KEYWORDS: 3D mask effects, Grayscale lithography, 3D modeling, Data modeling, 3D microstructuring, 3D acquisition, Semiconductors, Profilometers, Process control, Photoresist processing
Optical grayscale lithography offers the possibility to pattern 3D microstructures at large scale and high throughput for HVM semiconductor industry [1-4]. 3D structures uniformity is of importance to ensure homogeneous and at-best performances of several tens of millions of functional elements. This uniformity can be impacted in part by the optical mask variability. Impact of mask variability can be quantified in terms of Mask Error Enhancement Factor (MEEF) [5] for optical grayscale lithography which can be calculated by using resist contrast curve. It has been shown that MEEF is highly dependent on mask densities [5]. Once the mask is fabricated, the impact of mask variabilities on lithography can be controlled by process optimization. In this paper we evaluate the impact of process parameters on optical grayscale MEEF by theoretical and experimental means.
Silicon nitride (SiN) have become an essential material for integrated photonics. It is needed whenever very low optical losses, high optical power, low thermal sensitivity or broad wavelength range is required. SiN waveguides therefore present a great interest for applications as diverse as quantum photonics, data-communication, neuromorphic computing, LiDAR, sensing and microwave optic. We present our latest results on two distinct platforms: An 8” LPCVD SiN platform featuring ultra-low loss (5dB/m) and a 12” Physical Vapor Deposition (PVD) SiN platform allowing low loss in the C-band with a low-temperature deposition.
Impact of mask CD errors on microlens and pillar structures fabricated using grayscale lithography technique is studied. CD errors were evaluated from the mask SEM images using contour based metrology. Mask error enhancement factor for grayscale lithography is proposed based on mask (or design) chromium density for given 3D structure to be patterned. Impact of mean-to-target CD mask error and local CD variations on target critical parameters were studied separately. For grayscale lithography, the global mask error enhancement factor calculated to study impact of mask CD errors were found to be non linear and highly dependent on the mask (or layout) chromium density. Surface topography of given grayscale target was found to be highly dependent on the local CD variations. We also found that intentional local CD variation can be used to effectively tune certain target parameters.
The advance in microlithography has greatly helped the development of micro optical elements. Large array of microlenses can now be fabricated in the same fashion as manufacturing of integrated circuit at low cost and high yield [1-2].
Because microlens array requires well-defined and continuous surface relief profile, special methods are needed to supplement the normal microlithography to produce those spherical structures [3]. Various techniques have been developed, and the most widely used is multi-step photolithography with thermal resist reflow. However, the alternative grayscale photolithography technique appears to be the one as the most flexible and versatile method [4].
Indeed, this approach is a one-level lithography process enabling the development of 3D profiles in a photoresist masking layer. In addition, with the need to maintain or improve image quality at an ever-smaller pixel size, grayscale technic can offer one way to compensate the loss of the photosensitive area by achieving zero-gap microlens. One other advantage of grayscale is the possibility to have, from a single lithography, objects of different shapes, but also at the same time of different sizes (especially heights); which is possible with classical lithography only by doing multi-patterning.
There are several options for performing grayscale lithography, for example the HEBS mask (high energy beam sensitive) which darkens under exposure to electrons. The option that has been chosen is to use a grayscale reticle, with varying chromium features densities that locally modulate the intensity of transmitted UV light. Being non-uniformly exposed, this allows the creation of a relief structure in the resist layer after development. The resist height after development depends on the intensity of the incident light, the exposure time and the contrast of the resist. So contrary to conventional lithography where the goal is to achieve straight resist pattern profiles, grayscale lithography enables the realization of progressive profiles, which requires smooth resist contrast curve. The other specificity of these resists is that they must crosslink without flowing.
In this paper, we evaluate resists from different suppliers to generate microlenses smaller than 5μm via a grayscale mask. The study consists in establishing the contrast curves of these resists according to different process parameters, giving the designer great control of grayscale levels that can be achieved for a given resist. Then, pattern various microlenses shapes in these resists to evaluate the residual resist thickness according to the gray levels. With the final objective of establishing a relationship between these contrast curves and the profile variations at the microlens level to compute a suitable and accurate grayscale mask [5].
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.