This PDF file contains the editorial “What Can Semiconductor Fabs Do for the Green Earth?” for JM3 Vol. 7 Issue 02

We report on the observation of mask transmission resonances in line/space gratings on bilayer mask stacks, both experimentally and in rigorous electromagnetic field (EMF) lithographic simulations. The mask transmission resonances manifest themselves as a local enhancement in the dose-to-size curve through mask linewidth for a given pitch targeted to a fixed wafer CD. We relate this finding to a local transmission loss of the propagating diffraction orders. We investigate both by experiments and by simulations the dependency of this mask transmission resonance on various lithographic conditions such as mask material, grating pitch, and incidence angle and polarization state of the incoming light. The resonant-type anomaly has a large impact on the mask error enhancement factor (MEEF) behavior of these bilayer mask stacks. The dependence of the MEEF as a function of mask linewidth shows strong variations and even negative MEEF values around the position of this resonance, which stem from the local dose-to-size enhancement. More precisely, the behavior of the MEEF can be well predicted by the differential of the dose-to-size curve.

This PDF file contains the editorial “Silicon-Based MOEMS and Their Applications” for JM3 Vol. 7 Issue 02

Recently, there has been substantial progress in the development of ultracompact image projection systems. This has been enabled by the availability of electrically modulated laser sources for all three elementary colors and a 2D resonant microscanning mirror as a microopto-electro-mechanical system (MOEMS) device for light deflection. The laser beam formed by collimator optics is directed onto the microscanning mirror. Given the movement of the mirror, the laser beam scans the entire image area. By driving the mirror and electrically modulating the intensity of the laser beam in a synchronous manner, a projection of images can be achieved. In this contribution, we present the theoretical background of the projection system as well as the latest achievements in system design. Both monochrome and full-color systems are currently available. The latter uses a separate laser bank as an RGB light source, which is coupled with a projection head. For monochrome red systems, the laser diode can be integrated into the projection head as well, whose volume can be reduced to 15×7×5 mm. All systems have video graphics array (VGA) (640×480 pixels) resolution and operate with 8-bit color depth per pixel and 50 frames per second.

Recently developed MEMS micromirror technology provides an opportunity to replace macroscale actuators for laser beamsteering in lidar and free-space optical communication systems. Precision modeling of mirror pointing and its dynamics are critical to the design of MEMS beamsteerers. Beam jitter ultimately limits MEMS mirror pointing, with consequences for bit error rate and overall optical system performance. Sources of jitter are platform vibration, control voltage noise, and Brownian motion noise. This work relates the random jitter of the mirror facet to its originating sources via a multidimensional first-order Taylor expansion of a first-principles-derived analytic expression for the actuating torque. The input torque, consisting of deterministic and stochastic components, is related to the 2-D jitter through a pair of coupled damped harmonic oscillator differential equations. The linearized 2-D jitter model for the mirror is simulated using Matlab, while the full nonlinear torque model was simulated using Simulink. The work describes an experimental setup and methodology that is used to make precise micromirror measurements. Experimental measurements are in agreement with the jitter model, i.e., the linearized model is able to predict mirror facet jitter based on the measured power spectral densities for the sources of jitter.

Optical microelectromechanical systems (MEMS) technology or micro-optoelectromechanical systems (MOEMS) technology has proven to be an enabling technology for many components of optical networking applications. Due to their widespread applications, MEMS variable optical attenuators (VOA) have been one of the most attractive MEMS-based key devices in the optical communication market. Micromachined shutters and refractive mirrors assembled with 2D and 3D optics are the subjects of tremendous research activity. We conduct a comprehensive literature survey with respect to technologies such as fabrication processes, optical designs, actuators, and systems. Apparently, MEMS VOA technology is still evolving into a mature technology step by step. MEMS VOA technology is not only the cornerstone to support future optical communication technology, but also the best example for understanding the evolution of MOEMS technology and the commercialization of MOEMS devices.

We report the design, fabrication, and test results of a tunable pyroelectric detector with an integrated micromachined Fabry-Perot (FP) filter for gas analysis in the mid-wave infrared (MWIR). The new approach is based on a bulk micromachined Fabry-Perot interferometer with an air cavity, which is electrostatically tuned. Various types of movable reflectors and spring configurations are fabricated to determine the optimum solution with respect to maximum tuning range, low gravity influence on center wavelength, and suitable filter bandwidth. Short and long cavity filters are designed for the spectral ranges of 3 to 4.3 μm and 3.7 to 5.0 μm, respectively. The tunable filter is arranged on top of a current mode pyroelectric detector with a flat spectral response. It is shown that the main challenge is to achieve a high finesse in spite of nonperfect parallelism, mirror curvature, and the additional phase shift caused by the Bragg reflectors.

We present investigations of a new miniaturized NIR spectrometerwith a size of only 10×8×8 cm³, and a MOEMS-scanninggrating chip as a main element. It works currently in a spectral range of 1200 to 1900 nm with a resolution of less than 10 nm using only one single InGaAs diode as a detector. One entire spectral measurement is done within 6 msec, calculated by a digital signal processor, which is included in the spectrometer. The MOEMS-scanning-grating chip is resonantly driven by a pulsed voltage of up to 36 V, has a grating plate 3×3 mm², and reaches deflection angles of ±8 deg at 25 V. Control and investigation of the deflection angle, the static deformation, the spectralefficiency, and the mechanical shock resistance are key parameters toreach the spectrometer specifications. Results of these measurementsand their influence on the spectrometer are discussed. Special etch control structures to monitor the fabrication process of the grating structure in the nanometer range, which can be easily done by microscopic inspection, are also presented.

We present several types of translatory MOEMS actuators developed for fast optical-path-length modulation [e.g., in confocal microscopes or Fourier-transform infrared (FTIR) spectrometers] and their application on miniaturized FTIR spectrometers capable of performing time-resolved measurements from the near infrared to the mid infrared. The MOEMS devices are manufactured in a complementary metal oxide semi conductor compatible silicon-on-insulator process. They are electrostatically resonant, driven using in-plane comb drives. A first translatory 5-kHz MOEMS device is used in a first prototype of a miniaturized MOEMS-based FTIR spectrometer where the MOEMS actuator replaces the macroscopic mirror drive, enabling a miniaturized, robust, and low-cost FTIR system. The mirror plate of 1.65 mm² is suspended by bending springs. Due to the resonant operation, a 200-µm stroke can be achieved with low voltages (<40 V) at an ambient pressure below 500 Pa. Consequently, this yields a spectral resolution of 25 cm^-1 and an acquisition time of 200 µs per spectrum. In addition, we present a novel MOEMS device with an increased mirror aperture of 7.1 mm² and pantograph-like mirror suspension enabling up to a 500-µm stroke. This device is specifically optimized for miniaturized FTIR spectrometers to enable an improved spectral resolution of 10 cm^-1 and a signal-to-noise ratio of >1000:1.

Silicon gratings are fabricated using micromachining techniques. The gratings are used with fiber optic probes to measure high-resolution and long-range linear displacements. Different parameters of the fabrication process such as the etching solution, the concentration of the etchant, and the temperature are optimized to achieve a mirror-like surface quality of the grating steps. For each parameter set, the resulting roughness and flatness are analyzed and discussed. Finally, linear displacement measurements are performed with the optimized grating as a component of a long-range fiber optic sensor. A resolution better than 34 nm and a measurement range up to 8.7 mm are obtained.

We explore the use of deformable membrane mirrors for primary focus control and spherical aberration correction in confocal microscopesand optical coherence tomography instruments that are miniaturizedfor in-situ imaging. Membrane requirements to achieve specified system performance are explored for both high and low numerical aperture regimes. Examples of both confocal imaging and focus-tracking optical coherence tomography using varifocus silicon nitride MEMS mirrors are presented. It is envisioned that future instruments with enhanced-stroke adaptive membrane mirrors will provide high fidelity microscopy and optical coherence tomography for diagnostic imaging of intact, living tissue.

For a wide range of applications in biology, medicine, and manufacturing, the small field of view associated with high-resolution microscope systems poses a significant challenge in practice. To address this limitation, a novel optical microscope uses a micromachined MEMS deformable mirror working with a specially designed scan lens to achieve a two-order-of-magnitude increase in the field of view area. Called the adaptive scanning optical microscope (ASOM), the deformable mirror in the ASOM is an integral component of the optical system and the static (glass) optical elements are specifically designed to match the shape correcting capabilities of the deformable mirror itself. After describing the design and operating principle of the ASOM, experimental results from a low-cost prototype are presented. It is shown how an image-based optimization method can be used to first calibrate the electrical voltages to the MEMS deformable mirror. And once calibrated, we show how the deformable mirror can be used in an open loop control approach for very fast operation during run time. The methods for calibration of a MEMS deformable mirror and basic control structures demonstrated form the basis for a range of emerging adaptive-optics-enabled technologies and instrumentation.

We present a novel external cavity diode laser design developedfor applications in atomic physics that employs a micromachinedsilicon flexure to sweep the laser frequency and a volume holographicreflection grating (VHG) to provide the optical feedback. The advantages of using a silicon flexure are its simple microfabrication process and reduction of the overall size of the laser system. The results demonstrate the ⁸⁷Rb, ⁸⁵Rb (rubidium) D2 line absorption at 780 nm in an atomicoptics test experiment. Our novel laser system design has a size of28.76×20.65×12 mm. The wavelength can be tuned and swept from 780.2463 to 780.2379 nm equivalent to 4.14 GHz using piezoelectric transducer (PZT) actuators integrated on the silicon flexure. A frequency tuning range of 17.149 GHz can be obtained by changing the VHG temperature. The deflection of the silicon flexure is 129.19 nm. The advantage of combining a VHG and a silicon flexure is that the frequency can be coarsely tuned to 780.24 nm and swept at this center frequency with a range of 4.14 GHz by PZT. Moreover, the frequency fine tuning can be achieved by changing the VHG temperature to observe the rubidium spectrum. The laser output power is measured as 59 mW at 780.2474 nm.

We describe charging effects on spatial light modulators (SLM). These light modulators consist of up to one million mirrors that can be addressed individually and are operated at a frame rate of up to 2 kHz. They are used for deep ultraviolet (DUV) mask writing where they have to meet very high requirements with respect to accuracy. To be usable in a mask-writing tool, the chips have to be able to work under DUV light and maintain their performance with high accuracy over a long period of time. Charging effects are a problem frequently encountered with MEMS, especially when they are operated in an analog mode. In this work, the issue of charging effects in SLMs used for microlithography, their causes and methods of their reduction or elimination, by means of addressing methods as well as technological changes, is discussed. The first method deals with the way charges can accumulate within the actuator. It is a simple method that requires no technological changes but cannot always be implemented. The second involves the removal of the materials within the actuator where charges can accumulate.

Large micromechanical mirror arrays (MMA) with analog pixel deflection integrated onto active CMOS address circuitry require both high-quality planar reflective optical surfaces and a stable deflection versus voltage characteristic. However, for implementing a CMOS-compatible surface-micromachining process, certain obstacles such as a restricted thermal budget and a limited selection of suitable materials must be overcome. Amorphous TiAl is presented as a new actuator material for monolithical MEMS integration onto CMOS circuitry. TiAl films may be sputter deposited at room temperature, have an x-ray amorphous structure, and a low stress gradient. The glassy structure and high melting point make TiAl less vulnerable to stress relaxation, which makes TiAl an ideal spring material. One-level actuators with TiAl or Al-TiAl-Al structural layers and two-level actuators with separate TiAl spring and Al-alloy mirror layers were fabricated and tested with respect to their drift stability. The stability of TiAl-based actuators was found to be superior in comparison to one-level Al-alloy actuators. Two-level actuators with TiAl hinges emerge as the most promising design.

We describe two types of active optical devices developed for use as free-space optical interconnects (FSOIs) for chip-to-chip communications. The design of both types of devices—membrane and freestanding structures—includes both optical and mechanical components. The optical component contains porous silicon (PSi) with customized optical properties fabricated by electrochemical etching of silicon. The mechanical part of the devices is composed of metal/nitride bimorph thermal actuators. The membrane devices form concave mirrors when actuated, and can be used to focus the incoming optical signals and correct any optical misalignment within the input/output (I/O) fabric. The freestanding devices have out-of-plane optical components, whose tilting angle is controlled by the current applied to the actuator. These devices can function as either reflectors or tunable optical filters. By incorporating the developed PSi diffractive optical element (DOE) into the freestanding structure, another type of freestanding device is realized for beamsplitting applications. Details of the fabrication, testing, and integration of these PSi-based devices are presented.

We report on micromirror arrays being developed for use as reflective slit masks in multiobject spectrographs for astronomical applications. The micromirrors are etched in bulk single crystal silicon, whereas cantilever-type suspension is realized by surface micromachining. One micromirror element is 100×200 μm in size. A system of multiple landing beams is developed, which electrostatically clamps the mirror at a well-defined tilt angle when actuated. The mechanical tilt angle obtained is 20 deg at a pull-in voltage of 90 V. Measurements with an optical profiler show that the tilt angle of the actuated mirror is stable with a precision of one arc minute over a range of 15 V. This electrostatic clamping system provides uniform tilt angle over the whole array: the maximum deviation measured between any two mirrors is as low as one arc minute. The surface quality of the mirrors in the actuated state is better than 10 nm peak-to-valley and the local roughness is around 1-nm rms. Cryogenic testing shows that the micromirror device is functional at temperatures below 100 K.

A robust quantitative structure property relationship (QSPR) model with five parameters has been developed from 126 organic compounds for the prediction of refractive index at 589 nm. The model and the knowledge of the refractive index dispersion were used in the rational design of new materials for 193-nm immersion lithography. The significance of this model is that the structural descriptors can be readily calculated and the factors that significantly affect refractive index can be easily identified and used to guide the selection of candidates. Using this model, rapid screening of large structure databases is possible in order to find candidates. As an example of this approach, the synthesis of the copolymer of a trithiocyclane-methacrylate derivative, identified by the model, with 2-methyl adamantyl methacrylate is described. The measured refractive index of the copolymer at 589 nm agrees well with the value predicted by the model. The new polymer showed a 9.4% increase in refractive index at 193 nm compared with the standard ArF resist.

The use of a single figure of merit to judge resist performancewith respect to resolution, linewidth roughness (LWR), and sensitivity is proposed and evaluated. Chemically amplified photoresists used in advanced lithography nodes need to fulfill stringent requirements for a considerable number of resist and process characteristics. Along with resolution, linewidth roughness and resist sensitivity are important examples where the specifications have become very tight. Previously, it has been shown that resolution, linewidth roughness, and resist sensitivity are undamentally interdependent. Hence, when evaluating or optimizing resist performance, it is very important to take these three characteristics into consideration simultaneously. We propose to combine these characteristics into a single photoresist figure of merit K_LUP. This figure of merit, which is determined from sizing dose, imaging wavelength, resist thickness, exposure latitude, acid diffusion length, linewidth roughness, and pitch, allows for a direct comparison of very different resist formulations independent of the exposure tool used. Thus, K_LUP has great potential to assist in evaluating resist performance for the next lithography nodes, for both ArF and for EUV wavelengths.

Polydimethylglutarimide (PMGI)-based resists are finding increasing use in microelectromechanical systems (MEMS) as both sacrificial and structural materials. PMGI-based resists are commercially available and were originally designed for use in bilayer lift-off applications. Literature on deep-UV exposure and development of PMGI films is limited to films less than 2.5 μm in thickness, and use only tetramethylammonium hydroxide (TMAH)-based developers. We investigate the exposure and development of PMGI films greater than 6 μm in thickness using the two main classes of developer for PMGI, TMAH, and tetraethylammonium hydroxide (TEAH)-based developers. At these thicknesses, a nonuniform dose through the film due to the optical absorption of the PMGI leads to large gradients in the dissolution properties. We report etch rates as a function of surface dose and development time. Additionally a model is developed to provide a basic predictor of development depth and other important data for fabrication process planning and development.

The design of microstructures with a high quality factor (Qvalue) is of significant importance in many microelectromechanical system(MEMS) applications. Thermoelastic damping can cause an intrinsicenergy loss that affects the Q value of high-frequency resonance inthose devices such as MEMS mirrors. We deal with the simulation andanalysis of thermoelastic damping of MEMS mirrors based on the finiteelement method. Four designs of MEMS mirrors with different geometricshapes are studied. In each model, the dynamic responses of the systemsubjected to thermoelastic damping are compared to those of theundamped modes. Then we present a systematic parametric study onboth the resonant frequency and the Q value as functions of variousrepresentative parameters. These results are useful for early predictionof thermoelastic energy loss, not only restricted to the MEMS mirrors butalso applicable in more general MEMS resonators and filters design.

We discuss the development of a novel amperometric sensor to detect glucose concentrations in solution. Inorganic, vertically aligned nanowire arrays were employed as the sensing electrode in place of planar electrodes to utilize the unique properties of nanostructures, resulting in enhanced sensing signal from a smaller area. Heterostructured gold/platinum nanowires were used so that the dual functions of the nanoelectrodes for covalent immobilization of glucose oxidase and enhanced oxidation of hydrogen peroxide can be achieved using modified microfabrication methods. Two different enzyme immobilization methods—using self-assembled monolayers of alkanethiols and a porous conducting polypyrrole matrix—were investigated as methods for functionalizing the electrodes. Glucose sensing results were compared for planar and nanowire electrodes and the heterostructured nanowire electrodes. The results indicate that the unique structure of the sensing electrode delivers superior sensing performance from a smaller geometric area of electrodes, thus enabling further miniaturization of the sensor.

In our previous paper, we proposed a new method to obtain accurate pattern dimensions for correcting global critical dimension (CD) errors, which are defined as errors of CD uniformity in a region of several millimeters to several centimeters. The method is based on the pattern modulation method [a method of correcting CD errors by controlling figure sizes of large-scale integration (LSI) patterns]. An essential point of our method is to take into account the difference between the pattern density of the original LSI pattern and that of the corrected pattern (which has a pattern dimension different from original one after pattern modulation for correction) to provide an accurate correction. In this paper, we apply the proposed method to correct CD errors caused by flare and microloading effects. It is shown from numerical calculations that our method can suppress the correction error to less than 0.1 nm for both cases by three iterations. It is strongly recommended that our method be used for a wide range of applications to provide the necessary CD accuracies of the future.

Microwave performance of the anisotropic conductive film (ACF) and nonconductive film (NCF) interconnects was investigated by measuring the scattering parameters (S-parameters) of the flip chip modules employing the films. To compare the accurate intrinsic microwave performance of the ACF and NCF interconnects without lossy effect of chip and substrate, a de-embedding technique was employed. The effects of two chip materials, Si and quartz (SiO2), and of the metal pattern gap between the signal line and ground plane in the coplanar waveguide (CPW) on the microwave performance of the flip chip module were also investigated. The transmission properties of the quartz were markedly improved over those of the Si chip, which was not suitable for the measurement of the S-parameters of the flip chip interconnect. Extracted impedance parameters showed that the microwave performance of the flip chip interconnect with NCF was slightly better than that of the interconnect with ACF, mainly due to the inductive effect of the conductive particle surface and capacitance of the epoxy matrix in the ACF interconnect.

Reverse-tone step and flash imprint lithography (SFIL-R)shows promise as a cost-efficient, high-resolution patterning technique; however, the generation of satisfactory patterns requires the successful application of a planarizing topcoat over topography through spincoating. Photopolymerizable nonvolatile fluids are ideal topcoat materials because they planarize better than volatile fluids during spincoating and can continue to level after spincoating. Fluid mechanics analyses indicate that complete planarization using capillary force is slow. Therefore, defining the acceptable or critical degree of planarization (DOP_crit) becomesnecessary. Finite difference simulation of the spincoat and postspinleveling processes was used to determine the planarization time forvarious topographic and material property combinations. A new material, Si-14, was designed to have ideal planarization characteristics (low viscosity—15.1 cP; low shrinkage—5.1%) and satisfy SFIL-R processing requirements (oxygen etch resistance—33 wt% silicon, photocurable) and was used to validate our models through profilometry and interferometry experiments. During spincoating, minimizing the spin speed generates more planar films; however, this increases the spin time. To rectify this problem, a two-stage spincoating process—a first step with high spin speeds to achieve the target thickness quickly and a second step with low spin speeds to improve planarization—was proposed and experimentallydemonstrated.