Michael Lee McLain

Modeling ionizing radiation effects in solid state materials and CMOS devices

A comprehensive model is presented which enables the effects of ionizing radiation on bulk CMOS devices and parasitic structures to be simulated with closed form functions. The model adapts general equations for defect formation in uniform SiO2 films to facilitate analytical calculations of trapped charge and interface trap buildup in radiation sensitive shallow trench isolation (STI) oxides. An approach whereby defect distributions along the bottom and sidewall of the STI are calculated, incorporated into implicit surface potential equations, and ultimately used to model radiation-induced leakage currents in MOSFET structures and integrated circuits is described. The results of the modeling approach are compared to experimental data obtained on 130 and 90 nm devices and circuits. The features having the greatest impact on the increased radiation tolerance of advanced deep-submicron bulk CMOS technologies are also discussed. These features include increased doping levels along the STI sidewall.

Enhanced TID Susceptibility in Sub-100 nm Bulk CMOS I/O Transistors and Circuits

This paper evaluates the radiation responses of 2.5 V I/O transistors and regular-threshold MOSFETs from a 90 nm commercial bulk CMOS technology. The data obtained from Co ionizing radiation experiments indicate enhanced TID susceptibility in I/O devices and circuits, which is attributed to the p-type body doping. A quantitative model is used to analyze the effects of doping and oxide trapped charge buildup along the sidewall of the shallow trench isolation oxide. These effects are captured in the general electrostatic equation for surface potential, which can be correlated to off-state leakage current. Device simulations are used in concert with experimental measurements and the analytical model to provide physical insight into the radiation response of each device type.

Total ionizing dose effects in shallow trench isolation oxides

The peaked evolution of leakage current with total ionizing dose observed in transistors in 130 nm generation technologies is studied with field oxide field effect transistors (FOXFETs) that use the shallow trench isolation as gate oxide. The overall radiation response of these structures is determined by the balance between positive charge trapped in the bulk of the oxide and negative charge in defect centers at its interface with the silicon substrate. That these are mostly interface traps and not border traps is demonstrated through dynamic transconductance and variable-frequency charge-pumping measurements. These interface traps, whose formation is only marginally sensitive to the bias polarity across the oxide, have been observed to anneal at temperatures as low as 80 °C. At moderate or low dose rate, the buildup of interface traps more than offsets the increase in field oxide leakage due to oxide-trap charge. Consequences of these observations for circuit reliability are discussed.

Gate-Length and Drain-Bias Dependence of Band-to-Band Tunneling-Induced Drain Leakage in Irradiated Fully Depleted SOI Devices

The effects of gate length and drain bias on the off-state drain leakage current of irradiated fully-depleted SOI n-channel MOSFETs are reported. The experimental results are interpreted using a model based on the combined effects of band-to-band tunneling (BBT) and the trapped charge in the buried oxide. For negative gate-source voltages, the drain leakage current increases with the drain voltage because the electric field in the gate-to-drain overlap region is increasing. The off-state current in these devices increases with total ionizing dose due to oxide trapped charge build up in the buried oxide, enhanced by the BBT mechanism. The experimental data show that these effects are more significant for devices with shorter gate-lengths. Simulation results suggest that the BBT-generated holes are more likely to drift all the way from the drain to the source in shorter devices, enhancing the drain leakage current, while they tend to tunnel across the gate oxide in longer devices.

Modeling the Radiation Response of Fully-Depleted SOI n-Channel MOSFETs

A continuous analytical model for radiation-induced degradation in fully-depleted (FD) silicon on insulator (SOI) n-channel MOSFETs is presented. The combined effects of defect buildup in the buried oxide and band-to-band tunneling (BBT) have been shown to be the primary mechanisms that determine the radiation effects on the electrical characteristics. Closed-form expressions for the front and back-gate surface potential incorporate these effects, thereby enabling accurate modeling of the degraded current voltage characteristics that result from ionizing radiation exposure.

Reliability of high performance standard two-edge and radiation hardened by design enclosed geometry transistors

It was recently shown that radiation hardened by design (RHBD) annular-gate MOSFETs not only provide totaldose radiation tolerance, but can also improve the hot-carrier reliability of advanced CMOS circuits. In this paper, the hotcarrier reliability of standard two-edge and enclosed geometry transistors intended for use in space and strategic environments is demonstrated. Hot-carrier reliability measurements on standard two-edge, standard enclosed, gate under-lap enclosed, and annular transistors fabricated in the same 90 nm high performance technology indicate an improvement in hot-carrier lifetime in the enclosed geometry and multi-finger transistor designs when compared to a conventional single stripe MOSFET. Two-dimensional device simulations, along with experimental measurements, provide physical insight into the reliability response of each device type.

Analytical model of radiation response in FDSOI MOSFETS

It was recently shown that band-to-band tunneling (BBT), in combination with trapped charge buildup in the buried oxide, affects the radiation response in some fully-depleted silicon-on-insulator (FDSOI) MOSFET technologies. In this paper, an analytical model for these radiation response characteristics is proposed. The charge coupling between the front and back gates is demonstrated analytically using closed-form expressions for the back-gate threshold voltage as a function of trapped charge in the buried oxide and front gate voltage.

The susceptibility of TaOx-based memristors to high dose rate ionizing radiation and total ionizing dose

This paper investigates the effects of high dose rate ionizing radiation and total ionizing dose (TID) on tantalum oxide ( TaOx) memristors. Transient data were obtained during the pulsed exposures for dose rates ranging from approximately 5.0 ×107 rad(Si)/s to 4.7 ×108 rad(Si)/s and for pulse widths ranging from 50 ns to 50 μs. The cumulative dose in these tests did not appear to impact the observed dose rate response. Static dose rate upset tests were also performed at a dose rate of ~ 3.0 ×108 rad(Si)/s. This is the first dose rate study on any type of memristive memory technology. In addition to assessing the tolerance of TaOx memristors to high dose rate ionizing radiation, we also evaluated their susceptibility to TID. The data indicate that it is possible for the devices to switch from a high resistance off-state to a low resistance on-state in both dose rate and TID environments. The observed radiation-induced switching is dependent on the irradiation conditions and bias configuration. Furthermore, the dose rate or ionizing dose level at which a device switches resistance states varies from device to device; the enhanced susceptibility observed in some devices is still under investigation. Numerical simulations are used to qualitatively capture the observed transient radiation response and provide insight into the physics of the induced current/voltages.

Mapping of Radiation-Induced Resistance Changes and Multiple Conduction Channels in

The locations of conductive regions in TaOx memristors are spatially mapped using a microbeam and Nanoimplanter by rastering an ion beam across each device while monitoring its resistance. Microbeam irradiation with 800 keV Si ions revealed multiple sensitive regions along the edges of the bottom electrode. The rest of the active device area was found to be insensitive to the ion beam. Nanoimplanter irradiation with 200 keV Si ions demonstrated the ability to more accurately map the size of a sensitive area with a beam spot size of 40 nm by 40 nm. Isolated single spot sensitive regions and a larger sensitive region that extends approximately 300 nm were observed.