Systematic Studies of Fullerene Derivative Electron Beam Resists
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115 Mat. Res. Soc. Symp. Proc. Vol. 584 © 2000 Materials Research Society
EXPERIMENTS AND RESULTS The fourteen fullerene derivatives [14] studied are shown in fig. 1. Films of the derivatives were deposited by spin coating from chloroform, at room temperature, onto hydrogen terminated Si substrates HF. The film thickness, measured using a surface profiler (Detak, Sloan), could be controlled by varying the angular frequency of the spin coater, or the solution concentration as shown previously [11,15]. To measure the response of the derivatives to electron irradiation, derivative films of - 100 nm thickness were prepared and exposed to 20 keV electrons using a Hitachi S4500 scanning electron microscope (SEM). There was no measurable change in the thickness of the film immediately after irradiation. Upon immersion in the organic solvent monochlorobenzene (MCB) for 60 s it was observed that the films all demonstrated negative tone resist behaviour (i.e. the irradiated areas had become insoluble). This variation of
MAF2
,,,
MAF3
MAF4
~ nnn,r-,, l;i .M 2
MAF5
MAF7
MAF9 -t1-Jul1
THP3
THP2
THPI
4,
,U3,
THP5
THP4
-TeAF
TrAF-
Fig. 1 The fourteen methanofullerenes studied. See [14] for abbreviations.
the residual film thickness with electron dosage was measured using the surface profiler. The response curves of three of the derivatives are shown by way of illustration in fig. 2. (In order to make comparison of the properties of the film easier the curves have been offset along the y-axis and normalized.) The sensitivity of a derivative to electrons is defined as the dose at which the linear portion of the rising section of the response curve (fig. 2) reaches 50% of the original film thickness [16]. The measured sensitivities are plotted in fig. 3, together with the . . -. . sensitivity of the previously studied C60 resist, as a function of derivative mass. It *, SUm-U can be seen from fig. 3 that with one °a e* i exception (MAF3) there is a linear 0 / relationship between a derivative's sensitivity and its mass. The sensitivity of the 'fastest' derivative, THP5, is - 8.5 Ix l0-4 C/cm 2. In comparison the 2 sensitivity of C60 is - 1 x 10.2 C/cm [10] 0 * * *, - . *- .. MAF9 and the negative tone sensitivity of 2 / PMMA is - 5 x l0-3 C/cm [17]. • ••• • -i -, THP5 To investigate the reasons for the change in the derivatives solubilities after irradiation several derivatives were 0.0001 0.001 0.01 0.1 examined before and after irradiation Dose (Clcm2) with Raman spectroscopy. The Raman spectrum of C60 [18] shows a pentagonal Fig. 2 The response of three derivatives to e-beam pinch mode at 1469 cm"-. The same irradiation with 20 keV electrons. Shown is the film peak, albeit shifted to 1455 cm", is also thickness remaining after exposure and development seen in the spectrum of the derivative with monochlorobenzene
:
116
MAF8
TrAF [11,12]. Here unirradiated and irradiated films of the THP2, THP3 and THP4 (see fig. 1) were examined with Raman spectroscopy. The pentagonal pinch modes of THP2, THP3 and THP4 wer
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