Samuel Graham

Bio: Samuel Graham is an academic researcher from Georgia Institute of Technology. The author has contributed to research in topics: Thermal conductivity & Thermal resistance. The author has an hindex of 48, co-authored 347 publications receiving 9774 citations. Previous affiliations of Samuel Graham include Merck & Co. & United States Military Academy.

Integrated Copper Heat Slugs and EMI Shields in Panel Laminate (LFO) and Glass Fanout (GFO) Packages for High Power RF ICs

Abstract: This paper demonstrates, for the first time, ultra-thin, panel laminate fan-out (LFO) and glass fan-out (GFO) packages with embedded copper heat spreaders and electromagnetic shields for packaging high-power RF ICs in much smaller form factors and at potentially much lower cost than current ceramic and metal flange packages. This unique double-sided package addresses the thermal dissipation requirements of 30-100W power amplifiers by bonding the IC directly to a large copper heat spreader embedded in the substrate, using high thermal conductivity die-attach paste. It also addresses the RF, microwave and mm-wave performance requirements by utilizing low-loss tangent glass and polymer dielectrics, as opposed to lossy epoxy dielectrics or mold compounds. The combination of glass and high temperature polymers also enables superior harsh environment reliability with built-in stress buffer layers to mitigate the CTE mismatch induced stresses from large copper thermal structures.

Atomic layer deposited Al2O3 capping layer effect on environmentally assisted cracking in SiNx barrier films

Abstract: We investigated the effect of an atomic-layer-deposited alumina (Al2O3) capping layer (2 or 10 nm thick) on the environmentally assisted cracking (EAC) properties of 250-nm-thick, plasma-enhanced-chemical-vapor-deposited silicon nitride (SiNx) barrier films on polyethylene terephthalate polymer substrates, using in situ optical microscopy tensile tests and numerical modeling. The 10-nm-thick capping layer resulted in a 5% decrease in crack onset strain, corresponding to the cracking of the Al2O3/SiNx bi-layer. Even though the Al2O3 layer itself is immune to EAC, its use as a capping layer did not significantly improve the mechanical reliability of the Al2O3/SiNx bi-layer under strain in ambient conditions, except for a minor 30%-50% increase in the driving force threshold required to induce crack growth. An effective capping layer should remain un-cracked during the cracking of the underlying SiNx, and a parametric study showed that it was not possible with alumina. A high fracture energy, low elastic modulus (e.g., organic material) layer is required such that cracking only occurs in the SiNx layer, presumably expected to protect SiNx from EAC degradation.

Thermal Management of β -Ga₂O₃ Current Aperture Vertical Electron Transistors

Abstract: Beta-gallium oxide ( $\beta $ -Ga2O3) has attracted considerable attention for power devices due to its superior properties and the availability of device-quality native substrates compared to gallium nitride (GaN) technologies. In particular, devices such as the current aperture vertical electron transistor (CAVET) have a higher breakdown voltage compared to lateral transistors made from $\beta $ -Ga2O3. However, because of the low thermal conductivity of $\beta $ -Ga2O3, thermal management strategies at the device level are required in order to achieve high power operation. Here, we present a thermal modeling study of CAVET power transistors and analyze the impact of thermal management strategies on their thermal performance. Among the various cooling strategies, double-side cooling has the largest impact on device cooling. Double-side cooling in combination with a heat spreader can suppress the device’s thermal resistance from 24.5 to 4.86 mm $\cdot ^{\circ }\text{C}$ /W, allowing for a high-power-density CAVET device. The modeling and analysis results presented in this work can be utilized as a guide for improvement of the vertical $\beta $ -Ga2O3 device performance for future power electronics applications.

Experimental considerations of CVD diamond film measurements using time domain thermoreflectance

Abstract: Diamond has the highest known thermal conductivity of any known bulk material, but the properties of synthetic diamond films often fall far short of this high level. The DARPA program Thermal Transport in Diamond Films for Electronics Thermal Management brings together researchers from five universities to comprehensively characterize the thermal transport and material properties of CVD diamond thin films in an effort to better how to further improve the thermal transport properties and understand how accurately these properties can be measured using time domain thermoreflectance and Raman spectroscopy. Here we summarize the results of the thermal measurements of diamond conducted via time domain thermoreflectance (TDTR) using two different systems and discuss some difficulties of accurately measuring the thermal conductivity of micron-thick anisotropic films that often have high surface roughness. We also report that in certain cases the thermal conductivity and thermal boundary conductance of CVD diamond films has been improved to the point of making them highly attractive for thermal management of high power electronic devices.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Interface engineering of highly efficient perovskite solar cells

Abstract: Advancing perovskite solar cell technologies toward their theoretical power conversion efficiency (PCE) requires delicate control over the carrier dynamics throughout the entire device. By controlling the formation of the perovskite layer and careful choices of other materials, we suppressed carrier recombination in the absorber, facilitated carrier injection into the carrier transport layers, and maintained good carrier extraction at the electrodes. When measured via reverse bias scan, cell PCE is typically boosted to 16.6% on average, with the highest efficiency of ~19.3% in a planar geometry without antireflective coating. The fabrication of our perovskite solar cells was conducted in air and from solution at low temperatures, which should simplify manufacturing of large-area perovskite devices that are inexpensive and perform at high levels.

An electron acceptor challenging fullerenes for efficient polymer solar cells.

Abstract: A novel non-fullerene electron acceptor (ITIC) that overcomes some of the shortcomings of fullerene acceptors, for example, weak absorption in the visible spectral region and limited energy-level variability, is designed and synthesized. Fullerene-free polymer solar cells (PSCs) based on the ITIC acceptor are demonstrated to exhibit power conversion effi ciencies of up to 6.8%, a record for fullerene-free PSCs.