Chemical and Nanomechanical Characteristics of

Journal of the Korean Physical Society, Vol. 50, No. 4, April 2007, pp. 1113∼1118

Chemical and Nanomechanical Characteristics of Fluorocarbon Thin Films Deposited by Using Plasma Enhanced Chemical Vapor Deposition Nam-Kyun Kim, Nam-Goo Cha, Kyu-Chae Kim, Tae-Gon Kim and Jin-Goo Park∗ Division of Materials and Chemical Engineering, Micro Biochip Center, Hanyang University, Ansan 426-791 (Received 8 November 2006) In this study, the chemical and nanomechanical characteristics of fluorocarbon (FC) films deposited by using plasma-enhanced chemical vapor deposition on Al were evaluated to apply them as antistiction layers. The calculated deposition rate with C4 F8 was 325 nm/min at an optimized process condition of 10 sccm, 30 W, and 340 mTorr. The contact angles of the FC thin films on Al were around 110◦ , regardless of the deposition time. The surface energies were around 14 mN/m, and the contact angle hysteresis was lower than 35◦ . FTIR-ATR (Fourier transform infrared attenuated total reflection) spectra showed the presence of fluorocarbon groups. The adhesion and friction force after FC film deposition showed lower and more stable values than those of bare Al. Depositing the FC films at powers above 50 W increased the hysteresis, the adhesion force, the roughness and the friction force. The addition of Ar resulted in a decrease in the thickness of the FC film during deposition. The deposited FC films gradually decomposed at 200 ◦ C. PACS numbers: 77.84.Jd, 81.05.Lg, 81.15.Gh Keywords: MEMS, Antistiction layer, Adhesion force, Fluorocarbon film

I. INTRODUCTION Fluorocarbon (FC) thin films have been used for anti-adhesion layers in micro electro mechanical systems (MEMS) and nanoimprint lithography (NIL) because of their low coefficient of friction, low adhesion force, and high hydrophobicity [1,2]. The sticking problem is a notorious failure mechanism in these fields, and it causes malfunctions of structures when surfaces are brought into contact, thereby resulting in the significant yield and reliability problems associated with the systems [3]. MEMS devices are inherently sensitive to adhesion because of their relatively large surface areas and small offsets from adjacent surfaces [4]. NIL creates patterns by using heat or UV radiation during mechanical deformation of the polymer. At this time, stiction occurs between the mold and the polymer [5]. An understanding of the adhesion phenomenon is important for realizing dynamic applications for the structures. It has been suggested that van der Waals forces, electrostatic forces, and the condensation of water between the structures may be responsible for adhesion [4]. Hydrophobic film coating of the surface is effective in creating a hydrophobic and low energy surface that greatly reduces adhesion. These films have been commonly prepared by using a spin-on ∗ E-mail:

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method [6], a liquid self-assembled monolayer (SAM) [7], and a chemical vapor deposition (CVD) [8]. The spinon method allows easy deposition of thin films without any special tools, but cannot be applied to patterned structures. The liquid SAM method is relatively cheap, but cannot be used for coatings on nanostructures or large-area samples because it has a limited surface tension and low reproducibility. Also, considering the industrial applications, liquid SAM is not a suitable process due to its complexity, high cost, and environmental issues [9]. On the other side, a vapor deposition method like PECVD (plasma-enhanced chemical vapor deposition) can be applied to nanofabricated or complex superstructures because it can be used to coat any kind of material with thickness control, is compatible with semiconductor processes, and is more environmental benign because it does not use organic chemicals [10, 11]. The objective of this study was to characterize the chemical and the nanomechanical properties of FC polymer films deposited as anti-adhesion layers by using PECVD.

II. EXPERIMENTS 1. Sample Preparation and Procedure

Aluminum-coated wafers (300 nm in thickness) were prepared by using the sputtering technique. These -1113-

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Journal of the Korean Physical Society, Vol. 50, No. 4, April 2007

Fig. 1. Schematic diagram of the capacitance-type PECVD system.

wafers were cut into 20 × 15 mm2 rectangular shapes for the experiments, and the samples were kept in the oven for 6 hours to remove the moisture. The Al surface was cleaned by using an O2 plasma at an RF (radiofrequency) power of 200 W, an O2 flow rate of 30 sccm, and a chamber pressures of 200 mTorr for 3 minutes, followed by Ar plasma treatment at the same condition. C4 F8 (octafluorocyclobutane) was used as the precursor gas for the deposition of the FC thin films [12]. Figure 1 shows a schematic illustration of the PECVD reactor (FC-CVD, Sorona, Korea) for this experiment. The FC films were annealed in air by using a conventional thermal oven.

Fig. 2. FC film thickness as a function of the deposition time.

Chemical analysis of the FC layer was achieved by FTIR-ATR (Fourier-transform infrared attenuated total reflection, FTS6000, Bio-Rad, USA). Atomic and lateral force microscopy (AFM/LFM, CP research, Veeco, USA) was applied to measure the adhesion and the friction force between the AFM tip and the surfaces. A Si3 N4 tip with a force constant of 0.01 N/m was used for the AFM/LFM studies. All AFM/LFM measurements were done in a class-100 cleanroom at a constant 50 % relative humidity. Topography and friction images were obtained simultaneously across the sample surface by monitoring both the vertical and the lateral tip movements at a given load.

2. Characterization Methods

The static and dynamic contact angles were measured on the surfaces by using a contact angle analyzer (G10, Kr¨ uss, Germany). The surface energy was calculated from the contact angle results for DI (de-ionized) water and diiodomethane (CH2 I2 ) based on the geometric mean theory of Owens-Wendt [13]. Advancing and receding angles were measured by using a captive drop method with DI water on FC films. The difference between the advancing and the receding angles, defined as the contact angel hysteresis (∆H), may indicate the magnitude of surface heterogeneity. Variable-angle spectroscopic ellipsometry (VASE, J. A. Woollam, USA) with a rotating analyzer was used to measure the film’s thickness. The dynamic measurements were carried out at incidence angles from 70 to 75◦ and at wavelengths of light from 300 nm to 900 nm to maximize the sensitivity of the measurements. Optical modeling and data analysis were done by using a Cauchy model [14]. Field-emission secondary electron microscopy (FE-SEM, JSM7000F, Jeol, Japan) was used to confirm the thicknesses of the FC films as obtained from the VASE.

III. RESULTS AND DISCUSSION Figure 2 shows the film thickness as a function of the process time. The depositions of the FC films were carried out at an optimized condition of a 10 sccm flow of C4 F8 , a 30 W RF power and a 340 mTorr chamber pressure. The film thickness linearly increased with the deposition time. The calculated growth rate was 325 nm/min. Figure 3 shows the cross-sectional images of FC films grown on Al at different process times. Figure 3(a) shows the cross-sectional image of a 300-nm-thick Al wafer without a FC film. A 600-nm thickness of the FC film was achieved after a 5-minute deposition (Figure 3(b)), and an 1800-nm thickness of the FC film was achieved after a 15- minute deposition (Figure 3(c)). The contact angle analysis is a simple, but very powerful method for measuring monolayer changes in a surface. Figure 4 shows the contact angle, the contact angle hysteresis, and the surface energy of FC films deposited on Al. Not much change was found as a function of the process time. The DI water contact angles of FC films on

Chemical and Nanomechanical Characteristics of Fluorocarbon Thin Films· · · – Nam-Kyun Kim et al.

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Fig. 3. FESEM cross-sectional images of (a) a bare Al film, and of FC films deposited (b) for 5 minutes and (c) for 10 minutes.

Fig. 4. (a) Contact angle, (b) contact angle hysteresis, and (c) surface energy on deposited FC films as functions of time at a C4 F8 flow rate of 10 sccm, a plasma power of 30 W, and a chamber pressure of 0.34 Torr.

Al were around 110◦ (Figure 4(a)). It is well known that contact angle hysteresis (∆H) is a function of the surface roughness, surface polarity, heterogeneity, and molecular re-arrangement of functional groups in the interfacial region. Although the hydrophobicity of the FC films was excellent, a large hysteresis might indicate a poor coverage and a poor homogeneity of the films on the substrates [15]. The contact angle hysteresis was measured to be below 35◦ (Figure 4(b)). The surface energy of the FC films was calculated to be around 14 mN/m (Figure 4(c)). The contact angle and the surface energy were very similar or lower than those of bulk PTFE (polytetrafluoroethylene), which are known to be 108◦ and 18

mN/m, respectively [16]. In order to study the chemical composition of the FC films deposited by using PECVD, we obtained and analyzed FTIR-ATR spectra. Figure 5 shows the FTIR-ATR spectra of the FC films for various deposition times. A strong absorbance was measured at around 1200 cm−1 , which had been reported to be due to the symmetric stretch of CF2 group. Higher adsorption was observed as the deposition time increased. A relatively strong adsorption was observed near 1740 cm−1 , which represents the =C=CF2 group. Also, a very weak evolution of the adsorption at near 740 cm−1 was observed and is known to be due to the amorphous band of PTFE [17,18].

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Fig. 5. FTIR-ATR spectra of FC films for various deposition times.

Fig. 8. (a) Hysteresis and (b) thickness as functions of the RF power. Fig. 6. Adhesion force and roughness obtained by using AFM as functions of time.

Fig. 7. Friction force obtained by using LFM as a function of time.

Figure 6 shows the adhesion force using a Si3 N4 AFM tip as a function of the deposition time at a 30-W plasma power, a 10-sccm C4 F8 flow rate, and a 340-mTorr work-

ing pressure. The original adhesion force of bare Al was measured to be around 9 nN. The presence of the FC films on Al reduced it to around 4 nN. The surface roughness increased slightly with the film’s thickness. Figure 7 shows the temporal changes in the friction force measured in mV by using LFM mode. The friction force of bare Al was measured to be around 140 mV. After the depositions of the FC films, it decreased to almost onethird the friction force (∼50 mV) of Al. The adhesion and the friction force were reduced by FC film deposition and were not affected by the deposition time. Figure 8 shows the contact angle and its hysteresis for FC films as functions of the input plasma power for 3minute deposition time. The RF power of plasma was changed from 30 to 100 W. Slight changes in the contact angle were observed with increasing of plasma power (Figure 8(a)). However, the contact angle hysteresis increased from 30 to 50◦ , which might indicate poor surface coverage by FC molecules (Figure 8(b)). The contact angle, the surface roughness (Figure 9(a)) and the friction force (Figure 9(b)) drastically increased at plasma power over 50 W. The C4 F8 plasma at high powers may act as an etching agent and attack the deposited surface [19]. The deposited thickness was measured as a function

Chemical and Nanomechanical Characteristics of Fluorocarbon Thin Films· · · – Nam-Kyun Kim et al.

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Fig. 9. (a) Adhesion force and surface roughness, and (b) friction force as functions of the RF power.

Fig. 11. (a) Contact angle, (b) surface energy, and (c) hysteresis as functions of the annealing time in air at temperature of 100 and 200 ◦ C.

Fig. 10. Deposited FC film thickness as a function of the C4 F8 to Ar ratio.

of the gas ratio of Ar to C4 F8 by using VASE (Figure 10). The deposition conditions were a 10-sccm C4 F8 flow rate, and a 30-W plasma power for 3 minutes. The film thickness decreased with increasing gas ratio of C4 F8 to Ar. The addition of a small amount of Ar increases the

plasma stability at low plasma power without affecting the film’s deposition rate, but adding too much of Ar might decrease FC formation in the plasma and result in a lower film thickness [20]. FC films with 600-nm thicknesses were left in air at temperatures of 100 ◦ C and 200 ◦ C in order to observe the changes in the film properties (Figure 11). The contact angles did not change much with annealing time changed at 100 ◦ C but gradually decreased at 200 ◦ C

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(Figure 11(a)). Also, the surface energy showed almost the same values at 100 ◦ C but gradually increased with time at 200 ◦ C (Figure 11(b)). The contact angle hysteresis showed almost the same values regardless of annealing time (Figure 11(c)). The FC films deposited by using PECVD gradually decomposed at 200 ◦ C.

IV. CONCLUSION FC thin films deposited on Al by using PECVD were evaluated by using chemical and nanomechanical analyses. The cleaning of Al surface was performed in an O2 and Ar plasma at an RF power of 200 W for 3 minutes. C4 F8 were used as a precursor for depositing the FC films. The calculated deposition rate was 325 nm/min for the process conditions of a 10-sccm C4 F8 flow rate, a 30-W plasma power, and a 340-mTorr chamber pressure. The contact angles of the FC thin films on Al were around 110◦ , regardless of the deposition time and film’s thickness. The surface energy was calculated to be around 14 mN/m, and the contact angle hysteresis, which is an indicator of the surface heterogeneity, was lower than 35◦ . FTIR-ATR spectra showed the presence of CF2 , =C=CF peaks, like PTFE. The presence of the FC films decreased the adhesion force from 9 to 4 nN. The friction force of the films decreased around 3 times lower after the deposition. Plasma powers above 50 W increased the hysteresis, the adhesion force, the roughness, and the friction force. The deposited thickness gradually decreased with increasing gas ratio of Ar to C4 F8 . The deposited FC films gradually decomposed at 200 ◦ C.

ACKNOWLEDGMENTS This research was supported by a grant (code#: 06K1401-00214) from the Center for Nanoscale Mechatronics and Manufacturing (CNMM) under the 21st Century Frontier R&D Programs of the Korean Ministry of Science and Technology and by the fostering project of the Lab of Excellence in the Ministry of Education and Human Resources Development (MOE), the Ministry of Commerce, Industry and Energy (MOCIE), and

the Ministry of Labor (MOLAB), and by the post BK21 program.

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