1. Introduction

In recent years, the mobile phone camera industry has shown interest in DOEs (Diffractive Optics Elements), which are able to spread multi-points IR beam spots or patterns onto an object, as illustrated in Fig. 1(a). These spots due to the DOEs are used for the 3-D measuring of objects based on a structured light scheme [1–3]), autofocusing [4–6] of a phone camera lens module, and even by aligning the field of view of the dual cameras of a phone (when the phone has dual cameras instead of a single camera). On the other hand, the display industry also has a keen interest in POEs (Periodical Optics Elements), which block the outside lights from coming into a display module to enhance the contrast of the display.

 

Fig. 1 (a) A 3-D measuring scheme using a DOE with a structured light source, and (b) a 1.2 m × 0.7 m film type POE.

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During the development stage of new phone camera or display modules, various types of DOEs and POEs must be designed and tested. Therefore, the industry requires a fast and precise fabrication method to make prototypes of these optical elements. The electron beam lithographic system [7, 8] has a great resolution and precision but would be not proper for this work owing to its high cost, whereas the direct laser lithographic system [9–14] is superior to the electron beam system due to its easier operation (i.e. no vacuum environment required) and lower manufacturing cost. Of course resolution of the direct laser lithography would be hundreds times worse than the electron beam, but most of DOEs and POEs require relatively low resolution comparing with semiconductors and the direct laser lithography can meet the requirement. Moreover, when a large size POEs are required, as shown in Fig. 1(b), the electron beam method is hard to be applied because the size is over 1 m. Figure 2(a) shows a typical configuration of the direct laser lithographic system, which has a stabilized laser source, a lithographic head, and a two-axis linear-motor stage with an alignment tilt table. The target specimen is put on the linear stage and moves in two orthogonal directions. In our case, an Ar + laser was adopted as a lithographic source beam. Gaseous laser sources such as Ar + generally have some fluctuations at low frequencies ranging from 0 to hundreds of Hz, as well as relatively small fluctuations to several hundreds of kHz [15]. In order to decrease these fluctuations, an acousto-optic modulator (AOM) and a photodetector (PD) were used with a servo controller, as illustrated in Fig. 2(a). The lithographic head has an autofocusing function that keeps the focused lithographic beam spot on the top surface of the target specimen while the specimen moves in the x and y directions. On the other hand, for the fabrication of POEs, the lithographic head moves in the x and y directions while the target is fixed, as shown in Fig. 2(b).

 

Fig. 2 Schematic configuration of a direct laser lithographic system used to fabricate (a) DOEs and (b) POEs: AOM, acousto-optic modulator; BS, beam splitter; PD, photodetector; QD, quadrant detector; LD, laser diode.

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In any case, we suppose that this type of direct laser lithographic system is an absolutely useful tool to those who make various prototypes of DOEs and POEs. However, efforts to reduce the manufacturing time remain necessary because the direct laser lithographic system essentially fabricates DOEs or POEs in a line-by-line manner without any photo masks, thus requiring much time when the target element is composed of many lines. To address this requirement and shorten the developing durations of phone cameras or displays, we propose a new fabrication method that writes dual lines simultaneously using the laser ablation phenomenon [16]. The details of the proposed method and the experimental results are presented in Sections II and III, respectively.

2. Basic mechanism of dual-line fabrication

Fabrication mechanisms of direct laser lithography have been presented by several research groups [9–12]. A method using a thermochemical effect is one of the most effective techniques which can be used to fabricate a single chromium line. In this method, the lithographic source beam is focused onto a chrome layer directly and causes an oxidation on its surface, as illustrated in Fig. 3. As a result, a Cr₂O₃ layer is formed on the chrome surface. In our case, 90-nm-chromium layers were used. The exact thickness of the Cr₂O₃ layer is practically hard to be distinguished from the chromium layer, but the total height of the pattern (Cr + Cr₂O₃ layer) was measured about 94 nm in average value, so we suppose that the thickness of the Cr₂O₃ layer exists between 4 nm to 94 nm. After writing a complete pattern line by line, the target DOE is immersed into a liquid etchant consisting of six parts of a 25% solution of K₃Fe(CN)₆ and one part of a 25% solution of NaOH. This removes the bare chrome much more quickly than the Cr₂O₃. Therefore, if a user sets the proper etching time, the bare chrome will disappear while the written pattern covered with Cr₂O₃ will remain on the substrate, as shown in Fig. 3.

 

Fig. 3 Single-line fabrication mechanism.

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Various DOEs and POEs can easily be fabricated using this method. It, however, a considerable amount of time is required. Figure 4 shows a typical POE pattern required by a display company. To write one rectangular cell, the stage should move a distance of 2(Wx + Wy). To write 3 × 3 cells, the stage should move a minimum distance of 18(Wx + Wy). As shown in Fig. 4(b), the actual moving length will be 18(Wx + Wy) + 6(Wx + s) + 2(Wy + s) for 3 × 3 cells, but the stage is able to move very rapidly on the blue dotted line in Fig. 4(b). Thus, the actual length of the pattern (18(Wx + Wy)) may be a serious bottleneck from an industrial standpoint.

 

Fig. 4 (a) A photographic view of a typical POE pattern, and (b) the stage control scheme for the fabrication of a series of the rectangular patterns.

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To mitigate this bottleneck, we proposed the new idea of creating dual lines at once using the laser ablation phenomenon. When the laser intensity is increased to a sufficient level, the chrome layer is removed directly and this flied off Cr seems to be piled up again on the both sides, as shown in Fig. 5. This is the laser ablation part. Of course, the thermochemical effect is accompanied through this phenomenon. Oxidation, however, does not arise in the center area of the focused beam because there is no chrome, and the substrate is usually an anti-oxidation material such as glass. Therefore, the Cr₂O₃ layer forms only on the side wings. After the etching process, the dual lines are fabricated simultaneously. The space between the two lines is determined by the beam waist of the focused beam, as shown in Fig. 5. With the proposed technique, only 8 writing instances (four in the x direction and four in the y direction) are needed to create 3 × 3 cells. The total moving length of the stage is approximately 12(Wx + Wy) when the value of s is small enough. The proposed method has the effect of reducing the length by 33%. It, however, works much more efficiently at reducing the manufacturing time, as every stage must decelerate its speed to nearly zero at the corner. A previous method has 36 corners. See the circular dot in Fig. 4(b)] to fabricate 3 × 3 cells, while the proposed method has only eight cautious points [see the circular dot in Fig. 6(e)]. In fact, the manufacturing time of a 3 × 3 square pattern can be reduced when using the proposed method from 5.4 s to 2.6 s. This advantage is prominent when the number of cells increases. For a 1,000 × 1,000 cell, the previous method must process four million corners, whereas the proposed manufacturing method has only 10,201 ( = 101 × 101) cautious points. The actual time reduction is as high as 92% in such a case.

 

Fig. 5 Dual-line fabrication mechanism. The beam size is same as the Fig. 3.

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Fig. 6 (a) Two Cr lines are fabricated at once. (b) New lines just meet the previous lines. (c) The lithographic beam passes through and removes the previous line. (d) Final result. (e) A stage control scheme to fabricate the 3 × 3 rectangular pattern.

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3. Experimental results

The most important parameter when creating dual lines is the intensity level of the lithographic source beam. We wrote lines repeatedly with various intensity levels. First, we measured the intensity of the lithographic beam at the focal point, as shown in Fig. 2(a), using a commercial PD (photodetector). Next, we replaced the PD with a target specimen to fabricate a line. Lastly, we increased the intensity level of the source beam from 3 mW to 50 mW (interval: 1 mW) using the AOM, and repeated these processes. Figure 7 shows the sectioning profiles with various intensities obtained by an AFM (Atomic Force Microscopy). The single line was fabricated from 9 mW to 16 mW, while the dual lines initially appeared at 23 mW. In our experiments, the thickness of the chrome layer was 90 nm. The heights of the fabricated lines were slightly over 90 nm, as shown in Figs. 7(c) and 7(d). However, the heights of the dual lines clearly exceeded the original chrome thickness. We assume that this is due to the side wing effect, as illustrated in Fig. 5. The two (dual) lines were successfully fabricated from 23 mW to 47 mW.

 

Fig. 7 Sectioning profiles of the pattern formed with various intensities.

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The lithographic lens had a NA (Numerical Aperture) of 0.7 at 100X. The wavelength of the Ar + laser was 488 nm. The resolution of the linear stage was less than 20 nm in the x and y directions. The writing speed (stage speed) was about 250 mm/s on average. The autofocusing mechanism ensured that the focused beam was on the top surface of the target specimen within 1 μm of error, which was sufficient because the depth of focus of the lithographic lens was exceeded 1 μm. As a result, the dual lines were clearly fabricated with a line width of approximately 1 μm, as shown in Fig. 8(b). Moreover, several DOEs (rectangular, triangular, parallelogram, and diamond types) were also fabricated using the proposed method, as shown in Fig. 9. Figure 10 shows a SEM image of the diamond pattern. Their diffractive images were also captured when a light was illuminated onto the DOEs. The test setup is shown in Fig. 11.

 

Fig. 8 3D measuring results of the (a) single and (b) dual lines obtained by a commercial white-light scanning interferometer [17, 18].

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Fig. 9 Photographic views, 3D images, and their diffractive patterns of (a) rectangular, (b) triangular, (c) parallelogram, and (c) diamond DOEs.

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Fig. 10 SEM image of the diamond pattern. Hitachi SU6600 was used to take the image.

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Fig. 11 Experimental setup used to capture the diffractive image of DOEs.

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4. Conclusions

Based on the laser ablation phenomenon and the thermochemical effect of chrome, we suggested a new laser lithographic method that can fabricate dual lines simultaneously. In our study, when the intensity of the source beam ranged from 23 mW to 46 mW at the focal point, the dual lines were successfully fabricated. When fabricating a 3 × 3 rectangular pattern, the proposed method reduced the total lithographic length by about 33%. It, however, should be noted that the manufacturing time was reduced by nearly 52%. When fabricating a 1,000 × 1,000 rectangular pattern, the manufacturing time was reduced by more than 90%. Of course, the lithographic lens must be changed to change the space between the two lines. Certain complicated patterns may be difficult to fabricate using our method. Nevertheless, we believe that the proposed method offers a major advantage for industry.