PROC. 23rd INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2002), VOL 1, NIŠ, YUGOSLAVIA, 12-15 MAY, 2002
Thermally Stable Low Resistivity Ohmic Contacts for High Power and High Temperature SiC Device Applications R. Kakanakov, L. Kassamakova-Kolaklieva, N. Hristeva, G. Lepoeva, K. Zekentes
Abstract – Ti/Al/p-SiC and Ni/n-SiC ohmic contacts with improved electrical and thermal properties in respect to their application in high power and high temperature SiC devices are reported in this work. Contact resistivity as a function of annealing was investigated over the temperature range of 700 oC – 950 oC. The lowest resistivity of 1.42x10-5 Ω.cm2 for the Ti/Al contact was obtained after annealing at 900 oC while for the Ni -6 2 contact the lowest resistivity of 4.9x10 Ω.cm was achieved at o 950 C. The contact stability during prolonged ageing and at high operating temperatures and current density was studied as a criterion for their reliability. It was found that both contacts were thermally stable during ageing in an inert ambient (N2) at high temperature of 600 oC for 100 hours as well as at operating temperatures up to 450 oC in air and at current density of 103 A/cm2 passed through the contacts during the heating. The improved electrical and thermal properties of the Ti/Al/p-SiC and Ni/n-SiC ohmic contacts were demonstrated in the power p-n SiC diode developed.
The relatively high contact resistivity may cause an inadmissible voltage drop on the contacts. By reason of this, the ohmic contacts for high power and high temperature device applications should combine low contact resistivity with good stability at high operating temperatures and at high current densities. In this work we present the results from the improving of electrical and thermal properties of both n- and p-type ohmic contacts for a power p-n SiC diode by technology optimization. Ti/Al composition has been the subject of our investigation as an ohmic contact to the p-side of the diode while Ni metallization was used as ohmic contact to the ntype SiC substrate. Annealing conditions and their effect on the electrical properties of the contacts are discussed. The thermal stability of the contacts at temperatures as high as 600 oC and at a current density of 103 A/cm2 has been investigated in respect to their application in a power p-n SiC diode.
I. INTRODUCTION
II. EXPERIMENT
The requirements of modern industry with respect to semiconductor devices able to operate at high power and high temperatures are constantly increasing. Silicon carbide (SiC) is preferable microelectronic material for high power and high temperature device applications due to its inherent properties such as wide bandgap, high thermal conductivity, high saturation electron velocity, high breakdown field [1]. The wide bandgap of SiC results in a very high intrinsic temperature of 1650 oC for an extrinsic doping of 1016 cm-3 [2]. This great capacity of SiC predetermines high requirements to the metal contacts of the devices. The contact stability at high temperatures belongs to the main factors that restrict the high power and high temperature application of the SiC devices. The low resistivity of ohmic contacts is also a significant factor because of the high current density in the power devices.
The Ti/Al contact studied was formed on the upper ptype 4H-SiC layer of the diode structure. The latter was grown by liquid phase epitaxy (LPE) and had a thickness of 0.5 µm and a doping concentration of (3-5)x1019 cm-3. The ntype Ni ohmic contact has been developed on the n-type 4H-SiC substrate of the diode structure with a carrier concentration of (1-3)x1018 cm-3. The SiC surface was prepared using a standard cleaning procedure in organic solvents and subsequently etching in solutions of NH4OH:H2O2:H2O; HNO3:HF: H2O and HF: H2O. A premetallization etching in Ar+ discharge was performed to obtain a SiC surface with minimum contamination. The deposition of the p-metal films was performed by a subsequently sputtering of Al in argon at 3x10-3 torr pressure and by an electron beam evaporation of Ti in vacuum of 1x10-6 torr. The thickness of the layers was 40 nm and 60 nm respectively. These thicknesses correspond to the ratio Ti(70 w.%)/Al(30 w.%) which is a precondition for the formation of a stable contact composition during the annealing [3]. The Ni film was deposited by an electron beam evaporation in vacuum of 1x10-6 torr and had a thickness of 150 nm. The transmission-line model (TLM) test structures for electrical and thermal characterisation were obtained by a lift-off process on the mesa-etched
R. Kakanakov, L. Kassamakova-Kolaklieva, N. Hristeva, and G. Lepoeva are with the Institute of Applied Physics, Bulgarian Academy of Sciences, 59, St. Petersburg Blvd., 4000 Plovdiv, Bulgaria, E-mail:
[email protected] K. Zekentes is with Institute of Electronic Structure and Laser, FORTH - Hellas, PO Box 1527, 71110 Heraklion/Crete, Greece, E-mail:
[email protected]
0-7803-7235-2/02/$10.00 © 2002 IEEE
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areas. They consisted of six contact pads with 100 µm long and 150 µm wide. The distances between them varied from 6 µm to 64 µm.The ohmic property formation of the contacts was carried out in a furnace with resistance heating under a Ar+1%H2 gas flow. The 5 min annealing was performed at temperatures ranging from 700 oC to 950 o C in order to obtain the lowest resistivity. The stability of the contacts has been examined by investigation of their resistivity during different thermal treatments: ageing test, temperature-dependence test and temperature-current test. Ageing of the contacts for a long time has been performed at a constant temperature of 500 o C and 600 oC in an inert atmosphere (N2). In fixed time intervals the contacts have been cooled to the room temperature and the contact resistivity has been determined. The contact resistivity has always been measured at room temperature. In the temperaturedependence test the measurements have been proceeded at a temperature increasing smoothly from 25 oC to 450 oC in air. This study gives information on the contact reliability at the corresponding operating temperature as the contact resistivity has been measured during the heating. During the temperature-current test a current with a pre-set density (up to 103 A/cm2) is passed for a fixed time through the contacts at a constant temperature (up to 450 oC). This test has also been performed in air and contact resistivity has been measured at the corresponding temperature.
III. RESULTS AND DISCUSSION
contact at temperature as low as 700 oC caused ohmic properties of both n- and p-contacts but the contact resistivity was still high, 1.25x10-3 Ω.cm2 and 1.77x10-3 Ω.cm2, respectively. Further increase of the annealing temperature resulted in decrease of the contact resistivity as the degree of the reduction depended on the contact composition (Fig. 2). More significant decrease of the p-type Ti/Al contact resistivity was observed after annealing at 800 o C. The investigation of the Ti-Al phase diagram has found out that at this temperature a reaction between Ti and Al starts leading to the formation of TiAl3 alloy [3]. The lowest reproducible resistivity of 1.42x10-5 Ω.cm2 for Ti/Al contacts was measured after annealing at 900 oC. The high temperature stimulates the diffusion of Al atoms into the SiC. This process takes place together with the reactions in the contact layer. The diffused Al atoms may lead to enhanced field emission through the contact barrier by the creation of many hemispherical intrusions down into the SiC surface [4] which results in the contact resistivity decrease. The obtained value is in an order lower than the result (4x10-4 Ω.cm2) reported for the same contact configurations formed on the same polytype, 4H-SiC [5] and it is comparative with the resistivity (1.5x10-5 Ω.cm2) achieved by Crofton et al. [6] on 6H-SiC after annealing at 1000 oC. It should be noted that the 4H-SiC polytype has the widest bandgap due to which the formation of low resistivity ptype ohmic contacts to it is the most difficult. Further annealing temperature increase did not change essentially the resistivity of Ti/Al/SiC contacts.
A. Electrical Properties The I-V characteristics of as-deposited and annealed contacts were measured to check whether the contacts were ohmic in character (Fig. 1). Both Ti/Al and Ni as
0,010
Current, I [A]
Ti/Al as-deposited
0,005
Ti/Al / p-SiC Ni / n-SiC
2
Contact resistivity, [Ω.cm ]
-2
10
o
annealed at 900 C 0,000
-3
10
-4
10
-5
10
-6
10
Ni -0,005
700
750
800
850
900
950
o
Annealing temperature, T [ C]
as-deposited o
annealed at 950 C -0,010
-2
-1
0
1
2
Fig. 2. Dependence of the resistivity of Ti/Al/SiC and Ni/SiC contacts on the annealing temperature.
Voltage, U [V]
Fig. 1. I-V characteristics of as-deposited and annealed at optimum temperature Ti/Al/SiC and Ni/SiC contacts.
-deposited contact layers have demonstrated Schottky behaviour. The influence of the annealing conditions on the electrical properties of the contacts was estimated by their resistivity using TLM method. Annealing of the
The resistivity of the n-type Ni/SiC contact decreased slightly with annealing temperature increase up to 800 oC Smooth resistivity reduction from 6.28x10-4 Ω.cm2 to 5.63x10-5 Ω.cm2 was observed in the annealing interval 800 – 900 oC. Abrupt decrease of the contact resistivity was detected after annealing at 950 oC, at which temperature the lowest resistivity of 4.9x10-6 Ω.cm2 was measured. This
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value is low enough in comparison with the reported data [4]. The ohmic properties in the case of the Ni/SiC contact are caused by the reduction of the potential barrier height of the contact. This decrease is due to a change in contact composition as a result of the chemical reactions occurring during its formation. During annealing of the contacts at 950 oC in the presence of Ni atoms, dissociation of the silicon carbide surface proceeds. The liberated silicon atoms interact with the nickel from the contact layer forming Ni2Si. The presence of Ni2Si at the nickel/silicon carbide interface is the reason for the potential barrier height reduction, as a result of which ohmic properties are observed [7]. The I-V characteristics of both Ti/Al and Ni contacts corresponding to the lowest resistivities obtained are presented in Fig.1.
and 600 oC for 100 hours. The results are presented in Fig. 3. Very small changes from 1.42x10-5 Ω.cm2 to 2.1x10-5 Ω.cm2 were observed in the resistivity of the Ti/Al contacts between 1-8th hour ageing at 500 oC. After 8 hours the contact resistivity was stabilized and further heating at this temperature did not change it. The resistivity of Ni contacts remained practically the same during the whole time interval at this temperature. No changes in the contact resistivity were detected when both contacts were a subject of ageing at 600 oC: a criterion for stable contacts. We assume that this thermal stability is due to the formation of a chemically stable metal/SiC interface and a stable contact composition: TiAl3 alloy for the Ti/Al/SiC contact and Ni2Si for Ni/SiC one. Before performing the temperature-dependence test and temperature-current test an additional 100 nm thick gold layer was evaporated on the Ti/Al contacts to prevent the Ti oxidation in air. Fig. 4 illustrates
B. Thermal properties
500 C
2
Contact resistivity, [Ω.cm ]
o
2
Ti/Al / p-SiC Ni / n-SiC
-4
10
Ni/SiC Au/Ti/Al/SiC
J = 0 A/cm
-5
10
-6
10
100
200
300
400
500
o
Temperature, T [ C] -5
10
2 -6
10
0
20
40
60
80
100
Ageing time, t [h]
2
Contact resistivity, [Ω.cm ]
2
-4
10
0
Contact resistivity, [Ω.cm ]
Contact resistivity, [Ω.cm ]
The contact reliability at high temperature treatment is considered as the critical factor determining their power device applications. The effect of long-term high temperature ageing on the electrical properties of Ti/Al and Ni contacts has been investigated by heating at 500 oC
o
600 C 10
Ti/Al / p-SiC Ni / n-SiC
-4
3
2
Ni/SiC Au/Ti/Al/SiC
J = 10 A/cm -4
10
-5
10
-6
10
0
100
200
300
400
500
o
Temperature, T [ C]
10
-5
10
-6
Fig. 4. Dependence of the contact resistivity of Au/Ti/Al/SiC and Ni/SiC contacts on the operating temperature. 0
20
40
60
80
100
Ageing time, t [h]
Fig. 3. Resistivity values of the Ti/Al/SiC and Ni/SiC ohmic contacts after ageing at 500 oC and 600 oC in an inert (N2) ambient.
the results from the investigation of the contact behaviour at temperatures smoothly increasing from 25 oC to 450 oC without applied current and when current with density of 103 A/cm2 was passed through the contacts. For the Au/Ti/Al contact the resistivity decreased twofold as the temperature increased from 25 oC to 450 oC when a current was not applied. The Ni contact did not change the resistivity during this treatment. Passing the current with
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Forward current, IF [A]
density of 103 A/cm2 through the Au/Ti/Al/SiC contact caused a small change in resistivity at 50 oC, after that it remained practically the same up to 450 oC. No variations of the Ni contact resistivity were observed at the same test conditions. After each test was completed and the samples were cooled down the contact resistivity was measured again at 25 oC. The contact resistivity obtained did not differ from the values measured for each contact type before the test. The developed p-type Ti/Al/SiC and n-type Ni/SiC ohmic contacts have been applied as ohmic contacts in the power p-n SiC diode. The I-V characteristics of such diode at different operating temperatures are presented in Fig. 5.
16
o
25 C
14
o
100 C
12
o
range for the p-type contacts. That is why the lowest resistivity of 1.42x10-5 Ω.cm2 for Ti/Al/p-SiC contacts was achieved on epylayers with a carrier concentration of (35)x1019 cm-3 while a resistivity of 4.9x10-6 Ω.cm2 was obtained on n-type SiC substrate with a doping level of (13)x1018 cm-3 only. The study of the thermal stability of both contacts, Ti/Al and Ni, aimed at establishing their reliability for high power and high temperature applications. Ageing at temperatures as high as 500 oC and 600 oC for 100 h at each temperature was not found to degrade them. The Ti/Al and Ni contacts were stable after being subjected to treatment at temperature smoothly increasing up to 450 oC and high current density passed through the contact during heating. The results of this study show that the Ti/Al/p-SiC and Ni/n-SiC contacts combine low contact resistivity with good thermal stability. These contacts are appropriate for high power and high temperature applications which has been confirmed in the p-n SiC diode developed.
200 C
10
o
ACKNOWLEDGEMENTS
300 C
8 6 4 2 0 0,0
0,5
1,0
1,5
2,0
2,5
3,0
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4,5
5,0
Forward voltage, UF [V]
This work was supported financially by NATO project SfP-971 879 for that the authors express their gratitude. The authors wish to thank Dr. N. Kuznetsov from Crystal Growth Research Center, St. Petersburg, for preparing of the LPE layers of the diode structure. They also wish to thank Dr. G. Sarov from IAP, Plovdiv for the IV characteristic measurement of the diode.
Fig. 6. I-V characteristic at different temperatures of a power p-n SiC diode with Au/Ti/Al as a p-type and Ni as a n-type ohmic contacts.
III. CONCLUSION The investigation carried out in this work showed that Ti/Al and Ni films make low resistivity ohmic contacts on moderate doped p- and n-type SiC, respectively. The as-deposited contacts form a Schottky barrier to SiC. Annealing at temperature in the range 700 o C – 950 oC results in ohmic behaviour as the optimal temperature for a lowest contact resistivity obtaining depends on the contact composition. It was found that this temperature is 900 oC for the Ti/Al contact and 950 oC for the Ni contact. The experiments performed demonstrated the effect of the doping type of SiC on the electrical properties of the contacts. The wide bandgap of SiC restricts the obtaining of a contact resistivity in 10-6 Ω.cm2
REFERENCES [1] G.Pensl, H. Morkoc, B. Monemar, E. Janzén (Eds.), Silicon carbide, III-Nitrides and Related Materials, Part 2, Trans Tech Publications, Switzerland, 1998. [2] M. Meyer, Compound Semiconductor, 1996, vol. 2, pp. 6. [3] M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, or General Electric Co., Business Growth Services, Schenectady, NY (1958). [4] J. Crofton, L. M. Porter and J. R. Williams, Phys. Stat. Sol. (b), 1997, vol. 202, pp. 581-603. [5] Y. Luo, F. Yan, K. Tone, J. H. Zhao and J. Crofton, Mater. Sci. Forum, 2000, vol. 338-342, pp. 1013-1016. [6] J. Crofton, P. A. Barnes and J. R. Williams, Appl. Phys. Lett., 1993, vol. 62, p. 384. [7] Ts. Marinova, A. Kakanakova-Georgieva, V. Krastev, R. Kakanakov, M.Neshev, L.Kassamakova, O.Noblanc, C.Arnodo, S.Cassette, C.Brylinski, B.Pecz, G.Radnocy, and Gy.Vinze, Materials Science and Engineering, 1997, vol. B46, pp. 223-226.
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