Direct Wafer Bonding of GaN for Power Devices Applications
Authors: Dragoi Viorel , Razek Nasser , Guiot Eric , Caulmilone Raphael , Liao Michael , Wang Yekan Steven , Goorsky Mark S. , Yates Luke , Graham Samuel 
Published: February 2020

Gallium nitride is a semiconductor material with a high potential for diverse applications as optoelectronic devices (high brightness LEDs, lasers, displays), wireless communication (microwave amplifiers, RF filters), high power and high temperature electronics, power devices or space applications requiring radiation-hard components working over wide temperature range.

The economical advantages of GaN-based applications imposed the development of new manufacturing technologies. Wafer bonding was already adopted by semiconductor industry for consumer electronics, automotive or space applications so existing process solutions could be further developed for the new applications.

One of the most significant benefits of using wafer bonding for GaN-based applications is the possibility to combine GaN with various substrate materials offering a high flexibility by imposing strict quality requirements only to the surfaces to be joined together by bonding and not to their crystal structure.

The diversity of the new devices imposes different design criteria for the wafer bonding process. In order to provide a good platform for manufacturing the European research project AGATE aimed to create a substrate manufacturing pilot line to provide high quality substrates to device manufacturers. As main target, high quality substrates were produced by transferring GaN layers to different engineered substrates by a wafer bonding and layer transfer process.

The process flow was:

- The GaN layers on Sapphire substrates were coated with silicon dioxide layers and planarized by CMP

- The Molibdenum substrates/ Poly-AlN substrates were coated with silicon dioxide layers and planarized by CMP

- Both wafers were plasma activated using nitrogen plasma

- The wafers were bonded at room temperature and further thermally annealed at low temperatures

- The Sapphire substrate was removed.

In order to further expand the application field of the pilot line the oxide-free direct bonding of GaN on Si was performed. In this approach, the process flow used was as below:

- The GaN layers on Sapphire substrates were surface activated using ion beam sputtering for removing native oxides and contaminants from the surface.

- The Si wafers were surface activated using same principle and native oxide was removed.

- The GaN-Si direct bonding was performed in situ under high vacuum at room temperature, then thermal annealing was performed at low temperature.

This work presents experimental details on the two process flows described. The incoming substrates were inspected by measuring the surface microroughness by Atomic Force Microscopy (AFM), their geometry (bow measurement using white light interferometry) and the presence of oxides on their surfaces was investigated by elipsometry.

The bonded substrates were characterized in terms of bow and the bonding quality was inspected using visual inspection and Scanning Acoustic Microscopy (SAM).

The oxide free bonds were characterized by cross-section TEM and EDAX and their thermal and electrical performance were measured.

Figure 1