Daniel Nilsen Wright

Boron-Implanted 3C-SiC for Intermediate Band Solar Cells

Sublimation-grown 3C-SiC crystals were implanted with 2 atomic percent of boron ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 8.5×1016 atoms/cm2. The samples were then annealed at 1400, 1500 and 1600 °C for 1h at each temperature. The buried boron box-like concentration profile can reach ~2×1021 cm-3 in the plateau region. The optical activity of the incorporated boron atoms was deduced from the evolution in absorption and emission spectra, indicating possible pathway for achieving an intermediate band behavior in boron doped 3C-SiC at sufficiently high dopant concentrations.

Bending machine for testing reliability of flexible electronics

A novel bending machine has been designed and tested. It enables flexible electronics to be subjected to repeated bending with constant radius and tension. In-situ electrical characterization can give accurate analysis of lifetime distributions if sufficiently many samples are ran to failure, allowing reliability prediction models to be developed. Four sets of test samples with different combinations of substrate, routing, interconnect technology and components were examined. A poor level of reliability was observed when using anisotropic conductive paste to form interconnects, whereas a significantly higher level of reliability was observed when using a bismuth-tin solder paste. The assembly of larger components resulted in shortened time to failure, whereas increasing the bending radius prolonged the observed lifetimes.

Characterization of B-implanted 3C-SiC for intermediate band solar cells

Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×1017 atoms/cm2 in order to form intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 °C for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ~3×1021 cm-3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ~1.7-1.8 eV, which may be associated with B-precipitates having a sufficiently high density.

Reliability of plastic core solder balls in relation to formation of intermetallic compounds

The application of plastic core solder balls (PCSB) for the assembly of a silicon (Si) interposer die inside a ceramic package has been evaluated for a harsh application. Three different variants of assembly were compared. All variants survived the performed thermal shock cycling and centrifugation. Failures observed after thermal shock cycling indicated a reduced risk of crack formation for Ag-rich solder systems. Roughly 50% of each variant survived a performed application test. Significantly different failure modes were observed after thermal shock cycling compared to after the application test. Individual qualification tests are therefore recommended to evaluate reliability with respect to storage and application.

Thermomechanical reliability of gold stud bump bonding for large volume MEMS devices

Reliability of gold stud bump bonding must be questioned once process parameters are tweaked and dimensions shrunk to minimize costs for high volume MEMS assembly. 3D mock-up stacks representing a 0.5 x 1.0 mm MEMS die and its read-out integrated circuit were built using thermosonic and thermocompression chip-to-wafer bonding (TSB and TCB) with parameters targeting an optimized throughput. Electrical test circuits, daisy chains and Kelvin structures, were included in the design. Mechanical and electrical characterization was performed on samples directly after bonding and after environmental stressing in the form of thermal cycling and high temperature storage. Bond strength in the range of 40-60 MPa was measured after bonding. No degradation was observed after thermal cycling and an increased strength was measured after high temperature storage. Tilt in the range of 2.2 - 6.6 μm measured using white light interferometry was concluded to be a root cause of a limited yield for TSB. Fractography after shear testing, visual inspection of cross sections and electrical testing supported the hypothesis. On the other hand, tilt was measured to be in the range of 0.3 - 1.3 μm for TCB and this was concluded to be acceptable based on the same inspections and also based on stability of electrical properties measured in situ during environmental stressing. Tilt can be reduced by engineering of bond heads.

Metal coated polymer spheres for compliant fine pitch ball grid arrays

The further development of heterogeneous 3D integration involving electrical connections between layers of various materials demand compliant fine pitch interconnects to allow for the varying expansion as a function of temperature of the different materials involved. However, most available fine pitch interconnects, like micro bumps and copper pillars, are not particularly compliant whereas available compliant interconnects, like plastic core solder balls, are not fine pitch. In this work we present a novel process for achieving a fine pitch ball grid array comprised of compliant interconnects using singular 30 μm metal coated polymer spheres (MPS) in conjunction with nano-particle silver conductive ink. Results show that when an MPS attached to a chip is flip chip assembled onto a substrate with a 1.3 μm thick layer of wet conductive ink, the ink is drawn up along the MPS surface to such an extent that it a forms a bonding neck between the MPS and the chip surface. It also created a coating up to 1.5 μm on the MPS. Shear testing of the assembly gave values of up to 0.67 grams per MPS. This strength can be increased further by optimizing the process and to utilize such an interconnect in combination with underfill can result in sufficient reliability for several applications. The volume of ink required to form such bonds is in the same range as industrial ink jet nozzles can deliver. MPS was applied onto a substrate which had been jetted with a regular pattern of ink droplets. It was found that the MPS were observed to have had a certain extent of self-alignment along the jetted pattern.

A 2.5-D Integrated Data Logger for Measuring Extreme Accelerations

A very compact and rugged 2.5-D integrated data logger has been built and tested. The data logger is capable of measuring accelerations exceeding 70 000 g. Microcontroller and flash memory as bare dies have been mounted onto a silicon interposer with through silicon vias using anisotropic conductive film and Au stud bump bonding. A microelectromechanical system accelerometer is mounted onto the interposer, using a robust customized flip-chip mounting approach. The interposer is mounted into a 16-pin leadless chip carrier package using isotropic conductive adhesive, where the conductive part is made of metallized polymer spheres. The ceramic package was mounted onto an application printed circuit board (PCB) with filters, power management, and an interface contact, using soldered plastic core solder balls (PCSBs). The diameter of the data logger is less than 9 mm, and the height is approximately 5 mm. The data logger fits within 12.7-mm (0.50 cal.) projectile, and acceleration measurements have been performed during firing, flight, and recovery. The measured accelerations have been verified by comparing the calculated projectile muzzle velocities with Doppler radar measurements.

Soldering of components onto a smart tag - an evaluation of the process window

An experiment was designed using optimal design of experiments to identify the most relevant parameters in an assembly step for a smart tag; the soldering of 0402 resistors to a polyimide backplane. The parameter window was set to reflect natural variations in production, and parameter variations were investigated to ascertain whether these variations could lead to unacceptable values of electrical resistance or shear-strength of the soldered joints. All tests were performed before and after exposure to hygrothermal aging, i.e. exposure to 85 °C in combination with 85 % RH, and/or cyclic bending. All test results gave values well within the predefined failure criteria. It could therefore be concluded that all of the tested combinations of parameters spanning the tested process window can be considered as safe variations for the assembly of 0402 resistors in the production of the smart tags. The parameters that influenced the electrical resistance and shear-strength of these assemblies were identified.