- Email: [email protected]
Fabrication of high temperature moiré grating and its application
Optics and Lasers in Engineering 54 (2014) 255–262
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Fabrication of high temperature moiré grating and its application Huaixi Wang a, Huimin Xie a,n, Yanjie Li b, Peng Fang c, Xianglu Dai a, Lifu Wu a, Minjin Tang a a b c
AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, China Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2013 Received in revised form 27 June 2013 Accepted 29 June 2013 Available online 22 July 2013
The fabrication technique of high temperature moiré grating has been presented, which combines thermal nanoimprint process method and high temperature transferring technique. And the quality of grating was evaluated by analyzing its configuration through the Atomic Force Microscope (AFM) images. From the analysis, it was found that the optimal depth of grating will cause different stress condition in the process of vacuum deposition, and thicker deposited metal film on the grating may enhance the oxidation resistant ability. However, with the increase of the thickness of the metal film, cracks may occur in the film, In order to avoid this mechanical failure, the optimization of the depth of grating was performed. The optimized depth was obtained by analyzing the stress intensity using Finite Element Method (FEM). In order to verify the reliability of high temperature grating, the tensile experiment of high temperature alloy (GH3030) at 650 1C was carried out, and the experimental results demonstrated that fabricated high temperature grating was feasible. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Moire interferometry High temperature moiré grating Metal film Transferring technique GH3030
1. Introduction High temperature deformation measurement has drawn the attention of the researchers in the field of experimental mechanics during the recent years. Several methods including moiré interferometry, digital image correlation (DIC), electronic speckle pattern interferometry (ESPI) etc [1–5] have been developed to achieve this goal. Moiré interferometry [6–9] is an optical method for in plane deformation measurement, which has been widely applied in the mechanical characterization of materials and structures with the advantage of full filed and high sensitivity for displacement measurement(submicron level) [10–12]. As a basic component for deformation measurement, the quality of the grating will directly influence on the fringe contrast and intial field of the moiré pattern. And thus, preparation of specimen grating is a crucial issue for moiré interferometry measurement as well as the other advanced moiré techniques. D. Post proposed the grating fabrication method in combination holographic interferometry with photolithography techniques, but its process is complex, the main steps including setting up a holographic interferometry system, photoresist coating, exposure, developing and fixing of photoresist, as well as the metal layer deposition. As for the fabrication of high temperature moire interferometry grating, the process is even more complex. From the literatures we can find there are four kinds of the fabrication
n
Corresponding author. Tel.: +86 10 62792286; fax: +86 10 6 781 24. E-mail address: [email protected] (H. Xie).
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.06.021
techniques for high temperature grating including etching [13], zero-thickness grating [14], single deposited metal layer [15] and two deposited metal layer [16]. For two deposited metal layer, it can be expected that the depth of grating reaches its maximum in the development, but the development time will be too long so that the grating will dissolve in the developing solution, and the crack may appear if the metal coating is too thick, which makes it very difficult to produce high temperature moiré grating, the complex technique and difficulty of making high quality grating restrain its wide application. Nanoimprint lithography(NIL) [17–23] as a advanced technique of fabricating nanoscale patterns has received significant attention in recent years. With easier operations, it is feasible to fabricate the high frequency gratings on the sample or other substrates by using thermal nanoimprint, which offers an alternative method to fabricate high temperature moiré grating. In the study, the fabrication technique of high temperature moiré grating has been presented based on thermal nanoimprint process and high temperature transferring technique. By thermal nanoimprint technique, the holographic grating as a mould can be copied on the glass substrate with the photoresist on the surface, then the high temperature Cr film is coated on the surface of glass substrate. It is considered that the more thicker coating can improve the inoxidizability of specimen, but the coating may fracture if the thickness of coating is too thicker, so optimized depth was obtained by analyzing the stress intensity using FEM. The high temperature grating can be transferred on the surface of sample using high temperature adhesive. According to the chemical property of high temperature adhesive, the oxidation reaction will
256
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
take place and destroy grating if the temperature is higher than 750 1C, thus the high temperature moiré grating should be used in the deformation measurement under 700 1C.
yellow liquid with low viscosity (14.5 10 3 Pa s) and good fluidity at room temperature. Its main structural unit is shown in Fig. 2. The high temperature filler can be boron carbide powder, zirconium diboride powder or titanium powder. In this study, the boron carbide powder was added to the polysilazane.
2. High temperature Moiré interferometry Fig. 1 illustrates the experimental setup of high temperature moiré interferometer, it consist of laser, four-beam optical system, CCD, adjustable device, oven and testing machine. The green laser (λ ¼532 nm) was used, whose diffracted light can go though the green filter when the green filter was used to eliminate effect of infrared radiation on the image quality. Four-beam optical system can produce virtual reference grating. The adjustable device can adjust the position of moiré interferometer to match the specimen. Oven and testing machine were used to heat and load the specimen, respectively. The moiré fringe is captured by CCD. 2.1. The principle of moiré interferometry The schematic diagram of principle of high temperature moiré interferometry is shown schematically in Fig. 1. The crossed-line holographic grating is replicated on the surface of sample. Coherent laser beams 1 and 2 will produce a virtual reference grating of spatial frequency two times of specimen grating in their intersecting zone. When the specimen deforms, the diffracted wave front will produce interfering and form the moiré fringes. The moiré fringes represent u and v displacement fields along the x and y directions, respectively, which can be determined by [10]: NX NX ¼ f 2f s Ny Ny ¼ v¼ f 2f s u¼
∂u 1 ∂N x 1 ΔN x ¼ ≈ ∂x 2f s ∂x 2f s Δx ∂v 1 ∂N y 1 ΔNy ¼ ≈ εy ¼ ∂y 2f s ∂y 2f s Δy 1 ∂u ∂v 1 ΔN x ΔN y þ ≈ þ εxy ¼ 2 ∂y ∂x 4f s Δy Δx
ð1Þ
3.2. Curing As a high temperature adhesive, an optimal curing process is essential to ensure the good bonding property and quality of transferred grating. From several experiments, the optimal curing curve of high temperature adhesive can be seen from Fig. 3. 3.3. The mechanical property analysis Since the grating needs to be transferred to the surface of sample, the adhesion strength of high temperature adhesive should be measured. In the study, the adhesion property of two typical materials were analyzed, ceramic and high-temperature alloy. The materials was polished by using 400 sandpaper, and the two parts are bonded together with a high temperature adhesive. The adhesion property of ceramic and high-temperature alloy were attained from the compression shear and tensile shear experiment, respectively, as shown in Fig. 4. The experimental results showed that the shear strength of high-temperature alloy is about 5.8 MPa at room temperature. And the shear strength of ceramic is about 8.6 MPa. Nagata et al. [24] has studied the adhesion between photoresist and inorganic substrate, the results showed that the tensile strength was about 1 MPa. So the grating can be transferred to the surface of sample perfectly.
4. Fabrication of high temperature grating 4.1. Thermal nanoimprint system
εx ¼
ð2Þ
where f is the frequency of virtual reference grating, fs is the frequency of the specimen grating, Nx and Ny are fringe orders in the u and v field moiré fringes, respectively.
3. The mechanical property of high temperature adhesive 3.1. Characterization of high temperature adhesive High temperature adhesive was made by polysilazane (PSZ) and filler, and the appropriate mass ratio is 1:2. PSZ is a pale
Fig. 1. Schematic of high temperature moiré interferometer (M1, M2 are mirrors. C1, C2 are collimators).
The self-developed thermal nanoimprint system is shown in Fig. 5. In the paper [19], the first generation thermal nanoimprint system was presented. In the study, the advanced thermal nanoimprint system was applied in the fabrication of high temperature grating [20]. There was a further improvement in the parallelism of upper and lower plate, which can improve the uniformity of nanoimprint grating. The depth of grating can be controlled by the applied pressure and temperature. In the experiment, the temperature (T), pressure (p), and time (t) are as follows: p ¼1.5 MPa, t¼6 min, T ¼180 1C. 4.2. High temperature grating film Fig. 6 shows the schematic of the fabrication processes of high temperature grating using electroforming layer as a mould for the direct metal layer deposition on a glass substrate. The detailed process includes six steps: (1) Electroform holographic grating; (2) Produce electroforming grating mould by using Electroplating; (3) Deposit SU-8 photoresist on the glass
Fig. 2. The structural unit of Polysilazane.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
257
substrate by using spinning machine; (4) Imprint the electroforming grating mould on the photoresist by using thermal nanoimprint system; (5) Remove electroforming mould; (6) Coat the high temperature metal film using evaporation deposition.
Fig. 5. The thermal nanoimprint system. Fig. 3. The optimal curing curve of high temperature adhesive.
Fig. 6. The process of fabrication of high temperature grating.
4.3. Optimization of depth of grating
Fig. 4. The schematic diagram of shear test (a) Compression–shear (b) Tensile– shear.
It is known that the thickness of metal film will affect antioxidant ability of grating, the film may cause crack in the evaporation deposition if the film is too thick, so the thickness of metal film and the technical parameters of evaporation deposition should be optimized. In the simulation, the elastic modulus and Poisson's ratio of Cr film are 279 GPa and 0.21 [25], and the photoresist are 3.5 GPa and 0.30 [26]. Fig. 7 shows the stress distribution of different depth grating, the thickness of Cr film and photoresist are 0.1 μm and 1 μm, respectively. It can be seen that the Cr film is mainly subjected to tensile stress. From Fig. 8, we can see that the stress decreases first and then increases with
258
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
increasing depth of grating when the thickness of coating is less than 200 nm. And the stress decreases with increasing depth of grating when the thickness of coating is 200 nm. So the greater the depth is, the smaller the stress is in theory, but the depth of grating cannot be unlimitedly increased, as seen in Figs. 9 and 10. The depth of grating will not increase at 75 s exposure time when the development time is constant. The optical microscope image of grating captured from KEYENCE VHX-500F can be seen from Fig. 11. In the experiment, the intensity of interference surface of two beam laser is 280 lux and the development time is 30 s. The experiment showed that the Cr film will crack when the thickness is more than 100 nm. Fig. 12 shows the SEM image of grating with crack, it can be seen that the crack is at the wave trough of structure of grating and the profile of crack using KEYENCE VHX500F shows that the crack is caused by tensile stress from the morphology of the depression, which is consistent with the position of max stress by FEM. It can be verified that the Cr film will crack if the depth of grating is low in the experiment from Figs. 11 and 12, which also agrees with the result of FEM that the tensile stress is high when the depth of grating is low. It can be concluded that the depth of grating was 300 nm, and the thickness
Fig. 8. The stress comparison of different depth grating under different coating thickness.
of Cr film was 100 nm from analysis above, which will not cause crack and have good oxidation resistance.
Fig. 7. The model and stress distribution of different depth grating in the x direction (a) the model (b) 100 nm (c) 200 nm (d) 300 nm (e) 400 nm.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
Fig. 11. Optical microscope image of grating at 75 s exposure time.
Fig. 12. SEM image of grating with crack at 45 s exposure time.
Fig. 9. AFM images of grating under different exposure time (a) 45 s (b) 60 s (c) 75 s.
Fig. 10. Profile analysis by AFM of grating at different exposure.
Fig. 13. The flow chart of grating fabrication.
259
260
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
4.4. Replication of high temperature grating In the above section, the properties of high temperature adhesive have been studied. In order to make the surface grating smooth, the sample was roughly polished; the high temperature adhesive was coated on the surface of grating with glass substrate,
and adhered to the glass substrate on the surface of sample using adhesive with loading, then place them in the dying oven and cure as the curing condition mentioned above. After the adhesive is fully cured, the glass substrate was separated from the surface of sample, and the residual photoresist can be cleaned using acetone. The high temperature specimen grating has been transferred completely. The transferring process of high temperature sample grating is shown in Fig. 13. It is noted that the curing technique must be strictly executed, otherwise it is difficult to make the perfect high temperature specimen grating. Fig. 14 shows the sample with high temperature grating. The whole process of fabrication of high temperature sample grating can be concluded as the flow chart shown in Fig. 15.
5. Experiments and results In this section, the high temperature mechanical property of GH3030 was analyzed from the tensile experiment at 650 1C using high temperature moiré interferometry. The section area of sample was 8 mm 1 mm.The grating of sample surface was replicated by following the technique shown in Fig. 13. Fig. 16 shows the experimental setup of high temperature moiré interferometer. From Fig. 17(a), the zero field shows that the quality of grating is very good. The strain is 960 με according to Eqs. (1) and (2) from Fig. 17(b) and (c), and the elastic modulus is calculated as 130 GPa, which is close to 137 GPa from material handbook. This demonstrates the good reliability of the high temperature grating.
6. Discussions and conclusions
Fig. 14. Sample with high temperature grating.
A fabrication technique of high temperature moiré grating has been presented based on thermal nanoimprint process and high temperature transferring technique, which can be expediently applied in the field of high temperature deformation measurement.
Fig. 15. The flow chart of fabrication of high temperature sample grating.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
261
Fig. 17. Moiré fringes under different loads at 650 C (a) zero field (b) 1000 N (c) 1500 N.
high temperature of 700 1C under current conditions. The higher temperature tests can be expected with the development of high temperature adhesive in the future.
Acknowledgements The authors are grateful to the financial support from the National Basic Research Program of China (“973” Project) (Grant No.2010CB631005, 2011CB606105), the National Natural Science Foundation of China (Grant Nos. 11232008, 91216301, 11227801, 11172151), Tsinghua University Initiative Scientific Research Program. References
Fig. 16. The experimental setup of high temperature moiré interferometer.
In the experiment, it was found that if the metal film was very thin, the oxidation of the adhesive will take place much more easily at high temperature, and oxide will fill the space of adjacent grating lines. If the film is too thick, cracks may occur due to tensile stress. The depth of grating and thickness of metal film can be optimized using the FEM simulation. The optimum parameters are as follows: the depth of grating was 300 nm, and the thickness of Cr film was 100 nm. In the process of replication of high temperature grating, the high temperature adhesive should be dispersed uniformly. And the surface of sample should be polished evenly; otherwise there will be some air bubble when the adhesive was coated on the surface of sample, which can cause incomplete replication of grating. Due to the chemical property of high temperature adhesive, the powder of adhesive may oxidize at 750 1C, and then the grating will be destroyed. So it is recommended the grating be used within
[1] Wang RG, Tokuda T, Kido M. High temperature fatigue deformation measurement of ceramics thermally sprayed SUS304 steel using an ESPI system. Mater Sci Res Int 2003;9:69–74. [2] Shang HX, Gao JX, Ian Nicholson P, Kenny Steve. An investigation of reliability of solder joints in microelectronic packages by high temperature moiré method. Microelectron Reliab 2011;51:994–1002. [3] Pan B, Wu DF, Wang ZY, et al. High-temperature digital image correlation method for full-field deformation measurement at 1200 1C. Meas Sci Technol 2011;21:015701. [4] Liu XB, Kang B, Carpenter W, Barbero E. Investigation of the crack growth behavior of Inconel 718 by high temperature moiré Interferometry. J Mater Sci 2004;39:1967–73. [5] Kang B, Liu XB, Cisloiu C, Chang KM. High temperature moiré interferometry investigation of creep crack growth of inconel 783/environment and b-phase effects. Mater Sci Eng, A. 2003;347:205–13. [6] Post D. Developments in moiré interferometry. Opt Eng 1982;21(3):458–67. [7] Dai FL, McKelvie J, Post. D. An interpretation of moiré interferometry from wavefront interference theory. Opt Lasers Eng 1990;12(2–3):101–18. [8] Fang J, Dai. FL. A new real-time method of moiré interferometry for displacement derivative fringe patterns. Opt Lasers Eng 1990;13(1):51–8. [9] Post. D. Moiré interferometry-advances and applications. Exp Mech 1991;31 (3):276–80. [10] Post D, Han B, Ifju PG. High sensitivity moiré: experimental analysis for mechanics and materials. New York: Springer-Verlag; 1994. [11] Bastawros AF, Voloshin. AS. Thermal strain measurements in electronic packages through fractional Fringe Moiré Interferometry. J Electron Packag 1990;11:303–8. [12] Ifju PG, Han B. Recent applications of Moiré interferometry. Exp Mech 2010;50:1129–47. [13] Decluwe M. Etching Moiré Test Grid in Steel. Strain 1974;10:135–8. [14] Cloud G, Radke R, Peiffer J. moiré gratings for high temperature and long times. Exp Mech 1979;19(10):19N–21N. [15] Ifju PG, Post D. Zero-thickness specimen gratings for moiré interferometry. Exp Mech 1991;15(2):45–7. [16] Xie HM, Dietz P, Dai FL, Schmidt A. Fabrication of high frequency gratings for high temperature. Exp Tech 1995;19(5):28–9.
262
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
[17] Chou SY, Krauss PR, Renstrom PJ. Imprint of sub-25nm vias and trenches in polymers. Appl Phys Lett 1995;67(21):3114–6. [18] Li N, Wu W, Chou SY. Sub-20-nm alignment in nanoimprint lithography using moire´ oire fringe. Nano Lett 2006;6:2626–9. [19] Guo LJ. Nanoimprint lithography: methods and material requirements. Adv Mater 2007;19(4):495–513. [20] Tang MJ, Xie HM, Zhu JG, Li XJ, Li. YJ. The study of moiré grating fabrication on metal samples using nanoimprint lithography. Opt Exp 2012;20:2942–55. [21] Tang. MJ. The study of moiré grating fabrication method by nanoimprint lithography and the characterization of grating structures, [D]. Beijing: Tsinghua University; 2012. [22] Wang QH, Kishimoto S. Simultaneous analysis of residual stress and stress intensity factor in a resist after UV-nanoimprint lithography based on electron moire fringes. J Micromech Microeng 2012;22(10):105021.
[23] Wang QH, Kishimoto S, Tanaka Y, Kagawa Y. Micro/submicro grating fabrication on metals for deformation measurement based on ultraviolet nanoimprint lithography. Opt Lasers Eng 2013;51(7):944–8. [24] Nagata H, Kawai A. Characteristic of adhesion between photoresist and inorganic substrate. J Appl phys 1989;28:2137–41. [25] Cordill MJ, Fischer FD, Rammerstorfer FG, Dehm G. Adhesion energies of Cr thin films on polyimide determinedfrom buckling: experiment and model. Acta Mater 2010;58:5520–31. [26] Feng R, Farris. RJ. The characterization of thermal and elastic constants for an epoxy photoresist SU8 coating. J Mater Sci 2002;37:4793–9.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Fabrication of high temperature moiré grating and its application Huaixi Wang a, Huimin Xie a,n, Yanjie Li b, Peng Fang c, Xianglu Dai a, Lifu Wu a, Minjin Tang a a b c
AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, China Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, China
art ic l e i nf o
a b s t r a c t
Article history: Received 26 April 2013 Received in revised form 27 June 2013 Accepted 29 June 2013 Available online 22 July 2013
The fabrication technique of high temperature moiré grating has been presented, which combines thermal nanoimprint process method and high temperature transferring technique. And the quality of grating was evaluated by analyzing its configuration through the Atomic Force Microscope (AFM) images. From the analysis, it was found that the optimal depth of grating will cause different stress condition in the process of vacuum deposition, and thicker deposited metal film on the grating may enhance the oxidation resistant ability. However, with the increase of the thickness of the metal film, cracks may occur in the film, In order to avoid this mechanical failure, the optimization of the depth of grating was performed. The optimized depth was obtained by analyzing the stress intensity using Finite Element Method (FEM). In order to verify the reliability of high temperature grating, the tensile experiment of high temperature alloy (GH3030) at 650 1C was carried out, and the experimental results demonstrated that fabricated high temperature grating was feasible. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Moire interferometry High temperature moiré grating Metal film Transferring technique GH3030
1. Introduction High temperature deformation measurement has drawn the attention of the researchers in the field of experimental mechanics during the recent years. Several methods including moiré interferometry, digital image correlation (DIC), electronic speckle pattern interferometry (ESPI) etc [1–5] have been developed to achieve this goal. Moiré interferometry [6–9] is an optical method for in plane deformation measurement, which has been widely applied in the mechanical characterization of materials and structures with the advantage of full filed and high sensitivity for displacement measurement(submicron level) [10–12]. As a basic component for deformation measurement, the quality of the grating will directly influence on the fringe contrast and intial field of the moiré pattern. And thus, preparation of specimen grating is a crucial issue for moiré interferometry measurement as well as the other advanced moiré techniques. D. Post proposed the grating fabrication method in combination holographic interferometry with photolithography techniques, but its process is complex, the main steps including setting up a holographic interferometry system, photoresist coating, exposure, developing and fixing of photoresist, as well as the metal layer deposition. As for the fabrication of high temperature moire interferometry grating, the process is even more complex. From the literatures we can find there are four kinds of the fabrication
n
Corresponding author. Tel.: +86 10 62792286; fax: +86 10 6 781 24. E-mail address: [email protected] (H. Xie).
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlaseng.2013.06.021
techniques for high temperature grating including etching [13], zero-thickness grating [14], single deposited metal layer [15] and two deposited metal layer [16]. For two deposited metal layer, it can be expected that the depth of grating reaches its maximum in the development, but the development time will be too long so that the grating will dissolve in the developing solution, and the crack may appear if the metal coating is too thick, which makes it very difficult to produce high temperature moiré grating, the complex technique and difficulty of making high quality grating restrain its wide application. Nanoimprint lithography(NIL) [17–23] as a advanced technique of fabricating nanoscale patterns has received significant attention in recent years. With easier operations, it is feasible to fabricate the high frequency gratings on the sample or other substrates by using thermal nanoimprint, which offers an alternative method to fabricate high temperature moiré grating. In the study, the fabrication technique of high temperature moiré grating has been presented based on thermal nanoimprint process and high temperature transferring technique. By thermal nanoimprint technique, the holographic grating as a mould can be copied on the glass substrate with the photoresist on the surface, then the high temperature Cr film is coated on the surface of glass substrate. It is considered that the more thicker coating can improve the inoxidizability of specimen, but the coating may fracture if the thickness of coating is too thicker, so optimized depth was obtained by analyzing the stress intensity using FEM. The high temperature grating can be transferred on the surface of sample using high temperature adhesive. According to the chemical property of high temperature adhesive, the oxidation reaction will
256
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
take place and destroy grating if the temperature is higher than 750 1C, thus the high temperature moiré grating should be used in the deformation measurement under 700 1C.
yellow liquid with low viscosity (14.5 10 3 Pa s) and good fluidity at room temperature. Its main structural unit is shown in Fig. 2. The high temperature filler can be boron carbide powder, zirconium diboride powder or titanium powder. In this study, the boron carbide powder was added to the polysilazane.
2. High temperature Moiré interferometry Fig. 1 illustrates the experimental setup of high temperature moiré interferometer, it consist of laser, four-beam optical system, CCD, adjustable device, oven and testing machine. The green laser (λ ¼532 nm) was used, whose diffracted light can go though the green filter when the green filter was used to eliminate effect of infrared radiation on the image quality. Four-beam optical system can produce virtual reference grating. The adjustable device can adjust the position of moiré interferometer to match the specimen. Oven and testing machine were used to heat and load the specimen, respectively. The moiré fringe is captured by CCD. 2.1. The principle of moiré interferometry The schematic diagram of principle of high temperature moiré interferometry is shown schematically in Fig. 1. The crossed-line holographic grating is replicated on the surface of sample. Coherent laser beams 1 and 2 will produce a virtual reference grating of spatial frequency two times of specimen grating in their intersecting zone. When the specimen deforms, the diffracted wave front will produce interfering and form the moiré fringes. The moiré fringes represent u and v displacement fields along the x and y directions, respectively, which can be determined by [10]: NX NX ¼ f 2f s Ny Ny ¼ v¼ f 2f s u¼
∂u 1 ∂N x 1 ΔN x ¼ ≈ ∂x 2f s ∂x 2f s Δx ∂v 1 ∂N y 1 ΔNy ¼ ≈ εy ¼ ∂y 2f s ∂y 2f s Δy 1 ∂u ∂v 1 ΔN x ΔN y þ ≈ þ εxy ¼ 2 ∂y ∂x 4f s Δy Δx
ð1Þ
3.2. Curing As a high temperature adhesive, an optimal curing process is essential to ensure the good bonding property and quality of transferred grating. From several experiments, the optimal curing curve of high temperature adhesive can be seen from Fig. 3. 3.3. The mechanical property analysis Since the grating needs to be transferred to the surface of sample, the adhesion strength of high temperature adhesive should be measured. In the study, the adhesion property of two typical materials were analyzed, ceramic and high-temperature alloy. The materials was polished by using 400 sandpaper, and the two parts are bonded together with a high temperature adhesive. The adhesion property of ceramic and high-temperature alloy were attained from the compression shear and tensile shear experiment, respectively, as shown in Fig. 4. The experimental results showed that the shear strength of high-temperature alloy is about 5.8 MPa at room temperature. And the shear strength of ceramic is about 8.6 MPa. Nagata et al. [24] has studied the adhesion between photoresist and inorganic substrate, the results showed that the tensile strength was about 1 MPa. So the grating can be transferred to the surface of sample perfectly.
4. Fabrication of high temperature grating 4.1. Thermal nanoimprint system
εx ¼
ð2Þ
where f is the frequency of virtual reference grating, fs is the frequency of the specimen grating, Nx and Ny are fringe orders in the u and v field moiré fringes, respectively.
3. The mechanical property of high temperature adhesive 3.1. Characterization of high temperature adhesive High temperature adhesive was made by polysilazane (PSZ) and filler, and the appropriate mass ratio is 1:2. PSZ is a pale
Fig. 1. Schematic of high temperature moiré interferometer (M1, M2 are mirrors. C1, C2 are collimators).
The self-developed thermal nanoimprint system is shown in Fig. 5. In the paper [19], the first generation thermal nanoimprint system was presented. In the study, the advanced thermal nanoimprint system was applied in the fabrication of high temperature grating [20]. There was a further improvement in the parallelism of upper and lower plate, which can improve the uniformity of nanoimprint grating. The depth of grating can be controlled by the applied pressure and temperature. In the experiment, the temperature (T), pressure (p), and time (t) are as follows: p ¼1.5 MPa, t¼6 min, T ¼180 1C. 4.2. High temperature grating film Fig. 6 shows the schematic of the fabrication processes of high temperature grating using electroforming layer as a mould for the direct metal layer deposition on a glass substrate. The detailed process includes six steps: (1) Electroform holographic grating; (2) Produce electroforming grating mould by using Electroplating; (3) Deposit SU-8 photoresist on the glass
Fig. 2. The structural unit of Polysilazane.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
257
substrate by using spinning machine; (4) Imprint the electroforming grating mould on the photoresist by using thermal nanoimprint system; (5) Remove electroforming mould; (6) Coat the high temperature metal film using evaporation deposition.
Fig. 5. The thermal nanoimprint system. Fig. 3. The optimal curing curve of high temperature adhesive.
Fig. 6. The process of fabrication of high temperature grating.
4.3. Optimization of depth of grating
Fig. 4. The schematic diagram of shear test (a) Compression–shear (b) Tensile– shear.
It is known that the thickness of metal film will affect antioxidant ability of grating, the film may cause crack in the evaporation deposition if the film is too thick, so the thickness of metal film and the technical parameters of evaporation deposition should be optimized. In the simulation, the elastic modulus and Poisson's ratio of Cr film are 279 GPa and 0.21 [25], and the photoresist are 3.5 GPa and 0.30 [26]. Fig. 7 shows the stress distribution of different depth grating, the thickness of Cr film and photoresist are 0.1 μm and 1 μm, respectively. It can be seen that the Cr film is mainly subjected to tensile stress. From Fig. 8, we can see that the stress decreases first and then increases with
258
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
increasing depth of grating when the thickness of coating is less than 200 nm. And the stress decreases with increasing depth of grating when the thickness of coating is 200 nm. So the greater the depth is, the smaller the stress is in theory, but the depth of grating cannot be unlimitedly increased, as seen in Figs. 9 and 10. The depth of grating will not increase at 75 s exposure time when the development time is constant. The optical microscope image of grating captured from KEYENCE VHX-500F can be seen from Fig. 11. In the experiment, the intensity of interference surface of two beam laser is 280 lux and the development time is 30 s. The experiment showed that the Cr film will crack when the thickness is more than 100 nm. Fig. 12 shows the SEM image of grating with crack, it can be seen that the crack is at the wave trough of structure of grating and the profile of crack using KEYENCE VHX500F shows that the crack is caused by tensile stress from the morphology of the depression, which is consistent with the position of max stress by FEM. It can be verified that the Cr film will crack if the depth of grating is low in the experiment from Figs. 11 and 12, which also agrees with the result of FEM that the tensile stress is high when the depth of grating is low. It can be concluded that the depth of grating was 300 nm, and the thickness
Fig. 8. The stress comparison of different depth grating under different coating thickness.
of Cr film was 100 nm from analysis above, which will not cause crack and have good oxidation resistance.
Fig. 7. The model and stress distribution of different depth grating in the x direction (a) the model (b) 100 nm (c) 200 nm (d) 300 nm (e) 400 nm.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
Fig. 11. Optical microscope image of grating at 75 s exposure time.
Fig. 12. SEM image of grating with crack at 45 s exposure time.
Fig. 9. AFM images of grating under different exposure time (a) 45 s (b) 60 s (c) 75 s.
Fig. 10. Profile analysis by AFM of grating at different exposure.
Fig. 13. The flow chart of grating fabrication.
259
260
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
4.4. Replication of high temperature grating In the above section, the properties of high temperature adhesive have been studied. In order to make the surface grating smooth, the sample was roughly polished; the high temperature adhesive was coated on the surface of grating with glass substrate,
and adhered to the glass substrate on the surface of sample using adhesive with loading, then place them in the dying oven and cure as the curing condition mentioned above. After the adhesive is fully cured, the glass substrate was separated from the surface of sample, and the residual photoresist can be cleaned using acetone. The high temperature specimen grating has been transferred completely. The transferring process of high temperature sample grating is shown in Fig. 13. It is noted that the curing technique must be strictly executed, otherwise it is difficult to make the perfect high temperature specimen grating. Fig. 14 shows the sample with high temperature grating. The whole process of fabrication of high temperature sample grating can be concluded as the flow chart shown in Fig. 15.
5. Experiments and results In this section, the high temperature mechanical property of GH3030 was analyzed from the tensile experiment at 650 1C using high temperature moiré interferometry. The section area of sample was 8 mm 1 mm.The grating of sample surface was replicated by following the technique shown in Fig. 13. Fig. 16 shows the experimental setup of high temperature moiré interferometer. From Fig. 17(a), the zero field shows that the quality of grating is very good. The strain is 960 με according to Eqs. (1) and (2) from Fig. 17(b) and (c), and the elastic modulus is calculated as 130 GPa, which is close to 137 GPa from material handbook. This demonstrates the good reliability of the high temperature grating.
6. Discussions and conclusions
Fig. 14. Sample with high temperature grating.
A fabrication technique of high temperature moiré grating has been presented based on thermal nanoimprint process and high temperature transferring technique, which can be expediently applied in the field of high temperature deformation measurement.
Fig. 15. The flow chart of fabrication of high temperature sample grating.
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
261
Fig. 17. Moiré fringes under different loads at 650 C (a) zero field (b) 1000 N (c) 1500 N.
high temperature of 700 1C under current conditions. The higher temperature tests can be expected with the development of high temperature adhesive in the future.
Acknowledgements The authors are grateful to the financial support from the National Basic Research Program of China (“973” Project) (Grant No.2010CB631005, 2011CB606105), the National Natural Science Foundation of China (Grant Nos. 11232008, 91216301, 11227801, 11172151), Tsinghua University Initiative Scientific Research Program. References
Fig. 16. The experimental setup of high temperature moiré interferometer.
In the experiment, it was found that if the metal film was very thin, the oxidation of the adhesive will take place much more easily at high temperature, and oxide will fill the space of adjacent grating lines. If the film is too thick, cracks may occur due to tensile stress. The depth of grating and thickness of metal film can be optimized using the FEM simulation. The optimum parameters are as follows: the depth of grating was 300 nm, and the thickness of Cr film was 100 nm. In the process of replication of high temperature grating, the high temperature adhesive should be dispersed uniformly. And the surface of sample should be polished evenly; otherwise there will be some air bubble when the adhesive was coated on the surface of sample, which can cause incomplete replication of grating. Due to the chemical property of high temperature adhesive, the powder of adhesive may oxidize at 750 1C, and then the grating will be destroyed. So it is recommended the grating be used within
[1] Wang RG, Tokuda T, Kido M. High temperature fatigue deformation measurement of ceramics thermally sprayed SUS304 steel using an ESPI system. Mater Sci Res Int 2003;9:69–74. [2] Shang HX, Gao JX, Ian Nicholson P, Kenny Steve. An investigation of reliability of solder joints in microelectronic packages by high temperature moiré method. Microelectron Reliab 2011;51:994–1002. [3] Pan B, Wu DF, Wang ZY, et al. High-temperature digital image correlation method for full-field deformation measurement at 1200 1C. Meas Sci Technol 2011;21:015701. [4] Liu XB, Kang B, Carpenter W, Barbero E. Investigation of the crack growth behavior of Inconel 718 by high temperature moiré Interferometry. J Mater Sci 2004;39:1967–73. [5] Kang B, Liu XB, Cisloiu C, Chang KM. High temperature moiré interferometry investigation of creep crack growth of inconel 783/environment and b-phase effects. Mater Sci Eng, A. 2003;347:205–13. [6] Post D. Developments in moiré interferometry. Opt Eng 1982;21(3):458–67. [7] Dai FL, McKelvie J, Post. D. An interpretation of moiré interferometry from wavefront interference theory. Opt Lasers Eng 1990;12(2–3):101–18. [8] Fang J, Dai. FL. A new real-time method of moiré interferometry for displacement derivative fringe patterns. Opt Lasers Eng 1990;13(1):51–8. [9] Post. D. Moiré interferometry-advances and applications. Exp Mech 1991;31 (3):276–80. [10] Post D, Han B, Ifju PG. High sensitivity moiré: experimental analysis for mechanics and materials. New York: Springer-Verlag; 1994. [11] Bastawros AF, Voloshin. AS. Thermal strain measurements in electronic packages through fractional Fringe Moiré Interferometry. J Electron Packag 1990;11:303–8. [12] Ifju PG, Han B. Recent applications of Moiré interferometry. Exp Mech 2010;50:1129–47. [13] Decluwe M. Etching Moiré Test Grid in Steel. Strain 1974;10:135–8. [14] Cloud G, Radke R, Peiffer J. moiré gratings for high temperature and long times. Exp Mech 1979;19(10):19N–21N. [15] Ifju PG, Post D. Zero-thickness specimen gratings for moiré interferometry. Exp Mech 1991;15(2):45–7. [16] Xie HM, Dietz P, Dai FL, Schmidt A. Fabrication of high frequency gratings for high temperature. Exp Tech 1995;19(5):28–9.
262
H. Wang et al. / Optics and Lasers in Engineering 54 (2014) 255–262
[17] Chou SY, Krauss PR, Renstrom PJ. Imprint of sub-25nm vias and trenches in polymers. Appl Phys Lett 1995;67(21):3114–6. [18] Li N, Wu W, Chou SY. Sub-20-nm alignment in nanoimprint lithography using moire´ oire fringe. Nano Lett 2006;6:2626–9. [19] Guo LJ. Nanoimprint lithography: methods and material requirements. Adv Mater 2007;19(4):495–513. [20] Tang MJ, Xie HM, Zhu JG, Li XJ, Li. YJ. The study of moiré grating fabrication on metal samples using nanoimprint lithography. Opt Exp 2012;20:2942–55. [21] Tang. MJ. The study of moiré grating fabrication method by nanoimprint lithography and the characterization of grating structures, [D]. Beijing: Tsinghua University; 2012. [22] Wang QH, Kishimoto S. Simultaneous analysis of residual stress and stress intensity factor in a resist after UV-nanoimprint lithography based on electron moire fringes. J Micromech Microeng 2012;22(10):105021.
[23] Wang QH, Kishimoto S, Tanaka Y, Kagawa Y. Micro/submicro grating fabrication on metals for deformation measurement based on ultraviolet nanoimprint lithography. Opt Lasers Eng 2013;51(7):944–8. [24] Nagata H, Kawai A. Characteristic of adhesion between photoresist and inorganic substrate. J Appl phys 1989;28:2137–41. [25] Cordill MJ, Fischer FD, Rammerstorfer FG, Dehm G. Adhesion energies of Cr thin films on polyimide determinedfrom buckling: experiment and model. Acta Mater 2010;58:5520–31. [26] Feng R, Farris. RJ. The characterization of thermal and elastic constants for an epoxy photoresist SU8 coating. J Mater Sci 2002;37:4793–9.