Photonic integrATed circuiTs using advancEd hybRid iNtegration

Flip-chip Bonding

The development of ultra‐fast and highly sensitive InP photodetectors is one of PATTERN’s main goals. This goes hand in hand with the exploration of a heterogeneous integration method of InP photodetectors into the LNOI PIC platform. In this regard, the work will focus on thermo-compression flip-chip bonding.

The design of flip-chip compatible surface-illuminated InP photodiodes needs to be optimised to enable RF modulation frequency of 100 GHz and above, while maintaining sufficiently high responsivity at a wavelength of 1550 nm.

Flip-chip bonding has significant advantages:

  • InP photodetectors can be bonded anywhere on the PIC surface, as it does not require complex signal
    routing to the edge of the chip;
  • Negligible impact on the total environmental footprint of hybrid devices;
  • The same technology can be used for the co‐integration of electronic chips on the top surface of PICs
    with significantly higher speeds and lower RF losses in high frequencies compared to that of an
    electronic chip co‐packaged side‐by‐side and connected using electrical wires.
  • The process, once developed, can be extended to DC or low frequency ASIC control chips to reach
    ultimate compactness and low cost of a complete System-on-a-Chip co‐integrated on a single die.

Once the ideal alignment between the devices (indicated by maximum transmitted optical power) is determined, the optical facets are bonded together by means of an optical adhesive, resulting in a reduction of coupling losses due to the matching of the refractive index values between the waveguides. In this way, an irreversible mechanical fixation is created between the two structures, which could be PICs, fibre arrays, gain sections, lasers or photodetectors.

PATTERN partner PHIX will develop the process of optical attachment between fibre arrays and LNOI PICs. Fibre array (equipped with a spot-size converter, if necessary) will be aligned to the PIC either by alignment loops structures (preferred option) or by active alignment using photodetector, gain section or laser on the PIC. After successful alignment, an optical adhesive with matching refractive index will be used for permanent fixation.

The drawback is that flip-chip bonding is a process performed in series on die level (chip‐by‐chip) and requires expensive sub‐micron alignment.

PATTERN partner PHIX will implement the flip-chip bonding process and use thermo-compression bonding between the metal pads of the InP photodetector and LNOI PIC platforms to bond the two dies together. The void between the two chips (air gap) will be filled with an optical adhesive of matching refractive index to achieve optimum optical performance.


Flip-chip bonding of an InP Chip on a LNOI PIC platforma
is one of the heterogeneous integration methods that PATTERN investigates