Nicholas Allec

ThermalScope: multi-scale thermal analysis for nanometer-scale integrated circuits

Thermal analysis has long been essential for designing reliable, high-performance, cost-effective integrated circuits (ICs). Increasing power densities are making this problem more important. Characterizing the thermal profile of an IC quickly enough to allow feedback on the thermal effects of tentative design changes is a daunting problem, and its complexity is increasing. The move to nanoscale fabrication processes is increasing the importance of quantum thermal phenomena such as ballistic phonon transport. Accurate thermal analysis of nanoscale ICs containing hundreds of millions of devices requires characterization of thermal effects on length scales that vary by several orders of magnitude, from nanoscale quantum thermal effects to centimeter-scale cooling package impact. Existing chip-package thermal analysis methods based on classical Fourier heat transfer cannot capture nanoscale quantum thermal effects. However, accurate device-level modeling techniques, such as molecular dynamics methods, are far too slow for use in full-chip IC thermal analysis. In this work, we propose and develop ThermalScope, a multi-scale thermal analysis method for nanoscale IC design. It unifies microscopic and macroscopic thermal physics modeling methods, i.e., the Fourier and Boltzmann transport modeling methods. Moreover, it supports adaptive multi-resolution modeling. Together, these ideas enable efficient and accurate characterization of nanoscale quantum heat transport as well as chip-package level heat flow. ThermalScope is designed for full-chip thermal analysis of billion-transistor nanoscale IC designs, with accuracy at the scale of individual devices. ThermalScope enables accurate characterization of temperature-related effects, such as variation in leakage power and delay. ThermalScope has been implemented in software and used for full-chip thermal analysis and temperature-dependent leakage analysis of an IC design with more than 150 million transistors. It will be publicly released for free academic and personal use.

Multiscale Thermal Analysis for Nanometer-Scale Integrated Circuits

Thermal analysis has long been essential for designing reliable high-performance cost-effective integrated circuits (ICs). Increasing power densities are making this problem more important. Characterizing the thermal profile of an IC quickly enough to allow feedback on the thermal effects of tentative design changes is a daunting problem, and its complexity is increasing. The move to nanometer-scale fabrication processes is increasing the importance of thermal phenomena such as ballistic phonon transport. The accurate thermal analysis of nanometer-scale ICs containing hundreds of millions of devices requires characterization of heat transport across multiple length scales. These scales range from the nanometer scale (device-level impact) to the centimeter scale (cooling package impact). Existing chip-package thermal analysis methods based on classical Fourier heat transfer cannot capture nanometer-scale thermal effects. However, accurate device-level modeling techniques, such as molecular dynamics methods, are far too slow for use in full-chip IC thermal analysis. In this paper, we propose and develop ThermalScope, a multiscale thermal analysis method for nanometer-scale IC design. It unifies microscopic and macroscopic thermal modeling methods, i.e., the Boltzmann transport equation and Fourier modeling methods. Moreover, it supports adaptive multiresolution modeling. Together, these ideas enable the efficient and accurate characterization of nanometer-scale heat transport as well as the chip-package-level heat flow. ThermalScope is designed for full-chip thermal analysis of billion-transistor nanometer-scale IC designs, with accuracy at the scale of individual devices. ThermalScope enables the accurate characterization of various temperature-related effects, such as temperature-dependent leakage power and temperature-timing dependences. ThermalScope has been implemented in software and used for the full-chip thermal analysis and temperature-dependent leakage analysis of an IC design with more than 150 million transistors. ThermalScope will be publicly released for free academic and personal use.

Low Dark-Current Lateral Amorphous-Selenium Metal–Semiconductor–Metal Photodetector

We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal detector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-current-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the high dark current of previously reported lateral a-Se detectors. The proposed detector demonstrates the feasibility of low-cost lateral a-Se devices for indirect conversion digital X-ray imaging applications such as chest radiography, fluoroscopy, and computed tomography.

Spatiotemporal Monte Carlo transport methods in x-ray semiconductor detectors: Application to pulse-height spectroscopy in a-Se

PURPOSE The authors describe a detailed Monte Carlo (MC) method for the coupled transport of ionizing particles and charge carriers in amorphous selenium (a-Se) semiconductor x-ray detectors, and model the effect of statistical variations on the detected signal. METHODS A detailed transport code was developed for modeling the signal formation process in semiconductor x-ray detectors. The charge transport routines include three-dimensional spatial and temporal models of electron-hole pair transport taking into account recombination and trapping. Many electron-hole pairs are created simultaneously in bursts from energy deposition events. Carrier transport processes include drift due to external field and Coulombic interactions, and diffusion due to Brownian motion. RESULTS Pulse-height spectra (PHS) have been simulated with different transport conditions for a range of monoenergetic incident x-ray energies and mammography radiation beam qualities. Two methods for calculating Swank factors from simulated PHS are shown, one using the entire PHS distribution, and the other using the photopeak. The latter ignores contributions from Compton scattering and K-fluorescence. Comparisons differ by approximately 2% between experimental measurements and simulations. CONCLUSIONS The a-Se x-ray detector PHS responses simulated in this work include three-dimensional spatial and temporal transport of electron-hole pairs. These PHS were used to calculate the Swank factor and compare it with experimental measurements. The Swank factor was shown to be a function of x-ray energy and applied electric field. Trapping and recombination models are all shown to affect the Swank factor.

Fast Lateral Amorphous-Selenium Metal–Semiconductor–Metal Photodetector With High Blue-to-Ultraviolet Responsivity

A lateral metal-semiconductor-metal (MSM) photodetector with a layer of thin amorphous selenium (a-Se) is investigated to detect visible photons. A comparative study on detectors with different a-Se thicknesses is conducted. The thinner detector tends to have better optoelectronic performance, having a low dark current in the range of 40-180 fA under various lateral biases over 1000 s, high responsivity up to ~0.45 A/W toward a short wavelength of 468 nm, and high speed of photoresponse up to 2 kHz with signal rise time of 50 μs and fall time of 60 μs. The fast lateral a-Se MSM photodetector thus has enormous potential to be used in a variety of optical sensing applications, particularly in large-area digital indirect-conversion X-ray imaging.

SEMSIM: Adaptive Multiscale Simulation For Single-Electron Devices

Single-electron devices have drawn much attention in the last two decades. They have been widely used for device research and also show promise as a potential alternative to CMOS circuits due to their ultralow power dissipation. Three techniques have been used for single-electron device modeling in the past, including Monte Carlo (MC), master equation, and SPICE modeling. Among these, MC method provides accuracy, but lacks the time efficiency required for large-scale simulation. In this paper, we introduce an adaptive multiscale approach to single-electron device simulation using MC method as basis, which significantly improves time efficiency while maintaining accuracy. We have shown that it is possible to reduce simulation time up to nearly 40 times and maintain an average error of 3.3%. Going beyond simplistic approximations, we have modeled important secondary effects including cotunneling and Cooper pair tunneling, which are critical for device research.

Monte Carlo simulation of amorphous selenium imaging detectors

We present a Monte Carlo (MC) simulation method for studying the signal formation process in amorphous Selenium (a-Se) imaging detectors for design validation and optimization of direct imaging systems. The assumptions and limitations of the proposed and previous models are examined. The PENELOPE subroutines for MC simulation of radiation transport are used to model incident x-ray photon and secondary electron interactions in the photoconductor. Our simulation model takes into account applied electric field, atomic properties of the photoconductor material, carrier trapping by impurities, and bimolecular recombination between drifting carriers. The particle interaction cross-sections for photons and electrons are generated for Se over the energy range of medical imaging applications. Since inelastic collisions of secondary electrons lead to the creation of electron-hole pairs in the photoconductor, the electron inelastic collision stopping power is compared for PENELOPEs Generalized Oscillator Strength model with the established EEDL and NIST ESTAR databases. Sample simulated particle tracks for photons and electrons in Se are presented, along with the energy deposition map. The PENEASY general-purpose main program is extended with custom transport subroutines to take into account generation and transport of electron-hole pairs in an electromagnetic field. The charge transport routines consider trapping and recombination, and the energy required to create a detectable electron-hole pair can be estimated from simulations. This modular simulation model is designed to model complete image formation.

Direct-Conversion X-Ray Detector Using Lateral Amorphous Selenium Structure

In this paper, we propose to use a lateral metal-semiconductor-metal (MSM) structure with a thick amorphous selenium (a-Se) layer intended for direct-conversion X-ray detection. For the purposes of demonstration, a variety of single-pixel detectors with electrode spacing ranging from 2 to 10 μm were fabricated and characterized. Compared with the vertical structure, the MSM structure avoids the usage of high voltage, therefore eliminating a safety concern. However, the simulation results indicate that the electric field in such a structure is not uniformly distributed and only confined into a region near the bottom electrodes up to a thickness of ~20 μm. The charge collection is therefore undertaken in the bottom layer and the top layer where a majority of energy deposits instead plays a dominant role in charge generation and diffusion. We believe that the lateral MSM detector with thick a-Se will be feasible for direct-conversion X-ray detection.

Lateral amorphous selenium metal-semiconductor-metal photodetector for large-area high-speed indirect-conversion medical imaging applications

Thick amorphous selenium (a-Se) as an excellent photoconductor has been used in direct conversion X-ray imaging modalities such as mammography. However, due to substantial charge trapping, such detectors experience a long X-ray response time and as a result, suffer from a slow speed of operation. Therefore, its deployment to speed-required applications such as real-time fluoroscopy remains a challenge. In this work, we aim to investigate a lateral a-Se MSM photodetector as an indirect conversion X-ray imager and its utilization in high speed, high energy medical applications. The dark current density of the newly-fabricated detector is below 20 pA/mm2 for a 200 μm×50 μm pixel pitch at electric field strengths ranging from 6 to 12 V/μm. The photoresponsivity reaches up to 2.3A/W towards blue wavelength of 468 nm at an electric field strength of 20 V/μm. Furthermore, the photocurrent has a fast speed of photoresponse, demonstrating rise time, fall time and time constant of 50 μs, 60 μs and 30 μs, respectively. Given that low dark current and high photoresponsivity this detector holds, coupled with fast photoresponse, it is believed that lateral a-Se MSM photodetector is promising for indirect conversion X-ray imager integrated with either CMOS or TFT arrays.

Small-angle X-ray scattering method to characterize molecular interactions: Proof of concept

Characterizing biomolecular interactions is crucial to the understanding of biological processes. Existing characterization methods have low spatial resolution, poor specificity, and some lack the capability for deep tissue imaging. We describe a novel technique that relies on small-angle X-ray scattering signatures from high-contrast molecular probes that correlate with the presence of biomolecular interactions. We describe a proof-of-concept study that uses a model system consisting of mixtures of monomer solutions of gold nanoparticles (GNPs) as the non-interacting species and solutions of GNP dimers linked with an organic molecule (dimethyl suberimidate) as the interacting species. We report estimates of the interaction fraction obtained with the proposed small-angle X-ray scattering characterization method exhibiting strong correlation with the known relative concentration of interacting and non-interacting species.