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Measurement and analysis of EUV photoresist related outgassing and
contamination
Article in Proceedings of SPIE - The International Society for Optical Engineering · March 2009
DOI: 10.1117/12.814862
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Measurement and analysis of EUV photoresist related outgassing and
contamination
I. Pollentiera, G. Aksenovb, A-M. Goethalsa, R. Gronheida, R. Jonckheerea, and M. Leesonc
a
IMEC, Kapeldreef 75, 3001-Leuven, Belgium
b
Katholieke Univ. Leuven, Belgium
c
on assignment from Intel Corp.
ABSTRACT
Photoresist outgassing and the related risk for optics contamination in extreme ultraviolet (EUV) exposure tools are
concerns in the development of EUV lithography, especially towards the high volume manufacturing tools. The
measurement however of how much and what species are outgassing/contaminating, is still very challenging. Various
techniques are investigated worldwide, but there is still no consensus on which technique is most adequate. Moreover,
since the outgassing/contamination qualification of photoresists needs dedicated tool set-up, it is likely that the testing
configuration (with parameters such as exposure intensity, background vacuum quality, pumping speed, … ) can impact
the measurement result.
In this paper, we are comparing two candidates for outgassing/contamination measurement which are integrated in one
experimental set-up : RGA (Residual Gas Analysis) and witness plate testing. RGA is based on in situ mass
spectrometer measurements during photoresist EUV exposure and enables chemical identification of species that are
outgassing, but has limited information on the probability of mirror contamination. Results are shown on how the
measurement results can depend on the testing configuration. Witness plate testing is based on the evaluation of EUV
exposed mirror samples that are placed in the vicinity of EUV outgassing photoresist. Results are shown on how the
generated contamination can be affected by the tool configuration, and on how to measure/analyze the contamination.
Finally, since both techniques are integrated in one test-set-up, measurement results will be compared and correlated,
which should help in understanding the phenomena and lead to well defined measurement for photoresist qualification.
Keywords: EUV, outgassing, contamination, photoresist qualification, RGA, witness plate
1. INTRODUCTION
Currently, resist related outgassing is considered as a risk for optics degradation in EUV exposure tools. However, up to
now, there is no clear and agreed way of measuring this issue, in order to ensure that when the resist is exposed in an
EUV exposure tool the risk of contamination is limited to the foreseen lifetime of the EUV optics.
On one hand, investigation on this matter is done by measuring the amount of outgassing of the photoresist. Also here
there are several measurement techniques – such as pressure rise, gas chromatography mass spectroscopy (GC-MS), and
Residual Gas Analysis (RGA) – resulting in the fact that in a first attempt to check the outgassing in a round-robin test
on a model resist five orders of magnitude variations have been found in test results [1]. The main cause of the variation
was found to be related to the analysis of the outgassing and the related impact of the test set-up [2]. In a more recent
benchmarking of a model resist outgassing at CNSE, NIST, and IMEC, the variations in outgassing were found to be
much smaller, i.e. only a factor 3 difference across the three test sites [3].
Despite the progress in measuring the outgassing in a more accurate way, the technique still suffers from the uncertainty
which part of the resist related outgassing – i.e. which chemical species – results in contamination on a EUV mirror.
Therefore ASML as tool supplier is still encouraging witness plate evaluation [4] as the only way to qualify if a resist
Alternative Lithographic Technologies, edited by Frank M. Schellenberg, Bruno M. La Fontaine
Proc. of SPIE Vol. 7271, 727146 · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.814862
Proc. of SPIE Vol. 7271 727146-1
possibly will contaminate the mirror. Disadvantages of the technique are however that it requires a more complex and
expensive set-up to generate the contamination on a plate, a more complex and expensive metrology (e.g. reflectometry,
ellipsometry in combination with X-ray photoelectron spectroscopy (XPS), …), and that the background vacuum quality
of the test-set set-up might interfere with the contamination.
In this work, both approaches – i.e. measurement of the outgassing by RGA, and measurement by the contamination by
witness plate testing – has been investigated in one single test set-up, in order to compare and correlate both results.
2. EXPERIMENTAL
At IMEC, investigation on the outgassing and contamination is done using an experimental set-up, supplied recently by
EUV Technology (Martinez, US). The tool layout is shown in Fig. 1.
The EUV Technology outgassing tool uses an Energetiq EUV source, supplying 10W/2πsr EUV irradiation into the
system. Spectral filtering is achieved on one hand side by a thin Zr spectral purity filter (SPF), and secondly by a
multilayer (ML) mirror, that reflects the incoming irradiation towards the sample chamber. In this path a grazing
incidence mirror is present that partly reflects the light to the wafer position, while the rest can proceed to the witness
plate position. This grazing incidence mirror is also able to measure the amount of power incident on that mirror, which
was compared with a calibrated detector during tool set-up.
In the sample chamber a wafer stage enables moving a 200mm wafer relative to the EUV incident beam. The spot size
on wafer is ~ 10mm2 and the power density ~4.5mW/cm2. The wafers can be transferred in and out the sample chamber
by an automated load lock. In the chamber also a Pfeiffer QMG422 RGA is present to measure the outgassing. During
installation these measurements have been calibrated using a calibrated leak. Finally, the sample chamber contains also
a location where a witness plate can be mounted. This can again be done by a sample load lock, in order not to break the
sample chamber vacuum. The spot size at witness plate position is much smaller than at wafer and approximately 1mm
in diameter. The intensity is measured as Gaussian shaped, with maximum intensity in the spot center of about
80mW/cm2. A typical vacuum level that can be obtained in the sample chamber is ~1.2 10-8mbar.
In the remainder of this paper results will be discussed obtained by the RGA and witness plate testing.
Optical design by EUV Technology
ML
mirror
Xe-source
Energetiq
(10W/2πsr)
Zr SPF
RGA
Beam split mirror
& power detector
witness plate
200mm wafer
(mask) loadlock
Figure 1 : Schematical representation of IMEC’s outgassing tool (see text).
Proc. of SPIE Vol. 7271 727146-2
3. RESIST OUTGASSING TESTING BY RGA
In this section it will be outlined how the resist related outgassing is determined in the system described above.
Moreover, some trends in outgassing will be presented as well as results on our resist qualification for use on the EUV
ADT.
3.1 Determination of resist related outgassing
Resist related outgassing is determined by RGA in conditions that are representative for use on the scanner. Therefore
the outgassing test should be typically done at an exposure dose equivalent to 2.5*E0, where E0 is the dose to clear of the
resist. In the IMEC system this is achieved by scanning the 200mm wafer on a XY-stage with a speed according this
dose, and taking into account the power reading as obtained on the beam-split power mirror.
An RGA measurement performed on an EUV exposed resist wafer will not only reveal resist related partial pressures,
but also the partial pressures from the background. In order to make a distinction between background and resist, two
RGA measurements are done on the system, namely a measurement with a resist coated wafer and one with a bare Si
wafer. By subtraction of the two, the resist related outgassing is then identified. In our work, it has been found
important to do both the resist related measurement as well as the background measurement in identical conditions, i.e.
using EUV exposure and similar wafer scanning.
An example of such a resist related RGA measurement together with the background measurement is shown in Figure 2
(a). It can be seen that part of the resist related spectra is identical to the background within the noise level. However, in
the region 35-120amu, a clear difference is observed, which is due to resist outgassing. Taking into account the pumping
speed of the sample chamber, this difference in partial pressure can be converted into outgassing rate spectrum (in
molecules cm-2 s-1), as shown in Figure 2 (b). In this particular case the outgassing spectrum of Robert Brainard’s Open
Source resist is shown, where mainly isobutene (amu 27, 41 and 52), and tert-butylbenzene (amu 77, 91, 119) are
detected.
RGA spectra
Raaiat
Bac kg ro a a d
I.
II
I
0-a 3
0
20
40
60
80
100
120
140
160
180
200
amu (-)
(a)
(b)
Figure 2 : Resist related RGA outgassing measurement : (a) measurement of RGA partial pressure spectrum for both the background and resist; (b)
calculation of outgassing rate based on the difference in partial pressure.
3.2 Trends in resist related outgassing
Using the methodology described above to determine the outgassing rate for a specific resist, testing has been done using
various resists in order to understand where the outgassing is dependent on.
Proc. of SPIE Vol. 7271 727146-3
In Figure 3, the dependency of outgassing is depicted as function of resist thickness. Although the resist was only tested
at two thicknesses, it reveals that outgassing is fairly proportional to resist thickness, and therefore resist outgassing can
be considered as a bulk effect.
0
Cfl
Figure 3 : Outgassing rate as function of resist thickness.
N)
00
outgassing rate (molec cm-2 s-i
01
0
Moreover, it has been found the outgassing rate is slightly changing during long testing. This is illustrated in Fig. 4,
where outgassing has been measured by RGA while scanning the resist during almost 2hours. It was found that the
outgassing after ~1.5hr is about 50% higher that the outgassing rate as determined from the first 0.25hr. This might be
explained by either ongoing (de-protection) reactions in the resist, even when the exposure beam has left the exposed
area, and/or by insufficient capability of pumping away specific outgassing species.
Figure 4 : Outgassing rate as function of time during long-term testing.
In addition to full-spectrum measurements, such as illustrated in Figure 2, time dependent outgassing measurements have
been explored on limited amu’s, as illustrated in Fig. 5. In this test the resist has been exposed for ~1min, during which
five amu’s are followed. Amu18 is related to water, which clearly is insensitive to EUV exposure. Moreover it can be
seen that the time dependent behavior for some amu’s (such as amu 41 and 52, as well as for amu 77 and 91) are very
similar, which suggests that they are originated from the same chemical species. Therefore, it is believed that this time
dependent measurements can help in the identification of the outgassed chemicals.
amu
EUV
18
41
52
77
91
Figure 5 : Time dependent RGA measurements on limited amu’s shows that the time behavior of outgassing can be different for different amu’s. In
this case an EUV exposure is done on a wafer in a static way for 60s, as indicated by the rectangle.
Proc. of SPIE Vol. 7271 727146-4
3.3 Resist outgassing qualification
8E+14
8.E+14
7E+14
7.E+14
6E+14
5E+14
4E+14
3E+14
2e+14 molec cm-2 s-1
2E+14
1E+14
0
outgassing rate (molec cm-2 s-1)
outgassing rate (molec cm-2 s-1
Using the methodology described above to determine the resist outgassing rate, systematic testing has recently started at
IMEC in order to qualify if the resist can be used on the EUV ADT. For limited evaluation on the ADT, ASML has
currently a guideline that resist outgassing should not exceed 2 1014 molecules cm-2 s-1 @10mW/cm2. As can be seen in
Figure 6 (a), several resists have been tested in the last few months; the majority has past the test, but still a significant
number of resists exceed the guideline. In an attempt to estimate the measurement error on the outgassing result,
repeatability has been tested using two resist materials (Figure 6 (b)). In both cases the variations were limited and well
below 20%.
Repeatability
6.E+14
5.E+14
4.E+14
3.E+14
2.E+14
1.E+14
0.E+00
1
3
5
7
9
11
13
15
17
resist A
resist B
resist
(a)
(b)
Figure 6 : Results of resist outgassing qualifications at IMEC for the EUV ADT (a). In (b) the repeatability of the outgassing measurement is checked
on two resists.
4. CONTAMINATION TESTING BY WITNESS PLATE
In this section it will be outlined how the witness plate testing is done on the set-up including the metrology to measure
the contamination. Moreover witness plate test results will be discussed with primary focus on the contamination in
background, i.e. contamination without resist in the vicinity. Finally, preliminary witness plate results with a high
outgassing resist are compared to the background results.
4.1 Witness plate testing and metrology
Performing a witness plate test in the EUV Technology system is a fairly simple operation. The witness plate can be
brought into the sample chamber via a load lock without changing the vacuum significantly. By a proper position of the
beam split mirror the configuration allows then EUV irradiation both on witness plate and on the wafer. During the
witness plate testing the position of the witness plate is not changing, while the wafer can continuously be moved by the
XY-stage, e.g. to give the resist coated wafer the required exposure dose.
Initial screening of the witness plate testing during the factory acceptance demonstrated that due to the high intensity
irradiation on the witness plate position, it is fairly easy to generate contamination, even in background, i.e. when no
Proc. of SPIE Vol. 7271 727146-5
resist is present for outgassing. This is illustrated in Figure 7 (a), where the impact of the local exposure is detected by
reflectometer measurements, performed at LBNL-ALS using the synchrotron beam-line set-up.
Although these reflectometer measurements at EUV wavelength are very representative for EUV mirror contamination,
we have pursued alternative metrologies for measuring the contamination on witness plate mirrors. One reason for this is
to have a shorter turn-around time for measurement result. Moreover, the reflectivity measurement does not supply
chemical information of the contamination, and measurements can be susceptible to the substrate reflectivity swing
effect, and hide information on the real thickness.
A first measurement on witness contamination that was developed at IMEC is ellipsometry using a SOPRA GES-5. The
result on the same witness plate as Figure 7 (a), measured by reflectometry, is shown in Figure 7 (b). Good correlation is
found with respect to spot size and shape, and its thickness of ~1nm is in agreement with reflectivity calculation using
available optical constants for the materials involved. Having this good matching with reflectometry, the technique has
been used frequently in witness plate evaluation, as will be shown in the remainder of this paper.
In addition to ellipsometry, it is important to check the contamination on witness plates also towards composition.
Especially, when evaluating the contamination related to resist, the composition might be varying amongst resist
chemistries, and therefore thickness results from ellipsometry might be affected by that. In our work we have evaluated
angle resolved XPS to investigate the same contamination spot that previously was analyzed by reflectometry and
ellipsometry. This is illustrated in Figure 7 (c) where the material composition – normalized to a total of 100% – is
compared between a location in the contamination spot center and a location far outside the spot. As can be seen, part of
the detected elements such as Ru, O, Mo, and Si are related to the witness plate stack, and therefore are not related to the
contamination. Taking into account that the stack should be equal in the two cases, it can be concluded that
contamination in this case – i.e. factory acceptance period – is mainly carbon, and traces of fluor.
XPS (IMEC)
Reflectometry at EUV (ALSBerkeley)
Ellipsometry (IMEC)
67
Thickness mapping (551)
66.5
66
65.5
-2
65
-4
-6
-S
64.5
-8
10
20
30
35
40
45
64
F
Mo
SOPRA – GES-5
Refi (%)
S
S
O
Composition (%)
60
6
r
C
60
Ru
40
Si
20
x (mm)
1< ImmI
(a)
(b)
0
3220 off 78
outside
spot
on 78
on3220
spot
(c)
Figure 7 : Metrology of the localized contamination on the witness plate : (a) reflectometry at LBNL-ALS; (b) ellipsometry using SOPRA GES-5; (c)
XPS.
4.2 Trends in background witness contamination
Since the first measurements of background contamination, several tests have been performed to decrease the amount of
background contamination, and – more important – to understand the impact parameters of this.
One specific kind of testing is the impact of witness plate exposure time to the contamination thickness. In this case the
exposure time was varied from 0.5hr to 2hr, which is equivalent with respectively a dose of ~1.4 J/mm2 to 5.6 J/mm2 in
Proc. of SPIE Vol. 7271 727146-6
the center of the illumination spot. The result of ellipsometer measurements are shown in Figure 8. As can be seen the
main impact of the increasing exposure time is the increasing area of the contamination, while the thickness is fairly
constant to a value of ~0.3nm. Moreover, comparing the spot to the area related to the estimated FWHM of the
Gaussian-like witness plate irradiation profile, the contaminated area tends to be much larger. The reason for this
increase is not yet fully understood, but could be related with either a probability of self-diffusion or with non linearity in
contamination with illumination intensity.
Thickness msppincj nm)
Thickness msp p incj mm)
Thickness msppincj nm)
@5
1hr
0.5hr
2hr
4
4
4
3
2
-1
-2
4
-2
x imm
x imm
(a)
(b)
Q
4
C
x mm
(c)
Figure 8 : Behavior of background contamination as function of witness plate exposure time, measured by ellipsometry. The dashed circle is an
indication of the area corresponding to the FWHM of the Gaussian-like witness plate irradiation profile.
4.3 Resist related witness plate contamination
Recently preliminary resist related contamination tests were performed, in order to demonstrate the feasibility of this
testing. A high outgassing fluorinated PMMA resist [5] was used in an attempt to have an extreme case of resist
outgassing. For this resist the total outgassing was found to be 1.2 1016 molec cm-2 (1-200amu @ 10mW/cm2), which is
more than an order of magnitude larger than typical EUV resist chemistries currently used. In order to differentiate
between background and real resist related contamination, a background witness plate was carried out using a bare-Si
wafer. Moreover, as is described above, multiple witness exposures as function of time are done in both cases.
Results evaluated with ellipsometry are shown in Fig. 9 for both background and resist contamination. The background
contamination had a similar behavior as shown in Figure 8 – i.e. a fairly constant contamination thickness but gradually
increasing in area with exposure time – but over-all the contamination was less than in the previous test. The reason for
this is not yet understood. The resist related contamination, which was carried out just after the background shows very
similar behavior in terms of contamination area as function of exposure time. However, the spots related to the longer
exposure times suggest that the contamination thickness is slightly increased by the resist exposure. This encouraging
result will be investigated in detail further in future.
SUMMARY AND CONCLUSION
In this work, two EUV outgassing/contamination measurement techniques have been evaluated : RGA and witness plate
testing.
RGA is able to quantify the outgassing in terms of total amount, rate, and variations over time. Although the
identification of the outgassing species is not easy and it might not be possible to predict the probability of mirror
contamination, the technique can provide understanding of outgassing and should give consistent results in different setups.
Witness plate testing is more complicated, since it involves also the measurement of contamination, besides its
generation by EUV exposure. Ellipsometry in combination with XPS were found to be a trustworthy alternative to EUV
reflectometry. Witness plate testing in background conditions suggests either probability for self-diffusion or non-linear
Proc. of SPIE Vol. 7271 727146-7
behavior as function of exposure time. Distinction between background contamination and resist related contamination
is difficult, but preliminary tests suggest an increased contamination if the plate is exposed in the vicinity of a high
outgassing resist material.
Background
Thickness mapping (A)
Thickness rapping (nm)
15min
Thickness mppincj (nm)
4
6
30min
90min
S
4
4
4
E
E
E
E3
E3
E3
2
2
2
01
0
-4
-4
2
-2
U
2
-4
-2
LI
x mm)
x (mm)
x mm
Thickness mapping (nm)
Thickness mapping (nm)
Thickness mppincj (nm)
2
1__iO 4
15min
30min
90min
4
E
E3
2
-4
-2
x mm)
Ci
2
Resist
I
01
Figure 9 : Comparison of resist related witness plate contamination to the background, as measured with ellipsometry (see text)
AKNOWLEDGEMENTS
In this work we like to thank in first place R. Perera, D. Houser, J. Underwood, and A. Latzke of EUV Technology for
all support on the outgassing tool and for helpful discussions. Further on, we also greatly acknowledge M. Baklanov, T.
Conard, and D. Cheyns (IMEC) for helpful discussions on contamination metrology, and G. Claeys and F. Van Roey for
valuable help on tool and material support in the investigation. From ASML, Jens Steinhoff is acknowledged for tool
and metrology discussions. In the collaboration with CNSE, we thank R. Brainard for providing the Open Source resist,
and G. Denbeaux for helpful discussions. For providing the fluorinated PMMA resist material, we thank J. Strahan and
G. Willson (Univ. Texas). Finally, we acknowledge J. Van Wakkeren (TNO) and E. Gullikson (LBNL-ALS) for the
witness plate related material supply and reflectometer measurements respectively.
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Proc. of SPIE Vol. 7271 727146-8
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