Hydrogen Silsesquioxane Layer Research Articles

Resistive Switching Characteristics of Nonvolatile Memory with HSQ Film Using Microwave Irradiation.

In this study, we fabricated a resistive random access memory (ReRAM) of metal-insulator-metal structures using a hydrogen silsesquioxane (HSQ) film that was deposited by a low-cost solution process as a resistance switching (RS) layer. For post-deposition annealing (PDA) to improve the switching performance of HSQ-based ReRAMs, we applied high energy-efficient microwave irradiation (MWI). For comparison, ReRAMs with an as-deposited HSQ layer or a conventional thermally annealed (CTA) HSQ layer were also prepared. The RS characteristics, molecular structure modification of the HSQ layer, and reliability of the MWI-treated ReRAM were evaluated and compared with the as-deposited or CTA-treated devices. Typical bipolar RS (BRS) behavior was observed in all the fabricated HSQ-based ReRAM devices. In the low-voltage region of the high-resistance state (HRS) as well as the low-resistance state, current flows through the HSQ layer by an ohmic conduction mechanism. However, as the applied voltage increases in HRS, the current slope increases nonlinearly and follows the Poole-Frenkel conduction mechanism. The RS characteristics of the HSQ layer depend on the molecular structure, and when the PDA changes from a cage-like structure to a cross-linked network, memory characteristics are improved. In particular, the MWI-treated HSQ ReRAM has the largest memory window at the smallest operating power and demonstrated a stable endurance during the DC cycling test over 500 times and reliable retention at room (25 °C) and high (85 °C) temperatures for 10⁴ seconds.

Graphene nanoribbon field-effect transistors fabricated by etchant-free transfer from Au(788)

We report etching-free and iodine-free transfer of highly aligned array of armchair-edge graphene nanoribbons (ACGNRs) and their field-effect transistor (FET) characteristics. They were prepared by on-surface polymerization on Au(788) templates. The ACGNRs were mechanically delaminated and transferred onto insulating substrates with the aid of a nano-porous support layer composed of hydrogen silsesquioxane (HSQ). The key process in the mechanical delamination is the intercalation of octanethiol self-assembled monolayers (SAMs), which penetrate the HSQ layer and intercalate between the ACGNRs and Au(788). After the transfer, the octanethiol SAMs were removed with Piranha solution, enabling the reuse of the Au single crystals. The FETs fabricated with the transferred ACGNR array showed ambipolar behavior when the channel length was as long as 60 nm. Quasi-one-dimensional conductivity was observed, which implies a good alignment of GNRs after the transfer. In contrast, short-channel ACGNR FETs (channel length ∼20 nm) suffer from a geometry-dependent short-channel effect. This effect is more severe in the FETs with ACGNRs parallel to the channel, which is an ideal geometry, than in ones perpendicular to the channel. Since the ID-VD curve is well fitted by the power-law model, the short-channel effect likely stems from the space-charge limited current effect, while the wide charge-transfer region in the GNR channel can be another possible cause for the short-channel effect. These results provide us with important insights into the designing short-channel GNR-FETs with improved performance.

Air void optical scattering structure for high-brightness organic light emitting diodes

Abstract In order to increase the outcoupling efficiency of organic light emitting diodes (OLEDs), a hexagonal array of air void optical scattering structures was formed on a glass substrate. The air void structure was fabricated by first depositing a monolayer of polystyrene (PS) spheres having a diameter of 820 nm on the substrate. A layer of hydrogen silsesquioxane (HSQ) was then coated on the resulting monolayer and finally, annealing was carried out to burn out the PS spheres and to cure the HSQ layer. The air void optical scattering structure on the glass substrate interacted with the generated photons and suppressed the total internal reflection of light at the surface of the glass. As a result, the OLED fabricated on this glass exhibited a 10% increase in the optical output.

Novel germanium surface modification for sub-10 nm patterning with electron beam lithography and hydrogen silsesquioxane resist

Germanium is a promising high-mobility channel material for future nanoelectronic devices. Hydrogen silsesquioxane (HSQ) is a well known high-resolution electron beam lithography (EBL) resist, which is usually developed in aqueous based developers. However, this feature of HSQ causes troubles while patterning Ge surface as it is always shielded with native Ge oxides. GeO2 is a water soluble oxide, and since HSQ resist is developed in aqueous solvents, this oxide interferes with the patterning. After the EBL exposure, GeO2 is washed off during the development, lifting the patterned structures and making the high-resolution patterning impossible. To avoid this issue, it is necessary to either clean and passivate the Ge surface or use buffer layers between the native Ge oxides and the HSQ layer. In this article, a novel technique to clean the Ge surface prior to HSQ deposition, using simple “household” acids like citric acid and acetic acid, is reported. The acids are able to remove the native Ge oxides as well as efficiently passivate the surface. The acid passivation was found to hold the HSQ sturdily to the Ge surface, even during development with the aqueous salty solvent. Using this process, Ge nanowires having widths down to 5 nm were successfully patterned on germanium-on-insulator substrates. To the best of our knowledge, these are the smallest top-down fabricated Ge nanostructures reported till date.

Thin electron beam defined hydrogen silsesquioxane spacers for vertical nanowire transistors

A method to fabricate inorganic vertical spacer layers with well-controlled thickness down to 40 nm using electron beam exposure is demonstrated. These spacers are suitable in vertical nanowire transistor configuration. As spacer material, the authors use hydrogen silsesquioxane (HSQ), a material with low permittivity and high durability. They show that the resulting HSQ thickness can be controlled by electron dose used and it also depend on the initial thickness of the HSQ layer. To achieve good reproducibility, the authors found it necessary to fully submerge the nanowires beneath the HSQ layer initially and that the thickness of HSQ before exposure needs to be determined. Finally, they introduce these steps in an existing transistor process and demonstrate vertical nanowire transistors with high performance.

Nanoimprint Molds with Circumferentially Aligned Patterns Fabricated by Liftoff Process

A liftoff process was used to fabricate nanoimprint molds with dense patterns below 50 nm pitch. Circumferentially aligned patterns were defined by electron-beam lithography (EBL) using an electron-beam recorder with a rotary stage. Undercut profiles suitable for liftoff were fabricated by etching multilayered resist systems that use a hydrogen silsesquioxane (HSQ) layer as an etching mask for the underlying resin, and liftoff was performed by dissolving the HSQ layer on an insoluble resin layer. By using an undercut profile that was formed in a trilayered stack after EBL, a mold with high-density (42 nm pitch) and large-area (2.5 in.) pillar patterns was fabricated. Quartz replica molds were also fabricated by the liftoff process combined with UV nanoimprint. It was possible to fabricate molds with both a positive tone and a negative tone, and the fabrication of a replica mold with pillar patterns (49 nm pitch) was demonstrated.

Planarization of patterned magnetic recording media to enable head flyability

The fabrication and planarization of patterned magnetic recording media is investigated and the flyability of magnetic recording sliders on a patterned and planarized 65 mm glass disk is investigated a small coupon of patterned media with an array of nano pillars of 40 nm diameter and 60 nm height was first fabricated by e-beam lithography and reactive ion etching (RIE) to investigate the planarization process for patterned media. Since read/write flyability tests require a patterned disk rather than a small coupon area, we have prepared a bit patterned glass disks of 65 mm diameter (2.5 in.) using the so-called “Ag ball-up process” in combination with RIE. This “Ag ball-up process” permits the manufacturing of a nano-sized bit patterns on a large area, i.e., on a disk with 65 mm diameter. Planarization of the patterned area was performed with hydrogen silsesquioxane (HSQ) by spin coating. The HSQ layer was back-etched using RIE, resulting in a smooth surface. “Flyability testing” indicates significantly improved flying stability of typical magnetic recording sliders on the planarized glass disks, with the standard deviation of flying height fluctuations on the order of 0.1 nm. The latter value is comparable to that of “smooth” disks.

Fabrication of a 60‐nm‐Diameter Perfectly Round Metal‐Dot Array over a Large Area on a Plastic Substrate Using Nanoimprint Lithography and Self‐Perfection by Liquefaction

Typically, nanopatterning on plastic substrates has poor fidelity, poor adhesion, and low yield. Here the proposal of and the first experiment using a new fabrication method that overcomes the above obstacles and has achieved arrays of 60-nm-diameter, perfectly round metal dots over a large area on a polyethylene terephthalate (PET) substrate with high fidelity and high yield is reported. This new method is based on the use of a thin hydrogen silsesquioxane (HSQ) layer on top of PET, nanoimprint lithography, and self-perfection by liquefaction (SPEL). The HSQ layer offers excellent thermal protection to the PET substrate during SPEL, as well as good surface adhesion and etching resistance. Nanoimprinting plus a lift off created a large-area array of Cr squares (100 nm x 130 nm) on HSQ and SPEL changed each Cr square into a perfectly round Cr dot with a diameter of 60 nm, reducing the Cr footprint area by 78%. Compared to bare PET, the use of HSQ also reduced the variation in the diameter of the Cr dots from 11.3 nm (standard deviation) to 1.7 nm, an improvement of over 660%. This new technology can be scaled to much larger areas (including roll-to-roll web processing) and thus potentially has applications in various fields.

Room-Temperature Nanoimprint using Liquid-Phase Hydrogen Silsesquioxane with PDMS mold

We have newly developed an imprinting technique using liquid-phase hydrogen silsesquioxane (HSQ) with Poly(dimethyl siloxane) (PDMS) mold. The patterns with various linewidth were demonstrated by imprinting using liquid-phase HSQ. And then, after imprinting, the residual HSQ layer that remained in the compressed area was less than 10 nm thick, because the solvent in HSQ smoothly evaporated through the pores of PDMS mold.

Limiting factors for electron beam lithography when using ultra-thin hydrogen silsesquioxane layers

Isolated dots and lines with 6 nm width are written in 20-nm-thick hydrogen silsesquioxane (HSQ) layers on silicon substrates, using 100-keV electron beam lithography. The main factors that might limit the resolution, i.e., beam size, writing strategy, resist material, electron dose, and development process, are discussed. We demonstrate that, by adjusting the development process, a very high resolution can be obtained. We report the achievement of 7 nm lines at a 20-nm pitch written in a 10-nm-thick HSQ layer, using a potassium-hydroxide (KOH)-based developer instead of a classical tetra-methyl-ammonium hydroxide (TMAH) developer. This is the smallest pitch achieved to date using HSQ resist. We think that the resolution can be improved further, and is presently limited by either the beam diameter (which was not measured separately) or by the not-fully-optimized development process.

Electron-Beam Lithography Patterning of Ge2Sb2Te5 Nanostructures Using Hydrogen Silsesquioxane and Amorphous Si Intermediate Layer

We report the successful electron-beam patterning of nanostructures, in the form of both lines and dots, using hydrogen silsesquioxane (HSQ) resist. Although HSQ has proven to be a good resolution electron-beam resist, the adhesion between and the HSQ resist is problematic when trying to form fine patterns. To promote their adhesion, we introduced an amorphous Si (a-Si) layer between the and HSQ layers, and the layers were then sequentially removed using reactive ion etching and Ar etching with good selectivity of each layer. The selectivities for HSQ:a-Si etched by reactive gas and etched by Ar gas were 1:2.7 and 1:17, respectively. On the basis of this multilayer structure and a two-step dry etching process, various nanostructures as small as were successfully fabricated.

10 nm lines and spaces written in HSQ, using electron beam lithography

Hydrogen silsesquioxane (HSQ) is a high-resolution negative-tone inorganic resist with an established resolution below 10 nm. Using 100 keV electron beam lithography, we report the achievement of isolated 6 nm wide lines in 20 nm thick HSQ layers on silicon substrates. We also achieved 10 nm lines and spaces in a 10 nm HSQ layer. This is the smallest pitch (20 nm) achieved to date using HSQ resist. Experiments in order to investigate the effect of KOH based developer on ultimate resolution have been also performed and resulted in 7 nm wide lines. These results, in combination with the good etching resistance of HSQ, prove the versatility of HSQ for nanolithography.

Fabrication of Magnetic Nanostructures for MRAM using Electron Beam and Focused Ion Beam Exposure of HSQ

ABSTRACTGiant magnetoresistive (GMR) materials-based magnetic random access memory (MRAM) has become attractive due to non-volatility, speed and density1. The vertical MRAM (VMRAM) design model shows good signal level and high speed and density potential. The memory cell for the VMRAM model is a ring shaped magnetic material multilayer, which ensures high repeatability and low switching energy. This GMR structure, however, is difficult to pattern as it contains materials such as Ni, Fe, Co, and Cu, which are difficult to dry etch because they lack volatile etch products. This work shows that it is possible to overcome the difficulties associated with etching GMR materials by using hydrogen silsesquioxane (HSQ) as an etch mask. We have used HSQ in a direct-write electron-beam lithography system with a dose of 600 μC/cm2, and in a Ga+ ion focused ion beam (FIB) system with a dose of 12 μC/cm2. Both are followed by development and an argon plasma etch at 10mTorr and 100W RIE power. The HSQ layer provides high resolution as well as good etching resistance. Electron-beam exposed HSQ shows a 1:1.5 selectivity over the GMR film stack and the FIB exposed HSQ showed an improved etch selectivity of 1:1. Ring shaped GMR structures with a 75/225 nm (ID/OD) have been fabricated, which corresponds to a memory density of 4Gb/in2.

Room-Temperature Nanoimprint Lithography

ABSTRACTRoom-temperature nanoimprint lithography (RT-NIL) using spin-coated hydrogen silsesquioxane (HSQ) resin as the replication material was developed. HSQ pattern with 50 nm linewidth was successfully obtained by the RT-NIL. Postbaking temperature dependence of a HSQ imprinted depth on a mold linewidth was investigated. HSQ imprinted depth had a dependence on the mold linewidth. This revealed that the RT-NIL is suitable for the linewidths of below 1 mm. Furthermore, we have also developed a new imprinting technique that uses liquid-phase hydrogen silsesquioxane (HSQ) as an alternative to the spin-coated HSQ resin. The liquid-phase HSQ imprint technique enabled fabrication of various HSQ patterns with a wide range of linewidths from 25 nm to 300 mm. Arbitrary patterns, including both submicron and greater than 100 micron patterns, were simultaneously replicated with a one-step imprint process, something very difficult to accomplish with spin-coated HSQ. Moreover, after imprinting, the residual HSQ layer in the compressed area was less than 10 nm thick.

Reversing the hydrogen silsesquioxane image by silicon nitride and silicon oxide chemical mechanical polishing

Patterns, e-beam written in hydrogen silsesquioxane (HSQ), have been reversed in tone by chemical mechanical polishing of silicon nitride deposited on top of these HSQ patterns. First, high-resolution patterns have been written in an 80nm thick HSQ layer on a 6in. silicon wafer. After development, 200nm thick plasma enhanced chemical vapor deposited silicon nitride has been deposited at 300°C on top of these patterns. Next, the silicon nitride has been Syton polished down to 50nm thickness. Finally, the HSQ has been removed wet chemically from in between the nitride features using buffered oxide etch (7:1). Features of 50nm wide have been resolved, but even smaller isolated spaces are feasible. The same method has been tested with silicon oxide obtained from boron doped tetraethyl orthosilicate. Although the wet-chemical removal of HSQ is less selective in this case, still 50nm wide features have been resolved in silicon oxide this way.

A Comparative Study of Ti/Low-k HSQ (Hydrogen Silsesquioxane) and Ti/TEOS (Tetraethylorthosilicate) Structures at Elevated Temperatures

AbstractFor the benefit of reducing capacitance in multilevel interconnect technology, low-k dielectric HSQ (hydrogen silsesquioxane) has been used as a gapfill material in Al-metallization- based non-etchback embedded scheme. The vias are consequently fabricated through the HSQ layer followed by W plug deposition. In order to reduce the extent of via poisoning and achieve good W/Al contact, thin Ti/TiN stack films are typically deposited before via plug deposition. In this case, HSQ makes direct contact with the Ti layer. The reliability of the Ti/HSQ structures at elevated temperatures has been systematically studied in this work by using a variety of techniques. These results are also compared with those from Ti/TEOS (Tetraethylorthosilicate) structure, where TEOS is a conventional intra-metal dielectric. When the temperature is below 550 °C, a significant number of oxygen atoms are observed to diffuse into the titanium layer. The primary source of oxygen is believed to come from the HSQ film. When the temperature is above 550 °C, HSQ starts to react with Ti. At 700 °C, a TiO/Ti5Si3/HSQ stack structure forms. The Ti/HSQ system exhibits a higher reactivity than that of the Ti/TEOS system.