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Proceedings Paper

Grayscale lithography process study applied to zero-gap microlenses for sub-2μm CMOS image sensors
Author(s): S. Audran; J. Vaillant; V. Farys; F. Hirigoyen; E. Huss; B. Mortini; C. Cowache; L. Berthier; E. Mortini; J. Fantuz; O. Arnaud; L. Depoyan; F. Sundermann; C. Baron; J.-P. Reynard
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Paper Abstract

Microlens arrays are used on CMOS image sensors to focus incident light onto the appropriate photodiode and thus improve the device quantum efficiency. As the pixel size shrinks, the fill factor of the sensor (i.e. ratio of the photosensitive area to the total pixel area) decreases and one way to compensate this loss of sensibility is to improve the microlens photon collection efficiency. This can be achieved by developing zero-gap microlens processes. One elegant solution to pattern zero-gap microlenses is to use a grayscale reticle with varying optical densities which locally modulate the UV light intensity, allowing the creation of continuous relief structure in the resist layer after development. Contrary to conventional lithography for which high resist contrast is appreciated to achieve straight resist pattern profiles, grayscale lithography requires smooth resist contrast curve. In this study we demonstrate the efficiency of grayscale lithography to generate sub-2μm diameter microlens with a positive-tone photoresist. We also show that this technique is resist and process (film thickness, development normality and exposure conditions) dependent. Under the best conditions, spherical zero-gap microlenses as well as aspherical and off-axis microlenses, which are impossible to obtain with the conventional reflow method, were obtained with satisfying process latitude.

Paper Details

Date Published: 29 March 2010
PDF: 11 pages
Proc. SPIE 7639, Advances in Resist Materials and Processing Technology XXVII, 763910 (29 March 2010); doi: 10.1117/12.846507
Show Author Affiliations
S. Audran, STMicroelectronics (France)
J. Vaillant, STMicroelectronics (France)
V. Farys, STMicroelectronics (France)
F. Hirigoyen, STMicroelectronics (France)
E. Huss, STMicroelectronics (France)
B. Mortini, STMicroelectronics (France)
C. Cowache, STMicroelectronics (France)
L. Berthier, STMicroelectronics (France)
E. Mortini, STMicroelectronics (France)
J. Fantuz, STMicroelectronics (France)
O. Arnaud, STMicroelectronics (France)
L. Depoyan, STMicroelectronics (France)
F. Sundermann, STMicroelectronics (France)
C. Baron, STMicroelectronics (France)
J.-P. Reynard, STMicroelectronics (France)

Published in SPIE Proceedings Vol. 7639:
Advances in Resist Materials and Processing Technology XXVII
Robert D. Allen, Editor(s)

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