Jan 19, 2011 - Nickel silicide ohmic contacts to 4H n-SiC were investigated using electron ... are characterized by low resistivity but require high temper-.
Materials Transactions, Vol. 52, No. 3 (2011) pp. 315 to 318 Special Issue on New Trends for Micro- and Nano Analyses by Transmission Electron Microscopy #2011 The Japan Institute of Metals
TEM Characterisation of Silicide Phase Formation in Ni-Based Ohmic Contacts to 4H n-SiC Marek Wzorek1 , Andrzej Czerwinski1 , Andrian Kuchuk1;2 , Jacek Ratajczak1 , Anna Piotrowska1 and Jerzy Ka˛tcki1 1 2
Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland V. Lashkaryov Institute of Semiconductor Physics, NASU, Pr. Nauky 45, 03028 Kyiv, Ukraine
Nickel silicide ohmic contacts to 4H n-SiC were investigated using electron microscopy. Ni/Si multilayer structures were fabricated using magnetron sputtering technique. The Ni to Si layer thickness ratio was chosen to achieve the stoichiometry of Ni2 Si phase. The deposited structure was subjected to a two-step annealing procedure. First annealing step was performed at 600 C, the second at 1050 C or 1100 C. Microstructure and morphology after each annealing step were characterized using scanning and transmission electron microscopy. The specific voids and discontinuities of the layer were observed after annealing at high temperature. Phase compositions were investigated with electron diffraction technique. After annealing at 600 C the phases Ni2 Si, Ni3 Si2 and Ni31 Si12 were detected. High temperature annealing resulted in the presence of only Ni2 Si phase. The influence of phase transformations on the morphology of the contacts is discussed. Explanation of the origin of layer discontinuities is proposed. [doi:10.2320/matertrans.MB201014] (Received September 2, 2010; Accepted November 24, 2010; Published January 19, 2011) Keywords: transmission electron microscopy, silicon carbide, nickel ohmic contacts, electron diffraction
1.
Introduction
Silicon carbide, due to its unique properties, is a material with a high application potential for development of high power, high frequency and high temperature electronic devices. However, development of low resistivity ohmic contacts to SiC that could sustain their characteristics after long-term high power operation at high temperatures is critical for successful realization of SiC devices. Ni-based structures are considered as most promising for development of reliable contacts.1–5) Nickel ohmic contacts are characterized by low resistivity but require high temperature to form. During annealing Ni reacts with Si from the SiC substrate. The stable Ni2 Si phase is formed but redundant carbon atoms form precipitates in the contact layer. In order to avoid reaction of metal with the substrate, the Si can be deposited as the first layer onto the SiC wafer.3) This paper focuses on electron microscopy investigations on the Ni/Si/n-SiC contact structures. The Ni/Si multilayer structure was deposited on the 4H-SiC wafer and subjected to a two-step annealing procedure. The influence of phase transformations on the morphology of the contacts is discussed. 2.
Experimental
The n-type 4H-SiC substrate (n 2 1017 cm 3 ) was a Si-faced (0001) oriented wafer supplied by Cree Research Inc. Surface preparation procedure was described in previous work.6) The Ni/Si multilayer structure was deposited by magnetron sputtering technique, using Ni and Si targets and Ar plasma. The thickness of each deposited layer was chosen to achieve the overall stoichiometry of the Ni2 Si phase: 4H-SiC/Si(30.3 nm)/Ni(33.1 nm)/Si(30.3 nm)/ Ni(33.1 nm). The as-deposited structures were subjected to the annealing procedure in a nitrogen ambient that consisted of two
steps. The first step was performed at 600 C for 15 min. The next step was conducted subsequently at 1050 C or 1100 C for 3 min. The low temperature annealing was performed to initiate intermixing of Ni and Si layers and silicide phase formation, the high temperature step was necessary to achieve ohmic contact to the SiC substrate. The morphology and microstructure of the samples after each annealing step was examined with transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The silicide phases present in the material were characterized with electron diffraction and high resolution transmission electron microscopy (HRTEM). For TEM examination both cross-sectional and plan-view specimens were prepared. The electron microscopy experiments were performed using JEOL JEM 200CX transmission electron microscope, JEOL JEM-2100 high resolution transmission electron microscope and Philips XL-30 scanning electron microscope. 3.
Results
Transmission electron microscopy micrograph taken from the as-deposited sample is shown in Fig. 1. The layers of nickel and amorphous silicon are visible. The adjacent layers are separated by thin transitional regions which indicates that some intermixing of Ni and Si has occurred. Uniformity of the surface morphology was confirmed with SEM micrographs (not shown). 3.1 Annealing at 600 C The first annealing step was performed at low temperature in order to initiate Ni and Si reactions leading to silicide phase formation without contribution of Si atoms from the SiC substrate. The TEM micrographs of the sample annealed at 600 C (Fig. 2(a)) show that the interface with the substrate remains smooth. The Ni and Si layers have reacted forming uniform polycrystalline layer of the thickness around 90 nm.
316
M. Wzorek et al.
However, a thin amorphous layer of the thickness varying up to about 5 nm can be observed at the interface. SEM micrographs, which are shown in Fig. 2(b) also confirm the uniformity of the polycrystalline layer.
Fig. 1 TEM cross-sectional micrograph of the as-deposited sample.
Typical grain sizes can be estimated from plan view TEM micrographs as shown in Fig. 2(c). The (0001) direction is perpendicular to the plane of the figure. The grains sizes vary from 15 to about 150 nm. In order to characterize silicide phases, electron diffraction patterns and HRTEM micrographs from plan view specimens were obtained. Besides the preferable Ni2 Si phase, another phases were also observed. The exemplary HRTEM micrographs of Ni31 Si12 and Ni2 Si grains and their fast Fourier transforms (FFT) are presented in Figs. 2(d) and 2(e) respectively. Exemplary diffraction patterns taken from selected areas of the specimen are shown in Figs. 2(f) and 2(g). The ring pattern related to Ni2 Si grains and the diffraction spots from Ni3 Si2 grain are indicated in Fig. 2(f). The Ni31 Si12 diffraction spots are indicated in Fig. 2(g). 3.2 Two-step annealing: 600 C + 1050 C In Fig. 3(a) a TEM cross-sectional micrograph of the sample subjected to the additional high temperature annealing at 1050 C is shown. The high temperature annealing step was performed in order to achieve ohmic behavior of the contact. Indeed, in our previous work we reported on the
Fig. 2 Results obtained from the sample annealed at 600 C: (a) TEM cross-section, (b) SEM image of the surface, (c) TEM plan-view image, (d) HRTEM image of the exemplary Ni31 Si12 grain, the inset shows corresponding FFT pattern (e) HRTEM image of a Ni2 Si grain with its FFT pattern in the inset, (f) electron diffraction pattern obtained from selected area containing a Ni3 Si2 grain, (g) electron diffraction pattern of a Ni31 Si12 grain.
TEM Characterisation of Silicide Phase Formation in Ni-Based Ohmic Contacts to 4H n-SiC
317
Fig. 3 Electron microscopy micrographs from the sample annealed at 600 C and subsequently at 1050 C: (a) TEM cross-section, (b) TEM plan-view micrograph (c) SEM image of the surface, (d) low magnification TEM plan-view image. Specific defects were observed: ‘‘I’’—voids at the interface and ‘‘II’’—discontinuities that extend through the whole layer thickness.
Fig. 4 Micrographs from the sample annealed at 600 C and subsequently at 1100 C: (a) TEM plan-view image, (b) SEM image of the surface. ‘‘I’’ and ‘‘II’’ denotes voids at the interface and layer discontinuities respectively.
formation of Ni2 Si/n-SiC ohmic contact with specific contact resistivity rc 3 10 4 cm2 after annealing at 1050 C.7) The layer consists of large column grains and the height of the grains determines the layer thickness. The grains are larger than in the case of annealing at 600 C. Large voids at the interface of the layer with the substrate, were observed as indicated in Fig. 3(a) by the number ‘‘I’’. The plan view TEM image (Fig. 3(b)) shows that the voids are located mainly at grain boundaries. The size of these voids varies from 30 to about 250 nm. The thickness of the layer is around 100 nm but as it can be observed from SEM micrographs (Fig. 3(c)), the thickness varies locally. Annealing at 1050 C resulted in the presence of the discontinuities in the layer as indicated by the number ‘‘II’’ in this figure. These are discontinuities that extend through the whole layer thickness and make SiC substrate locally exposed. Plan-view TEM micrograph obtained at low magnification is shown in Fig. 3(d). The I-type regions as well as II-type regions are both visible. The density of I-type defects are estimated to be about 30/mm2 . Electron diffraction patterns (not shown) indicate the presence of Ni2 Si
phase. No other phase was detected. It correlates well with results of X-ray diffraction.7) 3.3 Two-step annealing: 600 C + 1100 C The morphology of the sample after annealing at 1100 C is similar to the case of 1050 C annealing. The only phase detected was Ni2 Si. The voids at the interface with the substrate (I-type defect) as well as the discontinuities of the layer (II-type defect) are present (Fig. 4). The density and size of the I-type defects are similar to the case of the sample annealed at 1050 C, however the II-type discontinuities are significantly larger as it can observed from plan-view TEM image in Fig. 4(a) and the SEM image shown in Fig. 4(b). The thickness of the layer is also less uniform than in the case of the sample annealed at 1050 C. 4.
Discussion
High temperature annealing at 1050 C or 1100 C resulted in the presence of one silicide phase: Ni2 Si. It is in agreement with the results of similar experiments,3) performed with
318
M. Wzorek et al.
During annealing at 1050 C or at 1100 C all of the Ni31 Si12 and Ni3 Si2 grains are transformed to Ni2 Si phase. The grains are larger than after the first annealing step. Presumably during high temperature annealing Ni2 Si grains grow consuming other phases. The location of the voids (I type defects) indicate that the process is probably connected with strong diffusion along grain boundaries. 5.
Fig. 5 Part of the Ni-Si phase diagram.11) The points corresponding to Ni3 Si2 stoichiometry at annealing temperatures 1050 C and 1100 C are marked on the figure.
single-step annealing at 950 C. It has also been reported that annealing at lower temperatures can result in the presence of various silicide phases.1,8–10) It agrees with the results reported in this paper, which indicate that in the sample annealed at 600 C, besides Ni2 Si phase, also Ni31 Si12 and Ni3 Si2 silicides are present. Phase composition after the first annealing step at 600 C may have impact on the contact morphology after subsequent high temperature annealing. The fragment of phase diagram of Ni-Si system11) is shown in Fig. 5. It can be seen that the presence of the phases with composition in the vicinity of the 964 C eutectic, like NiSi, will lead to a liquid phase at higher temperatures, like 1050 C or 1100 C. It will not occur in the case of Ni2 Si phase and the phases with lower Si content, like Ni31 Si12 , where the first eutectic appears at 1142 C. The points of Ni3 Si2 phase stoichiometry at 1050 C and 1100 C are marked on the diagram. It can be expected that at these annealing temperatures the mixture of a solid state and a liquid phase will be present. Furthermore, the volume percentage of the liquid phase will be higher at 1100 C than in the case of 1050 C annealing. The discontinuities of the contact layers (II-type defects) that can be observed in SEM images, could be the result of the occurrence of the liquid phase during high temperature annealing. The presence of the Ni3 Si2 phase after the first annealing step might be responsible for the deterioration of contact layer uniformity. The imperfection of the layer due to the occurrence of the liquid phase was observed also for other materials on SiC substrate.12)
Summary and Conclusions
Ni/Si contact structures to 4H n-SiC were examined using electron microscopy methods. Phase compositions of the samples after subsequent annealing steps were determined with electron diffraction technique. After the first annealing step, at 600 C, Ni2 Si, Ni31 Si12 and Ni3 Si2 silicides were detected. The influence of phase composition after the first annealing step on the morphology of the contacts was discussed. It is concluded that the presence of Ni3 Si2 phase is probably responsible for discontinuities of the contact layers (II-type defects) after high temperature annealing. Acknowledgement The research was partially supported by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG.01.03.01-00159/08, ‘‘InTechFun’’). REFERENCES 1) A. Ba¨chli, M.-A. Nicolet, L. Baud, C. Jaussaud and R. Madar: Mat. Sci. Eng. B 56 (1998) 11–23. 2) I. P. Nikitina, K. V. Vassilevski, N. G. Wright, A. B. Horsfall, A. G. O’Neill and C. M. Johnson: J. Appl. Phys. 97 (2005) 083709-1– 083709-7. 3) B. Pe´cz, G. Radno´czi, S. Cassette, C. Brylinski, C. Arnodo and O. Noblanc: Diamond Rel. Mater. 6 (1997) 1428–1431. 4) F. La Via, F. Roccaforte, A. Makhtari, V. Raineri, P. Musumeci and L. Calcagno: Microel. Eng. 60 (2002) 269–282. 5) S. Youn Han, J.-Yoon Shin, B.-Teak Lee and J.-Lam Lee: J. Vac. Sci. Technol. B 20 (2002) 1496–1500. 6) A. Kuchuk, V. Kladko, M. Guziewicz, A. Piotrowska, R. Minikayew, A. Stonert and R. Ratajczak: J. Phys: Conf. Ser. 100 (2008) 042003-1– 042003-5. 7) A. V. Kuchuk, V. P. Kladko, A. Piotrowska, R. Ratajczak and R. Jakiela: Mater. Sci. Forum 615–617 (2009) 573–576. 8) C. S. Lim, H. Nickel, A. Naoumidis and E. Gyarmati: J. Mat. Sci. 32 (1997) 6567–6572. 9) C. Torregiani, C. Van Bockstael, C. Detavernier, C. Lavoie, A. Lauwers, K. Maex and J. A. Kittl: Microel. Eng. 84 (2007) 2533–2536. 10) J. A. Kittl, M. A. Pawlak, C. Torregiani, A. Lauwers, C. Demeurisse, C. Vrancken, P. P. Absil, S. Biesemans, C. Coia, C. Detavernier, J. JordanSweet and C. Lavoie: Appl. Phys. Lett. 91 (2007) 172108-1–172108-3. 11) L. Bo¨rnstein: Group IV Physical Chemistry, vol. 5I, Ni-Np–Pt-Zr, (Springer Verlag, 1998). 12) J. S. Chen, E. Kolawa, M.-A. Nicolet, R. P. Ruiz, L. Baud, C. Jaussaud and R. Madar: J. Mater. Res. 9 (1994) 648–657.
