Dry Lithography Research Articles

Direct Laser Writing Dry Lithography of High‐Resolution Micro‐/Nanostructures in AgSb4Te Thin Film for Tunable Perfect Absorber

Direct laser writing is promising for fabrication of functional structures due to its high speed, low cost, and facile operation. However, high resolution is difficult to be implemented due to wet development‐induced structural collapse. Ag‐doped Sb4Te thin film is proposed for high‐resolution direct laser writing dry lithography. The crystalline structures and binding environment of AgSb4Te thin films are investigated. The dry lithographic characteristics suggest that the developing selectivity of exposed to as‐deposited film is as high as 17 under CHF3/O2 gas and the minimum linewidth of patterns can reach 80 nm via adjusting grating period. Developing selectivity mechanism is ascribed to exposure induced phase separation along with generation of Sb component and Ag7Te4 phases. The Sb component can be readily etched due to the lack of Ag atom protection and then etching of residual Ag7Te4 phase is further promoted while the uniformly distributed Ag atoms in as‐deposited film restrain its etching. In addition, nanohole array is fabricated onto AgSb4Te thin films and tunable absorbance can be realized by adjusting pattern period and phase‐change process. This work may provide a useful guide for the investigation of direct laser writing lithography mechanism and functional device fabrication.

Dry Lithography Patterning of Monolayer Flexible Field Effect Transistors by 2D Mica Stamping.

Organic field-effect transistors (OFETs) based on 2D monolayer organic semiconductors (OSC) have demonstrated promising potentials for various applications, such as light emitting diode (LED) display drivers, logic circuits, and wearable electrocardiography (ECG) sensors. To date, the fabrications of this class of highly crystallized 2D organic semiconductors (OSC) are dominated by solution shearing. As these organic active layers are only a few molecular layers thick, their compatibilities with conventional thermal evaporated top electrodes or sophisticated photolithography patterning are very limited, which also restricts their device density. Here, an electrode transfer stamp and a semiconductor patterning stamp are developed to fabricate OFETs with channel lengths down to 3 µm over a large area without using any chemicals or causing any damage to the active layer. 2D 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10 -DNTT) monolayer OFETs developed by this new approach shows decent performance properties with a low threshold voltage (VTH ) less than 0.5 V, intrinsic mobility higher than 10 cm2 V-1 s-1 and a subthreshold swing (SS) less than 100 mV dec-1 . The proposed patterning approach is completely comparable with ultraflexible parylene substrate less than 2 µm thick. By further reducing the channel length down to 2 µm and using the monolayer OFET in an AC/DC rectifying circuit, the measured cutoff frequency is up to 17.3 MHz with an input voltage of 4 V. The newly proposed electrode transfer and patterning stamps have addressed the long-lasting compatibility problem of depositing electrodes onto 2D organic monolayer and the semiconductor patterning. It opens a new path to reduce the fabrication cost and simplify the manufacturing process of high-density OFETs for more advanced electronic or biomedical applications.

Investigation of etching selectivity and microstructure of Ag-doped Sb2Te thin film for dry lithography

Dry lithography is a promising micro-/nanomanufacturing method owing to its advantages of being solution-free, its absence of undercut and resistance to swelling. However, heat-mode resists suitable for dry lithography are less reported. This work reports on the use of Ag-doped Sb2Te thin film as a heat-mode resist, and its etching selectivity and microstructures are investigated in detail. It is found that Ag1.44Sb2.19Te thin film possesses high etching selectivity in CHF3/O2 mixed gases and can act as a heat-mode resist. In order to elucidate the mechanism of high etching selectivity, the microstructures of the Ag-doped Sb2Te thin films are analyzed using x-ray diffraction, Raman spectra, x-ray photoelectron spectroscopy, and transmission electron microscopy methods. The results show that the etching selectivity is attributed to the phase separation of Ag1.44Sb2.19Te film and the formation of the Sb phase after laser exposure, leading to a reduction in etching resistance in CHF3/O2 mixed gases. In addition, pattern transfers from Ag1.44Sb2.19Te to SiO2 and Si substrates are achieved successfully and the etching selectivities of Si and SiO2 to Ag1.44Sb2.19Te are both higher than 2:1. This work may provide a useful guide for the research of dry lithography without wet development.

AgGeSbTe thin film as a negative heat-mode resist for dry lithography

AgGeSbTe thin film as a negative heat-mode resist for dry lithography

Nanoimaging of Low-Loss Plasmonic Waveguide Modes in a Graphene Nanoribbon.

Graphene nanoribbons are predicted to support low-loss and tunable plasmonic waveguide modes with an ultrasmall mode area. Experimental observation of the plasmonic waveguide modes in graphene nanoribbons, however, is challenging because conventional wet lithography has difficulty creating a clean graphene nanoribbon with a low edge roughness. Here, we use a dry lithography method to fabricate ultraclean and low-roughness graphene nanoribbons, which are then encapsulated in hexagonal boron nitride (hBN). We demonstrate low-loss plasmon propagation with a quality factor up to 35 in the ultraclean nanoribbon waveguide using cryogenic infrared nanoscopy. In addition, we observe both the fundamental and the higher-order plasmonic waveguide modes for the first time. All the plasmon waveguide modes can be tuned through electrostatic gating. The observed tunable plasmon waveguide modes in ultraclean graphene nanoribbons agree well with the finite-difference time-domain (FDTD) simulation results. They are promising for reconfigurable photonic circuits and devices at a subwavelength scale.

A non-contact planar microwave sensor for detection of high-salinity water containing NaCl, KCl, CaCl2, MgCl2 and Na2CO3

Abstract In this paper, the concentrations of chloride salts (sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl2), calcium chloride (CaCl2)) and a carbonate salt (sodium carbonate (Na2CO3)) were measured using a non-contact planar microwave sensor. The design of the sensor was based on a combination of a microstrip line and a split ring resonator (SRR) with a circular hole in the middle of the SRR. The sensor was fabricated using dry film photoresist lithography technique and its performance was validated through simulation and experiment. Chloride and carbonate salt solutions at concentrations from 40 mg/mL – 200 mg/mL were measured in the frequency range 0.5 GHz – 2.2 GHz with four resonance frequencies applied in sensing. The various salt concentrations were measured under ambient conditions (room temperature (T): 25 ± 1 ๐C and relative humidity (RH): 40 ± 1 %). The reflection coefficient (S11) and the resonance frequency (Fr) were recorded and analyzed by logarithmic regression and principal component analysis (PCA) methods to find correlations between salt concentration and classification group. The sensor was able to detect and classify water containing NaCl, KCl, CaCl2, MgCl2 and Na2CO3. The planar microwave sensor is suitable to be used in non-contact detection applications of inorganics.

Highly Efficient, Flexible Wireless‐Powered Circuit Printed on a Moist, Soft Contact Lens

AbstractContact lens with built‐in electronics is a next‐generation wearable product with potential applications such as biomedical sensing and wearable displays. However, fabricating a wireless‐powered circuit on a moist, soft contact lens, via common dry lithography, makes producing smart contact lenses challenging. Here, electrochemically (EC) printing a wireless‐powered circuit onto a moist, soft contact lens is demonstrated. EC printing involves adding a conductive polymer at the interface between a metal contact and a hydrogel‐based contact lens, resulting in strong adhesion of the circuit to the lens without losing high power transfer efficiency (50%) from an eyeglass transmitter to the printed receiver lens. The energy transfer characteristics during eye movement are modeled using the Neumann equation and Kirchhoff's voltage law for wireless power transfer. The energy transfer efficiency between the eyeglass transmitter and the printed receiver lens is derived, and illumination of a wireless‐powered single light‐emitting diode display as a function of eye rotation angle is demonstrated. This work opens the door to integrating more complex circuits at soft contact lens interface to produce smart contact lens with increased functionality.

Facile modification of freestanding silicon nitride microcantilever beams by dry film photoresist lithography

A photolithographic technique based on dry film photoresists for facile and low-cost patterning of microcantilever beams is presented. Dry film photoresists enable instantly homogenous photoresist coatings on flexible and 3D patterned substrate surfaces, represented here by microcantilever beams, which is otherwise challenging if conventional spin-coating of photoresist is utilized. Compared to alternative microtechnologies, such as focused ion beam milling or resist spray coating, our strategy is far less elaborate, fully compatible with routine additive and subtractive microfabrication processes and can be readily scaled. We show specifically microcantilever shape modification by CF4 reactive ion etching, localized metal deposition in combination with conventional lift-off procedures as well as a utilization of patterned dry film photoresists as permanent microstructural elements. These microstructural elements are in particular flat-ended cylindrical dry resist micropillars created at the freestanding end of the cantilever beam that can be employed as scanning probes. The resist pillars enabled imaging of a 3T3 mouse fibroblast cell culture surface to determine their elastic force constants. Alongside UV-exposure by a conventional mask aligner, we also demonstrate dry film photoresist exposure by contact-free laser lithography eliminating possible substrate damage by photomask contact.

Novel prism designs for solid immersion optical lithography in the ultra high-NA regime

A new prism design has been fabricated and tested experimentally for solid immersion optical lithography of high aspect ratio structures in the ultra high-NA regime using dielectric resonant underlayers. Such a prism design has been made possible through past work that investigated ultra high-NA imaging in the absence of high-force, intimate contact between the sample and solid-immersion medium; key to this is the use of a close index matched prism/IML combination. The work presented here demonstrates the use of a novel prism design for patterning both low and high aspect ratio structures in the ultra high-NA regime; robust and repeatable imaging with such a new prism design is now possible as the prism is not subjected to the large mechanical pressure associated with ultra-fine gap control in dry solid-immersion lithography. The prism presented here allows fabrication for ~58 nm half-pitch line structures over large exposure areas using a 405 nm wavelength exposure.

Plasma-etching of 2D-poled glasses: A route to dry lithography

The basis of a lithographic technique for producing glassy structures of diffractive optics, integrated optics, microfluidics, plasmonics, etc., is presented. The technique is based on the integration of two “dry” procedures: (1) glass polarization with structured (with relief surface) anodic electrode and (2) plasma-chemical etching of the poled glass. A pilot relief structure (that is, relief pattern 0.5 μm in depth) on the glass surface has been produced with the proposed technique.

Use of Microdischarge Lithography for Development of Organic Electronic Devices over Large Areas

The concept of 'displays everywhere', which may be extended to wearable sensors, signal tags or energy devices, implies ubiquity of electronic devices not only in walls or appliances but integrated in all kind of domestic objects, such as clothes, bags or wrappings, and is expected to preside over our daily lives in the near future. For this expectation to succeed, Electronics industry must face compatibility with three new paradigms: flexible substrates -to be adapted to different surfaces and geometries-, large areas (eventually up to square meters), and lightness. In any case, this revolution can only take place from a drastic reduction of the production costs. As it is well known, these paradigms are difficult to access solely for conventional microelectronics technology based on silicon or other crystalline materials, and many alternatives based in new materials and procedures have been proposed. Thus, a manufacturing facility based in the roll-to-roll (R2R) technology is considered a suitable candidate to achieve high yield over large areas in a cost effective way. In consequence, those patterning techniques compatible with R2R production lines are receiving enhanced attention. In this contribution, we exploit the possibilities of a dry and subtractive lithography based on electrical microdischarges to pattern both electrodes, cathode and anode, of organic devices, particularly displays based in organic light emitting diodes (Fig. 1). Using a homemade prototype rated at technology readiness level 4, we demonstrate that electrical discharge lithography may overcome many obstacles such as tilt correction and relocation after processing, avoiding the use of masks and many of the photolithographic steps. This is thanks to the electrical monitoring during operation as well as a precise electronic control of the position, with 1 µm reproducibility. Since sparks are very fast processes (range of few µs or less) features may be performed at speeds of tens mm/s, ensuring scalability to large areas (cms), and hence compatibility with high throughput manufacturing lines. Of particular interest results patterning of graphene as electrode, due to the practical absence of residues under electrical discharges. Other advantages and drawbacks will be discussed, as well as physical aspects involved. Figure 1

Fabrication of zigzag-edged graphene antidot lattice and its transport properties

Graphene nanostructures with defined edges are proposed as a promising platform for the realization of nano-electronics and spin-electronics. However, patterned graphene nanostructure can lead to extra damage and drastically reduce its charge carrier mobility due to the edge disorder. The high flexibility of a top-down patterning method with edge smoothness is extremely desirable. Hydrogen plasma enhanced anisotropic etching graphene is demonstrated to be an efficient method of fabricating zigzag-edge graphene nanostructures. In addition, boron nitride is shown to be an excellent substrate for graphene due to its atomic flatness. Here in this work, we fabricate zigzag edge graphene antidot lattices on a boron nitride substrate via dry transfer method and traditional electron beam lithography, and reactive ion etching followed by hydrogen anisotropic etching approach. At low magnetic fields, weak localization is observed and its visibility is enhanced by intervalley scattering on antidot edges. We observe commensurate features in magnetotransport properties which stem from carriers around one antidot, signifying the high quality of our patterned samples. At high magnetic field, crossover from Shubnikov-de Haas oscillation to quantum Hall effect can be clearly observed due to the high mobility of our zigzag edge graphene antidot lattices. The transport properties of our patterned samples suggest that our fabrication method paves the way for achieving high quality graphene antidot lattices. High quality zigzag edge graphene antidot lattice might be a great platform to study the transport properties of lateral superlattice potential modulation graphene.

Flow behavior control in immersion lithography

Over the past decade, immersion lithography has been the primary technology for exposure process in semiconductor manufacturing. Compared with traditional dry lithography method, this technology improves the exposure resolution greatly by inserting a high index liquid into the gap between lens and wafer surface. Keeping the immersion liquid pure and uniform as well as avoiding residual droplets during high speed scanning motion are two challenges faced by the development of immersion lithography. Contaminations, particles, bubbles, heating and stress in the liquid will destroy the continuity of the refractive index. High speed motion of the wafer during scanning may break the meniscus stability on the interface between the liquid and the gas surrounding it, and then generate residual droplets on the wafer. All above phenomena will affect the exposure performance of immersion lithography and corresponding flow behavior control methods are required to solve the problems. This paper gives a review of the studies on flow behavior control in immersion lithography, which contains two parts: studies for liquid purity and uniformity and studies for meniscus stability. In each part, the mechanism and character of the flow behaviors as well as their effects to exposure performance are presented firstly. Then, control approaches adopted by now, including material and surface modifications as well as immersion head utilization, are introduced. In addition, the challenges faced in future studies are also pointed out. The purpose of this review paper is to help the researchers to understand the flow control problems in immersion lithography and to provide better control approaches for exposure performance improvement.

ToF‐SIMS depth profiles on Argon‐implanted amorphous carbon. Damage effect and hydrogen characterization

Any noble gas characterization that includes chemical ionization for the detection process is hard to manage because of the very high ionization potential: the range is from 10 to 25eV; electronic affinity is also close to zero. So ionization processes used in secondary ion mass spectroscopy (SIMS) analysis have very poor cross section. It is known that amorphous carbon mechanical properties are changed by implant; sp2/sp3 ratio and H content in the film are modified, allowing high density and good transparency needed for dry etching and lithography steps. In this paper, we propose to monitor secondary ion clusters Cx–Hy by time of flight‐SIMS to obtain information on amorphous carbon layer implanted with Ar and to study Cn/C5 ratio to understand sp2/sp3 ratio behavior. Further consideration on 13CH as hydrogen representative will be addressed. A binary collision code Stopping and Range of Ions in Matter (SRIM) was used to estimate the depth of the region damaged by Ar+ implantation. In implanted samples, the profiles of Cx–Hy clusters have an in‐depth gradient. This is an indication that the film was involved in an energy release process. Therefore, these profiles give information on the implanted region even if the Ar profile is not collected. Although the SIMS technique uses ion beams that produce damages similar to those due to the implant process, a 13CN depth profile behavior was found able to discriminate between damage effects produced by that or the other. Copyright © 2016 John Wiley & Sons, Ltd.

Development of electrical-erosion instrument for direct write micro-patterning on large area conductive thin films.

We have developed a complete instrument to perform direct, dry, and cost-effective lithography on conductive materials, based on localized electrical discharges, which avoids using masks or chemicals typical of conventional photolithography. The technique is considered fully compatible with substrate transport based systems, like roll-to-roll technology. The prototype is based on two piezo nano-steppers coupled to three linear micro-stages to cover a large scale operation from micrometers to centimeters. The operation mode consists of a spring probe biased at low DC voltage with respect to a grounded conductive layer. The tip slides on the target layer keeping contact with the material in room conditions, allowing continuous electric monitoring of the process, and also real-time tilt correction via software. The sliding tip leaves an insulating path (limited by the tip diameter) along the material, enabling to draw electrically insulated tracks and pads. The physical principle of operation is based in the natural self-limitation of the discharge due to material removal or insulation. The so produced electrical discharges are very fast, in the range of μs, so features may be performed at speeds of few cm/s, enabling scalability to large areas. The instrument has been tested on different conducting materials as gold, indium tin oxide, and aluminum, allowing the fabrication of alphanumeric displays based on passive matrix of organic light emitting diodes without the use of masks or photoresists. We have verified that the highest potential is achieved on graphene, where no waste material is detected, producing excellent well defined edges. This allows manufacturing graphene micro-ribbons with a high aspect ratio up to 1200:1.

Nanoporous Silicon-Based Platforms for Biological Applications Fabricated by UV Laser Techniques

Nanostructured porous silicon (nanoPS) can be regarded as a complex network of silicon nanocrystals embedded in a porous matrix. NanoPS is fabricated by the electrochemical etch of silicon wafers in HF-based solutions. When crystalline Si is transformed into nanoPS, its intrinsic properties are altered owing to quantum confinement effects. That makes nanoPS an excellent candidate for the development of several applications in a broad range of fields including optoelectronics, photonics, and biomedicine. Surface micro- and nano-patterning is becoming an important way to enhance the performance of materials. In particular, nanoPS patterns have been proposed for the fabrication of photonic devices, high-sensitivity sensors, etc. Different techniques have been used for the fabrication of patterns on nanoPS including microstructuring the crystalline silicon substrate before the electrochemical etching process, dry soft lithography, stamp pressing, etc. However, none of these methods has the capability to offer flex-ibility in the pattern design in a time-efficient process, in large areas, and in a single step pro-cess.The present work reports on the fabrication of one- and two-dimensional platforms on nanoPS by phase-mask UV laser interference. This technique is a single-step and flexible approach to produce a large variety of patterns formed by alternate regions of almost untransformed nanoPS and regions where its surface has melted and transformed into Si nanoparticles.By changing the main experimental parameters such as nanoPS layer porosity, average pore size, laser fluence or pattern period, the properties of the resulting structures can be controlled. These patterns have been proved to be good candidates for such applications as sensing and for the development of biological platforms.

Print-to-pattern dry film photoresist lithography

Here we present facile microfabrication processes, referred to as print-to-pattern dry film photoresist (DFP) lithography, that utilize the combined advantages of wax printing and DFP to produce micropatterned substrates with high resolution over a large surface area in a non-cleanroom setting. The print-to-pattern methods can be performed in an out-of-cleanroom environment making microfabrication much more accessible to minimally equipped laboratories. Two different approaches employing either wax photomasks or wax etchmasks from a solid ink desktop printer have been demonstrated that allow the DFP to be processed in a negative tone or positive tone fashion, respectively, with resolutions of 100 µm. The effect of wax melting on resolution and as a bonding material was also characterized. In addition, solid ink printers have the capacity to pattern large areas with high resolution, which was demonstrated by stacking DFP layers in a 50 mm × 50 mm woven pattern with 1 mm features. By using an office printer to generate the masking patterns, the mask designs can be easily altered in a graphic user interface to enable rapid prototyping.

Photonic crystal structures on nonflat surfaces fabricated by dry lift-off soft UV nanoimprint lithography

The surface nonflatness induced from the material itself or the production atmosphere can lead to serious non-uniformity consequences in nanoimprint lithography (NIL) which is used for providing a low cost and high throughput nano-fabrication process. In this paper, soft UV NIL (SUNIL) processes are used for photonic crystal (PC) pattern transfer of a GaN-based light-emitting diode (LED) with patterned sapphire substrate (PSS). The results reveal a significant incompatibility between the conventional SUNIL and the nonflat p-GaN surface. Ellipse-shaped rather than circle-shaped PC structure is obtained on the p-GaN surface due the deformation of the soft mold in nonflat NIL. A dry lift-off (DLO) SUNIL is proposed to overcome the non-uniformity issue in nonflat NIL as well as the collapse problem of the free-standing pillar-shaped resist in wet lift-off. The photoluminescence enhancements of the LED fabricated by the DLO SUNIL method compared to those with conventional SUNIL and unpatterned LED are 1.41 fold and 3.48 fold, respectively. Further study shows that the DLO SUNIL is applicable in the fabrication of the PC structure with tunable duty cycle via one single initial PC mold.

Block-based mask optimization for optical lithography

Pixel-based optical proximity correction (PBOPC) methods have been developed as a leading-edge resolution enhancement technique (RET) for integrated circuit fabrication. PBOPC independently modulates each pixel on the reticle, which tremendously increases the mask's complexity and, at the same time, deteriorates its manufacturability. Most current PBOPC algorithms recur to regularization methods or a mask manufacturing rule check (MRC) to improve the mask manufacturability. Typically, these approaches either fail to satisfy manufacturing constraints on the practical product line, or lead to suboptimal mask patterns that may degrade the lithographic performance. This paper develops a block-based optical proximity correction (BBOPC) algorithm to pursue the optimal masks with manufacturability compliance, where the mask is shaped by a set of overlapped basis blocks rather than pixels. BBOPC optimization is formulated based on a vector imaging model, which is adequate for both dry lithography with lower numerical aperture (NA), and immersion lithography with hyper-NA. The BBOPC algorithm successively optimizes the main features (MF) and subresolution assist features (SRAF) based on a modified conjugate gradient method. It is effective at smoothing any unmanufacturable jogs along edges. A weight matrix is introduced in the cost function to preserve the edge fidelity of the printed images. Simulations show that the BBOPC algorithm can improve lithographic imaging performance while maintaining mask manufacturing constraints.

Capped carbon hard mask and trimming process: A low-cost and efficient route to nanoscale devices

Both sub-22 nm architecture design optimization and reliable, low-cost process development represent major challenges toward nanoscale device fabrication. In order to address the second of these two issues, the authors have demonstrated that it is possible to overcome current tool and process lithography limitations using a capped carbon hard mask process, without dramatically increasing device fabrication costs, as only existing tools are used in this process. Starting from 50 nm patterns, 25 nm fully depleted silicon-on-insulator (FDSOI) transistors with good reliability and acceptable electrical behavior are obtained. This patterning solution may be applied to existing lithography processes (dry or immersion ArF lithography) in order to enhance current resolution capabilities. Moreover, the use of a capping layer enables to set free from photoresist thickness limitations, which are becoming increasingly critical for sub-22 nm feature patterning. Indeed, for such dimensions, photoresist thickness generally needs to be lower than 66 nm in order to avoid pattern collapse effects. This trend can lead to serious integration problems especially for the fabrication of thick stack device architectures. Therefore, in addition to improving current lithography processes, our strategy may also be useful for novel lithography processes such as extreme ultraviolet lithography or maskless lithography. The authors have also demonstrated that the capped carbon hard mask process could enable the patterning of sub-11 nm FDSOI gates, with a current best result close to 7 nm, starting from 30 nm photoresist patterns. Note that all etching steps of the process have been performed in the same etching chamber, which is a key point for meeting industrial requirements. These results show that it is possible to bypass tool and process lithography limitations to pattern sub-22 nm devices without dramatically increasing fabrication costs while maintaining lithography throughput. The authors have therefore shown that the capped carbon hard mask process could be a high-performance and low-cost industry-compatible solution for nanoscale device fabrication.