Characterization of 32nm Node BEOL Grating Structures Using Scatterometry
2008
Implementations of scatterometry in the back end of the line (BEOL) of the devices requires design of advanced
measurement targets with attention to CMP ground rule constraints as well as model simplicity details. In this paper
we outline basic design rules for scatterometry back end targets by stacking and staggering measurement pads to
reduce metal pattern density in the horizontal plane of the device and to avoid progressive dishing problems along
the vertical direction. Furthermore, important characteristics of the copper shapes in terms of their opaqueness and
uniformity are discussed. It is shown that the M1 copper thicknesses larger than 100 nm are more than sufficient for
accurate back end scatterometry implementations eliminating the need for modeling of contributions from the buried
layers. AFM and ellipsometry line scans also show that the copper pads are sufficiently uniform with a sweet spot
area of around 20 μm. Hence, accurate scatterometry can be done with negligible edge and/or dishing contributions
if the measurement spot is placed any where within the sweet spot area. Reference metrology utilizing CD-SEM and
CD-AFM techniques prove accuracy of the optical solutions for the develop inspect and final inspect grating
structures. The total measurement uncertainty (TMU) values for the process of record line width are of the order of
0.77 nm and 0.35 nm at the develop inspect and final inspect levels, respectively.
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