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The characteristics of the hydrogenated amorphous carbon (HAC) films can be determined by the composition of sp³, sp², and sp¹hybridized structures depending on the bond type of four valence electrons. The C-C bonds with sp³ and sp² correspond to diamond and graphite, respectively, and a mixture of sp³ and sp² determines the material properties. The sp³ bond shows some advantages including mechanical strength, wide band gap, and chemical and electrochemical inertness, however it is unfavorable for a strip process after etching. The HAC films by adding hydrogen to a sp³/sp² could possibly present an etching resistance. The HAC films are characterized according to the composition ratios among a sp³, sp², and hydrogen. When the hydrogen content is large, polymer-like HAC films can be formed with low film density due to high content of sp³. When the hydrogen content is small, the content of sp³ becomes low, resulting in a diamond-like HAC with high film density and hardness. When the hydrogen content becomes smaller, the content of sp² is high and a graphite-like HAC can be obtained. Depending on the deposition conditions, the sp²/sp³ hybridization ratio can vary over a wide range of values.
Flexible HAC thin films were fabricated by plasma-enhanced chemical vapor deposition (PECVD) of cyclohexane (CHex, C₆H₁₂) precursor on the substrate of indium tin oxide on polyethylene naphthalate (ITO/PEN). The sheet resistance of ITO was about 12 Ω/square. The transparency of ITO was ≥ 75 % with a haze of 3 %. The PEN substrate was DuPont Teijin film. Thicknesses of ITO and PEN layers were >180 nm and 0.125 mm, respectively. Gas molecules vaporized from the CHex precursor were carried by argon (Ar) with a purity of 99.999 % gas and delivered to the reactor. A base pressure was 0.53 Pa before deposition. The HAC films were deposited at room temperature of 25 °C with an operating pressure of 26.7 Pa. The RF plasma power with 13.56 MHz was chosen from 20 to 100 W. To characterize etching properties of the HAC films, dry etching with Ar+CF₄+O₂-based gas chemistry was conducted in an inductively coupled plasma-reactive ion etching (ICP-RIE) system. Tetraethyl orthosilicate (TEOS) oxide was used as the reference for calculating the etch rate and etch selectivity. The etch selectivity of the HAC film was calculated by t_TAC/t_TEOS, where t_TEOS and t_TAC are the thicknesses of the TEOS and HAC removed after dry etching. The thickness and extinction coefficient of the HAC films were measured by ellipsometer. Surface morphology of the films was observed by atomic force microscopy (AFM). The functional groups of the films were identified using Fourier transform infrared (FTIR) spectroscopy. The chemical composition of the films was determined by X-ray photoelectron spectroscopy (XPS). The structural arrangement of the carbon bonds was analyzed by Raman spectroscopy.
The prominent IR absorption consists of C–H stretching modes at 3300-2800 cm^-1 and C=C and C–H bending modes below 2000 cm^-1. The C–H stretching mode can be deconvoluted into three: sp¹≡C–H modes centered at 3300 cm^-1, sp²=C–H_n modes at 3085-2975 cm^-1, and sp³–C–H_n modes at 2955-2850 cm^-1. The structural transformation could occur from the sp³ to sp²depending on deposition conditions. Two carbon allotropes – diamond and graphite – can be distinguished by their Raman spectra. Typical D and G peaks were observed around 1350 and 1590 cm^-1 in the Raman spectra, respectively. The Raman spectra of disordered carbons can be classified by the intensity ratio of the D and G modes, I(D)/I(G). To quantify the chemical bonding status, the ratio R is introduced as the peak area ratios of C–C to C–H_x. The etch-durability represented as the inverse of the film loss is related to the ratio R. Etching properties were strongly affected by deposition conditions and etching parameters.

Co-Author(s): Thomas Poche (University of Louisiana at Lafayette), Rajib Chowdhury (University of Louisiana at Lafayette)

Seonhee Jang

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