Kazunori Koga^1,2,Sunghwa Hwang¹,Shinjiro Ono¹,Daichi Yoshikawa¹,Takamasa Okumura¹,Naoto Yamashita¹,Kunihiro Kamataki¹,Naho Itagaki¹,Masaharu Shiratani¹,Jun-Seok Oh³,Susumu Takabayashi⁴,Tatsuyuki Nakatani⁵

Kyushu Univ.¹,National Institutes of Natural Sciences²,Osaka City University³,National Institute of Technology, Ariake College⁴,Okayama University of Science⁵

Abstract

Kyushu Univ.¹,National Institutes of Natural Sciences²,Osaka City University³,National Institute of Technology, Ariake College⁴,Okayama University of Science⁵

Hydrogenated amorphous carbon (a-C:H) film has remarkable features such as mechanical hardness, chemical inertness, and biological compatibility [1]. However, one of the major concerns in applications of the films was residual stress leading to wrinkling, cracking, and peeling off. There were three representative strategies for stress relaxation of the films: the control of incident ion energy based on subplantation model [2], a post-annealing to a rearrangement of C–C bond network [3], and incorporation of impulities into the films. So far, using a plasma chemical vapor deposition (PECVD) method, we have clarified the mechanism of the growth of carbon nanoparticles (CNP) [4] and a key factor of CNP deposition [5]. We found the incorporation of CNPs can reduce the stress by appling an a-C:H/CNP/a-C:H sandwitched structure [6]. Here we measured dependence of the thickness of the upper a-C:H layer lu on the stress as a pareter of surface coverage Cp of CNPs to reveal the mechanism of stress relaxation of a-C:H using CNPs. Experiments were performed by a Plasma CVD reactor having two plasma sources. The first and second plasmas were employed for CNP production and a-C:H film deposition, respectively. Si substrates were placed on the electrode for the second plasma generation which located 115 mm under the electrode for the first plasma generation. Ar and CH4 gases were introduced from the top of the reactore. The gas flow rate of Ar and CH4 was 19 sccm and 2.6 sccm, respectively. The gas pressure was kept at 0.3 Torr. The discharge power was 280 Vpp for each electrode. We used CNPs of 4.3 nm in size and a-C:H films with a mass density of 1.88 g/cm3. The thickness of lower a-C:H layer was 154 nm. The Cp was obtained from the size distribution of deposted CNPs measured with a transparent enectron microscope. We measured the lu dependence on the film stress at Cp= 0 % and 8.9 %. For Cp= 0 % (w/o CNP deposition), the stress linearly increased from 0.67 GPa at lu= 0 nm to 1.59 GPa at lu= 154 nm, but the film was broken at lu= 182 nm might due to high stress. For Cp= 8.9%, the stress increase shows the same as that for Cp=0% from 0.66 GPa at lu= 0 nm to 0.89 GPa at lu= 46 nm. The rate of stress increase was dropped for above lu= 50 nm, and the stress reached 1.07 GPa at lu= 182 nm without any cracks. To discuss the stress reduction, we observed the surface roughness using a atomic force microscope. The rms roughness of the films for Cp= 0% was constant at approximately 0.23 nm regardless of the lu. For Cp= 8.9%, the rms roughness sharply went up and down in a range of 0.3 nm to 0.6 nm below lu= 50 nm then it kept at small value of 0.33 nm. These implied the stress relaxation takes place soon after the transition from three-dimensional film growth to two-demensional growth for lu> 50 nm which corresponds of the mean distance of agglomerated CNPs. It suggests coupling between two agglomerates by a-C:H films relieves the stress. [1] K. Bewilogua et al., Surface & Coatings Technology 242, 214-225 (2014). [2] C. Davis, Thin Solid Films 226, 30-34 (1993). [3] A. Ferrari et al., Journal of Applied physics 85, 7191-7197 (1999). [4] R. Wang et al., Journal of Materials Chemistry A 5, 3717-3734 (2017). [5] S.H Hwang et al., Diamond and Related Materials 109, 108050 (2020). [6] S.H Hwang et al., Japanese Journal of Applied Physics 59, 100906 (2020).