Browsing by Author "Paulasto-Krockel, Mervi"

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  • Fatigue Crack Networks in Die-Attach Layers of IGBT Modules Under a Power Cycling Test
    (2024) Liu, Shenyi; Vuorinen, Vesa; Liu, Xing; Fredrikson, Olli; Brand, Sebastian; Tiwary, Nikhilendu; Lutz, Josef; Paulasto-Krockel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The die-attach layer is a vulnerable structure that is important to the reliability of an insulated-gate bipolar transistor (IGBT) module. A new failure mechanism named fatigue crack network (FCN) has been identified in the central area of the IGBT modules' solder layer. In this article, to investigate the formation mechanism of the FCN, a fast power cycling test (PCT) (current on 0.2 s and current off 0.4 s) was designed and performed on a commercial IGBT module. Subsequently, scanning acoustic microscopy and X-ray imaging were used for nondestructive inspection of the defects of the solder layer. The cross section was based on the nondestructive inspection results. Then, electron backscattered diffraction analysis was carried out on both observed vertical and horizontal cracks. As a result, both networked vertical cracks at the center and horizontal cracks at the edge of the solder layer were detected. The recrystallization occurred during the PCT. The voids and cracks emerged at high-angle grain boundaries. A finite element simulation was performed to understand the driving force of FCN qualitatively. The stress simulation results indicate that under time-dependent multiaxial stress at the center of the solder, the defects nucleated, expanded, and connected vertically to form the FCNs.
  • Finite element simulation of solid-liquid interdiffusion bonding process: Understanding process dependent thermomechanical stress
    (2022-05-01) Tiwary, Nikhilendu; Vuorinen, Vesa; Ross, Glenn; Paulasto-Krockel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Solid-liquid interdiffusion (SLID) bonding finds a wide variety of potential applications toward die-attach, hermetic encapsulation of microelectromechanical systems (MEMS) devices and 3-D heterogeneous integration. Unlike soft soldering technique, the solidification of intermetallic compound (IMC) formation in SLID bonding occurs during the process isothermally, making it difficult to predict and mitigate the sources of process-dependent thermomechanical stresses. Literature reports two dominant factors for the built-in stress in SLID bonds: volume shrinkage (due to IMC formation) and coefficient of thermal expansion (CTE) mismatch. This work provides a detailed investigation of the Cu-Sn SLID bonding process by finite element (FE) simulations. Specifically, the FE simulation of the SLID bonding process is divided into three steps: ramp-up, hold-time, and ramp-down stages to understand the stresses formed due to each individual step. Plastic material properties for Cu as well as temperature-dependent material parameters for different entities are assigned. Process-dependent thermomechanical stresses formed during the ramp-up and hold-time steps (IMC formation) were found not to be significant. The hold-time step is governed by the reaction and diffusion kinetics, which determines the bond line quality including defects, such as voids. The ramp-down step is the dominant phase influencing the final stress formations in the bonds. The results show an average of >30% decrease in the stress levels in Cu3Sn layer (IMC) when the bonding temperature is brought down from 320 °C to 200 °C, thus demonstrating the importance of low-temperature SLID process.
  • Novel low-temperature interconnects for 2.5/3D MEMS integration: demonstration and reliability
    (2024) Emadi, Fahimeh; Vuorinen, Vesa; Liu, Shenyi; Paulasto-Krockel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    To meet the essential demands for high-performance microelectromechanical system (MEMS) integration, this study developed a novel Cu-Sn-based solid-liquid interdiffusion (SLID) interconnect solution. The study utilized a metallization stack incorporating a Co layer to interact with low-temperature Cu-Sn-In SLID. Since Cu6(Sn,In)5 forms at a lower temperature than other phases in the Cu-Sn-In SLID system, the goal was to produce single-phase (Cu,Co)6(Sn,In)5 interconnects. Bonding conditions were established for the Cu-Sn-In/Co system and the Cu-Sn/Co system as a reference. Thorough assessments of their thermomechanical reliability were conducted through high-temperature storage (HTS), thermal shock (TS), and tensile tests. The Cu-Sn-In/Co system emerged as a reliable low-temperature solution with the following key attributes: 1) a reduced bonding temperature of 200 °C compared to the nearly 300 °C required for Cu-Sn SLID interconnects to achieve stable phases in the interconnect bondline; 2) the absence of the Cu3Sn phase and resulting void-free interconnects; and 3) high thermomechanical reliability with tensile strengths exceeding the minimum requirements outlined in the MIL-STD-883 method 2027.2, particularly following the HTS test at 150 °C for 1000 h.
  • Process Integration and Reliability of Wafer Level SLID Bonding for Poly-Si TSV capped MEMS
    (2018-11-26) Vuorinen, Vesa; Ross, Glenn; Viljanen, Heikki; Decker, James; Paulasto-Krockel, Mervi
     A4 Artikkeli konferenssijulkaisussa
    The objective of this study was to develop a fully integrated process for wafer level MEMS packaging utilizing PolySi through silicon via (TSV) capped MEMS devices. First, interconnection metallurgy and Solid Liquid Interdiffusion (SLID) bonding process was optimized. Then sc.'vias before bonding' capping process and contact metallizations for Poly-Si TSVs were developed. Finally, the process integration was demonstrated by using piezoelectrically driven MEMSactuators. However, several design and manufacturing related challenges were observed and detailed failure analysis were carried out to resolve these problems.
  • Study of Cu-Sn-In system for low temperature, wafer level solid liquid inter-diffusion bonding
    (2020-09-15) Hotchkiss, Joseph; Vuorinen, Vesa; Dong, Hongqun; Ross, Glenn; Kaaos, Jani; Paulasto-Krockel, Mervi; Wernicke, Tobias; Ponninger, Anneliese
     A4 Artikkeli konferenssijulkaisussa
    The Solid Liquid Interdiffusion (SLID) bonds carried out for this work take advantage of the Cu-In-Sn ternary system to achieve low temperature wafer-level bonds. The experiments were carried out across a range of temperatures and the results cover optimized wafer-level bonding process, the formation of the bond microstructure, mechanical performance, as well as the effects of thermal aging.
  • Wafer-Level AuSn/Pt Solid-Liquid Interdiffusion Bonding
    (2018-02) Rautiainen, Antti; Vuorinen, Vesa; Heikkinen, Hannele; Paulasto-Krockel, Mervi
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    In this paper, wafer-level AuSn/Pt solid-liquid interdiffusion bonding for hermetic encapsulation of microelectromechanical systems (MEMS) is evaluated. Although AuSn is used for bonding of ICs, the implementation of AuSn diffusion bonding in MEMS applications requires thorough understanding of its compatibility with the complete layer stack including adhesion, buffer, and metallization layers. Partitioning of the layer stacks is possible in MEMS devices consisting of several silicon wafers since the device wafer carrying functional structures and the encapsulation wafer have different restrictions on process integration and applicable metal deposition techniques. In this paper, CMOS/MEMS compatible sputtered platinum is utilized on the device wafer as a contact metallization for Au-Sn metallized cap wafer. The role of the platinum layer thickness as well as the nickel and molybdenum buffer layers on mechanical reliability were tested. The mechanical shear and tensile tests were performed for samples after bonding as well as after high-temperature storage and thermal shock tests. The results were rationalized based on the combined microstructural, thermodynamic, and fracture surface analyses. High-strength and thermodynamically stable bonds were achieved, exhibiting shear strength up to $~ $180 MPa and tensile strength up to $~ $80 MPa. Platinum was consumed completely during bonding and was observed to dissolve mainly into the (Au,Pt)Sn phase. Thicker platinum layer (200 versus 100 nm) increased the (Au,Pt)Sn phase thickness and resulted in higher strength. The molybdenum buffer layer under the platinum metallization increased the tensile strength significantly.