Isotropic Etching Process Research Articles

A Reworkable Adhesive-Free Interconnection Technology for Microfluidic Systems

In this paper, we present a simple adhesive-free interconnect technology for microfluidic systems. Reliable connection between microfluidic devices and macroscopic world is a critical issue in many lab-on-a-chip applications. In such interconnections, it is particularly advantageous to avoid liquid adhesives (e.g., epoxy, silicone, etc.) since they can create reliability (blockage or leakage) and reproducibility problems. Our adhesive-free interconnection technology is based on a micromachined silicon flange which is connected to a medical-grade silastic tube using a heat shrink tubing sleeve. The silicon part was fabricated by consecutive anisotropic and isotropic etching processes resulting in a through-hole surrounded by a connection socket ring created using the well known lag effect of deep reactive ion etching. Biocompatible polyolefin heat shrink tubing (expansion ratio of 2:1 at its shrink temperature of 143/spl deg/C) was used to connect and tightly seal the silicon flange to the silastic tube without application of any adhesive. A two-step heat treatment starting from the silicon flange and terminating at the silastic tubing was used to seal the connection. High temperature removability of the heat shrink tubing makes interconnects reworkable. Pull-out force was measured to test the mechanical strength of interconnects with different socket shapes (square, hexagon, octagon, and 32-side circle). Increasing the number of sides improves the mechanical strength with the circular interconnects having an average pull-out force of more than 3N. Leakage tests were also performed to characterize the seal quality. The circular connectors of two different flange sizes showed zero leakage up to the maximum test pressure of 200 kPa (29 psi).

Fabrication of sub-50 nm current-perpendicular-to-plane spin valve sensors

A new process is developed to fabricate sub-50 nm current-perpendicular-to-plane (CPP) spin valve sensors. This processing technology relates to a feature reduction method using hard mask technique. The feature size is first reduced by an isotropic wet etch process of hard mask, and then transferred to CPP sensor by an ion milling process. Using this approach, the feature size can be reduced from above 100 nm determined by lithography technology to several nanometers. An etch back process is developed for the sensor via opening. Sidewall-free sub-50 nm CPP sensors have been successfully fabricated. The sensors are characterized through magnetoresistance measurements and show typical CPP characteristics.

Development of Microneedle Array for Cell Surgery

In order to implement cell surgery on a chip-based system, we have been developing microneedle arrays capable of introducing desired biomolecules (nucleic acids, proteins, antibodies, etc.) into living cells and the parallel extracting biomolecules expressed in the cells. This paper presents a novel and simple bath fabrication process for out-of-plane, hollow silicon dioxide (SiO2) microneedles in array. The micromachining technique involves standard photolithography and a combination of an anisotropic deep reactive ion etching (DRIE) process for producing thorough holes into a Si substrate as a needle mold followed by wet oxidation and an isotropic XeF2 etching process for leaving SiO2 microneedle structures. This allows a good degree of flexibility with respect to different needle designs such as needle sizes, shapes, and spacing between each needle with a one-mask process. An array of microneedles with a circler tip shape (an inner diameter of 3.5 μm, an outer diameter of 6.0 μm, and a height of 90 μm) were successfully fabricated.

Fabrication of three-dimensional stamps for embossing techniques by lithographically controlled isotropic wet etching

The excellent characteristics that nanoimprint lithography (NIL) shows as a two-dimensional patterning technique, such as resolution and throughput, are shared with its three-dimensional (3D) structuring version. Nevertheless, the potentialities of NIL for 3D structuring remain largely unexplored and unexploited, hindered by the difficulties in the fabrication of high quality masters with throughput suitable for practical purposes. We present a technique that allows production of stamps with tightly controlled three-dimensional profiles in the 10nm–100μm scale range. This technique consists of a sequence of lithographic steps alternated with isotropic wet etching processes performed on a quartz substrate. Scanning electron microscopy and atomic force microscopy characterization shows that 3D structures with very accurate shape control and nanometer scale surface roughness can be formed with this method. Quartz stamps have been employed in nanoimprint, hot embossing, or casting processes to shape complex plastic elements. This technique is expected to have many applications in different nanotechnology-related fields such as optics, nanomechanics, microfluidics, and plasmon nano-optics.

Characterization of elastic moduli of Cu thin films using nanoindentation technique

The elastic moduli in perpendicular and parallel directions to surface of Cu thin film were investigated by nanoindentation test and micro-cantilever beam bending test and the elastic moduli were compared with the theoretical estimations of elastic moduli based on the texture analysis. The thickness of electroplated Cu thin film, characterized by surface profiler, was varied as 3–12 μm with varying the electroplating condition. The specimens for micro-cantilever beam bending test were fabricated by lithography and isotropic etching process. Elastic modulus in perpendicular direction of Cu thin film, measured by nanoindentation test, was obtained as 123–99 GPa and decreased with increasing the film thickness. Elastic modulus in parallel direction of Cu thin film, measured by micro-cantilever beam bending test using nanoindentation technique, was obtained as 121 GPa for 2.8 μm thick Cu thin film and 90 GPa for 10.5 μm thick Cu thin film. Texture of Cu thin film was analyzed from the orientation distribution function calculated from the pole-figures obtained by X-ray diffraction technique. Cu thin film with thickness of 3 μm showed strong 〈1 1 0〉 texture, while 〈1 1 3〉 texture increased with increasing the thickness of Cu thin film. The theoretical estimations of elastic moduli in both perpendicular and parallel directions to surface of Cu thin film based on the texture analysis showed a good agreement with experimental measurements based on the nanoindentation technique.

Application of polyimide sacrificial layers for the manufacturing of uncooled double-cantilever microbolometers

ABSTRACTIn this paper, a low-temperature surface micromachining module with two sacrificial layers of polyimide is developed for the manufacturing of double-cantilever microbolometer focal plane arrays. The use of spin-on polyimide allows an all-dry final release step overcoming stiction problems often encountered in wet sacrificial etching processes. For the patterning of the polyimide, a plasma-enhanced chemical vapor deposited silicon oxide is employed as a mask layer. Anisotropic etching of both the mask film and the polyimide layer is accomplished by reactive ion etching. After patterning structural layers, sacrificial etching of the polyimide is conducted using an isotropic dry etch process in high-density oxygen plasma.

Cantilever tip probe arrays for simultaneous SECM and AFM analysis

A cantilever transducer system with platinum tip electrodes in sub-micron regime has been fabricated for simultaneous Scanning Electrochemical Microscopy (SECM) and Atomic Force Microscopy (AFM) analysis. Sharpened High Aspect Ratio Silicon (HARS) tips are shaped combining isotropic and anisotropic deep-reactive etch processes and form the body of the transducer. Deposition of a silicon nitride followed by a back etch step allows embedding these silicon tips in a silicon nitride layer so that they protrude through the nitride. This way, embedded silicon tips with a diameter smaller than 600 nm, a radius smaller than 50 nm, and an aspect ratio higher than 20 can be achieved. Subsequently, a platinum layer and an insulator layer are deposited on these tip structures. Introducing a metal masking technology utilizing Focused Ion Beam (FIB) technology, a precise exposure of the buried metal layer can be achieved to form ultra micro-electrodes on top of the tip. Finally, cantilever structures are shaped and released by etching the silicon substrate from the backside. Electrochemical and mechanical characterization show full functionality of the transducer system. Due to the high aspect ratio topography of the tip structure and low spring constant of silicon nitride cantilevers, these probes are particularly suited for high resolution SECM and AFM analysis. Furthermore, this technology allows a production of both linear probe arrays and two-dimensional probe arrays.

Control of Selectivity between SiGe and Si in Isotropic Etching Processes

The selectivity between SiGe and Si has been investigated in chemical dry etching. Starting from a pure CF4 process that etches SiGe with a good selectivity to Si, the modification of the gas mixture was studied with the aim of understanding the way these materials are etched in the presence of O2, N2 and/or CH2F2. Passivation phenomena were used to show that invert selectivities can be obtained. Their combination leads to ultrahigh Si:SiGe selectivity.

Chip modules for generation and manipulation of fluid segments for micro serial flow processes

Micro serial processing of continuous sample streams in microfluidic environments provides a new, powerful and scalable approach for micro scaled chemical reactions and analytical procedures. It is based on sample generation by embedding sample liquid in a continuous stream of an immiscible carrier fluid. Segmented streams are processed in capillaries with integrated microfluidic devices for chemical and analytical processing. Devices and methods were developed for generation of segmented flows and for dosing of liquid into compartments of a segmented sample stream. Segmentation is realized with predefined compartment volume and a high degree of uniformity in size and frequency. Injectors are optimized for interoperatibility with segmented flow handling in HPLC-capillary systems. For fabrication of microfluidic devices two half channels were prepared in glass by means of an isotropic etch process, followed by anodic bonding mediated by a nickel chromium metalization. Segmented streams are generated in the injector module and guided into a dosing module, where controlled infusion of liquid into the compartments of the segment stream is realized. The system was used for a neutralization reaction in compartments containing formic acid with sodium hydrogen phosphate.

Fabrication and properties of PZT micro cantilevers using isotropic silicon dry etching process by XeF 2 gas for release process

Fabrication processes of PZT micro cantilevers using isotropic silicon dry etching process by XeF 2 gas of front side were newly proposed and investigated. The purpose of study was to compare the proposed processes using only front side for release process of PZT micro cantilevers with conventional processes using bulk silicon micromachining of backside, and so on. In case of the proposed method, it was not necessary to use double side aligner, and wet chemical solutions and other protection jigs for silicon bulk micromachining of backside. During release process using isotropic silicon dry etching by XeF 2 gas, patterned 1.2 μm-thick photo-resist film layers were used for protective film on upper Pt electrode and PZT film. Degradation of properties of PZT films after release process using XeF 2 gas was not found. In case of operating at 3.9 V and 1.9 kHz of fabricated PZT micro cantilevers with 650 μm length by 100 μm width, these were actuated with resonant and vibrated up to the width of 125.9 μm. Consequently, microstructures and electromechanical properties of PZT micro cantilever by new proposed method were compatible to other conventional methods. It was thought that proposed processes were more cost effective, stable and mass productive than other reported processes.

Isotropic etching of SiGe alloys with high selectivity to similar materials

An isotropic etching process was developed in order to remove the sacrificial SiGe layer in Si/SiGe/Si stacks and thus obtain a cavity between the Si layers. This process is shown to be selective versus silicon as long as some SiGe remains, but the Si etch rate increases suddenly when the SiGe disappears. A mechanism based on the preferential action of fluorine species is proposed. It is corroborated by the results obtained on advanced electrical devices.

A microfabricated hybrid device for DNA sequencing.

We have created a hybrid device of a microfabricated round-channel twin-T injector incorporated with a separation capillary in order to extend the straight separation distance for high speed and long readlength DNA sequencing. Semicircular grooves on glass wafers are obtained using a photomask with a narrow line-width and a standard isotropic photolithographic etching process. Round channels are made when two etched wafers are face-to-face aligned and bonded. A two-mask fabrication process has been developed to make channels of two different diameters. The twin-T injector is formed by the smaller channels whose diameter matches the bore of the separation capillary, and the "usual" separation channel, now called the connection channel, is formed by the larger ones whose diameter matches the outer diameter of the separation capillary. The separation capillary is inserted through the connection channel all the way to the twin-T injector to allow the capillary bore flush with the twin-T injector channels. The total dead-volume of the connection is estimated to be approximately 5 pL. To demonstrate the efficiency of this hybrid device, we have performed four-color DNA sequencing on it. Using a 200 microm twin-T injector coupled with a separation capillary of 20 cm effective separation distance, we have obtained readlengths of 800 plus bases at an accuracy of 98.5% in 56 min, compared to about 650 bases in 100 min on a conventional 40 cm long capillary sequencing machine under similar conditions. At an increased separation field strength and using a diluted sieving matrix, the separation time has been reduced to 20 min with a readlength of 700 bases at 98.5% base-calling accuracy.

Fabrication of controlled sidewall angles in thin films using isotropic etches

Angular structures in thin films have applications in microelectronics and optoelectronics. In this study we analyze isotropic etching of a two-layer system for the fabrication of angular sidewalls. Such a process is attractive because of its bulk processing capability. The isotropic etch process is demonstrated to provide good control of the sidewall angle. Theoretical analysis along with numerical simulation of the slope etching process is used to understand profile evolution. The numerical simulations showed that to obtain a flat angular sidewall face, reaction controlled etching is needed. Angular sidewalls are fabricated in plasma enhanced chemical vapor deposition (PECVD) silicon oxide with PECVD silicon nitride as the sacrificial layer using an isotropic, buffered hydrofluoric acid based, wet chemical etch. PECVD silicon nitride is shown to be a good material for use as the sacrificial layer because of its etch rate controllability through deposition conditions. Controlled angles ranging from 5° to 50° are demonstrated using this isotropic etch technique. Penetration of the etchant between the photoresist and the sacrificial silicon nitride layers is determined to be the cause of the limited range of the angles that could be etched. The rms roughness of the etched faces ranged from 79 to 389 Å.

Polyimide sacrificial layer and novel materials for post-processing surface micromachining

We present a low-temperature post-processing module, utilizing polyimide as a sacrificial layer and novel materials such as PECVD SiC and metals (sputtered aluminium and titanium) as structural layers. The use of spin-on polyimide allows an all-dry final release step overcoming stiction problems often encountered in wet sacrificial etching processes. The spinning and curing procedure has been tailored to the specific needs of the IC-compatible post-process module. For the patterning of the polyimide, thin films of aluminium, PECVD silicon oxide or silicon carbide are employed as a mask layer. Anisotropic etching of the mask film and of the polyimide layer is accomplished by RIE. After patterning the structural layer, sacrificial etching of the polyimide is done using an isotropic dry etch process in high-density oxygen plasma. An underetch rate of 4 μm min−1 is achieved. Compatibility with different structural materials is tested and test structures are designed and realized in a fully post-processing surface micromachining module.

Importance of fluorine surface diffusion for plasma etching of silicon

Dry etching of silicon is an important process in the manufacturing of integrated circuits and micromachined devices. Traditionally, the etch rate limiting step for an isotropic silicon etching process is considered to be the arrival of fluorine atoms directly from the gas phase onto the silicon surface, and the mechanism to promote anisotropy is the prevention of lateral etching by the formation of an inhibiting layer on the vertical walls. Furthermore, isotropic dry etching is considered to etch features in the same way as isotropic wet etching. Conventional mechanisms cannot explain, however, the perfect anisotropic etching of silicon with pure SF6, when no polymer is formed. Neither can it be understood how a deep (>50 μm), isotropic, dry etching process applied to silicon can result in structures with a pinched neck and sharp ridges, in contrast with a wet etching process, where the corners are rounded and no pinching of the neck is observed. It is proposed that long-range diffusion of fluorine atoms can precede the eventual binding to a silicon atom. The rate of binding increases if the silicon is bombarded with high energy ions. Tests were performed to corroborate this model which is also consistent with the findings of others.

Effects of Borophosphosilicate Glass Dopant Concentrations on Isotropic Etch Profile

AbstractIn the process of chemically etching contact openings, film characteristics of the Borophosphosilicate glass (BPSG) film strongly effect the formation of an ideal contact profile. Ideal contact profiles represent a wine glass shape to provide better metal step coverage into the contact opening in comparison to a vertical sidewall profile. The rounded shape of the wine glass is etched chemically (isotropic) allowing for etch in the vertical and horizontal direction. As the addition of dopants to the oxide film effect the etch rate, the profile of the isotropic etch will change which in turn effects the electrical device properties, down stream processes and device reliability.BPSG film characteristics are the area of investigation. Characteristics of concern are weight percent concentrations of boron and phosphorus, film density and dopant concentration profiles in relation to depth. To evaluate the entire contact formation module, chemical etch characteristics including etch rate in relation to film depth for thermal oxide films were also investigated. Experimentation in the form of a response surface design was used to model effects of previously discussed BPSG film characteristics.To achieve the desired and predictable wet etch rate of BPSG in a buffered oxide etch (BOE), calculated control limits were placed on the boron and phosphorus dopant concentrations. Results showed that dopant effects on the isotropic etch process exceeded the control capability of dopant concentrations in the deposition process. In relating process control capabilities to 6σ techniques, a Cpk above 1.5 is considered capable. To eliminate previously discussed defects created in the isotropic etch process, reject limits for dopant concentrations in a BPSG film deposition process would need to be set well within the typical 6σ giving a Cpk below 1.5

A monolithic inkjet print head: DomeJet

This paper describes a thermally driven monolithic inkjet print head, DomeJet, that consists of dome-shaped ink chambers and thin film nozzle guides. Nozzle guides are fabricated by anisotropic etching after PECVD oxide passivation of their side walls. Each of the ink chambers and channels are fabricated by isotropic dry etching process using XeF/sub 2/. Omega or ring type polysilicon heaters are integrated on top of each chamber. Fifty-six nozzles are embedded at two columns on each chip where their pitch is 1/150 inch at each column. A pulse voltage of 2.6 /spl mu/s has been used in initial printing tests, resulting in 36 /spl mu/m diameter dots on a HP transparent film that was placed 1 mm away from the nozzle 18 /spl mu/m wide. The drop volume and velocity are measured to be 4 pl and 12 m/s, respectively. A printed image is successfully obtained using the fabricated head installed to a Samsung inkjet printer.

Nanoscale patterning of self-assembled monolayers with electrons

We show the fabrication of gold nanostructures using self-assembled monolayers of aliphatic and aromatic thiols as positive and negative electron beam resists. We applied a simple and versatile proximity printing technique using focused ion beam structured stencil masks and low energy (300 eV) electrons. We also used conventional e-beam lithography with a beam energy of 2.5 keV and doses from 3500 to 80 000 μC/cm2. Gold patterns were generated by wet etching in KCN/KOH and characterized by atomic force microscopy and scanning electron microscopy. The width of the finest lines is ∼20 nm; their edge definition is limited by the isotropic etching process in the polycrystalline gold.

Multiscale, multifunction diffractive structures wet etched into fused silica for high-laser damage threshold applications

We combined functionalities of two diffractive optics with almost 100x lateral and vertical scale-length difference onto a single fused-silica surface. Fine-scale (2-mum-period) gratings for beam sampling were printed in photoresist by interference lithography and transferred to the substrate by a hydrofluoric acid etch. Subsequently, 115-mum-linewidth stairstep gratings for color separation at focus were proximity printed and wet etched in a two-mask process. Line shapes of the lamellar sampling grating are remarkably preserved following etching of the much deeper color separation grating structures with this nominally isotropic etch process. Model simulations of isotropic etching of topographical features show good agreement with the measured shape evolution of the sampling grating profiles, and the simulations reveal the sensitivity of the final feature shape to its initial aspect ratio. As a rule of thumb, lamellar grating profiles can be etched approximately 0.08A(-2) times their modulation depth, where A is their initial aspect ratio (height/width), before they evolve into a cusplike shape and begin to lose height.

Reduction in Contact Resistance with In Situ O 2 Plasma Treatment

A reduction in contact resistance due to reactive ion etching using an plasma is reported. This treatment can reduce the contact resistance to the same level as when chemical dry etching is used, which is an isotropic etching process using fluorine‐containing gases such as . The mechanism of this reduction was investigated using X‐ray photoelectron spectroscopy, transmission electron microscopy, and thermal wave modulated reflectance. Removal of a thick modified layer which contains carbon, fluorine, and oxygen reduces the contact resistance. Further, the influence of plasma treatment time and bias radio frequency power on contact resistance is reported. An increase in contact resistance at high radio frequency power shows that there is reformation of the damaged layer by plasma treatment, and that it is important to control the removal and reformation of the damaged layer.