High Aspect Ratio Holes Research Articles

Ion and neutral transportation consideration in etching of thin Si[sub 3]N[sub 4] in high aspect ratio structures for aspect ratio independent etching

We investigate the reactive ion etching (RIE) lag or aspect ratio dependent etching in etching of a thin Si3N4 layer in the regime of relatively low ion bombardment energy. The surface reaction is studied through the transportation of ions and radicals in high aspect ratio holes. The radical transportation is studied in the regime that the radical concentration is low and the ion flux is saturated. The key parameter for the radical transportation is the high substrate temperature, where radicals can reach the hole bottom, easily resulting in high surface coverage. On the other hand, in a regime of saturated surface coverage with abundant radicals, the ion flux is the reaction limiting parameter for the RIE lag. The comparison of RIE lag with the surface coverage shows that the transportation of depositive radicals is more sensitive to the temperature. Finally, in the range where the neutral and ion fluxes are balanced, we achieved a result of aspect ratio independent etching in a structure with an aspect ...

Etching of high aspect ratio features in Si using SF6∕O2∕HBr and SF6∕O2∕Cl2 plasma

We have investigated the etching of high aspect ratio holes (∼4μm deep, ∼0.2μm diameter) in silicon using plasmas maintained in mixtures of SF6, O2, and HBr or Cl2 gases. The etching experiments were conducted in a low pressure (25mTorr), high density, inductively coupled plasma etching reactor with a planar coil. Visualization of the profiles with scanning electron microscopy is used in conjunction with plasma diagnostics such as optical emission and mass spectroscopies to understand the key factors that control the feature profile shape and etch rate. HBr addition to SF6∕O2 mixture reduces the F-to-O ratio, increases sidewall passivation and reduces mask undercut. Addition of Cl2 to SF6∕O2 discharge also decreases the F-to-O ratio, but Cl-enhanced F chemical etching of silicon significantly increases the mask undercut and lateral etching. In both SF6∕O2∕HBr and SF6∕O2∕Cl2 mixtures, reduction of O2 flow rate and subsequent increase of the halogen-to-O ratio eventually results in significant lateral etching because of the lack of oxygen required to form a siliconoxyhalide passivating film on the sidewalls.

Three-dimensional thin-film Li-ion microbatteries for autonomous MEMS

Autonomous MEMS require similarly miniaturized power sources. In this paper, we present the first working three-dimensional (3-D) rechargeable Li-ion thin-film microbattery technology that is compatible with MEMS requirements. The technology has been developed, and full 3-D cells have been manufactured on both glass and silicon substrates. Our 3-D microbatteries have a sandwich-like structure of conformal thin-film electrodes, electrolyte and current collectors. The films are deposited sequentially on all available surfaces of a perforated substrate (e.g., silicon or a glass microchannel plate or "MCP") using wet chemistry. The substrate has thousands of high-aspect ratio holes per square cm, thereby providing more than an order of magnitude increase in surface area per given footprint (original 2-D substrate area). The full 3-D cell consists of a Ni cathode current collector, a MoO/sub y/S/sub z/ cathode, a hybrid polymer electrolyte (HPE) and a lithiated graphite anode that also serves as anode current collector. One 3-D cell with a roughly 1-/spl mu/m-thick cathode ran at C/10 to 2C charge/discharge rates and room temperature for 200 cycles with 0.2% per cycle capacity loss and about 100% Faradaic efficiency. The cell exhibited a capacity of 2 mAh/cm/sup 2/, about 30times higher than the capacity of a similarly built planar (2-D) cell with the same footprint and same cathode thickness.

Thick polymer cover layers for laser micromachining of fine holes

We have demonstrated that a thick polymer cover layer can significantly improve UV laser micromachining. The polymer cover layer acts as a wave-guide, concentrating the incoming laser beam onto the underlying substrate to be machined. With this method very low laser fluences can be used to drill fine, high aspect ratio holes in quartz, glass and other materials that are generally difficult to machine. We show that despite the improvements recorded, our experimental conditions are not optimized and the contribution of the cover layers in our experiments is less than what it could be. We suggest improvements to the cover layer material to improve the micomachining. The method has important advantages over conventional methods since a mask is not required and common laser equipment can be used to quickly and inexpensively form desired holes within the substrate.

Fabrication of Metal Nanohole Arrays with High Aspect Ratios Using Two‐Step Replication of Anodic Porous Alumina

Metal nanohole arrays with straight holes of high aspect ratio have been fabricated using a new replication process based on the preparation of polymer pillars, both sides of which are supported and can therefore be maintained upright. These nanopillar arrays are demonstrated as negatives for the preparation of metal (Ni) nanohole arrays (see Figure).

Double-pulse machining as a technique for the enhancement of material removal rates in laser machining of metals

Several nanosecond 0.53-μm laser pulses separated by several tens of nanoseconds have been shown to significantly enhance (three to ten times) material removal rates while minimizing redeposition and heat-affected zones. Economic, high-quality, high-aspect ratio holes (>10:1) in metals are produced as a result. A phenomenological model whereby the second laser pulse interacts with the ejecta produced by the first laser pulse and in close proximity to the material surface is consistent with the observations. Incident laser wavelengths of 1.05 and 0.35 μm also benefit from this pulse format.

A novel method for making high aspect ratio solid diamond tips

A novel method for making high aspect ratio solid diamond tips is presented. The new method is based on focused ion beam (FIB) milling of high aspect ratio inverted conical-shape holes in silicon, then the holes are filled with diamond by hot-filament chemical vapor deposition (HFCVD) method, the solid diamond tips are formed after removal of silicon by wet chemical etching. Inverted conical shape holes of high aspect ratio with controllable depth and curvature has been achieved by optimising the beam current, the diameter of the milled holes, the milling time and the scan scheme. SEM inspection has revealed that the diamond tip has up to 7.5:1 aspect ratio, and tip apex radius can be as small as sub-50 nm. The micro-Raman (mRm) spectrum from a diamond tip shows dominant diamond line at 1332 cm −1 and a slight peak at 1500 cm −1 from the sp 2 bonded carbon. The high aspect ratio solid diamond tips have potential applications in indenters and electron emitters due to the excellent mechanical and negative electron affinity (NEA) properties of diamond.

A statistical approach to determine process parameter impact in Nd:YAG laser drilling of IN718 and Ti-6Al-4V sheets

The numerous unique advantages afforded by pulsed Nd:YAG laser systems have led to their increasing utility for producing high aspect ratio holes in a wide range of materials. Notwithstanding the growing industrial acceptance of the technique, the increasingly tighter geometrical tolerances and more stringent hole quality requirements of modern industrial components demand that “defects” such as taper, recast, spatter etc., in laser-drilled holes are minimized. Process parameters like pulse energy, pulse repetition rate, pulse duration, focal position, nozzle standoff, type of gas and gas pressure of the assist gas are known to significantly influence hole quality during laser drilling. The present study reports the use of Taguchi design of experiments technique to study the effects of the above process variables on the quality of the drilled holes and ascertain optimum processing conditions. Minimum taper in the drilled hole was considered as the desired target response. The entire study was conducted in three phases:(a) screening experiments, to identify process variables that critically influence taper in laser drilled holes, (b) Optimization experiments, to ascertain the set of parameters that would yield minimum taper and (c) validation trials, to assess the validity of the experimental procedures and results. Results indicate that laser drilling with focal position on the surface of the material being drilled and employing low level values of pulse duration and pulse energy represents the ideal conditions to achieve minimum taper in laser-drilled holes. Thorough assessment of results also reveals that the laser-drilling process, optimized considering taper in the drilled hole as the target response, leads to very significant improvements in respect of other hole quality attributes of interest such as spatter and recast as well.

Etching characterization of shaped hole high density plasma for using MEMS devices

High aspect ratio microstructure technology is the key technology for advanced fabrication of microelectro mechanical systems (MEMS). High aspect ratio hole etching for silicon was investigated as a function of platen power, platen temperature, and coil power. Their effects on etch profile angle, scallops, and etch rate were also studied. As the platen power was increased from 5 to 30 W, the etch rate was increased from 0.37 to 3.18 μm/min. However, the etch rate was decreased at a platen power higher than 30 W. As the coil power was increased from 600 to 2400 W, the etch rate was increased from 2.95 to 3.19 μm/min, but the etch rate was decreased at the coil power higher than 1200 W. As the platen temperature was increased from 10 to 30 °C, the scallops decreased from 139 to 122 nm. Etched cross section was observed on 40-μm in diameter hole properties by scanning electron microscopy (SEM).

Etching of sub-micron high aspect ratio holes in oxides and polymers

Sub-micron holes have been successfully etched into: PMMA/DR-1 on Teflon, BCB, Ta2O5, Nb2O5 and SiO2, with the aim of forming photonic crystal structures. Pattern transfer from the NiCr masks into the material is achieved in an electron cyclotron resonance high-density plasma system, using a combination of the gases: CHF3, CF4, O2, Ar and SF6, and an RF bias on the sample. The etching of Ta2O5 and Nb2O5 is hindered by the high sublimation temperature, approximately 230 °C, of TaF5 and NbF5. This problem was resolved by heating the sample. For these materials aspect ratios of 5 were achieved, with holes of down to 200-nm wide being etched. Wall roughness was typically 30 nm, peak to trough. For SiO2, aspect ratios of 8 were achieved and the wall roughness was about 10 nm, peak to trough. For the polymers: PMMA/DR-1 on Teflon and BCB aspect ratios of 16 are possible for 250-nm wide holes, along with high, 1 μm/min etching rates.

Etching of sub-micron high aspect ratio holes in oxides and polymers

Sub-micron holes have been successfully etched into: PMMA/DR-1 on Teflon, BCB, Ta 2O 5, Nb 2O 5 and SiO 2, with the aim of forming photonic crystal structures. Pattern transfer from the NiCr masks into the material is achieved in an electron cyclotron resonance high-density plasma system, using a combination of the gases: CHF 3, CF 4, O 2, Ar and SF 6, and an RF bias on the sample. The etching of Ta 2O 5 and Nb 2O 5 is hindered by the high sublimation temperature, approximately 230 °C, of TaF 5 and NbF 5. This problem was resolved by heating the sample. For these materials aspect ratios of 5 were achieved, with holes of down to 200-nm wide being etched. Wall roughness was typically 30 nm, peak to trough. For SiO 2, aspect ratios of 8 were achieved and the wall roughness was about 10 nm, peak to trough. For the polymers: PMMA/DR-1 on Teflon and BCB aspect ratios of 16 are possible for 250-nm wide holes, along with high, 1 μm/min etching rates.

Recent Advances in Three Dimensional Thin Film Microbatteries

ABSTRACTAutonomous MEMS require similarly miniaturized power sources. We present the first three-dimensional (3D) working thin-film microbattery (MB) technology that is compatible with MEMS requirements. Our 3D MBs are formed in perforated substrates (silicon chip or glass microchannel plate), in which a sandwich-like thin-film battery structure is deposited conformally and sequentially on all available surfaces, using wet chemistry. The area of high aspect ratio holes in the substrate enhances the capacity and energy density per given substrate footprint by more than an order of magnitude. Full 3D cells were manufactured on both glass and silicon substrates. A 3D cell on a micro-channel plate substrate exhibited a capacity of ca. 2mAh/cm2, in good agreement with the ca. 23 times area gain provided vs an identical footprint 2D cell.

Formation of Complex Tungsten-Silica Microstructures by Ar[sup +] Laser Processing

Angewandte Physik, Johannes Kepler Universitat Linz, A-4040 Linz, Austria The possibility for localized, spatially separated laser-induced deposition and etching of tungsten from pure WF 6 gas is demonstrated. By employing a fused silica substrate covered with a tungsten layer, a threefold process is induced; deposition of tungsten in the center of the laser beam, etching of the tungsten-layer outside of the beam center and etching the excavated silica by fluorine radicals. This multiple process results in an effective drilling. The deposition of a conducting tungsten kernel inside a high aspect ratio hole, formed in the insulating silica, suggests the applicability to a single-step production of field emitter units.

Low-resistance polysilicon process in contact application for high density devices

For the lowest resistance, it is required to have the epitaxial silicon contact between the silicon plug and the substrate and good step coverage at the high aspect-ratio contact holes, simultaneously. In this work, a double polysilicon (DPS) deposition technique was proposed for the requirements. The first, thin silicon layer is deposited in a single-wafer process chamber with an in-situ H2-RTP (rapid thermal process) treatment for the epitaxial contact, and the second silicon layer is formed in a batch-type furnace for good step coverage. From chain resistance, Kelvin Rc, and current-voltage (I–V) measurement, the DPS process meets both low resistance and good uniformity, so that it suggests a breakthrough in the small-sized, semiconductor device application.

Formation ofWell-defined Nanocolumns by Ion Tracking Lithography

AbstractLow dimensional systems on the nanometer scale afford a wealth of interesting possibilities including highly anisotropic behavior and quantum effects. Nanocolumns permit electrical and mechanical contact, yet benefit from two confined dimensions. This confinement leads to new optical, mechanical, electrical, chemical, and magnetic properties. We construct nanocolumn arrays with precise definition and independent control of diameter, length, orientation, areal density and composition so that geometry can be directly correlated to the quantum physical property of interest. The precision and control are products of the fabrication technique that we use. The process starts with an ion of sufficient energy to “track” a dielectric such as a film applied uniformly onto a substrate. The energy loss of the ion alters chemical bonding in the dielectric along the ion's straight trajectory. A suitable etchant quickly dissolves the latent tracks leaving high aspect ratio holes of small diameter (∼10nm) penetrating a film as thick as several microns. These small holes are interesting and useful in their own right and can be made to any desired size by continuing the etching process. Moreover, they serve as molds for electrochemical filling. After this electrodeposition, the mold material can be removed leaving the columns firmly attached to the substrate at the desired orientation. A variety of structures can be envisioned with these techniques. As examples, we have created arrays of Ni and of Pt nanocolumns (∼ 60 nm diameter and ∼ 600 nm long) oriented perpendicular to the substrate. The high aspect ratio and small diameter of the columns enables easy observation of quantum behavior, namely efficient electron field emission and Fowler Nordheim behavior.

Copper device editing: Strategy for focused ion beam milling of copper

Focused ion beam (FIB) methodologies for successfully milling copper (U.S. Patent No. 6,322,672 B1) have been demonstrated. Approaches to milling copper (Cu) are required because standard FIB mill procedures produce rough, uneven cuts that are unsuitable for circuit edits, a principal FIB function. Efforts to develop gas assisted etching (GAE) processes which would smoothly mill Cu failed because Cu halides are not volatile and remain on the substrate as corrosive electrically conductive debris. Single crystal studies show that Cu grains with different crystal orientations vary in mill rate by as much as 4×. Moreover, the (110) crystal orientation, which mills most slowly, forms a Cu3Ga phase when milled with a focused Ga ion beam. This phase is particularly resistant to milling and, in polycrystalline Cu, propagates during the milling operation, contributing to the uneven trench profiles. CoppeRx, a novel scan strategy, cleanly and uniformly removes polycrystalline Cu with minimal damage to the underlying dielectric. CoppeRx minimizes the formation and propagation of the Cu3Ga phase and equalizes the etch rates of the Cu crystal orientations. The CoppeRx strategy includes the milling of an “egg crate” topography to minimize the propagation of the Cu3Ga phase and the creation of a heavy atom sacrificial layer of the Cu surface (U.S. Patent Application No. 20010053605) which scatters the incident Ga ion beam, thereby reducing the channeling influence on Cu milling rates. This heavy atom layer is created by flowing W(CO)6 vapor during the FIB milling process. The CoppeRx scan strategy is especially beneficial for milling thick (>0.8 μm) Cu structures with large, prominent grains. Because Cu interconnect lines are relatively thin (<0.4–0.5 μm), grain-related milling roughness is less of a problem. The CoppeRx egg crate topography and W scattering layer are not required. Instead, the successful cutting of 40 ohm Cu interconnect lines to produce >20 M ohm open circuits is achieved by flowing O2 or H2O during the milling process (U.S. Patent No. 6,322,672B1). The O2/H2O flow smoothes the Cu milling by producing an amorphous surface oxide, thereby reducing channeling, and by enhancing the etch selectivity for Cu relative to the surrounding and underlying SiO2 based dielectric. These interconnect cuts have been routinely done at the bottom of high aspect ratio holes (e.g., 1×1×9 μm).

Improved etch characteristics of SiO2 by the enhanced inductively coupled plasma

The generation of active species for silicon dioxide etch and the formation of passivation layers on Si or photoresist is important for etching contact holes of high aspect ratio in sub-half-micron technology. Some methods use a process gas with high C/F ratio chemistry, such as C4F8, and/or apply a pulsed plasma technique, or radical control by chamber-wall modification. As a simple novel method, enhanced inductively coupled plasma (E-ICP) is expected to control the plasma characteristics by changing its E-ICP frequency; we tested the feasibility of using E-ICP for the process of contact hole etching. Scanning electron microscope images of etched profiles for micropatterns of 1 and 0.3 μm are shown to compare to the results by CW-ICP, magnetized-ICP, and E-ICP.

Anisotropic etching of polymer films by high energy (∼100s of eV) oxygen atom neutral beams

An inductively coupled high density plasma source was used to generate an energetic (100s of eV), high flux (equivalent of ∼10s mA/cm2) oxygen atom neutral beam. Positive ions were extracted from the plasma and neutralized by a metal grid with high aspect ratio holes. High rate (up to 0.6 μm/min), microloading-free, high aspect ratio (up to 5:1) etching of polymer with straight sidewalls of sub-0.25 μm features was demonstrated. The polymer etch rate increased with power and showed a shallow maximum with plasma gas pressure. The etch rate increased roughly as the square root of the boundary voltage (which controls neutral beam energy), and was independent of substrate temperature. The latter observation suggests that spontaneous etching did not occur. The degree of neutralization of the extracted ions was estimated to be greater than 99% at the base case conditions used in this work.

水素イオンのチャネリングによる微細コンタクトホール底の欠陥

水素イオンのチャネリングによる微細コンタクトホール底の欠陥

Transport mechanisms of ions and neutrals in low-pressure, high-density plasma etching of high aspect ratio contact holes

Transport mechanisms of ions and neutrals in high aspect ratio contact holes during low-pressure, high-density plasma etching were investigated by both experiments and numerical simulations. Etching experiments were performed in electron cyclotron resonance plasmas with a C 4F 8/O 2 gas mixture, together with measurements of plasma parameters. The SiO 2 etch rate decreased, and the deposition rate of polymer film during over-etching increased with decreasing hole diameter. The trajectories of ions and neutrals were investigated by numerical simulations that took into account the local charging effect for ions and Knudsen transport for neutrals. The etch rate distribution on the hole bottom was estimated by an etching model that included ion sputtering and ion-assisted etching. A sub-trench profile of the etch rate was obtained for a low aspect ratio hole, while a flat profile was observed for a high aspect ratio hole. This numerical result was in good agreement with the experimental results. Moreover, the dependence of polymer film thickness on over-etch duration was explained by a deposition model that included both the polymer formation by ion-assisted deposition and polymer removal by oxygen atoms.