Construction of High Aspect Ratio structures for Acousto-electronic Devices and Electro-optical components on single-crystal Substrates – CHARADES

One goal of the CHARADES project is the implementation of efficient machining tools to confine the fields in volumes as low as a few µm3 while preserving low acoustic or optical losses for the achievement of compact components. It is very difficult to obtain from a technological point of view. As a matter of fact we have to produce structures with high aspect ratios, having almost vertical sidewalls with low roughness.

Particularly, a new method based on simultaneous sawing/polishing techniques is implemented for the fabrication of high aspect ratio structures. This kind of structure is defined in 2 steps: a first groove is made, then we translate some few micrometers which represent the width of the ridge and a second groove is made in order to define the thin ridge. This method offers the great advantages of enabling a resolution of one micrometer, an aspect ratio larger than 300, vertical and smooth walls. The developments made thus completely fulfilled the original objectives of the project.

In parallel, technologies based on fluorine-gases deep reactive ion etching (RIE/ICP-RIE) are implemented to allow for batch fabrication of devices on large diameter wafers which particularly interests our industrial partners. SEM images reveal a significant roughness of sidewalls due to LiF redeposition, etched angles close to 70° and very low etching rates (25nm/mn). By using this developed process, we have experimentally demonstrated the possibility to enhance the extinction ratio of Mach –Zehnder Interferometer modulators integrated in X-Cut LiNbO3 by etching a deep trench in between parallel arms. The main goal was to define low-loss optical ridge-waveguides with smooth walls but the persistence of defects, even after the high-temperature post-annealing process in order to smooth the sidewalls, did not meet the required specifications (collaboration Photline). However recent works with Ar-gas plasma etching method are very promising for future prospects.

The CHARADES project has succeeded in developing tools, breaking with the existing techniques, allowing the production of high aspect ratio structures with low roughness. These new components have the originality to allow the propagation of both optical and acoustic waves. This result opened the way to many achievements concerning different application areas:

- Fabrication of a guided polarization separator and an integrated optofluidic sensor based on self-trapped beams
- Fabrication of low-loss high confined optical ridge waveguides (< 1dB/cm both polarizations and a lateral confinement benefit greater than 2 compared to LiNbO3 standard waveguides)
- Fabrication of low-loss LiNbO3 tapered-ridge waveguides for efficient coupling between confined waveguides and single mode fibers: improvement of the total insertion losses by 3dB in presence of the taper
- Fabrication by combining optical-grade dicing and focused ion beam milling of High aspect ratio 1D photonic crystals on lithium niobate with a reflectivity of 53% at 1550nm and a photonic cavity with a quality factor of 3250
- 15dB-increase of the extinction ratios of Mach-Zehnder modulators thanks to a proper deep trench placed in between parallel arms without any additional losses at 1550nm
- Fabrication of acoustic resonators based on high aspect ratio periodically poled lithium niobate ridge-waveguides with large electromechanical coupling (20%) and large phase velocities (10000 m.s-1). First proof-of-principle of a band-pass filter at 250 MHz with a band pass of 2%.

The CHARADES project has clearly identified the technological issues for the implementation of high aspect ratio structures with low roughness on single-crystals substrates in order to define high confined optical waveguides and trapping structures for resonators and acoustic wave sensors.

Thus, the ability to define low-loss optical ridge waveguides with low roughness by plasma dry etching method always represents a technological step remaining to solve due to the transfer of mask defects on the sidewalls of the etched structures. However recent works with Ar-gas plasma etching method are very promising for future prospects. Photline and FEMTO-ST will continue to work on the subject.

The deposition of patterned electrodes on these highly non-planar optical structures also remains an issue to solve but many technological ways are being explored especially thanks to the acquisition of new thin film deposition equipments within FEMTO-ST and the collaboration on ALD technique developed with the University of Eastern Finland.

Regarding the electric field sensor (collaboration with Kapteos), the major issue consisting in achieving fibered and compact Fabry-Perot resonant cavities on lithium niobate has been solved: it remains to provide a final fibered prototype.

Finally, several techniques developed during the project opens the way to many achievements in the near future including thinning technique and fabrication of membranes with the saw, femtosecond laser machining (collaboration with Femto-Engineering), Ar plasma dry etching and the ability to define high aspect ratio nanostructures by FIB milling and allow to submit a new highly multidisciplinary project.

The CHARADES project was multidisciplinary and has allowed many works in different areas (micro-nanotechnologies, guided-wave optics, nano-photonics, nonlinear optics, acoustics ...). We published 8 articles in international journals (Optics Letters, IEEE Photonics Technology Letters, Optics Express, Optical Materials, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control and Applied Physics Letters) and 1 chapter in the book «Ferroelectric Crystals for Photonic Applications«. We presented our results 29 times abroad and in France with 7 invited papers. Distinction: best poster award at the Joint ISAF ECAPD PFM conference in 2012 by Fabien Henrot and also finalist in 2015.

During the project, two patents have also been registered:
- « Method of fabrication of an optical ridge waveguide with low coupling losses » by N. Courjal, A. Gerthoffer, F. Henrot, JY Rauch, C. Guyot and B. Guichardaz, January 29th 2015
- « Method of fabrication of a photonic waveguide » by N. Courjal, F. Henrot, C. Guyot and E. Fiziaine, December 19th 2014

Two PhD have also been defended:
- C. Guyot : « Study, fabrication and characterization of high aspect ratio photonic crystals on lithium niobate for ultra-compact electro-optical modulators »
- F. Henrot : « High aspect ratio structures for acousto-electrical resonators and for electro-optical devices on single crystals materials ».

Radio-frequency (RF) signal processing is largely based on various components built on single crystal materials generally patterned and structured using planar technologies on wafers for the manufacturing of acousto-electrical devices (surface or bulk acoustic wave devices), optical wave-guides or electro-optical components (modulators, interferometers). One can therefore achieve a large number of functions but some of the corresponding devices are submitted to functionnal flaws independant of the material properties and characteristics (these materials - quartz, lithium niobate or tantalate - exhibit a high level of maturity from an industrial point of view) but related to the usual machining or patterning limits for manufacturing guiding or excitation/detection structures, which prevents to overcome the corresponding functionnal limitations. The CHARADES project then is devoted to the development of technologies capable to upset the corresponding scientific domains and to allow for major evolutions of the manufacturing of new components exploiting the most advantages of single crystal materials required for achieving the targetted functions. Particularly, a new method based on simultaneous sawing/polishing techniques is implemented for the fabrication of high aspect ratio structures (ridges, aspect ratio in excess of 100) providing devices exhibiting unique guiding and trapping capabilites. New functionnalities hence are accessible and the demonstration of the corresponding innovative potential is achieved for acousto-electrical resonators used as sources, filters and sensors and in optics for waveguides, sensors and electro-optical modulators for telecommunication purposes. In parallel, technologies based on deep reactive ion etching are implemented to allow first for batch fabrication of devices on large diameter wafers and second to push ahead the operation limits of the considered devices (frequencies, compactness). Finally, a significant part of the project is dedicated to the theoretical analysis and the experimental investigation of cross-coupled physical phenomena, favored by the ultimate machining capabilities and the specific configuration of the structures developped in the CHARADES project.