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193nm binary photomasks are still used in the semiconductor industry for the lithography of some critical layers for the
nodes 90nm and 65nm, with high volumes and over long periods. However, these 193nm binary photomasks can be
impacted by a phenomenon of chrome oxidation leading to critical dimensions uniformity (CDU) degradation
with a pronounced radial signature. If not detected early enough, this CDU degradation may cause defectivity issues and
lower yield on wafers. Fortunately, a standard cleaning and repellicle service at the mask shop has been demonstrated as efficient to remove the grown materials and get the photomask CD back on target.Some detection methods have
been already described in literature, such as wafer CD intrafield monitoring (ACLV), giving reliable results but also
consuming additional SEM time with less precision than direct photomask measurement.
In this paper, we propose another approach, by monitoring the CDU directly on the photomask, concurrently with defect
inspection for regular requalification to production for wafer fabs. For this study, we focused on a Metal layer in a 90nm
technology node. Wafers have been exposed with production conditions and then measured by SEM-CD. Afterwards,
this photomask has been measured with a SEM-CD in mask shop and also inspected on a KLA-Tencor X5.2 inspection
system, with pixels 125 and 90nm, to evaluate the Intensity based Critical Dimension Uniformity (iCDU) option.
iCDU was firstly developed to provide feed-forward CDU maps for scanner intrafield corrections, from arrayed dense
structures on memory photomasks. Due to layout complexity and differing feature types, CDU monitoring on logic
photomasks used to pose unique challenges.The selection of suitable feature types for CDU monitoring on logic
photomasks is no longer an issue, since the transmitted intensity map gives all the needed information, as shown in this
paper.
In this study, the photomask was heavily degraded after more than 18,000 300mm wafers exposed and the cleaning
brought it back almost to its original state after manufacture. Wafer CD, photomask CD and iCDU results can be
compared, before and after a standard mask shop cleaning. Measurement points have be chosen in logic areas and
SRAM areas, so that their respective behaviours can be studied separately. Transmitted maps before and after cleaning
were analysed in terms of CD shift and CDU degradation. The delta map shows a nice correlation with photomask CD
shift. iCDU demonstrated the capability to detect a reliable CD range degradation of 5nm on photomask by a comparison
between a reference inspection and the current inspection. Die to die inspection mode provides also valuable data,
highlighting the degraded chrome sidewalls, more in the photomask centre than on the edges.
Ultimately, these results would enable to trigger the preventive cleanings rather than on predefined thresholds. The
expected gains for wafer fabs are cost savings (adapted cleanings frequency), increased photomask availability for
production, longer photomask lifetime, no additional SEM time neither for photomask nor on wafer.
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