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We report on the preparation of thin As35S65 films from their amine based solutions by a spiral bar coating technique on flexible PET substrates and the direct writing of subwavelength surface corrugated gratings with a period of 350 nm into these films by excimer laser interference lithography. The structural and optical properties of bar coated As35S65 glass are investigated in detail and compared with those of samples fabricated by vacuum thermal evaporation. The polarization dependent spectral filtering properties of the guided mode resonance devices built by the waveguiding thin films and the surface relief gratings written by a single laser pulse are presented finally.
© 2014 Optical Society of America
1. Introduction
Chalcogenide glasses (ChGs) are well known analogs of oxide glasses in which oxygen is replaced by one or more other chalcogen atoms (S, Se, Te). The changes in the glass structure caused by this substitution of oxygen by heavier atom(s) result in some new, unique properties of these glasses such as wide transmission window in the IR spectral range (up to wavelength λ = 12 – 15 µm) [1–3], high values of refractive index (commonly between 2 – 3.2) [3, 4], low glass transition temperature (100 – 350 °C) [5] or in some cases in their photosensitivity (e.g. ability to change their structure/properties being exposed by suitable radiation) [6]. All these properties make ChGs perspective materials for many applications in optoelectronics and photonics.
ChG thin films (TFs) of high optical quality are usually prepared via gaseous phase (e.g. vacuum thermal evaporation, sputtering, chemical vapor deposition) but the low cost effectiveness and necessity to work in high vacuum of these methods limits the broader exploitation of these materials. These problems can be avoided by production of TFs from their solutions [7] by methods not requiring high vacuum such as spin coating [8, 9] or sol-gel method [10]. However spin coating and sol-gel methods, widely used mainly in laboratories, are not appropriate similarly for mass production due to the limitations of the substrate’s size and the batch production. We believe that the application of coating methods, e. g. bar coating technique, slot die method or other coating methods, in combination with a roll-to-roll (R2R) process could be a suitable tool for the cost-effective mass production of ChG thin films with high optical quality.
In this work we tested the possibility to use the spiral bar coating technique to prepare thin As35S65 films from their amine based solutions which would have similar optical quality as vacuum evaporated TFs. We studied similarities and differences in the structure and optical properties of As35S65 TFs prepared by spiral bar coating (SBC) and vacuum thermal evaporation (VTE) techniques, respectively. Finally we demonstrate that deep UV interference lithography allows to fabricate high quality sub-wavelength surface corrugated diffraction gratings with a period of 350 nm in As35S65 thin films prepared by both techniques on PET (polyethylene terephthalate) substrates and report on the spectral and polarization filtering properties of samples written by a single laser pulse.
2. Experimental
The source bulk As35S65 glass was prepared by standard melt quenching method from high purity (5N) elements. TFs were prepared on 175 µm thick polyethylene terephthalate (PET) foil substrates (Melinex ST 504, DuPont) by two different methods. First method of As35S65 TFs preparation was standard vacuum thermal evaporation method (UP858, Tesla) with an evaporation rate of 1 - 1.5 nm/s and a thickness of 230 nm. Second type of TFs was prepared by spiral bar coating technique from propylamine based solution of As35S65 using a 10 µm spiral mounted in the motorized automatic film applicator (Elcometer 4340).
The structure of the TFs was investigated using a Raman spectrometer (IFS55/FRA106, Bruker) with excitation by Nd:YAG laser (1064 nm).
Gratings with an area of 4x4 mm2 were directly written into both types of prepared TFs using two beam interference lithography exploiting a KrF laser (pulse fluence 18 mJ/cm2 per beam, pulse duration 25 ns, pulse repetition rate 1 Hz). The period of prepared diffraction gratings was 350 nm. The interferometer setup with a phase mask based beam splitter, which was recently also used to write surface relief gratings with a period of 540 nm into thin films of different chalcogenide glasses for applications in the near infrared spectral range [11], is in detail described in the paper [12].
Transmission spectra of prepared TFs and polarization dependent transmission spectra of the gratings were measured using UV-VIS-NIR spectrometers (UV3600, Shimadzu; ANDO AQ-6315). The surface of prepared TFs and the topology of written diffraction gratings were investigated by atomic force microscopy (Solver NEXT, NT-MDT). The mean width, raggedness of the borders of the individual grating grooves and the standard deviation of their width were calculated by using an algorithm programmed in Matlab software (MathWorks).
3. Results and discussion
Both VTE and SBC As35S65 TFs were successively prepared on PET foil substrates. Both possessed good adhesion to this flexible substrate which is an important factor in the development of a cost effective production technology of various flexible low-loss thin film waveguides with grating couplers employing a high-volume industrial R2R process. The surface of the VTE TFs is very smooth, see Fig. 1(a)..But as the thickness of the film was 230 nm only, the topology of surface follows well the imperfections in the surface of the applied PET foil, compare Figs. 1(a) and 1(b). The SBC TF possesses a higher smoothness of the surface, see Fig. 1(c), but is not fully homogeneous in thickness along the sample. With a thickness of 750 nm the surface topology of this TF does not follow the small imperfections of the substrate surface.
Fig. 1 AFM scan of VTE TF (a), bare PET substrate (b), and SCB TF (c) deposited on the PET substrate.
The structures of the prepared TFs as well as of the source bulk material were investigated using Raman spectroscopy, see Fig. 2.The Raman spectrum of the source bulk As35S65 sample proves the presence of AsS3/2 pyramidal structural units and Sn chains as major structural units in the glassy matrix (bands at 345 cm−1 and 495 cm−1, respectively).
In the Raman spectrum of the VTE TF new, strong bands appear in the low frequency region (130 cm−1 – 250 cm−1) and also a strong band at 362 cm−1. They give straight evidence that As-As homopolar bonds containing structural units (such as As4S4 or As4S3) [13] are embedded in the glassy matrix of these S-rich thin films formed by AsS3/2 pyramidal units interlinked by Sn chains. It is well known that these As rich structural units have a cage like structure, see Fig. 3, similar to P4S4 and P4S3 compounds [14] which allows them to be relatively stable even being randomly spread in the S-rich glassy matrix. Due to their higher structural uniformity the Raman signal of the As rich clusters is stronger than that of the S-rich glassy matrix. It is well known that these cage like As-rich species can be formed together with other molecular fragments during the VTE process when the raw bulk material is quickly vaporized. Due to quick condensation on the cold substrate these fragments do not manage to react with each other and the final structure reflects this molecular like structure [6].
The structure of SBC TFs differs significantly from those of VTE TFs due to fully different conditions during their deposition and followed formation of the film. Instead in vacuum they are prepared at normal pressure and the starting raw glassy material does not go through vapour phase as during vacuum deposition process. Bulk material is firstly diluted in the volatile solvent which releases from glassy matrix of SBC TFs during consequent heating up at temperature above the evaporation temperature of the solvent (but still sufficiently below Tg of the given glass composition).
The Raman spectrum of SBC TFs gives evidence that the structure of these TFs has higher inordinance as all main bands in the spectrum are broadened, see Fig. 2. Beside AsS3/2 pyramidal units (band with maximum at 345 cm−1) and Sn chains (band at 495 cm−1) even separated S8 rings are present (a new band at 475 cm−1 appears) [13] within the structure of SBC As35S65 TFs. The existence of S8 rings is compensated by the presence of As-As containing structural units even in higher concentration (bands in the region 130 cm−1 – 250 cm−1 and mainly the band at 362 cm−1 is intensive). As stated above these As-rich structural units are also present in the structure of vacuum evaporated TFs. But the significant broadening of these bands in the Raman spectrum of coated TF gives evidence that As-rich structural units are not only in the form of separate cages but also as partially opened fragments of these cages, see Fig. 4, which are linked within the glassy matrix in which are randomly spread S8 rings.
We believe that a small amount of volatile solvent captured in the glassy matrix of the SBC TFs contributes to the higher degree of disorder. Raman spectroscopy gives direct evidence of this phenomenon, see Fig. 5.
Thus we can conclude from these structural studies that the structures of VTE and SBC TFs are significantly different which must result also in different physical and chemical properties as is demonstrated further.
The diffraction gratings written into VTE and SBC TFs were investigated using AFM. Examples for the gratings written by a single laser pulse together with average profiles of these gratings are given in Fig. 6.The quality of the gratings obviously depends on the method of TF preparation.
Fig. 6 AFM scans and average profiles of diffraction gratings written into VTE (A) and SCB TF (B) by 1 laser pulse.
The differences in regularity of diffraction gratings were studied by data analysis of AFM scans. The borders of the grooves were obtained as the threshold intersection of AFM scan in a half of the grating’s height. Using these data the raggedness of the borders of the grooves and the standard deviation of its width were calculated. The mean width of the grooves for the evaporated TF was 244 nm with a standard deviation of 7.2 nm. For spiral bar coated TF the mean width of the grooves was 271 nm with a standard deviation of 9.2 nm. Thus we can conclude that the diffraction grating in spiral bar coated TF has wider grooves with a slightly higher variation of the width.
The regularity of the groove borders represented by raggedness of the border was 6.7 nm for VTE, respectively 12.3 nm for the SBC TFs. The regularity of borders could be estimated from elongation of borders in comparison to straight distance as well, see Fig. 7.The groove border’s elongation for the VTE TFs was 2.9% more than ideal straight and smooth borders and the value found for SBC TF was 6.4%. Thus we can conclude that diffraction gratings prepared in evaporated TF’s are of higher structural quality than those prepared in spiral bar coated TFs.
Fig. 7 The groove quality analysis from the borders of the grooves in a half of diffraction grating’s height for VTE (A) and SBC TF (B) written by 1 laser pulse.
The dependences of the diffraction grating’s depth on the number of applied exposure laser pulses and on the method of TF’s preparation were also investigated. The depth of the structure written with a single laser pulse depends mainly on the penetration depth of the exposure beam. Data in Fig. 8 calculated for the extinction coefficient k = 1.2931 measured for evaporated TF at a wavelength of 248 nm show that 95% of the KrF laser intensity is absorbed within the upper 45 nm of the As35S65 TF. Thus grooves of approximately this depth can be expected after the first exposure pulse.
The real depth of the diffraction grating written into VTE TFs by a single laser pulse is about 42 nm, which is close to the expected value estimated from the penetration depth. Figure 9 proves that for evaporated TFs the depth limit of the diffraction grating is approached already after the second applied exposure pulse. Further exposures don’t increase this value significantly.
Fig. 9 Depth dependences of diffraction gratings written in VTE and SBC TFs on the number of exposure pulses.
Diffraction gratings written into SBC TFs exhibit a completely different behavior. After the first exposure pulse the depth of the written structure is 65 nm – significantly more than was expected from the penetration depth for VTE TFs. The cause of this phenomenon is probably the decreased rigidity of the glass structure because of the presence of the embedded residuals of PA within the glassy matrix, see Fig. 5.
The depth of the written grooves doesn’t increase with further exposure pulses (2, 4, 8 pulses) as in case of VTE TFs. Conversely the depth of written structures in SBC TF decreases with additional exposure pulses, see Fig. 9. One of possible explanations of this phenomenon is that the residuals of PA enclosed in the glass’ structure are vaporized and partially released during the exposure by additional laser pulses due to the locally increased temperature and thus higher fluidity of the glass. This process can cause local collapse of the structure, which results in lower value of the induced depth. After a certain number of exposed laser pulses the glass’ structure is completely PA free and thus exhibits similar behavior like evaporated TFs – diffraction gratings written by 16 pulses into SBC TFs have the same depth as diffraction gratings written by 16 pulses into VTE TFs.
Figure 10 shows the normal incidence transmission spectra of the bare PET substrate with a thickness of 175 µm and of the As35S65 TFs prepared respectively by VTE and by SBC on the PET substrate at places in close vicinity to the grating areas structured on these samples, respectively.
Fig. 10 Transmission spectra of the bare PET substrate (1) and the As35S65 TFs prepared by VTE (2) and SBC (3)
Evaluating the wavelength dependent interference-free transmission and reflection of a PET wafer it was found that the refractive index of PET in the considered spectral range can be well approximated by
and that the extinction coefficient kPET of PET is below 2·10−6 within the range 550 nm ≤ λ ≤ 1200 nm and increases from about 2·10−6 at 550 nm to about 5.5·10−6 at a wavelength of 400 nm.The refractive index of As35S65 prepared by VTE can be calculated from
where E is the photon energy and E0 = 4.78 eV is the single oscillator energy, and Ed = 19.86 eV is the dispersion energy [15–17].Figure 11 shows the experimental data and the calculated transmission spectra for three different thicknesses of the As35S65 layer prepared by VTE using the refractive index data for the substrate and the chalcogenide TF calculated from Eqs. (1) and (2), respectively, and neglecting losses applying Eq. (A1) given in [18]. Therefrom a TF thickness of 232 nm is concluded.
Fig. 11 Measured transmission spectra of the bare PET substrate (1), the As35S65 TF prepared by VTE (2) on the substrate and spectra calculated for a layer thickness of 220 nm (3), 230 nm (4), and 240 nm (5)
Applying the procedures described in [18] and [19] the following data for the As35S65 TF prepared by SBC were determined from its transmission spectrum shown as curve 3 in Fig. 10: Ed = 14.93 eV, E0 = 4.96 eV, and a layer thickness of 748 nm. The refractive index of the SBC ChG is thus about 12% less compared to vacuum evaporated, see Fig. 12. Unlike for high quality As35S65 TF prepared by VTE there exists a certain loss level (extinction coefficient of k ≈3·10−3) even sufficiently far away from the absorption edge all over the considered wavelength range.
Using the dielectric function of the substrate and the glasses prepared by VTE or SBC and the layer thicknesses the following number of guided modes were calculated using the software package ATSOS (DOOS-Design & optimization of optical systems, Tabarz, Germany).
The waveguide formed by the 232 nm thick chalcogenide TF prepared by VTE, the PET substrate and air guides 3 modes up to a wavelength of 450 nm, 2 modes for 450 nm ≤ λ ≤ 675 nm and is single mode for λ > 675 nm (all wavelengths λ are polarization dependent and given approximately).
Despite the reduced refractive index the 748 nm thick TF fabricated by SBC exhibits at a given wavelength much more modes: 6 modes up to λ ≈600 nm, 3 modes for 600 nm < λ < 900 nm und still 2 modes up to wavelengths of more than 1200 nm.
A well collimated either TE-polarized or TM-polarized white light beam with a diameter of 3 mm centered on the 4 x 4 mm2 grating area was used to measure the transmission properties of the gratings written each by 1 laser pulse with the parameters given above into the sample with the 232 nm thick TF prepared by VTE and the sample with the 748 nm thick layer prepared by SBC. Figure 13 shows the normal incidence grating transmission spectra for TE- (electrical field of incident wave is parallel to the grating lines) and TM-polarization (electrical field of incident wave is perpendicular to the grating lines) for the 232 nm thick sample. There are well pronounced declines of transmitted power at two different wavelengths for TE polarization () and for TM polarization (), respectively. They are the result of the resonant redistribution of the power of the incident wave between transmitted, reflected and coupled into the waveguide waves which may take place when the conditions for the grating excitation of a waveguide mode by the incident wave are fulfilled. This guided mode resonance effect has been exploited for the realization of a wide variety of integrated optical devices and sensors and is also the subject of intensive current research [20–24].
Fig. 13 Polarization dependent transmission spectra of a grating written by one laser pulse into a VTE TF of As35S65 with a thickness of 232 nm.
The normal incidence grating transmission spectrum for the sample prepared by SBC is shown in Fig. 14.The transmission is resonantly declined at 5 wavelengths in the range from 537 nm to 752 nm for TM polarized and at 4 wavelengths ranging from 600 nm to 700 nm for TE polarized light, respectively.
Fig. 14 Polarization dependent transmission spectra of a grating written by one laser pulse into a SBC As35S65 TF with a thickness of 748 nm.
Finally we performed also preliminary investigations regarding the properties of the samples as resonant waveguide gratings for monochromatic light and measured the angle dependent transmission and reflection properties applying a He-Ne laser as the probe beam source. Figure 15 shows one example for the grating written into the layer prepared by vacuum evaporation for small incidence angles at a wavelength of 594 nm for TM polarization.
Fig. 15 Angle dependent transmission (T) and reflection (R) of the grating written by one laser pulse into the 232 nm thick As35S65 TF prepared by VTE on a PET substrate for a 594 nm laser probe beam and TM polarization.
4. Summary and Conclusions
We have reported on the preparation of As35S65 thin films (TFs) by spiral bar coating technique (SBC) and investigated the resulting structural and optical properties of the glass in comparison with vacuum thermally evaporated (VTE) samples. Raman spectroscopy gave evidence that TFs prepared by SBC in compare with VTE TFs contain higher concentration of As rich clusters (such as As4S4 or As4S3) and additionally S8 rings. SBC TFs include small amount of the solvent (PA) in its structure. The refractive index of SBC glass is about 12% lower than that of VTE. Direct excimer laser interference patterning utilizing a two beam interferometer was applied to corrugate the surface of TFs with subwavelength gratings of 350 nm pitch. There is a distinct difference in the behavior of chalcogenide glasses prepared by SBC and VTE under pulsed 248 nm irradiation, which we tentatively attribute to the influence of remaining solvent molecules in the glass matrix and the reduced density of the bar coated glass. We also observed small topological differences between the gratings written under the same conditions into SBC and VTE samples.
The realized resonant waveguide grating structures composed of multimode waveguides and subwavelength gratings written by a single excimer laser pulse display pronounced spectral and polarization filtering properties. Although the optimization of the filter characteristics was not the aim of the present study, we observed sharp polarization dependent bands of reduced transmission with a spectral width of less than 10 nm for normally incident collimated white light for both kinds of TFs prepared by vacuum evaporation or bar coating. The described combination of two technologies both suitable for mass production offers thus a new perspective for the cost-effective fabrication of resonant waveguide gratings for a wide variety of applications in optics, optoelectronics and sensing.
Acknowledgment
This work was supported from the Czech Science Foundation by the grant P204/11/0832.
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