Material removal mechanism of precision grinding of ... - Springer Link

Int J Adv Manuf Technol (2010) 46:563–569 DOI 10.1007/s00170-009-2114-8

ORIGINAL ARTICLE

Material removal mechanism of precision grinding of soft-brittle CdZnTe wafers Zhenyu Zhang & Yaowu Meng & Dongming Guo & Lailei Wu & Yongjun Tian & Riping Liu

Received: 15 January 2009 / Accepted: 13 May 2009 / Published online: 28 May 2009 # Springer-Verlag London Limited 2009

Abstract Cd0.96Zn0.04Te (111) wafers were precisely ground by #800, #1500, #3000, and #5000 diamond grinding wheel. For comparison, Cd0.96Zn0.04Te (110) wafers were machined by lapping, mechanical polishing, and chemical mechanical polishing. High-resolution environmental scanning electron microscopy equipped with energy dispersive spectroscopy and optical interference surface profiler both were employed to investigate the surface quality and material removal mechanism. The results show that the material removal mechanism of #800 grinding wheel is abrasive wear, fatigue wear, and adhesive wear, and that of #1500 is abrasive wear and fatigue wear. Both the material removal mechanism of #3000 and #5000 grinding wheel are abrasive wear, leading to the excellent ductile removal precision grinding. While the material removal mechanism of CMP on CdZnTe wafers is firstly chemical resolving reaction and secondly mechanical carrying action. Moreover, precision grinding exhibits high-efficiency character and eliminates the imbedding of free abrasives of Al2O3 and SiO2. Z. Zhang (*) : Y. Meng : D. Guo Key Laboratory for Precision and Non-Traditional Machining Technology of Ministry of Education, Dalian University of Technology, Dalian 116024, China e-mail: [email protected] Z. Zhang : L. Wu : Y. Tian : R. Liu State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China Z. Zhang State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan 430074, China

Keywords Material removal mechanism . Precision grinding . Chemical mechanical polishing . CdZnTe wafer . Soft-brittle crystal

1 Introduction Precision grinding has been widely used for the hard-brittle materials, such as Si wafers [1], bearing steels [2, 3], Crbased alloy [4], silicon nitride [5], and so on, also for the soft-plastic materials, for example Ni-based alloys [6], stainless steel [7, 8], etc. while for the soft-brittle II–VI compounds crystals has not been reported. CdZnTe (CZT) is a kind of representative II–VI compound crystals and the most promising material for the room temperature radiation detector [9, 10], which is widely used as a substrate to grow the epitaxial layers of HgCdTe infrared detector focal plane arrays dominating military systems [11, 12]. However, the machining of CZT wafers is a challenge to the fields of precision and ultra-precision manufacture, due to its softbrittle nature. The hardness of CZT is about 1 GPa [13], which is obvious lower than 11–13 GPa of hard-brittle material Si and 5–7 GPa of soft abrasive CeO2 [14, 15]. So, the widely used abrasives in polishing the hard-brittle materials, such as CeO2 [16], Al2O3 [17], SiO2 [18], etc. are not suitable applied in that of CZT machining, otherwise, the imbedding on the machined surface of CZT crystals will occur, resulting in the dramatically decreasing of surface accuracy and integrity [19]. Moreover, CZT crystals are obviously different with traditional soft-plastic materials, for example Cu, Al, etc. The usual machining method of soft-plastic Cu is cutting, while the fluctuation of cutter will make CZT rupture, inducing from its intrinsic soft-brittle nature. This phenomenon will also occur in grinding, milling, and so on.

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Now, CZT wafers are mainly machined through lapping, mechanical polishing, and chemical corrosion manually or mechanically [20, 21], while with this traditional machining process, imbedding and scratches on the machined surface of CZT wafers are observed [10, 16], leading to the decrease of surface roughness and integrity. To improve the surface quality of CZT wafers after machining, Moravec et al. [22] have developed chemical polishing, whereas this method is very complex and the polished surface is usually convex, due to the higher removing rate at the edges than that in the center. In our recent research, we have developed a kind of chemical mechanical polishing (CMP) and obtained good surface accuracy and quality [10, 16]; however, this is not highly effective and the chemical polishing slurry is not environment-friendly. According to the present literature, precision grinding on CZT wafers has not been reported. For the precision grinding of CZT wafers with soft-brittle nature, there is a risk on the precision grinding machine tool, with the rupturing of CZT wafers during the grinding process. If this accident happens, the teeth of cup-shaped grinding wheel would fracture, and the air spindle would collide on the cylindrical wall, resulting in the failure of the precision grinding machine tool. Unfortunately, this phenomenon on the CZT wafers is easy to occur. In this study, as an exploring work, precision grinding experiments on CZT wafers are carried out and material removal mechanism is also analyzed.

2 Machining methods Cd0.96Zn0.04Te crystals grown by the modified Bridgman method were cut along the (111) and (110) faces into wafers with dimensions of 10×10×2 mm3. 2.1 Precision grinding Figure 1 shows the schematic diagram of precision grinding of CZT wafers. The air spindle rotated along its center and feeds vertically. As the precision of the whole machine tool mainly depends on the air spindle, it was the most important and precise part for the precision machine tool. Cup-shaped diamond grinding wheel with diameter of 350 mm was fixed on the holder. To decrease the grinding temperature and reduce the burning on the workpiece, the diamond grinding wheel was designed with several discrete teeth. The periphery of diamond grinding wheel was through the center of worktable spindle to keep the same contact area between diamond grinding wheel and workpiece for a circular workpiece during grinding. Because the diameter of vacuum chuck was 250 mm, a polyethylene covering panel with square holes was used to prevent

Int J Adv Manuf Technol (2010) 46:563–569

Fig. 1 Schematic diagram of precision grinding of CZT wafers

entering of air. The worktable revolved along the same direction of air spindle, resulting in the uniform grinding of workpiece. Precision grinding of CZT (111) wafers were performed on a precision grinding machine tool (VG401, Okamoto, Japan), and CZT wafers were automatically fixed on the vacuum chuck. When the diamond grinding wheel approached on the surface of CZT wafers, a magnified optical microscope was employed to adjust the distance between the diamond grinding wheel and CZT wafers to avoid the fracture of wafers. The rotation speed of air spindle was 2,800 rpm, and that of worktable spindle was 450 rpm. The grinding slurry is ionized water, and the flux was 200 ml/min during grinding. Firstly, the feed speed was 20 μm/min and grinding time is 10 min, thus for a revolution of worktable spindle, the removal rate is

Fig. 2 ZYGO surface morphology of CZT wafer after precision grinding with #5000 diamond grinding wheel


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#3000, and #5000 were selected as precision grinding tool in the experiments. 2.2 Chemical mechanical polishing

Fig. 3 Surface roughness Ra as a function of mesh sizes of diamond grinding wheel

40 nm/rev, leading to an ultra-precision grinding result. Then, the feed speed was adjusted to 5 μm/min for 5 min and the removal rate was 10 nm/rev. Finally, the feed speed was 1 μm/min for 5 min and the removal rate was 2 nm/rev. So, the whole grinding time is 20 min. Four kinds of diamond grinding wheel with mesh sizes of #800, #1500, Fig. 4 SEM surface topography of CZT wafers after precision grinding by a #800, b #1500, c #3000, and d #5000 diamond grinding wheel at a magnification of ×1,000

For a comparison with precision grinding, lapping, mechanical polishing, and chemical mechanical polishing were also conducted for the machining of CZT (110) wafers. Prior to lapping, CZT wafers were fixed on an aluminum plate with wax uniformly on the outer circumference and the load on CZT wafers was 20 kPa applied by the selfweight of aluminum plate. Lapping, mechanical polishing and CMP experiments were carried out on a ZYP200 rotating fluctuating weight lapping and polishing machine. The rotating speed of worktable was 90 rpm and that of workpiece was 60 rpm. For lapping of CZT wafers, a castiron plate with texture was used as lapping plate after dressing and lapping slurry was α type Al2O3 with sizes of 2–5 μm water solution. Lapping time was 120 min to remove the waviness produced by sawing on the surface of CZT wafers. Then, the cast-iron plate was replaced by a floss pad, mechanical polishing started, and the polishing duration was 30 min, the polishing slurry was also Al2O3 water solution. Eventually, the floss pad was substituted

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with a new one, polishing slurry was prepared by SiO2 nano-sphere with diameter of 5 nm, nitric acid, hydrochloric acid, and ionized water. The pH value was in the range of 2.5–3.2. The weight ratio of SiO2 and ionized water was 1:10. CMP time was 12 min, consequently the whole time of lapping, mechanical polishing, and CMP was 162 min.

3 Surface quality and material removal mechanism Surface roughness and morphology were measured by an optical interference surface profiler (ZYGO, USA). Surface topography was characterized using a field emission environmental scanning electron microscope (ESEM, Quanta 200 FEG, FEI, Netherlands) equipped with energy dispersive spectroscopy (EDS). 3.1 Precision grinding Figure 2 depicts the ZYGO surface morphology of CZT wafer after precision grinding with #5000 diamond grinding wheel. The surface roughness Ra is 5.726 nm and Rz is 59 nm, which is obviously lower than 8.752 nm and 101 nm reported by Zha et al. using traditional lapping, Fig. 5 SEM surface topography of CZT wafers after precision grinding by a #800, b #1500, c #3000, and d #5000 diamond grinding wheel at a magnification of ×6,000


mechanical polishing, and chemical corrosion method, respectively [17]. Furthermore, the whole grinding time is only 20 min, while the machining time of traditional method is about 4 h, indicating the high efficiency advantage of precision grinding. On the other hand, the scratches are obviously observed on the ground surface, exhibiting the obvious character of grinding. Figure 3 shows the surface roughness Ra as a function of mesh sizes of diamond grinding wheel. When the mesh sizes of diamond grinding wheel are #800, #1500, #3000, and #5000, the surface roughness Ra of CZT wafers is respectively about 125 nm, 50 nm, 19 nm, and 5.5 nm. The finer the diamond grinding wheel is, the lower the surface roughness is. When the mesh size increases from #800 to #1500, then the surface roughness decreases sharply. While with further increasing of mesh size, the surface roughness slowly fall, and the tendency is similar to linear decrease. Figure 4a and d show the SEM surface topography of CZT wafers after precision grinding by #800, #1500, #3000, and #5000 diamond grinding wheel at a magnification of ×1,000, respectively. Big and deep scratches appear on the surface of CZT wafer ground by #800 diamond grinding wheel, and a few cracks are observed. The material removal mechanism ground by #800 diamond grinding wheel is abrasive wear and fatigue wear, exhibit-


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Fig. 6 SEM surface topography of CZT wafers after precision grinding by a #800, b #1500, c #3000, and d #5000 diamond grinding wheel at a magnification of ×20,000

ing a lot of wear tracks and a few laminar delamination cracks, as seen in Fig. 4a. For #1500 diamond wheel grinding, the extent of laminar cracks is more severe compared with that of #800 grinding, thus the material removal mechanism is fatigue wear and abrasive wear, as shown in Fig. 4b, which resulted from the grinding temperature and force. The diamond grains of #1500 grinding wheel are finer than that of #800, while the number of the former at a unit area is higher than that of the latter. As #1500 grinding wheel is relatively big, grinding heat generates sharply and distributes uniformly, inducing in the difficulty of heat conduction and generating of fatigue wear. Whereas, for #800 grinding wheel, the grains are bigger than those of #1500, the amount of the former is less than that of the latter at a unit area, and the most important is the ununiformity of grain size of #800 grinding wheel, leading to the ununiformity of grinding hot and transmitting easily due to the larger volume among grains. So the fatigue wear for a #800 grinding wheel is not obvious as that of #1500. When the mesh size attains to #3000, the diamond grains are very small and the grinding heat enhances slowly compared with that of #1500 and under the condition of cooling water it conducts with water and air. As a result, the fatigue wear disappear and the surface turns into flat and smooth, as shown in Figs. 3 and

4d. Accordingly, the material removal mechanism of #3000 grinding wheel is abrasive wear. With the further finer of diamond grains to #5000, the surface is smoother, and only some subtle wear tracks are observed, therefore the material removal mechanism is also abrasive wear. Consequently, #3000 and #5000 grinding are ductile grinding, which is the basis of precision and ultra-precision grinding.

Fig. 7 SEM surface topography of CZT wafer after lapping, mechanical polishing, and CMP

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Fig. 8 SEM surface morphology of CZT wafer for a one particle and b its EDS spectrum at a magnification of ×8,000

To further investigate the material removal mechanism, ×6,000 and ×20,000 SEM surface topography of CZT wafers are shown in Figs. 5 and 6, respectively. Small scraps appear on the ground surface of #800 grinding wheel, which means the adhesive wear, as displayed in Figs. 5a and 6a. So the material removal mechanism of #800 grinding wheel is abrasive wear, fatigue wear, and adhesive wear. However, for the ground surface of #1500 grinding wheel, no scraps appear. A crack generates along the grinding direction and starts from the laminar delamination, implying the progress of fatigue wear, as seen in Figs. 5b and 6b. For precision grinding by a #3000 grinding wheel, obvious plastic deformation occurs, as depicted in Figs. 5c and 6c, exhibiting the distinct ductile grinding characters. For the #5000 grinding wheel, the plastic deformation is very small, and even in ×20,000 magnification, the scratches is very shallow and subtle, meaning the smooth surface, and excellent surface quality, as seen in Figs. 5d and 6d. 3.2 Chemical mechanical polishing Figure 7 shows the SEM surface topography of CZT wafer after lapping, mechanical polishing, and CMP. There is no any scratch on the ground surface and the surface is very Fig. 9 SEM surface morphology of CZT wafer for a another particle and b its EDS spectrum at a magnification of ×20,000

smooth and flat. The surface roughness Ra is 1.276 nm, and Rz is 10 nm, which is obviously lower than those of reports by Zha et al. [17], and slightly lower than those of recently research [10, 16]. As the material removal mechanism of CMP is mainly depend on the chemical resolving action, but for the mechanical force, so there is no any scratch appearing. Furthermore, floss pad is soft grinding material even for soft-brittle CZT wafers, whose action is to carry away the soluble salts from the surface of CZT wafers, thus the material removal mechanism is firstly the chemical reaction, and secondly mechanical carrying action. Figures 8a, b show the SEM surface morphology of CZT wafer for one particle and its EDS spectrum at a magnification of ×8,000, respectively. The white particle imbeds on the ground surface, and its elements are mainly Si, O, C, and Cl, as seen in Fig. 8b. According to the polishing slurry of CMP, the white particle is glomeration of SiO2 with diameter of 5 μm after evaporating of water. C element may derive from the adsorption of ground surface from air, or some impurities from SiO2 nano-sphere, and chlorine element comes from the ionized water. From this observation, it is concluded that SiO2 nano-sphere can imbed or leave on the surface during the CMP process. Figure 9 shows SEM surface morphology of another white particle and its corresponding EDS spectrum. The


chemical ingredients are mainly C, O, Mg, Al, Si, Fe, Ca, and K, as displayed in Fig. 9b. Considering the mechanical polishing slurry and CMP slurry, the white particle is composed of Al2O3 and SiO2, resulting in the higher concentration of O and Al. The white particle includes two Al2O3 grains and one glomeration of SiO2 nanospheres between two Al2O3 grains. C, Ma, K, and Ca elements come from impurities of industrial Al2O3 grains. NaCl origins from ionized water and Fe generates from the castiron lapping plate. So the free abrasives of Al2O3 and SiO2 imbed or leave on the ground surface, and they cannot be eliminated by the subsequent process and cleaning.

4 Conclusion In this study, Cd0.96Zn0.04Te (111) wafers are ground by four kinds of diamond grinding wheel, and surface roughness Ra after precision grinding by #5000 grinding wheel is 5.726 nm, indicating precision grinding character. Furthermore, the whole grinding time is only 20 min, which exhibits excellent high-efficiency property compared with traditional machining method on CdZnTe wafers. For the fixed abrasives of diamond grains on the grinding wheel, there is no any imbedding appearing on the ground surface of CdZnTe wafers, so precision grinding on soft-brittle CdZnTe wafers has broad prospect. Acknowledgments The authors appreciate the financial support from the National Basic Research Program of China (Grant No. 2009CB724306), the National Natural Science Foundation of China (Grant No. 50805017), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 200801411022), the Open Foundation of State Key Laboratory of Metastable Materials of Science and Technology of Yanshan University, and the Open Research Foundation of State Key Laboratory of Digital Manufacturing Equipment and Technology in Huazhong University of Science and Technology.

References 1. Tani Y, Okuyama T, Murai S, Kamimura Y, Sato H (2007) Development of silica polyvinyl alcohol wheels for wet mirror grinding of silicon wafer. Ann CIRP 56(1):361–364. doi:10.1016/ j.cirp. 2007.05.083 2. Chakrabarti S, Paul S (2008) Numerical modelling of surface topography in superabrasive grinding. Int J Adv Manuf Technol 39(1–2):29–38. doi:10.1007/s00170-007-1201-y 3. Salonitis K, Chondros T, Chryssolouris G (2008) Grinding wheel effect in the grind-hardening process. Int J Adv Manuf Technol 38 (1–2):48–58. doi:10.1007/s00170-007-1078-9

569 4. Aurich JC, Herzenstiel P, Sudermann H, Magg T (2008) Highperformance dry grinding using a grinding wheel with a defined grain pattern. Ann CIRP 57(1):357–362. doi:10.1016/j.cirp. 2008.03.093 5. Webster J, Tricard M (2004) Innovations in abrasive products for precision grinding. Ann CIRP 53(2):597–617. doi:10.1016/ S0007-8506(07) 60031-6 6. Liu Q, Chen X, Gindy N (2008) Robust design and optimisation of aerospace alloy grinding by different abrasive wheels. Int J Adv Manuf Technol 39(11–12):1125–1135 7. Zhang YH, Wu Q, Hu DJ (2008) Research on wear detection of wheel in precision NC curve point grinding. Int J Adv Manuf Technol 35(9–10):994–999 8. Xu XP, Yu YQ (2002) Adhesion at abrasive-Ti6Al4V interface with elevated grinding temperatures. Int J Adv Manuf Technol 21 (16):1293–1295 9. Mark A, Julie SL, Paul NL (2002) Electron trapping nonuniformity in high-pressure-Bridgman-grown CdZnTe. J Appl Phys 92 (6):3198–3206 10. Prokesch M, Szeles C (2006) Accurate measurement of electrical bulk resistivity and surface leakage of CdZnTe radiation detector crystals. J Appl Phys 100(1):014503 11. Tsen GKO, Sewell RH, Atanacio AJ, Prince KE, Musca CA, Dell JM, Faraone L (2008) Incorporation and activation of arsenic in MBE-grown HgCdTe. Semicond Sci Technol 23(1):015014 12. WangCZ WXJ, Zhao J, Chang Y, Grein CH, Sivananthan S, Smith DJ (2007) Microstructure of interfacial HgTe/CdTe superlattice layers for growth of HgCdTe on CdZnTe (211) B substrates. J Cryst Growth 309(2):153–157 13. Zhang ZY, Gao H, Jie WQ, Guo DM, Kang RK, Li Y (2008) Chemical mechanical polishing and nanomechanics of semiconductor CdZnTe single crystals. Semicond Sci Technol 23(10):105023 14. Peterson KE (1982) Silicon as a mechanical material. Proc IEEE 70(5):420–475 15. ShoreyAB KKM, Johnson KM, Jacobs SD (2000) Nanoindentation hardness of particles used in magnetorheological finishing. Appl Opt 39(28):5194–5204 16. KomanduriR UN, Kirtane T, Gerlick R, Jain VK (2006) A new apparatus for finishing large size/large batch silicon nitride(Si3N4) balls for hybrid bearing applications by magnetic float polishing (MFP). Int J Mach Tools Manuf 46(2):151–169 17. Cheng HB, Feng ZJ, Wang YW, Lei ST (2005) Magnetorheological finishing of SiC aspheric mirrors. Mater Manuf Process 20(6):917–931 18. Ng D, Liang H (2008) Comparison of interfacial forces during post-chemical-mechanical-polishing cleaning. J Tribol Trans ASME 130(2):021603 19. Zhang ZY, Guo DM, Kang RK, Gao H, Li Y (2008) Chemical mechanical polishing research of CdZnTe functional crystalline with soft brittle nature. Chin J Mech Eng 44(12):215–220 in Chinese 20. Zha GQ, Jie WQ, Li Q, Liu YQ (2006) Mechanical polishing of CdZnTe single crystal and measurement of surface damage layer. J Funct Mater 37(1):120–122 in Chinese 21. Zeng DM, Jie WQ, Zha GQ, Wang T, Yang G (2007) Effect of annealing on the residual stress and strain distribution in CdZnTe wafers. J Cryst Growth 305(1):50–54 22. Moravec P, Hoschl P, Franc J, Belas E, Fesh R, Grill R, Horodysky P, Praus P (2006) Chemical polishing of CdZnTe substrates fabricated from crystals grown by the vertical-gradient freezing method. J Electron Mater 35(6):1206–1213