The periodic structuring of sapphire, by using laser induced backside wet etching technique (LIBWE) and 266 nm, 150 ps Nd:YAG laser radiation, is demonstrated here for first time. Sub-micron period Bragg reflectors are successfully patterned in sapphire wafers using modest energy densities and number of pulses. The gratings are characterized using diffraction efficiency measurements, AFM, and SEM. Issues related to the ablation process and to the phase mask holography are presented and discussed. The experimental results presented depict that the applied method is capable to produce relief structures of depth as deep as 100 nm, while maintaining high resolutions, close to the thermal diffusion length of the material corresponding to the ultrashort pulse duration.
© 2007 Optical Society of America
The micro-/nano-structuring of high transparency crystalline materials by using laser beams remains a significant challenge in the field of material processing. Specifically, the use of high photon energy and intensity radiation for the direct ablative structuring of wide-band-gap, transparent crystalline dielectrics, has not provided promising results yet, concerning the surface quality and the reproduction of the etched features. In contrast, there are alternative laser based structuring approaches, which do not suffer from most of the drawbacks of direct ablation, and they have been successfully applied for etching crystalline dielectrics. Laser-induced backside wet etching (LIBWE) is a versatile method that has been successfully applied in the micro-patterning of single crystals such as CaF2, quartz, and BaF2 [1–3]. In such an approach, the transparent optical material is at their backside in contact with a highly absorbing liquid. The laser irradiation incidents through the frontside of the sample, whereas the inscription process takes place at the backside. The etch mechanism for LIBWE using nanosecond lasers at moderate fluences is supposed to be a sequence of heating a near surface region of the transparent material by the laser-heated liquid up to melting/softening point followed by mechanical removing of the heated/softened surface region by the high pressure/stresses at the solid-liquid interface. Recently, the application of LIBWE with femtosecond laser pulses was successfully demonstrated comprising a different etch mechanism primarily based on near surface defect generation and subsequent ablation of the modified surface layer . LIBWE method has provided excellent results when is applied for the surface structuring of amorphous glass materials like fused silica or silicate glasses  .
The patterning of sub-micron period Bragg reflectors in sapphire crystal wafers by using LIBWE is presented here. Sapphire is an important optical material with excellent thermal, chemical and mechanical properties, exhibiting resistance to radiation, and is used extensively as a substrate for GaN thin film deposition for lasers and for silicon-on-insulator technology. Also, compact lasing devices have been fabricated in sapphire mono-crystals by Ti ion indiffusion, wherein the diffused ion satisfies both index engineering and spectroscopic requirements . Therefore, the high yield periodic micro structuring of sapphire is of great significance for the development of miniaturised sensing and lasing devices. Several attempts have been carried out for the high yield direct photo structuring of sapphire using a great variety of combinations between wavelengths, pulse durations, and exposure conditions [8, 9], including laser backside wet etching with ns pulses . Nonetheless, even though most of the aforementioned techniques succeeded in ablating or etching sapphire, the etched pits were of rather low-quality, while none of the above was applied in the machining of features with sizes below few microns.
With the method presented here we succeeded in inscribing sub-micron period relief gratings in sapphire wafers, by employing phase mask interference in contact-mode and LIBWE technique with modest energy densities and number of pulses. Thereby, the application of picosecond laser pulses in conjunction with backside etching approach is demonstrated here for the first time.
The fourth harmonic at 266 nm, of a 150 ps pulsed Nd:YAG EKSPLA Lithuania laser, was used for the machining of 0.5 mm thick R-plane sapphire wafers of 1-102 orientation. The laser used was temporally and spatial single mode, having a coherence length longer than 20 mm. The wafers were face-attached on a special cell (CC) containing pure chlorobenzene (CB) (see Fig. 1). For creating the periodic pattern onto the sapphire back surface, a fused silica phase mask (PM) was placed in contact mode at the front surface of the sample (SAP). Thus, the split laser beam penetrates the sample and the interference pattern forms high quality fringes on the back surface of the sample. The phase mask period was 1060 nm and it was optimised for the 248 nm wavelength; thus, its 0th diffraction order was not of the lowest value for 266 nm illumination. The distance between the phase mask and the sapphire slab was adjusted at 100 μm, approximately.
After exposure, the samples were cleaned ultrasonically in acetone and traces of remaining organic layer due to the LIWBE process were removed by applying a gentle oxygen plasma etch. The diffraction efficiency of the gratings was measured using a linearly polarised 635 nm diode laser, probing the diffraction orders at Bragg angles. A 100 cm focal length spherical lens was placed before the grating sample, for adjusting the overlap between the laser beam and the grating spot size for achieving measurements of improved accuracy. From those diffraction efficiency measurements the grating depth was evaluated using standard coupled mode theory equations for thin gratings .
3. Results and discussion
Several exposures were performed, employing different energy densities and number of pulses. Not all of them ended with grating structuring or high surface quality results. We have measured most of those gratings but significant diffraction over a uniformly ablated area, without extensive broad scattering, was detected in few of those. In general, the upper limits of the exposures were defined by the non-linear absorption that occurred primarily in the sapphire slab; and sample cracking for LIWBE etching at high energy densities. For the envelope of conditions we attempted, the best results in terms of grating depth were obtained for an energy density of 260 mJ/cm2 corresponding to a machined area of the order of 4 mm2. Higher energy densities (namely ≈350 mJ/cm2) leaded to extensive microscopic material damage, by means of grating groove exfoliation; and those gratings diffracted less. Again, lower energy densities (namely ≈170 mJ/cm2) did not trigger high enough temperature gradient and shock wave for ablating sapphire successfully, using a modest number of pulses.
The grating depth results correspond to 260 mJ/cm2 energy density exposures are presented in Fig. 2. The results presented in Fig. 2 refer to the 1st, 2nd and 3rd diffraction orders scattered from the grating.
A non-monotonic grating depth growth is observed versus the number of pulses of the exposure for all the diffraction orders. For the energy density employed here, the maximum grating depth for all the scattered orders is obtained using exposures of 300 pulses. This typical behaviour of the grating strength has been observed previously for grating inscription in glasses or thin films using nanosecond UV laser radiation in direct ablation [12, 13] or LIBWE approach . Such effect is usually associated with groove damage, occur for prolonged exposures, where ablation or etching may also take place at the dark fringes of the interference, due to slightly reduced contrast. Such reduced contrast is due to the use of a non-optimised phase mask for the illumination wavelength, where the 0th diffraction order is increased (being 2.76%, here). In addition, the use of a non-optimised phase mask exhibiting a high 0th order  and slight misalignments between the phase mask and the sample , result in the preferential inscription of the fundamental periodicity of the phase mask –that of 1060 nm-, instead of the half periodicity of 530 nm.
This is clearly depicted from the diffraction efficiency evaluated grating depth results presented in Fig. 2, as well as, from SEM microscans (see Fig. 3).The grating pattern appears to be of high uniformity, nonetheless, a diffused parasitic droplet formation is observed, which may be attributed to the etching process. The thermal etching mechanism probably promotes melting of the surface sapphire, also for the case of ps-duration pulses . Analytical temperature calculations of the sapphire surface temperature, considering the laser absorption both at the interface and into the liquid  show that the melting point of sapphire of 2303 K is exceeded for the energy densities used, if an interface absorption of minimum 5% is assumed. Since surface modification and incubation effects occur for materials etched using LIBWE, such an enhanced absorption at the solid-liquid interface is highly possible . Due to the high thermal conductivity of sapphire , a substantially higher surface absorption is required for ablating using longer laser pulses, i.e., of several nanoseconds duration . From that point of view, this is the reason for the improved quality of sapphire etching with 150 ps laser pulses in comparison to ns pulses.
Under improved alignment conditions, a more symmetrical inscription of the 530 nm period is achieved, as that is clearly depicted in the close view SEM scan of Fig. 4.
The structuring capabilities of the specific LIBWE process are further revealed in the AFM morphological data of Fig. 5(a). In the grating structure presented in Fig. 5(a), a higher spatial frequency modulation grating, of shallow depth, has also been inscribed, in addition to the 530 nm and 1060 nm periodicities.
Using Fourier analysis of a line profile obtained from the AFM scan, and evaluating the power spectral density (PSD) of the grating, the periodicity of this higher frequency surface modulation is measured to be of the order of 357 nm ± 5 nm. The slight disagreement between the spatial frequencies evaluated using the Fourier analysis and these that are actually scattered from the phase mask is solely due to technical artefacts. The existence of such higher spatial frequency modulation has also been clearly observed in the diffraction efficiency measurements (see Fig. 2), exhibiting depths that are manifold smaller (< 10 nm) than those measured for the longer periods [see Fig. 5(a) and 5(b)]. The generation of such short period grating is clearly attributed to the 2nd diffraction order delivered from the phase mask resulting in a net periodicity of ≈353nm. The 2nd diffraction order has a scattering efficiency of 2%, approximately, figure that leads to the inscription of such a shallow relief structure.
The capability of inscribing such high-resolution periodic features in the sapphire surface results from the short pulse duration of the laser source used here. Since LIBWE is primarily a thermally driven etch process, the heat-affected zone (HAZ) will define the minimum size feature that is imprintable for specific pulse duration and thermal diffusivity of the machined material. The HAZ is defined as √2Dtpulse and in combination with the thermal diffusivity D of sapphire, which is 1.63 × 10-5 m2/sec and the laser pulse duration tpulse (150 ps), leads to a spatial figure of 70 nm. This HAZ length is well below the half period (≈ 160 nm) of the high frequency gratings. Furthermore, it is worthy to mention that the thermal diffusivity of sapphire has a rather high figure compared with other amorphous or crystalline dielectrics . Therefore, the ps-version of the LIBWE method presented here, can be straightforwardly used for the successful inscription of grating structures in several dielectrics with periods shorter than 200 nm, combing advantageous characteristics found in the ns- and fs- versions of LIBWE [1–4].
Summarising, the high yield inscription of sub-micron period (≈ 530 nm) Bragg gratings in sapphire wafers is reported here, using LIBWE method in combination with the output of a 150 ps, 266 nm Nd:YAG laser. The gratings imprinted were characterised using diffraction efficiency, as well as, AFM and SEM scans; while for the specific exposure conditions used, maximum grating depths, in average, of the order of 80 nm were obtained. The method presented here exhibited a great potential for the inscription of ultra-short period (≈ 200 nm) Bragg reflectors in sapphire wafers, fact, which greatly enhances its applicability in the microelectronics and photonics device fabrication. We are continuing our studies for investigating the application of the LIBWE process in greater depth and tune the etching conditions for the case of grating inscription in sapphire.
Experiments were carried out at the Ultraviolet Laser Facility operating at IESL-FORTH with support from the EU through the Research Infrastructures activity of FP6 (Project: Laserlab-Europe; Contract No: RII3-CT-2003-506350). Parts of this work were financially supported by the Deutsche Forschungsgemeinschaft Germany under contract DFG ZI660/3. SP would like to thank George Violakis and Kostas Pahis for performing optical measurements. The authors wish to acknowledge Dr. T. Höche for his skilled SEM investigations.
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