Substrate geometry effect on the uniformity of amorphous carbon films deposited by unbalanced magnetron sputtering

Substrate geometry effect on the uniformity of amorphous carbon films deposited by unbalanced magnetron sputtering

Thin Solid Films 461 (2004) 282 – 287 www.elsevier.com/locate/tsf Substrate geometry effect on the uniformity of amorphous carbon films deposited by ...

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Thin Solid Films 461 (2004) 282 – 287 www.elsevier.com/locate/tsf

Substrate geometry effect on the uniformity of amorphous carbon films deposited by unbalanced magnetron sputtering Xing-zhao Ding *, X.T. Zeng, Z.Q. Hu Surface Technology Group, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, Singapore 638075, Singapore Received 7 May 2003; received in revised form 5 February 2004; accepted 5 February 2004 Available online

Abstract A chromium doped amorphous carbon (a-C) film was deposited by an unbalanced magnetron sputtering. A special designed double-V shaped stainless steel model in simulating a plastic injection mold gateway was used as the substrate to investigate the geometric effect on the uniformity of the film. It was found that, on both the side wall and bottom plane of the double-V shaped substrate, the film properties strongly depended on a geometric parameter, geometric aspect ratio, defined as the depth over width of the simulated gateway at the points under measurement. With the increase of the aspect ratio, i.e. approaching to the narrow end and/or closer to the bottom plane of the gateway, the film thickness and hardness decreased and the intensity ratio of the Raman sub-bands D over G increased. With the increase of the aspect ratio, the micro hardness of the a-C film decreased far more significantly on the side wall than that on the bottom plane. With increasing working gas pressure, the film thickness decreased consistently, and the hardness uniformity on both the side wall and bottom plane was improved. When the substrate negative bias voltage was changed from 70 to 100 V, the film uniformity (for both the thickness and hardness) was improved on the bottom plane, but degraded on the side wall. D 2004 Elsevier B.V. All rights reserved. Keywords: Geometry effect; Uniformity; Unbalanced magnetron sputtering; Amorphous carbon film

1. Introduction Amorphous carbon films were extensively studied because of their excellent diamond-like properties and wide applications in microelectronics, optics, magnetic media, wear components and tools industries [1– 4]. The methods used to produce such films include physical vapor deposition and chemical vapor deposition, or their combination. Among them magnetron sputtering has been extensively explored because this method has been widely established in industry due to its advantage of easy handling and scalability. It was once commonly assumed that the unique properties of the diamond-like carbon films are derived from their high sp3 fraction. More recently, however, it has been shown that the sputtered amorphous carbon films with sp2-bonding dominated structure (graphite-like) also exhibited super hardness, excellent tribological properties, and high loadbearing capacity [5– 8]. These outstanding properties are

* Corresponding author. Tel.: +65-6793-8528; fax: +65-6792-2779. E-mail address: [email protected] (X. Ding). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.02.014

particularly required for high loading wear applications. The high hardness (up to 55 GPa) and elasticity (elasticrecovery of 85%) of these graphite-like films were attributed to the interlinking of sp2-bonded graphite-like planes with sp3 bonds [8,9]. Structural investigation indicated that they are composed of a highly compressed and dense sp2 network with reduced distance between the (002) graphite planes. In deposition of hard amorphous carbon films, either diamond-like or graphite-like in structure, appropriate ion bombardment on the growing film surface is required to achieve high film density and good mechanical properties. A negative bias voltage is normally applied (or induced when a radio frequency power is used) to the substrate to introduce such ion bombardment. Too low substrate bias cannot provide enough energy for dense film growth, while too high substrate bias will induce a high level of residual compressive stress in the films, which often causes film fracture or delamination failure particularly in high contact loading applications. Recently, it has been reported that carbon composite coatings incorporating other elements such as nitrogen, boron, silicon, fluorine, and some metals could reduce the internal stresses, thus improve the adhesion

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and mechanical and tribological properties [10 – 13]. Our previous work on magnetron sputtered hydrogenated and hydrogen-free amorphous carbon films also demonstrated that slightly doping chromium into the carbon composite film could effectively reduce the residual stress while maintaining the high hardness and excellent wear resistance [14,15]. Another challenge in practical applications comes from the fact that the actual workpieces to be coated such as molds, gears and cutting tools have various shapes and geometry, which will inevitably affect the electrical field and plasma distribution in the vicinity of the sample surface during deposition [16 –18]. As a consequence, the geometry of the workpieces controls the kinetics of the deposition process and influences the structure and properties of the film due to the non-uniform ion bombardment on the sample surface. This was clearly demonstrated in our recent experiments by applying sputtered amorphous carbon films as a protective coating to plastic injection molds for electronic packaging applications. The film on the mold injection gateways, through which the epoxy molding compounds (EMC) were injected into the mold cavities to encapsulate the IC-chips, had a much shorter lifetime compared with the other parts of the mold. As a consequence, the EMC would incline to stick on the surface of the gateways and cause blockage at the gateways, resulting in incomplete fill of the corresponding cavities. This is currently a prime issue in the IC encapsulation industry. We believed that the problem of the shorter lifetime of the coating and the consequent preferential sticking of EMC on the parts of the gateways resulted from the non-uniformity of the film arising from the geometry effect. The objective of the present work was to assess experimentally the effect of substrate geometry on the uniformity of structure and properties of amorphous carbon films deposited by unbalanced magnetron sputtering. For this purpose, a set of double-V shaped stainless steel model jigs was specially designed and used as the substrate to simulate the plastic injection mold gateway. The influence of working atmosphere pressure and substrate bias voltage on the film uniformity was also investigated.

2. Experimental details The chromium-doped hydrogen-free amorphous carbon film (containing approx. 5 – 8 at.% Cr) was deposited using a Teer UDP-550 unbalanced magnetron sputtering system with two chromium targets and two graphite targets installed around the side wall of a cylindrical chamber. The deposition process of the amorphous carbon film has been described in detail previously [14,15]. All of the four targets were sputtered with DC power sources. High purity argon was used as the discharge gas in the working chamber. A set of specially designed double-V shaped stainless steel model jigs, as schematically shown in Fig. 1, was used as the substrate to simulate the gateway of the plastic injection

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Fig. 1. A schematic diagram of the specially designed double-V shaped model substrate.

mold. Two semi-trapesoid stainless steel blocks (5 mm in thickness, 25 mm in length, and 10 mm in width) with a pair of side faces specially machined not perpendicular to the top and bottom faces were fixed onto a stainless steel disk (50 mm in diameter and 6 mm in thickness) to form a double-V shape configuration with the forming angles on both the bottom plane and cross-sectional plane of approximately 15j. At first, the jigs were ultrasonically cleaned in a series of alkaline solutions, washed in deionized water, and dried by blowing nitrogen gas. Before loading into the deposition chamber, the model jigs were assembled together and mounted on the sample holder with the top surface facing to the targets. Before deposition, the substrate was in situ plasma cleaned at a bias of 500 V for half an hour. During deposition, the sample holder was rotating continuously around the central axis at a speed of 10 rev./min, so the model substrate would be facing to the four targets successively. A pulsed DC power source was applied to the substrate to induce proper ion bombardment on the growing film surface to assist the deposition. In order to enhance the adhesion strength of the film onto the substrate, a multilayer structure was constructed [14,15]. It started with a thin chromium bonding layer (about one tenth of the total thickness of the film), followed by a graded Cr– C transition layer (about one quarter of the total thickness of the film) and finished with a chromium-doped amorphous carbon layer. The total processing time was 5 h, including the half an hour plasma cleaning, 5 min chromium bonding layer, half an hour of the graded Cr– C transition layer and 3 h 55 min of the top layer. After deposition, the model jigs were disassembled. The structure, thickness, and hardness of the film at nine selected points on the side wall (a,b,. . .. . .,i) and six points on the

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bottom plane (A,B,. . .. . .,F) of the double-V shaped model substrate (see Fig. 1) were characterized. Raman spectra (Rennishaw Ramanscope) were measured in the range of 800 – 2000 cm 1 at room temperature with 632.8 nm excitation wavelength of a He Ne laser to analyze the atomic bond structure. In the wave number region of 1000– 1800 cm 1, a broad asymmetric scattering band could be observed, which is a characteristic structure feature of amorphous carbon materials and can be generally deconvoluted into Gaussian D (centered at approx. 1360 cm 1) and G (centered at approx. 1570 cm 1) sub-bands by curvefitting. The integrated intensity ratio between the D and G sub-bands (ID/IG) was used to qualitatively estimate the sp3/ sp2 ratio with the higher ID/IG ratio corresponding to a lower sp3 content in the film [19,20]. The micro hardness of the film was measured using a Nanotest 550 nanoindenter. The maximum indentation depth was set at 50 nm, and the Oliver and Phar method was used for the hardness calculations [21]. The thickness of the film was measured by a ball-crater method, in which the film at the selected points was ground through to the substrate using a 30 mm diameter stainless steel ball with the presence of diamond paste containing 0.25 Am diamond particles. The film thickness of each layer was calculated from the diameters of the stainless steel ball and the concentric circles generated at the ball crater scar, which corresponded to the interfaces between neighboring layers, respectively.

3. Results and discussion For the convenience of comparison, we defined a parameter of geometric aspect ratio to study the geometric effect. The geometric aspect ratio is defined as the depth (d) over width (w) of the simulated gateway at each selected point shown in Fig. 1. Here d refers to the distance from a selected

Fig. 2. Thickness variation as a function of geometric aspect ratio over the side wall (—o—) and bottom plane (— —) surfaces of the double-V shaped substrate.

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Fig. 3. Variations of Raman ID/IG ratio (— —) and micro hardness (—o—) as a function of geometric aspect ratio over the side wall (a) and bottom plane (b) surfaces.

point to the top surface of the block, and w the width of the gateway at the point under measurement. For the points on the bottom plane, d is constant (5 mm). For the points nearer to the narrow end and/or closer to the bottom plane of the simulated gateway, the aspect ratio value is higher. Fig. 2 shows the measured thickness of the film as a function of the aspect ratio at different points on the side wall (a) and bottom plane (b). As expected, the thickness of the film decreased monotonically with the increase of the aspect ratio. In Fig. 2, it is noted that the coating thickness at all the measured points is greater than 0.5 Am. In the nanoindentation measurement for the coating hardness, the maximum indentation depth was set at 50 nm, which is less than one tenth of the coating thickness, therefore the effect of the substrate on the hardness can be neglected [21]. Fig. 3 shows the Raman ID/IG ratio and micro hardness of the film at the selected points on the side wall (a) and bottom plane (b) as a function of the aspect ratio. The relatively high ID/IG values (2.5 – 6.5) of the film for all the points indicated that the film had a sp2-bonding dominated structure. On both the sidewall and the bottom plane of the model substrate the ID/IG ratio increased with the increase of the aspect ratio, indicating a decrease of the sp3 content. In correspondence with the variation of sp3 content, the micro hardness of the film decreased with the increase of the aspect ratio on both the side wall and the bottom plane. It was noted that the hardness drop on the side wall was far more significant than that on

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Fig. 4. Indentation scar produced by the Rockwell diamond indenter impressed into the Cr-doped a-C coating on stainless steel substrate under a load of 150 N.

the bottom plane. With similar ID/IG ratios, the hardness of the film on the side wall was much lower than that on the bottom plane. This implied that the micro hardness of the amorphous carbon film was not directly related to the ID/IG ratio or sp3 content in the film. The decreases of both the coating thickness and hardness with increasing the aspect

Fig. 5. Dependence of film thickness (a) and micro hardness (b) over side wall surface on argon gas pressure: — — 0.27 Pa; —n— 0.51 Pa; — E— 0.74 Pa. The solid lines in the figure are fitting curves.

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ratio, i.e. approaching the narrow end and/or closer to the bottom plane of the gateway, should be directly responsible for the shorter lifetime of the film on the part of gateways during the application of plastic injection molding. Nanoindentation is now a routine in hardness testing, but there is not yet a standard method for quantitative evaluation of fracture toughness of a coating. It is claimed that the fracture toughness of films can be determined from the measured dependence of the length of ‘radial cracks’ on the applied diamond indenter [22]. We attempted using Rockwell diamond indenter to evaluate the toughness of the Crdoped a-C coating. Under a high load of 150 N, the indenter penetrated deeply into the coating. However, no radial, but only circular cracks are created around the indentation scar, as shown in Fig. 4, which might imply that the coating is very tough. Similar results were observed at all the selected measuring points. Moreover, in Fig. 4, no chiping of the coating can be observed. The good toughness of the coating was also confirmed in the scratch tests, in which there is not any coating failure, i.e. cracking, chipping, or delamination, can be observed when the normal load was increased up to 130 N. All these results indicated that the adhesion strength of the coating onto the substrate is excellent and the internal stress in the coating is low. In magnetron sputtering deposition processes, working gas pressure ( PAr) and negative bias voltage (Vb) applied on

Fig. 6. Dependence of film thickness (a) and micro hardness (b) over bottom plane surface on PAr: — — 0.27 Pa; —n— 0.51 Pa; —E— 0.74 Pa. The solid lines in the figure are fitting curves.

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the substrate are two important parameters influencing film properties. In this work, the effects of PAr and Vb on the uniformity of the amorphous carbon film were also investigated. Fig. 5 compares the variations of film thickness and micro hardness with the aspect ratio on the side wall for the samples deposited under three different PAr, 0.27, 0.51, and 0.74 Pa. With the increase of argon gas pressure, the film thickness decreased consistently on the side wall (Fig. 5a). The three thickness variation curves against the aspect ratio were almost parallel to each other, which implied that the gas pressure had little influence on the thickness uniformity of the film on the side wall surface. The decrease of deposition rate with increasing PAr might be due to the increased scattering effect among the sputtered particles and the working gas. In Fig. 5b, it was observed that with the increase of gas pressure, the micro hardness uniformity on the side wall surface was slightly improved. This improvement on the hardness uniformity was accompanied by the reduction of micro hardness at the points with lower aspect ratio, so it was not what we desired. The decrease of the micro hardness with increasing PAr could be attributed to the reduction of the energy of the depositing species due to their increased particle scattering at the higher pressure. Fig. 6 shows the PAr dependence of the film thickness and micro hardness on the bottom plane of the model Fig. 8. Dependence of film thickness (a) and micro hardness (b) over bottom plane surface on negative substrate bias voltage: — — 70 V; —o— 100 V.

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Fig. 7. Dependence of film thickness (a) and micro hardness (b) over side wall surface on negative substrate bias voltage: — — 70 V; —o— 100 V. The solid lines are fitting curves.

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substrate. Similarly to the variations in Fig. 5a, the thickness of the film (Fig. 6a) decreased consistently with the increase of PAr. However, the micro hardness of the films at different points did not change consistently with the increase of PAr (Fig. 6b). On the bottom plane the micro hardness of the film changed only slightly with the aspect ratio. With the increase of PAr the micro hardness uniformity over the bottom plane was favorably improved. Figs. 7 and 8 show the substrate bias voltage dependence of the film thickness and micro hardness on the side wall (see Fig. 7) and the bottom plane (see Fig. 8), respectively. It is interesting to note that when the negative bias voltage was changed from 70 to 100 V, the film uniformity (for both the thickness and micro hardness) was improved on the bottom plane, but degraded on the side wall. This phenomenon may be interpreted as follows. With the increase of bias voltage, the energy of the bombarding Ar ions and the depositing species was increased. These higher energy particles could reach the bottom plane surface relatively easier, even at the points with higher aspect ratio. While on the side wall surface, the film deposition took place predominantly through the scattered particles, especially at the points with higher aspect ratio. Higher bias voltage may lead to a reduced number of the scattered particles at these points, resulting in the degradation of the film uniformity.

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4. Conclusions

References

Amorphous carbon films were deposited by unbalanced magnetron sputtering. The substrate geometric effect on the uniformity of the film was investigated using a specially designed double V-shaped jig to simulate the gateway of the plastic injection mold. It was found that on both the side wall and the bottom plane of the double-V shaped substrate, the film properties strongly depended on the geometric aspect ratio. With the increase of the aspect ratio, i.e. approaching the narrow end and closer to the bottom plane of the simulated gateway, Raman spectrum ID/IG ratio increased, while the thickness and hardness of the film decreased, which is responsible for the shorter lifetime of the film on the parts of gateways of the IC mold. The hardness decrease was far more significant on the side wall than on the bottom plane. With the increase of the deposition pressure, the film thickness decreased consistently, and the hardness uniformity on both the side wall and the bottom plane was improved. When the substrate negative bias voltage was changed from 70 to 100 V, the film uniformity (for both the thickness and hardness) was improved on the bottom plane surface, but degraded on the side wall surface.

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Acknowledgements Authors would like to thank Mr Anthony Yeo and Ms Yuchan Liu for their technical assistance in this work.