,
the mechanism was long debated
n There are two basic models:
Each ion amorphizes
”pockets” in Si directly
Overlap of am. pockets
leads to full amorphization
n 1. Heterogeneous or
”Direct impact”
amorphization
n 2. Homogeneous or
”defect stimulated”
amorphization
Strålningsskador 2014 – Kai Nordlund
Irradiation produces point
defects homogeneously
When the defect density
exceeds some threshold,
the lattice collapses to an
amorphous state
The amorphization mechanism of Si?
n TEM experiments show that
direct amorphization of Si is
possible: amorphous pockets
produced by a single ion have
been seen, first clearly by
Ruault
n However, even Ruault
concludes that these pockets
can not fully explain
amorphization of Si
n Unfortunately point defects not
visible in TEM due to migration
Strålningsskador 2014 – Kai Nordlund
[Ruault et al, Phil. Mag A 50 (1984) 667]
The amorphization mechanism of Si?
n MD of prolonged irradiation can reproduce the amorphization
Strålningsskador 2014 – Kai Nordlund
[Nord et al, Phys. Rev. B 65, 165329]
The amorphization mechanism of Si?
n Detailed analysis of this, as well as other MD work, showed
that in fact that the amorphization of Si can usually not be put
into the single “heterogeneous” or “homogeneous” box, but
that numerous mechanisms are active at the same time
n I have counted up to 8 known mechanisms (detailed
understanding of these not necessary on this course:
Thermal Athermal
1.
2.
3.
4.
5.
6.
7.
8.
Direct impact amorphization
Defect-stimulated ”heterogeneous”
Cascade-induced amorphous zone growth at a-c interfaces
Cascade-induced partial recrystallization at a-c interfaces
Recoil-induced defect recombination
I-V recombination by thermal defect migration
Thermal annealing of damage
Recrystallization of a-c interfaces by mobile defects
Strålningsskador 2014 – Kai Nordlund
The Si implantation process physics, 3
n The desired dopants are in the implanted layers, but only
crystalline Si works electrically as desired => recrystallization
needed
n Standard recrystallization is nowadays a very rapid so called
rapid thermal annealing or flash anneal where the sample is
heated with a intense rapid light burst (flash) up to ~1000 oC
for timescales of a few seconds or less
n This recrystallizes the Si in a manner that the dopant end up
on perfect lattice positions at about the implantation depth
n
Although especially for boron a variety of radiation-enhanced
diffusion (RED) called transient-enhanced diffusion (TED) can
move the dopants beyond the implant depth.
Strålningsskador 2014 – Kai Nordlund
The Si implantation process physics, 4
n But the substages of annealing are also complex and fairly
well understood after intense research in the 1990’s and
2000’s
n Before the material is fully recrystallized, it undergoes several
stages or reduced defect density, illustrated on the following
slides
n
A few good more recent references: Cristiano et al, Nucl. Instr.
Meth. B 216 (2004) 46; Colombeau, Appl. Phys. Lett. 83 (2003)
1953; for BCA+KMC modelling Pelaz et al, J. Appl. Phys. 96
(2004) 5947; Marquez et al, Mater. Sci. Engr. B 124 (2005) 72
Strålningsskador 2014 – Kai Nordlund
The Si implantation process physics, 4
1. Implantation
Strålningsskador 2014 – Kai Nordlund
Si wafer
The Si implantation process physics, 5
1. Implantation
2. Amorphization
Strålningsskador 2014 – Kai Nordlund
Si wafer
The Si implantation process physics, 6
1. Implantation
2. Amorphization
3. Recrystallization by high T
=> small defect clusters remain
Strålningsskador 2014 – Kai Nordlund
Si wafer
The Si implantation process physics, 7
1. Implantation
2. Amorphization
3. Recrystallization by high T
4. Small clusters emit interstitials
which coalesce to form 311 rodlike
defects and stacking faults
Strålningsskador 2014 – Kai Nordlund
Si wafer
The 311 defect in Si
n The mentioned 311 defect, also known as the rod-like defect,
is a special defect in tetrahedral semiconductors
n It contains a zig-zag pattern of interstitial atoms ordered on
311 planes such that all atoms have tetrahedral coordination
and is hence pretty stable
n
It is an interstitial-type linear defect but not a dislocation!
Experimental TEM image
[Eaglesham et al, APL 65 (1995) 2305]
Strålningsskador 2014 – Kai Nordlund
Atomic structure deduced from HRTEM experiments
[Parisini and Bourret, Phil. Mag. A 67 (1992) 605;
Nordlund, J. Appl. Phys. 91 (2002) 2978]
The Si implantation process physics, 8
1. Implantation
2. Amorphization
3. Recrystallization by high T
4. 311’s and SF’s
5. 311’s emit interstitials and
stacking faults unfault to form
a low concentration of perfect
dislocations => device works
as intended finally!
Strålningsskador 2014 – Kai Nordlund
Si wafer
10.6. Optical properties
n As discussed in section 6, single point defects in ionic crystals
affect the colour of materials!
n
Atom-level effects have directly a macroscopic optical outcome
n Among the multitude of other optical effects of irradiation, we
will now review briefly another one relevant to modern
optoelectronics
Strålningsskador 2014 – Kai Nordlund
Example: speeding up VCSEL lasers
n Modern optical telecommunication relies to a great extent on
so called VCSEL (Vertical-cavity-surface-emitting-lasers)
lasers that allow sending GHz frequence pulses to optical
fibers
Metal contact
Upper Bragg reflector
Lower Bragg reflector
Substrate
Strålningsskador 2014 – Kai Nordlund
SESAM mirrors and their irradiation
n One of the crucial part of the VCSEL is the Bragg reflector
structure made with a Semiconductor saturable absorber
mirror (SESAM)
[Short intro to SESAM: http://www.ulp.ethz.ch/research/Sesam/WhatIsaSESAM]
n For increasingly high frequencies, it is crucial to achieve
ultrafast operation of the SESAM’s
n Ion irradiation can be used to speed up their operation!
2. InGaAs QW
3. GaAs QW
4. InGaAs QW
5. GaAs
6. AlAs DBR
Increased speed
1. GaAs
7. GaAs DBR
8. GaAs
9. n-GaAs sustrate
1 µm SESAM
Strålningsskador 2014 – Kai Nordlund
[Dhaka et al, Semic. Sci. and Techn. 21, 661 (2006); Electronics Lett. 41, 23 (2005)]
The reason to the speedup
n By introducing defects that act as traps for carriers, the
irradiation reduced the free carrier lifetime and hence speeds
up the possible operation speed of the SESAM
n How this happens on the molecular level is not clear, but
heavy ions give a stronger decrease in decay time
n
This indicates that defect clusters are more efficient in trapping
the charge carriers
Strålningsskador 2014 – Kai Nordlund
[C. Björkas, K. Nordlund, et al, J. Appl. Phys. 100, 053516 (2006).]
10.7. Magnetic effects
n Irradiation can also be used to modify the magnetic effects of
materials
n Defects can have magnetic
moments, and irradiation
Fluence
varying with
y coordinate
change of composition
(ion beam mixing) can lead
to changes in ferromagnetism
n Recent example: Pt / Co / Pt
multilayers structures were
irradiated with 30 keV Ga ions
and magnetic properties
measured
Strålningsskador 2014 – Kai Nordlund
[Maziewski et al, Phys. Rev. B 85, 054427 (2012)]
Magnetic effects: example for Pt/Co/Pt multilayers
n The irradiation in this case
changed the magnetisation
in several stages.
n E.g. for 3.3 nm Co
thickness, ferromagnetic
hysteresis loops opened up
twice(!)
n Other measurements (not
shown here) also showed
the magnetization changed
from the in-plane to the outof-plane direction
n Reasons not known but
likely related to ion beam
mixing
Strålningsskador 2014 – Kai Nordlund
10.8. Chemical effects
n Irradiation of materials tends to change their surface structure
and hence often also the surface chemistry
n An easy-to-understand outcome can be a change in the
hydrophobocity (hydrofobicitet / hydrofobisuus) of a surface,
i.e. how strongly it repels water
n Example: 25 eV Ar ion irradiation of mica after aging
n
Reasons attributed to sputtering of K+ ions from the mica
surface which affects the hydrocarbon and water abnsorption
- Detailed mechanism not known, though
Strålningsskador 2014 – Kai Nordlund
[Keller et al, J Chem Phys. (2011) 134(10):104705]
Plasma ion treatment of wood against termites
n Ion treatment can also change the surface properties of wood
n A particularly non-obvious possible advantage is for protection of
wood against termites. Blantocas and Al-Aboodi [Wood and fiber
science, 10/2011; 43(4): 449-456] report that 1 keV H2 ion
bombardment of pieces of wood
dramatically reduces the rate
at which termites eat the wood!
n
Reason attributed to increased
hydrophobicity making the wood
less wet and thus less attractive
to termites
Strålningsskador 2014 – Kai Nordlund
Chemical effects: radioactive ion implantation
into stents
n In treating heart and other diseases related to blood (and
other bodily fluids) flow, a standard technique is to use so
called stents (stent / stentti), a mesh typically of metal
inserted into blood vessels to prevent them from closing up
n This is a common
part of modern surgery
n However, in some
cases blood tends to clog
up back on the stent
http://en.wikipedia.org/wiki/File:SEMS_endo.jpg
Strålningsskador 2014 – Kai Nordlund
Chemical effects: radioactive ion implantation
into stents
n Somewhat startlingly, ion implantation or plasma immersion
ion implantation of radioactive isotopes (!) can be used to
improve on the issue
n The radioactive isotope slowly naturally decays, and the
radiation from the decay keeps the stent in a state to where
blood does not stick
[Example ref: Hehrlein, Circulation. 1996; 93: 641-645; Kelly Pike, US Patent US 6224536 B1 ]
Strålningsskador 2014 – Kai Nordlund
10.9 Biological effects of irradiation
n Irradiation is used in many ways in biology
n In addition to the cancer treatment (see section 7) it is
routinely used to kill bacteria, i.e. sterilize stuff
n
Including food [http://www.epa.gov/rpdweb00/sources/food_irrad.html]
n A more extreme example: since the anthrax letter terror
campaign in the U.S. in 2001, the postal service started
electron irradiating government mail sent in the Washington
capital district area up to doses of 56 kiloGray to kill any
anthrax spores
n
Huge dose, remember that 2 Grays is lethal to humans
n Has the downside that the letter paper itself tended to turn
yellowish…
n Official source:
http://www.epa.gov/rpdweb00/sources/mail_irrad.html
Strålningsskador 2014 – Kai Nordlund
Biological effects of irradiation
n A few groups in the world (including Surrey / Karen Kirkby
and the Bundeswehr University in Germany / Günther
Dollinger) have special facilities that allow irradiating living
cells
n By combining irradiation by single ions with biological labeling
techniques and microscopy, it is possible to follow in situ how
an ion irradiation event modifies living cells!
[http://www.e12.ph.tum.de/groups/rim/SNAKE_Zellmikrobestrahlung_englisch.htm]
Strålningsskador 2014 – Kai Nordlund
Biological effects of irradiation on living cells
n The experiments allow detecting how DNA is damaged and
repaired associated with ion irradiation
n One of the observations is that during pulsed beam
irradiation, the damage
and repair does not come
directly by the irradiation, but
a bit later and not at the same
position
n
Likely explanation: actual
DNA damage more likely to
be done by radicals formed
in the cell nucleus due to
the irradiation
n Repair done by biological processes much later (~15 minutes)
[Hauptner et al, Matematisk-fysiske Meddelelser 52, Royal Danish Academy of Sciences and
Letters, Copenhagen (2006) p. 59-85; Radiat Environ Biophys (2012) 51:23–32]
Strålningsskador 2014 – Kai Nordlund
Biological effects of irradiation on living cells
n Direct citation to give a taste:
n
Details not important for this course
“Left side: fluorescence micrographs (optical
slices) of HeLa cell nuclei irradiated with single
29 MeV 7Li or 24 MeV 12C ions, respectively.
The direction of the ion tracks encloses an angle
of 10◦ with the image plane. 15 min after
irradiation the cells were fixed and 53BP1 DNA
repair factor accumulations were visualised by
indirect immunofluorescence (green signal)…
Right side: Corresponding three-dimensional
reconstructions of image stacks were performed
for the immunofluorescence signal using
rendering software. The foci structures (red
colour) of the DNA repair factor accumulations
along the ion tracks reveal the distribution of DSB
sites at the time of cell fixation.”
[Hauptner et al, Matematisk-fysiske Meddelelser 52, Royal Danish
Academy of Sciences and Letters, Copenhagen (2006) p. 59-85; ]
Strålningsskador 2014 – Kai Nordlund
Irradiation of seeds to make mutations
n Irradiation of seeds is a fairly commonly used technique to
induce mutations that may be biologically useful!
n
Mutation breeding has long been standard part of biology, and
irradiation induces mutations, so this makes sense…
n
Cotton, rice, etc.
n
Ion irradiation may be more efficient
than gamma or e because it induces
bigger DNA strand breaks
n Can even produce new kinds of
fllower shapes!
[http://mitizane.ll.chiba-u.jp/metadb/up/thesis/Yamaguchi_Hiroyasu.pdf]
Strålningsskador 2014 – Kai Nordlund
[http://www.taka.jaea.go.jp/tiara/tiara/e_page/2Biology.html]
Final word: cost
n Cost is a serious issue in making practical applications…
n
And related to this time of processing => time is money
n Ion beams perceived to be extensive, but not necessarily:
n
Ion guns are cheap and small, and for many of the effects
presented, a few keV is enough
n Plasma immersion ion implantation (PIII, PI3) can achieve very
high fluxes and hence be fast, and works also for non-flat
systems
n Even swift heavy ions are used in application [Klas Hjort]:
fluence is small => time is small
n For applications demanding both high-energy and highfluence, cost remains an issue
n Nevertheless, as the examples in this sections have shown,
ion beams have a multitude of practical applications, and
many more can be coming!
Strålningsskador 2014 – Kai Nordlund