VUV and soft x-ray diffraction gratings fabrication by holographic ion beam etching

Transcription

1 VUV and soft x-ray diffraction gratings fabrication by holographic ion beam etching Xiangdong Xu, Yilin Hong, Shaojun Fu National Synchrotron Radiation Laboratory, University of Science and Technology of China/ P.O. Box 6022, Hefei, Anhui , China ABSTRACT Holographic and ion beam etching technique has become routine means for fabricating vacuum ultraviolet and soft x-ray diffraction gratings. A novel technique has been successfully, in which oxygen reactive ions etching was used to achieve resist ashing of the grating, to fabricate diffraction gratings with holographic ion beam etching. The new technique was used to fabricate a spherical blazed grating, 1200g/mm and 130 nm blazed wavelength, and some laminar gratings for monochromators in the beamline of National Synchrotron Radiation Laboratory. The results show that the new technique can considerably lower the stringent requirements of holographic exposure and development, and makes it controllable to make smooth grooves with desirable depth and duty cycle. A gold transmission grating is one of the critical elements in the soft x-ray spectrometer for plasma diagnostics. With holographic-ion beam etching technique, a number of self-supporting transmission gratings have been fabricated for inertial confinement fusion (ICF) diagnosis. Keywords: holographic lithography, ion beam etching, blazed grating, laminar grating, self-supporting gold transmission grating 1. BLAZED GRATINS 1.1. Introduction A spherical blazed grating is one of the most important optical components used for a Seya-Namioka monochromator in synchrotron radiation beamline in vacuum ultraviolet wavelength range. It must be replace after significant degradation in diffraction efficiency due to carbon contamination [1]. The grating used in Synchrotron radiation is usually expensive price, long delivery times and extremely variable quality, so National Synchrotron Radiation Laboratory invested considerable resources in instruments for holographic ion beam etching grating manufacture. The classical ruled grating has a triangular profile [2]. By using ruling diamonds with different facet angles it is possible to change the slope of the grooves and thereby obtain different blaze wavelengths. The holographic technique has various advantages over conventional ruling techniques, e.g. low cost, no ghost, less stray light, high groove positioning accuracy, etc. Until now three methods have been successfully applied to the production of blazed holographic gratings. The first method [3] is to utilize standing waves to form nodal planes in the photoresist. This method, however, has the disadvantage that the blaze angle is restricted by the available light wavelengths. The second method [4] is to expose the photoresist to a sawtooth variation of intensity by means of Fourier synthesis. The main difficulty in making blazed gratings in this way is to set the angles between two or more sets of beams much better than is practically attainable. The third method [5] is to make blazed holographic gratings by ion beam etching, in which a holographic grating with a sinusoidal profile is transferred into the substrate by oblique ion beam etching. Ion beam etching conditions is easily controlled electrically and the precise control of the groove profiles which promises higher groove efficiency is expected. In this paper a simple and useful method to make good blazed holographic gratings by combining holographic-ion beam etching with oxygen reactive ion etching technique is reported and its characteristics are described. *xxd@ustc.edu.cn; phone ; fax Holography, Diffractive Optics, and Applications II, edited by Yunlong Sheng, Dahsiung Hsu, Chongxiu Yu, Byoungho Lee, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2005) X/05/$15 doi: /

2 1.2. Fabrication The fabrication process of a blazed grating using holographic exposure and ion beam etching is shown schematically in Fig.1.The fabrication process steps are: (1) The preparation of substrates must be extreme care. After cleaning with acetone, the substrate is baked at 150ºCfor 1h.Then the substrate is spin-coated Shipley S1400 photoresist with a thickness of about 0.4µm. The sample is baked at 85ºC for 30min. to remove the solvent, which could cause erratic variation in resist sensitivity. (2) The grating pattern in S1400 is produced by holographic exposure using an argon laser (wavelength, nm). The sample is developed by 5 sodium hydroxide solution as developer for 3~5min., after which it was immediately rinsed in deioned water and dried. (3) The sample with grating relief is corrected using oxygen reactive ion etching (RIE) (4) A sawtooth shaped groove is obtained by permitting the argon ion beam to strike the mask at an angle consistent with the requirements of the final groove angle. After etching, the sample with a sawtooth shaped groove is cleaned carefully. (5) Ion beam etching grating is coated with aluminum film under optimal thermal evaporation conditions. (a) photoresist coating b) holographic exposure and development (c) O 2 reactive ion etching (d) Ar ion beam etching Fig.1. Fabrication process of a blazed grating using holographic and ion beam etching 1.3. Results The bare surface of the grating was characterized using a Dimension TM 3100 Scanning Probe Microscopy (SPM) from Digital Instruments. Figure 2 shows a triangular profile, indicating the capability of the ion beam etching technique to produce blazed triangular gratings. Figure 3 shows the performance of a 1200g/mm aluminum-coated ion beam etching grating measured at NSRL. 166 Proc. of SPIE Vol. 5636

3 Fig.2. SPM photograph of the bare blazed grating of 1200g/mm Relative Intensity (a.u.) Wavelength(nm) Fig.3. Relative intensity measured on the 1 m Seya-Namioka monochromator as a function of the photon wavelength at NSRL 1.4. Discussion The main parameters influencing the efficiency of a blazed grating are the constancy of the blaze angle and the flatness of the facets. In addition to lines of resist grating straight and smooth, its height must be appropriate for the certain blaze angle and the flatness. The relationship of the groove depth and the blaze angle as follows [6] : h dtan θ b (1) Where h is groove depth, d grating constant, θ b the blaze angle. The fabrication of high efficiency diffraction grating by holographic ion beam etching techniques is a multi-step operation, each step of which is subject to process variation that can affect reproducibility and consistent attainment of optimum efficiency. For maximum efficiency and quality, appropriate exposure and development are essential. Variations in the characteristic of the resist necessitate monitoring development as it proceeds. We obtain the two coherent interfering beams by the division of wave front, in which one half of the wavefront pursues a different optical path to the other half. The grating constant d depends on the angle of intersection of the beams and the wavelength of Proc. of SPIE Vol

4 light. The groove depth h and the ratio of land-to-groove depend on the initial resist film thickness, exposure dosage and development conditions. The blaze angle θ b depends on the angle of ion beam incidence Holographic exposure and development In principle, a holographic grating manufacture is easy. When two coherent beams of light intersect they generate a series of interference fringes. These may then be recorded in a photoresist and used to form the grooves of the grating. Most peoples are likely to be acquainted with the advantages of holographic grating absence of ghosts, higher SNR, a faster production process, etc. However, rarely is it pointed out that a number of specific problems exist in holographic recording that can readily reduce these advantages to zero. For example, a weak reflections from the Fourier lens interface could result in attaching unwanted fringes into the grating, because such reflections act as additional light sources coherent to the main recording beams, as illustrated in Fig.4. Figs.4 (a) and 4(b) are from same sample: 4(a) is an optical microscope image which focal plane is adjusted to the top surfaces of resist grating and we can only find out additional fringes. 4(b) is a SEM image, a considerable degree of bending is present especially at the sidewalls, which could be caused by the additional fringe intersection. (a) (b) Fig.4. Micrograph of resist grating (1200l/mm). (a) optical microscope photograph shows additional fringes. (b) SEM photograph of a resist grating. Groove shape is formed during the development of the exposed resist and line height or groove depth depends on the exposure dose, developer concentration, temperature, and developing time. These dependencies were investigated thoroughly and, in general, a desired groove depth value could be obtained by a proper choice of developing time with other conditions fixed. However, in practice, deviations in the exposing and developing process from the idea often alter results unpredictably. In our opinion, both exposure and development process control are most important in the photoresist grating fabrication. If exposure dose is appropriate, local defects may be dropped lowermost level. The criterion of appropriate exposure dose is large tolerance in development time. As overexposure, it is very difficult to determine endpoint because development time is very short, and etch depth is very sensitive to a minute difference of exposure dose over all exposed areas, so large-area and uniform grating could not be made. As underexposure, development time is very large, which results in discontinuous and roughness increasing over all grating areas. From observations, we worked out a complete real time monitoring criterion of endpoint detection of resist development for all areas exposed at one time 7]. This knowledge allows us to manufacture a large area and uniform grating O 2 RIE It is very difficult to eliminate the diffraction from dust particles and other blemishes on optical surfaces, bubbles or other inclusions within optical elements. These defects have a deleterious effect on the quality of the grating pattern. In order to obtain optimal groove shape resist relief grating as a mask, we apply oxygen RIE or a photoresist ashing process [8] to eliminate the resist grating defects. 168 Proc. of SPIE Vol. 5636

5 (a) (b) Fig.5 Comparison of resist grating before ashing (a) and after ashing (b). First, it was necessary to reduce the exposure or developing time for defining uniform grating relief pattern. Then, a plasma etching system (13.56MHz) was used to ash the patterned photoresist. Optimal controllability and uniformity of the ashing process was observed for an oxygen flow of 50SCCM and a RF power of 50 W. The merit of ashing effect is shown in Figs. 5(a) and 5(b), respectively. When the blaze angle of 1200l/mm grating is 4.5º, the groove depth of a resist grating mask ought to be 48 nm by equation (1). It is very difficulty to obtain directly the precise groove depth by wet erosion. However, the precise groove depth (shown in Fig.6) may be obtained by oxygen RIE after holographic exposure and development. (a) (b) Fig.6. SPM photograph of resist grating after ashing Ion beam etching The evolution of an asymmetric triangular groove profile by ion beam erosion has been investigated thoroughly by L.F.Johnson[ 9-10]. Ion beam etching or ion milling was performed using a 15 cm Kaufman type ion source at ion energy 400eV and ion current intensity 80mA for the argon etching. Three are three aspects of the problem needing attention in ion milling. First, a sample grating on the stage was cooled by water to prevent the sample from heating. Then, the direction of the ion beam was perpendicular to the groove direction and proper angle that selected to control grating profiles. Last, the real milling rates must be measured by experiments under enactment conditions due to angular dependence of the erosion rates of S1400 photoresist and substrate (K9 glass). It is one of the main difficulties in making blazed gratings in this way. The typical blazed holographic grating by the ion beam etching technique is shown in Fig. 2. Proc. of SPIE Vol

6 1.5. Conclusions The fabrication technique of VUV blazed gratings using holographic exposure, oxygen RIE and argon ion beam etching has been developed for synchrotron radiation application. The function of oxygen RIE is to improve resist grating mask edge roughness and to control the land-to-groove width ratio. The fabrication technique developed here is relatively simple because the conditions of RIE could be controlled easily and rigorous demands on holographic exposure and development could be reduced. We obtained blazed holographic grating of the size of about mm 2 by this technique. 2. LAMINAR GRATINGS 2.1. Introduction Metrology and Spectral Radiation Standard (U27) is one of eight beamline and station of Phase Project at NSRL. This beamline is designed to study light source transferring standard, detector transferring standard and optical element measurement. It includes two branches from one bending magnet. In one branch, SR is focused by a toroidal mirror and then monochromized by a spherical grating monochromator (SPM) for optical element test. The other branch, SR is monochromized by a 1m Seya-Namioka monochromator for light-source transferring standard. There are three pieces of laminar gratings at SPM. G 1 (1800g/mm) and G 2 (600g/mm) gratins for spectral ranges 4~36 nm were manufactured by Jobin-Yvon. G 3 (200g/mm) grating for spectral ranges 36~120 nm was fabricated in this work. Laminar or lamellar grating is a phase grating, which is an important dispersion component for VUV and soft x-ray region. This type of grating with rectangular profiles utilizes interference between waves diffracted from both the groove and the land, by appropriate choice of the groove depth h and the incidence angle, the two contributions can be caused to destructively interfere in the zero order so that more energy is available for the other orders [11] Fabrication The fabrication process of a lamellar grating using holographic ion beam etching is shown schematically in Fig.7.The fabrication process steps are: (1) The preparation of substrates must be extreme care. After cleaning with acetone, the substrate is baked at 150ºCfor 1h.Then the substrate is spin-coated Shipley S1400 photoresist with a thickness of about 0.4~0.8µm. The sample is baked at 80ºC for 30min. to remove the solvent, which could cause erratic variation in resist sensitivity. (2) The grating pattern in S1400 is produced by holographic exposure using Kr + laser (wavelength, nm). The sample is developed by 5 sodium hydroxide solution as developer for 3~5min., after which it was immediately rinsed in deioned water and dried. (3) The sample with grating relief is corrected using oxygen reactive ion etching (RIE) (4) A rectangular shaped groove is obtained by permitting the argon ion beam to strike the mask at vertical with substrate. After etching, the sample is cleaned carefully. (5) The grating is coated 40 nm thick Au by dual ion beam sputter deposition facility. 170 Proc. of SPIE Vol. 5636

7 (a)photoresist coating(b)holographic exposure and development (c) O 2 reactive ion etching (d) Ar ion beam etching (e) strip resist and coat Au Fig. 7 Fabrication process of a laminar grating using holographic and ion beam etching 2.3. Results and Discussion Fused silica laminar gratings of 200g/mm with mm 2 were fabricated by the process shown in Fig.7. Figure8 shows an example of SPM photographs of fused silica laminar grating, which has optically flat groove surfaces and smooth edges except non-straight as a result of a multiplicity of stripes, which produced by optics blemish, adding to interference fringes for grating. The efficiency at a given wavelength of a laminar grating is determined by the including angle between incident and diffracted beam, the groove depth and the duty cycle (the ratio of groove width to grating period).the optimized grating parameters for wavelength ranges from 36 nm to 120nm as follows: groove density 200g/mm, including angle 162Ü, groove depth 70nm, duty cycle 0.4f10%.So control of the duty cycle and groove depth is important in fabricating laminar gratings. Fig. 8 SPM photograph of soft x-ray laminar grating Proc. of SPIE Vol

8 Control of the duty cycle by UV preexposure, holographic exposure time and O 2 RIE in this work. The precise control of the groove depth can be obtained by controlling the etching time. The etch rate of fused silica is 28.3nm/min. at normal incidence for Ar at ion energy 400eV, beam current 80mA and a pressure of 1.5h10-2 Pa. 3. GOLD TRANSMISSION GRATINGS 3.1. Introduction A gold transmission grating is one of the critical elements in the soft x-ray spectrometer for plasma diagnostics. Transmission grating spectrometers can be designed to have flexibility, simplicity, spatial resolution, high band pass, modest spectral resolution, and time resolution. A major advantage of transmission gratings over reflection gratings is that their efficiency is not as easily degraded by contamination in the laser target environment. In order to prevent soft x- ray absorption grating not supported on a substrate, and the spaces between grating bars are truly empty, the grating structure is supported by a coarse gold grid aligned orthogonal to the fine bars, which known as gold self-supporting transmission grating (SSTG). Common SSTGs are available in the market but are very expensive, we have devoted much effort to fabricating SSTGs for the domestic users Fabrication The manufacture of gold transmission gratings by holographic ion beam etching is a multistep operation, each step of which is subject to process variation that can affect reproducibility and consistent of optimum efficiency. The primary process steps in grating fabrication are schematically shown in Fig. 9. The fabrication process steps [7] are: 1) Gold film,0.3~0.6 m thick, was sputtered onto a cleaned glass plate. Then the substrate was spin-coated with Shipley 1400 photoresist with a thickness of about 0.3 m on the Au film. To remove the solvent which could cause erratic variation in resist sensitivity the sample was backed at 80 for 30 min. 2) The grating pattern in resist was produced by holographic exposure using Kr + laser (wavelength, 413.1nm),exposure time about 7~20sec.The sample was developed by5 sodium hydroxide solution as developer for 2~6 min. after which it was immediately rinsed in deioned water and dried. 3) The gold grating pattern was formed by argon ion beam etching using a resist grating pattern mask. After which it was post baked at 100 for 1 h. 4) S photoresist was spin-coated onto the Au grating pattern again. The sample was backed at 80 for 30 min. 5) The supporting grid pattern in resist was produced by ultraviolet (UV) lithography using many larger period grid masks. 6) About 4 m thick gold supporting grid was superposed on the grating by microelectroplating.after gold plating the resist is removed with acetone followed by oxygen plasma descum step. 7) The grating with gold supporting grid was glued on supporting ring for 24h. After which its glass substrate was back eroded with hydrofluoric acid. 172 Proc. of SPIE Vol. 5636

9 Clean substrate Development Resist coat Electroplating Sputtered Au Hard bake Soft bake Strip resist Resist coat Ion beam etching UV exposure Ion beam etching Soft bake Strip resist Development Glued supporting ring Holographic exposure Seed plating Hard bake Erode substrate Fig. 9 Basic process flow chart for self-supporting gold transmission grating 3.3. Results and Discussion We have manufactured a number of SSTGs by the procedure as described above. Figure10 shows SEM photograph of a SSTG with 2000 g/mm pitch. With newly manufactured 2000g/mm SSTG and a soft x-ray CCD, high spectral resolution transmission grating spectrometer has been established at Research Center of Laser Fusion, CAEP [12-13]. Fig. 10 SEM photograph of 2000g/mm gold transmission gratin grating Proc. of SPIE Vol

10 The fabrication of high efficiency diffraction gratings by holographic ion beam etching is a multi-step operation, each step of which is subject to process variation that can affect reproducibility and consistent attainment of optimum efficiency. It is most important that resist relief gratings and support structure fabrication steps. The main factors influencing on high line density resist relief during exposure and development have been described in [7]. Excessive heating of the grating during ion beam etching steps will cause resist burnt, difficultly removed by acetone, and may induce stress changes in the gold, deforming the grating. This problem can be overcome by interval etching allowing the grating to cool down after each interval. Electroforming gold support microstructure is another critical step in the fabrication of SSTGs. The electrolyte for gold dc plating was a homemade gold sulfite solution with PH=7. The electrolysis was current controlled. Temperature in the ~40ÜC and current densities in 2.5mA/cm 2 were used. We observed many defects in the gold electroplating process, which caused failure of the structures. Figure 11 shows gold swarm up the surface of resist protects grating structure during microelectroplating. This problem can be partially overcome by interval removing gold overlay with fibre cotton for optics, such as Fig.11 (a) and (b). While gold particles accumulation become compact film cover on resist surface it is not removed, such asfig.11 (c) and (d). (a) (b) (c) (d) Fig.11 Optical microscope photograph of the gold plating of grating supporting microstructure 174 Proc. of SPIE Vol. 5636

11 Acknowledgements This work was supported by project 211 from the Ministry of State Education of China and by the major project in ninth five years from the Chinese Academy of Sciences. This research is supported in part by the National Natural Science Foundation of China (Grant No ). REFERENCES 1. T. Koide, M. Yanagihara, Y. Aiura et al. Resuscitation of carbon-contaminated mirrors and gratings by oxygendischarge cleaning. Appl. Opt, 1987,26: M. C. Hutley, Diffraction grating. London :Academic press, N. K. Sheridon. Production of blazed holograms. Appl. Phys. Lett. 1968,12(9): G. Schmahl. Holographically made diffraction gratings for the visible, UV and soft x-ray region. J.Spectrosc. Soc. Jpn. Suppl.11974, 23: Y. Aoyagi, S. Namba, Blazed ion-etched holographic gratings. Opt. Acta 1976,23(9): B.Shen. Experiment study on ion beam etching of holographic grating on aluminum substrate..micofabrication Technology, 1986,3:36-41 ( in Chinese). 7. X. Xu,Y. Hong, Y.Tian et al., Fabrication of self supporting transmission gratings for plasma diagnostics. Proc. SPIE, 1999,3766,: J. Chung, M. C. Jeng, J. E. Moon, et al. Deep-submicrometer MOS device fabrication using a photoresist-ashing technique. IEEE. Electr. Dev. Lett., 1988,9(4): L. F. Johnson. Evolution of grating profiles under ion-beam erosion. Appl. Opt. 1979,18(15): L.F.Johnson, K.A.Ingersoll. Asymmetric triangular grating profiles with 90Ü groove angles produced by ionbeam erosion. Appl. Opt. 1981,20(17): A. G. Michette. Optical systems for soft x-rays. New York: Plenum press, 1986, J. Yang, Y. Ding, Z. Zheng et al. High spectral resolution measurement of soft x-ray spectrum using transmission grating spectrometer. High Power Laser and Particle Beam, 2004, 15(1): J. Yang, Y. Ding, W. Zhang et al. Precise measurement technology of soft x-ray spectrum using dual transmission grating spectrometer. Res. Sci. Instrum. 2003, 74(10): Proc. of SPIE Vol