Aerial imaging for source mask optimization: mask and illumination qualification

Abstract

As the semiconductor industry moved to 4X technology nodes and below, low-k 1 ArF lithography approached the theoretical limits of single patterning resolution, a regime typically plagued by marginally small process windows. In order to widen the process window bottleneck, projection lithography must fully and synergistically employ all available degrees of freedom. The holistic lithography source mask optimization (SMO) methodology aims to increase the overall litho performance and achieve a robust process window for the most challenging patterns by balancing between the mask and illumination source design influences. The typical complexity of both mask and illumination source that results from a generic SMO process exceeds the current norm in the lithographic industry. In particular, the SMO literature reports on masks that fully operate as diffractive optical elements, with features that have little resemblance to the final wafer-level pattern. Additionally, SMO illumination sources are characterized by parametric or pixelated shapes and a wide range of transmission values. As a consequence of the new mask and source designs, qualifying the mask for printing and non-printing defects and accurate assessment of critical dimensions becomes one of the main mask inspection challenges. The aerial imaging technologies of Applied Material's Aera2™ mask inspection tool provide enabling solutions by separating out only the defects that matter and accurately measures aerial imaging critical dimensions. This paper presents the latest numerical and experimental SMO mask qualifications research results performed at Applied Materials with a mask containing two-dimensional DRAM production structures.

Aerial imaging qualification and metrology for source mask optimization

“…And we demonstrated experimentally that mask qualification tasks such as full-mask scale CDU measurement and process window measurement can be accomplished successfully using the aerial imaging capabilities of the Aera2 TM platform -Applied Materials' aerial imaging mask inspection tool. The current paper complements [6] in several important respects. It considers (in §3.a) a comparison of process windows for a DRAM-like 2D, low-k 1 pattern obtained under two implementations of a "freeform" exposure condition (EC) characteristic of SMO.…”

“…This is in accord with previous indications (c.f. [6]) that relative CD measurements are not significantly affected by the specific detail of SMO implementation on Aera2 (in [6] we considered CDU). Also notice that the PW analysis software introduces some degree of overfitting.…”

“…Applying the gray level threshold to these profiles finally yielded the requested CD measurements. Unlike the partial measurements described in [6], here a much larger sample was used for each CD extraction, guaranteeing a much smaller measurement error. Last, an emulation of scanner dose variations (in order to obtain a full FEM measurement) is simply obtained from the through-focus normalized aerial images by renormalizing the images by the required dose factor.…”

“…This range of defocus on the mask side corresponds to an effective wafer defocus range of ±95 nm. In order to extract CD's from the aerial images, all images were first properly normalized, and then a binarization threshold (per pattern and EC, of course) was determined, based on the image exhibiting highest pattern contrast (see [6] for a description of the threshold setting procedure]. The raw aerial images were digitally resampled (by a modest factor), and subsequently interpolated intensity profiles along the specified cut-lines (see below) were extracted.…”

“…The great opportunities offered by SMO come at the price of more complicated illumination pupil and mask pattern. In an earlier paper [6] we considered the challenges the SMO introduces in the context of mask inspection and qualification. We demonstrated numerically that mask defect detection is rather insensitive to the specifics of SMO illumination implementation on an aerial imaging mask inspection tool.…”