The radicals of capacitive plasmas actually used in mass production were analyzed
using various measurement systems. The composition of radicals in bulk plasma depends on the
gas chemistry, the dissociation process, and interaction with the wall. It is revealed that parent
gas (C4F8) is dissociated by multiple collision with electrons according to τ·n
e<σv>,
where τ is the residence time, n
e is the electron density, σ is the dissociation collision cross
section and v is the electron velocity. A high-performance etching process, which can realize
0.09 µmφ contact holes with aspect ratio of 11, was achieved using a short residence time to
suppress the excess dissociation and the control of deposition species through the addition of O2
to C4F8/Ar plasma as well as the reduction of the density of F radicals through the reaction with
the Si wall.
To investigate the mechanism of C4F8 dissociation in parallel-plate-type plasma, we used several of the latest diagnostic tools and made extensive measurements of electrons, radicals, and ions under conditions that greatly suppressed the effects of plasma-surface interaction. These measurements showed that the amount of light fluorocarbon radicals and ions increased with increasing electron density. The dissociation of C4F8 was analyzed by using rate equations, after confirming the stability and uniformity of the plasma. The total dissociation rate coefficient of C4F8 was 1×10−8 cm3/s, and CF2 radicals were mainly generated from products of C4F8 dissociation. F was mainly generated from CF2 by electron-impact dissociation and lost by pumping. We could estimate that the C2F4 density was roughly comparable to the densities of CF and CF3, and that the surface loss probability of C2F4 increased with increasing electron density. C2F4 might play an important role in the etching because of its rich polymerization characteristics.
A dual-frequency superimposed (DFS) 100 MHz and 3.2 MHz rf capacitive-coupled plasma etch process for sub-90 nm devices has been developed. The electron density of DFS reactive ion etching (RIE) plasma at 40 mTorr was controlled from 4.0×1010 to 3.6×1011 cm-3 by adjusting the 100 MHz rf power, and the self-bias voltage (-V
dc) was controlled from 20 to 760 V by adjusting the superimposed 3.2 MHz rf power. DFS RIE demonstrated independent control of electron density and self-bias voltage in a wide range. In the damascene etch process of SiOC film using Si3N4 as an etch mask, it was found that mask edge erosion is dependent on ion energy regardless of the selectivity of SiOC to Si3N4. DFS RIE offers the most suitable process for damascene etching of SiOC, which requires precise ion energy control.
The pattern size dependence of SiO2 and Si3N4 etch rates of contact holes (RIE-lag) in C4F8+CO plasma was studied. It was found that these etch rates can be characterized by the aspect ratio, regardless of the pattern size. SiO2 etch rate decreased with increasing aspect ratio and became 0 at an aspect ratio of 6. Si3N4 etch rate also decreased; however, etching still occurred at an aspect ratio of 30. From ion current measurements through capillary plates (CPs), it was deduced that etch rates decreased because of decreasing ion current. XPS analyses revealed that fluorocarbon film deposited on the Si3N4 surface at the bottom of a hole was more F-rich than that deposited on a flat Si3N4 surface. This explained why Si3N4 is etched even in high-aspect-ratio holes. A small amount of O2 addition to the C4F8+CO plasma resolved the RIE-lag. It was found that the ion current density at high aspect ratio increased with O2 addition, which would enhance SiO2 etching and contribute to suppressing RIE-lag.
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