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 Katcki1

1Institute of Electron Technology, Al. Lotnikow 32/46, 02-668 Warsaw, Poland2V. 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 usingmagnetron sputtering technique. The Ni to Si layer thickness ratio was chosen to achieve the stoichiometry of Ni2Si phase. The depositedstructure 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 specificvoids and discontinuities of the layer were observed after annealing at high temperature. Phase compositions were investigated with electrondiffraction technique. After annealing at 600�C the phases Ni2Si, Ni3Si2 and Ni31Si12 were detected. High temperature annealing resulted in thepresence of only Ni2Si phase. The influence of phase transformations on the morphology of the contacts is discussed. Explanation of the origin oflayer 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 materialwith a high application potential for development of highpower, high frequency and high temperature electronicdevices. However, development of low resistivity ohmiccontacts to SiC that could sustain their characteristics afterlong-term high power operation at high temperatures iscritical for successful realization of SiC devices.

Ni-based structures are considered as most promising fordevelopment of reliable contacts.1–5) Nickel ohmic contactsare characterized by low resistivity but require high temper-ature to form. During annealing Ni reacts with Si from theSiC substrate. The stable Ni2Si phase is formed but redundantcarbon atoms form precipitates in the contact layer. In orderto avoid reaction of metal with the substrate, the Si can bedeposited as the first layer onto the SiC wafer.3)

This paper focuses on electron microscopy investigationson the Ni/Si/n-SiC contact structures. The Ni/Si multilayerstructure was deposited on the 4H-SiC wafer and subjectedto a two-step annealing procedure. The influence of phasetransformations on the morphology of the contacts isdiscussed.

2. Experimental

The n-type 4H-SiC substrate (n � 2� 1017 cm�3) was aSi-faced (0001) oriented wafer supplied by Cree ResearchInc. Surface preparation procedure was described in previouswork.6) The Ni/Si multilayer structure was deposited bymagnetron sputtering technique, using Ni and Si targetsand Ar plasma. The thickness of each deposited layerwas chosen to achieve the overall stoichiometry of theNi2Si 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 anneal-ing procedure in a nitrogen ambient that consisted of two

steps. The first step was performed at 600�C for 15min. Thenext step was conducted subsequently at 1050�C or 1100�Cfor 3min. The low temperature annealing was performed toinitiate intermixing of Ni and Si layers and silicide phaseformation, the high temperature step was necessary toachieve ohmic contact to the SiC substrate.

The morphology and microstructure of the samples aftereach annealing step was examined with transmission electronmicroscopy (TEM) and scanning electron microscopy(SEM). The silicide phases present in the material werecharacterized with electron diffraction and high resolutiontransmission electron microscopy (HRTEM). For TEMexamination both cross-sectional and plan-view specimenswere prepared. The electron microscopy experiments wereperformed using JEOL JEM 200CX transmission electronmicroscope, JEOL JEM-2100 high resolution transmissionelectron microscope and Philips XL-30 scanning electronmicroscope.

3. Results

Transmission electron microscopy micrograph taken fromthe as-deposited sample is shown in Fig. 1. The layers ofnickel and amorphous silicon are visible. The adjacent layersare separated by thin transitional regions which indicates thatsome intermixing of Ni and Si has occurred. Uniformity ofthe surface morphology was confirmed with SEM micro-graphs (not shown).

3.1 Annealing at 600�CThe first annealing step was performed at low temperature

in order to initiate Ni and Si reactions leading to silicidephase formation without contribution of Si atoms from theSiC substrate. The TEM micrographs of the sample annealedat 600�C (Fig. 2(a)) show that the interface with the substrateremains smooth. The Ni and Si layers have reacted forminguniform polycrystalline layer of the thickness around 90 nm.

Materials Transactions, Vol. 52, No. 3 (2011) pp. 315 to 318Special Issue on New Trends for Micro- and Nano Analyses by Transmission Electron Microscopy#2011 The Japan Institute of Metals

However, a thin amorphous layer of the thickness varyingup to about 5 nm can be observed at the interface. SEMmicrographs, which are shown in Fig. 2(b) also confirm theuniformity of the polycrystalline layer.

Typical grain sizes can be estimated from plan view TEMmicrographs as shown in Fig. 2(c). The (0001) direction isperpendicular to the plane of the figure. The grains sizes varyfrom 15 to about 150 nm. In order to characterize silicidephases, electron diffraction patterns and HRTEM micro-graphs from plan view specimens were obtained. Besides thepreferable Ni2Si phase, another phases were also observed.The exemplary HRTEM micrographs of Ni31Si12 and Ni2Sigrains and their fast Fourier transforms (FFT) are presentedin Figs. 2(d) and 2(e) respectively. Exemplary diffractionpatterns taken from selected areas of the specimen are shownin Figs. 2(f) and 2(g). The ring pattern related to Ni2Si grainsand the diffraction spots from Ni3Si2 grain are indicated inFig. 2(f). The Ni31Si12 diffraction spots are indicated inFig. 2(g).

3.2 Two-step annealing: 600�C + 1050�CIn Fig. 3(a) a TEM cross-sectional micrograph of the

sample subjected to the additional high temperature anneal-ing at 1050�C is shown. The high temperature annealing stepwas performed in order to achieve ohmic behavior of thecontact. Indeed, in our previous work we reported on theFig. 1 TEM cross-sectional micrograph of the as-deposited sample.

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 Ni31Si12 grain, the inset shows corresponding FFT pattern (e) HRTEM image of a Ni2Si grain

with its FFT pattern in the inset, (f) electron diffraction pattern obtained from selected area containing a Ni3Si2 grain, (g) electron

diffraction pattern of a Ni31Si12 grain.

316 M. Wzorek et al.

formation of Ni2Si/n-SiC ohmic contact with specific contactresistivity rc � 3� 10�4 � cm2 after annealing at 1050�C.7)

The layer consists of large column grains and the height ofthe grains determines the layer thickness. The grains arelarger 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 voidsare located mainly at grain boundaries. The size of thesevoids varies from 30 to about 250 nm.

The thickness of the layer is around 100 nm but as it can beobserved from SEM micrographs (Fig. 3(c)), the thicknessvaries locally. Annealing at 1050�C resulted in the presenceof the discontinuities in the layer as indicated by the number‘‘II’’ in this figure. These are discontinuities that extendthrough the whole layer thickness and make SiC substratelocally exposed. Plan-view TEM micrograph obtained at lowmagnification is shown in Fig. 3(d). The I-type regions aswell as II-type regions are both visible. The density of I-typedefects are estimated to be about 30/mm2. Electron diffrac-tion patterns (not shown) indicate the presence of Ni2Si

phase. No other phase was detected. It correlates well withresults of X-ray diffraction.7)

3.3 Two-step annealing: 600�C + 1100�CThe morphology of the sample after annealing at 1100�C is

similar to the case of 1050�C annealing. The only phasedetected was Ni2Si. The voids at the interface with thesubstrate (I-type defect) as well as the discontinuities of thelayer (II-type defect) are present (Fig. 4). The density andsize of the I-type defects are similar to the case of the sampleannealed at 1050�C, however the II-type discontinuities aresignificantly larger as it can observed from plan-view TEMimage 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 caseof the sample annealed at 1050�C.

4. Discussion

High temperature annealing at 1050�C or 1100�C resultedin the presence of one silicide phase: Ni2Si. It is in agreementwith the results of similar experiments,3) performed with

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.

TEM Characterisation of Silicide Phase Formation in Ni-Based Ohmic Contacts to 4H n-SiC 317

single-step annealing at 950�C. It has also been reported thatannealing at lower temperatures can result in the presenceof various silicide phases.1,8–10) It agrees with the resultsreported in this paper, which indicate that in the sampleannealed at 600�C, besides Ni2Si phase, also Ni31Si12 andNi3Si2 silicides are present.

Phase composition after the first annealing step at 600�Cmay have impact on the contact morphology after subsequenthigh temperature annealing. The fragment of phase diagramof Ni-Si system11) is shown in Fig. 5. It can be seen that thepresence of the phases with composition in the vicinity of the964�C eutectic, like NiSi, will lead to a liquid phase at highertemperatures, like 1050�C or 1100�C. It will not occur in thecase of Ni2Si phase and the phases with lower Si content, likeNi31Si12, where the first eutectic appears at 1142�C.

The points of Ni3Si2 phase stoichiometry at 1050�C and1100�C are marked on the diagram. It can be expected thatat these annealing temperatures the mixture of a solid stateand a liquid phase will be present. Furthermore, the volumepercentage of the liquid phase will be higher at 1100�C thanin 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 ofthe occurrence of the liquid phase during high temperatureannealing. The presence of the Ni3Si2 phase after the firstannealing step might be responsible for the deterioration ofcontact layer uniformity. The imperfection of the layer due tothe occurrence of the liquid phase was observed also for othermaterials on SiC substrate.12)

During annealing at 1050�C or at 1100�C all of theNi31Si12 and Ni3Si2 grains are transformed to Ni2Si phase.The grains are larger than after the first annealing step.Presumably during high temperature annealing Ni2Si grainsgrow consuming other phases. The location of the voids (Itype defects) indicate that the process is probably connectedwith strong diffusion along grain boundaries.

5. Summary and Conclusions

Ni/Si contact structures to 4H n-SiC were examined usingelectron microscopy methods. Phase compositions of thesamples after subsequent annealing steps were determinedwith electron diffraction technique. After the first annealingstep, at 600�C, Ni2Si, Ni31Si12 and Ni3Si2 silicides weredetected. The influence of phase composition after the firstannealing step on the morphology of the contacts wasdiscussed. It is concluded that the presence of Ni3Si2 phase isprobably responsible for discontinuities of the contact layers(II-type defects) after high temperature annealing.

Acknowledgement

The research was partially supported by the EuropeanUnion within European Regional Development Fund,through grant Innovative Economy (POIG.01.03.01-00-159/08, ‘‘InTechFun’’).

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