Transient annealing of semiconductors by laser, electron beam and radiant heating techniques

79 Pages·2002·6.07 MB·English

Rep. Prog. Phys. 48 (1985) 1155-1233. Printed in Great Britain
Transient annealing of semiconductors by laser, electron beam
and radiant heating techniques
A G Cullis
Royal Signals and Radar Establishment, St Andrews Road, Malvern,
Worcestershire WR14 3PS, UK
Abstract
The annealing of semiconductors is of critical importance for successful electronic
device fabrication. The present review surveys the new field of transient annealing
and covers all timescales below those available with the conventional furnace. The
work outlined includes the use of techniques which rely upon transient energy deposi-
tion in semiconductors from laser, electron beam, ion beam and other radiant sources.
The many advances which have been achieved using these transient annealing methods
in both fundamental and applied areas of physics are described.
This review was received in its present form in February 1985.
+
0034-4885/85/081155 79$09.00 @ 1985 The Institute of Physics 1155
1156 A G Cullis
Contents
Page
1. Introduction 1157
2. Annealing techniques and mechanisms 1158
2.1. Q-switched laser annealing 1158
2.2. Scanning cw laser annealing 1166
2.3. Electron beam, ion beam and x-ray annealing 1168
2.4. Annealing by general radiant heating techniques 1171
3. Q-switched laser annealing phenomena 1172
3.1. Semiconductor surfaces 1172
3.2. Ion-implanted semiconductors 1174
3.3. Deposited and diffused semiconductor layers 1187
3.4. Metal layers on semiconductors 1190
3.5. Ultra-high-speed solidification phenomena 1192
4. Scanning cw laser annealing phenomena 1195
4.1. Ion-implanted semiconductors 1196
4.2. Deposited semiconductor layers 1201
4.3. Deposited metal layers on semiconductors 1205
5. Electron and related particle beam annealing phenomena 1206
5.1. Pulsed electron beams 1206
5.2. Scanning electron beams 1208
5.3. Ion and x-ray beams 1210
6. Radiant source and general multisecond regime annealing phenomena 1211
6.1. Solid phase processes 1211
6.2. Liquid phase processes 1216
7. Future directions 1219
Acknowledgments 1221
References 1221
Transient annealing of semiconductors 1157
1. Introduction
The annealing of semiconductors is an integral part of electronic device fabrication
technology. Applications range from the recrystallisation of lattice damage produced
by ion implantation to the promotion of chemical reactions such as oxidation or the
formation of metal-semiconductor compounds. Heat treatment timescales have gen-
erally been those available by use of conventional furnace systems, which limit
minimum process times to a significant fraction of an hour. However, in many cases
this constraint prevents the full optimisation of the processing in terms of the reaction
kinetics involved.
Despite the desirability of exploring short anneal timescales, only a relatively small
number of studies were carried out before the middle of the 1970s. Investigations then
received strong impetus from observations of the interaction of pulsed laser radiation
with semiconductors. Much of this early work was pioneered in the USSR, but since
approximately 1977 a large number of groups in laboratories world-wide have taken
up the challenge. Indeed, in the latter part of the decade the growth of research activity
was almost exponential as many new physical phenomena were discovered. During
the past few years of rapid advance, the work itself has diversified considerably. It
was soon recognised that the energy delivered by short laser pulses could also be
supplied by other means. Alternative energy deposition techniques, including the use
of radiant optical, electron beam and even ion beam processing, have been investigated
in great detail. The present article reviews the progress that has been made and, in
§ 2, describes the various techniques which have been employed: the different annealing
mechanisms are also outlined.
The range of timescales covered by transient annealing work is extremely large and
spans process durations differing by more than eleven orders of magnitude. The fastest
semiconductor annealing cycles induced by very short radiation pulses lie in the
nanosecond regime. Most studies have relied upon the use of Q-switched laser radiation
and the results obtained, including the discovery of a number of new, fast crystal-growth
phenomena, are described in § 3. The second distinct area of transient annealing work
lies in the millisecond regime. This is most commonly studied using scanning con-
tinuous-wave (cw) laser beams and the advances which have been made, including
significant achievements in Si-on-insulator fabrication, are reviewed in § 4. It should
be noted that the rapidly evolving topic of*l aser photochemistry lies outside the area
covered by the present article and is not considered: the interested reader is referred
to the review given by Osgood (1983).
Two groups of techniques, in particular electron beam and ion beam methods,
allow semiconductor annealing to be undertaken in a number of regimes, including
those outlined above. Specific results obtained in this area are covered in § 5. These
methods also allow work to be carried out in the third, multisecond annealing regime.
However, most studies have employed optical radiant heating and related approaches.
The developments that have taken place show particular device fabrication potential
and are discussed in P 6. Finally, the prospects for future progress in all subject areas
are briefly considered in 5 7.
1158 A G Cullis
2. Annealing techniques and mechanisms
2.1. Q-switched laser annealing
2.1.I . Experimental technique. The radiation produced by lasers has both high
intensity and high directionality so that it is well suited to the localised annealing of
semiconductors. Q-switched laser radiation pulses generally persist for times in the
nanosecond regime and can have a wide range of wavelengths-from approaching
0.1 pm in the ultraviolet for excimer lasers to about 10 pm in the infrared for CO2
lasers (see, for example, Gibson and Key 1980). In general, for efficient annealing to
be achieved it is desirable to select laser wavelengths which lie within an absorption
band exhibited by the material of interest. For example, the lattice absorption of
crystalline Si increases from about lo2c m-' to approximately lo6 cm-' as the radiation
wavelength is decreased from 2 pm to 0.4 pm. Therefore, significantly lower pulse
energy densities are required for onset of annealing at, for example, the ruby laser
wavelength (0.694 nm) than at the Nd-YAG laser wavelength (1.06 pm). Accordingly,
the efficiency of annealing at the latter wavelength can be significantly improved by
frequency doubling into the green region of the spectrum at least a portion of the
radiation in each infrared pulse (Auston et a1 1979b). If the material to be annealed
does not exhibit significant absorption at the radiation wavelength chosen, coupling
will still occur at very high radiation energy densities due to the effects of free-carrier
absorption or non-linear (multiphonon) processes (Hasselbeck and Kowk 1982).
However, under such conditions the energy density 'window' for controlled annealing
is very narrow and the surface of the material is often damaged with even small
excursions in energy density above the annealing threshold. This is the situation for
COz laser (10 pm) annealing of crystalline Si, although improvement in control of the
process can be achieved by moderate heating of the sample to promote free-carrier
generation and initiate stable radiation absorption (Celler et al 1979).
The absorption of laser radiation by a semiconductor sample can be substantially
increased if lattice disorder is present. For example, implantation of high-energy ions
can severely damage and even amorphise a surface layer which may then have an
absorption coefficient more than an order of magnitude higher than that of the
underlying crystal. Under laser irradiation, coupling is greatest in the implanted region,
which receives enhanced energy deposition and so exhibits a reduced annealing energy
density threshold (see § 2.1.3).
A typical laser system based upon a Q-switched ruby oscillator is shown in figure
1. The length of the radiation pulse at 694 nm given by the pumped oscillator would
typically lie in the range 20-5011s. At least 1 J of electromagnetic energy might be
delivered in this time. However, it is often necessary to shorten the pulse length or to
frequency-double the radiation (in this case, into the near-ultraviolet) for particular
applications. The system illustrated in figure 1 makes provision for this since pulses
can be chopped into sections with a second, beam-switching, Pockels cell and
frequency-doubled by second-harmonic generation in a crystal with non-linear optical
response. Provision for 694 nm pulse amplification is also made available on the laser
rail.
Most power is generated by the Q-switched laser cavity when it is allowed to
oscillate with multiple transverse modes excited. However, a multimode beam exhibits
an exceptionally non-uniform transverse intensity distribution (which takes the form
of a Gaussian multiplied by a Hermite polynomial). This non-uniformity is extremely
Transient annealing of semiconductors 1159
Figure 1. Diagram of typical ruby laser system with pulse chopper, amplifier and frequency doubler.
undesirable for laser annealing applications because it leads to gross changes in
annealed layer structure on both microscopic and macroscopic scales. Indeed, local
surface damage can be produced by individual intensity peaks. If an aperture is
inserted into the laser cavity, single mode (TEM 00) operation can be achieved, which
yields a beam with a smooth Gaussian envelope. This is satisfactory for some annealing
work, especially if only the relatively flat centre of the pulse is used. However, laser
efficiency is greatly reduced by this approach and the coherence of the beam may give
problems due to diffraction scattering and interference pattern formation at the sample
surface. Problems such as these can be avoided by use of a multimode beam which
has been suitably diffused to remove the pronounced mode pattern intensity variations
described previously. A simple diffuser plate (such as a ground-glass screen) has
serious disadvantages since it is not possible to eliminate beam speckle non-uniformities
without also greatly degrading the beam intensity by the introduction of wide-angle
light scattering (Lahart and Marathay 1975). This undesirable characteristic can be
overcome by employing a curved and tapered quartz light-guide diffuser (Cullis et al
1979b). The mode pattern in the laser output beam is broken up as the light passes
into the light-guide rod through a ground input face, which is often about 1 cm in
diameter. Most of the incident light is scattered close to the forward direction and
passes down the guide, being contained by total internal reflection. The multiple
reflections further diffuse the beam and progressively eliminate speckle. This process
is enhanced, especially for near-axis light, by a curve in the guide while a tapered
1160 A G Cullis
configuration increases the beam energy density. At the guide output face, which is
highly polished, excellent radiation uniformity of better than *5’/0 over most of the
area can be attained. Radiation energy densities of up to several J cm-’ can be used
without introducing damage in a quartz diffuser of this type. If it is necessary to use
a laser wavelength at which no suitable transparent glass can be found for light-guide
fabrication (for example, at 10pm) a hollow kaleidoscopic tube diffuser may be
satisfactory (Grojean et al 1980). It is also important to note that some excimer lasers
give a relatively uniform direct multimode output so that beam diffusion requirements
may be relaxed.
Single high-energy pulses from Q-switched lasers are often used to uniformly anneal
semiconductor surface areas with dimensions up to 1 cm at energy densities of up to
5 J cm-’. In an alternative mode of operation, a sequence of relatively low-energy
pulses can be focused to form small (10-100 pm) spots which are scanned over the
sample surface (Celler et a1 1978). Each spot has an approximately Gaussian intensity
distribution but a reasonably uniform annealed region is produced by overlapping
spot positions by approximately 50%. The rate of sample coverage is comparable to
the large-spot annealing mode since a laser repetitively Q-switched at greater than
lo4 Hz is generally used (such as a frequency-doubled cw-pumped, acousto-optically
Q-switched Nd-YAG laser). However, the final overall anneal uniformity is poorer
and some materials (such as oxide-covered Si) may be damaged by the intensity peaks
at the pulse centres.
2.1.2. Annealing mechanism. When laser light interacts with a semiconductor only the
electrons are involved since the atomic nuclei are too heavy to respond to the very
high-frequency electromagnetic field. If the photon energy is significantly less than
the band gap energy of the material, only weak intraband and phonon absorption are
possible. As mentioned in the last subsection, this is not usually a satisfactory Q-
switched laser annealing regime. If the photon energy is greater than the band gap
energy, strong interband excitation of electrons is possible. The exact strength of this
process depends, of course, upon the actual photon wavelength and the ‘direct’ or
‘indirect’ character of the semiconductor, but a typical Q-switched laser pulse results
in the formation of a dense electron-hole plasma in near-surface regions (more details
of carrier excitation processes are given by von Allmen (1982)).
When initially produced, excited free electrons generally have excess kinetic
energy. However, extremely rapid electron-electron and electron-plasmon collisions
would lead to equilibration of energy amongst conduction band electrons in times of
less than s: this is illustrated in figure 2 for the case of Si. A third process results
in the transfer of energy to the atomic lattice, namely electron-phonon collisions which
generate further phonons. The nearly vertical line in figure 2 shows (Dumke 1980)
that about 1 eV of energy should be lost by an excited electron to the lattice in about
lo-’* s. However, this expectation is based upon conventional theory and has been
the subject of much controversy, as will be outlined below. Whatever the situation,
these three processes will leave a high concentration of relatively ‘cool’ electrons at
the bottom of the conduction band. The electrons then decay by Auger recombination,
the time constant of which is dependent upon the square of the electron density (figure
2). The recombination energy released by each transition is transferred to another
electron (or hole) which can then release its energy to the lattice by further collisions
with phonons. In this way, rapid lattice heating is expected to occur.
Transient annealing of semiconductors 1161
T,tiSi
Figure 2. -, times for electron scattering and recombination processes in Si as a function of carrier
density. - -, thermal diffusion distances in Si at 77 K, room temperature (300 K) and the melting point
(1685 K) (adapted from Brown 1980).
Processes which take place during Q-switched laser irradiation of semiconductors
have been monitored traditionally by observations of surface reflectivity changes. Work
by Sooy er al (1964), Birnbaum and Stocker (1968) and Blinov et a1 (1967) on a range
of semiconductors has demonstrated that a brief high-reflectivity period shown by such
measurements is best explained by assuming that a thin, transiently molten surface
layer is formed above a particular threshold. This would indicate that laser pulse
energy is transferred rapidly to the semiconductor lattice, in agreement with the
previously presented theory and with measurements of carrier population decay
(Nilsson 1973). The time-resolved reflectivity measurements of Auston et a1 (1979a)
show particularly clearly the transiently enhanced surface reflectivity which would be
characteristic of the formation of molten surface layers on Si, Ge and GaAs under
normal laser annealing conditions. However, the transient surface melting model has
not enjoyed universal support and an alternative model was proposed in order to
account for the various observed phenomena. Van Vechten er a1 (1979) suggested
that, during Q-switched laser annealing, only a small amount of the deposited energy
might be communicated to the lattice, which would remain at a relatively low tem-
perature. Instead, these workers proposed that the initially formed electron-hole
plasma could persist for a very long time and lead to annealing by two somewhat
speculative mechanisms: (i) the substantial reduction in the number of bonding elec-
trons could ‘fluidise’ the irradiated material which would subsequently undergo a type
of recrystallisation; and (ii) the presence of the plasma could permit enhanced vacancy
diffusion (by a screening process) and promote defect structure changes. The essential
feature of this postulated non-thermal annealing mechanism is the required increase
in plasma lifetime (to more than 100 ns) by many orders of magnitude beyond that
conventionally predicted. Indeed, the presence of the plasma was proposed as the
real cause of the transient reflectivity increase. The theory, which has been subject to
modification (van Vechten and Compaan 1981), required that the plasma should be
contained in the annealed surface region by band gap reduction due to very limited
local heating (a few hundred degrees Kelvin only (van Vechten and Wautelet 1981)).
1162 A G Cullis
Such plasma confinement would be an important phenomenon since it would counteract
basic carrier diffusion (Yoffa 1980) but, unless carriers excited by Auger recombination
could not transfer their excess energy to the lattice, a modification of the heat generation
profile would be the only result (van Driel et a1 1982). However, the theory of van
Vechten and Compaan (1981) proposed that the electron-phonon scattering process
(the heat transfer route) could be strongly screened by Bose condensation of Frenkel
excitons, so that the lifetime of the plasma would be greatly extended (to greater than
100 ns), as required.
Much work has been carried out to determine whether Q-switched laser annealing
occurs by transient melting or by the proposed non-thermal mechanism. Indeed, the
theoretical basis of the latter has been strongly criticised (Dumke 1980) and it is not
supported by time-dependent optical reflectivity measurements over a range of
wavelengths (Nathan et a1 1980). The non-thermal mechanism had originally received
support from time-resolved Raman scattering measurements (Lo and Compaan 1981,
Compaan et a1 1982, von der Linde et a1 1983) which had indicated a Si lattice
temperature of only 600-700 K immediately after the high-reflectivity period in the
laser annealing cycle. However, much of the original data has been reinterpreted by
Wood et a1 (1982) who found no conflict with the ‘melting model’, and further physical
limitations of the Raman experiments have been outlined by Narayan et a1 (1981).
Argument concerning the significance of the Raman measurements has continued until
recent work has finally indicated (Wartmann et a1 1984, Compaan 1985) that substantial
lattice heating and surface melting do, indeed, occur.
The extended debate on this topic has stimulated many independent dynamic
time-resolved measurements of various Si physical properties during laser irradiation.
These investigations have given overwhelming support for the conventional, thermal
annealing model. Particularly detailed optical reflectivity and transmission measure-
ments in the femtosecond regime (Shank et a1 1983a,b, H u h e t a1 1984), the picosecond
regime (Liu et a1 1982, von der Linde and Fabricius 1982, LomprC et a1 1983, 1984,
van Driel et a1 1984) and the nanosecond regime (Murakami et a1 1980, Lowndes et
a1 1982, Preston and van Driel 1984) show transient behaviour characteristic of molten
layer formation after plasma decay on the picosecond timescale (other work by Aydinli
et a1 (1981) and Yamada et a1 (1981) has been shown by Lowndes et a1 (1982) to be
in doubt). The actual Si matrix temperature during annealing has been measured
(Stritzker et a1 1981) by determination of the velocities of atoms leaving the irradiated
surface: it was shown that temperatures up to and beyond the melting point were easily
attained. A similar result has been obtained by time-resolved x-ray diffraction studies
(Larson et a1 1982, 1983) which have measured the heat-induced variation in Si lattice
parameter. Direct measurement of the temperature rise in laser-irradiated Ge has also
been carried out (Baeri et a1 1984) by use of evaporated thin-film thermocouples. The
transient matrix electron temperature for irradiated Si has been measured (Liu et a1
1981c, Leung and van Driel 1984) and shown to be in accord with conventional
estimates of plasma decay. Dynamic measurement of electrical conductivity changes
in the irradiated area (Galvin et a1 1982), acoustic emission from the annealed sample
(Baltzer et a1 1983) and diffraction of low-energy electrons from the annealed surface
(Becker et a1 1984) have each provided further proof that Q-switched laser annealing
ordinarily proceeds by transient surface melting. Finally, it is important to note that
Baglin et a1 (1983b) carried out a critical test for non-thermal annealing effects in the
nanosecond regime by a pulsed ion beam experiment (described in 0 5.3) and con-
cluded, once again, that only normal thermal melting processes take place. This
Transient annealing of semiconductors 1163
conclusion is totally inescapable when the crystal growth and impurity segregation
phenomena described in § 3.2 are also taken into consideration.
2.1.3. Heat jlow calculations. When Q-switched laser radiation pulses deposit energy
in a semiconductor lattice it is important to theoretically model the temperature rise
produced. At the most fundamental level it has been confirmed (Lietoila and Gibbons
1982) that, since energy of electronic excitation is transformed into lattice phonons
on the timescale indicated by the dynamic reflectivity measurements (usually just a
small fraction of the laser pulse duration time), transient melting is the inevitable
dominant annealing mechanism. Therefore, in order to understand the materials
phenomena accompanying annealing, the behaviour of the molten layer must be
described.
Most work has focused on elemental Si. The characteristic diffusion length for
heat in the Si lattice on the timescale of a typical laser pulse (approximately lo-* s)
is a few microns-see figure 2. Thus, calculations must usually treat at least this depth
of material. If the laser radiation is spatially uniform across the sample surface (edge
effects neglected), the following one-dimensional heat equation applies (Ready 1971):
(2.1).
where c is the heat capacity, p is the mass density, T is the temperature, I(z, t) is the
radiation power density at depth z and time t, a is the absorption coefficient and K is
the thermal conductivity. Since several of the parameters are markedly temperature
dependent and phase changes must be considered (sometimes with different material
components along the z direction) there is serious difficulty in solving this equation
analytically. Nevertheless, with restricted temperature dependence taken into account,
analytical solutions have been derived both when melting does not occur (Lax 1978,
Meyer et a1 1980, Kwong and Kim 1983) and when melting takes place (Bertolotti
and Sibilia 1981). However, the most general solutions can only usefully be obtained
by numerical computation.
The numerical solution of equation (2.1) is usually carried out by dividing the
sample into slices parallel to the surface. The temperature of each slice is computed
in successive small time intervals by calculating the absorbed energy from the laser
pulse and the heat diffusing between adjacent slices. When the temperature of any
slice reaches the melting point, the latent heat of fusion of the slice is added before
its temperature is allowed to continue to rise. In the standard computational approach
(see Baeri and Campisano 1982), for any given slice thickness the maximum time
increment which can be chosen depends upon the values of c, p and K in order to
ensure stability of the successive solutions. This places a limit on the speed of
calculation, although various algorithms are available to ease this restriction (Morton
1967).
Numerical calculations of the heat flow during laser annealing have been carried
out for several materials at various levels of approximation (Baeri et a1 1979, Schultz
and Collins 1979, Auston et a1 1979a, Bell et a1 1979, Cullis et a1 198Oc, Wood and
Giles 1981, Bhattacharyya et a1 1981, Godfrey et a1 1981). For Gaussian, 30 ns ruby
laser pulses (694 nm) incident on crystalline Si, using the thermal and optical parameters
listed in table 1, calculations yield the melt depth as a function of time as shown in
figure 3. The threshold pulse energy density for surface melting is predicted to be
1164 A G Cullis
Table 1. Thermal and optical properties of siliconk.
Property Crystal Amorphous
-
Td, T,,(K) 1685" 1485- 11 85a,e*f*g*h
c (J g-' K-') 0.73 + 1.02b -0.74+ 1.15'
K (W cm-' K-') 1.4+0.23b -0.019
P (g cm-7 2.33 -2.3
AHm (J ~ m - ~ ) 4.06 x io3 2.9 x io3
R (694 nm) 0.34 + 0.44' 0.44
CY (cm-'; 694 nm) 2.7 x lo3+ 7 x lo4 -7 X lo4'
"It has been theoretically proposed (Bok 1981) that both T,, and T,, are
depressed by the presence of highly dense plasmas, although experimental
measurements indicate (Preston and van Driel 1984) that the magnitude of
this effect is not significant.
Touloukian (1967).
Algazin er a1 (1978).
Jellison and Modine (1982).
e Baeri et a/ (1980a) and Thompson et a/ (1984).
Donovan et al (1983).
Webber et a1 (1983).
Cullis et a1 (1984).
Bean et a1 (1979).
shows range from value at 300 K to value at melting point. R is the solid
--f
reflectivity.
I
0
--..
E
1 0
L
4a-
U
f
go
0 50 100 150
Time ins)
Figure 3. Computed melt depth as a function of time for 30 ns ruby laser pulses (hatched) with a range of
energy densities incident on Si: -, crystalline Si; - - -,1 200 A amorphous Si surface layer with T,, = 1385 K
(note short plateau at amorphous layer depth).
approximately 0.8 J cm-*, in excellent agreement with the results of calorimetry
measurements (Peercy and Wampler 1982). As the radiation energy density is increased
above this value, the melt front penetrates into the solid at a velocity of the order of
10 m s-' to progressively greater depths. The melt duration time also increases corre-
spondingly, with the final resolidification interface velocity often in the range 2-3 m s-l.
This sequence is in agreement with the transient conductivity measurements of Galvin
et a1 (1982) for pure Si, although it should be noted that the presence of large
concentrations of impurity can significantly modify the observed behaviour (Peercy
1985ksee 0 3.2.