study of single event effects by ultra-fast pulsed laser system
Date
2020-07-29
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ORCID
Type
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Degree Level
Doctoral
Abstract
Since early 60s of the last century, single event effects (SEEs) caused by particles from cosmic rays and package materials have been observed and studied in both cosmic and terrestrial environments. Driven by the Moore's Law, the feature size of transistors has been scaled down to 5 nm over the past 50 years along with the lower operating voltage and higher switching frequency, which make modern integrated circuits (ICs) more vulnerable to SEEs. In a well-designed IC, soft errors (transient and temporary errors) induced by SEEs appear to be the most troublesome both in terrestrial and high altitude or space environments. The soft-error rates caused by SEEs can be ten to thousand times higher than typical hard failure rates in ICs. For example, early in year 2000 the unusual high soft error rates in Sun Microelectronics Inc's flagship servers have caused tens of million dollars losses and brand image damages. Simulation models and tools have been developed to simulate and analyze SEEs and various design approaches were introduced to mitigate SEEs. The effectiveness of the designs and accuracy of the simulation models have to be verified with experimental results. Ion accelerator experiments using heavy ion, protons, or neutrons are traditional experimental methods for SEE testing. However, ion beams are broad and cannot provide temporal and physical information of the ion hits, which is critical to study and understand SEEs in the devices. In addition, accessing accelerators is generally expensive and difficult. There is always a demand to have a lower cost and more accessible facility for radiation effects research.
As early as the 1960s, researchers had revealed that a pulsed laser beam could generate extra electron-hole pairs (EHPs) in silicon. Since the 1980s, pulsed laser facilities have been used to induce SEEs in ICs, which provided a complementary tool for studying SEEs. Pulsed lasers can induce SEEs in ICs at a particular time and location. This brings significant benefits in studying the sensitive areas in an IC and can be used to validate the SEE simulation results. However, due to the smaller feature size of the transistors (in nanometer scales) and larger silicon chip area (in centimeter scales), it becomes more and more difficult to obtain the precise results from the advanced ICs laser experiments due the limit of the laser wavelength and other parameters. This research is to develop an ultra-fast laser facility and study the SEEs in advanced ICs. The facility includes the laser sources and a universal SEE sensitivity mapping system that has resolution high enough to scan a single SRAM cell with the dimension of about 1×1 µm, and velocity fast enough to scan the entire area of an operational amplifier of about 1×1 mm.
To provide the laser pulses for the final mapping system, an ultra-fast (in the femtosecond scale) pulsed laser system was built in the laboratory with some characteristics superior to similar laser systems. This ultra-fast pulsed laser facility can project a laser beam into a specifically drawn region of interest (ROI) with high scanning velocity. In addition, this system has a high output energy as well as the ability to monitor and control the laser energy precisely. Furthermore, a SEE sensitivity mapping system was developed based on this laser system, which can generate a real-time sensitivity map with the same size as a real ROI that was drawn by an operator for both analog and digital devices.
SEE sensitivity mapping experiments based on single SRAM cells are carried out after the completion of the laser and mapping system development. The size of the SRAM cells is only about 1×1 µm, so the system must have small enough spatial resolution to depict the distribution of SEE sensitivity inside that ROI. Two different SRAM cell structures were used in the testing, Quatro_10T and Regular_11T. The results show that the SEE sensitivity distribution can be depicted clearly for both of these two cells, even though the laser spot size is bigger than the ROI size in the testing. Furthermore, the laser testing results are verified by TFIT simulation results, which confirms the laser facility has enough spatial resolution to scan the SRAM cells and generate sensitivity maps. The results are significant since this is the first experiment which demonstrated that pulsed laser can achieve submicron resolution when generating SEE sensitivity maps of ICs, and the results can be used to experimentally validate the simulation results from software, which was a bottleneck in this research area.
SEE sensitivity mapping system is also used to test operational amplifiers (op-amps), which occupies much larger silicon areas, e.g. in millimeter scale or larger. Therefore, the scanning velocity is of much greater consideration than the spatial resolution. The large area means hundreds, or thousands of steps are required to scan the entire operational amplifier area. In the proposed system, the related sensitivity mapping results can be attained in less than 2 minutes. The mapping testing is based on two commercial operational amplifiers, AD844 and OP77. The mapping results have been analyzed and verified with the oscilloscope signals.
In conclusion, a SEE ultra-fast laser system is built that has the abilities of monitoring the laser pulse amplitude regularity, precisely controlling the laser energy, projecting the laser pulses into ROIs, generating real-time SEE sensitivity distribution maps, and obtaining real-time images of device under test (DUT) by an imaging laser. This laser system has been used to generate SEE sensitivity distribution maps with high spatial resolution in one single SRAM cell and with high scanning velocity in commercial op-amps. The developed mapping system has a high scanning velocity and fine resolution of less than half of one micron. Confirmed by experimental results on SRAM cells and operational amplifiers, the laser SEE mapping system demonstrated that it can be an effective tool to study SEE sensitivity distribution in advanced ICs.
Description
Keywords
single event effects, ultra-fast laser system, mapping technology, SRAM, opterational amplifier
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Electrical and Computer Engineering
Program
Electrical Engineering