Nanoimprint Lithography (NIL) remains a versatile technology for replicating optically functional patterns. However, height and roughness conservation over large areas during replication, remains a challenge. In order to identify UV-NIL process capabilities, this study propose to validate the replication of multi-height pillars present on the resist master manufactured by grayscale lithography. Multiple measurement were performed on all pillars before and after replication using profilometry and Atomic Force Microscopy (AFM) to evaluate height and roughness of all patterns. This achievement represents a novel approach to understanding and identifying the capabilities of the UV-NIL process.
The latest promising optoelectronic devices require complex 3D structures. In this study, optical grayscale microlenses and multi-height structures are etched into a polymer layer in a Capacitively Coupled Plasma reactor (CCP).
Etching can cause profile deformation and surface roughness, which may affect device performances. A parametric study is conducted to investigate 3D etching mechanisms.
We observed strong shape modulation by varying plasma parameters. Increasing chamber pressure, or decreasing High Frequency power, show similar tendencies, going from a rounded to a conic profile. Additional experiments suggest that 3D etching mechanisms rely on complex passivation processes and ion bombardment effects.
First, Grayscale I-Line lithography process developed in CEA-Leti allows to manufacture a variety of 3D patterns based on the well-known photolithography technology. Grayscale photolithography is an innovative and alternative approach to create 3D patterns such as microlenses for example. Exposure of a low contrast resist at different doses results in different thicknesses in the resist film. The variation of the intrinsic dose is obtained by using a binary mask that has different chromium densities, thus modulating the exposure intensity on the resist surface1. Secondly, the NanoImprint Lithography (NIL) is a technology capable of reproducing a wide morphological range. NIL is increasingly requested for the reproduction of 3D patterns. Initially, standard NIL process uses a hard master usually composed of Si or SiO2. The proposed work validates the quality of the replication by the NanoImprint process of a "resist master" created by grayscale lithography. This approach facilitates the manufacturing process of a master by avoiding the etching step and offering a cost-effective solution. The measurement of several types of 3D patterns are performed before and after replication during this study. CD preservation is evaluated for 32 types of microlenses simultaneously replicated. Finally, the combination of the Grayscale and NanoImprint technologies allows to considerably increasing the printing possibilities. By freeing the difficulties of multiple patterns morphology conservation during the etching, the replication of a resist master permits other potential applications, particularly in the optical field.
Since its beginning in the 90’s NanoImprint Lithography (NIL) has been continuously improved to target the different industry requirements. Using an intermediate soft stamp media was one of the main improvements and has now become a standard technology. Based on that technology, EVG introduces a full wafer imprinting solution, whereas the size of the stamp corresponds to the size of the wafer to imprint. Results obtained at CEA-Leti using this solution, with respect to uniformity, sub-50nm resolution, repeatability, and high aspect ratio patterns, are today state of the art and allow NIL to be considered as an HVM technology. Nevertheless, further development is carried out on different aspects such as overlay (OVL) which is the scope of this work.
Different contributors of OVL as translation, rotation but also distortion are dissociated and analyzed. Alignment repeatability is studied. Additionally, imprint to imprint OVL correction terms are applied. A dedicated methodology has been established and allows to obtain global OVL signature. According to the above, main process contributors are highlighted and studied in the paper to separate influence of each of them. Finally, different ways to improve overlay are discussed and some of them - which could be linked to hardware, process or both - are evaluated. Overall, the OVL status obtained and first improvements bring NIL technology closer to the alignment requirements of the industry.
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