Photonic Trimming is the operation of changing the phase of optical paths in photonic integrated circuits (PICs). This process recenters the PIC performance to the target or designed value.
FEMTUM has developed a non-contact laser process to perform post-fabrication phase correction of defective PICs, turning known bad dies into known good dies. The process is very fast, with a duration in the milliseconds range, and very well controlled. The laser and optical head are designed to be simply integrated into current semiconductor tools.
In this blog, we will review the key requirements and test results obtained by FEMTUM’s applications lab.
Photonic Circuits Manufacturing Residual Errors
Manufacturing PICs is challenging. Even though the lithography-based fabrication method evolved exponentially over the past decades and reached a mature stage, significant errors on the dimensions (height or width of the waveguide) of the waveguides remain. This translates into defective operation for phase-based components and circuits, such as Mach-Zehnder Interferometers or micro-rings. Thus, a post-fabrication correction is needed to increase the yield per wafer.
Image 1 shows wafer-level measurements of the spectral response of MRRs targeted to resonate at a wavelength of 1310 nm. We see the high variability of the resonances (dips) in the optical spectra.
Image 1. Spectral response of PIC micro-ring resonators1, with a high variability of the position of the resonances. Right image shows the wafer localization of the PICs.
Trimming, a Local Phase Correction
At our applications lab in Québec City, we have tested Mach-Zehnder Interferometers (MZI) and Micro-rings to demonstrate the key specifications of the trimming process :
- Linearity of the phase correction,
- low process induced loss,
- permanency and
- phase correction limit greater than a full π-SHIFT
Image 2 depicts demonstrations of the Photonic Trimming process on simple MZIs. We can see that the main resonances have been corrected to 1550 nm. The period of each MZI (free spectral range, FSR) remains unchanged.
Image 2: Photonics Trimming of different FSR MZI. Left is the FEMTUM approach. Top right shows the original spectra, as fabricated. Bottom right shows the trimmed spectra, with recentered resonances. FSR is unaffected.
The next sections present the phase correction experiment’s results in terms of linearity, added insertion loss, correction range and thermal stability.
Phase-shift Linearity
A well understood process is key to reaching the performance in the shortest time. A key metric to demonstrate this is to map the linearity of the trimming with regards to the number of pulses of the same energy. Image 3 shows the linearity of the phase change per pulse, using the same laser parameters for each pulse.
Image 3: Consecutive phase change for each pulse. Data extracted from the FEMTUM automation software LIT.
With such a high linearity, we can tailor the process to get the precise correction that brings the PIC as close to the target value as needed, by adapting laser parameters such as pulse power. High-resolution phase corrections can be achieved. It is up to the end-user to determine the tolerence, or how close to the target value the PIC needs to perform.
Low Induced Loss
We have shown that Photonics Trimming affects the phase of the optical path, but is the optical loss affected? The process is does not affect the optical loss by more than 0.1 dB. Table 1 shows the process induced optical loss for the tested MZI of Image 2, which are all below 0.1 dB.
| Circuit | Initial phase shift [rad] | Process-induced losses [dB] |
| 1 | 1.42 | 0.06 |
| 2 | 1.09 | 0.03 |
| 3 | 3.10 | 0.05 |
Table 1: Process-induced loss for three Mach-Zehnder Interferometers. Loss is kept under 0.1 dB for the three different phase changes.
Phase Correction up to π-shift
From the spectra in Image 1, we notice a periodicity in the curves. Each resonance or dips in the curves occur for phase differences of an integer number of 2π radians. Since Photonic Trimming can correct phase in both positive and negative directions, this means that no spectrum is further away than half of the period, or of a π-shift. Image 4 shows that the spectrum has been shifted by half the period.
Image 4 : Demonstration of phase correction on a MZI PIC.
FEMTUM’s Photonics Trimming is highly linear and shows low induced loss. The correction range is high enough so that the Trimming process can be applied to all the defective PICs on a manufactured wafer. Trimming can be implemented at the wafer level or after the dicing.
If you require more information on Femtum’s laser trimming solution, please contact us to discuss your challenges or view Femtum.ai.