  Low-pressure multi-frequency capacitively coupled plasmas are used for
numerous etch and deposition applications in the semiconductor industry.
Pulsing of the radio-frequency (RF) sources enables control of neutral and
charged species in the plasma on a millisecond timescale. The synchronous
(i.e., simultaneous, in-phase) pulsing of both power sources in a dual
frequency capacitively coupled plasma is examined in this article. Due to the
low gas pressure, modeling has been done using the electrostatic
Particle-in-cell/Monte Carlo collision method. The objective of this work is to
investigate the sensitivity of the plasma properties to small changes in timing
during synchronous pulsing of the 2 RF sources. It is demonstrated that small
deviations in the on and off times of the 2 RF sources can lead to major
changes in the plasma characteristics. This high sensitivity is of concern for
process repeatability but can be utilized to enable better control of the
dynamics of plasma-surface interaction. In the simulations, the pulsing
parameters (on and off times and ramp rates) are varied and the temporal
evolution of plasma characteristics such as electron density (ne), species
current at the electrode, and electron temperature are examined. It is
demonstrated that if the low-frequency (LF) source is turned off a few {\mu}s
before (or after) the high-frequency source, ne during the off-state is
significantly higher (or lower) due to the frequency coupling effect.
Similarly, turning on the LF source with a small delay results in a sharp
increase in the plasma density when the RF sources are turned on.
