A compact optical pickup head in blue wavelength with a single-axial actuator i.e. focusing, for laser thermal lithography was designed, fabricated, and tested. The numerical aperture of the objective lens was 0.85. The linear range of the focus error signal was 3 μm. A planar spring structure for improving the horizontal stability was designed and incorporated into the actuator. We applied a modified push-pull method together with a static Blu-ray re-writable disc to test the horizontal stability of the pickup head. We found that the in-plane jitter of the pickup head in two orthogonal directions were 0.34 nm and 1.59 nm, respectively. We demonstrated an example of applying the pickup head to write an inorganic photo-resist GeSbSnO film, and well-defined pattern was obtained with ~220 nm spot size.
© 2013 OSA
Recently, laser thermal lithography (LTL) has become a new technique for lithography [1–4]. Utilizing the thermal effect of in-organic photo-resists, spots or line-widths smaller than the diffraction limit can be achieved. Phase transition mastering (PTM) technology, another application of LTL, has become the standard mastering process for BD-ROM disc [5–8]. Yet other applications of using LTL to produce periodic patterns for photonics crystals have been proposed [9,10]. Although LTL has been successfully applied to those areas, system cost and device size could largely be reduced by using an optical pickup head (OPH) as the light source.
Comparing with other lithography technologies such as photo-lithography, electron beam lithography (EBL) and focused ion beam lithography (FIBL), the use of an OPH in LTL has its own advantages. Firstly, among all these lithography technologies, the system cost and thus the user cost is the lowest. Secondly, unlike EBL and FIBL, OPH operates in atmosphere, without the need of vacuum, therefore, the preparation time is short and the size of the work-piece can be larger. Thirdly, it is mask-less and free from proximity effect that often appears in the electron and ion beam lithography methods due to charge accumulation in the resist [11–13].
For DVD-RW, BD-ROM and BD-RW, the optical pickup head (OPH) uses a tri-axial actuator with six-wired suspension structure to cover the run-out of the spinning optical disc while it is reading or writing the disc. By adopting the six-wired structure, the actuator can easily move laterally, up-and-down and adjust the tilt of the objective lens. However, it is the six-wired structure that results in the actuator too soft to avoid the environmental perturbation. The jitter in lateral direction for this kind of OPH was usually several hundred nano-meters from the neutral positions, therefore the OPH is not suitable for applications where tracking servo mechanism is not available on the working pieces, e.g. in LTL application. Although Rothenbach et al  have tried to use a Blu-ray OPH as the light source for exposing the photo-resist on the substrate by fixing the actuator to avoid the jitters in lateral and vertical directions, consequently, they had to adopt another precision stage to keep the objective lens well focused on the surface of the substrate. Since there was no auto-focusing mechanism in their system, therefore, they had to spend efforts to adjust the focus on the substrate before the lithography process. And, it is difficult for this design to process a sample with large-area or with bent surface.
Therefore, for LTL application where there is no tracking-servo mechanism available, the actuator in the OPH should be redesigned to improve its horizontal stability while maintaining the auto-focus function. In this study, we propose to use an optical pickup head in blue wavelength for LTL. The OPH has a single-axial actuator for auto-focusing and has good horizontal stability by using a special designed planar spring to stabilize the in-plane jitter. We report the design, fabrication and performance results of this OPH. We also introduce a method that adopts a modified push-pull method for testing the horizontal stability of the OPH.
2. Optical design of the optical pickup head
2.1 Brief descriptions of the optics
Figure 1 shows schematically the optical structure of the OPH. For convenience of the drawing, the laser beam is drawn such that it is reflected by the fold mirror into the direction of the paper-plane instead of the paper-normal in the real structure. A blue-violet laser diode with 405nm wavelength was used as the light source. The polarization of the emitted laser beam is set to be parallel to the paper. A beam-shaper with magnification 1.5X is used in the optics to circularize the output beam. The polarization of the laser beam changes from linear polarization into circular polarization after passing through the QWP. After passing through the objective lens and cover glass, the laser beam is focused onto the substrate. The substrate is a silicon wafer coated with an in-organic photo resist. The numerical aperture (NA) of the objective lens is 0.85. Table 1 shows the optical specifications of the objective lens.
For the return path, after passing through the QWP, the beam becomes linear polarization again. But the orientation of the polarization becomes perpendicular to the paper, and the laser beam is then reflected by the PBS. After that, the beam passes the detector lens and the cylindrical lens, successively. Finally, it impinges onto the photo-detector integrated circuits (PDIC). The PDIC is a square quadrant-detector, 123 μm x 123 μm in size with 5 μm gap width between neighboring detectors. The cylindrical lens is used for the astigmatic focusing detection to generate the focus error signal (FES).
2.2Optical simulations of the optics
Figure 2(a) and 2(b) show the layout of the optical simulation and the result of the focus error signal of the OPH by using Zemax version 10.0 software. In Fig. 2(a), an aspheric lens with focal length 24.8 mm is used for the collimator. Both the beam shaper and PBS cube prism are made of BK7 with thickness 5 mm. The incident angle of the beam onto the beam shaper is 32.41 degrees. For the return path, the detector lens is a glass aspheric lens with focal length 20 mm. And the cylindrical lens is a plano-convex lens made of 1 mm thick BK7 with 25 mm radius of curvature. Figure 2(b) shows the simulated focus error signal (FES), the linear range is about 3.0 μm.
3. Design of the single-axial actuator
3.1 Brief descriptions of the actuator
The single-axial actuator is the most important component in our OPH. Unlike the traditional design, only the focusing capability is preserved in our design. The actuator adopts a planar spring structure , including an identical upper and a lower spring, to improve its horizontal stability. Figures 3 and 4 show the top view drawing and the exploded view drawing of the actuator, and the relevant important components are indicated in the figures.
The actuator is 20 mm in height. The outer diameter of the yoke is 40 mm. The upper spring is fixed to the yoke by the upper spring cover that has three fix-screws. Similarly, near the center of the spring, the inner ring of the spring is fixed to the lens holder by a small lens cover with three screws. The objective lens is glued in the center of the lens holder. There are four coils adhered on each side-wall of the lens-holder. And four magnets are fixed on the inner wall of the yoke to offer constant magnetic field. These magnets are made of NiBrFe alloy with 9.8 mm x 4.2 mm x 5.8 mm in size. At the bottom of the actuator, a lower planar spring is fixed to the bottom of the yoke using a lower spring cover. Similarly, near the center of the lower spring, the lower spring is fixed to the bottom of the lens holder by another small lens cover with three screws.
3.2 Design of the actuator spring
For designing the planar spring, there are three important aspects that should be considered. Firstly, in order to decrease the displacement in the radial direction, the spring constants in the radial direction should be larger than that in the axial direction. Secondly, since the positioning of the PDIC requires using a BD-ROM disc, therefore, the movable range in axial direction for the spring needs to be large enough to cover the run-out of the disc in this direction. Thirdly, since the numerical aperture of the objective lens is 0.85, the tilt of the lens should be well controlled to below 3′ to avoid aberrations.
Figure 5 shows the schematic diagram of the spring. The spring is made of 0.1 mm thick brass. Its outer diameter is 26 mm. The widths of the outer ring and the inner ring are 2.2 mm and 1.1 mm, respectively. There are three S-shaped stems designed to link inner and outer rings. The diameter of the center hole is 4.4 mm. For the outer ring, three 1.7 mm-diameter holes are drilled near the outer ends of the three stems. And for the inner ring, three 1.0 mm-diameter holes are drilled near the inner ends of the three stems. For improving the stability in the axial direction, the stems are designed to be S-shaped and with narrower width at both ends and wider width at the middle. In our design, the width of the stem is 0.85 mm near the inner ring, 1.67 mm in the middle, and 0.89 mm near the outer ring.
The first resonant frequency F0 of the actuator is 113 Hz as was obtained by using simulation software Ansoft. We found that the resonance frequency is very sensitive to the values of the structural parameters. The spring constants Kx, Ky, and Kz of the actuator were found to be 2.283 x 105 Nts/M, 2.298 x 105 Nts/M, and1.662 x 102 Nts/M, respectively. Kz is smaller than one thousandths of Kx and Ky. This result implies that the displacement in lateral direction is much smaller than that in axial direction. As an example, when the spring moves a full linear range axially, i.e. 3 μm, the displacements in lateral direction will be less than 3 nm as if the same force is exerted in the lateral directions. The movable range of the actuator is designed to be ± 0.3 mm, a range larger than the axial run-out of the disc, i.e. ± 0.15 mm. Finally, the simulated tilt of the lens is less than 1 minute to assure a low aberrations induced by the lens.
4. Basic measurement results of the optical head
Figure 6 shows photograph of the OPH. It is made of aluminum and a little larger than the normal blue OPH for the purpose of improving the thermal dissipation and stability. The size of the OPH is 95 mm in length, 78 mm in width and 29 mm in height.
There are three basic measurements, besides the horizontal stability test, need to be done to verify the performance. These include optical spot of the objective lens, bode plot of the actuator, and displacement versus drive voltage of the actuator.
4.1 Optical spot of the objective lens
Figure 7 shows the optical spot measurement results of the objective lens. The measurement was carried out by using an Opto-device BSX 001 Spot Checker. The Gaussian spot size was 0.26 μm in full width at half maximum (FWHM). It was a diffraction-limited spot. The ellipse ratio was found to be 0.957.
4.2 Mechanic measurement results of the actuator
Figure 8 shows the Bode plot of the actuator. The first resonance frequency F0 was measured to be 79 Hz, while the second resonance frequency was about 18 kHz. The measured first resonance frequency is less than the simulated result, and the difference may come from the small uncertainty in the value of the structural parameters. Table 2 shows the measurement results for the relevant parameters.
Figure 9 shows the measurement result of the axial displacement versus drive voltage. The displacement is linear with the input voltage. The linearity warrants a good focus servo performance.
4.3 Stability test of the focus servo
Figure 10(a) shows the focus error signal (FES), the peak-to-peak voltage is 7.4 V for the BD-RW disc. Figure 10(b) shows the residual FES when the focus servo is on and the residual FES is 70 mV, which is 0.95% of the peak-to-peak FES. Since the linear range of the FES is designed to be 3 μm, therefore, the jitter of the vertical direction is about 28 nm. If we define the depth of focus (DOF) to be the longitudinal range within which the beam size expands to 1.05 times of that for the focal point, a strict definition for DOF, then the DOF of our NA0.85 objective lens is 112nm. Apparently, jitter of the focus servo is much smaller than the DOF.
5. Horizontal stability test of the optical pickup head
5.1Modified push-pull method
It is important that a reliable and easy-implemented method for measuring the horizontal stability of the actuator be developed in this study. Here, we propose a “modified push-pull” method to measure the horizontal stability of the actuator. The advantage of the method is that it is built-in the OPH, and it is easy to be implemented by using the existing device together with a BD-RW disc (Blu-Ray rewritable disc).
The conventional push-pull method is that the quadrant-detector picks up the cross-track signal A-B, as shown in Figs. 11(a) and 11(b) while the disc is spinning . The signal is then used to activate the tracking-servo mechanism for tracking action. For a standard BD-RW disc, the track pitch is 320 nm with approximately 50% duty cycle [17,18]. The ratio between the half-track pitch and the peak to peak push-pull signal magnitude can be obtained by reading the push-pull signal from a spinning BD-RW disc with our OPH.
Our “modified push-pull” method is that keeping the disc remains static while the focus servo is activated, and the signal A-B that picked up by the quadrant detector is then proportional to the lateral jitter of the actuator. Therefore, the lateral jitter of the actuator can be obtained by multiplying the magnitude of the static push-pull signal with the ratio between the half-track pitch and the spinning push-pull signal. The magnitude of the lateral jitter obtained in this way represents the lateral stability of the actuator. We then rotate the actuator by 90° and repeat the measurement for the lateral jitter again to obtain the “lateral jitter” in the orthogonal direction. The “lateral jitter” of these two orthogonal directions is defined here as the “horizontal stability” of the actuator.
5.2Stability tests of the actuator
For the measurement of the stability of the actuator, an adjuster control box model 5531 from ACT Electronics Inc. and a mechanical stage with a spindle motor were used. Figure 12 shows the schematic layout of the experiment setup. A single-layered (SL) BD-RW disc is used for the experiments. The optical axis of the beam is slanted + 45°to the track tangent EE’. For convenience, the projection of the PDIC on the objective lens is also shown in the figure. The push-pull signal is (A + D)-(B + C). The experiment was used to measure the jitter of the actuator in the direction perpendicular to the EE’.
Figure 13(a) shows the push-pull signal with a spinning BD-RW disc, the averaged peak-to-peak voltage of the push-pull signal was about 155 mV. Figure 13(b) shows that of a static disc, the averaged peak to peak amplitude was about 1.53 mV, ~1 percent of that for the spinning disc. Since the half-pitch of the BD-RW is 160nm, the lateral jitter of the OPH in this direction was therefore 1.58 nm.
Figure 14 shows the 90°-rotated push-pull experiment setup while the optical axis of the beam was set in −45°to the track tangent FF’. The projection of the PDIC on the objective lens is also showed in the figure. The push-pull signal is (A + B)-(C + D). Figure 15 shows that for a static disc, the averaged peak to peak amplitude was about 2.75 mV, ~1.8 percent of that for the spinning disc. The lateral jitter of the OPH in this direction was therefore 2.83 nm.
Figure 16 shows the background push-pull signal when laser diode was turned off. The averaged peak to peak voltage was 1.23 mV and it was equivalent to ~1.24 nm of lateral jitter. Therefore, subtracting the background noise-equivalent jitter, the lateral jitter of two orthogonal directions that represent the horizontal stability of the actuator were 0.34 nm and 1.59 nm, respectively.
5.3 Exposure experiments
The OPH was used to expose the GeSbSnO in-organic photo-resist film that was deposited on a 4” Si wafer. The resist film consist of a first layer of 30 nm thick oxygen-rich GeSbSnO film on the substrate and a second layer of 65 nm thick low oxygen-content GeSbSnO film on top of the first layer. The oxygen-rich film has a lower optical absorption coefficient and a lower thermal conductivity coefficient than the low oxygen-content film to serve as a thermal insulator such that the laser can burn the second layer at lower power while the first layer remains intact. The wafer was clamped on an air-bearing R-θ stage for movement and exposure. The output power of the GaN diode laser at 405 nm wavelength for writing was 4.0 mW. The repetition rate of the pulse was 10 MHz and the pulse duration was 10 ns. Figures 17(a) and 17(b) shows the SEM images of the top-view and the side-view of the exposed spots. The size of the burned spots was ~220 nm in diameter. All the spots were well aligned and developed down to the interface of the first and the second layer.
We presented the design, fabrication, and tests of a compact blue OPH that has only a single-axial actuator for focusing but has a planar spring structure to facilitate the horizontal stability for laser thermal lithography in which no tracking-servo mechanism is available. In order to test the horizontal stability of the OPH, we introduced a modified push-pull method incorporated with a BD-RW disc to measure the lateral jitters of the actuator. And high horizontal stability, in terms of 0.34 nm and 1.59 nm in-plane jitter for the two orthogonal directions, for our OPH was demonstrated. Well-defined and under diffraction-limit spots burning was demonstrated on the GeSbSnO in-organic photo-resist by using of our OPH.
The authors wish to acknowledge the Ministry of the Economic Affairs of Taiwan, R.O.C., for their supporting of this work.
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