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Surface Acoustic Wave wireless sensors for High Operating Temperature environments

Final Report Summary - SAWHOT (Surface acoustic wave wireless sensors for high operating temperature environments)

Surface acoustic wave (SAW) technology has been applied for more than 20 years to develop sensors exhibiting unique capabilities. Their capability to be wirelessly operated without any on-board power supply has generated several developments yielding this technology ready for market. In parallel, langasite (LGS), a new piezoelectric has revealed its unique capability to operate at temperature up to 1000 °C. Combining both aspects, the SAWHOT project has been set to demonstrate wireless SAW sensors operating in an unprecedented temperature range.

The SAWHOT project consortium was set up on the basis of a bilateral Russian-European partnership generating a unique workforce cooperating within the Seventh Framework Programme (FP7) to address this challenge. Among several aspects, this project has brought in sustainable high-tech socio economic prospects: new markets and standards, improved cooperation between European Union (EU) and Russian organisations.

All parts of the sensor system have been deeply reconsidered to allow for harsh environment operations. Two systems have been implemented at an industrial level for monitoring cryogenic temperatures (down to liquid nitrogen conditions) and high temperatures (up to 800 °C) as well. The first was based on SAW-tags, a single-port reflective delay structure built on lithium niobate (LiNbO3) with identification capabilities, as the second used SAW resonators on LGS.

Nano-imprint lithography (NIL), a new manufacturing technology has been exploited for the fabrication of SAW resonators on LGS whereas standard ultraviolet (UV) lithography was used for SAW-tags. NIL has demonstrated its capability to perform devices with extreme fidelity considering electrode shape and width. Promising results showed that this technology is close to industrial maturity.

Substantial improvements have been achieved for temperature measurements on a wide temperature range for monitoring a carbon-nanotube production process and gas turbine instrumentation. Significant knowledge have been generated in nano-sciences and nano-technologies linked to SAW physical sensors and materials for industrial applications. Further work would be required to increase the time life of the sensors and to industrialise their production.

Demonstration of operability of the sensors in such harsh environments should consolidate the position of SAW solutions for temperature measurements and let envisage the use of SAW on LGS for measurements of other physical properties at such extreme temperatures (pressure, torque etc.). Thus, solutions developed should allow for improvement of position of SAW in sensors markets.

Project context and objectives:

The scope of the SAWHOT project was to develop an innovative wireless system capable to measure physical parameters in a wide range temperature, to pass a new limit in the sensor performances. This device based on SAW sensors was expected to be elaborated partly on a specific high quality refractory substrate called LGS (Russian contribution) and partly on LiNbO3 for low-temperature (cryogenic) applications. Different devices could be implemented such as SAW reflective delay lines called SAW-tags (integrating a identification code) and resonators, providing very narrow band responses with optimal compactness, well suited for high temperature applications according to literature.

SAW devices implement transducers sized at the micrometer level, electrodes are used to be manufactured relying on micro-replication technologies from the microelectronic industry. Stepper based photolithography allow for high throughput fabrication but the patterning accuracy is limited to near sub-micrometer range, which limit the performance of the SAW devices.

A new nano-manufacturing technology, the so-called NIL replication process was considered for device manufacturing, challenging standard stepper-based lithography processes to allow for ultimate accuracy fabrication of nanometre range transducer pattern details. On the contrary, LGS resonators specifically designed for high temperature measurements (up to 800 °C) have been fabricated.

The SAWHOT project did include industrial end users (on both European and Russian sides), in order to implement these high added value systems and assess their technological performance.

The development of a piezoelectric SAW physical sensor system operating in harsh environment (from -196 °C up to more than 650 °C) with wireless transmission of sensing energy and read-out signal was expected to combine at best:

1. substrates e.g. high quality single crystal (composition, architecture, production process) capable to operate on an extended temperature range (-200 °C to +900 °C)
2. design and fabrication of the nano-structured SAW sensors
3. advanced technologies for manufacturing nanometre sized SAW device parts
4. specific packaging and antennas capable to stand long term high temperature conditions
5. appropriate choice of the read-out system and interrogation strategy customised for the environment of the applications.

These developments had to be achieved in response to three main applications:

1. monitoring nano-based production process and more particularly carbon nanotubes (CNT) production
2. temperature measurement in gas turbine engines (or fuel cell systems)
3. monitoring temperature in high voltage connections of feeders (6 to 200 kV) ranging from room conditions up to 300 °C and above (thermal power station and power substation), a typical Russian end-user application.

To address such a challenge, two systems had to be considered at industrial level for monitoring cryogenic temperatures (down to liquid nitrogen conditions) and high temperatures (up to +900 °C) as well. The use of SAW-tags was imagined at a very early stage for low-temperature applications, based on a single-port reflective delay structure built on LiNbO3 with identification capabilities. For high temperature, only LGS could be capable to address the above-mentioned challenge. As the physical properties of LGS is poorly suited for long distance (more than 50 cm.) interrogation, SAW resonators operating near the 434 MHz-centred industrial, scientific and medical (ISM) band were the best solution, expected to yield highly compact structures.

This project was planned with two levels of risk, depending on temperature operating conditions:

1. an optimised system able to operate up to 650 °C
2. a challenge to reach a system operating up to 900°C.

The main scientific and technological goals were:

1. the substrate characterisation
2. the research of the best choice of the cut growth of the crystal
3. the selection of relevant material (substrate orientation, electrode metal, potential additional overlays) to operate at temperatures up to 900 °C
4. the state-of-the-art in wireless SAW physical sensors for industrial applications
4. the demonstration of a whole SAW device based system capable to operate and extract physical parameters in very harsh environments.

Progress had to be achieved as well for the design of SAW devices on LGS, considering the new nano-technology - NIL - features. Moreover, structures combining electrodes and groove gratings required significant simulation efforts to promote their use for such sensors.

Improved capabilities had to be provided for monitoring and sensing of processes in difficult environments with impact in nano-manufacturing (in the SAWHOT case production of CNT), the project has to contribute to the improvement of nano-science and nano-technologies as a significant activity within the developed tasks focus on monitoring the growth conditions of CNT.

The last technical challenge that can be recalled here is the demonstration of the developed system pertinence at an industrial level, proving its capability to monitor temperature and other physical parameters under extreme operating conditions (high pressures, temperatures, intense electromagnetic fields, accelerations / chocks and strong environmental aggressions).

Finally, the project was dedicated to reinforce cooperation between EU and Russian organisations, as 10 partners on the European Community (EC) side (half industry, half academics) had to strongly collaborate with four Russian partners from both industry and academic organisations. An important issue of the project was to set an exploitation scheme of the results, describing the rules for sharing foregrounds among the partners willing to exploit them.

Project results:

Four groups of scientific and technological results of SAWHOT have been identified when setting the exploitation strategy plan of the project. One can easily distinguish between:

1. specification and theoretical work (design) allowing for the definition of the sensor and system architectures
2. technical foreground related to technological developments, material growth, fabrication of the devices and packaging developments yielding assembled sensors ready to use
3. system development and demonstration of their operability in real environment (as close as possible to the end-user operating conditions)
4. promotion and dissemination of the foreground, along industrial approaches (patent application) as well as academic communication (peer-review journal articles, symposium proceedings papers, public communication).

The following paragraphs discuss more in detail the actual production of the project.

Specifications and design:

1. Specifications for the SAW material and enhancements have been set, as well as specifications for the SAW substrate.
2. Setting a complete collection of requirements has allowed for an accurate definition of sub-system specifically devoted to high or cryogenic temperature applications.
3. Technical specifications for antennas associated with SAW sensors operating either at very low (cryogenic) or very high (above 500 °C) temperature have been defined which can feed any comparable wireless sensor development.
4. Technical specifications have been set as well for the wireless SAW sensors devoted to high or cryogenic temperature applications, used for design purposes.
5. A well-detailed design process and results for both delay lines and resonators based on the previously mentioned specifications has been achieved, conforming the requirements for high temperature and cryogenic temperature purposes. Antennas able to work at such temperature have been designed as well.

A first step in the project was devoted to identify what could be the possible materials for addressing extreme temperature:

Piezoelectric materials are required in SAW devices to allow transduction of electrical signal applied to interdigitated electrodes into mechanical waves. A benchmark of piezoelectric materials has been achieved regarding operating conditions specified for the targeted applications. A comparison of the presented piezoelectric materials, evaluating their properties regarding the given application requirements, commercial availability, piezoelectric properties and current knowledge about the materials has been realised.

While gallium ortho-phosphate (GaPO4) exhibits exceptional low losses at high temperatures (above 500 °C), the availability and the current status of research regarding SAW applications prevents the use of this materials for commercial scale applications. AlN is suffering from the same issue but thin films may allow for alternative solutions in that matter. While the few existing high temperature measurements indicate good high temperature stability, there are currently no AlN bulk crystals suitable for SAW applications available on the market.

Compared to conventional LiNbO3, which exhibits only poor high temperature properties, stoichiometric LiNbO3 at temperature above 500°C is expected to exhibit a significantly higher stability at high temperatures. Although stoichometric LiNbO3 is currently not available in large amounts and wafers (2''), a growing demand for this material may change the situation. Existing technologies and processes can be used to process those wafers. However, the knowledge about the behavior of stoichiometric LiNbO3 at temperatures above 500 °C is currently limited and should be therefore subject of further research, before a commercial application of this material should be considered.

Despite the increasing availability of high temperature stable piezoelectric ceramics, their applicability for SAW devices is strongly limited by their poly-crystal structure.

Considering all these properties, the LGS-type crystals (LGS and langatate) are currently the most promising materials for high temperature SAW applications. Although they show higher losses than for example GaPO4, they are in general usable at very high temperatures due to the absence of phase transformations up to their melting point. An advantage compared to the other presented materials is the commercial availability of high quality wafers, especially of LGS, which can be obtained by different manufacturers also in larger scales. Since the piezoelectric properties of LGS, especially for SAW applications, are studied more in detail than those of Langatate, LGS was consequently identified as the preferred material to achieve the goals of the project.

Technological and technical foreground:

1. Significant improvement of the manufacturing process of LGS wafers have been performed within SAWHOT, from the growing process itself to the finishing steps connected to the wafers themselves (lapping, polishing). Characterisation procedures have also been developed, At the end of the project, the process allowed for manufacturing of homogeneous, high grade 4'' LGS wafers.
2. In SAWHOT, we have demonstrated fabrication on brittle LGS substrates, which was not demonstrated previously. Manufacturing methods based on UV stepper technology suffers from a larger line-width variance, thus the quality of the final SAW devices is impacted. As a consequence, the development of calibration-free sensor is favoured when using jet and flash imprint lithography (J-FIL) fabrication technology (NIL process).
3. The SAW device manufacturing relies on the following process: a first sacrificial photo-resist layer is deposited on the LGS substrate; a second resist is deposited and patterned via nano-imprint lithography; a dry etch process is implemented to transfer the pattern in the first resist which is anisotropicaly etched; metal is deposited on the whole structure, resist is eliminated, leaving metal only at the electrode location.
4. The capability to combine grooves and electrodes on LGS has been assessed. Groove etching using either an ion etching method or wet etch protocols has been investigated, yielding state-of-the-art results. This approach is expected to improve the Q-factor of the SAW devices and their robustness to temperature (reducing the amount of metal inside the device).
5. Technical approaches for characterising and subsequently control the quality and homogeneity of the substrate during and after growth have been developed, allowing for improving the device compliance with NIL standards.
6. A new SAW transducer based on thin films that can resist to high temperature environment (here AlN is promoted) has been imagined as a backup to LGS based solutions. This new transducer structure allows for a high control of the resonance frequency together with an effective resonance quality at frequency ranging from a few hundred MHz to several GHz. This transducer could possibly allow for the excitation of highly electromechanically coupled modes.
7. A set of SAW substrates have been identified as particularly well suited for temperature sensing and device combination has been successfully achieved for high temperature (HT) sensing application.
8. Electrodes capable to withstand temperature above 650 °C on long term have been identified and tested, the best metal combinations have been defined to avoid electro-migration and to keep the integrity of the metallisation even after several ten hours exposition to temperature equal or in excess of 650 °C. Exposure to such temperature leads to several degradation processes, including de-wetting of metals deposited on LGS. Stability of the electrodes has been increased implementing a passivation layer. Such stabilised electrodes have been capable to withstand temperature of 800 °C for more than 200 hours and temperatures of 900 °C for more than 20 hours.
9. Finite element modelling, coupled with materials properties measured, have been developed and implemented in order to optimise the design of the packaged sensors. The three-dimensional (3D) parametric model could be used to assess new designs and optimise the behaviour towards temperature.
10. A molecular bonding process allowing for assembly of two LGS wafers has been developed and optimised. Implementation of such assembly allows for LGS primary packages of the devices.
11. An innovative approach has been developed to allow for sensor packaging able to operate at temperature equal or larger than 650 °C. The process consists in embedding the assembled packaged parts within a ceramic sarcophagus. The devices are connected using wire bonding technologies to prepared ceramic packages and connected to the antenna using patterned boards. Once assembled, the whole assembly is drowned in a ceramic sarcophagus.
12. Antennas able to work at such temperature have been tested. The assembly process for combining the sensor and its antennas has been validated in the frame of the previous foreground.

System and demonstration:

1. A passive wireless measurement system compatible with a low temperature and/or rotating objects application has been developed and tested. A passive wireless measurement system compatible with an application at high temperature on fixed or rotating objects has been developed and adapted to high temperature purposes. This system has been deployed on end-user test rigs. Russian partners have validated the use of the system for power plant instrumentation and temperature measurement for physical vapour deposition (PVD) process. The system was successfully used for monitoring temperatures in chemical vapour deposition (CVD) reactors for a CNTs production. The application on a gas turbine engine was very challenging, due to the harshness of the environment.
2. The software was adapted to provide accurate information on the measured physical parameter including adapted guided user interface (GUI). These developments were carried out for both low and high temperature and tested on end-user test rigs.
3. The validation of the sensor operation at very high temperature has been tested. Encouraging results have been obtained at temperature up to 800 °C. However, the system still needs adaptation to operate from room to such a high temperature and to communicate through metallic shielding in realistic operation environments. These tests on a gas turbine are still ongoing.
4. Wireless temperature monitoring, where reaction/process of materials production takes place in very high temperatures and harsh environments, has been tested for CNT growth. This work was not totally completed, because of communication issues during the production process, but very encouraging results regarding the sensor's durability were obtained in that frame. Further improvements of the system antenna and packaging towards commercialisation are essential.
5. Development of a low temperature sensing system including identification capabilities will allow for numerous applications such as sensors compatible with permafrost operating conditions, space applications and cryogenic metrology developments.

Promotion and dissemination:

1. Patents applications have been generated concerning high temperature compatible packaging, the sensor itself and its structural elements. A large number of scientific papers have been produced during the project, promoting scientific and technologic contribution of SAWHOT to the knowledge.
2. Allowing for practical implementation of wireless system for high and low temperature, a possible HT sensing industrial training session can be implemented, based on the project experience. Development of a flyer / poster has been achieved as a 'reference' tool.
3. New materials, sensor concepts, fabrication technologies, wireless interrogation techniques and implementation procedures of high temperature sensing systems can be learned by the academic partners of the project as major scientific contributions to the art.

Potential impact:

The impact of the project on employment appears as soon as the beginning of the project. Within the European partners, more than 35 actors have been involved in the project, including six which have been specifically recruited for the project. The implementation of the project has effectively sustained existing employments or generated new ones as follows:

1. Recruitment of a project manager at IPV at the beginning of the project. The person left the organisation at the end of the first year of the project to work in a similar position in another organisation.
2. Recruitment of two advanced researchers specifically for the project, including a post-doc for duration of three years.
3. Involvement of nine PhD students, among whom two specifically recruited for the project.

The two last items demonstrate the project on development of skills and preparation of highly qualified employees. Two of the advanced researchers employed under post-doc positions have found new positions in the field of SAW devices, one in a public laboratory and the other one in a private company.

One can also notice an impact of the project in terms of indirect benefits for the European industry. Some works have effectively been supplied out of the consortium of SAWHOT, as the manufacturing of the delay-lines manufactured on LiNbO3 and the first devices produced on LGS, that has been realised by a European foundry implementing stepper based technology. Some European third parties specialised in crystal finishing have also been involved in the collaboration by the Russian partner FOMOS.

Considering the industrial applications and the connected markets, objectives of SAWHOT were very ambitious regarding the temperature range addressed, while the highest temperature level targeted (900 °C) hasn't been validated through a full operating demonstration, the target of 650 °C has been reached and operability of the system has been demonstrated up to temperature of 800 °C.

Such performances are a real breakthrough compared to temperatures addressed up to now and should allow for an extension of the use of SAW devices for temperature measurement in harsh environment, as required by a lot of industrial applications (chemical, industrial process control, transport, automotive etc.).

Technical challenges have been more difficult to overcome that it has been foreseen when writing the proposal, leading to extend most of the work packages (WPs). However results obtained consist in a significant step forward in application of SAW sensors for use in harsh environments. Work performed in SAWHOT and the foreground generated through the project has led to numerous publications at an international level and some of the results have already risen to patent applications. Intellectual protection measures should lead to other patent application after the end of the project, as described in the current document.

Some results of the project could generate business opportunities from now: LGS wafers could already been produced, some of the manufacturing processes could already be proposed for prototyping (NIL lithography for example), systems were supplied for research and development (R&D) activities or feasibility demonstration.

However, commercialisation of most of the results would need further works in order to reach a level of robustness and reliability required by markets. Industrialisation steps would then need to be led in order to satisfy industrial demand. Industrial partners are interested in sustaining business relationship and collaboration in order to improve developed solutions, promote application of SAW sensors for harsh environments and develop their business. SENSeOR especially is interested in improving solutions implemented for the high temperature sensors based on resonators. If led, industrialisation works would probably results in a time to market of at least three years.

Next actions for commercialisation will strongly depend on initiatives and means of industrial partners to lead the required improvement. This is both a strategic issue that would require significant means to be committed and one can imagine this subject will be discussed in each company involved in a short term. However such actions should emerge at a very short term.

Project website: http://www.sawhot-project.com

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