This PDF file contains the editorial “Editorial: Turning Proceedings Articles into Archival, Peer-Reviewed Papers” for JM3 Vol. 5 Issue 02

This PDF file contains the editorial “Editorial: BioMEMS and Microfluidics” for JM3 Vol. 5 Issue 02

We focus on the development and fabrication of SU8^TM-based microchannel networks, which can be integrated into microdevices for fast drug delivery and cell transport on chips. Instead of using sacrificial materials or wafer bonding, a new simplified fabrication method is developed. Single- and double-layered SU8^TM channels on silicon substrates are successfully achieved by using this new method, as well as integration of these SU8 channels with microelectrode arrays. A series of cell transport experiments is also successfully performed on these devices. This new fabrication approach and the resulting cell transport experiments are discussed in detail.

Hot embossing microfabrication has been used for the replication of a mold onto a polymer substrate for the past decade, with the molds usually made using techniques like wet etching, CNC machining, and laser writing. Fabricating a male mold using a laser and CNC machining consumes a lot of resources in mold design, mold fabrication time, and smooth surface finish of the mold. In this work, a novel approach called two-stage embossing that is a slight modification of the existing process is proposed. This process still requires a primary mold (of the same shape as the final desired part) that is used to emboss on a polymer of higher glass transition temperature than the substrate to be used for the second stage embossing. Finally this polymer secondary mold is used for the final replication on the desired substrate. Preliminary experimental results focusing on mold quality with respect to the number of embossing cycles of the secondary mold, the embossing quality of the final substrate as compared to the primary silicon mold, and the life of the secondary mold are presented. The experimental results confirm the viability of the process as a candidate process for hot embossing microreplication applications.

A unique evanescent-wave biosensor was designed and fabricated using simple and robust microfabrication technology. The sensor uses a microscale optical waveguide fabricated from NOA61 that is surface-altered with a custom chemical modification process, coupled with modified self-assembly of an oxygen-sensitive fluorescent dye and the enzyme glucose oxidase. To interface the analyte with the waveguide surface, a multilayer PDMS fluidic mold was designed to fit over the waveguide. Interfacing of both optics and fluidics was achieved using novel-generic methods. The entire device has been successfully fabricated and assembled, with analyte response testing completed using glucose solutions. The system also demonstrated inherent random noise insensitivity while making measurements.

We present simulation and experimental studies of a piezoelectrically actuated microdiaphragm air pump, which is characterized by thin structure, large flow, and low power consumption. A novel large-displacement actuation structure is designed for the air pump. A prototype of the micro air pump is fabricated by precise fabrication. Furthermore, studies of modeling, simulation, and experiments are carried out. The experimental and simulation results demonstrate that both the pump's flow and the amplitude of the actuation structure's motion depend on the frequency of the input voltage. The maximal values of the flow and the amplitude will be obtained when the frequency of the input voltage is equal to the actuation structure's first-order natural frequency. The diaphragm air pump has the best performance when it works in resonance mode. With 20-V input, the flow and the amplitude of the air pump are 4.5 ml/s and 0.00041 m, respectively, and the power consumption of the pump can be as low as 3.18 mW. With the advantages of large flow, thin structure, and low power consumption, the diaphragm air pump has great potential applications for air supply for microfuel cells or the cooling of electronic devices.

Although conventional optical lever technology typically used for scanning probe microscope applications has proven high sensitivity, accuracy, and is cost effective for most applications involving micromachined cantilever deflection measurements, the frequency limitations and space needs limit its applicability to emerging ultrasonic-based scanning probe microscopy (SPM) applications. Recently, the fabrication of cantilevers integrated with actuation and sensing components has opened avenues for feedback-based driving of micromachined cantilevers at higher order resonance frequencies, while sensing average deflection without the need of an optical deflection pathway for average deflection sensing. The work presented here reviews recent efforts by our group in fabricating micromachined cantilevers with integrated ZnO actuation layers to induce cantilever deflection. These cantilevers are being fabricated for use in a heterodyne force microscopy system (HFM) to enable SPM imaging contrast based on viscoelastic response of a surface in contact with a micromachined tip, wherein active-feedback technology is being applied to maintain ultrasonic tip excitation at higher order cantilever resonances. The first- and second-pass fabrication results are presented and reviewed regarding cantilever release and ZnO actuator (and electrode) fabrication. Dynamic response data from these structures, measured via laser Doppler vibrometry, reveal the expected resonance structure for a cantilever of these dimensions.

Line edge roughness (LER) and intrinsic bias of 193-nm photoresist based on two methacrylate polymers are evaluated over a range of base concentration. Roughness is characterized as a function of the image log slope of the aerial image, the gradient in photoacid concentration, and the gradient in polymer protecting groups. Use of the polymer protection gradient as a characteristic roughness metric accounts for the effects of base concentration. Results demonstrate that a methacrylate terpolymer exhibits an advantage over the copolymer resist by achieving lower roughness at smaller values for the polymer protection gradient, resulting in lower LER for patterning. Intrinsic bias is found to be a function of the concentration of base. Process window analysis demonstrates that a greater depth of focus can be achieved for resists with low intrinsic bias. However, a tradeoff in depth of focus with LER is found. Spectral analysis indicates resists with greater intrinsic bias exhibit greater correlation lengths. Systems with greater intrinsic bias demonstrate lesser roughness for patterned features, with a minimum roughness achieved at maximum intrinsic bias. Kinetics of deprotection are modeled to calculate the chemical contrast of each resist. Resists exhibiting the greatest chemical contrast are identified as materials that generate the least roughness.

The semiconductor industry has used optical lithography to create impressively small features. However, the resolution of optical lithography is approaching limits based on light wavelength and numerical aperture. Immersion lithography is a means to extend the resolution by inserting a liquid with a high index of refraction between the lens and wafer. This enables the use of higher numerical aperture optics. Several engineering obstacles must be overcome before immersion lithography can be used on an industry-wide scale. One such challenge is the deposition of the immersion liquid onto the wafer during the scanning process; any residual liquid left on the wafer is a potential defect mechanism. The residual liquid deposition is controlled by the details of the fluid management system, and is strongly dependent on the three-phase contact line. Therefore, this work concentrates on understanding the behavior of this contact line, specifically by measuring the dynamic contact angle and the critical velocity for liquid deposition. A contact angle measurement technique is developed and verified; the technique is subsequently applied to measure the dynamic advancing and receding contact angle for a series of resist-covered surfaces under conditions that are relevant to immersion lithography.

Extreme ultraviolet lithography (EUVL) is the leading next-generation lithography (NGL) technology to succeed optical lithography at the 32-nm node and beyond. The technology uses a multilayer-based reflective optical system, and the development of suitable, defect-free mask blanks is the greatest challenge facing the commercialization of EUVL. We describe recent progress toward the development of a commercial tool and process for the production of EUVL mask blanks. Using the resources at Mask Blank Development Center at SEMATECH-North in Albany, New York, we are able to decrease the mean multilayer-coating-added defect density on 6-in. square quartz substrates by almost an order of magnitude, from ~0.5 defects/cm² to ~0.055 defects/cm² for particles ≥80 nm in size (polystyrene latex equivalent). We also obtain a "champion" mask blank with an added defect density of only ~0.005 defects/cm². This advance is due primarily to a compositional analysis of the particles using focused ion beam and energy dispersive analysis of x-rays (EDX) followed by tool and procedural upgrades based on best engineering practices and judgment. Another important specification for masks blanks is the coating uniformity, and we have simultaneously achieved a centroid wavelength uniformity of 0.4% and a coating-added defect density of 0.06 def/cm².

The oxidation resistance of protective capping layers for extreme ultraviolet lithography (EUVL) multilayers depends on their microstructure. Differently prepared Ru-capping layers, deposited on Mo/Si EUVL multilayers, are investigated to establish their baseline structural, optical, and surface properties in an as-deposited state. The same capping layer structures are then tested for their thermal stability and oxidation resistance. The best performing Ru-capping layer structure is analyzed in detail with transmission electron microscopy (TEM). Compared to other Ru-capping layer preparations studied here, it is the only one that shows grains with preferential orientation. This information is essential to model and optimize the performance of EUVL multilayers.

We propose the use of square arrays of multiple atomic lenses, produced by interference of four nearly collinear optical beams in atom lithography using dipole force. Simulated lithographic patterns are reported for collimated as well as divergent rubidium atomic beam traveling in such square arrays of optical channels. The proposed configuration has the ability to write large numbers of periodic structures in square arrays in a single step.

A technique for measuring the profile of the illumination in the pupil of a lithography projector is presented. The technique is based on exposing pinhole patterns on a wafer at different dose and defocus settings, and processing the scanning electron microscopy (SEM) images of the printed pinholes. The latent image intensity at the edges of the resist patterns equals the dose-to-clear. This establishes a multitude of equations, each of which states that the latent image intensity at a particular field location, dose, and defocus is known. The intensity distribution in the pupil of the illuminator is obtained by solving a large system of such equations, subject to the constraint that the intensity distribution is non-negative. An image processing algorithm based on nonlinear diffusion is used for finding coordinates of points on the edges of resist in SEM images. The results of the inversion for 193-nm stepper with 0.55/0.85 annular illumination and numerical aperture of 0.75 at five exposure field locations are presented.

A polysilazane is investigated for a spin-on glass (SOG) used for a middle layer in a trilevel resist system. Higher film density is required for the middle layer to obtain higher etch resistance during the underlayer etching. The compositions of the polysilazane baked at 200°C and 300°C are Si_0.42O_0.34C_0.40N_0.20 and Si_0.29O_0.65C_0.10N_0.05, respectively, which are determined by x-ray photoelectron spectroscopy. The polysilazane is converted to silicon-oxide-like compositions by baking at 300°C. The film density and etch rate of SOG made from polysilazane are compared with that of polysiloxsane on condition that both films are baked at 300°C. The film density of the SOG made from the polysilazane is 2.07 g/cm³, which is higher than 1.87 g/cm³ of the conventional SOG made from a polysiloxsan. The etch resistance of the SOG made from the polysilazane is improved by 90% compared with that of the SOG made from the polysiloxsane due to the increased film density.

We present an experimentally verified governing equation, which provides designers some physical insights of rotary comb actuated microsystems. The degree of rotation depends on a single dimensionless parameter, including electric moment applied, central ring radius, and radial spring constant. Radial spring constants, which differ from rotational spring constants typically used in other models, can be independently obtained using radial forces from simulation only. The rotation angles and the resonant frequencies are determined to be linearly proportional to the ring radius power of -0.16 and 0.08, respectively. As the rings become larger, both the angular spring constants and the resonant frequencies increase.

There is a great need for techniques that will permit the evaluation of the micromechanical state of micro-electro-mechanical system (MEMS) devices, at all steps of manufacturing, with respect to material properties, as well as of lifetime and for monitoring mechanical performances of MEMS actuators. We propose a new approach, based on integrated optical read-out using a Mach-Zehnder interferometer (MZI), monolithically integrated on the top of an electrostatically rotatable micromirror loaded with the sensing arm of MZI. The working principle of MZI read-out is based on the local change of the effective refractive index of guided waves of MZI, induced by strains of the deformable structure. A single-mode buried channel waveguide based on the silica/silicon oxinitride/silica structure is used, presenting optical attenuation of 0.6 dB/cm. The coupling of the standard optical fiber to the waveguide is based on the V-groove technique, supplemented by micromechanical sawing of the silicon substrate. For TE polarization, the set of obtained parameters is a coupling efficiency of about 55% with a horizontal misalignment of ±0.5 µm, a vertical misalignment of ±0.7 µm, and angular precision of ±0.2 deg on <110> directions of the silicon substrate.

Optoelectronic techniques can be used in laser applications in biomedical instrumentation to achieve better flexibility and control of the available optical power. These techniques enable the processing and manufacturing of biomedical implants like cardiovascular stents and micrometallic components with reduced heat-affected zone (HAZ) and extremely precise edge cutting. The present investigations deal with the study of laser piercing routines and further profile cutting of thin stainless steel sheet tubes for biomedical implant manufacturing. The process is performed using an acousto-optic (AO) modulator-based pulsed Nd:YAG laser. The AO modulator used in the experiments is a Bragg diffraction device. During the piercing of holes, the focused laser power is gradually increased from a low value and finally reaches the maximum as the beam goes deeper into the material. The most suitable value of dwell time is found to be 50 ms. A nine step staircase modulating voltage with a maximum ac component of 800 mV is used, and the laser power is 4 W in TEM00 operation. The acousto-optic modulator-based pulsed Nd:YAG laser is capable of cutting extremely complex geometries of stents on 316LVM tubing. Laser cutting results in a kerf width of 20.5±0.5 µm. Precise strut dimensions of 115±15 µm for subsidiary strut, 150±15 µm for main strut, and 150±15 µm for the link are also obtained. With the piercing routine, extremely fine holes with reduced heat-affected zones are produced, and this quality is reflected during consequent machining of required profiles for implants.

Antireflective (AR) coating on waveguides with high refractive index is imperative to minimize insertion losses. In fabricating silicon, rectangular, suspended waveguides on silicon-on-insulator (SOI) wafers, a single Si₃N₄ layer is deposited on the waveguide walls. For the purpose of applying an optimum layer, we develop an integrated micromechanical gauge to determine the coating width by measuring the induced stress in the silicon. Gauges at different sites on a wafer produce results with a standard deviation of about 0.5%. The insertion loss due to the waveguides is measured directly by coupling a laser beam at 1550 nm from a single-mode fiber to the waveguide, then to another fiber and a detector. Tests are run on six wafers and two types of devices: a waveguide with two facets and a waveguide with a gap, presenting four facets. The optimal silicon-nitride thickness is found at 200 nm, featuring a fiber-waveguide-fiber insertion loss of about 1 dB for a two-facet device and 1.7 dB for a four-facet device.