D. Sarangi

Possible solution to the problem of high built-up stresses in diamond-like carbon films

The various issues relating to the nature of high built-up stresses in diamond like carbon (DLC) films are presented and analyzed and the utility of pulse plasma technique in growing low residual stress DLC films is emphasized. Subsequently, sufficiently thick (2.2 μm) and hard (2000 kg/mm2) DLC films of significantly low stress (≈0.1 GPa) were deposited by the pulse plasma enhanced chemical vapor deposition (PECVD) technique. Stress values were found to be less than 0.5 GPa even with wide variation in pulse parameters (power density 0.4–2.0 W/cm2, dwell time 10–150 ms and duty cycle 10%–70%). A possible growth mechanism operating during pulse plasma discharge of such low residual stress and hard DLC films appears to involve the three phenomena: (i) relaxation of adions/adatoms, (ii) control of the substrate temperature, and (iii) creation of a hard/soft multilayer structure. To examine the role of substrate heating during the pulse plasma discharge, films were also deposited on deliberately heated substr...

Diamond-like carbon films with extremely low stress

Abstract We in this paper report different ways to realise thick diamond-like carbon (DLC) films with stress values lower than 0.5 GPa, Thick DLC films grown by conventional r.f. self bias technique often delaminate from the substrates due to the presence of high compressive stresses of the order of 4–7 GPa. We have made an in-depth study of the delamination problem of DLC films at NPL and found that only for substrates kept away from the plasma (plume) it is possible to grow thick DLC films. This goes to show the heating of the substrates, when m contact with the plasma, appears to be one of the most important factors giving rise to the high stress values. Techniques that have produced consistently low stress values (0.2–0.5 GPa) in this laboratory are pulse plasma PECVD and the one using dc saddle field fast atom beam source. Electronic properties of the materials so produced have been estimated by evaluating Urbach energy using photothermal deflection spectroscopy (PDS) measurements. A correlation between the unbound hydrogen in these films, as measured by a nuclear technique (ERDA), and the stress levels has been found. Deposition rate, room temperature conductivity, optical bandgap and refractive index have also been measured for these films.

Diamond‐like carbon films grown using a saddle field source

This article reports the measurement of deposition rate, dark conductivity versus temperature, optical band gap, refractive index, extinction coefficient, hardness, adhesion, and internal stress of diamond‐like carbon (DLC) films grown by aspirating hydrocarbon gases (CH4 and C2H2) and C6H6 vapors into a saddle field source. The source operates at 0.7–1.0 kV in the pressure range of 1.0×10−4–7.0×10−4 Torr and is of a modular design to cover increasingly larger areas. DLC films have been grown for the first time using CH4 by this technique. The effect of source to substrate distance on the deposition rate and uniformity of the films has been studied at varying power to the source using C2H2 gas. The films are found to be hard and they adhere well to 7059 glass, quartz, silicon, Mo, and Mylar substrates. Though the deposition rate increases with increasing power and with increasing carbon to hydrogen ratio of the hydrocarbon feedstock, the material properties are relatively independent of the type of hydroc...

Characterization studies of diamond-like carbon films grown using a saddle-field fast-atom-beam source

In this article, we report results of an extensive characterization study involving scanning electron microscopy, spectroscopic ellipsometry (SE), photothermal deflection spectroscopy (PDS), x-ray photoelectron spectroscopy, x-ray Auger electron spectroscopy (XAES), current–voltage (I–V) measurements, hydrogen content evaluated from Fourier transform infrared spectroscopy and elastic recoil detection analysis, and also measurement of stress and hardness of diamond-like carbon (DLC) films. These films were grown using methane (CH4), acetylene (C2H2) gases, and benzene (C6H6) vapors into a saddle-field fast-atom-beam (FAB) source. DLC films formed by the saddle-field FAB source technique exhibit extremely low residual stress (0.12–0.26 GPa) and high Knoop hardness (9–22 GPa) measured at 50 g load. The values of optical constants (n, k, e1, e2) evaluated from SE, characteristic energy of band tail (Urbach energy, E0) evaluated from PDS studies, sp2 percentage evaluated from XAES data, the density of states [N(EF)] derived from space-charge-limited conduction, and the hydrogen content are found to decrease, and the sp3/sp2 ratio evaluated are found to increase with the increase of carbon-to-hydrogen ratio in the hydrocarbon gases/vapors used for growing DLC films by this technique. The values of E0, N(EF), hydrogen content, and sp3/sp2 ratio of these DLC films are found to be in the range of 180–280 meV, 1–6×1017 eV−1 cm−3, 3–8 at. % and 5.2–12.3, respectively, which are lower than the values of E0 (300–500 meV), N(EF) (∼1018 eV−1 cm−3), and hydrogen content (15–40 at. %) and higher than sp3/sp2 ratio (1.3–2.5) of DLC films grown by the more conventional rf self-bias technique reported in the literature.In this article, we report results of an extensive characterization study involving scanning electron microscopy, spectroscopic ellipsometry (SE), photothermal deflection spectroscopy (PDS), x-ray photoelectron spectroscopy, x-ray Auger electron spectroscopy (XAES), current–voltage (I–V) measurements, hydrogen content evaluated from Fourier transform infrared spectroscopy and elastic recoil detection analysis, and also measurement of stress and hardness of diamond-like carbon (DLC) films. These films were grown using methane (CH4), acetylene (C2H2) gases, and benzene (C6H6) vapors into a saddle-field fast-atom-beam (FAB) source. DLC films formed by the saddle-field FAB source technique exhibit extremely low residual stress (0.12–0.26 GPa) and high Knoop hardness (9–22 GPa) measured at 50 g load. The values of optical constants (n, k, e1, e2) evaluated from SE, characteristic energy of band tail (Urbach energy, E0) evaluated from PDS studies, sp2 percentage evaluated from XAES data, the density of states [...

Diamond‐like carbon films grown by very high frequency (100 MHz) plasma enhanced chemical vapor deposition technique

Diamond‐like carbon films were grown by VHF‐PECVD technique. Since the self‐bias potential developed in a VHF plasma is very low, sufficiently high negative dc voltage was applied to the substrates in order to make DLC film being grown reasonably hard. Also a comparative study of VHF grown films was made with rf (13.56 MHz) discharge grown films (grown in the same PECVD reactor). This made it possible to investigate the specific effects of excitation fre‐ quency while keeping other parameters constant. Deposition rate (rd) was found to be about 5 times higher for VHF grown films. Marginal variation in optical band gap (Eg) and refractive index (n) were observed in VHF grown films with variation in deposition parameters. Maximum value of hardness recorded was 1500 kg/mm2 in the case of rf and 902 kg/mm2 in the case of VHF grown films, within the range of deposition parameters. Stress values were in the range 1.7×109−2.9×109 Nm−2 for VHF and 3.6×109−4.6×109 Nm−2 for rf grown films.

Characterisation of a saddle field fast atom beam source and its application to the growth of diamond-like carbon films

Abstract The use of saddle field fast atom beam (FAB) sources to grow diamond-like carbon (DLC) films is attractive due to its dc and rf attributes. This paper reports the detailed analysis of the beam coming out from the FAB source using argon (Ar), methane (CH 4 ) and acetylene (C 2 H 2 ) as source gases. Three operation modes of the FAB source are found, viz. glow discharge mode , transition mode and oscillation mode . A new process parameter, applied power, is defined for characterisation of the FAB source. Energy distribution analysis shows that the mean energy of the radicals coming out of the FAB source lies within 50% of the applied voltage to the FAB source. The neutralisation coefficient of the beam was estimated to be more than 90% and was found to be almost independent of the discharge current of the FAB source. It will be seen through this characterisation study that a completely novel technique has been found to deposit DLC films, simultaneously by neutral and ionic radicals. The mechanical and opto-electronic properties of these DLC films are discussed. The role of ion assistance is seen in hardening the films. DLC films deposited by ionic radicals show better opto-electronic properties as measured by photothermal deflection spectroscopy (PDS).

Filtered saddle field fast atom beam deposition of diamondlike carbon films

An innovation to separate out ionic and neutral radicals from the beam coming out of a saddle field fast atom beam (FAB) source is reported. Diamondlike carbon (DLC) films were grown simultaneously by these ionic and filtered neutral radicals using methane (CH4) as the source gas in the FAB source, on to the substrates placed at different positions inside the deposition chamber. Faraday cup measurements carried out on the two beams confirm the separation of ionic and neutral radicals. It was found that the DLC films deposited by neutral radicals have higher hardness values (980–1070 kg/mm2) than those deposited by ionic radicals (810–970 kg/mm2), whereas, the values of optical band gap and refractive index are found to be higher in DLC films deposited by ionic radicals than those deposited by neutral radicals. It is emphasized that during normal operation of a saddle field source, to grow DLC films, one essentially has a mixed deposition with contributions both from neutral and ionized radicals. Higher ha...

Onset of photoconduction in hydrogenated amorphous carbon films prepared by RF asymmetric PECVD technique

Abstract Hydrogenated amorphous carbon (a-C:H) films grown on glass substrates by asymmetric RF plasma CVD process exhibit onset of photoconduction under certain range of self bias values. Log σ Vs 10 3 /T studies in these films indicate that twice the activation energy equals optical band gap at high temperature. At intermediate and lower temperatures, the difference of dark and photoactivation energies is ⋟ 0.15 eV. This indicates that the material has well defined trapping centers which are changed in the presence of light in a definite way.

Diamond-like carbon films formed by a filtered fast atom beam source

Abstract A method to filter the diamond-like carbon (DLC) film forming neutral and ionic radicals coming out from a saddle-field fast atom beam (FAB) source is suggested. By flowing different hydrocarbon gases (CH 4 and C 2 H 2 ) through this modified FAB source, DLC films have been grown simultaneously, by beams of neutral and ionic radicals at different locations of the reactor. Current density measurements of these two types of beams confirm the separation of neutral and ionic radicals. Energy distribution analysis of ions shows that the mean energy of the radicals lies within the limit of 50% of the applied voltage to the FAB source. The properties of the deposited DLC films grown by CH 4 and C 2 H 2 gases are compared. It is found that DLC films grown by C 2 H 2 gas have higher hardness values than CH 4 -deposited films. In both the cases, films deposited by neutral radicals have higher hardness values than those deposited by ionic radicals. The values of Urbach energy ( E 0 ) evaluated by photothermal deflection spectroscopy (PDS) technique are found to be less in the case of C 2 H 2 -deposited films than the CH 4 -deposited films, but neutral radical-deposited films have higher values of E 0 than ionic ones, in both cases.