Development of advanced methods for diamond encapsulation and heat spreading techniques for device applications

In the presented study, different aspects of low temperature growth of microcrystalline diamond thin films by the microwave plasma chemical vapour deposition (MPCVD) technique have been explored. Due to the high thermal conductivity of the diamond, it has a great potential in advanced applications such as in the field of electronics for thermal management, but it is also a well established fact that the low temperature growth of microcrystalline diamond (MCD) is a great experimental challenge. Since the microcrystalline diamond growth typically occurs at temperature higher than 823K, the direct deposition of MCD thin film on the thermally sensitive electronic devices can lead to their structural damage. Considering these factors, this study aimed to develop such a method that can be utilized for direct deposition of microcrystalline diamond thin films at low substrate temperatures for device encapsulation and thermal spreading.

There are various factors which are counted significant when it comes to low temperature growth of MCD thin films, one being the surface pre-treatment. The surface pre-treatment also known as seeding procedure is well explored field for nanocrystalline and ultrananocrystalline diamond thin films however, it is not a very well-explored field for MCD growth at low substrate temperature. 

The presented research work is comprised of two phases; Phase-I studies the conventional seeding technique and growth in this field to accurately establish the shortcomings in those methods. The growth parameters in this part of the research work were varied initially, such as gas atmosphere, gas pressure, microwave power, and gas flowrates. It was concluded that the presence of the oxygen plays a vital role in enhancing the quality of the film at low substrate temperature. The optimization of the oxygen content in the gas atmosphere was however required and performed to avoid the excessive carbon etching. The optimized gas composition was utilized for diamond growth for the rest of the experiments. In the Phase-II the established shortcomings of the conventional seeding methods were considered to develop a state-of-the-art seeding method, which is a combination of the ultrasonication and bead assisted sonication disintegration (BASD) techniques. In the BASD technique, the milling source is zirconia beads, but in presented study the milling source was chosen to be 3D metal printing Ti6Al4V microbeads (5-50um). Different ratios (1:3, 1:1, and 3:1) of microcrystalline diamond seeds and Ti6Al4V were used in the seeding slurry to seed the silicon substrates. Considering the diamond growth results, 1:3 was found to be the optimized seeding ratio. Morphological and structural analysis of the films confirmed that this seeding technique assists significantly in MCD growth at low substrate temperature (<773K). The use of a higher diamond content (1:1, and 3:1) in the seeding solution was found to be a source of deterioration in the film quality and growth. Since the aim of this study was to develop a technique, the optimized ratio is further explored for its potential in seeding the non-conventional surfaces, such as sapphire, GaN-based devices and Si3N4 passivation. Laser patterned silicon surfaces have also been seeded with the optimized seeding ratio to understand the seeding process at a micro-physical level and to explore its potential for the controlled and patterned seeding.