This PDF file contains the editorial “Editorial: Types of Articles in JM3” for JM3 Vol. 4 Issue 04

This PDF file contains the editorial “Editorial: MOEMS: Design, Technology, and Applications” for JM3 Vol. 4 Issue 04

1-D photonic crystals with defects such as (L₁H₁)^mL_CH₂(L₂H₂)ⁿ are investigated. The bandgap edge transmission peak is narrowed when the couple layer L_C and the cover layer H₂(L₂H₂)ⁿ are added to the photonic crystal (L₁H₁)^m. Three different structures are studied. Structure A is not suitable for the tunable filter, which has two discrete tuning regions. Structures B and C provide good optical tuning methods: large dynamic range (>150 nm), narrow bandwidth [full-width at half maximum (FWHM) is about 2 nm], and good tuning linearity. These structures can be easily fabricated by the surface micromachined technique.

A MOEMS fiber modulator/sensor is fabricated by depositing a lithium niobate sol-gel thin film between the core and cladding of a fiber preform. The preform is then drawn into 125-µm fibers. Such a MOEMS modulator design is expected to enhance existing lithium niobate undersea acousto-optic sound wave detectors. In our proposed version, the lithium niobate thin film alters the ordinary silica core/cladding boundary conditions such that, when a stress or strain is applied to the fiber, the core light confinement factor changes, leading to modulation of fiber light transmission. Test results of the lithium niobate embedded fiber with a 1550-nm, 4-mW laser source revealed a reduction in light transmission with applied tension. As a comparison, using the same laser source, an ordinary silica core/cladding fiber did not exhibit any reduction in transmitted light when the same strain was applied. Further experimental work and theoretical analysis is ongoing.

Micromachined tunable Fabry-Perot interferometers based on compound semiconductors have earlier been proposed for fiber optic communications employing wavelength division multiplexing (WDM) for wavelengths around 1.55 µm. The cavity length in micromachined interferometers is varied by displacing one of the two distributed Bragg reflector (DBR) mirrors by electrostatic actuation of supporting beams. The filter's optical response for varying cavity lengths is simulated by a transfer matrix method, and the optical tuning efficiency of the filter is 0.53. We investigate three conventional filter designs using the finite element method (FEM) and compare it with a new proposed filter design. Using a mathematical model, deflection is analytically calculated and compared with finite element analysis results. Due to the way in which the mirror is integrated with a suspending framework of beams, bending within the mirror during actuation cannot be averted. The filter's optical performance demands that the mirror remain so flat that the maximum bending deflection is 1 nm for the mirror of given dimensions. Using a criterion based on mechanical and optical considerations, the dimensions of the beams suspending the mirror are optimized for each filter design under investigation. Combining the optical and mechanical simulations by FEM, wavelength tuning characteristics for each filter design are determined.

A novel micromachined electromagnetic actuated variable optical attenuator is described. The attenuation level is adjusted by changing the lateral distance between two V-groove-aligned single-mode fibers. Based on the waveguide transmission theory, the relationship between the attenuation and the offset is analyzed. The fabrication of the device only enrolls common and well-known semiconductor technologies. The packaged volume of the device is 20×10×5 mm³. According to the experiment results, the insertion loss is less than 1 dB, the polarization dependent loss is less than 0.1 dB, dynamic range is larger than 50 dB, and the driving voltage is less than 5 V. It is proven to be a low-cost, high-performance passive device for future all-optical networks.

Microelectromechanical systems (MEMS) and optics are a natural match. There are several reasons: MEMS devices have dimensions and achievable actuation distances comparable to the wavelength of light; smooth-surfaced dielectrics, semiconductors, and metals can be used in various combinations; and, since redirection of photons does not require large forces, the relatively feeble MEMS actuators can easily manipulate them. Micro-optical-electromechanical systems (MOEMS) are those where optics merge with MEMS. Many MOEMS devices are based on mirror arrays that can be tilted using electrostatic actuation. This work, however, focuses on programmable diffraction gratings and their uses for projection displays, spectroscopy, and wavelength management in modern optical telecommunication systems.

We develop a multifunctional interferometric platform for testing MEMS/MOEMS, measuring 3-D out-of-plane deflections and providing both material properties and motion behavior of microdevices. Specific metrology procedures are demonstrated to determine respectively the residual stress of silicon membranes compressively prestressed by SiO_xN_y plasma-enhanced chemical vapor deposition (PECVD), the vibration modes of PZT microactuators, as well as the expertise of scratch drive actuators.

Through the application of a new approach to energy analysis to microelectromechanical systems, the flat plasma spectrometer (FlaPS) provides a solution to the investigation of plasma distributions in space. It is capable of measuring the kinetic energy and angular distributions of ions in the space environment for energies ranging from a few eV to 50 keV. A single pixel of a FlaPS instrument has been designed, built, and tested to occupy a volume of 1 cm³, and is characterized by a high throughput-to-volume ratio, making it an ideal component for small-scale satellites. The focus of this work is on the design, fabrication, simulation, and testing of the instrument front end that consists of a collimator, parallel plate energy analyzer, and energy selector mask. Advanced microfabrication techniques enable fabrication of the miniature plasma spectrometer with geometric factor 4.9×10^-5 cm²-sr per pixel and an entrance aperture area of 0.01 cm². Arrays of narrow collimator channels with 4-deg angular divergence and high transmission allow energy analysis of ions without the need for focusing, the key feature that enables large mass reduction. It is also shown that the large plate factors achievable with this approach to energy analysis offer definite advantages in reducing the need for excessively high voltages.

Light and electricity are two major sources leading technology advances into the future. Micro-opto-electro-mechanical-systems (MOEMS) devices combine these two sources in an ideal manner: they are electronically addressable devices comprising optical elements to modulate light temporally and/or spatially. Further, MOEMS devices take advantage of high integration density, high reliability, high bandwidth, and low cost fabrication for mass production. While in some cases MOEMS technology focuses on the replacement of conventional devices, the majority of developments uses the unique potential of this technology to create devices based on novel principles with extended or even new functionality for advanced applications. Products based on MOEMS technology have already entered or are only a few steps away from entering the market in various fields, e.g., in consumer, information, and communication technology, medicine, biology, and metrology. This work gives an overview of MOEMS development activities with special emphasis on devices for light beam deflection and modulation. Single micromirrors, e.g., for scanning or laser beam positioning, are also presented and discussed as micromirror arrays and membrane mirrors for image generation and phase modulation. Technology trends are derived from the current development activities and an outlook to future work is given.

A 1500-µm-diam silicon/silicon nitride 3-D scan mirror has been built using MOEMS technology. It is capable of static and dynamic beam scanning achieved with a biaxial gimbal. A gold-coated deformable membrane at the center of the device provides both focus control (z axis) and spherical aberration correction. This architecture is able to move the focus of a laser beam throughout a 3-D space with one single optical surface, and is referred to as a MOEMS 3-D scan mirror. This mirror will be incorporated into a miniature confocal laser scanning microscope for biomedical in-situ imaging applications. We describe the 3-D scan mirror design, fabrication, characterization, and briefly describe the target application. A confocal laser scanning microscope using this mirror to achieve beam scanning and focus control is also demonstrated.

We demonstrate a closed-loop adaptive control scheme for achieving accurate positioning and trajectory tracking of an electrostatically driven torsional micromirror. Compared to the conventional proportional-integral-derivative (PID) controller, the proposed adaptive self-tuning controller has advantages of on-line compensating parameter variations and model uncertainty of the torsional micromirror, resulting from fabrication imperfections. Numerical simulation results utilizing MATLAB indicate that the proposed adaptive controller has a better transient response and can more precisely follow the reference trajectory, compared to the PID control scheme. The torsional micromirror is designed and successfully fabricated using surface micromachining processes. Real-time experimental results demonstrate that the proposed adaptive control is feasible and can improve the performance of the micromirror.

Adaptive optics techniques have been demonstrated in both laboratory and field tests, with a great level of scientific satisfaction, especially in astronomical and surveillance communities. Such successes have sparked the interest for these techniques in other fields, like biomedical imaging and industrial applications. However, to decrease complexity and costs, both very important issues for applications other than astronomical and surveillance, new technologies have to be brought to fruition. MEMS are becoming a very important player in this arena. We describe a portable adaptive optics (AO) system based on a MEM device that has been tested in both laboratory and field experiments. Results of these tests are discussed. Capabilities and shortcomings of this technology are discussed. A look at future applications and trends is given.

We present a new method for the fabrication of diffractive and refractive micro-optical components. The method is suitable for high-quality rapid prototyping of optical components and allows the fast experimental test of designs for computer-generated holograms or refractive microstructures. Our method is based on employing a digital-multimirror device (DMD) as a switchable projection mask. The DMD is imaged into a photoresist layer using a Carl Zeiss lithography objective with a demagnification of 10:1 and a numerical aperture of 0.32 on the image side. The resulting pixel size is 1.368×1.368 µm. In comparison with laser direct writing with a single spot, our method is a parallel processing of nearly 800,000 pixels (1024×768 pixels). This fabrication method can be applied to all MOEMS components. The method adds a new dimension in MOEMS processing, reducing the fabrication complexity, and improves the flexibility of process simulation and design.

The investigation of dynamics of microelectromechanical systems (MEMS) and microoptoelectromechanical systems (MOEMS) is an important problem of engineering, technology, and metrology. Specifically, recent interest in applying MOEMS technology to the miniaturization of micromirrors, sensors, and actuators for variety of applications requires the design of appropriate testing and measurement tools for investigation of dynamic properties of those systems. Though MOEMS technology offers great promise in addressing the need for smaller dynamical systems, the development of new types of MOEMS structures is a very costly and complicated procedure. On the other hand, straight application of the principles of design of macromechanical systems is rather limited in MOEMS applications. Therefore, the application of measurement technologies capable of detecting the dynamic properties of microscale systems may help to understand and evaluate the functionality of the systems. Technological aspects of MOEMS design and methods of their characterization are presented.

We present a new photomask technology capable of forming a continuous rotationally symmetric microstructure in thick photoresist. This technique eliminates many of the drawbacks of grayscale and half-tone masking technology. A binary phase grating of pi phase depth on a transparent quartz mask plate is fabricated in PMMA resist using an e-beam direct writing technique. When the phase mask is used in the stepper, an analog intensity profile is created on the wafer. The period is constrained, allowing for control of the zero-order in the stepper. The duty cycle of the phase gratings can be varied in such a way to provide the proper analog intensity profile for a wide range of micro-optics on the photoresist. The design, analysis, and fabrication procedures of this technique are discussed. This processing technique can be applied to many MOEMS devices that require refractive elements for optical processing. The method greatly simplifies the device process, reducing the cost and improving the device yield.

In a mature industry, all elements of the supply chain are available and are more or less in balance. Mainstream technologies are defined and well supported by a chain of product differentiation companies. Those focus companies offer services ranging from consultancy to manufacturing, where subcontracting is an essential element in the industrialization. Their specialization and dedication to one or a few elements in the technology increases professionalism and efficiency. The MOEMS industry, however, is still in its growing stage. After forming many companies aimed at the development of products and the production of components and systems, we see now many companies concentrating on the delivery of services to this industry. These services are: design and engineering, foundries, assembly and packaging, processing, and design simulation software. For manufacturing suppliers and customers, the lack of industry standards and mainstream technologies are serious drawbacks. Insight into the availability and trends in technology is important to make the right choices in the field of industrialization and production. This awareness was the reason we performed a detailed study of the companies supplying commercial services in this field. This work focuses on one important part of this study: packaging and assembly. This tends to remain a bottleneck at the end of the design cycle, often delaying and sometimes preventing industrialization and commercialization. For nearly all MEMS/MOEMS products, literally everything comes together in packaging and assembly. This is the area of full integration: electrical, mechanical, optical fluidic, magnetic, etc., functionalities come together. The problems associated with the concentration of functionalities forms a big headache for the designer. Conflicting demands, of which functionality versus economics is only one, and technical hurdles have to be overcome. Besides that, packaging and assembly is by nature application-specific, and solutions are not always transferable from one product to another. However, designers can often benefit from experience from other and general available technologies. A number of companies offer packaging and assembly services for MEMS/MOEMS, and this work gives typical examples of those commercial services. The companies range from small start-ups, offering very specialized services, to large semiconductor packaging companies, having production lines for microsystem-based products. Selecting the proper packaging method may tip the scales toward a product success or failure, while it nearly always presents a substantial part of the cost of the product. This is therefore not a marginal concern, but a crucial part of the product design. We address technologies and provide sufficient levels of classification and categorization for various aspects for the technologies, in specific, and the industry, in general, to provide particularly useful insights into the activities and the developments in this market. With more than 50 companies studied and assessed, it provides an up to date account of the state of this business and its future potential.

The eventual widespread insertion of microoptoelectromechanical systems (MOEMS) into the marketplace rests fundamentally on the ability to produce viable components that maximize optical performance while minimizing power consumption and size. Active control of surface topology allows for one component to perform multiple functions, thus reducing cost and complexity. Based on the patented MEMS compound grating (MCG), extension of the research at the College of Nanoscale Science and Engineering (CNSE) at the University of Albany, New York, to novel designs, materials, and fabrication methods yielded low-power, high-performance prototypes. The main focus of this work is on the development of a polymer version (including a sacrificial layer, in some designs) of the MCG, which allows for ease of fabrication and a reduced electrostatic actuation voltage. Following a system design effort, several generations of the component are fabricated to optimize the process flow. Component metrology, electromechanical characterization, and initial results of optical tests are reported. A second example presented is the design and prototype fabrication of a spring micrograting using a customized SOI process. This highly flexible component builds on the MCG concept and yields an order of magnitude reduction in actuation voltage.

Some practical physics-based formulas have been derived to make quick estimates of imaging performances in lithographic systems. These formulas have a simple structure so that they can be used in mathematical worksheets or, if desired, on a pocket calculator. Formulas have been derived to predict the printing of dense lines, bright field isolated lines, and dark field isolated spaces in positive resist. They are for exposures from binary masks and give exposure latitude, focus depth, and change in critical dimension. These parameters are also derived as a function of fading of the aerial images in lateral and focal directions. Validity has been checked with simulations. It proved that, in cases of the aerial image fading and of focus depth evaluation, only a few adjustment parameters were needed to obtain a reasonably good match with simulations. A table with all basic imaging parameters and their dependence on disturbances such as fading is included.

It has been proposed that the impact of stray light (scattered light or flare) in lithographic exposure tools can be mathematically described by the use of a point spread function of scattering (PSF_sc). We present an experimental verification of this model as follows. First, we determined the PSF_sc at multiple positions in the exposure slit from experimental disappearing-pad-test data. It is found that a PSF_sc based on the fractal model fits these data very well. Next, we used an aerial-image simulator to quantitatively predict the impact of the fitted PSF_sc on the critical dimension (CD), for line-space (L/S) patterns with varying mask Cr coverage from light field to dark field. A dedicated mask was built to provide L/S patterns with the different Cr coverage, at the same slit positions where the stray-light measurements were done. Finally, we compared the predictions of the stray-light impact on CD with the CD measured from wafers printed with this mask. This comparison yielded good agreement, which further confirms the validity of the PSF_sc approach. We also experimentally investigated the impact of lens contamination on the PSF_sc and propose a double-sloped fractal-based PSF_sc to describe situations such as this.

The functional dependence of a resist critical dimension (CD) with respect to resist thickness for a general absorptive thin-film stack in the case of oblique incidence is derived analytically with the rigorous electromagnetic theory. Based on obtained results, we discuss those thin-film effects related to CD control, such as the swing effect, bulk effect, etc., especially in the regime of high numerical aperture optical lithography.

The methodology of the optimization of a thin-film stack for high-NA optical lithography is different from that of the conventional low-NA case, since there are many new factors that need to be considered simultaneously. These include polarization dependence of the reflection coefficient, impact from variation of the angle of incidence, low energy coupling efficiency at the air/resist interface for TE waves, and critical dimension (CD) variation resulting from the bulk effect. In this work, we show that in the extreme high-NA regime, it is hardly possible with existing materials to realize a perfect top antireflection coating that satisfies simultaneous requirements of minimizing the swing effect as well as maximizing throughput and image contrast. Fortunately, for immersion lithography, it is very probable to realize such a top antireflection coating, even when NA is close to the physical limit. We also show that the impact from variation of the angle of incidence can be overcome by implementing top/bottom antireflection coating optimized at low/high angles of incidence, or vice versa. We finally show that CD variation resulting from the bulk effect can be compensated in half a swing period by fine tuning the swing effect.

A microfabrication technique specifically applicable for making structures with thin, flat air gaps is presented. The technique relies upon heating a polymer causing it to contract and form a stretched micromembrane across stationary platforms. To validate and quantify this process, structures were made using SU8, a spin-on photosensitive epoxy, as the polymer attached to a silicon substrate. The stretching of the SU8 in these structures for different heat treatments was characterized using a mechanical profilometer. Optical interference effects were also used to evaluate the overall flatness of these structures based on the reflected colors across their surfaces. SU8 membranes up to 200 micrometers wide were successfully stretched flat with variations in the underlying air-gap thickness of less than 20 nm.

The development of a miniature angular rate sensor based on silicon-on-insulator (SOI) microfabrication technology is presented. The design, fabrication, integration, and inertial testing of a MEMS-based angular rate sensor with large dynamic range were the driving forces behind this research. The design goals of 10-deg/h bias stability while operating through 2000-deg/s roll environments are presented. The sensor design is based on a straightforward single-mask fabrication approach that utilizes deep reactive ion etching of a 100-µm-thick device layer, with a buried 2- to 3-µm oxide layer used as the sacrificial layer, in an SOI substrate. To date, the data show demonstrated bias drift performance of 60 deg/h over this fast-roll environment.

We propose a technique using high-energy scanning electron microscope (SEM), which has the advantage of measuring 3-D structures and underlayer structures when compared to conventional low-energy SEM, to meet future metrology requirements. At first, we demonstrate that a technique using high-energy SEM has the advantages of measuring gate structures with a spatial resolution of a few nanometers. For example, a notched gate structure was most clearly visible when the beam energy is at 200 keV. Another example of a polyside gate with a sidewall spacer was most clearly visible at 100 keV. In addition, we studied the relationship between the thickness of the upper layer and beam energy at which the structure of the underlayers can be observed. The beam energy should be high enough to pass through the upper layer without the incident beam becoming broader, but low enough for the incident electrons to be backscattered at the structures in the underlayer. We could observe the line structures at a depth of 800 nm or less using an incident beam with energy from 50 to 100 keV.

Patterning thick SU-8 with conventional photolithography facilities is important for fabricating various MEMS structures. However, the fabrication of thick SU-8 MEMS has experienced severe problems such as cracks, distortions, or delaminations during the fabrication process and/or postservices, due to the large internal stress generated during the photolithography process. In this work, an in-depth finite element analysis (FEA) is performed to investigate the causes and effects of the internal stresses. Analytical results show that the post-exposure bake (PEB) temperature is the main factor in developing the resulted internal stress. Under the guidance of analytical results, an optimized UV photolithography process for the fabrication of ultra-thick low-stress SU-8 patterns is developed with conventional (simple) equipment. A low PEB temperature of 55°C reduces the internal stresses by more than 70% compared to those fabricated with the recommended procedure. Experimental results indicate that cracks, distortions, and delaminations are eliminated from the fabricated SU-8 structures using the newly developed procedure. In addition, the patterned SU-8 has a Young's modulus of 2.5 GPa and an ultimate strength of 50 MPa, which is about 50% higher than previous reported values.

A series of different fluorinated polymer platforms used for early and current 157-nm photoresists is investigated with regard to blanket etch properties and surface roughness. Besides methacrylic-based polymers applied for 193-nm lithography, fluorine containing norbornene homopolymers, fluorinated cycloolefines, and tetrafluoroethylene (TFE) norbornene copolymers are chosen. Etch rates in different plasmas used for several applications, such as poly, SiN mask open, and selective/nonselective SiO₂ etch, are determined and compared to standard 193-nm platforms currently used for DRAM manufacturing. Looking at various base resins, significant differences can be found using HBr- or Cl₂-based poly etch conditions and various fluorocarbon-based oxide etch chemistries. Up to 2.4 times higher etch rates in Cl₂ and the different C_xF_y oxide etch recipes are observed for the highly fluorinated cycloolefines and the TFE norbornenes, showing a strong correlation between fluorine content and etch rate. After stress by different etch conditions, the polymer surfaces are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Surprisingly, the surface roughness of the methacrylic platforms and the norbornene base resin (determined by AFM) are found to be substantially higher than that of the highly fluorinated platforms. These results can directly be correlated to pictures obtained by optical methods (SEM).

A new method for simulating the post-exposure bake (PEB) of optical lithography is presented and applied to modeling the reaction-diffusion processes in a chemically amplified resist (CAR). The new approach is based on a mesoscopic description of the photoresist, taking into account the discrete nature of resist molecules and inhibitor groups that are attached to the resist polymers, but neglecting molecular details on an atomistic (microscopic) level. As a result, the time- and space-dependent statistical fluctuations of resist particle numbers, the correlations among them, and their effect on the printing result can be accounted for. The less molecules that are present in the volume of interest, the more important these fluctuations and correlations will become. This is the case for more and more shrinking critical dimensions (CD) of the lithographic structures but unchanged molecular sizes of the relevant resist species. In particular, the new PEB simulation method allows us to predict the behavior of statistical defects of the printed lithographic structures, which may strongly contribute to printing features like line edge roughness (LER).

Molecular ruler nanolithography is a unique process capable of extending the achievable resolution of conventional lithography techniques. This is accomplished by patterning functional host structures whose dimensions can be tailored by systematically assembling engineered organic molecules of precise length on their surface, creating a template for producing features with reduced dimensions. We introduce a novel method for implementing molecular ruler lithography using a new sacrificial multilayer host structure. In particular, we demonstrate, using contact lithography, that host structures incorporating an underlying sacrificial resist can be used in conjunction with chemical solution phase self-assembly of a multilayer molecular ruler film, consisting of alternating layers of mercaptodecanoic acid molecules linked with copper ions. This new approach provides several advantages, including the potential for high yield processing, simplified removal of the parent host material, isolation of the host material from the substrate surface, and the capability to pattern daughter features whose thickness exceeds the thickness of the assembled molecular film. This new process can be implemented to create a wide variety of nanoscale structures and devices.