Abstract
The temporal behavior of the molecular etching product SiF4 in fluorocarbon-based plasmas used for the dry etching of ultra low-k (ULK) materials has been brought into connection with the polymer deposition on the surface during plasma treatment within the scope of this work. For this purpose, time-resolved measurements of the density of SiF4 have been performed by quantum cascade laser absorption spectroscopy. A quantification of the non-linear time dependence was achieved by its characterization via a time constant of the decreasing SiF4 density over the process time. The time constant predicts how fast the stationary SiF4 density is reached. The higher the time constant is, the thicker the polymer film on top of the treated ultra low-k surface. A correlation between the time constant and the ULK damage was also found. ULK damage and polymer deposition were proven by Variable Angle Spectroscopic Ellipsometry and X-ray Photoelectron Spectroscopy. In summary, the observed decay of the etching product concentration over process time is caused by the suppressed desorption of the SiF4 molecules due to a more dominant adsorption of polymers.
Highlights
Langmuir probe and self-excited electron resonance spectroscopy (SEERS) in different gas discharges,1 opening the possibility to measure this electrical plasma parameter noninvasively in process plasmas
It was observed that the F/C ratio has a significant influence on the temporal behavior of the SiF4 density during the plasma process
Ultra low-k dry etching processes were analyzed by quantum cascade laser absorption spectroscopy
Summary
The requirements of plasma etching processes in semiconductor chip manufacturing become more and more challenging. The SEERS sensor is widely applied on etch reactors and can be used as the fault detection system as it was already proposed by Baek et al in 2005.2 In their study, inductive coupled plasma (ICP) etching processes of tungsten-silicide were investigated, and they elaborated the link between the etched profile and the electron collision rate This is a very good example of combined analysis of in situ and ex situ process results. Instead of measuring electrical plasma parameters, Lang et al applied an absorption measurement method on fluorocarbonbased plasmas for dry etching of porous dielectric materials.5 They were able to detect the conditioning state of the process chamber by comparing the concentration of the etching by-product carbon monoxide of cleaned and non-cleaned process chambers. To characterize the temporal behavior of the SiF4 density, an exponential time constant could be introduced to quantify its decay and to explain the interrelation between the sample properties and the stable etching product SiF4
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