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

Application of Molecular Vapour Deposited Al2O3 for Graphene-Based Biosensor Passivation and Improvements in Graphene Device Homogeneity

Graphene-based point-of-care (PoC) and chemical sensors can be fabricated using photolithographic processes at wafer-scale. However, these approaches are known to leave polymerresidues on the graphene surface, which are difficult to remove completely. In addition, graphenegrowth and transfer processes can introduce defects into the graphene layer. Both defects and resistcontamination can affect the homogeneity of graphene-based PoC sensors, leading to inconsistentdevice performance and unreliable sensing. Sensor reliability is also affected by the harsh chemicalenvironments used for chemical functionalisation of graphene PoC sensors, which can degrade partsof the sensor device. Therefore, a reliable, wafer-scale method of passivation, which isolates thegraphene from the rest of the device, protecting the less robust device features from any aggressive chemicals, must be devised. This work covers the application of molecular vapour depositiontechnology to create a dielectric passivation film that protects graphene-based biosensing devicesfrom harsh chemicals. We utilise a previously reported “healing effect” of Al2O3 on graphene toreduce photoresist residue from the graphene surface and reduce the prevalence of graphene defects to improve graphene device homogeneity. The improvement in device consistency allows formore reliable, homogeneous graphene devices, that can be fabricated at wafer-scale for sensing andbiosensing applications

Integrated silicon pressure sensors using wafer bonding technology

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1997.Includes bibliographical references (p. 151-156).by Lalitha Parameswaran.Ph.D

Back-end processing of scanning mirrors with scratch drive actuators

The use of micro-electro-mechanical system (MEMS) processes in fabrication of micr- electro-mechanical devices for optical applications has been widespread. The invention of digital light processing (DLP) technology by Texas Instruments Inc. popularized the use of micro-mirrors. One of the applications of the micro-mirrors is in the form of scanners for biomedical imaging. The small size of mirrors and actuators makes them a good candidate for in vivo measurements/imaging. Various techniques in bulk- and surface-micromachining, and various actuators have been used to fabricate scanning mirrors. The scanning mirrors used in this work make use of scratch drive actuators (SDA) for their scanning motion. Both surface- and bulk-micromachining technologies are used to fabricate the devices. Surface-micromachining (Multi User MEMS Processes or MUMPs®) is used to fabricate scanning mirrors, and bulk-micromachining is used to separate dies, to create sloped sidewalls for efficient packaging and to make grooves for optical fibers. This work describes the techniques used in the post-MUMPs processing of the devices. The main features of the post-MUMPs processing are substrate etching and formation of insulating links. The presence of devices on the front side necessitates the use of a protection method in the substrate etching step. Polymers have drawn attention in recent years as protective materials due to their chemical stability as well as the ease of use. This thesis work utilizes poly dimethylsiloxane (PDMS) as a protection material, and examines the effect of PDMS process conditions on the quality of the protection. Protection for at least 10 hours was achieved in this work. The scanning mirror devices of this thesis work require non-conductive links between the actuators and the mirror. Thick photoresists such as AZ 4620, AZ 9260, SU-8 etc, are a good choice due to the ease in their patterning as well as their excellent insulating properties. This thesis documents the processes followed to achieve optimal link structures using thick photoresist (AZ 9260). The resist was optimally hard baked to make it chemically inert to common organic solvents