Microstructure of Al contacts on GaAs

Article abstract of DOI:10.1063/1.120255

The microstructure of Al films deposited on GaAs(100) 2×4 surfaces through chemical vapor deposition from dimethylethylamine alane in the 100-160°C temperature range exhibits a dominant (111) texture which is not encountered in evaporated films. Such a texture has been associated with enhanced electromigration resistance in related systems. Growth of (111)-oriented grains is observed when the deposition rate is limited by the surface reaction of the impinging precursor molecules, while at higher temperatures (160-400°C) only the conventional texture is observed

Full text of DOI:10.1063/1.120255

Microstructure of Al contacts on GaAs
I. Karpov, A. Franciosi, C. Taylor, J. Roberts, and W. L. Gladfelter
Citation: Applied Physics Letters 71, 3090 (1997); doi: 10.1063/1.120255
View online: http://dx.doi.org/10.1063/1.120255
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/71/21?ver=pdfcov
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Microstructure of Al contacts on GaAs
I. Karpov and A. Franciosia)
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis,
Minnesota 55455, and Laboratorio Nazionale TASC-INFM, Area di Ricerca, Padriciano 99,
I-34012 Trieste, Italy
C. Taylor, J. Roberts, and W. L. Gladfelter
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
͑Received 6 June 1997; accepted for publication 23 September 1997͒
The microstructure of Al films deposited on GaAs͑100͒ 2ϫ4 surfaces through chemical vapor
deposition from dimethylethylamine alane in the 100–160 °C temperature range exhibits a
dominant ͑111͒ texture which is not encountered in evaporated films. Such a texture has been
associated with enhanced electromigration resistance in related systems. Growth of ͑111͒-oriented
grains is observed when the deposition rate is limited by the surface reaction of the impinging
precursor molecules, while at higher temperatures ͑160–400 °C͒ only the conventional texture is
observed. © 1997 American Institute of Physics. ͓S0003-6951͑97͒01147-9͔
Aluminum-based vias are widely used in modern inte-
grated circuit ͑IC͒ technology, but are well known to be sus-
ceptible to failure by electromigration.1 Some of the impor-
tant microstructural variables affecting electromigration
lifetime, i.e., the mean time to failure, are grain size, grain
size distribution, and texture.1,2 For example, the presence of
Al grains with ͑111͒ texture was found to increase the elec-
tromigration lifetime of metallizations fabricated on Si͑100͒
wafers by evaporation or sputtering.2
In principle, Al nucleation and the resulting film micro-
structure might be affected by the specifics of the deposition
process, but relatively little is known about microstructure
evolution during evaporation or chemical vapor deposition
͑CVD͒ of metals on semiconductors. Among the precursors
currently been considered3–8 for Al CVD, dimethylethy-
lamine alane ͑DMEAA͒ may offer several advantages be-
cause of its nonpyrophoric character, high vapor pressure
and low pyrolysis temperature.6–8
We report here major differences in the microstructure
of Al films deposited by evaporation and CVD from
DMEAA on atomically clean GaAs͑100͒ 2ϫ4 surfaces.
We employed a multichamber ultrahigh vacuum ͑UHV͒
system ͑base pressure pϳ4ϫ10Ϫ10 Torr) that includes a
UHV-compatible CVD reactor with reflection high-energy
electron diffraction ͑RHEED͒ capabilities, and an analysis
chamber with an Auger electron spectrometer, to deposit the
films in the 100–400 °C substrate temperature range and pre-
cursor partial pressures (pD) in the 2ϫ10Ϫ4 –2ϫ10Ϫ5 Torr
range.8–10
eter operating at a wavelength of 2.1 ␮m. The final
GaAs͑001͒ rms surface roughness ͑1.4–1.7 Å͒ was compa-
rable to that of as-grown GaAs surfaces.9
The DMEAA precursor was synthesized using the pro-
cedure described elsewhere7 and admitted to the reactor
through a leak valve after repeated freeze-pump-thaw
cycles.8,10 The precursor partial pressure pD was monitored
by means of a nude ion gauge calibrated to take into account
the ionization probability of DMEAA.8 The final Al film
thickness was measured ex situ by means of a profilometer
using a diluted HF etch to remove the film from selected
areas of the substrate.
For comparison, evaporated Al films on GaAs͑100͒ 2
ϫ4 substrates were also obtained using the same experimen-
tal system on the same type of substrates, for similar values
of the substrate temperature and Al deposition rate ͑0.5–1.0
Å/s͒. We employed a BN crucible surrounded by a W resis-
tor coil filled with 99.999 wt. % Al and determined the depo-
sition rate using a quartz cystal thickness monitor.
The film microstructure was characterized by x-ray dif-
fraction ͑XRD͒, atomic force microscopy ͑AFM͒, and scan-
ning electron microscopy ͑SEM͒. We used a single-crystal
Siemens D-500 diffractometer or a Rigaku D-Max B diffrac-
tometer with a pole figure attachment. For the pole figure
measurements, the Rigaku diffractometer was set at a 2⌰
value of 38.5° to monitor reflections from Al ͕111͖ planes.
The sample was simultaneously tilted and rotated to record
diffracted x-ray intensities up to a maximum tilt angle of 80°
using the Schultz reflection technique.11
The GaAs substrates for Al deposition were prepared by
molecular beam epitaxy ͑MBE͒. Si-doped, epitaxial
GaAs͑100͒ 2ϫ4 layers 1 ␮m thick were grown at 580 °C by
solid source MBE on GaAs͑100͒ wafers and protected by a
thick ͑ϳ0.5 ␮m thick͒ amorphous As cap layer during trans-
fer in air to the reactor chamber.9 The cap layer was ther-
mally desorbed in-situ to produce a sharp 2ϫ4 RHEED pat-
tern prior to Al deposition.9,10 The sample holder
temperature was monitored by means of an optical pyrom-
The AFM studies were performed in air, using a com-
mercial instrument which included a cantilever with spring
constant of 0.58 N/m and a scanner with a scan area of
125 ␮mϫ125 ␮m. The SEM studies were performed on
cleaved ͕110͖ cross sections of the Al/GaAs complex, using
an Hitachi S800 microscope operated at 20 kV.
Films deposited by CVD exhibited, in general, a colum-
nar microstructure under cross-sectional SEM examination.
The grain size distribution, as studied by AFM, had a single
maximum. The average grain size increased with film thick-
ness and was about 0.2–0.3 ␮m for 0.3–0.5 ␮m thick films
deposited at 200 °C.
a͒
Also with Dipartimento di Fisica, Universita’ di Trieste, I-34127 Trieste,
Italy.
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3090 Appl. Phys. Lett. 71 (21), 24 November 1997 0003-6951/97/71(21)/3090/3/$10.00 © 1997 American Institute of Physics
On: Sat, 20 Dec 2014 09:42:08

FIG. 1. X-ray diffraction patterns recorded from Al films fabricated by
chemical vapor deposition ͑CVD͒ from ͑a͒ dimethylethylamine alane
͑DMEAA͒ and ͑b͒ evaporation 0.4 and 0.35 ␮m thick, respectively, depos-
ited at TDϭ400 °C on GaAs͑100͒ 2ϫ4 surfaces.
FIG. 2. X-ray diffraction patterns recorded from ͑a͒ CVD and ͑b͒ evapo-
rated Al films 0.35 and 0.3 ␮m thick, respectively, deposited at TD
ϭ150 °C on GaAs͑100͒ 2ϫ4 surfaces. In addition to the Al͑200͒ and
Al͑220͒ reflections, the latter visible only as a shoulder on the left side of the
GaAs͑400͒ peak, Al films fabricated by CVD from DMEAA at 150 °C
exhibit an important Al͑111͒ contribution.
Representative XRD patterns obtained from Al films
grown by CVD from DMEAA and evaporation are shown in
Fig. 1 for a deposition temperature TDϭ400 °C and in Fig. 2
for TDϭ150 °C. The film thickness was 0.40 and 0.35 ␮m
for CVD and evaporation, respectively, in Fig. 1, and 0.35
and 0.30 ␮m for CVD and evaporation, respectively, in Fig.
2. Comparison of Fig. 1 and Fig. 2 clearly indicates that
while films produced by CVD and evaporation at 400 °C
exhibit virtually identical XRD patterns, there are substantial
differences in the microstructure of films fabricated by CVD
and evaporation at 150 °C. The major Al-related peaks in
Figs. 1͑a͒ and 1͑b͒ correspond to the Al͑220͒ reflection, i.e.,
to Al crystallites with ͕110͖ planes oriented parallel to the
GaAs͑100͒ substrate. A much weaker Al͑200͒ reflection is
also observed, suggesting the presence of at least some crys-
tallites with ͕100͖ planes parallel to GaAs͑100͒ surface. The
relative intensities and width of the diffraction peaks for the
two films fabricated at 400 °C are compellingly similar. The
similarity of the two microstructures for films produced by
CVD and evaporation at 400 °C is also supported by ͕111͖
pole figures ͑not shown͒, which reflect ͕111͖ planes of Al
crystallites arranged with ͕110͖ plane parallel to the ͑100͒
2͑b͔͒, show only major Al-related peaks corresponding to the
Al͑220͒ and Al͑200͒ reflections.
A more quantitative analysis was performed by record-
ing a series of XRD patterns for films of similar thickness
͑0.3–0.5 ␮m͒ fabricated for different values of TD , and
monitoring the ratio of the integrated diffraction intensities
of the Al͑111͒ and Al͑200͒ peaks. The results are shown in
Fig. 3͑a͒ as a function of the deposition temperature TD . Our
analysis show that little or no Al͑111͒ contribution is ob-
served for TDϾ160 °C, while the same contribution in-
creases rapidly with decreasing deposition temperature be-
low 160 °C. The same type of qualitative dependence on TD ,
only magnified, would be observed in the Al͑111͒/Al͑220͒
intensity ratio ͑not shown͒, since the Al͑200͒/Al͑220͒ inten-
sity ratio increases per se with decreasing deposition tem-
perature.
A complete analysis of the film texture using pole fig-
ures reveals the presence of several minority domains with
different orientations in addition to the major ͑111͒ textured
component in films fabricated by CVD at low temperature.
ʈ
ʈ
The main additional orientations were: ͑a͒ Al͑100͓͒001͔
substrate, and a dominant Al͑011͓͒100͔ GaAs͑001͒͗110͘ ep-
ʈ
GaAs͑100͓͒001͔; ͑b͒ Al͑100͓͒001͔ GaAs͑100͓͒011͔; ͑c͒
itaxial relation.
ʈ
ʈ
Al͑110͓͒001͔ GaAs͑100͓͒011͔; ͑d͒ Al͑110͓͒001͔ GaAs͑100͒
In addition to the Al͑200͒ and Al͑220͒ reflections, Al
films fabricated by CVD from DMEAA at 150 °C ͓see Fig.
2͑a͔͒ exhibited an important Al͑111͒ contribution. The
Al͑111͒ reflection is instead completely absent from the
XRD patterns recorded for Al films evaporated on
GaAs͑100͒ 2ϫ4 at 150 °C ͓see Fig. 2͑b͔͒, 400 °C ͓see Fig. 1
͑b͔͒, or for that matter even at room temperature.12 Al films
fabricated by evaporation at 150 °C, for example ͓see Fig.
¯
͓011͔. Orientation ͑a͒ above was also recently observed in Al
films grown on GaAs͑100͒ by chemical beam epitaxy from
trimethylamine alane,13 while orientations ͑b͒-͑d͒ above have
also been reported in previous studies of MBE grown Al
films on GaAs͑100͒.14
With increasing deposition temperature the main ͑111͒-
textured contribution becomes weak and for TDϾ300 °C a
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Appl. Phys. Lett., Vol. 71, No. 21, 24 November 1997 Karpov et al. 3091
On: Sat, 20 Dec 2014 09:42:08

ture, further suggesting that a reduced surface mobility might
be the key to the nucleation of ͑111͒-oriented Al crystallites.
We therefore correlate the appearance of an increasing
͑111͒ contribution in films fabricated by CVD at decreasing
deposition temperatures below 150–160 °C with a reduction
in the surface mobility of the Al adatoms. We tentatively
associate such a reduction with the presence of an increasing
number of precursor molecules or reaction by-products ad-
sorbed nondissociatively onto the the surface, i.e., to an in-
crease in the average residence time of the corresponding
molecules. The decrease in the overall CVD rate for TD
Ͻ150–160 °C in Fig. 3͑b͒ is a clear manifestation of a re-
duction in the surface reaction rate of the precursor mol-
ecules, and we suggest that the adsorbed molecules might
compete with Al adatoms for the available surface sites and
lead to an important reduction in surface diffusion.
This work was supported in part by NSF under Grant
No. DMR-9525758. The authors thank Lucia Sorba for her
invaluable help with the growth of the GaAs substrates and
Kai-Ann Yang for the synthesis of the DMEAA precursor.
FIG. 3. ͑a͒ Ratio of Al͑111͒ to Al͑200͒ diffraction intensities for CVD films
of similar thickness ͑0.3–0.5 ␮m͒ deposited on GaAs͑100͒ 2ϫ4 surfaces as
a function of the deposition temperature TD . ͑b͒ Logarithmic growth rate
for Al films fabricated by CVD from DMEAA on GaAs͑100͒ 2ϫ4 surfaces.
We show results as a function of the substrate temperature TD for a constant
partial pressure of DMEAA pDϭ2ϫ10Ϫ4 Torr. The growth rate is limited
by the temperature-activated surface reaction rate only for TD
Ͻ150–160 °C.
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ʈ
dominant Al͑110͓͒001͔ GaAs͑100͓͒011͔ orientation is ob-
served for both CVD and evaporation, yielding a completely
analogous microstructure.
6 M. Simmonds and W. L. Gladfelter, in The Chemistry of Metal CVD,
edited by T. Kodas and M. H. Smith ͑VHH, Weinheim, 1994͒, Chap. 2.
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The results of Figs. 1–2 suggest that at low enough
deposition temperatures, the specifics of the CVD process
may start to affect the film microstructure. This suggestion is
supported by a comparison of the results in Fig. 3͑a͒ with the
kinetics of the CVD process from DMEAA. In Fig. 3͑b͒ we
show ͑from Ref. 8͒ the measured Al growth rate on
GaAs͑100͒ 2ϫ4 as a function of TD in the 100–500 °C
range, for a fixed value of pDϭ2ϫ10Ϫ4 Torr. The results
show that for TDϾ160 °C the films grew at a similar rate of
about 8 Å/s, while for TDϽ150–160 °C the growth rate de-
creased rapidly, dropping to about 0.3 Å/s for TDϳ100 °C,
and hinting at an Arrhenius-type dependence of the growth
rate. The growth rate was also found to be directly propor-
tional to pD for TDϾ160 °C ͑not shown͒. Therefore for TD
Ͼ160 °C the deposition process is flux limited, while for
TDϽ150–160 °C the growth rate is limited by the surface
reaction rate of the impinging precursor molecules.8
Since the Al͑111͒ reflection has not been observed in
XRD patterns from Al films evaporated at room temperature
on GaAs͑100͒—see Fig. 2͑b͒ and Ref. 12—while it has been
detected in films evaporated at 0 °C on the same surface,15 it
would appear that a substantial decrease in the surface mo-
bility of the Al atoms is required to favor the growth of
Al͑111͒ crystallites during evaporation. Al films evaporated
at room temperature on amorphous or highly disordered ma-
terials such as glass,16 or polymers,17 also exhibit ͑111͒ tex-
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