Breakthroughs in Silicon Grism and Immersion Grating Technology at Penn State 1

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1 Breakthroughs in Silicon Grism and Immersion Grating Technology at Penn State 1 Jian Ge, Dan McDavitt, Shane Miller, John Bernecker, Abhijit Chakraborty & Junfeng Wang 525 Davey Lab, Department of Astronomy & Astrophysics, Penn State University, University Park, PA Abstract Fabrication of silicon grisms up to 2 inches in dimension has become a routine process at Penn State thanks to newly developed techniques in chemical etching, lithography and post-processing. The newly etched silicon grisms have typical rms surface roughness of ~ 9 nm with the best reaching 0.9 nm, significantly lower than our previous attempts (~ nm). The wavefront quality of the etched gratings is high. Typical wavefront error is ~ wave at micron, indicating diffraction-limited performance in the entire infrared wavelengths ( microns) where silicon has excellent transmission. These processes have also significantly eliminated visible defects due to grating mask breaks during chemical etching. For the best grisms, we have less than 1 defect per cm 2. The measured total integrated scatter is less than 1% at micron, indicating similar or lower scatter in the IR when grisms are operated in transmission. These new generation grisms are being evaluated with our Penn State near IR Imager and Spectrograph (PIRIS) in cryogenic temperature. We are applying the new techniques in etching an 80x40 mm 2 grating on 30 mm thick substrate to make an anamorphic silicon immersion grating, which can provide a diffraction-limited spectral resolution of R = 220,000 at 2.2 micron. We plan to put this immersion grating in a modified PIRIS to measure magnetic field strength using the Fe I line at 1.56 micron among hundreds of nearby solar type stars to investigate the probability of the Maunder Minimum using the Mt. Wilson 100inch with adaptive optics in Key Words: Silicon grism, immersion grating, Infrared, Spectroscopy, high resolution, anamorphic grating 1. Introduction Moderate to high resolution (R = λ/ λ ~ ) IR spectroscopy plays a critical role in astronomical studies with both ground-based and space-based telescopes. The applications of mid- (R > 1000) and high- (R > 10,000) resolution IR spectroscopy include astrophysical studies of atomic and molecular emission and absorption lines (e.g. Br-γ, Pa-α, H 2 O, CO, H 2, HD, methane, etc.), interstellar dust grain features (e.g. polyaromatic hydrocarbons (PAHs), water ices, etc.), circumstellar disks, planetary nebulae, and extra-galactic sources. High resolution IR spectroscopy has also important applications in studies of laboratory chemical reactions, combustion and materials processing research. In the UV and optical, high resolution spectrographs (R 50,000) combined with the large format detectors enable the simultaneous sampling of multi-order spectra (e.g. STIS aboard HST, Woodgate et al. 1998; HIRES at Keck, Vogt et al. 1994). Such results have significantly advanced our understanding of the dynamics and energetics of the Universe. As infrared detector arrays approach the current dimensions of optical arrays, similar spectroscopic instruments at high spectral resolutions in the infrared are possible. Several ground-based instruments are 1 Send correspondence to Jian Ge, jian@astro.psu.edu, Tel: , Fax:

2 already working in the near-ir region. For example, the NIRSPEC at Keck is pioneering high resolution IR spectroscopy with R 25,000 (Mclean et al. 1998). However, by simply scaling the design of optical instruments to longer IR wavelengths, the IR instruments with conventional grating technology become large and massive. Weight and volume limitations are always a challenge for spectroscopic IR instruments since they have to be operated in cryogenic temperatures. For all space missions, weight and volume are expected to be the limiting factors. Therefore, to enable higher spectral resolution IR instruments for both ground-based telescopes and space missions a new instrument technology is needed to design compact IR spectrometers with smaller volumes and less weight. The key approach to reduce instrument size is to immerse a portion of the optical path in materials with high indices of refraction, n. In instruments where wavelengths are scaled from µm to 1-5 µm or beyond a suitable approach is to consider materials with n 3-4. Therefore, the speed of light is C = C/n such that high n actually slow down the propagation of radiation, the wavelength of the incident light in the high n material is shortened. In these materials, one can view this as an instrument operating at shorter wavelengths. Immersion enables higher dispersion gratings. For shorter wavelengths caused by the high n materials, the grating looks bigger and bigger gratings yield higher dispersion (the dispersion n). IR grating dispersion power greatly benefits from the available very high n materials, as opposed to the optical where almost all glasses have n < 2. For instance, the most significant IR materials such as Ge, Si, and GaAs have room temperature indices of n = 4.07, 3.45 and 3.32 at 2.2 µm, respectively. Therefore, by employing these materials, one can reduce the IR spectrometer linear length by more than a factor of 3, and reduce the volume and weight by an order of magnitude. Silicon is especially interesting because grating grooves can be etched on silicon substrates by certain chemical reagents, e.g. potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH) (e.g. Tsang & Wang 1975; Ge et al. 2001), which etch the silicon (100) planes hundreds of times faster than the (111) planes to form very sharp and smooth V- shaped grating grooves. The size of the grooves is determined by the initial groove mask size. Hence, very coarse gratings can be fabricated by careful design of grating masks and control of the chemical etching process. Coarse gratings will allow us to build cross-dispersed spectrometers with large continuous wavelength coverage on 2-D detectors to achieve a multiplex advantage in IR spectroscopy (i.e. increase instantaneous wavelength coverage). This Silicon anamorphic immersion grating new grating technique permits the 73.8º production of gratings with much coarser 70.5º grooves than are possible with 54.7º d = 16 mm conventional ruling techniques. Currently 36 mm no echelle gratings are commercially Regular reflection echelle grating available with fewer than 23.2 grooves per R = 250,000 millimeter (or grooves larger than about 43 µm). 95 mm 328 mm A silicon immersion grating operated in transmission is called a silicon grism, which can also be considered as a combination of a diffraction grating and a prism that is designed to have no deviation for the central wavelength. The spectral resolving power of a grism is described as d = 189 mm Figure 1. Comparison between a silicon anamorphic immersion grating and a conventional reflection grating.

3 ( 1) R = n tanα, where α is the prism wedge angle. Silicon is an ideal grism material due to its β very high refractive index. A silicon grism offers six times the dispersion power of a conventionally made CaF 2 grism (n ~ 1.4), and nearly twice that of a KRS-5 grism (n ~ 2.4). These CaF 2 and KRS-5 grisms can be made using conventional mechanical ruling, but can only be operated in low dispersion orders because of the difficulty of making coarse grooves on these materials by diamond ruling. In addition, a silicon grism can still offer a factor of 1.2 times the dispersion of a reflection grating of equal length and blaze angle. Therefore, silicon grisms enable medium and high spectral resolution spectroscopy in IR camera systems. By operating silicon immersion gratings in an anamorphic immersion mode, the increase in spectral resolving power can be up to a factor of n 2 or ~ 12 times at Brewster s angle (Dekker 1987). In Figure 1, a direct comparison between a silicon anamorphic grating operated at Brewster s incident angle (73.8 ) and a regular reflection grating demonstrates the potential of this new grating for reducing instrument collimator beam size. For the same diffraction-limited resolution (R ~ 250,000) at 2.2 µm, a new kind of spectrometer with the anamorphic silicon immersion grating with a 54.7 blaze angle (natural blaze for silicon immersion gratings) requires only a 16 mm diameter beam, about 12 times smaller than a conventional spectrometer with a regular echelle at the same blaze angle. This can lead to orders of magnitude reduction in size and weight of the new spectrometer, resulting in much less cost and time to construct IR cryogenic instruments New Breakthroughs of Silicon Grating Technology Development A breakthrough in developing silicon immersion grating technology was made by a team led by Jian Ge at LLNL in High quality prototype silicon immersion gratings were successfully fabricated through new micromachining techniques (Ge et al. 1999a,b; 2000; 2001). Instead of using them in an immersed reflection mode we operated them in an immersed transmission mode. The devices are called silicon grisms. They can be easily evaluated in an IR camera by inserting them at the camera pupil location. A spectrum of Ne I emission line at µm with the best silicon grism obtained with the UC Berkeley IRCAL near-ir camera provides a diffraction-limited spectral resolving power, R = 4900, with a 0.15 arcsec entrance slit and a 5 mm diameter camera pupil. For the best silicon grism, the total integrated scattered light is 8% in K, and the grating peak efficiency is 45% (Ge et al. 2001). Figure 2. Spectral format of the cross-dispersed spectral format in the IRCAL camera with a silicon grism and a low resolution CaF2 grism cross-disperser (left panel) and cross-dispersed silicon echelle grism spectrum of T Tau N in the K band obtained with the IRCAL camera at the Lick 3m in 2000 (right side). Both completely cover µm. The scientific demonstration of silicon grisms was carried out in the IRCAL near-ir camera at the Lick 3m telescope in September Complete K band spectra were obtained for six young stellar objects (YSOs) and their companions (T Tauri,

4 AS 353, XY Perseus, LKHα 234, XZ Tau, and HK Orionis). The initial results were presented in previous meetings (Ge et al. 2000; 2001). Figure 2 shows a cross-dispersed silicon echelle grism spectrum of the T Tauri N companion. This is achieved by silicon grisms operated in orders with very coarse grooves, ~ 13 l/mm, a factor of two coarser than commercially available echelles. With a low dispersion cross-disperser, ~16 cross-dispersed orders of spectra are packed on the PICNIC array in the IRCAL camera. Hence, silicon grisms or immersion gratings allow large instantaneous continuous wavelength coverage on 2-D detectors of any size. Observation efficiency can be correspondingly increased. This demonstrates that the grating technique allows echellogram of a silicon Figure 3. Moderate resolution (R = 2000) K band spectra of BD The dashed line is the single Gaussian best fit. grism match with any two dimensional arrays and also IR high resolution spectroscopy can be achieved by simply inserting a silicon echelle grism and a low resolution cross-dispersed grism in an IR camera filter wheel. Silicon echelle grisms provide a new costeffective, convenient and efficient way for making moderate and high resolution IR spectroscopic instruments within existing IR cameras. Data analysis shows that Brγ emission lines are associated with both T Tauri N and S Figure 4. The etched silicon wafer gratings with the TMAH process without AP (left) and with AP dose (middle). On the right, a commercially made fused-silica transmission grating with 10 µm grooves is shown for comparison. Irregularity and surface roughness can be clearly seen in this commercial grating. components, similar to what have been found by the Keck AO IR spectroscopy (Duchene et al. 2002; Ge et al. 2002). The spectrum of BD shows an enormous broad Brγ absorption with a FWHM of 550 ± 50 km/s (~ 90% of stellar brake-up velocity), indicating this star is perhaps undergoing an early phase of slowing the stellar rotation down to become a normal Be star (Ge et al. 2002a). These observations demonstrate the scientific capabilities of silicon grisms. Our main focus in silicon grating technology development over last two years is developing new fabrication processes for reducing grating surface roughness and increase grating efficiency. We have used TMAH instead of the KOH as our main chemical etchant, which has produced better grating patterns, smoother grating surface, and less wavefront distortion. Our recently developed etching process with TMAH, ammonium persulfate (AP) and a thin silicon dioxide mask shows great success in reducing surface roughness and increasing grating efficiency. Figure 4 shows scanning electron micrograph (SEM) pictures of etched gratings with and without AP processes and also a commercially made grating. The grating with the AP process has much smoother

5 grating facet surfaces and less defects. Details of optical performance of the etched silicon gratings are reported in the following. (a). Surface roughness measurements by the Atomic Force Microscope (AFM) November 2001 March 2002 June 2002 Figure 5. Atomic Force Microscope scanning of the best etched silicon gratings in November 2001, March 2002 an June The RMS surface roughness over 2 µm in randomly chosen grating surfaces are 9 nm, 3 nm and 0.9 nm from the right to the left, respectively. An atomic force microscope is used for measuring the RMS surface roughness. The results from November 2001, March 2002, and June 2002 are shown in Figure 5. The surface quality has been significantly improved over a period of 8 months. In November 2001, the typical RMS surface roughness for the best etched silicon gratings with the TMAH and AP process over ~ 1 µm scale was 9 nm. In March 2002, the RMS surface roughness for the best etched gratings was down to 3 nm. In June 2002, the rms roughness was further reduced to 0.9 nm. The surface roughness for these gratings is lower than that for the Hubble Space Telescope mirror, 12 nm. (b). Integrated scattered light measurements An etched grating with ~ 10 nm rms roughness has been evaluated with our optical spectrograph. The total measured integrated scatter at µm is less than 1% scatter as shown in Figure 6. This level of scatter is already a factor of three times better than that of a commercial 23.2 l/mm echelle grating measured by our collaborators, Kuzmenko & Ciarlo in It is also a factor of ~ 10 times better than previous silicon grating results by other groups (Kuzmenko et al. 1998; Keller et al. 2000). The integrated light levels from silicon grisms with silicon grating surface quality are also being evaluated with Figure 6. Cross-section of a HeNe laser at µm, indicating <1% integrated scattered light for the TMAH etched grating. our PIRIS near IR spectrograph. We expect to reach ~1% or less scatter level in the entire near- IR.

6 (c). Wavefront quality of etched gratings Wavefront quality of the etched gratings has been evaluated by a Zygo interferometer at Lawrence Livermore National Lab through collaborating with Dr. P. Kuzmenko. Figure 7 shows the wavefront over mm 2 etched grating area at µm in reflection. The RMS wavefront distortion is waves, indicating diffraction-limited performance once the grating is operated in both immersed reflection and transmission. The high wavefront quality was achieved using a very thin oxide grating mask layer. Our previous study shows that a sharp mask pattern is the critical step for maintaining wavefront quality (Ge et al. 2001). This is why etched gratings with plasma etching have better quality than those with wet etching. However, it is challenging to make masks on thick silicon substrates in the plasma chamber. Our thin mask layer made by wet processing can maintain the grating pattern as sharp as a plasma-etched grating mask, resulting in high wavefront quality. Maintaining high wavefront quality is critical for achieving the highest dispersion power provided by silicon immersion gratings. Figure 7. Wavefront map of a silicon grating with mm 2 etched area at µm measured by a Zygo interferometer. The grating was made with the new TMAH+AP etching. (d). Silicon Grisms Made through New Processes The new etching technique, TMAH + AP process, has been applied in fabricating new generation silicon grisms. Figure 8 shows etched gratings on two silicon substrates with 100 mm in diameter and 20 mm in thickness. One has 16 gratings. Each grating has mm 2 etched area, a 54.7 deg blaze angle and 66 µm period grating grooves. The other has 4 gratings. Each has mm 2 etched area, a 54.7 blaze angle, and 13 µm grating grooves. The surface roughness of one Figure 8. Etched gratings on thick silicon substrates with the newly developed TMAH and AP etching process at Penn State. The gratings on the left have mm 2 etched area and 66 µm grating period. The gratings on the right have mm 2 etched area and 13 µm grating period. Figure 9. New generation of silicon grisms made of TMAH plus AP processes are mounted in holders and ready for observing with PIRIS (the left three, with etched areas of ~ 10x10 mm 2 ) and ARIES (the right one, with etched area of 15x15 mm 2 ).

7 of the gratings with 66 µm grooves has been measured by the profilometer and shows ~ 15 nm rms roughness. This is a factor of two times lower than the last silicon grism we made at LLNL, which produces ~ 10% integrated scattered light in the near-ir (Ge et al. 2001). We expect these new grisms should have ~ 1% integrated scattered light in the near-ir. These gratings have been cut and polished in an optical company. Figure 9 shows 4 finished grisms installed in mechanical mounts. The three smaller grisms are for testing and observing with our Penn State near IR Imager and Spectrograph (PIRIS) in the lab and at the Mt. Wilson 100 inch telescope. The larger one is for testing and observing at the Arizona Imager and Echelle Spectrograph (ARIES) at the MMT 6.5-m telescope. One of the silicon grisms with 13 µm period grooves has been successfully installed in PIRIS and has produced the first light cross-dispersed echelle spectrum of a continuum tungsten lamp in the H band at the end of August 2002 (Figure 10). Further testing with a narrow and short entrance slit ( µm), a near-ir 1.53 µm laser and a neon emission lamp is being planned and will be conducted late in Figure 10. Cross-dispersed echelle spectrum of a tungsten lamp in the H band by a silicon grism with 13 µm period grooves and a low resolution fused-silica grism cross-disperser. The top is the short wavelength and the bottom is the long wavelength. The hot pixels in the picture are caused by the wrong bias setting. 3. Grating Efficiency Modeling Grating efficiency was modeled using commercially available G-solver software in Figure 11 shows grating efficiency 7.3 Micr on Gr ism Dif f r action Ef f iciencies of a silicon grism with a 7.3 µm groove period and a deg blaze angle in the J, H, 0.8 K, L and M bands. The 0.7 grism is operated in the first order at the M band, the 2 nd 0.6 order for the L band, the 3 rd 0.5-5th Tr ansmission Or der order for the K band, the 4 th -4th Tr ansmission Or der 0.4-3r d Tr ansmission Or der 0.3-2nd T r ansmi ssi on Or der order for the H band and the -1st Tr ansmission Or der 5 th 0.2-3r d T r ansmi ssi on Or der, Coated order for the J band. No 0.1-4th T r ansmi ssi on Or der, Coated scattered light loss is -1st T r ansmi ssi on Or der, Coated nd T r ansmi7.1 ssi on Or der, Coated assumed in the modeling. Without anti-reflection coating, the peak efficiency is about 60% for all these bands. With a single layer silicon nitride coating, the peak efficiency can reach 80%, which is beyond any -0.1 Wavelength (micr ons) -5th T r ansmi ssi on Or der, Coated Figure 11. Modeling of a silicon grism with 7.3 µm groove in the J, H, K, L and M bands. This grism is designed for Goddard Space Flight Center Rapid Visible and IR Multi-object Spectrograph (RIVMOS) (Ge et al. 2002b). commercially available low resolution grisms (Rayner 1998; about 45% efficiency for the KRS-5 grisms in the Gemini NIRI camera, Simons 2001, private communications). This grism is designed for the NGST multiple object demonstration instrument, called Rapid Infrared and

8 Visible Multiple Object Spectrograph (RIVMOS), to provide R = 2000 in µm (Ge et al. 2002). We are about to purchase thick silicon substrates and make these grisms later in We have also modeled the efficiency for a silicon grism with 13 µm grating grooves (Figure 12). The peak grating efficiency is about 45% without AR coating. The efficiency is about 60% with a single layer silicon nitride AR coating. The grism is operated at quite high orders in the near IR, e.g. at the 20 th order at 1.55 µm. Therefore, this grism serves an echelle operating in the relatively low diffraction orders. Since part of the diffraction energy at the peak of the blaze is spread to the neighbor orders, the peak efficiency for each order is lower than that for the finer groove grism shown in Figure 11. This modeling is extremely useful since we can directly compare the modeling results with our lab measurements. If the measured values match the theoretical results, then there will be no room for grating performance improvement. If there is any difference, then we will investigate what the real causes are and try to improve our techniques later. 4. Implementation of Special Equipment for Etching Thick Silicon Substrates (a). UV flood source Special tools for handling thick silicon substrates are being developed by us at Penn State Nanofabrication Facility in An OAI UV flood source has been purchased and installed in the class 10 clean room. This tool provides UV exposure for making grating patterns on silicon substrates, one of the most important steps for making high quality silicon immersion gratings. Figure 13 shows the setup of the UV flood source, which can handle thick silicon substrates up to 6 inches in diameter and thickness. We have calibrated the UV flood source with the existing mask aligner exposure tool, which we utilized developing grating masks on silicon substrates less than 30 mm thick. Diffraction Efficiency H Band Diffraction Efficiencies Wavelength (microns) Figure 14 shows a comparison of mask patterns made by our UV flood source and the mask aligner. The UV flood source produces the same quality masks on photoresist as the aligner. -17T -18T -19T -20T -21T -22T -20T coated Figure 12. Efficiency of silicon grisms with 13 µm grating grooves. Figure 13. UV flood source, purchased by us, is the critical tool for patterning gratings on silicon substrates with thicknesses up to 150 mm.

9 (b). Temperature Controlled Ultra-sonic Etching Tank A temperature controlled BECO etching tank was purchased to conduct wet chemical etching in Figure 14. Comparison of exposed grating patterns produced by the UV flood source (left) and the mask aligner (right). Both machines produce grating mask patterns on photoresists of equal quality In 2002, we purchased a used ultra-sonic tank with a randomly switching frequency and adjustable power. This enabled us to convert the BECO into a temperature controlled ultra-sonic etching tank. The setup of the ultra-sonic and the BECO etching bench is shown in Figure 15. This setup serves two purposes: it can maintain etching temperature within 0.1 C for over 24 hours and the ultra-sonic helps to break hydrogen bubbles during the etching, which in return helps to keep a constant etching rate, resulting in smooth grating surfaces. In summary, we have developed new lithography and anisotropic chemical etching techniques and also special tools for fabricating high quality silicon immersion gratings with an etched grating size up to 6 inches. A RMS surface roughness of only 0.9 nm has already been reached with our silicon gratings. We are in the middle of applying our new techniques for fabricating silicon immersion gratings and excellent optical performance is expected. Figure 15. Ultra-sonic etching tank for smooth etching. ACKNOWLEDGEMENTS We thank Paul Kuzmenko and Dino Ciarlo for many stimulating discussions and help on silicon grating fabrication and testing. We also thank Stephen Fonash for his advice and assistance at Penn State Nanofab. This work was supported by NASA grant NGA , NAG , NAG , NSF AST and the Penn State Eberly College of Science.

10 References Dekker, H., An immersion grating for an astronomical spectrograph, in Instrumentation for ground-based optical astronomy, ed. L.B. Robinson (Springer Verlag, 1987), 183 Duchêne, G.; Ghez, A. M.; McCabe, C. 2002, Resolved Near-Infrared Spectroscopy of the Mysterious Pre-Main-Sequence Binary System T Tauri S, ApJ, 568, 771 Ge, J., et al., 1999a, Etched Silicon Gratings for NGST, in Proc. Next Generation Space Telescope Science and Technology, ASP Conference Series, Edited by Eric Smith and Knox Long, Vol. 207, 457 Ge, J., et al., 1999b, The First Light of the World's First Silicon Grisms, BAAS, 195, 1504 Ge, J., et al. 2000, High spectral and spatial resolution spectroscopy of YSOs with a silicon grism and adaptive optics, BAAS, 197, 5201 Ge, J. et al. 2001, Development of Silicon Grisms for High-resolution IR spectroscopy, Proc. SPIE, 4485, 393 Ge, J., Kuzmenko, P., Ciarlo, D., & Chakraborty, A., 2002a, First Astronomical IR Spectroscopy with an Etched Silicon Echelle Grism, ApJ Letters, to be submitted Ge, J. et al. 2002b, Optical design of rapid infrared-visible multi-object spectrometer: a NGST demonstration instrument, Proc. SPIE, 4850, in press Keller, L.D., et al. 2000, Fabrication and testing of chemically micromachined silicon echelle gratings, Appl. Optics, 39, 1094 Kuzmenko, P.J., & Ciarlo, D.R., 1998, Improving the optical performance of etched silicon gratings, Proc. SPIE, 3354, 357 McLean, I.S. et al. 1998, Design and development of NIRSPEC: a near-infrared echelle spectrograph for the Keck II telescope, Proc. SPIE, 3354, 566 Tsang, W.T., & Wang, S., 1975, Preferentially etched diffraction gratings in silicon, J. Appl. Phys., 46, 2163 Vogt, S.S. et al. 1994, HIRES: the high-resolution echelle spectrometer on the Keck 10-m Telescope, Proc. SPIE, 2198, 362 Woodgate, B.E. et al. 1998, The Space Telescope Imaging Spectrograph Design, PASP, 110, 1183