Wet Etching Vs Dry Etching Processes for Current and Next-Generation MEMS Devices

Dry etching vs wet etching - dry etching example from memsstar

 

The worldwide micro electromechanical systems (MEMS) market is steadily increasing driven primarily by consumer demand for new wearable technology followed by the desire in the medical industry to reduce costs whilst improving the speed of diagnosis with emphasis on point of care testing. This has resulted in the rapid production of new devices with ever decreasing dimensions.

With these next-generation devices, established processing techniques such as wet etching are finding incompatibilities with new device geometries. In today’s manufacturing environments, dry etching methods with vapor has led to the development of new techniques. This article describes some of the current methods for MEMS manufacturing from an etch perspective, from wet etching and its evolution to the current suite of dry release technologies.

Wet Etching

Wet etching has been commonplace among MEMS manufacturers due to the characterized nature of many of the etch solutions. It is a relatively low cost approach for the removal of materials in structures with large feature sizes. Depending on the materials to be etched, a number of options are available.

For the removal of silicon dioxide (SiO2) films, hydrofluoric acid (HF) is the most common approach. The water contained within the acid solution acts as the process catalyst and can etch oxide films at etching rates in excess of 1 µm/min. This solution is also capable of etching silicon nitride (Si3N4) films, albeit at reduced etching rates. Buffered HF solutions have been implemented as a means of adding further control to the etch solution by adding ammonium fluoride (NH4F).

Etching of silicon based films (bulk, poly and amorphous) is approached by a number of different solutions: KOH, TMAH, EDP, and numerous others. Each have their advantages and disadvantages when it comes to processing. While etching rates vary with these solutions, there are other, more important concerns: KOH is known to leave metal contamination, but is used to etch in specific crystal planes. TMAH is less orientation dependant and does not leave metal contamination and EDP is similar to TMAH with the added benefit of using metal etch masks in some designs.

Dry Etching

The increased utilisation of vapor phase etching systems has come about due to smaller feature sizes, increased selectivities between materials and a requirement for more complex materials.  These requirements highlight some of the limitations of a wet etching process.

Dry etching using vapor phase processes was developed as a means of moving existing wet etching processes into the vapor phase to improve performance and material compatibility. This has allowed for a substantial improvement in the etch, increasing yield and enabling the release of smaller and complex devices. The two most common systems now available for dry etching are vapor phase oxide and silicon etch systems.

Oxide etching systems utilize anhydrous hydrogen fluoride (HF) (compared with HF acid) now supplied in the gas phase where further process parameters can be adjusted for optimum etch performance.

Silicon etching systems now exist with the use of xenon difluoride (XeF2), a white crystalline material, which is introduced to the material to be etched in a gaseous state. It subsequently reacts with silicon (bulk, poly and amorphous), and other specific materials to remove them and release the final device.

Dry Etching Vs Wet Etching

Early MEMS processing utilised wet etching for the device fabrication. MEMS devices have evolved to such an extent that these solutions have begun to cause issues primarily in the form of stiction: a near permanent adhesion of one layer to another by one of a number of forces. In modern MEMS process, wet etching solutions are still used, but only in cases where it will not lead to stiction. Iterative rinse processes have been created to remove liquid from these processes, with a final critical point drying (CPD) method used to ensure liquid removal. These added process steps increase production time and reduce overall throughput.

Dry etching processes have become the preferred method for advanced structures due to smaller feature sizes, deep etch undercuts, high-aspect ratio features, or where the avoidance of stiction is critical. These have appeared in the form of vapor phase etch systems which are capable of releasing devices with zero stiction effects resulting in a marked increase in yield.

The advent of vapor phase etching systems have allowed manufacturing groups to pass over wet etching options in favour of dry processes, which provide increased controls for the etchants, pressure and temperatures in which the etching will occur. This has allowed increasingly complex structures to be realised.

memsstar can help you with your Dry Etching requirements

memsstar provide advanced etch, deposition and surface coatings to support MEMS manufacturing offering a variety of process solutions for R&D through to high volume manufacturing facilities. Our etching processes are detailed below.

XERICTM Silicon Etch

Used for dry processing, the XERIC Silicon Etch system utilises vapor phase dry release etching with Xenon Difluoride providing continuous flow design for the etch material. The patented memsstar XERIC sacrificial vapor release XeF2 process is highly selective to a range of materials including SiO2, Si3N4. This process means that large undercuts of structures can be performed with no degradation in etch rate and a wide range of films including all forms of silicon can be etched.

Find out more about the XERICTM Silicon Etch.

 XERICTM Oxide Etch

This is a patented process which is compatible with a wide range of metals utilising vapor phase dry release with Anhydrous Hydrogen Fluoride (HF) and has market leading selectivities towards Si3N4. The XERIC Oxide Etch HF sacrificial release system offers a variety of benefits to companies who are engaged in MEMS manufacturing and development. Find out more about the XERIC Oxide Etch.

AURIXTM

The AURIX system for surface preparation and deposition is the ideal solution for advanced surface coatings for the delivery of reliable, cost-effective protection for sensitive devices. The vacuum deposition environment of the AURIX system provides vapor-phase self-assembled monolayer (SAM) coating capabilities while eliminating the effects of moisture which is crucial in creating repeatable and robust surface coatings. Precision-tuned processes with patented control reduce the amount of chemicals used and offer superior surface properties and improved surface energy control. Find out more about the AURIX process.

Dry Etching Enquiries

If you have requirements for dry vapour phase etching, we would be delighted to work with you to provide the best solution to suit your needs. To discuss your requirements simply contact us on T: +44 1506 243203 or email us at info@memsstar.com. We look forward to being of service.

Memsstar: Dry Etching Solutions for MEMS manufacturing and R&D. 

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