Theses supervised by Doç. Dr. Burhanettin Erdem Alaca
10 theses · Koç University
Küçük ölçekte mekanik test için eyleyici ve kuvvet algılayıcısı geliştirilmesi
This study is geared towards providing a uniaxial tension test platform for silicon nanowires, which includes an actuator, a force sensor and a single Si nanowire as the sample. Actuator pulls the Si nanowire at one of its tips, whereas the force acting on the Si nanowire is traced by the sensor attached at the opposing tip of the Si nanowire. The whole assembly is intended to be fabricated monolithically to overcome sample/device alignment and interface issues. This study reports design, fabrication and characterization of MEMS actuators and force sensors to meet the needs of such a testing platform. A set of four electrostatic actuators and four tri-plate force sensors based on differential capacitive readout are designed for Si nanowires of different sizes. Actuators are designed to generate forces ranging from 3 µN to 32 µN to enable the fracture of each Si nanowire while having a maximum actuation voltage of 10V - 25V. The minimum obtained ratio of the horizontal pull-in voltage to the actuation voltage is 2.6 for this set of actuators. Force sensors are designed to have a measurement range from 1.0 µN to 28.5 µN corresponding to the onset of fracture in Si nanowires. Sensitivity values, taken as the ratio of readout voltage to Si nanowire strain, are obtained as 0.65, 3.26, 5.46 and 3.74 V/(µm/µm). The corresponding displacement ranges of the sensors are in the submicron region ranging from 0.209 µm to 0.377 µm. Furthermore, non-linearity of the sensors is designed to be less than %5. First-generation actuator fabrication is accomplished through surface micromachining of SOI wafers with 10 µm device layer, 1 µm BOX layer and 380 µm handle layer, whereas force sensor fabrication failed during final release step of backside etching of the handle layer. To prevent the failure, we foresee the usage of a protective and conformal material which will enhance the rigidity of the device and will be removed easily after etching. Characterization of the first-generation actuators is accomplished by carrying out CV measurements. Results indicate that MOS capacitance is dominant over MEMS capacitance. The following conclusions are drawn based on fabrication and characterization of the first-generation actuators, which are taken into account in the design phase of the new set of devices, whose specifications were indicated above. 1. Release of actuators is problematic. A back-side etch is necessary; 2. A protective and conformal material is necessary for back-side etching; 3. With decreasing displacement values, MOS capacitance effect becomes increasingly important and has to be taken into account in the electromechanical design.
Silisyum nanotellerin kalin silisyum katmaninda yekpare üretimi ve nanomekaniksel testi
Nanowires have gained prominence in the fields of nanoelectronics and nano electromechanical systems thanks to continuous miniaturization of components and their utilization in day-to-day applications. Unique properties are exhibited by nanoscale structures due to quantum size confinement and an extremely large surface-to-volume ratio compared to their bulk-counterparts. Hence, mechanical characterization of these novel nanostructures is crucial in order to fulfill reliability requirements of these devices. A significant work still lies ahead in the mechanical domain due to challenges encountered inconsistent nanowire fabrication, manipulation, alignment and attachment in a test setup. The primary aim of this study is to develop a top-down fabrication approach allowing monolithic fabrication of silicon nanowires in a thick silicon layer, either a silicon wafer or the device layer of an SOI substrate. This will ensure a good level of control on nanowire dimensions and orientation along with a permanent attachment to surrounding support structures. The second aim of this study is to characterize the mechanical integrity of resulting nanowires through a high-resolution testing approach. For these purposes a fabrication technology is developed based on high-resolution lithography, directional etching, sidewall passivation and controlled undercut through deep reactive ion etching. Resulting nanowires exhibit widths (in-plane dimensions) between 20-80 nm, where a thickness (out-of-plane dimension) of 100 nm is demonstrated over very deep (10 um) trenches etched in silicon. A wide range of aspect ratios with nanowire lengths ranging from 200 nm to 12 um is achieved. Following the successful demonstration of the fabrication approach, three-point bending experiments are conducted on silicon nanowires using atomic force microscopy. Tests are conducted under ambient conditions by using a PeakForce Quantitative Nanomechanical Property Mapping module. Both elastic and fracture behavior of nanowires are captured. Results interpreted through analytical and numerical tools indicate the existence of a tensile intrinsic stress within nanowires. This work provides a significant contribution to the monolithic fabrication of silicon nanowires where crystalline orientation, location and dimensions are solely determined by the layout design. Although an etch depth of 10 um is demonstrated, guidelines are as well provided to increase the depth to 50 um and beyond. The developed technology is unique and has important implications for future integration of nanowires with microsystems.
Hücrelerin mekanik özelliklerinin ve hücre-alttaş etkileşimlerinin ölçümü i̇çin orta ve mikro ölçekte i̇ki yöntem geliştirlmesi
Mechanical factors that affect cells play key roles in many human diseases such as myopathies, neurodegenerative diseases, and cancer metastasis. Mechanical characterization of cells and cell-substrate interactions is therefore significant to the diagnosis and treatment of such diseases. Moreover, understanding cell mechanics can facilitate the integration of cells into machine design at the microscale. In this study, a mesoscale and a microscale method are developed for the mechanical characterization of cells and cell-substrate interactions. The mesoscale approach is to integrate uniaxial stretching with optical microscopy. A custom built stretcher device is developed for deforming an elastic substrate and adherent cells. Microscopy is used to image both the substrate and the cells. Substrates with embedded fluorescent microspheres are prepared to demonstrate the measurement of local in-plane strain fields. This method enables establishing accurate correlations between substrate deformation and cell response. The microscale approach is based on the idea of building a smart surface comprising an array of MEMS sensors to measure traction forces between adherent cells and the surface itself. Electromechanical design of a single MEMS traction sensor utilizing piezoresistive silicon nanowires is reported. The design process is geared towards optimizing both the piezoresistive response, and the mechanical sensitivity. The design of a suitable fabrication flow is also reported and the initial fabrication results are presented. In concluding, a method for the characterization of MEMS sensor, and design methodology and fabrication approach for a sensor array are proposed for future research.
Mikromekanik bir çınlacın manyetik devinim ve mekanik gerilme tabanlı akordu
The principle attribute of mechanical resonators is ruled by their resonance frequencies. As the main components of resonating Micro- and Nanoelectromechanical Systems (MEMS and NEMS), mechanical resonators determine the sensitivity, accuracy, and operation range. Therefore the tuning capability of such systems is crucial for design and development of NEMS and MEMS. In this study, a transversally and axially tunable microresonator is fabricated and studied. Magnetomotive actuation and optical read-out techniques are employed to excite microresonator and measure the output. Two active tuning mechanisms, magnetomotive tuning and tension-based tuning, are used to tune the resonance frequency of the micromechanical resonator. A chip-bending method is introduced to realize tension–based tuning mechanism. The variation in the effective stiffness of the micromechanical resonator due to softening effect and hardening effect is investigated. The resonance frequency is tuned in the range of -13% with the magnetomotive tuning mechanism and the resonator exhibits a resonance frequency shift from 789 kHz to 683 kHz. The resonance frequency tuning range for chip-bending method is found to be 8%, from 789 kHz to 854 kHz. Simultaneous use of both techniques results in minor deviations from the aforementioned values. The combination of both tuning mechanisms represents one of the few examples in the literature. The approach is especially promising for use in sensor and RF filter applications.
Sitoiskelet iplikçiklerin mems kuvvet algılayıcı ve floresan mikroskopisi kullanılarak gerçek zamanlı incelenmesi
The ability of cells to resist and exert mechanical forces allows them to perform many essential tasks such as cell growth, orientation, migration, and division. Cytoskeleton is the protein network of the cells that provides this ability as well as mechanical strength. In the absence of cytoskeletal components, cells experience significant embrittlement and this causes various human diseases. Understanding the mechanisms that determine how cells withstand and exert the mechanical stimuli is therefore significant to the diagnosis and treatment of cytoskeleton-related diseases. For this purpose, two different technologies with the major objective of observing the behavior of cytoskeletal filaments under local loads are developed. The first technology is based on the idea of building a smart surface comprising of MEMS sensors that can measure in-plane forces acting on the cytoskeletal filaments. The design of force sensors is geared towards optimizing both the mechanical sensitivity and piezoresistive response. Fabrication work is carried out through submicron patterning and doping by ion implantation. The initial fabrication results on silicon test and SOI wafers are presented. The second technology developed within the scope of this study is to construct a miniaturized stretcher integrated with a fluorescence microscope, which can load very thin diaphragms and measure associated minute forces. The integration is also enhanced with a synchronization enabling us to keep a certain area in the field of view during the entire duration of stretching. By applying sub-pixel particle tracking algorithm to the deformed micrographs, deformation and in-plane strains in the cells or in the substrates can be determined with a high spatiotemporal resolution. This work addresses two current shortcomings of non-contact strain measurement of soft matter: i) real-time measurement, and ii) high spatial resolution. The proposed technique can overcome this challenge and provides a displacement measurement resolution of 116 nm and a strain resolution of 0.04% over a gage length of 300 µm.
Integration of silicon nanowires with 3D devices: Fabrication and modeling
Silicon nanowires as fundamental building blocks in many nanoelectromechanical systems find applications in sensors and electronic devices. Extensive use of silicon nanowires in modern electronic devices faces integration challenges with the high-order architectures such as three-dimensional integrated circuits and microelectromechanical systems. A monolithic integration of silicon nanowires with microstructures can address the shortcoming of available techniques marred by the lack of batch techniques compatible with semiconductor manufacturing. In this study, a monolithic incorporation of silicon nanowires in bulk silicon with an etch depth of 40-um is demonstrated through a combination of high-resolution lithography and deep etching technology. This technique opens up new possibilities for modern sensors by replacing capacitance-based motional detection by high-sensitivity piezoresistive system via silicon nanowire use. Such monolithic approach to downscaling silicon piezoresistive sensors is presented in this study by spanning an electrostatic comb-drive actuator and a micromechanical amplifier by a single nanowire. Similar contribution of piezoresistive silicon nanowire electromechanical resonators to 3D devices is depicted based on frequency down-mixing approach as well. As this multi-scale integration is accomplished in a batch-compatible fashion, the presented technique carries significant implications for physical and biological miniaturized piezoresistive sensor technologies. In addition to integration challenges, the full potential of the miniaturized silicon nanowires in the field of physical sensors is yet to be realized. Mechanical modeling is one of the main challenges in design of nanowires with scale dependence of transport properties. Surface stress is considered as the major effect of size dependence in mechanical properties of nanowire. A sound guideline in mechanical modeling can address the successful engineering design of such unique architecture. In this work, a guideline for selecting the optimum engineering tool for the frequency response modeling of flexural nanowire resonators is introduced through a benchmarking study among analytical and numerical techniques. The limited applicability due to the associated computational cost in available approaches to analyze thermomechanical behavior of nanowires with surface stress contribution are addressed in this study by using a temperature-dependent interatomic potential in the standard Cauchy-Born theory. In this respect a comprehensive study on crystal orientation and geometrical parameters can shed light on the role played by the surface stress in determining silicon nanowire mechanical behavior. This is accomplished by linking the local surface stress at the atomic scale to the overall behavior of the continuum system. Addressing the existing controversy regarding the contribution of surface stress in the mechanical behavior of silicon nanowires, results in this work can provide a guideline for the interpretation of existing and the design of future experimental investigations.
Kuplajlanmmış dikey nanotel dizileri tabanlı yeni bir kompozit çınlaç mimarisi
A new composite resonator architecture is developed at the nanoscale by stacking nanowires (NWs) perpendicular to the surface of a substrate and coupling the NWs in the resulting vertical array mechanically by means of a membrane-type coupling element called coupling membrane (CM). Employment of CMs corresponds to a significant expansion to rather limited mechanical coupling toolkit at the nanoscale due to connections even between non-adjacent resonators. Electromechanical characterization of resonators is performed using electrostatic actuation and piezoresistive readout to ensure the best integration possibility with on-chip electronics in the future. Electrical measurements are performed not only for coupled vertical NW arrays but also for single clamped-clamped NWs to examine effects of mechanical coupling and multiple resonators in an array on frequency spectrum characteristics. Coupled NW arrays consisting of 8 NWs designed based on finite element modeling (FEM) modal analysis simulations are fabricated using the developed top-down fabrication flow from the 1 um thick device layer of a silicon-on-insulator (SOI) wafer. Effects of resonator and coupling element on frequency spectrum response are studied separately by changing the lengths of NW and CM systematically. Two piezoresistive frequency downmixing techniques of the two-source/1ω and two-source/2ω schemes are compared in terms of detected signal amplitudes and their dependence on gate voltages. In general, frequency spectrum measurements provide mechanical modes up to 130 MHz in high vacuum. Electromechanical characterization of piezoresistive single Si NW samples fabricated in the 10 um thick device layer of a SOI substrate is demonstrated for the first time in the literature. Detected mechanical modes from 18 MHz to 101 MHz for single NW samples are successfully associated to the first flexural in-plane and out-of-plane modes using FEM simulations thanks to the determination of an actual NW cross section by TEM measurements. Moreover, NW array measurements result in multiple mechanical modes both dispersed in a wide frequency range and in close proximity such that being reminiscent of typical spectrum response of a bandpass filter. However, inevitable nonuniformity of NWs in an array and absence of accurate spatial measurements complicate implementation of FEM simulations in the NW array case. Thus, FEM simulations are conducted to investigate more fundamental issues such as proving existence of mechanical coupling between NWs in a stack by distinguishing collective array modes from an individual NW resonance. To this end, experimental resonance frequency shifts of single NW and NW array samples with respect to electrode voltages are compared between each other and with FEM results. These comparisons between simulations and measurements reveal similar and distinct characteristics simultaneously for different spectrum modes.
PDMS ince filmlerin viskoelastik karakterizasyonu
Despite the fact that a small uncertainty in the PDMS Poisson's ratio leads to significant errors in traction force microscopy, there is a clear lack of data for PDMS films at the scale of 100 μm, a relevant size scale frequently employed in cell mechanics studies. Equally important is the need for the consideration of the viscoelastic nature of PDMS, as no mechanical property–including the Poisson's ratio can be taken as a time-independent constant. The foremost challenge for addressing these issues is the difficulty of carrying out stress relaxation tests on miniature PDMS samples accompanied by non-contact strain measurement with a very high spatiotemporal resolution. This study introduces such a stress relaxation platform incorporating i) the proper means for the application of necessary boundary conditions, ii) a high-precision in load measurement, and iii) a non-contact, local strain measurement technique based on single particle tracking. During stretching, images are recorded at a rate of 18 Hz with a 40 μm spatial resolution. Microsphere-embedded PDMS films as thin as 125 and 155 μm are prepared to study the Poisson's ratio by a local strain microscope. After tracing the displacement of microspheres by a single particle tracking method and using a strain mapping, Poisson's ratio for 155-μm-thick PDMS is measured to decrease from 0.483±0.034 to 0.473±0.040 over a period of 20 mins. For 125-μm-thick PDMS, this reduction takes place from 0.482±0.041 to 0.468±0.038. Moreover, a non-monotonic reduction is observed in both cases. This negative correlation between Poisson's ratio and relaxation time is found to be statistically significant for both thicknesses with p<0.001. The viscoelastic behavior is further characterized through the Burgers model. With a measurement field of 597x550 μm2, this study emphasizes the importance of the local investigation of mechanical properties. Furthermore, the dependence of transverse strain on a film thickness difference of 30 μm is measured to determine the sensitivity of local strain tracking. The inherent high resolution of the proposed approach enables one to measure deformations more precisely and to observe the temporal evolution of the Poisson's ratio that has not been observed before. In addition to the high-precision determination of PDMS Poisson's ratio, this work also offers a promising pathway for the accurate and time-dependent determination of the mechanical properties of other soft materials, where similar ambiguities exist regarding the mechanical behavior. The technique can also be used to establish a link between the conditions PDMS is exposed to during cell growth process and its mechanical properties.
Sodyum aljinat filmlerinin gerilme ve gevşeme özelliklerinin incelenmesi
Mechanical behavior of alginate films plays significant roles in many applications, hence good understanding and control of its mechanical properties are necessary for optimal fulfillment of these functions. In this work stress-strain and relaxation behavior of alginate films were investigated in tensile tests. In the first part tensile tests were performed on alginate films with different glycerol and calcium chloride concentrations. For tensile testing, strain was applied to the samples with constant speed until failure. Elastic modulus, ultimate strain and tensile strength values were obtained for each film composition. It was found that, calcium chloride and glycerol significantly affected elastic modulus, tensile strength and ultimate strain. The highest strength and strain values obtained when both calcium chloride and glycerol concentrations were at moderate concentrations and consequently a saturation point was seen for both parameters In the second part, the viscoelastic response to increasing glycerol was examined in relaxation tests. For relaxation tests, the samples were subjected to a step displacement, and the resulting stress was monitored for 9 h during relaxation process. The experiments were performed in two different relative humidity's, 38∓1% and 51∓1%. It was found that increasing glycerol and humidity decreased the final normalized stress of the films. Moreover, humidity had a synergic effect on the glycerol concentration. The relaxation behavior was modeled with three different models,1- three Maxwell elements, 2- two Maxwell elements plus an isolated spring, 3- a stretched exponential Kohlrausch-Williams-Watts model plus an isolated spring. The correlation of strain and cross-head displacement was acquired through finite element modeling and image processing. For this purpose, a model of 9319 tetrahedral elements was constructed with 61970 degrees of freedom. The crosshead displacement was imposed as a boundary condition, and the corresponding strain was computed. The strains computed with finite element modeling were verified through image processing, where the paths of surface irregularities were tracked from consecutive images obtained during the experiments.
Akım kontrollü, alan salımlı taramalı sonda litografisinde çizgi kenar pürüzlülüğünün incelenmesi: imge tersinimi ile doğrudan yazmanın karşılaştırılması
Current-controlled field-emission scanning probe lithography offers unique capabilities to achieve sub-10 nm feature sizes with a high resolution in ambient conditions without any vacuum or special gas requirements. Patterned features can be scanned directly after the exposure using the same exposure setup and tip. With linewidths thus reduced from microns to nanometers, the line edge roughness has recently become even more important, since it does not scale with the decreasing feature size having a direct effect on the critical dimension of the printed features, thereupon, also on the speed and functionality of fabricated devices. For this purpose, lines patterned with a 200-nm nominal pitch through direct-write in scanning probe lithography are compared to those obtained by image reversal. Negative-tone exposure before development (latent image) is also considered thanks to the resolving power of imaging through non-contact AFM mode and used for validation purposes. Images collected are analyzed using edge detection techniques with the average linewidths for image reversal and direct-write found to be 50.01 nm and 26.81 nm, respectively. We report 16.44 nm average line edge roughness for the latent image, 16.78 nm for image reversal and 10.95 nm for direct-write. Presenting a thorough comparison of patterning through image reversal of calixarene molecular glass resist from negative-tone to positive-tone as well as direct-write, this thesis discusses various aspects of the quality of resulting patterns.