Densities of standard materials used for the analysis of the BEMA density.
Abstract
Diamond-like carbon (DLC) films have been spreading from their theoretical basis to worldwide industrial applications because of their unique properties. Since their properties depend strongly on the conditions of synthesis, the effective classification of DLC films becomes quite necessary. From the ternary phase diagram to the Japan New Diamond Forum standard, the classification attempts are also accompanied by the continuous development of their applications. Generally, the hydrogen content and sp3/(sp2 + sp3) ratio are the primary parameters for their classification. However, researchers are afraid that currently sp3/(sp2 + sp3) ratio estimated included not only network sp3 but also sp3 hybrid carbons in the hydrogen-terminated cluster. Simultaneously, the above classification methods need to use the large equipment, such as the synchronous radiation source. Therefore, to realize more straightforward to classify DLC films efficiently, the optical constants (refractive index (n) and extinction coefficient (k)) have been proposed in 2013 to be effective method to classify the DLC films, for which a lot of considerable discussion in the past ISO/TC-107 meetings has been made. The purpose of this chapter is to introduce the latest developments of optical constants on the classification of DLC films and explore their relationship with the current standard.
Keywords
- spectroscopic ellipsometry
- diamond-like carbon film
- classification
1. Introduction
Diamond-like carbon (DLC) film is one of the attractive carbon materials due to its outstanding properties which have wide applications in mechanical, electrical, optical, and chemical fields [1]. In general, DLC films have complex structures composed of the “diamond-like”
Nowadays, not only the structural analysis but also the physical, chemical, and mechanical properties have been used to distinguish the types of DLC films. Especially, the optical constants in terms of the refractive indices (
In this chapter, we discuss the application of SE in the current classification of DLC films through two parts of experiments. In part one, DLC films were assumed to be represented by the superposition of the standard materials, for which the BEMA theory was applied to reproduce the experimental result of SE. From these analyses, the different types of DLC films can be represented by varying the superposition coefficients of the standard materials. We have selected five standard materials to build ten kinds of optical models after considering the feasibility of each model. In order to verify the reliability of the results, we used the X-ray reflectivity (XRR) method to measure the film densities as well as the NEXAFS method to evaluate the
2. Experimental details
In the part one experiment, 13 DLC films were prepared by different deposition methods which included samples provided by the other research groups upon the request by the authors. All the samples were deposited on a (100)-oriented p-type single silicon wafer substrate. Samples #01–03 were the provided samples which were deposited by a filtered cathodic vacuum arc (FCVA) method, and the detail conditions were unclear (to obtain ta-C films). Samples #04–06 were made by a FCVA method, where the graphite target (The Nilaco Co., Ltd., purity of 99.9%) was used as the carbon precursor at a negative bias voltage in the range of 1.0–2.0 kV, a deposition time of 10 min with a working pressure of 0.5 Pa, and an arc voltage of 800 V (to obtain a-C or ta-C:H films). Samples #07 and #08 were prepared by a radio-frequency (RF) magnetron sputtering method at a negative bias voltage of 0.3 kV, a deposition time of 5 and 3 min with a working pressure of 10 Pa, and a RF power of 150 W (to obtain ta-C:H or a-C:H films). Samples #09 and #10 were deposited by an electron-cyclotron-resonance chemical vapor deposition (ECRCVD) method at a negative bias voltage of 0.3 and 0.5 kV, a deposition time of 10 min with a working pressure of 0.5 Pa, and an RF power of 100 W (to obtain a-C:H or PLC films). Samples #11–13 were prepared by a plasma-enhanced (PE)-CVD method at the same 10-min deposition time and applied negative bias voltage in the range of 0.0–0.5 kV (to obtain a-C:H or PLC films). In the part two experiment, various types of DLC films were also deposited on a (100)-oriented p-type single silicon wafer substrate. Samples A–C were prepared by the FCVA deposition which is provided by the other research group upon the request of the authors. Samples D and E were deposited by RF magnetron sputtering at a negative bias voltage of 0.3 kV, a deposition time of 3 and 5 min with a working pressure of 20 Pa, and an RF power of 150 W. Samples F–K were synthesized by RF-PE-CVD methods at the same 10-min deposition time and applied negative bias voltage in the range of 0.0–0.5 kV.
All the DLC samples and the selected standard materials of PG and GC plates were measured with an ellipsometer (HORIBA, Jobin-Yvon, UVISEL NIR 23301010I). The incident angle of the source radiation was set to 70°; each measurement was carried out in the spectral range between 0.6 and 4.8 eV with a step of 0.05 eV at 293 K. The values of Tauc-band gap (
3. SE-BEMA analysis
3.1. Selection of standard materials
As described above, the main constituents in the structure of DLC films are C(
3.2. Optical model establishment
SE is an optical technique for investigating the dielectric properties of thin films, from which the film properties such as thickness, roughness, optical constants, electrical conductivity, compositions, and other material properties can be obtained. SE is an indirect method, that is, the measured amplitude component (
The simulation analyses of standard materials (PG and GC) were carried out before that of DLC films. For the PG, as shown in Figure 1(a), the substrate was modeled by TL + Drude and the layer on the substrate was (TL + Drude) + void (50%). For the simulation of GC, which is considered to consist a certain amount of void and three-dimensional structural
In the simulation of DLC films, we have assumed ten optical models as shown in Figure 1(c)–(l). The crystalline silicon (c-Si) was used as the substrate in each optical model. The first layer of each optical model was assumed to consist of a mixed phase which was composed of several standard materials. The diamond or PE was assumed to be the standard materials of C(
In Eq. (1),
Standard material | PG | GC | Diamond | PE | void |
---|---|---|---|---|---|
Density (g/cm3) | 2.26 | 1.69 | 3.52 | 0.95 | 0 |
4. Results and discussion
4.1. Results
In the part one experiment, the commercial software (Rigaku, GXRR) was used to simulate the logarithmic data of the reflection intensity, from which the density and thickness of GC and DLC films can be obtained. Figure 3 shows the XRR profiles of the standard material of GC on a semilogarithmic scale. The solid curve represents the experimental profile, and the dotted curve the simulation analysis. In the high angle side (> 0.5°), there is a large discrepancy between the simulation and experimental curves. Nonetheless, the true density obtained from XRR method is generally determined by the critical angle, whose simulated value 0.358° is in good agreement with the experimental value 0.360°. The true density can be determined by the critical angle, and the thickness analyses by the fringe pattern which are obtained with GXRR software based on the Parrat’s method [28]. Therefore, the discrepancy mentioned above in the large-angle region has no effect on the determination of the GC density. From the above measured critical angle of GC plate, the density is estimated to be 1.67 g/cm3. Another measurement point yielded the density to be 1.71 g/cm3. Then, the final density of standard GC was the arithmetic average of the two densities as 1.69 g/cm3 in this chapter.
The examples of the XRR profiles and simulation results for the samples of #06, #08, #09, and #12 made by FCVA, sputtering, ECRCVD, and PECVD methods, respectively, are shown in Figure 5. The critical angles of these samples were 0.43, 0.40, 0.38, and 0.35°, from which the XRR densities (
For all the other samples,
Sample | Method | H content (at.%) | |||
---|---|---|---|---|---|
#01 | FCVAI | 181.0 | 3.10 | 3.23 | 0.3 |
#02 | FCVAI | 66.3 | 3.02 | 3.14 | 0.5 |
#03 | FCVAI | 210.8 | 3.25 | 3.17 | 1.0 |
#04 | FCVAII | 29.8 | 2.53 | 2.30 | 4.5 |
#05 | FCVAII | 26.9 | 2.01 | 2.01 | 4.5 |
#06 | FCVAII | 21.4 | 2.62 | 2.39 | 5.0 |
#07 | Sputtering | 312.0 | 2.17 | 2.15 | 19.0 |
#08 | Sputtering | 188.8 | 2.21 | 2.26 | 19.0 |
#09 | ECRCVD | 256.0 | 2.00 | 2.05 | 26.0 |
#10 | ECRCVD | 250.0 | 1.26 | 1.19 | 33.0 |
#11 | PECVD | 104.8 | 1.74 | 1.73 | 31.0 |
#12 | PECVD | 220.0 | 1.50 | 1.45 | 36.0 |
#13 | PECVD | 50.7 | 1.23 | 1.23 | 42.0 |
Figure 5 shows the typical RBS and ERDA spectra of DLC films deposited by (a) FCVA #06, (b) sputtering #08, (c) ECRCVD #09, and (d) PECVD #12 methods. For the RBS spectra, the peaks that He+ ions backscattered according to carbon atoms in DLC films are observed around 200–450 ch. For the ERDA spectra, the peaks of hydrogen atoms recoiled from the sample by the irradiation of He+ ions emerged around 180–420 ch. The C and Si peaks on RBS spectra of the DLC films and the substrates are profiled using an RBS fitting calculation package. The H peaks on ERDA spectra of the DLC films are profiled using an ERDA fitting calculation package to compare the peak intensities of C and H. The estimating error of the present fitting process is around 5%. The hydrogen contents of DLC films of #06, #08, #09, and #12 were 5.0, 19.0, 26.0, and 36.0 at.%, respectively. Table 2 lists the other results obtained from the above peak fittings. The hydrogen content of DLC film of type made by FCVA method was in the range of 0.3–1.0 at.% and that of type in the range of 4.5–5.0 at.%. The hydrogen content of DLC films obtained from sputtering method was 19.0 at.%. The hydrogen contents obtained from the ECRCVD and PECVD methods were in the range of 26.0–42.0 at.%.
Figure 6 shows the examples of the carbon
Sample/group | Film type | H (at.%) | (%) | (eV) | (eV) | (eV) | (nm) | (g/cm3) | |||
---|---|---|---|---|---|---|---|---|---|---|---|
A | I | t | 0.3 | 47.9 | 2.65 | 0.13 | 3.45 | 0.70 | 0.68 | 207 | 3.25 |
B | I | t | 0.5 | 45.6 | 2.66 | 0.22 | 2.55 | 0.65 | 0.62 | 66 | 3.14 |
C | I | t | 1.0 | 44.3 | 2.75 | 0.30 | 2.00 | 0.75 | 0.71 | 181 | 3.23 |
D | II | 19 | 56.6 | 2.34 | 0.31 | 1.60 | 0.65 | 0.61 | 312 | 2.17 | |
E | II | 19 | 59.1 | 2.42 | 0.29 | 1.90 | 0.80 | 0.76 | 186 | 2.21 | |
F | III | 31 | 58.5 | 2.17 | 0.17 | 2.40 | 1.05 | 0.71 | 491 | 1.73 | |
G | III | 32 | 67.1 | 2.15 | 0.19 | 2.20 | 1.00 | 0.89 | 501 | 1.72 | |
H | III | 36 | 62.0 | 2.04 | 0.07 | 2.65 | 1.30 | 1.05 | 476 | 1.49 | |
I | III | 37 | 64.9 | 1.97 | 0.05 | 2.30 | 1.15 | 0.92 | 457 | 1.43 | |
J | III | 41 | 57.5 | 1.86 | 0.05 | 3.35 | 1.95 | 1.36 | 350 | 1.35 | |
K | IV | PLC | 45 | 63.0 | 1.65 | 0.00 | 4.80 | 2.60 | 2.55 | 254 | 1.21 |
The commercial software (HORIBA DelaPsi2) was used to simulate the SE data of standard materials and DLC films. Figure 7 shows the typical fitting results of Ψ and Δ spectra of SE method with the application of the BEMA equations with different optical models. In the first stage, the optical models (c)–(h) using diamond as one of the basic standard materials were applied to the DLC films. The simulation results of sample #06 which has low-hydrogen content are shown in Figure 7(a) and (b). The solid lines represent the experimental data, and the dashed lines the fitted data in each optical model. Except for
In part two experiment, the RBS/ERDA, XRR, and NEXAFS analysis are also performed on all samples. All the results are summarized in Table 3. The film thicknesses are in the range of 66–501 nm. Samples A–C are hydrogen free (under 1.0 at.%) DLC films with high true density (exceed 3.10 g/cm3). Samples D and E have the same hydrogen content of 19 at.% and almost the same true density of 2.20 g/cm3. As the substrate bias decreases from 0.5 to 0 kV, the hydrogen contents of samples E and F increase from 31 to 45 at.% and their densities decrease from 1.73 to 1.21 g/cm3. The estimated errors of these present fitting process are around 5% (e.g., sample F is 31 ± 2 at.%), ± 0.05 g/cm3, and ±5 nm, respectively. According to our previous classification scheme, the present films can be well classified into three types: t
Figure 8(a) shows the results of spectra of
Except for sample K with a maximum value of
4.2. Discussions
4.2.1. Discussions from experiment part one
Figure 9 shows the comparisons of
Table 4 shows the
Sample | Type | Optical model | Volume fraction (%) | ||||
---|---|---|---|---|---|---|---|
C( | C( | Void | BEMA | NEXAFS | |||
#01 | I | c | 77 | 23 | – | 0.77 | 0.65 |
#02 | I | c | 70 | 30 | – | 0.70 | 0.58 |
#03 | I | c | 72 | 28 | – | 0.72 | 0.71 |
#04 | II or III | d | 39 | 61 | – | 0.39 | 0.49 |
#05 | II or III | f | 26 | 65 | 9 | 0.29 | 0.38 |
#06 | II or III | d | 32 | 68 | – | 0.32 | 0.47 |
#07 | II or IV | e | 34 | 42 | 24 | 0.44 | 0.50 |
#08 | II or IV | e | 39 | 39 | 22 | 0.50 | 0.57 |
#09 | IV | h | 33 | 35 | 32 (PE) | 0.49 | 0.29 |
#10 | VI | k | 68 (PE) | 24 | 8 | 0.74 | 0.42 |
#11 | IV | f | 21 | 58 | 21 | 0.27 | 0.38 |
#12 | IV | e | 7 | 54 | 40 | 0.11 | 0.42 |
#13 | VI | k | 49 (PE) | 34 | 17 | 0.59 | 0.44 |
4.2.2. Discussion from experiment part two
Figure 10(a) shows the relationship between the present classification scheme based on
5. Conclusions
We analyzed various types of DLC films with different structure and hydrogen contents in the range of 0.3–42% fabricated by FCVA, sputtering, ECRCVD, and PECVD techniques. The structural analysis of a variety of DLC films was performed by using a combination of SE, BEMA, RBS/ERDA, NEXAFS, and XRR. In part one, from the comparisons of the density (
References
- 1.
Bewilogua K, Hofman D. History of diamond-like carbon films – From first experiments to worldwide applications. Surface and Coatings Technology. 2014; 242 :214-225 - 2.
Zhou XL, Tunmee S, Suzuki T, Phothongkam P, Kanda K, Komatsu K, Kawahara S, Ito H, Saitoh H. Quantitative NEXAFS and solid-state NMR studies of sp 3/(sp 3 +sp 2) ratio in the hydrogenated DLC films. Diamond and Related Materials. 2016;73 :232-240 - 3.
Ferrari AC, Libassi A, Tanner BK, Stolojan V, Yuan J, Brown LM, Rodil SE, Kleinsorge B, Robertson J. Density, sp3 fraction, and cross-sectional structure of amorphous carbon films determined by x-ray reflectivity and electron energy-loss spectroscopy. Physical Review B. 2000; 62 (16):11089-11103 - 4.
Robertson J. Diamond-like amorphous carbon. Materials Science and Engineering R. 2002; 37 (4-6):129-281 - 5.
Jacob W, Möller W. On the structure of thin hydrocarbon films. Applied Physics Letters. 1993; 63 (13):1771-1773 - 6.
Gui WG, Lai QB, Zhang L, Wang FM. Quantitative measurements of sp3 content in DLC films with Raman spectroscopy. Surface and Coatings Technology. 2010; 205 (7):1995-1999 - 7.
Tunmee S, Photongkam P, Euaruksakul C, Takamatsu H, Zhou XL, Wongpaya P, Komatsu K, Kanda K, Ito H, Saitoh H. Investigation of pitting corrosion of diamond-like carbon films using synchrotron-based spectromicroscopy. Journal of Applied Physics. 2016; 120 :195303 - 8.
Tunmee S, Supruangnet R, Nakajima H, Zhou XL, Arakawa S, Suzuki T, Kanda K, Ito H, Komatsu K, Saitoh H. Study of Synchrotron Radiation Near-Edge X-Ray Absorption Fine-Structure of Amorphous Hydrogenated Carbon Films at Various Thicknesses. Journal of Nanomaterials. 2015; 2015 :1-7 - 9.
VDI2840. Carbon films basic knowledge, film types and properties. Düsseldorf: Verein Deutscher Ingenieure; 2005 - 10.
Kim SW, Kim SG. Prospects of DLC coating as environment friendly surface treatment process. Journal of Environmental Sciences. 2011; 23 :S08-S13 - 11.
Hiramatsu M, Nakamori H, Kogo Y, Sakurai M, Ohtake N, Saitoh H. Correlation between Optical Properties and Hardness of Diamond-Like Carbon Films. Journal of Solid Mechanics and Materials Engineering. 2012; 7 (2):187-198 - 12.
DIN 50989-1: 2017-04-Draft, Ellipsometry-Part 1: Principles; Text in German and English, Beuth Verlag; 2017 - 13.
Weiler M, Sattel S, Jung K, Ehrhardt H, Veerasamy VS, Robertson J. Highly tetrahedral, diamond‐like amorphous hydrogenated carbon prepared from a plasma beam source. Applied Physics Letters. 1994; 64 :2797-2799 - 14.
Zhou XL, Arakawa S, Tunmee S, Komatsu K, Kanda K, Ito H, Saitoh H. Structural analysis of amorphous carbon films by BEMA theory based on spectroscopic ellipsometry measurement. Diamond and Related Materials. 2017; 79 :46-59 - 15.
Niklasson GA, Granqvist CG, Hunderi O. Effective medium models for the optical properties of inhomogeneous materials. Applied Optics. 1981; 20 (1):26-30 - 16.
Chen ZY, Yu YH, Zhao JP, Wang X, Liu XH, Shi TS. Determination of the sp3/sp2 ratio in tetrahedral amorphous carbon films by effective medium approximation. Journal of Applied Physics. 1998; 83 (3):1281-1285 - 17.
Zhou XL, Suzuki T, Nakajima H, Komatsu K, Kanda K, Ito H, Saitoh H. Structural analysis of amorphous carbon films by spectroscopic ellipsometry, RBS/ERDA, and NEXAFS. Applied Physics Letters. 2017; 110 :201902 - 18.
Safaie P, Eshaghi A, Bakshi SR. Optical properties of oxygen doped diamond-like carbon thin films. Journal of Alloys and Compounds. 2016; 672 :426-432 - 19.
Papadopoulos AD, Anastassakis E. Optical properties of diamond. Physical Review B. 1991; 43 (6):5090-5097 - 20.
Tajir D, Tougaard S. Electronic and optical properties of selected polymers studied by reflection electron energy loss spectroscopy. Journal of Applied Physics. 2012; 111 (5):054101 - 21.
Guo WS, Wong SP, Yu YH. Spectroscopic ellipsometry characterization of diamond-like carbon films formed by filtered arc deposition. Nuclear Instruments and Methods in Physics Research Section B. 2000; 169 (1-4):54-58 - 22.
Battie Y, Broch L, En Naciri A, Lauret J-S, Guézo M, Loiseau A. Diameter dependence of the optoelectronic properties of single walled carbon nanotubes determined by ellipsometry. Carbon. 2015; 83 :32-39 - 23.
Fujiwara H. Spectroscopic Ellipsometry: Principles and Applications. London: John Wiley & Sons Ltd; 2007 - 24.
Palik ED. Handbook of Optical Constants of Solid. London: Elsevier; 1997 - 25.
Williams MW, Arakawa ET. Optical properties of glassy carbon from 0 to 82 eV. Journal of Applied Physics. 1972; 43 (8):3460-3463 - 26.
Iwaki M, Terashima K. Change in atomic density of glassy carbon by Na ion implantation. Surface and Coatings Technology. 2000; 128-129 :429-433 - 27.
Kawai T, Keller A. On the density of polyethylene single crystals. Philosophical Magazine. 2006; 8 (91):1203-1210 - 28.
Sauro JP, Bindell J, Wainfan N. Some Observations on the Interference Fringes Formed by X Rays Scattered from Thin Films. Physics Review. 1966; 143 :439-443 - 29.
Tamor MA, Haire JA, Wu CH, Hass KC. Correlation of the optical gaps and Raman spectra of hydrogenated amorphous carbon films. Applied Physics Letters. 1989; 54 :123-125 - 30.
Cho G, Yen BK, Klug CA. Structural characterization of sputtered hydrogenated amorphous carbon films by solid state nuclear magnetic resonance. Journal of Applied Physics. 2008; 104 :013531 - 31.
Tillmann W, Hoffmann F, Momeni S, Heller R. Hydrogen quantification of magnetron sputtered hydrogenated amorphous carbon (a-C:H) coatings produced at various bias voltages and their tribological behavior under different humidity levels. Surface and Coatings Technology. 2011; 206 :1705-1710 - 32.
Kaplan S, Jansen F, Machonkin M. Characterization of amorphous carbon‐hydrogen films by solid‐state nuclear magnetic resonance. Applied Physics Letters. 1985; 47 :750-753 - 33.
Ito H, Yamamoto K, Masuko M. Thermal stability of UBM sputtered DLC coatings with various hydrogen contents. Thin Solid Films. 2008; 517 :1115-1119 - 34.
Schneider D, Schwarz T. A photoacoustic method for characterising thin films. Surface and Coatings Technology. 1997; 91 :136-146