MEMS positioners with silicon nitride waveguides for photonic integrated circuits alignment

Abstract

The integration of active chips such as lasers and semiconductor optical amplifiers into silicon photonics is essential in photonic integrated circuits. Silicon as the main platform in the semiconductor industry, is not the right substrate for efficient active chips. Thus, externally fabricated active chips are currently integrated using flip-chip bonding. However, vertical and lateral misalignments, and horizontal gaps between the waveguides of the active chips and the waveguides of the silicon chips are the major sources of optical losses. This work proposed five MEMS positioner devices to provide dynamic alignment of active chips in photonic integrated circuits. Multi-degrees of freedom (MDOF) devices were fabricated and tested. The first device has an aluminum nitride piezoelectric actuator to provide downward and upward displacements in the z-axis. The second and third devices used hybrid piezoelectricelectrostatic actuations to provide 2DOF and 3DOF motions, respectively. All the devices were equipped with capacitive combs to track the displacements, where good correlation between the sensed capacitance and measured displacements were achieved. In addition, the dynamic alignment is provided by activating more than one actuator at a time, and suspended waveguides were aligned to the fixed ones by compensating in-plane and out-of-plane misalignments. The fourth device used three chevron thermal actuators to provide 3DOF motions in the x, y and z axes. A large z-axis displacement was achieved as a result of buckling caused by two opposite in-plane thermal forces. The fifth device provides 2DOF in-plane motions in x- and y- axes using thermal actuators. Prototypes of the devices show that in the x-axis, the third device provides total displacement of 300 nm at ±100 V, whereas the fourth device achieves a total of 6.7 μm displacement at 105 mW. In the y-axis, displacements of 3.16 μm at 120 V, 10.9 μm at 140 V, 4.5 μm at 140 mW and 6.92 μm at 189 mW were achieved by the second, third, fourth and fifth devices, respectively. In the z-axis, the first, second, third and fourth devices give 1.3 μm, 3.16 μm, 0.63 μm, at ±60 V, and 7 μm at 210 mW, respectively. Unlike the first four positioners that used 10 μm-thick silicon-on-insulator (SOI) layer, the fifth positioner utilized a 59 μm-thick SOI layer. Also, this positioner is fabricated with a suspended functional waveguide made of a stack of silicon dioxide-silicon nitride-silicon dioxide layers. The suspended waveguide is separated from another identical fixed waveguide by 6.92 ± 0.01 μm. Closing this gap achieved -1.60 ± 0.06 dB minimum insertion loss from 1550 to 1620 nm, whereas closing the gap while maintaining ± 2 μm lateral displacement can provide an attenuation of up to 20 dB. Thus, the device can operate as an attenuator or an on/off switch.

Date	5 Mar 2024
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Frédéric Nabki (Supervisor) & Michaël Ménard (Co-supervisor)