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.
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.
NanoImprint Lithography (NIL) is not a novel technology anymore1 but huge progress has been achieved for its industrial introduction since its first reporting. One of the main evolutions concerns the use soft stamp media2 ,which is now a standard technology. EVG introduced this technology with a full wafer imprint solution (the size of the stamp corresponds to the size of the wafer to print)3 and results obtained since five years are at the state of the art. Repeatability, uniformity, sub-50nm resolution and high aspect ratio patterns are addressed at the same time4–6 . Nevertheless, some challenges still remain, as e.g overlay7 and in particular the distortion phenomenona 8 , which contribute to the remaining overlay next to global translation and rotation. This study is focused on distortion effect which appears during NIL process using flexible backplanes and its minimization by using different materials. A polymer backplane is compared with a glass backplane which are used as carrier to the soft stamp material. A dedicated methodology to precisely measure this distortion is implemented to remove global alignment signature. Distortion signature is firstly evaluated with a standard soft stamp material and process of reference already established. Distortion fingerprint mapping is obtained for each wafer. Thanks to this mapping, a monitoring distortion plot is extracted, in order to follow the evolution of the distortion depending on wafers (wafer-to-wafer) and lots (lot-tolot). This study highlights that the use of a glass backplane developed by EVG clearly allows to improve the distortion in terms of magnitude but also of stability.
The NanoImprint Lithography (NIL) technology by using a soft stamp is today ready for high volume manufacturing (HVM) with the global solution proposed by EVG1. This UV-based imprint, using a transparent stamp is now a standard technology and the most common option for the full wafer imprint, meaning the size of the stamp correspond to the size of the wafer to print. Previous work has shown promising results with strong repeatability and uniformity in terms of critical dimension (CD)2. In 2017, larges features, bigger than 500 nm period, and shallow aspect ratio were qualified3. Latter in 20194, lithography and etching through a Si/SiO2 stack were demonstrated for 25 wafers imprinted in a single run:
- Depending on several diameters contact (from 100 to 50 nm) and densities (from 1:3 to 1:15).
- For line and space arrays with a density of 1:4 and variable spaces widths (from 100 to 50 nm).
In this paper we demonstrate that the limit of the patterns dimension can be pushed to sub-50 nm features thanks to EVG SmartNIL technology, the optimized EVGNIL-UV/AS2 soft stamp material with matching resist as well as the improvement of pattern transfer by dry etching. Based on CDSEM metrology, and SEM cross-sections, high fidelity and reproducibility were demonstrated, with 25 replications in a single run using the same soft stamp. Transfer compatibility of the imprint material was validated until 45 nm line, with 1:4 density. Furthermore, the process window of this NIL technology and its compatibility with applications as photonics and 3D patterning are discussed. The specific developments achieved around stripping of the substrates and the perspectives for low defectivity process are pointed out.
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