- Categories:Technology and R&D
- Time of issue:2021-08-20 09:39:23
The generation method of diamond heat sink and its application in the field of microwave and radio frequency
For more than 50 years, synthetic diamond manufactured by high pressure high temperature technology (HPHT) has been widely used in grinding applications, giving full play to the extremely high hardness and strong wear resistance of diamond. In the past 20 years, a new diamond generation method based on chemical vapor deposition (CVD) has been put into commercial application, which enables the generation of single crystal and polycrystalline diamond at a lower cost. These new synthesis methods support the comprehensive development and utilization of the optical, thermal, electrochemical, chemical, and electronic properties of diamond.
At present, diamond has been widely used in optics and semiconductor industries. This article mainly discusses the thermal advantages of diamond, introduces the working principle of diamond heat sink, briefly shows the diamond generation method, summarizes some common applications of diamond (including application methods) and takes the future application prospects of diamond as a conclusion. First, let's briefly introduce the reason and principle of diamond being the best heat conductor among all solid materials at room temperature.
Diamond heat conduction principle
Diamond is a cubic crystal, formed by covalent bonding of carbon atoms. Many of the extreme properties of diamond are the direct result of the sp³ covalent bond strength that forms a rigid structure and a small amount of carbon atoms.
Metal conducts heat through free electrons, and its high thermal conductivity is associated with high electrical conductivity. In contrast, heat conduction in diamond is only accomplished by lattice vibration (ie, phonons). The extremely strong covalent bond between diamond atoms makes the rigid crystal lattice has a high vibration frequency, so its Debye characteristic temperature is as high as 2,220°K. Since most applications are much lower than the Debye temperature, the phonon scattering is small, so the heat conduction resistance with the phonon as the medium is extremely small. But any lattice defect will produce phonon scattering, thereby reducing thermal conductivity, which is an inherent characteristic of all crystal materials. Defects in diamond usually include point defects such as heavier ˡ³C isotopes, nitrogen impurities and vacancies, extended defects such as stacking faults and dislocations, and 2D defects such as grain boundaries.
As a component dedicated to thermal management, natural diamond was used in some early microwave and laser diode devices. However, the availability, size and cost of suitable natural diamond plates limit the market application of diamonds. With the emergence of microwave-assisted CVD polycrystalline diamond with thermal properties similar to type IIa natural diamond (Figure 1), the availability issue has been resolved. Currently, many suppliers provide a series of ready-made thermal grade diamonds. Since the free-standing polycrystalline diamond is produced using a large wafer with a diameter of 140 mm (Figure 1), the size is no longer limited to a single device or a small array, and the array size can be expanded to several centimeters. Based on the above reasons, the practicability of CVD diamond has been verified, and it has been widely used in various devices since the 1990s.
Figure 1. Thermal conductivity and temperature comparison between layers measured by IIa natural diamond laser flash methodAs shown in Figure 1, TM200 (TM means heat, 200 means thermal conductivity> 2,000 Wm¯ˡKˡ) The thermal conductivity at room temperature is 2,200 Wm¯ˡKˡ, which is 5 times higher than the thermal conductivity of copper (see Table 1) . Element Six provides a series of products, so the thermal conductivity and cost can be customized according to technical requirements and budget. Since the thermal conductivity at room temperature is >1,000 Wm¯ˡ Kˡ, TM100 exceeds ceramic materials such as aluminum nitride by 4 to 6 times.
The thermal performance of advanced products is more advantageous under the condition of lower than room temperature, and the thermal conductivity is significantly improved when the temperature is as low as 100°K. The performance and temperature trend of the TM180 and TM200 grades shown in Figure 2 are similar to those of type IIa natural diamonds.
We used characterization techniques to analyze the microstructures of different levels in detail. Within the scope of the study, the conductivity of TM100 is less sensitive to temperature. The grain size in CVD diamond increases with the increase in thickness, which has a significant impact on the conductivity. For CVD diamond with the same grain size, the point defect density in TM100 and TM180 are similar, but the dislocation density in TM100 is three orders of magnitude higher than that in TM180. This difference plays a major role in phonon scattering and has a significant impact on conductivity. The dislocation density measured in TM180 and TM200 are similar, but the slight difference in conductivity at lower temperatures can be explained by the grain size and the point defect density of TM200 that is 5 times lower than that of TM180. The following paragraphs of this article will discuss other production technologies. The grain size, purity, and dislocation of each production technology are significantly different, so there are also large differences in thermal performance.
Table 1. Green = significant advantage, yellow = medium advantage, red = negative impact
In the semiconductor market, the power density of power converters or solid-state RF power amplifiers is constantly increasing, making local thermal management more and more burdensome. CVD diamond also has extreme properties such as high thermal conductivity and electrical insulation, and is an ideal choice to solve the above problems. Our measurement results show that the ratio of the conductivity between the inside and the inside of the microwave-assisted CVD diamond layer is less than 10%, which is almost the same as the measurement uncertainty. Isotropic thermal properties and electrical insulation are important properties of heat sinks in many thermal applications. This is in sharp contrast to materials such as highly oriented pyrolytic graphite, which has electrical conductivity and anisotropic thermal conductivity, as shown in Table 2.
Table 2. Comparison of thermal properties of polycrystalline diamond synthesized by different CVD technologies
Diamond generation method
Synthetic diamonds are manufactured using a series of different technologies. Synthetic diamond grain size, large single crystal and sintered polycrystalline diamond products are all synthesized using high pressure and high temperature pressing technology. The highest purity single crystal diamond products are made by microwave-assisted CVD, but polycrystalline CVD diamond can be manufactured by different technologies. As shown in Table 2, the properties of diamonds manufactured by different technologies are different. Generally speaking, diamond CVD can be divided into three categories: microwave assisted CVD, hot filament CVD and DC arc or DC torch CVD.
In various CVDs, the same point is the small amount of gas phase carbon component in hydrogen, and the gas temperature exceeds 2,000°K, which promotes the decomposition of H2 into extremely reactive H× groups. The precipitation diameter of the hot filament reactor is usually as high as 300 mm, but the balance between the precipitation area, uniformity (attributes such as purity) and output is very important, and is as important as the overall performance. The phase purity (affected by the reduction of sp² content) can be controlled by two methods: (1) reduce the input methane flow rate and generation rate (but will increase the generation time and cost), (2) increase the H2 decomposition rate by increasing the gas temperature. Microwave and DC arc spray reactors make it easier to increase the gas temperature. The use of microwave-assisted CVD can achieve the best impurity content control, because this method does not require cathodes or filaments, so that the purity, light transmission performance and thermal conductivity of microwave-assisted CVD diamond can reach the maximum.
CVD diamond heat dissipation application
Factors to be considered when integrating CVD diamond in a thermal system. To successfully integrate thermal management components into devices, the complete thermal conduction path as well as electrical requirements and thermomechanical stress must be considered. Although CVD diamond is extremely rigid and has a small thermal expansion coefficient (about 1 ppm/K), it is an ideal choice for high-power transmission window applications, but it is compatible with Si (2.6 ppm/K), GaAs (5.7 ppm/K) and There are obvious differences in commonly used semiconductor materials such as GaN (3.2 – 5.6 ppm/K), which poses greater challenges to thermal design engineers. Unless it is considered at the beginning of the design, the stress generated by thermal cycling can adversely affect device life and reliability. Two methods to control these stresses are composite semiconductor pre-cracking  and diamond interlayer; in diamond interlayer, the upper layer is used to balance the stress. When integrating diamond into a device package, the ideal geometry depends on many factors such as power density and cooling channel location, but the model design is relatively simple.
Figure 2. Metallized CVD diamond heat sink
CVD diamond can be widely integrated into the heat dissipation solution in the following three ways: (i) Independent individual diamond units are joined by metallization and welding, see Figure 3 (for example, using Ti/Pt/Au sputtering metal deposition and AuSn eutectic Soldering); (ii) Prefabricated wafers support multiple devices, enabling device manufacturers to process wafers in large quantities (such as metallization and placement). After such additional steps are completed, these wafers can be used as substrates for individual sub-assemblies. (iii) Direct use of diamond coating.
Laser Diode Array
Using CVD diamond as the interface between the laser diode array and the microchannel cooling copper block, the device temperature rises from 22°C to 16°C, as shown in Figure 4, which significantly extends the product life.
Laser diode arrays (100 W/mm2 peak power density at 200 μm intervals) change the geometry of the CVD diamond on the simple cooling copper block, indicating that 300 μm thick and 3 mm wide diamonds are required instead of thin diamond coatings. It should be noted that the modeling comparison of experimental results shows that metallization is also an important part of the heat conduction path. The typical metallization is Ti/Pt/Au, and the total thickness is about 1 µm. The titanium layer is a key factor for adhesion, and a carbonized layer is formed at the junction with diamond. The gold layer provides a low resistance connection and serves as a base layer for subsequent soldering or wire bonding. The platinum layer acts as a barrier to prevent the diffusion of copper to form excess intermetallic compounds.
Figure 3. (a) RF package simulation of integrating discrete RF devices on a BeO heat sink bonded to a CuW flange.
Another example is an RF package composed of discrete RF components connected to a 1 mm thick beryllium oxide heat sink mounted on a CuW flange. Beryllium oxide is toxic and its thermal conductivity is only about 200 Wm¯ˡKˡ. The thermal model (Figure 5) shows that replacing beryllium oxide with 300 µm thick TM100 CVD diamond can reduce thermal resistance by 30%. The measurement result of the temperature drop of the entire package collected by the infrared camera shows the maximum temperature drop of the entire device and the CuW flange, and also shows that the temperature drop of the diamond layer is almost negligible. At present, the product has been mass-produced, with the help of CVD diamond to increase the output by 40% at the same junction temperature.
Figure 4. (b) The temperature drop measurement result of the entire package captured by the infrared camera
The application prospects of diamond
The use of CVD diamond as a heat sink in the semiconductor industry is still in its infancy. The growing demand for superior thermal management solutions in optoelectronics, power and radio frequency devices will drive the widespread adoption of this engineering material in the next ten years. Combined with the growth rate of these market areas, the increase in diamond adoption will drive a large amount of investment into the synthetic diamond manufacturing industry. Such investment will promote economies of scale and enable diamond to occupy a place in the semiconductor market that was previously unable to compete.
In the next ten years, we can expect semiconductor manufacturers to integrate diamond as a substrate in devices for commercialization. For example, the combination of GaN and diamond theoretically provides 120 Wcm² power consumption, compared to 40 Wcm²² for SiC.
At the same time, we also expect that CVD diamond, which has both high thermal conductivity and high breakdown voltage characteristics, will be used as an active semiconductor. Initially, it may be mainly used in the field of high-voltage switches, but as the technology matures, other applications will continue to appear.
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