Pattern Transfer Process Research Articles

Single-Digit Nanometer Electron-Beam Lithography with an Aberration-Corrected Scanning Transmission Electron Microscope.

We demonstrate extension of electron-beam lithography using conventional resists and pattern transfer processes to single-digit nanometer dimensions by employing an aberration-corrected scanning transmission electron microscope as the exposure tool. Here, we present results of single-digit nanometer patterning of two widely used electron-beam resists: poly (methyl methacrylate) and hydrogen silsesquioxane. The method achieves sub-5 nanometer features in poly (methyl methacrylate) and sub-10 nanometer resolution in hydrogen silsesquioxane. High-fidelity transfer of these patterns into target materials of choice can be performed using metal lift-off, plasma etch, and resist infiltration with organometallics.

High-Definition Nanoimprint Stamp Fabrication by Atomic Layer Etching

Nanoimprint lithography (NIL) has the potential for low-cost and high-throughput nanoscale fabrication. However, the NIL quality and resolution are usually limited by the shape and size of the nanoimprint stamp features. Atomic layer etching (ALE) can provide a damage-free pattern transfer with ultimate etch control for features of all length scales, down to the atomic scale, and for all feature geometries, which is required for good quality and high-resolution nanoimprint stamp fabrication. Here, we present an ALE process for nanoscale pattern transfer and high-resolution nanoimprint stamp preparation. This ALE process is based on chemical adsorption of a monoatomic layer of dichloride (Cl2) on the silicon surface, followed by the removal of a monolayer of Cl2-modified silicon by argon bombardment. The nanopatterns of different geometries, loadings, and pitches were fabricated by electron beam lithography on a silicon wafer, and ALE was subsequently performed for pattern transfer using a resist as an etc...

Enhanced light extraction of plastic scintillator using large-area photonic crystal structures fabricated by hot embossing

Plastic scintillators are widely used in various radiation measurement systems. However, detection efficiency and signal-to-noise are limited due to the total internal reflection, especially for weak signal detection situations. In the present investigation, large-area photonic crystals consisting of an array of periodic truncated cone holes were prepared based on hot embossing technology aiming at coupling with the surface of plastic scintillator to improve the light extraction efficiency and directionality control. The experimental results show that a maximum enhancement of 64% at 25° emergence angle along Γ-M orientation and a maximum enhancement of 58% at 20° emergence angle along Γ-K orientation were obtained. The proposed fabrication method of photonic crystal scintillator can avoid complicated pattern transfer processes used in most traditional methods, leading to a simple, economical method for large-area preparation. The photonic crystal scintillator demonstrated in this work is of great value for practical applications of nuclear radiation detection.

(Invited) Exploration of Process Techniques for Ga2O3 Based Electronics

Monoclinic β-phase Ga2O3 has outstanding potential for power electronics, and high quality, large diameter bulk crystals and epitaxial layers of Ga2O3 are already available with a range of controllable n-type doping levels by edge-defined film-fed (EFG) growth using iridium crucibles, by Czochralski or by float zone. The direct energy bandgap of Ga2O3, ∼4.9 eV, yields a very high theoretical breakdown electric field (∼8 MV/cm). For power electronics, the Baliga figure-of-merit proportional carrier mobility, critical electric field and breakdown voltage, is almost four times higher for Ga2O3 than for GaN. Currently, the major limitation for Ga2O3 based device fabrication is the lack of low resistance Ohmic contacts, low damage dry etching process, and thermally stable Schottky contacts. In this work, we report a technique by employing Aluminum Zinc Oxide (AZO) to improve Ohmic contacts on Ga2O3, studies of etch rates and etching induced damages with Cl2/Ar and BCl3/Ar based discharge as well as Ni/Au and Pt/Au Schottky contacts. High quality Ohmic contacts are a prerequisite for any device to deliver low contact resistance at moderate anneal temperatures. Additional contact resistance leads to slower device switching speeds as well as reliability issues due to local contact heating from high contact resistance during device operation. AZO thin films have been widely studied for transparent and flexible device applications such as liquid crystal displays, plasma display panels, electronic paper displays, organic light emitting diode, solar cells, touch panels, gas sensors and other optoelectronics devices. To be used as a contact layer on Ga2O3, a band alignment to ensure a less barrier to electron transport from the contact metal layer via AZO into the Ga2O3.is a prerequisite. The AZO/Ga2O3 heterojunction was realized with a nested gap alignment of band offsets with a valence band offset of 0.61eV ± 0.06eV and a conduction band offset of 0.79 eV ± 0.80 eV determined from x-RayPhotoelectron Spectroscopy (XPS) measurements. This is useful information for Ohmic contact schemes on n-type Ga2O3 devices. A minimum transfer resistance and specific contact resistance of 0.42 Ω-mm and 2.82 ´ 10-5 Ω-cm2 were achieved after a relative low annealing temperature of 400°C. It is essential to generate high resolution pattern transfer processes on semiconductor with wet chemical or dry etching. While numerous wet etchants been reported, these etching processes involve strong acids and elevated temperatures, and the standard positive photoresists will not be able to use as the etching mask. For dry etching, although, the etching rates are generally low, and the resultant oxygen deficient surfaces, standard device patterning and process are compatible with Ga2O3 based device fabrication. Both Cl2/Ar and BCl3/Ar discharges were used to etch high quality bulk Ga2O3 with a Plasma-Therm Inductively Coupled Plasma (ICP) system in this work. The etching is ion-enhanced and leads to a reduction of Schottky barrier height and diode reverse breakdown voltage in diodes fabricated on the etched surfaces depending on the rf and IPC powers. For low rf and ICP power conditions, the reverse breakdown voltage decreased around 6%, while barrier height reduced from 1.2 to 1.01 eV. Under higher power conditions, the reverse breakdown voltage decreased around 35%, and the barrier height was reduced to 0.86 eV. For Schottky diodes, Ni/Au or Pt/Au were used to form Schottky contact on bulk or epitaxial β- Ga2O3. The barrier heights were 1.07 and 1.04 eV for Ni/Au and Pt/Au, respectively, at 25°C. The diode on-state resistances (Ron) decreased with increasing temperature. The temperature coefficient of reverse breakdown voltage (VB), β, was −4 mV/K for Ni/Au and −0.1 mV/K for Pt/Au. The figure-of-merit (VB 2/RON) was above 3 MW/cm2 at 25°C for Ni/Au diodes and was still ∼1 MW/cm2 at 200°C. The reverse recovery times were also measured as a function of temperature and were of the order of 21–28 ns over the range 25–150°C.

Enhancing the imaging quality and fabrication efficiency of bionic compound eyes using a sandwich structure

This paper puts forward a novel method for fabrication of sandwich-structured BCE using a detachable micro-hole array (MHA) prepared by 3D printing. Compared with most traditional methods, 3D printing enables effective implementation of direct micro-fabrication for curved BCE without the pattern transfer and substrate reshaping process. This 3D fabrication method allows rapid fabrication of the curved BCE and automatic assembly of the detachable MHA using a custom-built mold under negative pressure. The formation of a multi-focusing micro-lens array (MLA) was realized by adjusting the parameters of the curved detachable MHA. The imaging performance was effectively enhanced by the sandwich structure that consist of the multi-focusing MLA, the outer detachable MHA and the inner solidified MHA. This method is suitable for mass production due to its advantages as a time-saving, cost-effective and simple process. Optical design software was used to analyze the optical properties, and an imaging simulation was performed.

Bimodal phase separated block copolymer/homopolymer blends self-assembly for hierarchical porous metal nanomesh electrodes.

Transparent conducting electrodes (TCEs) are essential components in various optoelectronic devices. Nanostructured metallic thin film is one of the promising candidates to complement current metal oxide films, such as ITO, where high cost rare earth elements have been a longstanding issue. Herein, we present that multiscale porous metal nanomesh thin films prepared by bimodal self-assembly of block copolymer (BCP)/homopolymer blends may offer a new opportunity for TCE. This hierarchical concurrent self-assembly consists of macrophase separation between BCP and homopolymer as well as microphase separation of BCP, and thus provides a straightforward spontaneous production of a highly porous multiscale pattern over an arbitrary large area. Employing a conventional pattern transfer process, we successfully demonstrated a multiscale highly porous metallic thin film with reasonable optical transparency, electro-conductance, and large-area uniformity, taking advantage of low loss light penetration through microscale pores and significant suppression of light reflection at the nanoporous structures. This well-defined controllable bimodal self-assembly can offer valuable opportunities for many different applications, including optoelectronics, energy harvesting, and membranes.

Role of the dense amorphous carbon layer in photoresist etching

The development of new photoresists for semiconductor manufacturing applications requires an understanding of the material properties that control the material's plasma etching behavior. Ion bombardment at ion energies of the order 100 s of eV is typical of plasma-based pattern-transfer processes and results in the formation of a dense amorphous carbon (DAC) layer on the surface of a photoresist, such as the PR193-type of photoresist that currently dominates the semiconductor industry. Prior studies have examined the physical properties of the DAC layer, but the correlation between these properties and the photoresist etching behavior had not been established. In this work, the authors studied the real-time evolution of a steady-state DAC layer as it is selectively depleted using an admixture of oxygen into an argon plasma. Observations of the depletion behavior for various DAC layer thicknesses motivate a new model of DAC layer depletion. This model also correlates the impact of the DAC layer thickness with the etch rate of the bulk photoresist. The authors find that up to a 40% depletion of the DAC layer thickness does not have a significant impact on the bulk photoresist etch rate. However, further depletion results in an exponential increase in the etch rate, which can be up to ten times greater at full depletion than for the fully formed DAC layer. Thus, with these trends the authors show that the photoresist etch rate is controlled by the thickness of the DAC layer. Furthermore, thickness loss of the DAC layer in an O2-containing plasma coincides with a chemical modification of the layer into an oxygen-rich surface overlayer with properties that are intermediate between those of the DAC layer and the bulk photoresist. Support for this interpretation was provided via x-ray photoelectron spectroscopy characterization. Atomic force microscopy was used to gauge the impact on surface roughness as the DAC layer is formed and depleted. The trends established in this work will provide a benchmark in our development of new photoresists, which will be suitable for pattern transfer processes that will ultimately be a part of enabling smaller semiconductor device feature sizes and pitches.

Rapid fabrication of curved microlens array using the 3D printing mold

In this paper, we presented a new method to directly fabricate 3D microlens array (MLA) without the pattern transfer and substrate reshaping process. Gas-assist deformed concave Polydimethylsiloxane (PDMS) film was used as the molding template. The template was fixed on the curved micro-hole array with a micro cavity during the injection molding process. The curved micro-hole array was fabricated by 3D printing in order to avoid the interference from microlens nearby. In addition, the micro-hole array is detachable, thus allowing it to be assembled onto the MLA automatically. Less than few minutes, the formation of 3D MLA can be realized by which the polymeric lens material is capable of being cured within dozens of seconds. Multi-focusing MLA was designed to reduce the defocus. And the focal length of each microlens varied according to the different diameter of micro-holes. This novel method is time-saving, cost-effective and precision. A single microlens has a radius of 250–500 μm, and the accuracy can reach 30 μm. Performance of the 3D MLA has been optically characterized. The introduction of micro-holes array can effectively reduce the crosstalk between the eyes and the influence of stray light. The curved MLA have has a FOV over 70°.

Spray-coatable negative photoresist for high topography MEMS applications

In microsystem technology, the lithographical processing of substrates with a topography is very important. Interconnecting lines, which are routed over sloped topography sidewalls from the top of the protecting wafer to the contact pads of the device wafer, are one example of patterning over a topography. For structuring such circuit paths, a photolithography process, and therefore a process for homogeneous photoresist coating, is required. The most flexible and advantageous way of depositing a homogeneous photoresist film over structures with high topography steps is spray-coating. As a pattern transfer process for circuit paths in cavities, the lift-off process is widely used. A negative resist, like ma-N (MRT) or AZnLOF (AZ) is favoured for lift-off processes due to the existing negative angle of the sidewalls. Only a few sprayable negative photoresists are commercially available. In this paper, the development of a novel negative resist spray-coating based on a commercially available single-layer lift-off resist for spin-coating, especially for the patterning of structures inside the cavity and on the cavity wall, is presented.A variety of parameters influences the spray-coating process, and therefore the patterning results. Besides the spray-coating tool and the parameters, the composition of the resist solution itself also influences the coating results. For homogeneous resist coverage over the topography of the substrate, different solvent combinations for diluting the resist solution, different chuck temperatures during the coating process, and also the softbake conditions, are all investigated. The solvent formulations and the process conditions are optimized with respect to the homogeneity of the resist coverage on the top edge of the cavities. Finally, the developed spray-coating process, the resist material and the process stability are demonstrated by the following applications: (i) lift-off, (ii) electroplating, (iii) the wet and (iv) the dry chemical etching of metals on substrates with topographies.

A flexible two dimensional force sensor using PDMS nanocomposite

This paper presents a two dimensional force sensor fabricated on PDMS nanocomposite, with patterned carbon nanotubes (CNTs) acting as a force sensing unit. A novel fabrication method is composed of inkjet printing of CNTs onto polyethylene terephthalate (PET) and subsequent transfer of the CNT patterns to PDMS, resulting in a CNT-elastomer nanocomposite that is flexible and conductive. This approach allows patterning of a large-area conductive carbon nanotube pattern on PDMS. The achieved sheet resistance of the transferred patterns on PDMS was 1.2kΩ/□ when printed 35 times, using an office inkjet printer. The fabricated sensor changes its resistance when force is applied perpendicularly to the sensor. A two dimensional force sensor, working on the principle of compression-induced deformation was fabricated and characterized with achieved resolution of four sensing cells per cm2. Additionally, we demonstrate a two dimensional flexible force sensor capable of creating a pressure map of the applied force. Together with inkjet printing, this pattern transfer process represents a highly effective patterning technique for embedding carbon nanotubes in PDMS.

In-depth TEM characterization of block copolymer pattern transfer at germanium surfaces

Dry plasma etching for the pattern transfer of mask features is fundamental to semiconductor processing and the development of device and electrically conducting elements becomes more challenging as features reach the deep nanoscale regime. In this work, high resolution transmission electron microscopy (TEM) coupled with energy dispersive x-ray (EDX) characterization were used to analyze the pattern transfer of graphoepitaxially aligned block copolymer (BCP) features to germanium (Ge) substrates as a function of time. The BCP patterns were converted into metal oxide hardmasks in order to affect good aspect ratios of the transferred features. An unusual interface layer between metal oxide nanowires and the germanium-on-insulator substrate was observed. EDX analysis shows that the origin of this interface layer is a result of the presence of a negative tone e-beam resist material, HSQ (hydrogen silsesquioxane). HSQ was employed as a guiding material to align line-space features of poly(styrene)-block-poly(4-vinylpyridine) (PS-b-P4VP) BCP with 16 nm half-pitch topography. Additionally, the existence of a metal oxide layer (from the initial PS-b-P4VP film) is also shown through ex situ TEM and EDX characterization. Three dimensional modeling of features is also provided giving a unique insight into the arrangement and structure of BCP features prior to and after the pattern transfer process. The results presented in this article highlight the accuracy of high resolution electron microscopy and elemental mapping of BCP generated on-chip etch masks to observe and understand through-film features affecting pattern transfer.

Electroplating through Fluidic Channels as Production Technology for 3D Interconnect Devices and Sensing Structures

Electroplating is a common process in micro technologies and in large scale application to create e.g. corrosion protected surfaces (1). New technologies as three dimensional printing and vacuum casting enable new methods to create surfaces. This provides the opportunity to merge micro structures [e.g. conductors] with macro structures [e.g. curvature].The created textures might be implemented in an electroplating based pattern transfer process. The results indicate the possibility of creating functional planar and three dimensional structures using the described technology.

Sono-electrodeposition transfer of micro-scale copper patterns on to A7 substrates using a mask-less method

A patterned anode tool was used to transfer electrodeposited microstructures on to an un-patterned A7 sized cathode by maintaining very narrow separation (300μm) between the two electrodes and agitating the fluid in the inter-electrode gap by ultrasonic means. A non-acidic copper solution with a low content of metal ions and additives was used. Limiting current experiments were initially performed to demonstrate that improved and uniform agitation could be maintained within the inter-electrode gap at relatively low ultrasonic powers of 5 to 30WL−1. The best pattern definition was obtained at a US power of 5WL−1 and a current density of 20mAcm−2. Importantly, the results obtained were comparable to those obtained by conventional through-mask plating. A single anode tool could be used to pattern up to five substrates, substantially minimising the amount of lithographic processing required. These results suggest that the proposed technique is a useful mask-less microfabrication process for pattern transfer on to large substrates.

Creation of High-Density Nanostructures for Use in Energy Conversion Applications

Nanostructures have garnered much interest in recent years for their potential in increasing performance of alternative energy devices, such as solar cells, thermoelectrics, batteries, and piezoelectrics. Many methods exist for the fabrication of nanostructures, but one of the most promising methods is the use of nanoporous aluminum oxide templates due to its extremely high density and self-ordering through a hexagonal, close-packed structure. However, the initial aluminum growth substrate is generally not suitable for use in energy conversion devices so a major challenge becomes the practical use of aluminum oxide templates. In this presentation, we demonstrate how to effectively utilize the unique advantages of aluminum oxide templates for a variety of applications. We introduce a method to transfer aluminum oxide templates to a substrate of interest. Using this process, we are able to pattern highly-ordered nanostructured materials with dimensions below 10 nm that may be suitable for use in thermoelectric devices. We also use this pattern transfer process to fabricate porous templates on complex multilayered substrates. These multilayered nanostructures can be used in many applications. Here, we present the growth of highly-ordered, catalyzed carbon nanotubes. These methods highlight an effective approach for creating the highly-dense, highly-ordered nanostructures required for many energy conversion applications.

Photon nanojet lens: design, fabrication and characterization

In this paper, a novel nanolens with super resolution, based on the photon nanojet effect through dielectric nanostructures in visible wavelengths, is proposed. The nanolens is made from plastic SU-8, consisting of parallel semi-cylinders in an array. This paper focuses on the lens designed by numerical simulation with the finite-difference time domain method and nanofabrication of the lens by grayscale electron beam lithography combined with a casting/bonding/lift-off transfer process. Monte Carlo simulation for injected charge distribution and development modeling was applied to define the resultant 3D profile in PMMA as the template for the lens shape. After the casting/bonding/lift-off process, the fabricated nanolens in SU-8 has the desired lens shape, very close to that of PMMA, indicating that the pattern transfer process developed in this work can be reliably applied not only for the fabrication of the lens but also for other 3D nanopatterns in general. The light distribution through the lens near its surface was initially characterized by a scanning near-field optical microscope, showing a well defined focusing image of designed grating lines. Such focusing function supports the great prospects of developing a novel nanolithography based on the photon nanojet effect.

Fabrication of resonator–quantum well infrared photodetector focal plane array by inductively coupled plasma etching

Inductively coupled plasma (ICP) etching has distinct advantages over reactive ion etching in that the etching rates are considerably higher, the uniformity is much better, and the sidewalls of the etched material are highly anisotropic due to the higher plasma density and lower operating pressure. Therefore, ICP etching is a promising process for pattern transfer required during microelectronic and optoelectronic fabrication. Resonator-quantum well infrared photodetectors (R-QWIPs) are the next generation of QWIP detectors that use resonances to increase the quantum efficiency (QE). To fabricate R-QWIP focal plane arrays (FPAs), two optimized ICP etching processes are developed. Using these etching techniques, we have fabricated R-QWIP FPAs of several different formats and pixel sizes with the required dimensions and completely removed the substrates of the FPAs. Their QE spectra were tested to be 30 to 40%. The operability and spectral non- uniformity of the FPA is ∼99.5 and 3%, respectively. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. (DOI: 10

Aspect ratio effect on the prediction of boundary layer interference in steady natural convection inside heterogeneous enclosures

Flow interference, caused by solid obstacles placed within an enclosed fluid undergoing natural convection, can have a tremendous impact on the flow pattern and heat transfer process. Disconnected, conductive, solid square blocks uniformly distributed inside a rectangular, horizontally heated enclosure are used as a platform to investigate this flow interference, which can be caused: (1) adjacent to the enclosure walls – when the (hot or cold) vertical fluid stream boundary layers grow enough to reach the solid blocks; and, (2) along the enclosure horizontal surfaces – when the top (or bottom) horizontal fluid streams get impeded by the adjacent blocks. Analytical equations, based on scale analysis, are derived for predicting the minimum number of blocks beyond which vertical and horizontal interferences take place, as function of the Rayleigh number (Ra), fluid volume fraction (porosity ϕ), and the enclosure length-to-height aspect ratio (A). The analytical predictions are validated against numerical simulation results, covering the ranges 105⩽Ra⩽108, 0.36⩽ϕ⩽0.84, and 0.25⩽A⩽4. The predictions prove the horizontal interference predominates in relation to the vertical interference in shallow (A>1) enclosures.

Surface Modification of Block Copolymer Through Sulfur Containing Plasma Treatment

Some of the important issues of block copolymer (BCP) as an application to the potential low cost next generation lithography are thermal stability and deformation during pattern transfer process in addition to defect density, line edge/width roughness, etc. In this study, sulfur containing plasma treatment was used to modify the BCP and the effects of the plasma on the properties of plasma treated BCP were investigated. The polystyrene hole pattern obtained from polystyrene polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) was initially degraded when the polystyrene hole was annealed at 190 °C for 15 min. However, when the hole pattern was treated using sulfur containing plasmas using H2S or SF6 up to 2 min, possibly due to the sulfurization of the polystyrene hole surface, no change in the hole pattern was observed after the annealing even though there is a slight change in hole shapes during the plasma treatment. The optimized plasma treated polystyrene pattern showed the superior characteristics as the mask layer by showing better thermal stability, higher chemical inertness, and higher etch selectivity during plasma etching.

Electrical yield verification of half-pitch 15 nm patterns using directed self-assembly of polystyrene-block-poly(methyl methacrylate)

A novel half-pitch (HP) 15 nm line pattern multiplication process with simple process steps and low cost-of-ownership using a polystyrene-block-poly(methyl methacrylate) lamellar block copolymer was developed to carry out process verification of directed self-assembly lithography on a 300 mm wafer for practical semiconductor device manufacturing. Electrical yield verification of HP 15 nm metal wire circuits fabricated by the HP 15 nm line pattern multiplication process was carried out on a 300 mm wafer. The electrical yield verification revealed the viability of the HP 15 nm line pattern multiplication process from the perspective of the total practical performance including critical dimension control, defect control, local placement error, line width roughness, line edge roughness, and process windows in the pattern transfer process.

Lift-Off Patterning of Nano-Crystalline Quantum Dot Films

The paper is concerned with the effectiveness of the lift-off process in pattern definition of ultra-thin nanocrystalline quantum dot (NQD) films. The lift-off process is implemented and investigated in conjunction with mist deposition and spin coating of NQD films. The lift-off process is preformed in gas-phase (oxygen plasma exposure) and liquid-phase (acetone immersion). The results of NQD patterned films using lift-off process are presented and discussed in terms of performance of the lift-off medium (dry and wet), resolution of the pattern transfer process, as well as photoresist thickness. It demonstrates that while fully compatible with the mist deposition, the patterning scheme proposed is not compatible with spin coating of NQDs films.