Strength degradation of silicon diffusion-doped with gold

Article abstract of DOI:10.1134/S0020168509040013

Diffusion doping with gold is shown to reduce the microhardness of silicon single crystals. Oxygen precipitation suppresses this process because diffusing interstitial gold atoms, Au_i, interact with oxygen and become captured by growing precipi

Full text of DOI:10.1134/S0020168509040013

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 4, pp. 343–346. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © D.I. Brinkevich, S.A. Vabishchevich, N.V. Vabishchevich, V.S. Prosolovich, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 4,
pp. 389−392.
Strength Degradation of Silicon Diffusion-Doped with Gold
a
b
b
a
D. I. Brinkevich , S. A. Vabishchevich , N. V. Vabishchevich , and V. S. Prosolovich
a
Belarussian State University, pr. Nezavisimosti 4, Minsk 220030, Belarus
Polotsk State University, ul. Blokhina 29, Novopolotsk, 211440 Belarus
b
e-mail: brinkevich@bsu.by
Received January 24, 2008
Abstract—Diffusion doping with gold is shown to reduce the microhardness of silicon single crystals. Oxygen
precipitation suppresses this process because diffusing interstitial gold atoms, Au , interact with oxygen and
i
become captured by growing precipitates.
DOI: 10.1134/S0020168509040013
INTRODUCTION
apical angle of 136°. The indentation load was varied
from 0.5 to 2 N.
Silicon is the basic material of modern microelec-
tronics. In addition, it is a key component in the fabri-
cation of sensors, micro- and nanoelectromechanical
systems, and other hybrid nanodevices. In this context, out surface effects.
the study of the influence of various processing steps on
its mechanical properties (Young’s modulus, hardness,
wear resistance, and others) is of special importance.
The choice of the load was dictated by two factors:
1. The indent depth must be sufficiently large to rule
2
. Indentation must produce no cracks.
At a 0.50-N load, the indent depth was ~1µm, which
minimized the effect of surface processing. At the high-
est load, the fraction of cracked indents, unsuitable for
H determination, was within 10%. In each experiment,
at least 50 indents were placed on the wafer surface. We
measured the two indent diagonals, and the average
was used to determine H [3].
Gold doping is widely used to reduce the carrier
lifetime in silicon and, accordingly, to enhance the
speed of semiconductor devices [1]. Gold-doped sili-
con is the key material for high-sensitivity IR detectors
[
2]. Its strength, however, is essentially unexplored.
Such studies are of high current interest because, in
subsequent steps of semiconductor device fabrication
The results were analyzed using statistical methods
[
4]. The microhardness data were found to follow a nor-
(
oxidation, diffusion, chip bonding, and others), silicon
mal (Gaussian) distribution. The uncertainty in our H
measurements was 2% (at a 95% confidence level).
wafers are exposed to thermal and mechanical influ-
ences, which give rise to buckling and microcracking of
the wafers. These processes reduce the yield of devices
As a gold diffusion source, we used a film produced
to specification and are determined in large part by the on the surface of a 2-mm-thick silicon wafer by chem-
strength of the implanted wafers.
ical deposition from a potassium dicyanoaurate solu-
tion. Diffusions were carried out in a hydrogen atmo-
sphere at 925°C over a period of 5 h. Next, the surface
The objective of this work was to study the effect of
Au diffusion doping on the microhardness of silicon.
Effect of heat treatment at 925°C for 5 h on the interstitial
oxygen concentration in silicon
EXPERIMENTAL
Silicon wafers were prepared from Czochralski-
grown ingots. In our experiments, we used dislocation-
free KEF-20 (phosphorus-doped, nominal resistivity of
NO × 10–17, cm–3
NAu × 10–14, cm–3
Sample
no.
2
0 Ω cm) silicon wafers differing in oxygen concentra-
as-pre-
pared
heat-treated
Au-diffused
tion (table). The interstitial oxygen concentration was
–1
evaluated from the strength of the 1106-cm absorp-
tion band. The dislocation density was determined
1
2
3
4
6.9
8.9
6.9
8.2
5.9
4.0
6.9
9.8
4.0
2.8
3.1
5.3
7.0
9.5
2
–2
using selective etching and was within 10 cm in all of
our samples.
Microhardness H was measured in the [100] direc-
tion by a standard technique using a PMT-3 tester fitted
with a square-pyramidal diamond indenter tip with an
9.6
10.1
3
43

3
44
BRINKEVICH et al.
1
1
1.2
0.8
1
0.0
1
'
1
2
9
9
.6
.2
1
1
0.4
0.0
2
'
2
'
2
1
1
'
0
.5
1.0
Load, N
1.5
2.0
0.5
1.0
Load, N
1.5
2.0
Fig. 1. Microhardness as a function of indentation load for
1, 1') as-prepared and (2, 2') heat-treated silicon wafers at
Fig. 2. Microhardness as a function of indentation load for
(1, 1') as-prepared and (2, 2') gold-diffused silicon wafers at
(
17
17
oxygen concentrations of (1, 2) 6.9 × 10 and (1, 2) 10.1 ×
oxygen concentrations of (1, 2) 6.9 × 10 and (1, 2) 10.1 ×
17
–3
17
–3
10
cm .
10 cm .
damage layer up to 100 µm in thickness was ground (~8% [8]) but exceeded that produced by low-temperature
away, and the wafer surface was polished.
(150°ë) annealing (1.5–2%) [7].
The gold concentration in the samples was deter-
mined by neutron activation analysis (table). A number
of samples (controls) were heat-treated under the same
conditions without gold diffusion. The Au impurity
concentration in those samples was insensitive to heat
treatment and was close to that in the as-prepared
wafers (~5 × 10 cm ). The electroactive gold concen-
tration was inferred from Hall effect measurements by a
standard technique at temperatures from 78 to 300 K in dc
electric and magnetic fields [5].
Heat treatment of control samples with interstitial
17
–3
oxygen concentrations above 8 × 10 cm led to active
oxygen precipitation (table) [9]. The process was sub-
stantially more effective at higher interstitial oxygen
17
–3
concentrations: at N = 10.1 × 10 cm , the fraction
O
of precipitated oxygen reached ~75%. At N = 6.9 ×
12
–3
O
1
7
–3
1
0
cm , heat treatment had little or no effect on the
interstitial oxygen concentration (table). Note that, in the
case of active oxygen precipitation, the increase in H was
smaller (Fig. 1, curve 2'). Thus, we are led to conclude that
oxygen precipitation reduces the microhardness of silicon.
RESULTS AND DISCUSSION
The present experimental data can be interpreted as
The microhardness of the as-grown silicon single follows: Heat treatment of single-crystal silicon near
crystals was about 9.8 GPa (at a 1-N load) and 900°C leads to a transformation of Ç-defects—aggre-
depended very little on the oxygen concentration in the gates of silicon self-interstitials, Si , stabilized by car-
i
17
–3
range N = (6.9–10.1) × 10 cm (Fig. 1). The plots of bon atoms [10], which are capable of forming intersti-
O
tial-type point defects (and/or defect complexes),
thereby raising the strength of the crystal [11]. Heating
causes Ç-defects to emit self-interstitials. The liberated
carbon atoms, which have a smaller covalent radius
microhardness versus indentation load for all of our sam-
ples are typical of nonplastic (hard) crystals (Figs. 1, 2):
increasing the load from 0.5 to 1 N reduces the microhard-
ness; at higher loads, H varies insignificantly. According
to Gerasimov et al. [6], the increased hardness in the sur-
face layer of silicon single crystals is due to bond dimer-
ization on the semiconductor surface and the development
of surface microroughness.
(
0.77 Å [11]) compared to Si atoms (1.17 Å [12]), com-
press the silicon lattice, thereby reducing the bond length
and, accordingly, raising the strength of the crystal. These
processes seem to be responsible for the observed influ-
ence of heat treatment on the hardness of silicon.
Gold diffusion was found to reduce the microhard-
ness of the silicon single crystals (Fig. 2). The strength
Oxygen impurities also have a significant effect on
loss was less pronounced at higher oxygen concentrations the mechanical properties of single-crystal silicon.
(
9
Fig. 2, curves 2, 2'). At the same time, heat treatment at Oxygen atoms in silicon are known [13] to impede dis-
25°C for 5 h with no gold diffusion increased the micro- location propagation and multiplication during defor-
hardness of control samples (Fig. 1, curves 2, 2').A similar mation and must, therefore, increase the strength of the
effect was observed earlier at lower annealing tempera- material. The hardening effect of interstitial oxygen is
tures, 150–800°ë [7, 8]. The change in microhardness in also evidenced by the fact that the microhardness of
our experiments (2–5%) was smaller than that at 800°C oxygen-free float-zone silicon is substantially (5–10%)
INORGANIC MATERIALS Vol. 45 No. 4 2009

STRENGTH DEGRADATION OF SILICON DIFFUSION-DOPED WITH GOLD
345
lower than that of Czochralski Si. Thus, oxygen precip-
CONCLUSIONS
itation, accompanied by the removal of oxygen atoms
from interstitial positions, must reduce the strength of sil-
icon single crystals. Moreover, oxygen precipitation is
accompanied by generation of self-interstitials [14], which
would be expected to assist in suppressing the above-men-
tioned transformation of Ç-defects during heating and,
accordingly, to reduce the microhardness.
Diffusion doping with gold was shown to reduce the
microhardness of silicon single crystals. Oxygen pre-
cipitation suppresses this process because diffusing
gold atoms, Au , interact with oxygen and become cap-
i
tured by growing precipitates.
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