Two-photon lithography: breaking through the packaging bottleneck of optoelectronic chips

Dr. Shaoliang Yu’s team from Zhijiang Lab and collaborators from the Massachusetts Institute of Technology (MIT) published a review article titled Two-photon lithography for integrated photonic packaging in Light: Advanced Manufacturing.

As a rising star in the semiconductor industry, optoelectronic chips have important application prospects in the fields of optical interconnection, optical computing, optical sensing, and lidar, and have become the focus of attention from all walks of life, opening up a new track for the development of the post-Moore’s Law era. Benefiting from the mature semiconductor CMOS process, the fabrication capacity of optoelectronic chips has been rapidly developed.

However, there are still key technical bottlenecks in the packaging process of optoelectronic chips. In addition to the well-known interconnection of electrical signals, the coupling of optical signals between different modules needs to be taken into account in the packaging process of optoelectronic chips, which requires solving the optical interconnection problems between optical fiber-chip and chip-chip. There are two main challenges:

1. Optical chips involve a variety of material systems and structures, and there are great differences in the size and distribution of different beam mode fields, so the efficient coupling between multiple material systems and structures can be achieved by cleverly solving the problem of mode field mismatch in the packaging process.

2. The beam size of the waveguide on the chip is in the micron range, which requires high-precision alignment to achieve efficient coupling, which puts forward higher requirements for the alignment accuracy of the packaging process.

Figure 1: Schematic diagram of optoelectronic chip packaging. Source: Research group

Two-photon lithography based on two-photon absorption process, as a three-dimensional printing process at micro-nano size, can prepare arbitrary three-dimensional structures with high precision, which is expected to solve the bottleneck of the optoelectronic chip packaging process.

On the one hand, a three-dimensional curved surface or gradient waveguide structure can be integrated on the chip, and the beam can be shaped by reflection or adiabatic compression to achieve the mode field transformation of the ultra-wide waveband.

On the other hand, the topography of the 3D structure has a high degree of geometric freedom, which increases the flexibility of the manipulation of the on-chip tool field, resulting in more efficient coupling interconnection.

In addition, two-photon lithography can also prepare the connection structure after the submodule is assembled, which effectively reduces the alignment accuracy requirements in the packaging process.

Therefore, in the packaging of optoelectronic chips, two-photon lithography technology has important application value and has been widely explored, and there are currently three main technical routes.

Optical packaging method based on two-photon lithography

1. Photonic wire bonding

Drawing on the widely used wire bonding and photonic wire bonding technology in microelectronics, the two-photon lithography process is used to directly print polymer waveguides between the waveguides to be connected. Through the gradual change of the waveguide cross-section, the adiabatic transformation of the mode field is completed, so as to realize the efficient interconnection between different waveguides. This method has been verified in optical interconnection and coherent communication, and is suitable for a variety of application scenarios such as fiber-chip, chip-chip, etc.

2. Miniature freeform surfaces

A miniature optical freeform surface is printed on the end surface of the waveguide, and the light field emitted by the waveguide is shaped in the form of reflection or refraction, and the mode field distribution and propagation direction are adjusted to complete the mode field transformation. The structure used has low dispersion and is not wavelength sensitive, and ultra-wideband coupling from visible to near-infrared has been validated, and it is compatible with wafer-level test and packaging, enabling high-density interconnect packaging.

3. Mechanical alignment and guidance structure

Two-photon lithography can also be used to print mechanical alignment guide structures to assist in high-precision alignment of the coupling process. Printing an inverted cone structure on the grating coupling region to guide the alignment process of the fiber can achieve sub-micron alignment accuracy without introducing significant additional losses, which is expected to be applied in pluggable devices.

Figure 2: (a), Schematic diagram of two-photon lithography. (b), Photon leads. (c), Freeform surface. (d), alignment of the guidance structure. Source: Light: Advanced Manufacturing 4, 32(2023)

Commercialization exploration

With the gradual development of optoelectronic chips to the market, packaging technology based on two-photon lithography has also begun to be commercialized. For large-scale commercial applications, more factors need to be considered in addition to coupling characteristics such as bandwidth and insertion loss. For example, whether two-photon lithography can stably and reliably produce high-quality 3D structures, whether it can meet the industry’s processing speed and accuracy requirements, and whether it has user-friendly ease of use and maintenance.

At present, several companies have opened up the commercial market of two-photon lithography. NanoScribe, Vanguard, Heidelberg and other companies have launched commercial two-photon lithography equipment, and have made great progress in scanning speed, processing accuracy, alignment accuracy, etc., while Dream Photonics, PHIX, etc., mainly provide process services, and can directly provide packaging services based on two-photon lithography. The application of two-photon lithography technology in optoelectronic chip packaging has taken a solid step towards large-scale commercialization.

Figure 3: Three types of slicing: uniform slicing, adaptive slicing, and smart slicing. Source: Light: Advanced Manufacturing 4, 32(2023)

Future outlook

After more than ten years of exploration, the packaging method based on the two-photon lithography process has made a lot of progress and has been widely recognized by all walks of life. In the era of explosive growth in communication capacity, it is necessary to judge whether the two-photon lithography process can occupy an important position in the packaging of optoelectronic chips, and whether it can meet the needs of large-scale applications in the future. Based on this, the author also sorts out the future development trend of this field.

1. Greatly improve the preparation efficiency

The current point-by-point scanning method has a slow preparation speed and is difficult to meet the efficiency requirements of large-scale production. On the one hand, new two-photon exposure methods such as multi-beam and layer-by-layer can be used to improve the preparation speed. On the other hand, other fabrication processes can also be explored, such as nanoimprint, which can upgrade the serial processing method to parallelism to meet the fabrication needs at the wafer level.

Figure 4: Three exposure methods: point-by-point, layer-by-layer, and multi-beam exposure. Source: Light: Advanced Manufacturing 4, 32(2023)

2. Develop multiple types of lithography materials

Two-photon lithography mostly acts on photopolymer materials. Compared with conventional semiconductor or dielectric materials, polymer materials have a large coefficient of thermal expansion, a limited range of refractive index options, and poor long-term stability. At the same time, the shrinkage of polymers in the cross-linking process also brings certain challenges to the morphology control of three-dimensional structures. Exploring organic-inorganic hybrid composite photosensitive materials can solve the above problems to a certain extent.

3. Optimize the design and modeling methods

The geometric degree of freedom of the three-dimensional structure is high, which brings great convenience to the wavefront control. However, the design regulates many parameters, which brings a lot of pressure to the simulation design process. It is necessary to combine geometric optics and wave optics methods to explore computational exploration and construct a new modeling method. Data-driven and physics-driven machine learning methods can also play an important role in the design and characterization of 3D miniature optical structures.

4. Develop new methods for structural characterization

The miniature three-dimensional optical structure, the scale is between the macro and the micro, the structure is small, the curvature is large, and the conventional measurement methods, such as white light interferometry, electron microscopy, atomic force microscopy, etc., are difficult to carry out effective measurements, and new characterization methods are urgently needed. The three-dimensional reconstruction of electron microscopy based on multi-quadrant is expected to achieve accurate measurement of the topography of microfreeform surfaces. X-ray microtomography is also a promising characterization method.


Two-photon lithography technology can accurately prepare three-dimensional structures and integrate them on optoelectronic chips, which can build large-bandwidth and low-loss optical signal links between fiber-chip and chip-chip, realize the efficient interconnection of optical signals, reduce the alignment accuracy of the packaging process, and bring new opportunities to the packaging process of optical chips. With the iterative evolution of technology and the further development of the industry, we expect that the packaging architecture of optoelectronic chips based on two-photon lithography will be applied on a large scale to solve the packaging problems of optoelectronic chips. (Source: Advanced Manufacturing WeChat public account)

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