1
Outgassing of water vapour and hydrocarbon species in extreme ultraviolet (EUV) lithography places a strict outgassing specification on all materials entering the vacuum environment. This is
especially true in the projection optics environment where tight control of contaminant partial pressures is required to preserve multilayer mirror life, consequently outgassing will be an
important consideration of photo-resist materials. The level of this outgassing from the photo-resist is likely to be increased upon irradiation with the EUV light and with freshly coated wafers
passing through the tool at over 100 wafers per hour, this EUV stimulated photo-resist outgassing may be the most significant contributor of hydrocarbon contaminants to the system. The level
of stimulated outgassing has the potential to greatly influence the design of EUVL vacuum systems and contamination mitigation schemes.
We have performed EUV stimulated vacuum outgassing measurements, using a quadrupole mass spectrometer (QMS), from 2 EUV type photo-resist coated wafers and a blank silicon wafer as
control sample. These are some of the first to be made using a high power pulsed EUV source. Stimulated outgassing was recorded for two different incident EUV intensities on each sample
and a measured dependence of the outgassing on incident EUV intensity was observed. Specific vacuum outgassing has been observed from the resist coated wafers and identified as originating
from the resist as opposed to the solvent.
2 Experimental set up
3 Source Conditions
• Outgassing measurements were performed using a 200
amu residual gas analyser.
• Measurements were performed using EUV from a z-pinch gas
discharge source.
• Exposure conditions Power 1
1.4 kW input power
14 W EUV in band into 4π sr
=0.5% CE into 2π sr
power density achieved 200 mW/cm 2
• Exposure conditions Power 2
1 kW input power
9 W EUV in band into 4π sr
=0.5% CE into 2π sr
power density achieved 135 mW/cm 2
• The pumping speed at the chamber was approximately
70 l/s. The base pressure of the chamber was better than
5 x 10-5 Pa, rising to 1 x 10-4 Pa during exposure. The
sensitivity limit of the RGA was 1 x 10-10 Pa.
• The wafer samples were positioned 90cm from the
plasma, at an angle of incidence 45o to both the EUV
beam and the RGA.
• EUV light was filtered by a 200nm thick zirconium
foil, size 1cm2
Actual sample exposure
• The exposure area was 1.4cm 2 and exposures lasted
for 300 seconds
Sample Exposure
False colour image showing
sample exposure and image
of mesh supporting the Zr
filter.
4 RGA Results
Resist A
Resist A power 1
Resist A power 2
blank Si
1E-05
Outgassing rate (Pa.m 3/s)
1E-06
C6H6
Xe2+
78 amu
Xe+
64-68 amu
1E-07
CxHy
CxHy
128-136 amu
alicyclic acrylate type
solvent: PGMEA
CxHy
1E-08
•Resist A
1E-09
1E-10
1
5
9
13 17 21 25
29 33 37 41 45 49 53 57 61 65
69 73 77 81 85 89 93 97 101 105 109 113 117 121 125 129 133 137 141 145 149 153 157 161 165 169 173 177 181 185 189 193 197
amu
Resist B
Resist B power 1
Resist B power 2
blank Si
1E-05
Outgassing rate (Pa.m 3/s)
1E-06
Xe+
Xe2+
1E-07
•Resist B
128-136 amu
• The figures compare RGA spectra from a blank silicon wafer with 2 separate
measurements of a photo-resist coated wafer with different incident EUV
power densities. The spectra displayed represent the peak outgassing during
the exposure.
• The blank Si wafer was subjected to the same EUV exposure conditions as
the resist coated samples and no increase in outgassing was observed.
• Varying the EUV exposure conditions has no effect on the level of outgassing
of H2, H2O, N2 , CO, O2 or CO2.
• The peak outgassing rate is observed to increase with increasing power
density incident on the sample (note that the blue spectra are at a higher
power density than the red). Resist A also shows a bigger dependence on
the incident EUV power density than resist B
64-68 amu
CxHy
1E-08
CxHy
C6H12
acrylate type
solvent: PGMEA, PGME
84 amu
CxHy
1E-09
1E-10
1
5
9
13 17 21 25 29
33 37 41 45 49 53 57 61 65 69 73 77 81
• Different outgassing is observed between the two resist samples, and specific
EUV stimulated outgassing is observed at the following amu:
Resist A: 37-39, 47, 50-52, 63, 74, 76-78
Resist B: 47, 50-52, 55, 63, 77, 78, 84, 91-93
85 89 93 97 101 105 109 113 117 121 125 129 133 137 141 145 149 153 157 161 165 169 173 177 181 185 189 193 197
amu
5 Preliminary Conclusions
• The main outgassing species for resist A occurs at 78 amu and for resist B at 78 and 84
amu. There is other specific CxHy outgassing which is so far undetermined and analysis is
ongoing.
Resist A: 78 amu is possibly due to C6H6 outgassing
this is identified from the resist structure and the cracking pattern as
probably Benzene outgassing.
6 Acknowledgements, Further information
Further work:
• Identification of outgassed species.
• Experiments to quantify the effect of power scaling on resist outgassing.
• Comparison of outgassing rates from pulsed EUV sources with EUV from a synchrotron.
• The authors would like to acknowledge the following people/organisations: