Evaluating the Performance of c-C4F8, c-C5F8, and C4F6
for Critical Dimension Dielectric Etching
B. Ji, P. R. Badowski, S. A, Motika, and E. J. Karwacki, Jr.
Introduction:
One of the many challenges IC manufacturing faces with the advancement of technology
nodes is reactive ion etching [RIE] of dielectric materials to produce smaller feature with
higher aspect ratios. Compounding the difficulty is the introduction of 193 nm
photoresist materials, whose chemical composition makes them less resistant to RIE and
their depth of focus requires the use of thinner films to pattern during lithography.
Another trend that is also beginning to factor in is the use of lower k dielectric materials.
Many of these materials are carbon doped silicon oxide. The challenge they presents is a
possible reduction in etch selectivity between the resist and dielectric being etched,
because of organic forms of carbon being included in the structure of the material.
A process control parameter that is being looked at to assist with these RIE problems is
tailoring of the etch chemistry.[1,2] Figure 1 illustrates what is being asked of RIE
chemistries:
1. Thinner resist films require they be protected from etching by ion
bombardment, and attack by oxygen ions and radicals [3].
2. To maintain the integrity of the sidewall, a protective polymer film that is
sufficiently crosslinked is needed to assure the walls are kept smooth [4].
3. The polymer along the trench bottom must be sufficiently fluorinated, and
there must be an adequate supply of CF2 ions and radicals within the plasma
to promote rapid etching of the trench [5].
4. Finally, all of this chemistry must function in a manner that enables uniform
etching across dense and isolated features of varying dimensions [i.e.,
minimizes microloading].
1
.
Figure 1: Overview of RIE process. Numbered areas indicate where chemistry plays a
critical role in etch process.
To address these process requirements RIE etching chemistry has undergone a number of
transitions. Figure 2 illustrates a trend in RIE chemistry for etching silicon oxide. An
early route to reducing the fluorine to carbon ratio was substituting hydrogen for a
fluorine atom on CF4. The weak bond strength of a carbon to hydrogen bond helped
promote fragmentation within the plasma and formation of free radicals of CF and CF2.
[6-8] These species are believed to play critical roles in promoting RIE. However, these
species are not capable of producing polymeric films at high deposition rates.
To enhance polymer film formation molecules with higher levels of “unsaturation”
within its structure were investigated. The adoption of c-C4F8 is a good example of
incorporating unsaturation into the molecule. Here the ring structure of the molecule
contributes one degree of unsaturation. The net result of this is a concomitant reduction
in its fluorine to carbon ratio. This molecule has been extensively used for etching
silicon oxide for many years.
However, starting at the 180 nm technology node another shift in chemistry began to
occur to lower fluorine to carbon ratios. Two molecules that have received considerable
attention over the recent technology nodes are hexafluorobutadiene [C4F6] and
octafluorocyclopentene [c-C5F8]. Both of these molecules have two degrees of
unsaturation, and almost identical fluorine to carbon ratios. A number of the most
advanced critical dimension dielectric etching processes now employ these two etching
chemistries.
2
1
Fluorine:Carbon Ratio
4.0
F
F
F
C
3.0
CF4
CHF3
F
H
F
F
F
F
C
F
F
2
F
F
F
F
F
c-C4F8
3
2.0
F
C
1.0
F
CF2
C
F
F2C
4
F
F
F
F
C5F8
F
F
F
C4F6
Selectivity [Oxide:Resist]
Figure 2: General trend in the evolution of RIE chemistries for anisotropic etching of
SiO2.
To better understand the performance differences between c-C4F8, C4F6, and c-C5F8, we
performed a number of characterization studies to learn more about how these molecules
behave in RIE processes. In this application note we present findings associated with
polymer film formation, etch selectivity, and composition of polymer film produced from
c-C4F8, C4F6, and c-C5F8. These studies were carried out on two etching platforms within
our applications laboratories. A Gaseous Electron Conference [GEC] reactor was used
for studying polymer film formation and identification of species present within the
plasma. Anisotropic etching studies were performed on a 200 mm Mark II MERIE
reactor attached to an Applied Materials P-5000 platform within our facility.
We attempted to evaluate the relative performance of the three aforementioned
fluorocarbons. Process recipes for all three were optimized to obtain the maximum oxide
etching rate and highest oxide to resist selectivity. Figure 3 shows the results for this
study. Both c-C5F8 and C4F6 result in significantly higher oxide etch rates and etch
selectivity when compared to the incumbent c-C4F8. Of the two more unsaturated
molecules, C4F6 stands out as the better option.
3
Figure 3: Summary of RIE optimization study.
Using mass spectrometry we attempted to learn more about how these molecules may
fragment within a plasma. Our initial work used low energy (30 eV) electron impact
ionization as a surrogate for plasma excitation. Figure 4 summarizes this study. The data
suggest C4F6 and c-C5F8 produce more, larger-sized fragments with F/C less than 2.0
(purple label peaks). Furthermore, C4F6 produces a more abundant number of fragments
with F/C less than 1.5 (blue label peaks). These larger fragments are believed to be
responsible for polymer film formation, and unsaturated fragments (low F:C ratios) help
facilitate cross-linking of the polymer film. Another interesting trend observed is that
C4F6 produces more CF and CF2 fragments. These species are thought to be positive
contributors to the etching rate of SiO2.
4
Figure 4: Summary of 30 eV electron impact fragmentation study of c-C4F8, C4F6, and
c-C5F8.
Measuring the rate of polymer film deposition during RIE and the composition of the
film are very challenging measurements we continue to pursue in our studies. A
screening study we often employ to evaluate the propensity for a fluorocarbon molecule
to polymerize within a plasma is to measure the rate of film deposition from a plasma
comprised of just the fluorocarbon molecule. Figure 5 summarizes the results of this
study, and an evaluation of the chemical composition of the polymer film by X-ray
Photoelectron Spectroscopy (XPS). The results show how “reactive” C4F6 and c-C5F8 are
when activated within in a plasma environment. In contrast to c-C4F8, polymeric films
can be readily produced from these two materials. Again, C4F6 is found to be the more
reactive material. By XPS we observe that C4F6 produces a film that contains
approximately 10% more cross-linking. As mentioned above the conditions employed
here to deposit these films differ from those used to etch oxide. In a future note we will
discuss work now underway to characterize polymeric films created during RIE.
5
Figure 5: Deposition rate and chemical composition by XPS of polymer films made
from C4F6, c-C5F8, and c-C4F8.
In conclusion, benefits of higher oxide etching rate and oxide to resist etch selectivity we
observe for critical etching using C4F6 can be attributed to:
• propensity for polymerizing when activated by plasmas
• formation of highly unsaturated molecular fragments that lead to a high
degree of cross-linking with the films deposited
• production of CF and CF2 fragments that assist in the RIE process.
Currently, we are continuing our plasma studies by employing mass spectrometry using
very low energy impact ionization to probe the composition and distribution of neutral
species within the plasma. Our surface studies are directed towards a better
understanding of the chemical composition of the protective films that grow along the
resist surface and sidewall of the etched features.
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