The aperture in the AMOL system with a matched photoresist case is shaped as a funnel which is constantly narrowing throughout the AML. With the introduction of the PRL the competition between wavelengths in the AML, a standing wave in , the transmit- ted and the from the PRL, has encouraged the aperture to reach a similar narrow size but to broaden faster at a null in the standing wave. This change in shape of the AML aperture increases the amount of light transmitted at the edges of the main aperture in the PRL case, hence creating a weaker differentiation between light passing through the more transparent centre of the AML aperture and the aperture fringes. This will reduce the NILS and increase the FWHM of the intensity waveform in the resist as was seen in the metric results (Fig 7.9). A considerable change in the AMOL system has been shown by the introduction of the PRL, in particular in the improvement of the DOF and large differences in both the photoresist and AML. The impacts of the interactions with the PRL of and are different; these are now modeled separately to allow the different cases to be observed and understood more clearly.
7.6
EFFECTS OF ITRODUCIG THE PRL
In this section the effects of the PRL on the AMOL system are discussed through an examination of the role of each of the wavelengths separately, showing that the reflec- tion of is much more important to the performance improvement than that of . This effect can be explained by the propagating nature of and the importance of the evanescent field of in exposing the photoresist. Here the FEM model of the AMOL system is used to model different reflective parameters for and in a fictitious PRL placed at 50 nm beneath the photoresist. In each case the comparisons are made at the / intensity ratio giving the best NILS. The two cases which have already examined:
A. and with a matched substrate (Fig 7.6a, reproduced in Fig 7.15a), B. and with a PRL (Fig 7.6b, reproduced in Fig 7.15b),
are compared with two fictitious systems which model part of an AMOL system with a PRL:
C. is reflected by a PRL and has a matched substrate (Fig 7.15c), D. has a matched substrate and is reflected by a PRL (Fig 7.15d).
Figure 7.15 Comparison of the four cases for the intensity ratios `a/`b giving the best NILS showing (a) A and (b) B from previous considered cases and the fictitious (c) C(`b has a PRL, `a matched substrate) and (d) D (`b has a matched substrate, `a a PRL) showing the absorbance in the AML and the `b intensity in the photoresist.
7.6 EFFECTS OF INTRODUCING THE PRL
Considering first the new case (C, Fig 7.15c), where is reflected by an ideal PRL but has a matched substrate; here the reflection of in the resist is negligible allowing the focus to be placed on the reflection of and how this affects the image. First, comparing the intensity in the photoresist of C to the matched substrate case (A) the most noticeable change is the much improved lateral confinement of in the photores- ist, combined with a large increase in intensity transmitted into the resist. If the aperture of C is compared to the matched case (A, Fig 7.15a) there is a widening of the aperture near the resist/AML boundary as was noted in the PRL case (B). The aperture widening can be accounted for by considering the reflected from the PRL; although is an evanescent wave in the resist the closeness of the PRL allows the wave to be partially reflected and the effect of this reflection is to increase the intensity in the base of the AML widening the aperture. This effect is noticeable for ~20nm into the photoresist but increased with aperture width as the lower spatial frequencies have slower decay.
Comparing the same case (C) to the ideal PRL case (B, Fig 7.15b) there are also impor- tant differences noted. Although the intensity remains narrowly confined in the photoresist for both cases the larger aperture of the system without the PRL (C) has led to both a wider aperture in the PML and intensity waveform in the photoresist. The lack of any reflection is also clear, with no standing wave present in the AML with a funnel shaped aperture being formed. From these comparisons the conclusion may be drawn that it is the interaction with the PRL which creates the observed, well con- fined, aperture in the AMOL system, however without some reflection of from the PRL the effect can be less effective due to widening of the aperture at the AMOL/resist boundary. It is also noted that the intensity of is reduced when is not reflected, this is likely due to the aperture widening causing a reduction in the amplification of by the surface plasmons.
The second new case in which has a matched substrate and has an ideal PRL (D, Fig 7.15d) creates a similar intensity profile in the photoresist as the matched case (A), although with some improvement in intensity, again due to change in optimum intenisty ratio. Without interacting with the PRL there is no, or limited, improvement in the
confinement in the photoresist and imaging capability of the system. Comparing D with the ideal PRL case (B, Fig 7.15b) reveals the similarity of the inverted hour-glass pattern created in the AML. The hourglass shape created is even narrower in case D due to the being reflected from the PRL and interacting with the AML again, thus narrowing the
aperture further; this leads to a lower optimum intensity ratio being able to be used. Having investigated the two cases in which only one of the two wavelengths in the simulation is affected by the PRL and comparing these to the matched and ideal PRL cases the effect of the PRL on each of the wavelengths is as they are transmitted in the AML and photoresist is seen more clearly. The reflection of has a larger effect at longer distances, as it is a propagating wave, and hence creates a strong standing-wave feature in the AML, it also acts to reduce the aperture at the AMOL/resist boundary and reduce the optimum intensity ratio needed. Fortunately this aperture reduction is coun- tered, to allow similar sized apertures to form, by the reflection of which leads to a slightly wider aperture, but similar absorbance pattern inside the AML. Most effectively when considering the ability of the system to provide lithographic results the horizontal confinement of the transmitted inside the photoresist layer is much improved when a PRL is introduced. It has also become apparent in examining the range of simulations that were run that many of the effects being observed and explained are dependent on the resist depth between the AML and PRL and hence how these differences affect the system are now examined more systematically.