Aspect Ratio Patterns Research Articles

A new UV sensitive positive resist for X-ray masks manufacture

LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall roughness by means of X-ray lithography and electroplating. The availability of a precise X-ray mask is a precondition for the final precision of the manufactured micro parts. Typical mask substrate materials, e.g. beryllium, carbon based foils, Si3N4 or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high thermal expansion or surface porosity causing X-ray scattering. For the fabrication of X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified, negative tone photoresist exhibits high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly hinders the inspection of the fabricated X-ray mask. We present the use and suitability of an UV sensitive, chemically amplified, viscous, aqueous-alkaline developable, and easy removable positive tone photoresist, XP mr-P 15 AV, exhibiting high aspect ratio patterns with vertical sidewalls for the fabrication of X-ray masks by means of UV lithography on vitreous carbon substrates.

Effects of Aspect Ratio of Micro-sized Photoresist Patterns on Bond Strength between a Si Substrate with AFM Fracture Observation

AbstractBond strength between three dimensional micro-sized cylindrical patterns and Si substrate has been evaluated to clarify the effects of the cylinder length vs. diameter ratio, i.e. the aspect ratio, on the bond strength. Cylindrical shape was employed for avoiding ambiguity of loading point under bend conditions. Multiple cylindrical specimens of an epoxy type photoresist, SU-8 with various lengths were fabricated on a silicon substrate under the same photolithographic condition. Bond strength between micro-sized SU-8 and Si substrate under bend loading mode was measured by a mechanical testing machine for micro-sized materials. The maximum bend moment is 9.6 × 10−6 Nm in average and lineally increases with increasing the aspect ratio. On the other hand, the maximum load, i.e. maximum shear load is 106 mN in average and almost constant with increasing aspect ratio. This result suggests that the shear stress near the interface may cause the initiation of delamination. This phenomenon is discussed with three dimensional fracture observation and quantitative analysis of the line profile around the initiation site by AFM.

Time dependent analysis of the resist deformation in thermal nanoimprint

Time evolution of the resist deformation process in the thermal nanoimprint lithography (NIL) has been investigated by both experiment and simulation study. For the numerical simulation, the authors newly developed a simulator using generalized Maxwell model as the viscoelastic constitutive model in conventional finite element method, which handles the complete NIL operations including resist pressing, cooling, and demolding processes. The dependency on the linewidth of the filling rate was investigated by both experiments and simulation and they sufficiently agreed with each other. It is confirmed that narrow pattern (high aspect ratio pattern) was difficult to be filled in a short period under the same imprinting pressure.

Research on optimization of the hot embossing process

The hot embossing of polymer was studied to improve product quality. The finite element method (FEM) was used to analyze the embossing step, the cooling step and the demolding step with respect to product quality. The numerical simulation results show that profile precision is largely influenced by the topological structure of the mold during the embossing step. Inadequate holding time will result in low pattern fidelity. Inappropriate demolding temperature could induce a large thermal stress at the bottom of the micro pattern, while keeping imprint pressure during the cooling step will aggravate this phenomenon. Fracture, necking and pattern distortion can easily be induced by adhesion between the polymer and the mold when high aspect ratio patterns are demolded. The hot embossing process was optimized and an improved hot embossing system with an automatic demolding device was designed based on the simulation analysis. Successful fabrication of fine patterns with a high aspect ratio verified the improvements.

Glass nanostructures fabricated by soft thermal nanoimprint

A one-step, fast, and potential low cost process has been developed to fabricate silicalike glass nanostructures by combining sol-gel chemistry and thermal nanoimprint. An inorganic-organic sol-gel thin film is patterned at low pressure and temperature with flexible stamps. Various geometries are achieved with a patterning resolution of about 150nm and pattern aspect ratio higher than 4. To obtain pure silica structures a thermal annealing at high temperatures is required. During this step most of the structures collapse due to fluidization of the sol-gel material. It is shown that if a suitable condensation level is obtained during imprinting, the nanostructures are thermally stable.

The adsorption of cationic surfactants on photoresist surfaces and its effect on the pattern collapse in high aspect ratio patterning

The collapse of photoresist patterns in high aspect ratio patterning is a defect that may reduce the production yield drastically. Recently, a novel method to reduce the pattern collapse was successfully applied: the rinse of the photoresist patterns with a cationic surfactant solution. In a previous study it was shown that one chosen surfactant adsorbs on both untreated photoresist surfaces and photoresist films that had undergone the whole photolithographic process. For this surfactant best pattern collapse reduction coincides with the formation of surfactant aggregates on the photoresist surface and with a maximum receding contact angle on processed photoresist. The present study investigates the influence of the hydrocarbon chain length of the surfactant on the pattern collapse reduction. The adsorption of surfactants of various chain length on unexposed and exposed photoresist was observed using null-ellipsometry and zeta potential measurements. Increasing the surfactant chain length shifts the adsorption domains to lower concentrations. It leads to an increased adsorbed amount and maximum contact angle as well as to a better pattern collapse reduction.

Adhesion Enhancement through Micropatterning at Polydimethylsiloxane−Acrylic Adhesive Interfaces

Adhesion at polydimethylsiloxane (PDMS)-acrylic adhesive interfaces is shown to be enhanced through micropatterning of the PDMS substrate. By varying the geometry of the patterns (groves and hexagonal arrays of pillars of micrometer sizes, obtained through soft lithography techniques) and comparing rigid and deformable substrates, the respective roles of the geometry and the size and flexibility of the pattern features on the level of adhesion have been analyzed. For cylindrical pillars, two regimes are clearly identified: for a relatively low aspect ratio (h/r < 3, with h and r, respectively, the height and the radius of the pillars), soft patterned substrates are more efficient than rigid ones at increasing adhesion, pointing out the role of the elastic energy associated with the deformation of the pattern that is lost when the adhesive detaches from the substrate. Using scaling laws, the predominant contribution to that elastic energy can be further identified: deformation of the substrate underlying the pillars for h/r < 1.6 or bending of the pillars for h/r > 1.6.; for a high aspect ratio (h/r > 3), only rigid patterned substrates enhance adhesion, then the only possible contribution to energy dissipation comes from the enhanced viscoelastic losses associated with the pattern that induce modifications of the strain field within the adhesive layer. Soft, high aspect ratio patterns lose their efficiency even if still bent under the effect of the peel forces. This is because when bent, some of the pillars touch each other and remain stuck together, lying flat on the surface after the passage of the peel front. The bending elastic energy of the pillars (which is still lost) is then balanced by the corresponding gain in surface energy of the substrate in the peeled region. These systematic experiments demonstrate that the ability of the patterned surface to be deformed plays a crucial role in enhancing adhesion and allow us to propose a way to fine tune the level of adhesion at PDMS-acrylic adhesive interfaces, independently of the chemistry of the adhesive.

High-speed processing with high-energy SF6 cluster ion beam

In order to realize high-speed processing, high-energy SF6 cluster ion beam was generated and Si substrates were irradiated with the SF6 cluster ions at acceleration energies of 10–80keV. The sputtering yield increased with acceleration energy and was about 2600 times higher than that of Ar monomer ions. The sputtering yield of SF6 cluster ions was about 3300atoms/ion at acceleration energy of 80keV. The etching ratios of Si/Ni were about 25 at acceleration energy of 20keV. The selectivity of Ni was sufficiently high to use as a mask material for Si etching. Both surfaces of etched material (Si) and mask material (Ni) remained smooth surfaces after high-energy SF6 cluster ion beam irradiation. These results show that the high-energy SF6 cluster ion beam can fabricate patterns of high aspect ratio with high-speed keeping the surfaces smooth.

Molecular dynamics study of pattern transfer in nanoimprint lithography

A molecular dynamics simulations model of nanoimprint lithography (NIL) is proposed in order to study the pattern transfer and its related phenomena. The proposed model is similar to a real NIL process imprinting an α-quartz stamp with a rectangular line pattern into an amorphous poly-(methylmethacrylate) (PMMA) film. The polymer deformation behavior and the adhesion and friction effects between the stamp and the polymer film are investigated and their dependency on the pattern aspect ratio is discussed. Force fields including bond, angle, torsion, van der Waals, and electrostatic potentials are used to describe intermolecular and intramolecular interacting forces. Nose-Hoover thermostat is used to control the temperature of the polymer film and cell multipole method is adopted to treat long range interactions. The deformation of the polymer film is observed for two stamps having different aspect ratio patterns. The distributions of density and stress in the polymer film are calculated for the detail analysis of deformation behavior. For a high aspect ratio pattern (aspect ratio = 2.5, imprint depth = 8.0 nm), large amount of springback of the residual polymer film is observed, which is mainly due to the residual compressive stress left in the polymer film. However, for a low aspect ratio pattern (aspect ratio = 1.0, imprint depth = 3.0 nm), the springback is not observed. In addition, adhesion and friction forces are obtained by dividing the polymer film into subregions and calculating the interacting force between each subregion and the stamp. While the adhesion force is nearly constant regardless of the pattern aspect ratio, the friction force increases as the pattern aspect ratio grows, so the friction force becomes larger than the adhesion force when the pattern aspect ratio increases.

Sub-10-nm high aspect ratio patterning of ZnO in a 500μm main field

We demonstrate the fabrication of sub-10-nm high aspect ratio electron beam patterning of ZnO at the center as well as corners of the 500μm square main deflection field using a negative tone zinc naphthenate resist. After electron beam exposure, the resist was developed in toluene, resulting in high-resolution patterns as small as 7nm with an aspect ratio of ∼10 in the center as well as at the corners of the main field. The line edge roughness (3σ) of these lines shows roughness with fluctuations of about 2.8nm; thus making it the smallest value of the line edge roughness measured so far for any electron beam resist. Heating the structures at 500°C resulted in reduction of feature sizes to ∼5nm and in the formation of crystalline ZnO. Such sub-10-nm high aspect ratio patterning within the main deflection field is an important step to enable full-wafer-level nanofabrication.

Multi-layered resist process in nanoimprint lithography for high aspect ratio pattern

Multi-layered resist process for nanoimprint lithography is newly proposed to fabricate high aspect ratio pattern under low pressured condition. A low molecular weight polymer is coated on a high molecular weight polymer and a mold is pressed to the bi-layered resist structure over the glass transition temperature of the polymers. The upper layered resist easily flows into the mold groove under low pressured condition and the lower layered resist stays near the base area of the pattern, which makes the base area strong. As the result, the defect at the mold releasing process is eliminated for the high aspect pattern fabrication. The deformation process is confirmed by numerical simulation and the thickness of each layer is optimized. Using the multi-layered resist process, high aspect ratio pattern is successfully fabricated under low pressured condition and the fracture defect at the mold releasing step is eliminated.

Effects of line width and dose on electron beam nano-patterning of ma-N 2403

The effects of dose on pattern shape were evaluated for electron beam nano-lithography of a negative photoresist, ma-N 2403, under an electron beam condition of 20 keV with a probe current of 9 pA. With sufficient design width, i.e. 250 nm, there was negligible loss of pattern height, which was attributed to the combined dose of the incident electron beam and the back scattered electrons. The dose for narrow design width should be adjusted to obtain patterns of high aspect ratio with no loss of height and reduced pattern width difference from the design width.

A large format membrane-based x-ray mask for microfluidic chip fabrication

X-ray lithography is a very good option for the fabrication of micro-devices especially when high aspect ratio patterns are required. Membrane-based x-ray masks are commonly used for high-resolution x-ray lithography. A thin layer of silicon nitride (Si3N4) or silicon carbide (SiC) film (1–2 µm) is normally used as the membrane material for x-ray mask fabrication (Wells G M, Reilly M, Nachman R, Cerrina F, El-Khakani M A and Chaker M 1993 Mater. Res. Soc. Conf. Proc. 306 81–9; Shoki T, Nagasawa H, Kosuga H, Yamaguchi Y, Annaka N, Amemiya I and Nagarekawa O 1993 SPIE Proc. 1924 450–6). The freestanding membrane window of an x-ray mask, which defines the exposing area of the x-ray mask, can be obtained by etching a pre-defined area on a silicon wafer from the backside (Wang L, Desta Y, Fettig R K, Goettert J, Hein H, Jakobs P and Chulz J 2004 J. Micromech. Microeng. 14 722–6). Usually, the window size of an x-ray mask is around 20 × 20 mm because of the low tensile stress of the membrane (10–100 MPa), and the larger window dimension of an x-ray mask may cause the deformation of membranes and lower the mask quality. However, x-ray masks with larger windows are preferred for micro-device fabrication in order to increase the productivity. We analyzed the factors which influence the flatness of large format x-ray masks and fabricated x-ray masks with a window size of 55 × 55 mm and 46 × 65 mm on 1 µm thick membranes by increasing the tensile stress of the membranes (>300 MPa) and optimizing the stress of the absorber layer. The large format x-ray mask was successfully applied for the fabrication of microfluidic chips.

Microzone plates with high-aspect ratio fabricated by e-beam and x-ray lithography

We introduce a combined e-beam and x-ray lithographic method to fabricate microzone plates (MZP) on free-standing silicon nitride films. An automatic design program is developed to draw the complex layout of MZP with very smooth boundaries. A gold MZP master mask with a minimum ring width of 250 nm is fabricated by e-beam lithography. The master mask is replicated using x-ray lithography (XRL) and nickel electroplating to obtain the final MZP. The combined lithographic technique produces a MZP with a pattern aspect ratio of 4.4:1.

Nano-oxidation of Si using ac modulation in atomic force microscope lithography

In atomic force microscope (AFM) anodization lithography, voltage modulation is an important factor leading to enhanced oxide growth rate, increased aspect ratio of oxide patterns and greater control of oxide features. Through the reduction of space charge buildup in the oxide, ac modulation overcomes the self-limiting character of the oxide. When ac modulation was applied to substrates, the aspect ratio of protruded oxide patterns increased five-fold compared to dc pulse. Controlling electron exposure time between the positive and negative voltages is an important factor for controlling the aspect ratio of oxide patterns. This showed the dependence of applied voltage types and various electron exposure times in ac modulation. By adjusting electron exposure time and reducing space charge in the oxide, this study revealed that ac modulation is superior for obtaining high aspect ratios compared to dc pulse.

Pattern preserving deposition: Experimental results and modeling

In this work we discuss pattern-preserving growth during metal deposition from the vapor on micro/nano-structured metal substrates. Experimental results for Cu deposition on patterned Cu substrates show pattern preserving growth or pattern destruction depending on the incident angle. We introduce a mesoscopic 1+1 dimensional model including deposition flow (directed and isotropic), surface diffusion and shadowing effects that account for the experimental growth data. Moreover, simulations on post-deposition annealing, for high aspect-ratio patterns show departures from the predictions of the linear theory for surface diffusion.

Pattern‐Transfer Fidelity in Soft Lithography: The Role of Pattern Density and Aspect Ratio

AbstractUsing high‐aspect‐ratio nanostructures fabricated via two‐photon laser‐scanning lithography, we examine the deformation of elastomeric stamps used in soft nanolithography and the fidelity of patterns and replicas made using these stamps. Two‐photon laser‐scanning lithography enables us to systematically regulate the aspect ratio and pattern density of the nanostructures by varying laser‐scanning parameters such as the intensity of the laser beam, the scanning speed, the focal depth inside the resist, and the scanning‐line spacing. Two commercially available stamp/mold materials with different moduli have been investigated. We find that the pattern‐transfer fidelity is strongly affected by the pattern density. In addition, we demonstrate that true three‐dimensional structures can be successfully replicated because of the flexible nature of elastomeric poly(dimethylsiloxane).

Atomic force microscope anodization lithography using pulsed bias voltage synchronizedwith resonance frequency of cantilever

An applied bias voltage between the atomic force microscope tip and the substrate is one ofthe important factors related to the growth of oxide patterns. A pulse modulator was usedto apply a pulsed bias voltage that synchronizes with the resonance frequency of thecantilever between the tip and the substrate in tapping mode. The height of the protrudedoxide structure was increased for short duration times of the pulsed bias due to thereduction of built-up space charge in oxide. The aspect ratio of patterns using pulsed biasvoltage was about two times higher than that using continuous bias voltage. This studyrevealed that the pulsed bias has an advantage for obtaining a higher aspect ratiopattern than the continuous bias by reducing the effect of space charge in oxide.

Fine pattern transfer by nanocasting lithography

A novel pattern transfer technique has been demonstrated by simple procedure based on a nanoimprint lithography. A polymer is coated on a mold by a spin coater and cast into fine grooves on the mold. After evaporation of a solvent, an adhesive resin is coated on the polymer and put a substrate plate. After releasing the mold, fine pattern is successfully transferred to the polymer. Applying this process, fine pattern transfer using various polymers, high aspect ratio pattern and sub-100 nm patterns have been demonstrated on various substrates.

Nano-Fabrication of Nano-Crystalline Diamond Films Grown by Microwave Plasma-Assisted Chemical Vapor Deposition

In order to produce nano-scaled ultra-fine patterns into diamond films, a nano-crystalline diamond film with very flat surface has been synthesized. The fabrication process for a nano-scaled patterning in the nano-crystalline diamond film has also been developed. The nano-crystalline diamond film was grown on a silicon wafer by the microwave plasma-enhanced chemical vapor deposition with 1.0 % nitrogen addition in the gas phase. The grown diamond film consisted of nanometer-sized crystals has a smaller surface roughness of Rms=18 nm compared to nitrogen-undoped polycrystalline diamond films. The nano-crystalline diamond film was fabricated into ultra-fine patterns by the e-beam lithography and the reactiveion etching technique. In these processes, one of the key factors for the ultra-fine patterning was a selection of the mask material. The amorphous silicon nitride thin-film was revealed to be appropriate for the etching mask. As the nano-crystalline diamond film etching gas, a mixture of oxygen and a small amount of tetrafluorocarbon was effective for obtaining a higher aspect ratio pattern with little residue at the etched surfaces. We have succeeded in producing nano-scaled ultra-fine patterns into nano-crystalline diamond films with a minimum line-width of 100 nm.