Hydrogen Silsesquioxane Structures Research Articles

3D Volumetric Energy Deposition of Focused Helium Ion Beam Lithography: Visualization, Modeling, and Applications in Nanofabrication

AbstractIn this paper, 3D volumetric energy deposition and local crosslinking of hydrogen silsesquioxane (HSQ) are experimentally and numerically explored in focused helium ion beam lithography (HIBL). In particular, a through‐membrane exposure method is developed to make visible and subsequently to measure the 3D interaction volume and energy deposition of helium ions in HSQ. By comparing the actual dimensions of the crosslinked HSQ structures with Monte Carlo modeling of the spatial distribution of the energy deposition, the critical energy density for crosslinking HSQ is obtained. Finally, 3D nanofabrication of complex crosslinked HSQ nanostructures such as embedded nanochannels and suspended grids is demonstrated using two different exposure configurations. The proposed method expands the 2D point spread function of HIBL into three dimensions, thus opening a new avenue for nanoscale 3D fabrication.

Evaluation of Young’s modulus of imprinted hydrogen silsesquioxane pillar after residual layer removal by reactive ion etching

Nanoimprint lithography has two basic steps. The first is the imprint step in which a mold with nanostructures on its surface is pressed into a resin film on a substrate, followed by removal of the mold. The second step is the residual layer removal by a reactive ion etching (RIE). There is no report whether the properties of the imprinted structure after RIE change or not. In this work, the authors evaluated the Young's modulus of the imprinted pillar after residual layer removal by RIE. In this experiment, hydrogen silsesquioxane (HSQ), a type of spin-on-glass, was used as an imprint material. The residual layer was etched by RIE using CHF3 gas. The Young's modulus of imprinted pillar after RIE was measured via cantilever method. The Young's modulus of HSQ pillar after RIE was twice as much as that of HSQ pillar before RIE. From the Fourier transform infrared measurement, it was founds the chemical structure of HSQ was changed by forming network structure due to heating by RIE plasma energy. These results indicate that the mechanical property of imprinted structure was changed in the residual layer removal step by using RIE.

3D Nanostructuring of hydrogen silsesquioxane resist by 100 keV electron beam lithography

The authors investigated the three-dimensional nanostructuring of hydrogen silsesquioxane (HSQ) resist by multiple-step 100 keV electron beam lithography. Consecutive overlay exposures were used to create two- and three-levels in high aspect ratio HSQ structures with lateral dimensions down to 30 nm and resist thicknesses of about 1 μm. The HSQ resist was developed by a high contrast solution and supercritically dried in a carbon dioxide environment after each exposure step. The three-dimensional HSQ patterning has potential applications in the fabrication of performance enhanced devices such as photonic crystals, nanoelectromechanical systems, and diffractive X-ray lenses.

Hydrogen silsesquioxane double patterning process for 12nm resolution x-ray zone plates

Soft x-ray zone plate microscopy is a powerful nanoanalytic technique used for a wide variety of scientific and technological studies. Pushing its spatial resolution to 10nm and below is highly desired and feasible due to the short wavelength of soft x rays. Instruments using Fresnel zone plate lenses achieve a spatial resolution approximately equal to the smallest, outermost zone width. In this work, a double patterning zone plate fabrication process is developed. based on a high resolution resist, hydrogen silsesquioxane (HSQ), to bypass the limitations of conventional single exposure fabrication to pattern density, such as finite beam size, scattering in resist, and modest intrinsic resist contrast. To fabricate HSQ structures with zone widths on the order of 10nm on gold plating base, a surface conditioning process with (3-mercaptopropyl) trimethoxysilane, 3-MPT, is used, which forms a homogeneous hydroxylation surface on gold surface and provides good anchoring for the desired HSQ structures. Using the new HSQ double patterning process, coupled with an internally developed, subpixel alignment algorithm, the authors have successfully fabricated in-house gold zone plates of 12nm outer zones. Promising results for 10nm zone plates have also been obtained. With the 12nm zone plates, they have achieved a resolution of 12nm using the full-field soft x-ray microscope, XM-1.

In situ synthesis and direct immobilization of ssDNA on electron beam patterned hydrogen silsesquioxane

In addition to being a high-resolution negative-tone electron beam resist, hydrogen silsesquioxane (HSQ) has chemical properties similar to glass, making it useful for integration with biodevice fabrication. The authors demonstrate the use of electron beam patterned HSQ as a solid support for light-directed in situ ssDNA synthesis and ssDNA immobilization, creating submicron HSQ structures (ranging from 1μmto40nm) that are functionalized with ssDNA. After ssDNA synthesis, the hybridization of Cy-3 labeled complementary strands reveals that the synthesis is indeed localized to the HSQ. They observed relatively low background fluorescence from the supporting silicon substrate or from HSQ where no DNA synthesis was performed. In the course of the experiment they surveyed several materials as support for the HSQ patterning. In addition, the support substrate must be resistant to DNA synthesis. They found that piranha cleaned silicon, glassy carbon, hydrogen plasma treated glassy carbon, and hexamethyldisilazane primed silicon allow little or no synthesis when examined by hybridization with fluorescent labeled complement DNA. This work is relevant to the fabrication of devices that may require submicron patterns of structures functionalized with ssDNA for hybridization assays or DNA self-assembly applications and demonstrates a novel use of a commonly used negative-tone resist.

Adhesion of proton beam written high aspect ratio hydrogen silsesquioxane (HSQ) nanostructures on different metallic substrates

Hydrogen silsesquioxane (HSQ) behaves as a negative resist under MeV proton beam exposure. HSQ is a high-resolution resist suitable for production of tall (<1.5 μm) high aspect ratio nanostructures with dimensions down to 22 nm. High aspect ratio HSQ structures can be used in many applications, e.g. nanofluidics, biomedical research, etc. Isolated HSQ nanostructures, however, tend to detach from substrates during the development process due to the weak adhesive forces between the resist and the substrate material. Larger proton fluences were observed to promote the adhesion. To determine an optimal substrate material and the proton irradiation doses for HSQ structures, a series of 2 μm long and 60–600 nm wide free-standing lines were written with varying fluences of 2 MeV protons in 1.2 μm thick HSQ resist spun on Ti/Si, Cr/Si and Au/Cr/Si substrates. The results indicate that the Ti/Si substrate is superior in terms of adhesion, while Au/Si is the worst. Cr/Si is not suitable as a substrate for HSQ resist because debris was formed around the structures, presumably due to a chemical reaction between the resist and Cr.

Proton beam written hydrogen silsesquioxane (HSQ) nanostructures for Nickel electroplating

Hydrogen silsesquioxane (HSQ) behaves as a negative resist under MeV proton beam exposure. HSQ is a high-resolution resist suitable for production of tall ( < 1.5 μ m ) high-aspect-ratio nanostructures with dimensions down to 22 nm. High-aspect-ratio HSQ structures are required in many applications, e.g. nanofluidics, biomedical research, etc. Since P-beam writing is a direct and hence slow process, it is beneficiary to fabricate a reverse image of the patterns in a metallic stamp, e.g. by Ni electroplating. The Ni stamp can then be used to produce multiple copies of the same pattern. In this study we investigate the possibility to produce Ni stamps from p-beam written HSQ samples. HSQ high aspect ratio nanostructures, however, tend to detach from Au/Si substrates (typically used in electroplating) during the development process due to the weak adhesive forces between the resist and the substrate material. To determine an optimal substrate material and the proton irradiation doses for HSQ structures, a series of 2 μ m long and 60–600 nm wide free-standing lines were written with varying doses of 2 MeV protons in 1.2 μ m thick HSQ resist spun on Ti/Si, Cr/Si and Au/Cr/Si substrates. The results indicate that both Ti/Si and Cr/Si substrates are superior in terms of adhesion. The adhesion of high aspect ratio HSQ nanostructures to Au/Cr/Si is poor with a maximum aspect ratio of the adhering structures not exceeding two. Cr/Si is not suitable as a substrate for HSQ resist as debris is formed around the structures, presumably due to a chemical reaction between the resist and Cr.

The comparative study of thermal stability among silicon dioxide, fluorinated silicate glass, hydrogen silsesquioxane, and organosilicate glass dielectrics on silicon

This study investigates the thermal stability of thermally grown silicon dioxide (thermal SiO 2), fluorinated silicate glass (FSG), hydrogen silsesquioxane (HSQ), and organosilicate glass (OSG). Samples with and without sputtered copper films, as-prepared and after annealing at 200 ∼ 800 °C in vacuum, have been investigated by various methods of analysis. For the as-prepared dielectrics, it is found that the densities of HSQ and OSG are less than those of SiO 2 and FSG due to the greater roughness and more porous structure of HSQ and OSG. After annealing at 800 °C, the HSQ structure converts to a SiO 2-like structure, which is mainly due to decomposition of HSQ. Furthermore, the methyl groups and oxygen atoms are also liberated from OSG after high temperature annealing. A plateau in the Cu profile near the Cu/dielectric interface is observed in the results of secondary ion mass spectrometry after annealing at 450 °C. On the other hand, penetration of Cu atoms across the interface between Cu and HSQ, and across the interface between Cu and OSQ, are observed for Cu/HSQ and Cu/OSG systems after annealing at 450 °C. The thermal stability of these four dielectrics are discussed and compared.

Simple fabrication of UV nanoimprint templates using critical energy electron beam lithography

Ultraviolet nanoimprint lithography (UV-NIL) is a promising nanoscale patterning method, but the template fabrication processes are expensive and time consuming because a chrome layer is necessary both as a charge dissipation layer to minimize pattern distortion during electron beam patterning and as a physical mask during subtractive quartz etching. In this paper, the authors propose a simple UV-NIL template fabrication scheme based on the direct electron beam patterning of hydrogen silsesquioxane (HSQ) on quartz substrates using critical energy electron beam lithography (CE-EBL). By operating at the critical energy (E2) where the charge balance between incoming and outgoing electrons leaves the surface neutral, charge induced pattern distortion typically seen on quartz is practically eliminated. This template fabrication process eliminates conventional deposition and etching of charge dissipation layers. Quartz with sub-100-nm HSQ structures was used as a UV-NIL template, and SU-8 polymer structures were successfully replicated by a UV nanoimprint process. This simple template fabrication approach may lead to the development of new biological devices, nanoelectronics, and optoelectronics.

Enhancing etch resistance of hydrogen silsesquioxane via postdevelop electron curing

In this work, the authors enhanced the etch resistance of the negative-tone electron resist, hydrogen silsesquioxane (HSQ) to CF4 reactive ion etching (RIE) by curing HSQ after development. They fabricated superconducting nanowires that were 15nm wide by pattern transfer into a 6-nm-thick layer of NbN using cured HSQ as the etch mask. HSQ was cured using a postdevelop electron-beam exposure step prior to RIE in CF4 chemistry. This curing step was shown not to impact the resolution of the HSQ structures while increasing their etch resistance. The results of the authors demonstrate that the etch resistance of HSQ can be tuned after development, which is a desirable resist property of HSQ in addition to its high resolution and low line-edge roughness.

Nonaqueous development of silsesquioxane electron beam resist

While the primary use of hydrogen silsesquioxane (HSQ) is as a dielectric in microelectronics fabrication, this material is also capable of forming high resolution, negative-tone features with low roughness when patterned with an electron beam. Unfortunately, under common processing conditions HSQ is relatively insensitive to electron beam exposure; poor reproducibility has also been observed. HSQ postexposure processing typically consists of development via immersion in an industry-standard aqueous solution of base, followed by rinsing with water or isopropanol. While other resist materials have been specifically designed for compatibility with aqueous base processing, HSQ is known to be chemically unstable in the presence of base. We report that several organic solvents that are not reactive towards HSQ are less aggressive at removing the exposed regions of the film. As a result, it is possible to successfully image HSQ with markedly reduced exposure dose. The considerable difference in exposure dose can be largely attributed to the difference in reactivity toward HSQ between organic solvents and aqueous base, but other factors must also be considered. For example, HSQ structures can be formed at low doses when developed with isopropanol, but the structures are very rough and irregular. This paper reports an alternative approach to HSQ processing that provides insight into mechanistic phenomena while offering certain advantages over standard processing techniques.

Investigation of N2 Plasma Effects on the Density Profile of Hydrogen Silsesquioxane Thin Films

ABSTRACTThe density depth profile and chemical bond structure of hydrogen silsesquioxane (HSQ) thin films treated with an N2 plasma with varying power and exposure time were measured using specular x-ray reflectivity (SXR) and Fourier transform infrared (FTIR) spectroscopy. The SXR data indicate that the density profile of an untreated HSQ film is not uniform and four layers with different electron densities were required to fit the SXR data. For HSQ films treated with either increasing plasma power or plasma exposure time, the film roughness increased and a densified layer was observed at the film/air interface. The thickness of the densified layer increased with both plasma power and plasma exposure time. As a result, up to seven distinct layers were used to model the experimental SXR data from plasma treated films. The FTIR data show that the plasma transforms the Si-H bonds in the HSQ film into Si-O bonds leaving more oxygen atoms around a Si atom. These data are also consistent with the densification observed in the SXR measurements. In general, the HSQ film is more sensitive to increasing plasma power than to increasing plasma exposure time.