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Abstract
Interference Lithography (IL) is a powerful and inexpensive tool for large area precision nanoscale patterning of periodic structures. In this work we extend IL’s capability to create features in arbitrary shapes and locations through the use of binary contact masks with wavefront division deep-UV interference lithography. Grating couplers for use in a streak measurement system and a focal plane division polarimeter are created to demonstrate the viability and versatility of the technique. Simultaneous fabrication of 90nm and 20μm features proves the potential of this process to simplify and streamline common fabrication processes in research and in industrial applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Interference lithography (IL) is a powerful tool for creating devices that require nanoscale periodic patterns such as grating couplers, photonic crystals and wiregrid polarizers [1–6]. Unlike traditional lithography methods, interference lithography is a maskless technique that relies on multibeam interference to generate an intensity pattern to expose the photoresist [7, 8]. Interference Lithography systems are capable of producing extremely regular large-area patterns of lines, posts, or holes with features as small as λ/4 (half-width of λ/2 period) without the need for complicated multi-step pattern doubling processes used in conventional projection lithography. These systems can operate at a significantly higher resolution than standard research level i-line contact mask lithography, and are simpler and cheaper to operate in a lab setting. Critical dimensions depend only on wavelength and interference angle, allowing it to keep pace with the resolution limit of production line technologies given proper laser sources. Sub-10nm features have also been demonstrated using EUV interference [9]. The pattern dimensions for two beam interference are determined by Eq. (1)
where Λ is the pattern pitch, λ is the illuminating wavelength, and θ is the angle of the beams with respect to normal [10].Despite its many advantages, IL’s inability to place arbitrary geometries limits its scope significantly. Due to the high incident angles needed to generate intensity patterns at the pitch required for visible/IR light diffraction, interference lithography is incompatible with normal masking techniques. Placement of arbitrary geometry thus far has required the use of traditional lithography to create liftoff or masking, which adds significant complexity to the process [11, 12]. This is especially true for devices that require multiple grating angles, each of which require a minimum of two lithography steps making such devices extremely time and labor intensive [13]. In this paper we present a novel method to combine binary contact masks with interference lithography to provide a single step technique to generate arbitrary grating coupler geometries with IL and demonstrate its effectiveness through a few application cases.
2. Method
The interference lithography system used in this work (illustrated in Fig. 1) consists of a diode-pumped continuous-wave YAG laser, doubled to 532nm which is then doubled again to 266nm in a Coherent MBD-266 monolithic block doubler which provides a stable CW deep-UV source. The laser is spatially filtered using a Thorlabs KT310 Spatial Filter then fed through a Newport GBS-UV-H UV Beam Shaper to produce a highly uniform (rated at 8.14% RMS average intensity) top-hat profile, which is expanded and collimated to cover a 4” diameter circle at the Lloyd’s mirror stage. Exposure was controlled using a pickoff to monitor laser power and a timed shutter to turn the beam on and off to provide the proper dose.
The Lloyd’s mirror is used to create a second virtual DUV source to generate the interference pattern in a wavefront division scheme. One arm holds a UV mirror to divide and reflect the beam, while a mount is used in the other arm to hold the sample in place [14]. The Lloyd’s mirror provides several advantages over using a beam splitter and mirrors in an amplitude division scheme to produce interference. With the mirror and sample physically connected by the stage, vibration between the two beams is considerably reduced, allowing for longer exposure times without compromising the pattern sharpness [15]. Additionally this scheme has an advantage over single diffraction element wavefront division methods, which are locked to a single diffraction angle or pattern based on the diffraction mask [16]. With a Lloyd’s mirror system the incident angle of the two beams is controlled easily by simply rotating the entire stage. The geometry of the setup ensures equal intensities from both beams as well as equal and opposite angles, which in turn controls the period of the interference fringes according to Eq. (1) where λ is 266nm in our system.
Fig. 2 Binary mask is held in contact with sample due to vacuum pressure. Mask holder is mounted in the Lloyd’s mirror used to generate an interference pattern.
To create the arbitrary features, a binary contact mask is made on a semi-flexible fused silica wafer using traditional i-line lithography followed by a metal liftoff. As shown in Fig. 2, a custom vacuum chuck mounts into the Lloyd’s mirror stage where the vacuum is used to pull the mask into contact with the sample while also holding the sample securely in place. A raised o-ring seals the vacuum and allows the mask to bow slightly, the center of which is brought into contact with the sample. The interference lithography is then performed, resulting in one- or two-dimensional patterns being created within the open geometries of the contact mask. Although a Lloyd’s mirror method was used for this paper, this technique is compatible with other IL schemes with minor modifications.
A basic two-dimensional array of diffracting gratings was fabricated to demonstrate the ability to place several gratings at specific locations. A checker board pattern mask with 20μm by 20μm squares was prepared using a thin fused silica wafer. The pattern was created with traditional i-line lithography using Megaposit SPR 955-CM photoresist and anhydrous ammonia image reversal. The metal was deposited with e-beam evaporated chromium, then a liftoff was performed using an acetone bath resulting in a binary contact mask.
Fig. 3 Square pixel gratings fabricated using the hybrid method. a: Wide view of the 20μm by 20μm pixels in checkerboard pattern. b: detail of pixel grating corner.
Samples were fabricated on silicon substrates using a positive-tone Sumitomo Chemical© PEK-635A8 chemically-amplified deep-UV photoresist diluted 3:1 with PGMEA (propylene glycol methyl ether acetate) to create a 350nm thick layer. Due to the reflectivity of the substrate, Brewer Science DUV42P-6 was used as a Backside Anti-Reflection Coating (BARC) to prevent vertical standing waves within the photoresist. A 25°angle of incidence was chosen for the interference lithography step, resulting in a 315nm pitch and approximately 50% duty cycle. After exposure, the silicon was dry etched in a fluorinated plasma containing SF6, C4F8 and Ar. The resulting gratings were imaged using a Hitachi S-4800 High Resolution Scanning Electron Microscope (HRSEM) in Fig. 3, showing the uniformity over a large number of pixels, and the fidelity of the individual grating lines.
3. Results
3.1. Optical grating coupler
Grating couplers are a convenient noncontact technique to quickly and easily couple light into waveguides. As a simple example of an application of the masked IL technique, photoresist optical grating couplers were fabricated for use in a streak loss measurement system. A fiber collimator mounted to a rotation stage was used to match the input angle of a surface grating to couple 1550nm laser light into slab waveguides. The optical losses in the waveguide material were calculated by using an infrared imaging system to measure the light scattered through the film surface [17, 18].
The mask for the Optical Grating Coupler was fabricated using the same method described above with only a 1mm rectangular slit open in the mask. A commercially purchased Low Pressure Chemical Vapor Deposition (LPCVD) silicon nitride film on a silicon wafer with an silicon oxide buffer layer formed a slab waveguide. PEK-635A8 photoresist was exposed at a 5.5°exposure angle to create a grating period of 1392nm. This period was chosen to create a photoresist grating coupler designed to couple 1550nm light at 45 degrees into the silicon nitride film based on the grating equation Eq. (2)
where Λ is once again the grating period, λ0 is the input wavelength, neff is the waveguide mode effective index, and ϕ is the input angle [19]. The guided mode effective index neff is calculated by a shooting method mode finding algorithm based on material index and film thickness.The localized nature of the grating (visible in Fig 4 by the orange diffracted light) prevented the mode from coupling back out of the waveguide as it propagates. After use, the photoresist grating was washed off of the sample using an acetone, methanol, and isopropyl alcohol rinse. The nondestructive nature of the fabrication and removal of the grating coupler meant that the same waveguide could be used before and after the annealing steps, making it possible to directly measure the effects on the loss characteristics. This method has been used successfully in the continued operation of the streak measurement setup with many different waveguides and coupling angles.
Fig. 4 Photoresist optical grating coupler designed for 1550nm light at 45°input on silicon nitride slab waveguide. a: grating coupler is visible by orange diffracted light. b: SEM image of photoresist grating on silicon nitride waveguide with silicon oxide buffer layer beneath.
3.2. Division of focal plane array polarimeter
Division of focal plane array polarimeters provide the ability to take images which record polarization information of a scene. Superpixels of different orientation polarizers operate similarly to Bayer color filter to capture the Stokes parameters of light in a single image [20]. Commonly a set of four wire grid polarizers oriented 0°, 45°, 90°, and 135°are used to measure the first three Stokes parameters [21].
For a more advanced application of the masked IL technique the first set of sub pixels in a division of focal plane wire grid polarimeter superpixel array were fabricated. The mask used for this device is the same as for the example diffraction grating array discussed above. Samples were made on fused silica substrates using PEK-635A8 photoresist, this time diluted 1:1 with PGMEA to reduce the film thickness to roughly 200nm, with a BARC layer to avoid standing waves. A 45°angle was chosen resulting in a 188nm pitch and roughly 50% duty cycle for a line width of 89nm. After exposure, 50nm of aluminum was deposited onto the sample using e-beam evaporation, after which the photoresist was lifted off using acetone to create the polarizers. The fused silica substrate makes it difficult to produce good quality SEM images of the pixel filters. Therefore, a companion sample of photoresist on a silicon wafer was created in parallel to monitor the process. The HRSEM image of this sample coated in gold shown in Fig. 5 demonstrates the precise lines and small feature size this process generates.
The polarization performance was measured using the system shown in Fig. 5 modeled after the system described in [22]. A Red Cree XLamp XP-E2 LED illuminated an integrating sphere, the output of which was filtered through an aperture and collimated. A Glan-Thompson polarizing prism in a rotation mount provided the polarization.
The polarizing filter was imaged onto a CCD sensor using an optical relay system. Data was collected per-pixel from the CCD output, with polarizing pixels normalized to neighboring clear pixels, an average of which is shown in Fig. 6. The maximum average transmission through the polarizer was measured at 70.8% and the minimum transmission was 6.7%. The extinction ratio measured is lower than other devices created using similar techniques, likely due to evaporated metal density and larger grating pitch, but could be optimized for comparable performance [23]. This technique uses common metals, such as aluminum, and avoids use of exotic materials such as dichroic dyes, birefringent crystals, or doped polymer sheets [24–27]. Although a single polarizing direction was fabricated for this example, the addition of alignment capability would allow the creation of entire super pixel arrays using back to back exposures while shifting the mask to expose each sub pixel in turn.
Fig. 6 left: Polarization transmission of pixel polarizers at 625nm wavelength, normalized to blank pixels. right: Photoresist grating created as companion sample to polarizing filter.
4. Conclusion
In this paper we have successfully developed a process to create localized periodic structures in arbitrary geometries and locations through hybridizing deep UV interference lithography with binary contact mask lithography. ∼90nm features were fabricated simultaneously with 20μm features in a single step much more rapidly and easily than would have been possible with traditional techniques. By fabricating several functional optical devices utilizing this capability we have demonstrated that this technique is a flexible method for a wide variety of applications. Further development of the technique to include alignment optics will make multiple exposures during a single lithographic step possible. This will extend the capability to include fabricating complicated devices which require multiple grating orientations.
Funding
Defense Associated Graduate Student Innovators (RX16-10)
Acknowledgments
I. Agha and J. Burrow for fruitful discussion, G. Sevison for the system diagram, K. Hirakawa for the use of imaging optics, and A. Gariepy and B. Stauffer for proofreading.
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