Preprint – Manuscript submitted to Materials Science in Semiconductor Processing (November 20, 2018) Morphological and electrical properties of Nickel based Ohmic contacts formed by laser annealing process on n-type 4H-SiC S. Rascunà 1*, P. Badalà 1, C. Tringali 1, C. Bongiorno 2, E. Smecca 2, A. Alberti 2, S. Di Franco 2, F. Giannazzo 2, G. Greco 2, F. Roccaforte 2, M. Saggio 1 1 STMicroelectronics SRL, Stradale Primosole 50, 95121 Catania, Italy 2 Consiglio Nazionale delle Ricerche – Istituto per la Microelettronica e Microsistemi (CNR-IMM), Strada VIII, n.5 Zona Industriale, I-95121 Catania, Italy (*) Corresponding author: [email protected], For the n-type SiC, annealed Ni-films are Abstract. This work reports on the commonly used to form nickel silicide (Ni 2Si) morphological and electrical properties of Ni- back-side Ohmic contacts. Typically, rapid based back-side Ohmic contacts formed by thermal annealing (RTA) exceeding 900°C laser annealing process for SiC power diodes. are used to achieve an Ohmic behavior [3]. Nickel films, 100 nm thick, have been However, today there is the need to replace sputtered on the back-side of heavily doped the conventional thermal annealing by laser 110 µm 4H-SiC thinned substrates after annealing processes carried out on the back- mechanical grinding. Then, to achieve Ohmic side of thinned wafers at the end of the behavior, the metal films have been irradiated fabrication flow [4]. with an UV excimer laser with a wavelength In Fig. 1, a schematic of a standard flow of 310 nm, an energy density of 4.7 J/cm 2 and chart (with and without grinding step) for the pulse duration of 160 ns. The morphological fabrication of Junction Barrier Schottky (JBS) and structural properties of the samples were diode is shown in comparison with a new analyzed by means of different techniques. laser annealing process flow. In particular, in Nanoscale electrical analyses by conductive the standard manufacturing process flow (Fig. Atomic Force Microscopy (C-AFM) allowed 1a,b), a high temperature RTA process (900 - correlating the morphology of the annealed 1000 °C) is typically required for the metal films with their local electrical formation of the Ni 2Si back-side Ohmic properties. Ohmic behavior of the contacts contact [5]. This process must be carried out fabricated by laser annealing have been before defining the front side metal to prevent investigated and compared with the standard undesired interface reactions and electrical Rapid Thermal Annealing (RTA) process. degradation of the Schottky barrier. Hence, Finally, it was integrated in the fabrication of this standard silicidation process represents 650V SiC Schottky diodes. today a technological bottleneck, due to the complexity of the process flow integration that requires multiple flip over of the wafers Introduction and reliability of back-side layer due to the Silicon carbide (4H-SiC) is one of the key exposition to many processes remaining in the materials to fabricate high-power and low flow. ON-resistance (R ON ) devices in the next In this context, the introduction of wafer generation of power electronics systems [1, grinding, to reduce the resistive contribution 2]. One of the most important issues in SiC of the substrate (Fig.1b), has to deal with technology is the fabrication of reliable low- consequent limitation in step processes and resistance back-side Ohmic contacts to SiC the increasing risk of wafer breakage. devices. Preprint – Manuscript submitted to Materials Science in Semiconductor Processing (November 20, 2018) Fig. 1. Schematic flow charts for the fabrication of a JBS diode: standard wafers with back-side ohmic contact by RTA process (a), thinned wafers with back-side ohmic contact by RTA process (b), thinned wafers with back-side ohmic contact by laser annealing process (c). formation enables the possibility to complete However, such process can bring great the device front side first, and then to process advantages in terms of device power the back-side contact without detrimental dissipation. In fact, as can be seen in the effects for the Schottky barrier and without calculation shown in Fig. 2a, in a 650 V SiC limitation on the thinning of the wafers. The Schottky diode fabricated onto a 350 µm thick possibility to obtain Ohmic contact formation substrate, about 70 % of the total R ON is by the use of laser annealing has been already represented by the SiC substrate contribution, demonstrated in Silicon [7, 8, 9]. In the recent Rsub [6]. On the other hand, thinning the years, several works reported on the formation substrate to 110 µm allows reducing this of Ohmic contacts to SiC thinned substrate resistive contribution down to 44 % of the using laser annealing (LA) processes, suitable total R ON (Fig. 2b) [6]. This latter explains both for diode and MOSFET technology [4, why for medium voltage applications (600- 10, 11, 12, 13, 14, 15]. 1200 V), the wafer grinding step has become In this paper a morphological, structural mandatory in SiC technology to reduce the and electrical characterization of Ni-based substrate thickness and, hence, to minimize back-side Ohmic contacts formed by laser the total device R ON . annealing on 4H-SiC has been reported. In From this point of view, laser annealing particular, the electrical behavior showed the (Fig. 1c) represents an alternative valid possibility to obtain an Ohmic behavior under solution for achieving the silicidation with a certain process condition. In these cases, limited heat transfer. Moreover, the use of formation of nickel silicide has been laser annealing in back Ohmic contact confirmed by Transmission Electron Preprint – Manuscript submitted to Materials Science in Semiconductor Processing (November 20, 2018) Microscopy (TEM) analysis and X-ray acquired using a semiconductor device diffraction (XRD) analysis. Moreover, a parameter analyzer (Agilent B1500A). The combination of nanoscale morphological and back-side morphologies and microstructures electrical analysis allowed correlating the have been investigated by Scanning Electron occurred Ni-SiC reaction with the achieved Microscopy (SEM – Fei 865 dual beam) and Ohmic contact behavior. Transmission Electron Microscopy (TEM – Jeol JEM 2010F), while X-ray diffraction patterns were acquired using a D8 Discover (Bruker AXS) diffractometer equipped with a Cu source and thin films attachment to obtain information about the crystalline structure of materials. Nanoscale electrical analyses by conductive Atomic Force Microscopy (C- AFM – DI3100 with Nanoscope V controller) using Pt coated Si tips allowed to correlate the morphology of the annealed metal films with their local electrical properties. Finally, the laser annealing process has been integrated in the fabrication of a 4H-SiC power JBS diode, and the I-V characteristics of this diode were collected using a high power curve tracer (Sony Tektronix 371A). Results and discussion Fig. 2. Resistive contributions (in terms of Electrical measurements on dedicated test contact resistance, R c, epitaxial-layer pattern have been performed on the heavily resistance, R drift , and substrate resistance, doped substrate surface (0.02 Ωcm). As can Rsub ) of the total R ON in a 650 V 4H-SiC be observed in Fig. 3, linear I-V Schottky diode for two different substrate characteristics measured on adjacent pads thickness of 350 µm (a) and 110 µm (b). For indicated the achievement of the Ohmic this calculation, substrate resistivity of 0.02 behavior. On the other side, Ni silicide, Ωcm, specific contact resistance of backside formed by standard RTA annealing (1000 °C), contact of 5 ×10 -5 Ωcm 2, epilayer doping of exhibits a higher current, indicating a better 1×10 16 cm -3 have been used. contact resistance. It must be pointed out that the test patterns used for the electrical measurements have been fabricated on the Experimental back-side of thick (350 µm) heavily doped Nickel films, 100 nm thick, have been substrates. Hence, the lack of a vertical sputtered on the back-side of heavily doped isolation of the TLM patterns, together with 4H-SiC substrates, whose thickness had been the low sheet resistance of the substrate, did reduced down to 110 µm by mechanical not allow to quantitatively determining the grinding. Then, the metal films have been value of the specific contact resistance. irradiated with an ultraviolet (UV) excimer laser with a wavelength of 310 nm, an energy density of 4.7 J/cm 2 and pulse duration of 160 ns. Patterned samples, as Transmission Line Model (TLM) structure, have been defined to evaluate the electrical properties of the contacts. The current voltage (I-V) measurements on the TLM test patterns were Preprint – Manuscript submitted to Materials Science in Semiconductor Processing (November 20, 2018) Fig. 4. XRD pattern of the back-side layer after laser treatment (black line) compared Fig. 3. I-V curves acquired on adjacent TLM with a reference sample (RTA): (green line) patterns of Ni-based Ohmic contacts formed grazing incidence profile; (red line) 2theta- by standard RTA (1000°C) and Laser omega profile. The labelled peak positions in annealing processes (at an energy density of blue identify the orthorhombic Ni 2Si phase. 4.7J/cm 2). A deep structural analysis of the LA treated Ni films have been performed. In particular, X-ray diffraction analyses (Fig. 4) revealed 1617 the formation of the Ni 2Si phase [ 16, 17] with main peaks as those detected in the reference sample (RTA). The comparison of acquisitions by symmetric diffraction (2theta- omega) and by grazing incidence highlights a wide distribution of the growth axes in the Ni 2Si polycrystals array. As a further insight, the large full width at half maximum of the Fig. 5. SEM in plan view of the back-side peaks compared to the reference case, that layer after 100 nm Ni sputtering and 310 nm causes the peak to overlap, marks the laser treatment at an energy density of 4.7 presence of a fine-grained material as reaction J/cm 2.