Downstream ammonia plasma passivation of GaAs

Applicants have discovered that gallium arsenide surfaces can be dry passivated without heating or ion bombardment by exposing them downstream to ammonia plasma formation. Specifically, a workpiece having exposed gallium arsenide surfaces is passivated by placing the workpiece in an evacuable chamber, evacuating in the chamber, generating an ammonia plasma removed from the immediate vicinity of the workpiece, and causing the plasma products to flow downstream into contact with the workpiece. Preferably the plasma gas pressure is 0.5 to 6.0 Torr, the substrate temperature is less than 100.degree. C. and the time of exposure is in excess of 5 min. The plasma should be generated at a location sufficiently removed from the workpiece that the workpiece surface is not bombarded with ions capable of damaging the surface (more than about 10 cm) and sufficiently close to the workpiece that reactive plasma products exist in the flow (within about 30 cm). The workpiece should also not be placed within line-of-sight of the plasma to avoid radiation (UV, visible and X-ray) induced damage. The result is fast, stable, room temperature passivation, compatible with clustered dry processing techniques for integrated circuit manufacture.

FIELD OF THE INVENTION 
This invention relates to surface passivation of III-V semiconductors by 
plasma processing. In particular it concerns downstream ammonia plasma 
passivation of GaAs. The process not only passivates GaAs surfaces with 
minimal surface damage but also passivates without heating. 
BACKGROUND OF THE INVENTION 
A major block to the advancement of GaAs device technology is the 
difficulty of passivating exposed GaAs surfaces. Unlike silicon, whose 
oxide passivates the silicon surface leaving a low density of interface 
states, gallium arsenide does not have a passivating native oxide. Indeed, 
the interface between GaAs and its native oxide can be laden with defects 
which create a large interface state density in the mid band gap. This 
large density produces a high recombination velocity and Fermi level 
pinning which, in turn, deteriorate device performance. Specifically, the 
large interface state density produces excessive leakage currents in field 
effect transistors and photodiodes. 
Considerable efforts have been made to improve the poor electronic 
properties of GaAs surfaces and GaAs-insulator interfaces, but no wholly 
acceptable technique has yet been achieved. Wet processing techniques are 
generally incompatible with the trend in integrated circuit manufacturing 
toward clustered dry processing. The dry processing techniques thus far 
proposed, require either elevated temperatures incompatible with prior 
formed device structures or ion bombardment which is deleterious to the 
long term stability of the treated surface. Accordingly, there is a need 
for an improved process for dry plasma passivation of gallium arsenide 
surfaces without heating and ion bombardment. 
SUMMARY OF THE INVENTION 
Applicants have discovered that gallium arsenide surfaces can be dry 
passivated without heating or ion bombardment by exposing them downstream 
to ammonia plasma formation. Specifically, a workpiece having exposed 
gallium arsenide surfaces is passivated by placing the workpiece in an 
evacuable chamber, evacuating the chamber, generating an ammonia plasma 
removed from the immediate vicinity of the workpiece, and causing the 
plasma products to flow downstream into contact with the workpiece. 
Preferably the plasma gas pressure is 0.5 to 6.0 Torr, the substrate 
temperature is less than 100.degree. C. and the time of exposure is in 
excess of 5 min. The plasma should be generated at a location sufficiently 
removed from the workpiece that the workpiece surface is not bombarded 
with ions capable of damaging the surface (more than about 10cm) and 
sufficiently close to the workpiece that reactive plasma products exist in 
the flow (within about 30cm). The workpiece should also not be placed 
within line-of-sight of the plasma to avoid radiation (UV, visible and 
X-ray) induced damage. The result is fast, stable, room temperature 
passivation, compatible with clustered dry processing techniques for 
integrated circuit manufacture.

DETAILED DESCRIPTION 
Referring to the drawings, FIG. 1 is a schematic view of apparatus useful 
for treating a gallium arsenide surface in accordance with the invention. 
In essence, the apparatus comprises an evacuable chamber 10 disposed 
downstream from a gas input tube 11 enclosed within a microwave cavity 12. 
A workpiece 13 having an exposed gallium arsenide surface is disposed 
within chamber 10 where the surface is advantageously monitored by 
photoluminescence measuring equipment comprising a laser arrangement 14 
and a photoluminescence detecting arrangement 15. In this embodiment the 
chamber 10 is conveniently a 6-way Pyrex cross. Input tube 11 is a 2.54cm 
diameter quartz tube enclosed in a 5cm microwave cavity resonant at 
2.45GHz. The tube 11 has a bend 16 immediately after the microwave cavity 
to reduce the amount of radiation received by workpiece 13. The cavity 
should be sufficiently far removed from the workpiece that the plasma glow 
does not reach the workpiece. 10-30cm downstream is believed to be 
acceptable spacing. 
Laser arrangement 14 preferably comprises a pulsed laser 14A, such as a 
Molectron 20 Hz N.sub.2 pumped dye laser (600 mJ/cm.sup.2 at 500 nm). 
Light from the laser passes through beam splitter 14B and one output beam 
is focussed by lens 14C onto workpiece 13. 
Detecting arrangement 15 can comprise lens 15A for focussing 
photoluminescence from workpiece 13 into an optical fiber 15B which 
directs the light through a bandpass filter (not shown) to a 
photomultiplier tube 15C connected to a gated integrator 15D. The gate is 
controlled via photodiode 15E responsive to the second output beam from 
beam splitter 14B and the integrated output can be displayed on a personal 
computer 15F. 
The workpieces 13 are typically semi-insulating or doped GaAs wafers. In 
preliminary steps, which are advantageous but not necessary for the 
success of passivation, the workpieces are dipped in HF for 30s, washed 
with dionized water, blow dried with nitrogen before loading into chamber 
10. 
After loading, the chamber is evacuated to a low pressure less than about 
10.sup.-6 Torr in order to remove potential contaminants. Ammonia is fed 
into tube 11 at a pressure of 0.5 to 6 Torr and microwave power is applied 
at 140 W. Gas flow is sufficient to maintain a steady supply of ammonia 
(10 sccm is adequate). The photoluminescent intensity ("PL intensity") is 
then observed to increase by a factor of ten within a minute of plasma 
turn-on and then to slowly increase to approximately twenty-five times the 
initial level after a few minutes depending on plasma gas pressure and 
power. The plasma and gas flow is then turned off, and the 
photoluminescent intensity increases abruptly to reach its final value. A 
typical PL intensity versus time plot for plasma gas at 1 Torr and 
microwave power at 140 W is shown in FIG. 2. 
While the theory underlying this operation is not necessary for this 
invention, applicants' best current understanding is that hydrogen atoms 
are created in the plasma via electron impact dissociation of ammonia. 
These atoms are transported downstream via convection and diffusion to the 
GaAs surface where they chemisorb onto the surface and react with As and 
As.sub.2 O.sub.3 to form AsH.sub.3. The removal of As and As.sub.2 O.sub.3 
reduces arsenic antisite defects leading to the observed immediate 
increase in PL intensity. 
The slow increase, following the initial rapid increase in PL, is believed 
to be due to plasma-assisted modification or formation of gallium oxide. 
The sudden increase in PL intensity when the plasma is gated off is 
believed attributable to the rapid desorbtion of H atoms from the surface. 
The dynamic equilibrium between the surface and the H-containing gas over 
the surface is disturbed when the plasma is gated off, resulting in 
desorption of H atoms and reduction in any surface states due to the H 
atoms. 
A variety of experiments were conducted to determine the effect of plasma 
exposure time, pressure, microwave power and ammonia flow rate. FIG. 3 is 
a graphical plot showing PL intensity versus exposure time for exposures 
of 30s (curve a), 240s (curve b), and 900s (curve c). The plasma was 
generated at 2 Torr and 140 W with 10 sccm ammonia flow rate. The graph 
shows that irrespective of when the plasma is turned off, the PL intensity 
jumps to the same final value. However, other tests show that the samples 
exposed a short time were less stable than the longer exposed samples. It 
is believed that during long exposure (in excess of about 5 minutes) 
gallium oxide forms on the surface, reducing the ability of H atoms 
chemisorbed on the surface to maintain surface states. At the end of about 
15 minutes the gallium oxide reaches a final self-limiting thickness and 
passivates the GaAs surface. It is further believed that the presence of 
atomic hydrogen on the surface plays an essential role in the gallium 
oxide formation via the reaction: 
EQU As.sub.2 O.sub.3 +2GaAs+12H.fwdarw.Ga.sub.2`O.sub.3 +4AsH.sub.3 
FIG. 4 is a graphical illustration of the PL intensity as a function of 
time for several different plasma pressures ranging from 0.5 Torr to 6 
Tort when GaAs samples are treated with a downstream microwave ammonia 
plasma operating at 140 W and 10 sccm. As can be readily observed, 
passivation takes longer for higher pressures. This result is surmised to 
be due to a decrease in hydrogen atom concentration with increasing 
pressure. At higher pressures, fewer hydrogen atoms are brought downstream 
to the sample, and the passivation process therefore takes longer. 
FIG. 5 shows the effect of ammonia flow rate (ranging from 2 sccm to 50 
sccm) on passivation at 2 Torr and 140 W. As can be seen, passivation is 
faster at higher flow rates. This is consistent with the fact that at 
higher flow rates the flux of hydrogen atoms to the GaAs surface is 
higher. 
FIG. 6 shows the effect of microwave power on passivation. In the 
experimental ranges, the higher the power, the faster the passivation. 
This effect is believed due to the higher flux of hydrogen atoms at higher 
powers. Power in the range 75-140 W is believed to be optimal. 
FIG. 7 is a graphical illustration showing the PL intensity for several 
samples as a function of time after treatment. Each of the samples was 
treated for 9 minutes or more, and the samples were kept in a dry box 
purged with dry nitrogen. The samples lost only about 40% of their PL 
intensity over a month, and this reduced intensity is still ten times the 
PL intensity for unpassivated GaAs. The PL intensity for samples kept in 
ambient air decays faster than for samples kept in a dry box. 
The downstream ammonia passivation process offers numerous advantages over 
RF parallel plate passivation using hydrogen or ammonia. In the parallel 
plate process the GaAs surface is damaged by bombarding ions with the 
consequence that timing can be critical. Ion damage occurs rapidly after 
an initial maximum in PL, and even a few seconds delay in turning the 
plasma off reduces the final PL level attainable. In parallel plate 
passivation low pressures below about 0.5 Torr permit high energy ion 
bombardment of the GaAs surface which, in turn, damages the surface 
introducing new surface states. In addition, the ion bombardment damage is 
reflected in reduced stability. In two weeks RF NH.sub.3 passivated 
samples kept in a dry box lost 70% of their PL intensity. This instability 
is believed attributable to voids and defects in the gallium oxide formed 
under ion bombardment. Such voids and defects permit oxygen atoms from 
ambient air to diffuse to the GaAs interface to form As.sub.2 O.sub.3. 
Thus downstream ammonia passivation can be carried out at room temperature 
without heating. As compared with RF parallel plate passivation, 
downstream ammonia passivation is less sensitive to exposure time and 
provides an enhanced stability passivation surface. Another advantage is 
that the sample can be overexposed to plasma effluents to ensure uniform 
passivation over larger areas (2"-3" wafers). This is unlike RF where 
overexposure would damage surface. In addition downstream passivation does 
not require electrodes which can be a source of contamination. The 
downstream process provides an inexpensive and efficient technique for 
passivation which can be clustered with existing dry processes. 
It is to be understood that the above-described embodiments are 
illustrative of only a few of the many possible specific embodiments which 
can represent applications of the principles of the invention. Numerous 
and varied other methods can readily be devised in accordance with these 
principles by those skilled in the art without departing from the spirit 
and scope of the invention.