In recent years, gallium nitride (GaN) is widely used in high-frequency, high-power microwave, millimeter wave devices, etc., because it has excellent performance of large band gap, high thermal conductivity, high electron saturation drift speed, and easy formation of heterostructures. At the same time, various fields have put forward higher requirements in the power, frequency, efficiency and reliability of GaN-based microwave power devices. The low heat dissipation capability of GaN HEMT devices developed with greater power and higher efficiency has become an important factor restricting the improvement of device performance, and the heat dissipation capability is mainly determined by the substrate material of the device. Since the diamond thermal conductivity is excellent, the diamond based devices are coming into people’s eyes.
Compared with commonly used SiC-based GaN microwave power devices, diamond-based GaN power devices have higher heat dissipation capabilities. From the realization of smaller size and higher power density power devices to the promotion of future RF power devices and related systems miniaturization, integration and high-power applications, there are more and more researches on high thermal conductivity in diamond.
1. Diamond High Thermal Conductivity
Currently, diamond is the substrate material with the highest thermal conductivity in nature (the thermal conductivity of Si, SiC and diamond are respectively 150, 390 and 1200～2000W·m-1·K-1), and it has nearly perfect heat dissipation in high heat devices, so more and more attention is paid on the diamond materials, especially on the diamond thermal conductivity.
As a wide bandgap semiconductor material, diamond can be used to prepare power devices, optoelectronic devices, diamond-based detectors, sensors, microelectromechanical and nanoelectromechanical devices, etc. The heat transfer mechanism of diamond is to transfer heat through lattice vibration, and the quantum energy of vibration generated by carbon atoms is relatively large. Therefore, diamond heat conductivity is higher than any materials in nature, and it has great potential in the application of heat dissipation. As a substrate material, diamond can be deposited in the GaN channel with a size of hundreds of nanometers, so that the transistor device can effectively dissipate heat during operation.
Obviously, thermal conductivity of a diamond in single crystal and polycrystalline is higher than common substrates, like SiC and Si. The diamond substrate can effectively solve the heat dissipation problem that affects the performance improvement of GaN power devices, and it can fabricate GaN-based power devices with greater power density under the same size.
The size of polycrystalline diamond is no longer limited to a single device or a small array, and the array size can be expanded to several centimeters. It is widely used in various devices. When used in a phased array chip, it can significantly improve the reliability of the system and reduce the size and cost of the system; when used in a solid-state power amplifier, it can significantly reduce the size, cost and improve efficiency; when used in broadband communications, it can reduce chip size and cost, improving reliability.
2. Methods for Growing High Quality Diamond Materials
In order to obtain higher growth rate, higher quality and larger size diamond substrate materials, growth methods are constantly innovating and improving. Methods for growing synthetic diamond thermal conductivity and CVD diamond thermal conductivity are briefly introduced below:
2.1 Methods for Growing Synthetic Diamond
The method of synthetic diamond is divided into high pressure and high temperature method (HPHT) and chemical vapor deposition (CVD). The diamond prepared by HPHT method has many problems, such as small synthetic size, low purity and single shape, which cannot meet the requirements in various industries, restricting its application. The CVD method can produce single diamond crystal thermal conductivity, polycrystals thermal, and thin film doping. In theory, the size of CVD diamond is not limited.
2.2 Methods for Growing CVD Diamond
There are three main methods for the preparation of CVD diamond: hot filament CVD (HFCVD) method, direct current plasma jet CVD (DC-PJ CVD) method, microwave plasma CVD (MPCVD) method. The hot filament CVD method is the first method of synthesizing thermal conductivity diamond film in history. Its equipment structure is simple and easy to operate with low investment cost. The process is characterized by a faster growth rate of diamond, a wide range of deposition parameters and less stringent requirements. But the pollution of the filament material directly limits the further improvement of the deposition quality of the diamond film. In the 1990s, foreign researchers made creative breakthroughs in the preparation of large-area high-quality diamond films. Through both the DC-PJ CVD method and the MPCVD method, great progress has been made in various aspects, such as equipment power improved, deposition area expanded, and the quality of diamond film improved.
Recently, MPCVD is the most widely used in the industry and is considered to be the most ideal method for preparing large-area, high-quality IC diamond thermal conductivity in the future.
There is no internal electrode in the resonant cavity of MPCVD (Microwave Plasma Chemical Vapor Deposition Technology), which can avoid the pollution caused by electrode discharge. The operating pressure range is relatively wide, and the plasma is generated with high density, large area and high stability, without contacting the vacuum vessel wall, thereby avoiding the contamination of the film by the vessel wall.
3. Applications of Diamond Thermal Properties
So far, CVD diamond thermal conductivity can be widely integrated into heat dissipation solutions in the following three ways:
Independent single diamond unit is joined by metallization and welding (for example, using Ti/Pt/Au sputtering metal deposition and AuSn eutectic welding);
Prefabricated wafers support multiple devices, enabling device manufacturers to process wafers in large quantities (such as metallization and placement;
Direct use of diamond coating.