Monolithic Silicon Photonics Interferometer for Ultra-sensitive MEMS Sensors

Lead Research Organisation: University of Glasgow
Department Name: School of Physics and Astronomy

Abstract

Interferometers are devices which can provide accurate position measurements. They work by comparing the distance travelled by light in two (typically) perpendicular arms. When the light from the two beams is combined and interfered, the change in intensity can be compared to a fraction of the wavelength of light. If both light waves are in-phase the light output is a maximum, while if the light waves are out of phase the intensity will be zero.
This project will develop a miniaturised on-chip interferometer suitable for use in Micro-Electro-Mechanical System (MEMS) sensors. The entire interferometer will occupy a size below 10 mm x 10 mm. The light source will be an off-chip 1550 nm laser which is coupled to on-chip silicon waveguides, polarising beamsplitters and Ge photodetectors which are fabricated by reactive ion etching of commercial silicon-on-insulator (SOI) wafers. The interferometer utilises the polarisation of the incoming laser beam to operate two separate interferometers within the same Si components. This is achieved by shifting one of the beams by 90 degrees with respect to the other one with a phaseplate. By plotting the two outputs from the interferometer we achieve a circular pattern whereby the circle is traversed every time the mirrors are moved a relative separation equal to one half the wavelength of light (or lambda/2). If the circle can be subdivided into 1000 sections, the sensitivity is of the order 700 picometres for 1550 nm laser light. In principle it is possible to do even better by accurate modelling of the shape of the resulting circular pattern and we propose to demonstrate a sensitivity of 100 picometres.
Current MEMS sensors are typically monitored with simple electrostatic sensors which measure the capacitance between micro-structured electrodes. Whilst being a relatively simple component, this method has limitations in the achievable sensitivity, and also challenges as electrostatic voltage causes spurious forces which can destabilises the delicate MEMS device and cause it to lock. The on-chip interferometer which is described in this proposal will be combined with a MEMS accelerometer in order to provide an ultra-sensitive device able to sense tiny changes in gravity. To give quantitative measure of the sensitivity, MEMS accelerometers found in iPhones typically have an acceleration sensitivity of 0.015 ms-2 and are able to sense the direction of the screen. The accelerometer we propose here has sensitivity approximately 1 million times better than an iPhone accelerometer. This will allow the opportunity to utilise the optical readout MEMS to monitor the gravitational effects of a variety of bodies. This could include dense objects being smuggled through ports of entry, hidden subterranean tunnels, the location of carbon sequestered under the ground or the location of buried nuclear waste. The device thus has a wide range of applications in the areas of precision sensing and monitoring, detecting hidden dense masses and buried objects/tunnels, and the monitoring of carbon/nuclear waste in geological repositories.

Planned Impact

Academic impacts: We expect a broad range of academic impacts to be generated from this project. Once all IP has been protected we expect that a significant output of high quality academic peer reviewed papers suitable for inclusion in future REF exercises would be generated. Furthermore, the investigators will ensure that a high profile is established through visible presence at conferences and workshops which are aligned to the subject area (e.g. photonics, gravity and geophysics, defence). Talks and posters will be utilised to identify the impact of the work within the global challenge areas. This will directly enhance the health of physics and engineering as academic disciplines. We further aim to train highly skilled researchers (1 PhD student and undergraduate project students) in topics which include MEMS fabrication, mechanical design, optical and electronic design and fabrication. The skills learnt during this project will be of the highest quality and significant breadth and will allow the students to excel in their future career path. The project will provide the successful applicant with a high level of individual ownership and offer the opportunity to perform cutting edge research & development. Moreover, the studentship incorporates knowledge transfer aspects and industrial collaboration together with high quality core research training. There will be opportunity for the students to present their work at national/international conferences, publish in high quality journals, and interact with industrial end users, ultimately enhancing the student teaching and learning exerience.

Economic and Societal Impacts: The project will enhance the commercialisation and exploitation opportunities for this technology. Glasgow University spin out company Kelvin Nanotechnology, who provide commercial access to the James Watt Nanofabrication Centre (JWNC), will contribute to the project exploitation with initial low volume manufacture and by providing advice and support for the transfer of the technology to larger MEMS foundries if successful. The technology will be made available to STFC personnel and academics through the Kelvin-Rutherford partnership. In addition we will engage with industry and government bodies to understand their precise needs and enhance knowledge of their applications. This will be achieved through engagement with partners including MoD, SELEX ES, Thales, BP Alternative Energy, Micro-g Lacoste, Bridgeporth, and DEFRA. Prof Paul sits on the Cabinet Office High Impact Threats and Home Office CBRN scientific advisory panels and has sat on MOD DSAC until the end of 2013. He therefore has the required DV (Developed Vetting) clearance to know the operational requirements, and knows the appropriate people related to the UK national security who would be interested in using this technology. The on-chip amplifier and MEMS device will further enhance the ability of the businesses to enhance their economic sustainability via the potential of bringing to market a novel and disruptive technology, with wide ranging applications on the global challenge areas.

Publications

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Middlemiss RP (2017) Field Tests of a Portable MEMS Gravimeter. in Sensors (Basel, Switzerland)

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Middlemiss RP (2018) Microelectromechanical system gravimeters as a new tool for gravity imaging. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

 
Description During this project we have designed, fabricated and integrated a MEMS (MicroElectroMechanicalSensor) based optical interferometer. This has been realised by a Deep Reactive Ion Etching (DRIE) process developed in the James Watt Nanofabrication Centre at University of Glasgow. The interferometer uses the same processes developed for our MEMS gravimeter, and the two systems have recently been integrated on a single 220 micron thick silicon-on-insulator wafer (see figure 1). The interferometer is illuminated by a collimated optical input fiber via a 10mW telecommunications laser working at 1550 nm. A fibre coupler picks off 50% of the light to monitor power variations in the interferometer and provide long term stable operation.

For the integrated package (figure 1) the MEMS proof mass bottom surface acts as one of the mirrors, while a second arm with an equivalent length forms the interferometric readout. During the project we have also done etch tests on micro-mirrors realised by creating deep etched Bragg structures with different periods and grating orders. This will be used to realise high reflectivity mirrors in a later version. A series of test structures have already been designed and fabricated and we are currently in the process of testing several designs and comparing versus thermal stability (changes in temperature) and laser stability (changes in wavelength).

The MEMS gravimeter which has been integrated together with the interferometer has recently performed a measurement of the Earth tides. The earth tides are the change in local gravitational acceleration due to the stretching and shrinking of the earth due to the relative motion of the moon and sun (a change of 2x10-6 m/s^2 compared to the local acceleration which is 9.81 m/s^2). This is the first time an effect has been measured with a MEMS and this opens up a wide range of potential applications in the area of oil & gas exploration, environmental monitoring and security application. Indeed we have had significant company engagement from Bridgeporth (http://www.bridgeporth.com/ survey company) and Tullow Oil (http://www.tullowoil.com/ oil company).

Figure 1. Scanning Electron Microscope image of the interferometer with thin beamsplitter and MEMS. The grooves labelled "input" and "output" identify where optical fibres are positioned. The second arm goes to the bottom right of the image where there is another mirror. The material under the label "Arm 1" is removed with a vacuum chuck when the device is released from the etching structure.

We have performed significant modelling to assess the topology which provides the best performance for the interferometer. This includes both a thin beamsplitter as shown in figure 1 and also a thick beamsplitter (see figure 2). The thick beamsplitter has the benefit that it is insensitive to changing reflection at the beamsplitter due to temperature changes. Temperature changes results in both a change in the length of the beamsplitter and also changes in the refractive index, both which change the optical path length and thus change the reflected and transmitted intensities. Our modelling has traded off these different contributions to the overall long term stability of the device with the aim to provide the most stable output.

Figure 2. Top: Design mask for the MEMS interferometer fabrication Bottom: The thin and and thick beamsplitter designs

The interferometer was first aligned in July 2015 using the thick beamsplitter design. The initial tests were performed with the entire device fabricated on a single wafer with fixed mirrors. The entire interferometer is 3mm x 3mm in size and as a result we developed a microscope setup to align the fibre and interferometer (shown in figure 3).

Figure 3. Left: A microscope is used to align the fire in the input groove while a tip/tilt/translation stage allows the relative motion of the fibre with respect to the interferometer Right: a close-up of the interferometer and the inserted optical fibre. This chip includes 4 interferometers.

Figure 4 shows the output fringes of the interferometer. The main spot is clearly seen, while secondary spots come from multiple reflections within the silicon. In future designs these will be mitigated by designing suitable angles of the interferometer so that reflections are guided away from the output port (these reflections are shown by the red arrows in figure 3). The intensity coming from the output port was monitored with a 1550nm sensor and it was noted that the device remained at a constant intensity even during heating with a temperature change of approximately 40 C (measured with a platinum resistance thermometer). The reason for this is that the Michelson interferometer is a differential device and these heating effects apply to both arms equally, thus the difference in arm length which is correlated with the output power, is to first order insensitive to temperature variations.

Figure 4. Output fringes from the output measured with a 1550nm sensor.

Notes on original objectives:

1. Interferometer design

Significant theoretical modelling has been performed to understand the performance of interferometers based on both thin and thick beamsplitters. The effects of thermal variation, coupling through thermal expansion and refractive index changes, has been considered in terms of output power variations. We have further considered the effect of wavelength fluctuations due to the 1550nm telecommunications laser.

2. Interferometer development and fabrication

A DRIE process has been developed at the James Watt Nanofabrication Centre which is capable of fabricating both the MEMS interferometer and gravimeter on the same chip. We have performed etch tests and also optimised mirror reflectivity via the use of Bragg stacks. A robust technique to align the interferometer has been developed and fringes produced at the output.

3. Interferometer characterisation

We have characterised the interferometer via thermal tuning (heating of the substrate) and also current tuning to vary the wavelength of the input laser. As the Michelson interferometer is a differential device we see excellent immunity to these input variations. For the monolithic device with fixed mirrors we have shown performance at the level of 100pm in 1s.

4. Field testing with the MEMS device

A MEMS device has been tested and provided a first measurement of Earth tides. This has enabled us to secure a Quantum technology Hub partnership resource project to make a field trialable unit. This is currently being led by one of our industry partners/end-users (Bridgeporth) and will enable us to test the gravimeter against an industry standard unit (early 2016).

5. Company engagement

The MEMS and interferometer work has further enabled the research group to secure additional funding via the Glasgow led Quantum technology Hub in Quantum Enhanced Imaging. This has expanded our company and end user engagement including:
Clydespace: Discussions with a local satellite company for developing optical sensors for Cubesats to enable attitude control. This will lead to an industrial led EngD studentship starting in March 2016.
Bridgeporth: Hosting a joint scoping meeting with Bridgeport to identify applications for low power high sensitivity miniaturised MEMS gravimeters. This company is now leading a partnership resource project.
Tullow Oil: this company was also present at the scoping meeting with Bridgeporth and have provided expertise on application with potential links to MEMS on drones for surveys
Schlumberger: We met with the company at the Royal Society of London showcase and they are very interested in our technology for gravity sensing. A 2nd meeting will be held in December 2015 to discuss ways forward
BAE Systems: We met with the company at the Royal Society of London showcase and they are very interested in our technology for gravity sensing of submarines. A 2nd meeting will be held in December 2015 to discuss ways forward

Notes on URL:

An electronic labbook is kept at https://arran.physics.gla.ac.uk/wp/mems/. A username and password can be provided if necessary, although a printout of the topics might be the easiest method of dissemination

Update March 2017:

We are now working on an STFC Impact Acceleration Award to package the MEMS gravimeter for field trials. The packaging will be done via the company OptoCap, and the plan is to build 25 units for engagement with end users.
Exploitation Route We will work with our end users to consider best opportunities for taking this work forward. At the current time we are working with Bridgeporth on the development of a field portable unit. During a recent Royal Society of London Quantum technology showcase there was also significant interest from Schlumberger (http://www.slb.com/) and BAE systems submarine division (http://www.baesystems.com/en/our-company/our-businesses/maritime)
Sectors Aerospace, Defence and Marine,Environment

URL https://arran.physics.gla.ac.uk/wp/mems/
 
Description We have used the findings from this project to inform the future direction of our novel MEMS gravimeter with optical readout. We propose to use the industrial links developed during the project to further the development of a field prototype. Via the Glasgow led Quantum Technology Hub we have bene successful in securing an industry led partnership resource project. We are working with a Glasgow based company (DaLEK) to miniaturise the drive electronics and develop a getter based system that does not require mains power. This will increase the Technology Readiness Level of the device from 3 to approximately 5. We will also look at the possibility of: (a) setting up a spin out company, (b) work with our local innovation office to discuss possibility for venture capital support, (c) discuss opportunities with MEMS foundries for device fabrication on large scale wafers. Over the next 3 years we will also be developing rotation based sensors (gyroscopes) using DRIE silicon rings to provide 3 axis acceleration and 2 axis rotation sensing. We have developed significant company engagement with end users (Bridgeporth, Tullow Oil, ClydeSapce, BAE Systems, Schlumberger) and this will continue to generate impact in the coming years. The MEMS gravimeter is now a major deliverable in the UK Quantum technology Hub programme and central to this is the interferometer. There is significant benefit in describing how fundamental core research in gravitational wave detection has spun into an applied field of MEMS gravimeters, and we have utilised opportunities at the Royal Society showcase, a visit from the UK science minister and public talks to make this link clear. We will continue to develop the MEMS/interferometer work via the Glasgow led Quantum Technology hub. At the same time we will consider opportunities to spin out a gravimeter based company as all feedback we have from end users is that this is a novel and potentially transformative technology. We will further look at other funding opportunities to support this activity (and maintain our patent on the technology) via venture capital funding and funding opportunities from DSTL (Defence Science and Technology Laboratory). The latter could well be in collaboration with the Birmingham Quantum Technology hub where the possibility for combined MEMS and atoms based gravimeters offer a new sensing technology in inertial navigation systems. Update March 2018: Since last submission, we have secured an Innvate UK grant to develop a squeezed light interferometer in silicon. We have further secured 27% of a 3.5 million EURO grant to install MEMS gravimeters onto Mount Etna.
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Environment
Impact Types Societal,Economic

 
Description FET OPEN H2020
Amount € 3,500,000 (EUR)
Organisation European Commission H2020 
Sector Public
Country Belgium
Start 03/2018 
End 03/2022
 
Description Innovate UK call in Quantum Technologies
Amount £500,000 (GBP)
Organisation Innovate UK 
Sector Public
Country United Kingdom
Start 03/2018 
End 03/2019
 
Description Partnership development fund via the Quantum Technology Hub
Amount £29,000 (GBP)
Organisation University of Glasgow 
Sector Academic/University
Country United Kingdom
Start 11/2015 
End 04/2016
 
Description QuantIC phase 2 partnership resource project
Amount £300,000 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 06/2017 
End 05/2018
 
Description STFC Impact Acceleration Account
Amount £25,000 (GBP)
Organisation University of Glasgow 
Sector Academic/University
Country United Kingdom
Start 02/2017 
End 08/2017
 
Description DSTL drone based gravimetry 
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom 
Sector Public 
PI Contribution Development of a MEMS gradiometer that can be deployed onto a UAV platform for aerial gravity surveys. This activity has developed within the Quantum Technology Hub (QuantIC), and originally seeded by the original STFC award.
Collaborator Contribution Development of a MEMS gradiometer, including design, fabrication, packaging and testing
Impact The project started Jan 2019, so outputs in 2019 will be focussed on MEMS fabrication
Start Year 2019
 
Description FET-OPEN H2020 
Organisation US Geological Survey
Country United States 
Sector Public 
PI Contribution We have recently secured a FET-OPEN grant (as Co-I) to deliver 70 MEMS gravimeters onto Mount Etna. This work will be part of a 4 year grant (2018-2022).
Collaborator Contribution We will be providing packaged MEMS devices for this new grant activity.
Impact This is a new collaboration starting in March 2018
Start Year 2018
 
Description Field trials with BP 
Organisation BP (British Petroleum)
Country United Kingdom 
Sector Private 
PI Contribution We will be testign our MEMS gravimeter and gradiometer on a drone-based platform as part of the BP Quantum Technology programme. This will also feature a lab-based test of the silicon interferometer developed under the original STFC award
Collaborator Contribution Device fabrication and testing within the University of Glasgow
Impact We will be performing field trials in Q3 of 2019
Start Year 2019
 
Description Quantum technology Hub in Quantum Enhanced Imaging 
Organisation University of Glasgow
Country United Kingdom 
Sector Academic/University 
PI Contribution This Glasgow led Hub includes Universities of Glasgow, Heriot-Watt, Edinburgh, Bristol, Oxford and Warwick. The work in this grant has been included under the WP3 via gravity imaging (total value £950k) using MEMS with optical readout.
Collaborator Contribution The othe workpackage partners (Bristol and Warwick) are working on the development of squeezed light sources for sub-shot noise imaging. The combination of these activities with our work on MEMS interferometers will enable a new transformative technology in precision sensing.
Impact Development of ultra sensitive MEMS gravimeters and interferometers. Multidisiplinary activities across Engineering and Physics
Start Year 2014
 
Title MEMS Gravimeter PCT 
Description This patent covers the development of a novel MEMS gravimeter based on geometric anti-springs to provide the worlds lowest frequency MEMS device 
IP Reference MJN/BP7136252 
Protection Patent application published
Year Protection Granted 2015
Licensed No
Impact This patent was filed in August 2015 and we will work with end users to consider licensing/spin out opportunities. We are currently engaging with a design & manufacturing company to build a field unit, and a micro-nanofabrication company to develop wafer scale processing
 
Description Demonstration of the MEMS gravimeter to the science minister 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Discussed our research activities with policy makers

Highlighting the importance of the research to societal challenges
Year(s) Of Engagement Activity 2015
 
Description Display at the Quantum Technology showcase at the Royal Society in London 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact A public event for press, industry, funding agencies and politicians, enabling dissemination of research outputs

Allowing the opportunity to discuss the research/instrumentation via to companies on how to get field trialable units and some of the potential applications of miniaturised interferometers
Year(s) Of Engagement Activity 2015
 
Description Exhibit in the community display case at the Kelvingrove Museum 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Data unavailable as the items were installed in a display case with a poster describing the research object

Highlighting novel technology which can provide state-of-the-art gravity sensing
Year(s) Of Engagement Activity 2015
 
Description Quantum Technology showcase 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact Showcasing the MEMS gravimeter at the annual Quantum technology showcase
Year(s) Of Engagement Activity 2016
 
Description Quantum Technology showcase 2018 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Third sector organisations
Results and Impact Annual quantum technology showcase, where we dislayed work on our MEMS gravimeters
Year(s) Of Engagement Activity 2018
 
Description Quantum teacher conference 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? Yes
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Talked sparked discussion on my core research activities (Gravitational wave astronomy)

Having the opportunity to describe how core research can feed into the societal challenges (e.g. gravity sensing for environmental monitoring or earthquake detection), feeding through STFC global challenge and QT Hub
Year(s) Of Engagement Activity 2015
 
Description Talk at American Geophysical conference 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Talk at the american geophysical conference
Year(s) Of Engagement Activity 2017
 
Description Talk at Ayr astronomical society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Talked led to many questions on gravity sensing and precision measuring.

This provided a chance to describe how core research can feed into the societal challenges
Year(s) Of Engagement Activity 2015
 
Description Talk at York astronomical society 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Talk at a local astronomical society
Year(s) Of Engagement Activity 2017