Lauren E. S. Rohwer

Intrinsic broad-band white-light emission by a tuned, corrugated metal-organic framework.

Herein we report on the broad-band direct white-light originating from a single component emitter, namely a novel three-periodic metal-organic framework (MOF). This material features an unprecedented topology with (3,4)-connected nodes. The structure-function relationship in this system is driven by two complementary unique structural features: corrugation and interpenetration. Good correlation between simulated and experimental emission spectra has been attained, resulting in optimized color properties that approach requirements for solid-state lighting (SSL). Guided by the optimized calculated spectra, the tunability of the assembly was proven by the successful in-framework co-doping of Eu(3+). This resulted in significantly improved color properties, opening new paths for the rational design of alternative materials for SSL applications.

Development of solid state light sources based on II-VI semiconductor quantum dots

Solid state light sources based on integrating commercial near-UV LED chips with encapsulated CdS quantum dots are demonstrated. Blue, blue-green, and white quantum dot LEDs were fabricated with luminous efficiencies of 9.8, 16.6, and 3.5 lm/W, respectively. These are the highest efficiencies reported for quantum dot LEDs. Quantum dots have advantages over conventional micron-sized phosphors for solid state lighting, such as strong absorption of near-UV to blue wavelengths, smaller Stokes shift, and a range of emission colors based on their size and surface chemistry. Alkylthiol-stabilized CdS quantum dots in tetrahydrofuran solvent with quantum yields (QYs) up to 70% were synthesized using room temperature metathesis reactions. A variety of emission colors and a white spectrum from monodisperse CdS quantum dots (D~2 nm) have been demonstrated. The white emission was obtained from the CdS quantum dots directly, by altering the surface chemistry. When incorporated into an epoxy, the high solution phase QY was preserved. In contrast to other approaches, the white LED contains monodisperse CdS quantum dots, rather than a blend of different-size blue, green, and red-emitting quantum dots. The concentration of CdS quantum dots in epoxy can be increased to absorb nearly all of the incident near-UV light of the LED.

Encapsulation of Nanoparticles for the Manufacture of Solid State Lighting Devices

Solid state lighting devices that utilize semiconducting nanoparticles (quantum dots) as the sole source of visible light emission have recently been fabricated. The quantum dots in these devices have been demonstrated to have quantum efficiencies similar to those of conventional phosphors. The dispersion and concentration of the nanoparticles within a suitable polymeric matrix was found to be critical to device performance. Yet achieving suitable concentrations and adequate dispersion implies chemical compatibility between the nanoparticles and the matrix, which must be achieved without detrimental effect on either the physical/optical properties of the matrix or the stability/surface state of the quantum dots. A number of encapsulation strategies have been identified and are discussed with regard to their effect on nanoparticle dispersion and concentration within silicone and epoxy matrices.

Wafer Level Micropackaging of MEMS Devices Using Thin Film Anodic Bonding

A wafer level packaging technique that involves anodic bonding of Pyrex wafers to released surface micromachined wafers is demonstrated. Besides providing a hermetic seal, this technique allows full wafer release, provides protection during die separation, and offers the possibility of integration with optoelectronic devices. Anodic bonding was performed under applied voltages up to 1000 V, and temperatures ranging from 280 to 400°C under vacuum (10 -4 Torr). The quality of the bonded interfaces was evaluated using shear strength testing and leak testing. The shear strength of Pyrex-to-polysilicon and aluminum bonds was ∼10-15 MPa. The functionality of surface micromachined polysilicon devices was tested before and after anodic bonding. 100% of thermal actuators, 94% of torsional ratcheting actuators, and 70% of microengines functioned after bonding. The 70% yield was calculated from a test sample of 25 devices.

Thin gold to gold bonding for flip chip applications

We have demonstrated a solderless flip chip bonding process that utilizes electroless nickel / palladium, immersion gold pad metallization. This mask-less process enables higher interconnect densities than can be achieved with standard solder bump reflow. The thin (100nm) immersion gold surfaces were coated with dodecanethiol self-assembled monolayers. Strong gold to gold bonds were formed at 185°C with shear strengths that exceed Mil-Std 883 requirements. Gold stud bumps are also promising for flip chip applications, and can be bonded at 150°C when the gold surfaces are properly pre-treated — dilute piranha solution, argon plasma, and dodecanethiol SAM treatments work equally well.

Optimization of anodic bonding to MEMS with self-assembled monolayer (SAM) coatings

This work describes full wafer encapsulation of released, self-assembled monolayer (SAM) coated, multi-level polysilicon surface micromachines using the anodic bonding technique. This process has been utilized to protect fragile surface micromachines from damage due to particles, moisture contamination, and post-release die handling. The anodic bonding process was optimized to ensure strong glass-to-wafer bonds, while maintaining the effectiveness of liquid-phase and vapor-phase deposited SAM coatings. The temperature, time, and voltage effects on each SAM coating was analyzed. Glass-to-silicon and glass-to-SAM coated silicon had shear strengths of approximately 18 MPa. Glass-to-polysilicon bonds had lower shear strengths of approximately 10 MPa. Bonds were hermetic to 5 X 10-8 atm-cm3/s.

Biocompatible MOFs with high absolute quantum yield for bioimaging in the second near infrared window

Here we detail a study highlighting the correlation between particle size and absolute quantum yield (QY) in novel mixed metal near-infrared (NIR) emitting metal–organic frameworks (MOFs) materials. The nanoscale analogue in this series presents a QY of 6.3%, the highest of any NIR emitting MOFs reported to date.

Platelet Composite Coatings for Tin Whisker Mitigation

Reliable methods for tin whisker mitigation are needed for applications that utilize tin-plated commercial components. Tin can grow whiskers that can lead to electrical shorting, possibly causing critical systems to fail catastrophically. The mechanisms of tin whisker growth are unclear and this makes prediction of the lifetimes of critical components uncertain. The development of robust methods for tin whisker mitigation is currently the best approach to eliminating the risk of shorting. Current mitigation methods are based on unfilled polymer coatings that are not impenetrable to tin whiskers. In this paper we report tin whisker mitigation results for several filled polymer coatings. The whisker-penetration resistance of the coatings was evaluated at elevated temperature and high humidity and under temperature cycling conditions. The composite coatings comprised Ni and MgF2-coated Al/Ni/Al platelets in epoxy resin or silicone rubber. In addition to improved whisker mitigation, these platelet composites have enhanced thermal conductivity and dielectric constant compared with unfilled polymers.

Development of ultra dense edge interconnects for die to die connections based on immersion solder bridging

A high density 2-D integration process that involves linking multiple die along their edges using a linear array of solder bridges was explored. Solder bridging is a versatile approach that is compatible with a range of interconnect geometries and metallizations. We have demonstrated this approach using copper plated nodules that were fabricated on the surface of the die and extend beyond the edge of the die. These nodules were 25 microns (μm) thick with 10, 20, and 50 μm widths. The formation of solder bridges was accomplished using immersion soldering, where the entire part was dipped into a molten solder bath. Due to surface energy effects, the solder selectively wets and flows along all wettable metal surfaces to form a strong solder bond. The solder can even flow across gaps (15 microns).

Laser ablation of polyetheretherketone films for reversible wafer bonding

A reversible wafer bonding method has been developed that enables high-temperature processing of thinned silicon wafers. The silicon wafers are bonded to Pyrex carriers using a polyetheretherketone (PEEK) film, which melts at 343 °C, and provides a very strong bond. Debonding is accomplished by UV laser ablation through the Pyrex carrier and can be facilitated by coating the Pyrex wafer with Teflon. Most of the PEEK film remains on the silicon wafer after debonding and is removed with solvents. Precoating the silicon with germanium/tetraethyl orthosilicate (TEOS) might enable PEEK removal without solvents. This germanium/TEOS layer lifts off with the PEEK film during laser ablation.