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GaAs HBT's with tungsten N and P type ohmic contacts
Microelectronic Engineering 15 (1991) 161-164 Elsevier
161
SELF-ALIGNED AIGaAs/GaAs HBT's WITH TUNGSTEN N AND P TYPE O H M I C CONTACTS
P.LAUNAY, B.BAMUENI, A.M.DUCHENOIS, P.BLANCONNIER Centre National d'Etude des Trlrcommunications Laboratoire de bagneux, 196 av. Henri Ravera, 92220 BAGNEUX ABSTRACT A self-aligned GaA1As/GaAs HBT technology with tungsten emitter and base ohmic contacts is presented. The HBT's layers are grown by MBE, the emitter contact layer is Ga0.35In0.65As. A Zinc diffusion allows for contacting the extrinsic base layer. Self-alignment is obtained by the diffusion around the emitter mesa; nitride spacers are used to isolate the intrinsic area during the diffusion process. I) I N T R O D U C T I O N Although widely used for the fabrication of HBT circuits, the mesa technology is facing several difficulties: Firstly it is difficult to reach the base layer, especially when this layer is thin, and to obtain a good base contact. In particular if a thin AIGaAs layer is left after the emitter mesa etching, the specific contact resistivity is degraded (1). Secondly it is well known that gold based ohmic contacts are not stable when the temperature exceeds 500"C. So it is impossible to use high temperature processes during the HBT fabrication. Moreover the reliability of this kind of contact is still an open question. Thirdly plasma etching cannot be used with gold metallizations. These two last points are also a problem for an ion-implanted technology (2). In this paper we present a way to overcome these difficulties with a simple self-aligned technology. A zinc diffusion at low temperature is used to contact the base layer and to realize the self-alignment. Emitter and base ohmic contacts are made with tungsten. We will describe the diffusion process and the electrical base contact in a first part. In a second part the whole HBT process and the electrical results on self-aligned transistors will be presented. II) T H E T U N G S T E N BASE O H M I C C O N T A C T PROCESS. The diffusion step relies on the semi open box technique using a solid source of ZnAs2 (3). After investigation of different diffusion processes as function of time and temperature to minimise the diffusion depth, we have chosen the following process: the temperature is 580°C during 1 mn after a ramp of 13 nan, the diffusion depth is about 2000 A in GaAs. A complete characterization has been made on several layers with different doping levels (see table 1). For this purpose epitaxial layers of 1 gm depth have been grown by MBE and MOCVD. After Zn diffusion, each sample is separated in two parts; on one part a slight chemical etch ( 20 nm ) is performed and nothing is done on the other part. Finally the tungsten metal is deposited and lifted-off. 0167-9317191]$3.50 © 1991 -Elsevier Science Publishers B.V. All fights reserved.
P. Launay et aL / Self-alignedAlGaAs/GaAs HBT's
162
The electrical results are obtained on test patterns by the Transmission Line Model (TLM); SIMS and Polaron analysis are used to determine the Zinc surface concentration. The zinc diffusion creates a highly p-type doped area on which the ohmic contact is made. So we evaluate electricaly the possibility on the different samples to realize this p-type area.
Table 1: Tungsten ohmic contact resistivity on different layers Layer
% A1
Doping
Resistivity
( Dxzmz )
Resistivity After 200 A
(f~cm2 ) etching
( cm 3 )
Mean
Sigma
Mean
Sigma
MBE
0
Be (2 10 TM)
8.9 10.7
1.3 10v
MBE
0
Be ( 1019)
1.7 10 .6
2.6 10 -7
1.2 10 .6
5.2 10.7
MOCVD
0
Zn (2 1017)
1.9
2.4 10-7
1.6
10 -6
5.1 10 .7
MOCVD
0
Zn (4 10 TM)
2.1 10-5
8.3
10 -6
1.1 10 .6
2.1 10 -7
MOCVD
0
Zn (1.7 1019)
1.4 10 -6
2.1 10 -7
1.9 10 .6
3.9 10.7
MBE
30
Si (2 1017)
1.2 10.5
5.4 10 -6
1.7 10.5
3.7 10.6
MBE
20
Si (2 1017)
3.4
10 .6
8 10 -6
4.1 10 .6
MBE
10
Si (2 1017)
1.2 10.6
3.3 10 -7
7.4 10 -7
10 -6
10 -6
9.2
10 -7
In the case of GaAs layers the specific contact resistivity, between 8 x 1 0 -7 and 2xl0-6f~cm z, is independant of the initial p-type doping level. This indicates for these samples a good zinc diffusion process whatever the initial dopant or doping level are. This is confirmed by SIMS and Polaron analysis which give Zinc Surface concentration after diffusion higher than 1020cm 3. These results are in agreement with the model of W.Dingfen and al (4): They evaluate a specific contact resistivity of 10 .6 ~-]cm2 for a high surface doping of 10z° cm -3. In each case the etched sample gives better results, particularly for one of the samples. This indicates that surface pollutions can occur during the diffusion process or in the other process steps. Electrical results on GaAIAs layers, doped with Si ( 2x1017cm -3 ) to simulate the emitter layer, show an increase of the specific resistivity with the aluminum content. The low Zn surface concentration ( 5x1019cm 3 ) for the layer with 30 % aluminum, by comparaison with higher surface doping ( 102°cm3 ) in the other layers, a higher gap and other physical parameters explains these results. Two conclusions must be pointed out: In the HBT process it will be necessary to etch the GaA1As before tungsten deposition to contact thebase layer ( see part III ). Secondly in the case of aluminum graded base, where the optimum aluminum percentage is around 10%, the zinc diffusion process to contact the base with a tungsten ohmic contact should be applicable. In the HBT process the zinc diffusion proceeds through a thin GaAIAs ( 100 n m ) and the GaAs base layer. To study the diffusion in the actual situation we have grown by MBE all the layers of the HBT with an initial base doping of 3x1018cm 3. After a partial emitter mesa etching the zinc diffusion proceeds in the base extrinsic area.
P. Launay et a L / Self-aligned AlGaAs/GaAs HBT's SIMS profile in the intrinsic area, not shown here, indicates that the temperature of the diffusion process is low enough to avoid Beryllium outdiffusion from the base to the emitter in the transistor intrinsic area. Figure 1 shows the SIMS profile in the extrinsic base area. The zinc concentration at the GaA1As surface is low, but this concentration in the base layer defines a P* zone of 10Z°cm3 which allows the use of a tungsten metallization to contact the base after a thin GaA1As etching. We can also observe a Beryllium out-diffusion. Zinc atoms diffuse through an interstitial-substitutional mechanism and would replace the Be in the substitutional site as P.A.Houston and al (5) suggest, via the reaction:
10"1020.
~ ~0"-~
~.~0,o
f
~ 10rl AI
~°~
Be
~
Zn
..base..collector
0'.z
Zni÷ + Bes = Zn~ + Bei+ In the case of zinc as initial dopant (MOCVD), zinc atoms can occupy the vacancies in the layer during the diffusion process.
163
,.subcoilector
d.i
o'.~
o'.B
OEPTH(~m)
Figure 1: SIMS profile after Zn diffusion in the HBT.
III) THE SELF-ALIGNED AIGaAs/GaAs HBT Figure 2 shows the final HBT structure and the transistor layers. A tungsten film is deposited by RF sputtefi.ng followed by plasma deposition of 5000 A of SiOz on the whole wafer. The emitter mesa is made by dry etching, which is stopped 1000 A before reaching the base layer. So the emitter ohmic contact is aligned on the mesa. Then nitride spacers of 2000 A width are realized, followed by the zinc diffusion step to contact the base layer. Before the tungsten deposition by RF sputtering to contact the base layer, a slight chemical etch is performed to eliminate the A1GaAs; this is also possible with thin base layers because the diffused region is deep enough.
~
X Xll ~it/
IKXXL
l
"
E1
Sl:~a~r
~7] Tungsten
Emittercontact
~
I
D~usionZn
~
ImplantationH+,B+
n+ Galq.~As 500A n+ Gain,As 500 A n+ GaAs 500 A
1E19 1E19 4E18
Emitter
n GaAIAs
1500 A 2E17
Base
p+ GaAs
1000 A 5E19
Collector
n GaAs
4500 A 2E16
Collectorcontact n+ GaAs
5000 A 2E18
Figure 2: HBT cross section and epitaxial layers.
164
P. Launay et aL / Self-aligned AlGaAs/GaAs HBT's
With this procedure a thin A1GaAs layer is left under the spacers, on the transistor extrinsic base layer which is not exposed to air or to the etch process. By doing so surface recombination can be minimized. After base mesa etching and isolation by H+,B+ implantation, the AuGeNi-Ag-Au collector contact is lifted-off. First results on large transistors (2grnx201.tm emitter-base and 8~mx201arn base-collector junctions) give cutoff frequency of 37.5 GHz and maximun oscillation frequency of 37 GHz for a current density of 6 . 5 X 1 0 4 Acm 2 ( Figure 3 ). These can be compared to results reported by T.Nittono and al (6). They obtained on a self-aligned transistor with small dimensions a cutoff frequency of 82 GHz and a maximum oscillation frequency of 37 GHz with a similar technology.
40 38 ,-, N
"r
36 34
v E tl. 3 2 ~"
\ ID
Ft Fm
30 28
IV) C O N C L U S I O N
O
,
4
I
5
~
I
6
-
~
I
7
~
I
,
8
I
9
A new self-aligned A1GaAs/GaAs HBT Currentdensity(xl000 Acm-2) process has been developed. Both emitter and base ohmic contacts are made with the same refractory metal: tungsten. The self-alignment is obtained by means of SiN Figure 3: Cutoff frequency and spacers and zinc diffusion in a simple way. maximum oscillation frequency against This process allows to contact very thin base collector current density. layers and to minimize surface recombination. First transistors gave very promising electrical results: high quality junctions and current gain cutoff frequency of 37.5 GHz and maximum oscillation frequency of 37 GHz . Transistors with small dimensions are now being processed to assess the dynamic performances of this new technology. ACKNOWLEGEMENTS The authors will like to thank F.Alexandre and J.Riou for supplying the samples, A.Scavennec and C.Dubon Chevallier for fruitful discussions. REFERENCES
1
5
C.Dubon-Chevallier, A.M.Duchenois, A.C.Papadopoulo, L.Bricard, F.H61iot, P.Launay, ESSDERC 90, p 133. K.Daoud-Ketata, J.F Bresse, C.Dubon-Chevalier, Gallium Arsenide and Related Compounds, 1986,p 301. P.E.Hallali, P.Blanconnier, JP.Praseuth, J.Elect.Soc. 135(11) 2869. W.Dingfen, W.Dening, K.Heime, Solid-State Electronics 29(5) 489. P.A.Houston, F.R.Shepherd, A.J.SpringThorpe, P.Mandeville,
6
T.Nittono,K.Nagata,O.Nakajima, T.Ishibashi EDL,10(11) 506.
2 3 4
A.Marittai, Appl.Phys.Lett. 52(15)1219.
161
SELF-ALIGNED AIGaAs/GaAs HBT's WITH TUNGSTEN N AND P TYPE O H M I C CONTACTS
P.LAUNAY, B.BAMUENI, A.M.DUCHENOIS, P.BLANCONNIER Centre National d'Etude des Trlrcommunications Laboratoire de bagneux, 196 av. Henri Ravera, 92220 BAGNEUX ABSTRACT A self-aligned GaA1As/GaAs HBT technology with tungsten emitter and base ohmic contacts is presented. The HBT's layers are grown by MBE, the emitter contact layer is Ga0.35In0.65As. A Zinc diffusion allows for contacting the extrinsic base layer. Self-alignment is obtained by the diffusion around the emitter mesa; nitride spacers are used to isolate the intrinsic area during the diffusion process. I) I N T R O D U C T I O N Although widely used for the fabrication of HBT circuits, the mesa technology is facing several difficulties: Firstly it is difficult to reach the base layer, especially when this layer is thin, and to obtain a good base contact. In particular if a thin AIGaAs layer is left after the emitter mesa etching, the specific contact resistivity is degraded (1). Secondly it is well known that gold based ohmic contacts are not stable when the temperature exceeds 500"C. So it is impossible to use high temperature processes during the HBT fabrication. Moreover the reliability of this kind of contact is still an open question. Thirdly plasma etching cannot be used with gold metallizations. These two last points are also a problem for an ion-implanted technology (2). In this paper we present a way to overcome these difficulties with a simple self-aligned technology. A zinc diffusion at low temperature is used to contact the base layer and to realize the self-alignment. Emitter and base ohmic contacts are made with tungsten. We will describe the diffusion process and the electrical base contact in a first part. In a second part the whole HBT process and the electrical results on self-aligned transistors will be presented. II) T H E T U N G S T E N BASE O H M I C C O N T A C T PROCESS. The diffusion step relies on the semi open box technique using a solid source of ZnAs2 (3). After investigation of different diffusion processes as function of time and temperature to minimise the diffusion depth, we have chosen the following process: the temperature is 580°C during 1 mn after a ramp of 13 nan, the diffusion depth is about 2000 A in GaAs. A complete characterization has been made on several layers with different doping levels (see table 1). For this purpose epitaxial layers of 1 gm depth have been grown by MBE and MOCVD. After Zn diffusion, each sample is separated in two parts; on one part a slight chemical etch ( 20 nm ) is performed and nothing is done on the other part. Finally the tungsten metal is deposited and lifted-off. 0167-9317191]$3.50 © 1991 -Elsevier Science Publishers B.V. All fights reserved.
P. Launay et aL / Self-alignedAlGaAs/GaAs HBT's
162
The electrical results are obtained on test patterns by the Transmission Line Model (TLM); SIMS and Polaron analysis are used to determine the Zinc surface concentration. The zinc diffusion creates a highly p-type doped area on which the ohmic contact is made. So we evaluate electricaly the possibility on the different samples to realize this p-type area.
Table 1: Tungsten ohmic contact resistivity on different layers Layer
% A1
Doping
Resistivity
( Dxzmz )
Resistivity After 200 A
(f~cm2 ) etching
( cm 3 )
Mean
Sigma
Mean
Sigma
MBE
0
Be (2 10 TM)
8.9 10.7
1.3 10v
MBE
0
Be ( 1019)
1.7 10 .6
2.6 10 -7
1.2 10 .6
5.2 10.7
MOCVD
0
Zn (2 1017)
1.9
2.4 10-7
1.6
10 -6
5.1 10 .7
MOCVD
0
Zn (4 10 TM)
2.1 10-5
8.3
10 -6
1.1 10 .6
2.1 10 -7
MOCVD
0
Zn (1.7 1019)
1.4 10 -6
2.1 10 -7
1.9 10 .6
3.9 10.7
MBE
30
Si (2 1017)
1.2 10.5
5.4 10 -6
1.7 10.5
3.7 10.6
MBE
20
Si (2 1017)
3.4
10 .6
8 10 -6
4.1 10 .6
MBE
10
Si (2 1017)
1.2 10.6
3.3 10 -7
7.4 10 -7
10 -6
10 -6
9.2
10 -7
In the case of GaAs layers the specific contact resistivity, between 8 x 1 0 -7 and 2xl0-6f~cm z, is independant of the initial p-type doping level. This indicates for these samples a good zinc diffusion process whatever the initial dopant or doping level are. This is confirmed by SIMS and Polaron analysis which give Zinc Surface concentration after diffusion higher than 1020cm 3. These results are in agreement with the model of W.Dingfen and al (4): They evaluate a specific contact resistivity of 10 .6 ~-]cm2 for a high surface doping of 10z° cm -3. In each case the etched sample gives better results, particularly for one of the samples. This indicates that surface pollutions can occur during the diffusion process or in the other process steps. Electrical results on GaAIAs layers, doped with Si ( 2x1017cm -3 ) to simulate the emitter layer, show an increase of the specific resistivity with the aluminum content. The low Zn surface concentration ( 5x1019cm 3 ) for the layer with 30 % aluminum, by comparaison with higher surface doping ( 102°cm3 ) in the other layers, a higher gap and other physical parameters explains these results. Two conclusions must be pointed out: In the HBT process it will be necessary to etch the GaA1As before tungsten deposition to contact thebase layer ( see part III ). Secondly in the case of aluminum graded base, where the optimum aluminum percentage is around 10%, the zinc diffusion process to contact the base with a tungsten ohmic contact should be applicable. In the HBT process the zinc diffusion proceeds through a thin GaAIAs ( 100 n m ) and the GaAs base layer. To study the diffusion in the actual situation we have grown by MBE all the layers of the HBT with an initial base doping of 3x1018cm 3. After a partial emitter mesa etching the zinc diffusion proceeds in the base extrinsic area.
P. Launay et a L / Self-aligned AlGaAs/GaAs HBT's SIMS profile in the intrinsic area, not shown here, indicates that the temperature of the diffusion process is low enough to avoid Beryllium outdiffusion from the base to the emitter in the transistor intrinsic area. Figure 1 shows the SIMS profile in the extrinsic base area. The zinc concentration at the GaA1As surface is low, but this concentration in the base layer defines a P* zone of 10Z°cm3 which allows the use of a tungsten metallization to contact the base after a thin GaA1As etching. We can also observe a Beryllium out-diffusion. Zinc atoms diffuse through an interstitial-substitutional mechanism and would replace the Be in the substitutional site as P.A.Houston and al (5) suggest, via the reaction:
10"1020.
~ ~0"-~
~.~0,o
f
~ 10rl AI
~°~
Be
~
Zn
..base..collector
0'.z
Zni÷ + Bes = Zn~ + Bei+ In the case of zinc as initial dopant (MOCVD), zinc atoms can occupy the vacancies in the layer during the diffusion process.
163
,.subcoilector
d.i
o'.~
o'.B
OEPTH(~m)
Figure 1: SIMS profile after Zn diffusion in the HBT.
III) THE SELF-ALIGNED AIGaAs/GaAs HBT Figure 2 shows the final HBT structure and the transistor layers. A tungsten film is deposited by RF sputtefi.ng followed by plasma deposition of 5000 A of SiOz on the whole wafer. The emitter mesa is made by dry etching, which is stopped 1000 A before reaching the base layer. So the emitter ohmic contact is aligned on the mesa. Then nitride spacers of 2000 A width are realized, followed by the zinc diffusion step to contact the base layer. Before the tungsten deposition by RF sputtering to contact the base layer, a slight chemical etch is performed to eliminate the A1GaAs; this is also possible with thin base layers because the diffused region is deep enough.
~
X Xll ~it/
IKXXL
l
"
E1
Sl:~a~r
~7] Tungsten
Emittercontact
~
I
D~usionZn
~
ImplantationH+,B+
n+ Galq.~As 500A n+ Gain,As 500 A n+ GaAs 500 A
1E19 1E19 4E18
Emitter
n GaAIAs
1500 A 2E17
Base
p+ GaAs
1000 A 5E19
Collector
n GaAs
4500 A 2E16
Collectorcontact n+ GaAs
5000 A 2E18
Figure 2: HBT cross section and epitaxial layers.
164
P. Launay et aL / Self-aligned AlGaAs/GaAs HBT's
With this procedure a thin A1GaAs layer is left under the spacers, on the transistor extrinsic base layer which is not exposed to air or to the etch process. By doing so surface recombination can be minimized. After base mesa etching and isolation by H+,B+ implantation, the AuGeNi-Ag-Au collector contact is lifted-off. First results on large transistors (2grnx201.tm emitter-base and 8~mx201arn base-collector junctions) give cutoff frequency of 37.5 GHz and maximun oscillation frequency of 37 GHz for a current density of 6 . 5 X 1 0 4 Acm 2 ( Figure 3 ). These can be compared to results reported by T.Nittono and al (6). They obtained on a self-aligned transistor with small dimensions a cutoff frequency of 82 GHz and a maximum oscillation frequency of 37 GHz with a similar technology.
40 38 ,-, N
"r
36 34
v E tl. 3 2 ~"
\ ID
Ft Fm
30 28
IV) C O N C L U S I O N
O
,
4
I
5
~
I
6
-
~
I
7
~
I
,
8
I
9
A new self-aligned A1GaAs/GaAs HBT Currentdensity(xl000 Acm-2) process has been developed. Both emitter and base ohmic contacts are made with the same refractory metal: tungsten. The self-alignment is obtained by means of SiN Figure 3: Cutoff frequency and spacers and zinc diffusion in a simple way. maximum oscillation frequency against This process allows to contact very thin base collector current density. layers and to minimize surface recombination. First transistors gave very promising electrical results: high quality junctions and current gain cutoff frequency of 37.5 GHz and maximum oscillation frequency of 37 GHz . Transistors with small dimensions are now being processed to assess the dynamic performances of this new technology. ACKNOWLEGEMENTS The authors will like to thank F.Alexandre and J.Riou for supplying the samples, A.Scavennec and C.Dubon Chevallier for fruitful discussions. REFERENCES
1
5
C.Dubon-Chevallier, A.M.Duchenois, A.C.Papadopoulo, L.Bricard, F.H61iot, P.Launay, ESSDERC 90, p 133. K.Daoud-Ketata, J.F Bresse, C.Dubon-Chevalier, Gallium Arsenide and Related Compounds, 1986,p 301. P.E.Hallali, P.Blanconnier, JP.Praseuth, J.Elect.Soc. 135(11) 2869. W.Dingfen, W.Dening, K.Heime, Solid-State Electronics 29(5) 489. P.A.Houston, F.R.Shepherd, A.J.SpringThorpe, P.Mandeville,
6
T.Nittono,K.Nagata,O.Nakajima, T.Ishibashi EDL,10(11) 506.
2 3 4
A.Marittai, Appl.Phys.Lett. 52(15)1219.