Cavity enhanced absorption spectroscopy for optical flux monitoring during thin film growth – CEAS-OFM
Broadband cavity enhanced optical flux monitoring
The CEAS-OFM project is a PRCE project coordinated by Guillaume Saint-Girons (INL Lyon). It also involves ILM Lyon and RIBER. The project began in October 2021 and lasted 36 months. It has received ANR funding of 376k€ for a total cost of around 816k€
A reliable and accurate sensor to control thin layer deposition processes
Thin film deposition processes are at the heart of a variety of technologies that require constant improvement of their accuracy, reliability and reproducibility. In most practical cases growth rate control is carried out using dedicated calibrations samples, further growths being performed under the hypothesis of stable sources. Source stability is however limited, and deposition process drift represent significant cost and time expense. Correcting such drift requires the use of a sensor able to measure the growth rate in real time, coupled to feedback loops on the sources, to actively compensate growth rate variations during the process. Despite significant efforts developed for long, such sensor is not available yet. Among the existing solutions, so called optical flux monitoring sensors (OFM) present several advantages (unsensitivity to ambient gases, no substrate shadowing) but suffer from too low accuracy and significant drift. The purpose of CEAS-OFM is to develop a new concept of broadband cavity enhanced optical flux monitoring sensor (BBCE-OFM) whose uncertainty outperforms that of conventional OFM sensors, and whose operating principle is more robust against drift.
OFM sensors use hollow cathode lamps (HCLs) as light sources, because the spectrum of the latter is spectrally very close to the absorption lines to be measured. This overcomes the issue of the narrow spectral width of these lines, which is difficult to resolve with standard monochromators. However, HCLs are very unstable and introduce strong drift OFM sensors. Besides, they are quite expensive and bulky, and hence complex to integrate in optical setups.
Our sensor includes an echelle monochromator whose spectral resolution is close to the broadening of the absorption lines to be measured. This allows to replace the HCLs by broadband sources such as LEDs. This configuration gives access to the spectral shape of the signal which confers on our sensor a number of advantages over conventional OFM sensors, including a strong improvement of their stability. Additionally, an optical cavity, similar to that used in high sensitivity gas sensors, allows to considerably increase the interaction length between the atomic beams and the light, which boosts the sensor sensitivity.
Development of a sensor architecture robust against drift, with an optimized signal-to-noise ratio.
-Integration of an optical cavity in a deposition reactor: development of an alignment procedure, demonstration of the mechanical stability and development of a CRDS tool for cavity characterization.
-Development of a procedure for acquiring and analyzing the sensor signal, and development of a software interface for signal acquisition.
-Demonstration of an SNR up to 10 times larger than that of conventional OFM sensors, making accurate measurements possible even at low growth rates.
-Implementation of a valorization process (including the submission of a CNRS prematuration project)
The project has led to the filing of 3 patents, currently under review, protecting the various innovations implemented in the sensor (optical cavity, echelle spectrometer, use of a broadband source). Now that these patents have been filed, scientific publications will follow. Two articles have recently been submitted for publication: one on the spectral characterization of the absorption lines (Optics Express) and the other on the performance of a BBCE-OFM sensor for monitoring SrTiO3 growth (Journal of Applied Physics). Several other articles will follow, including at least one on the calibration of the echelle spectrometer, and another on the growth rate extraction. In addition, the results of the project have given rise to an invited lecture at an international workshop, an oral presentation at another international workshop, and poster presentations at two international conferences.
Prematuration project : application under progress
Towards the development of a commercial product
Thin film deposition processes are key in many technologies. Developing a sensor to precisely control in-situ, operando and on line the growth rate/composition of thin layers is thus a critical issue, which remains unsatisfactorily addressed by existing solutions. The purpose of the project is to develop such a sensor, with beyond state of the art accuracy, reproducibility and robustness. Our solution, named CEAS-OFM, relies on standard optical flux monitoring sensors, but embeds an optical cavity (as in well proven pollutant traces measurement systems) that enables considerable sensitivity enhancement, and an optical path drift correction system for unrivalled stability and robustness. The idea is currently being patented by CNRS and RIBER (world leader in the fabrication of MBE growth reactors and partner of the project), and the project aims is to end up with a marketable product, with the support of a third partner, ILM, specialist of cavity enhanced absorption spectroscopy.
Project coordination
Guillaume Saint-Girons (INSTITUT DES NANOTECHNOLOGIES DE LYON)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
INL INSTITUT DES NANOTECHNOLOGIES DE LYON
ILM INSTITUT LUMIERE MATIERE
RIBER / R&D
Help of the ANR 376,156 euros
Beginning and duration of the scientific project:
- 36 Months
