Optical Lithography Methods Research Articles

5 GHz Band n79 wideband microacoustic filter using thin lithium niobate membrane

Microacoustic resonators made on suspended continuous membranes of LiNbO3 were recently shown to have very strong coupling and low losses at ≥5 GHz, suitable for high-performance filter design. Employing these simple resonator structures, the authors have designed, fabricated, and measured a 4.7 GHz bandpass ladder-type filter having 1 dB mid-band loss and 600 MHz bandwidth to address the 5G Band n79 requirements. The filter is fabricated on a monolithic substrate using standard i-line optical lithography and standard semiconductor processing methods for membrane release, starting with commercially available ion-sliced wafers having 400 nm thickness crystalline LiNbO3 layers. The filter is well-matched to a 50 Ω network and does not require external matching elements. Through accurate resonator engineering using our finite element method software filter design environment, the passband is spurious-free, and the filter provides better-than 30 dB rejection to the adjacent WiFi frequencies. This filter demonstrates the performance and scalable technology required for high-volume manufacturing of microacoustic filters >3.5 GHz.

Fabrication of superconducting nanowire single-photon detectors by nonlinear femtosecond optical lithography

This paper describes a new approach to the fabrication of superconducting nanowire single-photon detectors from ultrathin NbN films on SiO2 substrates. The technology is based on nonlinear femtosecond optical lithography and includes direct formation of the sensitive element of the detector (the meander) through femtosecond laser exposure of the polymethyl methacrylate resist at a wavelength of 525 nm and subsequent removal of NbN using plasma-chemical etching. The nonlinear femtosecond optical lithography method allows the formation of planar structures with a spatial resolution of ~50 nm. These structures were used to fabricate single-photon superconducting detectors with quantum efficiency no worse than 8% at a wavelength of 1310 nm and dark count rate of 10 s−1 at liquid helium temperature.

Study of multiplexer based on surface plasmon-polaritons for communication devices

Surface plasmon-polaritons provide a unique opportunity to create devices for signals localization and control on an optical subwavelength scale. They can be used as promising data carriers in highly integrated nanooptical transmission systems. Dielectric waveguides based on surface plasmon-polaritons (SPP) arise a particular interest in devices that will run in ultra high-speed data transmission ranges. The paper demonstrates the samples of the four-channel multiplexer based on SPP that works with ultra high-speed pulses. The multiplexer samples are developed using quite simple, but an extremely accurate method of phased optical lithography (POL). For excitation of the SPP, the 800 nm Ti: sapphire laser with a pulse frequency of 27 fs is used. We have shown the ultra high-speed distribution of SPP on the 10×5 µm multiplexer. Experimental studies are tested in the simulation by a finite difference method in the time domain. Good agreement between the experimental results and numerical simulation is obtained.

Surface plasmon lithography beyond the diffraction limit

Due to the light’s diffraction feature, conventional optical lithography methods have been suffering from the diffraction limit of resolution, which imposes great obstacles for obtaining nano patterns with ultra-violet and deep ultra-violet light source. Surface plasmons (SPs) are the free electron density waves confined at the interface of metal and dielectric medium, which exhibits unique sub-diffraction optical characteristic in SP propagation, focusing and imaging performances. In recent years, benefiting from SPs’ features beyond the diffraction limit, researchers have proposed a variety of SP lithography methods in the manner of interference, imaging and direct writing, demonstrating both in theoretical simulations and experiments that the sub-diffraction resolution could be readily obtained in SP lithography just using ultra-violet light source and single exposure process. The SPs interference lithography is simple and easy to realize nano periodic patterns with short wavelength of SPs, but usually deliver some shortcomings, like greatly restricted interference area size and complex fabrications of exciting structures. Bulk plasmon polaritons formed by greatly coupled SPs was proposed and demonstrated in our work for deep subwavelength large area interference lithography without the necessity of subwavelength exciting structures. SPs lithography in direct writing manner mainly originates from localized surface plasmon resonance behaviors of nano particles or metallic tips, further investigations of bow-tie structure and SP focusing lens with specific nano structures deliver enhanced resolution and intensity efficiency and help to realize point to point writing lithography with much more flexibility. Plasmonic lens imaging lithography seems to be particular interesting for applications as projecting nano irregular patterns in one exposure step. Superlens, as a simplified version of perfect lens, provides the physical prototype of SP imaging lithography. Further, the contributions of metal-dielectric-metal cavity lens and SP wave front engineering methods, like off axis illumination and near field proximity corrections, deliver great quality improvement of lithography patterns, including resolution, aspect ratio and fidelity. Half pitch resolution down to 22 nm (~1/17 light wavelength) has been obtained in SP imaging lithography experiments. Also some functional devices, like meta-lens and polarizer, have been successfully fabricated by plasmonic lens imaging lithography. It shows that SP helps to give a potentially promising access for high resolution, efficient, large area and low cost nano lithography tools. Besides the review and analysis of research progress, representative achievements of SP lithography, the remained problems and outlook of further developments are presented as well in this review.

Forming aluminum electrodes by screen printing and electron-beam evaporation for high performance interdigitated back contact solar cells

Interdigitated back contact (IBC) solar cell is one of the most efficient silicon solar cells commercially available to date. This paper studies the application of aluminum electrodes, deposited by electron-beam evaporation, instead of commonly used silver paste. Furthermore, screen printing instead of optical lithography method was used to generate etching masks. Together, we develop a low-cost technology for production. The specific contact resistivity of Al was found to be even lower than that of traditionally used Ag paste. The specific contact resistivity (ρc) and the recombination parameter (Jo) were found to be dependent on the doping level. Therefore, there is a design trade-off between a low Jo obtained with a low surface doping concentration, and a low ρc obtained with a high surface doping concentration. This process produced an IBC solar cell with a size of 4cm2, an open circuit voltage of 680mV, a conversion efficiency (η) of 22.72%, a fill factor of 0.80, and a short circuit current density of 41.69mA/cm2. The optimal design for the Al contact IBC cell was found by comparing the 3D Quokka© simulations of the Al contact and Ag paste contact IBC cells. The simulation indicated that further decrease in the Al contact size and spacing using screen printing and optimize the doping level could result in η up to 23.4% in production, comparable to the performance achieved in small cells using optical lithography.

A Method to Fabricate Thin Micro Silicon Cantilever Based on Optical Lithography with AZ 1518

AZ 1518 is a standard thin positive photoresist in AZ1500series which has been widely used in photolithography process, beside this, as a perfect semiconductor material, micro sized silicon cantilevers have been widely used in micro electromechanical systems (MEMS). In This paper, we have presented the process of fabricate micro silicon cantilever, including the selection of the photoresist, developer and substrate, optical lithography method and PLD method. After these, AFM is used to study the cantilever pattern and the angle of inclined planes. The research results show the exposure time and experimental environment can significantly influent the quality of cantilever pattern.

Wafer-scale metamaterials for polarization-insensitive and dual-band perfect absorption.

Mid-infrared (IR) perfect absorbers have great potential in practical applications such as biomedical sensing and thermal energy and have been successfully demonstrated in a number of plasmonic metallic nanostructures. However, all the experimental realizations of perfect absorbers are strongly dependent on nanofabrication techniques, which usually require high costs and a long time to fabricate a wafer scale device. Here we propose and experimentally demonstrate a wafer scale, polarization independent, wide angle, and dual-band IR perfect absorber. By fabricating double "E"-shaped metallic structures on a ZnSe coated gold film, a dual-band metamaterial absorber has been uniformly realized on a 2'' silicon wafer. Two absorption peaks have been realized at 18 and 27 THz, which are well consistent with the designs. We believe that our research will boost the applications of metamaterial perfect absorbers.

Optical generation of a spatially variant two-dimensional lattice structure by using a phase only spatial light modulator

We propose a simple and straightforward method to generate spatially variant lattice structures by optical interference lithography method. Using this method, it is possible to independently vary the orientation and period of the two-dimensional lattice. The method consists of two steps which are: numerical synthesis of corresponding phase mask by employing a two-dimensional integrated gradient calculations and experimental implementation of synthesized phase mask by making use of a phase only spatial light modulator in an optical 4f Fourier filtering setup. As a working example, we provide the experimental fabrication of a spatially variant square lattice structure which has the possibility to guide a Gaussian beam through a 90° bend by photonic crystal self-collimation phenomena. The method is digitally reconfigurable, is completely scalable, and could be extended to other kind of lattices as well.

Quantum optical lithography from 1nm resolution to pattern transfer on silicon wafer

Abstract Many attempts have been made to break the diffraction limit, a major problem in optical lithography. Here, we report and demonstrate a lithography method, quantum optical lithography, able to attain 1 nm resolution by optical means using new materials (fluorescent photosensitive glass–ceramics and QMC-5 resist). The performance is several times better than that described for any optical or electron beam lithography (EBL) methods. The written patterns on resist were transferred to Si wafer. SEM measurements show 5 nm line widths.

Continuous phase-shift lithography with a roll-type mask and application to transparent conductor fabrication

We report the development of a near-field optical nanolithography method using a roll-type phase-shift mask. Sub-wavelength resolution is achieved using near-field exposure of photoresist through a cylindrical phase mask, allowing dynamic and high throughput continuous patterning. As an application, we present the fabrication of a transparent electrode in the form of a metallic wire grid by using the roller-based optical lithography method. To fabricate a mesh-type metal pattern, a specific phase-shift mask was designed and critical experimental parameters were also studied. As a result, a transparent conductor with suitable properties was achieved with a recently built cylindrical phase-shift lithography prototype designed to pattern on 100 mm2 of substrate area.

Ultrabright source of entangled photon pairs

A source of triggered entangled photon pairs is a key component in quantum information science; it is needed to implement functions such as linear quantum computation, entanglement swapping and quantum teleportation. Generation of polarization entangled photon pairs can be obtained through parametric conversion in nonlinear optical media or by making use of the radiative decay of two electron-hole pairs trapped in a semiconductor quantum dot. Today, these sources operate at a very low rate, below 0.01 photon pairs per excitation pulse, which strongly limits their applications. For systems based on parametric conversion, this low rate is intrinsically due to the Poissonian statistics of the source. Conversely, a quantum dot can emit a single pair of entangled photons with a probability near unity but suffers from a naturally very low extraction efficiency. Here we show that this drawback can be overcome by coupling an optical cavity in the form of a 'photonic molecule' to a single quantum dot. Two coupled identical pillars-the photonic molecule-were etched in a semiconductor planar microcavity, using an optical lithography method that ensures a deterministic coupling to the biexciton and exciton energy states of a pre-selected quantum dot. The Purcell effect ensures that most entangled photon pairs are emitted into two cavity modes, while improving the indistinguishability of the two optical recombination paths. A polarization entangled photon pair rate of 0.12 per excitation pulse (with a concurrence of 0.34) is collected in the first lens. Our results open the way towards the fabrication of solid state triggered sources of entangled photon pairs, with an overall (creation and collection) efficiency of 80%.

Storage of Ag nanoparticles in pore-arrays of SU-8 matrix for antibacterial applications

Silver nanoparticles (NPs) stored in pore-arrays (pa) SU-8 photoresist layer were utilized as an antibacterial nanocomposition against E. coli bacteria. The pa-SU-8 matrix was fabricated by an optical interference lithography method resulting in small pores with a diameter of ∼50 nm and a depth of ∼100 nm. The Ag NPs were deposited on the soft polymeric matrix at different drying temperatures of 50 and 90 °C. X-ray photoelectron spectroscopy showed that the deposited silver NPs were substantially in the metallic state, independent from the drying condition. However, the concentration of the immobilized Ag NPs on the film surface increased (by a factor of 2.5) at the higher drying temperature. This increase was consistently observed in the strengthening of the surface plasmon resonance absorption peak of the Ag NPs. The size of the Ag NPs released from the Ag NPs/pa-SU-8 film was found to be in the range 7–15 nm. Antibacterial activity of the Ag NPs/pa-SU-8 films was investigated at different temperatures (37, 55 and 75 °C) of the solution containing the film. The relative rate of reduction of the viable bacteria was calculated to be 4.5 × 10−2 (2.8 × 10−2) min−1 at 37 °C, for the film dried at 90 (50) °C. By increasing the temperature to 75 °C, the rate of reduction increased to 8.4 × 10−2 (4.1 × 10−2) min−1, corresponding to 60 (120) min required time to kill all the bacteria. Silver ion release measurements showed that the higher drying temperature (90 °C) provided more diffusion of Ag NPs in the porous soft matrix to store a considerable amount of Ag NPs in it, resulting in a lasting antibacterial activity. The amount of silver released from the Ag NPs/pa-SU-8 film, and thus its antibacterial activity, can be controlled by small variation (∼20 °C) of the environmental temperature.

Manipulation of Magnetic Beads by Magnetic Fields of Narrow Metallic Wires for Use in Biosensors

In this paper, we investigated a means to manipulate small magnetic beads using the magnetic fields of current-carrying wires. Narrow metallic wires were fabricated with optical lithography and vacuum deposition methods. As we applied 20-40 mA of current, the magnetic beads could be collected on the wire. Depending on the magnitude of the currents, the velocity of the beads was 2-20 mum/s. In addition to the motion of the beads by the interaction with the fields, we also observed drift of the beads caused by the local heating of the wire and the resulting temperature gradient. We also demonstrated transfer of beads between wires by switching the current between the wires. We consider that this simple device can be used to transfer the beads to the locations of the sensors and that it can increase the reaction probability between beads and magnetic sensors.

Nanoscale thermal lithography by local polymer decomposition using a heated atomic force microscope cantilever tip

Nanopatterning of polymer thin films is the basis for the vast majority of current microlithography processes used in integrated circuit manufacturing. Future scaling of such polymer patterning methods will require innovative solutions to overcome the prohibitively high tool and mask costs associated with current optical lithography methods, which will prevent their use in many applications. Scanning probe-based methods for surface modification are desirable in that they offer high resolution patterning while also offering the ability to perform in situ metrology. We report a new scanning probe lithography method that uses heated atomic force microscope cantilevers to achieve nanoscale patterning in thin polymer films via the local thermal decomposition of the polymer and in situ postdecomposition metrology. Specifically, cross-linked polycarbonate thin films are developed in this work and are shown to be excellent writing media for this process. This new method has the advantage that the tip can be heated and cooled on microsecond time scales and thus material can be removed and patterned without need for the disengagement of the tip from the polymer surface. This ability to write while the tip is constantly engaged to the surface offers significantly higher writing speeds for discontinuous patterns relative to other scanning probe techniques.

Optimization of the electron‐beam‐lithography parameters for the moth‐eye effects of an antireflection matrix structure

AbstractIn this study, we mainly used the characteristics of electron‐beam lithography in measurement control and direct‐write technology to improve the physical restrictions and production processes of optical lithography and other nanopattern production methods. We did this by using a silicon wafer as a substrate, coating a negative‐tone photoresist, and using scattering and the reflection produced by the collision of an electron beam with the wafer lattice and the proximity effect of a secondary electron inside the electron‐beam photoresist to produce an antireflection matrix structure with a moth‐eye effect. In addition, we used the Taguchi quality method with an orthogonal array to plan the experiment and the signal‐to‐noise ratio to analyze the experimental data, and in the experimental process, we produced a full factorial equivalent experiment, using very few experiment repetitions and deriving optimum conditions. Also, we used back‐propagation neural networks to fine‐tune significant factors, allowing the production of the deepest process control parameters and thereby imparting to the antireflection matrix structure the best effect. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5303–5313, 2006

Near-field optical lithography method for fabrication of the nanodimensional objects

A new method of contact scanning near-field optical lithography has been developed to enable fabrication of elements with characteristic dimensions of 30–50 nm. The method involves the deposition of a thin-layer polymer-metal coating, the thermal destructive deformation of a top metal layer with a probe of scanning near-field optical microscope (SNOM), the transfer of the pattern through the polymer by using dry etching and the formation of various nanoelements through this prepared mask. The method is applicable to any material, e.g. metal, dielectric, light/heavy doped semiconductors for the formation of nanometre objects.

Microfabrication methods of magnetic tunnel junctions with high tunneling magnetoresistance

In this work, on the one hand, the contact-shadow-mask method and technique were used to micro-fabricate the magnetic tunnel junction (MTJ) and optimize the experimental conditions. The width of the gap for the long and narrow top or bottom magneto-electrode is 100 μm, which can be used to deposit MTJs and form a cross strip with the tunnel section of 100 μm×100 μm. The MTJs with tunneling magnetoresistance (TMR) ratio of 30%—48％ can be directly obtained for the structure of Ta(5 nm)/Cu(25 nm)/Ni79Fe21(5 nm)/Ir22Mn78(10 nm)/Co75Fe25(4 nm)/Al(08 nm)-O/Co75Fe25(4 nm)/Ni79Fe21(20 nm)/Ta(5 nm). On the other hand, t he MTJs with high TMR ratio and small active area from 20 μm×40 μm down to 4 μm×8 μm were fabricated using two optical lithography methods of milling contact hole and lift-off resist, combined with Ar ion-beam etching or CF4 reactive etching technique s. Then, the TMR ratio from 22% up to 50% can be achieved before and after annealing at around 300 ℃ for 1 h. Our investigation shows that the patterned MTJs, which were microfabricated using the two optical lithography methods stated above, can b e used as the fundamental element of magnetoresistive random access memory, magn etic read-heads in hard disk drives and the field sensitive sensor.

NiSi Nanowires and Nanobridges Formed by Metal-Induced Growth

AbstractNickel monosilicide (NiSi) nanowires (NWs) were fabricated by metal-induced growth at 575 °C. The solid-state reaction of Ni and Si provides linear grown NWs. The parallel grown NW forms a nanobridge (NB) across a trench, patterned with a simple optical lithography and metal lift-off method. The Ni pads gave a good Ohmic contact without affecting the I-V transport characteristics through a NB. The metallic NB, 2.73 µm in length and 50 nm in diameter, gave a low resistance of 148 . The self-assembled nanobridge can be applied to form nanocontacts at relatively low temperatures. The MIG NB is a promising 1 dimensional nanoscale building block to satisfy the need of ‘self and direct’ assembled ‘bottom-up’ fabrication concepts.

Predicting air entrainment due to topography during the filling and scanning process for immersion lithography

Current optical lithography methods are nearing theoretical limits that prevent their use in the production of circuits for future nodes. A proposed solution is to increase the index of refraction of the transmission medium between the final lens of the exposure system and the wafer. When a liquid is used in this lens–wafer gap, the process is known as immersion lithography. A major concern is air bubbles in the liquid, since they are sources of index discontinuities. This article investigates the potential for trapping air as the free surface of the fluid front moves over features associated with wafer topography during the filling and scanning process. Optical simulations have shown that even very small bubbles located near or on the resist can significantly impact the imaging process. Therefore, the ability to predict the characteristics of the flow, liquid, and features that lead to air entrainment during filling is important. Modeling techniques were developed in order to create models that were capable of resolving the flow characteristics over 100nm scale features without the need to simulate the entire macroscopic flow through the lens–wafer gap. Results of these models show that bubble formation occurs only for extreme geometries and flow conditions. In actual production the velocities, contact angles, and feature profiles are well out of these extreme ranges, and will not cause bubble formation.

Microstructured Au/TiO2 model catalyst systems.

The preparation of microstructured Au/TiO2 model catalysts as a first step toward micrometer-scale parallel studies on model catalysts and toward studies of mesoscopic effects in catalytic reactions was investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The model systems, which consist of micrometer-size active areas covered with Au nanoparticles that are separated by similarly sized inactive areas free of Au particles, are fabricated by combining optical lithography methods for microstructuring and ultrahigh vacuum evaporation for Au nanoparticle deposition and by applying suitable cleaning steps. It is demonstrated that practically perfect microstructures with Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) on a clean TiO2 substrate can be produced this way and that the processing steps do not affect the deposited Au nanoparticles, neither in size nor in lateral distribution.