Mechanical Properties and Fatigue of Polycrystalline Silicon under Static and High Frequency Cyclic Loading

10 and 20µm thick n-doped, epitaxially deposited, Bosch polysilicon layers of columnar structure were investigated. In order to characterize the fracture strength and fatigue,specimens with different sizes and geometries were designed and tested under static and high frequency cycling loading. Fractographic analysis has given an insight into fracture mechanism and helped to identify typical fracture modes and defects types

Optimization of the Geometry of a Microelectromechanical System Testing Device for SiO2—Polysilicon Interface Characterization

Microelectromechanical systems (MEMSs) are small-scale devices that combine mechanical and electrical components made through microfabrication techniques. These devices have revolutionized numerous technological applications, owing to their miniaturization and versatile functionalities. However, the reliability of MEMS devices remains a critical concern, especially when operating in harsh conditions like high temperatures and humidities. The unknown behavior of their structural parts under cyclic loading conditions, possibly affected by microfabrication defects, poses challenges to ensuring their long-term performance. This research focuses on addressing the reliability problem by investigating fatigue-induced delamination in polysilicon-based MEMS structures, specifically at the interface between SiO2 and polysilicon. Dedicated test structures with piezoelectric actuation and sensing for closed-loop operation were designed, aiming to maximize stress in regions susceptible to delamination. By carefully designing these structures, a localized stress concentration is induced to facilitate the said delamination and help understand the underlying failure mechanism. The optimization was performed by taking advantage of finite element analyses, allowing a comprehensive analysis of the mechanical responses of the movable parts of the polysilicon MEMS under cyclic loading

Micro-Electro-Mechanical-Systems (MEMS) and Fluid Flows

The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators. These micro transducers are able to be integrated with signal conditioning and processing circuitry to form micro-electro-mechanical-systems (MEMS) that can perform real-time distributed control. This capability opens up a new territory for flow control research. On the other hand, surface effects dominate the fluid flowing through these miniature mechanical devices because of the large surface-to-volume ratio in micron-scale configurations. We need to reexamine the surface forces in the momentum equation. Owing to their smallness, gas flows experience large Knudsen numbers, and therefore boundary conditions need to be modified. Besides being an enabling technology, MEMS also provide many challenges for fundamental flow-science research

Standalone Tensile Testing of Thin Film Materials for MEMS/NEMS Applications

The microelectronics industry has been consistently driven by the scaling roadmap, colloquially referred to as the Moore’s law. Consequently, during the past decades, integrated circuits have scaled down further. This shrinkage could have never been possible without the efficient integration and exploitation of thin film materials. Thin film materials, on the other hand, are the essential building blocks of the micro- and nano-electromechanical systems (MEMS and NEMS). Utilization of thin film materials provides a unique capability of further miniaturizing electromechanical devices in micro- and nano-scale. These devices are the main components of many sensors and actuators that perform electrical, mechanical, chemical, and biological functions. In addition to the wide application of thin film materials in micro- and nano-systems, this class of materials has been historically utilized in optical components, wear resistant coatings, protective and decorative coatings, as well as thermal barrier coatings on gas turbine blades. In some applications, thin film materials are used mainly as the load-bearing component of the device. Microelectromechanical systems (MEMS) are the example of these applications. Thin film materials carry mechanical loads in thermal actuators, switches and capacitors in RF MEMS, optical switches, micro-mirror hinges, micro-motors, and many other miniaturized devices. In these applications, one of the main criteria to choose a specific material is its ability to perform the mechanical requirements. Therefore, a clear understanding of the mechanical behavior of thin film materials is of great importance in these applications. This understanding helps better analyze the creep in thermal actuators (Tuck et al., 2005; Paryab et al., 2006), to investigate the fatigue of polysilicon (Mulhstein et al., 2001; Shrotriya et al., 2004) and metallic micro-structures (Eberl et al., 2006; Larsen et al., 2003), to scrutinize the relaxation and creep behavior of switches made of aluminum (Park et al., 2006; Modlinski et al., 2004) and gold films (Gall et al., 2004), to study the hinge memory effect (creep) in micro-mirrors (Sontheimer, 2002), and to address the wear issues in micro-motors. (van Spengen, 2003) In some other applications, the thin film material is not necessarily performing a mechanical function. However, during the fabrication process or over the normal life, the device experiences mechanical loads and hence may suffer from any of the mechanical failure issues. Examples of these cases are the thermal fatigue in IC interconnects (Gudmundson & Wikstrom, 2002), strain ratcheting in passivated films (Huang et al., 2002; He et al., 2000), the fracture and delamination of thin films on flexible substrates (Li & Suo, 2006), the fracture of porous low-k dielectrics (Tsui et al., 2005), electromigration (He et al., 2004), the chip-package-interaction (CPI) (Wang & Ho, 2005), and thin film buckling and delamination (Sridhar et al., 2001). In order to address the above-mentioned failure issues and to design a device that has mechanical integrity and material reliability, an in-depth knowledge of the mechanical behavior of thin film materials is required. This information will help engineers integrate materials and design devices that are mechanically reliable and can perform their specific functions during their life-time without any mechanical failure. In addition to the tremendous industrial and technological driving force that was mentioned earlier, there is a strong scientific motivation to study the mechanical behavior of thin film materials. The mechanical behavior of thin film structures have been known to drastically differ from their bulk counterparts. (Xiang, 2005) This discrepancy that has been referred to as the length-scale effect has been one of the main motivations in the scientific society to study the mechanical behavior of thin film materials. In order to provide fundamental mechanistic understanding of this class of materials, old problems and many of the known physical laws in materials science and mechanical engineering have to be revisited from a different and multidisciplinary prospective. These investigations will not be possible unless a concrete understanding of the mechanical behavior of thin film materials is achieved through rigorous experimental and theoretical research in this area.Natural Sciences and Engineering Research Council (NSERC) of Canad

Built-In test of MEMS capacitive accelerometers for field failures and aging degradation.

Postprint (published version

Doctor of Philosophy

dissertationThis dissertation describes the design, fabrication, testing, reliability, and harsh environment performance of single-device Micro-electro-mechanical-system (MEMS)- based digital logic gates, such as XOR and AND, for applications in ultra-low-power computation in unforgiving settings such as high ionizing radiation and high temperatures. Within the scope of this dissertation are several significant contributions. First, this work was the first ever to report the evolution in logic design architecture from a CMOS-paradigm to a MEMS architecture utilizing a single functional device per logic, as opposed to multiple relays per logic. This novel approach reduces the number of devices needed to implement a logic function by approximately 10X, leading to better reliability, yield, speed, and overall better characteristics (subthreshold characteristics, smaller turn-on/off voltage variations, etc.) and it simplifies implementation of MEMSbased circuits. The logic gates illustrate ~1.5V turn-on voltage at 5MHz with >109 cycles of reliable operations and low operational power consumption (leakage current and power <10-9A, <1^W). Second, this work is the first ever to report an intensive study on the cycle-bycycle evolution of contact resistance (Rc) up to 100,000 cycles, on materials such as, Ir, Pt, W, Ni, Cr, Ti, Cu, Al, and graphite, which are materials commonly used in MEMS switches. Adhesion forces between contacts were also studied using a contact-modeAFM, force vs. displacement, experiment. Results show that materials with high Young's modulus, high melting temperatures, and high density show low initial contact resistances and low adhesion forces (such as Ir, Pt, and W). Third, the devices were interrogated separately in harsh environments where they were exposed to high doses of ionizing radiation (90kW) in a nuclear reactor for a prolonged time (120 min) and, separately, at high temperatures (409K). Here, results show that solid-state devices begin to deteriorate almost immediately to a point where their gate can no longer control the drain-to-source current, whereas MEMS switches survive such ionizing radiation and temperatures portraying clear ON and OFF states for far longer. In terms of the applications empowered and the breadth of topics covered to accomplish these results, the work presented here demonstrates significant contributions to an important and developing branch of engineering

Condition Assessment and Fault Prognostics of Microelectromechanical Systems.

International audienceMicroelectromechanical systems (MEMS) are used in different applications such as automotive, biomedical, aerospace and communication technologies. They create new functionalities and contribute to miniaturize the systems and reduce their costs. However, the reliability of MEMS is one of their major concerns. They suffer from different failure mechanisms which impact their performance, reduce their lifetime and their availability. It is then necessary to monitor their behavior and assess their health state to take appropriate decision such as control reconfiguration and maintenance. These tasks can be done by using Prognostic and Health Management (PHM) approaches. This paper addresses a condition assessment and fault prognostic method for MEMS. The paper starts with a short review about MEMS and presents some challenges identified and which need to be raised to implement PHM methods. The purpose is to highlight the intrinsic constraints of MEMS from PHM point of view. The proposed method is based on a global model combining both nominal behavior model and degradation model to assess the health state of MEMS and predict their remaining useful life. The method is applied on a microgripper, with different degradation models, to show its effectiveness

Polycrystalline silicon thermal actuators integrated with photodiode

A thermal actuator (TA) is the thermal compliment of electrostatic actuators. A TA has several advantages over other microactuation methods. They can provide relatively large forces (^N) and large displacements (\u3e10 jam) at CMOS compatible voltages and currents. The main disadvantage of a TA is it large power consumption. A TA can be used in basic building-block MEM devices such as stepper motors, optical-component positioners, and grippers. The TA shown below in Figure 36 converts electrical energy into mechanical via ohmic heating and deflects due to asymmetric heating. The disparity in the widths of the Cold (wide) beam and the Hot (thin) beam causes an uneven current density to flow through the TA when an electrical bias is placed across the ends of the two beams. The higher current density in the Hot beam causes it to expand, due to thermal expansion, more than the Cold beam. This results in the sweeping of an arc in the plane of the wafer by the free end of the TA. Figure 37 shows a TA in its electrically biased state. Figure 36: Thermal actuator in the steady-state position. Figure 37: Thermal actuator in biased state. I propose to design, develop a fabrication process, fabricate, and test microsystems that integrate a polycrystalline silicon TA with a photodetector. The microsystem uses a photodetector as a position sensor to indicate the TA position in real-time. The process flow simulation will be accomplished using Silvaco Athena Process Simulator as a part of the design process. Various polycrystalline silicon TA will be fabricated in order to verify the effects of the design parameters. The design parameters are the length and width of the Hot Arm. The chip design will incorporate test structures to aid in the analysis of the structures and photodiodes and will use the Mentor Graphics IC layout editor. The design will not incorporate on-chip signal amplification. The fabrication process used will incorporate standard integrated circuit technologies, processes and standard surface micromachining processes. Optical and Scanning Electron Microscope photographs of the devices will be taken after important processing steps. Tests will include actuator position determination via photodetector current, maximum deflection, photodiode current-voltage characteristics, and TA cyclic fatigue. Video of the electrical testing will be taken. An empirical model of the relationship between the deflection and the current will be developed