Au nano-layer in atmospheric air

Materials Science in Semiconductor Processing 114 (2020) 105069

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Investigation on the bonding quality of GaN and Si wafers bonded with Mo/ Au nano-layer in atmospheric air Kang Wang , Kun Ruan , Haiyang Bai , Wenbo Hu *, Shengli Wu , Hongxing Wang Key Laboratory for Physical Electronics and Devices of the Ministry of Education, School of Electronic Science and Engineering, Xi’an Jiaotong University, No. 28, Xianning West Road, Xi’an, 710049, China

A R T I C L E I N F O

A B S T R A C T

Keywords: GaN-on-silicon Mo/Au nano-layer Bonding process Ohmic contact

A room-temperature bonding process based on Mo/Au nano-layer was applied to bond GaN on Si wafers in atmospheric air. The analytical test results show that a low bonding defect density (0.2%) and a strong bonding strength (10.0 MPa) were achieved for the GaN/Si sample bonded with Mo/Au nano-layer. The bonding defect density of the bonded GaN/Si sample remained mostly unchanged and each of Au/Au, Au/Mo/GaN and Au/Mo/ Si interfaces had a strong adhesion strength after 1000-cycles thermal cycling testing. In addition, Mo/Au ohmic contact on n-type GaN was realized by Arþ ion beam treatment on the GaN surface.

1. Introduction Gallium nitride (GaN) material has been widely used in high-power and optoelectronic devices due to its advantages of wide band gap, high electrical breakdown field and high saturation electron velocity [1–3]. For GaN-based devices, epitaxial GaN films are mainly grown on sap­ phire, SiC and Si substrates. The most common and convenient substrate for the growth of GaN is sapphire, but which has several disadvantages such as low thermal conductivity (0.5 W/cmK) and large lattice mismatch (16%) [3–6]. The low thermal conductivity of sapphire sub­ strate hinders the effective heat dissipation of GaN-based devices during operation, which results in the deterioration of lifetime and performance of the devices [6,7]. SiC is also frequently used substrate material for high-power GaN-based devices due to its high thermal conductivity (4.0 W/cmK) and low lattice mismatch (~3.5%), but its cost is relatively high [6,8]. Si is the most ideal substrate material for the integration between Sibased and GaN-based devices because of its large size, high quality, low cost and controllable electrical conductivity [4,5,9]. However, due to the large thermal expansion coefficients difference (56%) and lattice mismatch (17%) between GaN and Si [4,10,11], the direct growth of epitaxial GaN on Si leads to high threading dislocation density (1010 cm 2), large diffusion of Si atoms into GaN films and cracks [5,9,11,12], which seriously affects the performance of the devices and results in low yield [4,9,13,14], while the growth method based on buffer layer re­ quires a high temperature (600–1000 � C) and the preparation process is

complicated [5,11,12]. In order to optimize the preparation process and improve the performance of GaN-based devices, wafer bonding tech­ nology has been developed. For the bonding of GaN and Si, GaN epitaxial layers and Si substrates can be fabricated respectively in par­ allel. F. Mu et al. successfully realized the direct bonding of GaN on Si wafers by using surface activated bonding (SAB) [9], but SAB technol­ ogy must be carried out in ultra-high vacuum (UHV) environment [15–19]. E. Higurashi et al. demonstrated the bonding of GaN on Si wafers by using Cr/Au layer at low-temperature (150 � C) [20]. 2-inch GaN films were transferred from original free-standing substrates onto sapphire wafers via SiO2 bonding layers by using Smart Cut™ (TM: Trade Mark of S.O.I.T.E.C. S.A.) technology [21]. Moreover, GaN on sapphire transferred on silicon with hydrophilic metallic bonding as another bonding technique has attracted much attention [22]. In this work, Mo layer was used as an underlayer of Au film because Mo had higher thermal conductivity, lower diffusion coefficient and more stable chemical properties than Ti or Cr [23]. We realized the room temperature bonding of GaN and Si wafers in atmospheric air by using Mo/Au nano-layer and Mo/Au ohmic contact on n-type GaN was ob­ tained after Arþ ion beam treatment on the GaN surface. 2. Experimental ~5-μm-thick n-type GaN layers grown on sapphire substrates (10 mm � 10 mm � 0.5 mm) were bonded on intrinsic Si wafers (10 mm � 11 mm � 0.5 mm) at room temperature. The carrier concentration of n-

* Corresponding author. E-mail address: [email protected] (W. Hu). https://doi.org/10.1016/j.mssp.2020.105069 Received 25 November 2019; Received in revised form 11 March 2020; Accepted 13 March 2020 Available online 29 March 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.

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Materials Science in Semiconductor Processing 114 (2020) 105069

Fig. 1. Schematic illustration of the bonding process.

Mo/Au contacts deposited after Arþ ion beam treatment on the GaN surfaces for different times were measured. The schematic illustration of the bonding process is illustrated in Fig. 1. The experimental conditions are shown in Table 1. For these as-prepared samples, their surface roughnesses, film thicknesses, bonding defect density, bonding strength and chemical compositions were analyzed by atomic force microscopy (AFM, Dimension Icon), scanning electron microscopy (SEM, Zeiss GeminiSEM 500), scanning acoustic microscope (SAM, D9500), electronic universal testing machine (MTS 858 Mini Bionix II) and X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB Xiþ), respectively, and their I–V characteristics were examined by semiconductor character­ ization system (Keithley 4200 scs).

Table 1 Experimental conditions. Processes

Conditions

Wafer treatment by Arþ ion bombardment Film deposition Bonding

RF power: 30 W; Duration:100 s Mo: ~10 nm; Au: ~19 nm 2000 N (load), room temperature

type GaN was >1 � 1018 cm 3. GaN and Si samples were ultrasonically cleaned in turn with acetone, ethanol and deionized water. Before deposition of Mo/Au layer, the surfaces of GaN and Si samples were treated with Arþ ion beam for removing the native oxides. And then, Mo/Au layers were successively deposited on the activated surfaces of GaN and Si samples in a magnetron sputtering system. It should be noted that there is no air break between Arþ activation and the Mo/Au deposition. Finally, both samples were transferred from the magnetron sputtering system to the wafer bonding equipment and bonded imme­ diately in atmospheric air. In addition, I–V characteristic curves of the

3. Results and discussion The typical AFM images of original Si wafer, GaN layer and Mo/Au layers deposited on Si wafer and GaN layer are shown in Fig. 2(a)–(d). Their root-mean-square (RMS) roughnesses were 0.251 nm, 0.502 nm,

Fig. 2. AFM images of (a) an original Si wafer, (b) an Mo/Au layer deposited on Si wafer, (c) an original GaN layer, (d) an Mo/Au layer deposited on GaN layer.

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Fig. 3. (a) Cross-section SEM images of the bonded GaN/Si sample and (b) its EDS mappings: Si map in purple, Ga map in cyan, N map in yellow, Al map in red and O map in green.

0.273 nm and 0.513 nm for the scanned area 2 � 2 μm2, respectively. Because deposited Mo/Au layers were composed of nano-grains [23], the surface roughnesses of Mo/Au layers deposited on Si wafer and GaN layer were slightly higher than that of original Si wafer and GaN layer, but they (<1 nm) still fulfilled the requirements of bonding for surface roughness [24]. It is well known that bonding techniques required a smooth surface of atomic level, while the deposited Mo/Au film remained an atomically smooth surface. Metallic bonds can be formed between the surfaces of metal layers even at room temperature through atomic diffusion and grain growth across bonding interface [15,16,19, 23]. Since Au has a good ductility, a high self-diffusion coefficient (3.3 � 10 29 m2/s at 300 K) and oxide layer cannot be formed on the surface of Au film [16,23], Au films as bonding layers can be bonded together in atmospheric air and even with a so rough surface: 0.5 nm. Fig. 3(a) and (b) show the cross-section SEM image and energy dispersive spectroscopy (EDS) mappings of the bonded GaN/Si sample. Si, Ga, N, Al and O elements are highlighted in purple, cyan, yellow, red and green, respectively. It can be obviously seen that the Si/GaN/Al2O3 multilayer structure is clearly visible and the Mo/Au/Mo bonding layer (thickness: ~58 nm) is continuous and uniform without defects at the interface. However, Mo and Au elements can not be observed, which may be due to the extremely thin Mo/Au layer. To evaluate the bonding quality of the bonded GaN/Si sample, SAM was used to measure the bonding defect density in detail, as shown in Fig. 4(a). In the SAM observation, a low bonding defect density of about 0.2% was located on the near edge of the sample, which was caused by tiny particles adsorbed on the GaN or Si surface prior to bonding. Therefore, despite the existence of unbonded area, most of areas of the sample were bonded together and the proportion of bonded area reached 99.8%. The tensile test was performed to confirm the bonding strength. The sample was fixed to the attachments of electronic uni­ versal testing machine by using adhesive glue before tensile test. When the pulling force increased to 1000 N (10.0 MPa) during tensile test, the bonded Si suddenly broke into two small pieces, one of piece existed independently, and the other broken Si piece was still bonded to GaN layer and the bonded GaN/sapphire was intact, as shown in Fig. 4(b) and (c). The fracture occurred not only at the bonding interface but mainly at the bulk Si, this is due to the formation of a strong bonding strength between Au/Au interface [15,16,23]. Furthermore, the chemical com­ positions in the region A (GaN side) shown in Fig. 4(c) were analyzed by

XPS. In the XPS spectra shown in Fig. 4(d)–(f), the existences of Mo and Au in this region indicate that the fracture mainly occurred at the Mo/Si and Au/Au interfaces. And the existence of Si on the fractured region A of the GaN side provided a new evidence for the formation of a three-dimensional fracture inside the Si wafer. At present, self-heating effect of GaN-based devices is still the main factor restricting device reliability [1,25,26]. Hence the thermal-impact-resistance performance of the bonded GaN/Si sample was evaluated by thermal cycling testing. Fig. 5 shows the schematic diagram of thermal cycling testing. The thermal cycling testing was performed using another bonded GaN/Si sample. This sample was chosen for thermal cycling testing because three unbonded areas were distributed to the edge (unbonded area I), center (unbonded area II) and near edge (unbonded area III) of the sample, so that the effects of thermal cycling testing on the unbon­ ded areas could be studied more intuitively. Although there were three unbonded areas (I, II, III) caused by particles and the bonding defect density was about 2.5%, the sample was substantially bonded together, as shown in Fig. 6(a). After 1000-cycles thermal cycling testing, the proportion of unbonded areas remained mostly unchanged, as shown in Fig. 6(b). For unbonded area I located on the edge of the sample, because the bonding defect seems connected with the sample edge, so a certain amount of oxygen molecules and water vapor could be adsorbed in the area during thermal cycling testing. After 1000-cycles thermal cycling testing, even if some diffusion of water or oxygen could occur at the bonding interface, no evolution of this interface could be seen due to high bonding energy between Au/Au layers. The thermal cycling testing result of bonded GaN/Si sample is better than that of bonded GaN/ diamond sample we previously studied [27]. The tensile strength of the sample after 1000-cycles thermal cycling testing was tested. After tensile test, the tensile strength of the sample was 4.1 MPa. Although the tensile strength was lower than that before 1000-cycles thermal cycling testing, the fracture of the sample only occurred in a small part of the bonding area. It can be obviously seen from Fig. 7(a) that a small piece of Si broke from the bonded GaN/Si sample and the sample as a whole was intact, and the proportion of fractured area was approximately 13%. The chemical compositions in the region A shown in Fig. 7(a) were analyzed by XPS. The characteristic peaks Mo 3d, Au 4f and Si 2p were observed in region A, as shown in Fig. 7(b)–(d). The weak Mo, Si and strong Au characteristic peaks

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Fig. 4. (a) SAM image of the bonded GaN/Si sample, surface profiles of (b) Si side and (c) GaN side of the fractured sample after tensile test, XPS spectra of (c) Mo 3d, (d) Au 4f and (f) Si 2p in the region A on the surface of the GaN side of the fractured sample.

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rectifying behavior. When the surface of the GaN was bombarded with an Arþ ion beam for 15 s, however, the Mo/Au contact changed from rectifying to ohmic contact and the contact resistance increased with Arþ ion beam treatment time. The transition from rectifying to ohmic contact after Arþ ion beam treatment is considered to be a highly doped interfacial layer formed by nitrogen vacancies caused by Arþ ion beam treatment [20]. The micro-mechanism of the transition from rectifying to ohmic contact was explained by using band bending. Fig. 9(a) shows the band diagram prior to contact of Mo and n-type GaN, where Wm (4.6 eV) and Ws (4.2 eV) are the work functions of Mo and GaN, respectively, χ (4.1 eV) is the electron affinity of GaN and Eg (3.4 eV) is the band gap of GaN at room temperature [30]. Before contact, the bands of Mo and GaN are not bend. After close contact, because the work function of Mo is higher than that of GaN ((EF)s>(EF)m), resulting in band upward bending. So free electrons in metal Mo encountered a Schottky barrier (qΦns ¼ 0.5 eV) at the Mo/GaN interface and a space charge region with a width of d was formed on the GaN side, as shown in Fig. 9(b). Since the Schottky barrier was weak, the Mo/Au contact was a slightly rectifying behavior for the GaN without Arþ ion beam treatment. The result is also consis­ tent with the results in the literature [31]. Because Arþ ion beam treatment on n-type GaN surface can lead to an increase in carrier concentration [32], which results in the narrowing of the barrier. Meanwhile, the degradation in the Schottky diode characteristics is thought to be caused by the formation of high density of donor-like traps on the n-GaN surface treated with Arþ ion beam [30,32]. The increase in surface state density is said to result in a reduction of the barrier [30,32]. Therefore, the carriers can pass through the barrier in a tunnel manner to form an ohmic contact, as shown in Fig. 9(c).

Fig. 5. Schematic diagram of thermal cycling testing.

indicate that the fracture in region A mainly occurred at the Au/Au interface rather than at the Mo/Au and Mo/Si interfaces. Therefore, Mo/ Au layer can be used as a buffer layer material to release the internal stress formed by the large thermal expansion coefficients difference (56%) [10] between GaN and Si. Since GaN is a very important semiconductor material, its electrical properties have been widely studied. E. Higurashi et al. researched the ohmic properties of GaN/Si wafers before and after bonding using Cr/Au contact layer [20]. And many studies confirmed an increase in surface nitrogen vacancies after plasma treatment on the surface of n-type GaN [20,28,29]. Therefore the I–V characteristic curves of the Mo/Au con­ tacts deposited after Arþ ion beam treatment on the GaN surfaces for different times were investigated, as shown in Fig. 8. For the GaN without Arþ ion beam treatment, the Mo/Au contact was a slightly

4. Conclusions A room-temperature bonding process based on Mo/Au nano-layer was successfully applied to bond GaN on Si wafers in atmospheric air.

Fig. 6. SAM images of the sample (a) before and (b) after 1000-cycles thermal cycling testing.

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Fig. 7. (a) Surface profiles (a whole part and a small piece of Si of the fractured sample) and XPS spectra of (b) Mo 3d, (c) Au 4f and (d) Si 2p in the region A of the fractured sample after 1000-cycles thermal cycling testing.

The results demonstrated that Mo/Au nano-layer can not only be used as a suitable adhesive layer material to bond GaN and Si, but also have a good thermal-impact-resistance performance. In addition, Mo/Au con­ tact deposited on n-type GaN changed from rectifying to ohmic contact after the GaN surface was bombarded with Arþ ion beam for 15 s. Therefore, Mo/Au nano-layer is expected to be useful for heterogeneous integration of high-power GaN-based devices. Declaration of competing interest The authors declare no conflict of interest in this manuscript. CRediT authorship contribution statement Kang Wang: Methodology, Writing - original draft. Kun Ruan: Investigation. Haiyang Bai: Investigation. Wenbo Hu: Writing - review & editing. Shengli Wu: Writing - review & editing. Hongxing Wang: Writing - review & editing.

Fig. 8. Effects of the Arþ ion beam treatment time on the I–V characteristics of as-deposited Mo/Au (40/140 nm) contacts (3.52 mm � 9.80 mm) on n-type GaN at a gap spacing of 2.88 mm.

Fig. 9. Band diagram of contact between metal Mo and n-type GaN.

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Acknowledgments [14]

This work was supported by Dongguan Introduction Program of Leading Innovative and Entrepreneurial Talents and also supported by National Key Research and Development Program (Grant No. 2017YFB0402802). We thank Mr. Ren and Miss. Liu at Instrument Analysis Center of Xi’an Jiaotong University for their assistance with SEM and XPS analysis.

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Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.105069.

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