Electron Beam Resists Research Articles

Functionalization dependence of the electron beam sensitivity for a conformationally immobile calix[6]arene resist

Calixarenes form a group of compounds that have been studied as high resolution negative electron beam resists. In this work, resists based on a conformationally immobile calix[6]arene molecule were synthesized and their sensitivities measured while the number of attached functional groups was systematically varied. This provided a direct test of the effect of the degree of functionalization on the electron beam resist sensitivity. The synthesis produced two basic molecular conformations: a cone conformer and a 1,2,3-alternate conformer. The number of attached allyl groups was deliberately varied from 0 to 8 allyls in steps of 2 allyls per synthesis. The resulting nine different resists were exposed and contrast curves were measured by atomic force microscopy. The least sensitive were the unfunctionalized calix[6]arenes with threshold doses of about 2 mC/cm2. The fully functionalized conformers had threshold doses around 0.1 mC/cm2. The sensitivities of both conformers were found to scale directly with th...

Fabrication of Less than 20-nm-Diameter Nanodot Arrays Using Inorganic Electron Beam Resist and Post Exposure Bake

Next-generation NanoImprint Lithography (NIL) includes photolithography and high-voltage-acceleration (>50 kV) Electron-Beam Lithography (EBL) both of which are expensive, which is why Low-Acceleration-Voltage (LAV) EBL has attracted attention. We fabricated nanodot arrays with a diameter of 20 nm or less combining inorganic resist and LAV EBL. Using Post-Exposure Bake (PEB), we reduced dot-array pattern size and conducted Ultra-Violet (UV) NIL using these nanodot molds. The results we obtained using nanodot arrays 10 nm in diameter fabricated by EBL at an acceleration voltage of 4 kV with PEB yielded a checkerboard design 10 nm in diameter spaced at 20 nm intervals using 10 kV EB. This density corresponds to nanodot-arrays of 1 Tb/in2.

Transparent target for ultrafast electron beam tomography

In electron beam tomography a rotating X-ray source is generated by electromagnetic scanning of a high-power electron beam across a spatially fixed circular metal target. The X-rays passing the object of investigation are registered by a circular fast X-ray detector array. A basic problem associated with this scanning principle is the angular overlap of the X-ray focal spot path and the detector, which is required for image reconstruction. Therefore, the X-ray production target and the detector array are commonly arranged with some axial displacement. This displacement may produce image artifacts because X-rays are not longer confined to a transversal plane. We propose a novel transparent X-ray target, which fully solves this problem. The new target concept has been practically tested with respect to thermal behavior and electron beam resistance. Its successful application in an experimental tomography setup is demonstrated.

Manipulation of Nanoscale Phase Separation and Optical Properties of P3HT/PMMA Polymer Blends for Photoluminescent Electron Beam Resist

A novel photoluminescence electron beam resist made from the blend of poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA) has been successfully developed in this study. In order to optimize the resolution of the electron beam resist, the variations of nanophase separated morphology produced by differing blending ratios were examined carefully. Concave P3HT-rich island-like domains were observed in the thin film of the resist. The size of concave island-like domains decreased from 350 to 100 nm when decreasing the blending ratio of P3HT/PMMA from 1:5 to 1:50 or lower, concurrently accompanied by significant changes in optical properties and morphological behaviors. The lambda(max) of the film absorption is blue-shifted from 520 to 470 nm, and its lambda(max) of photoluminescence (PL) is also shifted from 660 to 550 nm. The radiative lifetime is shorter while the luminescence efficiency is higher when the P3HT/PMMA ratio decreases. These results are attributed to the quantum confinement effect of single P3HT chain isolated in PMMA matrix, which effectively suppresses the energy transfer between the well-separated polymer chains of P3HT. The factors affecting the resolution of the P3HT/PMMA electron beam resists were systematically investigated, including blending ratios and molecular weight. The photoluminescence resist with the best resolution was fabricated by using a molecular weight of 13 500 Da of P3HT and a blending ratio of 1:1000. Furthermore, high-resolution patterns can be obtained on both flat silicon wafers and rough substrates made from 20 nm Au nanoparticles self-assembled on APTMS (3-aminopropyltrimethoxysilane)-coated silicon wafers. Our newly developed electron beam resist provides a simple and convenient approach for the fabrication of nanoscale photoluminescent periodic arrays, which can underpin many optoelectronic applications awaiting future exploration.

Surface characterization of poly(methylmethacrylate) based nanocomposite thin films containing Al 2O 3 and TiO 2 nanoparticles

Poly(methylmethacrylate) (PMMA) based nanocomposite electron beam resists have been demonstrated by spin coating techniques. When TiO 2 and Al 2O 3 nanoparticles were directly dispersed into the PMMA polymer matrix, the resulting nanocomposites produced poor quality films with surface roughnesses of 322 and 402 nm respectively. To improve the surface of the resists, the oxide nanoparticles were encapsulated in toluene and methanol. Using the zeta potential parameter, it was found that the stabilities of the toluene/oxide nanoparticle suspensions were 7.7 mV and 19.4 mV respectively, meaning that the suspension was not stable. However, when the TiO 2 and Al 2O 3 nanoparticles were encapsulated in methanol the zeta potential parameter was 31.9 mV and 39.2 mV respectively. Therefore, the nanoparticle suspension was stable. This method improved the surface roughness of PMMA based nanocomposite thin films by a factor of 6.6 and 6.4, when TiO 2 and Al 2O 3 were suspended in methanol before being dispersed into the PMMA polymer.

Dynamics of Radical Cation of Poly(4-hydroxystyrene) and Its Copolymer for Extreme Ultraviolet and Electron Beam Resists

The use of ionizing radiation such as extreme ultraviolet (EUV) radiation is expected in next-generation lithography. Poly(4-hydroxystyrene) (PHS), which has been used in KrF excimer laser (248 nm) lithography as a backbone polymer, is a promising material for chemically amplified EUV and electron beam resists. In this study, the dynamics of the radical cation of poly(styrene-ran-4-hydroxystyrene) [P(S-HS)] was investigated. It was found that the hole transfer reaction in the matrix plays an important role in the sensitization of the resist. The hole transfer range is estimated to be ∼2.5 units (∼1.0 nm).

Difference of Spur Distribution in Chemically Amplified Resists upon Exposure to Electron Beam and Extreme Ultraviolet Radiation

In chemically amplified resists, acid generators are sensitized mainly by secondary electrons upon exposure to an electron beam (EB) or extreme ultraviolet (EUV) radiation. Therefore, the reaction mechanisms of EB and EUV resists are analogous. In addition to the difference in the absorption coefficients and related phenomena, the major difference between EB and EUV resists is the energy spectrum of secondary electrons generated by incident radiation. Both an EB and EUV radiation generate ion pairs through ionization in resist films. The space that an ion pair occupies is called a spur. When spurs overlap, the electron dynamics changes and affects the chemical yield and distribution. In this study, the distribution of intermediate species in EB and EUV resists was investigated by a Monte Carlo simulation. The difference between their spur distributions and its effects were clarified. To calculate the acid distribution in chemically amplified resists, the single-spur model is generally sufficient for EB resists, while the multispur model is required for EUV resists.

Superconducting nanostructures in a single step process

Focussed ion beam-induced chemical vapor decomposition of W(CO)6 allows single-step fabrication of nanoscale superconducting structures without lithography. Nanobridges have room temperature resistivity of around 200μΩ cm and a Tc above 5 K. We present a study of superconductivity of this amorphous material depending on the deposition conditions and dimensions down to 50 run across, in correlation with its normal-state properties. We also demonstrate more sophisticated structures, such as SQUIDs. We have fabricated superconducting devices operating at liquid helium temperatures on substrates and surfaces unaccessible to conventional technologies involving photo- or electron beam resists, for example around the tip of an AFM cantilever with potential applications in high-resolution magnetometry.

Improved etch resistance of ZEP 520A in reactive ion etching through heat and ultraviolet light treatment

The authors have developed a treatment process to improve the etch resistance of an electron beam lithography resist (ZEP 520A) to allow direct pattern transfer from the resist into a hard mask using plasma etching without a metal lift-off process. When heated to 90 °C and exposed for 17 min to a dose of approximately 8 mW/cm2 at 248 nm, changes occur in the resist that are observable using infrared spectroscopy. These changes increase the etch resistance of ZEP 520A to a CF4/O2 plasma. This article will document the observed changes in the improved etch resistance of the ZEP 520A electron beam resist.

Effect of Post Exposure Bake in Inorganic Electron Beam Resist and Utilizing for Nanoimprint Mold

A high-aspect-ratio structural mold has been fabricated using a simple process that comprises of electron beam lithography (EBL) and post exposure bake (PEB) of inorganic resist. Using developed inorganic resist for mold, pattern transfer to a photo-curable polymer was carried out by ultraviolet nanoimprint lithography (UV-NIL). The developed inorganic resist has enough hardness to be used for mold because this resist structure is almost equivalent to that of quartz. The aspect ratio of the fabricated mold with PEB was 2.59; this value is twice the size of aspect ratio of fabricated mold without PEB (1.16). Thus, PEB is very effective in fabricating high-aspect ratio pattern in inorganic resist. The size of the fabricated mold with PEB was 1004 nm developed depth and 388 nm line and 252 nm space. Using the mold, UV-NIL was carried out. The size of replicated pattern was 942 nm height and 278 nm line and 241 nm space. The replicated height and line width were slightly smaller than mold depth and line width because of volume shrinkage of photo-curable polymer by UV irradiation. The aspect ratio of the replicated pattern using the mold with PEB was 3.39; thus, high-aspect-ratio transfer was also possible. This process is useful for rapid fabrication of quarter wavelength plates.

High resolution, high sensitivity inorganic resists

New inorganic electron-beam resists have been developed on the basis of the aqueous chemistries of Zr and Hf. With a 30-keV electron-beam, resist sensitivities as low as 8 μC/cm 2 have been realized. At higher exposure doses, 15-nm lines and 36-nm dense features have been written with line-width roughness near 2 nm. In reactive-plasma etching, the resists exhibit etch resistance >7× that of thermal SiO 2.

Preparation of Substituted N-Methacryloxy Phthalimides for E-Beam Lithography

Poly (methyl methacrylate) (PMMA) is a very good option for electron-beam resists, but suffers from poor radiation sensitivity and poor plasma etch resistance. These problems can be offset, at least in part, by functionalizing the PMMA with photo-labile, plasma etch-resistance groups. Toward this end, a series of N-methacryloxy phthalimides with different substitutuents (methyl, nitro, tetrahydrogen and dichloro) were synthesized as possible useful radiation-sensitive resists. The work discussed here focuses on the cycloimido group, which can be easily attached to methacrylic acid to form a radiation-sensitive resist material for use in electron beam lithography. The labile N–O bond of the cycloimido group, upon exposure to radiation, cleaves and forms a radical on the main chain of the polymer, which leads to polymer degradation. Cursory evaluations showed that these new materials are deserving of further study.

Glasses for lithography

The highest resolution in a lithographic process is often determined by the properties of the resist material. With the currently used polymeric resists, a resolution of better than 100 nm has been achieved under manufacturing conditions, but the future nanoscale devices will require a 10 times superior resolution. In this paper we present an overview of the resist materials, especially with regard to limiting resolution. In principle, inorganic resists should have higher limiting resolution than polymer resists due to smaller fundamental structural units and stronger bonds in the former. However, compositional and/or structural inhomogeneities may limit their ultimate resolution. New results are presented that indicate chalcogenide glasses as promising photo and electron beam resists, which also have the advantages of greater hardness, resistance to acids, easy fabrication in thin film form, and the unique phenomena like radiation enhanced diffusion.

Novel Patterning of Gold Using Spin-Coatable Gold Electron-Beam Resist

Conventional lithography methods of gold patterning are based on deposition and lift-off or deposition and etching. In this letter, we demonstrate a novel method of gold patterning using spin-coatable gold electron-beam resist which is functionalized gold nanocrystals with amine ligands. Amine-stabilized gold electron beam resist exhibits good sensitivity, 3.0 mC/cm2, compared to that of thiol-stabilized gold electron beam resists. The proposed method reduces the number of processing steps and provides greater freedom in the patterning of complex nanostructures.

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.

New resists for proton beam writing

To explore the full capabilities of proton beam writing (PBW) as a lithographic tool it is important to investigate potential new resist materials. In PBW the interactions of the protons with the resist are comparable to the electron interactions with the resist in electron beam writing. In both techniques the induced secondary electrons will modify the molecular structure of the resist, therefore electron beam resists are potential candidates for PBW. Here we discuss resist properties such as contrast and sensitivity of two new negative resists for PBW. The first resist is a spin-coatable TiO 2 resist for which sub 10 nm resolution has been reported using electron beam writing. In PBW smooth side walls have been observed for this resist. Despite a relative low sensitivity of this resist for PBW (8000 nC/mm 2) it has potential applications in the area of integrated optical components such as waveguides and gratings because of its high refractive index. WL-7154 is a UV-sensitive negative resist and shows high sensitivity for PBW (4 nC/mm 2). This resist could function as a mold for Ni electroplating to fabricate Ni stamps for nanoimprint- and soft-lithography.

Suppression of pinhole defects in fullerene molecular electron beam resists

Molecular resists, such as fullerenes, are of significant interest for next generation lithographies. They utilize small carbon rich molecules, giving the potential for higher resolution and etch durability, together with lower line width roughness than conventional polymeric resists. The main problem with such materials has historically been low sensitivity, but with the successful implementation of chemical amplification schemes for several of the molecular resist families this is becoming less of a concern. Aside from sensitivity the other main obstacle has been the difficulty of preparing good quality thin films of non-polymeric materials. Here we present a study of pinhole defect density in fullerene films as a function of substrate cleanliness, post-application bake, and incorporation of chemical amplification components. Ultrathin (sub 30 nm) films of the previously studied fullerene resist MF03-01, and the polymeric resist PMMA were prepared on hydrogen terminated silicon by spin coating and the density of pinhole defects in the films was studied using atomic force microscopy. It was seen that pinhole density was strongly affected by the quality of the substrates, with the lowest densities found on films spun on freshly cleaned substrates. Aging of the film subsequent to spin coating was seen to have less effect than similar aging of the substrate prior to spin coating. Additionally, the use of a post-application bake significantly degraded the quality of the films. The addition of an epoxy crosslinker for chemical amplification was found to reduce defect density to very low levels.

Effect of cold development on improvement in electron-beam nanopatterning resolution and line roughness

This article presents the results about the effect of cold development on the improvement in patterning resolution and line edge roughness for electron-beam lithography resists. Cold development shows improved resolution and contrast for resists that are exposed by polymer chain scission. It does not work for chemically amplified resists. A discussion on why this works for resists like ZEP 520 and PMMA and not for positive chemically amplified resists (such as UV113) is presented. Results for 13 nm structures obtained after metal liftoff using a 30 kV e-beam tool using ZEP 520 resist are shown. These results have impact in the photomask industry and other manufacturers that require squeezing out as much resolution out of their existing tools and materials. It is found that, even with the improvement by cold development, there is a “shot noise” of 2% uncertainty limit that is not surpassed for resists exposed at 100kV. This explains why high throughput and high resolution electron-beam nanolithography is not possible.

Acid generation efficiency in a model system of chemically amplified extreme ultraviolet resist

A highly sensitive extreme ultraviolet (EUV) resist is strongly needed to reduce the development costs of high power EUV sources. Although highly sensitive chemically amplified resists based on acid-catalyzed reactions have been used in mass production lines, the chain length of acid catalytic reactions must be suppressed within several nanometers to meet the resolution requirement below the 32nm technology node. Under such circumstances, the initial acid yield produced by EUV exposure is critical to the formation of ultrafine patterns. Using an acid sensitive dye, the authors evaluated the number of acid molecules generated by an EUV photon. The observed acid yield was well explained by the ionization model for acid generation originally proposed for chemically amplified electron beam resists.

Relationship between Acid Generator Concentration and Acid Yield in Chemically Amplified Electron Beam Resist

Acids generated upon exposure play the most important role in the pattern formation of current standard resists, called chemically amplified resists. It is well known that acid yield depends on the acid generator concentration. However, the precise relationship between acid yield and acid generator concentration has not been investigated thus far. Understanding the details in acid generation is important for the development of resist materials and the process simulation. In this work, the dependence of acid yield on the acid generator concentration was studied using poly(4-hydroxystyrene) (PHS) and poly(methyl methacrylate) (PMMA) as a matrix. Both results showed a nonlinear dependence. With increase in acid generator concentration, acid yield showed a saturation tendency. The dependence differed between PHS and PMMA. This reflects the fact that protons of acids originate from the radical cation of polymers generated by ionization.