| Popis: |
For capacity reasons, it is interesting for us to have the flexibility of switching lithography processes between DUV and I-line steppers. The following discussion concentrates on high energy tilted implants of CMOS technology, critical enough to be worth running on the more expensive DUV equipment. As far as the differences are understood at the level of the printing, as well as the dissimilarities during the following implantation steps, it is possible using the same reticle and with minor target adjustments, to switch between the 2 tools/processes when required. This paper investigates the most important differences between the functionality of a same implant layer making use of the two wavelengths. Taken as high energy implant mask for several successive ion implantations, the resist film considered here is 1.6 pm thick. The taper profiles of I-line and DUV resist are shown after development, and after the successive implantation steps. Both wavelengths provide straight profiles after development, with one main difference: a slight footing for the I-line resist. This can be very well seen on the corresponding top down pictures revealing more tapers for the I-line process than for DUV. The first implantation step following development influences the profiles the most. In fact the profile of the DUV resist changes considerably while the one with I-line resist remains unchanged. That can be explained by the fact that the aliphatic structure of DUV photoresist is less resistant to degradation by ion bombardment as compared to the highly aromatic chemical structure of I-line photoresist. The subsequent implant steps of lower energy do not further influence the tapers, not even in the case of the I-line film. Therefore the biggest shrinkage occurs during the first implantation and all the next ion sequences will see this first deformation without changing it. Finally, simulation show that, an adjustment of the reticle OPC by adding serifs can be beneficial to the I-line layer to diminish corner rounding where the footingkapering can be worse. |
