Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - TOP HIT (Transfer-print operations for heterogeneous integration)

Teaser

Project description and objectives:Smart systems work best when built with the highest performing components, i.e. those built from the most suitable materials using the best processes. As these processes are generally not compatible with a universal platform, heterogeneous...

Summary

Project description and objectives:

Smart systems work best when built with the highest performing components, i.e. those built from the most suitable materials using the best processes. As these processes are generally not compatible with a universal platform, heterogeneous integration of the components is needed to realise the best system performance. Semiconductor based components (electronics, sensors, lasers) have their essential functionality in thin layers (< 10 m thick) near the surface of the processed wafers. This opens up the opportunity for integration onto a new platform using only thin coupons of the material. After printing, wafer scale processing can be used to connect the coupons with metal tracks, thereby forming arrays of integrated modules on the target wafer surface. The “Transfer-print OPerations for Heterogeneous InTegration” (TOP-HIT) project set out to develop micro-transfer-print as a versatile, scalable, and reliable technology for the compact heterogeneous integration of thin functional materials and devices.

Figure 1 shows a schematic overview of the printing process, while figure 2 shows semiconductor device coupons ready for pick up from a source wafer. The transfer process relies on the ability to release the components from their initial substrate – generally through undercut etching - together with a patterned stamp and an associated tool to pick and place arrays of components in a parallel process. The accuracy of the printing is better than ±1.5 microns across 200mm diameter wafers. The use of array printing means that, depending on the application, ten of thousands of devices can be printed per minute.

In TOP-HIT we sought to advance the micro-transfer-print technology for compact, heterogeneous integration by exploring different materials and device structures. We validated the device integration on two different technology platforms: one in the photonics, the other in the electronics domain.

The consortium consisted of two research performing organizations (UCC/Tyndall and imec), an SME – X-Celeprint, a foundry (X-FAB), and two end users namely Huawei (Caliopa and CIP divisions) and Seagate who are leaders in the silicon photonics and magnetic read-write head industries respectively. The strong collaboration between the partners contributed to the achievements of the project. Figure 3 illustrates the roles of the different partners within TOP-HIT.

Work performed

Description of the work and key results:

The project has considerably advanced the transfer print technology. The technological processes for tethering, undercut etching and transfer print have been thoroughly studied. New release technologies for photonic and electronic materials based on InP, GaAs and Ge and Si have been developed. The design of the epitaxial structures has been optimized to provide highly selective undercut etching and also smooth surfaces for bonding. Pre- and post-processing of the materials into functional devices has been performed. With the pre-processing scheme the performance of the devices can be validated prior to printing. Components (coupons of materials or devices) have been printed to target substrates with and without adhesives. Processes to ensure strong bonding of coupons have been implemented. Stress management in pre-fabricated components is seen to be important thus requiring special consideration both in the design and fabrication work. Processes for accurate positional alignment between the components and the target wafers have been implemented. The components have been interconnected on a wafer scale. Completed components have passed initial reliability tests. Interestingly, we have seen that compared with conventional technology, the overall yield of devices from the starting wafer is increased as the saving on the area for the interconnections outweighs the area needed for the tethering. Based on the success of these studies we are confident that the micro-transfer-print technology can also be applied to a vast array of other materials by appropriate engineering.

Specifically, TOP-HIT has undertaken the first investigations of transfer printing of InP materials to silicon photonic circuits. Specific target circuits using silicon photonics were designed and fabricated on multi-project wafer runs. Using the technology the consortium has demonstrated
• Distributed feedback lasers using evanescent coupling between InP and Si waveguides
• Widely tuneable III-V-on-Si lasers
• Si photonics circuits with >20GHz bandwidth
• Butt coupling between InP laser chips and silicon waveguides through placement of lasers in recesses
• First demonstration of the transfer-printing of Ge photodiodes on a passive silicon photonic integrated circuit
• Transfer printing of substrate-illuminated III-V photodiodes on a silicon photonic IC

TOP-HIT has demonstrated high-efficiency GaAs based power converting cells designed for laser illumination. A power efficiency of 50% has been achieved with an open circuit voltage of >1.2V. InGaP was shown to be a very effective release layer.

TOP-HIT has demonstrated that high-performance, highly compact silicon electronic circuits can be designed for transfer printing. The consortium has determined the optimum tether materials and arrangements to create releasable and printable CMOS chips. The associated design rules were established for creating releasable chips on 200mm diameter wafers. Two different CMOS release processes were implemented by the consortium with first-time-right design and on-time fabrication of the CMOS integrated circuits.
Reliable mass transfer of one of the world’s smallest electronic circuits was demonstrated. This demonstration provides an integration path for the addition of more functionality further enhancing the performance.

Final results

Impact of the project:

The consortium has pioneered the research into the use of transfer printing for integrating active photonic devices with silicon photonics platforms. TOP-HIT has been first to demonstrate a wide range of integrated functional components using the technique described above. The work has been presented at conferences and in peer-reviewed publications. A comprehensive range of publications describing the work is listed on the TOP-HIT website: http://www.tophit-ssi.eu/dissemination.

The maturity of the technology has increased at the partner organizations. The technology can be accessed through partnerships. The results of the project have led to the establishment of the MICROPRINCE project which is focused on the creation of a pilot line for heterogeneous integration of smart systems by micro-transfer-printing. This pilot line will be in a semiconductor foundry manufacturing environment. Functional components like processed III/V devices, optical filters, and special sensors will be transferred to target wafers to demonstrate the capabilities of the technology. This program is being co-funded by H2020 and by the industry partners.

In summary, TOP-HIT has demonstrated that transfer printing has the potential to be a game changing technology for the manufacturing of scalable smart photonic systems based on the heterogeneous integration of optimised components.

Website & more info

More info: http://www.tophit-ssi.eu/.