Binary Photomasks Research Articles

Silicon nanocavity with a quality factor of 6.7 million fabricated by a CMOS-compatible process.

Here, we report on the increase of the quality-factors of photonic crystal nanocavities fabricated by a CMOS-compatible process. We fabricated nanocavities with the same cavity design but used either a binary photomask or a phase-shift photomask in the photolithography step to assess the impact of the photomask-type on the fabrication accuracy of the air holes. We characterized 62 cavities using time-resolved measurements and the best cavity had a quality-factor of 6.65 × 106. All cavities exhibited a quality-factor larger than 2 million and the overall average was 3.25 × 106. While the estimated magnitude of the scattering loss due to the air hole variations in the 33 cavities fabricated with the phase-shift photomask was slightly lower than that in the 29 cavities fabricated with binary photomask, the phase-shift photomask did not provide a significant improvement in the fabrication accuracy. On average, the scattering loss in these samples is more than 3 times larger than that of nanocavities fabricated using electron-beam lithography, which indicates room for further improvement.

Long-period fiber gratings based on blue phase liquid crystal/polymer composites

ABSTRACT A relatively simple and cost-effective method to realise a novel long-period fibre grating (LPFG) based on blue phase liquid crystal (BPLC) composites was proposed and experimentally demonstrated by implementing photopolymerisation-induced phase separation (PIPS) through a binary photomask. The BPLC-polymer composite zones were obtained by a process of periodic ultraviolet light with phase separation of the BPLC molecules and UV-curable monomer mixture. The appearance and optical characteristics of the proposed LPFGs have been studied experimentally. Experimental results show that optimum PIPS process conditions are critical for achieving a reliable LPFG-based fibre device. Compared with the previously reported methods, this new technique for designing and making an LPFG is more cost-effective and highly flexible due to the ease in manufacturing of the grating composite structure in the all-fibre system, which can provide great potential for photonic applications.

Assessing stability of metal tellurides as alternative photomask materials for extreme ultraviolet lithography

Tellurium (Te) is one of the elements with highest extinction coefficient κ at the 13.5 nm extreme-ultraviolet (EUV) wavelength. It is being considered as an alternative absorber material for binary photomasks in EUV lithography. The absorber material is required to remain chemically stable during EUV exposure, at elevated temperatures up to 150 °C, during mask cleaning, and in the low hydrogen pressure environment that is present in the EUV scanner. However, Te is known to react with oxygen and hydrogen, forming less EUV absorbing TeO2 and more volatile H2Te, respectively. Since the melting temperature of Te is only 449.5 °C at normal pressure, alloying Te with a more stable metal might result in a high κ material that will remain thermally and chemically stable over a wider range of operating conditions. In this paper, the authors report on the stability assessment of metal telluride (M-Te) alloys for the EUV absorber material. They combined Te with high κ metals, noble metals, and etchable metals. High κ and noble M-Te materials are both thermally more stable than etchable M-Te, but they cannot be patterned easily for use in an EUV photomask. High κ M-Te exhibits polycrystal morphology at room temperature compared to quasiamorphous noble M-Te though both can crystallize at a higher temperature. Hydrogen stability and cleaning solution stability of M-Te materials are improved considerably compared to Te, but their higher surface reactivity cannot be completely mitigated without the addition of an inert capping layer. Furthermore, etchable M-Te alloys are easily oxidized during deposition, resulting in lower electron density and hence lower κ. Nevertheless, M-Te alloys may be a way to stabilize Te for usage as the EUV absorber material.

Mask-aligner lithography using a continuous-wave diode laser frequency-quadrupled to 193 nm.

We present a mask-aligner lithographic system operated with a frequency-quadrupled continuous-wave diode laser emitting at 193 nm. For this purpose, a 772 nm diode laser is amplified by a tapered amplifier in the master-oscillator power-amplifier configuration. The emission wavelength is upconverted twice, using LBO and KBBF nonlinear crystals in second-harmonic generation enhancement cavities. An optical output power of 10 mW is achieved. As uniform exposure field illumination is crucial in mask-aligner lithography, beam shaping is realized with optical elements made from fused silica and CaF2 featuring a diffractive non-imaging homogenizer. A tandem setup of shaped random diffusers, one static and one rotating, is used to control speckle formation. We demonstrate first experimental soft contact and proximity prints for a field size of 1 cm2 with a standard binary photomask and proximity prints with a two-level phase mask, both printed into 120 nm layers of photoresist on unstructured silicon substrates.

Electrically tunable polarisation-independent blue-phase liquid crystal binary phase grating via phase-separated composite films

ABSTRACTA simple method for fabricating polarisation-independent blue-phase liquid crystal phase grating is demonstrated by implementing photopolymerisation-induced phase separation through a binary photomask. The dynamic focusing property of the proposed liquid crystal grating is independent of the polarisation state of incident light. The efficiency of various diffraction orders for the phase grating was measured as a function of the applied voltage. Experimental results show that the maximum diffraction efficiency reaches 36% for the ±1 order, which approaches the theoretical limit ~41%. The measured rise time is 1.4 ms and fall time is 2.2 ms. Such a tunable grating has great potential for photonic applications.

Photomask displacement technology for continuous profile generation by mask aligner lithography

Mask aligner lithography is one of the most widely used technologies for micro-optical elements fabrication. It offers a high throughput with high-yield processing. With different resolution enhancement technologies shadow printing is a mature alternative to the more expensive projection or electron-beam lithography. We are presenting a novel mask aligner tool that allows shifting the photomask with high accuracy between sequential exposure shots. It offers possibilities such as double patterning or gray tone lithography by applying different light doses at different locations. Within this publication, we show the first results for high resolution blazed grating structures generated in photoresist by multiple exposures using a conventional binary photomask.

High spatio-temporal resolution video with compressed sensing.

We present a prototype compressive video camera that encodes scene movement using a translated binary photomask in the optical path. The encoded recording can then be used to reconstruct multiple output frames from each captured image, effectively synthesizing high speed video. The use of a printed binary mask allows reconstruction at higher spatial resolutions than has been previously demonstrated. In addition, we improve upon previous work by investigating tradeoffs in mask design and reconstruction algorithm selection. We identify a mask design that consistently provides the best performance across multiple reconstruction strategies in simulation, and verify it with our prototype hardware. Finally, we compare reconstruction algorithms and identify the best choice in terms of balancing reconstruction quality and speed.

Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

Comparison of Measurement Techniques for Linewidth Metrology on Advanced Photomasks

This paper compares electrical, optical, and atomic force microscope (AFM) measurements of critical dimension (CD) made on a chrome on quartz photomask. Test structures suitable for direct, on-mask electrical probing have been measured using the above three techniques. These include cross-bridge linewidth structures and pairs of Kelvin bridge resistors designed to investigate dimensional mismatch. Overall, the results show very good agreement between the electrical measurements and those made with a calibrated CD-AFM system, while the optical metrology system overestimates the measured width. The uncertainty in each of the measurements has been considered, and for the first time an attempt has been made to describe the levels and sources of uncertainty in the electrical measurement of CD on advanced binary photomasks.

SUB-100 nm LITHOGRAPHY WITH PATTERN AND PARTIAL COHERENCE CONTROL

This paper describes optical patterning of sub-lithographic wavelength features using only conventional chrome-on-glass binary photomasks without phase-shift features. The sub-100 nm patterns were obtained through manipulation of masks bias designs and partial coherence control of the lithographic radiation. The key factors in the design of experiments are the density and design of masks patterns and the partial coherence of exposure radiation system used. Based on the study, smallest resolution of 36% that of designed value can be obtained for features of 75 nm using a 200 nm designed line width. Besides indication of proposed process' resolution limits for further applications, advantages of the method allows its application to applications requiring sub-100 nm critical dimensions through use of biased pattern designs, especially feasible in devices such as deep sub-micrometer comb drives actuators in nano-microelectromechanical structures; metal–semiconductor–metal photodetectors, and nanophotonic crystal structures, etc.

Synthesized processing techniques for monolithic integration of nanometer-scale hole type photonic band gap crystal with micrometer-scale microelectromechanical structures

This article reports the synthesized fabrication process design and module development that enabled the monolithic integration of deep submicrometer size, two dimensional hole-type photonic band gap crystals (PhCs) with microelectromechanical system (MEMS) actuators and optical testing structures (OTS). Techniques enabling sublithographic wavelength patterning using only conventional chrome-on-glass binary photomasks without phase shift features were achieved through the manipulation of mask bias designs and the partial coherence control of the lithographic exposure system. Together with the development of time multiplexed reactive ion etching and focus ion beam milling techniques, such design of the process allows the realization of highly dense PhC and MEMS actuators physically released from the buried oxide layer. Here, disparate pattern dimensions [with PhC critical dimensions (CDs) of only 175nm, MEMS typical dimensions of 2μm, and OTS openings more than 400μm wide], varied etch depth (3μm for the PhC and MEMS, 61μm for the OTS), and the requirement of a sufficient process latitude for exposure and etch processes are some of the key challenges that were overcome for a successful integration of air-bridge-type PhC CDs with movable MEMS actuators. Hence, the works described in this article enable MEMS tunable PhC properties with potential application in next generation dynamic optical communication networks and photonic integrated circuits.

Development of a two-dimensional phase-grating mask for fabrication of an analog-resist profile

Fabrication of a thick analog profile with photoresist is a difficult task in photolithography. We demonstrate that a binary phase-grating photomask with an appropriate period and duty cycles is capable of manipulating the exposure illumination in an analog fashion and can be used for fabrication of the desired analog micro-optics profiles on the surface of a thick photoresist. By choosing the proper period and variation of duty cycle of the phase-grating mask, one can create the desired analog intensity of exposure illumination for an optical stepper. This allows the formation of a wide range of analog micro-optics profiles with an SPR 220-7 photoresist. The numerical convolution of the diffraction efficiency curve and resist exposure characteristics is used to predict the final resist profile and also to design the appropriate duty-cycle distribution for the binary phase grating. As a demonstration of this technology, we fabricated a variety of micro-optical elements, such as a positive lens, ring lens, prism, and vortex of approximately 100-200 microm diameter, by using a phase-grating mask fabricated in a poly(methyl methacrylate) electron-beam resist.

Femtosecond fabricated photomasks for fabrication of microfluidic devices

This paper describes the direct write laser fabrication of a photolithography mask for prototyping of microfluidic devices in polydimethylsiloxane. An amplified femtosecond pulse laser is used to selectively remove the aluminium metal layer from the poly(methyl methacrylate) photomask substrate. The use of a femtosecond pulse laser to selectively etch a metal layer has several advantages over other conventional methods for binary photomask fabrication, namely rapid prototyping of microfluidic devices using soft lightography. Control of the energy density and defocus position of the focusing objective lens results in the etching of features with widths ranging from 2 microm to 35 microm when using an objective lens with numerical aperture of 0.25.

Tunable Metallization by Assembly of Metal Nanoparticles in Polymer Thin Films by Photo- or Electron Beam Lithography

The technique of patterning of surfaces with metal-rich structures on micro- or nanoscales was developed by assembling metal nanoparticles into a thin film of polymer in a controllable way. Palladium (Pd) nanoparticles were incorporated into a thin film of poly(methyl methacrylate) (PMMA) using palladium (II) bis(acetylacetonato), Pd(acac)(2), as a precursor vaporized in a nitrogen atmosphere. Depending upon its dose, the irradiation of a PMMA film by UV light or an electron beam (EB) enhances its reducing capability against Pd(acac)(2). This dependency on dose can be used to control the formation and assembly of Pd nanoparticles. Using this technique, binary patterns consisting of metal-rich and metal-poor regions in the polymer film can be created simply by irradiating the surface of the polymer through a binary photomask. Besides the creation of binary patterns, it is also possible to create grayscale patterns where the density of Pd nanoparticles can be tuned to provide shades of gray by the use of light with continuously modulated intensity. Because the electron beam also enhances the reducing power of PMMA against Pd(acac)(2), it is thus possible to obtain highly metallized films with nanoscale pattern features. The PMMA film can be selectively removed by oxygen plasma treatment or by pyrolysis. Thus, highly metallized surfaces with binary or grayscale patterns can be obtained by selective removal of the PMMA films. The metallized regions possess relatively high resistivity against CF(4) plasma compared to the bare silicon surface; therefore, the metallized surface patterns can be transferred onto the underlying silicon substrate by CF(4) plasma treatment. Because of the nanosize effect of metal nanoparticles, the thermal treatment at 900 degrees C, which is significantly lower than the melting temperature of the bulk Pd, yields continuous metallic features by binding the assembled nanoparticles.

Reflow technique for the fabrication of an elliptical microlens array in sol-gel material.

A simple reflow technique is employed for the fabrication of elliptical refractive microlens arrays (MLAs) on a low-cost inorganic-organic SiO2/ZrO2 sol-gel glass. The measured results show that the fabricated elliptical microlenses in a 256 x 512 array have excellent surface and dimensional qualities in terms of smoothness and uniformity. It is also shown that the optical parameters of the MLAs, such as the focal length and aperture dimension, can be accurately controlled. The proposed technique requires only an ordinary binary photomask for pattern transfer. Furthermore, the sol-gel material is found to be feasible for high-volume production, because the fabrication of MLAs can be implemented by use of cheap sol-gel materials without an etching step.

Femtosecond laser ablation of thin films for the fabrication of binary photomasks

Femtosecond lasers have considerable advantages over conventional laser sources for microfabrication. Even features with sizes very much less than the focused beam spot size of the laser can be fabricated using these lasers. Considering the potential of the laser for microfabrication, a valuable technique has been developed for the fabrication of binary photomasks by using femtosecond laser ablation of thin films. This is a single-step process and is faster compared to the complex multi-step lithography process currently used for photomask fabrication. A dispersion compensated acousto-optic scanning system has been developed for scanning the beam to transfer the required pattern to the mask blank. The applicability of this technique for microfabrication has been proven conclusively by fabricating microfeatures with the photomask fabricated by this technique. This can be made industrially viable by incorporating femtosecond laser sources with high pulse repetition rates.

Direct photolithographic deforming of organomodified siloxane films for micro-optics fabrication.

Direct photolithographic deforming of hybrid glass films is used to fabricate optical structures. The structure is fabricated in polyethylene-oxide-acrylate modified hybrid glass films with (1) binary and gray-scale photomasks using a mercury UV-lamp exposure and (2) maskless UV-laser patterning. Fabrication of isolated lenslets, lens arrays, and gratings is presented, including the associated exposure patterns. The hybrid glass material yields light-induced deformation peak-to-valley (p.v.) heights up to 12.8 microm with mercury UV-lamp exposure and p.v. deformation heights up to 6.8 microm with 365-nm UV-laser exposure. The fabricated lenslets' surface data are presented as Zernike-polynomial fit coefficients. Material synthesis and processing-related aspects are examined to understand and control the material's deformation under exposure. The hybrid glass material exhibits a maximum spectral extinction coefficient of 1.6 x 10(-3) microm(-1) at wavelengths ranging from 450 to 2,200 nm and has a refractive index of 1.52 at 632.8 nm. The fabricated structures exhibit rms surface roughness between 1 and 5 nm.

Fabry-perot-type antireflective coating for deep-ultraviolet binary photomask applications.

We demonstrate an antireflective coating structure, which is based on the three-layer metal interference called the Fabry-Perot structure, for a deep-ultraviolet binary mask. The antireflective coating structure is composed of a metal-oxide-metal stack. By addition of different optimized structures, reflectances of less than 1.5% at both 248 and 193 nm have been achieved. At the three-layer Fabry-Perot structure, the bottom chrome layer provides suitable absorption. By controlling the thickness of the intermediate silicon oxide layer, we can tune the minimum-reflection regime to the desired exposure wavelength. The top metal layer can prevent charge accumulation during e-beam writing.

Fabrication of micro-optical structures by applying negative tone hybrid glass materials and greyscale lithography

Direct lithographic patterning of lenslet arrays with lens sags up to 75 µm and opto-mechanical structures with thickness up to 118 µm is demonstrated, using negative tone hybrid glass materials and greyscale and binary photomasks. The hybrid glass material features a maximum extinction coefficient of 2.0×10−4 µm−1 between 450 and 1600 nm, a refractive index of 1.531 at 632.8 nm and an estimated Abbe number of 45. The patterned structures exhibit rms surface roughness between 10 to 45 nm.

Near-field optical photomask repair with a femtosecond laser.

We present a high-resolution near-field optical tool designed for repair of opaque defects in binary photomasks. Both instrument design and near-field imaging and patterning results will be presented. Designed for ablative processing of thin metal films, the MR-100 incorporates an industrial amplified femtosecond laser, third harmonic generator and built-in autocorrelator. The ultrashort duration of the femtosecond pulses enables the tool to remove chrome layers with negligible damage to the surrounding metal or the underlying quartz substrate. The micropipette based near-field writing head can deliver power densities of hundreds of GW/cm2 to spots of several hundred nanometres and below. Repairs on sample masks will be presented and the repair quality will be discussed.