Characteristics of chlorine, transformer-coupled pulsed plasmas are reported. Time dependencies of electron (ne), positive ion (ni+), and negative ion (ni-) densities and electron temperatures (Te) were measured with a Langmuir probe and microwave interferometry at 240 and 500 W input powers, and pressures between 3 and 20 mTorr. During the OFF portion of the power modulation, ne decreases rapidly as Cl- is formed by dissociative attachment of Cl2. The formation of Cl- is accelerated at high Cl2 densities (at high pressures and low powers). At 10 mTorr and higher pressures, an ion–ion plasma forms near the end of the OFF portion of the cycle, the sheath collapses, and Cl- reaches the wafer. Te decays rapidly in the OFF period and increases with a similar time constant at the beginning of the ON cycle if electrons are present at a sufficiently high level. If ne is very low at the beginning of the ON cycle, such as at high pressure (10 mTorr), then Te exhibits a spike at the beginning of the ON period. In a comparison study, plasma induced damage is reduced when aluminum is etched under similar source power modulation conditions. © 1999 American Institute of Physics.