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Plasma enhanced chemical vapour deposition of tungsten and tungsten silicide thin films
Applied
Surface Science 73 (lYY3) 58-63
applied surface science
North-Holland
Plasma enhanced chemical vapour deposition of tungsten and tungsten silicide thin films S. McClatchie, School
of Electrical,
Received
2Y March
H. Thomas Electronic
1993; accepted
The plasma enhanced
and D.V. Morgan
and Systems Engineering,
for publication
14 April
chemical vapour deposition
over a range of deposition
conditions
of
Urkersify
W&s.
P. 0. BO.Y 017, Curdiff
CF_’ IXH,
1993
of tungsten and tungsten silicide thin films onto GaAs
using a modified
UK
parallel
plate radial flow reactor.
has been investigated
The tungsten films deposited
from WF,
and HZ were typically smooth and adherent with as-deposited resistivities in the range 20-100 /~iL.crn and a grain size in the 0 range 200-2000 A. Schottky properties for diodes fabricated from the as-deposited layers showed CurrentCvoltage characteristics governed smooth
by thermionic and adherent
properties. the WF,
emission with ideality factors (n) as low as 1.04. WSi, with
a grain
size
< 200 A. Schottky
diodes
fabricated
films deposited from
from WF,
and SiH,
these layers also exhibited
were typically good Schottkq
It was possible to deposit films ranging from nearly pure tungsten to nearly pure silicon using this technique
: SiH,
by varying
gas flow ratio
1. Introduction
particular emphasis placed on the electrical erties of the deposited layers.
Plasma enhanced chemical vapour deposition (PECVD) has been used extensively for the deposition of insulating and semi-insulating films for use in the electronics industry. The deposition of tungsten and tungsten silicide thin films using PECVD has also been reported [l-4]. The majority of the previous studies have centred on the use of these materials for silicon based technology, where they have been used as interconnect metallisations and for via hole filling applications. A particular problem with films deposited by this technique was the need to carry out a post deposition annealing step to obtain a good quality film. Films deposited by this technique have therefore not generally found favour, with films deposited using industry standard techniques such as sputtering and CVD remaining predominant. Comparatively little work has been carried out on the deposition of these materials by PECVD onto GaAs. That which has, has been concerned with the deposition process only. This paper presents the results of studies into the PECVD of tungsten and tungsten silicide onto GaAs, with 0169.4332/93/$06.00
‘C, IYY3
Elsevier
Science Publishers
2. Experimental
prop-
procedure
The tungsten and tungsten silicide films were deposited by PECVD onto the GaAs substrates using a modified commercially available parallel plate radial flow reactor, operating at a radio frequency of 100 kHz. Samples were loaded onto the resistively heated lower electrode via a nitrogen glove box. The system was then evacuated to a base pressure of < 1 x 1K5 Torr prior to deposition, using a turbo molecular pump mounted directly below the chamber. Subsequent plasma processing was carried out using a close coupled roots/ rotary combination pump with cxtcrnal oil filter. Process gases were fed to the reactor via separate mass flow controlled chemically cleaned stainless steel pipelines. The WF, line was maintained at a temperature of approximately 2S’C in order to prevent the possibility of the condensation of gaseous WF, in the pipeline. Control of chamber pressure was achieved using an auto-
B.V. All rights reserved
S. McClatchie
Ed al. / PECVD
of tungsten
matic pressure controlled throttle valve mounted in the pumping port. Two types of GaAs substrate material were used in this study. Semi-insulating HB material was used for measurement of the deposition rate, sheet resistance and surface morphology, and Si doped 1 x 10lh cm-j material for the fabrication of Schottky diodes. The wafers were cleaved into pieces of approximately 1 cm2 and cleaned using a standard process in boiling solvents. The samples were then deoxidised in a 3% ammonia solution just prior to loading into the deposition chamber. The as-deposited films were characterised using a variety of measurement techniques. The thickness of the layers was measured by masking a portion of the sample prior to deposition and measuring the resultant step height using a Dektak surface profiler. The sheet resistance of the films was measured using a standard four point probe technique. Surface morphology was studied using both optical and scanning electron microscopy (SEMI. The average grain size was estimated by dividing the size of the field of view of the SEM images by the number of grains observed in that field. A measure of the adhesion of the films to the GaAs substrates was obtained by subjecting them to a “Sellotape” test, where poor adhesion was denoted by the film being removed from the substrate as the Sellotape was removed. An estimate of the film stress was also obtained by measuring the wafer bow of a thin strip of GaAs material both before and after deposition using a calibrated optical microscope. In the case of the silicide films the tungsten/silicon ratio was measured using electron microprobe analysis (EMPA). Subsequent rapid thermal processing of the films was carried out in a Heatpulse rapid thermal annealing system. Schottky diodes were fabricated from the deposited layers, with the ohmic back contacts being formed by alloying evaporated Au/Ge/Ni layers to the backside of the wafers at a temperature of 385°C for 3 min. The current-voltage (Z-V’) and capacitance-voltage (C-V) measurements, carried out on Schottky diodes fabricated from the deposited layers, were measured under computer control using a Keithley Instruments
59
and tungsten silicide thin films
236 source measure unit (SMU) and a Hewlett Packard multi-frequency LCR meter, respectively. Where measurements were made at temperatures other than room temperature, a liquid nitrogen cryostat was used.
3. Results and discussion 3.1. Tungsten Tungsten was deposited from tungsten fluoride (WF,) and hydrogen (H,). WF,(g)
+ H*(g)
e W(s)
hexa-
+ HF(g).
Addition of hydrogen to the plasma was necessary, as it was found that no deposit was observable from pure WF, discharges. It was found that for such a discharge, etching of the film was favoured over deposition. The addition of hydrogen results in the scavenging of the etchant fluorine species, with the result that deposition becomes the dominant process. The HZ: WF, ratio was fixed at 3 : 1 throughout these studies. Due to the problems associated with the successful transfer of plasma processes from one type of system to another, we have investigated the effect of plasma pressure, temperature, gas flow, and power on the deposition rate over a range of possible process condition. The range of conditions studied was: WF, flow: 5-15 seem, pressure: 50-350 mTorr, temperature: 250-350°C and RF power: O-75 W. Initially, an experiment was designed utilising an orthogonal matrix in order that the effect of each of the parameters on the overall deposition could be ascertained. These experiments were implemented, and followed up by further in-depth investigations into the effect of each of the parameters. The purpose of these experiments was to determine the optimum reactor conditions necessary to maximise the deposition rate. 3.1.1. Deposition rate Typical results showed that the films smooth and adherent with the deposition increasing with plasma power, pressure, gas and temperature. Fig. 1 shows the effect of
were rate flow pres-
sure on the deposition rate, whilst fig. 2 shows the effect of plasma power. It is noted that no deposit was observed in the absence of the plasma. The effect of temperature on the deposition rate is shown in fig. 3. Using thcsc results an optimum process was determined. A typical deposition rate under these optimum conditiots, with the WF, gas flow set at 15 seem, was 100 A min ‘. This rate was approxi-
mately twice that of the previous non-optimiscd process. A lower rate was observed whcrc powder deposits were formed on the chamber walls.
The as-deposited sheet resistance of the films was dependent on all of the process paramctcrs. Sheet r&stance increased with increasing prcssure but fell with increasing power. gas flow and temperature. The resistivity of films deposited under the optimum conditions as dctcrmincd above, were calculated from the sheet resistance and film thickness measurements. A typical value of 20 ,LL~ cm was obtained.
SEM analysis of the as-deposited films showed that they were granular in nature. The average grain size increased with increasing substrate temperature and plasma power. However, grain size was found to be critically dependent on the integrity of the vacuum immcdiatcly prior to dcposition. The presence of even small leaks of the in the formation order of 2-3 mT min ‘, result4 of small grains of < 200 A. This was accompanied by the deposition of a black powder onto the cool surfaces of the chamber walls and top plate. The powder deposits were identified using X-rag
S. Md’lutchie
et cd. / PECVD of turzgs~er~ md tungsrrn divide
61
thin films
analysis as a variety of tungsten oxides. Films deposited under such conditions exhibited multiple tungsten peaks in their X-ray spectra. In contrast, films deposited under the adopted optimum conditions, with good vacuum integOrity, exhibited a grain size in the region of 2000 A. X-ray analysis of these films revealed a single large tungsten peak, indicating the presence of a single tungsten phase for these films. 3. I. 4. Adhesion All of the films in the as-deposited state showed no signs of peeling on removal from the deposition chamber. However, films deposited at plasma powers above 25 W showed selective peeling following the Sellotape adhesion test. These higher power films also showed signs of peeling during subsequent wet processing. Adhesion was critically dependent on the quality of the surface preparation, with bad adhesion resulting from a less than stringent pre-deposition cleaning step. The use of a hydrogen plasma cleaning step, prior to the deposition, resulted in very poor adhesion. Films deposited with this pre-treatment step, peeled away from the substrate on removal from the deposition chamber. This behaviour is in contrast to that observed on silicon substrates, where a hydrogen plasma pre-treatment significantly improved adhesion.
8-t
,’
L
;m ;
~oI.‘I’tlGl:
,J
1~
,,,
(1)
plot for diode D I.
Fig 3. Current-voltage
tained. The I-V characteristics were also obtained as a function of temperature. A plot of log(Z,/T*) versus 1000/T for the same diode (Dl) is shown in fig. 5. This plot gave a consistent barrier height of 0.67 eV. The effect of increasing the plasma power to the system, was to cause an increase in the ideal-
3.1.5. Stress Stress measurements revealed only qualitative results due to the limitations of the available equipment. Films deposited at high plasma powers exhibited tensile stress, whilst the stress for those deposited at lower powers was too low to be measured. 3.1.6. Electricul properties A typical I-V plot for a diode (Dl), which was fabricated from a film deposited using the optimised process, is shown in fig. 4. The device shows good Schottky behaviour, with the characteristic governed by thermionic emission. Diode parameters for this device were determined using a computer based curve fitting routine. The solid line in the plot shows this fit. An ideality factor of 1.04 and a barrier height of 0.67 eV were ob-
0
:
1000
Fig 5. Richardson
9
G
‘I (I\
I .!
‘j
plot for diode D 1.
I ‘-1
and that this modification was dependent on the plasma power used for the deposition. Notwithstanding, good Schottky diodes can be deposited by PECVD using low plasma powers.
400 KHz 1 200 KHz .I 100 KHz 40 KHz . 20 KHZ
3.2. Tungsten silicide WSi, was deposited from tungsten hexa-fluoride and dilute (5% in Nz) silane (SiH,). WF,(g)
‘-’
1
0
I
REVERYE Fig 6. Capacitance-voltage
2
:?
3
7
RllS (I) plot for diode Dl.
ity factor of the diodes, with the y1 value increasing to 1.2 for those deposited at a power of 75 W. Fig. 6 shows the l/C’ versus V plot, as a function of frequency, for the same Dl device. The plots show some departure from linearity, but this is consistent with a variation in Nd within the wafer manufacturers specification. Furthermore, a slight frequency dispersion of capacitance was observed over the range 20 kHz-1 MHz. The complex impedance and the phase angle 4 were monitored for these measurements and showed 4 > 80” for all deposition conditions except for those at the higher power of 75 W. A conventional zero bias thermally stimulated current (TSC) experiment was performed, and no difference in the trap levels present for the tungsten (Dl) diode and a control Au evaporated diode was observed. To investigate the region at the interface further, a surface emission technique was used [5]. Surface emission was found to occur for the tungsten diode (Dl) but not for the Au control diode. Diodes deposited at higher plasma powers exhibited larger emission currents, but these layers peeled away from the substrate during the thermal cycling necessary for the surface emission technique. These results indicate that a modification at the W/GaAs interface has taken place as a result of the PECVD process,
+ SiH,(g)
ti WSi,( s) + HF( g).
Preliminary studies have indicated that the deposition process was non-uniform with tungsten rich deposits occurring at the centre of the lower electrode and silicon rich deposits at the edge. Typical results showed that deposited films were smooth and adherent with the deposition rate increasing with plasma power, pressure, gas flow and temperature. The as-deposited sheet resistance of the films decreased with increasing temperature and gas flow and increased with increasing plasma power. In contrast to the case of the tungsten films discussed earlier, SEM anaysis revealed no observable surface features under any of the deposition conditions. Diodes fabricated using films deposited by PECVD. showed good Schottky properties, as was the case for the tungsten diodes discussed earlier, and remained stable following RTA processing at temperatures of up to 900°C. The effect of the process conditions on the stoichiometry of the deposited layers is currently being investigated, in conjunction with a modification to gas inlet system used in the PECVD equipment.
4. Conclusions It has been shown that good tungsten Schottky contacts to &AS can be formed using PECVD at low plasma powers. Under these conditions, and with the remaining process parameters optimised to give a deposition rate of the order of 100 A min-‘, the uniformity of this process across the lower electrode was k 596, with a repeatability of +5% run to run. WSi, films have also be deposited by this technique, with initial results also indicating that
S. McClatchie et al. / PECVD of tungsten and tungsfen silicide thin films
good electrical characteristics tegrity are obtained.
and
surface
in-
Acknowledgements The authors would like to thank the staff at Oxford Plasma Technology for all of their help during these investigations and R. Jones for the EMPA experiments. This work was supported by the Science and Engineering Research Council.
63
References [l] K. Akimoto and K. Watanabe, Appl. Phys. Lett. 39 (1981) 445. [2] J.K. Chu, C.C. Tang and D.W. Hess, Appl. Phys. Lett. 41 (1982) 75. [3] C.C. Tang and D.W. Hess, Appl. Phys. Lett. 45 (1984) 633. [4] Y.T. Kim, S. Min, J.S. Hong and C.K. Kim, Appl. Phys. Lett. 58 (1991) 837. [S] P.D. Taylor and D.V. Morgan, Solid State Electron. 19 (1973) 473.
Surface Science 73 (lYY3) 58-63
applied surface science
North-Holland
Plasma enhanced chemical vapour deposition of tungsten and tungsten silicide thin films S. McClatchie, School
of Electrical,
Received
2Y March
H. Thomas Electronic
1993; accepted
The plasma enhanced
and D.V. Morgan
and Systems Engineering,
for publication
14 April
chemical vapour deposition
over a range of deposition
conditions
of
Urkersify
W&s.
P. 0. BO.Y 017, Curdiff
CF_’ IXH,
1993
of tungsten and tungsten silicide thin films onto GaAs
using a modified
UK
parallel
plate radial flow reactor.
has been investigated
The tungsten films deposited
from WF,
and HZ were typically smooth and adherent with as-deposited resistivities in the range 20-100 /~iL.crn and a grain size in the 0 range 200-2000 A. Schottky properties for diodes fabricated from the as-deposited layers showed CurrentCvoltage characteristics governed smooth
by thermionic and adherent
properties. the WF,
emission with ideality factors (n) as low as 1.04. WSi, with
a grain
size
< 200 A. Schottky
diodes
fabricated
films deposited from
from WF,
and SiH,
these layers also exhibited
were typically good Schottkq
It was possible to deposit films ranging from nearly pure tungsten to nearly pure silicon using this technique
: SiH,
by varying
gas flow ratio
1. Introduction
particular emphasis placed on the electrical erties of the deposited layers.
Plasma enhanced chemical vapour deposition (PECVD) has been used extensively for the deposition of insulating and semi-insulating films for use in the electronics industry. The deposition of tungsten and tungsten silicide thin films using PECVD has also been reported [l-4]. The majority of the previous studies have centred on the use of these materials for silicon based technology, where they have been used as interconnect metallisations and for via hole filling applications. A particular problem with films deposited by this technique was the need to carry out a post deposition annealing step to obtain a good quality film. Films deposited by this technique have therefore not generally found favour, with films deposited using industry standard techniques such as sputtering and CVD remaining predominant. Comparatively little work has been carried out on the deposition of these materials by PECVD onto GaAs. That which has, has been concerned with the deposition process only. This paper presents the results of studies into the PECVD of tungsten and tungsten silicide onto GaAs, with 0169.4332/93/$06.00
‘C, IYY3
Elsevier
Science Publishers
2. Experimental
prop-
procedure
The tungsten and tungsten silicide films were deposited by PECVD onto the GaAs substrates using a modified commercially available parallel plate radial flow reactor, operating at a radio frequency of 100 kHz. Samples were loaded onto the resistively heated lower electrode via a nitrogen glove box. The system was then evacuated to a base pressure of < 1 x 1K5 Torr prior to deposition, using a turbo molecular pump mounted directly below the chamber. Subsequent plasma processing was carried out using a close coupled roots/ rotary combination pump with cxtcrnal oil filter. Process gases were fed to the reactor via separate mass flow controlled chemically cleaned stainless steel pipelines. The WF, line was maintained at a temperature of approximately 2S’C in order to prevent the possibility of the condensation of gaseous WF, in the pipeline. Control of chamber pressure was achieved using an auto-
B.V. All rights reserved
S. McClatchie
Ed al. / PECVD
of tungsten
matic pressure controlled throttle valve mounted in the pumping port. Two types of GaAs substrate material were used in this study. Semi-insulating HB material was used for measurement of the deposition rate, sheet resistance and surface morphology, and Si doped 1 x 10lh cm-j material for the fabrication of Schottky diodes. The wafers were cleaved into pieces of approximately 1 cm2 and cleaned using a standard process in boiling solvents. The samples were then deoxidised in a 3% ammonia solution just prior to loading into the deposition chamber. The as-deposited films were characterised using a variety of measurement techniques. The thickness of the layers was measured by masking a portion of the sample prior to deposition and measuring the resultant step height using a Dektak surface profiler. The sheet resistance of the films was measured using a standard four point probe technique. Surface morphology was studied using both optical and scanning electron microscopy (SEMI. The average grain size was estimated by dividing the size of the field of view of the SEM images by the number of grains observed in that field. A measure of the adhesion of the films to the GaAs substrates was obtained by subjecting them to a “Sellotape” test, where poor adhesion was denoted by the film being removed from the substrate as the Sellotape was removed. An estimate of the film stress was also obtained by measuring the wafer bow of a thin strip of GaAs material both before and after deposition using a calibrated optical microscope. In the case of the silicide films the tungsten/silicon ratio was measured using electron microprobe analysis (EMPA). Subsequent rapid thermal processing of the films was carried out in a Heatpulse rapid thermal annealing system. Schottky diodes were fabricated from the deposited layers, with the ohmic back contacts being formed by alloying evaporated Au/Ge/Ni layers to the backside of the wafers at a temperature of 385°C for 3 min. The current-voltage (Z-V’) and capacitance-voltage (C-V) measurements, carried out on Schottky diodes fabricated from the deposited layers, were measured under computer control using a Keithley Instruments
59
and tungsten silicide thin films
236 source measure unit (SMU) and a Hewlett Packard multi-frequency LCR meter, respectively. Where measurements were made at temperatures other than room temperature, a liquid nitrogen cryostat was used.
3. Results and discussion 3.1. Tungsten Tungsten was deposited from tungsten fluoride (WF,) and hydrogen (H,). WF,(g)
+ H*(g)
e W(s)
hexa-
+ HF(g).
Addition of hydrogen to the plasma was necessary, as it was found that no deposit was observable from pure WF, discharges. It was found that for such a discharge, etching of the film was favoured over deposition. The addition of hydrogen results in the scavenging of the etchant fluorine species, with the result that deposition becomes the dominant process. The HZ: WF, ratio was fixed at 3 : 1 throughout these studies. Due to the problems associated with the successful transfer of plasma processes from one type of system to another, we have investigated the effect of plasma pressure, temperature, gas flow, and power on the deposition rate over a range of possible process condition. The range of conditions studied was: WF, flow: 5-15 seem, pressure: 50-350 mTorr, temperature: 250-350°C and RF power: O-75 W. Initially, an experiment was designed utilising an orthogonal matrix in order that the effect of each of the parameters on the overall deposition could be ascertained. These experiments were implemented, and followed up by further in-depth investigations into the effect of each of the parameters. The purpose of these experiments was to determine the optimum reactor conditions necessary to maximise the deposition rate. 3.1.1. Deposition rate Typical results showed that the films smooth and adherent with the deposition increasing with plasma power, pressure, gas and temperature. Fig. 1 shows the effect of
were rate flow pres-
sure on the deposition rate, whilst fig. 2 shows the effect of plasma power. It is noted that no deposit was observed in the absence of the plasma. The effect of temperature on the deposition rate is shown in fig. 3. Using thcsc results an optimum process was determined. A typical deposition rate under these optimum conditiots, with the WF, gas flow set at 15 seem, was 100 A min ‘. This rate was approxi-
mately twice that of the previous non-optimiscd process. A lower rate was observed whcrc powder deposits were formed on the chamber walls.
The as-deposited sheet resistance of the films was dependent on all of the process paramctcrs. Sheet r&stance increased with increasing prcssure but fell with increasing power. gas flow and temperature. The resistivity of films deposited under the optimum conditions as dctcrmincd above, were calculated from the sheet resistance and film thickness measurements. A typical value of 20 ,LL~ cm was obtained.
SEM analysis of the as-deposited films showed that they were granular in nature. The average grain size increased with increasing substrate temperature and plasma power. However, grain size was found to be critically dependent on the integrity of the vacuum immcdiatcly prior to dcposition. The presence of even small leaks of the in the formation order of 2-3 mT min ‘, result4 of small grains of < 200 A. This was accompanied by the deposition of a black powder onto the cool surfaces of the chamber walls and top plate. The powder deposits were identified using X-rag
S. Md’lutchie
et cd. / PECVD of turzgs~er~ md tungsrrn divide
61
thin films
analysis as a variety of tungsten oxides. Films deposited under such conditions exhibited multiple tungsten peaks in their X-ray spectra. In contrast, films deposited under the adopted optimum conditions, with good vacuum integOrity, exhibited a grain size in the region of 2000 A. X-ray analysis of these films revealed a single large tungsten peak, indicating the presence of a single tungsten phase for these films. 3. I. 4. Adhesion All of the films in the as-deposited state showed no signs of peeling on removal from the deposition chamber. However, films deposited at plasma powers above 25 W showed selective peeling following the Sellotape adhesion test. These higher power films also showed signs of peeling during subsequent wet processing. Adhesion was critically dependent on the quality of the surface preparation, with bad adhesion resulting from a less than stringent pre-deposition cleaning step. The use of a hydrogen plasma cleaning step, prior to the deposition, resulted in very poor adhesion. Films deposited with this pre-treatment step, peeled away from the substrate on removal from the deposition chamber. This behaviour is in contrast to that observed on silicon substrates, where a hydrogen plasma pre-treatment significantly improved adhesion.
8-t
,’
L
;m ;
~oI.‘I’tlGl:
,J
1~
,,,
(1)
plot for diode D I.
Fig 3. Current-voltage
tained. The I-V characteristics were also obtained as a function of temperature. A plot of log(Z,/T*) versus 1000/T for the same diode (Dl) is shown in fig. 5. This plot gave a consistent barrier height of 0.67 eV. The effect of increasing the plasma power to the system, was to cause an increase in the ideal-
3.1.5. Stress Stress measurements revealed only qualitative results due to the limitations of the available equipment. Films deposited at high plasma powers exhibited tensile stress, whilst the stress for those deposited at lower powers was too low to be measured. 3.1.6. Electricul properties A typical I-V plot for a diode (Dl), which was fabricated from a film deposited using the optimised process, is shown in fig. 4. The device shows good Schottky behaviour, with the characteristic governed by thermionic emission. Diode parameters for this device were determined using a computer based curve fitting routine. The solid line in the plot shows this fit. An ideality factor of 1.04 and a barrier height of 0.67 eV were ob-
0
:
1000
Fig 5. Richardson
9
G
‘I (I\
I .!
‘j
plot for diode D 1.
I ‘-1
and that this modification was dependent on the plasma power used for the deposition. Notwithstanding, good Schottky diodes can be deposited by PECVD using low plasma powers.
400 KHz 1 200 KHz .I 100 KHz 40 KHz . 20 KHZ
3.2. Tungsten silicide WSi, was deposited from tungsten hexa-fluoride and dilute (5% in Nz) silane (SiH,). WF,(g)
‘-’
1
0
I
REVERYE Fig 6. Capacitance-voltage
2
:?
3
7
RllS (I) plot for diode Dl.
ity factor of the diodes, with the y1 value increasing to 1.2 for those deposited at a power of 75 W. Fig. 6 shows the l/C’ versus V plot, as a function of frequency, for the same Dl device. The plots show some departure from linearity, but this is consistent with a variation in Nd within the wafer manufacturers specification. Furthermore, a slight frequency dispersion of capacitance was observed over the range 20 kHz-1 MHz. The complex impedance and the phase angle 4 were monitored for these measurements and showed 4 > 80” for all deposition conditions except for those at the higher power of 75 W. A conventional zero bias thermally stimulated current (TSC) experiment was performed, and no difference in the trap levels present for the tungsten (Dl) diode and a control Au evaporated diode was observed. To investigate the region at the interface further, a surface emission technique was used [5]. Surface emission was found to occur for the tungsten diode (Dl) but not for the Au control diode. Diodes deposited at higher plasma powers exhibited larger emission currents, but these layers peeled away from the substrate during the thermal cycling necessary for the surface emission technique. These results indicate that a modification at the W/GaAs interface has taken place as a result of the PECVD process,
+ SiH,(g)
ti WSi,( s) + HF( g).
Preliminary studies have indicated that the deposition process was non-uniform with tungsten rich deposits occurring at the centre of the lower electrode and silicon rich deposits at the edge. Typical results showed that deposited films were smooth and adherent with the deposition rate increasing with plasma power, pressure, gas flow and temperature. The as-deposited sheet resistance of the films decreased with increasing temperature and gas flow and increased with increasing plasma power. In contrast to the case of the tungsten films discussed earlier, SEM anaysis revealed no observable surface features under any of the deposition conditions. Diodes fabricated using films deposited by PECVD. showed good Schottky properties, as was the case for the tungsten diodes discussed earlier, and remained stable following RTA processing at temperatures of up to 900°C. The effect of the process conditions on the stoichiometry of the deposited layers is currently being investigated, in conjunction with a modification to gas inlet system used in the PECVD equipment.
4. Conclusions It has been shown that good tungsten Schottky contacts to &AS can be formed using PECVD at low plasma powers. Under these conditions, and with the remaining process parameters optimised to give a deposition rate of the order of 100 A min-‘, the uniformity of this process across the lower electrode was k 596, with a repeatability of +5% run to run. WSi, films have also be deposited by this technique, with initial results also indicating that
S. McClatchie et al. / PECVD of tungsten and tungsfen silicide thin films
good electrical characteristics tegrity are obtained.
and
surface
in-
Acknowledgements The authors would like to thank the staff at Oxford Plasma Technology for all of their help during these investigations and R. Jones for the EMPA experiments. This work was supported by the Science and Engineering Research Council.
63
References [l] K. Akimoto and K. Watanabe, Appl. Phys. Lett. 39 (1981) 445. [2] J.K. Chu, C.C. Tang and D.W. Hess, Appl. Phys. Lett. 41 (1982) 75. [3] C.C. Tang and D.W. Hess, Appl. Phys. Lett. 45 (1984) 633. [4] Y.T. Kim, S. Min, J.S. Hong and C.K. Kim, Appl. Phys. Lett. 58 (1991) 837. [S] P.D. Taylor and D.V. Morgan, Solid State Electron. 19 (1973) 473.