P.N. Dixit

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.

Effect of pulse parameters on the deposition rate of hydrogenated amorphous silicon in a modified pulsed plasma discharge

Hydrogenated amorphous silicon films were deposited from 100% silane and disilane gases by a pulsed radio‐frequency plasma chemical vapor deposition method in which a nonzero low power level was maintained. The pulse parameters were varied to study their effect on the deposition rate. It was found that the deposition rate depends both on the high power level and the dwell time. For a given high power level, the deposition rate is less than that of a continuous wave discharge up to a certain dwell time and increases beyond this value in both silane and disilane discharges. Onset of powder formation was observed beyond this crossover point.

Effect of power on the growth of nanocrystalline silicon films

Nanocrystalline silicon thin films were grown using a gaseous mixture of silane, hydrogen and argon in a plasma-enhanced chemical vapor deposition system. These films were deposited away from the conventional low power regime normally used for the deposition of device quality hydrogenated amorphous silicon films. It was observed that, with the increase of applied power, there is a change in nanocrystalline phases which were embedded in the amorphous matrix of silicon. Atomic force microscopy micrographs show that these films contain nanocrystallite of 20–100 nm size. Laser Raman and photoluminescence peaks have been observed at 514 cm−1 and 2.18 eV, respectively, and particle sizes were estimated using the same as 8.24 nm and 3.26 nm, respectively. It has also been observed that nanocrystallites in these films enhanced the optical bandgap and electrical conductivity.

Optoelectronic properties of hydrogenated amorphous silicon films grown using a modified pulsed plasma discharge

Hydrogenated amorphous silicon (a‐Si:H) films were deposited by a modified pulsed plasma decomposition of silane and disilane in which a nonzero low power level was maintained throughout the discharge. Deposition rate (rd), optical band gap (Eg), dark and photoconductivity (σD and σph), and photosensitivity (σph/σD) were investigated as a function of pulse parameters. By combining dwell time (τ) and the high power level (HPL) it has been shown that Eg can be tailored over a fairly wide range. Similarly, σD and σph/σD have been shown to depend, in addition to HPL, on τ as well, thereby proving the possibility of using τ as an additional process parameter. High band‐gap a‐Si:H material of quality (σph=4.4×10−6 Ω−1 cm−1, σph/σD≊105) comparable to that of device quality a‐Si:C:H has been deposited by this technique.

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.

High rate deposition of diamond like carbon films by very high frequency plasma enhanced chemical vapor deposition at 100 MHz

Diamond like carbon (DLC) films were grown using 13.56 and 100 MHz plasma as excitation frequencies in the same plasma enhanced chemical vapor deposition (PECVD) system. Deposition rate, stress, hardness, optical band gap, refractive index, Urbach energy, electrical conductivity, and hydrogen content of these films have been measured. It was found that just by changing the excitation frequency from 13.56 to 100 MHz, deposition rates of DLC films were enhanced about five times. Thus, very high frequency (100 MHz) PECVD process, with imposed dc bias, is capable of producing reasonably hard DLC films at high growth rates.

Correlation of residual stress with optical absorption edge in diamond-like carbon films

Abstract A correlation has been observed between the residual stress and the optical absorption edge of diamond-like carbon films and on the basis of this correlation, an empirical relation has been established between the Urbach energy ( E 0 ) and the residual stress ( S ) in the films given by E 0 = E 00 + mS , where E 00 =140 meV and m =37 meV/GPa. The residual stress and the optical absorption edges of diamond-like carbon films, grown by different techniques, are then discussed in terms of amount of disorder in the network.

Infrared studies of hard hydrogenated amorphous carbon (aC:H) film and the effect of argon plasma treatment

Abstract The hard hydrogenated amorphous carbon (aC:H) films were deposited on both sides of well polished germanium (Ge) substrate by RF plasma decomposition of benzene vapours. Infrared spectroscopy was used to measure the transmission and Sp3/Sp2 ratio of diamond-like carbon (DLC) film as a function of plasma treatments. It has been demonstrated that the partial etching of aC:H film by argon plasma treatment may be a suitable process (1) to improve the film quality and also (2) to tailor at required wavelengths the infrared transmission of aC:H coated Ge optics.

EFFECT OF HYDROGEN DILUTION ON THE DEPOSITION RATE OF HYDROGENERATED AMORPHOUS SILICON FILMS IN A MODIFIED PULSED PLASMA DISCHARGE

Hydrogen dilution effects on the growth of a‐Si:H films in a modified pulsed plasma discharge are studied for two different silane flow conditions with hydrogen dilution ranging from 0% to 80%. The increase of deposition rate (rd) due to hydrogen dilution is attributed mainly to changes in electron density (ne) and electron decay time constant (τe). Concurrently, it appears that hydrogen dilution mitigates the deleterious effects of secondary plasma reactions. Increasing the dwell time of the high power period under hydrogen dilution also increases rd, accompanied by an enhancement of photoconductivity (σph) as compared to films grown under similar conditions without dilution. The decrease in rd at dilution ≳25% is seen as the onset of a process of etching, presumably by atomic hydrogen, and this limits rd irrespective of the availability of the number of silane molecules at such dilutions. Time‐resolved optical emission spectroscopy results for H* emission supports the above views.