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Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator
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CIRP-1343; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2015) xxx–xxx
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CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator Yasuhiro Kakinuma (2)*, Shunya Azami, Takasumi Tanabe Department of System Design Engineering, Faculty of Science & Technology, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultra-precision Surface integrity Optical micro-resonator
The optical micro-resonator, which stores light at a certain spot, is essential in next-generation optical signal processing. Single-crystal calcium fluoride (CaF2) is the most suitable material for this element. Ultra-precision turning is a feasible fabrication process for CaF2 optical micro-resonators. In this study, the influence of subsurface damage on the resonator’s Q factor is investigated. TEM observation shows that the subsurface layer of up to several tens of nanometers thickness changed from single-crystal to polycrystalline morphology due to ultra-precision turning. A diamond tool with 08 rake angle results in lower damage than one with negative rake angle, which enhances the resonator’s performance. ß 2015 CIRP.
1. Introduction For extremely high signal processing and reducing energy loss in electronic devices, conventional signal processing circuits must be replaced by optical versions. The optical micro-resonator, which enables the localization of light at certain spots, is an essential component for optical signal processing. Although semiconductor materials such as Silica or SiNb3 are generally employed in research on optical micro-resonators [1], single-crystal CaF2 [2] is expected to be the most suitable material for the device from an absorption coefficient viewpoint. Etching and irradiating with a CO2 laser are generally employed for the manufacturing of semiconductor materials into optical micro-resonators. However, these processes are prohibited with CaF2, because anisotropic etching is inadequate for the fabrication of a bulge-shaped resonator [3] and a single-crystal structure will be broken by laser heat. As a result, the actual performance of CaF2 optical micro-resonators fabricated by these processes is less than ideal. The most feasible prospective fabrication technique for CaF2 resonators is ultra-precision turning and polishing, as shown by Maleki [3]. While the polishing process is required to make the surface of a resonant part smoother [4], prolonged polishing deteriorates the accuracy of the part’s form, which affects the resonator’s performance. To achieve not only finer surface quality but also sharper resonant parts, the total material amount removed by the polishing process should be reduced to maintain the fine shape machined by turning. Hence, it is necessary to produce a smoother surface during the ultra-precision turning process. Single-crystal CaF2’s characteristics include high brittleness and crystal anisotropy. Previous CaF2 machining studies have focused on face turning [5], in which the crystalline plane was constant. For cylindrical turning, the fabrication technique used for bulgeshaped resonators, the crystalline plane and cutting direction vary * Corresponding author. E-mail address: [email protected] (Y. Kakinuma).
continuously. Here, we experimentally investigate the turning performance of CaF2 to find the most appropriate cutting conditions. This study deeply analyzes the influence of cutting conditions on both the surface quality and sub-surface damage created by CaF2 turning via TEM observation. 2. Experimental setup and procedure Ultra-precision cylindrical turning (UPCT) of CaF2 was carried out by an ultra-precision aspheric surface machine tool. Conical CaF2 workpieces with a small cylindrical tip, 1 mm in diameter and 1 mm in length, with end face orientations (1 1 1), (1 1 0), and (1 0 0) were prepared. Figs. 1 and 2 show the experimental setup for the UPCT and the shapes of two single-crystal diamond tools designed to investigate the influence of rake angle on the cylindrical surface quality and degree of subsurface damage. The tool represented as tool #1 has a 208 rake angle, 0.2 mm nose radius, and 88 clearance. This shape was adopted because a negative rake angle and large nose radius are generally preferable when performing ductile-mode cutting in large-sized optical
Fig. 1. Experimental setup for the ultra-precision cylindrical turning (UPCT) of the single-crystal CaF2.
http://dx.doi.org/10.1016/j.cirp.2015.04.076 0007-8506/ß 2015 CIRP.
Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076
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CIRP-1343; No. of Pages 4 Y. Kakinuma et al. / CIRP Annals - Manufacturing Technology xxx (2015) xxx–xxx
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Fig. 2. Tool shape of the utilized single-crystal diamond tools.
Fig. 4. The observed surfaces at points a and b in Fig. 3 (a) after turning the CaF2 with end face (1 1 1).
Fig. 3. Crystallographic image of the observation point viewed from each end face.
materials. The tool represented as tool #2 has a 08 rake angle, 0.05 mm nose radius, and 158 clearance. Theoretically, since thrust force decreases with rake angle, a sharper tool with a 08 rake angle would make the cutting process more stable in cylindrical turning, and the resulting machined surface quality would be enhanced. The cylindrical machined surface quality after UPCT was evaluated from the perspective of surface roughness and crack initiation, which were measured by scanning white light interferometry and differential interference microscopy, respectively. Crystallographic images of the observation point as viewed from each end face are represented in Fig. 3. It is clear that each cycle of crystal structure at the cylindrical surface in the end face of (1 1 1), (1 1 0), and (1 0 0) corresponds to 1208, 1808, and 908, respectively. In order to investigate crack initiation with regard to the crystal’s anisotropy, the whole cylindrical surface after UPCT was observed at 158 intervals. In addition, the sub-surface damage under various cutting conditions was analyzed by TEM observation. 3. Influence of tool shape on surface quality Previous orthogonal cutting tests of single-crystal CaF2 showed that the minimum critical depth of cut is 50 nm to produce a homogeneous fine surface on all crystallographic orientations [6]. Based on this result, the cutting conditions of UPCT were determined as shown in Table 1. The tool edge radius of both tool #1 and tool #2 is approximately 20–30 nm as measured by SEM observation. Therefore, a cut depth of 50 nm is applicable to this cutting process. Table 1 Cutting conditions of UPCT for both tools. Cutting speed m/min Rotation speed min 1 Feed per revolution mm/rev (micrometer/rev) Depth of cut nm End face orientation
3.14 1000 0.5 50 (1 1 1), (1 1 0), (1 0 0)
When tool #1 is employed, it was found that cracks initiated periodically in the case of end faces (1 1 1) and (1 1 0). Fig. 4 shows the observed surfaces at points a and b in Fig. 3(a), after turning the CaF2 with end face (1 1 1). It was found from previous orthogonal cutting tests of the (1 1 0) plane that the critical depth of cut in the [ 1 1 1] direction was over 500 nm [6]. On the (1 1 0) plane, this is the same cutting direction at the point a. Hence, a fine surface with no cracking was obtained. However, cracks suddenly appeared along the cleavage plane’s side line at point b. This is because the cutting force mainly affects cleavage rather than slip planes. To investigate the influence of crystal anisotropy on surface quality after UPCT, the cylindrical surface was evaluated by measuring its surface roughness. Fig. 5 shows the results of surface roughness Ra
Fig. 5. Results of surface roughness Ra on the turned surface with end face (1 1 1), (1 1 0), and (1 0 0).
on the whole cylindrical surface turned with end faces (1 1 1), (1 1 0), and (1 0 0). For the result of the sample with end face (1 1 1), the surface became coarser at 608 intervals, in spite of the 1208 cycle of the crystal structure. Taking into account the crystallographic model, the crystal structure at the cylindrical surface is horizontally flipped when CaF2 with end face (1 1 1) rotates at 608 [6]. This is why surface roughness varies at 608 intervals for the (1 1 1) sample. Regarding the end face (1 1 0), the cylindrical surface quality deteriorated considerably compared to
Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076
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Fig. 6. Chatter vibrations with tool #1 during UPCT.
Fig. 8. Subsurface damage with tool #1 and tool #2.
Fig. 7. Differences in tool contact area with the CaF2 workpiece.
the others. The end face (1 1 0) is thus inappropriate for UPCT. To consider anisotropy, there is 1808 periodicity in the surface roughness as well as crystal structure. From both results, it is clear that the surface quality machined after UPCT strongly depends on the cycling of the crystal structure. Unlike the end faces (1 1 1) and (1 1 0), the end face (1 0 0) remarkably enhanced the surface quality. No cracks were observed over the whole cylindrical surface. The crystallographic model suggests that the symmetric crystal structure, along with the cutting direction, advantageously distributes the cutting force toward the cleavage plane, which usually causes the crack initiation. However, chatter vibration occurred irregularly with tool #1 despite the ultra-precision turning with nanometer-scale depth of cut, as shown in Fig. 6. From the wavenumber of chatter mark, the frequency is calculated to be around 4.9 kHz. Both low stiffness in the small-diameter microoptical part and small force fluctuations caused by crystal anisotropy could lead to unstable vibrations during UPCT. In future work, the chatter vibration generated in ultra-precision cutting will be analyzed by measuring the variation in small cutting force. When UPCT was carried out with tool #2 under the same cutting conditions in Table 1, the whole machined surface became much finer than that with tool #1, regardless of which crystal plane was selected as the end face. No chatter vibration occurred in using tool #2. Two possible reasons for the enhanced surface quality are considered: the 08 rake tool reduces the thrust force below that of the negative rake tool, so that the UPCT could become stable; the difference in tool contact area with the CaF2 workpiece, as shown in Fig. 7, may also contribute. In ultra-precision cutting, the cutting force is assumed to be greatly dependent on the plastic flow area, related to the contact area of the tool with the workpiece because of size effects. As the tool contact area with tool #2 was one-fourth less than that with tool #1, the cutting force would be reduced as well. With the adaptation of Colwell’s law, the chip flow angle is calculated as 89.48 and 88.88 in tools #1 and #2, respectively. The slight difference in chip flow angle may contribute to the reduction of thrust force in UPCT. As to tool wear, no significant change after the experiments was observed for both tools because the cutting length was not so long. For the following chapter, CaF2 with end face (1 0 0), which is appropriate for UPCT, is utilized for TEM observation. 4. Evaluation of sub-surface damage 4.1. Influence of tool shape In order to investigate the influence of tool shape on the degree of sub-surface damage, TEM observation was conducted for the fine cylindrical surface created by tool #1 and tool #2. The TEM sample was cut from point a in Fig. 3(c) and thinned to 50 nm using a FIB laser. Fig. 8(a) and (b) shows the TEM micrographs corresponding to tool #1 and tool #2, respectively. Although dislocation generally
Fig. 9. Close-up view of subsurface damage area in the tool #2.
occurs in the depth direction in ultra-precision cutting of Si [7], a unique dislocation line parallel to the surface was confirmed in UPCT of CaF2. Regarding damage depth, significant differences can be seen according to the tool shape. While two dislocation lines were observed at 18 and 46 nm depths in the sample cut with tool #1, a single dislocation line appeared at a depth of 21 nm in the sample cut with tool #2. Tool #1 involved a larger plastic flow area, wider tool contact area, and larger negative rake angle, which would produce a larger cutting force and eventually deteriorate the surface integrity. A magnification of the TEM image from tool #2 is shown in Fig. 9. Above the dislocation line, the crystal orientation was found to be disarranged and collapsed. FFT analysis on the disarranged area indicates that the crystal structure changed from mono- to polycrystalline due to UPCT. The crystal transformation cannot be confirmed below a depth of 35 nm. We concluded that tool #2 produced less subsurface damage than tool #1 as well as improved surface quality. 4.2. Relation between cut depth and damage depth The influence of cutting depth on the subsurface damage depth was evaluated through TEM observation. The cutting conditions of the turning tests are listed in Table 2. The results of the turning tests show no significant difference in external surface, and fine surfaces Table 2 Cutting condition with different depth of cut. Cutting speed m/min Rotation speed min 1 Feed per revolution mm/rev (micrometer/rev) Depth of cut nm End face orientation Crystal plane for TEM sample
3.14 1000 0.5 50, 200 (1 0 0) Point a (in Fig. 3(c))
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Fig. 10. Subsurface damage with 200 nm cut depth.
with no cracks could be produced under both cutting conditions. To compare subsurface damage, a TEM sample at a 200 nm depth of cut was prepared, cut from point a in Fig. 3(c). As shown in Fig. 10, the resulting TEM image indicates that an increase of cutting depth leads to a larger subsurface damage area. Moreover, it confirms an increase in the number of dislocation lines. 4.3. Relation between anisotropy and damage depth Differences in subsurface damage at points a, d, and e in Fig. 3(c) were investigated from the anisotropy perspective. Fig. 8(b) and Fig. 11 show TEM micrographs at points a, d, and e. The degree of subsurface damage varied according to crystal orientation. Compared to points a and e, point d has two dislocation lines and shows deeper damage. Point d corresponds to the (1 1 0) plane, which is the most difficult plane in which to promote plastic deformation [6]. As a result, relatively deep subsurface damage up to a depth of approximately 40 nm was left at point d.
Fig. 11. Subsurface damage at points d and e with end face (1 0 0).
200 nm and 50 nm were adopted, respectively. The diameter of the resonator was around 400 mm and no cracks were found on the entire cylindrical surface. The quality of an optical micro-resonator is generally characterized by the Q factor, which is defined as the continuous time to store light in a resonator. The trial-manufactured resonator by UPCT without polishing successfully achieved a Q factor on the order of 106, as measured by using a tunable laser. The time to localize light was approximately 1.0 ns. An ideal singlecrystal CaF2 resonator has a theoretical Q factor on the order of 1013 [3]. Therefore, even small amounts of subsurface damage in the range of tens of nanometers deep caused the loss of light absorption and performance degradation. In this study, the subsurface damage layer can be reduced to the range of tens of nanometers in depth. To achieve a higher Q factor, the removal of the reduced sub-surface damage layer is necessary, by surface treatment such as laser recovery and polishing processes. If such a surface treatment is applied, the removal of a very small amount of the subsurface layer enables the maintenance of a very fine form accuracy, which is considered an important factor in determining the resonator’s performance [8]. 6. Conclusion For the CaF2 optical micro resonator, the influence of cutting conditions on both the surface quality and subsurface damage in the ultra-precision cylindrical turning of single-crystal CaF2 was experimentally analyzed. The following results were obtained through this study. (1) Compared with the end faces (1 1 1) and (1 1 0), the end face (1 0 0) remarkably enhances the surface quality. In the case of the (1 0 0) end face, the symmetry of the crystal structure along with the cutting direction distributes the cutting force toward the cleavage plane in an advantageous manner. (2) From TEM observation, the subsurface crystal structure after UPCT changes from mono- to polycrystalline for a depth of tens of nanometers. A larger tool nose radius and larger negative rake angle produce multiple dislocation lines and deeper subsurface damage. (3) The increase of cutting depth leads to the extension of the subsurface damage area, with an increase in the number of dislocation lines, even if the external surface quality remains fine. In addition, subsurface damage on the cylindrical surface varies according to the crystal’s orientation. Especially on the (1 1 0) plane, relatively deep subsurface damage remains. (4) A trial CaF2 optical micro-resonator manufactured by UPCT only achieved a Q factor on the order of 106. The subsurface damage layer can be reduced to the range of tens of nanometers in depth. The removal of a very small amount of the subsurface damage layer could retain fine form accuracy if polishing is applied after turning.
5. Trial manufacture of the CaF2 optical micro resonator References Fig. 12 shows the appearance of the CaF2 resonator manufactured by UPCT and a magnified image of the bulge-shaped portion. For the rough cutting and finishing cutting, depths of cut of
Fig. 12. Trial-manufactured CaF2 optical micro resonator.
[1] Tanabe T, Notomi M, Kuramochi E, Shinya A, Taniyama H (2007) Trapping and Delaying Photons for One Nanosecond in an Ultra-small High-Q Photoniccrystal Nanocavity. Nature Photonics 1:49–52. [2] Savchenkov A-A, Ilchenko V-S, Matsko A-B, Maleki L (2004) Kilohertz Optical Resonances in Dielectric Crystal Cavities. Physical Review A 70:051804(R). [3] Grudinin I-S, Matsko A-B, Savchenkov A-A, Strekalov D, Ilchenko V-S, Maleki L (2006) Ultrahigh Q Crystalline Microcavities. Optics Communications 265:33–38. [4] Namba Y, Ohnishi N, Yoshida S, Harada K, Yoshida K (2004) Ultra-precision Float Polishing of Calcium Fluoride Single Crystals for Deep Ultra Violet Applications. CIRP Annals 53(1):459–462. [5] Yan J, Syoji K, Tamaki J (2004) Crystallographic Effects in Micro/Nano-machining of Single-crystal Calcium Fluoride. Journal of Vacuum Science and Technology B 22(1):46–51. [6] Azami S, Kudo H, Mizumoto Y, Tanabe T, Yan J, Kakinuma Y (2014) Experimental Study of Crystal Anisotropy based on Ultra-precision Cylindrical Turning of Single-crystal Calcium Fluoride. Precision Engineering 40:172–181. [7] Yan J, Asami T, Harada H, Kuriyagawa T (2012) Crystallographic Effect on Subsurface Damage Formation in Silicon Microcutting. CIRP Annals 61(1):131–134. [8] Lee H, Chen T, Li J, Yang K-Y, Jeon S, Painter O, Vahara K-J (2012) Chemically Etched Ultrahigh-Q Wedge-Resonator on a Silicon Chip. Nature Photonics 6:369–373.
Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076
CIRP-1343; No. of Pages 4 CIRP Annals - Manufacturing Technology xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
CIRP Annals - Manufacturing Technology jou rnal homep age : ht t p: // ees .e lse vi er . com /ci r p/ def a ult . asp
Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator Yasuhiro Kakinuma (2)*, Shunya Azami, Takasumi Tanabe Department of System Design Engineering, Faculty of Science & Technology, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultra-precision Surface integrity Optical micro-resonator
The optical micro-resonator, which stores light at a certain spot, is essential in next-generation optical signal processing. Single-crystal calcium fluoride (CaF2) is the most suitable material for this element. Ultra-precision turning is a feasible fabrication process for CaF2 optical micro-resonators. In this study, the influence of subsurface damage on the resonator’s Q factor is investigated. TEM observation shows that the subsurface layer of up to several tens of nanometers thickness changed from single-crystal to polycrystalline morphology due to ultra-precision turning. A diamond tool with 08 rake angle results in lower damage than one with negative rake angle, which enhances the resonator’s performance. ß 2015 CIRP.
1. Introduction For extremely high signal processing and reducing energy loss in electronic devices, conventional signal processing circuits must be replaced by optical versions. The optical micro-resonator, which enables the localization of light at certain spots, is an essential component for optical signal processing. Although semiconductor materials such as Silica or SiNb3 are generally employed in research on optical micro-resonators [1], single-crystal CaF2 [2] is expected to be the most suitable material for the device from an absorption coefficient viewpoint. Etching and irradiating with a CO2 laser are generally employed for the manufacturing of semiconductor materials into optical micro-resonators. However, these processes are prohibited with CaF2, because anisotropic etching is inadequate for the fabrication of a bulge-shaped resonator [3] and a single-crystal structure will be broken by laser heat. As a result, the actual performance of CaF2 optical micro-resonators fabricated by these processes is less than ideal. The most feasible prospective fabrication technique for CaF2 resonators is ultra-precision turning and polishing, as shown by Maleki [3]. While the polishing process is required to make the surface of a resonant part smoother [4], prolonged polishing deteriorates the accuracy of the part’s form, which affects the resonator’s performance. To achieve not only finer surface quality but also sharper resonant parts, the total material amount removed by the polishing process should be reduced to maintain the fine shape machined by turning. Hence, it is necessary to produce a smoother surface during the ultra-precision turning process. Single-crystal CaF2’s characteristics include high brittleness and crystal anisotropy. Previous CaF2 machining studies have focused on face turning [5], in which the crystalline plane was constant. For cylindrical turning, the fabrication technique used for bulgeshaped resonators, the crystalline plane and cutting direction vary * Corresponding author. E-mail address: [email protected] (Y. Kakinuma).
continuously. Here, we experimentally investigate the turning performance of CaF2 to find the most appropriate cutting conditions. This study deeply analyzes the influence of cutting conditions on both the surface quality and sub-surface damage created by CaF2 turning via TEM observation. 2. Experimental setup and procedure Ultra-precision cylindrical turning (UPCT) of CaF2 was carried out by an ultra-precision aspheric surface machine tool. Conical CaF2 workpieces with a small cylindrical tip, 1 mm in diameter and 1 mm in length, with end face orientations (1 1 1), (1 1 0), and (1 0 0) were prepared. Figs. 1 and 2 show the experimental setup for the UPCT and the shapes of two single-crystal diamond tools designed to investigate the influence of rake angle on the cylindrical surface quality and degree of subsurface damage. The tool represented as tool #1 has a 208 rake angle, 0.2 mm nose radius, and 88 clearance. This shape was adopted because a negative rake angle and large nose radius are generally preferable when performing ductile-mode cutting in large-sized optical
Fig. 1. Experimental setup for the ultra-precision cylindrical turning (UPCT) of the single-crystal CaF2.
http://dx.doi.org/10.1016/j.cirp.2015.04.076 0007-8506/ß 2015 CIRP.
Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076
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CIRP-1343; No. of Pages 4 Y. Kakinuma et al. / CIRP Annals - Manufacturing Technology xxx (2015) xxx–xxx
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Fig. 2. Tool shape of the utilized single-crystal diamond tools.
Fig. 4. The observed surfaces at points a and b in Fig. 3 (a) after turning the CaF2 with end face (1 1 1).
Fig. 3. Crystallographic image of the observation point viewed from each end face.
materials. The tool represented as tool #2 has a 08 rake angle, 0.05 mm nose radius, and 158 clearance. Theoretically, since thrust force decreases with rake angle, a sharper tool with a 08 rake angle would make the cutting process more stable in cylindrical turning, and the resulting machined surface quality would be enhanced. The cylindrical machined surface quality after UPCT was evaluated from the perspective of surface roughness and crack initiation, which were measured by scanning white light interferometry and differential interference microscopy, respectively. Crystallographic images of the observation point as viewed from each end face are represented in Fig. 3. It is clear that each cycle of crystal structure at the cylindrical surface in the end face of (1 1 1), (1 1 0), and (1 0 0) corresponds to 1208, 1808, and 908, respectively. In order to investigate crack initiation with regard to the crystal’s anisotropy, the whole cylindrical surface after UPCT was observed at 158 intervals. In addition, the sub-surface damage under various cutting conditions was analyzed by TEM observation. 3. Influence of tool shape on surface quality Previous orthogonal cutting tests of single-crystal CaF2 showed that the minimum critical depth of cut is 50 nm to produce a homogeneous fine surface on all crystallographic orientations [6]. Based on this result, the cutting conditions of UPCT were determined as shown in Table 1. The tool edge radius of both tool #1 and tool #2 is approximately 20–30 nm as measured by SEM observation. Therefore, a cut depth of 50 nm is applicable to this cutting process. Table 1 Cutting conditions of UPCT for both tools. Cutting speed m/min Rotation speed min 1 Feed per revolution mm/rev (micrometer/rev) Depth of cut nm End face orientation
3.14 1000 0.5 50 (1 1 1), (1 1 0), (1 0 0)
When tool #1 is employed, it was found that cracks initiated periodically in the case of end faces (1 1 1) and (1 1 0). Fig. 4 shows the observed surfaces at points a and b in Fig. 3(a), after turning the CaF2 with end face (1 1 1). It was found from previous orthogonal cutting tests of the (1 1 0) plane that the critical depth of cut in the [ 1 1 1] direction was over 500 nm [6]. On the (1 1 0) plane, this is the same cutting direction at the point a. Hence, a fine surface with no cracking was obtained. However, cracks suddenly appeared along the cleavage plane’s side line at point b. This is because the cutting force mainly affects cleavage rather than slip planes. To investigate the influence of crystal anisotropy on surface quality after UPCT, the cylindrical surface was evaluated by measuring its surface roughness. Fig. 5 shows the results of surface roughness Ra
Fig. 5. Results of surface roughness Ra on the turned surface with end face (1 1 1), (1 1 0), and (1 0 0).
on the whole cylindrical surface turned with end faces (1 1 1), (1 1 0), and (1 0 0). For the result of the sample with end face (1 1 1), the surface became coarser at 608 intervals, in spite of the 1208 cycle of the crystal structure. Taking into account the crystallographic model, the crystal structure at the cylindrical surface is horizontally flipped when CaF2 with end face (1 1 1) rotates at 608 [6]. This is why surface roughness varies at 608 intervals for the (1 1 1) sample. Regarding the end face (1 1 0), the cylindrical surface quality deteriorated considerably compared to
Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076
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CIRP-1343; No. of Pages 4 Y. Kakinuma et al. / CIRP Annals - Manufacturing Technology xxx (2015) xxx–xxx
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Fig. 6. Chatter vibrations with tool #1 during UPCT.
Fig. 8. Subsurface damage with tool #1 and tool #2.
Fig. 7. Differences in tool contact area with the CaF2 workpiece.
the others. The end face (1 1 0) is thus inappropriate for UPCT. To consider anisotropy, there is 1808 periodicity in the surface roughness as well as crystal structure. From both results, it is clear that the surface quality machined after UPCT strongly depends on the cycling of the crystal structure. Unlike the end faces (1 1 1) and (1 1 0), the end face (1 0 0) remarkably enhanced the surface quality. No cracks were observed over the whole cylindrical surface. The crystallographic model suggests that the symmetric crystal structure, along with the cutting direction, advantageously distributes the cutting force toward the cleavage plane, which usually causes the crack initiation. However, chatter vibration occurred irregularly with tool #1 despite the ultra-precision turning with nanometer-scale depth of cut, as shown in Fig. 6. From the wavenumber of chatter mark, the frequency is calculated to be around 4.9 kHz. Both low stiffness in the small-diameter microoptical part and small force fluctuations caused by crystal anisotropy could lead to unstable vibrations during UPCT. In future work, the chatter vibration generated in ultra-precision cutting will be analyzed by measuring the variation in small cutting force. When UPCT was carried out with tool #2 under the same cutting conditions in Table 1, the whole machined surface became much finer than that with tool #1, regardless of which crystal plane was selected as the end face. No chatter vibration occurred in using tool #2. Two possible reasons for the enhanced surface quality are considered: the 08 rake tool reduces the thrust force below that of the negative rake tool, so that the UPCT could become stable; the difference in tool contact area with the CaF2 workpiece, as shown in Fig. 7, may also contribute. In ultra-precision cutting, the cutting force is assumed to be greatly dependent on the plastic flow area, related to the contact area of the tool with the workpiece because of size effects. As the tool contact area with tool #2 was one-fourth less than that with tool #1, the cutting force would be reduced as well. With the adaptation of Colwell’s law, the chip flow angle is calculated as 89.48 and 88.88 in tools #1 and #2, respectively. The slight difference in chip flow angle may contribute to the reduction of thrust force in UPCT. As to tool wear, no significant change after the experiments was observed for both tools because the cutting length was not so long. For the following chapter, CaF2 with end face (1 0 0), which is appropriate for UPCT, is utilized for TEM observation. 4. Evaluation of sub-surface damage 4.1. Influence of tool shape In order to investigate the influence of tool shape on the degree of sub-surface damage, TEM observation was conducted for the fine cylindrical surface created by tool #1 and tool #2. The TEM sample was cut from point a in Fig. 3(c) and thinned to 50 nm using a FIB laser. Fig. 8(a) and (b) shows the TEM micrographs corresponding to tool #1 and tool #2, respectively. Although dislocation generally
Fig. 9. Close-up view of subsurface damage area in the tool #2.
occurs in the depth direction in ultra-precision cutting of Si [7], a unique dislocation line parallel to the surface was confirmed in UPCT of CaF2. Regarding damage depth, significant differences can be seen according to the tool shape. While two dislocation lines were observed at 18 and 46 nm depths in the sample cut with tool #1, a single dislocation line appeared at a depth of 21 nm in the sample cut with tool #2. Tool #1 involved a larger plastic flow area, wider tool contact area, and larger negative rake angle, which would produce a larger cutting force and eventually deteriorate the surface integrity. A magnification of the TEM image from tool #2 is shown in Fig. 9. Above the dislocation line, the crystal orientation was found to be disarranged and collapsed. FFT analysis on the disarranged area indicates that the crystal structure changed from mono- to polycrystalline due to UPCT. The crystal transformation cannot be confirmed below a depth of 35 nm. We concluded that tool #2 produced less subsurface damage than tool #1 as well as improved surface quality. 4.2. Relation between cut depth and damage depth The influence of cutting depth on the subsurface damage depth was evaluated through TEM observation. The cutting conditions of the turning tests are listed in Table 2. The results of the turning tests show no significant difference in external surface, and fine surfaces Table 2 Cutting condition with different depth of cut. Cutting speed m/min Rotation speed min 1 Feed per revolution mm/rev (micrometer/rev) Depth of cut nm End face orientation Crystal plane for TEM sample
3.14 1000 0.5 50, 200 (1 0 0) Point a (in Fig. 3(c))
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Fig. 10. Subsurface damage with 200 nm cut depth.
with no cracks could be produced under both cutting conditions. To compare subsurface damage, a TEM sample at a 200 nm depth of cut was prepared, cut from point a in Fig. 3(c). As shown in Fig. 10, the resulting TEM image indicates that an increase of cutting depth leads to a larger subsurface damage area. Moreover, it confirms an increase in the number of dislocation lines. 4.3. Relation between anisotropy and damage depth Differences in subsurface damage at points a, d, and e in Fig. 3(c) were investigated from the anisotropy perspective. Fig. 8(b) and Fig. 11 show TEM micrographs at points a, d, and e. The degree of subsurface damage varied according to crystal orientation. Compared to points a and e, point d has two dislocation lines and shows deeper damage. Point d corresponds to the (1 1 0) plane, which is the most difficult plane in which to promote plastic deformation [6]. As a result, relatively deep subsurface damage up to a depth of approximately 40 nm was left at point d.
Fig. 11. Subsurface damage at points d and e with end face (1 0 0).
200 nm and 50 nm were adopted, respectively. The diameter of the resonator was around 400 mm and no cracks were found on the entire cylindrical surface. The quality of an optical micro-resonator is generally characterized by the Q factor, which is defined as the continuous time to store light in a resonator. The trial-manufactured resonator by UPCT without polishing successfully achieved a Q factor on the order of 106, as measured by using a tunable laser. The time to localize light was approximately 1.0 ns. An ideal singlecrystal CaF2 resonator has a theoretical Q factor on the order of 1013 [3]. Therefore, even small amounts of subsurface damage in the range of tens of nanometers deep caused the loss of light absorption and performance degradation. In this study, the subsurface damage layer can be reduced to the range of tens of nanometers in depth. To achieve a higher Q factor, the removal of the reduced sub-surface damage layer is necessary, by surface treatment such as laser recovery and polishing processes. If such a surface treatment is applied, the removal of a very small amount of the subsurface layer enables the maintenance of a very fine form accuracy, which is considered an important factor in determining the resonator’s performance [8]. 6. Conclusion For the CaF2 optical micro resonator, the influence of cutting conditions on both the surface quality and subsurface damage in the ultra-precision cylindrical turning of single-crystal CaF2 was experimentally analyzed. The following results were obtained through this study. (1) Compared with the end faces (1 1 1) and (1 1 0), the end face (1 0 0) remarkably enhances the surface quality. In the case of the (1 0 0) end face, the symmetry of the crystal structure along with the cutting direction distributes the cutting force toward the cleavage plane in an advantageous manner. (2) From TEM observation, the subsurface crystal structure after UPCT changes from mono- to polycrystalline for a depth of tens of nanometers. A larger tool nose radius and larger negative rake angle produce multiple dislocation lines and deeper subsurface damage. (3) The increase of cutting depth leads to the extension of the subsurface damage area, with an increase in the number of dislocation lines, even if the external surface quality remains fine. In addition, subsurface damage on the cylindrical surface varies according to the crystal’s orientation. Especially on the (1 1 0) plane, relatively deep subsurface damage remains. (4) A trial CaF2 optical micro-resonator manufactured by UPCT only achieved a Q factor on the order of 106. The subsurface damage layer can be reduced to the range of tens of nanometers in depth. The removal of a very small amount of the subsurface damage layer could retain fine form accuracy if polishing is applied after turning.
5. Trial manufacture of the CaF2 optical micro resonator References Fig. 12 shows the appearance of the CaF2 resonator manufactured by UPCT and a magnified image of the bulge-shaped portion. For the rough cutting and finishing cutting, depths of cut of
Fig. 12. Trial-manufactured CaF2 optical micro resonator.
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Please cite this article in press as: Kakinuma Y, et al. Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF2 optical micro resonator. CIRP Annals - Manufacturing Technology (2015), http://dx.doi.org/10.1016/j.cirp.2015.04.076