This PDF file contains the editorial “The Microlithography Symposium and JM3” for JM3 Vol. 2 Issue 04

This PDF file contains the editorial “SURFACE MICROMACHINING” for JM3 Vol. 2 Issue 04

Optimization of the photonic bandgap in finite-height photonic crystal (PhC) slab structures requires high-fill-factor lattices. We present a method for fabrication of high-fill-factor PhC devices in silicon-on-insulator (SOI) substrates using electron-beam lithography and high-aspect-ratio reactive-ion etching (RIE). We achieve 8:1 aspect-ratio PhC structures with 60-nm vertical membrane walls using a custom deep reactive-ion etching process in a conventional low-end RIE with patterned resist as the only etch mask. We present examples of various PhC devices fabricated using this method including a high-efficiency coupling structure for PhC waveguides.

Microsystems often require switches or mechanisms to provide two stable states. In answer to this need, we design a novel micromechanical bistable switch based on the locking mechanism commonly used in extension ladders. This switch was designed and fabricated through the multiuser MEMS processes (MUMPs). Actual performance of the switch was videotaped, analyzed, and compared with theory and simulation. This design was fully functional on the first design iteration, and can easily be extended to provide multiple stable states. We outline the design, simulation, and results from the testing of the microfabricated system.

The residual strain of amorphous and polycrystalline SiC films deposited using a single precursor 1,3-disilabutane is characterized as a function of deposition temperature ranging from 700 to 850°C. SiC microstrain gauges and cantilever beam arrays fabricated by micromachining are employed to characterize directly the average residual strain and strain gradient. The residual strain of SiC films changes from compressive to tensile as the deposition temperature increases. The strain gradient is also found to depend on the deposition temperature, and can be adjusted between positive and negative values to fabricate flat, curling-up, and curling-down micromechanical structures.

Ultra-widely tunable microcavity devices implemented by surface micromachining are studied. We model, fabricate, and characterize 1.55-μm vertical-resonator-based optical filters and vertical cavity surface emitting lasers (VCSELs) capable of wide, monotonic, and kink-free tuning by a single control parameter. Our devices are comprised of single or multiple horizontal air gaps in the dielectric and InP-based material system. Distributed Bragg mirrors with multiple air gaps are implemented. Due to the high refractive index contrast between air (n = 1) and InP (n = 3.17), only three periods are sufficient to guarantee a reflectivity exceeding 99.8% and offer an enormous stop-band width exceeding 500 nm. Unlike InGaAsP/InP or dielectric mirrors, they ensure short penetration depth of the optical intensity field in the mirrors and low absorption values. Stress control of the suspended membrane layers is of utmost importance for the fabrication of these devices. By controlling the stress, we are able to fabricate InP membranes that are extremely thin (357 nm thick) and at the same time flat (radius of curvature above 5 mm). Micromechanical single parametric actuation is achieved by both thermal and electrostatic actuation. Filter devices with a record tuning more than 142 nm with 3.2 V are presented.

We present the design, fabrication, and characterization of surface micromachined on-chip 3-D air-core arch-shape solenoid microinductors. Combinations of unique surface micromachining fabrication process techniques, such as deformation of polymeric sacrifical molds and conformal electrodeposition of photoresist molds on nonplanar sacrificial polymer mounds, are utilized. An air gap inserted between the inductor's body and the substrate is used to reduce the degradations of high-frequency inductor performances. Fabricated inductors are characterized and modeled at high frequencies from S-parameter measurements. ABCD parameters, derived from measured S parameters, are translated into a simplified physical π model. The resulting 2-, 3-, and 5-turn arch-shape suspended air-core solenoid inductors have inductances between 0.62 to 0.79 nH, peak quality (Q) factors between 15.42 to 17 at peak-Q frequencies between 4.7 to 7.0 GHz, and self-resonant frequencies between 47.6 to 88.6 GHz.

This PDF file contains the editorial “Micro-Optics for Photonic Networks” for JM3 Vol. 2 Issue 04

Tandem chirped grating couplers for spectral measurement applications in optical communications are developed. The current devices are designed to monitor data/telecom dense wavelength-division multiplexing (DWDM) channels in the spectral range from 1528 to 1561 nm (C-Band). A replication process provides the diffractive structures, on the gratings a high-index waveguide material is deposited. Design parameters and fabrication tolerances are discussed in detail, and measurement results of the fabricated devices are presented.

Techniques for analyzing symmetric and asymmetric polarized modes in coupled photonic crystal waveguides are presented. The techniques employed include the plane wave method (PWM) and the finite-difference time-domain (FDTD) method. Two variants of a photonic crystal directional coupler consisting of air holes in silicon are discussed. Additionally, fabricated photonic crystal directional couplers are characterized and experimental results are presented. Applications for photonic crystal directional couplers include frequency-selective filters, dispersion compensators, and optical switches.

We propose novel designs of optical vias to be used in high-density photonic integrated circuits (HDPICs), where photonic crystal (PhC) optical networks are used for spatial optical signal distribution and routing through multiple planes and in different directions. These optically conducting networks with their ability to guide light are an optical analogy to electrical circuits, in particular printed circuits boards (PCBs), which transport electrons through electrical networks. Different techniques for in- and out-of-plane broadband as well as narrow-band coupling between various PhC devices and layers are discussed. Numerical analyses were performed using the finite-difference time-domain (FDTD) method with appropriate boundary conditions.

Two-dimensional (2-D) microlens arrays have been fabricated with silica-on-silicon planar lightwave circuit (PLC) technology. Several experimental techniques and computer simulation methods are applied to characterize properties of single and double microlens arrays, with one and two refracting surfaces, respectively. Systematic comparison of the measured and simulated beam propagation profiles enables optimization of the lens and module design resulting in higher input-output coupling efficiency. The insertion losses of the lens-slab-lens optical modules with 90-mm-long slab waveguides are measured to be 2.1 and 3.5 dB for the double and single lens modules, respectively. Comprehensive analysis reveals the major loss contributions. Excess losses of the modules caused by variations of the lens curvatures, material refractive indexes, light wavelength, etc., can be controlled within the acceptable limits. Further possibilities for the module loss reduction are discussed. Fairly weak wavelength dependence as well as overall stability of the module properties indicate that the microlens arrays are suitable for dense wavelength division multiplexing (DWDM) photonic networks.

Diffractive optical elements (DOEs) offer the ability to boost fill factors of high-speed (field-of-view limited) near-infrared detectors. In this context, we have investigated the design and fabrication of a system that involves integration of DOEs with avalanche photodetectors (APDs). These APDs are implemented in the antimonide material system for operation around a 2.1-μm wavelength. Consequently, such systems could be used to reduce the required threshold power at free-space photonic receivers. To this end, we present the design and fabrication technologies for the DOEs, APDs, and their integration using polymer-based flip-chip interconnections.

An innovative high-resolution maskless lithography system is designed employing a combination of low- and high-numerical-aperture (NA) projection lens systems along with integrated micro-optics, and using Texas Instruments' super video graphic array (SVGA) digital micromirror device (DMD) as the spatial and temporal light modulator. A mercury arc lamp filtered for the G-line (λ = 435.8 nm) is used as the light source. Exposure experiments are performed using data extraction and transfer software, and synchronous stage control algorithms derived from a point array scrolling technique. Each exposure scan produces a field width (W) of approximately 8.47 mm with a field length (longitudinal field) limited only by onboard memory capacity. DMD frame rates of up to 5 kHz (kframes/s), synchronized to the stage motion, are achievable. In this experiment, TSMR-8970XB10 photoresist (PR), diluted to 3.8 cP with PR thinner is prepared. The PR is spin-coated onto a chrome-coated glass substrate to 1.0-μm thickness with 0.1-μm uniformity. A 0.4-μm scan step is used and 27,000 DMD data frames are extracted and transferred to the DMD driver. Results indicate consistent 1.8-μm line space (L/S) resolved across the entire field width of 8.47 mm. Given optimized exposure and development conditions, 1.5-μm L/S is also observed at certain locations. The potential of this maskless lithography system is substantial; its performance is sufficient for applications in microelectromechanical systems (MEMS), photomasking, high-resolution LCD, high-density printed circuit boards (PCBs), etc. Higher productivity is predicted by a custom H-line (λ = 405 nm) lens system designed and used in conjunction with a violet diode laser systems and the development of a real-time driver.

Biotechnology, in conjunction with semiconductor and microelectronics, would have a tremendous impact on new solutions in gene and drug discovery, point-of-care systems, pharmacogenomics, and environmental and food safety applications. A combination of microfabrication techniques and molecular biology procedures have the potential to produce powerful, inexpensive, and miniature analytical devices (e.g., microfluidic lab chips), aiding further development of genetic analysis. Microfluidics for biotechnology applications require development of inexpensive, high-volume fabrication techniques and reduction of biochemical assays to the chip format. We discuss design, fabrication, and testing of plastic microfluidic devices for on-chip genetic sample preparation and DNA microarray detection. Plastic microfabrication methods are being used to produce components of a complete microsystem for genetic analysis. A detailed discussion on the development of micromixers, microvalves, cell capture, micro-polymerase chain reaction (PCR) devices, and biochannel hybridization arrays is given. We also describe a path to further individual component integration.

An experimental method is presented to maximize the replication quality of UV-molded micro-optical components. It is important to maximize the replication quality to obtain the replicated micro-optical components with the desired properties by accurate control of the shape. We suggest a simple technique to avoid micro air bubbles. The effects of the UV-curing dose and the compression pressure on the replication quality of UV-molded structures are examined experimentally. Finally, as a practical application of the process design method, microlens arrays with diameters between 30 and 230 μm were fabricated by the presented method, and the replication quality and the optical properties of the replicated microlens are measured and analyzed.