Surface-activated ALD for wafer bonding beyond silicon

Recently, a promising technique called surface-activated bonding (SAB) has been developed for bonding Al₂O₃ thin films deposited using atomic layer deposition (ALD) at room temperature, without the need for heating. It was reported by the group of Ryo Takigawa from Kyushu University, Japan. It was published in Nature Scientific Reports, March 3, 2023.

When two flat and smooth surfaces are brought together, they can stick spontaneously. This process is often found when two silicon wafers are brought together, and called wafer bonding or direct bonding.

This video shows a typical wafer bonding process. When a glass substrate is placed on a silicon wafer, they can bond spontaneously. The bond front progresses as interface energy is reduced and air is pushed out. However, for hermetic sealing, this initial process is followed by a high temperature fusing step. In the case of the ALD bonding discussed here, the fusing step is not needed!

Typical wafer bonding of a glass wafer onto a silicon wafer

The ALD bonding technique is particularly useful in the semiconductor industry, where heating can cause thermal stress and degrade bond alignment accuracy. 

surface-activated bonding (SAB) involves using an argon fast atom beam (FAB) to remove organic contaminants and the native oxide layer from the wafer surfaces. The activated wafer surfaces are then brought into contact at room temperature, creating a strong bond. 

While researchers have studied the SAB of single-crystal Al₂O₃ and Al₂O₃ deposited by mist CVD and hydrophilic bonding, the SAB of ALD Al₂O₃ thin films on Si thermal oxide wafers has shown the most promise due to ALD's precise growth, atomic-scale thickness control, and good uniformity on large-scale wafers. 

The bonded wafers were diced into dimensions of 0.5 mm x 0.5 mm, and the surface energy was estimated to be approximately 1.5 J/m², indicative of strong bonds.

Image of the diced 0.5 mm × 0.5 mm chips, showing dicing without debonding. From Takakura, R., Murakami, S., Watanabe, K. et al. Room-temperature bonding of Al₂O₃ thin films deposited using atomic layer deposition. Sci Rep 13, 3581 (2023). link

SOI (Silicon-On-Insulator) wafers

SOI (Silicon-On-Insulator) wafers are widely used in the semiconductor industry for making electronic devices like microprocessors, memory chips, power devices, and high-frequency devices like amplifiers and filters. 

These wafers are particularly useful because they are better at reducing unwanted effects in electronic devices, like capacitance, which slows them down, and leakage current, which wastes power. They are ideal for making devices that need to be fast and energy-efficient, such as computers, phones, and other electronic gadgets we use daily. 

Companies that produce SOI wafers include Soitec, Shin-Etsu Handotai, GlobalWafers, SUMCO, and WaferPro.

Other applications of wafer bonding can be found in stacking and packaging of computer chips. An interesting video about how stacking is essential to bringing memory and processors closer together can be found here. 

Perspectives

The room-temperature bonding of insulating materials is a significant breakthrough in the semiconductor industry, paving the way for future room-temperature heterogeneous integration and wafer-level packaging. This development can significantly impact the optoelectronics, MEMS, RF, and CMOS device fields because this new room-temperature bonding technique allows bonding and hermetic sealing of materials that don’t grow native oxides (like SiO₂ on a silicon wafer), such as GaAs, GaN, and other types of substrates.

You can check more about the research in their original paper.

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The ALD Bond video:

References:

Takakura, R., Murakami, S., Watanabe, K. et al. Room-temperature bonding of Al2O3 thin films deposited using atomic layer deposition. Sci Rep 13, 3581 (2023). link