Thermodynamics and Transport Model of Charge ... - IEEE Xplore

BOULAIS et al.: THERMODYNAMICS AND TRANSPORT MODEL OF CHARGE INJECTION IN SILICON 2729
Fig. 1. (a) Schematic of a ring oscillator composed of four inverters and an AND gate. All inverters are equidistant to the target. (b) Typical result of the FVC
output after a 950-mW 100-ns pulsed laser irradiating the center of the ring oscillator situated at 30 μm.
II. EXPERIMENTAL SETUP AND RESULTS
As described in [10], the sensitive circuits used to detect
injected charges in the experimental work are CMOS ring
oscillators, manufactured by TSMC in a 0.18-μm technology
that has a low-resistivity p-type substrate. Oscillating at a
stable frequency, the ring oscillator, as previously presented by
Wild [10], is composed of four inverters and a gating NAND acting
as an inverter during the oscillation operation. A frequencyto-voltage
converter (FVC) is used to drive the chip output at
a voltage that represents the oscillator’s frequency, as shown
in Fig. 1. The on-chip oscillation of about 380 MHz is first
divided by eight using a frequency divider (FD), then each
period is converted by the FVC to a dc voltage that drives the
output of the chip through an analog buffer. The laser target
is a laser diffused resistor (LDR) implanted in the center of
the ring oscillator and located at various distance d to the
ring oscillator, ranging from 20 to 40 μm. A test setup has
been developed as described in [10] using a frequency-doubled
continuous-wave Nd:YAG laser (532 nm) having an adjustable
diameter and power. A pulse is selected from the continuous
beam using an acoustooptic modulator and is focalized on
the target. In all experiments, a single pulse is used with a
pulse duration of 100 ns. The beam diameter is about 7 μm,
and the emitted laser power P L is varying up to 1.5 W. Note
that, throughout the text, the emitted laser power refers to
the average laser power of the continuous-wave laser, taking
into consideration losses due to the acoustooptic modulator.
It is difficult to evaluate the optical losses due to the multiple
reflections and the absorption in the interdielectric layers;
the exact laser power arriving on the silicon is not known.
A typical FVC variation for P L = 950 mW and d = 30 μm
is given in Fig. 1(b). The measured maximum of the FVC
output variation [FVCOV max defined in Fig. 1(b)] is shown in
Fig. 2 as a function of the laser power P L . Estimations based
on diffused-resistor creation experiments and in situ reflectivity
measurements give a value for the melting power threshold of
around P L,th−m =(1.1 ± 0.1) W. One can distinguish three
main regions in these measured results: 1) In the first region,
for P L < 0.375 W, the FVC output increases with P L ;2)in
the second region (0.375 W 0.7 W), the FVC output increases slightly with scattered
results. An increase in charge injection with increasing power
was expected. The presence of region II was not expected.
To explain these results, the effects of heating on the charge
transport must considered.
III. CONSTANT-TEMPERATURE ELECTRONIC
TRANSPORT MODEL
Many models describing the interaction between a laser pulse
and a semiconductor developed over the past four decades focus
on emulating the impact of single-event effects by ionizing
radiation on static memories [9], [11], [12]. In these existing
models, relatively low laser powers are assumed, as any heating
phenomena are excluded from the calculation. However, the
laser used for creating LDRs is different in terms of power
level and pulse duration, since significant heating and melting
of silicon is required. As a first approximation, the substrate’s
temperature was assumed to remain constant throughout the
process. It will be shown that this assumption is not accurate.
It is important to note that the laser is not directly focused
on the active area of a transistor under test, but rather on the
LDR region, which introduces additional specificity into the