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Investigation of a Self-Aligned Cobalt Silicide Process for Ohmic Contacts to Silicon Carbide

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Investigation of a Self-Aligned Cobalt Silicide Process for Ohmic Contacts to Silicon Carbide Journal of ELECTRONIC MATERIALS, Vol. 48, No. 4, 2019 https://doi.org/10.1007/s11664-019-07020-0 Ó 2019 The Author(s) Investigation of a Self-Aligned Cobalt Silicide Process for Ohmic Contacts to Silicon Carbide MATTIAS EKSTRO¨ M ,1,2 ANDREA FERRARIO ,1 and CARL-MIKAEL ZETTERLING 1 1.—Department of Electronics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, 164 40 Kista, Sweden. 2.—e-mail: [email protected] Previous studies showed that cobalt silicide can form ohmic contacts to p-type 6H-SiC by directly reacting cobalt with 6H-SiC. Similar results can be achieved on 4H-SiC, given the similarities between the different silicon car- bide polytypes. However, previous studies using multilayer deposition of sil- icon/cobalt on 4H-SiC gave ohmic contacts to n-type. In this study, we investigated the cobalt silicide/4H-SiC system to answer two research ques- tions. Can cobalt contacts be self-aligned to contact holes to 4H-SiC? Are the self-aligned contacts ohmic to n-type, p-type, both or neither? Using x-ray diffraction, it was found that a mixture of silicides (Co2Si and CoSi) was reliably formed at 800C using rapid thermal processing. The cobalt silicide mixture becomes ohmic to epitaxially grown n-type (1  1019cmÀ3) if annealed at 1000C, while it shows rectifying properties to epitaxially grown p-type (1  1019cmÀ3) for all tested anneal temperatures in the range 800–1000C. À4 2 The specific contact resistivity (qC)ton-type was 4:3  10 X cm . This work opens the possibility to investigate other self-aligned contacts to silicon car- bide. Key words: Cobalt (Co), rapid thermal processing (RTP), self-aligned silicide, silicon carbide (4H-SiC), transfer length method (TLM) there is currently missing a single-metal process for INTRODUCTION self-aligned ohmic contacts to p-type. It is known The finding that nickel silicide (Ni2Si) can be self- from the mature silicon technology that, in addition aligned to contact holes to silicon carbide (4H-SiC)1,2 to nickel silicide,9–11 titanium-,10–13 cobalt-10,11,13 has opened up new research opportunities. An and platinum-silicide14 can be self-aligned. Given example of such a study was the finding that the the similarities between silicon and 4H-SiC, some or self-aligned Ni2Si can be turned from rectifying to all of the silicides that can be self-aligned to silicon ohmic to p-type 4H-SiC by alloying it with alu- can be self-aligned to SiC. Thus, it is of interest to minum at relatively low temperatures.3 A compar- investigate the other silicide-forming systems. ison between self-aligned nickel silicide and lift-off Lundberg and O¨ stling15 showed that cobalt sili- process for power devices was recently presented by cides (CoSix) form ohmic contacts to p-type 6H-SiC. Sledziewski at the conference of silicon carbide and The SiC polytypes have similarities and differences. related materials (ECSCRM 2018). Nickel-based The bandgap energy is similar between the two contact processes give good ohmic contacts to n- polytypes (3.02 eV and 3.26 eV at 300 K for 6H and type 4H-SiC,2,4,5 but often gives rectifying contacts 4H, respectively.16) The energy difference between 6–8 to p-type, with some notable exceptions. As such, vacuum energy (E0) and valence band edge (EV )is approximately the same between different poly- 17–19 types (E0 À EV 7:1 eV). If the CoSix/6H-SiC contact that Lundberg and O¨ stling found was a (Received October 29, 2018; accepted January 29, 2019; Schottky–Mott-like ohmic contact, it would be published online February 12, 2019) 2509 2510 Ekstro¨m, Ferrario, and Zetterling reasonable to assume that it would also be ohmic to The second set was used to determine the specific 4H, albeit with a slightly higher Schottky barrier. contact resistivity to both n-type and p-type 4H-SiC. As such, self-aligned CoSix was considered as a The epitaxy was grown by Norstel, and was, from candidate for low resistance ohmic contacts to p- top to bottom: 1 lm n-type of 1019 cmÀ3 doping type 4H-SiC. However, it is known that contacts to concentration and 1 lm p-type of 1018 cmÀ3 doping 4H-SiC are not perfect Schottky–Mott contacts. 4H- concentration. The epitaxy was grown on a 100 mm SiC contacts have at least partial Fermi-level 4 off-axis cut n-type substrate wafer. The p-type 20 pinning, and as such it can be inferred that layer provides a pn-junction isolation for the top n- similar valence band energy does not guarantee type layer, and the p-type layer is isolated from the similar hole Schottky barrier height. The close n-type substrate by the formed pn-junction. These chemical similarity between cobalt (atomic number doping levels correspond to those used in our in- 27) and nickel (atomic number 28) could imply house bipolar junction transistor (BJT) technology, similar characteristics. As such, the CoSix/4H-SiC and as such it would be an interesting finding if the system could instead give low resistance ohmic cobalt contacts would provide low resistance ohmic contacts to n-type 4H-SiC, just like the Ni2Si/4H- contacts to both n-type and p-type at the same time. SiC system. CoSi2/4H-SiC, formed by multilayer The third and final set had highly doped p-type deposition of silicon and cobalt, has been previously epitaxy. The epitaxy was grown by Norstel and was, studied and been shown to provide low resistance 19 À3 21–23 from top to bottom: 200 nm p-type of 10 cm ohmic contact to n-type 4H-SiC, and the result doping concentration, 800 nm n-type 1016 cmÀ3 to p-type 4H-SiC is unclear.24 doping concentration, and 1 lm p-type of 5  We investigated the CoSix/4H-SiC system to 18 À3 answer the following two research questions25: 10 cm doping concentration. The epitaxy was grown on a 100 mm 4 off-axis cut n-type substrate 1. Can CoSix be self-aligned to 4H-SiC? wafer. The different pn-junctions provide isolation 2. Does self-aligned CoSix form ohmic contacts to between the different layers. This third set has a n-type, p-type, both or neither? higher p-type doping than the second set, which should enhance field emission (FE) by having a narrower depletion region. If the CoSix would form In this work, we present the results and conclusions low resistance ohmic contacts to p-type, it was of our investigations. The difference from previous expected that this third set would have lower contact studies to 4H-SiC is that only cobalt is contact resistance to p-type than the second set. deposited instead of the silicon/cobalt multilayer 21–23 The samples with epitaxy (sets 2 and 3) were stacks. Moreover, interface reactions between prepared to have transfer length method (TLM) cobalt and 4H-SiC are studied using rapid thermal structures by forming TLM mesas. These mesas processing (RTP) to anneal cobalt/4H-SiC instead of 26,27 limit the current flow to laminar flow between furnace annealing cobalt/6H-SiC. contacts. By limiting the current flow, the accuracy The annealing temperature range was limited in of the extraction of the specific contact resistivity is this study up to 1000 C. It was considered that any improved.31 The mesas were patterned by standard contact process requiring higher annealing temper- optical lithography and were dry etched by magnet- atures than 1000C would not be competitive with 2,3,21–23,28–30 ically enhanced reactive ion etching (MERIE). The already established contact processes. second set was etched twice in order to provide mesa Higher anneal temperatures could potentially isolation for both n-type and p-type, whereas the degrade other device related properties, like the third set was etched once to provide isolation for the silicon dioxide (SiO2)/4H-SiC interface of metal top p-type layer. Since dry etching induces surface oxide semiconductor field-effect transistors damage, the wafers underwent sacrificial oxidation, (MOSFETs). where the surface is oxidized and removed by stripping the grown oxide with hydrofluoric acid. EXPERIMENTAL DETAILS At this point, all three wafers could be diced into pieces for further processing. Three different sets of 4H-SiC samples All three sets of samples were cleaned in Piranha (10 mm  10 mm pieces) were prepared. The first solution (H2SO4 mixed with H2O2) for 5 min to set was used for x-ray diffraction (XRD) character- remove any organic material, followed by diluted ization. These pieces came from a 100 mm 4 off-axis HF (dHF) for 5 min to remove any chemical SiO2. cut n-type substrate wafer (SiCrystal, 0.0175 X cm, To enable the self-aligned process, a 50 nm SiO2 corresponding to 3  1018 cmÀ3 nitrogen doping) film was deposited by atomic layer deposition (ALD) with (0001) orientation. Although there may be at 350 C with ozone (O3) and a commercial precur- subtle differences in silicide formation between sor (AP-LTO-330) on the epi-layer samples. This substrate pieces and epitaxy pieces (through differ- ALD-SiO2 film has a very low density of pinholes ences in doping and mechanical properties), we and provides excellent isolation to prevent assume that the substrate pieces are a good model unwanted Co/4H-SiC reactions. The SiO2 film also for silicide formation on epitaxy. provides some surface passivation. In preparation Investigation of a Self-Aligned Cobalt Silicide Process for Ohmic Contacts to Silicon Carbide 2511 for a future dHF-dip, the SiO2 was hardened by annealing it in a RTP tool (a Mattson 100 RTP system) for 30 s at 1000 CinN2 ambient.
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