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Hydrogenated Amorphous Silicon Microstructuring for 0th-Order Polarization Elements at 1.0–1.1 µm Wavelength Volume 3, Number 6, December 2011 Thomas Kämpfe Svetlen Tonchev Guillaume Gomard Christian Seassal Olivier Parriaux

DOI: 10.1109/JPHOT.2011.2175444 1943-0655/$26.00 ©2011 IEEE

IEEE Photonics Journal

a-Si:H Microstructuring

Hydrogenated Amorphous Silicon Microstructuring for 0th-Order Polarization Elements at 1.0–1.1 m Wavelength Thomas Ka¨mpfe, 1 Svetlen Tonchev, 1 Guillaume Gomard, 2 Christian Seassal, 2 and Olivier Parriaux 1 1

Universite´ de Lyon, Laboratoire Hubert Curien, UMR CNRS 5516, 42000 Saint-Etienne, France 2 Universite´ de Lyon, Institut des Nanotechnologies de Lyon, UMR CNRS 5270, 69134 Ecully Cedex, France DOI: 10.1109/JPHOT.2011.2175444 1943-0655/$26.00 Ó2011 IEEE

Manuscript received September 17, 2011; revised November 2, 2011; accepted November 3, 2011. Date of publication November 9, 2011; date of current version December 2, 2011. This research was sponsored by the Lyon Science Transfert within the project LST768. Corresponding author: T. Ka¨mpfe (e-mail: [email protected]).

Abstract: Dedicated photolithographic and reactive ion etching processes applied to the plasma-enhanced chemical vapor deposition (PECVD) hydrogenated amorphous silicon layers of solar cells have been developed in the objective of the low-cost manufacturing of efficient, depth-limited subwavelength gratings transforming a linearly polarized beam into a radially and azimuthally polarized beam in the 1.0–1.1-m wavelength range. Index Terms: Holography, image analysis.

1. Introduction Spatially resolved polarization states, particularly radially and azimuthally polarized distributions, have become a subject of increasing interest recently owing to their widespread application possibilities in areas like e.g., laser material processing [1], optical tweezers [2], focus shaping [3], or higher harmonic generation [4]. There are essentially two approaches for the creation of such polarization distributions: The first approach consists of a polarization selective laser mirror imposing the amplification of a transverse mode of given polarization pattern [5]. The present paper belongs to the second approach, which is a polarization transformation performed on a linearly polarized beam outside the laser resonator. Several solutions with their pros and cons have been proposed: elements based on segmented plates of anisotropic crystals [6] have a very high damage threshold and can be specified over a broad wavelength range, but they require high precision engineering and are not compatible with cost efficient mass manufacturing technologies. Liquid crystal based elements [7] are slightly less demanding regarding their fabrication costs but are limited in their damage threshold and long-term stability. Furthermore, they face the inherent problem of a  phase jump at one azimuthal abscissa. Also, recently, the technique of femtosecond-structuring of glass was put forward to create polarization transforming elements [8], yet extremely long exposure times are required for a reasonably sized element given the typically low writing speed (1 mm2 in 1 h [9]). Subwavelength grating based elements for polarization transformation [10] are of fundamentally different nature and potentially mass replicable via the batch planar microstructuring technologies borrowed from microelectronics. The polarization transformation effect relies upon achieving a -phase shift between the E-field components parallel and normal to the lines (TE and TM polarization, respectively; see Fig. 1) of a high index contrast, subwavelength grating. The binary

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a-Si:H Microstructuring

Fig. 1. Polarization rotation by a form birefringent grating (left) and layout of a grating for linear to radial polarization transformation (right).

structures reported so far consisting of a corrugation at the surface of a high index substrate [11] require extremely deep and narrow grooves; in GaAs ðn ¼ 3:5Þ, the groove height to width ratio must be as large as 5, which is difficult to achieve technologically. The structure fabricated here is the result of a conceptual endeavor aimed at finding out the shallowest possible high index contrast corrugation achieving, simultaneously, the needed -phase shift between polarization and close to 100% transmission of both polarization components [12]. The theoretically optimized structure is an unusually large period, wide groove, narrow line corrugation in a high index layer on a low index substrate, lending itself to high-end but available batch manufacturing technologies such as nanoimprint or mask projection using a step and repeat camera with a KrF excimer laser source at 248 nm wavelength, followed by silicon reactive ion etching (RIE). The present paper makes the fabricability demonstration by means of laboratory equipment: the grating lines are defined in a chromium mask fabricated by e-beam lithography. The mask is transferred into a resist layer on top of a hydrogenated amorphous silicon (a-Si:H) coated fused silica substrate; then, the resist lines are submitted to a slimming process to achieve the prescribed line width. RIE is used to transfer the grating all through the a-Si:H layer.

2. Design of the Polarization Transformation Grating 2.1. Grating Layout for Linear to Radial Polarization Transformation A -phase shift between TE and TM polarization causes a mirroring of the polarization relative to the birefringence axis which is here represented by the grating line direction as illustrated in Fig. 1. A linear to radial polarization transformation implies that the grating line direction characterized by an angle  relative to the x -axis must fulfill the condition  ¼ =2, where  is the angular coordinate on the beam cross section. As can be seen from Fig. 1, the grating period must be changed over the beam cross section to fulfill this condition. Depending on the tolerance of the grating operation to period changes, the function describing the grating lines has to be adapted and sectorized, which can be done in different ways, as detailed, e.g., in [14]. In the present work, the grating pattern is decomposed in pie-slice shaped sectors of constantperiod and straight lines. The fixed period grating has a considerable wavelength tolerance and allows a more flexible application of the element, which was valued higher than a perfect theoretical single wavelength optimization of the element transformation efficiency, achievable with bended, period-changing grating lines and a minimized amount of sectors. A number of 20 sectors was considered as an appropriate practical choice with a circular grating area of 6 mm diameter.

2.2. -Phase Shift Between TE and TM in a High Index Grating on Low Index Substrate Amorphous silicon is a very interesting low loss optical material down to the red side of the visible spectrum. It is actually a unique high index material for the near-IR range. The possibilities it offers

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Fig. 2. Parameters of a half-wave form birefringent corrugation in a high index substrate (left, data approximated from [5]) and of the high index grating layer on low index substrate (right) of the present paper.

for polarization processing has long been recognized [15]. a-Si:H was chosen here to implement the conceptual solution derived in [12] for linear to radial/azimuthal polarization transformation. This solution is based on a phenomenological analysis using a true-mode representation [16] of the TE and TM mode interplay in the corrugation to perform a phase management in the TE and TM Fabry–Perot transmission conditions. As detailed in [12], changing the setup from a grating etched directly into a high index substrate to a high index grating on a low index substrate allows implemention of the lowest physically possible multiple of 2 for the phase of one roundtrip in the TE and TM Fabry–Perots under transmission resonance conditions, which results in a possibility to fulfill the  TE-TM phase difference as well as the maximum transmission condition in a much shallower, technologically well-manufacturable structure. The analysis in [12] delivers the normalized parameters of this general solution in the form of heuristic analytical expressions, which have been used to determine of the geometrical parameters of the grating of the right-hand side of Fig. 2. The comparison in Fig. 2 illustrates strikingly and at scale the structural differences between a polarization transformer made as a corrugation of a GaAs substrate, according to [10] and that of the present design with an a-Si:H layer with a refractive index of naSi ¼ 3:7 at a design wavelength of  ¼ 1064 nm. The substrate is fused silica with an index of nSiO2 ¼ 1:45. In comparison with a grating etched directly into a high index substrate, the necessary ridge aspect ratio drops from about 5 to 2.3, while the period is more than twice as large, resulting in significantly reduced fabrication requirements. It must furthermore be pointed out that the structure of the left cannot fulfill the -phase shift and high TE and TM transmission conditions without undesired compromise, whereas that of the right does, achieving a transmission of 98% for TE as well as for TM polarization with a phase shift of 1:03 at 1064 nm wavelength. The transmission stays above 90% and the phase shift within 0:851:15 for the wavelength range 1.0–1.1 m.

3. Fabrication Processes The set of fabrication processes is summarized in Fig. 3. As a preliminary step, the definition of the grating lines is made in a chromium mask by e-beam lithography. An electron beam writer SB350 OS (Vistec Electron Beam GmbH) was used, which utilizes a shaped beam, allowing for rapid writing of the necessary area. Since the line width of the grating is at the limit of the capabilities of the subsequent process steps, a range of masks with different line widths from 80 to 300 nm was fabricated. The first step is the coating of the fused silica substrates with the a-Si:H layer by plasmaenhanced chemical vapor deposition (PECVD). The process is exactly that used for the fabrication of solar cells [17]. The layers were measured by ellipsometry to have a thickness of 249 nm and a refractive index of naSi ¼ 3:68 at 1064 nm wavelength. The next step is the transfer of the chromium mask into an approximately 270-nm-thick, spincoated layer of Shipley 505A photoresist. The transfer is performed in an ad hoc UV mask aligner at 360 nm wavelength in a hard contact mode under Toluene ðn ¼ 1:514Þ immersion. It was observed that the line width of the transferred resist profile depends strongly on the applied pressure and the cleanliness of substrate and mask. The distance between mask and substrate is thus a crucial factor in the transfer process, which is confirmed by a simulation of the diffracted field behind the

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Fig. 3. Radial polarizer fabrication steps.

Fig. 4. Simulation of the near field directly behind the Chromium mask, showing the modulus of the E-field for three different ridge widths r . Linearly polarized light, oriented at 45 with respect to the grating lines is incident normally from the negative z-direction. Regions I and II indicate approximately the areas creating the ridges and grooves in the resist.

chromium mask, as illustrated in Fig. 4. It shows the modulus of the E-field vector for an incoming plane wave of linear polarization oriented at 45 with respect to the grating lines to simulate the effect of the random polarization emitted by the UV-lamp of the mask aligner. Rapid changes of the E-field with the distance from the mask are observed. For a Cr line width of 200 nm, two areas of about 250 nm  700 nm (x - and z-direction, respectively) with sufficiently constant, low and high E-field can be observed, allowing a definition of approximately 250 nm wide exposed lines in the resist. Different line widths of the Cr lines lead to more distorted diffraction patterns, resulting in a more pronounced double frequency characteristic and in stronger variations with the masksubstrate distance. Based on the simulations and experimental test exposures an e-beam mask with 200 nm Cr-line width was selected, allowing the fabrication of resist line of 250 nm width with acceptable reproducibility. In order to reduce the line width to the desired value of 109 nm, an isotropic resist slimming process was applied. It consists of a controlled homogeneous UV exposure and an additional chemical development step. With a well-defined slimming rate of 7 nm/s a developing time of about 10 s is required, depending on the exact SEM-measured line width after the mask transfer. The line height is reduced to about 200 nm, which is still sufficient for the following etching process step. This combination of the optical mask transfer at its resolution limit, verified by SEM inspection, with the purely chemical process of isotropic slimming at a calibrated rate defines the required resist mask with a sufficient reproducibility.

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Fig. 5. SEM images showing the resist pattern of the central region of the polarizer after the slimming process (On the left central circle with 10 m diameter is intentionally left unstructured; it is negligible for the optical function.), a detailed view of the ridges (right, top), and a cross section view of the etched grating in the a-Si:H layer (right, bottom, cut out created by focused ion beam).

The final step is the transfer of the resist mask into the a-Si:H layer by RIE, which was carried out in an Alcatel Nextral NE110 reactor. It was found that the photoresist thickness of 200 nm was sufficient to stop the etching of the a-Si:H lines without erosion; thus, the structured resist layer could be used directly as etch mask. A mixture of Ar and SF6 was used as etching gas. The parameters gas flow, pressure and power were optimized with regard to the obtained line profile and the etching ratio between resist and a-Si:H, resulting in 20-sccm Ar and 5-sccm SF6 flow with a chamber pressure of 15 mTorr and an RF-power of 50 W. The resulting etching speed is 1.8 nm/s for a-Si:H and 1.1 nm/s for the resist, which results in about 135 s etching time, confirming that the remaining resist height of about 200 nm after mask transfer and slimming is sufficient for the etching. No end-point detection for the etching was available in the setup. Simulations showed that even a very thin a-Si:H layer at the bottom of the grooves results in an extreme decrease of the element’s transmission, whereas deeper grooves (up to 50 nm) have no significant impact on the optical function; thus a slight over-etching into the substrate was accepted in order to ensure the complete removal of all remaining a-Si:H in the grooves (which is visible in Fig. 5). Fig. 5 shows SEM images of a-Si:H gratings, confirming the line width and smoothness of the resist lines and the rectangular shape of the cross section with very good verticality of the sidewalls.

4. Results The optical function of the fabricated elements was measured in two ways: firstly, with a beam diameter small enough to illuminate only one sector of the polarizer, thus providing information about the performance of the form-birefringence of the linear grating. Second, the whole element was illuminated with a collimated beam of linear polarization, and the resulting output beam was measured with a beam profiler after filtering by another linear polarizer (see Fig. 6). In the single sector measurements, the beam at 1064 nm wavelength was focused with a 20-mm focal length lens to a spot size of approximately 0.5 mm, fitting in the outer part of one of the sectors. A setup of crossed polarizers was used. The axis of input and output polarizers were oriented 90 to each other, whereas the grating lines were arranged at 45 relative to the polarizers. The transmission Tcross through this setup represents a valid and experimentally conveniently measurable merit function for the desired polarization transformation property, since it monotonously reaches 1 if and only if the TE-TM phase difference approaches  and the transmission approaches 100%. A maximum of Tcross ¼ 96% (disregarding the transmission losses at the substrate backside) was measured in one of the sectors, whereas in other sectors, Tcross dropped down to a minimum of 85%. This reflects the slightly different line widths of the grating in the different sectors caused by imperfectly uniform contact during the mask transfer over the area of the element, as well as by slight variations of the line width already present in the mask (the e-beam lithography with shaped

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Fig. 6. Polarization transformer (left: For illustration purposes illuminated by white light) and beam profile at 1064 nm wavelength after the polarization transformer filtered by a linear polarizer (right), showing the typical bow tie pattern which is the signature of a beam of radial or azimuthal polarization.

beam causes an angle dependence of the line width due to different composition of the elementary rectangular shapes defining the lines). The measurement of the whole element was carried out with a collimated Gaussian beam of 4 mm diameter (FWHM) at 1064 nm wavelength. The incident linear polarization was oriented as depicted in Fig. 1 to give rise to radial polarization. A linear polarizer oriented in the x -direction was positioned directly after the element. The resulting beam shape resembling a bow tie (see Fig. 6) confirms a circularly symmetrical polarization distribution. The overall power in the beam without output polarizer is 77% of the incoming beam. A small amount of the losses can be attributed to scattering caused at the sector borders, whereas the main cause lies in the slight deviation of the corrugation profile and the grating parameters from strict uniformity over the area of the element. Optimization of the fabrication process, as well as in the optical setup, e.g., by adapting the intensity distribution to a Laguerre–Gaussian distribution before illuminating the transformer as described in [18], can lead to an increase in the conversion efficiency.

5. Technological Outlook The present work has demonstrated that the monolithic 0th-order polarization transformer made in an a-Si:H layer on a low index substrate having the shallowest possible grating depth, relatively large period and groove width, and a-Si:H ridges of not too large aspect ratio, is fabricable using academic laboratory processing tools which can now be extrapolated to industrial manufacturing facilities. While the developed RIE process can straightforwardly be adopted for larger scale fabrication, the lithography cannot and has brought the following interesting information: Contact photolithography for the definition of 600 nm period and 100 nm line width is not a repeatable and generalizable fabrication technology. However, a very important lessons about the endeavors of the authors is the extreme vulnerability of the contact uniformity on the presence of dust particles trapped between the substrate and the a-Si:H layer. This feature is likely to also adversely affect the well-adapted production technology of nanoimprint since any trapped particle will alter the thickness uniformity of the resist rest at the bottom of the grooves. This is a major handicap because it is not likely that a-Si:H solar cells will ever be produced in a class 100 environment. The present work has also demonstrated the functionality of the proposed polarization transformer, although it is imperfect because of the above identified and underlined factors. The achieved results confirm that the proposed polarization transformer has a definite potential for close to 100% transmission. The batch process which appears to be the best adapted for the definition of the resist corrugation before the RIE etching step is most probably KrF step and repeat with controlled overexposure. This is standard technology and is bound to lead to very low-cost elements. The same design and technology with a-Si:H can be used at 800 nm wavelength and

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very easily in the C-band (in the BErbium window,[ 1530–1565 nm) and toward the far infrared with a large number of available transparent layer materials of very high index if a-Si:H does not suffice.

Acknowledgment The authors thank U. Zeitner and H.-C. Eckstein from the Fraunhofer Institute for Applied Optics and Precision Engineering, Jena, Germany, for the fabrication of the Chromium mask, as well as X. Meng and R. Mazurczyk from the Institute of Nanotechnology Lyon for their support with the RIE and the SEM imaging. The authors also thank S. Reynaud, Laboratoire Hubert Curien, for extensive SEM and AFM imaging and D. Troadec, Institut d’e´lectronique de microe´lectronique et de nanotechnologie, Villneuve d’ascq, for the cross section SEM imaging. Kroll Thin Film Technologies, Corcelles, Switzerland, is gratefully acknowledged for the supply and deposition of a-Si:H layers. Finally, the interest concretely shown by Amplitude Systemes and ALPhANOV, Bordeaux, for the application of the developed element to femtosecond laser beams at 1030 nm wavelength is gratefully acknowledged.

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