The influence of annealing on fluorene-type thin film produced by biphenyl and methane RF plasma system

Fluorene (C
₁₃
H
₁₀
)-type thin films were produced under the capacitively coupled single radio frequency (rf) system by using the biphenyl (C
₁₂
H
₁₀
)/methane (CH
₄
) plasma. The circular parallel plate electrodes were used; the upper electrode was connected to 13.56 MHz rf power while the lower electrode was grounded. To control the positive ion bombardment, a negative bias voltage was applied between the holder of the substrate and the wall of the chamber. The properties of the fluorene-type films were explored under the variable plasma parameters of input rf power (150, 200 W), CH
₄
flow rate (3, 5 sccm), negative bias voltage (-66 V), and deposition time (20, 30 min) at a constant pressure value of 0.3 mbar. The deposited films were annealed for 1 hour at different temperatures; 200, 300, 400, and 500 C̊. The changes of plasma chemistry during the deposition were investigated by optical emission spectroscopy (OES). The chemical structural properties were examined by fourier transform infrared (FTIR) spectroscopy. After annealing the sp
³
/sp
²
ratio showed a decrease with increasing temperature due to the degassing of CH
_x
from the surface and hydrogen release with temperature
¹
. The reconfiguration of chemical bonds in the film structure with annealing caused the changes in the intensities of band peaks
²
. Additionally, the annealing increased the amount of sp
¹
carbon bonding (carbyne) in the deposited film and led to decrease in the degree of cross-linking. The morphology of the films was analyzed by X-ray diffraction (XRD) and atomic force microscope (AFM). The crystallography of thin film was effectively changed at high rf power and the annealing caused a shifting in diffraction peaks. An increase was observed in crystalline size of the films after the annealing. The surface of thin film indicated significant nanostructures and peaks at high rf power and CH
₄
flow rate. After annealing, the intensity of peaks showed a decrease as a result of the decomposition and compaction of films as well as the reformation of the amorphous covalent network that is occurring smoother surfaces
^2,3
. The thickness was measured by ellipsometer, and since the annealing temperature caused to an effective thermal decay and a disruption of microvoids in the films
²
, the thickness of thin films had a decrease as increased the temperature. The contact angle measurement was used for the determination of hydrophobic behavior of the thin films.