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Effect of scratching speed on phase transformations in high-speed scratching of monocrystalline silicon
Materials Science & Engineering A 772 (2020) 138836
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Short communication
Effect of scratching speed on phase transformations in high-speed scratching of monocrystalline silicon Bing Wang *, Shreyes N. Melkote, Swagath Saraogi, Peizhi Wang George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase transformations Raman spectroscopy Silicon High-speed scratching
This paper investigates the phase transformation of (100) monocrystalline silicon in high-speed scratching tests. Various phase transformations of silicon are detected using micro-Raman spectroscopy over a wide range of speeds from 1 mm/min to 25 m/s. Wurtzite silicon (Si-IV) phase with strong Raman intensity is observed in all scratches. The Raman intensity ratio of amorphous silicon to Si-IV in the scratched groove center increases exponentially beyond a scratching speed of 105 mm/min, resulting in a decreased residual scratched depth. The tests provide a basic understanding of the effect of scratching speed on the phase transformation of silicon.
1. Introduction It is well known that the diamond cubic phase of crystalline silicon (Si–I) can be transformed to other phases, including crystalline and amorphous phases, under high hydrostatic pressures. It has been found that, upon loading, Si–I transforms to a metastable metallic phase (Si-II) with β-tin structure at a hydrostatic pressure of ~12 GPa [1]. Si-II transforms to crystalline phases of Si-III and Si-XII at slow unloading rates, or to an amorphous phase (a-Si) at high unloading rates [2–4]. The reason for formation of the amorphous phase is insufficient time for recrystallization of silicon under high unloading rates. An important phenomenon is that Si-III, Si-XII, and a-Si occur within the ductile scratching regions, which indicates that such phases have a significant effect on the ductile deformation of silicon [5]. Apart from traditional diamond anvil cell tests, nanoindentation and scratch tests are the two most commonly used methods to investigate the mechanical response and phase transformation of silicon. Much research has been reported on the nanoindentation of silicon over a wide loading/ unloading rates (10 2/s-107/s), wherein phase transformations have also been studied [6–8]. Compared to nanoindentation, the scratch test can provide more insights into the mechanical response of silicon in material removal processes. Gassilloud et al. [9] investigated the deformation mechanism of (100) monocrystalline silicon at two scratching speeds of 2 μm/s and 100 μm/s, and detected amorphous silicon and Si-XII in the scratched groove at 2 μm/s while only amor phous silicon was detected at 100 μm/s. Chavoshi et al. [10] performed high temperature nanoscratching of (110) Si wafer under reduced
oxygen conditions at two scratching speeds of 0.1 μm/s and 10 μm/s. They found the scratch morphology was greatly affected by the scratching speed and temperature, although the speed was varied in a small range in their research. Gogotsi et al. [5] studied pressure-induced metallization in scratching of (111) monocrystalline silicon using micro-Raman spectroscopy at a constant scratching speed of 10 μm/s with Vickers and conical indenters. They reported that the phase transformations and associated material removal mechanisms were dependent on the indenter shape. Other researchers who have also performed scratching tests on monocrystalline silicon include Zarudi et al. [11], Choi et al. [12], Vodenitcharova et al. [13], Ravindra et al. [14], and Kovalchenko et al. [15]. However, most of the current research on the scratching/scribing behavior of silicon is limited to a low speed range (10 μm/s-2.67 mm/s) [3,5,9–15]. Existing knowledge about the effects of the scratching parameters, the scriber/indenter shape, and the crystallographic orientation of silicon on the material removal behavior is mainly derived from low speed scratching experiments. As introduced above, there is still a large gap in the fundamental understanding of the deformation behavior of silicon under high scratching speeds. Whether the phase transformation of silicon is sen sitive to the scratching speed is therefore still unclear. The insufficient knowledge has limited the complete understanding of material removal mechanisms in practical applications of high-speed processing of silicon, e.g., diamond wire sawing (DWS) of silicon wafers routinely performed at scratching speeds of 10 m/s-20 m/s. Several researchers including Pala et al. [16], Zhang et al. [17] and Alreja and Subbiah [18] have attempted high-speed scratching experiments on silicon, but the speeds
* Corresponding author. E-mail addresses: [email protected] (B. Wang), [email protected] (S.N. Melkote). https://doi.org/10.1016/j.msea.2019.138836 Received 25 October 2019; Received in revised form 16 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
B. Wang et al.
Materials Science & Engineering A 772 (2020) 138836
used in these studies including 50 m/s [16], 40 m/s [17] and 1 m/s [18] were either significantly higher or lower than the speeds used in prac tical applications such as DWS, lapping and polishing, etc. In addition, the scratching speed in these studies was kept constant and therefore the effect of scratching speed on material deformation and phase trans formation was not reported. Therefore, a specialized setup for high-speed scratching of silicon was developed in the present work to investigate the effect of scratching speed, which was varied in two wide ranges of 1 mm/min to 10,000 mm/min and from 10 m/s (6 � 105 mm/min) to 25 m/s (1.5 � 106 mm/min). The scratching speed range of 10 m/s-25 m/s is 4–8 orders of magnitudes higher than the commonly researched speed range [3,5,9–15]. This paper provides new insights into phase transformations of monocrystalline silicon during an ultrahigh-speed scratching process.
A high-speed spindle (Fraureuth FS 33–60) with a speed range of 5000–60,000 RPM was used to achieve the high scratching speeds. Scratching was performed using a precisely manufactured aluminum disk [Fig. 1(b)] attached to the high-speed spindle, on which a conical tip diamond indenter with 120� included angle and a tip radius of 3 μm [Fig. 1(c)] was mounted with set screws at a radius of 19 mm. All highspeed scratching experiments with scratching speeds ranging from 10 m/s to 25 m/s were conducted with high-speed rotation of the spindle motor. The feeding speed was fixed at 2500 mm/min to avoid inter ference between adjacent scratching circles. The scratching direction and the scratching parameters are presented in Fig. 1(d) and (e). Although the scratching direction changes contin uously along the scratching path, the region of the scratches selected for subsequent micro-Raman spectroscopy and surface observation is located in the middle of the silicon coupon as indicated by the small blue rectangle [Fig. 1(d)] where the scratching direction is [110]. In com parison, the low speed scratching experiments were performed with the diamond indenter mounted directly to the spindle. The scratching speeds of 1 mm/min to 10,000 mm/min were obtained from the linear movement of the X–Y motion stages. The scratching direction of interest in each case was [110]. Scratching depths of 100 nm and 200 nm were used to ensure the mode of deformation in all scratching tests was pri marily ductile. Experiments under each set of input conditions were performed five times to ensure repeatability of the results. Micro-Raman spectroscopy was performed on selected scratches using a Raman spectrometer (Renishaw Invia) to determine the silicon phases produced in the scratched grooves. An argon ion laser with a wavelength of 488 nm was used. The residual depths of the scratched
2. Experimental procedure The experimental setup for high-speed scratching of silicon was shown in Fig. 1. Microelectronic grade polished monocrystalline (100) silicon wafers were used in the scratching experiments. The silicon wafer was sliced into rectangular coupons with dimensions of 10 mm � 25 mm. The silicon coupons were then fixed to a vacuum chuck mounted on a three-axis force dynamometer (Kistler 9256C), which was used to detect contact between the diamond indenter and the silicon coupon, thereby establishing the reference for specifying the scratching depth. The dynamometer was mounted on X-Y-Z motion stages (Aerotech ANT4V) with a positioning resolution of 1 nm in the scratching depth (Z) direction.
Fig. 1. Experimental setup of high-speed scratching (a) overall setup, (b) scratching disk, (c) diamond indenter, (d) schematic of scratching test, and (e) crosssectional view of I–I in Fig. d. 2
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Materials Science & Engineering A 772 (2020) 138836
grooves were measured using a 3D optical profiler (Bruker ContourGT-I, 50X objective). High resolution images of the scratched grooves were obtained using a scanning electron microscope (Zeiss Ultra60).
This indicates that the metastable Si-II phase was almost completely transformed to the amorphous phase. At a scanning distance of 600 nm from the groove centerline, no Si-IV was detected at both 10 m/s and 20 m/s. Furthermore, it can be seen from Fig. 2 that all Raman shifts of Si–I near the scratched edge (800 nm) are larger than 520 cm 1, which in dicates the presence of compressive residual stress [19]. Another sig nificant finding is that Si-IV was formed within all grooves independent of the scratching speed, and the Raman peak value of Si-IV in the 500 cm 1 – 509 cm 1 band always had the highest intensity. In addition, a Raman peak shift at 290 cm 1 was identified in the 1 mm/min scratched groove. Kailer et al. [6] also found a Raman peak shift of 290 cm 1 in optically darker regions of their nanoindentation impression, and they classified this peak as a Si-IV phase. The formation mechanism of Si-IV has been attributed to the inter section of twins produced during plastic deformation of Si–I [20]. The 500 cm 1 – 509 cm 1 Raman band corresponding to Si-IV indicates that the transition from Si–I to Si-IV is rather continuous, while the Raman intensity of Si-IV depends on the density of intersecting twins within the plastically deformed region. Using transmission electron microscopy (TEM) analysis of indentations made on monocrystalline silicon, Pirouz et al. [21] observed Si-IV appearing as ribbons or platelets embedded in Si–I around the indented area. Twins of Si–I were also observed un derneath the amorphous layer in the scratched grooves reported by Zhang et al. [17]. The twin density was especially high under the center of the indenter tip, and the Si-IV phase was probably formed when two twins intersected with each other. It can also be deduced from the Raman spectra in Fig. 2 that the stress state under the center of the indenter tip is favorable for the formation of Si-IV. Kobliska and Solin [22] proposed that the wurtzite silicon structure (i.e. Si-IV) is central to the microcrystalline theory of formation of a-Si. It has also been suggested that approximately half of the a-Si atoms are randomly arranged while the other half appear as wurtzite microcrys talline silicon, which implies a close relationship between the Si-IV and a-Si phases. In recent years, the Si-IV phase, which has a hexagonal crystal structure, has been recognized as a promising material for the
3. Results and discussion It was found that different phases of silicon were produced under different scratching speeds due to the variable loading and unloading rates. The phase components were also different across the scratched groove, which can be attributed to the varying stress states underneath the indenter. Gogotsi et al. [5], Trachet and Subhash [8], and Kailer et al. [6] also reported the variation of phase components in different positions after nanoindentation or low speed scratching of mono crystalline silicon. Consequently, line scanning micro-Raman spectros copy was performed across the scratched grooves. Fig. 2 shows a comparison of the phase components detected across the scratched grooves produced under different scratching speeds. Line scanning was performed at an interval of 200 nm from the scratched groove center as the inserted image in Fig. 2(a) shows. The phases were composed of a-Si and Si-IV in the scratched groove center for all scratching speeds investigated. Nevertheless, different phases were detected along the scanned line within the scratched grooves. From the centerline of the scratched groove to the groove edge, Si-III and Si-XII were detected at a scratching speed of 1 mm/min [Fig. 2(a) and (b)]. With the test position moving from the center to the groove edge, the phase components evolved from a-Si and Si-IV to Si-III and SiXII. At test positions near the groove edge, Si–I was also detected because either the Raman laser spot overlapped the undisturbed silicon surface near the groove edge or the phase transformation layer was thinner at this location. The presence of Si-III and Si-XII indicates the unloading rate is not high enough for the formation of pure a-Si at a scratching speed of 1 mm/min. In contrast, there was no Si-III and Si-XII in the grooves produced at scratching speeds greater than 1 mm/min (Note: only the Raman spectra of scratched grooves made at 10 m/s and 20 m/s are presented in Fig. 2).
Fig. 2. Raman spectra of different positions within the scratched grooves made at (a) 1 mm/min, (b) 1 mm/min with a smaller scale of Raman spectra intensity than Fig. a, (c) 10 m/s, and (d) 20 m/s (the 0–800 nm in all legends indicates the distance from the scratch centerline). 3
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fabrication of Si nanowires or nanotubes [23]. The findings of the cur rent paper suggest that scratching of monocrystalline silicon with the appropriate indenter geometry and scratching parameters can poten tially provide a low-cost approach to produce Si-IV. Zeng et al. [24] also observed that Si-IV can be formed through shear-induced phase trans formation from Si–I using nanoscratching, but the effect of scratching speed on the phase transformations was not reported. The Raman spectra in Fig. 2 also show that the intensities of different phases in the scratched grooves vary greatly with scratching speed. Selecting the scratched groove center as a reference position, the Raman spectra measured at different scratching speeds were compared. Considering the groove center is characterized by only a-Si and Si-IV, a parameter defined as the ratio of the Raman intensities of a-Si to Si-IV is proposed to quantify the relative amounts of amorphous phase produced at different scratching speeds. The total Raman intensity of a particular phase is calculated by integrating over the full width at half-maximum of the corresponding Raman peaks. In order to quantitatively describe the Raman intensities corresponding to the a-Si and Si-IV peaks, the Gaussian distribution was fit to the a-Si peaks while the Lorentzian distribution was fit to the Si-IV peaks [25]. Fig. 3 plots the Raman in tensity ratios as a function of the scratching speed. It can be seen from Fig. 3 that the Raman intensity ratio increases slightly as the scratching speed increases from 1 mm/min to 10,000 mm/min, but it increases exponentially beyond a scratching speed of 10 m/s (or rather 105 mm/min as determined from the fitted curve in Fig. 3). This indicates that more a-Si is formed at higher scratching speeds due to the higher unloading rates at these speeds. Based on the variation of the Raman intensity ratio, the scratching speed can be subdivided into three speed ranges of 1 mm/min – 10 mm/min (low speed range), 10 mm/min – 10,000 mm/min (medium speed range), and 10,000 mm/min – 20 m/s (high speed range). Transformation from Si–I to other phases is accompanied by volume change due to the different lattice structures of the phases. It is wellknown that the formation of Si-II from Si–I is accompanied by ~22% volume reduction due to the denser body-centered tetragonal structure of Si-II [26]. Formation of Si-XII due to phase transformation from Si-II under low rate decompression induces about 9% volume expansion, while a 2% volume recovery occurs during gradual phase trans formation from Si-XII to Si-III at lower hydrostatic pressures [27]. In contrast, formation of Si-IV, which has a hexagonal diamond structure and the same atomic density as Si–I, does not induce volume change. Moreover, the transformation from Si–I to an amorphous phase is accompanied by 3%–10% volume expansion due to the lower atomic density of amorphous silicon [28]. Consequently, the effect of material volume change due to phase transformation during scratching of silicon must be evident in the groove depth measurements after scratching.
Fig. 4 presents the variation of the residual depth of the scratched grooves with scratching speed. The residual depths of each groove were measured at five different locations along the groove and the measure ments were averaged. It can be seen from the figure that the residual depth has a negative linear correlation with the logarithm of the scratching speed in the 1 mm/min – 10 m/s range at both scratching depths of 100 nm and 200 nm. Comparatively, a negative exponential correlation is observed in the 10 m/s – 25 m/s range. From this result, it can be deduced that a larger amount of a-Si produced at higher scratching speeds leads to a larger volume expansion of the deformed material. Consequently, the residual depth of the groove after scratching decreases with increase in the scratching speed. Although the quanti tative relationship between the phase components and the amorphous layer thickness within the scratched grooves requires further analysis (e. g., using TEM), the influence of material volume expansion due to phase transformation can still be discerned from Figs. 3 and 4. In high-speed scratching, the phase transformations of mono crystalline silicon are induced by the effects of different factors including the applied stress and strain rate. Because of the small length scale (nanometers) of the scratching process and the lack of knowledge of the emissivity properties of the metastable high-pressure metallic phase of silicon, it is very difficult to reliably measure the temperatures generated underneath the scriber during high-speed scribing [29]. Ravindra et al. [30] performed micro-laser assisted machining of silicon and reported the effect of temperature induced by laser heating on the phase transformations. They found that a laser power of 45 W resulted in temperatures as high as 1000 � C, which produced an annealing effect on the machined surface. As a result, all high-pressure phases were annealed and recrystallized to the original diamond cubic structure (Si–I). Based on their findings, it would be expected that the amorphous silicon formed at the high scratching speeds used in our work would be annealed and transformed to Si–I. Consequently, less amorphous silicon would be present at higher scratching speeds. However, our results do not support this expectation. In addition, Zarudi et al. [11] studied the effect of temperature and stress on plastic deformation of mono crystalline silicon induced by scratching. In their work, the temperature was varied from room temperature to 196 � C. Their results showed that the size of the amorphous transformation zone in the sample sub surface was primarily dependent on the stress field applied, while the influence of temperature variation on the phase transformation was surprisingly small. Based on the foregoing discussion, although higher temperatures may occur at higher scratching speeds, the effects of temperature on the phase transformations of monocrystalline silicon are expected to be insignificant. In contrast, the high unloading rates, associated with the
Fig. 3. Variation in Raman intensity ratio of a-Si to Si-IV in the center of scratched grooves with scratching speed.
Fig. 4. Variation of residual depth of scratched grooves with scratching speed. 4
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high scratching speeds, play a more dominant role on the phase trans formations of monocrystalline silicon.
[2] J. Yan, H. Takahashi, X. Gai, H. Harada, J. Tamaki, T. Kuriyagawa, Load effects on the phase transformation of single-crystal silicon during nanoindentation tests, Mater. Sci. Eng. A 423 (2006) 19–23. [3] H. Wu, S.N. Melkote, Effect of crystallographic orientation on ductile scribing of crystalline silicon: role of phase transformation and slip, Mater. Sci. Eng. A 549 (2012) 200–205. [4] S. Ruffell, J.E. Bradby, J.S. Williams, High pressure crystalline phase formation during nanoindentation: amorphous versus crystalline silicon, Appl. Phys. Lett. 89 (2006), 091919. [5] Y. Gogotsi, G. Zhou, S.S. Ku, S. Cetinkunt, Raman microspectroscopy analysis of pressure-induced metallization in scratching of silicon, Semicond. Sci. Technol. 16 (2001) 345–352. [6] A. Kailer, Y. Gogotsi, K. Nickel, Phase transformations of silicon caused by contact loading, J. Appl. Phys. 81 (1997) 3057. [7] M. Budnitzki, M. Kuna, Stress induced phase transitions in silicon, J. Mech. Phys. Solids 95 (2016) 64–91. [8] A. Trachet, G. Subhash, Microscopic and spectroscopic investigation of phase evolution within static and dynamic indentations in single-crystal silicon, Mater. Sci. Eng. A 673 (2016) 321–331. [9] R. Gassilloud, C. Ballif, P. Gasser, G. Buerki, J. Michler, Deformation mechanisms of silicon during nanoscratching, Phys. Status Solidi A 202 (2005) 2858–2869. [10] S.Z. Chavoshi, S.C. Gallo, H. Dong, X. Luo, High temperature nanoscratching of single crystal silicon under reduced oxygen condition, Mater. Sci. Eng. A 684 (2017) 385–393. [11] I. Zarudi, T. Nguyen, L.C. Zhang, Effect of temperature and stress on plastic deformation in monocrystalline silicon induced by scratching, Appl. Phys. Lett. 86 (2005), 011922. [12] D.H. Choi, J.R. Lee, N.R. Kang, T.J. Je, J.Y. Kim, E.C. Jeon, Study on ductile mode machining of single-crystal silicon by mechanical machining, Int. J. Mach. Tool Manuf. 113 (2017) 1–9. [13] T. Vodenitcharova, O. Borrero-L� opez, M. Hoffman, Mechanics prediction of the fracture pattern on scratching wafers of single crystal silicon, Acta Mater. 60 (2012) 4448–4460. [14] D. Ravindra, M.K. Ghantasala, J. Patten, Ductile mode material removal and highpressure phase transformation in silicon during micro-laser assisted machining, Precis. Eng. 36 (2012) 364–367. [15] A.M. Kovalchenko, S. Goel, I.M. Zakiev, E.A. Pashchenko, R. Al-Sayegh, Suppressing scratch-induced brittle fracture in silicon by geometric design modification of the abrasive grits, J. Mater. Res. Tech. 8 (2019) 703–712. [16] U. Pala, S. Süssmaier, F. Kuster, K. Wegener, Experimental investigation of tool wear in electroplated diamond wire sawing of silicon, Proc. CIRP 77 (2018) 371–374. [17] Z. Zhang, F. Meng, J. Cui, B. Wang, Z. Wang, Y. Lu, H. Hassan, D. Guo, Deformation induced complete amorphization at nanoscale in a bulk silicon, AIP Adv. 9 (2019), 025101. [18] C. Alreja, S. Subbiah, Low pressure phase transformations during high-speed, hightemperature scratching of silicon, J. Micro Nano-Manuf. 6 (2018), 041001. [19] P.K. Kulshreshtha, K.M. Youssef, G. Rozgonyi, Nano-indentation: a tool to investigate crack propagation related phase transitions in PV silicon, Solar Energy Mater. Sol. Cells 96 (2012) 166–172. [20] U. Dahmen, C.J. Hetherington, P. Pirouz, K.H. Westmacott, The formation of hexagonal silicon at twin intersections, Scr. Metall. 23 (1989) 269–272. [21] P. Pirouz, R. Chaim, U. Dahmen, K.H. Westmacott, The martensitic transformation in silicon-I experimental observations, Acta Metall. Mater. 38 (1990) 313–322. [22] R.J. Kobliska, S.A. Solin, Raman spectrum of wurtzite silicon, Phys. Rev. B 8 (1973) 3799. [23] R.A. Minamisawa, M.J. Süess, R. Spolenak, J. Faist, C. David, J. Gobrecht, K. K. Bourdelle, H. Sigg, Top-down fabricated silicon nanowires under tensile elastic strain up to 4.5%, Nat. Commun. 3 (2012) 1096. [24] G. Zeng, B. Krick, N. Tansu, Shear-induced phase transformation: from singlecrystal silicon to Si-IV, APS March Meet. (2015). Abstract id. L9.008. [25] J. Yan, T. Asami, T. Kuriyagawa, Nondestructive measurement of machininginduced amorphous layers in single-crystal silicon by laser micro-Raman spectroscopy, Precis. Eng. 32 (2008) 186–195. [26] M.T. Yin, M.L. Cohen, Theory of static structural properties, crystal stability, and phase transformations: application to Si and Ge, Phys. Rev. B 26 (1982) 5668. [27] B.G. Pfrommer, M. Cote, S.G. Louie, M.L. Cohen, Ab initio study of silicon in the R8 phase, Phys. Rev. B 56 (1997) 6662. [28] S.C. Moss, J.E. Graczyk, Evidence of voids within the as-deposited structure of glassy silicon, Phys. Rev. Lett. 23 (1969) 1167. [29] L. Dong, J.A. Patten, Laser heating and thermally enhanced deformation of silicon, ASME 2007 Int. Manuf. Sci. Eng. Confer. (2007) 305–309. [30] D. Ravindra, M.K. Ghantasala, J. Patten, Ductile mode material removal and highpressure phase transformation in silicon during micro-laser assisted machining, Precis. Eng. 36 (2012) 364–367.
4. Conclusions The paper presented the results of phase transformation and material volume expansion during high-speed scratching of monocrystalline sil icon. The effect of scratching speed over a wide range (from 1 mm/min to 25 m/s) on phase transformation of monocrystalline silicon was revealed. The phase components across the scratched groove presented great variation due to different stress states underneath the indenter. Raman spectra measurements revealed that a large amount of Si-IV was formed under the center of the indenter tip. Si-III and Si-XII were formed at the low scratching speed of 1 mm/min, while only a-Si and Si-IV were detected at higher scratching speeds. The Raman intensity ratio of a-Si to Si-IV in the center of scratched grooves showed an increasing trend with scratching speed. A larger volume expansion was induced by the greater amount of a-Si formed at high scratching speeds, which led to a decrease in the residual depth of the scratched grooves. These results contribute to advancing the physical understanding of phase transformation behavior of monocrystalline silicon at high scratching speeds, and serve to guide the industrial application of high-speed scratching (e.g. DWS) of silicon wafers. Author contribution statement Bing Wang: Methodology, Investigation, Writing- Original draft preparation. Shreyes N. Melkote: Supervision, Conceptualization, Writing- Reviewing and Editing. Swagath Saraogi: Visualization, Investigation. Peizhi Wang: Investigation, Validation. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of the National Science Foundation (CMMI Grant No.1538293) for funding the study. Parts of the work reported in the paper were performed at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant No. ECCS-1542174). The first author would also like to acknowledge the support of the International Postdoctoral Exchange Fellowship Program of China (20180064). References [1] J.Z. Hu, L.D. Merkle, C.S. Menoni, I. L Spain, Crystal data for high-pressure phases of silicon, Phys. Rev. B 34 (1986) 4679.
5
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Short communication
Effect of scratching speed on phase transformations in high-speed scratching of monocrystalline silicon Bing Wang *, Shreyes N. Melkote, Swagath Saraogi, Peizhi Wang George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Phase transformations Raman spectroscopy Silicon High-speed scratching
This paper investigates the phase transformation of (100) monocrystalline silicon in high-speed scratching tests. Various phase transformations of silicon are detected using micro-Raman spectroscopy over a wide range of speeds from 1 mm/min to 25 m/s. Wurtzite silicon (Si-IV) phase with strong Raman intensity is observed in all scratches. The Raman intensity ratio of amorphous silicon to Si-IV in the scratched groove center increases exponentially beyond a scratching speed of 105 mm/min, resulting in a decreased residual scratched depth. The tests provide a basic understanding of the effect of scratching speed on the phase transformation of silicon.
1. Introduction It is well known that the diamond cubic phase of crystalline silicon (Si–I) can be transformed to other phases, including crystalline and amorphous phases, under high hydrostatic pressures. It has been found that, upon loading, Si–I transforms to a metastable metallic phase (Si-II) with β-tin structure at a hydrostatic pressure of ~12 GPa [1]. Si-II transforms to crystalline phases of Si-III and Si-XII at slow unloading rates, or to an amorphous phase (a-Si) at high unloading rates [2–4]. The reason for formation of the amorphous phase is insufficient time for recrystallization of silicon under high unloading rates. An important phenomenon is that Si-III, Si-XII, and a-Si occur within the ductile scratching regions, which indicates that such phases have a significant effect on the ductile deformation of silicon [5]. Apart from traditional diamond anvil cell tests, nanoindentation and scratch tests are the two most commonly used methods to investigate the mechanical response and phase transformation of silicon. Much research has been reported on the nanoindentation of silicon over a wide loading/ unloading rates (10 2/s-107/s), wherein phase transformations have also been studied [6–8]. Compared to nanoindentation, the scratch test can provide more insights into the mechanical response of silicon in material removal processes. Gassilloud et al. [9] investigated the deformation mechanism of (100) monocrystalline silicon at two scratching speeds of 2 μm/s and 100 μm/s, and detected amorphous silicon and Si-XII in the scratched groove at 2 μm/s while only amor phous silicon was detected at 100 μm/s. Chavoshi et al. [10] performed high temperature nanoscratching of (110) Si wafer under reduced
oxygen conditions at two scratching speeds of 0.1 μm/s and 10 μm/s. They found the scratch morphology was greatly affected by the scratching speed and temperature, although the speed was varied in a small range in their research. Gogotsi et al. [5] studied pressure-induced metallization in scratching of (111) monocrystalline silicon using micro-Raman spectroscopy at a constant scratching speed of 10 μm/s with Vickers and conical indenters. They reported that the phase transformations and associated material removal mechanisms were dependent on the indenter shape. Other researchers who have also performed scratching tests on monocrystalline silicon include Zarudi et al. [11], Choi et al. [12], Vodenitcharova et al. [13], Ravindra et al. [14], and Kovalchenko et al. [15]. However, most of the current research on the scratching/scribing behavior of silicon is limited to a low speed range (10 μm/s-2.67 mm/s) [3,5,9–15]. Existing knowledge about the effects of the scratching parameters, the scriber/indenter shape, and the crystallographic orientation of silicon on the material removal behavior is mainly derived from low speed scratching experiments. As introduced above, there is still a large gap in the fundamental understanding of the deformation behavior of silicon under high scratching speeds. Whether the phase transformation of silicon is sen sitive to the scratching speed is therefore still unclear. The insufficient knowledge has limited the complete understanding of material removal mechanisms in practical applications of high-speed processing of silicon, e.g., diamond wire sawing (DWS) of silicon wafers routinely performed at scratching speeds of 10 m/s-20 m/s. Several researchers including Pala et al. [16], Zhang et al. [17] and Alreja and Subbiah [18] have attempted high-speed scratching experiments on silicon, but the speeds
* Corresponding author. E-mail addresses: [email protected] (B. Wang), [email protected] (S.N. Melkote). https://doi.org/10.1016/j.msea.2019.138836 Received 25 October 2019; Received in revised form 16 December 2019; Accepted 16 December 2019 Available online 17 December 2019 0921-5093/© 2019 Elsevier B.V. All rights reserved.
B. Wang et al.
Materials Science & Engineering A 772 (2020) 138836
used in these studies including 50 m/s [16], 40 m/s [17] and 1 m/s [18] were either significantly higher or lower than the speeds used in prac tical applications such as DWS, lapping and polishing, etc. In addition, the scratching speed in these studies was kept constant and therefore the effect of scratching speed on material deformation and phase trans formation was not reported. Therefore, a specialized setup for high-speed scratching of silicon was developed in the present work to investigate the effect of scratching speed, which was varied in two wide ranges of 1 mm/min to 10,000 mm/min and from 10 m/s (6 � 105 mm/min) to 25 m/s (1.5 � 106 mm/min). The scratching speed range of 10 m/s-25 m/s is 4–8 orders of magnitudes higher than the commonly researched speed range [3,5,9–15]. This paper provides new insights into phase transformations of monocrystalline silicon during an ultrahigh-speed scratching process.
A high-speed spindle (Fraureuth FS 33–60) with a speed range of 5000–60,000 RPM was used to achieve the high scratching speeds. Scratching was performed using a precisely manufactured aluminum disk [Fig. 1(b)] attached to the high-speed spindle, on which a conical tip diamond indenter with 120� included angle and a tip radius of 3 μm [Fig. 1(c)] was mounted with set screws at a radius of 19 mm. All highspeed scratching experiments with scratching speeds ranging from 10 m/s to 25 m/s were conducted with high-speed rotation of the spindle motor. The feeding speed was fixed at 2500 mm/min to avoid inter ference between adjacent scratching circles. The scratching direction and the scratching parameters are presented in Fig. 1(d) and (e). Although the scratching direction changes contin uously along the scratching path, the region of the scratches selected for subsequent micro-Raman spectroscopy and surface observation is located in the middle of the silicon coupon as indicated by the small blue rectangle [Fig. 1(d)] where the scratching direction is [110]. In com parison, the low speed scratching experiments were performed with the diamond indenter mounted directly to the spindle. The scratching speeds of 1 mm/min to 10,000 mm/min were obtained from the linear movement of the X–Y motion stages. The scratching direction of interest in each case was [110]. Scratching depths of 100 nm and 200 nm were used to ensure the mode of deformation in all scratching tests was pri marily ductile. Experiments under each set of input conditions were performed five times to ensure repeatability of the results. Micro-Raman spectroscopy was performed on selected scratches using a Raman spectrometer (Renishaw Invia) to determine the silicon phases produced in the scratched grooves. An argon ion laser with a wavelength of 488 nm was used. The residual depths of the scratched
2. Experimental procedure The experimental setup for high-speed scratching of silicon was shown in Fig. 1. Microelectronic grade polished monocrystalline (100) silicon wafers were used in the scratching experiments. The silicon wafer was sliced into rectangular coupons with dimensions of 10 mm � 25 mm. The silicon coupons were then fixed to a vacuum chuck mounted on a three-axis force dynamometer (Kistler 9256C), which was used to detect contact between the diamond indenter and the silicon coupon, thereby establishing the reference for specifying the scratching depth. The dynamometer was mounted on X-Y-Z motion stages (Aerotech ANT4V) with a positioning resolution of 1 nm in the scratching depth (Z) direction.
Fig. 1. Experimental setup of high-speed scratching (a) overall setup, (b) scratching disk, (c) diamond indenter, (d) schematic of scratching test, and (e) crosssectional view of I–I in Fig. d. 2
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grooves were measured using a 3D optical profiler (Bruker ContourGT-I, 50X objective). High resolution images of the scratched grooves were obtained using a scanning electron microscope (Zeiss Ultra60).
This indicates that the metastable Si-II phase was almost completely transformed to the amorphous phase. At a scanning distance of 600 nm from the groove centerline, no Si-IV was detected at both 10 m/s and 20 m/s. Furthermore, it can be seen from Fig. 2 that all Raman shifts of Si–I near the scratched edge (800 nm) are larger than 520 cm 1, which in dicates the presence of compressive residual stress [19]. Another sig nificant finding is that Si-IV was formed within all grooves independent of the scratching speed, and the Raman peak value of Si-IV in the 500 cm 1 – 509 cm 1 band always had the highest intensity. In addition, a Raman peak shift at 290 cm 1 was identified in the 1 mm/min scratched groove. Kailer et al. [6] also found a Raman peak shift of 290 cm 1 in optically darker regions of their nanoindentation impression, and they classified this peak as a Si-IV phase. The formation mechanism of Si-IV has been attributed to the inter section of twins produced during plastic deformation of Si–I [20]. The 500 cm 1 – 509 cm 1 Raman band corresponding to Si-IV indicates that the transition from Si–I to Si-IV is rather continuous, while the Raman intensity of Si-IV depends on the density of intersecting twins within the plastically deformed region. Using transmission electron microscopy (TEM) analysis of indentations made on monocrystalline silicon, Pirouz et al. [21] observed Si-IV appearing as ribbons or platelets embedded in Si–I around the indented area. Twins of Si–I were also observed un derneath the amorphous layer in the scratched grooves reported by Zhang et al. [17]. The twin density was especially high under the center of the indenter tip, and the Si-IV phase was probably formed when two twins intersected with each other. It can also be deduced from the Raman spectra in Fig. 2 that the stress state under the center of the indenter tip is favorable for the formation of Si-IV. Kobliska and Solin [22] proposed that the wurtzite silicon structure (i.e. Si-IV) is central to the microcrystalline theory of formation of a-Si. It has also been suggested that approximately half of the a-Si atoms are randomly arranged while the other half appear as wurtzite microcrys talline silicon, which implies a close relationship between the Si-IV and a-Si phases. In recent years, the Si-IV phase, which has a hexagonal crystal structure, has been recognized as a promising material for the
3. Results and discussion It was found that different phases of silicon were produced under different scratching speeds due to the variable loading and unloading rates. The phase components were also different across the scratched groove, which can be attributed to the varying stress states underneath the indenter. Gogotsi et al. [5], Trachet and Subhash [8], and Kailer et al. [6] also reported the variation of phase components in different positions after nanoindentation or low speed scratching of mono crystalline silicon. Consequently, line scanning micro-Raman spectros copy was performed across the scratched grooves. Fig. 2 shows a comparison of the phase components detected across the scratched grooves produced under different scratching speeds. Line scanning was performed at an interval of 200 nm from the scratched groove center as the inserted image in Fig. 2(a) shows. The phases were composed of a-Si and Si-IV in the scratched groove center for all scratching speeds investigated. Nevertheless, different phases were detected along the scanned line within the scratched grooves. From the centerline of the scratched groove to the groove edge, Si-III and Si-XII were detected at a scratching speed of 1 mm/min [Fig. 2(a) and (b)]. With the test position moving from the center to the groove edge, the phase components evolved from a-Si and Si-IV to Si-III and SiXII. At test positions near the groove edge, Si–I was also detected because either the Raman laser spot overlapped the undisturbed silicon surface near the groove edge or the phase transformation layer was thinner at this location. The presence of Si-III and Si-XII indicates the unloading rate is not high enough for the formation of pure a-Si at a scratching speed of 1 mm/min. In contrast, there was no Si-III and Si-XII in the grooves produced at scratching speeds greater than 1 mm/min (Note: only the Raman spectra of scratched grooves made at 10 m/s and 20 m/s are presented in Fig. 2).
Fig. 2. Raman spectra of different positions within the scratched grooves made at (a) 1 mm/min, (b) 1 mm/min with a smaller scale of Raman spectra intensity than Fig. a, (c) 10 m/s, and (d) 20 m/s (the 0–800 nm in all legends indicates the distance from the scratch centerline). 3
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fabrication of Si nanowires or nanotubes [23]. The findings of the cur rent paper suggest that scratching of monocrystalline silicon with the appropriate indenter geometry and scratching parameters can poten tially provide a low-cost approach to produce Si-IV. Zeng et al. [24] also observed that Si-IV can be formed through shear-induced phase trans formation from Si–I using nanoscratching, but the effect of scratching speed on the phase transformations was not reported. The Raman spectra in Fig. 2 also show that the intensities of different phases in the scratched grooves vary greatly with scratching speed. Selecting the scratched groove center as a reference position, the Raman spectra measured at different scratching speeds were compared. Considering the groove center is characterized by only a-Si and Si-IV, a parameter defined as the ratio of the Raman intensities of a-Si to Si-IV is proposed to quantify the relative amounts of amorphous phase produced at different scratching speeds. The total Raman intensity of a particular phase is calculated by integrating over the full width at half-maximum of the corresponding Raman peaks. In order to quantitatively describe the Raman intensities corresponding to the a-Si and Si-IV peaks, the Gaussian distribution was fit to the a-Si peaks while the Lorentzian distribution was fit to the Si-IV peaks [25]. Fig. 3 plots the Raman in tensity ratios as a function of the scratching speed. It can be seen from Fig. 3 that the Raman intensity ratio increases slightly as the scratching speed increases from 1 mm/min to 10,000 mm/min, but it increases exponentially beyond a scratching speed of 10 m/s (or rather 105 mm/min as determined from the fitted curve in Fig. 3). This indicates that more a-Si is formed at higher scratching speeds due to the higher unloading rates at these speeds. Based on the variation of the Raman intensity ratio, the scratching speed can be subdivided into three speed ranges of 1 mm/min – 10 mm/min (low speed range), 10 mm/min – 10,000 mm/min (medium speed range), and 10,000 mm/min – 20 m/s (high speed range). Transformation from Si–I to other phases is accompanied by volume change due to the different lattice structures of the phases. It is wellknown that the formation of Si-II from Si–I is accompanied by ~22% volume reduction due to the denser body-centered tetragonal structure of Si-II [26]. Formation of Si-XII due to phase transformation from Si-II under low rate decompression induces about 9% volume expansion, while a 2% volume recovery occurs during gradual phase trans formation from Si-XII to Si-III at lower hydrostatic pressures [27]. In contrast, formation of Si-IV, which has a hexagonal diamond structure and the same atomic density as Si–I, does not induce volume change. Moreover, the transformation from Si–I to an amorphous phase is accompanied by 3%–10% volume expansion due to the lower atomic density of amorphous silicon [28]. Consequently, the effect of material volume change due to phase transformation during scratching of silicon must be evident in the groove depth measurements after scratching.
Fig. 4 presents the variation of the residual depth of the scratched grooves with scratching speed. The residual depths of each groove were measured at five different locations along the groove and the measure ments were averaged. It can be seen from the figure that the residual depth has a negative linear correlation with the logarithm of the scratching speed in the 1 mm/min – 10 m/s range at both scratching depths of 100 nm and 200 nm. Comparatively, a negative exponential correlation is observed in the 10 m/s – 25 m/s range. From this result, it can be deduced that a larger amount of a-Si produced at higher scratching speeds leads to a larger volume expansion of the deformed material. Consequently, the residual depth of the groove after scratching decreases with increase in the scratching speed. Although the quanti tative relationship between the phase components and the amorphous layer thickness within the scratched grooves requires further analysis (e. g., using TEM), the influence of material volume expansion due to phase transformation can still be discerned from Figs. 3 and 4. In high-speed scratching, the phase transformations of mono crystalline silicon are induced by the effects of different factors including the applied stress and strain rate. Because of the small length scale (nanometers) of the scratching process and the lack of knowledge of the emissivity properties of the metastable high-pressure metallic phase of silicon, it is very difficult to reliably measure the temperatures generated underneath the scriber during high-speed scribing [29]. Ravindra et al. [30] performed micro-laser assisted machining of silicon and reported the effect of temperature induced by laser heating on the phase transformations. They found that a laser power of 45 W resulted in temperatures as high as 1000 � C, which produced an annealing effect on the machined surface. As a result, all high-pressure phases were annealed and recrystallized to the original diamond cubic structure (Si–I). Based on their findings, it would be expected that the amorphous silicon formed at the high scratching speeds used in our work would be annealed and transformed to Si–I. Consequently, less amorphous silicon would be present at higher scratching speeds. However, our results do not support this expectation. In addition, Zarudi et al. [11] studied the effect of temperature and stress on plastic deformation of mono crystalline silicon induced by scratching. In their work, the temperature was varied from room temperature to 196 � C. Their results showed that the size of the amorphous transformation zone in the sample sub surface was primarily dependent on the stress field applied, while the influence of temperature variation on the phase transformation was surprisingly small. Based on the foregoing discussion, although higher temperatures may occur at higher scratching speeds, the effects of temperature on the phase transformations of monocrystalline silicon are expected to be insignificant. In contrast, the high unloading rates, associated with the
Fig. 3. Variation in Raman intensity ratio of a-Si to Si-IV in the center of scratched grooves with scratching speed.
Fig. 4. Variation of residual depth of scratched grooves with scratching speed. 4
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high scratching speeds, play a more dominant role on the phase trans formations of monocrystalline silicon.
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4. Conclusions The paper presented the results of phase transformation and material volume expansion during high-speed scratching of monocrystalline sil icon. The effect of scratching speed over a wide range (from 1 mm/min to 25 m/s) on phase transformation of monocrystalline silicon was revealed. The phase components across the scratched groove presented great variation due to different stress states underneath the indenter. Raman spectra measurements revealed that a large amount of Si-IV was formed under the center of the indenter tip. Si-III and Si-XII were formed at the low scratching speed of 1 mm/min, while only a-Si and Si-IV were detected at higher scratching speeds. The Raman intensity ratio of a-Si to Si-IV in the center of scratched grooves showed an increasing trend with scratching speed. A larger volume expansion was induced by the greater amount of a-Si formed at high scratching speeds, which led to a decrease in the residual depth of the scratched grooves. These results contribute to advancing the physical understanding of phase transformation behavior of monocrystalline silicon at high scratching speeds, and serve to guide the industrial application of high-speed scratching (e.g. DWS) of silicon wafers. Author contribution statement Bing Wang: Methodology, Investigation, Writing- Original draft preparation. Shreyes N. Melkote: Supervision, Conceptualization, Writing- Reviewing and Editing. Swagath Saraogi: Visualization, Investigation. Peizhi Wang: Investigation, Validation. Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors acknowledge the support of the National Science Foundation (CMMI Grant No.1538293) for funding the study. Parts of the work reported in the paper were performed at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant No. ECCS-1542174). The first author would also like to acknowledge the support of the International Postdoctoral Exchange Fellowship Program of China (20180064). References [1] J.Z. Hu, L.D. Merkle, C.S. Menoni, I. L Spain, Crystal data for high-pressure phases of silicon, Phys. Rev. B 34 (1986) 4679.
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