Ni ohmic contacts on 4H-SiC

Ni ohmic contacts on 4H-SiC

Materials Science and Engineering B80 (2001) 370– 373 www.elsevier.com/locate/mseb Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic conta...

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Materials Science and Engineering B80 (2001) 370– 373 www.elsevier.com/locate/mseb

Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic contacts on 4H-SiC K. Vassilevski a,b,*, K. Zekentes a, K. Tsagaraki a, G. Constantinidis a, I. Nikitina b a

Microelectronics Research Group, Foundation for Research and Technology-Hellas, P.O. Box 1527, Heraklion, Crete 71110, Greece b Ioffe Institute, 26 Polytechnicheskaya strasse, St. Petersburg 194021, Russia

Abstract Ohmic contacts to the top p-type layers of 4H-SiC p+ –n –n+ epitaxial structures having an acceptor concentration lower than 1× 1019 cm − 3 were fabricated by the rapid thermal anneal of multilayer Al/Ti/Pt/Ni metal composition. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to the formation of duplex cermet composition containing Ni2Si and TiC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. The contacts exhibited a contact resistivity Rc of 9×10 − 5 V cm − 2 at 21°C, decreasing to 3.1 × 10 − 5 V cm − 2 at 186°C. It was found that thermionic emission through the barrier having a height of 0.097 eV is the predominant current transport mechanism in the fabricated contacts. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Silicon carbide; Ohmic contacts; Cermet; Aluminum; Thermionic emission

1. Introduction Silicon carbide (SiC)-based semiconductor electronic devices have demonstrated the ability to function in extremely high-temperature and high-power conditions. Significant improvements in epitaxial and bulk crystal SiC growth processes were achieved recently. This paved the way for the manufacturing of high-power and high-frequency SiC devices, and caused an increasing interest in the development of appropriate device processing in order to use in the best possible way the advantages originating from the SiC inherent material properties. One of the most critical issues of SiC device processing is the fabrication of ohmic contacts to pdoped 4H-SiC. Although contacts with resistivity Rc  1× 10 − 4 V cm − 2 have been fabricated to p-doped 4H-SiC, reproducible low-resistivity contacts were formed only to epitaxial layers having an acceptor concentration higher than 5×10 − 19 cm − 3, grown either by chemical vapor deposition [1], by Liquid Phase * Corresponding author. Present address: Microelectronics Research Group, Foundation for Research and Technology-Hellas, P.O. Box 1527, Heraklion, Crete 71110, Greece. Tel.: + 30-81-394105; Fax: + 30-81-394106. E-mail address: [email protected] (K. Vassilevski).

Epitaxy (LPE) [2], or by means of ion doping [3]. All these kinds of p-doped 4H-SiC materials have not been commercially available until now. Thus, the employment of these contact fabrication techniques to produce SiC devices based on commercially available epitaxial structures requires additional processing steps like the overgrowth by LPE of a heavily doped layer or its formation by ion doping. In this paper, we report on the fabrication of ohmic contacts to commercial 4H-SiC p+ –n–n+ epitaxial structures having an acceptor concentration lower than 1019 cm − 3.

2. Sample preparation The contacts to p+ 4H-SiC epitaxial layer were fabricated by rapid thermal annealing (RTA) of multilayer Al/Ti/Pt/Ni metal composition deposited by electronbeam evaporation. 4H-SiC p+ –n–n+ epitaxial structures grown on production grade n-type 4H-SiC substrate with 0.015 –0.028 V cm bulk resistivity were purchased from Cree, Inc. The doping profile of p+ – n–n+ 4H-SiC epitaxial structure was chosen to be convenient for subsequent electrical characterization of the diodes. The 1 mm thick heavily doped n+ layer was

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used as the buffer layer. The n base layer was 2 mm thick and had a donor concentration of  1.3× 1017 cm − 3. Finally, the acceptor concentration in the 1 mm thick p+ layer was chosen to be the highest possible for commercially available epitaxial layers. The thickness and the doping levels of the layers were verified by secondary ion mass spectrometry (SIMS). In particular, the aluminum atom concentration in the p+ layer was found to be 1.5 ×1019 cm − 3, while the acceptor concentration in the p layer was equal to (6 – 8)× 1018 cm − 3, according to Hg-probe capacitance –voltage measurements. Therefore, about 50% of aluminum atoms are electrically activated in the p+ layer, as deduced from the comparison of the Hg-probe measurement results with the data obtained by SIMS. Prior to contact fabrication, the samples were cleaned sequentially by: (1) degreasing in organic solvents, (2) bathing in de-ionized water (DI) and, finally, (3) by use of the standard RCA cleaning procedure [4] and of dipping the sample in 10% HF for 2 min at room temperature between the steps. The geometry of the contacts to the p+ epitaxial layer (pads size, 40×80 mm2 with distances 4, 8, 12, and 16 mm between them)

Fig. 1. (a) TLM mesa structure formed on 4H-SiC p+ –n–n+ epitaxial wafer and (b) the resistance dependencies on the distance between contact pads for various temperatures.

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was defined by contact UV lithography. Immediately prior to placing the samples in the vacuum chamber, the samples patterned with AZ 5214 photoresist were immersed in 10% HF for 10 s at room temperature, followed by rinsing in DI water and blow drying with nitrogen. The metal deposition was made by electronbeam evaporation at a residual pressure B5×10 − 7 mbar without heating of the substrate during the process. The metals were deposited in a single run in the following sequence: A1(50 nm)/Ti(100 nm)/Pt (25 nm)/ Ni (50 nm). After excess metal removal by lift-off in acetone, the RTA process was performed by lamps at 1000°C for 120 s in a commercial rapid thermal annealing chamber pumped down to 4× 10 − 5 Torr. A detailed description of the contact fabrication procedure is given elsewhere [2,5,6]. Then, a gold overlayer 200 nm thick was deposited for improving the current spreading in the contact metallization and the electrical contact between the contact pads and the tips of the probe station. Next, a 200 nm aluminum mask was deposited on the p+ epitaxial layer. A commercial plasma system (Vacutec AB 1350 series) was used to form the mesa structures for the measurement of the contact resistivity by transmission line model (TLM). The chamber was pumped down to 2 × 105 mbar before the etch process. The temperature of the RF electrode was kept at 200°C. The process was optimized to obtain a contamination-free smooth surface. The etch rate was approximately 100 nm min − 1 at 200 W r.f. power, 20 sccm flow of SF6, 20 sccm flow of Ar, and a pressure of 30 mbar. Uncovered SiC was etched away down to the n+ epitaxial layer. After the SiC etching, the Al mask was selectively removed in KOH solution. The TLM mesa structure formed on the 4H-SiC p+ – n–n+ epitaxial wafer is shown in Fig. 1a.

3. Experimental results

3.1. Structural properties of the contacts

Fig. 2. AES depth profile of ohmic contact to p-type 4H-SiC. 1, Ni2Si-containing contact layer; 2, TiC-containing contact layer; 3, SiC.

At first, the contact composition was investigated by Auger electron spectroscopy (AES) depth profiling. It was found that the RTA process led to formation of a layered contact structure having two clearly separated regions: (1) close to the surface enriched by Ni and Si; and (2) close to the SiC interface enriched by Ti and C, as shown in Fig. 2. Phase analysis of the contacts was performed by X-ray diffraction. It was found that the contact layer contained Ni2Si and TiC components after annealing. Evidently, the creation of this ceramic containing metal (cermet) contact layer was progressing simultaneously with the decomposition of SiC. This process is undesirable for fabrication of ultrahigh frequency devices usually having shallow p–n junctions. At least the depth of the SiC decomposition should be

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Fig. 3. AFM observations. (a) 3D view and (b) cross-section profile of the surface of 4H-SiC surface after removal of the ohmic contacts: (A) surface after contact removal; (B) free surface. The step height between the two arrows in (b) is equal to 110 nm. The steps along the y axis are due to 4H-SiC growth process.

regia and HNO3:HF (3:1) several times. The morphology of the 4H-SiC after contact removal was investigated by AFM on test epitaxial layers grown by LPE [2]. Fig. 3 shows the AFM image (3D view and crosssection profile) of the sample in the region of the contact edge. The growth steps typical for the LPE process oriented parallel to the y direction are clearly seen on the free surface as well as on the surface after contact removal. The contact formation caused a SiC decomposition to a depth of about 110 nm, as is evident from the height of the step parallel to the x axis. The mean roughness of the recess floor measured between growth steps was estimated to be about 30 nm, while the mean roughness of the free surface measured between growth steps was no more than 3 nm. Al was not detected by the AES analysis of the annealed samples. However, it was shown elsewhere [2] by SIMS profiling of these contacts that part of the Al diffused through Ti towards the surface while another part remained in its initial position at the interface of the contact and silicon carbide. The introduction of the platinum layer in the contact metal composition had no effect on the formation of the cermet structure. Creation of Ni2Si and TiC was also discovered when Pd was used instead of Pt layer [2]. Nevertheless, introducing the Pt layer in the interface between Ni and Ti led to reduction of contact surface roughness (see Fig. 4) and to the improvement of the gold overlay adhesion.

3.2. Electrical characteristics of the contacts

Fig. 4. Nomarski photos of Al/Ti/Pt/Ni contact (a) before annealing and (b), (c), (d) annealed by RTA at 1000°C for 120 s. Contact diameter is 40 mm. The thickness of the Pt layer is (a) − 25 nm, (b) − 10 nm, (c) −20 nm, and (d) −25 nm.

well defined. To estimate this, the contacts were removed by sequential dipping of the sample in aqua

Electrical characterization of the contacts was performed by linear TLM [7] in the temperature range from 21 to 186°C. The I–V characteristics measured between two contacts pads were linear up to the current value of about 6 mA, at which the voltage drop was exceeding 3 V and current spreading in the n-layer occurred. The contact resistivity at room temperature was measured before and after sample heating and was found to be the same. The measurements were carried out in air ambient. The resistance dependencies on the distance between contact pads for various temperatures are shown in Fig. 1b. The contacts revealed specific a contact resistance Rc of 9× 10 − 5 V cm2 at 21°C, decreasing to 3.1×10 − 5 V cm2 at 186°C. The strong dependence of the contact resistivity on the temperature is a characteristic for contacts formed on relatively low doped semiconductors when a thermionic emission is the predominant current transport mechanism. In the thermionic emission regime, the contact resistivity is given by [8]: Rc =

 

k qƒB exp qA*T kT

(1)

K. Vassile6ski et al. / Materials Science and Engineering B80 (2001) 370–373

Fig. 5. Dependencies of contact specific contact resistance and sheet resistivity of a p-doped 4H-SiC layer on the reciprocal temperature value.

where k is the Boltzmann constant, q is the electron charge, T is the temperature, A* is the Richardson constant, and ƒB is a height of potential barrier between the contact and semiconductor. Fitting experimental results with Eq. (1) as it is shown in Fig. 5 gives the value of the potential barrier height ƒB =0.097 eV. Dependence of sheet resistivity on reciprocal temperature is also shown in Fig. 5. The slope of the best fit of experimental data with a function B exp(EA/2kT), where B is the constant independent of the temperature, gives the value EA =230 meV. This value is close to the Al acceptor ionization energy in 4H-SiC published elsewhere (180 meV) [9].

4. Conclusion Ohmic contacts to p-type layers of commercial 4HSiC p+ –n –n+ epitaxial structures having an acceptor concentration lower than 1019 cm − 3 were fabricated. The contacts exhibited a contact resistivity Rc of 9× 10 − 5 V cm2 at 21°C decreasing to 3.1× 10 − 5 V cm2 at 186°C. Measurements of the contact resistivity at various temperatures allowed one to define the current transport mechanism in the fabricated contacts. It was found that the thermionic emission through the barrier having a height of 0.097 eV is predominant. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to formation of two-layer cermet composition

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containing Ni2Si and TIC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. This contact fabrication technique may be considered as device processing compatible, since the SiC decomposition depth was sufficiently small in comparison with typical thickness of epitaxial layers for ultrahigh frequency devices. Furthermore, the contact formation procedure does not include additional epitaxial or ion doping steps, as well as a treatment in aggressive chemicals prior to metal deposition. It utilizes the deposition of the metals on the substrate at room temperature and allows one to use the lift off procedure for sample patterning. It could be useful for the fabrication of high-frequency and high-power bipolar 4H-SiC devices, when low-series resistance and thermal stability are critical factors for efficient device operation.

Acknowledgements This work was supported by NATO SfP 971879(98) and INTAS-CNES 97-1386 grants. One of the authors (K.V.) is grateful to N. Belousov for useful discussion.

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