While microelectronics and photonics share a number of common underpinning materials, systems and fabrication technologies, they have traditionally been seen as two distinct disciplines. Integration of photonics devices with microelectronic control tends to be addressed at the macro-scale, incorporating discrete “blocks” with minimal interdependence.

Devices that integrate MEMS made from silicon with electronic circuits (such as airbags and computer projectors) are already in existence, but there are very few combined MEMS and photonics systems outside of laser sources for telecoms and similar devices. Systems which include MEMS, electronic control and photonics have not yet been adopted by the market to any great degree.

The Scottish Consortium in Integrated Micro-Photonic Systems (SCIMPS) brings together expertise in optical sources, MEMS, microfluidics, electronics and biomedicine to tackle a range of challenges in diagnostics and optical instrumentation. The objective of the SCIMPS initiative is to bring true MEMS and micro-electronic integration and control to instrumentation based on advanced photonics technology. Funded by the Scottish Funding Council, the project brings together the work of Glasgow, Heriot-Watt and Strathclyde Universities, the Institute of Photonics and the Institute for System Level Integration.
Performance in miniature

As part of the SCIMPS project, the research team at iSLI is working on silicon MEMS devices and MEMS/electronic subsystems to enable the next generation of high-performance miniaturised and highly integrated photonic systems. An early focus of the research work involved developing micro-scanning devices for use within miniaturised dynamic optical systems being developed by other members of the consortium. 

iSLI developed an electro-thermal MEMS scanner that is actuated by a single structural layer. The original three-beam electro-thermal out-of-plane actuator comes from work carried out in the Centre for Microsystems and Photonics, also a SCIMPS partner, at the University of Strathclyde.

The actuator is fabricated through SOIMUMPs, a multi-user process provided by French company Memscap. Both the mirror and the actuation mechanism are formed in the same layer, which is easy to fabricate, cost-effective and ensures reliability. The “upward” direction of movement is defined by the residual stress of a metal layer deposited on top of the silicon. The current scanner can be operated from DC to low frequencies (80Hz) with large optical scanning angle (10˚). Both the CMP and the Institute of Photonics are now actively using this scanner within development systems for laser scanning imaging and microscopy.

“Typically, the axial position of an electrostatically driven MEMS scanner will vary as the angle of the mirror changes; something which is commonly undesirable in an optical system,” says Dominik Weiland, one of the MEMS designers.

“At iSLI we have developed an analytical solution for a device with an additional electrode to control this z‑axis movement. This presents the opportunity to manufacture a scanner that is highly stabilised along the optical axis, which will be of particular value in imaging systems,” says Weiland.

Simple structure
Through further work with the Micro-Scale Sensors Group within the University of the West of Scotland, iSLI proposed a simple structure for an optical microscanner, composed of two piezoelectric bimorph beams that are used to actuate a mirror plate. The complete analytical model of the piezoelectric device has been developed, leading to accurate calculation of the profile of the structure and the tilt of the mirror.

The model can be used very rapidly to study the effects of geometrical parameters and material properties on the performance of the micro-scanner, without having to revert to the use of computationally expensive finite element modelling at the outset. This understanding forms the basis of further behavioural modelling and VHDL integration, allowing rapid system level simulation and optimisation of these structures.

The microscanner is just one device that has been developed as an early demonstrator. The focus of the consortium is systems which integrate the multiple technologies available to target new areas. It is also working on a range of technologies and applications (such as a micro zoom lens and an optics toolkit for medical applications) which are much larger than this one-device concept.

Rising to the challenge
Advancement of optical systems and instrumentation into the micro domain brings with it many new opportunities, but at the same time presents significant technical challenges. The teams on the SCIMPS project are developing their understanding of how a compact single-unit system can be built that consists of several different technologies: optics, MEMS and electronics.

The system must be highly accurate, have electrical connections and needs to be realised through an assembly process that all of the components are able to survive. Within the MEMS domain, one target of the project is to “invent” MEMS devices that will allow the movement of optical elements in the number of axes and over the distances required by new optical systems.

There are many applications for this type of technology merge, including high-quality and flexible imaging of teeth in dental surgery, probing and testing individual cells as they pass through a microchannel, and processing and reading multi-analyte diagnostic slides for disease detection. This early stage technology integration therefore has significant potential for development. MEMS alone has opened up access to an array of physical sensing measurands not easily available to electronics-only approaches.

In a similar fashion there are also a lot of emerging applications being opened up by MEMS. This has presented a number of opportunities where photonics may be able to provide functions that will allow MEMS to address a whole new set of applications that could not be tackled without photonics.

Although this is very early stage technology integration and in the short term it may be too costly for commercial development, there could be rapid progress in medical applications. Once the teams are successful in developing the integration framework, the potential to move forward is enormous.

“All strands of technology are moving towards higher levels of integration and greater heterogeneity. An application that might be difficult to integrate today may well be in the sweet spot for development tomorrow. Only by developing common problem framing, understanding and cooperation across disciplines can these new technologies be developed,” comments Ian Henry, senior design engineer at iSLI.

Dr Mark Begbie is technology group director at iSLI