25 January 2011
Lab fab: Fabrication technology
Extending the capabilities of base cmos with 'More than Moore' technologies through Lab Fabs. By Ann Witvrouw.
As the pace of fabrication technology quickens, not just at the most advanced geometry nodes, but also within mainstream and legacy processes where cost, mechanical or electrical constraints require chip manufacturers to take a different approach.
Those companies which don't have in house manufacturing capability, or who are in specialist markets where the volumes are too low to fully invest in manufacturing development, are turning to centres of excellence – or Lab Fabs, like those at imec – to help them realise advances in fabrication technology that can then be manufactured around the world.
The pressure on the European semiconductor industry to develop innovative devices has driven the need to set up Lab Fabs to extend the capabilities of base cmos process technologies and these 'More than Moore' technologies and to allow incubation and germination of ideas that can then be transferred to mass production. Broadening the capabilities of integrated cmos is crucial, not just for start ups, but also for some large IDMs who have evolved beyond their current capabilities or who may not want to develop a particular product because it requires materials that are incompatible with their internal processes.
To develop such devices needs a much wider set of expertise, ranging from cmos process and design to packaging and interconnect. Smart devices and systems are developed using a cmos design methodology and process, integrating sensor or actuator functions. The processes can be modified or optimised for power efficiency, operating voltages and drive currents. And thin film technology for integrated passives, MEMS and other analogue functionality can be added.
To illustrate the potential, imec has produced three demonstrators: a15µm wide SiGe micromirror; a grating light valve; and a SiGe accelerometer – all of which have been built on its SiGeMEMS platform, which supports the monolithic integration of MEMS devices directly on top of a standard cmos metallisation layer. The demonstrators were built in the frame of a Flemish Strategic Basic Research (SBO) project called Gemini by imec and its project partners, Ghent University (Photonics and CMSTgroups) and Katholieke
Universiteit Leuven, ESAT-MICAS. They were chosen to illustrate the broad applicability of the technology platform. Imec's SiGeMEMS technology is based on a 'MEMS last' approach, where the MEMS are processed after and on top of the cmos circuits (Fig 1).
It enables monolithic integration of cmos and MEMS, leading to better performance compared with other integration schemes, including: better signal to noise ratio through reduced interconnect parasitic resistance and capacitance; smaller die size and package; and lower power consumption.
The SiGeMEMS platform is versatile. It consists of standard and optional modules that can be processed at 450°C above standard cmos, with possibilities to tune and optimise the modules. The standard modules provide, for example, a cmos protection layer, MEMS via and poly SiGe electrode, an anchor and poly SiGe structural layer, and poly SiGe packaging. Optional modules, such as optical, piezoresistive or probes, can be added depending on the application.
The platform's flexible and modular approach allows application specific tuning and optimisation. For example, the thickness of the MEMS structural layer can vary between 300nm and 4µm. A 300nm thick layer allows the manufacture of optical MEMS, such as the micro mirror and grating demonstrators. For such devices, the process is extended to add coatings with specific reflective properties. For example, a 4µm structural layer is used to create inertial sensors or actuators, such as the Gemini accelerometer (Fig. 2).
The Gemini mirror design uses an actuation mechanism which relies on six electrodes (using two possible electrode thicknesses of the SiGeMEMS platform). Two of the electrodes serve as landing electrodes, the other four are attracting electrodes, driven by two antiphase saw tooth signals and two fixed analogue voltage signals. By applying this signal scheme, the duty cycle of the mirror is modulated in an analogue fashion. Laser Doppler vibrometer measurements have confirmed the feasibility of analogue pulse width modulation for 15µm wide SiGe micromirrors, which have already been designed for use in a display system.
The actuation mechanism enables the display of a large range of greyscale values. The six electrode design combines the advantages of analogue driving (which gives no contouring effects) with that of a full swing mirror movement (simpler optical system and higher response speeds).
Next, the grating light valve which is a MEMS reflection grating that produces bright and dark pixels in a display system by controlled diffraction of incident light due to electrostatic deflection of micro beams. In the Gemini grating light valves, clamped beams suspended over an electrode can modulate the intensity of the diffracted light when an actuation voltage is applied to half of the beams. Display systems featuring such a technology provide high contrast ratio, high resolution and high brightness. Both the mirrors and grating light valves are realised with a 300nm thick SiGe structural layer.
In the third demonstrator, the Gemini accelerometer, in plane and out of plane low g designs have been explored. Measurements of a fabricated out of plane accelerometer show this device senses the gravitation projection to the main sensing axis with an average sensitivity of 0.5mV/g; comparable to the state of the art. But, because the technology can be used for above cmos integration, this will greatly improve the noise performance of accelerometers and simplify their integration. The accelerometers have been built with a 4µm thick SiGe structural layer to obtain an improved capacitive readout of the in plane devices.
Imec includes feasibility studies, design and technology development, prototyping and low-volume manufacturing and recently announced a new SiGeMEMS foundry service which is based on the SiGeMEMS platform, fixing the options in a baseline process with a 4µm SiGe mechanical layer. It is supported by mature design kits for the most important commercial MEMS design tools, allowing interested parties to develop their own MEMS designs for rapid prototyping.
In addition, for universities and research centres, there is a multiproject wafer (MPW) service. By gathering the designs of multiple customers on a single mask set, MPWs enable the fabrication of test structures and prototype devices at low cost. An initial imec MPW run, with a tape out deadline at the end of January 2011, will be processed on a wafer with a single metal layer for initial prototyping. A second run with full capability and populated with SiGeMEMS devices on top of TSMC's 0.18 µm cmos is scheduled for the second half of this year.
Imec provides a CMORE toolbox which contains device technologies including high voltage technologies, CMOS imagers, photonics and MEMS. The toolbox also covers packaging capabilities such as through silicon vias and MEMS capping.
We have seen how the cmos processes currently used to make logic and drams can drive an entire new industry. These are now being integrated with new types of devices with innovative functionalities, impossible to realise with cmos alone.
Ann Witvrouw is principal scientist, MEMS, at Imec.
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