Wavefront sensor sampling plane fabricated by maskless grayscale lithography

Transcription

1 Wavefront sensor sampling plane fabricated by maskless grayscale lithography G.A. Cirino * a, F.T. Amaral b, S.A. Lopera c, A.N. Montagnolil a, A. Arruda d, R.D. Mansano c, T.M-Brahim e, D.W.L. Monteiro b, a DEE - Univ. Federal de Sao Carlos, SP, Brazil; b DEE Univ. Federal de Minas Gerais, MG, Brazil; c PSI Univ. de Sao Paulo, SP, Brazil; d Independent Consulting; e IETR - Univ. Rennes I, Rennes, FR ABSTRACT In this work we report on the design and characterization of Shack-Hartmann wavefront sampling plane based on a microlens array (MLA) with 12 X 12 hexagonal contiguous diffractive lenslets, a pitch of 355 µm, a focal length of 4.5 mm, and lateral dimensions 4.3 X 4.3 mm 2. The device was fabricated by maskless grayscale lithography. Preliminary optical characterization was carried out using a He-Ne laser source (λ = 633 nm), by evaluating the intensity distribution of all spots generated at the MLA focal plane, I MAX, as well as their sharpness by measuring full width at half maximum (FWHM) intensity values. The average resulting values were FWHM AVG = 16 ± 1.4 µm and I MAX_AVG = 0.83 ± 0.05 a.u. AFM characterization was performed within a region 10 X 10 mm 2 comprising the center of a microlens and the resulting RMS roughness was 6.87 nm (λ /92). A comparison between theoretical and measured intensity profiles at the MLA focal plane was also carried out. A good correspondence between the results was found. An effective optical characterization was carried out (also at λ=633 nm) in order to determine wavefront aberrations from Zernike polynomials by introducing a wavefront with a well-known induced aberration, such as defocus or spherical aberration. For the wavefront reconstruction, the modal approach was used, in which the first derivatives of Zernike polynomials are used as the set of orthogonal basis functions. The corresponding polynomial coefficients up to the first 10 Zernike terms were obtained and the resulting reconstructed wavefront presents an RMS reconstruction error compliant to most optical systems of interest. Keywords: Digital Light Projector, Wavefront sensor, Microlens Array, Fresnel microlens, Maskless Lithography, Shack-Hartmann Sensor 1. INTRODUCTION In order to address a well-controlled surface microrelief for micro-optical components such as microlenses, diffraction gratings and computer-generated holograms, a suitable tool for 3-D pattern generation is required. In these fabrication processes it is clear that photolithography is a key technology for generation of 3D profiles. One important demand is the fabrication of MicroLens Array (MLA). Digital Light Projector (DLP) technology has been employed for microstructures pattern transfer based on expensive commercial equipment [1,2]. There are a great number of techniques for MLA fabrication reported in the literature, once this subject has been studied since the middle eighties, such as photoresist reflow [9-13], inhomogeneous exposure of photoresist [5], focused ion beam on hard material that act as mold for plastic replication [14], lithography in proximity mode on thick photoresist exploring the diffraction properties of light beneath the photomask [15-17], soft lithography [18], use of a low-cost technique based on 35-mm slide imagers and subsequent photo reduction [19], among others. * gcirino@ufscar.br; phone ; ufscar.br

2 Digital Light Projector (DLP) technology has also been employed for microstructures pattern transfer. Binary line-space features of 1.8 µm have been consistently resolved by Chan et al [20], which enables the fabrication of micro-optic devices with binary surface relief, such as binary-phase modulation computer-generated holograms or Fresnel zone plates. Totsu et al reported the fabrication of smooth refractive noncontiguous MLA of 100 µm in diameter on photoresist [21] and also on silicon after reactive ion etching [22]. These works are based on relatively expensive, commercially available equipment and the generated phase profiles are binary or a refractive version of MLA generated by the superposition dose from 16 layers. In this work a diffractive continuous-phase modulation structure consisting of contiguous 8X8 Fresnel microlens array, fabricated by the employment of a relatively high-speed process, low-cost home-built maskless lithography exposure system is presented. Grayscale exposure patterns were digitally generated, binarized to obtain a 256-layered set of subimages and superposed on a photoresist-coated substrate, layer-by-layer, by the maskless exposure system. Another important aspect, which is mandatory for mass production, is the employment of low-cost replication techniques to fabricate inexpensive MLAs on polymer materials. PolyDiMethylSiloxane (PDMS) is the most widely used siliconbased organic polymer. It is suitable in photonics applications because it is optically clear, and can be synthesized in a range of refractive index between 1.4 and 1.65 depending on its chemical structure engineering [3]. An effective optical characterization was carried out (also at λ=633 nm) in order to determine wavefront aberrations from Zernike polynomials by introducing a wavefront with a well-known induced aberration, such as defocus or spherical aberration. For the wavefront reconstruction, the modal approach was used, in which the first derivatives of Zernike polynomials are used as the set of orthogonal basis functions. The corresponding polynomial coefficients up to the first 10 Zernike terms were obtained and the resulting reconstructed wavefront presents an RMS reconstruction error compliant to most optical systems of interest. 2. LENS DESIGN AND FABRICATION Each microlens of the Fresnel MLA proposed in this work present a pitch of 355 µm and focal distance of 4.5 mm. The complex-valued thin converging lens optical transmittance, t L, has a phase distribution, generated by using paraxial approximation with a quadratic phase factor given by [24, 25]: k t L (x, y) = exp[ jφ L (x, y)] = exp j 2 f (x2 + y 2 ) ; k = 2π λ (1) where (x,y) is the two-dimensional coordinates of the lens, f is its focal distance, k is the wave number, φ L (x,y) is the lens phase modulation function and λ is the operating wavelength. In order to implement a diffractive version of the lens, the phase function φ L (x,y) is wrapped to an interval between 0 and an integer multiple of 2π, given by [25] φ N 2π (x, y) = [φ L (x, y)]mod N.2π (2) where φ Ν2π (x,y) is the wrapped phase function and N is a positive integer (0 < φ Ν2π < N.2π). The surface relief profile was implemented by generating a variation of the thickness, h(x,y), on PDMS material with known refractive index at the operating wavelength, n PDMS. The thickness variation h(x,y) is related to the phase profile φ Ν2π (x,y) by

3 h(x, y) = φ N 2π (x, y) λ 2π (n PDMS 1) (3) Figure 1a shows the phase distribution of the central part of an element of the lens array and the cross section of the surface relief profile; figure 1b shows the entire lens array elements arranged in hexagonal configuration. The maximum height of the microrelief is 1.12 µm, considering equation 3 with N=1 and n PDMS = 1.56 at the visible region of spectrum [23]. Fig. 1 (a) The phase distribution of an element of the diffractive continuous-phase lens array elements arranged in hexagonal configuration. The maximum height of the microrelief is 1.12 µm. The schematic view of a low-cost home-built maskless exposure system used in this work is shown in figure 2. The system is composed by a 200 W arc mercury lamp as the UV light source, a set of mirrors, lenses and other optical and electronic components. The key component of this system is a Digital Micromirror Device (DMD) 0.55 XGA DMD from Texas Instruments, which has a 0.55-inch diagonal spatial light modulator of aluminum-coated micro-mirrors. Fig. 2 Schematic view of the employed low-cost home-built grayscale maskless exposure system.

4 It continuously generates the image frames by reflecting the UV light pixel-by-pixel and exposes the two-dimensional UV dot array patterns on a photoresist-coated substrate. The substrate lies on top of a 3-axis computer-controlled stage. The DMD consists of 1024 X 768 pixels array, in a square grid pixel arrangement, having a pitch size of 10.8 µm. To achieve the grayscale photolithography regime, superposition of multiple UV dose generated by layered patterns was adopted. Figure 3 shows the schematics of the proposed grayscale photolithography scheme with 256 layers, generated by a bitmapped image on a positive resist. The exposure data consisting of a series of the UV image frames were exposed layer by layer. A photoresist layer (AZ1518 from Clariant) is deposited on top of a Si substrate and patterned by using the proposed home-built maskless grayscale lithography system. The photoresist was spin coated on a 3-inch silicon substrate at 2500 rpm during 20 s, and soft backed, at 105 o C during 90 s resulting in a 1.2 µm thick resist layer. The sample was then submitted to the exposure step, as described in previous section. 256 frames were exposed sequentially at a rate of 39 msec/frame. The total exposure time for each element of the lens array was 9.98 s. The X-Y stage takes on average 200 ms to repositioning and aligns for a new lens of the array. Therefore the total exposure time for an array, as shown in figure 1b, was 651 s. This exposure time is relatively short, enabling high throughput or fast prototyping. The maximum superposed UV dose was 450 mj/cm 2. Table 1 shows the cumulative dose along exposure time. After exposure, the sample was submitted to development step with the HPRD-402 positive resist developer (from OCG Microelectronics Materials Inc.), diluted in de-ionized water during approximately 50 s. Table 1 Cumulative dose as a function of exposure time. % Exposure Time Cumulative UV Dose [ mj/cm 2 ] Figure 3 shows a micrograph of the MLA fabricated as described above. Fig. 3 Micrograph of the fabricated MLA employing the maskless exposure lithography tool.

5 3. INTRAOCULAR LENS CHARACTERIZATION The fabricated MLA was used as a sampling plane for a Shack-Hartmann wavefront sensor, depicted schematically in figure 4. FELIPE, POR FAVOR, INSERIR AQUI UMA EXPLICAÇÃO DO SETUP E UM DESENHO ESQUEMÁTICO DO MESMO. In order to determine the dioptric power of PMMA intraocular lenses fabricated by Mediphacos was submitted to evaluation of its dioptric power. The results was compared with a comercial equipment IOLA from Rotlex. The lens dioptric power in the air, DP AIR, is determined by DP AIR = M L λ(4c 20 12C 40 ) (D DEC / 2) 2 (4) where λ is the operating wavelength, C 20 and C 40 are the Zernike polynomial coefficients representing defocus and spherical aberration, respectivelly. The parameter D DEC is the decomposition diameter (in milimeters), which represents the diameter the wavefront will be reconstructed. Five different IOL was characterized, each one was measured five times. Table 3 shows the comparative results. One can note that all comparative measurements present error less than 1%, which is quite satisfactory. 4. CONCLUSION This work presents the design, fabrication and characterization of contiguous hexagonal Fresnel f/# = f/15 MLA, with 300 µm diameter and 4.5 mm focal length. The device was fabricated by employing a grayscale maskless projection system. A mold in photoresist was generated followed by a replication process in PDMS elastomer. The mold generation takes 10.8 minutes to fabricate a 4.3 X 4.3 mm 2 device with 2.5 µm resolution, enabling rapid prototyping. Phase relief characterization for both photoresist mold as well as PDMS replica showed that the resulting dimensions agree with the designed lens. Such MLA can be used as wavefront sampler of a Shack-Hartmann wavefront sensors, in optical interconnects and to enhance the efficiency of detector arrays. The fabricated MLA was used in an optical setup in order to determine the dioptric power of PMMA intraocular lenses. The comparative measurements present error less than 1%.

6 Table 3 Comparative measurements between the proposed setup and a comercial equipment IOLA from Rotlex.

7 REFERENCE LINKING SPIE is able to display the references section of your paper in the SPIE Digital Library, complete with links to referenced journal articles, proceedings papers, and books, when available. This added feature will bring more readers to your paper and improve the usefulness of the SPIE Digital Library for all researchers. Denote reference citations within the text of your paper by means of a superscript number. List references at the end of the paper in numerical order, and enclose the reference number in square brackets. Include the following information (as applicable). If you use this formatting, your references will link your manuscript to other research papers that are in the CrossRef system. Exact punctuation is required for the automated linking to be successful. book: [1] Booth, N. and Smith, A. S., [Infrared Detectors], Goodwin House Publishers, New York & Boston, (1997). journal paper: proceedings paper: [2] Davis, A. R., Bush, C., Harvey, J. C. and Foley, M. F., "Fresnel lenses in rear projection displays," SID Int. Symp. Digest Tech. Papers 32(1), (2001). [3] Van Derlofske, J. F., "Computer modeling of LED light pipe systems for uniform display illumination," Proc. SPIE 4445, (2001). website: [4] Myhrvold, N., Confessions of a cybershaman, Slate, 12 June 1997, < (19 October 1997). REFERENCES [1] Booth, N. and Smith, A. S., [Infrared Detectors], Goodwin House Publishers, New York & Boston, (1997). [2] Davis, A. R., Bush, C., Harvey, J. C. and Foley, M. F., "Fresnel lenses in rear projection displays," SID Int. Symp. Digest Tech. Papers 32(1), (2001). [3] Van Derlofske, J. F., "Computer modeling of LED light pipe systems for uniform display illumination," Proc. SPIE 4445, (2001). [4] Myhrvold, N., Confessions of a cybershaman, Slate, 12 June 1997, < (19 October 1997). [5] Jones, C. J., Director, Miscellaneous Optics Corporation, interview, Sept [6] FamilyName, GivenName Initial., "Title," Source, pg# (year).