The profile distortion, notch, in the etching of gate lines with high-density plasmas is becoming more deleterious as ULSI devices are being further scaled down. Dependence of the etch profiles on the polysilicon conductivity was studied for various spacings of line-and-space patterns. The notch depth was deeper for polysilicon with higher conductivity. The image potential was employed to account for the dependence of notch on conductivity. To measure the net positive charges which are known to contribute to the notch phenomenon, we observed the ion and electron current waveforms from patterned and unpatterned wafers as a function of bias power. The results showed that the surface of the patterned wafer is more positively charged than that of the unpatterned wafer, and the net charge can be reduced by raising the bias power. However, an increase in the bias power lowers the selectivities over both thin gate oxide and photoresist.
We developed a new method to enhace the photoresist selectivity in SiO2 etching by
modulating both the source and bias powers and by controlling the phase difference between the
modulation functions. Enhancement of mask selectivity was observed in the pulse plasma,
especially in the out-phase condition. To understand the heavy polymerization in the out-phase
pulse plasma, we analyzed the ion energy distributions of CF
x
+(x=1, 2, 3) ions using the
energy-spectroscopic quadrupole mass spectrometer (QMS) and measured the waveforms of the bias
power with a high-voltage probe which was connected directly to the wafer. Two distinct
plasma potential distributions were obtained in the pulse plasma and the dc bias voltage (V
DC)
was maximum in the out-phase condition. The heavy polymerization in the out-phase condition
was explained as a result of high V
DC. We also investigated the emission intensity of the C2
(516.5 nm) line, and found that C2 species were precursors of the polymerization and
contributed to the heavy polymerization in the out-phase condition.
The instabilities caused by the reflected rf power in a pulsed-plasma operation employing modulated rf power was studied. By suppressing the side-band modes in the frequency domain, the pulsed plasma became more stable and produced less reflected power. The mode-suppressed pulsed plasma showed almost the same plasma characteristics as the conventional step-function-modulated pulsed plasma. The mode-suppressed plasma was applied to etch a polysilicon pattern. The etched polysilicon profile showed no charge-up defects, suggesting that the mode-suppressed plasma can be utilized for controlling the electron temperature in a more stable operation.
The self-starting condition for the additive pulse mode locking in an arc lamp pumped cw Nd:YLF laser was investigated experimentally. With variation in the coupling between the main cavity and the external cavity, the threshold power for self-starting additive pulse mode locking was measured. A comparison is made between the experimental results and the previously reported results for the cavity mode correlation time.
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