Diamond Heat Sinks for High Performance Electronics Packaging Heat Dissipation

High-performance chips have become a common necessity for artificial intelligence (AI), deep learning, cloud computing, supercomputers and other cutting-edge technologies. However, with Moore's Law slowing down in the mainstream development direction and chip processes approaching their physical limits, advanced packaging technology has become an important way to continue chip performance improvement. In 2.5D/3D packaging technology, diamond heat sink with its excellent thermal conductivity (up to 2000 W·m^-1K^-1 ) has become an ideal solution to solve the heat dissipation problem of high-performance chips.

Currently, the common semiconductor materials such as Si, SiC and GaN have relatively low thermal conductivity, generally not more than 500 W·m^-1K^-1

, while the power density of high-power electronic devices can be up to 1000 W·cm^-2. Meanwhile, the difference in power density between different functional areas causes uneven temperature distribution inside the chip, and the local hot spots are even 5~10 times the average heat power density of the chip.

Diamond sheet or film is the highest thermal conductivity heat sink material existing in nature at present, expected to effectively export the accumulated heat to achieve the ideal heat dissipation effect, and has been widely recognized as one of the future solutions to improve the heat dissipation capability of semiconductor devices. Both monocrystalline and polycrystalline diamonds have much higher thermal conductivity than other substrate materials and can be used as a superior alternative.

The connection method between diamond and semiconductor device determines the efficiency of heat dissipation effect. If diamond can be directly connected with semiconductor materials, the characteristics of high thermal conductivity of diamond can be fully utilized, so the research of direct connection process has been a research hotspot. The primary ways of direct connection between diamond and semiconductor are: 1) direct connection between diamond and semiconductor through deposition process; 2) direct connection between diamond and semiconductor through low-temperature bonding.

Depositing a diamond film directly on the prepared semiconductor device or a diamond passivation layer on the front side of the device can improve the upward heat dissipation ability of the device, but the thermal expansion adaptation problem can still lead to cracking of the epitaxial layer. At the same time, the CVD process for depositing diamond heat dissipation layer generally requires high temperature (>700°C) and high concentration of hydrogen plasma atmosphere, which can seriously etch semiconductors such as Si, SiC and GaN, leading to a serious degradation of their electrical and other properties.    

In order to avoid the high temperature and hydrogen plasma environment required for direct epitaxial growth, the method of first depositing semiconductor materials on a substrate using an epitaxial growth process, then removing the substrate and low-temperature bonding with a diamond substrate has been widely studied. Both polycrystalline diamond and monocrystalline diamond can be used as a heat-sinkable substrate for low-temperature bonding, which greatly reduces the difficulty of preparing diamond substrates; and the semiconductor epitaxial layer and diamond heat-sinkable substrate can be prepared independently before bonding, which streamlines the process of diamond-based semiconductor devices.

The low-temperature bonding process circumvents the difficulties of epitaxial growth, but requires the diamond heat-sinkable substrate and semiconductor epitaxial layer surface flatness, small warpage, low surface roughness (<1 nm), which is a greater challenge to the current processing technology. In addition, it is difficult to effectively control the pressure size and holding time during direct bonding, resulting in the specimen in the bonding process is easy to break, especially large-size specimens is difficult to realize, is still in the laboratory exploration stage, only in the millimeter scale of the small-size chip on the success, can not be large-scale applications.

Although the most ideal application of diamond heatsinks is direct connection to the chip, indirect connection packaging between the chip and the substrate using metal is a more mature process in the semiconductor industry. Commonly used processes include soft brazing using solder (tin-lead or lead-free), transient liquid-phase diffusion soldering using low-melting-point intermediate layers (such as gold-tin eutectic alloys), and nanosilver low-temperature sintering.

CSMH uses the MPCVD method to prepare large-sized and high-quality diamonds, and currently has mature products such as diamond heat sinks, diamond wafers, diamond windows, diamond hetero junction integrated composite substrates, etc. Among them, the thermal conductivity of diamond heat sinks is 1000-2200W/(m.k), and the surface roughness of diamond wafer Ra<1nm. lt has been applied in aerospace, high-power semiconductor lasers, optical communication, chip heat dissipation, nuclear fusion and other fields.