Browsing by Author "Paulasto-Kröckel, Mervi, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland"

Filter results by typing the first few letters
Now showing 1 - 7 of 7
  • Contact Metallization Design for Low-Temperature Interconnects in MEMS Integration
    (2024) Emadi, Fahimeh
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Advanced electronic interconnects must meet various criteria, including low-temperature (LT) processing, miniaturization, and a stable microstructure with optimal electrical, mechanical, and thermomechanical properties. Specific application requirements, such as those for micro-electromechanical systems (MEMS) combining mechanical and electrical parts on a micrometer scale, must also be considered. Cu-Sn solid-liquid interdiffusion (SLID) interconnects show potential for utilization in MEMS integration. However, challenges persist, such as high processing temperatures and the complexity of wet chemical electroplating materials (e.g., Cu and Sn) on wafers containing fragile movable MEMS devices. Addressing these challenges involves employing LT Sn-In instead of Sn, isolating the electroplating processes to passive structures, and using a thin film contact metallization deposited via physical vapor deposition (PVD) on the device's wafer side, providing an alternative to wet chemical processes. Therefore, this thesis aimed to identify suitable contact metallizations for Cu-Sn-based SLID systems, design the metallization stack, accordingly, investigate the microstructural evolution and mechanical properties of the interconnects, assess their reliability, and demonstrate the utilization of LT-SLID, connected with TSVs, to create three-dimensional (3D) interconnects with a specific focus on 3D MEMS integration. Cobalt emerged as the most viable contact metallization option to interact with Cu-Sn-based SLID interconnects. The results showed that the microstructure of the Cu-Sn/Co bond line evolves over time at elevated temperatures or longer bonding times; hence, the Co-to-Sn thickness ratio must be controlled to prevent bond failures. No such concerns were observed with the Cu-Sn-In/Co joints. Moreover, the Cu-Sn-In/Co system exhibited promising results that met the interconnects' requirements, such as the tensile strength requirements of MIL-STD-883 method 2027.2. The bonding process was demonstrated at temperatures as low as 200 °C, resulting in a void-free stable microstructure even after a high-temperature storage (HTS) test. Furthermore, the tensile strength of the bonds improved after the HTS test. The compatibility of the developed interconnects with TSVs was confirmed, enabling the fabrication of 3D SLID-TSV interconnects for the advanced integration of MEMS devices. These interconnects demonstrate better performance than Cu-Sn SLID-TSV interconnects, which have faced challenges such as silicon cracking and void formation. This finding highlights the effectiveness of the 3D LT-interconnects.
  • Deposition and characterization of aluminum nitride thin films for piezoelectric MEMS
    (2020) Österlund, Elmeri
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Piezoelectric actuation and sensing would improve many sensors based on microelectromechanical systems (MEMS), such as gyroscopes used in inertial measurement units. Inertial MEMS sensors, especially gyroscopes, require in-plane actuation and sensing, in addition to out-of-plane actuation and sensing in order to measure movement in all three directions. Effective piezoelectric implementation requires the deposition of high crystal quality material on the vertical sidewalls of MEMS structures, which can then move in the in-plane direction. Currently used methods for thin film deposition and processing do not have adequate conformal coverage on vertical sidewalls. Previous research on the deposition of piezoelectric aluminum nitride (AlN) has focused on using AlN in applications, such as resonators, where conformal coverage is not needed, using line-of-sight physical vapor deposition (PVD), which generally has poor conformal coverage and results in tilted crystallites when deposited on vertical surfaces. Chemical vapor deposition (CVD) on the other hand, should result in better conformal coverage on the vertical sidewalls of the three-dimensional structures needed for optimum in-plane actuation and sensing, and metalorganic CVD (MOCVD) and atomic layer deposition (ALD) were used to deposit AlN thin films on vertical sidewalls. The conformal coverage of the MOCVD and ALD processes were studied by growing AlN thin films on patterned silicon substrates, and the crystal quality and microstructure of the films were studied using a combination of X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM). Furthermore, the mechanical properties, reliability, and microstructural stability of AlN and Sc-alloyed AlN (AlScN) thin films were studied. The purpose of this dissertation was to the find a suitable method and process parameters for the deposition of high crystal-quality piezoelectric AlN on vertical sidewalls that can then be used in the fabrication of piezoelectric inertial MEMS sensors and enable the use of AlN and AlScN in new applications by studying their mechanical reliability and microstructural stability.
  • Development of piezoelectric microelectromechanical systems for multiaxial motion and sensing
    (2024) Bespalova, Kristina
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Piezoelectric materials offer several advantages for MEMS applications due to their superior direct electromechanical coupling and low voltage consumption, especially when compared to electrostatic-based MEMS. Integrating piezoelectric thin films in MEMS also allows for a significantly smaller chip footprint than devices employing other transduction techniques. Furthermore, thin piezoelectric films can be integrated into the fabrication of multifunctional devices capable of three-dimensional motion (3D motion). Such 3D piezoMEMS enable driving and sensing along the x, y, or z-axes using components of a single element. This distinguishes 3D piezoMEMS from conventional MEMS that utilize elements that often facilitate motion in only one direction. This dissertation investigates the development of a new fabrication approach and adapting and optimizing existing fabrication techniques for 3D piezoMEMS fabrication. Pure lateral motion of a single MEMS element is implemented by placing metal organic chemical vapour deposited aluminium nitride (MOCVD AlN) thin films on the vertical surfaces of the Si cantilever. The fabrication approach demonstrated in the work unlocks the piezoelectric and electrode material deposition potential on vertical sidewall structures in the fabrication of advanced 3D piezoMEMS.
  • Effects of accelerated lifetime test parameters and failure mechanisms on the reliability of electronic assemblies
    (2014) Hokka, Jussi
     School of Electrical Engineering | Doctoral dissertation (article-based)
    This dissertation presents the results of accelerated reliability assessment methods employed on lead-free component board assemblies. Temperature cycling and mechanical shock tests are commonly used to assess the reliability of portable electronic products. Higher acceleration factors can be achieved by exposing the devices under test to higher loadings than those experienced in operation conditions or producing the loadings more frequently. Recently there has been an increasing interest towards the optimization of test parameters in order to minimize the time required for testing. However, if the effects of acceleration procedures, especially on solder interconnection microstructures, are not well-understood, misleading conclusions can be made that can lead to poor product reliability having disastrous consequences in the worst case. The results of this work demonstrate that the highly accelerated test conditions can lead to excessive lifetime acceleration and misleading failure mechanisms. It is shown that relaxation of the residual stresses has a significant effect on the shock impact lifetime of component boards while the failure mechanism(s) do not change with the increased impact repetition frequency. Relaxation of the residual stresses in load bearing materials takes place during the time between the impacts. The extent to which they can operate affects the way how the stresses/strains in the solder interconnections develop during further impacts. Similarly, lifetimes and failure mechanisms of component boards under thermomechanical cyclic conditions are shown to be dependent on the accelerated test parameters. In the highly accelerated tests, the microstructural evolution (recrystallization) controls the propagation of cracks, while in the real-use conditions, significantly less microstructural evolution takes place and the rate of crack propagation through the solder is notably lower. Re-assessment of the standardised test parameters and lifetime prediction models is therefore necessary in order to achieve better correlation between test conditions and real-use conditions. This work discusses different ways of achieving this target.
  • Intermetallic Void Formation in Cu-Sn Micro-Connects
    (2019) Ross, Glenn
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The compatibility of new materials and their interfaces are key components in the pursuit of highly integrated and reliable systems. An extensive understanding is required of the behaviour and stability of the materials not only during device fabrication but over the entire functional lifetime of a device. A microstructural defect, and the focus of this thesis, that threatens the mechanical and electrical performance of 3D-intergrated systems is Cu-Sn intermetallic void formation. Results of this work has been separated as follows: (i) understanding the sporadic behaviour of void formation, (ii) understand the key parameters influencing voiding formation, (iii) examine the microstructural and chemical properties associated with void formation, (iv) present a void formation hypothesis and (v) discuss void reduction and detection methods for the microelectronic industry. Historically void formation has been sporadic and uncontrolled and led this author, in additional to several other authors, to question whether formation is only due to the Kirkendall effect (interdiffusion imbalance between two joined metals). The Cu electroplating process and the parameters used play a large role in effecting the voiding propensity. These parameters, including the additive chemistry and current density, influence the microstructural properties and chemical composition of the deposited film such as, grain structure, residual stresses, crystal defect density and trace impurities. A new intermetallic void formation hypothesis is proposed based on microstructural and chemical state of the Cu-Sn system. Intermetallic voids can be suppressed when the Cu electroplating process is well controlled, which requires careful observation of the electroplating parameters. This requires cooperation and understanding of the process between the semiconductor fabrication companies and the Cu process suppliers. However, it is difficult to control voiding completely, therefore non-destructive void detection methods need to be developed.
  • Quality, Microstructural Refinement and Stability of Atomic-layer-deposited Aluminum Nitride and Aluminum Oxide Films
    (2018) Broas, Mikael
     School of Electrical Engineering | Doctoral dissertation (article-based)
    High-quality, stable ALD films are required in microelectronics when the films are exposed to further processing during device manufacturing, or if the films are exposed to a demanding environment. For example, front-end-of-line processing exposes the deposited materials to high temperatures and aggressive chemicals during process steps such as dopant activation and wafer cleaning. Furthermore, a protective film against humidity and corrosion may need to maintain its structural integrity for the lifetime of the device which can be several years. Therefore, engineering the film quality and understanding the effects of high-temperature processing on thin films are required for the successful integration of the films to a semiconductor device. The goal of this thesis was to study the quality, microstructural refinement, and the stability of ALD AlN and Al2O3 films. The results were divided to the process development of ALD AlN and Al2O3 films, the examination of their microstructural development due to high-temperature thermal treatments, and the resulting stability of the ALD films. Film stability was understood to encompass thermal stability (e.g. oxidation) and chemical stability (ability to resist dissolution and corrosion). Film quality comprised of attributes such as the amount of impurities, stoichiometry, and crystallinity which were characterized for the as-deposited films and after the high-temperature treatments. The emphasis on ALD AlN was in process development. Trimethylaluminum (TMA) -based AlN was amorphous and contained a high amount of hydrogen when deposited at 200 °C. The hydrogen outgassed during high-temperature treatments and the AlN films began to oxidize at and above 800 °C. AlCl3-based AlN films, processed closer to 500 °C, had less impurities and a polycrystalline microstructure as opposed to the TMA-based films deposited at 200 °C. The AlN film residual stress was also tunable in the plasma-enhanced AlCl3 process by adjusting the plasma time of the nitrogen precursor. ALD AlN studied in this thesis and the literature review show promise of the film quality continuously improving. The main challenges are in improving the crystalline quality and minimizing the amount of impurities, such as hydrogen, in the AlN films. The focus on ALD Al2O3 was in understanding the effects of the high-temperature treatments. As-deposited ALD Al2O3 was amorphous and dissolved into wet chemical cleaning solutions. Heat treatments at and above 800 °C crystallized the films. However, high vacuum annealing caused blistering of the alumina films, whereas atmospheres with hydrogen and nitrogen produced crystalline films without blisters. The fully-crystallized alumina films were stable in SC-1 and HF cleaning solutions. The crystallized alumina films are demonstrated to be suitable for technologies such as silicon on insulator. Furthermore, crystallized ALD alumina could be utilized as a protective layer in a variety of applications that withstand the crystallization temperature.
  • Solid-liquid interdiffusion bonding for MEMS device integration
    (2018) Rautiainen, Antti
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Microelectromechanical systems (MEMS) have been widely utilized in many developed applications including safety critical systems in automotive and aerospace. Several MEMS devices require controlled environment for operation and protection against volatile compounds i.e., hermetic sealing. Heretofore, this has been attained typically with anodic or glass-frit bonding methods. However, metal bonding offers several benefits compared to these, such as, possibility to combine easily hermetic sealing and creation of electrical interconnections, enhanced device performance and smaller footprint. Solid-liquid interdiffusion (SLID) bonding is one of the developed low bonding temperature methods, which is studied in this thesis by utilizing copper-tin, gold-tin and zinc-aluminum based metallurgical systems by investigating their thermodynamic-kinetic behavior as well as reliability performance. Interfacial reactions are explained based on the microstructural analysis and the mechanical reliability is evaluated with shear and tensile tests. In addition, bonds are subjected to different stresses by utilizing standard environmental tests, such as high temperature storage, thermal shock and mixed flow gas tests. This thesis presents SLID bonds having high mechanical strength and robustness in the environmental tests. The effect of voiding level in the Cu3Sn phase on the cracking propensity is reported, and the voiding level is observed to link to copper electroplating bath condition. Zinc-aluminum based alloys are examined as a cost-effective material system for metal bonding. Interfacial reactions between these alloys and common base metals are investigated and rapid intermetallic compound formation is observed in a soldering procedure that simulates a wafer bonding process. Furthermore, platinum-based contact metallization stacks are presented for Cu-Sn and Au-Sn bond metallizations. These contact metallizations are CMOS-compatible, and thus, applicable easily on e.g., MEMS device or application specific integrated circuit (ASIC) wafer in order to simplify the process integration and to increase the device performance. In high strength Au-Sn/Pt SLID bonds, the thermodynamic data is utilized to reason the rise of the re-melting temperature as a function of original platinum layer thickness. In addition, the difference in reliability performance is connected to failure mode analysis. In case of Cu-Sn/Pt bond, platinum is observed to participate into IMC formation reactions at Cu/Sn interface during soldering and to stabilize the high temperature hexagonal crystal structure of the Cu6Sn5 phase to room temperature. The thermodynamic data and reliability evaluation related to SLID bonding provided in this thesis can be directly utilized in electronics industry in 0- and higher-level integration design.