A Comprehensive Review on Convex and Concave Corners in Silicon Bulk Micromachining based on Anisotropic Wet Chemical Etching

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique . In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed

Wafer level chip scale packaging using wafer bonder

An in-house processing capability is developed in this research for silicon-glass bonding for microfabrication and wafer level chip scale packaging (WLCSP) using a wafer bonder. New masking technology for wet etching of glass to a depth of more than 430 µm is reported in this research work along with development of an anodic bonding process that permits electrical feedthroughs for connections to outside world. Three novel masks were developed in this work for deep wet etching of glass. They were multilayers of metals Mo/Cr/Au (mask 1) and Cr/Au/electroplated Ni (mask 2) both in combination with 20 µm thick AZ® P4620 photoresist and anodically bonded silicon (mask 3). Etch depths greater than 600 µm in glass has been achieved using anodically bonded silicon mask 3 above. It may be currently the only method available to achieve etch depths of 1 mm in glass. Earlier barrier of 300 µm etch depth in glass using multilayer metal mask has effectively been broken in this work with an etch depth of 430 µm achieved using electroplated Ni mask 2) above. A high value of 0.88 for the aspect ratio, defined as the ratio of the vertical etch depth to the lateral etch distance, was achieved using mask 1) above. The problem of etched surface roughness observed in glass with undiluted HF etching has been alleviated by use of a combination of 50:5:1 by volume HF:HCl:HNO3. Etch depths of 355 µm has been achieved in silicon using 45 % KOH solution at 50 °C with 1 µm thick oxide mask. The above etch parameters also resulted in smooth etched mirror like surfaces, sharp edges in etched pits and deep trenches in silicon. The decontaminated etched glass and silicon substrates were aligned in-situ and bonded using an AML 402 wafer bonder. The corner areas of the glass wafer were diced to expose the metal lines permitting electrical communication from the anodically bonded packaged chip to the outside world. The concept of WLCSP using anodic bonding has been developed and demonstrated in this research

Polymer-based fluidic devices integrated with perforated micro- and nanopore membrane for study of ionic and DNA transport

This study aims to develop a process, allowing a low-cost and high-throughput fabrication technique to produce freestanding polymer membranes having perforated micro- and nanopores, and also to design 3D micro/nanofluidic devices with the membrane, enabling a study of ions and DNA transport through nanopores. Technically, we have designed and fabricated high quality silicon stamp. Then, they have been used as molds for modified nanoimprint lithography that takes advantages of a sacrificial layer to obtain freestanding polymer membrane. This technique allows easy fabrication of large area, fully released polymer membranes containing perforated micro- and sub-micropores. The membrane with perforated micropores has been successfully integrated with microfluidic channels and used for in situ formation of lipid bilayer. The membrane with nanopores (\u3c 10 nm diameter) has been directly fabricated using modified nanoimprint lithography with silicon microneedle stamp. Also, the pore size was reduced further (down to 10 nm) with a subsequent process such as pore reduction by using polymer reflowing. Then, it was utilized for sensing and characterizing the ions and DNA transport through pores

EUROSENSORS XVII : book of abstracts

Fundação Calouste Gulbenkien (FCG).Fundação para a Ciência e a Tecnologia (FCT)

Passive devices for terahertz frequencies

Terahertz technology is a relatively new field of electromagnetic study and interest is rapidly growing in the wake of dramatic imaging demonstrations. Other applications are expected to follow, and they will need passive devices with functionality already found in more familiar microwave and visible regions of the electromagnetic spectrum, but presently missing in the terahertz region. Two fundamental devices in particular are variable polarisation compensators, and tunable frequency-selective filters. This work represents the first demonstration of a variable polarisation compensator using subwavelength patterned features (artificial dielectrics). Following on from the original proposal, this work contains a complete and thorough investigation including the development of a bulk silicon micromachining fabrication process, full characterisation of the device performance in the W-band (70 – 110GHz) and comprehensive simulations of the device, including detailed simulation of three distinct new designs with improved performance (continuously-variable retardance with maximum in excess of quarter- and half wave). The third of the three designs is capable of extremely low insertion loss (<0.6 dB) and overcomes a difficulty of the original design that prevented zero retardance in a practical device. Secondly, a new tunable photonic crystal filter is proposed and demonstrated. Easily accessible external control surfaces integrated into the interlocking plates of a layer-by-layer photonic crystal allow unprecedented contol over the number and type of defects within the structure, all of which may be tuned "on-the-fly". Devices are initially investigated with a full-vector electromagnetic finite-difference time-domain technique, to reveal the influence of the design dimensions on the band gap as well as the effect of the defects. A two-plate metal device having four layers of rods is constructed and measured in the W-band. In good agreement with the simulations, it is experimentally determined that a moveable passband is centered at 81 GHz, with a quality factor of 11, and a tuning shift of 1.7 GHz for a plate movement of 450 µm

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies