Pellicle (photolithography)
In the field of semiconductor technology, a pellicle is a transparent membrane for photo masks to protect against contamination.
background
In modern photolithographic structuring, a pattern on a photomask is transferred into a photoresist layer on a wafer . The structure pattern on the photomask is "stored" in the form of a structured opaque or partially transparent layer on a solid, transparent substrate. Especially in systems based on the stepper principle , in which the mask is transferred multiple times and in a grid pattern, it becomes clear that a defect on the photo mask can massively increase the number of defects on the wafer and thus drastically reduce the yield . Above all, particles from various sources (e.g. dust) that can accumulate on the structured layer should be mentioned as defects. In contrast to the unstructured side of the mask substrate, cleaning the structured side is more complicated and always involves the risk of layer damage, another form of defect. For this reason, attempts are made to prevent the structured side from being soiled by means of a membrane, the pellicle, which is transparent to the radiation used.
Layout and function
A pellicle is an approx. 1 µm thick polymer film that is stretched over a frame made of plastic or aluminum that is firmly attached to the photomask . The frame height and thus the distance between the pellicle and the structured layer is generally 5-10 mm, that is to say in the area of the mask substrate thickness. This creates a more or less isolated area - the frame contains small ventilation holes to compensate for pressure fluctuations and prevent the pellicle from sagging - above the structured layer, in which there are no particles and therefore cannot get onto the layer.
Particles from the environment, which are always present in a clean room of the highest class, cannot get through the pellicle directly onto the structured layer, but fall onto the pellicle or the unstructured back of the mask. Pellicles offer two advantages: On the one hand, particles on the unstructured surfaces can be blown off relatively easily with nitrogen without damaging the mask, on the other hand they are several millimeters away from the mask layer. The latter is beneficial if a particle should have newly deposited between the inspection steps. Because of the depth of field used (approx. 1 mm), the particle is therefore out of focus and has very little influence on the exposure.
conditions
Pellicles must be hard, thin (<1 µm) and sufficiently transparent (> 90%) for the illuminating radiation even after prolonged use with high lighting doses. The starting material for pellicles are organic compounds, e.g. B. cellulose nitrate for g-line or i-line lithography or amorphous fluoropolymers for DUV lithography. The membrane thickness must be set in such a way that thin-film interference results in maximum transmission.
literature
- Harry J. Levinson: Principles Of Lithography . SPIE Press, 2005, ISBN 978-0-8194-5660-1 , pp. 262–268 (main source of the article).
Individual evidence
- ^ Roger H. French, Hoang V. Tran: Immersion Lithography: Photomask and Wafer-Level Materials . In: Annual Review of Materials Research . tape 39 , no. 1 , 2009, p. 93-126 , doi : 10.1146 / annurev-matsci-082908-145350 .