Grayscale Lithography Research Articles

3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.

Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52 GPa and 300 MPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems.

Grayscale lithography—automated mask generation for complex three-dimensional topography

Grayscale lithography is a relatively underutilized technique that enables fabrication of three-dimensional (3-D) microstructures in photosensitive polymers (photoresists). By spatially modulating ultraviolet (UV) dosage during the writing process, one can vary the depth at which photoresist is developed. This means complex structures and bioinspired designs can readily be produced that would otherwise be cost prohibitive or too time intensive to fabricate. The main barrier to widespread grayscale implementation, however, stems from the laborious generation of mask files required to create complex surface topography. We present a process and associated software utility for automatically generating grayscale mask files from 3-D models created within industry-standard computer-aided design (CAD) suites. By shifting the microelectromechanical systems (MEMS) design onus to commonly used CAD programs ideal for complex surfacing, engineering professionals already familiar with traditional 3-D CAD software can readily utilize their pre-existing skills to make valuable contributions to the MEMS community. Our conversion process is demonstrated by prototyping several samples on a laser pattern generator—capital equipment already in use in many foundries. Finally, an empirical calibration technique is shown that compensates for nonlinear relationships between UV exposure intensity and photoresist development depth as well as a thermal reflow technique to help smooth microstructure surfaces.

Chromatic-aberration-corrected diffractive lenses for ultra-broadband focusing

We exploit the inherent dispersion in diffractive optics to demonstrate planar chromatic-aberration-corrected lenses. Specifically, we designed, fabricated and characterized cylindrical diffractive lenses that efficiently focus the entire visible band (450 nm to 700 nm) onto a single line. These devices are essentially pixelated, multi-level microstructures. Experiments confirm an average optical efficiency of 25% for a three-wavelength apochromatic lens whose chromatic focus shift is only 1.3 μm and 25 μm in the lateral and axial directions, respectively. Super-achromatic performance over the continuous visible band is also demonstrated with averaged lateral and axial focus shifts of only 1.65 μm and 73.6 μm, respectively. These lenses are easy to fabricate using single-step grayscale lithography and can be inexpensively replicated. Furthermore, these devices are thin (<3 μm), error tolerant, has low aspect ratio (<1:1) and offer polarization-insensitive focusing, all significant advantages compared to alternatives that rely on metasurfaces. Our design methodology offers high design flexibility in numerical aperture and focal length, and is readily extended to 2D.

ZEP520A — A resist for electron-beam grayscale lithography and thermal reflow

The usability of ZEP520A as resist for thermally activated selective topography equilibration (TASTE) was investigated. It was found that (i) a high-contrast resist such as ZEP520A is well suitable for grayscale electron-beam lithography, (ii) a selective thermal reflow is possible with ZEP520A and (iii) reflow is governed by the same energy minimization principle as known from poly (methyl methacrylate) (PMMA), another linear thermoplastic resist. The high contrast of ZEP520A does not play a role for the step-to-step sidewall angle as this is only governed by the design of the step. ZEP520A and similar positive tone resists on the market provide the same reflow-feature variety as PMMA including stable and unstable, concave and convex, arbitrary stepped and sloped features as well as combinations of all these features. The advantage of ZEP520A is a reduced writing time due to its high sensitivity. Finally, the transfer of the reflow process to structures being much smaller than typical TASTE structures so far was demonstrated.

Experimental Study of Numerical Optimization for 3-D Microstructuring Using DMD-Based Grayscale Lithography

Digital micromirror device (DMD)-based grayscale lithography is a promising tool for 3-D photolithography of thick photoresists, because this technique provides a patterning solution for free-form 3-D microstructures. Among the numerous process parameters in DMD-based grayscale lithography, the exposure dose pattern corresponding to the grayscale mask pattern, the focal position in the photoresist, and the development time most strongly influence the final profile of the 3-D microstructure. However, finding the best combination of the three process parameters is a difficult and time-consuming task. In this paper, we propose a process optimization method for DMD-based grayscale lithography that is based on the 3-D photolithography simulation and sensitivity analysis. This optimization tool provides not only automatic process optimization for all three process parameters, i.e., the exposure dose pattern, the focal position, and the development time, but also a solution to improve fabrication accuracy for critical features by adopting a variable weight factor. Through a series of experiments, using a 20- $\mu \text{m}$ -thick positive photoresist, the validity and effectiveness of the proposed optimization method and the improvement of the fabrication accuracy were successfully demonstrated. [2015-0055]

Fabrication of 3-D Silicon Microneedles Using a Single-Step DRIE Process

Fabrication of 3-D microstructures is one of the most challenging aspects of silicon micromachining. In this paper, we present a novel microfabrication method using one single-step deep reactive ion etching process with gray-scale e-beam lithography mask that offers deeply etched (>350- $\mu \text{m}$ deep) dual-angle 3-D microneedles with control over the height and shape of the structures. Moreover, we found that the shape of the e-beam lithography patterns can determine the general configuration and features of the final etched microneedles, and that the etching process parameters have the most impact on the microneedles’ shape, such as size and vertical base angle. Large arrays of $20\times 20$ microneedles with height uniformity of better than 3% are fabricated. [2014-0209]

Three dimensional polymer waveguide using hybrid lithography.

A three dimensional polymer waveguide with taper structure was demonstrated and fabricated by a reliable and effective hybrid lithography. The hybrid lithography consists of lithography to fabricate a polymer waveguide and gray scale lithography to fabricate a polymer taper structure. Laser ablation and shadow aluminum evaporation were designed for gray scale lithography. The length of the gray scale region ranging from 20 to 400 μm could be controlled by the laser power, the ablation speed, and the aluminum thickness. The slope angle was determined by the length of the gray scale region and the thickness of the photoresist. The waveguide taper structure could be transferred to the lower layer by the etching method. The taper structure can be used for integration of the waveguide with different dimensions.

Toward ultimate nanophotonic light trapping using pattern-designed quasi-guided mode excitations

In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely pattern-wise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes.

Near Field Investigation of a Plasmonic Lüneburg Lens

In this work, we investigate the interaction of surface plasmons with a plasmonic Luneburg lens using near field scanning optical microscopy. Gray-scale electron beam lithography is used to prepare a dome-shaped resist structure on top of a gold film. This particular shape yields the effective refractive index profile of a Luneburg lens for surface plasmons propagating at the film surface at an energy of $\hbar \omega ={1.72}$ eV. Next to the Luneburg lens a grating coupler is milled into the gold film with focused ion beam. The surface plasmons are launched to propagate through the lens and the near field pattern is scanned. We clearly identify a focal spot in the near field signal at the outer perimeter of the lens. In addition, we observe a beating pattern arising from further plasmon waves excited by higher orders of the grating coupler. The emergence of this beating pattern allows the detection of the plasmon’s wave fronts. An analytical model was used to retrieve the properties of the participating wave components. The measured near-field pattern could be modeled very well with electromagnetic simulations applying the effective refractive index approach.

Removal of drawing traces of grayscale laser lithography by reflow process

In a conventional method where patterned photoresist structures are in direct contact with the metal-cylindrical roller’s surface employing a relief printing, the adhesion area normally does not have a sufficient strength to hold against a subsequent contact pressure and a moment of force that are encountered during the rotation of the roller. Therefore, to improve the robustness of the protuberant photoresist structures, we introduced a residual layer of photoresist (not exposed to any UV light), where the protuberant photoresist structures with sloped sidewalls were cemented onto the residual layer. And then this whole system was used to cover around the surface of the metal-cylindrical-roller. It has been well observed, and reported that a laser drawing system using grayscale lithography leaves traces on photoresist surfaces. However, these traces can now be removed by performing a reflow process under the optimal conditions. As a result of investigating the shape’s transition of the protuberant photoresist structures at different reflow times, and of the durability experiment of 10 times press operations using a flat press-plate, the practicality of our technique combining the grayscale laser lithography and the reflow process was confirmed.

Bio-inspired 3D funnel structures made by grayscale electron-beam patterning and selective topography equilibration

Electron beam grayscale lithography and selective thermal polymer reflow were combined to realize replication master having a bio-inspired surface topography. It consists of an array of asymmetric denticles with two-directional gradients on different length-scales. Each denticle has a sharp pin of 2μm height on one and a smooth taper on the other side. In contrast to funnel-like structures having only vertical sidewalls, master with a sidewall angle smoothly changing between 15° and 70° were prepared. Discrete patterns, predefined by electron beam lithography, were transformed by reflow into well-defined, continuous, final device structures. Starting from stepped structures with only a few levels, this new approach of tapering allowed a very smooth transition from steep, sub-500nm regions with aspect ratios up to four towards very broad and shallow regions within the same structure. This enabled the origination of master structures to be used for replication as low-frictional, durable, bio-inspired surfaces by ceramic powder injection molding.

Performance of SU-8 Membrane Suitable for Deep X-Ray Grayscale Lithography

In combination with tapered-trench-etching of Si and SU-8 photoresist, a grayscale mask for deep X-ray lithography was fabricated and passed a 10-times-exposure test. The performance of the X-ray grayscale mask was evaluated using the TERAS synchrotron radiation facility at the National Institute of Advanced Industrial Science and Technology (AIST). Although the SU-8 before photo-curing has been evaluated as a negative-tone photoresist for ultraviolet (UV) and X-ray lithographies, the characteristic of the SU-8 after photo-curing has not been investigated. A polymethyl methacrylate (PMMA) sheet was irradiated by a synchrotron radiation through an X-ray mask, and relationships between the dose energy and exposure depth, and between the dose energy and dimensional transition, were investigated. Using such a technique, the shape of a 26-μm-high Si absorber was transformed into the shape of a PMMA microneedle with a height of 76 μm, and done with a high contrast. Although during the fabrication process of the X-ray mask a 100-μm-pattern-pitch (by design) was enlarged to 120 μm. However, with an increase in an integrated dose energy this number decreased to 99 μm. These results show that the X-ray grayscale mask has many practical applications. In this paper, the author reports on the evaluation results of SU-8 when used as a membrane material for an X-ray mask.

Multiple Patterning with Process Optimization Method for Maskless DMD-Based Grayscale Lithography

We report a multiple patterning approach utilizing digital-micromirror-device (DMD)-based grayscale lithography, providing a solution to improve fabrication accuracy for entire target three-dimensional structure. Because DMD-based lithography system consists a projection lens system, better resolution can be obtained around focal position comparing to the outer region of depth of focus. Thus, for thick-film resist micro structuring, exposing with multiple focal positions with separate grayscale masks leads to improvement of fabrication accuracy. In order to find the best combination of the multiple focal positions and their grayscale masks, the computational optimization is combined to the multiple patterning approach. Through a several experiments, effectiveness of the proposed approach was successfully demonstrated.

Degradation and nano-patterning of ferroelectric P(VDF-TrFE) thin films with electron irradiation

Low energy electrons were found to degrade the ferroelectricity of P(VDF-TrFE) and a new grayscale lithography was hence proposed.

Interfacial Profile and Propagation of Frontal Photopolymerization Waves

We investigate the frontal photopolymerization of a thiol–ene system with a combination of experiments and modeling, focusing on the interfacial conversion profile and its planar wave propagation. We spatially resolve the solid-to-liquid front by FT-IR and AFM mechanical measurements, supplemented by differential scanning calorimetry. A simple coarse-grained model is found to describe remarkably well the frontal kinetics and the sigmoidal interface, capturing the effects of UV light exposure time (or dose) and temperature, as well as the front position and resulting patterned dimensions after development. Analytical solutions for the conversion profile enable the description of all conditions with a single master curve in the moving frame of the front position. Building on this understanding, we demonstrate the design and fabrication of gradient polymer materials, with tunable properties along the direction of illumination, which can be coupled with lateral patterning by modulated illumination or grayscale lithography.

Lithographically tuned one dimensional polymeric photonic crystal arrays

One-dimensional polymeric photonic crystal (PC) arrays with tuned bandgap properties are designed and fabricated on the same substrate by employing mainstream micropatterning technologies and appropriate photopatternable materials. The two photopatternable materials are based on poly (2-hydroxy ethyl methacrylate) and epoxy polymers with the addition of appropriate photoacid generators and are diluted in orthogonal solvents, ethyl lactate and PGMEA respectively. The tuning of the photonic band gap of each photonic crystal is achieved through carefully tuned lithographic processing conditions with emphasis in low-lithographic contrast operation and gray scale lithography. By applying sequential lithographic steps in orthogonal developers one-dimensional photonic crystal arrays with 10 bi-layers are fabricated with very well defined patterns. With this approach, PC arrays with four distinct areas are realized on the same Si substrate with tuned reflectance spectrum covering the whole visible spectral regime.

Enhancing photovoltaic output power by 3-band spectrum-splitting and concentration using a diffractive micro-optic.

We report the enhancement of photovoltaic output power by separating the incident spectrum into 3 bands, and concentrating these bands onto 3 different photovoltaic cells. The spectrum-splitting and concentration is achieved via a thin, planar micro-optical element that demonstrates high optical efficiency over the entire spectrum of interest. The optic (which we call a polychromat) was designed using a modified version of the direct-binary-search algorithm. The polychromat was fabricated using grayscale lithography. Rigorous optical characterization demonstrates excellent agreement with simulation results. Electrical characterization of the solar cells made from GaInP, GaAs and Si indicate increase in the peak output power density of 43.63%, 30.84% and 30.86%, respectively when compared to normal operation without the polychromat. This represents an overall increase of 35.52% in output power density. The potential for cost-effective large-area manufacturing and for high system efficiencies makes our approach a strong candidate for low cost solar power.

On-chip enucleation of an oocyte by untethered microrobots

We propose a novel on-chip enucleation of an oocyte with zona pellucida by using a combination of untethered microrobots. To achieve enucleation within the closed space of a microfluidic chip, two microrobots, a microknife and a microgripper were integrated into the microfluidic chip. These microrobots were actuated by an external magnetic force produced by permanent magnets placed on the robotic stage. The tip of the microknife was designed by considering the biological geometric feature of an oocyte, i.e. the oocyte has a polar body in maturation stage II. Moreover, the microknife was fabricated by using grayscale lithography, which allows fabrication of three-dimensional microstructures. The microgripper has a gripping function that is independent of the driving mechanism. On-chip enucleation was demonstrated, and the enucleated oocytes are spherical, indicating that the cell membrane of the oocytes remained intact. To confirm successful enucleation using this method, we investigated the viability of oocytes after enucleation. The results show that the production rate, i.e. the ratio between the number of oocytes that reach the blastocyst stage and the number of bovine oocytes after nucleus transfer, is 100%. The technique will contribute to complex cell manipulation such as cell surgery in lab-on-a-chip devices.

Holographic patterning of graphene-oxide films by light-driven reduction.

We report on the patterning and reduction of graphene-oxide films by holographic lithography. Light reduction can be used to engineer low-cost graphene-based devices by performing a local conversion of insulating oxide into the conductive graphene. In this work, computer-generated holograms have been exploited to realize complex graphene patterns in a single shot, different from serial laser writing or mask-based photolithographic processes. The technique has been further improved by achieving speckle noise reduction: submicron and diffraction-limited features have been obtained. In addition we have also demonstrated that the gray-scale lithography capability can be used to obtain different reduction levels in a single exposure.

Focused ion beam high resolution grayscale lithography for silicon-based nanostructures

Nanofabrication techniques providing a fine control over the profile of silicon structures are of great importance for nanophotonics, plasmonics, sensing, micro- and nano fluidics, and biomedical applications. We report on the applicability of focused ion beam for the fine grayscale lithography, which yields surface profiles that are customized at nanoscale. The approach is based on a correlation between the ion beam irradiation dose of inorganic resist and the mask etching rate in the reactive ion etching. An exceptional property of this method is the number of gray tones that are not limited by the resist characteristics. We apply the process to fabricate unique periodic nanostructures with a slope angle varying across the structure and a period as small as 200 nm.