High-Density Optical Rewiring in CPO

Dry Film Polymer Waveguide Materials (Under Development)

Category:
Semiconductor/Electronic materials
Tag:
- High integration

Applicable Business AreasCPO (Co-Packaged Optics) / Silicon Photonics

Target ApplicationsOptical rewiring inside CPO

With the increasing demand for higher speed and larger capacity in AI servers and data centers, Co-Packaged Optics (CPO) is attracting significant attention. CPO requires optical wiring technology that achieves even higher speed and lower power consumption compared to conventional pluggable optics.

When directly connecting optical fibers to Photonic Integrated Circuits (PICs), constraints on fiber pitch (spacing) make high-density mounting difficult. In addition, glass waveguides—which are drawing attention as a technology for achieving high-density optical rewiring—still face challenges in processability.

To address these challenges, Resonac offers a new option for forming high-precision optical waveguides on substrates with its "Dry Film Polymer Waveguide Materials (Under Development)," which excel in processability. This product under development enables high-density optical rewiring and pitch conversion inside CPO, supporting the further advancement of CPO.

Fig 1. The Need for Waveguides in Co-Packaged Optics

Co-Packaged Optics will be required to achieve higher speed and lower power consumption than pluggable optics.
Because of fiber routing constraints and pitch limitations between the PIC and optical fiber, pitch conversion and high-density optical rewiring by optical waveguide will be necessary.

※The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

Solution

High-Density CPO Optical Rewiring via Integrated Core–Cladding Photolithography

In conventional pluggable optics, the ASIC and optical module are placed apart, resulting in long electrical wiring between them. This leads to increased signal loss during high-speed transmission, along with higher power consumption and heat generation, making it difficult to achieve further integration and lower power consumption. To address these challenges, CPO—which places the ASIC and optical transceiver in close proximity—is attracting attention. By significantly shortening the electrical wiring, CPO is expected to achieve both higher speed and lower power consumption.

In one CPO implementation approach, where optical fibers are directly connected to PICs, fiber routing constraints around the package and fiber pitch limitations pose challenges in terms of high-density mounting. Meanwhile, glass waveguides, which are being explored as a method for optical rewiring inside CPO, face challenges in processability. Therefore, there is a need for technology that absorbs the pitch difference between PICs and optical fibers, achieves high-density optical rewiring and pitch conversion, and also attains a high level of optical stability.

Resonac's "Dry Film Polymer Waveguide Materials (Under Development)" allows both the core and cladding layers to be formed by photolithography. Forming optical waveguides using this product under development is expected to enable high-density optical rewiring and pitch conversion for CPO. In addition, since this product under development can be processed using existing back-end equipment, the barrier to adoption in terms of new capital investment is reduced.

This clears the challenges of space constraints and high-precision connectivity that were difficult to resolve with conventional technologies.

Features

Fine Pitch Formability by Photolithography

Both the core and cladding layers are photosensitive, enabling formation by photolithography. Compatible with both multimode and single-mode configurations, formation of rectangular patterns has been confirmed through cross-sectional SEM observation and near field pattern (NFP) evaluation.

Table 1. Core Pattern Shape and Near Field Pattern (NFP)

Mode	SEM image	Cross-section	NFP
Multi mode
Single mode

※The data shown are representative values that represent examples of the results of measurements, calculations, etc., and are not guaranteed values.

Film Thickness Accuracy & Substrate Compatibility

The dry film format enables high film thickness accuracy and flatness. Applicable to both organic and inorganic substrates, and with alkaline developability（1 wt% Na₂CO₃）, the material offers high compatibility with the optical waveguide formation process. We are also exploring varnish-type waveguides to ensure compatibility with a variety of processes.

Fig 2. Form of Polymer Waveguide Materials

Fig 3. Formation Process of Polymer Waveguide

Cladding film lamination

Exposure, Thermal curing

Core film lamination

Exposure, PEB

Alkaline development

Thermal curing

Cladding film lamination

Exposure, Thermal curing

Optical Characteristics Suited for CPO Applications

In single-mode evaluation, we confirmed small wavelength dependence and polarization dependence loss (PDL) in the O-band. Since the variation in transmission loss and PDL with temperature changes is also small, this material is effective for stable optical transmission design, which is a priority in CPO applications.

Fig 4. Wavelength Dependence and Temperature Dependence of Single-Mode Waveguide

Wavelength dependence