For many years, optical emission spectroscopy (OES) has been successfully used for the measurement and control of plasma products in industrial plasma reactors. We have extended this technique using phase resolved optical emission spectroscopy (PROES), in a industrial electron cyclotron resonance (ECR) plasma etcher.

 

Experiments were conducted in a pure argon discharge with a SiO2 wafer on the biased electrode. Argon ion bombardment of the wafer liberates oxygen atoms to the discharge. Therefore, oxygen is only present at the beginning of a discharge in solid state, i.e. in the SiO2 lattice. The ECR etcher used in this experimental study has a 2.45 GHz microwave generator with a maximum power of 2kW, variable magnetic field of up 90 mT and 2MHz RF bias with maximum power of 250 W. The SiO2 wafer is mechanically clamped to the chuck to which the RF bias is applied.

 

In order to study the behavior of oxygen in the sheath region above the wafer an iCCD camera and high resolution spectrograph are employed and the iCCD camera is gated with respect to the 2MHz RF bias frequency. The production of oxygen is mostly due to RF voltage oscillation across the wafer induced by argon ion bombardment of its surface. The atomic oxygen spectral line intensity, from the 777 triplet, is monitored with respect to phase of the RF bias. Ion energy distribution functions, at the wafer surface, are measured using a floating retarding field energy analyzer (RFEA). The floating RFEA is is placed on the rf biased wafer surface and signal cabling is taken out through the reactor vacuum pump tunnel. This prevents the need for any modification to the reactor configuration. The RFEA sensor is 7 cm in diameter and the wafer on which it sits is 200 mm in diameter resulting in significant exposure of the wafer to ion bombardment. Phase resolved measurements are made using the iCCD camera which is operated with a repetition rate of 2 MHz synchronously (triggered) with the RF bias. The integration gate of 3.90625 ns is locked to a fixed phase position within the RF cycle (500 ns). This gives exactly 128 intervals over the 2Pi RF cycle. A variable delay between the fixed phase and the gate allows one to cover the complete RF cycle. We record strong correlation between the ion-flux and the PROES data.

This work was a partly funded by SFI under the Precision project.