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Bypassing and cutting through 1D photonic crystals by ultra-shallow wet-etched resonant gratings
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Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656 www.elsevier.com/locate/photonics
Bypassing and cutting through 1D photonic crystals by ultra-shallow wet-etched resonant gratings Svetlen Tonchev a,b, Olivier Parriaux a,* a b
University of Lyon, Lab. H. Curien UMR CNRS 5516, F-42000 Saint-Etienne, France Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
Received 20 March 2012; received in revised form 21 June 2012; accepted 21 June 2012 Available online 2 July 2012
Abstract The electromagnetic field in subwavelength resonant diffractive optical elements is so concentrated that a very shallow surface corrugation obtained by wet chemical micro–nano-etching is sufficient to give rise to high contrast diffraction effects allowing a high wavelength-, polarization- or transverse-mode-selectivity which is not achievable by conventional diffractive elements. Two examples of polarizing laser mirrors at both extremes of the optical spectrum with wet-etched grating bypassing and cutting through the 1D photonic crystal are demonstrated. # 2012 Elsevier B.V. All rights reserved. Keywords: Wet etching; Resonant grating; Photonic crystal; Multilayer mirror; Radial polarization
1. Introduction The development of micro–nanostructuring in diffractive optics, and more generally in microoptics, owes much to the steady and so far predictable development of microelectronics towards ever smaller feature sizes. The technologies used in microelectronics which can be borrowed by microoptics include lithography (either by direct e-beam writing or by step and scan), etching and deposition of silicon-based layers, such as amorphous silicon, silicon oxide (SiO2), silicon oxinitride (SiON) and silicon nitride (Si3N4), and more recently, high-k gate oxides, such as Hafnium and Tantalum oxides (HfO2 and Ta2O5) [1]. Well-documented Reactive Ion Etching (RIE) processes and related equipment have been developed for silicon-based
* Corresponding author. Tel.: +33 477915819; fax: +33 477915781. E-mail address: [email protected] (O. Parriaux). 1569-4410/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.photonics.2012.06.005
substrates and layer materials [2], whereas high refractive index metal oxide layers can be etched by Reactive Ion Beam Etching (RIBE) [3], which combines chemical etching and the kinetic energy of an ion beam to remove the non-volatile products of the reaction. This is a complex process with a large number of parameters and high processing costs. Metal layers can be etched by chlorine RIE [4]; this is a well-developed technology, but it requires strict safety precautions and costly infrastructure. Isotropic wet etching for microstructuring objectives in microelectronics was abandoned long ago for its incapability of matching with the demands on the characteristic dimension (CD), the sidewall etching spoiling the resolution needed by today’s photolithography. Wet etching is currently mainly used in microelectronics for anisotropic shaping of singlecrystal silicon and removal of relatively large areas of metal and dielectric layers. Wet microetching has found limited use in diffractive optics either since vertical
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S. Tonchev, O. Parriaux / Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656
walls are often required and sidewall etching would further limit the angular aperture of the element as the typical depth of fused quartz-based transmission diffractive optical elements (DOE) is roughly between half and one operation wavelength. However, there is currently an emerging field within diffractive optics where wet microetching is bound to play an important role as an alternative manufacturing technology to dry etching. This field concerns resonant diffractive elements, and more particularly, diffraction gratings, where the surface corrugation is located in the field of an electromagnetic surface wave. This surface wave can be that of a plasmon propagating at a metal– dielectric interface [5] or a guided [6] or leaky mode [7] of a dielectric layer or multilayer. Fig. 1 illustrates one resonant grating structure with the transverse field of the surface wave involved which is here a true waveguided mode. The mechanism of operation and typical effects of one type of resonant element (a resonant mirror) are symbolized in Fig. 2. In this case, the grating couples an incident wave to the surface wave, which acts as an energy reservoir. The surface wave propagates over some distance along the corrugated surface and then reradiates away into the incident and transmission media, where it interferes with the component of the field that was not coupled to the surface wave, giving rise to interference effects whose contrast, destructive or constructive character are determined by the optogeometrical parameters of the structure [8]. It is of particular interest that the diffraction effects can be made highly selective with regard to wavelength, angle and polarization, unlike in standard non-resonant corrugations, the selectivity of the created interference effects being conferred by the surface wave itself. From a photonic crystal point of view, the resonant gratings sketched in Fig. 1, sometimes named 2.5D photonic crystals if composed of holes instead of lines [9], are 1D waveguide mode photonic crystals operating according
Fig. 1. Resonant reflection of a free-space wave from a grating waveguide upon mode coupling (transverse electric field profile of the TE0 mode and wavelength selectivity represented).
Fig. 2. Symbolic representation of the operation of a waveguide grating resonant mirror: the grating splits the incident field into a 1st order coupled waveguide mode field with energy accumulation then outside re-radiation, and the 0th order transmitted field. The latter and the former recombine in the transmission half-space with designadjusted relative amplitude and phase for destructive interference, ensuring 100% reflection theoretically.
to the intraguide 2nd grating order, addressed by the incident free space wave via the 1st grating orders. However, the photonic crystal representation is here less meaningful physically than a coupled wave phenomenology since the lateral modal field confinement is weak as will be seen in the two examples where the grating permits to bypass or cut through the stop-band of the 1D photonic crystal of a dielectric multilayer. What matters most in the scope of the present paper is that the electromagnetic field of the surface wave can be so strong that highly selective control over the contrast of the said interference in the incident or transmission media can be achieved using a very shallow corrugation whose depth is between one tenth and one hundredth of the period depending on the refractive index of the corrugated layer material. This arises because the strength of a resonant grating is the product of three factors: the corrugation height h, the normalized modal field strength in the corrugation, and the permittivity difference between groove and ridge [10]. Interest in ultra-thin gratings has also been fuelled recently by demonstrations of such gratings with engineered subwavelength gold V-shaped structures forming socalled metasurfaces giving rise to phase discontinuities by means of surface steps height of no more than one hundredth of a wavelength [11]; this is presently made by a lift-off technique and could easily be wet etched. The etching of such shallow features is difficult to achieve using a dry chemistry process involving a plasma or reactive ion beam: as the etching rate is usually rather high, the depth and uniformity of the corrugation relief is determined by the very first seconds of a short process where the plasma is unstable and nonuniform. This is where wet microetching offers a solution: although such resonant gratings are often ‘‘zeroth order elements’’, and thus subwavelength, i.e., clearly submicron in visible and near-IR applications, the required aspect ratio of the corrugation is so small
S. Tonchev, O. Parriaux / Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656
that sidewall etching is negligible relative to the characteristic dimension of the etched microstructure. This means that structures characterized by a CD of a few tens of nanometers can be obtained by wet etching. As to the problem of the onset of the etching process, a wet etching solution can easily be diluted to the point that the first few seconds of the reaction where the liquid penetrates into the photoresist grooves are negligible compared to the total reaction time. To the question of what wet chemistry is the most appropriate, silicon- and III–V based optoelectronic technologies privilege acidic solutions to prevent alkaline contamination. This is not necessarily the best choice for diffractive optics since alkaline contamination is generally not an issue and acids tend to form bubbles, to render an oxide surface hydrophilic (thus ‘‘photoresist-phobic’’) which eases the acid creeping between the substrate and photoresist etch-stop. Microetching in photonics can therefore employ a wide range of solutions belonging to different types of wet chemistry, e.g., basic, acidic and of the exchange type [12]. It is worth mentioning in relation to the wet etching examples given hereunder, that manufacturing costs may be an issue. If the described micro-optical elements would have to be fabricated by the dry etching processes and equipments of microelectronics, their cost would be prohibitive for two reasons. First, the microoptics market is much more diverse than that of ICs, which means that for most products, the economy of scale does not operate. The second reason is that, because of the variety of substrate shapes, dimensions, and specificity of materials, dry etching equipment cannot be easily adapted to a batch process scheme, but is instead suited for manufacturing a single substrate at a time, which amounts to high fabrication costs. We have used a number of wet microetching processes for various metallic, semiconductor and dielectric layers which the next section will illustrate by giving two examples of subwavelength resonant structures. Both examples are laser polarization-control mirrors. They are chosen to illustrate the two extremes of the optical domain: one operates in the far-IR at 10.6 mm wavelength in a CO2 laser, the second one in the deep-ultra-violet (DUV) at 193 nm in an Argon fluoride excimer laser. The operation of both polarization control elements involves an optical resonance, but the materials are radically different. Nevertheless, both are etched wet, which demonstrates how versatile this technology can be, although the DUV element is currently at a less advanced development stage compared to that of the IR-element.
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2. Results and discussion Both polarizing mirror elements comprise a multilayer mirror as in standard laser mirrors, ensuring close to 100% reflection. The reflection of a dielectric multilayer is polarization independent under normal incidence, therefore a polarizing element – usually a Brewster plate – is placed in the laser resonator to induce selective loss on the transverse electric (TE) polarization which results in the lasing of the sole transverse magnetic (TM) linear polarization. In the resonant grating approach, the polarization selectivity is integrated into the multilayer mirror in the form of a periodic corrugation, typically in the final layer at the active medium side, which decreases the reflection of one polarization [13]. One advantage of this monolithic approach is that the laser beam can be endowed with almost any polarization distribution by simply adjusting the orientation of the grating lines over its cross-section, e.g., a radial or azimuthal polarization, provided the laser resonator accepts such a field distribution as one of its eigenmodes. 2.1. Far-IR grating mirror with TE stopband bypass The far-IR element presented here comprises a circular grating of radial period close to 6 mm etched in a layer of amorphous germanium deposited on top of a ZnSe-ThF4 multilayer mirror. Fig. 3a shows the microscope top-view of a few grooves of etched germanium corrugations, which are designed to allow radiation of the local TE polarization to bypass the mirror by the mediation of its first diffraction orders, whereas that of local TM polarization experiences high reflection. From a photonic crystal viewpoint none of the polarizations should leak through the multilayer mirror since with 6.5 mm period and 10.6 mm wavelength both local TE and TM 1st diffraction orders are directed into the high and low index layers under an angle which is larger than the TE and TM stop bands of the multilayer angular spectra. The solution to provoke a leakage loss through the multilayer, thus to bypass its stop band, is to design the grating period and the multilayer so that the 1st TE diffraction orders excites a leaky mode of the multilayer. Fig. 3b shows the grating pattern corresponding to an alternative polarization filtering concept whereby a ca. 9 mm period permits the local TM polarization to leak through the multilayer by refraction close to the Brewster angle whereas this angle is within the multilayer’s TE-stopband. Generating the radially polarized mode of a laser imposes here the grating
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Fig. 3. Optical microscope top-views of two different germanium polarizing gratings on a CO2 laser mirror ensuring high reflectivity of the radially polarized mode and damping of the reflectivity of the azimuthally polarized mode. (a) Circular line grating of 6.5 mm radial period inducing leakymode-mediated damping of the reflection of the local TE polarization. (b) Piecewise constant, ca. 9 mm azimuthal period radial grating inducing the leakage through the multilayer mirror of the local TM polarization under the Brewster angle between high and low refractive index layers, and forbidding the diffraction of the local TE polarization.
lines to be radial; this is possible to achieve by means of a segmented radial grating of approximately constant azimuthal period as illustrated since the polarization filtering principle is angularly and spectrally broadband. The corrugation depth that gives the TE/TM reflection spectra shown in Fig. 4 by means of the polarizing scheme in Fig. 3a is about 200 nm for 3250 nm line and groove widths, which corresponds to an aspect ratio of only 7/100, i.e., the duty cycle is hardly affected by the sidewall etching. The double-dip feature of the local TE reflection spectrum illustrates how the resonant coupling mechanism can be tailored: in order to broaden the tolerances on the ZnSe/ThF4 multilayer, the multilayer was engineered so as to bring two TE leaky modes in coalescence, which considerably widens the reflection dip. The relative location of the resonant grating multilayer and the operation principle of the
TE-leaky-mode tunneling through the mirror are also shown in Fig. 4. Fig. 5a shows an AFM scan of a few grooves of the grating in Fig. 3a etched in the germanium layer by a chemically neutral ion beam, exhibiting U-shaped grooves due to normal ion beam incidence. This single sample process lasts several minutes all together with about 7 interruptions to permit the substrate to cool down. In comparison, Fig. 5b is an AFM scan of the same element obtained by wet etching in a slightly acidic reduction–oxidation reaction [14,15] at a rate of 100 nm per minute, showing that the roughness of the etched groove bottom is similar to that of the photoresistprotected tops. The very flat groove bottom results from the use of an ultra-thin etch-stop layer. Remarkably, the wet etching procedure gives rise to groove flanks that are near vertical. This element was used as the back mirror of a high power CO2 laser for the generation of radial polarization [16] up to a power of 5 kW. 2.2. Deep-UV grating mirror with cut through the TE stopband
Fig. 4. Fabrication-tolerant double-dip TE reflection spectrum of a multilayer CO2 laser mirror with 1st order bypass leakage into the substrate. The TM reflection is unaffected by the presence of the shallow grating.
The second example of a laser polarization-control mirror operates in the DUV region of the optical spectrum. It is developed for the polarization of an ArF laser by means of a monolithic element in place of a set of cascaded intra-cavity Brewster prisms [17]. The element comprises a shallow grating etched into the last LaF3 layer of an AlF3/LaF3 multilayer mirror. The period is 136 nm with a corrugation depth of 15 nm. So shallow a groove depth is not beyond reach for a dry etching technology and was actually achieved by RIE into a PECVD silicon nitride layer used as a deep-UV
S. Tonchev, O. Parriaux / Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656
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Fig. 6. TE and TM reflection spectra under normal incidence of a 39 layer AlF3/LaF3 mirror with 15 nm deep, 136 nm period, corrugation in a LaF3 waveguide layer.
Fig. 5. AFM scan of a 6.5 mm period, 200 nm deep germanium corrugation by (a) Argon ion RIBE and (b) reduction–oxidation wet etching reaction.
biosensor waveguide-grating platform [18]. However, the difficulty of shallow dry etching increases in cases the products of the reaction are not volatile whereas a wet solution requiring a duration of the order of 1 min can be easily controllable. Fig. 6 shows the polarizing mirror structure and the corresponding TE and TM reflection spectra. Of the polarizing solutions described in Ref. [19], the principle which is most adequate in this system of low index contrast system 1.39/1.66 (standing for AlF3/LaF3) is to combine a standard quarter wave submirror with a waveguiding superstructure of a few layers, comprising a corrugated LaF3 layer, exhibiting close to 100% resonant reflection by TE0 mode coupling at 193 nm wavelength. Whereas the waveguide grating structure is designed for TE mode coupling at 193 nm wavelength under normal incidence, the TM polarization remains practically unaffected by the waveguide superstructure and only experiences the wide band reflection of the
Fig. 7. AFM scan of a binary grating of less than 0.05 aspect ratio in a LaF3 layer with photoresist etch-mask.
multilayer. Between the multilayer mirror and the waveguide reflector there is a low index buffer layer whose optical thickness is adjusted so as to represent the inside of a first order TE Fabry-Perot resonator at 193 nm wavelength. Thus, in the neighborhood of 193 nm wavelength the TE transmission has a maximum whereas the TM polarization is still highly reflected. Although the grating with its 136 nm period could in principle diffract the 1st orders into the substrate through the multilayer, it does not because the field of the latter in the low refractive index layers is evanescent, which forbids their transmission through the multilayer. From a photonic crystal viewpoint, the principle of operation of this polarizing DUV element relies upon creating a defect in the 1D TE photonic crystal by creating a resonant submirror exclusively for the TE polarization separated from the multilayer submirror by a certain spacing that permits a FabryPerot resonance between the two submirrors, allowing
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the TE polarization to cut through the mirror’s stop band. Fig. 7 shows an AFM scan of an 11 nm-deep corrugation of slightly larger period used for the purpose of process calibration. Once again, the bottoms of the grooves are as smooth as the resist-protected tops. In this process, an exchange reaction [12] was used at a rate of 20 nm per minute under alkaline conditions with a complex-forming agent to remove fluorine from the reaction as a soluble compound.
[4]
[5]
[6]
3. Conclusions [7]
Wet etching is shown to unexpectedly have the potential of a photonic manufacturing nanotechnology despite the inherent lateral broadening as a result of the development of resonant diffractive structures where subwavelength corrugations no deeper than a few tens of nanometers give rise to high efficiency selective effects thanks to the field concentration in the associated surface wave. This unforeseen come back can be based on chemistries which microelectronic has had to proscribe. The extent of its applicability depends on several technological issues being resolved, such as passivation of a standard photoresist etch stop against alkaline chemistry, promotion of strong adhesion between substrate and photoresist to prevent any lateral creeping of the etchant, the ability to perform bubblefree etching, immediate onset of the chemical reaction, self-smoothing of etch-front irregularities and the continuous supply of fresh etchant. The application examples given above have shown how ultra-shallow gratings can shun or cut through a 1D photonic crystal. Although the chosen examples mainly relate to the processing of highly coherent light waves and beams, this is not a limitation. The availability of high to very high refractive index layer materials enables resonant functional elements to be extended to light beams of wider wavelength and angular spectra.
[8]
[9]
[10]
[11]
[12] [13]
[14] [15]
[16]
[17]
References [1] M.S. Rahman, Reliability of advanced dielectrics in gate oxide and device level packaging in MEMS, January, 2009. Available electronically from http://hdl.handle.net/10106/2014. [2] H. Janseny, H. Gardeniers, M. de Boer, M. Elwenspoek, J. Fluitman, A survey on the reactive ion etching of silicon in microtechnology, Journal of Micromechanics and Microengineering 6 (1996) 14–28. [3] A. Talneau, F. Lemarchand, A.L. Fehrembach, J. Girard, A. Sentenac, Deeply-etched two-dimensional grating in a Ta2O5
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guiding layer for very narrow spectral filtering, Microelectronic Engineering 87 (2010) 1360–1362. K. Nakamura, T. Kitagawa, K. Osari, K. Takahashi, K. Ono, Plasma etching of high-k and metal gate materials, Vacuum 80 (2006) 761–767. Y. Jourlin, S. Tonchev, A.V. Tishchenko, C. Pedri, C. Veillas, O. Parriaux, A. Last, Y. Lacroute, Spatially and polarization resolved plasmon mediated transmission through continuous metal films, Optics Express 17 (2009) 12155–12166. G. Golubenko, A. Svakhin, V. Sychugov, A.V. Tishchenko, Total reflection of light from a corrugated surface of a dielectric waveguide, Soviet Journal of Quantum Electronics 15 (1985) 886–887. A.V. Tishchenko, V.A. Sychugov, High grating efficiency by energy accumulation in a leaky mode, Optical and Quantum Electronics 32 (2000) 1027–1031. D. Pietroy, O. Parriaux, T. Epalle, S. Tonchev, Contactless functional testing of grating-coupled evanescent wave (bio)chemical sensors, Sensors and Actuators B – Chemical 159 (2011) 27–32. P. Viktorovitch, B. Ben Bakir, S. Boutami, J.L. Leclercq, X. Letartre, P. Rojo-Romeo, C. Seassal, M. Zussy, L. Di Cioccio, J. Fedeli, 3D harnessing of light with 2.5D photonic crystals, Laser & Photonics Reviews 4 (2010) 401–413. , http://dx.doi.org/ 10.1002/lpor.200910009. O. Parriaux, V.A. Sychugov, A.V. Tishchenko, Coupling gratings as waveguide functional elements, Pure and Applied Optics 5 (1996) 453–469. P. Genevet, N. Yu, F. Aieta, J. Lin, M.A. Kats, R. Blanchard, M.O. Scully, Z. Gaburro, F. Capasso, Ultra-thin plasmonic optical vortex plate based on phase discontinuities, Applied Physics Letters 100 (2012) 013101–13111. D. Harvey, Modern Analytical Chemistry, McGraw-Hill Inc., Science/Engineering/Math, 2000 ISBN: 0072375477. O. Parriaux, A.V. Tishchenko, F. Pigeon, Associating a lossless polarizing function in multilayer laser mirrors by means of a resonant grating, SPIE Proceeding Series 6187 (2006) 72–79. K. Williams, R. Muller, Etch rates for micromachining processing, Journal of Microelectronic Systems 5 (1996) 256–269. K. Williams, K. Gupta, M. Wasilik, Etch rates for micromachining processing – part II, Journal of Microelectronic Systems 12 (2003) 761–778. M. Abdou Ahmed, J. Schulz, A. Voss, O. Parriaux, J.-C. Pommier, T. Graf, Radially polarized 3 kW beam from a CO2 laser with an intracavity resonant grating mirror, Optics Letters 32 (2007) 1824–1826. D. Basting, K. Pippert, U. Stamm, History and future prospects of excimer laser technology, RIKEN Review No. 43, in: Focused on 2nd International Symposium on Laser Precision Microfabrication (LPM2001), January 2002. K. Bougot-Robin, J.-L. Reverchon, M. Fromant, L. Mugherli, P. Plateau, H. Benisty, 2D label-free imaging of resonant grating biochips in ultraviolet, Optics Express 18 (2010) 11472–11482. O. Parriaux, A.V. Tishchenko, F. Pigeon, Associating a lossless polarizing function in multilayer laser mirrors by means of a resonant grating, Photonics Europe Proceedings of SPIE 6187 (2006) 72–79.
Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656 www.elsevier.com/locate/photonics
Bypassing and cutting through 1D photonic crystals by ultra-shallow wet-etched resonant gratings Svetlen Tonchev a,b, Olivier Parriaux a,* a b
University of Lyon, Lab. H. Curien UMR CNRS 5516, F-42000 Saint-Etienne, France Institute of Solid State Physics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
Received 20 March 2012; received in revised form 21 June 2012; accepted 21 June 2012 Available online 2 July 2012
Abstract The electromagnetic field in subwavelength resonant diffractive optical elements is so concentrated that a very shallow surface corrugation obtained by wet chemical micro–nano-etching is sufficient to give rise to high contrast diffraction effects allowing a high wavelength-, polarization- or transverse-mode-selectivity which is not achievable by conventional diffractive elements. Two examples of polarizing laser mirrors at both extremes of the optical spectrum with wet-etched grating bypassing and cutting through the 1D photonic crystal are demonstrated. # 2012 Elsevier B.V. All rights reserved. Keywords: Wet etching; Resonant grating; Photonic crystal; Multilayer mirror; Radial polarization
1. Introduction The development of micro–nanostructuring in diffractive optics, and more generally in microoptics, owes much to the steady and so far predictable development of microelectronics towards ever smaller feature sizes. The technologies used in microelectronics which can be borrowed by microoptics include lithography (either by direct e-beam writing or by step and scan), etching and deposition of silicon-based layers, such as amorphous silicon, silicon oxide (SiO2), silicon oxinitride (SiON) and silicon nitride (Si3N4), and more recently, high-k gate oxides, such as Hafnium and Tantalum oxides (HfO2 and Ta2O5) [1]. Well-documented Reactive Ion Etching (RIE) processes and related equipment have been developed for silicon-based
* Corresponding author. Tel.: +33 477915819; fax: +33 477915781. E-mail address: [email protected] (O. Parriaux). 1569-4410/$ – see front matter # 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.photonics.2012.06.005
substrates and layer materials [2], whereas high refractive index metal oxide layers can be etched by Reactive Ion Beam Etching (RIBE) [3], which combines chemical etching and the kinetic energy of an ion beam to remove the non-volatile products of the reaction. This is a complex process with a large number of parameters and high processing costs. Metal layers can be etched by chlorine RIE [4]; this is a well-developed technology, but it requires strict safety precautions and costly infrastructure. Isotropic wet etching for microstructuring objectives in microelectronics was abandoned long ago for its incapability of matching with the demands on the characteristic dimension (CD), the sidewall etching spoiling the resolution needed by today’s photolithography. Wet etching is currently mainly used in microelectronics for anisotropic shaping of singlecrystal silicon and removal of relatively large areas of metal and dielectric layers. Wet microetching has found limited use in diffractive optics either since vertical
652
S. Tonchev, O. Parriaux / Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656
walls are often required and sidewall etching would further limit the angular aperture of the element as the typical depth of fused quartz-based transmission diffractive optical elements (DOE) is roughly between half and one operation wavelength. However, there is currently an emerging field within diffractive optics where wet microetching is bound to play an important role as an alternative manufacturing technology to dry etching. This field concerns resonant diffractive elements, and more particularly, diffraction gratings, where the surface corrugation is located in the field of an electromagnetic surface wave. This surface wave can be that of a plasmon propagating at a metal– dielectric interface [5] or a guided [6] or leaky mode [7] of a dielectric layer or multilayer. Fig. 1 illustrates one resonant grating structure with the transverse field of the surface wave involved which is here a true waveguided mode. The mechanism of operation and typical effects of one type of resonant element (a resonant mirror) are symbolized in Fig. 2. In this case, the grating couples an incident wave to the surface wave, which acts as an energy reservoir. The surface wave propagates over some distance along the corrugated surface and then reradiates away into the incident and transmission media, where it interferes with the component of the field that was not coupled to the surface wave, giving rise to interference effects whose contrast, destructive or constructive character are determined by the optogeometrical parameters of the structure [8]. It is of particular interest that the diffraction effects can be made highly selective with regard to wavelength, angle and polarization, unlike in standard non-resonant corrugations, the selectivity of the created interference effects being conferred by the surface wave itself. From a photonic crystal point of view, the resonant gratings sketched in Fig. 1, sometimes named 2.5D photonic crystals if composed of holes instead of lines [9], are 1D waveguide mode photonic crystals operating according
Fig. 1. Resonant reflection of a free-space wave from a grating waveguide upon mode coupling (transverse electric field profile of the TE0 mode and wavelength selectivity represented).
Fig. 2. Symbolic representation of the operation of a waveguide grating resonant mirror: the grating splits the incident field into a 1st order coupled waveguide mode field with energy accumulation then outside re-radiation, and the 0th order transmitted field. The latter and the former recombine in the transmission half-space with designadjusted relative amplitude and phase for destructive interference, ensuring 100% reflection theoretically.
to the intraguide 2nd grating order, addressed by the incident free space wave via the 1st grating orders. However, the photonic crystal representation is here less meaningful physically than a coupled wave phenomenology since the lateral modal field confinement is weak as will be seen in the two examples where the grating permits to bypass or cut through the stop-band of the 1D photonic crystal of a dielectric multilayer. What matters most in the scope of the present paper is that the electromagnetic field of the surface wave can be so strong that highly selective control over the contrast of the said interference in the incident or transmission media can be achieved using a very shallow corrugation whose depth is between one tenth and one hundredth of the period depending on the refractive index of the corrugated layer material. This arises because the strength of a resonant grating is the product of three factors: the corrugation height h, the normalized modal field strength in the corrugation, and the permittivity difference between groove and ridge [10]. Interest in ultra-thin gratings has also been fuelled recently by demonstrations of such gratings with engineered subwavelength gold V-shaped structures forming socalled metasurfaces giving rise to phase discontinuities by means of surface steps height of no more than one hundredth of a wavelength [11]; this is presently made by a lift-off technique and could easily be wet etched. The etching of such shallow features is difficult to achieve using a dry chemistry process involving a plasma or reactive ion beam: as the etching rate is usually rather high, the depth and uniformity of the corrugation relief is determined by the very first seconds of a short process where the plasma is unstable and nonuniform. This is where wet microetching offers a solution: although such resonant gratings are often ‘‘zeroth order elements’’, and thus subwavelength, i.e., clearly submicron in visible and near-IR applications, the required aspect ratio of the corrugation is so small
S. Tonchev, O. Parriaux / Photonics and Nanostructures – Fundamentals and Applications 10 (2012) 651–656
that sidewall etching is negligible relative to the characteristic dimension of the etched microstructure. This means that structures characterized by a CD of a few tens of nanometers can be obtained by wet etching. As to the problem of the onset of the etching process, a wet etching solution can easily be diluted to the point that the first few seconds of the reaction where the liquid penetrates into the photoresist grooves are negligible compared to the total reaction time. To the question of what wet chemistry is the most appropriate, silicon- and III–V based optoelectronic technologies privilege acidic solutions to prevent alkaline contamination. This is not necessarily the best choice for diffractive optics since alkaline contamination is generally not an issue and acids tend to form bubbles, to render an oxide surface hydrophilic (thus ‘‘photoresist-phobic’’) which eases the acid creeping between the substrate and photoresist etch-stop. Microetching in photonics can therefore employ a wide range of solutions belonging to different types of wet chemistry, e.g., basic, acidic and of the exchange type [12]. It is worth mentioning in relation to the wet etching examples given hereunder, that manufacturing costs may be an issue. If the described micro-optical elements would have to be fabricated by the dry etching processes and equipments of microelectronics, their cost would be prohibitive for two reasons. First, the microoptics market is much more diverse than that of ICs, which means that for most products, the economy of scale does not operate. The second reason is that, because of the variety of substrate shapes, dimensions, and specificity of materials, dry etching equipment cannot be easily adapted to a batch process scheme, but is instead suited for manufacturing a single substrate at a time, which amounts to high fabrication costs. We have used a number of wet microetching processes for various metallic, semiconductor and dielectric layers which the next section will illustrate by giving two examples of subwavelength resonant structures. Both examples are laser polarization-control mirrors. They are chosen to illustrate the two extremes of the optical domain: one operates in the far-IR at 10.6 mm wavelength in a CO2 laser, the second one in the deep-ultra-violet (DUV) at 193 nm in an Argon fluoride excimer laser. The operation of both polarization control elements involves an optical resonance, but the materials are radically different. Nevertheless, both are etched wet, which demonstrates how versatile this technology can be, although the DUV element is currently at a less advanced development stage compared to that of the IR-element.
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2. Results and discussion Both polarizing mirror elements comprise a multilayer mirror as in standard laser mirrors, ensuring close to 100% reflection. The reflection of a dielectric multilayer is polarization independent under normal incidence, therefore a polarizing element – usually a Brewster plate – is placed in the laser resonator to induce selective loss on the transverse electric (TE) polarization which results in the lasing of the sole transverse magnetic (TM) linear polarization. In the resonant grating approach, the polarization selectivity is integrated into the multilayer mirror in the form of a periodic corrugation, typically in the final layer at the active medium side, which decreases the reflection of one polarization [13]. One advantage of this monolithic approach is that the laser beam can be endowed with almost any polarization distribution by simply adjusting the orientation of the grating lines over its cross-section, e.g., a radial or azimuthal polarization, provided the laser resonator accepts such a field distribution as one of its eigenmodes. 2.1. Far-IR grating mirror with TE stopband bypass The far-IR element presented here comprises a circular grating of radial period close to 6 mm etched in a layer of amorphous germanium deposited on top of a ZnSe-ThF4 multilayer mirror. Fig. 3a shows the microscope top-view of a few grooves of etched germanium corrugations, which are designed to allow radiation of the local TE polarization to bypass the mirror by the mediation of its first diffraction orders, whereas that of local TM polarization experiences high reflection. From a photonic crystal viewpoint none of the polarizations should leak through the multilayer mirror since with 6.5 mm period and 10.6 mm wavelength both local TE and TM 1st diffraction orders are directed into the high and low index layers under an angle which is larger than the TE and TM stop bands of the multilayer angular spectra. The solution to provoke a leakage loss through the multilayer, thus to bypass its stop band, is to design the grating period and the multilayer so that the 1st TE diffraction orders excites a leaky mode of the multilayer. Fig. 3b shows the grating pattern corresponding to an alternative polarization filtering concept whereby a ca. 9 mm period permits the local TM polarization to leak through the multilayer by refraction close to the Brewster angle whereas this angle is within the multilayer’s TE-stopband. Generating the radially polarized mode of a laser imposes here the grating
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Fig. 3. Optical microscope top-views of two different germanium polarizing gratings on a CO2 laser mirror ensuring high reflectivity of the radially polarized mode and damping of the reflectivity of the azimuthally polarized mode. (a) Circular line grating of 6.5 mm radial period inducing leakymode-mediated damping of the reflection of the local TE polarization. (b) Piecewise constant, ca. 9 mm azimuthal period radial grating inducing the leakage through the multilayer mirror of the local TM polarization under the Brewster angle between high and low refractive index layers, and forbidding the diffraction of the local TE polarization.
lines to be radial; this is possible to achieve by means of a segmented radial grating of approximately constant azimuthal period as illustrated since the polarization filtering principle is angularly and spectrally broadband. The corrugation depth that gives the TE/TM reflection spectra shown in Fig. 4 by means of the polarizing scheme in Fig. 3a is about 200 nm for 3250 nm line and groove widths, which corresponds to an aspect ratio of only 7/100, i.e., the duty cycle is hardly affected by the sidewall etching. The double-dip feature of the local TE reflection spectrum illustrates how the resonant coupling mechanism can be tailored: in order to broaden the tolerances on the ZnSe/ThF4 multilayer, the multilayer was engineered so as to bring two TE leaky modes in coalescence, which considerably widens the reflection dip. The relative location of the resonant grating multilayer and the operation principle of the
TE-leaky-mode tunneling through the mirror are also shown in Fig. 4. Fig. 5a shows an AFM scan of a few grooves of the grating in Fig. 3a etched in the germanium layer by a chemically neutral ion beam, exhibiting U-shaped grooves due to normal ion beam incidence. This single sample process lasts several minutes all together with about 7 interruptions to permit the substrate to cool down. In comparison, Fig. 5b is an AFM scan of the same element obtained by wet etching in a slightly acidic reduction–oxidation reaction [14,15] at a rate of 100 nm per minute, showing that the roughness of the etched groove bottom is similar to that of the photoresistprotected tops. The very flat groove bottom results from the use of an ultra-thin etch-stop layer. Remarkably, the wet etching procedure gives rise to groove flanks that are near vertical. This element was used as the back mirror of a high power CO2 laser for the generation of radial polarization [16] up to a power of 5 kW. 2.2. Deep-UV grating mirror with cut through the TE stopband
Fig. 4. Fabrication-tolerant double-dip TE reflection spectrum of a multilayer CO2 laser mirror with 1st order bypass leakage into the substrate. The TM reflection is unaffected by the presence of the shallow grating.
The second example of a laser polarization-control mirror operates in the DUV region of the optical spectrum. It is developed for the polarization of an ArF laser by means of a monolithic element in place of a set of cascaded intra-cavity Brewster prisms [17]. The element comprises a shallow grating etched into the last LaF3 layer of an AlF3/LaF3 multilayer mirror. The period is 136 nm with a corrugation depth of 15 nm. So shallow a groove depth is not beyond reach for a dry etching technology and was actually achieved by RIE into a PECVD silicon nitride layer used as a deep-UV
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Fig. 6. TE and TM reflection spectra under normal incidence of a 39 layer AlF3/LaF3 mirror with 15 nm deep, 136 nm period, corrugation in a LaF3 waveguide layer.
Fig. 5. AFM scan of a 6.5 mm period, 200 nm deep germanium corrugation by (a) Argon ion RIBE and (b) reduction–oxidation wet etching reaction.
biosensor waveguide-grating platform [18]. However, the difficulty of shallow dry etching increases in cases the products of the reaction are not volatile whereas a wet solution requiring a duration of the order of 1 min can be easily controllable. Fig. 6 shows the polarizing mirror structure and the corresponding TE and TM reflection spectra. Of the polarizing solutions described in Ref. [19], the principle which is most adequate in this system of low index contrast system 1.39/1.66 (standing for AlF3/LaF3) is to combine a standard quarter wave submirror with a waveguiding superstructure of a few layers, comprising a corrugated LaF3 layer, exhibiting close to 100% resonant reflection by TE0 mode coupling at 193 nm wavelength. Whereas the waveguide grating structure is designed for TE mode coupling at 193 nm wavelength under normal incidence, the TM polarization remains practically unaffected by the waveguide superstructure and only experiences the wide band reflection of the
Fig. 7. AFM scan of a binary grating of less than 0.05 aspect ratio in a LaF3 layer with photoresist etch-mask.
multilayer. Between the multilayer mirror and the waveguide reflector there is a low index buffer layer whose optical thickness is adjusted so as to represent the inside of a first order TE Fabry-Perot resonator at 193 nm wavelength. Thus, in the neighborhood of 193 nm wavelength the TE transmission has a maximum whereas the TM polarization is still highly reflected. Although the grating with its 136 nm period could in principle diffract the 1st orders into the substrate through the multilayer, it does not because the field of the latter in the low refractive index layers is evanescent, which forbids their transmission through the multilayer. From a photonic crystal viewpoint, the principle of operation of this polarizing DUV element relies upon creating a defect in the 1D TE photonic crystal by creating a resonant submirror exclusively for the TE polarization separated from the multilayer submirror by a certain spacing that permits a FabryPerot resonance between the two submirrors, allowing
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the TE polarization to cut through the mirror’s stop band. Fig. 7 shows an AFM scan of an 11 nm-deep corrugation of slightly larger period used for the purpose of process calibration. Once again, the bottoms of the grooves are as smooth as the resist-protected tops. In this process, an exchange reaction [12] was used at a rate of 20 nm per minute under alkaline conditions with a complex-forming agent to remove fluorine from the reaction as a soluble compound.
[4]
[5]
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3. Conclusions [7]
Wet etching is shown to unexpectedly have the potential of a photonic manufacturing nanotechnology despite the inherent lateral broadening as a result of the development of resonant diffractive structures where subwavelength corrugations no deeper than a few tens of nanometers give rise to high efficiency selective effects thanks to the field concentration in the associated surface wave. This unforeseen come back can be based on chemistries which microelectronic has had to proscribe. The extent of its applicability depends on several technological issues being resolved, such as passivation of a standard photoresist etch stop against alkaline chemistry, promotion of strong adhesion between substrate and photoresist to prevent any lateral creeping of the etchant, the ability to perform bubblefree etching, immediate onset of the chemical reaction, self-smoothing of etch-front irregularities and the continuous supply of fresh etchant. The application examples given above have shown how ultra-shallow gratings can shun or cut through a 1D photonic crystal. Although the chosen examples mainly relate to the processing of highly coherent light waves and beams, this is not a limitation. The availability of high to very high refractive index layer materials enables resonant functional elements to be extended to light beams of wider wavelength and angular spectra.
[8]
[9]
[10]
[11]
[12] [13]
[14] [15]
[16]
[17]
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