Abstract
3D pixel sensors aimed at the upgrades of the ATLAS and CMS experiments at the High Luminosity LHC have small pixel size and pretty dense layouts. In addition, modified 3D designs with small pixel size and trenched electrodes in place of columnar electrodes are being developed to optimize the pixel timing performance in view of the LHCb upgrade. The fabrication of these advanced 3D pixels is challenging from the lithographical point of view. This motivated the use of stepper lithography at Fondazione Bruno Kessler in place of a standard mask aligner. The small minimum feature size and high alignment accuracy of stepper allow a good definition of the sensor geometries also in the most critical layouts, so that a higher fabrication yield can be obtained. In this paper, we will present the main design and technological issues and discuss their impact on the electrical characteristics of 3D pixel sensors of different geometries.
Highlights
The fabrication of 3D pixels is becoming more challenging, for the etching of vertical electrodes, which relies on the Deep Reactive Ion Etching (DRIE) process, and from the lithographical point of view, with pretty dense layouts calling for small minimum feature sizes and high alignment accuracy
Despite the problems occurred during the fabrication of the first stepper batch, due to the choice to accommodate many design variants and the related complication in the exposure strategy, the overall results are satisfactory
The yields for the other non-critical geometries 50 × 50–1E and 25 × 100–1E remained roughly the same, about 60% on average, that is good enough considering the complex fabrication technology of 3D sensors and the very large density of columnar electrodes involved in small-pitch designs
Summary
The fabrication of 3D pixels is becoming more challenging, for the etching of vertical electrodes, which relies on the Deep Reactive Ion Etching (DRIE) process, and from the lithographical point of view, with pretty dense layouts calling for small minimum feature sizes and high alignment accuracy. In these conditions, mask aligners, which have been the workhorse for radiation sensor fabrication in most laboratories for decades, show some clear limitations in their performance, with detrimental effects on the device quality and on the production yield. This motivated the choice to switch to a step and repeat lithography equipment (so called stepper) for the development of the latest generations of 3D pixels at Fondazione Bruno Kessler (FBK), Trento, Italy
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