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Effect of plasma interactions with low- κ films as a function of porosity, plasma
chemistry, and temperature
Marcus A. Worsley, Stacey F. Bent, Stephen M. Gates, Nicholas C. M. Fuller, Willi Volksen, Michelle Steen, and
Timothy Dalton
Citation: Journal of Vacuum Science & Technology B 23, 395 (2005); doi: 10.1116/1.1861038
View online: http://dx.doi.org/10.1116/1.1861038
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/23/2?ver=pdfcov
Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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Effect of plasma interactions with low-
films as a function of porosity,
plasma chemistry, and temperature
Marcus A. Worsleya兲and Stacey F. Bent
Department of Chemical Engineering, Stanford University, Stanford, California 94305-5025
Stephen M. Gates and Nicholas C. M. Fuller
T. J. Watson Research Center, IBM, Yorktown Heights, New York 10598
Willi Volksen
Almaden Research Center, IBM, San Jose, California 95120-6099
Michelle Steen and Timothy Dalton
T. J. Watson Research Center, IBM, Yorktown Heights, New York 10598
共Received 20 August 2004; accepted 27 December 2004; published 9 March 2005兲
Integration of new low-
interlayer dielectrics 共ILD兲with current damascene schemes is a
continuing issue in the microelectronics industry. During integration of the ILD, processing steps
such as plasma etching, resist strip, and chemical-mechanical planarization are known to chemically
alter a layer of the dielectric. Here, porous organosilicate glass 共OSG兲ILD films, which—according
to the 2004 edition of the International Technology Roadmap for Semiconductors—are projected for
use in the 65 and 45 nm nodes, are investigated. spectroscopic ellipsometry, x-ray photoelectron
spectroscopy, and Fourier transform infrared spectroscopy are used to characterize the modified
layer of the ILD after exposure to O2or H2resist strip plasmas. The effects of the two types of
plasma etch chemistries on the formation of the modified layer were found to differ significantly.
These effects include both the degree of modification 共i.e., chemical composition兲and depth of the
modified layer. A key difference between the O2and H2plasmas is that silicon hydride groups are
present in the modified layer after exposure to H2plasma but not after exposure to the O2plasma.
In addition, the influence of OSG porosity on the etch rate and modified layer thickness was
investigated for porosities ranging from 0–45 %. As expected, the etch rate was found to increase
rapidly with porosity. Finally, conditions including reactive gas concentrations and substrate
temperature for the H2plasma were varied. These parameters produced considerable changes in the
chemistry of the modified layer, especially in the amount of hydrogen incorporated into the film.
Details of these results will be discussed in the context of the mechanism by which modification and
etching occurs as well as which process variables dominate those phenomena. © 2005 American
Vacuum Society. 关DOI: 10.1116/1.1861038兴
I. INTRODUCTION
New materials are necessary to continue the existing trend
toward smaller feature sizes in future computer chips. In the
past, chip speed was limited by the size of a transistor, but in
the immediate future the limitation will be defined by the RC
delay in global interconnects.1–4 As a first step to reduce the
RC delay, many chip manufacturers have replaced aluminum
wiring with lower resistance copper. In an effort to reduce
the capacitance between lines, the next step the industry is
taking is to find materials with a lower dielectric constant
to replace silicon oxide as an interlayer dielectric 共ILD兲.
There are a wide variety of low-
materials5–10 being re-
searched, and many issues have arisen involving the integra-
tion of these materials.2–4,11–20 The current challenge has be-
come one of designing low-
materials with mechanical and
chemical properties such that they can be successfully used
in current integration schemes. These films must have suffi-
cient mechanical strength to withstand numerous physical
stresses and high enough chemical stability to remain unal-
tered by integration processes such as photoresist strip.
For the forthcoming 65 nm technology node, much of the
industry is focused on incorporating porosity into a Si–O
network to further reduce the
-value.2–4,11,21,22 A likely can-
didate for a low-
ILD is the so-called organosilicate glass
共OSG兲or SiCOH films deposited by either chemical vapor
deposition or spin-on processes. Previous work has shown
that exposing films of this composition to O2plasma envi-
ronments can cause damage12–20 manifested as undesirable
chemical modifications.13,15,16,20 This result has been fairly
consistent across a range of OSG films.13,15,20 H2plasma
environments, however, have been shown to have a range of
effects. Some groups report that it has no effect on the film,14
others that it enhances the properties of the film,23,24 and yet
others indicate that H2plasmas do indeed cause damage.13
Therefore, the goal of this study is to characterize the depth
and degree of plasma damage on a blanket OSG film 共hori-
zontally exposed surface兲as a function of plasma chemistry
共O2and H2兲, porosity, and substrate temperature. We then
a兲Electronic mail: worsleym@stanford.edu
395 395J. Vac. Sci. Technol. B 23„2…, Mar/Apr 2005 0734-211X/2005/23„2…/395/11/$19.00 ©2005 American Vacuum Society
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discuss the probable mechanisms dominating the processes
altering the film.
II. EXPERIMENTAL DETAILS
The OSG samples used in this study were blanket films of
Dendriglass, a spin-on porous methyl silsesquioxane that has
been widely characterized.5,21,25–30 To prepare the 7000 Å
thick films, the Dendriglass solution was spun onto 8 in. Si
wafers at 3000 rpm, hot plate baked at 75 °C, and furnace
cured at 450 °C. Porogen loadings of 0%, 10%, and 40%
were used and corresponding values of
were 2.86, 2.59,
and 1.86, respectively. The dielectric constant,
was mea-
sured using evaporated aluminum dots in metal-insulation-
semiconductor structure. Values of volumetric porosity as
measured by spectroscopic ellipsometry 共SE兲were 0%, 8%,
and 45% respectively. These porosities were chosen because
they sampled three distinct categories of films: dense 共0%兲,
closed porosity 共8%兲, and open porosity 共45%兲. Closed po-
rosity refers to a pore morphology in which the pores are
isolated and separate from one another. Open porosity refers
to a pore morphology with significant interconnectivity be-
tween pores. The samples were then exposed to resist strips
composed of Ar/O2and Ar/H2.31 The time of each of these
exposures was calibrated to replicate conditions in which
4000 Å of photoresist would be removed. Thus, time of ex-
posure ranged from 41 s in an 88% O2plasma to 690 s in a
55% H2plasma. In this way, the exposures were normalized
based on resist strip rate. The different resist strip processes
were carried out in a commercial dual frequency capacitive
共DFC兲etch tool and are identified by the species in the
plasma.
Discharge parameters for the two plasma chemistries are
shown in Table I. The higher pressures, gas flow, and el-
evated powers in the Ar/H2plasma were chosen to achieve
appreciable resist strip rates. Also, in order to sustain a stable
plasma with 91% H2flow, a chamber pressure of 500 mTorr
共instead of 120 mTorr used at lower H2flows兲was required.
When investigating the effect of porosity and plasma chem-
istry, the highest O2flow of 88% and H2flow of 91% were
used. For all experiments the substrate temperature was held
at 20 °C except when temperature was the focus of the study;
then it was taken as high as 77 °C 共tool limit兲. In addition,
when the substrate temperature was varied, the 91% H2flow
condition was used and all other variables, including time,
were held constant 共i.e., no normalization兲.
The elevated power settings for the hydrogen plasmas
should not adversely affect the study. Preliminary work32
共not shown兲on a DFC tool distinct from the one used in this
study suggest that all these plasmas are weakly ionized
共109–1010 cm−3兲with a sublinear increase with source rf
power so the increased power would provide a relatively
small increase in ion flux. There would, however, be some
marginal difference in the associated ion energy given that
the bias rf power settings for the oxygen and hydrogen based
processes are 0 and 50 W, respectively.The net ion current to
the wafer 共⬃source power xbias power兲should only be
marginally affected and as such physical etching of the film
should not be significantly affected by these power setting
differences between the two plasmas. The results also indi-
cate that at the same source power the degree of dissociation
for hydrogen is substantially lower than that for oxygen. It
implies that at the elevated power the radical density of hy-
drogen is probably still less than or equal to that of oxygen.
Therefore the evaluation of chemical etching and modifica-
tion is also not significantly hindered.
To characterize the modified layer, several ex situ tech-
niques were employed. Refractive index 共RI兲at 633 nm and
thickness before and after plasma exposure were determined
by spectroscopic ellipsometry 共SE兲. Data was collected over
a range of 300–1000 nm at two angles of incidence 共65° and
75°兲on a J.A. Woolam Variable Angle Spectroscopic Ellip-
sometry instrument and analyzed using WVASE32 software.
Cauchy models with wavelength-dependent optical constants
were used to simulate both the as-deposited and modified
OSG layers. Further details of the modeling are described in
previous work.13 Physical measurements of thickness were
also performed using a KLA-TencorAlpha Step P-11 surface
profiler to confirm the accuracy of the thickness measure-
ments determined by SE. The thickness measurements al-
lowed the extraction of etch depth, modified layer depth, etch
rate, and modified layer growth rate 关Fig. 1共a兲兴 to define the
depth of plasma damage. Thus plasma damage could be
quantified in terms of the material removed 共etch damage兲
and material modified 共modified layer兲. The composition of
the samples was measured by x-ray photoelectron spectros-
copy 共XPS兲and Fourier transform infrared 共FTIR兲spectros-
copy. XPS spectra were obtained on a Surface Science In-
struments 共SSI兲S-Probe Monochromatized XPS
Spectrometer using Al 共K
␣
兲radiation. FTIR spectra were
measured by a Mattson spectrometer in transmission mode.
TABLE I. Discharge parameter.
Parameter Oxygen plasma Hydrogen plasma Argon plasma
Upper electrode 共rf兲300 W 共27 MHz兲600 W 共27 MHz兲300 W 共27 MHz兲
Lower electrode 共rf兲0W共2 MHz兲50W共2 MHz兲0W共2 MHz兲
Total gas flow 400 sccm 550 sccm 400 sccm
Chamber pressure 100 mT 120 mT 500 mT 100 mT
%O212305070880000 0 0
%H20000055596477 91 0
%Ar 88 70 50 30 12 45 41 36 23 9 100
Calibrated exposure time 共s兲66 52 46 42 41 560 690 536 507 329 66
396 Worsley
et al.
: Effect of plasma interactions with low-
films 396
J. Vac. Sci. Technol. B, Vol. 23, No. 2, Mar/Apr 2005
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RI and composition were used to define the degree of plasma
damage.
III. RESULTS AND DISCUSSION
Etching of the OSG film during exposure to a resist strip
plasma proceeds similarly to that of any other film in a
plasma ambient. A dynamic equilibrium condition is set up
between the formation of volatile adsorbates due to the pres-
ence of reactive sites and chemical reactivity of active
etchant species with such sites 共film modification or chemi-
sorption兲and ion induced desorption of these volatile adsor-
bates from the film surface 共etching or desorption兲. The latter
effect is dependent on the ion current incident on the film
surface. In the present study, it is assumed that the dominant
active etchant species are O and H radicals and the dominant
ion for inducing desorption is Ar+. It is also assumed that
both atomic and molecular species diffuse into the film.
Thus, both the modification and etching rates are functions
of film porosity and plasma conditions. The first section ad-
dresses the effect of porosity. The following two sections
will examine the effect of changing certain plasma condi-
tions.
A. Porosity
The effect of porosity on the film’s response to the plasma
treatment is observed in four measurements: the depth of the
modified layer remaining after exposure, the modified layer
growth rate, the etch depth, and the etch rate. In addition, the
sum of etch depth and modified depth, designated “total
depth of damage,” is also a useful metric. The modified layer
depth and etch depth after Ar/O2and Ar/H2plasma expo-
sure as a function of porosity are shown in Fig. 1. As ex-
pected, increasing porosity increased the total depth of dam-
age. For the Ar/O2plasma, the introduction of porosity
reduced the modified layer remaining while changing the
porosity did not significantly affect the modified layer re-
maining for the Ar/H2plasma. However, for both films it
appears that the key contributor to the increased total depth
of damage with porosity is a rapidly increasing etch depth.
As these films were exposed to the same conditions, this
result would suggest a rapid increase in etch rate with poros-
ity. Figure 2, a plot of the etch rate as a function of porosity
for both plasmas, supports this claim. The etch rate was de-
termined by dividing the etched thickness by the exposure
time and hence represents an average etch rate. Figure 2
shows that increasing the porosity increases the etch rate for
both plasmas. In fact, the factor by which the etch rate is
enhanced with increasing porosity for both plasmas is the
same. The etch rate is increased by a factor of about 2.5 and
7 in the 8% and 45% porosity film, respectively, over the
dense film.
The data above indicate that porosity has a large affect on
the total depth of damage sustained by the film. Looking
strictly at the mechanical properties of porous films versus
dense films, and considering the physical sputtering element
of etching, an increase in etch rate is expected. Adding po-
rosity to these films fundamentally weakens their structure
共e.g., elastic modulus, hardness兲3making them easier to
physically sputter. Thus, at least a portion of the increased
etch rate can be attributed to a mechanically weaker film at
higher porosities. The increased etch rate is also due in part
to the fact that there is less material per volume to etch in the
porous films. To account for this, calculations were per-
formed to normalize the etch rate by removing the difference
in material per volume caused by the porosity. However,
normalization based on volume of material etched did not
eliminate the general trend of etch rate increasing with po-
rosity. Therefore additional factors must be considered.
There are several other causes that likely contribute to the
increased etch rate at increased porosity. One is the increased
FIG.1.共a兲Schematic of damage effects. Dendriglass etch depth and modi-
fied layer depth after 共b兲Ar/ O2and 共c兲Ar/ H2plasma as a function of
porosity.
FIG. 2. Dendriglass etch rate in plasma as a function of porosity.
397 Worsley
et al.
: Effect of plasma interactions with low-
films 397
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surface area and access to reactive sites found in porous me-
dia. In the dense film, the sites exposed to etching and, to a
lesser extent, modification, are largely limited to the top sur-
face, since ion bombardment primarily occurs at the surface
and the diffusion into the film is relatively slow. However in
a porous film, the exposed surface area is many times greater
allowing modification and etching to occur on pore walls in
addition to the top surface. Increased porosity allows ions to
impact deeper beneath the film surface which likely en-
hances both the modification and etching processes. In addi-
tion, higher concentrations of reactive species from the
plasma can more easily diffuse into the film, due to the po-
rosity, increasing the rate of modification and etching.
These data also allow for some conclusions to be drawn
with respect to open and closed porosity. Recall that the cho-
sen porosities sample three distinct categories of films: dense
共0%兲, closed porosity 共8%兲, and open porosity 共45%兲.25 Fig-
ure 1共b兲shows that for the Ar/O2plasma, the total depth of
damage is approximately equal for the 0% and 8% porous
films and only increased with the 45% porous film. This
suggests that the open porosity found in the 45% porous film
may play a key role in facilitating the diffusion of oxygen
species into the film as the total depth of damage only in-
creased in the open porosity sample. However, Figure 1共c兲
suggests the hydrogen species are not limited by pore con-
nectivity as the total depth of damage steadily increased with
each porosity increase, and was not correlated with closed or
open porosity. Therefore, the depth of damage caused by the
H2plasma was more likely due to the decreased density of
the film rather than pore connectivity.
Another interesting result is that, for the Ar/O2plasma
关Fig. 1共b兲兴, the etch rate is enhanced relative to the modifi-
cation rate with the introduction of porosity. We speculate
that this may be the combined effect of the limited oxygen
species diffusivity and the increased surface area. Whereas
the modification, due to diffusion of reactive species, can
occur at both surface and subsurface sites, etching is princi-
pally limited to the surface regions which are exposed to ion
bombardment. A closed porosity film, due to increased sur-
face roughness, would represent a significant increase in sur-
face area exposed to ion bombardment at the top surface.
However, if the closed pore structure significantly inhibited
diffusion for the oxygen species, as suggested earlier, a com-
parable increase in modification rate would not be observed.
Therefore, the etch front would progress further into the
modified layer decreasing the modified thickness. Then with
the open porosity film, comparable increases in etch and
modification rate are seen resulting in no change in the modi-
fied layer thickness.
In general, these data suggest that introducing porosity
makes the film more susceptible to modification and etching.
This is consistent with previous reports on other porous OSG
films14,16 and highlights one of the challenges of ultra low-
integration.
B. Plasma chemistry
In the previous section, the effect of plasma chemistry
was explored by investigating the different ways in which
Ar/O2plasmas and Ar/H2plasmas interacted with films of
different pore structures. In this section, additional effects of
plasma chemistry will be presented and discussed. Results on
the depth of damage and the degree of damage will be
treated separately.
1. Depth of damage
The results shown in Figs. 1 and 2 can be used to compare
the effect of different plasma chemistries on the film. Figure
1 shows that the Ar/H2plasma leaves a modified layer 1.5–2
times as deep as the Ar/O2plasma, independent of porosity.
We consider several observations in explaining the difference
in the modified layer depth. First, as mentioned in the experi-
mental section, the Ar/H2plasma exposure times were
longer than those for the Ar/O2plasma. This longer expo-
sure was set in order to equalize the photoresist strip times.
However, if the OSG etch rate for the Ar/H2plasma is not
comparably lower than that of the Ar/O2plasma, the modi-
fied and etched depths will differ. The increased time would
allow any reactive species in the Ar/H2plasma to diffuse
further, pushing the modification front deeper into the film.
In addition, hydrogen, being a smaller species than oxygen,
will have a higher diffusivity, and can thus react with and
modify material farther ahead of the etch front. Finally, if the
ion-induced etch process is slower in the Ar/H2plasma than
in the Ar/O2plasma, thicker modified layers will result un-
der steady state conditions.
Figure 2 shows that the Ar/O2plasma gives an etch rate
approximately three times as fast as the Ar/H2plasma across
the porosity range. Nagai et al.19 attributed the aggressive
etching of O atoms, relative to N atoms, on OSG films to
their efficiency in removing carbon atoms from the surface.
This statement suggests that the modification process is criti-
cal in determining the etch rate. Thus, a detailed investiga-
tion of the mechanisms involved in modification is necessary
to understand the differences in how Ar/H2plasmas and
Ar/O2plasmas interact with the OSG film.
As proposed by Chang et al.,15 modification of the OSG
in O2plasma occurs via the overall reaction
⬅Si − CH3+4O→⬅Si − OH + CO2+H
2O, 共1兲
where ⬅Si designates Si bonded to three lattice O’s. They
also proposed that some portion of the Si–OH would further
react via the reaction
2⬅Si − OH →⬅Si−O−Si⬅+H
2O. 共2兲
Thus modification would evolve H2O and CO2. Note that the
only reaction that changes the structure is at the site of the
original Si–CH3. The etching process is subsequently com-
pleted by physical sputtering of remaining products.
For the H2plasma we propose that the modification oc-
curs via the following reactions:
⬅Si − CH3+2H→⬅Si−H+CH
4,共3兲
398 Worsley
et al.
: Effect of plasma interactions with low-
films 398
J. Vac. Sci. Technol. B, Vol. 23, No. 2, Mar/Apr 2005
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⬅Si−O−Si⬅+2H→⬅Si−H+ ⬅Si − OH, 共4兲
⬅Si − OH + 2H →⬅Si−H+H
2O. 共5兲
Thus modification would evolve H2O and CH4and could
involve multiple reaction pathways. Note that, unlike for the
Ar/O2plasma, the hydrogen species can react at any of the
four silicon bonds, including Si–O, to change the structure.
This also means that a greater range of products are possible
共i.e., Si–H, Si–H2,Si–H3兲for the case of hydrogen than for
oxygen 共Si–OH兲. Again, physical sputtering of remaining
products would complete the etching process.
Differences in the thermodynamic driving force of these
reactions may influence the modification rate, and thus the
etch rate. Ranking reactions 1–5 in terms of ⌬Hr共calculated
at 298 K兲puts reaction 1 first at −994 kJ/mol, then reactions
3, 4, and 5 at −411,−325, and −325 kJ/mol, respectively.
Reaction 2 is thermoneutral 共⌬Hr=0 kJ/mol兲suggesting it
may not be a key reaction. This reveals that the thermody-
namic driving force for modification by O atoms is more
than twice as great as that for H atoms. Although thermody-
namics does not dictate kinetics, it could contribute to the
difference in etch rates 共and modification rates兲exhibited by
the O2and H2plasmas.
2. Degree of damage: Film composition
An in-depth analysis of the composition of the modified
layer will give insight into modification reaction products
and allow us to confirm reactions 1–5. Figure 3 shows XPS
analysis comparing the effect of different plasma chemistries
on the modified layer in the 8% porous film. Due to the
surface sensitivity of XPS, the information obtained is spe-
cific to the surface of the modified layer for the plasma-
exposed samples. High resolution XPS spectra of the as-
deposited film and modified layers after Ar, Ar/H2, and
Ar/O2plasma exposure were analyzed.
TheC1speak 共not shown兲indicated atomic carbon con-
tent of 19%, 10%, 5%, and 2% for the as-deposited film, Ar,
Ar/H2, and Ar/O2exposed films, respectively. The drastic
reduction of carbon content seen with Ar/O2plasma is con-
sistent with previous reports of O2plasmas interacting with
OSG films. Ryan et al.20 showed that an oxygen ash could
remove as much as 100% of the Si–C bonds in an OSG film.
Liu et al.16 observed large decreases in Si–CH3and C–H
bonds in FTIR spectra after O2plasma ash. Therefore, the
observation that the O2plasma removes substantial carbon
content is consistent with literature.
These data also illustrate how the other plasma chemis-
tries reduce the carbon content to varying extents, with Ar
extracting the least carbon and Ar/O2extracting the most.
This is likely related to the modification mechanism, as men-
tioned above. Oxygen is more efficient at removing carbon-
aceous species than is hydrogen.14,23 Ar is the least efficient
as it etches by simple physical sputtering. In general, carbon
is depleted disproportionately because the Si–C bond is
the weakest bond in the OSG structure. The average
bond enthalpy for Si–C共451 kJ/mol兲is less than
H–CH2共462 kJ/mol兲and substantially less than
Si–O共800 kJ/mol兲.33 Another factor that may influence the
difference between the Ar/H2and Ar/O2plasma is the de-
sorption of products formed by these plasmas. Since COxis
more volatile than CH4it is possible that the difference in the
carbon content left behind by the O2and H2plasma is also a
function of the desorption product volatility. This idea will
be further investigated when discussing substrate tempera-
ture.
The significant reduction of carbon in the surface region,
as indicated by XPS, also suggests that reactions 1 and 3
play an especially prominent role in the modification pro-
cess. The reason for this is twofold: the faster reaction rates
and the higher concentrations of reactive sites in the film
associated with these two reactions. First, as mentioned
above, the Si–C bond is the weakest bond in the OSG struc-
ture, thus facilitating removal of the methyl groups. Second,
reactions 1 and 3 are favored because the majority of the
OSG surface is methyl-group terminated. In previous work,34
water contact angle studies were conducted on OSG films
before and after plasma treatments. The data showed that the
pristine OSG surface was hydrophobic 共methyl-terminated兲
and only became hydrophilic after removal of the C content
by plasma treatment. In addition, it is proposed that the sur-
face of the pores also contain substantial carbon content.
This conclusion is based on low moisture uptake of highly
porous OSG films suggesting the pore surfaces are hydro-
FIG.3. 共a兲XPS spectra at four different plasma conditions of the Si 2ppeak.
“x” designates the ratio of O to Si. 共b兲Schematic of how OSG film is altered
by plasma.
399 Worsley
et al.
: Effect of plasma interactions with low-
films 399
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phobic as well.35 As these surfaces represent a large portion
of the reactive sites for modification, reactions 1 and 3 and/or
permutations thereof should be dominant.
In Fig. 3共a兲, the Si 2ppeak obtained after four different
plasma conditions is presented, showing how the Si bonding
is altered depending on the plasma used. The ratio of oxygen
to silicon 共x兲in the film is also shown as a function of
plasma chemistry. The ratio of hydrogen to silicon could not
be measured since hydrogen is not detected by XPS. The
Ar/O2plasma appears to enrich the film in oxygen compared
to the as-deposited sample. As shown in the figure, xin-
creases from 1.3 to 2 indicating that the modified layer is
very oxidelike 共i.e., SiO2stoichiometry兲. The observation of
the modified layer consisting chiefly of SiO2supports the
proposed reactions 共reactions 1 and 2兲as the process by
which modification occurs. Based on the stoichiometry, the
Si 2ppeak energy would be expected to shift 1.1 eV between
the as-deposited film 共SiO1.3兲and the Ar/O2treated film
共SiO2兲.36 However, because the amount of charging varied
between samples, precise comparison of peak binding ener-
gies between spectra is difficult.
Both the Ar/H2and Ar plasmas introduce the Si to sub-
stantially different environments as evidenced by the features
in the spectra in Fig. 3共b兲. The lower-energy shoulder visible
after the Ar plasma exposure is shifted −2.5 eV from the
SiO1.3 peak. In the literature36 it is shown that SiO1.35 should
be shifted +2.0 eV from SiO0.5 and +2.9 eV from pure Si–Si
bonding. This suggests that the shoulder be attributed to SiOx
where x⬃0.3. This assignment is supported by XPS results
showing a 13% decrease in oxygen content for the modified
layer 共not shown兲. Also, as no reactive species are being
supplied in the Ar plasma, it is assumed that the dangling
bond left by the broken Si–C bond, in the absence of other
reactive species, may be satisfied with an adjacent Si or O
atom giving the two peaks observed.
In the case of Ar/H2, a new peak shifted −3.5 eV from
the SiO1.3 peak appears. Oxygen content was reduced by
40% in this case. The 1 eV difference in binding energy
between the new peak and that created by the Ar plasma
共SiO0.3兲peak, indicates bonding to a species less electrone-
gative than either O and Si. Therefore we attribute this peak
to Si–Hxspecies, with x=2–3,present in the modified layer.
This assignment is consistent with the modification reactions
proposed earlier. A schematic suggesting the film’s structure
after plasma treatment is shown in Fig. 3共b兲. It shows that the
Ar/O2plasma creates an oxidelike modified layer and the
Ar/H2plasma incorporates hydrogen into the film while re-
moving carbon less aggressively.
In Fig. 4 an XPS sputter depth profile of Dendriglass
taken after Ar/H2plasma exposure is shown. In this plot, the
peak attributed to silicon hydride relative to the SiO1.3 peak
is shown to drop exponentially with depth and to asymptoti-
cally approach a minimum value in the bulk film 共as indi-
cated by the dotted line at 450 Å兲. These data suggest that
there is a gradient of damage in the modified layer with the
most severe damage at the surface. The data also suggest that
the hydrogen species have penetrated and modified even the
bulk film to some degree, as seen in the nonzero value at
larger sputter depths. This result is in contrast to the step-like
profiles seen by some groups for O2plasmas14,37 where the
bulk film is not modified at all. In those results, the step
profile is attributed to densification of the oxidized layer,
which slows diffusion and thus inhibits further modification
of the film. The depth to which hydrogen species modify the
film in the Ar/H2plasma may be attributed to their greater
diffusivity and lower reactivity relative to the oxygen spe-
cies. Diffusion of hydrogen species deep into the film is re-
quired for bulk modification. Additionally, the modification
reactions proposed would break up the film structurally as
opposed to densifying it, allowing further diffusion. This sce-
nario would produce a modification profile resembling the
concentration of the reactive species due to diffusion.
Results from FTIR studies of the plasma-treated film 共not
shown兲support the assignment of the new Ar/H2XPS peak
as Si–Hx. The FTIR transmission spectra of the 8% porous
film as-deposited, after Ar/O2plasma and after Ar/H2
plasma were analyzed. The as-deposited film, the film after
Ar/O2plasma, and the film after Ar plasma all show the
characteristic peaks associated with OSG materials: C–H
stretch 共2978 cm−1兲,CH
3deformation 共1275 cm−1兲,
O–Si–O stretch 共1044 cm−1兲, and Si–C stretch 共781 cm−1兲.
However, the film after Ar/H2plasma has some interesting
features: O–H stretch 共⬎3100 cm−1兲, Si–H stretch
共2251 cm−1兲, Si–OH deformation 共891 cm−1兲. These features
confirm the presence of Si–Hxin the modified layer after
Ar/H2plasma exposure. In addition it reveals the presence
of OH species in the film. Also of note for the Ar/H2treated
film is the decrease in absorbance for the peaks associated
with carbon 共2978, 1275, and 781 cm−1兲.
Whereas changes were observed by FTIR in the OSG
samples exposed to the Ar/H2plasma, no significant
changes were seen for the Ar/O2-treated samples. It is likely
that the thickness of the modified layer relative to the bulk
film facilitated the observation of changes in the Ar/H2
plasma treated film while masking the changes in the Ar/O2
plasma treated film. The Ar/H2plasma produced a modified
layer consisting of about 6% of the total film. In addition, a
FIG. 4. XPS sputter depth profile of Dendriglass 共8% porosity兲after Ar/H2
plasma.
400 Worsley
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small amount of modification has occurred in the bulk as
well. On the other hand, the Ar/O2plasma produced a modi-
fied layer less than half that of the Ar/H2plasma. Because
FTIR samples the entire film, it is likely that the changes
produced by the Ar/O2plasma were below our detectable
limits. Thus no conclusions were drawn from the Ar/O2
plasma treated film from FTIR studies.
Data presented in this section clearly show that there are
different mechanisms by which H2and O2plasmas modify
the film. These differences are likely responsible for the dis-
parities seen in the etch rate and modified layer after expo-
sure.
C. Reactive species concentration
The influence of concentration of the reactive species 共H
or O兲on the depth and degree of modification was observed
by varying the fraction of H2or O2flow relative to total gas
flow. Figure 5 tracks the changes for Ar/O2and Figures 6
and 7 follow Ar/H2. Figures 5 and 6 highlight the depth of
damage by showing etch and modified layer growth rates.
The modified layer growth rate was determined by dividing
the modified layer depth remaining after etch by the expo-
sure time, and hence represents an average growth rate. Fig-
ure 7 focuses on the degree of damage caused to the OSG by
examining the refractive index 共RI兲and chemical composi-
tion of the modified layer formed by the Ar/H2plasma.
Plasma etching occurs via the combination of two mecha-
nisms: chemical adsorption of reactive species and physical
desorption 共sputtering兲of volatile adsorbates. In the case of
the plasmas investigated in this study, it is assumed that the
dominant reactive species, H or O radicals, are responsible
for chemical etching while primarily Ar+ions 共and
FIG. 5. Plots of Dendriglass 共a兲etch rate and 共b兲modified layer growth rate
as a function of oxygen flow.
FIG. 6. Plots of Dendriglass 共a兲etch rate and 共b兲modified layer growth rate
as a function of hydrogen flow.
FIG. 7. Dendriglass 共8% porosity兲共a兲refractive index and XPS SiHx:SiOx
peak ratio, and 共b兲Si 2pXPS spectra.
401 Worsley
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O+,O
2+,H
+, and H2+ions to a lesser extent38兲are respon-
sible for physical sputtering. For the Ar/O2plasma, Fig. 5共a兲
shows that the highest etch rate is achieved with 88% O2
flow, i.e, only 12% Ar, in the chamber. This suggests that
only a small amount of Ar, and therefore physical sputtering,
is necessary for etching to occur and that chemical etching is
more important.
The reactive species concentration also gives insight into
how porosity affects the kinetics of these processes. In Figs.
5共a兲and 5共b兲, note that although the reactive species concen-
tration does appear to play a role in determining the etch rate
and modified layer growth rate, the porosity of the film de-
termines the significance of that role. For the etch rate 关Fig.
5共a兲兴, the 0% porous film exhibits no dependence on O2flow
and the 8% and 45% porous films show clear dependence on
O2flow. Recalling the earlier discussion of the dependence
of etch rate on surface area, these data support the idea that
for the 0% porosity film, reactive sites are saturated at low
O2flows. Therefore increasing the O2flow would not in-
crease the etch rate. As porosity is increased to 8% and 45%,
the surface area and diffusivity of the reactive species also
increase. Increasing the surface area provides more reactive
sites so that saturation no longer occurs at low O2flows.
Thus, with the porous films, increasing the O2flow results in
an increase in etch rate.
In Fig. 5共b兲, the modified layer growth rate showed a
similar trend. Again, porosity plays a key role in determining
the significance of changing the reactive species concentra-
tion. There is a general trend of decreasing modified layer
growth rates with increasing porosity regardless of O2flow.
The exception for this is the high O2flow on the 45% film
which will be discussed shortly. This general trend, however,
is most likely a consequence of the drastically increasing
etch rate with porosity 关Fig. 5共a兲兴. The sharp increase in etch
rate with porosity 共almost tenfold from 0% to 45%兲means
the modified layer is consumed at an increasing rate. Thus,
the growth rate of a remaining modified layer decreases with
increasing porosity at steady state.
The porosity also appears to determine the kinetics of the
modification process as well. For the 0% and 8% porous
films, the growth rate was not significantly affected by O2
flow, while the 45% porous film showed a threefold increase
in growth rate with O2flow. Recall similar results in Fig.
1共a兲, where the total depth of damage remained constant for
the 0% and 8% porosity films then increased significantly for
the 45% porosity film. As discussed earlier, the 45% porosity
film represents a significant increase in both reactive sites
共surface area兲and access to the subsurface region 共open po-
rosity兲over the lower porosity films. If either of these factors
limits the modification rate in the lower porosity films, the
45% porous film may provide a regime where these are no
longer limiting. Thus the modification rate can be dependent
on reactive species concentration. We propose that the pres-
ence of significant open porosity may be responsible for the
concentration dependent behavior for the O2plasma. This
result may be another clue as to how strongly interconnec-
tivity can affect the processing of ILD films.
The degree of damage sustained by the OSG in the Ar/O2
plasma 共not shown兲was also monitored and found to be
independent of O2concentration. The refractive index 共RI兲
of the modified layer, though increased from that of the as-
deposited film 共1.33兲, remains relatively unchanged 共1.42–
1.43兲with increasing O2flow. This increase in RI is mostly
attributed to a loss of organic groups, though densification
may play a role as well. Also, the RI of the modified layer is
close to that of pure SiO2共1.47兲, the substance it closely
resembles. The Si 2pandC1sXPS spectra also exhibit no
peak shift with increasing O2flow, though the C 1sspectrum
shows significant carbon loss with any O2flow.
Figure 6 shows the depth of damage sustained for the
OSG in the Ar/H2plasma. Etch rate as a function of H2flow
for the Ar/H2plasma is shown in Fig. 6共a兲. This plot reveals
a similar trend as in the Ar/O2plasma. Etch rate shows some
dependence on porosity 共fourfold increase from 0% to 45%
film兲regardless of concentration, though not as significant as
with the Ar/O2plasma. The etch rate on porous films in-
creases with increasing H2flow and the etch rate on the
dense film 共0%兲showed little to no change with H2flow.
This result is similar to that observed for the Ar/O2plasma.
The fact that Ar/H2and Ar/O2etching processes show simi-
lar trends suggests that similar processes 共i.e., ion-induced
desorption of adsorbates兲are occurring though at signifi-
cantly different rates.
The modified layer growth rate as function of H2flow
关Fig. 6共b兲兴 is relatively unchanged for all three porosities
studied. This situation is analogous to the 0% and 8% porous
films in the Ar/O2plasma indicating that the presence of
open porosity 共at 45%兲does not as significantly affect the
film’s behavior in the Ar/H2plasma. Also of note is that all
three porosities collapse onto the same line indicating modi-
fied layer growth rate is independent of porosity. This result
is a direct function of the fact that modified layer depth is
independent of porosity 关Fig. 1共b兲兴. Because the modification
rate tracks with the etch rate as porosity is increased, no
change in the modified layer depth 共or modified layer growth
rate兲was observed.
Figure 7 shows the degree of damage sustained by the
OSG in the Ar/H2plasma. In Fig. 7共a兲, the RI and ratio of
Si–Hx:Si–Ox共determined by XPS兲as a function of H2flow
are plotted. Both of these values increase at higher H2flow.
The increase in RI is likely due to the increasing presence of
the silicon hydride species. According to Fukuda et al.39 the
Si–H bond, with a representative refractive value of 3.201, is
much more polar than the Si–O bond, which has a represen-
tative refractive value of 1.75. The Si–H bond is also more
polar than the Si–C bond.4Therefore, replacing Si–O or
Si–C bonds with Si–H bonds, as shown in reactions 3–5, to
any appreciable amount would increase the RI of the mate-
rial. The Si 2pXPS spectra 关Fig. 7共b兲兴 shows the increase in
Si–Hx:Si–Oxclearly. The C 1sXPS spectra 共not shown兲
indicated carbon content also fell slightly with increasing H2
flow. The lower carbon content could also contribute to the
increased RI.
402 Worsley
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These data highlight key differences in the way that dam-
age induced by H2and O2plasmas depends upon reactive
species concentration. Porosity played a dominant role in
how depth of damage was manifested.
D. Substrate temperature
Substrate temperature was also varied in the case of the
Ar/H2plasma to observe its effect on the modified layer in
the 8% porous film. Figure 8 is a plot of both etch rate and
modified layer thickness as a function of substrate tempera-
ture. Both etch rate and modified layer thickness are shown
to increase with temperature, though the modified layer
thickness increases twice as fast as the etch rate. The in-
crease in modified layer thickness may be attributed to in-
creased rates of modification as well as greater diffusivity on
the higher temperature substrate. As etch rate is limited by
the modification reactions, an increase in the rates of the
modification reactions would translate to an increased etch
rate. Thus it is suggested that increased reaction rates for
reactions 3–5 play a significant role in the greater etch rate at
elevated substrate temperatures. In addition, the increase in
etch rate with increasing temperature may be expected be-
cause desorption of the etch products would increase with
temperature.
XPS spectra and RI of the film after plasma exposure are
shown as a function of temperature in Fig. 9. In Fig. 9共a兲the
Si 2pspectra show a decrease in the Si–Hxpeak with tem-
perature. The Si–Hx:Si–Oxratio is shown with RI as a func-
tion of temperature in Fig. 9共b兲. Both the RI and the
Si–Hx:Si–Oxratio decrease with increasing temperature. In
addition, the carbon and oxygen content was measured by
XPS as function of temperature. It was found that carbon
content more than doubled in the modified film going from
20 °C to 77 °C 共though it is still at a level less than half the
amount present in the as-deposited film兲. Oxygen content
also showed a significant increase over this range. The in-
creased carbon and oxygen content also contributed to the
reduction in RI with increasing temperature. These data sug-
gest that the degree to which the film is modified is mini-
mized at elevated substrate temperatures.
Examination of the temperature data can be used to gain
insight into whether the desorption or the formation of
Si–Hxis the rate limiting step 共RLS兲in the modification of
the OSG film by the Ar/H2plasma. The experimental data
clearly show that Si–Hxcontent decreases and carbon and
oxygen content increases with increasing temperature. Reac-
tions 3–5 describe generation reactions for Si–H groups. The
lower presence of silicon hydrides could therefore be due to
a decreased rate of Si–Hxformation 共reactions 3–5兲.A
slower rate of Si–Hxformation with increasing temperature
共negative apparent activation energy兲could be interpreted as
a lower sticking coefficient for hydrogen species at higher
temperatures. Alternatively, reactions 3–5 may not change
significantly with temperature, and the lower concentration
of Si–H may result instead from facile desorption of Si–Hx
after formation at higher temperatures. Such an effect may
occur due to increased vapor pressure at elevated substrate
temperatures, for example. However, the data do not support
this second possibility. Reaction 3 links the formation of
Si–H groups with removal of carbon 共in the form of CH3兲.
Because the carbon content increases with temperature while
Si–H decreases, these data suggest that the rate of reaction 3
is reduced with increasing temperature. Furthermore, reac-
tions 4–5 link the formation of Si–H groups with the re-
moval of oxygen 共in the form of H2O兲. Because the oxygen
content also increases with temperature while Si–H de-
creases, we conclude that the rates of reactions 4–5 are also
FIG. 8. Dendriglass 共8% porosity兲etch rate and modified layer thickness as
a function of substrate temperature.
FIG. 9. Dendriglass 共8% porosity兲共a兲Si 2pXPS spectra and 共b兲refractive
index and XPS SiHx:SiOxpeak ratio as a function of temperature.
403 Worsley
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: Effect of plasma interactions with low-
films 403
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reduced with increasing temperature. Given these two obser-
vations, we speculate that the formation of Si–Hxis the RLS
in the modification process. The lower sticking coefficient of
hydrogen species at elevated temperatures would increase
their effective diffusivity, allowing diffusion farther into the
film. Though the modification is less severe 共etch rate shows
only moderate increases and SiHxcontent is decreased兲,itis
still detectable by our methods and occurs much deeper in
the film. Thus the apparent effect at elevated temperatures is
decreased chemisorption and increased diffusivity of hydro-
gen species in the OSG.
The temperature studies also may explain the variety of
results published by different researchers with H2
plasmas.13,14,23,24 As stated earlier, some groups report that
H2plasmas have no effect on the film,14 others that it en-
hances the properties of the film,23,24 and yet others indicate
that H2plasmas do indeed cause damage.13 From the results
of the present study one may conclude that at low tempera-
tures H2plasmas are damaging, and they become less so at
high temperatures. While it is likely that temperature is not
the only factor that may have been different in previous
work, its effect, as shown here, is significant.
Overall these data indicate that increasing the temperature
significantly reduces the degree to which the film is damaged
while simultaneously increasing the total depth of the dam-
aged layer. Formation of silicon hydride species appears to
play a dominant role in this process.
As a note, these temperature-dependent studies were all
performed under the same conditions, including strip time.
Though this does not change our results, some of the conclu-
sions would change depending on the photoresist strip rate’s
dependence on temperature. For example, if the strip rate
increased rapidly from 20–80 °C, then much shorter strip
times could be used resulting in shallower modified layers.
This would lead one to conclude that higher substrate tem-
peratures may decrease plasma damage in terms of both
depth and degree instead of just degree as we conclude. To
account for this, temperature effects on photoresist strip rates
should be addressed in future experiments.
IV. CONCLUSIONS
Through these experiments both the depth and degree of
plasma damage in OSG films were observed as a function of
porosity, plasma chemistry, reactive species concentration,
and substrate temperature. It was found that the depth of
plasma damage, which was characterized by etch depth,
modified layer depth, etch rate, and modified layer growth
rate was strongly influenced by porosity and plasma chemis-
try. Highly porous films in O2plasmas showed the highest
etch rate, dense films in O2plasmas had the highest modified
layer growth rates, while highly porous films in H2plasmas
had the deepest modified layers and etch depth. For H2plas-
mas, it was shown that even the bulk film exhibited nonzero
levels of modification.
The surface area 共or reactive sites兲accessible to the reac-
tive species and the diffusivity played key roles in determin-
ing how the plasmas interacted with films of different porosi-
ties. Pore interconnectivity played an important role with
regard to the O2plasma. Differences in etch rate and modi-
fied layer depths for the O2and H2plasmas were attributed
to the variations in the diffusivity and modification mecha-
nisms of the reactive species. Modification reactions for both
species were proposed and discussed in terms of the reaction
pathways and thermodynamics. Reactive species concentra-
tion data supported the claim that interconnectivity plays a
dominant role when dealing with the O2plasma and showed
how porosity in general dictated the magnitude of the etch
rate and its dependence on reactive species concentration.
In the case of both plasmas, the etch rate had an increas-
ing dependence on reactive species flow with porosity. The
fact that Ar/H2and Ar/O2plasmas show similar trends sug-
gested similar processes 共i.e., ion induced desorption of ad-
sorbates兲are occurring though at significantly different rates.
The modified layer growth rate showed no dependence on
reactive species flow except in the case of the 45% porous
film in O2plasma in which it increased. This reinforced data
suggesting that an open porosity film is especially suscep-
tible to damage by the O2plasma, a result which is particu-
larly relevant for ultra low-
materials. The substrate tem-
perature data showed increases in both etch rate and
modified layer thickness with temperature.
The degree of plasma-induced damage, characterized by
changes in chemical composition and RI, was clearly depen-
dent on plasma chemistry. This was seen in the varying de-
grees of carbon depletion and changes in Si bonding domi-
nant in the modified layer. Ar/O2plasmas were the most
efficient at removing carbon, while Ar/H2plasmas intro-
duced silicon hydride species into the film. These data sup-
ported the modification reaction scheme proposed and sug-
gested that carbon abstraction by O and H species 共reactions
1 and 3 and/or permutations thereof兲play dominant role共s兲in
the modification process. Reactive species concentration data
supported the claim that O species efficiently remove carbon
共only small amounts were needed to significantly deplete the
film兲.
When focusing on the H2plasma chemistry, more changes
in the degree of damage were found as a function of reactive
species concentration and substrate temperature. It was ob-
served that increasing the H2concentration increased both
the amount of Si–Hxand the RI, while increasing the sub-
strate temperature reduced both. In addition, the increased
carbon and oxygen content and decreased Si–Hxcontent in
the modified layer at elevated temperatures suggest that for-
mation of Si–Hxis the rate limiting step in this dynamic
equilibrium process. This final result is useful as it helps
explain the differing results published concerning the effect
of H2plasmas on OSG films by highlighting the temperature
dependence of H2plasma-induced damage.
ACKNOWLEDGMENTS
The authors would like to thank Willi Volksen and Robert
Miller for supplying the Dendriglass solutions and for dis-
cussions concerning the film, Ed Sikorski for discussions in-
volving the plasma tool, and Stefanie Chiras and Maurice
404 Worsley
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McGlashan-Powell for discussions involving XPS. This
work was partially supported by an IBM PhD Fellowship
共M.A.W.兲. M.A.W. gratefully acknowledges the National
Science Foundation and General Electric for funding. One of
the authors is a Ford Fellow 共M.A.W.兲.
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