Carl L. Axness

Latent interface-trap buildup and its implications for hardness assurance (MOS transistors)

Long-term anneals at temperatures from 25 degrees C to 135 degrees C were performed on irradiated MOS transistors. Following the normal saturation of interface-trap density (within 10/sup 2/ to 10/sup 5/ s after irradiation), large increases in the number of interface traps were observed for both commercial and radiation-hardened transistors at very long times after irradiation (>10/sup 6/ s at 25 degrees ). This latent buildup of interface traps can be significant, up to a factor of four times larger than the normal saturation value. The latent buildup is thermally activated with an activation energy of 0.47+or-0.08 eV. As a natural consequence of the delay between the normal and the latent buildup, there is a window in time in which little or no interface-trap buildup occurs. Two possible mechanisms for the latent buildup are explored: (1) the direct conversion of oxide traps into interface traps or border traps and (2) the diffusion of molecular hydrogen into the gate oxide from adjacent structures. The latent buildup of interface traps can degrade the performance of ICs in space systems and may cause IC failure at long times. Recommendations are provided for characterizing latent interface-trap buildup. >

An SEU Tolerant Memory Cell Derived from Fundamental Studies of SEU Mechanisms in SRAM

A new single event upset (SEU) hardening concept, an LRAM cell, is demonstrated theoretically and experimentally. Decoupling resistors in the LRAM are used only to protect against the short n-channel transient; longer persisting pulses are reduced in magnitude by a voltage divider, a basically new concept for SEU protection. In such a design, smaller resistors provide SEU tolerance, allowing higher performance, hardened memories. As basis for the LRAM idea, techniques were developed to measure time constants for ion induced voltage transients in conventional static random access memories, SRAM. Time constants of 0.8 and 6.3 nsec were measured for transients following strikes at the n- and p-channel drains, respectively--primary areas of SEU sensitivity. These data are the first transient time measurements on full memory chips and the large difference is fundamental to the LRAM concept. Test structures of the new design exhibit equivalent SEU tolerance with resistors 5-to-10 times smaller than currently used in SRAM. Our advanced transport-plus-circuit numerical simulations of the SEU process predicted this result and account for the LRAM experiments, as well as a variety of experiments on conventional SRAM.

SEU simulation and testing of resistor-hardened D-latches in the SA3300 microprocessor

The SEU tolerance of the SA3300 microprocessor with feedback resistors is presented and compared to the SA3300 without feedback resistors and to the commercial version (NS32016). Upset threshold at room temperature increased from 23 MeV-cm/sup 2//mg with no feedback resistors to 60 MeV-cm/sup 2//mg and 180 MeV-cm/sup 2//mg with feedback resistors of 50 k Omega and 160 k Omega , respectively. The performance goal of 10 MHz over the full temperature range of -55 degrees C to +125 degrees C is exceeded for feedback resistors of 160 k Omega and less. Error rate calculations for this design predict that the error rate is less than once every 100 years when 50 k Omega feedback resistors are used in the D-latch design. Analysis of the SEU response using a lumped-parameter circuit simulator imply a charge collection depth of 4.5 mu m. This is much deeper than what one would expect for prompt collection in the epi and funnel regions and has been explained in terms of diffusion current in the heavily doped substrate. >

Single event upset in irradiated 16 K CMOS SRAMs

The single-event-upset (SEU) characteristics of a CMOS SRAM cell irradiated under conditions that simulate the total-dose degradation anticipated in space applications were experimentally and theoretically investigated. Simulations of SEU sensitivity utilizing a 2-D circuit/device simulator, with measured transistor threshold-voltage shifts and mobility degradations as inputs, are shown to be in good agreement with experimental data at high total dose. Both simulation and experiment show a strong SRAM cell SEU imbalance resulting in a more SEU-tolerant preferred state and a less tolerant nonpreferred state. The resulting cell imbalance causes an overall degradation in SEU immunity, which increases with increasing total dose and should be taken into account in SEU testing and part characterization. >

Modeling the time-dependent transient radiation response of semiconductor junctions

Analytical one-dimensional time-dependent photocurrent models are developed from new solutions to the ambipolar transport equation. The p-n junction model incorporates the effects of an electric field in the quasi-neutral region, finite diode length, and an arbitrary generation function g=f(x,t). It provides improved accuracy over the Wirth-Rogers and Enlow-Alexander models. An approximate photocurrent solution for p-n-n/sup +/, n-p-p/sup +/, and p-i-n diode junctions is developed considering high-injection effects. Comparison with experimental data shows that a single set of physical parameters is adequate to characterize the model with respect to dose rate, pulse width, and geometry. >

SEU characterization and design dependence of the SA3300 microprocessor

A detailed characterization is presented of the single-event upset (SEU) sensitivity of the SA3300 microprocessor focusing specifically on the internal general-purpose registers. SEU response is explored as a function of temperature and logic state of the registers. The effects of two different design variations on SEU vulnerability are outlined. Microprobe measurements using a pulsed Nd:YAG laser suggest that the observed pattern dependence for both design revisions is due to bipolar photocurrent in a vertical n/sup +/pn transistor. A slight temperature dependence was observed in both design revisions. This is consistent with the use of oversized restoring transistors to minimize SEU vulnerability rather than polysilicon feedback resistors. More recent data show threshold above 120 MeV-cm/sup 2//mg with 80-k Omega feedback resistors. >

The 2D Steady Hydraulic Head Field Surrounding a Pumping Well in a Finite Heterogeneous Confined Aquifer

We present a second-order analytic solution [in terms of a heterogeneous log-transmissivity Y(r) = ln T(r)] for the hydraulic head field in a finite 2D confined heterogeneous aquifer under steady radial flow conditions assuming fixed head boundary conditions at the well and at a circular exterior boundary. The solution may be used to obtain the gradient used in calculation of solute transport to a well in a heterogeneous transmissivity field. The solution, obtained using perturbation methods coupled with Greens function techniques, leads us to postulate a more general form of the head for arbitrarily large-variance fields and may be used to obtain moment relations between the log-transmissivity and head under convergent flow conditions when Y(r) is expressed as a random space function. We present expressions for the mean head field when the log-transmissivity is Gaussian and conditioned on the transmissivity value at the well for an arbitrary ln T covariance. Finally, we look at the effect of parameter variations on the mean head behavior and present numerical simulations verifying the second-order mean head expressions.

Analytic 1-D

Circuit simulation codes, such as SPICE, are invaluable in the development and design of electronic circuits in radiation environments. These codes are often employed to study the effect of many thousands of devices under transient current conditions. Device-scale simulation codes are commonly used in the design of individual semiconductor components, but computational requirements limit their use to small-scale circuits. Analytic solutions to the ambipolar diffusion equation, an approximation to the carrier transport equations, may be used to characterize the transient currents at nodes within a circuit simulator. We present new analytic transient excess carrier density and photocurrent solutions to the ambipolar diffusion equation for 1-D abrupt-junction pn diodes. These solutions incorporate low-level radiation pulses and take into account a finite device geometry, ohmic fields outside the depleted region, and an arbitrary change in the carrier lifetime due to neutron irradiation or other effects. The solutions are specifically evaluated for the case of an abrupt change in the carrier lifetime during or after, a step, square, or piecewise linear radiation pulse. Noting slow convergence of the Fourier series solutions for some parameters sets, we evaluate portions of the solutions using closed-form formulas, which result in a two order of magnitude increase in computational efficiency.

pn

We transform the n-dimensional ambipolar transport equation to the n-dimensional nonhomogeneous heat equation, which has been solved for most common initial and boundary conditions. Thus, general solutions to the nonhomogeneous heat equation, obtained in a robust form through finite Fourier transforms, provide an easy approach to solving the ambipolar transport equation, which previously had been solved with more difficulty through Laplace transform techniques. We then obtain a general analytic one-dimensional time-dependent solution to the excess carrier and current densities in a pn junction diode in response to a transient radiation or light pulse under low-injection conditions. We derive most of the known analytic solutions to this problem and we examine the limiting behavior of these solutions to show that they are consistent. The model includes the effects of a constant electric field in the quasineutral region, a finite diode length, and an arbitrary generation function in terms of space and time. ...

Junction Diode Photocurrent Solutions Following Ionizing Radiation and Including Time-Dependent Changes in the Carrier Lifetime From a Nonconcurrent Neutron Pulse

The radiation characteristics of polysilicon and SIPOS resistors are compared. SIPOS is being considered as a replacement material for polysilicon in feedback resistors in rad-hard ICs. Both materials show little change in resistivity to gamma radiation and are much more neutron-radiation resistant than bulk silicon. >