Characterization of platinum lift-off technique

In micro electromechanical systems (MEMS) and micro electronic devices there has been a strong demand for the fabrication of electrodes. Platinum (Pt) is a good candidate for this, because it combines some attractive properties: low electrical resistance, high melting point and high chemical stability. However, the latest leads to very difficult for patterning Pt by wet chemical or dry etching. Besides, etching damages the surface making wafer bonding impossible. Lift-off seems to be a solution to this problem. A big problem in using lift-off is that platinum particles or ears may remain at the edges after lift-off. These ears protrude from the surface and may cause electrical shortcuts with an opposite electrode. Some authors reported shortly about a modified lift-off technique to overcome this problem. Before deposition of the metal, a small cavity is etched in the insulator, which is mostly SiO2, thereby breaking the metal during deposition. In this paper the effect of cavity depth and metal thickness on ear forming is investigated. A surface roughness and a resistance of the asdeposited metals are measured. The results of method have been applied successfully for Load Cell sensors in our lab

Laser-based packaging of micro-devices

In this PhD thesis the development of laser-based processes for packaging applications in microsystems technologies is investigated. Packaging is one of the major challenges in the fabrication of micro-electro-mechanical systems (MEMS) and other micro-devices. A range of bonding processes have become established in industry, however, in many or even most cases heating of the entire package to the bonding temperature is required to effect efficient and reliable bonding. The high process temperatures of up to 1100°C involved severely limit the application areas of these techniques for packaging of temperature sensitive materials. As an alternative production method, two localised heating processes using a laser were developed where also the heat is restricted to the joining area only by active cooling. Silicon to glass joining with a Benzocyclobutene adhesive layer was demonstrated which is a typical MEMS application. In this laser-based process the temperature in the centre of the device was kept at least 120°C lower than in the bonding area. For chip-level packaging shear forces as high as 290 N were achieved which is comparable and some cases even higher than results obtained using conventional bonding techniques. Furthermore, a considerable reduction of the bonding time from typically 20 minutes down to 8 s was achieved. A further development of this process to wafer-level packaging was demonstrated. For a simplified pattern of 5 samples the same quality of the seal could be achieved as for chip-level packaging. Packaging of a more densely packed pattern of 9 was also investigated. Successful sealing of all nine samples on the same wafer was demonstrated proving the feasibility of wafer-level packaging using this localised heating bonding process. The development of full hermetic glass frit packaging processes of Leadless Chip Carrier (LCC) devices in both air and vacuum is presented. In these laser-based processes the temperature in the centre of the device was kept at least 230°C below the temperature in the joining region (375°C to 440°C). Testing according to MIL-STD-883G showed that hermetic seals were achieved in high yield processes (>90%) and the packages did withstand shear forces in excess of 1 kN. Residual gas analysis has shown that a moderate vacuum of around 5 mbar was achieved inside the vacuum packaged LCC devices. A localised heating glass frit packaging process was developed without any negative effect of the thermal management on the quality of the seal

Monolithic MEMS quadrupole mass spectrometers by deep silicon etching

Published versio

Adhesion and wear resistance of materials

Recent studies into the nature of bonding at the interface between two solids in contact or a solid and deposited film have provided a better understanding of those properties important to the adhesive wear resistance of materials. Analytical and experimental progress are reviewed. For simple metal systems the adhesive bond forces are related to electronic wave function overlap. With metals in contact with nonmetals, molecular-orbital energy, and density of states, respectively can provide insight into adhesion and wear. Experimental results are presented which correlate adhesive forces measured between solids and the electronic surface structures. Orientation, surface reconstruction, surface segregation, adsorption are all shown to influence adhesive interfacial strength. The interrelationship between adhesion and the wear of the various materials as well as the life of coatings applied to substrates are discussed. Metallic systems addressed include simple metals and alloys and these materials in contact with themselves, both oxide and nonoxide ceramics, diamond, polymers, and inorganic coating compounds, h as diamondlike carbon

Phase 1 of the automated array assembly task of the low cost silicon solar array project

The state of technology readiness for the automated production of solar cells and modules is reviewed. Individual process steps and process sequences for making solar cells and modules were evaluated both technically and economically. High efficiency with a suggested cell goal of 15% was stressed. It is concluded that the technology exists to manufacture solar cells which will meet program goals

An Experimental and Numerical Study on Glass Frit Wafer-to-Wafer Bonding

A thermo-mechanical wafer-to-wafer bonding process is studied through experiments on the glass frit material and thermo-mechanical numerical simulations to evaluate the effect of the residual stresses on the wafer warpage. To experimentally characterize the material, confocal laser profilometry and scanning electron microscopy for surface observation, energy dispersive X-ray spectroscopy for microstructural investigation, and nanoindentation and die shear tests for the evaluation of mechanical properties are used. An average effective Young’s modulus of 86.5 ± 9.5 GPa, a Poisson’s ratio of 0.19 ± 0.02, and a hardness of 5.26 ± 0.8 GPa were measured through nanoindentation for the glass frit material. The lowest nominal shear strength ranged 1.13 ÷ 1.58 MPa in the strain rate interval to 0.33 ÷ 4.99 × 10 (Formula presented.) s (Formula presented.). To validate the thermo-mechanical model, numerical results are compared with experimental measurements of the out-of-plane displacements at the wafer surface (i.e., warpage), showing acceptable agreement

Plasma-activated fusion bonding for vacuum encapsulation of microdevices

A fabrication process for vacuum-encapsulating PZT microcantilevers was designed in this dissertation. Initially, a low temperature wafer-bonding recipe was optimized with the help of plasma-activation. Conventional direct fusion bonding temperature was reduced from 400°C to 85°C, and final thermal annealing temperature and time of 1000°C for 4 hours (hr) were significantly reduced to 300°C and 1 hr respectively. Tensile tests conducted on dies diced from the bonded wafer stack revealed bond strengths of 22.15 MPa, which was close to the bulk fracture strength of 24 MPa for silicon. Near infrared images of the wafer stack showed no debonded regions at the interface. Surface and interface chemistry of oxygen plasma-activated wafers before, during, and after bonding were investigated. Significance of wet chemical activation technique, like RCA (Radio Corporation of America) cleaning, was studied. The time interval between plasma-activation and fusion bonding was varied, and its effect on the bond quality and bond strength was investigated. Decrease in the bond-quality and strength was observed with an increase in storage time. However, an unexpected increase in the bond quality was observed after 48 hr, and was attributed to the increase in the interfacial oxide layer. Further investigations revealed that the interfacial oxide layer was capable of absorbing gas molecules released as a byproduct of ongoing reactions at the interface of the two wafers. Gettering capability of the interfacial oxide layer was confirmed through the bonding of plasma-activated and 48 hr stored silicon (Si) and silicon dioxide (SiO2) wafers. Infrared images showed a good bond for the wafer stack. Since designing a fabrication process flow for vacuum-encapsulation of microdevices was the primary objective, lead zirconate titanate microcantilevers were fabricated onto a silicon substrate. The microdevices were actuated in ambient air pressure as well as in a vacuum environment. Broadening of the resonance curve was observed with an increase in the magnitude of ambient pressure, and is a result of increased air-damping. Experimental results obtained were compared to theoretical results from finite element modeling analyses. Vacuum cavities were fabricated between two Si wafers. Optical lid-deflection method of measuring internal cavity pressure was explored and employed with the help of high aspect ratio pressure diaphragms on a capping wafer. An investigation of seal integrity of the vacuum package revealed real/virtual leaks. The gettering capability of the SiO2 layer was employed in order to preserve the vacuum-level in the cavities. Two types of gettering patterns were investigated. It was concluded that an SiO2 getter layer at the interface increased the seal-integrity of the vacuum packages, while getter rings still showed signs of real leaks. In addition, it was observed that the internal vacuum-level was higher for cavities with getter rings as compared to cavities without getters. It was concluded that getter rings were capable of preventing virtual leaks but not real leaks. A thick interfacial getter layer, however, prevented both the real and virtual leaks. Finally, a vacuum-packaging fabrication method to encapsulate lead zirconate titanate microcantilevers was proposed. In addition, more accurate methods of measuring package vacuum pressure magnitudes were proposed

Molecular level interactions of large area 2D materials

textTwo-dimensional materials such as self-assembled monolayers (SAMs), graphene, etc. are candidate materials for improving the performance of microelectronics components and MEMS/NEMS devices. In view of their relatively large in-plane dimensions, surface forces are likely to dominate their behavior. The purpose of the current work was to extract not only the adhesion energy (or steady state fracture toughness) but also the traction-separation relation associated with interactions between various two-dimensional materials and substrates. In particular, interactions between SAMs terminated by carboxyl and diamine (COOH/NMe2) groups, hydroxylated silicon surfaces, graphene and silicon, graphene and its seed copper and graphene and epoxy over large areas was considered. Traction-separation relations, which are a continuum description of such molecular interactions, were determined by a direct method, which makes use of measurements of crack tip opening displacements; an inverse approach where the key parameters are extracted by comparing measured global parameters with finite element solutions and a hybrid approach in which the direct method was supplemented by finite element analysis. Furthermore, the surface free energy of graphene was measured by contact angle measurements. The most striking observation across all the interactions that were considered is that the interaction ranges were much larger than those attributed to van der Waals forces. While van der Waals models might have been at play between graphene and its seed copper foil and graphene and epoxy, the adhesion energies were surprisingly high. This coupled with the long interaction range suggests that roughness effects modulated the basic force field. Interactions between graphene and silicon and hydroxylated silicon surfaces may have been due to capillary and/or electrostatic again possibly modulated by roughness. The interactions between COOH and NMe2 SAMs became stronger under vacuum, which may have induced chemical bonding, and tougher under mixed-mode loading.Engineering Mechanic

Design and fabrication of a microneedle for the implantation of floating microstimulators

Neural prosthetics are used to stimulate any remaining functional nervous tissue in order to restore function in visual, auditory, or other physiological components associated with the nervous system. Microelectrodes have been used in stimulating and detecting electrical activity in neurons. When stimulating or recording neural tissue activity using microelectrodes, there are usually many problems and difficulties in obtaining the correct functionality and results. There is great difficulty in placing these microelectrodes in a specific location in the central nervous system due to various problems with the methodology used. Many different techniques such as the traditional interconnected microelectrodes have been an arduous process and often times ineffective. A promising technique using the technology of micro electro mechanical systems (MEMS) eliminates the difficulty of placing and delivering microelectrodes in desired areas. MEMS technology enables the use of micro/nanometer sized features by micromachining and microfabrication. The reduced size of the devices produced allows the development of many products used for several different applications. Placing electrodes in the central nervous system have been a very difficult process. This thesis focuses on the design and fabrication of a microneedle that will have the capability of penetrating the neural tissue and delivering floating microstimulators pumped out by fluid through a microfluidic channe