03-05-2012, 12:37 PM
GaAs PHOTOCONDUCTIVE SEMICONDUCTOR SWITCH FABRICATION FOR IMPROVED RELIABILITY
[attachment=21144]
Pulsed power systems designed with many switches or
many different switch trigger times are heavily dependent
upon the triggering systems. Conceptual pulsed power
systems for fusion and isentropic compression are being
designed with hundreds to hundreds of thousands of switches.
Some have to be triggered simultaneously and some will
need to be programmable and staggered in time. Trigger
system cost, performance, and lifetime will be driving factors
in these systems. For example, designers of repetitive pulsed
power systems considered for fusion are seeking switch
lifetimes >107 shots. A solution that addresses the
performance requirements is the use of optically triggered
photoconductive semiconductor switches (PCSS), which
excel at delivering low jitter, fast-rise-time pulses. Lifetimes
for these devices have been demonstrated to be in the >>106
shot range at lower currents (tens of amps), but drop to the
103 shot range when the current per filament is hundreds of
amps. To increase the switch reliability at higher current
levels necessitates further investigation of device reliability
and failure mechanisms.
Results from lifetime studies on GaAs PCSS devices
indicate that the majority of failures occur near the anode.
However, switch fabrication variables can shift this failure
mechanism to a short-life, cathode-limited condition. A
statistical sample population of PCSS switches in a circuit
designed to deliver 500V/100A, lOns pulses is shown to have
a single log-normal distribution of shot life over a range of
testing temperatures and voltages. Switches for higher
operating voltage (70 kV) were fabricated in multiple split
lots to identify the influence of design and processing
parameters on the switch lifetime. It was found that these
larger-gap 70kV switches exhibit the same anode-limited
lifetime as the 500V devices. This paper will discuss results
from the lifetime studies, including variables in the
fabrication process (annealing temperature, pad-metal
thickness, length of contact-metal extension beyond the padmetal,
and inclusion of a silicon nitride passivation/antireflection
coating) that affect reliability, and provide
strategies for improving shot life.
* Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the United
States Department of Energy's National Nuclear Security
Administration under contract DE-AC04-94AL85000.
[attachment=21144]
Pulsed power systems designed with many switches or
many different switch trigger times are heavily dependent
upon the triggering systems. Conceptual pulsed power
systems for fusion and isentropic compression are being
designed with hundreds to hundreds of thousands of switches.
Some have to be triggered simultaneously and some will
need to be programmable and staggered in time. Trigger
system cost, performance, and lifetime will be driving factors
in these systems. For example, designers of repetitive pulsed
power systems considered for fusion are seeking switch
lifetimes >107 shots. A solution that addresses the
performance requirements is the use of optically triggered
photoconductive semiconductor switches (PCSS), which
excel at delivering low jitter, fast-rise-time pulses. Lifetimes
for these devices have been demonstrated to be in the >>106
shot range at lower currents (tens of amps), but drop to the
103 shot range when the current per filament is hundreds of
amps. To increase the switch reliability at higher current
levels necessitates further investigation of device reliability
and failure mechanisms.
Results from lifetime studies on GaAs PCSS devices
indicate that the majority of failures occur near the anode.
However, switch fabrication variables can shift this failure
mechanism to a short-life, cathode-limited condition. A
statistical sample population of PCSS switches in a circuit
designed to deliver 500V/100A, lOns pulses is shown to have
a single log-normal distribution of shot life over a range of
testing temperatures and voltages. Switches for higher
operating voltage (70 kV) were fabricated in multiple split
lots to identify the influence of design and processing
parameters on the switch lifetime. It was found that these
larger-gap 70kV switches exhibit the same anode-limited
lifetime as the 500V devices. This paper will discuss results
from the lifetime studies, including variables in the
fabrication process (annealing temperature, pad-metal
thickness, length of contact-metal extension beyond the padmetal,
and inclusion of a silicon nitride passivation/antireflection
coating) that affect reliability, and provide
strategies for improving shot life.
* Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the United
States Department of Energy's National Nuclear Security
Administration under contract DE-AC04-94AL85000.