High optical density photomasks for large exposure applications

Microlithography applications such as advanced packaging, micromachining and thin film head (TFH) production frequently require the use of thick photoresists and large exposure doses for successful pattern transfer onto substrates. When thick negative acting photoresists are used, exposures as high as 5000mJ/cm(2) may be required to maintain the pre-exposure photoresist thickness after develop. However, even a small exposure dose of a negative photoresist can cause a thin film to remain after development. This photoresist scumming can have a detrimental effect on subsequent processing of the substrates. Standard chromium (Cr) photomasks use an optical density (OD) of 3.0 to block light transmission. This specification was established with the introduction of projection lithography and is effective for most applications over a wide range of dosages. However, the combination of extremely high exposure values and thick negative photoresists raises the possible need for higher OD Cr to eliminate the risk of potentially damaging scumming from light leakage. In this study, light transmission through photomasks with standard (OD3) and high-density (OD4) Cr films was measured through the ultraviolet spectrum to determine leakage thresholds and evaluate the risk of unwanted exposure with highly sensitive photoresists. Because the higher OD photomasks are the result of an increase in Cr film thickness, photomask process differences, resolution capability and Critical Dimension (CD) uniformity issues were also evaluated. The thicker Cr film could also affect pattern transfer to the wafer. Therefore, resolution and CD uniformity were compared on wafers patterned from both OD3 and OD4 Cr reticles.