Negative Tone Resists Research Articles

Single-Electron Transistor Operation of a Physically Defined Silicon Quantum Dot Device Fabricated by Electron Beam Lithography Employing a Negative-Tone Resist

We have proposed and demonstrated a device fabrication process of physically defined quantum dots utilizing electron beam lithography employing a negative-tone resist toward high-density integration of silicon quantum bits (qubits). The electrical characterization at 3.8 K exhibited so-called Coulomb diamonds, which indicates successful device operation as single-electron transistors. The proposed device fabrication process will be useful due to its high compatibility with the large-scale integration process.

Solution‐Processed High‐Performance ZnO Nano‐FETs Fabricated with Direct‐Write Electron‐Beam‐Lithography‐Based Top‐Down Route

AbstractZinc oxide (ZnO) has been extensively investigated for use in large‐area electronics; in particular, the solution‐processing routes have shown increasing promise towards low‐cost fabrication. However, top‐down fabrication approaches with nanoscale resolution, towards aggressively scaled device platforms, are still underexplored. This study reports a novel approach of direct‐write electron‐beam lithography (DW‐EBL) of solution precursors as negative tone resists, followed by optimal precursor processing to fabricate micron/nano‐field‐effect transistors (FETs). It is demonstrated that the mobility and current density of ZnO FETs can be increased by two orders of magnitude as the precursor pattern width is decreased from 50 µm to 100 nm. These nano‐FET devices exhibit field‐effect mobility exceeding ≈30 cm2 V−1 s−1 and on‐state current densities reaching 10 A m−1, the highest reported so far for direct‐write precursor‐patterned nanoscale ZnO FETs. Using atomic force microscopy and parametric modeling, the origin of such device performance improvement is investigated. The findings emphasize the influence of pre‐decomposition nanoscale precursor patterning on the grain morphology evolution in ZnO and, consequently, open up large‐scale integration, and miniaturization opportunities for solution‐processed, high‐performance nanoscale oxide FETs.

Electron beam lithography using a PMMA/P(MMA 8.5 MAA) bilayer for negative tone lift-off process

The authors demonstrate a high resolution lift off process for electron beam lithography using a PMMA/P(MMA 8.5 MAA) bilayer as negative tone resists. Fifty-nanometer features were achieved for metal deposition up to 100 nm thick. The process was tested on silicon substrates as well as indium tin oxide on glass transparent substrates in order to prove the applicability of the patterns for extraordinary optical transmission.

Wet Chemical Processing with Megasonics Assist for the Removal of Bumping Process Photomasks

A new generation of negative tone resists by TOK, JSR, and Dow Chemical is gaining momentum in advanced packaging applications. Resist thickness requirements are actually increasing to the 50-100 um range as Cu pillars are adopted to accommodate the tighter pitches required in advanced packaging. In order to form pillars the resist must be thicker to contain the entire bump structure. Thick, negative tone resists are more transparent to exposure light wavelengths than positive tone resists and can be exposed by the lithography process much faster (~1 x 104 cross-linking chemical events are driven by 1 photochemical event vs. 100 – 1000 for positive tone resist).1 Therefore exposure times can be shorter, post-exposure bake steps can be shorter, and delay before or after exposure is not necessary, saving photolithography time and CoO. And, due to more complete cross-linking throughout the resist, the resist mask profiles are truer (from Flack et.al, “A Comparison of New Thick Photoresists for Solder Bumping”, SPIE 2005).1 The major drawback to negative tone resists is that the solvent strip times of the highly cross-linked resist masks are much longer than for positive tone. Flack et.al. noted resist strip times of 5 minutes for two positive resists used in their experiments vs. 50 minutes for the AZ-100nXT negative tone resist, using AZ400T at 80°C (most likely in an immersion tool). Long strip times or special stripping requirements are noted on the data sheets of the other majority suppliers of negative resists as well. Akrion engineers have developed a novel, single-wafer, negative PR strip processes using organic solvents plus Goldfinger megasonics, to provide 30 - 70% reductions in process times and the associated chemical consumption when compared to processes that do not utilize a megasonics assist. The multi-step process can be accomplished in a single process chamber, with each step optimized for time and temperature based on the resist thickness and solvent stripping chemistry used. In the Akrion process, a period of solvent exposure at low wafer spin speed is used initially to begin swelling and dissolving the thick photoresist layer. This is followed by a second solvent exposure step using aggressive frontside megasonic energy to promote polymer chain scission throughout the bulk of the photoresist layer. Following this step A DI water rinse and spin dry step are sufficient to completely remove the solvent, dissolved photoresist and any polymer residues. This paper reviews the development of the Akrion process and several examples of improvement in stripping time and results from work with customers.

A novel processing procedure for T/UV-NIL with negative tone resists

The use of a conventional negative tone resist, like SU-8, for UV nanoimprint lithography (UV-NIL) allows the exploitation of the wide knowledge available for this material from micromechanical applications for nanoimprint. For this purpose a novel processing procedure is defined. SU-8 is used as a spin-coated layer; it is first flood-exposed and then thermally imprinted, the imprint simultaneously serving as the post exposure bake required for the cross-linking of the material. By an evaluation of the residual layer height obtained after imprint the changes of the material properties upon exposure and upon temperature treatment are investigated. An optimized T/UV-NIL processing sequence is defined for SU-8 in terms of a two-step process, where, after flood exposure, the imprint is performed at 50^oC followed by heating to 100^oC and cross-linking, within the non-transparent Si stamp.

Cross-linking control during imprint for hybrid lithography

Hybrid lithography combining thermal nanoimprint with optical lithography is an effective means to prepare structures with differing geometries (nanometer, micrometer) over large areas, e.g., for sensor applications. For the realization of this technique, the suitability of photoresists for this process has to be investigated. In case of negative tone resists, the cross-linking induced or provided by exposure may be amplified in the imprint step due to heating times in the minute range. As cross-linking may impede the imprint, a subtle adjustment of the exposure dose with respect to the imprint temperature envisaged is required for successful processing. With an imprint temperature of 100 °C, good results for hybrid lithography could be obtained with two commercially available negative tone resists: ma-N 405 and ARN 4410. The latter could successfully be used for a subsequent liftoff. The best results were obtained when the resists were exposed before the imprint step.

Fabrication of luminescent nanostructures by electron-beam direct writing of PMMA resist

We report on the conversion of non-luminescent conventional poly(methylmethacrylate) (PMMA)-based electron-beam resists into luminescent materials when used as negative-tone resists, that is, when exposed to high electron irradiation doses. Raman spectroscopy reveals the chemical transformation induced by electron irradiation which is responsible for the observed luminescence in the visible (blue) region. The emission intensity from exposed PMMA-based patterns can be controlled by the electron irradiation dose employed to create them.

Performances of the Negative Tone Resist AZnLOF 2020 for Nanotechnology Applications

We present electron-beam lithography results on an AznLOF2020 UV negative tone resist. This study aims to define electron-beam lithography parameters compatible with microelectromechanical systems/nanoelectromechanical systems fabrication. Usually, one of the properties of the resist material is improved to the detriment of another. Resist properties have been investigated to form small features and high-aspect ratio lines. It has been demonstrated that thickness, postbaking time, development time, and dose effects can be varied and adjusted to form smallest patterns reproducible. Various writing strategies making use of test patterns comprising different nominal feature size were also studied. Additionally, high resolution and dense pattern fabrication with high-aspect ratios are demonstrated. Fifty-nanometer-wide lines spaced out at 100-nm intervals were achieved in a 200-nm-thick resist, and high-aspect ratio 200-nm-wide lines spaced out at 400-nm intervals were realized in a 1-μm-thick resist.

Fast and blister-free irradiation conditions for cross-linking of PMMA induced by 2MeV protons

For soft lithography, the conventional negative tone resists, such as SU-8, that are used to create the mold have a number of drawbacks. PMMA, which is normally used as a positive tone resist, can be used as a negative resist by using high-fluence irradiation conditions. In this report, we outline optimization of the irradiation conditions for PMMA thin films using 2 MeV H+ ions to exploit their ability to work as a negative tone resist at ion fluences above 1.0 × 1015 ions cm−2. The main aim was to induce cross-linking while maintaining the exposed regions free of blisters and maintaining short irradiation times. We found that by using a two-step process with a low-flux irradiation, followed by a high-flux irradiation, the exposure time could be shortened by ∼50%. We also found that ion fluences greater than 5.0 × 1015 ions cm−2 minimized the distortion in stitched regions.

微細加工用レジスト向け新素材・材料の開発

We have recently developed novel resist materials for micro-lithographic patterning. One is a hydroxystyrene derivative, which is a key compound for KrF excimer laser resists. We found that microwave-assisted decarboxylation of hydroxycinnamic acids smoothly proceeded under the presence of a catalytic amount of amine base, to afford hydroxystyrene derivatives in good yield.The other is positive and negative-tone molecular resists utilizing the unique character of a furan ring. The synthesized compounds showed relatively high glass transition temperature and readily formed uniform amorphous films on a silicon wafer. The sensitivity as an EB resist of both positive and negative-tone resists was below 10 µC/cm2 and line and space patterns of 200 nm could be fabricated. The promising feature of the positive-tone resist is that no outgassed products from base matrixes are theoretically produced under the exposure and post-exposure bake procedure.

Deep x-ray lithography processing for batch fabrication of thick polymer-based antenna structures

Deep x-ray lithography is applied for the first time to fabricate polymer-based antenna structures with different portions of ceramic contents. To produce successful and viable antenna structures, three different methods are proposed using positive and negative tone resists. In the first method the structures are lithographically fabricated avoiding an intermediate molding step using SU-8 as a photosensitive resist filled with fine ceramic powder with particles in the submicron range. In the second and third methods a polymethylmethacrylate (PMMA) mold is first fabricated by x-ray lithography, and then SU-8/MMA mixed with the high ceramic powder content is injected into the mold. In these methods a final step of crosslinking for SU-8 and polymerization for MMA is also required. Optimized fabrication parameters allow the production of high quality antenna structures as thick as 2.3 mm. X-ray lithography capabilities in fabrication of antennas and other passive microwave components with special features reinforce the idea of fabricating integrated passive microwave circuits along with active circuits using this emerging technology.

High resolution patterning – Preparation of VSB systems for 22 nm node capability

Microelectronic technology inevitably continues to develop towards ever shrinking, smaller geometries. For the year 2016, minimum feature sizes requiring 22 nm half pitch (hp) patterning lithography are predicted. To keep abreast of this challenging development, Vistec’s latest variable shaped beam (VSB) column – already described elsewhere [1] – was applied and the electron energy level increased to 100 keV for exposure of such fine pattern dimensions. Based on the electron-optical simulations that were performed, resolving power can be forecast to improve by a factor of 1.4 compared to results reported earlier. This is what 22 nm hp exposure requires as a compulsory precondition. This paper discusses four types of resist to demonstrate the increase in resolution capability. These are: positive and negative tone resists, conventional resists and chemically amplified resists. To provide a sound evaluation, several test patterns containing single lines, groups of seven lines and spaces, as well as extensive line/space areas and contact arrays were exposed. Top-down SEM images and cross-sectional views were both taken into account for a final assessment. It turned out that only the conventional resists are able to deliver the resolution capability for hp 22 nm.

Creation of super-hydrophobic siloxane-modified SU-8 microstuctures

Recently, the need for super-repellent surfaces has been increasing, with applications ranging from material science to Lab-on-a-Chip diagnostics and microfluidics. We propose a novel and simple method to engineer super-hydrophobic surfaces using commercially available negative-tone resists, namely SU-8 and the photodefinable spin-on silicone elastomer, PDSE. The surface energies of fabricated materials were investigated and applied to the transition from the super-hydrophobic to hydrophilic states, occurring during liquid phase evaporation.

Characteristics of negative electron beam resists, ma-N2410 and ma-N2405

We have characterized the electron beam lithography (EBL) properties of the new negative tone resists, ma-N2410 and ma-N2405. These negative resists reacts under low electron dose values from 10 to 140μC/cm2, tested using 10, 20, 28keV electron beam. There was negligible loss of pattern height, which was attributed to the combined dose of the incident electron beam and the backscattered electrons. Experimental tests were performed under various EBL writing conditions of dose value, developing time, line-width and resist thickness. Our investigation showed that these new commercially available resists have high resolution and high contrast with non-chemical amplification, useful for micro-fabrication application.

Electron-beam patterning and process optimization for magnetic sensor fabrication

Electron-beam lithography has been widely applied to define critical features in magnetic recording thin-film heads. To be a viable resist candidate for read sensor fabrication, e-beam resists need to be optimized in three major aspects—resolution, etching resistance, and strippability. Here the authors investigated four typical resists for their applicability in the production of sub-50nm read sensors and extendibility into sub-30nm regime, including a negative-tone chemically amplified resist (CAR), a negative-tone non-CAR, a positive-tone CAR, and a positive-tone non-CAR. Pattern design for positive-/negative-tone resists, resist resolution capability, sloped resist pole profile, underlayer preparation for Si-containing imaging layer, hybrid lithography for low-sensitivity non-CARs, and process issues in ion-beam etching of sensor stacks are discussed in detail. Finally, 20nm giant magnetoresistive sensor fabrication was demonstrated.

Molecular Glass Resists for High-Resolution Patterning

This paper describes a series of photoresists constructed from glass-forming, low-molecular-weight organic compounds, also known as molecular glasses. Compared with traditional polymeric resists, molecular glass resists are composed of smaller and more-uniform molecular building blocks. In this work, both positive-tone and negative-tone molecular glass photoresists with a range of core structures were designed and synthesized for study in advanced lithography. These molecular glass resists have asymmetric, rigid cores, which is important for producing glassy materials with glass transition temperatures substantially above room temperature. For positive-tone molecular glass photoresists, amorphous films could be obtained by partial protection of the core structure. Images were produced using photoacid-generator-catalyzed deprotection chemistry. Amorphous negative-tone resists were obtained by mixing molecular glass core structures with another minor resist component such as a photo cross-linker. It was shown by SEC that the molecular weight of the exposed and cross-linked negative-tone resist was less than 2000 g/mol, thus indicating that the solubility change is largely due to a molecular-weight increase. Both types of materials exhibited high sensitivity and resolution. Several resist characteristics were studied to assess their potential as high-resolution resists. These molecular glasses showed high fluorocarbon etch resistance that is comparable to that of poly(hydroxystyrene). Lower line edge roughness was obtained for the negative-tone molecular glass compared to a negative-tone polymeric e-beam resist. The resulting materials exhibited high sensitivity and resolution close to the tool limit under 248 nm exposures when using a chemical amplification process. A well-resolved pattern as small as 50 nm was obtained for the negative-tone molecular glass by e-beam exposure, indicating the excellent potential of using low molecular molar mass molecular glasses to form high-resolution structures.

Fabrication of arrays of sub-wavelength nano-apertures in an optically thick gold layer onglass slides for optical studies

We report on the fabrication of two-dimensional arrays of nano-optical apertures ingold layers by electron beam lithography (EBL) on a transparent glass substrate.30 × 30 µm2 large arrays of high aspect ratio sub-wavelength cylinders (400 nm diameter with period of1.81 µm) and annular apertures (diameters 250/330 nm and 310/330 nm inner/outer with period of600 nm) were patterned in a 750 nm thick resist layer using a high contrast negative toneresist. The resist structures show sharp and vertical edges after development.The 150 nm thick deposited gold layer ensures optical transmission of less than1.1 × 10−4 at 633 nm wavelength. White light based optical characterizations agreed with theorypredictions and prove the good quality of the structures.

Dendrimer Monolayers as Negative and Positive Tone Resists for Scanning Probe Lithography

A new scanning probe lithography scheme based on a self-assembled dendrimer monolayer on thin Ti films is presented. The method relies on the versatility of the functionalized dendrimer molecules to effectively function as etch resists by forming a densely packed self-assembled protective monolayer on a Ti film. Patterning of the Ti surface is accomplished using an AFM tip either as an ultra sharp scribe or as an electrical field point source to modify the monolayer. This, coupled to carefully selected etching conditions, allows the use of the dendrimer monolayers as both negative and positive tone resists. Facile formation of TiO2 features ca. 25 nm wide and 12 nm tall on silicon oxide and ca. 50 nm wide gaps in a thin Ti film can easily be achieved. The dendrimer resist approach can be further developed in order to improve the minimum feature size to the single dendrimer molecule level.

Characterization and simulation of surface and line-edge roughness in photoresists

The problem of surface and line-edge roughness characterization and prediction is discussed. Different roughness parameters, such as the root mean square deviation (rms or σ), the fractal dimension, and the Fourier spectrum, are presented and compared. These roughness parameters for three negative tone resists (wet and plasma developed) are analyzed versus exposure dose, photoacid generator concentration, and plasma development conditions. Finally, a molecular type simulator is used to predict the experimental roughness behavior.

Quantitative evaluation of electron beam writing in passivated gold nanoclusters

We report a quantitative investigation of direct electron beam writing in monolayer films of passivated gold nanoclusters. In this process, the passivating organic ligands are (partially) removed to create gold-based nanostructures. We report the fabrication of lines with width as narrow as 26 nm, while measurements of the linewidth as a function of dose allow us to obtain a quantitative measure of the sensitivity, for comparison with established negative tone resists.