Surface roughness of optical quartz substrate by chemical mechanical polishing

Transcription

1 Vol. 35, No. 11 Journal of Semiconductors November 2014 Surface roughness of optical quartz substrate by chemical mechanical polishing Duan Bo( 段波 ), Zhou Jianwei( 周建伟 ), Liu Yuling( 刘玉岭 ), Sun Mingbin( 孙铭斌 ), and Zhang Yufeng( 张玉峰 ) Institute of Microelectronics, Hebei University of Technology, Tianjin , China Abstract: In order to achieve a high-quality quartz glass substrate and to improve the performance of TiO 2 antireflection coating, chemical mechanical polishing (CMP) method was used. During CMP process, some process parameters including pressure, polishing head speed, platen speed, slurry flow rate, polishing time, and slurry temperature were optimized to obtain lower quartz surface roughness. According to the experiment results, when pressure was 0.75 psi, polishing head speed was 65 rpm, platen speed was 60 rpm, slurry flow rate 150 ml/min, slurry temperature 20 ı C, and polishing time was 60 s, the material removal rate (MRR) was 56.8 nm/min and the surface roughness (Ra) was 1.93 Å (the scanned area was m 2 /. These results were suitable for the industrial production requirements. Key words: quartz substrate; surface roughness; removal rate; CMP; process parameters DOI: / /35/11/ EEACC: Introduction Owing to the stable chemical properties and excellent physical properties, quartz substrates are widely used in aerospace, optoelectronics, nuclear energy and other advanced technology areas, especially as a substrate of anti-reflection layer applied to the optical system. The surface roughness will affect the performance of the optical system. Furthermore, with the rapid development of the optical system in recent years, the requirements for the quartz substrate surface roughness have become tighter, and have been improved from the nanoscale to sub-nanoscale. For example, high energy laser reflector Œ1 and laser gyro mirror Œ2 surface roughness are required to be less than 1 nm. There are many methods to get a polished surface of quartz substrates, such as classical polishing, float polishing Œ3, chemical mechanical polishing Œ4 6 (CMP), magnetic rheological polishing (Kurobe et al., 1986, Tani and Kawata, 1984, Umehara and Kato, 1996, Kordonski and Golini, 1999 and Yamaguchi et al., 2003) Œ7, and ion beam polishing Œ8; 9. As long as they are used reasonably, those methods can produce a nanoscale surface and even an ultra smooth surface of subnanoscale. However, the classical polishing belongs to slow polishing and surface quality of the wafer is easily affected by the human factor. In addition, such as float polishing, magnetic rheological polishing and ion beam polishing belong to contact-free polishing methods, which have some disadvantages including complex process, expensive equipment, and low material removal rate. Compared to all above, chemical mechanical polishing has been widely applied to wafer surface processing, with which the material removal rate is faster, and the process is simple and easy to control. Chemical mechanical polishing is a process of smoothing a wafer surface by using a combination of chemical reactions and mechanical forces Œ10. Numerous parameters are involved in the material removal process, such as the type of abrasive, pressure on the wafer, relative velocity between the polishing pad and the wafer, polishing pad, and substrate characteristics Œ11. Hed and Edwards studied the relationship between surface roughness and subsurface damage of optical glass Œ12. He et al. investigated the machined surface microstructure of optical glasses Œ13. Li et al. discussed the characteristics of optics polished with a polyurethane pad Œ14. However, in the previous work, the polishing process parameters of optical quartz glass (as a substrate of TiO 2 thin film), CMP was not investigated. In this paper, in order to analyze polishing characteristics and mechanism during the CMP process, the relationship between the polishing parameters and surface roughness was investigated. 2. Experiment All quartz substrates were polished by an E460E CMP polisher of Alpsitec Company. Rohmand Haas IC 1000TM pad was used, and the ex-situ pad conditioning was conducted with a TBW diamond conditioner. The samples material removal rate was calculated basing on the weight loss method, and the weight loss measured by an analytical balance (Mettler Toledo AB204-N). Surface roughness parameters were measured using by Agilent 5600LS AFM. The samples were supplied by Institute of Optics and Electronics, Chinese Academy of Sciences. The schematic diagram of the CMP operation processes is illustrated in Fig. 1. The structure of quartz substrates (99.99% purity) is shown in Fig. 2, and the diameter of the sample was 30 mm. The removal rate was calculated from the following formulas Œ15 : * Project supported by the Natural Science Foundation of Hebei Province (No. E ), the Science and Technology Plan Project of Hebei Province (Nos. Z , ), and the Hebei Province Department of Education Fund (No ). Corresponding author @qq.com Received 16 March 2014, revised manuscript received 19 May Chinese Institute of Electronics

2 Fig. 1. Schematic diagram of CMP processing. Fig. 2. Schematic of quartz substrates structure. MRR D m R 2 t : (1) In the formula, MRR is the removal rate; m is the weight change before and after polishing; is the material density; R is the radius; t is the polishing time. In this research, the slurry consisted of chelating agent, surfactant and silica sol (mean particle size 30 nm). The ph value of the slurry was Results and discussion Figure 3 shows the changing of quartz substrates MRR and Ra under the different pressure. Experimental results indicated that the Ra approached a constant at lower pressure ( psi) and then increased with further increasing of pressure. At low pressure, the slurry played a role of lubricant between the sample and polishing pad, so the friction would be reduced, which made the surface roughness better and without scratches. However, with the pressure increasing the polishing temperature increased, which caused the quartz surface defects such as scratches, orange peel and subsurface damage layer. Figure 3 also shows the MRR increased with the increasing of pressure. Furthermore, the MRR of quartz substrates was nm/min of the lowest value at 0.5 psi and nm/min of the highest value at 2.0 psi; such a result revealed that the MRR was mainly affected by the mechanical parts of the polishing process. From the above results, the optimum pressure of quartz substrate polishing was 0.75 psi considering both the surface roughness and Fig. 3. Effect of pressure on Ra and MRR. the removal rate. In the following a removal mechanism is proposed based on possible interactions of the hydroxyl ion and sample surface (as shown in Fig. 4). In the presence of hydroxyl ions, the Si O bonds are converted to Si OH and the degree of conversion increases as the ph is increased. Therefore, the hydroxyl ions could attack the Si atom in Si O bond, and weaken the Si O bonds. The hydration layer was carried by the abrasive and polishing pad, and left in the slurry. Figure 5 shows the MRR and Ra under different rotation speed conditions (polishing head/platen speed). The Ra slowly decreased and then increased with the increasing of rotation speed. Experimental results indicated that the minimum Ra of 1.92 nm was obtained under the condition of 65/60 rpm. At a lower speed, such as 35/30 rpm, most of the quartz surface layer was removed by chemical dissolution into the slurry and less of it was removed by mechanical action of the slurry particles. However, chemical corrosion was serious and it wasn t favorable to an ameliorated degree of finish. To further increase the speed, the mechanical action would improve, which made the slurry uniformly distribute in the polishing platen and the chemical reaction become full. In the mean time, the reaction resultants were carried away by abrasives and the polishing pad. Figure 6 shows a schematic diagram of the polishing process. As a result, the higher removal rate with good roughness was achieved. If the rotation speed was too high, the friction would increase and increase the surface scratches. The MRR by changing the rotation speed showed the linear increasing tendency. The highest MRR of nm/min was obtained in 103/97 rpm condition. But the high rotation speed made the slurry too rapidly separate from the sample surface. So the slurry could not play a role of lubricant between the sample and polishing pad. The surface scratches of the quartz substrate increased after polishing. For all of the above, the rotation speed 65/60 rpm was selected. In Fig. 7, experimental results revealed that the MRR didn t show a great difference in relation to the slurry flow rate. The MRR was about nm/min in all slurry flow rate conditions. When the flow rate reached 100 ml/min, slurry between the pad and the sample surface had completely reacted, therefore, chemical reaction resultants could be removed rapidly from the sample surface. Hence, the MRR basically reached saturation. In addition, research shows that the slurry

3 Fig. 4. The removal mechanism is proposed based on interactions of the hydroxyl ion with sample surface. Fig. 5. Effect of polishing head/platen speed on Ra and MRR. Fig. 7. Effect of slurry flow rate on Ra and MRR. Fig. 6. Schematic diagram of the polishing process. flow can also influence surface roughness. If the flow was too small, friction force would increase, which made the surface quality decrease obviously. The large flow could make chemical reaction resultants rapidly separate from the sample surface and reduce the part of the higher temperature of the sample surface resulting from friction. However, as the flow rate increased to 300 ml/min, the flow would not be favorable to an ameliorated surface topography. This was because the slurry between the pad and the sample surface had been lost before it completely reacted, which made the slurry non-uniformly distributed in the polishing pad, and result in intense local chem- Fig. 8. Effect of slurry temperature on Ra and MRR. ical reaction. In this reaction process, SiO 2 and the chelating agent can be changed into soluble amine salts. The molecular volumes of amine salts were easier to separate from the surface under the friction of abrasive and polishing pad. The possible

4 Fig. 9. Comparison surface morphology images of quartz substrate after CMP by different slurry temperature. Fig. 10. Effect of polishing time on Ra and MRR. Fig. 11. Surface roughness and removal rates after quartz substrate polishing at optimized process parameters. chemical reactions were shown in the following equations: R.NH 2 / 4 C 4H 2 O! ŒR.NH 3 / 4 4C C 4OH ; SiO 2 C 2OH!.SiO 3 / 2 C H 2 O; ŒR.NH 3 / 4 4C C 2.SiO 3 / 2! ŒR.NH 3 / 4.SiO 3 / 2 : For all of the above, the improved Ra of 0.86 nm was obtained in 150 ml/min of slurry flow rate conditions. Figure 8 shows the changing of MRR and Ra with an increase of slurry temperature. The MRR improved when the temperature changed from 20 to 30 ı C, but when the temperature was over 40 ı C the MRR decreased. The MRR mainly included two rates, one was the chemical reaction rate and the other was the desorption rate of resultants. The balance and synthetical effects of two rates decided the total removal rate. Hence, even if the chemical reaction was very rapid, if desorption was very slow, the total removal rate was not high. Furthermore, if the temperature was too high, the slurry was easy to volatilize and chemical reaction was too rapid, which easily formed a polishing haze. Figure 9 shows comparison surface morphology images of quartz substrate after CMP by different slurry temperature. The Ra increased with increasing slurry temperature. With slurry temperature increased, the chemical reaction of the slurry would be enhanced, most of the reaction resultants could not be removed rapidly from the sample surface, which would not be favorable to an ameliorated degree of finish. Considering the surface roughness, the appropriate slurry temperature was selected as being 20 ı C. Figure 10 shows the Ra and MRR under different polishing time conditions. Experimental results indicated that the MRR increased rapidly from to over nm/min after 60 s of

5 Table 1. The process conditions of quartz substrate CMP. 4. Conclusions Polishing process Parameter Pressure (psi) 0.75 Polishing head speed (rpm) 65 Polishing platen speed (rpm) 60 Slurry flow rate (ml/min) 150 Slurry temperature ( ı C) Polishing time (s) In this paper, the quartz substrate was polished by CMP process under different process parameters. According to the process analysis and experiment results, the appropriate process parameters were established as follows: pressure, polishing head speed, platen speed, slurry flow rate, slurry temperature, and polishing time were 0.75 psi, 65 rpm, 60 rpm, 150 ml/min, 20 ı C, and 60 s, respectively. The removal rate and the surface roughness were 56.8 nm/min and 1.93 Å (the scanned area was m 2 / in such conditions. References Fig. 12. Before polishing. Fig. 13. After polishing. polishing time. This revealed that the removal rate in the initial polishing process was mainly affected by the mechanical action of the CMP process without the adequate effects of slurry. However, the increasing MRR slowly declined after 60 s. This revealed that the abrasives on the polishing pad hindered the increase of removal rate by the inadequate flow of slurry. Figure 10 shows the Ra of sample was 0.22 nm of the lowest value at 60 s. To obtain the better characteristics of sample, the Ra has to be below 5 Å. Experimental results indicated that the proper polishing time was 60 s for the higher removal rate and better surface. The appropriate process parameters in this experiment are shown in Table 1. The removal rate and the surface roughness were 56.8 nm/min and 1.93 Å at the condition. Figure 11 shows surface roughness values and removal rate of quartz substrate. Figures 12 and 13 show AFM images of sample surface morphologies before polishing and after polishing, respectively. [1] Yao J J, Xu C, Ma J Y, et al. Effects of deposition rates on laser damage threshold of TiO 2 /SiO 2 high reflectors. Appl Surf Sci, 2009, 255(9): 4733 [2] Wang Z G, Long X W, Wang F. Bias characteristics of a multioscillator ring laser gyro with consideration of differential losses. Optics & Laser Technology, 2013, 48: 285 [3] Umehara N, Kirtane T, Gerlick R, et al. A new apparatus for finishing large size/large batch silicon nitride (Si 3 N 4 / balls for hybrid bearing applications by magnetic float polishing (MFP). International Journal of Machine Tools and Manufacture, 2006, 46(2): 151 [4] Wang Chenwei, Liu Yuling, Tian Jianyin, et al. Planarization properties of an alkaline slurry without an inhibitor on copper patterned wafer CMP. Journal of Semiconductors, 2012, 33(11): [5] Dettoni F, Rivoire M, Gaillard S, et al. High resolution nanotopography characterization at die scale of 28 nm FDSOI CMOS front-end CMP processes. Microelectron Eng, 2014, 113: 105 [6] Vasilev B, Bott S, Rzehak R, et al. A method for characterizing the pad surface texture and modeling its impact on the planarization in CMP. Microelectron Eng, 2013, 104: 48 [7] Furuya T, Wu Y, Nomura M, et al. Fundamental performance of magnetic compound fluid polishing liquid in contact-free polishing of metal surface. Journal of Materials Processing Technology, 2008, 201(1 3): 536 [8] Zoethout E, Louis E, Bijkerk F. Real-space insight in the nanometer scale roughness development during growth and ion beam polishing of molybdenum silicon multilayer films. Appl Surf Sci, 2013, 285(Part B): 293 [9] Anopchenko A, Jergel M, Majková E, et al. Effect of substrate heating and ion beam polishing on the interface quality in Mo/Si multilayers X-ray comparative study. Physica B: Condensed Matter, 2001, 305(1): 14 [10] Lee H S, Jeong H D. Chemical and mechanical balance in polishing of electronic materials for defect-free surfaces. CIRP Annals- Manufacturing Technology, 2009, 58: 485 [11] Lee H S, Jeong H D, Dornfeld D A. Semi-empirical material removal rate distribution model for SiO 2 chemical mechanical polishing (CMP) processes. Precision Engineering, 2013, 37: 483 [12] Hed P P, Edwards D F. Optical glass fabrication technology: relationship between surface roughness and subsurface damage. Appl Opt, 1987, 26(21): 4677 [13] He Xingyang, Su Ying. Machined surface microstructure of optical silica glasses. Journal of Wuhan University of Technology, 2010, 32(13): 34 [14] Li Y, Hou J, Xu Q, et al. The characteristics of optics polished with a polyurethane pad. Opt Express, 2008, 16: [15] Wei Wenhao, Liu Yuling, Wang Chenwei, et al. Study of a novel alkaline barrier slurry applied in copper chemical mechanical planarization. Journal of Functional Materials, 2012, 43(23):