Materials Science Forum, ISSN: 1662-9752, Vol. 1159, pp 61-66

doi: 10.4028/p-Zdgj7m

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2024-09-20

Revised:
2025-04-30

Accepted:
2025-04-30

Online:
2025-09-09

Formation of Structured Low-Ohmic p-Type Contacts on AI-Implanted 4H-SiC by Laser Annealing

The original version of this paper is available on https://www.scientific.net/MSF.1159.61.pdf

Carsten Hellinger , Michael Thum and Mathias Rommel

Fraunhofer Institute of Integrated Systems and Device Technology (IISB)

Schottkystrasse 10, 91058 Erlangen, Germany

carsten.hellinger@iisb.fraunhofer.de, michael.thum@iisb.fraunhofer.de, mathias.rommel@iisb.fraunhofer.de

Keywords: Laser annealing, ohmic contact, structured contact, p-type, 4H-SiC

Abstract

In this work, a method is presented to form structured low-ohmic p-type contacts on Alimplanted by laser annealing. A metal layer sequence for p -type contacts suitable for UV laser treatment was developed. Furthermore, a method to protect thermosensitive layers from damages by laser treatment is presented. By structuring the top metallization layer, it is possible to use the metal stack as a self-aligned mask. That makes it possible to laser the entire surface of the wafer, whereby thermosensitive layers are protected from damages by laser annealing by utilizing the optical properties of the individual metal layers. To evaluate the method, TLM structures were electrically characterized. TLM structures were electrically characterized successfully demonstrating the feasibility of the presented approach.

Introduction

The original version of this paper is available on https://www.scientific.net/MSF.1159.61.pdf

The ohmic contact between the metal contact layer and the semiconductor is of utmost importance for low power electronic devices [1]. Therefore, the fabrication of low ohmic contacts is one of the essential aspects in the manufacturing and development of highly energy efficient SiC power semiconductor devices, especially for vertical devices. For low p-type ohmic contacts on the frontside usually titanium-aluminum compounds are being used for p-type contacts [1,2,3]. A following temperature treatment of the metal layer is necessary for the titanium-aluminum layer to react with the SiC substrate. Thereby the semiconductor is formed, which results in a low-ohmic contact on p-type . This temperature step is generally performed by RTA (rapid thermal annealing) [1], although laser processing of full-surface n-type backside contacts is now established [6, 7, 8]. However, laser annealing for alloying structured frontside contacts is difficult. Either only the ohmic contact areas are annealed, which requires good alignment of metallized areas and laser processing, or the entire surface is annealed, which can lead to damage or degradation of the layers outside the contact area. Another challenge is the laser processing of the titanium-aluminum layer itself. According to literature, a commonly used metal stack for the fabrication of p-type contacts is , resulting in a layer after an annealing step [1]. However, aluminum has a reflectivity of over at a wavelength of 355 nm [9], which is the wavelength of commonly used short-pulse lasers [8]. As a result, only a small part of the laser radiation is absorbed and the metal layer hardly heats up. Therefore, a different layer composition or layer sequence must be used to produce p-type contacts using laser processing.

In this work, a solution was developed to produce structured ohmic contacts on p-type SiC using laser annealing. For this purpose, metal stacks were investigated for p-type contact formation by laser annealing. To characterize resulting contacts electrically, TLM structures were prepared, measured and analyzed.

Experimental

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TLM structures were manufactured on a commercial wafer with a n-type epitaxial layer of thickness and doping concentration. The layout of the TLM test structures is shown in fig. 1. Al was implanted in the area of the TLM structures (see fig. 1 with a surface
concentration of approx. 5E19 using three individual ion implantation steps (see table 1). To activate the dopants the wafers were subsequently annealed at for 30 minutes in Ar atmosphere.

Fig. 1. TLM test structures in top view, where Z is the width and L the length of the TLM pattern, W the wide of the implanted tub and d the distance between the TLM pattern (dark grey: contact structures, light grey: -doped area).

Fig. 1. TLM test structures in top view, where Z is the width and L the length of the TLM pattern, W the wide of the implanted tub and d the distance between the TLM pattern (dark grey: contact structures, light grey: -doped area).

Table 1. Parameters of p-type implantation.

Energy [keV] | Dose [1/cm²]
90 | 2.8E14
60 | 1.8E14
30 | 1.4E14

Afterwards the metallization layer was deposited. Therefore, a stack of 20 nm Titanium and 75 nm Aluminum was evaporated and a photoresist was spin coated and structured, before the uppermost 20 nm Ni layer was deposited (see fig. 2). Then the uppermost Ni layer was structured by lift-off (see fig. 3a) and the whole wafer was laser annealed with different energy densities. Therefore a Sumitomo SWA20US-M laser annealing tool with a frequency-tripled Nd:YVO4 laser with 355 nm wavelength, a beam size of and a pulse duration of 50 ns was used. The energy density was varied from 1.4 to , whereby the overlap in x-direction was varied from 67 to and in y -direction from 50 to .

Fig. 2. Process flow to produce TLM structures.

Fig. 2. Process flow to produce TLM structures.

Since the used stack has only a reflectivity of compared to Aluminum with over at 355 nm [9], significantly more radiation is absorbed in the ohmic contact areas with Ni layer on top, causing a reaction of the metal layers with the SiC substrate (see fig. 3b). In the area between the contacts, however, where aluminum is the top layer, most of the radiation is reflected and no reaction takes place until very high energy densities are being used for the laser annealing (see fig. 3b). After irradiation the non-reacted Al-Ti-layer between the ohmic contact areas was removed wet-chemically (see fig. 3c and fig. 4). Finally, a titanium and platinum layer is deposited and structured as contact metal.

Fig. 3. Images of TLM structures after (a) lift-off, (b) laser annealing with different energy densities and overlaps and (c) wet chemical etching of unreacted Aluminum and Titanium.

Fig. 3. Images of TLM structures after (a) lift-off, (b) laser annealing with different energy densities and overlaps and (c) wet chemical etching of unreacted Aluminum and Titanium.

Fig. 4. Microscope images of lasered TLM structures depending on energy densities after wet chemical etching of unreacted aluminum and titanium.

Fig. 4. Microscope images of lasered TLM structures depending on energy densities after wet chemical etching of unreacted aluminum and titanium.

Fig. 5. Microscope images of lasered TLM structures depending on energy densities and laser spot overlap in x - and y-direction after wet chemical etching of unreacted aluminum and titanium.

Fig. 5. Microscope images of lasered TLM structures depending on energy densities and laser spot overlap in x - and y-direction after wet chemical etching of unreacted aluminum and titanium.

Results and Discussion

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The optical analysis of the samples (fig. 4 and 5) shows that the reflectivity of the aluminum layer is high enough at 355 nm wavelength for energy densities up to . The non-reacted metal can be completely removed after thermal treatment. For higher energy densities, the metallization between the TLM structures reacts with the silicon carbide substrate and could not be etched wetchemically. This means that the TLM patterns are short-circuited. For energy densities of 2.2 and it is noticeable that the lateral spread of the contacts is increased beyond the original contact length L after laser treatment (see fig. 4 and 5). For , the contacts up to apart and for the contacts up to apart are short-circuited. Also, for a lateral spread can be seen in fig. 4. This observation and the associated reduction in the distance between the TLM patterns must be considered when evaluating the I(V) characteristics. Fig. 6 exemplary shows the resistance as a function of the distance between the TLM patterns for different energy densities and a linear fit. Only the non-short-circuited TLM patterns were used for the evaluation and the distance between the TLM patterns was corrected according to the lateral spread. The fit line shows a high coefficient of determination. It can be used to calculate the sheet resistance and the specific contact resistance.

Fig. 6. R(d) plot for an exemplary TLM test structure as a function of energy density.

Fig. 6. R(d) plot for an exemplary TLM test structure as a function of energy density.

Fig. 7 shows the specific contact resistivity and sheet resistance for different energy densities. The sheet resistance depends on the dopant concentration [11]. Since all test structures were produced according to the implantation values in table 1, the sheet resistance should be independent of the energy densities during the laser treatment. Due to the significantly large lateral spread, only the values for energy densities lower than are plotted. The specific contact resistivity shows the lowest values with for 1.9 and for in average. At , the energy density does not appear to be high enough to produce low-resistance contacts as the average specific contact resistivity is . For , the average specific contact resistivity is and is more than eight times higher than for . At the same time, the test structures with have significantly higher standard errors. Due to the manual insertion of the wafer into the laser tool, it is possible that the transitions between the sectors with and do not correspond exactly to the mask layout, whereby structures in the transition were processed with the surrounding energy densities (see fig. 5) and consequently falsify the measurement results.

Fig. 7. Determined specific contact resistivity and sheet resistance depending on energy density.

Fig. 7. Determined specific contact resistivity and sheet resistance depending on energy density.

To further narrow down the process window for the laser parameters and metal stack thicknesses to optimize the specific contact resistance, further tests are necessary. To find out which reaction products, i.e. and compounds, are responsible for the ohmic contact formation additional XRD and TEM analyses are essential.

Summary

The original version of this paper is available on https://www.scientific.net/MSF.1159.61.pdf

In this work, titanium-based ohmic contacts were produced on p-doped using a UV short pulse laser. Due to the optical properties of the commonly used metal stack a novel Ni ( 20 nm / Al / Ti stack optimized for UV lasers was used, whereby the Ni layer was structured by lift-off. The use of the structured metal stack as a self-aligned mask makes it possible to laser the entire surface of the wafer, whereby thermosensitive layers are protected from damages by laser annealing by utilizing the high reflectivity of the top Al layer, where the Ni was removed. With this method contact pads with a size between and could be realized. It was shown that structured contacts with a specific contact resistance of for and for on average on p-doped 4 H -SiC could be produced using this method.

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The original version of this paper is available on https://www.scientific.net/MSF.1159.61.pdf

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