Manufacturing technologies
Power device, Heat spreader, Diamond tool

High-efficiency ultra-precision polishing techniquefor diamond substrates using plasma
Research Center for Precision Engineering, Graduate School of Engineering
Professor Kazuya Yamamura
https://researchmap.jp/read0014013?lang=en

Manufacturing technologies▍ Background & Results
Currently, Scaife polishing using diamond abrasive is commonlyused to polish diamond substrates, but this results in the formation ofscratches and damaged layers, which deteriorates thermal conductivity and electronic properties. Furthermore, although chemical mechanical polishing (CMP) does not introduce damage, the polishing rateis too low to be practical. Therefore, we investigated the applicationof plasma-assisted polishing (PAP) as a new damage-free and highly
efficient planarization/smoothing method for diamond substrates.
Figure 1(a) shows a 20 mm square single-crystal diamond substratefabricated by the mosaic method, and large waviness with a height ofover 100 μm can be seen, but after PAP, as shown in Figure 1(b), a
flatness of less than 0.5 μm was achieved. At this time, under roughpolishing conditions, a polishing rate of up to 13 μm/h, more than 10times higher than that of CMP, was obtained. Furthermore, after PAP, asurface roughness on the order of 0.3 nm was obtained.
Polycrystalline diamond (PCD) substrates are being considered foruse as heat spreaders for power devices because they can be madelarger in size than single crystals. However, when mechanical polishing techniques are applied to PCD polishing, it is difficult to obtaina smooth surface because grain boundary steps are formed due tothe dependence of the polishing rate on crystal plane orientation andcrystal plane direction. Figure 2 shows the results of polishing a 2-inch
PCD substrate using PAP. After PAP for 2 hours, the convex structures
observed on the CMP surface disappeared, and after PAP for 5 hours,a surface roughness of 0.16 nm with no grain boundary steps wasachieved. Therefore, by applying PAP processing, even a polycrystalline substrate can achieve a surface roughness on the order of subnanometers, providing a surface sufficient for bonding power devices atroom temperature.
Figure 3 is a scanning transmission electron microscope (STEM)image of the (100) surface of single-crystal diamond polished by PAP.
It can be seen that the crystal structure is maintained all the way to theoutermost surface, and there is no damage at all.

100
[μm]

Diamond is a promising material in the field of next-generation
semiconductor power devices because it has excellent electricaland thermal properties such as low dielectric constant, high breakdown voltage, and high thermal conductivity. However, diamond
is difficult to process because of its high hardness and chemicalinertness. Our group has developed a new process for finishingdiamond substrates with high efficiency and without damage usinga ``Plasma-assisted Polishing technique’’ that combines ``chemicalaction’’ and ``mechanical action.’’ This process achieved a polishing rate that is more than 10 times higher than that of conventional
chemical mechanical polishing (CMP), and successfully achieved
subnanometer-order surface roughness with no grain boundary
steps on polycrystalline diamond substrates.

power devices has been conducted for several years. However,since diamond is a difficult-to-process material, a process thatachieves both a high polishing rate and a high-quality surface has
not yet been realized. The ``Plasma-assisted Polishing technique’’has made it possible to manufacture high-quality diamond substrates at low cost, and it has the significance of breaking throughthe conventional limits toward the widespread use of diamond substrates for semiconductor device applications. It is also expected
to improve the performance of power-saving, high-performance
power devices and high-frequency devices, leading to the early
realization of a low-carbon society.

500-50-100(a) Before PAP(b) After PAP

Fig. 1 Flatness of single crystal diamond substrate

Fig.
１ Flatness of single crystal diamond substrate
3020Height (nm)Abstract

After CMP, Ra: 3.2 nm Rz: 24 nm

100Main Figure １

After PAP 2h, Ra: 0.54 nm Rz: 3.6 nm-10After PAP 5h, Ra: 0.16 nm Rz: 1.0 nm

-20
-30

01020304050607080 (µm)

Fig. 2 Surface roughness of polycrystalline diamond substrate

Fig. 2 Surface roughness of polycrystalline diamond substrate
Main Figure 2▍ Significance of the research and Future perspective
Research into using diamond substrates as heat spreaders forFig. 3 STEM image of SCD after PAP

P a t e n t Japanese Patent Application No.2022-125662, Japanese Patent No.5614677
Liu, Nian; Yamamura, Kazuya et al. Highly efficient finishing of large-sized single crystal diamond substrates by combining nanosecond pulsed laser
trimming and plasma-assisted polishing. Ceram. Int. 2023, 49, 19109–19123. doi: 10.1016/j.ceramint.2023.03.038
Liu, Nian; Yamamura, Kazuya et al. Effects of polishing pressure and sliding speed on the material removal mechanism of single crystal diamond in
Tr e a t i s eplasma-assisted polishing. Diam. Relat. Mater. 2022, 124, 108899. doi: 10.1016/j.diamond.2022.108899
Liu, Nian; Yamamura, Kazuya et al. Damage‑free highly efficient plasma‑assisted polishing of a 20‑mm square large mosaic single crystal diamondsubstrate. Sci. Rep. 2020, 10, 19432. doi: 10.1038/s41598-020-76430-6
UR
L http://www-nms.prec.eng.osaka-u.ac.jp/en/
K e y w o r d plasma-assisted polishing, single crystal diamond substrate, polycrystalline diamond substrate