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The influence of ohmic metal composition on the characteristics of ohmic contacts
Vacuum/volume40/numbers 1/2/pages 179 to 181/1990
0042-207X/90S3.00+.00 Pergamon Press plc
Printed in Great Britain
The i n f l u e n c e of o h m i c m e t a l c o m p o s i t i o n on the c h a r a c t e r i s t i c s of o h m i c c o n t a c t s B K o v ~ c s , I M o j z e s , R V e r e s e g y h f i z y , M N 6 m e t h - S a l l a y , S B i r b a n d B P 6 c z , Research Institute for Technical Physics of the Hungarian Academy of Sciences, H- 1325 Budapest, Ujpest 1, PO Box 76, Hungary
The A uGe/Ni/A u metallization is one of the most widely used contact structures in compound semiconductor devices. Many laboratories use a metal structure consisting of a AuGe eutectic (88:12) layer with, typically, 5% Ni. The influence of Ni proportion on the electrical parameters of contacts, the surface morphology and the volatile component loss during annealing were investigated. The results show that the effect of Ni proportion on the minimum value of specific contact resistance and on the surface morphology is much more significant than its effect on the optimum heat treatment temperature. 1. Sample preparation and measuring techniques The diagnostic pattern, applied in this investigation, was designed for use in a computer-controlled wafer tester system to control the G a A s M E S F E T technology. To minimize the number of measuring probes the different test patterns have an identical pad arrangement. The test patterns were formed by mesa etching and a subsequent metal evaporation onto G a A s wafers. The G a A s wafers which were used have a 0.25/~m thick n-type epitaxial layer on the Cr-doped semi-insulator substrate. The carrier concentration of the epitaxial layer is 1 x 1017 c m - 3. The carrier concentration profile was measured using both electrolytic and metal-semiconductor C - V profiling techniques. The agreement was very good taking into account the decrease of epitaxial layer thickness due to the chemical processing. The sheet resistances of the epitaxial layers were about 700 f~ s q - ~. The mesa etching was controlled by resistance measurement of the residual layer. The etching process was stopped when the measured resistance increased up to 10 MD. The metallization was prepared by the sequential deposition of A u G e (eutectic), Ni and Au. The applied layer thicknesses are shown in Table 1. The metal layers were evaporated in the same vacuum cycle at a pressure lower than 10-4 Pa. The metallization structures were formed by a special lift-off process. To complete the diagnostic patterns the Schottky gate metallization was prepared in the similar way, applying a C r / A g / C r / A u metal sequence. The heat treatments of the samples were carried out in an open tube furnace applying a relatively slow heat pulse with 3 rain rise time. The alloying atmosphere was forming gas with
Table l. The layer thicknesses of the investigated contact materials Ni %
1st layer AuGe
2nd layer Ni
3rd layer Au
0 5 7 10 13
75 nm 75 nm 75 nm 75 nm 75 nm
0 nm 6 nm 9 nm 12.4 nm 16.1 nm
20 nm 20 nm 20 nm 20 nm 20 nm
30% H 2 : 7 0 % N2 composition. The studied temperature range was from 300-500°C. To determine the electrical parameters o f ohmic contacts three different measuring methods were used ; the van der Pauw technique for the sheet resistance measurements ~; the Kelvin resistor technique for the calculation of specific contact resistance'-; and the modified Transmission Line Model (TLM) method for the evaluation of specific contact resistance and the sheet resistance under the contact metallization 3'4. The different metallizations were investigated by the so-called evolved gas analysis method 6. The E G A measurements were carried out in a vacuum chamber. The arsenic loss of the heated samples was monitored by a quadrupole mass spectrometer. The arsenic yield depending on the actual temperature of heat treatment characterizes the metal interaction with GaAs. To check the surface morphology and the lateral redistribution of metal components, S E M investigations were carried out.
2. Results When determining the sheet resistance of the epitaxial layer after the contact annealing no characteristic increase was observed.
100
//•
8 ,,J
i,.,.Z / (~1
e-
[] 13% Ni • 10% Ni • 10% Ni o 7% Ni z~ 5% Ni • 0% Ni
1
300
400 Peak temperature (°C)
500
Figure l. The sheet resistance of annealed ohmic metallizations (f~ sq- *) vs the heat treatment temperature (°C). 179
B Kovdcs et al. The influence of ohmic metal composition on the characteristics of ohmic contacts
E-
1 n13% Ni 10 -~ 1°-2
E
v10% Ni • 10% Ni
{~ I~.I~~
07% Ni ~ 5% Ni
• 0%
,/!
10 -3
8 10 .4 Q. ~g
{
10-5 30O
400
500
Peak temperature (°C)
Figure 2. The specific contact resistances (fl cm 2) obtained on the cross bridge Kelvin resistor patterns.
The measured sheet resistance of some samples was relatively high but it was attributed to the original inhomogeneity of the VPE grown epitaxial G a A s layer. The sheet resistance of the ohmic contact metallization increased with the increasing alloying temperature (Figure 1), Two possible reasons for this are the Ga diffusion into the metallization and the island formation, which is known as the "bailingup" effect. The minimum value of specific contact resistance evaluated by any of the applied methods was in the range 370-440~C (Figures 2 and 3). The best specific contact resistance was obtained on the samples prepared by contact material containing 10% Ni, but this result was not reproducible. A more convincing result, obtained by the modified T L M method, showed that the contact version with 13% Ni was found to have the lowest minimum (Figure 3(a)).
1000 n 13% Ni
o 13% Ni
07% Ni
10-2
"~
100
._
10
m
1
O 7% Ni
A5% Ni
5% Ni
• 0% Ni
10 -3
3 10-4
• 0% Ni
.L_g
I
u}
5o0
400 (a)
ol 3OO
, 4O0
(b)
Peak temperature (°C)
" -500
Peak temperature (°C)
Figure 3. The contact parameters obtained on the modified TLM (Transmission Line Model) patterns : (a) the specific contact resistance (£2 cm 2) and (b) the epitaxial sheet resistance (f~ sq- ') beneath the contact, vs the heat treatment temperature (°C).
0% Ni
7% Ni
<
<
I
I O
(:3 x ¢,3 -o >, <
X -13
I
I
I ~
1
I
I
I
~-~'1
I
I
I /i tl[ ~ 1
I
I
100 200 300 400 500 600 700 Temperature (°C) 10% Ni
100 200 300 400 500 600 700 Temperature (°C) 5% Ni <
X
x ¢o -13
I
100 200 300 400 500 600 700 Temperature (°C)
Figure 4. The arsenic loss during the heat treatment. 180
<
/ i
100 200 300 400 500 600 700 Temperature (°C)
B Kov#cs et al: The influence of ohmic metal composition on the characteristics of ohmic contacts
The value ofepitaxial sheet resistance under the contact metallization vs the heat treatment temperature has a minimum at the same temperature region where the specific contact resistance has its lowest value (Figure 3(b)). The evolved gas analysis ( E G A ) was carried out according to the method described in ref 5. A heating rate of 150°C m i n - J was used. Similar to our previous results ~, the evolution of arsenic showed a two peak behaviour as a function of annealing temperature (Figure 4). N o evolution o f other species could be observed. Both the surface morphology and the E G A curve of the samples with 5% Ni in its contacts showed significant differences compared with the other samples. An attempt to determine the lateral inhomogeneity of species was unsuccessful in the case of 10-13% Ni.
3. Summary Studying the influence o f ohmic contact composition on the contact parameters a set o f g o l d - g e r m a n i u m based ohmic contacts with different Ni proportion was prepared. A very simple heat treatment method was chosen which is advantageous for device production.
The different composition of ohmic contact produce different optimum values of specific contact resistance but an unchanged optimum heat treatment temperature. This result is contrary to ref7. We suppose it is due to the different technical environment. The best electrical parameters and surface morphology were obtained in the case o f 10-13 % Ni in the contact material, alloyed at 370°C.
References L J van der Pauw, Philips Res Repts, 13, 1 (1958). z S J Proctor, L W Linholm and J A Mazer, IEEE Trans Electron Devices, ED-30, 1535 (1983). 3H H Berger, J Electrochem Soc, 119, 507 (1972). 4B Kov~ics and I Mojzes, IEEE Trans Electron Devices, ED-33, 1401 (1986). 5D Szigethy, I Mojzes and T Sebesty~n, Int J Mass-Spectrom, Ion Phys, 52, 179 (1983). 6B KovS_cs, I Mojzes, R Veresegyh~izy and B P~cz, 16th European Solid State Device Research Cop~ Cambridge (1986). 7W Patrick, W S Mackie, S P Beaumont and C D W Wilkinson, Appl Phys Lett, 48, 986 (1986).
181
0042-207X/90S3.00+.00 Pergamon Press plc
Printed in Great Britain
The i n f l u e n c e of o h m i c m e t a l c o m p o s i t i o n on the c h a r a c t e r i s t i c s of o h m i c c o n t a c t s B K o v ~ c s , I M o j z e s , R V e r e s e g y h f i z y , M N 6 m e t h - S a l l a y , S B i r b a n d B P 6 c z , Research Institute for Technical Physics of the Hungarian Academy of Sciences, H- 1325 Budapest, Ujpest 1, PO Box 76, Hungary
The A uGe/Ni/A u metallization is one of the most widely used contact structures in compound semiconductor devices. Many laboratories use a metal structure consisting of a AuGe eutectic (88:12) layer with, typically, 5% Ni. The influence of Ni proportion on the electrical parameters of contacts, the surface morphology and the volatile component loss during annealing were investigated. The results show that the effect of Ni proportion on the minimum value of specific contact resistance and on the surface morphology is much more significant than its effect on the optimum heat treatment temperature. 1. Sample preparation and measuring techniques The diagnostic pattern, applied in this investigation, was designed for use in a computer-controlled wafer tester system to control the G a A s M E S F E T technology. To minimize the number of measuring probes the different test patterns have an identical pad arrangement. The test patterns were formed by mesa etching and a subsequent metal evaporation onto G a A s wafers. The G a A s wafers which were used have a 0.25/~m thick n-type epitaxial layer on the Cr-doped semi-insulator substrate. The carrier concentration of the epitaxial layer is 1 x 1017 c m - 3. The carrier concentration profile was measured using both electrolytic and metal-semiconductor C - V profiling techniques. The agreement was very good taking into account the decrease of epitaxial layer thickness due to the chemical processing. The sheet resistances of the epitaxial layers were about 700 f~ s q - ~. The mesa etching was controlled by resistance measurement of the residual layer. The etching process was stopped when the measured resistance increased up to 10 MD. The metallization was prepared by the sequential deposition of A u G e (eutectic), Ni and Au. The applied layer thicknesses are shown in Table 1. The metal layers were evaporated in the same vacuum cycle at a pressure lower than 10-4 Pa. The metallization structures were formed by a special lift-off process. To complete the diagnostic patterns the Schottky gate metallization was prepared in the similar way, applying a C r / A g / C r / A u metal sequence. The heat treatments of the samples were carried out in an open tube furnace applying a relatively slow heat pulse with 3 rain rise time. The alloying atmosphere was forming gas with
Table l. The layer thicknesses of the investigated contact materials Ni %
1st layer AuGe
2nd layer Ni
3rd layer Au
0 5 7 10 13
75 nm 75 nm 75 nm 75 nm 75 nm
0 nm 6 nm 9 nm 12.4 nm 16.1 nm
20 nm 20 nm 20 nm 20 nm 20 nm
30% H 2 : 7 0 % N2 composition. The studied temperature range was from 300-500°C. To determine the electrical parameters o f ohmic contacts three different measuring methods were used ; the van der Pauw technique for the sheet resistance measurements ~; the Kelvin resistor technique for the calculation of specific contact resistance'-; and the modified Transmission Line Model (TLM) method for the evaluation of specific contact resistance and the sheet resistance under the contact metallization 3'4. The different metallizations were investigated by the so-called evolved gas analysis method 6. The E G A measurements were carried out in a vacuum chamber. The arsenic loss of the heated samples was monitored by a quadrupole mass spectrometer. The arsenic yield depending on the actual temperature of heat treatment characterizes the metal interaction with GaAs. To check the surface morphology and the lateral redistribution of metal components, S E M investigations were carried out.
2. Results When determining the sheet resistance of the epitaxial layer after the contact annealing no characteristic increase was observed.
100
//•
8 ,,J
i,.,.Z / (~1
e-
[] 13% Ni • 10% Ni • 10% Ni o 7% Ni z~ 5% Ni • 0% Ni
1
300
400 Peak temperature (°C)
500
Figure l. The sheet resistance of annealed ohmic metallizations (f~ sq- *) vs the heat treatment temperature (°C). 179
B Kovdcs et al. The influence of ohmic metal composition on the characteristics of ohmic contacts
E-
1 n13% Ni 10 -~ 1°-2
E
v10% Ni • 10% Ni
{~ I~.I~~
07% Ni ~ 5% Ni
• 0%
,/!
10 -3
8 10 .4 Q. ~g
{
10-5 30O
400
500
Peak temperature (°C)
Figure 2. The specific contact resistances (fl cm 2) obtained on the cross bridge Kelvin resistor patterns.
The measured sheet resistance of some samples was relatively high but it was attributed to the original inhomogeneity of the VPE grown epitaxial G a A s layer. The sheet resistance of the ohmic contact metallization increased with the increasing alloying temperature (Figure 1), Two possible reasons for this are the Ga diffusion into the metallization and the island formation, which is known as the "bailingup" effect. The minimum value of specific contact resistance evaluated by any of the applied methods was in the range 370-440~C (Figures 2 and 3). The best specific contact resistance was obtained on the samples prepared by contact material containing 10% Ni, but this result was not reproducible. A more convincing result, obtained by the modified T L M method, showed that the contact version with 13% Ni was found to have the lowest minimum (Figure 3(a)).
1000 n 13% Ni
o 13% Ni
07% Ni
10-2
"~
100
._
10
m
1
O 7% Ni
A5% Ni
5% Ni
• 0% Ni
10 -3
3 10-4
• 0% Ni
.L_g
I
u}
5o0
400 (a)
ol 3OO
, 4O0
(b)
Peak temperature (°C)
" -500
Peak temperature (°C)
Figure 3. The contact parameters obtained on the modified TLM (Transmission Line Model) patterns : (a) the specific contact resistance (£2 cm 2) and (b) the epitaxial sheet resistance (f~ sq- ') beneath the contact, vs the heat treatment temperature (°C).
0% Ni
7% Ni
<
<
I
I O
(:3 x ¢,3 -o >, <
X -13
I
I
I ~
1
I
I
I
~-~'1
I
I
I /i tl[ ~ 1
I
I
100 200 300 400 500 600 700 Temperature (°C) 10% Ni
100 200 300 400 500 600 700 Temperature (°C) 5% Ni <
X
x ¢o -13
I
100 200 300 400 500 600 700 Temperature (°C)
Figure 4. The arsenic loss during the heat treatment. 180
<
/ i
100 200 300 400 500 600 700 Temperature (°C)
B Kov#cs et al: The influence of ohmic metal composition on the characteristics of ohmic contacts
The value ofepitaxial sheet resistance under the contact metallization vs the heat treatment temperature has a minimum at the same temperature region where the specific contact resistance has its lowest value (Figure 3(b)). The evolved gas analysis ( E G A ) was carried out according to the method described in ref 5. A heating rate of 150°C m i n - J was used. Similar to our previous results ~, the evolution of arsenic showed a two peak behaviour as a function of annealing temperature (Figure 4). N o evolution o f other species could be observed. Both the surface morphology and the E G A curve of the samples with 5% Ni in its contacts showed significant differences compared with the other samples. An attempt to determine the lateral inhomogeneity of species was unsuccessful in the case of 10-13% Ni.
3. Summary Studying the influence o f ohmic contact composition on the contact parameters a set o f g o l d - g e r m a n i u m based ohmic contacts with different Ni proportion was prepared. A very simple heat treatment method was chosen which is advantageous for device production.
The different composition of ohmic contact produce different optimum values of specific contact resistance but an unchanged optimum heat treatment temperature. This result is contrary to ref7. We suppose it is due to the different technical environment. The best electrical parameters and surface morphology were obtained in the case o f 10-13 % Ni in the contact material, alloyed at 370°C.
References L J van der Pauw, Philips Res Repts, 13, 1 (1958). z S J Proctor, L W Linholm and J A Mazer, IEEE Trans Electron Devices, ED-30, 1535 (1983). 3H H Berger, J Electrochem Soc, 119, 507 (1972). 4B Kov~ics and I Mojzes, IEEE Trans Electron Devices, ED-33, 1401 (1986). 5D Szigethy, I Mojzes and T Sebesty~n, Int J Mass-Spectrom, Ion Phys, 52, 179 (1983). 6B KovS_cs, I Mojzes, R Veresegyh~izy and B P~cz, 16th European Solid State Device Research Cop~ Cambridge (1986). 7W Patrick, W S Mackie, S P Beaumont and C D W Wilkinson, Appl Phys Lett, 48, 986 (1986).
181