Heterogeneous integration of thick GaN and polycrystalline diamond

 

Heterogeneous integration of thick GaN and polycrystalline diamond at room temperature through dynamic plasma polishing and surface-activated bonding

Direct heterogeneous integration of GaN on diamond enhances device performance and reliability for high-power applications, while the implementation on thick GaN films and polycrystalline diamond (p-diamond) substrates remains challenging. In this study, the direct bonding between thick GaN films (∼370 μm) and p-diamond substrates was achieved. A dynamic plasma polishing (DPP) technique was adopted on the diamond to flatten surficial spikes from maximum 15 nm to 1.2 nm, obtaining a smooth surface with 0.29 nm Ra, achieving a robust GaN/diamond bonding with high bonding rate of ∼92% at room temperature combining the surface-activated bonding (SAB) method. The chemical status, thermal stress, and interfacial microstructures of GaN/diamond heterostructures were analyzed, revealing a residual stress of ∼200 MPa at the GaN/diamond interface, and the asymmetric increase of interfacial stress at rising temperature demonstrates the effectiveness of amorphous interlayer to release the stress.

Graphical Abstract

Gallium Nitride (GaN) high electron mobility transistors (HEMTs) have emerged as a crucial technology for radio frequency (RF) applications in both commercial and military domains. However, with the escalating demands for heightened operating voltage and power density, their performance and long-term reliability have been challenged as considerable heat and high electric field concentrate within the narrow gate area [1]. Heat dissipation, therefore, has become an exigent issue to be addressed for the broader deployment of GaN devices. Current research endeavors focus on the integration of GaN with substrates owning higher thermal conductivity to improve devices’ thermal budget. Diamond substrates thereby have garnered considerable attention for their potential to break through the heat dissipation bottleneck of GaN high-power devices [2], especially with the development of diamond growth techniques that have significantly reduced the cost in the past few years.

The most common strategy to achieve heterogeneous integration is heteroepitaxial growth, either of diamond on GaN [3], [4] or GaN on diamond [5], [6]. However, due to a significant lattice mismatch (∼11.8%) and thermal mismatch, the generating GaN/diamond interface is prone to issues such as shear strain, crack, and bowing, which make it challenging to obtain thick and high-quality GaN-on-diamond integration [7]. Several studies have applied metal interlayers to enhance the bondability of diamond substrate [8], [9], while the strategy inevitably causes increase in thermal boundary resistance (TBR). Additionally, TBR has been reported generally to show an inverse correlation to interface bonding strength, as thermal transport across interfaces can be regarded as phonon energy exchange [10], thus raising the concern of bonding rate and bonding strength. Direct bonding [11], [12], [13], as an alternative approach that allows separate growth of GaN and diamond, followed by transferring and bonding, thereby maintaining the quality of both GaN devices and diamond substrates, has been reported to barely cause damage to the active region of HEMTs [14].

Nevertheless, direct GaN-diamond bonding is currently most successful on thin monocrystalline diamond substrates because the bonding requires surfaces' extremely low roughness and high flatness. Therefore, although direct bonding theoretically can be scaled to wafers of any size, the limited growth rate of monocrystalline diamond and its difficulty in growing as large-size wafers significantly restrict its application. Polycrystalline diamond (p-diamond) shows the advantages of lower cost and the availability of larger wafers, but its high hardness makes it difficult for chemical-mechanical polishing (CMP) to achieve the desired roughness Ra below 0.5 nm for successful direct bonding [15]. Furthermore, current studies focus on the bonding thin GaN film to diamond due to the inverse correlation between bondability and film thickness, as indicated in the formula:�=3���13��232�4Where the energy γ needed to achieve bonding increases proportionally to the square of film thickness t_b[16]. Thick GaN films are preferred for high-quality epitaxial growth and facilitating complex structure fabrication via techniques such as SmartCut [17], [18], [19], thus emphasizing the significance of direct bonding between p-diamonds on thick GaN films.

Previously, surface-activated bonding method (SAB) has been adopted to achieve direct bonding between dissimilar semiconductors such as GaN/SiC [20] and GaN/Si [21], and the possibility of GaN/monocrystalline diamond [11], [12], [22] structure by direct bonding has also been reported. As the SAB achieves heterogeneous structures at room temperature, decreasing the interfacial stress induced by temperature change during and after the process, both bonding rate and bonding strength are well controlled. This led to the conviction that the SAB process works on a p-diamond with a proper surface. In this study, we introduce a dynamic plasma polishing (DPP) strategy to pretreat p-diamond, which was successfully directly bonded to a GaN film using the SAB method.

Section snippets

Material and methods

The p-diamond substrate was grown via the microwave-plasma chemical vapor deposition method with a thickness of ∼660 μm, while the GaN single crystal substrate (bought from Shanghai GaNova Electronic Information Co., Ltd) features a thickness of ∼370 μm with a roughness Ra of 0.3 nm. The relatively thick diamond and GaN film used in this study is to amplify interfacial stress at the bonding interface induced at low temperatures for an effective estimation. Both the as-received GaN and diamond

Results and discussion

The surface morphology of the diamond substrate before and after the DPP treatment was evaluated by atomic force microscopy (AFM) and scanning electronic microscopy (SEM), as shown in Fig. 2. The presence of striped protrusions and depressions on the surface of original p-diamond substrate normally leads in its high surface roughness [8], as presents in Fig. 2(a, b, c). Even after a preliminary chemical-mechanical polishing (CMP), a roughness Ra of 1.01 nm was obtained, which is still much

Conclusions

We have successfully bonded thick p-diamond and GaN film using the surface-activated bonding method, by applying a dynamic plasma polishing process to pretreat the diamond surface. We confirm that the DPP pretreatment significantly reduces the surface roughness of the p-diamond, changes its surficial chemical state, and enables room-temperature bonding. Our evaluation reveals that residual interfacial stress exists in heterogenous structures obtained via the SAB method even at room temperature,