The above-IC SiGe micro-electromechanical systems (MEMS) technology (see section SiGe MEMS technology platform and design platform) is one of the CMORE technology platforms. It relies on imec's high-end flexfab facility which is operated 24/7, runs 130/90nm CMOS, and is fully statistical process control (SPC) controlled and ISO certified. A design platform consisting of a design kit, layout rules, models, basic library cells and tool interfaces has been developed and the technology has been successfully used in several MEMS application designs and customer specific CMORE prototyping projects. In these projects, the customer can be actively involved in co-design and co-development with imec or can decide to outsource the design & development to imec. CMORE projects range from building proof-of-concept demonstrators to fully qualified product chips. In addition, imec offers low-volume production services for qualified products or will transfer the product to a high-volume manufacturer of the customer's choice (see section Strategy CMORE: a platform to turn novel concepts into products).
Advantages of the SiGe MEMS technology
Monolithic integration of MEMS and CMOS yields very compact and cost-effective solutions, especially when the die size of MEMS device and circuits are comparable. The SiGe MEMS technology allows to postprocess MEMS structures on top of CMOS circuits with a large number of fine-pitch interconnections between the MEMS devices and the underlying circuits. This makes the SiGe MEMS platform very well suited for applications that need large arrays of MEMS devices, which need to be individually connected to interfacing circuits (such as micro-mirror arrays, bolometers or other imaging applications).
At the same time these interconnections are very short and have very small unwanted parasitics associated to them, relaxing the requirements on the interfacing circuits for applications that are highly sensitive to noise or parasitics (such as very sensitive inertial sensors or high-Q resonators). Building further on imec's CMOS capabilities in the 200mm flexfab with access to advanced litho capabilities, the SiGe platform is also very well suited for MEMS applications that require nano-dimensions.
The above-IC SiGe MEMS technology has been successfully used for a large number of MEMS application design projects, illustrated by the following (non-exhaustive) list of examples:
Cantilever array for scanning probe storage
Imec used its poly-SiGe MEMS platform to fabricate a cantilever array carrying a read/write tip for a scanning probe storage memory. The cantilever is made in the structural SiGe layer and can be tilted using an underlying electrode around a torsional hinge, and move in plane driven by comb drives. In this cantilever array, the dual thickness of the structural layer was used to tune the stiffness of the cantilever and the stiffness of the torsional hinges independently. The tip is formed on top of the cantilever and connected using an additional suspended metal trace.
Figure 1: MEMS cantilever array for scanning probe storage.
Imec used its poly-SiGe MEMS platform to fabricate a very reliable CMOS-integrated 10cm² 11MPixel SiGe-based micromirror array. The array is to be used as a spatial light modulator and can be used in various applications such as video projection, mask writers, optical maskless lithography, head-up displays, microscopy, metrology... The array consists of 8μm x 8μm pitch pixels which can be individually addressed by an analog voltage to enable accurate tilt angle modulation. The pixel density is almost double compared to the state-of-the-art. Stable average cupping below 7nm, root mean square (RMS) roughness below 1nm and long lifetime (>1012 cycles, no creep) have been demonstrated. Poly-SiGe was chosen as structural material for the mirrors, instead of the commonly-used Al, as it solves many of the reliability issues of Al-based mirrors.
Figure 2: MEMS micromirror array.
Further, poly-SiGe is used in RF-MEMS switching devices. Here, the excellent mechanical and reliability properties of the SiGe are exploited to build the armature (or moving part) of the switching device. A dual thickness structural layer is implemented on the one hand to realize an extremely flat armature (defection <200nm over a distance of 800µm), and on the other hand to have flexible springs enabling low voltage actuation.
Figure 3: RF-MEMS switch.
Thin-film MEMS cap (and pressure sensors)
Zero-level packaging of MEMS is done to prevent damage to the fragile MEMS device during dicing and assembly, and to ensure a good operation and lifetime. Zero-level packaging is traditionally accomplished by bonding capping dies or a capping wafer to the wafer with the MEMS structures. Imec has developed an alternative approach based on surface micromachining that can be used to process a thin-film membrane above the MEMS device (see section MEMS packaging). After sacrificial etching of the sacrificial layer under and above the device, the membrane is sealed to enclose the required pressure and atmosphere in the cavity. The advantage of this approach is a reduced thickness and area of the packaged device compared to the traditional approach.
The thin-film packaging process flow with SiGe membranes can also be used to process pressure sensors above CMOS. Both capacitive and piezoresistive pressure sensors are possible (see section MEMS packaging).
MEMS resonator based timing devices
Imec's above-IC SiGe MEMS technology can also be used to build MEMS resonator based timing devices and RF components that have the potential to replace quartz-based components with smaller and potentially lower-cost devices. By optimizing the mechanical design of the resonator using the SiGe structural layer and high-aspect ratio gaps for high transduction efficiency, high-Q SiGe MEMS resonators have been demonstrated C18772, C19202, C19203.
Figure 4: MEMS resonator.
An important challenge for building MEMS resonator based timing devices face is the stability of the resonance frequency over a wide temperature range. By application-specific finetuning of the SiGe technology combined with a careful co-design of interfacing circuits and MEMS device, imec is targeting MEMS-based devices that meet the stringent requirements of quartz-based temperature-compensated crystal oscillators for wireless applications.