We studied on the thermal annealing effect on the residual stress and the mechanical properties in thin compressive stressed diamond-like carbon film on Si ...
Mat. Res. Soc. Symp. Proc. Vol. 795 © 2004 Materials Research Society
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The Thermal Annealing Effect On The Residual Stress And Interface Adhesion In The Compressive Stressed DLC Film.
Heon Woong Choi1, Myoung-Woon Moon1, Tae-Young Kim1,2, Kwang-Ryeol Lee2, and Kyu Hwan Oh1, 1 School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea 2 Thin Film Technology Research Center, Korea Institute of Science and Technology, P.O.Box 131, Cheongryang, Seoul 136-791, Korea
ABSTRACT
We studied on the thermal annealing effect on the residual stress and the mechanical properties in thin compressive stressed diamond-like carbon film on Si substrate. Annealing experiments were carried out with Rapid Thermal Procedure system at 200-600 °C, and the stress change with annealing temperature was investigated by in-situ stress measurement system. The apparent stress reduction occurred with minimal structure changes. In order to measure the change of chemical structure of diamond-like carbon film by annealing, we used Raman spectrometer. The adhesion deterioration in interface has been detected as annealing temperature increased. In the compressive stressed DLC film, we observed the dramatic evolution of interface delamination at certain high temperature using in-situ heating stage built in Environment SEM. The quantitative change of adhesion affected by annealing process was also measured with scratch testing. For exploring the interface structure affected by the thermal annealing process at high temperature, the cross section of annealed film has been observed with HR TEM. Keywords: Thermal annealing, Adhesion, Residual stress
INTRODUCTION
Diamond-like carbon (DLC) film has been a candidate for various applications due to its excellent tribological and mechanical properties. The extraordinary combination of high hardness, optical transparency, low coefficient of friction, chemical inertness and high electrical resistivity have stimulated for various applications. However, poor adhesion on most of the substrates limits the practical applications of the DLC films. This poor adhesion is mainly owing to high intrinsic compressive stress which prevents the growth of thick films. High residual compressive stress of the films will be a weak point of DLC films if the DLC films are being employed as wearresistant protective coatings for electronic components, hard disk and machining drills. Therefore it is extremely important to reduce residual stress in the compressive stressed DLC films. Numerous researches suggested that multilayer [1], incorporating metals [2], silicon [3] or thermal annealing [4-7] are practical method to improve the film property. But, metal incorporation has drawbacks such as removing the optical transparency of film. And in case of doped DLC such as Ni, W and Ti also has an improvement of film’s property but also reduction
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Fig. 1. The observation of delamination between the substrate and film during annealing under in-situ. (a) at 400°C (b) at 406°C (c) at 409°C (d) at 410°C (e) at 412°C (f) at 415°C in coating wear resistance [8, 9]. In recent times, Ferrari etc all [6] observed that post deposition annealing of ta-C provides a complete stress relief of stress at 600 °C and Friedman reported that complete stress relief of ta-C films with high-quality diamond-like properties could be achieved by a short-term anneal after a 2min anneal at 600 °C[4]. Grill reported that complete stress relaxation of the films was achieved after a 4h anneal at 440°C [10]. However, the mechanisms that improve thin film properties are still unclear. In this paper, we studied on the relationship between thermal annealing effect on the residual stress and the mechanical properties in thin compressive stressed diamond-like carbon film using a variety of measurement techniques such as home built laser curvature diffraction and Raman spectroscopy. Furthermore we tried to find the adhesion behaviors during annealing process using scratch test. And Environment SEM was also used to observe the evolution of interface delamination using in-situ heating stage. Accordingly we observed the interfacial delamination such a buckling and blister during in-situ annealing processing as shown in Fig1.
EXPERIMENTAL METHOD
Films were deposited using a radio frequency plasma-assisted chemical vapor deposition (PACVD) technique [11-12]. Prior to deposition, the chamber was evacuated to a pressure of less than 1.4 × 10-5 Pa. Deposition was made a thickness of 500µm on Si which had been ultrasonically cleaned in ethanol. Just before deposition, in order to remove the negative oxylayer on the Si substrate was rf sputter etched using Ar process gas for 1 min at a bias voltage of -400V. With no interruption to the plasma, Ar was replaced by CH4 at a flow rate of approximately 10 sccm and held for 2 min. As a results of that processing, We produced a 140 nm of thickness a-c:H film which is measured by Atomic Force Measurement. Annealing procedure was carried out in Rapid Thermal Procedure system to investigate the effect of heat treatment on interface adhesion and mechanical properties. We used a Silicon
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substrate to minimize the thermal stress and the possibility of interfacial oxidation [13] during annealing procedure. First, the as-deposited film was measured its original stress, adhesion, surface morphology and Raman spectrum. After inserting the specimen to the furnace of RTP, the furnace was sealed at 5m Torr pressure in vacuum and consequently, heated for 10 min at the annealing temperatures from 200 to 600 °C. A temperature periodic time for increasing annealing temperature is 1 min and it also applied 1 min for decreasing annealing temperature, respectively. A second series of annealing experiments was thermal cyclic annealing and followed same annealing conditions except maintaining anneal temperature. We investigated two heating cycle. One is 10minutes and the other is 60minutes. The samples were then allowed to cool fully before opening to air, to avoid oxidation. The films were characterized in terms of their intrinsic stress. A micro-Raman spectroscope with a Spectra Physics Ar-ion laser ( France Jobin- Yvon 64000) was used to characterize the coating structure. The laser output power was in the range of 5W, and the laser beam was focused on the sample surface using an optical microscope with a magnification of 100 × (laser spot size ~2µm). By using a home built radius of curvature technique, the relative change in the stress was calculated. The intrinsic compressive stress in the film ( σs ) is computed by means of Stoney’s equation [14], given by:
Es t s2 1 1 σs = 6(1 - V s ) t c R R 0 Where Es, Vs. and ts are the Young’s modulus. Poisson ratio and thickness of the substrate, and tc is the thickness of film. R and R0 is the radius of curvatures of the film-substrate composite and bare substrate, respectively, and are measured using a surface profilometer. In order to obtain the thickness of film, a line is marked on the sample surface with a marker before it is placed in the deposition chamber and the sample is deposited. Subsequently, the marker line is erased with alcohol and the AFM is used to measure across the boundary of the coating and uncoated surfaces of the un-etched portion of the sample. The mechanical properties of annealed films such as Young’s modulus and hardness were calculated by using a Nano-indenter ( TriboscopeⅡ). We observed the dramatic evolution of interface delamination using in-situ heating stage built in Environment SEM (FEI XL-30 FEG / PHILIPS) which has resolution analysis ability of 2nm and 40 - 500,000× range of magnification. First we placed the specimen on holder. And then, the chamber was sealed and evacuated to a pressure of 1 Torr under H2. After that we set up the increasing temperature at 500°C and applied increasing temperature time is 1minute and maintaining time is 10 minutes. Scratch adhesion testing was performed using a commercial automatic scratch tester-CSEM equipped with a Rockwell C diamond stylus (cone apex angle 120°, tip radius 500 µm). The failure modes of the coatings in the tests were examined by means of optical reflection microscopy. Scratches were made under linearly increasing load. The linearly increasing load is applied from 0 to 30N with the horizontal speed of the sample table of 10 mm min-1. During the scratch, the forces both tangential and normal to the surface were recorded automatically and the critical loads which occurred to fail of coating [15] were detected very precisely by means of an acoustic sensor attached to the indenter holder, the frictional force and by optical microscopy. All
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scratches were performed at ambient temperature. We also investigated the interface structure affected by the thermal annealing process at high temperature using HR TEM.
RESULTS
We used to Raman spectroscopy, HR TEM and a home built radius of curvature technique in order to study the relationship between the stress change and structural change. Fig.2 shows the before and after residual stress reduction ratio of annealed film as a function of the annealing time. As increasing annealing temperature, the residual stress values are reduced. A slight decrease of the stress value in DLC films observed from 0°C to 200°C. And then, the stress decreased significantly with increasing annealing temperature over 200 °C. Fig.3 shows the Raman spectra of annealed DLC films at a various temperatures in air. In the Raman spectrum of a-c:H film, a D peak appears in the 1350 cm-1 region due to disordering of ordered graphite structure [10,16]. A G peak appears in the 1580 cm-1 region of Raman shift owing to graphite [17] .Here D means disordered and G means graphite. In the DLC films, the typical broad asymmetric peaks appeared in the Raman spectra and its shape was not significantly changed until annealing at 400°C. However, the observable separation of the D and G peaks appeared at 500 °C. And then the D and G peaks were apparently separated with increasing of the annealing temperature. Fig.4 shows the stress reduction by thermal cyclic annealing at 200°C and 300°C with 10 minutes and 60minutes (cycle time). We observed that the short annealing cycle time is more effective than long annealing cycle time at same temperature. The major part of the stress relief is obtained in an interval time less then 2 hour and the stress value maintains an almost constant level in following annealing steps. Fig.5 shows the intensity ratio of the D and G peaks ( ID / IG ) in Gaussian analysis of the Raman spectra for the annealed DLC films. The ID / IG ratio in the DLC films started to increase significantly at 200°C. In addition to, the rapidly stress reduction was also observed at the same temperature. Such an increase of ID / IG ratio in the DLC films means that the increase of sp2 – bonded clusters in the films, indicating the increase of graphite micro crystallites in its volume [10,17]. 1.2 1.2 0
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Fig2. The stress reduction ratio of Fig3. The Raman results of annealed film vs annealing temperature. annealed film. (σ b: stress of before annealing , (a) As-deposited, (b) 600 °C. σ af : stress of after annealing )
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Fig4. The stress reduction by thermal cycle annealing from 200°C to 300°C with holding temperature for 10min or 60minutes.
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The G-band position also changes monotonously with annealing temperature. Both of ID / IG and G-peak position shift follows the general tendency of annealed DLC films [17]. Therefore, it is assumed that not only the graphitization of the films but also stress reduction rapidly starts from 200 °C in the DLC films. In order to investigate the relationship between the adhesion strength and annealing time, we applied to scratch testing, thermal annealing under insitu and HR TEM. Fig.1 shows the delamination under in-situ during annealing processing. Delamination was observed for samples on glass only for temperatures higher than 400 °C under in-situ as an evidence of adhesion deterioration. We furthermore observed that the circular type of buckling on annealing DLC surface is starting to develop at 400°C and then it gradually developed its growth as a telephone-cord type of buckling from 401 to 412 °C. It may be due to nucleation of Ar gas bubble at interface [13]. After that it enlarged its shape. We also observed rapidly growth of buckling on annealed strip at 400 °C. These indicate that adhesion property of annealed film were became deteriorated from 400 °C in spite of stress reduction during annealing processing. Fig.6 shows the optical micrograph of a scratch by nano-scratch testing. The scratch trace is initially smooth and shallow, then progressively broadens and deepens with increasing load. And also length of scratch is increased with increasing annealing temperature. In addition, the critical load of annealed films which occurred to fail of coating decreased with the increasing annealing temperature from 16.804N to 9.575N. It means that it clearly deteriorated its adhesion strength with increasing temperature. Large amounts of films have been pulled off, and the trace resembles a string of beads: this may result from the high brightness and intrinsic stress of the films [18]. The hardness and Young’s modulus of annealed a-c:H film at 200, 300, 400, 500 and 600 °C on Si are given in Fig.7 as a function of annealing temperature. Both of hardness and Young’s modulus are gradually decreased with increasing annealing temperature and abruptly decreased at 600 °C. Also surface morphology, stress reduction and adhesion are rapidly reduced at 600°C. It is evident from Fig7 that the post-heating temperature strongly influences the mechanical properties of the films. The Films which are as-deposited and annealed at 500°C still display high hardness and modulus value [19]. 10
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Fig 5. The Raman spectra intensity of I (D) / I(G) ratio vs annealing temperature.
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Fig 7. (a) Hardness (b) Elastic modulus Fig 6. The optical micrograph of a of annealed a-c:H film on a Si as a scratch created on annealed a-c:H film. function of annealing temperature.
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CONCLUSION
We observed not only stress reduction but also adhesion reduction of a-c:H film with increasing annealing temperature by thermal annealing. As increasing annealing temperature, The ID / IG ratio in the DLC films increase significantly from 200°C to 600°C which indicates the increasing of sp2-bonded clusters in the films. The stress reduction of annealed a-c:H films was observed from 200°C to 600°C. And the length and depth of scratch mark gradually are widen and enlarged with increasing annealing temperature. The critical loads of our coatings reduced with increasing temperature as an evidence of adhesion deterioration. However thermal annealed DLC films maintained their mechanical properties until 500°C such an Elastic modulus and hardness. A stress relief of a-c:H films can be achieved by thermal annealing until 500°C. Over 500°C, we observed graphitic property in the annealed a-c:H films. We also observed that the major part of the stress relief is obtained in an interval time less then 2 hour and the short cycle time is effective than long cycle time at same annealing temperature.
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