There is no such thing as a standard MEMS process. memsstar takes a look at why this is the case for a wide range of modern CMOS and MEMS devices.
At the birth of early semiconductor devices, only a small range of materials and processes were required to make the greatest proportion of electronic devices. The main reason a small number of processes could do so much was in part, due to the small number of common materials used for devices in early development in silicon, silicon dioxide, silicon nitride and aluminium. This range of materials has substantially increased in recent years and continues to do so as industry and academia look to solve the ever-evolving challenges of modern consumer and medical devices.
The use in recent times of more ‘exotic’ materials such as graphene, silicon carbide and the resurgence of aluminium gallium nitride and arsenide (AlGaN, AlGaAs) has led to the development of some truly unique devices. Yet the fundamental materials remain; oxides and nitrides are still key functional materials due to their long-time characterised nature, their reliability and their relative ease of growth or deposition. The same can be said for other materials. Aluminium has remained a mainstay for its own material properties while slowly being joined by other equally common metals in today’s devices: tungsten, titanium, titanium nitride, copper and gold. In fact, as these metals have been introduced and their own requirements developed and imposed on device designs, the onset of PECVD deposition processing with lower temperature depositions have been required which has also led to variations in the oxide and nitride films. With the further increase in MEMS devices and the dawn of the ’Internet of Things’, we consider the aspects of making a MEMS or CMOS device function from a manufacturing position.
Typically, MEMS devices are built from a range of layers. Some of these are electronic connections between the front and back end sections, some are the functional layers, which include the movable portion of the device itself and in between these layers are the sacrificial materials. The choice of materials comes down to a number of key concerns. There are many ways to design a sensor such as an accelerometer, however, depending on the output requirements, certain materials will provide either greater stability, higher levels of conduction or improved chemical resistance for lifetime and environmental exposure.
MEMS Manufacturing Processes
So what does this mean in terms of processes for MEMS manufacturing? This leads to a high level of bespoke processes and in turn, systems, to deal with the more varied devices now under development. CVD systems now have to cater for a wider range of material deposition processes. Where a system would provide two or three standard films, a CVD chamber can now provide eight to ten material deposition options with custom hardware configurations depending on the requirement of the company. This further evolves when dealing with wafer sizes. As 200 mm wafers continue to deal with the bulk of the market in standard manufacturing, these bespoke materials and designs are currently more common at the 150 mm level. These systems will ultimately require a step up to 200 mm, with yet more specific requirements being developed in time.
In terms of etching, either wet or the now more prominent vapour phase etching, these bespoke material combinations also require catered processes to accommodate the materials providing function or structural roles, while leaving them unaffected at the same time. Vapour phase etching certainly has the biggest benefit here due to the ability to more strictly control all the process conditions (pressure, gas flows, temperature and so on), which leads to yet more variation in the overall process flow as these etch processes are used to remove key area of the wafer to realise functional devices.
This trend continues now, and shows no sign of returning processes back to the “one process fits all” days of early manufacturing. Gone are the days of a standard best-known method (BKM) for a wide range of devices, with bespoke systems and device requirements continuing to dictate non-standard process development routes for the time being.
Can you run your standard process on this device? Depends on the device I’m afraid. Tell me more about the materials used…
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