AlN-AlN Wafer Bonding and Its Thermal Characteristics

Introduction

Wafer bonding allows heterogeneous integration of two materials with similar or very different lattice parameters and thermal properties, via an intermediate bonding layer or direct contact. One of its most prominent applications is silicon-on-insulator (SOI) [1], and it can also be extended to germanium-on-insulator (GOI) or other III-V materials integration. SOI fabricated from wafer bonding is gaining ground from the conventional Si substrate in ultra-large scale integrations (ULSI) and micro-electro-mechanical systems (MEMS) as the starting substrate [2], which benefits from its on-insulator structure. The “on-insulator” advantages make it possible to attain mechanical stability close to Si substrate and excellent electrostatic control. However, the advantages of SOI are degraded due to the presence of a conventional SiO₂ insulator layer with thermal conductivity of ~1.4 Wm^-1K^-1. The squeezed heat dissipation path deteriorates its heat transfer efficiency to the underlying bulk Si layer, leading to severe self-heating effect. The self-heating effect is magnified by device scaling, which limits the applicability of SOI in electronics especially in the cases where high temperature and power dissipation are expected. In order to address this problem, Al₂O₃-Al₂O₃ bonding had been demonstrated and appeared to have favorable thermal characteristics [3]. In this paper, a more promising intermediate bonding layer is introduced. It shows the possibility of fabricating bonded wafer with enhanced thermal conductivity through room temperature AlN-AlN fusion bonding. Its strong heat dissipation capability is verified by COMSOL simulation and experimental results obtained from resistance thermal detector (RTD).

Experiment, Results and Discussion

Si wafers used for bonding were pre-cleaned by the standard RCA cleaning method. Then, a 10 nm AlN thin film was sputtered onto the pre-cleaned Si surface. While for comparative study, a 10 nm of SiO₂ and Al₂O₃ was deposited via PECVD and ALD system, respectively. After deposition, a surface activation step was provided under N₂ plasma to enhance the surface hydrophilicity. The activated wafers were cleaned by de-ionized (DI) water rinse and dried with spin dryer. AlN-AlN bonding was initiated at room temperature, and the bond was strengthened by annealing at 300 °C in N₂ ambient for a duration of 3 hrs. Next, Si on one side of the bonded structure was removed by grinding and wet etching. Au line Kelvin structures for thermal characteristics verification were patterned and formed.

    The IR imaging results have shown that AlN thin film exhibits very good bondability when two of them are bonded face-to-face. The good bondability can be attributed to the low surface roughness, which was as low as 0.16 nm in a 5 μm × 5μm AFM scan area. Only very few unbonded areas were observed due to the presence of particles, which were trapped during wafer handling and can be eliminated with careful control of contamination sources. After annealing, no obvious bubble enlargement was found, which indicates that out-gassing is not significant. This led to a respectable AlN-AlN bonded interface with bond strength of ~1359.9 mJ/m² which is sufficient for subsequent process steps.

    According to the COMSOL simulation results, AlN has shown great advantage as the insulator layer in term of heat dissipation capability as compared with conventional insulator materials SiO₂ and the recently widely studied Al₂O₃. Their heat dissipation capability was investigated and compared by studying the temperature profile across bonded structure when the material stack was subjected to a heat flux of 1 × 10⁸ W/m². The simulated surface temperature of 20 nm (Si, SiO₂, Al₂O₃, AlN) on Si structures are 32.403, 33.818, 32.446, and 32.402 °C respectively. 20 nm AlN on Si appeared to have the similar temperature profile as direct Si-Si bonded wafer, because they have close thermal conductivity values (AlN: 134 Wm^-1K^-1 [4] and Si: 131 Wm^-1K^-1). Thus, it can be shown that the presence of AlN thin film does not degrade the thermal property or hinder heat from spreading to the underlying Si substrate.

Conclusion

In conclusion, a successful void-free AlN-AlN wafer bonding was demonstrated. The bonded AlN-AlN layer showed superior heat dissipation capability, which is identical to that of Si-Si direct bonding.

Reference

[1]  U. Gösele et al., Annu. Rev. Mater. Sci., 28, p. 215-241, 1998.

[2]  R. L. Puurunen et al., in TRANSDUCERS 2011,pp. 978-981. 2011.

[3]  J. Fan et al., ECS J. Solid State Sci. and Technol.,2, p. 169-174, 2013.

[4]  M.-H. Park et al., Mater. Sci. Semicond. Process., 15, p. 6-10, 2012.