Diagnostics and control of radicals in an inductively coupled etching reactor

Abstract

Comprehensive measurements of charged particles and neutral radicals in an inductively coupled plasma (ICP) are performed to understand and control the etching process using a CF4/H2 gas. The electron density in the ICP reactor decreases exponentially in a downstream region while the most abundant ionic species CF+ increases in proportion to the rf power with the CF+3 density almost constant. The neutral radical diagnostics by appearance mass spectrometry indicate 10 times more F atoms and somewhat fewer CFx radicals (x=1–3) in ICP, compared with a high-pressure capacitively coupled plasma diode. Such a small ratio of the CFx density to the F density is possibly a cause of the low etch selectivity of SiO2 to Si in ICP etching. Two innovative methods to achieve the high selectivity in ICPs are demonstrated. One is wall heating (100–200 °C), which leads to a drastic increase in CFx densities with the F density almost constant. The other is a pulse modulation of rf power at 30–50 μs durations where the time-resolved measurement of electron temperatures indicates a 6 μs rise time and a 70 μs decay time. The density ratio CFx/F (x=2,3) increases with decreasing rf-on time. The mechanisms of the radical composition change and the resultant high selectivity in pulsed plasmas are discussed.