Why Do We Need Optical Design Kits for Flat Optics?

Emilie Viasnoff

Jun 30, 2023 / 7 min read

By Emilie Viasnoff, Business Development Director, Synopsys Optical Solutions; Maryvonne Chalony, Applications Engineer, Synopsys Optical Solutions; and Larry Melvin, R&D Engineer, Mask Solutions and Smart Manufacturing

Metalenses Concept | Synopsys Blog

Smartphones, smartwatches, assisted and autonomous vehicles, security systems, and VR/MR/AR headsets drive the pervasiveness of cameras and, more globally, optical sensors used in LiDAR, scanners, and other applications. In 2025, nearly 10 billion cameras will be produced, scaling to unprecendented volumes the fabrication of optical components, such as ultrasmall optical sensors and cameras, metalenses, microdisplays, microlasers, and combiner optics.

The semiconductor industry has long handled mass production with over a trillion units shipped worldwide each year. With pervasiveness, miniaturization, and integration becoming strong drivers for optical components, significant efforts have been put into leveraging existing semiconductor processes for their production. Emerging optical components are often referred to as flat optics, since they are manufactured on flat silicon or glass substrates. One advantage is that they can be manufactured using semiconductor processes instead of traditional injection molding or glass polishing techniques. Since flat optics structures are patterned over large areas with complex nanostructures, the fidelity of these patterns may significantly impact optical performance.

Read on to learn more about the flat optics domain along with associated manufacturing challenges. We will describe a new workflow to address these challenges using optical design and manufacturing process simulations. The aim is to pave the way to Optical Process Design Kits, which will enable higher manufacturing yields, efficiency, and an improved understanding of how manufacturing affects optical component performance. We will present an achromatic lens example to illustrate the proposed virtual fab flow.

Flat Optics: An Emerging Technology Leveraging Semiconductor Processes

Optical devices interact with light to modify its path, direction, polarization, or intensity. The elementary component of electronics is the transistor that acts as a gate for electrons. The elementary component of optics is a lens that deviates the ray of light by changing the optical path. Multiple lenses often deviate a light beam with as few chromatic and geometric aberrations as possible. Standard lens manufacturing approaches include polymer injection and glass-turning, grinding, and polishing, resulting in a 3D curved lens.

The pervasiveness of cameras has resulted in a 10x increase of imaging systems in the last 15 years, putting pressure on traditional manufacturing. Flat lenses leveraging semiconductor processes on (flat) substrates has emerged as an alternative to traditional polymer injection and glass-turning. This new manufacturing technique leverages UV replication or hot embossing to manufacture lenses, and more broadly optics. It also paves the way for new shapes of lenses; instead of traditional spherical or aspherical 3D shapes, nanostructured flat lenses have emerged.

When nanostructures patterned on a substrate are smaller than the wavelength of the light beam, the light interacts with a new artificial material, acting as a 3D lens on the optical path. In addition to metalenses, flat optics and semiconductor processes also open infinite possibilities in optical device engineering for refractive and  diffractive optical elements. This in turn paves the way for innovations in light guides, metalenses, IR, UV filters, and nanostructured light sources (such as microLED microdisplays for AR). Driven by IR 3D-sensing illuminators, the first mass-produced application for flat optics, this domain has grown at an impressive 55% CAGR over the past five years.

Evolution of Optics Manufacturing: Why We Need Optical PDKs

Advanced optical designs that leverage flat optics are pushing manufacturing volumes to new limits and device sizes to disruptive compactness. Meanwhile, other technical requirements such as image performance and power consumption are becoming more challenging to achieve. Many optical manufacturers are shifting from traditional optical processes to high-volume manufacturing using semiconductor processes to meet advanced requirements and optimize costs. As optics evolve, optical tolerancing must also evolve. Today, optical tolerancing is done by estimating material properties and representing manufacturing variations as parameters. The manufacturing process is not completely considered, which can result in low yields, low performance, and redesigns. These design-manufacturing-test loops are costly in terms of time and money for optical designers, integration teams, and test engineers.

Over many years, microelectronics designers have developed an ecosystem, tools, and methodologies to account for manufacturing variations through accurate simulations during the design phase. As a result, semiconductor foundry process design kits (PDKs) are widely available to electronics designers. Although targeted performance parameters include resistance and capacitance for electronics, other manufacturing effects — such as line edge roughness, variances in sidewall angle, optical proximity correction strategies, and mask stitching — would also impact flat optics performance such as reflection, transmission, diffraction efficiency, color attenuation, dispersion, and yield. To efficiently design a nanostructured flat optical device, such as a miniaturized lens for a camera, waveguide combiner, or microLED display, it is more critical than ever to account for the capabilities and limitations of the manufacturing process. This analysis is performed in integrated circuit (IC) designs using design rules (DRs) and PDKs. Similar to microelectronics, flat optics need optical process design kits (OPDKs) to guide optical designers and ease the integration engineer’s work. Let’s review the methodology to build OPDKs.

Optical Design and Manufacturing Simulation for Flat Optics

The designs, processes, materials, and applications of flat optics are evolving at a fast pace, given the novelty of this technology. Simulating a flat optical device’s manufacturing process can provide insights into how the final device will perform, enabling design adjustments to compensate for any manufacturing impacts before reaching the first fabrication loop. Moreover, closing the gap between optical design and process simulations will pave the way for co-optimization, optical DRs and OPDKs.

Synopsys provides a complete set of tools to develop flat optics-based systems with design to manufacturing simulations. This portfolio includes CODE V® for imaging optics design and tolerancing, LightTools® for stray light analysis and waveguide combiners optimization, and RSoftTM Photonic Device Tools for microdisplay, holographic and diffractive gratings, and metaoptics design. It also includes the Synopsys TCAD (Technology Computer-Aided Design) portfolio with the SentaurusTM suite of tools, which include electro-optical device and manufacturing process simulations, mask synthesis and data preparation tools, and yield exploration and management tools.

Similar to a design-to-manufacturing simulation flow, a flat optics flow would start with the optical design. This first step aims to optimize the nanostructure patterns toward the desired optical performances. A GDS file of the flat optics is then produced and post-processed, leveraging the manufacturing simulation tools to correct optical proximity effects, accounting for the photoresist’s behavior, and simulating the impact of a chosen etch process. The flow outputs a modified GDS of the structures, which can be simulated with optical design tools to weigh the impact of the simulated process on the optical performances. Once fully integrated and automated, the flow produces OPDKs suited to any flat optics component.

The Metalens Example

Let’s see an example of a flat optics design-to-manufacturing flow. In optical systems, achromaticity is targeted to avoid a focus shift with various wavelengths. This type of lens is widely used in any visible camera. For “classic” refractive optics, the chromatic aberration is due to the dispersion of the lens material and can be compensated through a combination of various refractive optics. On top of material dispersion, the nanostructure of a flat achromatic lens exhibits a wavelength-dependent optical behavior. Metalens achromaticity compensation is achieved by selecting the suitable materials, arrangement, and shapes of the nanostructures in the metalens. It can be designed using Synopsys’ MetaOptic Designer tool, which optimizes meta-surface arrangement for diverse and complex target types.

We designed an achromatic metalens using MetaOptic Designer with an Airy spot at a focal distance of 20µm and optimization at a 500 nm wavelength. A second case was designed and optimized for three wavelengths [400nm, 500nm, and 600nm] to the same focal distance of 20µm. Both design cases use the same meta-atom library (nano-pillar of Si3N4 on a SiO2 substrate). The initial design target geometry is then simulated in a manufacturing flow. The manufacturing simulation begins with the target pattern. The photolithography mask is then generated by correcting the target pattern with an Optical Proximity Correction (OPC) technique (Proteus OPC or Proteus ILT) to create a mask pattern that compensates for problems such as image fidelity loss and line-end shortening during photolithography. The corrected mask is next imaged using rigorous photolithography simulation (S-Litho) to produce a photoresist pattern. Finally, the photoresist pattern is transferred into the substrate material in an etch process (Sentaurus Process Explorer), leaving the designed pattern’s corresponding representation on the wafer. The resulting physical device is fed back to optical simulations for optical performance assessment, which allows the design team to adjust the design before freezing the fabrication flow.

Fabrication Process Steps for Metalens

Designing for the Future: OPDKs

The proposed design-to-manufacturing simulation flow paves the way to designing flat optics anticipating the impact of the manufacturing process ahead of freezing the fabrication flow. Ultimately, it would enable co-optimization across the simulation chain, providing optical DRs and OPDKs that integration engineers could provide to optical designers before starting any new flat optics design.

Synopsys offers an industry-leading platform—optical design, device modeling, and lithography simulation tools—in a unified environment that can help improve time to market for next-generation flat optics devices. If you are creating a flat optics optical design, and want to reduce your time to market by accounting for complex manufacturing effects at reduced manufacturing costs, we are ready to help you.

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