5-1 Introduction
Fluorinated amorphous carbon (a-C:F) films were deposited by RF bias assisted microwave plasma electron cyclotron resonance chemical vapor deposition ( ECR-CVD ) with tetrafluoromethane (CF4) and acetylene (C2H2) as precursors. The deposition process was performed at flow ratios from R = 0.90 and R = 0.98, where R = CF4/(CF4+C2H2). The substrate temperature after finishing deposition was around 50oC, according to the thermal couple placed under the substrate. The thickness of as-deposited films ranged from ~ 400 Å (for R = 0.98) to ~ 700 Å (for R
= 0.90). Subsequent annealing treatments of the deposited a-C:F films were carried out at 300oC for 30 min in pure nitrogen atmosphere. The higher flow ratio R led to lower deposition rate, and the hardness of a-C:F films decreased with uprising flow ratio R. The radicals of plasma were detected by OES equipment. AFM, nano-indenator and TDS were used to observe the morphology and thermal stability of a-C:F films, and FESEM was used to measure the film thickness in as-deposited condition and after annealing.
5-2 Result and Discussion
The plasma composition depends on various chemical pathways in the plasma, which again depends on the plasma parameters such as microwave power, RF power, electron temperature, electron density, gas
flow rate, and degree of ionization. To illustrate how these collisions could result in the radical production, Table 5-1 shows a set of reactions which is the possible sheath radical production in the C2H2 and CF4 plasma1-5. The flow rate of the source gases were kept at flow ratio R (R = CF4/[(CF4+C2H2)]), 0.90, 0.95, 0.97 and 0.98. Figure 5-1 shows the optical emission spectra (OES) of the ECR excitation plasma at R = 0.90 and 0.97.
There were C2, C3, CF2, CH, F2, H2 and HF radical species, and C2-, F2+ and HF+ ionic species in the plasma6. The C2 radicals will construct the main skeleton of the a-C:F films, and the fluorine atoms will be replaced by hydrocarbons to form fluorocarbon bonds or HF bonds. Figure 5-2shows CF and CF2 species found in the plasma with the spectra for the wavelength range 200-300 nm. The F2, F2+, CF, CF2 and CF3 radicals are both sinks and sources7 at the same time during the deposition of the a-C:F films.
Table 5-1. Primary electron collisions
200 300 400 500 600 700 800
Figure 5-1. Optical emission spectra obtained in C2H2 and CF4 mixed gas discharge at 600W source power -200V rf bias, and 20 m Torr in the ECR-CVD.
200 220 240 260 280 300
CF2
Figure 5-2 Comparisons of the plasma optical emission spectra for R = 0.97 and R = 0.90.
HRSEM was used to measure the thickness of a-C:F films. The behavior of the deposition rate as a function of the flow ratio of the CF4/C2H2 gas mixture is presented in Figure 5-3. A steep decline in the deposition rate from 56 to about 3 nm/min was observed as the flow ratio increased from ~ 0.90 to ~0.98. This indicates that the etching effect of F radicals strengthens rapidly in the narrow region. The thermal desorption of the fragments in the a-C:F films is one of the major factors causing film shrinkage. The film thickness loss is proportional to the flow ratio R (Fig.
5-4) in a fashion similar to that of F concentration in the films, a result consistent with the lower thermal stability for the films of higher F content.
Besides outgassing effect, the concurrent film structure relaxation during heat treatment may also induce shrinkage. The evolution of film microstructures, in particular, the voids observed in the films of high F content, would contribute to the change of thickness. Since the higher fluorine concentration a-C:F film has more sp3 bounds in the film, it results in thermal instability at high temperature. The TDA result of R=0.90 a-C:F film is shown in Figure 5-5. No hydrocarbon molecular signals were found in the TDA result. There were only CF, CF2 and CF3 gases which terminated the carbon skeleton in the films released from the film during heating process.
Figure 5-3 shows the deposition rate of a-C:F films.
Figure 5-4. The shrinkage of film thickness is negatively proportional to flow ratio R
0.88 0.90 0.92 0.94 0.96 0.98 1.00
10 20 30 40 50 60
nm/min
R
3 .2 3 .4 3 .6 3 .8 4 .0
-2 4 -2 1 -1 8 -1 5
Film thickness change ∆d/d as dep.(%)
G a s m ix ture F /C ra tio
Figure 5-5 shows the TDA result of R = 0.90 a-C:F film. 31, 50 and 69 mean the molecule weight of CF, CF2 and CF3.
The topography of a-C:F films can be observed by AFM, as shown in Figure 5-6, and the surface roughness (root mean square) of the a-C:F films prepared by ECR-CVD is less than 0.8 nm, which is good for mechanical and electric applications. The relevant data are listed in table 5-2. The hardness of a-C:F films is also shown in Figure 5-7, and the a-C:F films is softer than copper film (~ 1 GPa). The higher flow ratio generates more linear fluorocarbon bonds, which will be annihilated after annealing. In contrast, the cross-linking bonds will be increased when the flow ratio is reduced. Consequently, the hardness of the a-C:F film depends on the fluorine content and the thermal treatment. Therefore the higher CF4 flow ratio (R=0.98) produced more sp3 linear structure, which made the a-C:F films smoother and softer.
(a) (b)
5 nm
Figure 5-6 shows the AFM images of R = 0.90 a-C:F films. (a) is as-deposited, and (b) is after annealing.
Table 5-2. The hardness of a-C:F films in as-deposited and after annealed conditions.
Roughness (nm)
R = 0.90 as-deposited 0.78 R = 0.90 after annealed 0.79 R = 0.95 as-deposited 0.60 R = 0.95 after annealed 0.76 R = 0.97 as-deposited 0.43 R = 0.97 after annealed 0.47 R = 0.98 as-deposited 0.42 R = 0.98 after annealed 0.47
Figure 5-7 shows the hardness of the a-C:F films in as-deposited and after annealed condtions.
5-3 Summary
The a-C:F films of high fluorine content were obtained by the ECR-CVD method with tetrafluoromethane (CF4) and acetylane (C2H2) as precursor gases. The ECR-CVD can get flat a-C:F film, and it is important to evaluate the dielectric constant of a-C:F films. The mechanical properties of a-C:F film depend on the plasma precursors. The fluorine concentration of a-C:F film increases with flow ratio R, and the higher flow ratio will form more linear fluorocarbon bounds in the film. As a result, at a higher flow ratio, the a-C:F films will have smoother and softer structure.
The fluorocarbon molecules of a-C:F films will be released, starting at 150
℃, and the a-C:F films will be entirely destroyed around 1000℃. With fluorine atoms replaced by hydrocarbons to form fluorocarbon bonds or HF bonds, no hydrocarbon molecular signals will be found in TDA results.
0.90 0.92 0.94 0.96 0.98
Reference
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