Flexible mask illumination setup for serial multipatterning in Talbot lithography Daniel Thomae,1,* Jacqueline Maaß,2 Oliver Sandfuchs,3 Alexandre Gatto,2 and Robert Brunner1 1
Ernst-Abbe-Fachhochschule Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany 2 3
Carl Zeiss Jena GmbH, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
Hochschule Hamm-Lippstadt, Marker Allee 76-78, 59063 Hamm, Germany *Corresponding author: daniel.thomae@fh‐jena.de
Received 6 December 2013; revised 6 February 2014; accepted 6 February 2014; posted 10 February 2014 (Doc. ID 202640); published 14 March 2014
A flexible illumination system for Talbot lithography is presented, in which the Talbot mask is illuminated by discrete but variable incidence angles. Changing the illumination angle stepwise in combination with different exposure doses for different angles offers the possibility to generate periodic continuous surface relief structures. To demonstrate the capability of this approach, two exemplary micro-optical structures were manufactured. The first example is a blazed grating with a stepsize of 1.5 μm. The second element is a specific beam splitter with parabolic-shaped grating grooves. The quality of the manufacturing process is evaluated on the basis of the optical performance of the resulting micro-optical elements. © 2014 Optical Society of America OCIS codes: (050.0050) Diffraction and gratings; (070.6760) Talbot and self-imaging effects; (220.3740) Lithography; (220.4000) Microstructure fabrication. http://dx.doi.org/10.1364/AO.53.001775
1. Introduction
For the manufacturing of spectroscopic master gratings, two main technologies have been established over the years [1]. The first of them is interference lithography (IL), where the grating is generated by exposing a resist coated substrate by an interference fringe pattern and a subsequent development process by which the latent structure is transferred into a continuous surface profile. IL has the advantage to produce a high-quality master grating in a single exposure step, but it is limited in the diversity of attainable structures. In particular, IL allows the creation of blaze-like grooves but depends on the grating period and exposure wavelength; it is limited in profile depth, which is typically in the order of 150 nm [2,3]. 1559-128X/14/091775-07$15.00/0 © 2014 Optical Society of America
The second well-established manufacturing technology of spectroscopic master gratings is classical ruling [1]. Here a cut diamond is directed line by line across the substrate, where deep blazed profiles are also engraved in the covering metal layer. Ruled blazed gratings offer higher diffraction efficiency compared to gratings manufactured by IL, but, due to the remaining roughness of the material and small local variations of the grating period, they also cause an increased amount of disturbing stray light. As an alternative, Talbot lithography offers the potential to overcome the limitations of both mentioned classical technologies, especially the limitation of attainable groove shapes with IL and the time consumption of ruling. Talbot lithography is based on the Talbot effect [4,5], in which a plane, monochromatic wave is used to illuminate a periodic amplitude mask. Behind the mask an extended intensity distribution (Talbot carpet) is formed, which was measured and documented by Case et al. in [6]. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1775
For exposing the resist coated substrate, especially the self-imaging planes may be used, which are located at approximately z ≈ n · p2 ∕λ behind the mask [5], with p as the mask period, λ as the wavelength, and n 1; 2; 3; …. When using an amplitude line mask, the exposed and developed structure is a simple phase line grating. More complex structures can be generated by a variation of the illumination angle, which shifts the lateral position of the exposure pattern with respect to the photoresist. One of the first experiments that made use of this approach was reported by Bryngdahl in 1972 [7]. Bryngdahl illuminated an S-shaped source aperture, which created a specific angular spectrum of illuminating plane waves impinging on a two-dimensional periodic pinhole array mask. In accordance with the angle distribution the “S” form reappeared in the mask’s Talbot distances as a superposition of laterally shifted self-images. Recently, this basic principle was also transferred to lithography by Harzendorf et al. [8] and Stürzebecher et al. [9], where a commercial mask aligner with an advanced illumination system (AIS) [10] was applied. Due to a structured aperture stop within the AIS, a predefined plane wave angular spectrum illuminates the mask. This creates simultaneously a set of laterally shifted Talbot self-images forming an intensity distribution that is used to expose a photoresist coated on a substrate. Due to the simultaneous use of laterally shifted Talbot images for the resist exposure, this procedure is characterized as a parallel multipatterning technique. The parallel multipatterning technique requires a complex illumination system to ensure a homogeneous intensity distribution and homogeneous angular spectrum at each point of the mask. This requirement is, however, accompanied by a complex and expensive setup. For clarity in the designations we will use in the following the term “mask” for the periodic structure generating the Talbot pattern and “grating” for the exposed and developed final photoresist structure. In this contribution we present a simple to implement alternative approach for Talbot lithography, which is based on an adjustable and flexible illumination setup with subsequent illumination steps, which we call “serial multipatterning.” This approach allows a simplification of the setup, which now contains mostly standard optical parts. Since it is advisable to fit the dimensions of the illumination system to the mask period, using standard optical parts makes modifications of the system very easy. Furthermore, the serial approach allows easy fine-tuning of the grating’s groove shape without physical modifications of the exposure tool itself. 2. Concept and Implementation of Flexible Illumination System for Talbot Lithography
The basic setup comprises the illumination system, the mask, and a resist coated substrate (Fig. 1). Mask and substrate are integrated in a mechanical subunit 1776
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
Fig. 1. Schematic experimental setup consisting of source, collimator (filters), a tilting mirror, mask, and resist coated substrate: the source’s radiation is collimated by a condenser lens and may get additionally filtered. The remaining light is deflected by a mirror onto the mask. The Talbot carpet adjacent to the mask is used to expose the resist coated substrate. Due to different tilting angles of the mirror, the radiation’s incident angle onto the mask is varied. This shifts the exposure pattern (Fig. 2).
in which both are adjustable in distance and orientation with respect to each other. The structured mask and the resist plane have to be separated by the Talbot distance and have to be aligned parallel to each other. A crucial aspect of the setup concerns the illumination system: a point source of light in combination with a collimating lens generates a plane wavefront. This wavefront is directed toward a tiltable plane mirror, which then reflects the plane wave onto the mask. By changing the mirror inclination, the impinging angle on the mask can be varied, which leads to a lateral shift of the exposing spots in the resist plane. This principle is demonstrated schematically in Fig. 2. On the left of Fig. 2 the exposure step is shown at its beginning. The primarily selected inclination angle is associated with specific positions of the exposure spots in the resist. After the first exposure the inclination angle is changed step by step, and for each angle a precalculated exposure dose is applied to the resist. The dose applied to each lateral spot position is controllable by the exposure time; the intensity remains constant. This principle offers multiple degrees of freedom. With different ‘recipes’ comprising illumination angles and their corresponding exposure time, a broad diversity of continuous periodic surface relief
Fig. 2. Sequential exposure process—in each step the corresponding illuminating angle shifts the foci of the Talbot carpets and, therefore, subsequently exposes different positions on the resist. The dose per position is varied over the applied exposure time.
structures such as triangular-shaped diffractive gratings (Fig. 2 right) are accessible. From the generated dose distribution, a permanent surface structure is unveiled in the subsequent development process. Additionally, the exposing wavefront can be filtered by spectral band-pass filters, polarization filters, or further optical elements to tailor the illumination radiation to the desired demands. As the principal ray is fixed, the use of angle-sensitive elements such as interference filters is no problem. Although this setup exposes the resist in a serial process and step by step, it has to be mentioned that this approach allows the processing of all grating grooves, simultaneously. Especially time-consuming and failure-causing movements or stepping procedures between mask and substrate during the exposure process are not necessary. The serial approach requires a longer net exposure time as compared to the parallel process, but offers a high flexibility to adapt the illumination conditions to the response curves of the photoresist during the process. For example, on the base of a first lithographic result the exposure time for each illumination angle can be optimized with respect to the accurate resist-response curve, without any change in the system’s physical setup. As a first conclusion, this setup offers a flexible lithographic process to manufacture a wide variety of periodic continuous surface relief profiles. A.
Dimensioning of the Illumination System
The illumination setup, consisting of source, collimator, and tilting mirror, has to be dimensioned to the mask period p and the illumination wavelength λ. Figure 3 presents the illumination setup schematically highlighting specific geometrical details. In Fig. 3(a) the illuminated mask with the attached Talbot carpet and the exposing spots inside the resist layer is depicted. Due to the oblique illumination, with its principal ray impinging the mask under an angle βmax , the exposing spot pattern is deflected downward by the distance Δx relative to the optical axis. The lateral spot deflection Δx can be described in good approximation by the triangle formed by the
separation of mask and resist zMR , the illumination angle β, and the right angle between Δx and zMR . For generating arbitrary profile shapes, each position within one period has to be reached by an exposing spot. This means a lateral spot deflection of half of the mask’s period has to be guaranteed for both upward and downward directions. The corresponding angle βmax of the principal ray for maximum elongation is the largest illuminating angle that is needed to allow a complete and continuous structuring of the resist over the whole period. In the exemplary case, when the mask’s first selfimage plane at half the Talbot distance is used, it corresponds to a distance between mask and resist surface of zMR ≈ p2 ∕λ. For the angle βmax follows: p λ βmax ≈ arctan : (1) arctan 2 · zMR 2·p Figure 3(b) depicts the illumination system with the emitting area of the source on the left, the collimating lens on the right, and the principal rays originating from the opposite edges of the source. Due the source’s lateral extension L and the focal length f of the collimator, the illumination radiation is also associated with a cone angle α for the angular spectrum yielding L α arctan : (2) f In order to allow a high resolution in the lithographic process and to avoid disturbing broadening effects, it is important to choose a source extension and a focal length, such that the angle α is much smaller than βmax (α ≪ βmax ). Decreasing α by increasing the focal length f of the collimator reduces the intensity of the collimated wavefront approximately proportional to 1∕f 2 . In our experimental setup a light-emitting diode (LED) with a wavelength of around 400 nm (ProLight Opto PM2L-1LLx-LC) and a chip dimension of ≈1 mm2 was chosen. To approximate a point source with emitting spherical waves, it is important to use a LED with only a single chip. These spherical waves are transferred into plane waves by a collimating lens (300 mm plano–convex with 300 aperture diameter). The source’s edge length and the focal length result in an illumination angular spectrum extended about α 0.2° versus an angle βmax 1.15°. Although the concept of the setup is very simple, it allows the generation of high-quality plane wavefronts with a homogeneous intensity distribution and with sufficiently small wavefront aberrations. B. Potential Laser Sources
Fig. 3. (a) Maximum principal ray angle βmax for Talbot lithography and (b) angular spectrum’s extent of the illumination radiation formed by the collimator.
In principle, the introduction of a laser source in combination with a pinhole would allow a reduction of the emitting source area. Laser sources also have a higher technical potential as they are available 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1777
in a wavelength range below 300 nm, a spectral region where LEDs have not (yet) reached sufficient power and acceptable lifetimes. Shorter wavelengths reduce the aberrations in the intensity distribution of the Talbot carpet [11] and, therefore, offer a higher lithographic resolution in the resist plane [8]. Besides that, a shorter wavelength increases the Talbot distance and, therefore, reduces adjustment requirements [8]. The serial multipatterning concept, on the one hand, allows, in principle, the use of coherent sources such as lasers for Talbot lithography, and on the other hand, the coherence properties of laser light may negatively influence the exposed grating. In particular, the appearance of speckles has to be avoided. Another disadvantage of laser sources is the fine structured Talbot carpet produced by strictly monochromatic radiation. This induces a very small tolerance against adjustment errors of the distance between mask and resist. The use of polychromatic sources superimposes different Talbot carpets of many wavelengths and therefore smoothes the fine structure of the carpets shown in [6]. Due to the challenges involved with a laser, we used as a proof of concept the far more economical LED solution. 3. Experimental Results
To demonstrate the usability of the flexible illumination system for Talbot lithography, we applied the system to manufacture deep blazed spectral gratings and a diffractive beam splitter grating. In general, for the fabrication of high-quality spectral gratings some challenges have to be addressed. First, deviations from the grating’s fundamental periodicity generate diffraction ghosts, and roughnessinduced random errors are the origin for diffuse scattering effects. In particular, the occurrence of ghost diffraction orders is attenuated when using the grating in higher diffraction orders [12,13]. As a rule of thumb, diffractive gratings working in higher diffraction orders exhibit deeper profiles. In Talbot lithography deviations from the addressed fundamental periodicity occur because of the mask’s boundary that causes edge effects in the Talbot carpet [6,14]. Additionally, a variation of the periodicity is also related to the mask’s manufacturing technology where, e.g., disturbing stitching effects have to be minimized. Finally, particle contamination on the mask’s surfaces is an additional source for point defects. For our experiments we have used a 10 μm period dense line amplitude chromium mask created by electron beam lithography. A further challenge in the grating manufacturing process arises from the need to create a desired groove shape and keep this groove shape constant over the whole area of the grating. Using Talbot lithography, local variations in the attained groove shape are mainly caused by deviations in the distance between mask and photoresist—the maximum tolerable distance error is dependent on 1778
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
the specific mask structure and the illumination wavelength spectrum, and can be estimated by calculating the intensity carpet behind the mask [6,14], as was done in, e.g., [8]. Besides the residual tilting error between mask and substrate, which is below 2 arcmin in our setup, the minimal attainable distance error between mask and resist surface is given by the flatness of the mask, the substrate, and the resist layer. A. Manufacturing of Deep Blazed Gratings
For a proof of concept we concentrated on manufacturing deep blazed gratings in a 2.7 μm thick photoresist (MicroChemicals, AZ-1518) on a glass substrate (SCHOTT Borofloat 33). The target-groove shape was attained by eight exposures under different illumination angles at the first self-image distance of zMR ≈ p2 ∕λ. The gratings were developed and afterward coated with a thin layer of aluminum. An atomic force microscopy (AFM) scan of the groove profile after aluminum coating is shown in Fig. 4. Here, a distinct blaze-like profile is clearly observable with a periodicity of 10 μm and a depth of approximately 1.55 μm. The presented profile reveals a distinct sawtoothlike shape, but there are still further improvements of the processing details necessary. For example, effort can be applied to linearize the slope of the active blaze facet and create a sharper step on the passive blaze facet. Although a similar sawtooth-like profile could also be manufactured by IL, the profile depth of such an IL structure would be limited to a maximum depth smaller than approximately 200 nm [2,3]. And, therefore, it will either show a very low diffraction efficiency in the visible or near-infrared spectral range or has to be further treated, e.g., by reactive ion-etching processes. To evaluate the optical properties of the manufactured grating, we measured the diffraction efficiency in dependence on different diffraction orders. This was done in the setup shown in Fig. 5. The grating was illuminated by slightly converging laser light (He–Ne at 633 nm) under a chief ray angle of 22.45° relative to the grating normal in the classical mount (no conical illumination) (see dashed arrow in Fig. 5).
Fig. 4. AFM scan showing the topography of the blaze-like grating structure manufactured by serial Talbot lithography. The grating shows a lateral period of 10 μm and a depth of approximately 1.55 μm. The resist grating was coated with aluminum.
Fig. 5. Setup for efficiency measurement—only principal rays are drawn. The numbers denote the different diffraction orders.
The laser itself emitted nearly unpolarized radiation. The converging illumination (NA 0.03) forms a spot for each diffraction order, which was captured by an optical powermeter. The astigmatism induced for higher diffraction orders by this configuration increases the spot size, but it was still possible to capture the whole spot within the detector area. The diffraction orders, which are directed in angles close to the incident beam, were not accessible, because the housing of the measuring photodiode would shadow the incoming ray. Figure 6 shows the measured power for different diffraction orders, relative to the power incident to the grating. It has to be mentioned that the occurring faint ghosts are located very close to the main diffraction order and were also captured by the photodiode. About 46% of the incident power is diffracted into the target-diffraction orders (sixth and seventh), almost 8% at one diffraction order close to it (eighth) and around 24% at other main diffraction orders. The remaining 22% of the incident power is not assignable. This 22% may be located in the missing diffraction orders and gets lost in ghost orders, diffuse scatter, and absorption and transmission of the coating. Linearizing the active flank of the profile due to an increased number of exposure steps should result in a higher efficiency in the sixth and seventh orders. Besides the efficiency, the uniformity of the groove shape over the area of the grating is another quality criterion. For example, a height variation of the profile ranging from one side to the other would manifest in an inhomogeneous distribution of the diffraction efficiency. Therefore, we used the same illumination setup but with a plane wave instead of a converging wave. The far-field diffraction images at a diffusing screen were captured with a camera (Fig. 7). Since their intensity varies over more than one order of
Fig. 6. Diffraction efficiency of manufactured blaze grating.
Fig. 7. Far-field diffraction images of the grating with marked diffraction orders—the lower image was captured with eight times the exposure time of the upper one.
a magnitude, it was not possible to capture them all in one shot. The upper row was captured with a short exposure time, the lower one with a long exposure time. In this row the intensity distribution within the weak diffraction orders is also visible, but the bright orders are overexposed. The zeroth diffraction order shows as a characteristic bright circle with a dark square in its center. This feature results from the circular illumination of an area that slightly exceeds the dimensions of the square-shaped grating. The unstructured aluminized surface surrounding the grating is directly reflecting the incident light (bright frame), and the grating diffracts light into higher diffraction orders (dark square). A prominent feature (in Fig. 7) is the concentration of the diffracted power into the sixth, seventh, and eighth diffraction orders. Regarding inhomogeneity a small spot in the grating’s center appears in the fifth, seventh, and eighth diffraction orders, which originates from a reflex focused at the resist layer during pre-exposure. The spot is slightly brighter than the surrounding region for the fifth and sixth orders and darker in the seventh order. In the far-field pattern of the seventh order an intensity gradient is visible that shows a reduced diffraction efficiency in the lower part of the grating. In other diffraction orders, such as the ninth or the fifth, even stronger intensity gradients are observed. The applied inspection of the grating’s far-field distribution is not appropriate for a precise conclusion of the origin of the disturbing effects, but the method allows a simple identification of the trouble-causing area. As a last quality criterion the wavefronts of the gratings’ diffraction orders were measured. For illumination, again, a plane wave originating from a He–Ne laser was applied. The grating was tilted so that the selected diffraction order was deflected at an angle of about 22° relative to the incident ray. The wavefront of this diffraction order was then captured by an Optocraft SHSCam HR-159-GE Shack–Hartmann sensor with a 6.25× telescope in front of it. Hereby difficulties, which occur in the presence of wavefronts with distinct intensity fluctuations, have to be handled. Especially, it has to be avoided that some microlens spots are overexposed while others have not yet reached a sufficient signal level. For our analysis we examined the first, fourth, sixth, seventh, and eighth diffraction orders. From the measured wavefront values the three Zernike 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1779
Fig. 8. Measured wavefront for the order noted above, peak-tovalley (PV) and RMS in micrometers. The black dot in the images’ center is caused by the wavefront sensor.
coefficients tilt-x, tilt-y, and defocus were subtracted, since they are easily correctable by adjustment. The corrected wavefronts for different diffraction orders are displayed in Fig. 8. The measured peakto-valley (PV) error is in the range of 1.3–1.5 μm and for the RMS wavefront error a value of about 200 nm was found. Both values rise moderately for higher diffraction orders. The wavefront error of a reflection grating is separable into two influence factors, first, the flatness of the substrate and, second, the global spacing error of the grating grooves over the entire substrate. The substrate contribution causes an error that is independent of the diffraction order; the latter error increases linearly [15] with increasing diffraction orders. A comparison of all diffraction orders shows RMS and PV wavefront aberrations that are all in the same range, and also the shape of the wavefront error is similar in all of these cases. Moreover, in each displayed wavefront error the faint spot in the center and also the heights and depths near the borders of the grating remain at the same position. Hence, we assume that for this grating the wavefront aberrations are mainly caused by the flatness error of the substrate and its resist layer. B.
Manufacturing Diffractive Beam Splitters
Besides spectroscopy, gratings can also be used as diffractive beam splitters for monochromatic radiation. A special light distribution characteristic is offered by a beam splitter with a parabolic groove profile. Such an element allows the uniform distribution of the incident light across several diffraction orders, e.g., from the −10th to the 10th order. To manufacture such a beam splitter, we applied Talbot lithography with the flexible mask illumination setup. As a basic mask, again, the 10 μm periodic amplitude structure was used. To generate a parabolic groove shape, we exposed the resist coated substrate in nine illumination steps, where the exposure time was calculated from the dose-dependent resist-response curve. As a photoresist AZ1518 (MicroChemicals) was selected and the exposure source was again the 400 nm power LED. The AFM cross section of the resulting groove profile with an approximate height of 2.5 μm is displayed in Fig. 9. To minimize disturbing or distorting convolution effects between the sample profile and the AFM tip, a high-aspect-ratio probe (Olympus AC 160 TS) was used for this specific measurement. 1780
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
Fig. 9. AFM cross section showing the topography of the parabolic-shaped grooves of the beam splitter grating manufactured by serial Talbot lithography. The grating possesses a lateral period of 10 μm and a depth of approximately 2.5 μm.
In essence, the topography shows that the target parabolic shape is almost achieved. Significant deviations occur in the turning points; e.g., the top area is flattened too much and the lower turning points show rather a rounded shape compared to the tapered transition of intersecting neighboring parabolas. For an optical characterization the diffraction efficiency of this structure was measured in transmission by using a 633 nm He–Ne laser of approximately 1 mm beam diameter and at normal incidence. Figure 10 displays the diffraction efficiency relative to the incident power, separately, for TE and TM polarization. TE polarization is defined by the electrical field oscillating in the direction of the grating grooves. The 1st and 7th orders show efficiencies around 10%, the 3rd, 4th, 6th, and 8th around 6%, and all other orders are below 2.5%. With respect to the target of a uniformly distributed efficiency, especially the values for the zeroth, 2nd, and 5th orders are too low. The deviation may be attributed to the imperfect ‘parabolic’ profile, which may be further optimized to overcome this inaccuracy. To prove that the measured efficiency distribution is attributed to the achieved topographical accuracy, we applied methods of rigorous-coupled-wave analysis (RCWA) to calculate the theoretical diffraction
Fig. 10. Measured diffraction efficiency of the beam splitter grating for two polarization directions (“measured TM” and “measured TE”) and the related rigorous-coupled-wave analysis (RCWA) simulated efficiencies (“Unigit TM” and “Unigit TE”).
efficiencies. Therefore, as a base for the periodic continuation, the profile of the center groove shown in Fig. 9 was selected. As the complex refractive index of the processed resist material a value of about n 1.67 0 · i was assumed. The resulting efficiencies of the RCWA calculations for the different diffraction orders are also printed in Fig. 10 (upward and downward oriented triangles). Overall simulation and measurement show good agreement with an average difference of 1% of the incident power and a maximum error of 4.6%. 4. Conclusions
In this contribution we present an alternative experimental setup for Talbot lithography, which reduces complexity, lowers cost, and allows an easy adaptation of the illumination system to different grating periods. Our ‘serial multipatterning method’ enables fine-tuning of the grating’s groove shape without physical changes in the setup. A useful enhancement would be automatic control of mirror tilt and the exposure shutter to let the machine perform the exposure procedure on its own. As has been shown before [8], Talbot lithography allows the generation of deep gratings that can be used directly in the visible spectral range. An additional proportional transfer via techniques such as reactive ion beam etching (RIBE) [2] is not necessary to obtain sufficient diffraction efficiency. This makes the presented tool suitable for the generation of custom-built gratings and a working tool for testing new grating geometries for optical devices. We express our deep acknowledgements to M. Burkhardt and R. Steiner from the company Carl Zeiss Jena for their intensive and valuable advisory. The authors thank the BMBF for sponsorship within the project “FREE” (grant 13N10823). D. Thomae expresses thanks to the Ernst-Abbe-Fachhochschule Jena for the possibility to take part in the Ph.D. program of the University.
References 1. E. G. Loewen, Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997). 2. R. Brunner, R. Steiner, K. Rudolf, H.-J. Dobschal, R. Fechner, and A. Schindler, “Deep-UV microscopy based on a hybrid diffractive/refractive lens system,” in Frontiers in Optics 2004/Laser Science XXII/Diffractive Optics and MicroOptics/Optical Fabrication and Testing (Optical Society of America, 2004), paper DSuC3. 3. R. Brunner, R. Steiner, K. Rudolf, and H.-J. Dobschal, “Diffractive-refractive hybrid microscope objective for 193 nm inspection systems,” Proc. SPIE 5177, 9–15 (2003). 4. H. F. Talbot, “Facts relating to optical science,” Philos. Mag. 9 (56), 401–407 (1836). 5. L. Rayleigh, “On copying diffraction-gratings, and on some phenomena connected therewith,” Philos. Mag. 11(67), 196– 205 (1881). 6. W. B. Case, M. Tomandl, S. Deachapunya, and M. Arndt, “Realization of optical carpets in the Talbot and Talbot-Lau configurations,” Opt. Express 17, 20966–20974 (2009). 7. O. Bryngdahl, “Image formation using self-imaging techniques,” J. Opt. Soc. Am. 63, 416–419 (1973). 8. T. Harzendorf, L. Stuerzebecher, U. Vogler, U. D. Zeitner, and R. Voelkel, “Half-tone proximity lithography,” Proc. SPIE 7716, 77160Y (2010). 9. L. Stuerzebecher, T. Harzendorf, U. Vogler, U. D. Zeitner, and R. Voelkel, “Advanced mask aligner lithography: fabrication of periodic patterns using pinhole array mask and Talbot effect,” Opt. Express 18, 19485–19494 (2010). 10. R. Voelkel, U. Vogler, A. Bich, P. Pernet, K. J. Weible, M. Hornung, R. Zoberbier, E. Cullmann, L. Stuerzebecher, T. Harzendorf, and U. D. Zeitner, “Advanced mask aligner lithography: new illumination system,” Opt. Express 18, 20968– 20974 (2010). 11. P. M. Mejías and R. Martínez Herrero, “Diffraction by onedimensional Ronchi grids: on the validity of the Talbot effect,” J. Opt. Soc. Am. A 8, 266–269 (1991). 12. E. G. Loewen, “11.2 spectral purity,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 402–413. 13. E. G. Loewen, “14.4.4 periodic errors,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 509–510. 14. J. Maass, O. Sandfuchs, D. Thomae, A. Gatto, and R. Brunner, “Effective and flexible modeling approach to investigate various 3D Talbot carpets from a spatial finite mask,” J. Eur. Opt. Soc. 8, 13004 (2013). 15. E. G. Loewen, “15.3 two-beam symmetrical recording,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 538–541.
20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1781
Ernst-Abbe-Fachhochschule Jena, Carl-Zeiss-Promenade 2, 07745 Jena, Germany 2 3
Carl Zeiss Jena GmbH, Carl-Zeiss-Promenade 10, 07745 Jena, Germany
Hochschule Hamm-Lippstadt, Marker Allee 76-78, 59063 Hamm, Germany *Corresponding author: daniel.thomae@fh‐jena.de
Received 6 December 2013; revised 6 February 2014; accepted 6 February 2014; posted 10 February 2014 (Doc. ID 202640); published 14 March 2014
A flexible illumination system for Talbot lithography is presented, in which the Talbot mask is illuminated by discrete but variable incidence angles. Changing the illumination angle stepwise in combination with different exposure doses for different angles offers the possibility to generate periodic continuous surface relief structures. To demonstrate the capability of this approach, two exemplary micro-optical structures were manufactured. The first example is a blazed grating with a stepsize of 1.5 μm. The second element is a specific beam splitter with parabolic-shaped grating grooves. The quality of the manufacturing process is evaluated on the basis of the optical performance of the resulting micro-optical elements. © 2014 Optical Society of America OCIS codes: (050.0050) Diffraction and gratings; (070.6760) Talbot and self-imaging effects; (220.3740) Lithography; (220.4000) Microstructure fabrication. http://dx.doi.org/10.1364/AO.53.001775
1. Introduction
For the manufacturing of spectroscopic master gratings, two main technologies have been established over the years [1]. The first of them is interference lithography (IL), where the grating is generated by exposing a resist coated substrate by an interference fringe pattern and a subsequent development process by which the latent structure is transferred into a continuous surface profile. IL has the advantage to produce a high-quality master grating in a single exposure step, but it is limited in the diversity of attainable structures. In particular, IL allows the creation of blaze-like grooves but depends on the grating period and exposure wavelength; it is limited in profile depth, which is typically in the order of 150 nm [2,3]. 1559-128X/14/091775-07$15.00/0 © 2014 Optical Society of America
The second well-established manufacturing technology of spectroscopic master gratings is classical ruling [1]. Here a cut diamond is directed line by line across the substrate, where deep blazed profiles are also engraved in the covering metal layer. Ruled blazed gratings offer higher diffraction efficiency compared to gratings manufactured by IL, but, due to the remaining roughness of the material and small local variations of the grating period, they also cause an increased amount of disturbing stray light. As an alternative, Talbot lithography offers the potential to overcome the limitations of both mentioned classical technologies, especially the limitation of attainable groove shapes with IL and the time consumption of ruling. Talbot lithography is based on the Talbot effect [4,5], in which a plane, monochromatic wave is used to illuminate a periodic amplitude mask. Behind the mask an extended intensity distribution (Talbot carpet) is formed, which was measured and documented by Case et al. in [6]. 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1775
For exposing the resist coated substrate, especially the self-imaging planes may be used, which are located at approximately z ≈ n · p2 ∕λ behind the mask [5], with p as the mask period, λ as the wavelength, and n 1; 2; 3; …. When using an amplitude line mask, the exposed and developed structure is a simple phase line grating. More complex structures can be generated by a variation of the illumination angle, which shifts the lateral position of the exposure pattern with respect to the photoresist. One of the first experiments that made use of this approach was reported by Bryngdahl in 1972 [7]. Bryngdahl illuminated an S-shaped source aperture, which created a specific angular spectrum of illuminating plane waves impinging on a two-dimensional periodic pinhole array mask. In accordance with the angle distribution the “S” form reappeared in the mask’s Talbot distances as a superposition of laterally shifted self-images. Recently, this basic principle was also transferred to lithography by Harzendorf et al. [8] and Stürzebecher et al. [9], where a commercial mask aligner with an advanced illumination system (AIS) [10] was applied. Due to a structured aperture stop within the AIS, a predefined plane wave angular spectrum illuminates the mask. This creates simultaneously a set of laterally shifted Talbot self-images forming an intensity distribution that is used to expose a photoresist coated on a substrate. Due to the simultaneous use of laterally shifted Talbot images for the resist exposure, this procedure is characterized as a parallel multipatterning technique. The parallel multipatterning technique requires a complex illumination system to ensure a homogeneous intensity distribution and homogeneous angular spectrum at each point of the mask. This requirement is, however, accompanied by a complex and expensive setup. For clarity in the designations we will use in the following the term “mask” for the periodic structure generating the Talbot pattern and “grating” for the exposed and developed final photoresist structure. In this contribution we present a simple to implement alternative approach for Talbot lithography, which is based on an adjustable and flexible illumination setup with subsequent illumination steps, which we call “serial multipatterning.” This approach allows a simplification of the setup, which now contains mostly standard optical parts. Since it is advisable to fit the dimensions of the illumination system to the mask period, using standard optical parts makes modifications of the system very easy. Furthermore, the serial approach allows easy fine-tuning of the grating’s groove shape without physical modifications of the exposure tool itself. 2. Concept and Implementation of Flexible Illumination System for Talbot Lithography
The basic setup comprises the illumination system, the mask, and a resist coated substrate (Fig. 1). Mask and substrate are integrated in a mechanical subunit 1776
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
Fig. 1. Schematic experimental setup consisting of source, collimator (filters), a tilting mirror, mask, and resist coated substrate: the source’s radiation is collimated by a condenser lens and may get additionally filtered. The remaining light is deflected by a mirror onto the mask. The Talbot carpet adjacent to the mask is used to expose the resist coated substrate. Due to different tilting angles of the mirror, the radiation’s incident angle onto the mask is varied. This shifts the exposure pattern (Fig. 2).
in which both are adjustable in distance and orientation with respect to each other. The structured mask and the resist plane have to be separated by the Talbot distance and have to be aligned parallel to each other. A crucial aspect of the setup concerns the illumination system: a point source of light in combination with a collimating lens generates a plane wavefront. This wavefront is directed toward a tiltable plane mirror, which then reflects the plane wave onto the mask. By changing the mirror inclination, the impinging angle on the mask can be varied, which leads to a lateral shift of the exposing spots in the resist plane. This principle is demonstrated schematically in Fig. 2. On the left of Fig. 2 the exposure step is shown at its beginning. The primarily selected inclination angle is associated with specific positions of the exposure spots in the resist. After the first exposure the inclination angle is changed step by step, and for each angle a precalculated exposure dose is applied to the resist. The dose applied to each lateral spot position is controllable by the exposure time; the intensity remains constant. This principle offers multiple degrees of freedom. With different ‘recipes’ comprising illumination angles and their corresponding exposure time, a broad diversity of continuous periodic surface relief
Fig. 2. Sequential exposure process—in each step the corresponding illuminating angle shifts the foci of the Talbot carpets and, therefore, subsequently exposes different positions on the resist. The dose per position is varied over the applied exposure time.
structures such as triangular-shaped diffractive gratings (Fig. 2 right) are accessible. From the generated dose distribution, a permanent surface structure is unveiled in the subsequent development process. Additionally, the exposing wavefront can be filtered by spectral band-pass filters, polarization filters, or further optical elements to tailor the illumination radiation to the desired demands. As the principal ray is fixed, the use of angle-sensitive elements such as interference filters is no problem. Although this setup exposes the resist in a serial process and step by step, it has to be mentioned that this approach allows the processing of all grating grooves, simultaneously. Especially time-consuming and failure-causing movements or stepping procedures between mask and substrate during the exposure process are not necessary. The serial approach requires a longer net exposure time as compared to the parallel process, but offers a high flexibility to adapt the illumination conditions to the response curves of the photoresist during the process. For example, on the base of a first lithographic result the exposure time for each illumination angle can be optimized with respect to the accurate resist-response curve, without any change in the system’s physical setup. As a first conclusion, this setup offers a flexible lithographic process to manufacture a wide variety of periodic continuous surface relief profiles. A.
Dimensioning of the Illumination System
The illumination setup, consisting of source, collimator, and tilting mirror, has to be dimensioned to the mask period p and the illumination wavelength λ. Figure 3 presents the illumination setup schematically highlighting specific geometrical details. In Fig. 3(a) the illuminated mask with the attached Talbot carpet and the exposing spots inside the resist layer is depicted. Due to the oblique illumination, with its principal ray impinging the mask under an angle βmax , the exposing spot pattern is deflected downward by the distance Δx relative to the optical axis. The lateral spot deflection Δx can be described in good approximation by the triangle formed by the
separation of mask and resist zMR , the illumination angle β, and the right angle between Δx and zMR . For generating arbitrary profile shapes, each position within one period has to be reached by an exposing spot. This means a lateral spot deflection of half of the mask’s period has to be guaranteed for both upward and downward directions. The corresponding angle βmax of the principal ray for maximum elongation is the largest illuminating angle that is needed to allow a complete and continuous structuring of the resist over the whole period. In the exemplary case, when the mask’s first selfimage plane at half the Talbot distance is used, it corresponds to a distance between mask and resist surface of zMR ≈ p2 ∕λ. For the angle βmax follows: p λ βmax ≈ arctan : (1) arctan 2 · zMR 2·p Figure 3(b) depicts the illumination system with the emitting area of the source on the left, the collimating lens on the right, and the principal rays originating from the opposite edges of the source. Due the source’s lateral extension L and the focal length f of the collimator, the illumination radiation is also associated with a cone angle α for the angular spectrum yielding L α arctan : (2) f In order to allow a high resolution in the lithographic process and to avoid disturbing broadening effects, it is important to choose a source extension and a focal length, such that the angle α is much smaller than βmax (α ≪ βmax ). Decreasing α by increasing the focal length f of the collimator reduces the intensity of the collimated wavefront approximately proportional to 1∕f 2 . In our experimental setup a light-emitting diode (LED) with a wavelength of around 400 nm (ProLight Opto PM2L-1LLx-LC) and a chip dimension of ≈1 mm2 was chosen. To approximate a point source with emitting spherical waves, it is important to use a LED with only a single chip. These spherical waves are transferred into plane waves by a collimating lens (300 mm plano–convex with 300 aperture diameter). The source’s edge length and the focal length result in an illumination angular spectrum extended about α 0.2° versus an angle βmax 1.15°. Although the concept of the setup is very simple, it allows the generation of high-quality plane wavefronts with a homogeneous intensity distribution and with sufficiently small wavefront aberrations. B. Potential Laser Sources
Fig. 3. (a) Maximum principal ray angle βmax for Talbot lithography and (b) angular spectrum’s extent of the illumination radiation formed by the collimator.
In principle, the introduction of a laser source in combination with a pinhole would allow a reduction of the emitting source area. Laser sources also have a higher technical potential as they are available 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1777
in a wavelength range below 300 nm, a spectral region where LEDs have not (yet) reached sufficient power and acceptable lifetimes. Shorter wavelengths reduce the aberrations in the intensity distribution of the Talbot carpet [11] and, therefore, offer a higher lithographic resolution in the resist plane [8]. Besides that, a shorter wavelength increases the Talbot distance and, therefore, reduces adjustment requirements [8]. The serial multipatterning concept, on the one hand, allows, in principle, the use of coherent sources such as lasers for Talbot lithography, and on the other hand, the coherence properties of laser light may negatively influence the exposed grating. In particular, the appearance of speckles has to be avoided. Another disadvantage of laser sources is the fine structured Talbot carpet produced by strictly monochromatic radiation. This induces a very small tolerance against adjustment errors of the distance between mask and resist. The use of polychromatic sources superimposes different Talbot carpets of many wavelengths and therefore smoothes the fine structure of the carpets shown in [6]. Due to the challenges involved with a laser, we used as a proof of concept the far more economical LED solution. 3. Experimental Results
To demonstrate the usability of the flexible illumination system for Talbot lithography, we applied the system to manufacture deep blazed spectral gratings and a diffractive beam splitter grating. In general, for the fabrication of high-quality spectral gratings some challenges have to be addressed. First, deviations from the grating’s fundamental periodicity generate diffraction ghosts, and roughnessinduced random errors are the origin for diffuse scattering effects. In particular, the occurrence of ghost diffraction orders is attenuated when using the grating in higher diffraction orders [12,13]. As a rule of thumb, diffractive gratings working in higher diffraction orders exhibit deeper profiles. In Talbot lithography deviations from the addressed fundamental periodicity occur because of the mask’s boundary that causes edge effects in the Talbot carpet [6,14]. Additionally, a variation of the periodicity is also related to the mask’s manufacturing technology where, e.g., disturbing stitching effects have to be minimized. Finally, particle contamination on the mask’s surfaces is an additional source for point defects. For our experiments we have used a 10 μm period dense line amplitude chromium mask created by electron beam lithography. A further challenge in the grating manufacturing process arises from the need to create a desired groove shape and keep this groove shape constant over the whole area of the grating. Using Talbot lithography, local variations in the attained groove shape are mainly caused by deviations in the distance between mask and photoresist—the maximum tolerable distance error is dependent on 1778
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
the specific mask structure and the illumination wavelength spectrum, and can be estimated by calculating the intensity carpet behind the mask [6,14], as was done in, e.g., [8]. Besides the residual tilting error between mask and substrate, which is below 2 arcmin in our setup, the minimal attainable distance error between mask and resist surface is given by the flatness of the mask, the substrate, and the resist layer. A. Manufacturing of Deep Blazed Gratings
For a proof of concept we concentrated on manufacturing deep blazed gratings in a 2.7 μm thick photoresist (MicroChemicals, AZ-1518) on a glass substrate (SCHOTT Borofloat 33). The target-groove shape was attained by eight exposures under different illumination angles at the first self-image distance of zMR ≈ p2 ∕λ. The gratings were developed and afterward coated with a thin layer of aluminum. An atomic force microscopy (AFM) scan of the groove profile after aluminum coating is shown in Fig. 4. Here, a distinct blaze-like profile is clearly observable with a periodicity of 10 μm and a depth of approximately 1.55 μm. The presented profile reveals a distinct sawtoothlike shape, but there are still further improvements of the processing details necessary. For example, effort can be applied to linearize the slope of the active blaze facet and create a sharper step on the passive blaze facet. Although a similar sawtooth-like profile could also be manufactured by IL, the profile depth of such an IL structure would be limited to a maximum depth smaller than approximately 200 nm [2,3]. And, therefore, it will either show a very low diffraction efficiency in the visible or near-infrared spectral range or has to be further treated, e.g., by reactive ion-etching processes. To evaluate the optical properties of the manufactured grating, we measured the diffraction efficiency in dependence on different diffraction orders. This was done in the setup shown in Fig. 5. The grating was illuminated by slightly converging laser light (He–Ne at 633 nm) under a chief ray angle of 22.45° relative to the grating normal in the classical mount (no conical illumination) (see dashed arrow in Fig. 5).
Fig. 4. AFM scan showing the topography of the blaze-like grating structure manufactured by serial Talbot lithography. The grating shows a lateral period of 10 μm and a depth of approximately 1.55 μm. The resist grating was coated with aluminum.
Fig. 5. Setup for efficiency measurement—only principal rays are drawn. The numbers denote the different diffraction orders.
The laser itself emitted nearly unpolarized radiation. The converging illumination (NA 0.03) forms a spot for each diffraction order, which was captured by an optical powermeter. The astigmatism induced for higher diffraction orders by this configuration increases the spot size, but it was still possible to capture the whole spot within the detector area. The diffraction orders, which are directed in angles close to the incident beam, were not accessible, because the housing of the measuring photodiode would shadow the incoming ray. Figure 6 shows the measured power for different diffraction orders, relative to the power incident to the grating. It has to be mentioned that the occurring faint ghosts are located very close to the main diffraction order and were also captured by the photodiode. About 46% of the incident power is diffracted into the target-diffraction orders (sixth and seventh), almost 8% at one diffraction order close to it (eighth) and around 24% at other main diffraction orders. The remaining 22% of the incident power is not assignable. This 22% may be located in the missing diffraction orders and gets lost in ghost orders, diffuse scatter, and absorption and transmission of the coating. Linearizing the active flank of the profile due to an increased number of exposure steps should result in a higher efficiency in the sixth and seventh orders. Besides the efficiency, the uniformity of the groove shape over the area of the grating is another quality criterion. For example, a height variation of the profile ranging from one side to the other would manifest in an inhomogeneous distribution of the diffraction efficiency. Therefore, we used the same illumination setup but with a plane wave instead of a converging wave. The far-field diffraction images at a diffusing screen were captured with a camera (Fig. 7). Since their intensity varies over more than one order of
Fig. 6. Diffraction efficiency of manufactured blaze grating.
Fig. 7. Far-field diffraction images of the grating with marked diffraction orders—the lower image was captured with eight times the exposure time of the upper one.
a magnitude, it was not possible to capture them all in one shot. The upper row was captured with a short exposure time, the lower one with a long exposure time. In this row the intensity distribution within the weak diffraction orders is also visible, but the bright orders are overexposed. The zeroth diffraction order shows as a characteristic bright circle with a dark square in its center. This feature results from the circular illumination of an area that slightly exceeds the dimensions of the square-shaped grating. The unstructured aluminized surface surrounding the grating is directly reflecting the incident light (bright frame), and the grating diffracts light into higher diffraction orders (dark square). A prominent feature (in Fig. 7) is the concentration of the diffracted power into the sixth, seventh, and eighth diffraction orders. Regarding inhomogeneity a small spot in the grating’s center appears in the fifth, seventh, and eighth diffraction orders, which originates from a reflex focused at the resist layer during pre-exposure. The spot is slightly brighter than the surrounding region for the fifth and sixth orders and darker in the seventh order. In the far-field pattern of the seventh order an intensity gradient is visible that shows a reduced diffraction efficiency in the lower part of the grating. In other diffraction orders, such as the ninth or the fifth, even stronger intensity gradients are observed. The applied inspection of the grating’s far-field distribution is not appropriate for a precise conclusion of the origin of the disturbing effects, but the method allows a simple identification of the trouble-causing area. As a last quality criterion the wavefronts of the gratings’ diffraction orders were measured. For illumination, again, a plane wave originating from a He–Ne laser was applied. The grating was tilted so that the selected diffraction order was deflected at an angle of about 22° relative to the incident ray. The wavefront of this diffraction order was then captured by an Optocraft SHSCam HR-159-GE Shack–Hartmann sensor with a 6.25× telescope in front of it. Hereby difficulties, which occur in the presence of wavefronts with distinct intensity fluctuations, have to be handled. Especially, it has to be avoided that some microlens spots are overexposed while others have not yet reached a sufficient signal level. For our analysis we examined the first, fourth, sixth, seventh, and eighth diffraction orders. From the measured wavefront values the three Zernike 20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1779
Fig. 8. Measured wavefront for the order noted above, peak-tovalley (PV) and RMS in micrometers. The black dot in the images’ center is caused by the wavefront sensor.
coefficients tilt-x, tilt-y, and defocus were subtracted, since they are easily correctable by adjustment. The corrected wavefronts for different diffraction orders are displayed in Fig. 8. The measured peakto-valley (PV) error is in the range of 1.3–1.5 μm and for the RMS wavefront error a value of about 200 nm was found. Both values rise moderately for higher diffraction orders. The wavefront error of a reflection grating is separable into two influence factors, first, the flatness of the substrate and, second, the global spacing error of the grating grooves over the entire substrate. The substrate contribution causes an error that is independent of the diffraction order; the latter error increases linearly [15] with increasing diffraction orders. A comparison of all diffraction orders shows RMS and PV wavefront aberrations that are all in the same range, and also the shape of the wavefront error is similar in all of these cases. Moreover, in each displayed wavefront error the faint spot in the center and also the heights and depths near the borders of the grating remain at the same position. Hence, we assume that for this grating the wavefront aberrations are mainly caused by the flatness error of the substrate and its resist layer. B.
Manufacturing Diffractive Beam Splitters
Besides spectroscopy, gratings can also be used as diffractive beam splitters for monochromatic radiation. A special light distribution characteristic is offered by a beam splitter with a parabolic groove profile. Such an element allows the uniform distribution of the incident light across several diffraction orders, e.g., from the −10th to the 10th order. To manufacture such a beam splitter, we applied Talbot lithography with the flexible mask illumination setup. As a basic mask, again, the 10 μm periodic amplitude structure was used. To generate a parabolic groove shape, we exposed the resist coated substrate in nine illumination steps, where the exposure time was calculated from the dose-dependent resist-response curve. As a photoresist AZ1518 (MicroChemicals) was selected and the exposure source was again the 400 nm power LED. The AFM cross section of the resulting groove profile with an approximate height of 2.5 μm is displayed in Fig. 9. To minimize disturbing or distorting convolution effects between the sample profile and the AFM tip, a high-aspect-ratio probe (Olympus AC 160 TS) was used for this specific measurement. 1780
APPLIED OPTICS / Vol. 53, No. 9 / 20 March 2014
Fig. 9. AFM cross section showing the topography of the parabolic-shaped grooves of the beam splitter grating manufactured by serial Talbot lithography. The grating possesses a lateral period of 10 μm and a depth of approximately 2.5 μm.
In essence, the topography shows that the target parabolic shape is almost achieved. Significant deviations occur in the turning points; e.g., the top area is flattened too much and the lower turning points show rather a rounded shape compared to the tapered transition of intersecting neighboring parabolas. For an optical characterization the diffraction efficiency of this structure was measured in transmission by using a 633 nm He–Ne laser of approximately 1 mm beam diameter and at normal incidence. Figure 10 displays the diffraction efficiency relative to the incident power, separately, for TE and TM polarization. TE polarization is defined by the electrical field oscillating in the direction of the grating grooves. The 1st and 7th orders show efficiencies around 10%, the 3rd, 4th, 6th, and 8th around 6%, and all other orders are below 2.5%. With respect to the target of a uniformly distributed efficiency, especially the values for the zeroth, 2nd, and 5th orders are too low. The deviation may be attributed to the imperfect ‘parabolic’ profile, which may be further optimized to overcome this inaccuracy. To prove that the measured efficiency distribution is attributed to the achieved topographical accuracy, we applied methods of rigorous-coupled-wave analysis (RCWA) to calculate the theoretical diffraction
Fig. 10. Measured diffraction efficiency of the beam splitter grating for two polarization directions (“measured TM” and “measured TE”) and the related rigorous-coupled-wave analysis (RCWA) simulated efficiencies (“Unigit TM” and “Unigit TE”).
efficiencies. Therefore, as a base for the periodic continuation, the profile of the center groove shown in Fig. 9 was selected. As the complex refractive index of the processed resist material a value of about n 1.67 0 · i was assumed. The resulting efficiencies of the RCWA calculations for the different diffraction orders are also printed in Fig. 10 (upward and downward oriented triangles). Overall simulation and measurement show good agreement with an average difference of 1% of the incident power and a maximum error of 4.6%. 4. Conclusions
In this contribution we present an alternative experimental setup for Talbot lithography, which reduces complexity, lowers cost, and allows an easy adaptation of the illumination system to different grating periods. Our ‘serial multipatterning method’ enables fine-tuning of the grating’s groove shape without physical changes in the setup. A useful enhancement would be automatic control of mirror tilt and the exposure shutter to let the machine perform the exposure procedure on its own. As has been shown before [8], Talbot lithography allows the generation of deep gratings that can be used directly in the visible spectral range. An additional proportional transfer via techniques such as reactive ion beam etching (RIBE) [2] is not necessary to obtain sufficient diffraction efficiency. This makes the presented tool suitable for the generation of custom-built gratings and a working tool for testing new grating geometries for optical devices. We express our deep acknowledgements to M. Burkhardt and R. Steiner from the company Carl Zeiss Jena for their intensive and valuable advisory. The authors thank the BMBF for sponsorship within the project “FREE” (grant 13N10823). D. Thomae expresses thanks to the Ernst-Abbe-Fachhochschule Jena for the possibility to take part in the Ph.D. program of the University.
References 1. E. G. Loewen, Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997). 2. R. Brunner, R. Steiner, K. Rudolf, H.-J. Dobschal, R. Fechner, and A. Schindler, “Deep-UV microscopy based on a hybrid diffractive/refractive lens system,” in Frontiers in Optics 2004/Laser Science XXII/Diffractive Optics and MicroOptics/Optical Fabrication and Testing (Optical Society of America, 2004), paper DSuC3. 3. R. Brunner, R. Steiner, K. Rudolf, and H.-J. Dobschal, “Diffractive-refractive hybrid microscope objective for 193 nm inspection systems,” Proc. SPIE 5177, 9–15 (2003). 4. H. F. Talbot, “Facts relating to optical science,” Philos. Mag. 9 (56), 401–407 (1836). 5. L. Rayleigh, “On copying diffraction-gratings, and on some phenomena connected therewith,” Philos. Mag. 11(67), 196– 205 (1881). 6. W. B. Case, M. Tomandl, S. Deachapunya, and M. Arndt, “Realization of optical carpets in the Talbot and Talbot-Lau configurations,” Opt. Express 17, 20966–20974 (2009). 7. O. Bryngdahl, “Image formation using self-imaging techniques,” J. Opt. Soc. Am. 63, 416–419 (1973). 8. T. Harzendorf, L. Stuerzebecher, U. Vogler, U. D. Zeitner, and R. Voelkel, “Half-tone proximity lithography,” Proc. SPIE 7716, 77160Y (2010). 9. L. Stuerzebecher, T. Harzendorf, U. Vogler, U. D. Zeitner, and R. Voelkel, “Advanced mask aligner lithography: fabrication of periodic patterns using pinhole array mask and Talbot effect,” Opt. Express 18, 19485–19494 (2010). 10. R. Voelkel, U. Vogler, A. Bich, P. Pernet, K. J. Weible, M. Hornung, R. Zoberbier, E. Cullmann, L. Stuerzebecher, T. Harzendorf, and U. D. Zeitner, “Advanced mask aligner lithography: new illumination system,” Opt. Express 18, 20968– 20974 (2010). 11. P. M. Mejías and R. Martínez Herrero, “Diffraction by onedimensional Ronchi grids: on the validity of the Talbot effect,” J. Opt. Soc. Am. A 8, 266–269 (1991). 12. E. G. Loewen, “11.2 spectral purity,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 402–413. 13. E. G. Loewen, “14.4.4 periodic errors,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 509–510. 14. J. Maass, O. Sandfuchs, D. Thomae, A. Gatto, and R. Brunner, “Effective and flexible modeling approach to investigate various 3D Talbot carpets from a spatial finite mask,” J. Eur. Opt. Soc. 8, 13004 (2013). 15. E. G. Loewen, “15.3 two-beam symmetrical recording,” in Diffraction Gratings and Applications, Vol. 58 of Optical Engineering (Dekker, 1997), pp. 538–541.
20 March 2014 / Vol. 53, No. 9 / APPLIED OPTICS
1781
