Oxides formation on hydrophilic bonding interface in plasma-assisted InP/Al₂O₃/SOI direct wafer bonding

In recent years, significant breakthrough has been made in the field of silicon photonics, and various Si-based integrated photonic devices have been developed, making Si-based optical systems promising for future information processing and communications, especially in data center and super-computing.^1–4 However, silicon is not an efficient light emitting material due to its indirect band gap. As a result, bonding III-V compound semiconductors to silicon-on-insulator (SOI) wafers has become a key technology to realize hybrid integrated light sources in silicon photonics.^1,5–7 Hybrid integration can be realized either by wafer bonding through of a thin layer of adhesive organic medium such as benzocyclobutene (BCB),^8,9 or through direct wafer bonding.^9–14 In direct wafer bonding, two wafers of different materials are bonded covalently. Thus it not only allows improved optical coupling due to the proximity between the III-V active material and the silicon waveguide, but also facilitates heat dissipation through the bonding interface. Recently, successful direct wafer bonding using Al₂O₃ as the intermediate layer has been demonstrated.¹⁵ In this Letter, plasma-assisted InP/Al₂O₃/SOI direct wafer bonding is carried out, and excellent bonding result is confirmed by transmission electron microscopy (TEM). To clarify the bonding mechanism, contact angle (CA) measurement is carried out to investigate the influence of surface treatment on the hydrophilicity of the wafers. Furthermore, X-ray photoelectron spectroscopy (XPS) is employed to reveal the changes on the InP and Al₂O₃/SOI interface. These results provide a better insight to the physical/chemical model of InP/Al₂O₃/SOI direct wafer bonding.

In our experiment, 345-μm-thick boron-doped n-InP wafer and 545-μm-thick SOI wafer with 670 nm n-type Si top layer are adopted for wafer bonding. Firstly, vertical outgassing channels are etched in the top silicon layer of the SOI wafer, so that gaseous by-products such as water vapor can be absorbed during bonding, and no bubbles or voids will form at the bonding interface.^16–19 A layer of 20-nm-thick Al₂O₃ is then deposited on the SOI wafer by atomic layer deposition (ALD) at 300°C. On the other hand, the InP wafer is soaked in 30% ammonia solution for 3 minutes after standard organic cleaning. Oxygen plasma surface activation for both InP and Al₂O₃/SOI wafers are carried out in a reactive ion etching (RIE) chamber with RF power of 85 W, O₂ gas flow of 28 sccm, and a chamber pressure of 40 mTorr for 28 seconds. Spontaneous bonding between InP and SOI occurs when the two wafers are pressed against each other. To complete the bonding process, the sample is further annealed at 280°C for 8 hours with an applied pressure of 1.5 MPa.

In our experiment, InP wafer with a size of 1cm×1cm is bonded to 1cm×1cm SOI wafer following the above-mentioned procedures. High bonding strength is achieved for the bonded InP/Al₂O₃/SOI sample, as confirmed by crack opening test. Figure 1 is the TEM image of the bonded sample. It is evident that InP and SOI wafers are boned tightly through a 20-nm-thick Al₂O₃ intermediate layer. As can be seen, the Al₂O₃ layer is uniformly bonded with no sign of a micro-void which confirms that a seamless bond at the microscale has been successfully achieved.

Plasma-assisted wafer bonding is considered to be a hydrophilic bonding.^20,21 Water contact angle test is adopted to investigate the hydrophilicity of the wafer surfaces after pre-bonding treatment. According to Table I, the contact angle of InP wafer decreases from 54.5° to 16.2° after plasma activation, whereas that of SOI wafer drops from 32.5° to 10.8°. The decrease in contact angle implies improved hydrophilicity of the wafer surface. This can be attributed to the increased density of -OH containing groups, which are known to interact and polymerize even at room temperature, provided that they are in close proximity.^22,23 Thus, the contact angle measurement confirms the plausibility of hydrophilic bonding.

In order to understand the bonding mechanism, XPS is employed to characterize the wafer surfaces before and after the bonding process. To facilitate the analysis of bonded wafer, special samples are prepared by using SOI wafer without outgassing channels. For such samples, the InP wafer can be de-bonded from the SOI wafer with ease, as a result of the bubbles formed during bonding process. Samples without vertical outgassing channels are submitted to the same pre-bonding treatment and bonding process as the normal ones, so that information concerning the bonding interface could be collected from the de-bonded wafer pairs.

Figure 2 depicts the XPS data of the InP wafer, and it is found that chemical substances on the InP surface include InP, In₂O₃ and InPO₄. Comparing Figs. 2(a)–2(b) with Figs. 2(c)–2(d), it is evident that the percentage of oxidized indium on the InP surface increases after oxygen plasma activation. For direct wafer bonding, oxides on the wafer surfaces can absorb bonding by-products (H₂O etc.) so as to reduce formation of voids at the bonding interface.²⁴ The increase in hydrophilic oxides is corroborated by the reduced contact angle after plasma activation, as shown in Table I. Figures 2(c) and 2(d) show that oxygen plasma activation leads to formation of InPO₄ and In₂O₃. The hydrophilicity of InPO₄ and In₂O₃ on the InP surface enables them to absorb water molecules, so that they can be bonded with the hydrophilic Al₂O₃/SOI wafer. Meanwhile, it is known that oxygen plasma surface activation results in opened dangling bonds of molecules on the surface of InP, thus enabling them to be covalently attracted to the molecules on the surface of Al₂O₃/SOI wafer.²⁵ Figures 2(e) and 2(f) are the XPS data of the de-bonded InP wafer. Comparing Fig. 2(d) with Fig. 2(f), it is noticed that the relative percentage of InP has drastically dropped, whereas the percentages of oxides have increased. On the other hand, it deserves special attention that the relative percentage of phosphate radical in oxides is significantly increased after bonding, as revealed by Figs. 2(c) and 2(e). The increase in oxides implies that a water thermal oxidation process has occurred at the bonding interface. In fact, water trapped at the interface between the hydrophilic InP and Al₂O₃/SOI surfaces may form oxides during the bonding process. Figure 2(e) indicates that indium element exists in three forms, namely, In-P, In-O and In-PO₄, whereas Fig. 2(f) reveals that aside from InP and InPO₄, new substances with binding energies of 133.3 eV and 130.3 eV have emerged in the P 2p XPS spectrum. To identify such substances formed during the bonding process, we need the XPS data taken on the surface of the de-bonded Al₂O₃/SOI sample.

Figure 3 shows the XPS data of the de-bonded Al₂O₃/SOI wafer. Figure 3(a) depicts the Al 2p XPS spectrum, indicating that in addition to Al₂O₃, there is Al element on the Al₂O₃/SOI surface in the form of AlPO₄. This means that Al element on Al₂O₃/SOI and P element on InP have combined to form covalent bonds. Therefore, it can be concluded that the substance with a binding energy of 133.3 eV in the P 2p XPS spectrum shown in Fig. 2(f) is AlPO₄. The core level O 1s XPS data in Fig. 3(b) shows that O element not only exists in the form of Al₂O₃, but also as In₂O₃ and AlPO₄. This is an evidence that oxygen on the Al₂O₃/SOI surface has combined with In and P on the surface of InP. All the above data corroborate that SOI and InP wafers do bond with each other through the thin layer of Al₂O₃. It can also be inferred that complex chemicals exist on the bonding interface, such as In₂O₃, InPO₄ and AlPO₄, which help bond InP and Al₂O₃/SOI tightly to form bonding system with high bonding strength and thin bonding interface. As for the substance with a binding energy 130.3 eV, it has not been identified in any literature so far. A possible candidate might be P-O-In-Al, but further proof is required.

Meanwhile, indium element is detected on Al₂O₃/SOI surface. According to the core level In 3d XPS data in Fig. 4, indium element on the surface of Al₂O₃/SOI only exists in the form of In₂O₃ and InPO₄⋅nH₂O, which proves that indium oxide, together with Al₂O₃/SOI wafer, actually comes off the InP wafer during the de-bonding process. It also confirms the high bonding strength between InP and Al₂O₃/SOI wafers.

In summary, the interface of the bonded InP/Al₂O₃/SOI sample is studied in detail. Tight bonding between InP and SOI is obtained with Al₂O₃ as the intermediate layer, as confirmed by the TEM measurement. XPS data taken with wafers before and after bonding process reveals the chemical substances involved in the bonding process, and provide a better insight to the bonding mechanism. These studies can be of great aid to the study of building physical/chemical model of InP/Al₂O₃/SOI direct wafer bonding.

This work was supported in part by the National Basic Research Program of China (2014CB340002), the National Natural Science Foundation of China (61210014, 61621064, 61574082 and 51561165012), the High Technology Research and Development Program of China (2015AA017101), Tsinghua University Initiative Scientific Research Program (20131089364, 20161080068, 20161080062), and the Open Fund of State Key Laboratory on Integrated Optoelectronics (IOSKL2014KF09).