Chem. Mater. 2010, 22, 15–17 15 DOI:10.1021/cm903313j
Two-Photon Acid Generation in Thin Polymer Films. Photoinduced Electron Transfer As a Promising Tool for Subwavelength Lithography Paul S. Billone,† Julie M. Park,† James M. Blackwell,‡ Robert Bristol,‡ and J.C. Scaiano*,† † Department of Chemistry, Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Canada K1N 6N5 and ‡Components Research, Intel Corporation, 2511 NW 229th Avenue, Hillsboro, Oregon 97124
Received October 29, 2009 Revised Manuscript Received November 26, 2009
When Gordon Moore first predicted more than 40 years ago that the number of transistors per computer chip would double every 2 years,1 an edict now referred to as “Moore’s Law”, the typical dimension of lithographic features were many times larger than the wavelengths employed to record the images. In stark contrast, current state-of-the-art lithographic processes can pattern features with widths of 590 nm) of film prepared as in Figure 1. The left side of the film was exposed to 280 mJ of 193 nm irradiation at 19.8 mJ/cm2 per pulse and the right was exposed to the same total dose at 5.5 mJ/cm2 per pulse. The brighter red, the result of protonated C6, on the left side of the image as compared to the right indicates that significantly more acid is generated with the higher pulse energy, in spite of the fact that the same total energy was delivered.
higher frequencies (e90 Hz), indicated that this had no effect under our experimental conditions (see the Supporting Information). The spectral results showing changes to the C6 probe for a series of experiments, delivering a total dose of ∼175 mJ/cm2, are shown in Figure 1. We were careful to only go to relatively low conversion to avoid any saturation or significant bleaching effects. The changes at ∼520 nm in Figure 1 reflect the formation of acid, as reported by the protonation of C6. It is clear that different yields of acid are obtained for different forms of energy delivery, with better yields when the individual 193 nm pulses are of higher energy (Figure 2). An alternate way of displaying the data is presented in Figure 3, where the protonated C6 signals are plotted against the energy per pulse, for the same total energy of ∼175 mJ/cm2. If only “linear” photochemistry was involved in acid production, the plot of Figure 3 would give a horizontal line; clearly this is not the case. The positive slope of the plot suggests the involvement of a multiphoton, most likely two-photon, process. The fact that the y-intercept is not zero indicates that some acid is also formed in a onephoton process. This is not surprising since the PAG used
Communication
Chem. Mater., Vol. 22, No. 1, 2010
Figure 3. Plot showing the yield of protonated C6 in PMMA films prepared as in Figure 1 with an almost equal dose (∼175 mJ/cm2) delivered to each film but with varying energy per pulse. The increasing acid yield with increasing pulse energy is indicative of a multiphoton sensitization mechanism. The values associated with each point indicate the exact dose, in mJ/cm2, delivered to that film.
Scheme 1
has some absorbance at 193 nm (ε = 860 M-1 cm-1) and is known to yield some acid by direct photolysis (see the Supporting Information). We have tested mPT with some other trialkylsulfonium PAGs and did not observe any nonlinearity in acid formation. We tentatively attribute this to the relative energies of donor and acceptor orbitals; in the other trialkylsulfonium systems, we believe that the energy of the first excited state of mPT (before a second photon is absorbed) is high enough to transfer an electron to the PAG. We are in the process of carrying out electrochemical and computational studies to verify this. It must also be noted that at more lithographically relevant pulse energies (
