Effect of Sample Configuration on Droplet-Particles of TiN Films Deposited by Pulse Biased Arc Ion Plating

J. Mater. Sci. Technol., Vol.25 No.5, 29 681 Effect of Sample Configuration on Droplet-Particles of TiN Films Deposited by Pulse Biased Arc Ion Plating Yanhui Zhao 1), Guoqiang Lin 2), Jinquan Xiao 1), Chuang Dong 2) and Lishi Wen 1) 1) Institute of Metals Research, Chinese Academy of Sciences, Shenyang 1116, China 2) Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Ministry of Education, Dalian University of Technology, Dalian 11685, China [Manuscript received October 13, 28, in revised form May 8, 29] Orthogonal experiments are used to design the pulsed bias related parameters, including bias magnitude, duty cycle and pulse frequency, during arc ion deposition of TiN films on stainless steel substrates in the case of samples placing normal to the plasma flux. The effect of these parameters on the amount and the size distribution of droplet-particles are investigated, and the results have provided sufficient evidence for the physical model, in which particles reduction is due to the case that the particles are negatively charged and repulsed from negative pulse electric field. The effect of sample configuration on amount and size distribution of the particles are analyzed. The results of the amount and size distribution of the particles are compared to those in the case of samples placing parallel to the plasma flux. KEY WORDS: Arc ion plating; Pulsed bias; TiN film; Droplet-particles 1. Introduction Arc ion plating is one of the most important methods to deposit hard films such as TiN due to its high ionization (7% 8%), fast deposition rate, high film density and strong film/substrate adhesion. But droplet-particles (DPs) pollution problem restricts its further application to deposit high-quality fine thin films. Therefore, how to confine and reduce DPs is always becoming an important issue in the development of arc ion plating technique. In recent years, some experiments [1 4] show that negative unipolar pulsed bias could obviously reduce DPs. Although it cannot eliminate all of DPs, its advantages lie in not requiring additional equipments and not obviously decreasing deposition rate. Because the magnitude of bias voltages with pulse mode could be much higher than that in d.c. mode, it has been speculated that the reason for DPs reduction lies in improved ion bombardments sputtering [4 5]. Our group [6 7] has studied the surface morphology of TiN films deposited by arc ion plating, and the results show that the amount of DPs on the surface of TiN films deposited using unipolar pulsed bias is fewer than that using d.c. bias at the same magnitude of bias. The results show that ion sputtering is not the main reason for particles elimination. In this case, we have given a physical model to qualitatively explain the experimental phenomenon. The model suggests that DPs could be negatively charged due to the sheath oscillation under pulsed bias more than from the stable sheath in d.c. bias mode, and hence the repulsive force acting on DPs from the substrate under the pulsed bias is higher than that under the d.c. one. As a result, DPs are significantly reduced. Then, we [8 9] have applied an orthogonal design to investigate the effects of unipolar pulse bias related parameters including pulsed bias magnitude, duty cycle and frequency on the amount and the size Corresponding author. Prof.; Tel.: +86 24 2397178; E-mail address: yhzhao@imr.ac.cn (Y.H. Zhao). distribution of DPs, and the experimental results obtained agree well with this physical model. It should be noted that the samples are placed parallel to the plasma flux in the above-mentioned experiments. But the sample configuration also put an important effect on the amounts of DPs and the samples placing parallel to the plasma flux have less DPs content [1]. For most tools, there is not always only one placing configuration, various planes with different configurations need to be coated so that the film uniformity should be emphasized. In the present paper, orthogonal designs were used to investigate the effects of the pulsed bias related parameters on the amount and the size distribution of DPs and to analyze the weight of each parameter when the coated samples were placed normal to the plasma flux. And the present results would be compared with those of placing parallel to the plasma flux, which was available to supply experimental evidence for DPs reduction level with different placing configuration. 2. Experimental TiN films were synthesized using a Russian Bulat- 6 arc deposition system (as shown in Fig. 1). Other than the sample configuration normal to the plasma flux, all the experimental details were similar to the previous work [8]. According to unipolar pulse bias related parameters (noting: all the following-mentioned pulsed bias only refers to an unipolar case), each experiment has three levels, that is, pulsed bias magnitude (U p ): V, V, 1 V; frequency (f): 2 khz, 3 khz, 4 khz; duty cycle (D): 1%, 3%, 5%, respectively. Then an L9(34) orthogonal table is used to design the nine experiments from No. 1 to No. 9. TiN films were also deposited using the same bias magnitude of V but at dc mode. JSM-5LV scanning electron microscopy (SEM) was used to observe surface morphology of TiN films, and then SEM pictures were input into QIW type

682 J. Mater. Sci. Technol., Vol.25 No.5, 29 Fig. 1 Schematic diagram of the Bulat6 arc ion plating system 11 1 96 (a) 7 2 136 1 3 6 2 1 1 (b) 7 655 2 1 38 4 Fig. 2 SEM surface morphology of TiN film at the same magnitude of negative bias of V and its corresponding histogram of DPs distribution under d.c. bias (a), pulsed bias (b) image analyzer to analyze the number and the size distribution of DPs. The obtained results were arranged to an orthogonal table, and then minor difference principle would be applied to analyze the weight of each parameter. 3. Results and Discussion 3.1 DPs distribution under the same magnitude of d.c. bias and pulsed bias in the case of samples placing normal to the plasma flux Figure 2 shows an SEM surface morphology of TiN films and the particle size distribution deposited under the same magnitude of V with d.c. bias and pulsed bias (U p = V, D=1% and f=2 khz), respectively. The histograms show that the amount under pulsed bias is much less than that under d.c. bias. Their corresponding area distribution densities at d.c. bias and pulsed bias are 115365 and 7857 mm 2, respectively. Since the mean bombardment power under pulsed bias is less than that under d.c. ones, the particle reduction is mainly related to the pulse mode. In the case of samples placing parallel to the plasma flux [8 9], their area distribution densities with d.c. bias and pulsed bias under the same magnitude of V with d.c. bias and pulsed bias (D=4% and f=3 khz) are 4283 and 3435 mm 2, respectively, which also shows that the amount under pulsed bias is much less. It should be noted that the area distribution densities in the case of samples placing normal to the plasma flux are much more than those in the

J. Mater. Sci. Technol., Vol.25 No.5, 29 683 Table 1 Orthogonal analysis table of the effect of pulsed bias related parameters on the distribution density of DPs in the case of samples placing normal to the plasma flux Expt. Pulse peak Duty cycle Frequency Area distribution density of DPs No. -U p /V D/% f/khz /(number/mm 2 ) 1 1 2 7857 2 3 3 69522 3 5 4 47814 4 1 3 63253 5 3 4 4812 6 5 2 43487 7 1 1 4 9924 8 1 3 2 57723 9 1 5 3 38811 1 188193(U p= V) 23335 (D=1%) 17267(f=2 khz) 11 15486(U p = V) 175365(D=3%) 171586(f=3 khz) 12 195774(U p = 1V) 13112(D=5%) 195174(f=4 khz) 13 4914 13238 23588 case of parallel direction. 3.2 DPs distribution under pulsed bias using orthogonal design in the case of samples placing normal to the plasma flux An orthogonal design is used to deposit TiN films by changing the pulsed bias related parameters, as shown in Table. 1. A surface area distribution density of the DPs can be obtained by changing the amount of the DPs under the same view into the amount per square millimeter, and the results of different surface area distribution density are also listed in the right column of Table 1. The influence from different pulse parameters are analyzed using the data displayed in Table 1, which shows the particle distribution density of an orthogonal experiment. The data listed in lines 1, 11 and 12 are the sum of the particle distribution density values at a constant influence parameter. For instance, at a constant U p of V but at different D and f, there are three area distribution density values, 7857, 69522 and 47814 mm 2, as listed in the right column. The sum of these three values, 188193, is presented in column -U p, line 1. There are a total of three such sums, 188193, 15486 and 195774, for U p =, and 1 V, respectively. Then, the range, which is the difference of the largest sums and the smallest one, 4914, is listed in line 13. The range magnitude reflects the influence of variation at a particular parameter. Accordingly, the influence from duty cycle is the strongest, followed closely by frequency and the magnitude of the pulsed bias voltages. Among the nine samples in the orthogonal experiment, sample 9 ( 1 V, 5%, 3 khz) is the most reduced one with a particle distribution density of 38811 mm 2. While sample 7 ( 1 V, 1%, 4 khz) exhibited the maximum amount of particle distribution densities of 9924 mm 2, and their SEM images and corresponding histograms are shown in Fig. 3. It should be noted that the amount of the particle distribution densities are all less than that observed for the sample deposited at a dc bias of V. In the case of samples placing parallel to the plasma flux, the nine area distribution densities of experimental results using the same orthogonal design were 1934 to 34351 mm 2, in which the influence from the magnitude of the pulsed bias voltages, followed closely by duty cycle and frequency [8 9]. It is also noted that all the area distribution densities of DPs in the case of samples placing normal to the plasma flux are much more than those in the case of parallel direction and the reason will be analyzed in section 3.3. 3.3 Results comparison of the amount and size distribution of DPs for different sample configuration in the case of samples between normal and parallel to the plasma flux DPs contamination in films depends on DP generation, DP transport to the substrate and the DPsubstrate interaction [11]. In this present work, all the other parameters, such as arc current, the targetsubstrate distance, gas partial pressure, deposition time are kept constant, only pulsed bias related parameters and samples configuration are variable. In this case, both bias mode and samples configuration correlate in a significant manner with the DP-substrate interaction. Furthermore, a physical model [7] based on the charging on the DPs in the sheath at d.c. bias and pulsed bias was established. The model suggests that DPs could be negatively charged due to the sheath oscillation under pulsed bias more than from the stable sheath in d.c. bias mode, and hence the repulsive force acting on DPs from the substrate under the pulsed bias is higher than that under the d.c. one, and therefore, DPs are significantly reduced. On the other hand, pulsed bias magnitude could be much higher than that in d.c. bias. The higher negative charges on the DPs and the higher repulsive forces acting on the DPs are also benefitial to particles reduction. From the orthogonal experimental results it can be noted that the particle area distribution densities and size distribution are relevant to the pulse bias related parameters in the following ways. The higher is the bias voltage, the stronger the electric field and hence the fewer the particles; a higher duty cycle leads to a more intense electric field and hence more particles are repulsed from the substrate. However, too large duty cycle (e.g. larger than 7%) approaches the less effective d.c. case; and a higher frequency reduces the average electron density inside the plasma sheath and hence fewer particles are obtained. So the physical model coincides with the experimental results.

684 J. Mater. Sci. Technol., Vol.25 No.5, 29 1 (a) 691 7 2 1 82 15 3 1 1 1 (b) 7 326 2 1 44 11 1 Fig. 3 SEM image (upper) and particle size distribution (lower), under pulsed bias U p= 1 V, D=1%, f=4 khz (a) and U p= 1 V, D=5%, f=3 khz, (b) in the case of samples placing normal to the plasma flux Fig. 4 Schematic illustrations of the forces acting on a DP in plasma sheath at dc bias (a) and pulsed bias (b)

J. Mater. Sci. Technol., Vol.25 No.5, 29 685 In the case of sample placing parallel to the plasma flux, DPs are negatively charged during their flying in the plasma space and the negatively charged particles, once entering into the plasma sheath, receive a combination of different forces, including the repulsive forces from the negatively biased substrate F e, gravity force F g, ion drag force F [12] i. Applying the plasma sheath models given by Edelberg and Aydil [13], the charging effect and forces acting on DPs with different radius were analyzed and calculated in pulse biased arc ion plating. The results showed that with smaller radius, F e is larger than the sum of F g and F i. The smaller is the DP radius, the larger F e is. Thus, DPs with small radius were easily inhibited to land on the substrate surface and DPs reduction was obtained, which could be clearly shown in the former results [8 9]. In comparison to d.c. bias, the DP entering into the pulsed sheath is charged with more electrons and hence becomes more negative. Since the electric field intensity increases when the particle approaches the substrate, the particle receives a stronger repulsive force. Therefore those particles receiving a sufficiently high repulsive force would float above the sheath and the amount of DPs was significantly reduced. In the case of samples placing normal to the plasma flux, the forces acting on DPs changed and should be analyzed again. In this case, we just consider the forces parallel to the plasma flux. The forces acting on a particle parallel to the plasma flux include F e and F i. Figure 4 shows the schematic illustrations of the forces acting on a DP in plasma sheath at d.c. bias and pulsed bias. It has been demonstrated that DPs are emitted preferentially, however, the DP flux in the direction normal to the cathode plane is still high and it is partially due to DP bouncing from the vacuum chamber wall [1]. DPs velocities were measured in the range from 1 to 1 m/s [14 18]. So, in the normal direction to the substrate surface, a DP has a very high initial kinetic energy E =D p v 2 p /2. If a DP cannot penetrate the plasma sheath, it will be eliminated. Therefore, the criteria are: E >Q p U b F i d, DPs deposit onto the surface; E <Q p U b F i d, DPs do not deposit onto the surface, where U b is the bias voltage, d is the sheath thickness, and D p, v p, Q p the D P s, mass, velocity and charges [19], respectively. A DP ejected by arc discharge at the cathodic arc target enters the sheath with certain kinetic energy. Thus, when R p L d, where R p is the DP radius and L d the Debye length, the negative charge Q p of the DP in the plasma can be expressed as [12] : Q p = 4πε au d (r) where ε is the dielectric constant, a is a factor describing the diameter of the D P, U d (r) the potential voltage of the DP as affected by the local floating potential U(r). Since U(r) changes markedly with the position r within the sheath, Q p is a function of position r. The quantitative calculation is quite difficult because the non-linear functions U d (r) are very complicated [12,2]. However, the above criteria can help to understand the mechanism of the effect of biases on DP contamination. According to calculation results of Guo et al. [21], we can roughly and quantitatively calculate the sum of all forces acting on the DPs. Some assumption should be made: the DPs have sphere shape, and F e in the sheath is constant. Considering a particle with a radius of.5 µm, suppose its velocity is 1 and 1 m/s, respectively. Their initial kinetic energies are expressed as E 1 and E 2 and they can be transferred into a constant force F 1 and F 2 supposing the plasma thickness is constant. So, E 1 =F 1d and E 2 =F 2 d. According to Huang s results [22], d µm when U p = 1 V, D=5%, f=25 khz. So, F 1 =E 1 /d, F 2 =E 2 /d. E =DP vp/2=ρv 2 vp/2=.5 4/3 πr 2 3 ρv 2 p, where V, ρ are the DP volume and density, respectively. Then, the calculated results are obtained as F 1 =1.31 1 1 N and F 2 =1.31 1 6 N. According to Guo s calculation results, the sum values of forces were the order of magnitude of 1 1 N when the radius of DP was selected as.5, 2 and 4 µm, which is lower than the constant force from the initial kinetic energy. So, the sum of forces acting on the DPs in the sheath is very difficult to overcome the initial kinetic energy and DPs contaminations are thus formed. It should be noted that the above-mentioned calculated results are quite rough, dependent on the specific condition (e.g. DP radius, initial velocity, initial angel with the substrate surface, plasma sheath U p (r)). Due to large initial kinetic energies of DPs normal to the substrate surface, the repulsive electric forces acting on DPs must do much larger work to overcome the initial kinetic energies, or even cannot overcome the energies, than in the case of samples placing parallel to the plasma flux. This is why the amount of DPs in the case of normal direction to the substrate is much more than that in the case of parallel direction. In the case of samples placing parallel to the plasma flux [8 9], all the area distribution density values obtained under different pulsed biases are 1934 to 3435 mm 2, which are much less than those values in the range of 38811 to 9924 mm 2 in the case of normal direction. These results may be related to the initial velocity direction of DPs. Since the DPs flux in the direction normal to the cathode plane is still high, the velocity component in the normal direction to the substrate surface is much larger than that in the parallel direction. The higher the velocity of DPs, the larger the initial kinetic energies of the DPs and therefore, it is easier for the DPs to overcome the repulsive forces to pass through the plasma sheath and land on the substrate surface. That is why much more DPs appear on the TiN film surface in the case of samples placing normal to the plasma flux than that in the case of parallel direction. Furthermore, whatever parallel and normal to the plasma flux, pulsed bias parameters put much stronger effects on DPs reduction with smaller radius. In summary, sample configuration plays an important role in DPs cleaning. DPs could be obviously reduced when samples are placed parallel to the plasma flux than that in the normal direction. It is helpful to select particular process such as coated tools rotating to ensure uniformity of different surface for tools with various shape. Pulsed bias contributes to stronger DPs cleaning than d.c. bias and improve the morphology of film surfaces. However, it should be

686 J. Mater. Sci. Technol., Vol.25 No.5, 29 pointed out that the pulsed bias alone cannot completely prevent the DPs. The complete removal of the DPs could be obtained by applying magnetron filtering process. 4. Conclusions In the present work, an orthogonal experiment was designed to verify a physical model for particle cleaning in pulsed bias arc deposition in the case of samples placing normal to the plasma flux. The obtained results were compared to those in the case of samples placing parallel to the plasma flux. (1) The pulsed bias related parameters affect evidently the amount and the size distribution of the DPs on the surface of TiN films deposited by arc ion plating, and the weight of effects decreases in the order of duty cycle, pulsed bias and frequency. The higher the duty cycle and the higher the magnitude of pulsed bias, the better the reduction results of the DPs. (2) The experimental results prove the correctness of the physical model for samples placing normal to the plasma flux, in which the DPs would be repulsed from electric force and thus be reduced because the DPs are negatively charged. (3) It is quite difficult for pulsed electric field to overcome so large initial kinetic energies of DPs, and hence much more DPs appear for samples placing normal to the plasma flux than in the case of parallel direction. Acknowledgements This work was supported by the National Natural Science Foundation of China under grant No. 58162. Authors would like to thank Prof. Yuzhou Gao in Dalian Maritime University, for the help of SEM measurements. REFERENCES [1 ] J. Fessman, W. Olbrich and G. Kampschulte: Mater. Sci. Eng., 1991, A14, 83. [2 ] W. Olbrich and G. Kampschulte: Surf. Coat. Technol., 1993, 59, 274. [3 ] M. Kumagai, K. Yukimura, E. Kuze, T. Maruyama, M. Kohata, K. Numata, H. Saito and X.X. Ma: Surf. Coat. Technol., 23, 169/17, 41. [4 ] R.R. Aharonov, M. Chhowalla, S. Dhar and R.P. Fontana: Surf. Coat. Technol., 1996, 82, 334. [5 ] Z.Y. Li, W.B. Zhu, Y. Zhang, G.Y. Li and E.Y. Cao: Surf. Coat. Technol., 2, 131, 158. [6 ] M.D. Huang, G.Q. Lin, C. Dong, C. Sun and L.S. Wen: Acta Metall. Sin., 23, 39, 51. (in Chinese). [7 ] M.D. Huang, G.Q. Lin, Y.H. Zhao, C. Sun, L.S. Wen and C. Dong: Surf. Coat. Technol., 23, 176, 19. [8 ] Y.H. Zhao, G..Q. Lin, C. Dong and L.S. Wen: J. Mater. Sci. Technol., 25, 21, 423. [9 ] G.Q. Lin, Y. H. Zhao, H.M. Guo, D.Z. Wang and C. Dong: J. Vac. Sci. Technol., 24, A22, 1218. [1] M. Keidar, R. Aharonov and I.I. Beilis: J. Vac. Sci. Technol., 1999, A17, 367. [11] M. Keidar, I. Beilis, R.L. Boxman and S. Goldsmith: Surf. Coat. Technol., 1996, 86/87, 415. [12] T. Nitter: Plasma Sources Sci. Technol., 1996, 5, 93. [13] A. Edlberg and E.S. Aydil: J. Appl. Phys., 1999, 86, 479. [14] M. Keidar, I. Beilis, R.L. Boxman and S. Goldsmith: IEEE Trans. Plasma Sci., 1996, 24, 226. [15] I.I. Aksenov, V.A. Belous, V.G. Padalka and V.M. Khoroshikh: Sov. J. Plasma Phys., 1978, 4, 425. [16] A.W. Baouchi and A.J. Perry: Surf. Coat. Technol., 1991, 49, 253. [17] H. Wang, J. Zou, Z. Cheng and L. Cheng: In Proc. 15th ISDEIV, 1992, 315. [18] S. Shalev, R.L. Boxman and S. Goldsmith: J. Appl. Phys., 1995, 58, 253. [19] M. Keidar: IEEE Trans. Plasma Sci., 1995, 23, 92. [2] T. Nitter, T.K. Aslaksen, F. Melands and O. Havnes: IEEE Trans. Plasma Sci., 1994, 22, 159. [21] H.M. Guo, G. Q. Lin, M.Y. Sheng, D.Z. Wang and C. Dong: Acta Metall. Sin., 24, 4, 164. (in Chinese). [22] M.D. Huang: PhD Thesis, Dalian University of Technology, 22. (in Chinese)