On the basis of Abbe's diffraction theory of optical imaging, the appearance of a concave mirror with arbitrarily shaped small deviations in Foucault's knife-edge test and the new phase-contrast test is evaluated. The orthogonal "circle polynomials" are found and applied to the diffraction phenomena of circular mirrors.
As subhalf wavelength optical lithography is pursued, the variety of imaging requirements increases. This can result in complex combinations of various resolution enhancement techniques. Optimization based on simple standards or rules is not possible. Problems often arise as analysis is carried out in a spatial domain, where mask and image properties are evaluated using sizing or dimensional evaluations. A more appropriate perspective for image optimization is that of the lens pupil, in a spatial frequency domain. In this paper, we describe the common characteristics of resolution enhancement, beyond the historical comparisons of alternating phase shifting masking (PSM) and strong OAI. Enhancement techniques including assist feature optical proximity correction (OPC), custom illumination, attenuated PSM, and pupil filtering are described from a spatial frequency standpoint where each can be utilized to take advantage of strengths and avoid weaknesses. As a result of this type of analysis, we will also describe an alternative OPC method in which assist features of varying tone, referred to as gray bars, provide significant image improvement.
Characterization of linewidth variation by a three-step methodology is presented. Via electrical linewidth measurement, sources of linewidth variation with distinct spatial signatures are first isolated by spatial analysis. Causes with similar spatial signatures are then separated by contributor-specific measurements. Unanticipated components are lastly identified by examination of the residuals from spatial analysis. Significant sources include photomask error, flare, aberrations, development nonuniformity, and scan direction asymmetry. These components are then synthesized to quantify the contributions from the three modules of the patterning process: photomask, exposure system, and postexposure processing. Although these modules are independent of one another, their effects on linewidth variation may be correlated. Moreover, the contributions of the modules are found to vary with exposure tool, development track, and lithography strategy. The most effective means to reducing the overall linewidth variation depends on the relative importance between these components. Optical proximity correction is efficacious only for a well-controlled process where proximity effect is the predominant cause of linewidth variation.
An investigation of the micromachining of trenches in lithium niobate (LiNbO3) using direct imaged 248 nm KrF excimer laser ablation is presented. Trenches 20 μm wide and 0.5-7.5 μm deep have been produced. These trenches are assessed and are deemed suitable for the machining of integrated electro-optic structures. Based on the characterized ablation conditions, trenches of high quality have been successfully fabricated on an X-cut optical modulator which is potentially important for the realization of efficient broadband optical intensity modulators.
The development of microbowtie structures for a next-generation optical probe called the Wave Interrogated Near-Field Array (WINFA) is presented. The WINFA combines the sensitivity of near-field detection with the speed of optical scanning. The microbowties are designed to act as resonant elements to provide spatial resolution well below the diffraction limit with a transmission efficiency approaching unity. Following an introduction of the concept and background information, the design of the microbowtie is presented. A numerical electromagnetic scattering model is developed and used for better designs of the bowtie structures. The electron-beam lithography process is then used to fabricate the final designed bowties structure. Special fabrication procedures have been developed to cope with the charge dissipation problem that arises when lithographing an insulating substrate as is required in the present probe design. Two types of substrates and two types of resists are considered in the present study. The fabricated microstructures have 40 nm bowtie gaps that are more than 200 000 times smaller than the one built previously. All fabricated bowtie microstructures are examined and the results are compared. It has been found that, in addition to the relative ease in fabrication, the bowties on indium-tin-oxide coated glass substrate can not only minimize the charge accumulation in a glass substrate, but also satisfy the functional requirement of optical transparency to the incident wave. Recommendations for making a bowtie structure in the even smaller bowtie array are also included.
A direct means of measuring the image blur of electron beam projection lithography (EPL) tools is described. We developed an aerial image sensor using a Si membrane knife-edge and a transmitted electron detection technique. The aerial image sensor is designed to increase signal amplitude and signal contrast in order to yield a large signal to noise ratio even under a low beam current density condition. The image blur can be quantified accurate to a few nanometers because the measurement error due to the sensor is extremely small. The aerial image sensor was installed in Nikon's electron beam projection experimental column and was evaluated. The measured image blur, defined as the distance between the 12% and 88% points of the beam edge profile, under the optimum condition was 13 nm, and the measurement repeatability was 3 nm (3 sigma). The application of this technique to a system calibration is demonstrated. Focus and astigmatism were measured and the optimum settings of focus coils and stigmators were determined with excellent repeatability. The potential for this technique to provide an automated self-calibration system on EPL tools is clearly shown.
An efficient spiral-type magnetic microactuator design, composed of an enclosed core and a magnetic plate, is presented to produce a high magnetic flux density, a large force, and a large displacement. The design allows a large variation in area of the Permalloy plate and then the area of the poles can optimally enlarge with the fixed area of the Permalloy plate. Verification by the magnetic path analysis and the finite element method yields a theoretical actuator design for the magnetic microactuator. An integrated magnetic microactuator is fabricated and tested to demonstrate the capability of the improved design. The microactuator consists of an electromagnet and a four-suspended-beam structure, bound together with a 28-μm-thick spacer between them. A series of experiments determines that the measured stiffness of the four-beam structure is approximately 45 μN/μm. Notably, the Permalloy plate on the four-suspended-beam structure is moved by 27.6 μm at a current and voltage of 292 mA and 4.5 V, respectively. The estimated force produced by the microactuator is around 1240 μN. These results show the microactuator with an enclosed core is efficient in producing magnetic force and has flexibility in application.
In this paper we present a range of organic micromachining techniques that enable a planar membrane-based printed circuit to be fabricated on a 5 μm thick organic membrane in the absence of any steps involving thermal oxidation and low pressure chemical vapor deposition. The technologies are suitable for mass production of millimeter-wave or submillimeter-wave components. Transmission losses of a membrane-supported E-plane component are typically less than 0.5 dB/cm.
A new technique to remove the silicon from beneath a large structure, by micromachining for making a suspended microstructure both for thermal, as well as electromagnetic, isolations on a complementary metal-oxide-semiconductor (CMOS) chip is reported. Conventional methods require two step front-side etching, an isotropic step, followed by an anisotropic etching step. An alternative technique is based on the backside etching process, which requires extra masks, and processing steps. In order to keep the postprocessing steps to a minimum, a simple technique has been developed that exploits the front-side anisotropic etching to create both undercuttings as well as deep etching in one single step. This method uses the gate oxide and polysilicon layer in CMOS technology as the sacrificial layer for initiating the undercutting needed to make a freestanding microstructure. The microsuspension thickness, width and length of 2 μm, 150 μm×150 μm, respectively, are made out of low-pressure chemical vapor deposition oxide and have been fabricated.
This paper investigates the use of electrical conductivity monitoring in silicon-based capillaries and the inherent problems therein. In comparison to reference glass devices, the conductance waveforms from the silicon devices were significantly distorted. This has been shown to be due to the profiles of the ends of the capillaries where single-sided etching was employed, and the silicon dioxide capacitance. Double-sided processing provides a solution to tapering of channel inlets, by reducing the time that the front side is exposed to the KOH solution. Models are developed for the devices, which identify degradation of the oxide isolation as another source of distortion. Matching of the experimental and simulated characteristics enables an estimation of the capacitance between the silicon and the bulk solution. Silicon nitride layers are shown to provide more effective isolation and greatly reduce the distortion observed during conductivity monitoring.
The application of precision grinding for the formation of a silicon diaphragm is investigated. The test structures involved 2-6 mm diam diaphragms with thicknesses in the range of 25-150 μm. When grinding is performed without supporting the diaphragm, bending occurs due to nonuniform removal of the silicon material over the diaphragm region. The magnitude of bending depends on the final thickness of the diaphragm. The results demonstrate that the use of a porous silicon support can significantly reduce the amount of bending, by a factor of up to 300 in the case of 50 μm thick diaphragms. The use of silicon on insulator (SOI) technology can also suppress or eliminate bending although this may be a less economical process. Stress measurements in the diaphragms were performed using x-ray and Raman spectroscopies. The results show stress of the order of 1×107-1×108 Pa in unsupported and supported by porous silicon diaphragms while SOI technology provides stress-free diaphragms. Results obtained from finite element method analysis to determine deterioration in the performance of a 6 mm diaphragm due to bending are presented. These results show a 10% reduction in performance for a 75 μm thick diaphragm with bending amplitude of 30 μm, but negligible reduction if the bending is reduced to <10 μm.