MATERIALS SCIENCE & ENGINEERING
Materials Science and Engineering B46 (1997) 223-226
Nickel based ohmic contacts on SIC Ts. Marinova a, A. Kakanakova-Georgieva a, V. Krastev a, R. Kakanakov b, M. Neshev b, IL. Kassamakova b, 0. Noblanc c3 C. Arnodo c, S. Cassette c, C. Brylinski c, B. Pecz d, G. Radnoczi d, Gy. Vincze d
a I. G. I. C., Bulgarian Academy 0s Sciences, SoJ?a 1113, Bulguriu b Insiitute of Applied Physics, Ploodiil 4000, Bulgaria c Thomson-CSFILCR, 91404 Orsay Cede,?, France Hungarian Academy of Sciences, P.O. Box 76, H-1325 Budapest.
Abstract We have comparedthe chemicaland structural propertiesof Ni/SiC and Ni,Si/SiC interfaces.In the caseof Ni/SiC, the contact formation is initiated by the dissociationof Sic, due to the strong reactivity of nickel at 950°C. Ni,Si is formed and carbon accumulates,both at the interface and throughout the metal layer. At the interface,many Kirkendall voids are observedby TEM. Despitethis poor interface morphology, low contact resistanceshave beenmeasured.But the presenceof carbon in the contact layer and at the interface is a potential source of contact degradation at high temperature. In the caseof Ni/Si multilayers evaporated on Sic insteadof pure Ni, the contact formation is precededby Ni and Si mutual diffusion in the depositedlayer yielding Ni,Si. Therefore, a smalleramount of carbon is releasedfrom SIC. Low carbon segregation,abrupt interface and low contact resistancecharacterizethis contact. The thermal stability of Ni,Si contacts is illustrated with ageingexperimentscarried out at 500°C.0 1997Elsevier ScienceS.A. ~~ Keywords:
Nickel contact; Ohmic contact; Silicon carbide
1. Introduction Recent progress in crystal growth of silicon carbide (Sic) has led to the availability of commercial wafers and epitaxial structures [l]. Moreover, Sic electrical and thermal properties make Sic devices suitable in the fields of high power, high temperature and high frequency electronics [2,3]. However, it is widely appreciated that the optimal performance of microwave devices is related to the quality of ohmic contacts. Low contact resistance and high temperature stability are required. Nickel is one of the most popular materials used in advanced aerospace systems  and good nickel ohmic contacts on n-Sic have been already demonstrated . In this work, we compare the interface properties of standard Ni and improved Ni,Si contacts. XPS depth profiling was used to determine the reaction mechanism and the diffusion process occurring at the metal/Sic interface. The contact morphology was observed by TEM and electrical measurementswere made to determine the specificic contact resistance. Thermal 0921-5107/97/$17.00 8 1997 Elsevier Science S.A. All rights reserved. P1Is0921-5107(96)01981-2
tests have been undertaken to assessthe stability of these contacts.
2. Experimental procedure The samples used were either n+ 6H-SiC substrates (Nd = (1 - 1.8) x lo’* cmp3) or n-type 4H-SiC epitaxial structures made of a heavily doped (Nd = lOi cm - ‘) 0.2 pm thick cap layer on a 0.5 Ltrn thick active layer (Nd = 1 x 10” cm --3). A 10 ,um thick p-layer as a buffer between the active layer and the n + substrate is part of the structure. All the samples were purchased from Cree Research (N.C.). Pure Ni and NijSi multilayer structures of adequate NijSi ratio to obtain the Ni,Si composition were deposited using electron-beam evaporation at a pressure of 5 x lo- * Torr. Prior to the evaporation, the surface was slightly etched using argon ion milling. The NijSi structures were prepared by deposition sequencesstarting on SIC either with the Ni or with the Si layer. The
T. Mat?noea et al. /‘Materials Science and Engineering B46 (1997) 223-226
overall thickness was 1500 A. The contacts were annealed for 10 min at 950°C in N, atmosphere. XPS studies were carried out in an ESCALAB MkII (VG Scientific) electron spectrometer with an Al K, (1486.6 eV) X-ray source. The depth profiling was made by using a 3 keV Ar’ ion beam with a 10 uA cm - * current density. The interface morphology was examined in cross section on thinned samples using a Philips CM20 transmission electron microscope equipped with an EDS Noran system. Electrical tests were carried out using an automatic prober allowing measurements in air from room temperature to 450°C. The samples were cooled down to room temperature prior to the measurements. The specific contact resistance was determined using the transmission line model . The investigations were carried out on as-deposited samples, on samples after annealing and on some samples after exposure at 500°C in nitrogen for 100 h.
3. Results and discussion 3.1. Interface properties of NijSiC
The as-deposited polycrystalline nickel layer is homogeneous and a smooth surface is observed. The interface is chemically abrupt with a very thin amorphous layer, probably due to the ion bombardment prior to evaporation. Fig. 1 shows the XPS profile of a Ni/GHSic contact after annealing and storage test at 500°C for 100 h in nitrogen. The element distribution is similar to the distribution obtained directly after the contact formation at 950°C (not shown). The Ni2p/Si2p peak ratio as well as the binding energy of these peaks (respectively 853.2 and 99.4 eV) indicate the formation of a nickel silicide with a composition close to Ni,Si . Carbon in graphite state (Cls at 284.2 eV) is present in the whole contact layer with a maximal concentration at the interface. At the interface, the Ni 2p peak Ni /GHSiC agedat500”CforlOOhinN, 80 Ni
400 600 Sputtering time [minj
Fig. 1. X-ray photoemission profiles of Ni/SiC contact after an annealing and storage test at 500°C for 100 h in N2 atmosphere.
Fig. 2, TEM micrograph of the Ni/SiC interface after annealing.
remains at the same position while the maxima of the Si 2p and C 1s peaks are shifted towards the binding energies corresponding to Sic. The TEM cross section of the annealed specimen is presented in Fig. 2. It is clear that all the nickel layer has reacted to form a nickel silicide. The contact layer contains a lot of Kirkendall voids and its thickness has been increased substantially. The interface is shifted into the Sic part of which has been consumed to supply Si for Ni,Si formation, In the area of the original interface, an extremely high number of voids can be found. Quantitative EDS analysis indicates a composition close to Ni,Si and a strong carbon incorporation. Diffraction patterns from different grains could be indexed as the [email protected]
orthorhombic phase. These results suggest the following mechanism to describe the NijSiC contact formation: (1) Sic dissociates due to the strong reactivity of nickel above 400°C; (2) at 950°C, the Ni$i stable phase is formed leading to carbon accumulation both at the interface and in the metal layer ; (3) a part of dissociated Si atoms diffuse through the nickel layer and simultaneously Ni atoms diffuse towards Sic until the complete consumption of the deposited nickel layer. 3.2. Interface properties of M&/Sic
Silicon was introduced in the nickel layer in order to prevent the Sic dissociation during the contact formation. NijSi multilayers, in the ratio Ni,:Si, were evaporated instead of pure nickel. Fig. 3(a) and 3(b) show the depth distribution of the different elements for the two different deposited structures, after annealing at 950°C for 10 min. Similar profiles are observed for the two samples. A Ni,Si silicide layer is obtained as indicated by the binding energies and the ratio of Ni 2p to Si2p signal intensity. There is no carbon contained in the silicide layer. At the interface, carbon is still observed but the amount is lower than for the previous Ni contacts. Fig. 4 is a bright field TEM image of the contact obtained with Si as first deposited layer. The sample was annealed. The contact layer is uniform, polycrystalline and the 6-Ni,Si orthorhombic phase is
et al. /Materials
Si /Ni I6H-W at 950% for 10 min ---. c .I o- 0 - o- Si ._-v Ni
0 - - substrate no41
at 950% for 10 min .-~.. c
$ P g
.Q 0 .t
0 Si Ni
Ageing time (h)
i g 5
Fig. 3. X-ray photoemission profiles of Si-NijSiC structure after annealing at 950°C for 10 min. (a) First evaporated Ni layer; (b) first evaporated Si layer.
identsed. Some Kirkendall voids are still present at the interface but not in the contact layer itself. In the case of the nickel interfacial layer, the contact morphology is similar while a greater number of voids is observed. These results suggest that intentional silicon incorporation in the nickel layer modifies the diffusion pro-
Fig. 5. Time dependence of the specific contact resistance of the NijSiC ohmic contact during ageing experiment at 500°C in N, atmosphere.
cesses which are responsible for the contact formation. In the case of NijSi multilayers, Ni and Si mutual diffusion occurs, leading to the stable phase of the nickel silicide, Ni,Si. The reaction between Ni and Sic at the interface is limited since almost all the nickel is already bonded to the silicon atoms. As a consequence, the SIC decomposition is reduced and only a small amount of carbon is released. 3.3. Stability of 7he nickel based ohmic contacts NilSiC: Specific contact resistance as low as as 2.8 x 10m6 R cm2 was measured on epi-structure after the optimisation of evaporation and annealing conditions. After exposure at 5OO”C, small variations on the specific contact resistance were evidenced (Fig. 5). Ageing tests have been carried out on substrates with different doping levels (no. 41: Nd = 1.8 x 1O1’ cm- 3 and no. 43: Nd = 1 x lOI* cmP3 in Fig. 5). Higher values were obtained due to the lower doping level. Moreover the TLModel is not really suitable to give accurate values of the specific contact resistance when the measurements are made on substrates. As in the case of epi-structures very slight variations were observed after 100 h at 500°C. Ni,Si/SiC: Measurements performed on epi-structures gave specific contact resistance values between 1.2 and 2.7 x 10 - 5 Q cm2. These values are one order of magnitude higher than with Ni ohmic contacts. Further optimisation is necessary and will be the subject of a future paper. Ageing tests have also been performed on substrates. In-sample 44-1, Ni is the fist deposited layer while in sample 44-2, Si is the first deposited layer. The specific contact resistance seems to be higher when Si is at the interface. However this result should be confirmed on epi-structures. Both samples show a good stability at
et al. /Materials
Si I Ni IGH-SiC (noA4-1) Ni I si IGH-SiC (no.44~2)
j, 2 P :z .” B
-0 = Si I Ni / GH-SC -A = Ni/8/6KSiC(noM-2) . Ni /6H-S1C
0.1 I 0
time, t b]
Fig. 6. Time dependence of the specific contact resistance of the Ni-SijSiC ohmic contact dnring ageing experiment at 500°C in Nz atmosphere. Sample no 44-l: Ni first evaporated layer; sample no 44-2: Si first evaporated layer.
500°C (Fig. 6). Measurements as a function of temperature have been made in air with Ni and Ni,Si contacts on substrates using TiPtAu overpads. Very slight variations from the room temperature values were evidenced with increasing temperature (Fig. 7). These electrical results are in agreement with .
Fig. 7. Temperature dependence of the specific contact resistance of Ni-Si/SiC ohmic contacts,
The better interface quality observed with Ni,Si makes this contact attractive for high temperature and high power operation.
This work has been supported by the European Economic Community through the Copernicus Project 940603.
Even if electrical stability of Ni contacts seems sufficient in these experimental conditions, the carbon segregation at the interface and in the silicide layer is a potential source of electrical instability. Moreover: the voids created at the interface could cause internal stress and delamination of the contact layer. The structural and chemical properties of the interface have been substantially improved by using NijSi instead of pure nickel as ohmic contact on Sic. Low carbon concentration was observed at the interface and within the contact layer. The number of Kirkendall voids decreased significantly leading to an enhancement of the contact reliability. Further improvement has to be achieved. The Si/Ni ratio has to be optimised. In particular, the influence of the thickness of the interfacial Si layer will be studied.
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