Browsing by Author "Torkkeli, Altti"

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  • Droplet microfluidics on a planar surface
    (2003-10-03) Torkkeli, Altti
     Doctoral dissertation (monograph)
    This work reports on the modelling of, and experiments on, a method in which liquid is transported as droplets on a planar hydrophobic surface with no moving parts, merely through electrostatic forces generated by the underlying electrodes. Two-directional transportation along a straight electrode path and across a junction, fusion of two droplets and methods for importing, exporting and filtering of water droplets were demonstrated, and can be used as basic functions of a lab-on-a-chip type microfluidic system. In this work, the electrostatic droplet actuation is for the first time demonstrated on super-hydrophobic surfaces. Such surfaces are composed of air-filled pores and exhibit a very low droplet sliding resistance due to reduced contact angle hysteresis and a high water contact angle (usually > 150°). This work shows that superhydrophobic surfaces can be used to reduce the minimum voltage and to increase the maximum speed under certain conditions, but there are some harmful side-effects. First of all, the electrostatic pressure can push water into the surface pores, which hinders actuation. The phenomenon can also be treated as a vertical electrowetting effect. Another drawback is that the use of superhydrophobic surfaces makes actuation more critical to the properties of the liquid. For example, actuation of biological buffer solutions was not successful. For these reasons, it is concluded that it is more beneficial to use a smooth surface with low hysteresis than a superhydrophobic surface in droplet actuation. Electrostatic droplet actuation is a potential method for manipulating liquid on a microscopic scale, but there is still work to do. This work contains a detailed examination of the droplet actuation mechanism, and trapping of charges in the solid-liquid interface is found to be the most severe problem that needs to be solved.
  • Mechanical properties and reliability of aluminum nitride thin films
    (2019-01-25) Österlund, Elmeri; Kinnunen, Jere; Rontu, Ville; Torkkeli, Altti; Paulasto-Kröckel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Knowledge of the mechanical properties and fatigue behavior of thin films is important for the design and reliability of microfabricated devices. This study uses the bulge test to measure the residual stress, Young's modulus, and fracture strength of aluminum nitride (AlN) thin films with different micro-structures prepared by sputtering, metalorganic vapor phase epitaxy (MOVPE), and atomic layer deposition (ALD). In addition, the fatigue behavior is studied under cyclic loading. The results indicate that the fracture strength and Young's modulus of AlN are mainly determined by the film microstructure, which is consecutively influenced by the deposition method and conditions. A microstructure with a higher order of crystallinity has increased fracture strength and Young's modulus. Additionally, the strength limiting defects are located at the film-substrate interface. The measured residual stresses were 249, 876, 1,526, and 272 MPa for two sputtered films of different thicknesses, MOVPE and ALD films, respectively. The fracture strengths were 1.42, 1.54, 2.76, and 0.61 GPa, and Young's moduli were 335, 343, 346, and 272 GPa. No clear signs of fatigue were observed after 10,000 cycles at a load corresponding to 83% of the fracture strength. (C) 2018 Elsevier B.V. All rights reserved.
  • Metalorganic chemical vapor deposition of aluminum nitride on vertical surfaces
    (2020-02-01) Österlund, Elmeri; Suihkonen, Sami; Ross, Glenn; Torkkeli, Altti; Kuisma, Heikki; Paulasto-Kröckel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Metalorganic chemical vapor deposited (MOCVD) aluminum nitride (AlN) on vertical sidewalls can be used to implement piezoelectric in-plane actuation and sensing in microelectromechanical system (MEMS) sensors. The AlN films should optimally cover conformally the sidewalls and have good crystal quality with c-axis oriented microstructure for optimal piezoelectric properties. Previous MOCVD AlN research has focused on using AlN as a buffer layer for other III-nitrides and so far, AlN growth has not been studied on large vertical surfaces. In this study, AlN thin films were grown using MOCVD on vertical sidewalls of fabricated templates and the conformality and crystal quality was characterized. The growth template fabrication was optimized with respect to surface roughness, the conformal coverage was analyzed by measuring the thickness profiles of the films, and the crystal quality was investigated using in-plane XRD and TEM. The AlN films have good crystal quality (FWHM 1.70°–3.44°) and c-axis orientation on vertical Si(1 1 1) sidewalls. However, the thicknesses of the films reduce approximately at a rate of 0.8–1.2 nm/μm down the sidewall. Lowering the reactor pressure improved the conformal coverage while changing the growth mode from columnar to step-flow, which also improved the film morphology.
  • Säädettävä infrapunainterferometri
    (1994) Torkkeli, Altti
     Helsinki University of Technology | Master's thesis
  • Stepper lithography optimization
    (2013-12-03) Leppäniemi, Jarmo
     Kemian tekniikan korkeakoulu | Master's thesis
    In this work we found optimum exposure parameters and determined a process window for a 1X Stepper tool used in MEMS fabrication. Compared to older full-wafer imaging, Stepper lithography has a smaller exposure field that enables to maintain high Depth of Focus (DOF) even at submicron dimensions. Even though its resolution limit is far from the modern step-and-scan tools used in IC industry today, its higher depth of focus makes it a suitable tool for MEMS, where lithography over severe topography is often mandatory. For the optimization of our Stepper tool, photoresist profiles obtained with different exposure parameters and test masks were analysed with Scanning Electron Microscope (SEM) and optical microscope delegated to measurement of Critical Dimensions (CD). Both profiles on top of thermal oxide and on top of bare silicon were investigated. A Swing Curve was constructed to determine photoresist thickness by RPM. Photoresist profiles were analysed with different focus and exposure energies to construct Bossung plots. These Bossung plots were used to determine our process window. Different types of submicron structures were investigated with the optimum parameters found. Optimum photoresist thickness obtained in Swing Curve analysis was 1,67 μm for planar surfaces. The process window of our Stepper tool is quite large, with DOF of 4 μm. The optimum exposure energy of our Stepper tool was 230 mJ and optimum focus was -1,0 μm. The isofocal CD was about 150 nm lower than CD on mask. The resolution limit of our Stepper tools is slightly lower than 1,0 μm: submicron lines are exposed and defined clearly, but they are sloped and thus quite narrow at top. After final oxide etching, photoresist profiles were almost identical on top of bare silicon and oxide. Our Stepper tool has sufficient resolution and high DOF suitable for MEMS fabrication. The optimal exposure parameters found in this work allow for smaller safety tolerances in chip design and thus more efficient use of silicon area. This allows reducing the size and cost of the MEMS without reducing device performance. In a farther future with more strict resolution requirements, either creative solutions or new kind of lithography tools are needed.
  • Vertical Piezoelectric Structures for in-plane Actuation in MEMS Sensors
    (2015-12-14) Österlund, Elmeri
     Sähkötekniikan korkeakoulu | Master's thesis
    The purpose of this Master's Thesis is to investigate the feasibility of in-plane piezoelectric actuation and to identify suitable materials and methods. The focus is on the selection of piezoelectric material and its growth method. The theory of piezoelectricity is presented and piezoelectric actuation and sensing are discussed along with piezoelectric materials. In addition, crystal growth and different growth methods are presented. MEMS gyroscopes are discussed as well. Piezoelectricity is the electrical response that results from mechanical stress. Piezoelectric materials are used in actuation and sensing in MEMS and in other devices. Piezoelectric actuation offers advantages over other methods, including enhanced performance and lower power use. This thesis presents an in-plane actuator with vertical sidewall structure that is more effective than other methods. This technology can be used for example in gyroscopes and other MEMS devices. AlN is selected as the material due to its advantages over other materials, such as compatibility and availability. AlN is required to grow on vertical sidewalls and have adequate crystal quality. Out of the considered growth methods MOVPE is the only one that can meet these requirements. Experiments show that the presented sidewall actuator structure works in in-plane actuation and has increased performance compared to current methods. The developed MOVPE process is suitable for AlN growth considering the crystal quality.