New methods of fabricating gratings for deformation measurements: A review

Optics and Lasers in Engineering 92 (2017) 48–56

Contents lists available at ScienceDirect

Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng

New methods of fabricating gratings for deformation measurements: A review Xianglu Dai, Huimin Xie

MARK

⁎

AML, Department of Engineering Mechanics, Tsinghua University, Beijing 100084, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Deformation grating fabrication Grating Moiré Deformation measurement

Gratings have been widely accepted as practical and effective deformation carriers/sensors and are commonly used in many deformation measurement methods. Since the deformation measurement sensitivity is directly proportional to the grating frequency, and the measurement accuracy is strongly affected by the grating quality. Thus, it is crucial to prepare an appropriate grating on the specimen surface that is to be measured. Over the past few decades, an increasing number of grating fabrication methods have been developed, including holographic photolithography, electron beam lithography, focused ion beam etching, nanoimprinting, soft lithography, and others. Although substantial literature regarding grating fabrication can be found, a comprehensive review is still necessary to promote the application of these methods. This review introduces the technical details and characteristics of recently developed grating fabrication methods and provides suggestions of which grating fabrication methods to use in correspondence with different deformation measurement methods. Emphasis is placed on the introduction of grating fabrication processes and the quality and applicability of the resulting gratings.

1. Introduction Full-field measurement of the deformations of materials and structures subjected to various loads is an important task in experimental mechanics. Aside from the widely used pointwise strain gauge technique, various full-field non-contact optical methods [1], including both interferometric techniques (such as moiré interferometry [2,3], holographic interferometry [4,5], speckle interferometry [6,7], and coherent gradient sensing [8,9]) and non-interferometric techniques (such as the geometric moiré method [10,11], the grid method [12,13], GPA1 [14,15], and DIC [16,17]) have been developed and applied for this purpose. Among the abovementioned methods, moiré interferometry, the geometric moiré method, the grid method, and GPA, which are referred to as grating-based methods, employ gratings as deformation carriers/sensors; these gratings are referred to as specimen gratings or deformation gratings. Benefitting from the use of specimen gratings, the grating-based methods exhibit advantages of high sensitivity, low noise, stability against rigid-body displacements, and flexibly in terms of their abilities to determine the in-plane and off-plane deformation fields of specimens over a broad range of scales.

A grating is a periodic structure consisting of lines or dots with contrasting thicknesses or that produce light of contrasting intensities. In order to satisfy the increasing accuracy requirements in scientific research and engineering, various grating-based methods must be developed to ensure applicability and accuracy. However, grating preparation on a specimen surface is challenging, and several technical parameters must be considered to ensure a successful test. For example, a grating that is to be used in a moiré interferometry test should have a highly uniform frequency, large working area (with dimensions on the order of millimetres or larger), and high diffraction efficiency. A grating that is employed in SEM moiré(a variation of the geometric moiré method) should possess high contrast, be thin, and have an appropriate electrical conductivity. The gratings used in the grid method and GPA should exhibit fine micromorphology under the selected imaging system, since error could be induced when extracting deformation information from an indistinct grating image. Since different grating characteristics are required in different deformation measurement methods, many studies have been conducted in order to establish appropriate techniques to meet the requirements of each method. To date, many techniques for producing high-frequency specimen

⁎

Corresponding author. E-mail address: [email protected] (H. Xie). 1 GPA: geometric phase analysis; DIC: digital image correlation; SEM: scanning electron microscope; NIL: nanoimprint lithography; SL: soft lithography; FIB: focused ion beam; EBL: electron beam lithography; FLA: femtosecond laser ablation; HP: holographic photolithography; TG: transfer grating; ZTG: zero-thickness grating; DMLG: deposited metal layer(s) grating; MEMS: micro-electromechanical systems; TBC: thermal barrier coating; T-NIL: thermal NIL; UV-NIL: ultraviolet NIL; SAMIM: solvent-assisted microcontact molding; 3S: solute-solvent separation; PDMS: polydimethylsiloxane; MPS: medium polymer substrate http://dx.doi.org/10.1016/j.optlaseng.2016.12.011 Received 20 June 2016; Received in revised form 1 December 2016; Accepted 11 December 2016 0143-8166/ © 2016 Elsevier Ltd. All rights reserved.

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

photoresist in the developer occurs. A parallel grating can be fabricated by using a single exposure, and a crossing grating can be obtained by using an additional exposure after rotating the substrate by 90°. Generally, the substrate surface is perpendicular to the internal bisector of the two laser beams, and the obtained grating has the same frequency as that of the interfere fringes. Obviously, the grating frequency is proportional to the incident angle and inversely proportional to the laser wavelength (see Eq. (1)). Thus, there are two ways to increase the grating frequency: by shortening the laser wavelength and by enlarging the incident angle. However, it is inconvenient to align an optical system when the laser wavelength is extremely short or is outside of the visible light range, and the optical components of shortwavelength lasers are expensive. On the other hand, it is straightforward and common to increase the grating frequency by enlarging the incident angle. Since the incident angle cannot exceed 90°, the maximum grating frequency is theoretically limited to several thousand lines per millimetre. Regardless of this limitation, HP is widely used for grating preparation, particularly the preparation of gratings for moiré interferometry, due to its advantageous characteristics, such as its ability to produce large gratings with appropriate frequencies, negligible distortion, and uniform diffraction. However, the fringes formed by HP gratings in SEM moiré methods exhibit weak contrast, owing to the smooth sinusoidal topography of the grating lines. Many techniques have been derived to improve the quality and to extend the applicability of gratings fabricated by HP. For example, Post et al. replaced the static light source with a moving point light source to eliminate the coherent noise [32]. Chen et al. developed a threedirectional grating (using exposures at 0°, 45°, and 90°) to improve the residual measurement precision [34]. Shi et al. developed the refractive medium exposure method and grating frequency multiplication technique to improve the grating frequency to 6000 lines/mm using a laser wavelength of 457.9 nm [35]. In general, two types of methods can be employed to fabricate gratings on specimen surfaces: replication methods (TG [2]) and direct exposure methods (ZTG or etch grating [2,36] and DMLG [37–39]), as shown in Fig. 2. TGs are widely used in tests that are performed at temperatures below 120 °C due to its advantages of simplicity, versatility in terms of specimen size and material, and low cost. In contrast, the direct exposure methods were developed to produce gratings that are usable in extreme environments, particularly at high temperatures. However, fabricating a grating directly on a specimen surface is complicated and expensive. First, the specimen material should be mirror-polished, which seems impossible for porous materials; in addition, the resist should be removed from the window, implying that no residual resist may remain in the grooves, which places quite strict demands on the exposure and developing processes. Thus, direct exposure methods are not utilized unless required for a particular application [33].

gratings have been developed. These techniques can be categorized as masked or maskless. In a masked technique, a mask is prepared and is used to shield or mould the adjacent material; as a result, the grating structure on the mask is duplicated on the material by using physical or chemical means, or both. General photolithography [18], mask deposition [19], NIL [20–22], and SL [23–25] are masked techniques. In a maskless technique, a controllable beam (e.g. a laser beam, electron beam, or FIB) is used to write a pattern on a special material. The beam is generally controlled by computer programs and writes according to the pre-set track point by point; thus, maskless techniques are flexible and convenient. EBL [26–28], FIB lithography [29,30], and FLA [31] are maskless methods. HP (or laser interferometry photolithography) [32–34] is another maskless technique and is highly efficient; in this method, the photoresist is exposed to an intensity field formed by laser beams, and the produced grating has a very large area, with dimensions on the order of centimetres. Although substantial literature related to grating fabrication is available, a comprehensive review that introduces the technical details and advantages of all the currently used grating fabrication techniques is still lacking, though it would facilitate appropriate fabrication technique selection and grating application. This paper provides such a review by introducing the technical details and characteristics of recently developed grating fabrication methods and suggesting which grating fabrication methods are appropriate for use in combination with different deformation measurement methods. Emphasis is placed on introducing the grating fabrication processes and the quality and applicability of the resulting gratings. 2. Grating fabrication techniques Since micromachining is the fundamental and most important step in the grating preparation process, each grating fabrication method is named after the employed micromachining technique. After performing the abovementioned steps, several typical micromachining procedures (post-processing techniques) are generally needed to produce different types of gratings; these steps include etching, film coating, and film transferring. In this section, the basic steps in grating fabrication via HP, EBL, FIB etching and depositing, NIL, SL, and FLA are introduced in detail, and the typical micromachining processes that are subsequently employed are briefly described. 2.1. Holographic photolithography A schematic diagram of grating fabrication by HP is presented in Fig. 1. When two coherent collimated laser beams (L1 and L2) overlap in air, a series of periodic fringes (intensity peaks and valleys) is formed due to the interference of the beams. For laser beams of wavelength λ and incident angle φ, and fringe frequency f can be calculated using

2 sin φ f= . λ

2.2. Electron beam lithography EBL is a standard technique that is widely used in micromachining. In this method, a focused beam of electrons is scanned to draw custom shapes on a surface that is covered with an electron-sensitive resist film. The electron beam can change the solubility of the resist and enable subsequent, selective removal of either the exposed or non-exposed regions of the resist during the developing process. EBL, which is schematically illustrated in Fig. 3, is similar to HP except in terms of the beam source and the resist. Owing to the use of an ultra-shortwavelength electron beam, EBL has a very fine resolution (20 nm); thus, it can be employed to fabricate gratings with ultra-high frequencies (more than 10,000 lines/mm). The steps of grating preparation by EBL are listed in Fig. 4. EBL is a serial manufacturing method, and the resist is exposed point by point, which leads to a very low efficiency. It takes on the order of several hours and several thousands of dollars to prepare a grating with an area of 1 cm2. Therefore, EBL is suitable for the

(1)

If a photoresist-coated substrate is exposed in the interference region and is subsequently developed and fixed, a grating structure can be fabricated on the photoresist. Because the laser irradiation can change the stability of the photoresist, selective dissolution of the

Fig. 1. Schematic diagram of grating fabrication by HP.

49

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

Fig. 2. Schematic diagrams of fabrication of grating with two deposited metal layers (type A), grating with one deposited metal layer (type B), ZTG (type C), and TG (type D). Bottom film in type A is used to improve surface optical and chemical features, e.g. prevents oxidation, and can also be pre/post-coated on type C for the same purpose.

Fig. 4. EBL grating preparation process.

fabricate gratings at high temperatures, and the ability to resist temperatures of up to 1100 °C was verified [40,41]. Xing developed the multi-scanning method to reduce the pitch error and enhance the quality of gratings produced via EBL, and ultra-fine gratings were achieved by using a normal SEM and EBL [28,42]. In summary, as a high-frequency, maskless, and flexible but lowefficiency grating fabrication method, EBL is a suitable choice for the production of gratings to be used in the SEM moiré method, GPA, and the grid method. It is especially appropriate for producing gratings to be used in the SEM moiré method, as demonstrated by extensive research.

Fig. 3. Schematic illustration of EBL.

preparation of gratings with areas on the order of square micrometres (microregion gratings), and such gratings are quite appropriate for use in the SEM moiré method, GPA, and the grid method, in combination with a high-resolution microscope. Numerous researchers have already focused on improving the quality and extending the applications of gratings produced using EBL. Kishimoto et al. first introduced SEM moiré for micro-deformation testing and reported on the use of EBL to prepare an SEM moiré grating in 1991 [11]. Many researchers subsequently expanded upon this work and achieved significant improvements. In 1993, Dally et al. reported that they wrote an SEM moiré grating with a frequency in excess of 10,000 lines/mm [27]. Xie et al. combined the deposited metal layer(s) method with EBL to

2.3. Focused ion beam etching and depositing FIB etching is a powerful technique that has been widely applied for micro/nanostructure production, mask repair, device modification, failure analysis, and integrated circuit debugging in the semiconductor industry [43]. As shown in Fig. 5, the FIB system is similar to that employed in EBL; the primary difference is the use of a gallium ion (Ga+) beam instead of an electron beam. There are two modes of FIB machining: etching and depositing. In FIB etching, the ions hit the 50

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

Fig. 5. Schematic illustrations of (a) FIB etching and (b) FIB assist depositing.

In summary, FIB etching possesses almost all of the advantages and disadvantages of EBL, although the FIB method is more straightforward and simpler. Similarly to gratings produced via EBL, those fabricated by employing FIBs are recommended for use in the SEM moiré method, GPA, and the grid method. 2.4. Nanoimprint lithography NIL was initially proposed and developed by Chou et al. in 1995 [50] and can be employed to create micro/nanoscale patterns in a resist material rapidly and with high throughput over a large area. NIL has emerged as one of the most promising technologies for nanoscale patterning, and many derivative techniques have been developed. Among these derivative techniques, T-NIL and UV-NIL, whose processes are illustrated schematically in Fig. 7, are regarded as the most important two. In T-NIL, a heat-softened imprint resist is moulded by a quartz or silicon stamp by applying pressure so that the relief pattern on the stamp is copied onto the resist. In UV-NIL, an ultravioletsensitive imprint resist is moulded by a stamp and is then cured by ultraviolet exposure, causing the relief pattern on the stamp to be copied onto the resist. Although the key component of NIL is an expensive stamp, the stamp can be re-used, so the cost of NIL is very low.

Fig. 6. FIB grating preparation process.

specimen surface, and both the secondary electrons and the sample materials are emitted due to the large mass of the incoming ions. In FIB assist depositing, a precursor gas is released and agglomerates on the specimen surface, the energy of the emitted Ga+ ions is partially absorbed by the organic metal molecules, and, consequently, the molecules are decomposed. The decomposed metal atoms are deposited onto the target region of the specimen surface, and the residual gas is simultaneously pumped out. Compared with EBL, the FIB method is more convenient and straightforward, since specimens are processed directly and the resist coating and developing steps are avoided, as shown in Fig. 6. In 2003, Xie et al. first applied FIB etching to produce a specimen grating, thereby demonstrating a new method of fabricating gratings for deformation testing [44]. Later, many studies were conducted with the objective of enhancing the grating fabrication process by increasing the maximum obtainable grating frequency and extending the applications of this method. In 2004, Dong et al. fabricated nanogratings for moiré measurement using FIB etching and succeeded in obtaining a grating with a pitch of 40 nm [45]. Li et al. reported on the fabrication of mosaic gratings using FIB etching and demonstrated that perfect alignment usually cannot be realized in the mosaic grating, causing the initial moiré to be nonuniform [29]. In addition, the applications of FIB etching gratings have been extended to the deformation measurement of MEMS [46], TBC [29,47], grain boundaries [48], optical fibres [49], and so on. FIB etching can be employed to fabricate microscale gratings for microscale deformation measurement, while fabrication by direct etching would damage the specimen surface and disturb the stress state around the grating. With the objective of reducing this damage, FIB assist depositing was proposed by Wu et al. in 2014 [30]. FIB assist depositing is suitable for fabricating thin gratings on various substrates, especially for micro/nanofilms.

Fig. 7. Schematic illustrations of (a) T-NIL and (b) UV-NIL.

51

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

In 2012, Tang et al. first reported fabricating a high-frequency moiré grating by using T-NIL [20] and UV-NIL [51], and these works drew great attention in the following years. For example, Wang et al. developed a robust method of fabricating a grating on a rough metal surface by using modified UV-NIL in 2013 [22], and in 2014, Wang et al. enabled the use of gratings produced via T-NIL for hightemperature moiré interferometry testing by combining T-NIL with the TG technique and a high-temperature adhesive [52]. In addition, Wang et al. analysed the residual stress in a resist film, estimated the stress intensity factor of a crack induced during UV-NIL, and proposed a method of evaluating the quality of a grid fabricated by UV-NIL [53]. Unlike HP, NIL relies upon physical contact between the stamp and the resist (on the specimen surface) to duplicate the grating, so a high temperature (~200 °C) and high pressure (~0.2 MPa) are required. In addition, NIL is sensitive to defects, such as imperfections in the stamp and the presence of extrinsic contaminants (dust particles and bubbles), since the affected area is always larger than the defect itself. Many methods of improving the contact uniformity [20,22,54], cleaning the stamp [55], and detecting the affected area [56,57] have been attempted. Based on the our experience, it is convenient to prepare a TG by using NIL (see Fig. 2), since a resist that is sufficiently thick to guarantee complete contact can be employed and since residual resist material is allowed. It would be challenging to manufacture a residualfree or an ultra-thin resist grating for film coating or etching on a specimen surface, since complete contact is difficult to realize if the resist layer is thin.

Fig. 8. Schematic illustrations of (a) SAMIM and (b) 3S-SL.

2.5. Soft lithography SL is a non-photolithographic strategy based on self-assembly and replica molding that is used for micro- and nanofabrication. It provides a convenient, effective, and low-cost method of micro- and nanostructure manufacturing [58]. SL and NIL are parallel methods that both have ultra-high efficiencies, enabling microstructures with larger areas to be fabricated rapidly. An elastomeric block with patterned relief structures on its surface is the key component in SL and also distinguishes SL from NIL. Because the stamp that is employed in NIL is stiff and fragile, its ability to adapt to uneven substrates is poor, and it can easily be broken during the demoulding process. Thus, since SL involves an elastomeric stamp rather than a stiff one, it is more versatile than NIL, has less stringent target surface flatness requirements, and involves an easier demoulding process. However, the resolution of SL is lower than that of NIL owing to the low stiffness of the stamp in the former technique. So far, several different SL methods have been proposed, such as microcontact printing [59], microtransfer molding [60], micromoulding in capillaries [61], SAMIM [62], and 3S-SL [24]. SAMIM and 3S-SL have already been extended to specimen grating preparation, owing to their simple procedures. The principles of SAMIM and 3S-SL are illustrated in Fig. 8. In SAMIM, a resist layer is softened by a proper solvent, and a PDMS stamp is then used to cover the mixed liquid. The mixed liquid is filled into the structure of the PDMS stamp, and the resist hardens as the solvent dissipates. Finally, the resist solidifies with a copy of the negative pattern from the PDMS stamp. The operational principles of 3S-SL are similar to those of SAMIM, but the resist and solvent are premixed to form a solution; then, the solution is dropped onto the specimen and covers the PDMS stamp. The stamp subsequently absorbs the solvent and filters the resist, and the negative resist pattern is transferred onto the specimen surface. The key materials in SAMIM and 3S-SL are the PDMS stamp and a proper solvent. The solvent must be able to wet the PDMS and must simultaneously and thoroughly dissolve (or swell) the resist. Thus, the basic mechanism in SAMIM and 3S-SL is that the stamp absorbs the solvent and filters the resist, as shown in Fig. 9. In 2014, Dai et al. first reported a TG fabrication technique based on SAMIM [23], which is as follows: a grating is imprinted onto an MPS by

Fig. 9. Basic mechanism of SAMIM and 3S-SL.

SAMIM; a thin metal film is coated onto this MPS; and the metal film is transferred onto the specimen surface. This method can be employed to fabricate high-frequency and large-area TGs easily. Afterwards, Dai et al. introduced 3S-SL into the specimen grating preparation technique, enabling the fabrication of both TGs and residual-resist-free gratings for subsequent film coating or etching [25]. Based on our experiments, the SAMIM-based grating fabrication technique is simpler than 3S-SL, since full contact can easily be achieved in SAMIM when using an MPS, and the transfer process is quite mature, but this method can only be employed to fabricate TGs. The 3S-SL-based grating fabrication technique is more versatile, since it can be used to fabricate all of the kinds of specimen gratings that are shown in Fig. 2, by adjusting the resist–solvent ratio (i.e. the concentration of the solution). Although SAMIM and 3S-SL involve minimal specimen surface requirements and can be performed easily, they exhibit a common disadvantage compared to NIL: gratings fabricated via SAMIM and 3S-SL are distorted more than those produced using NIL due to the low stiffness of the PDMS stamp and its shrinkage during curing. Generally, this distortion can be decreased by reducing the stamp deformation by using a hybrid stamp; the details of this technique can be found in Refs. [25] and [57]. Coincidentally, the use of a hybrid stamp represents an attempt to modify NIL to overcome the drawbacks of employing a stiff stamp, so the boundaries between SL and NIL are gradually becoming blurred. 2.6. Femtosecond laser ablation Writing patterns by laser ablation is a normal processing method, 52

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

also large-area machining methods and can be employed to fabricate gratings with dimensions on the order of centimetres. FLA has a relatively small machining area owing to the sharp decrease in power with increasing working area; the only currently available report shows that gratings with dimensions on the order of millimetres can be produced using this method. EBL and FIB etching have smallest machining areas among the abovementioned methods, since they are serial machining methods that fabricate patterns point by point; the gratings fabricated using these methods have dimensions on the order of micrometres.

but it is limited owing to the thermal diffusion that occurs inside the processed material during the duration of the laser pulses (tens of microseconds). Thus, it is necessary to shorten the laser pulses duration, as well as to increase the laser power. A femtosecond laser is capable of satisfying this requirement, since it has an ultra-short duration (tens of femtoseconds); thus, the deterioration of the machined region due to thermal effects can be avoided, and microstructures can be generated. Three manufacturing methods that employ femtosecond lasers have been reported to date: single-beam scanning, multi-beam interference, and mask exposure. Single-beam scanning is the initially designed manufacturing technique; this method is simple but has a low efficiency. Multi-beam interference and mask exposure are newly developed techniques; these are similar to HP and general photolithography but are extremely difficult to manipulate owing to the ultra-high power of a femtosecond laser. Nakata et al. used a femtosecond laser to fabricate a periodic structure on a material surface by employing multi-beam interference [63,64]. After that, Nakata et al. developed the mask exposure technique, by which a complex microstructure pattern can be generated in a single shot [65]. Kishimoto et al. first reported the fabrication of a grating on a specimen surface by using a scanning femtosecond laser and applied the grating to perform moiré measurements, including measurements of charge-coupled device moiré fringes, scanning laser moiré fringes, and electron moiré [31]. Since the grating is directly ablated onto the specimen surface when a femtosecond laser is employed, femtosecond laser techniques can be employed in extreme environments, as long as no specimen damage is caused by the extreme conditions. Although research on grating fabrication using femtosecond lasers is in its infancy, it is expected that more work will be conducted to demonstrate the characteristics and superiority of such methods.

3.3. Surface roughness The roughness of the specimen surface is an essential consideration when selecting an appropriate grating fabrication process. If a grating needs to be fabricated on a specimen surface directly, all of the presented methods must be feasible on a surface with a roughness of less than 0.01 µm (Ra < 0.01), which means that the specimen surface must be precisely polished. If NIL is to be used, the surface must be especially flat in order to ensure complete contact between the specimen surface and the stamp. In addition, the reflection of the laser or electron beam from the surface should be considered in HP and EBL, since it can disturb the exposure of the resist. Generally, in order to improve the surface flatness, coating the surface with an appropriate film before grating fabrication is a suitable option. However, if a TG is to be used, the specimen surface need not be exceedingly flat (since the adhesive can compensate for a rough surface), as long as the adhesive can form a strong connection between the specimen surface and the metal film on the grating substrate. A grating can easily be transferred to a surface of average roughness (such as the surface produced by wire electrical discharge machining), which is the main advantage that has made TGs become widely used.

3. Characteristics and applications of grating fabrication methods

3.4. Working conditions

Before selecting a grating fabrication method, several factors should be carefully considered, including the grating frequency, grating area, surface roughness, working conditions, and cost. In the following section, each of these factors will be discussed in detail.

Since grating-based deformation measurement techniques are remote sensing methods, and the measurement equipment can be spatially separated from the tested specimens. Thus, grating-based deformation measurement techniques are frequently used for deformation testing in extreme environments, especially those involving high temperatures. Since the measurement equipment is not exposed to harsh conditions in an extreme environment experiment, the only requirement is that the specimen grating must be durable. In the abovementioned grating fabrication methods, the post-processing techniques, including etching and film coating, are most common methods of preparing hightemperature-resistance gratings, such as DMLGs and ZTGs [37–39]. TGs are also usable, if the temperature is below 700 °C [52].

3.1. Grating frequency The grating fabrication method should be carefully determined according to the frequency of the grating produced. HP is generally used to prepare gratings with frequencies ranging from 600 lines/mm to 6000 lines/mm. EBL and FIB etching are two maskless methods that can be employed to produce gratings with high frequencies of up to 10,000 lines/mm. NIL can be used to fabricate ultra-high-frequency gratings, where the frequency is determined by the selected stamp, and the stamp itself is usually fabricated by HP and EBL. SL is similar to NIL, and the grating frequency is determined by the stamp. However, it is difficult for a PDMS prepolymer to fill in spaces in structures that have dimensions of less than 100 nm; thus, SL is only suitable for preparing gratings with frequencies of less than 5000 lines/mm. FLA is a new grating fabrication method, so few studies of this technique have been conducted, and the frequencies of gratings fabricated using this method are less than 1000 lines/mm.

3.5. Cost The cost of grating fabrication is directly proportion to the grating area and the grating frequency. If a serial machining method (such as EBL or FIB etching) is used, it is quite expensive and time-consuming to fabricate a grating with dimensions on the order of centimetres (several thousand dollars per piece); however, it is easy to prepare a highfrequency or even multi-frequency grating with dimensions on the order of micrometres. HP and NIL have roughly similar costs of about several hundred dollars for a grating with an area of 1 cm×1 cm. SL is a cheap method; based on our research, the cost of a grating with an area of 1 cm×1 cm is on the order of tens of dollars. Since the maskless FLA system is serial processing and high cost, the price of the FLA grating is the same as FIB grating. Table 1 summarizes the characteristics of each of the grating fabrication methods to facilitate technique selection.

3.2. Grating area The grating area is a crucial factor in the deformation measurement, since the grating area is related to the fabrication time, cost, and measurement method selection. For a TG, a large master grating can be divided into smaller pieces to be transferred to multiple specimens. Among the presented grating fabrication methods, HP has the largest fabrication area due to its use of a high-power laser, which can produce gratings with dimensions on the order of decimetres. NIL and SL are 53

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

Table 1 Characteristics of each grating fabrication method. Technique

Mask

Processing mode

Processing efficiency

Processing area

Processing target

Frequency

Cost

Grating type

HP EBL FIB NIL SL FLA

No No No Yes Yes –

Parallel Serial Serial Parallel Parallel –

High Low Low High High Middle

Large Small Small Large Large Middle

Resist Resist Specimen surface Resist Resist Specimen surface

Middle High High High Middle Low

Middle High High Middle Low High

TGDMLGZTG DMLGZTG ZTG TG DMLG ZTG TG DMLG ZTG ZTG

4. Grating quality assessment and characterization methods

is too small to form an effective fringe pattern. Thus, checking the fullfield grating pitch by examining the grating image using the Fourier transform is recommended. This method is detailed in Ref. [23] and can be summarized as follows: a 2D Fourier transform of the grating image is performed to obtain the Fourier spectrum; filtering and a 2D inverse Fourier transform are conducted to drive out the phase field; the zero points of the phase are located by employing linear fitting; finally, the full-field pitch information is obtained based on the distance between adjacent zero points. Numerical simulations have verified that the pitch error of this method is less than 0.01 pixels.

Since grating quality is an important component in optical engineering, there is a comprehensive evaluation system that can be employed to assess it. For example, for a diffraction grating, a series of evaluation indices is available to rank the quality; these indices include the diffraction efficiency, wavefront aberration, pitch error, amount of stray light, strength and number of the ghost line, and distinguishability. However, for a deformation grating, the evaluation indices that are employed in optical engineering are meaningless, while some new evaluation indices have been developed. In this section, four important evaluation indices and the corresponding characterization methods are introduced, with the objective of guiding grating preparation.

4.2. Grating thickness The objective of deformation testing is to measure the deformations of the specimen surface, but only the grating deformations could be recorded generally. The difference between the deformation of the grating and that of the subjacent specimen can be non-negligible, if the deformation gradient is large and the grating layer is overly thick. The reason for this difference is the shear lag, whereby the shear stress in the grating attenuates as it propagates through the grating material [2]. Based on our experience, the thicknesses of DMLGs and ZTGs are around several hundred nanometres (or much thinner) and the shear lag is extremely weak, so the only factor that must be considered is the depth/height of the grating structure, which should be less than 1/10 of the specimen thickness, especially for a thin film specimen [20]. However, it is always advisable to assess the compatibility between the TG and the measured deformation before selecting the TG, since a TG that is dozens of micrometres thick may conceal errors. In our previous work, we demonstrated that a 25-µm-thick TG could obscure micro cracks in the specimen surface, as shown in Fig. 10 [25]. Thus, TGs are not suitable for use in conditions involving a large strain gradient or fracture failure.

4.1. Frequency uniformity In moiré testing, a grating that lacks frequency uniformity will yield an extremely irregular initial fringe pattern that could fail the test, as the specimen deformations will be occluded by the irregular fringes. Thus, it is essential to determine the frequency uniformity of the grating before using it. Two kinds of characterization methods are generally used: those that involve checking the null field of the grating [25] and those that involve checking the full-field grating pitch [23]. Which type of characterization method is used depends upon the selected deformation measurement method. When it is planned to use a particular grating in a moiré measurement method, such as the scanning moiré method or moiré interferometry, it is a routine step to obtain the null field before testing. Thus, using the null field as an evaluation index is perfectly logical and reasonable. From the null field, the grating can be directly judged to be satisfactory or unsatisfactory. If two fringes or fewer fringes appear in the null field, the characterized grating is considered to be satisfactory; on the other hand, the grating is deemed unsatisfactory if the fringes are messy and dense. In our previous study [25], we characterized grating frequency uniformity based on the null field that was obtained using moiré interferometry. If a grating is to be used in GPA or the grid method, the grating area

4.3. Diffraction efficiency The diffraction efficiency is used to show the diffraction ability of gratings. There are two ways of specifying the diffraction efficiency: the

Fig. 10. SEM images of (a) TG cross-section and (b) epoxy–Ni base alloy interface. Impacted indentation method was used to induce severe deformations (micro cracks) around indentations on Ni base alloy, as shown in (b).

54

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

single DMLG shown in Fig. 2. In the classic and scanning moiré methods, the opening ratio of the grating, which is defined as the ratio of the width of the white/black bar (or groove or ridge bar) to the grating pitch, can be designed to enhance the fringe contrast. By analysing the results of a numerical simulation, Tang et al. concluded that an opening ratio of 0.5 generates an optimal fringe pattern [20]. To further enhance the contrast of moiré fringes obtained using an SEM, the abovementioned methods of increasing the grating contrast in an SEM image can also be employed. In moiré interferometry, using a sine-wave grating with fewer micrometre-sized defects and a fine coherent laser (in single longitudinal mode) can effectively enhance the fringe contrast. 5. Conclusion Since grating-based deformation measurement methods first appeared, grating fabrication techniques have been improved to satisfy the diverse requirements of the various types of deformation measurement tests under different conditions. A standard, widely accepted grating fabrication technique is still lacking, which reflects the variety of requirements and applications of the grating-based measurement methods and also reveals that it is impossible for one fabrication method to possess all of the desired characteristics. Therefore, grating fabrication methods will continue to be studied to meet the growing demands. This review focused on the technical details and advances of the different grating fabrication techniques and suggested which methods are appropriate for fabricating gratings that are to be used for different deformation measurement methods. The grating fabrication processes were introduced, and the quality and applicability of the various types of gratings were discussed. Currently, the methods of fabricate gratings for room-temperature testing are becoming mature, and numerous options are available. However, many issues related to how to manufacture gratings for use at high temperatures must still be addressed, such as the complexity of the manufacturing process, low qualified rate, and high cost. Nevertheless, due to the strong demand for high-temperature deformation testing and with the rapid development of advanced micromachining techniques, the authors believe that methods of fabricating gratings for hightemperature testing will receive widespread attention and will be further developed in the near future.

Fig. 11. Device for obtaining high-diffraction-efficiency gratings.

absolute diffraction efficiency, which is defined as the ratio of the power of a particular diffractive order to the incident power, and the relative diffraction efficiency, which is defined as the ratio of the power of a particular diffractive order to the power that would be reflected by a mirror with the same coating as the grating. In moiré interferometry, the absolute diffraction efficiency is employed, and a high diffraction efficiency requires a low laser power and can allow high-frame-rate imaging to be performed, which is strongly desirable in dynamic measurements. The diffraction efficiency is mainly affected by the groove profile of the grating microstructure, grating frequency, laser wavelength, incident angle, and diffractive order. The first-order diffracted beams always possess the highest diffraction efficiencies, and they are therefore the most commonly used beams in moiré interferometry. TGs fabricated by HP are the most frequently used gratings in moiré interferometry, so here we introduce a method that can be employed to detect and maximize the diffraction efficiency of a TG. The device that is used in this method is shown in Fig. 11 [66]; in this device, a laser beam is normally incident on the grating (on a transparent substrate), which is fixed in position in the developer. The intensity of the zero-order beam from the grating is compared with that of the first-order beam as development proceeds. A neutral density filter is placed in the zeroth-order beam, in order to compensate for the difference between the refractive indices of the aqueous developer and the air (in which the grating is presumed to be used subsequently). If a filter with a neutral density slightly greater than 2.0 is used and development is stopped when the zero- and first-order beams have equal intensities, a reasonable compromise between efficiency and linearity can be obtained [66].

Acknowledgements The authors are grateful to the financial support from the National Natural Science Foundation of China (Grant Nos. 11232008, 11227801), Tsinghua University Initiative Scientific Research Program.

4.4. Contrast

References

High contrast is desirable in a grating image or fringe pattern to facilitate data processing. In general, the contrast K is given by

[1] Rastogi PK. Photomechanics (topics in applied physics). Berlin: Springer; 2000. [2] Post D, Han B, Ifju P. High sensitivity moiré: experimental analysis for mechanics and materials. Berlin: Springer; 1994. [3] Ifju PG, Han B. Recent applications of moiré interferometry. Exp Mech 2010;50(8):1129–47. [4] Jones R, Wykes C. Holographic and Speckle Interferometry, 6. Cambridge: Cambridge University Press; 1989. [5] Kreis T Holographic interferometry: Principles and methods. in: Simulation and Experiment in Laser Metrology: Proceedings of the International Symposium on Laser Applications in Precision Measurements Vol. 2; 1996 June 3-6; Balatonfüred, Hungary. Hoboken, NJ: John Wiley & Sons, Inc., ; 1996. [6] Leendertz JA. Interferometric displacement measurement on scattering surfaces utilizing speckle effect. J Phys E: Sci Instrum 1970;3(3):214. [7] Rastogi PK, editor. Digital speckle pattern interferometry and related techniques.. Weinheim: Wiley-VCH; 2000. [8] Tippur HV, Krishnaswamy S, Rosakis AJ. A coherent gradient sensor for crack tip deformation measurements: analysis and experimental results. Int J Fract 1991;48(3):193–204. [9] Ma K, Xie H, Zhu J. Two modified coherent gradient sensing methods for slope measurement of reflective surfaces. Meas Sci Technol 2014;25(12):125202. [10] Durelli A, Parks V. Moiré analysis of strain.strain. Upper Saddle River, NJ: Prentice Hall; 1970.

K=

Imax − Imin , Imax + Imin

(2)

where Imax and Imin denote the maximum and minimum values, respectively, of the intensity of the grating or fringe in the image. In GPA and the grid method, the image is typically obtained using an SEM, so to enhance the grating contrast, it is generally necessary to obtain a high-contrast SEM image. Depending on the SEM imaging mechanism, either of two methods can be adopted to enhance the contrast. One is to increase the angularity of the grating profile; for example, a square-wave grating exhibits high contrast, while a sinewave grating yields low contrast. The other is to change the conduction characteristics between grating microstructures, for example, by coating an electric film onto an insulated specimen surface, or by coating an insulated film onto an electric specimen surface, as was done for the 55

Optics and Lasers in Engineering 92 (2017) 48–56

X. Dai, H. Xie

[39] Forno C. High-temperature-resistant gratings for moiré interferometry. In: proceedings of the International Conference on Interferometry-94; 1994 May. Warsaw, Poland. Bellingham, WA: International Society for Optics and Photonics; 1994. [40] Xie H, Kishimoto S, Shinya N. Fabrication of high-frequency electron beam moiré grating using multi-deposited layer techniques. Opt Laser Technol 2000;32(5):361–7. [41] Kishimoto S, Xie H, Shinya N. Electron moiré method and its application to microdeformation measurement. Opt Laser Eng 2000;34(1):1–14. [42] Zhao YR, Lei ZK, Xing YM, Hou XH, Bai PC. Fabricating parameters optimization of high frequency grating by multi-scanning electron beam method. Exp Mech 2014;54(1):45–55. [43] Reyntjens S, Puers R. A review of focused ion beam applications in microsystem technology. J Micro Micro 2001;11(4):287. [44] Xie H, Li B, Geer R, Xu B, Castracane J. Focused ion beam Moiré method. Opt Laser Eng 2003;40(3):163–77. [45] Yan D, Cheng J, Apsel A. Fabrication of SOI-based nano-gratings for Moiré measurement using focused ion beam. Sens Actuator A: Phys 2004;115(1):60–6. [46] Liu Z, Lou X, Gao J. Deformation analysis of MEMS structures by modified digital moiré methods. Opt Laser Eng 2010;48(11):1067–75. [47] Zhu JG, Xie H, Li Y, Hu Z, Luo Q, Gu C. Interfacial residual stress analysis of thermal spray coatings by miniature ring-core cutting combined with DIC method. Exp Mech 2014;54(2):127–36. [48] Li Y, Xie H, Tang M, Zhu J, Luo Q, Gu C. The study on microscopic mechanical property of polycrystalline with SEM moiré method. Opt Laser Eng 2012;50(12):1757–64. [49] Jiang M, Tang M, Wu H, Li Y, Xie H. FIB moiré gratings and their application in the measurement of optical fibers' mechanical properties. Chin Phys Lett 2012;29(3):034210. [50] Chou SY, Krauss PR, Renstrom PJ. Imprint of sub-25nm vias and trenches in polymers. Appl Phys Lett 1995;67(21):3114–6. [51] Tang M, Xie H, Li Y, Li X, Wu D. A new grating fabrication technique on metal films using UV-nanoimprint lithography. Chin Phys Lett 2012;29(9):098101. [52] Wang H, Xie H, Li Y, Fang P, Dai X, Wu L, et al. Fabrication of high temperature moiré grating and its application. Opt Laser Eng 2014;54:255–62. [53] Wang Q, Kishimoto S. Simultaneous analysis of residual stress and stress intensity factor in a resist after UV-nanoimprint lithography based on electron moiré fringes. J Micro Micro 2012;22(10):105021. [54] Schift H. Nanoimprint lithography: an old story in modern times? A review. J Vac Sci Technol B 2008;26(2):458–80. [55] Lin P, Pi S, Jiang H, Xia Q. Mold cleaning with polydimethylsiloxane for nanoimprint lithography. Nanotechnol 2013;24(32):325301. [56] Li H, Ding Y, Tang Y, Liu H, Lu B. Distortion reduction by load release for imprint lithography. Micro Eng 2006;83(3):485–91. [57] Dai X, Xie H, Dai F, Kishimoto S. Characterizing macroscopic lateral distortion in nanoimprint lithography using moiré interferometry. Appl Phys Lett 2016;108(5):053109. [58] Xia Y, Whitesides GM. Soft lithography. Ann Rev Mater Sci 1998;28(1):153–84. [59] Amit K, Whitesides GM. Features of gold having micrometer to centimeter dimensions can be formed through a combination of stamping with an elastomeric stamp and an alkanethiol ‘‘ink’’ followed by chemical etching. Appl Phys Lett 1993;63:2002. [60] Zhao XM, Xia Y, Whitesides GM. Fabrication of three-dimensional micro-structures: microtransfer molding. Adv Mater 1996;8:837. [61] Kim E, Xia Y, Whitesides GM. Polymer microstructures formed by moulding in capillaries. Nature 1995;376:581. [62] Kim E, Xia Y, Zhao XM, Whitesides GM. Solvent-assisted microcontact molding: a convenient method for fabricating three-dimensional structures on surfaces of polymers. Adv Mater 1997;9:651. [63] Nakata Y, Okada T, Maeda M. Fabrication of dot matrix, comb, and nanowire structures using laser ablation by interfered femtosecond laser beams. Appl Phys Lett 2002;81(22):4239–41. [64] Nakata Y, Okada T, Maeda M. Lines of periodic hole structures produced by laser ablation using interfering femtosecond lasers split by a transmission grating. Appl Phys A 2003;77(3–4):399–401. [65] Nakata Y, Okada T, Maeda M. Lithographical laser ablation using femtosecond laser. Appl Phys A 2004;79(4–6):1481–3. [66] Beesley MJ, Castledine JG. The use of photoresist as a holographic recording medium. Appl Opt 1970;9(12):2720–4.

[11] Kishimoto S, Egashira M, Shinya N, Carolan RA. Local micro-deformation analysis by means of microgrid and electron beam moiré fringe method. In: Jono M, Inoue T, editors. Proceedings of the 6th International Conference on Mechanical Behavior of Materials Vol. 4; 1991 July 29-Aug 2. Kyoto, Japan, Maryland Heights, MO: Elsevier Ltd; 1991. p. 661–6. [12] Parks VJ. The grid method. Exp Mech 1969;9(7):27N–33N. [13] Sirkis JS. System response to automated grid methods. Opt Eng 1990;29(12):1485–93. [14] Hÿch MJ, Potez L. Geometric phase analysis of high-resolution electron microscopy images of antiphase domains: example Cu3Au. Philos Mag A 1997;76(6):1119–38. [15] Dai X, Xie H, Wang Q. Geometric phase analysis based on the windowed Fourier transform for the deformation field measurement. Opt Laser Technol 2014;58:119–27. [16] Peters WH, Ranson WF. Digital imaging techniques in experimental stress analysis. Opt Eng 1982;21(3):427–31. [17] Pan B, Quian K, Xie H, Asundi A. Two-dimensional digital image correlation for inplane displacement and strain measurement: a review. Meas Sci Technol 2009;20(6):062001. [18] Dai X, Xie H, Wang H, Li C, Liu Z, Wu L. The geometric phase analysis method based on the local high resolution discrete Fourier transform for deformation measurement. Meas Sci Technol 2014;25(2):025402. [19] Jin H, Haldar S, Bruck HA, Lu WY. Grid method for microscale discontinuous deformation measurement. Exp Mech 2011;51(4):565–74. [20] Tang M, Xie H, Zhu J, Li X, Li Y. Study of moiré grating fabrication on metal samples using nanoimprint lithography. Opt Exp 2012;20(3):2942–55. [21] Tang M, Xie H, Zhu J, Wu D. The failure mechanisms of TBC structure by moiré interferometry. Mater Sci Eng A 2013;565:142–7. [22] Wang Q, Kishimoto S, Tanaka Y, Kagawa Y. Micro/submicro grating fabrication on metals for deformation measurement based on ultraviolet nanoimprint lithography. Opt Laser Eng 2013;51(7):944–8. [23] Dai X, Xie H, Wang H. Deformation grating fabrication technique based on the solvent-assisted microcontact molding. Appl Opt 2014;53(30):7037–44. [24] Dai X, Xie H. A simple and residual-layer-free solute–solvent separation soft lithography method. J Micromech Microeng 2013;25(9):095013. [25] Dai X, Xie H. Versatile specimen-grating fabrication technique for moiré method based on solute-solvent separation soft lithography. Opt Mater Express 2016;6(5):1530–44. [26] Kishimoto S, Egashira M, Shinya N. Microcreep deformation measurements by a moiré method using electron beam lithography and electron beam scan. Opt Eng 1993;32(3):522–6. [27] Dally JW, Read DT. Electron beam moiré. Exp Mech 1993;33(4):270–7. [28] Xing YM, Kishimoto S, Shinya N. Multiscanning method for fabricating electron moiré grating. Exp Mech 2004;44(6):562–6. [29] Li Y, Xie H, Guo B, Luo Q, Gu C, Xu M. Fabrication of high-frequency moiré gratings for microscopic deformation measurement using focused ion beam milling. J Micromech Microeng 2010;20(5):055037. [30] Wu D, Xie H, Dai X, Wang R. A novel method to fabricate micro-gratings applied for deformation measurement around a crack in a thin film. Meas Sci Technol 2014;25(2):025012. [31] Kishimoto S, Tanaka Y, Tomimatsu T, Kagawa Y, Nagai K. Fabrication of micromodel grid for various moiré methods by femtosecond laser exposure. Opt Lett 2009;34(1):112–4. [32] Post D, McKelvie J, Tu M, Dai F. Fabrication of holographic gratings using a moving point source. Appl Opt 1989;28(16):3494–7. [33] Guo Y, Ifju P, Boeman R, Dai F. Formation of specimen gratings for moiré interferometry applications. Exp Tech 1999;23(5):28–32. [34] Chen J. Three-directional grating and application in measuring residual stresses. Int J Mech Eng Educ 2003;31(4):310–6. [35] Shi L, Dai F, Zhou X, Daum W High-sensitivity moiré grating fabrication. in: Proceedings of the International Conference on Experimental Mechanics: Advances and Applications; 1996 Dec 4–6; Singapore. Bellingham, WA: International Society for Optics and Photonics; 1997. p. 524–528. [36] Ifju P, Post D. Zero-thickness specimen gratings for moiré interferometry. Exp Tech 1991;15(2):45–7. [37] Kearney A, Forno C. High temperature resistant gratings for moiré interferometry. Exp Tech 1993;17(6):9–12. [38] Huimin X, Dietz P, Fulong D, Schmidt A. Fabrication of high frequency gratings for high temperature. Exp Tech 1995;19(5):28–9.

56