Diagnostics of inductively coupled chlorine plasmas: Measurements of the neutral gas temperature

We report measurements of the bulk, neutral gas temperature in a chlorine transformer-coupled plasma. A trace amount (2%–5%) of ${\mathrm{N}}_{\mathrm{2}}$ was added to the discharge and the rotational temperature of the ${\mathrm{C}}^3{\Pi }_u$ state was determined from the ${\mathrm{C}}^3{\Pi }_u\to {\mathrm{B}}^3{\Pi }_g$ emission in the ultraviolet. This temperature has been shown by others to be equal to the rotational temperature of ground-state ${\mathrm{N}}_{\mathrm{2}},$ which is the thermally equilibrated (translational and rotational) gas temperature ${\mathrm{(T}}_g\mathrm{).}$ The gas temperature 3 cm above the wafer is equal to, or only slightly above the wall temperature (300 K) throughout the low-power, capacitively coupled regime (<60 W, 0.024 W/cm³). Between the lowest (130 W, 0.053 W/cm³) and highest (900 W, 0.36 W/cm³) inductively coupled mode powers investigated, $T_g$ increases sublinearly with power (and electron density). The high-power $\text{(900\hspace{0.05em}}\mathrm{W})$ $T_g$ increases with increasing pressure (650, 750, 900, and 1250 K at 2, 5, 10, and 20 mTorr, respectively). Mechanisms of neutral gas heating are discussed. The energy released in dissociation of ${\mathrm{Cl}}_{\mathrm{2}}$ appears to be the dominant heating mechanism.