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Reticle critical dimension uniformity (CDU) is an important criterion for the qualification of mask layer processes.
Normally, the smaller the three sigma value of reticle CDU is, the better is the reticle CDU performance. For
qualification of mask processes, the mask layers to be qualified should have a comparable reticle CDU compared to the
process of record (POR) mask layers. Because the reticle critical dimension (CD) measurement is based on algorithms
like “middle side lobe measurement”, evaluation of the reticle CD-values can not reflect aspects like the sidewall angle
of the reticle and variation in corner rounding which may be critical for 45nm technology nodes (and below). All
involved tools and processes contribute to the wafer intra-field CDU (scanner, track, reticle, metrology). Normally, the
reticle contribution to the wafer CDU should be as small as possible. In order to reduce the process contributions to the
wafer intra-field CDU during the mask qualification process, the same toolset (exposure tool, metrology tool) should be
applied as for the POR. Out of the results of these investigations the correlation between wafer measurement to target
(MTT) and reticle MTT can be obtained in order to accurately qualify the CDU performance of the mask processes.
We will demonstrate the correlation between reticle MTT and wafer MTT by use of multiple mask processes and
alternative mask blank materials. We will investigate the results of four process-layers looking at advanced binary maskblank
material from two different suppliers (moreover the results of a 2X-via layer as an example for a phase-shift maskblank
is discussed). Objective of this article is to demonstrate the distribution between reticle MTT and wafer MTT as a
qualification criterion for mask processes. The correlation between wafer CD-difference and reticle CD-difference of
these mask processes are demonstrated by having performed investigations of dense features of different 28nmtechnology
process layers (poly-, active-, contact-, 1X-metal-, 2X-via layers). Referring to the correlation between wafer
and reticle MTT, the contribution of the reticle CD-difference to the wafer CD-difference can be used as an evaluation
method for the transfer-process of different mask sites.
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