Browsing by Author "Tiwary, Nikhilendu"

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Achieving low-temperature wafer level bonding with Cu-Sn-In ternary at 150 °C
(2023-01-01) Golim, Obert; Vuorinen, Vesa; Ross, Glenn; Wernicke, Tobias; Pawlak, Marta; Tiwary, Nikhilendu; Paulasto-Kröckel, Mervi
Letter
In this work, a low-temperature wafer-level bonding process at 150 °C was carried out on Si wafers containing 10 µm-sized microbumps based on the Cu-Sn-In ternary system. Thermodynamic study shows that addition of In enables low-melting temperature metals to reach liquid phase below In melting point (157 °C) and promotes rapid solidification of the intermetallic layer, which are beneficial for achieving low-temperature bonding. Microstructural observation shows high bonding quality with low amount of defect. SEM and TEM characterization concludes that a single-phase intermetallic formed in the bond and identified as Cu6(Sn,In)5 with a hexagonal lattice. Mechanical tensile test indicates that the bond has a mechanical tensile strength of 30 MPa, which are adequate for 3D heterogeneous integration.
Design and characterization of in-plane piezoelectric-actuated microcantilevers
(2021-08-24) Nieminen, Tarmo
Kemian tekniikan korkeakoulu | Master's thesis
This thesis set out to study the deflection of in-plane piezoelectric-actuated mi- crocantilevers through finite element method modelling, and to create a method for measuring the in-plane motion. Piezoelectric microelectromehcanical systems (MEMS) are potential improvement on electrostatic MEMS. In-plane piezoelectric- actuated microcantilever is a novel piezoelectric MEMS for high amplitude resonators. These structures create complicated two dimensional in-plane motion and currently no good method exists for characterizing this motion. The modelling showed that the in-plane deflection of the microcantilevers is in the range of hundreds of nanometers, and over 60 times larger than the out-of-plane deflection. An algorithm, called the blurred image, based on approximating the deflection from motion blur, was developed to study the in-plane deflection. The algorithm was able to detect complicated two dimensional motion of a metronome to a high accuracy. The results of the modelling show that the sidewall cantilevers are excellent at creating in-plane deflection. However, these results should be confirmed by manufacturing them and testing them in practice. The blurred image algorithm gave very promising results about its capabilities in measuring complicated in-plane motion, but the algorithm should be tested with nanometer scale motion.
Detection of In-Plane Movement in Electrically Actuated Microelectromechanical Systems Using a Scanning Electron Microscope
(2023-03-22) Nieminen, Tarmo; Tiwary, Nikhilendu; Ross, Glenn; Paulasto-Kröckel, Mervi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
The measurement of in-plane motion in microelectromechanical systems (MEMS) is a challenge for existing measurement techniques due to the small size of the moving devices and the low amplitude of motion. This paper studied the possibility of using images obtained using a scanning electron microscope (SEM) together with existing motion detection algorithms to characterize the motion of MEMS. SEM imaging has previously been used to detect motion in MEMS device. However, the differences in how SEM imaging and optical imaging capture motion, together with possible interference caused by electrical actuation, create doubts about how accurately motion could be detected in a SEM. In this work, it is shown that existing motion detection algorithms can be used to detect movement with an amplitude of 69 nm. In addition, the properties of SEM images, such as bright edges, complement these algorithms. Electrical actuation was found to cause error in the measurement, however, the error was limited to regions that were electrically connected to the actuating probes and minimal error could be detected in regions that were electrically insulated from the probes. These results show that an SEM is a powerful tool for characterizing low amplitude motion and electrical contacts in MEMS and allow for the detection of motion under 100 nm in amplitude.
Electromigration Reliability of Cu3Sn Microbumps for 3D Heterogeneous Integration
(2024) Tiwary, Nikhilendu; Grosse, Christian; Kögel, Michael; Windemuth, Thilo; Ross, Glenn; Vuorinen, Vesa; Brand, Sebastian; Paulasto-Kröckel, Mervi
A4 Artikkeli konferenssijulkaisussa
3D heterogeneous integration (HI) and advanced packaging (AP) technologies require small volume, high-density interconnects for stacking discrete chips for which the reliability of interconnects becomes crucial. Intermetallic compounds (IMCs) based μbumps have been shown to outperform solder-based μbumps concerning their resistance to electromigration (EM) related failures, which is a key index to assess the interconnect reliability. Cu-Sn solid-liquid interdiffusion (SLID) bonding is an attractive low-cost wafer-level bonding technology for rapid manufacturing of full Cu3Sn IMC μbumps, However, SLID requires melting of Sn during the bonding process which poses risks and design challenges in manufacturing. Due to Sn squeeze-out during the bonding process, Sn melt could react with redistribution layers (RDLs) or metallization layers and form IMCs at undesired locations resulting in early failures thereby compromising the reliability. The Sn-squeeze out issue during bonding is addressed in this work by designing test structures with equal and unequal lateral dimensions of μbumps in the top and bottom wafers. The effects of Sn-squeeze out on the EM resistance and reliability are compared in both designs. Significant improvement in the Sn-squeeze out and corresponding EM resistance was observed in the test structures manufactured with unequal lateral dimensions of μbumps in the top and bottom wafers. FE element simulations were carried out to gain insights and assess the impact of Sn squeeze-out on the reliability and functionality of the Cu3Sn μbumps.
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.
Impact of Inherent Design Limitations for Cu–Sn SLID Microbumps on Its Electromigration Reliability for 3D ICs
(2023-01-01) Tiwary, Nikhilendu; Ross, Glenn; Vuorinen, Vesa; Paulasto-Kröckel, Mervi
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Continuous scaling of package architectures requires small volume and high-density microbumps in 3D stacking, which often result in solders fully transforming to intermetallic compounds (IMCs). Cu-Sn solid-liquid interdiffusion (SLID) bonding is an attractive technology where the μ bumps are fully composed of IMCs. In this work, test structures made up of Cu3Sn IMC μ bump with a lateral dimension of 25 μm × 25 μm and 50 μm × 50 μm, respectively, were manufactured on a pair of 4-inch Si wafers demonstrating wafer-level bonding capability. Electromigration (EM) tests were performed for accelerated conditions at a temperature of 150 °C for various current densities ranging from ≈2 × 104 to 1 × 105 A/cm2. Scanning electron microscopy (SEM) and elemental dispersive spectroscopy (EDS) were employed to characterize the as-fabricated test structures. Due to Sn squeeze out, Cu3Sn was formed at undesired location at the upper Cu trace. Both nondestructive [lock-in thermography (LiT)] and destructive techniques were employed to analyze the failure locations after EM tests. It was observed that the likelihood of failure spots is the current crowding zone along the interconnects in 3D architectures, which gets aggravated due to the formation of Cu3Sn in undesirable locations. Thermal runaway was observed even in Cu3Sn, which has been shown to be EM-resistant in the past, thus underlining inherent design issues of μ bumps utilizing SLID technology.
Low-temperature Metal Bonding for Optical Device Packaging
(2021-11-13) Golim, Obert; Vuorinen, Vesa; Tiwary, Nikhilendu; Ross, Glenn; Paulasto-Kröckel, Mervi
A4 Artikkeli konferenssijulkaisussa
Low-temperature solid-liquid interdiffusion (SLID) bonding is an attractive alternative for the packaging of optical devices. It reduces global residual stress build up caused by differences in coefficient of thermal expansion (CTE) at elevated temperatures. This work applied the Cu-Sn-In-based SLID bonding method to bond silicon and optically transparent materials at 200 °C. Experimental results show a successful bonding with minor unavoidable misalignment from the CTE mismatch and major misalignment from the bonding alignment process. Microstructural analysis shows the intermetallic compound consists only of Cu6(Sn,In)5 on the bond that is thermally stable up to 600 °C