S3-3

Microwave Plasma-Assisted Synthesis and Applications ofDiamond Films
マイクロ波プラズマによるダイヤモンド膜の合成と応用
K. Kobashi, T. Tachibana, Y. Yokota, K. Hayashi, N. Kawakami and T. Kobori
小橋宏司，橘武史，横田嘉宏，林和志、川上信之、古保里隆
Kobe Steel, Ltd., Electronics Research Laboratory
1-5-5 Takatsuka-dai, Nishi-ku, Kobe 651-2271, Japan（株）神戸製鋼所 電子技術研究所，〒651-2271 神戸市西区高塚台1-5-5

Diamond films were synthesized by microwave plasma-assisted chemical vapor deposition (CVD) toinvestigate the possibility of controlling the film morphology by changing the CVD conditions and theCVD reactors. It was found that the film morphology was strongly influenced by the CVD conditions andreactors. In terms of the film area, it is now possible to deposit diamond films over a 6-inch area using a60-kW, 915-MHz reactor. There are a number of applications for diamond films, and particularly,
heteroepitaxial films were found to be very useful for practical applications such as UV sensors.

1. Introduction
The diamond film technology has advanced
significantly during the past 5 years or so, partly
because of the use of semi-production type CVD
reactors that can generate a large plasma using the915 MHz microwave with the input power of 60 –100 kW. As a result, a uniform diamond film
deposition over a 6-inch area became possible. Thisis a remarkable advancement from the past
technology where 2.45-GHz, ≤ 8-kW reactors were
use to coat diamond films over 2 – 3 inch areas. For
diamond film production, it is necessary toinvestigate to what degree the film morphology canbe controlled by the large reactor to synthesize thebest film for a specific application. As a topic forsuch applications, a recent development of UV
sensors will be presented.
2. CVD Reactor and Film Morphology
The reactor used is shown in Fig. 1. It isnormally operated under conditions of (source gas:c) 1-5% CH4/H2, (gas flow rate) 2000 sccm, (gaspressure: P) 60 – 200 Torr, and (substratetemperature: Ts) 800 - 1200 °C. The growth rate is 1– 10 µm/h, but generally, the film quality is betterfor a low growth rate. Under standard CVD
conditions with a standard substrate pretreatment, apolycrystalline diamond film was deposited on thesubstrate, where the orientations of the diamondgrains in the film are random. The grain size
depends on the growth time and the input power Pm,
and normally 5 – 10 µm.
To investigate the film morphology, CH4+CO2/H2 was used as the source gas. The reason for theaddition of CO2 is that in a precedent study using

CH4/H2 as the source gas, the film morphology was
not satisfactorily controlled. Note that the film
morphology is expressed by the α-parameter that isdefined by α = 3 (v111/v100), [1] where v111 andv100 are the growth rate of (111) and (100) faces of
diamond along the direction normal to each face,respectively. The α-parameter normally takes avalue between 1 and 3, and Fig. 2 shows thecorrespondence between the α-values and the
diamond crystal shapes.

Fig. 1. 915-MHz microwave plasma CVD reactor for
diamond film synthesis. It can coat a film over a 6-inchsubstrate.

Fig. 2. The α-parameter and the crystal shape. [1]Figure 3 shows the film morphology achieved byusing the source gas, CH4+CO2/H2, [2] and thenumbers in the figure are the α-values. The SEM
images, a – d, correspond to the surfaces of thefilms synthesized under conditions indicated by a –d in the figure, respectively. To express the“effective” carbon concentration, C*, it is definedby C* = [c(CH4) – c(CO2)]/[c(CH4) – c(CO2) +c(H2)], where c(X) is the vol% of molecule X in thesource gas. For C* < 0, no diamond was deposited.
It was thus found that by using CH4+CO2/H2 as thesource gas, the α -values was attained not only
between 1 and 3 but also beyond this range bychanging C* and Ts. Based on the data, it waspossible to grow uniaxially <100>-oriented
diamond film over a 6-inch substrate.

4. Conclusion
It seems that for commercial production of
diamond films, it is necessary to construct even alarger CVD reactor than that shown in Fig.1. The
new reactor must have a capability of coating
diamond uniformly over a 40-cm substrate with abiasing capability. Generation and control of such alarge plasma at a pressure of 100 – 200 Torr is a bigchallenge. To this end, further research of scienceand technology of microwave plasma will beimportant.

Fig. 4. Photoresponses of the diamond UV sensor for
ArF (193 nm) and dye (313nm) laser irradiations. The
response to the former is approximately 100 times more
intense than that to the latter. Note that the laser peakwidth is ∼ 5 ns. [3]Fig. 3. Morphology of the films synthesized by thereactor shown in Fig. 1. [2]3. Diamond UV sensors
Diamond has a large bandgap of 5.45 eV so thatit exhibits a photoconductivity to the light with thewavelength λ ≤ 220 nm, deep ultraviolet (UV).
Recently, a UV sensor was made using a highlyoriented diamond (HOD) film, in which (100) facesof diamond are parallel to the film surface andazimuthally aligned in the same direction. [3] On
the surface of the HOD film, interdigited electrodesof Pt was fabricated with the gap length of 5 – 10µm. Figure 4 shows the photoresponse of thefabricated UV sensor to the laser irradiation. TheUV sensor was operational only for the laser lightwith λ ≤ 220 nm, insensitive to the solar light, andcould follow the laser pulse with ns time scale.
There was no significant damage on the sensor evenby the direct irradiation of the laser light. Figure 5
shows the packaged sensors. The solar-insensitive,
robust UV sensors are expected to be widely used
to monitor the UV lights from lumps and lasers in
the coming years.

Fig. 5. Packaged diamond UV sensors.
References[1] Ch. Wild, P. Koidl, W. Müller-Sebert, H. Walcher,
R. Kohl, N. Herres, R. Locher, R. Samlenski, andR. Brenn, Diamond and Related Mater. 2 (1993)
158.
[2] Y. Yokota, Y. Ando, K. Kobashi, T. Hirao, and K.
Oura, Diamond and Related Mater. 12, (2003) 295.
[3] J. Achard, A. Tardieu, A. Kanaev, A. Gicquel, M.
C. Castex, Y. Yokota, K. Hayashi, T. Tachibana,and K. Kobashi, Diamond and Related Mater. 11,(2002) 423.