The initial gyro design was integrated into SINTEF's design rules and optimised to meet the Topic Manager's (TM) specifications with respect to electrical characteristics. A suitable ceramic package was identified for packaging purposes.
The next tasks concentrated on setting up the industrially viable processes that would be used for manufacturing the MEMS gyrometer:
- accomplishing high resolution photolithography for the patterning of the active MEMS element
- direct wafer bonding to enable wafer level vacuum sealed MEMS units
- Through silicon vias to enable electrical connection to the device
High resolution patterning was realised by optimalisation of the photolithography process and the demanding etch process that defines the working element of the MEMS.
Typically, direct bonding is done as early in the processing as possible since it requires a very flat and clean surface to succeed. In MUPIA, however, the direct bonding would be done after substantial processing. For this reason it was necessary to have a contingency design that was not dependent on successful direct bonding. The project decided, therefore, to manufacture two variants of the MUPIA MEMS,
- MUPIA Open Design (MOD)
- MUPIA Closed Design (COD)
The MOD variant was processed such that the active MEMS element was exposed to the ambient and therefore needed to be vacuum sealed at package level. The vacuum sealing for the COD variant would be done at wafer level and therefore did not demand vacuum sealing at package level.
The first deliverable of gyrometers consisted of the MOD variant. Due to Covid19, the full characterisation of the MEMS the TM's facilities was delayed significantly. However, simplified testing showed promising results. The main issues were due to insufficient vacuum in the package which in turn lowers the Quality factor and decreases the performance potential of the MEMS gyrometer.
The final delivery was intended to consist of the COD variant due to its inherent advantages. However, the vacuum sealing at wafer level proved more challenging than initially anticipated resulting in significant delay in the processing of the COD variant. In addition, a processing error led to the TSVs not having electrical contact with the device. As a result, the MOD variant was invoked as a contingency for the final delivery of the MEMS gyrometer.
Detailed characterisation was carried by the TM out on a subset of the final delivery. The results showed at least a 70% functionality yield with resonance frequencies and x-axis quality factor within the expected ranges. Other measured parameters suggested that the MEMS processing and packaging could benefit from further optimisation in etching uniformity and vacuum sealing, respectively.
Dissemination activities:
• Through Silicon Vias in MEMS packaging, a review, Guido Sordo, NordPac (IMAPS Nordic / IEEE EPS), June 11-13, 2019, Copenhagen, Denmark
• Development of mechanically compliant flip chip interconnect using single metal coated polymer spheres, Daniel Nilsen Wright, 22nd Microelectronics and Packaging Conference & Exhibition (EMPC, IMAPs Europe), September 16-19, 2019 ,Pisa, Italy
• Wafer bonding process for zero level vacuum packaging of MEMS, Guido Sordo, European Systems-Integration Technology conference (ESTC, IEEE EPS), September 15-18, 2022, Online
Exploitation:
• Based on the experience from MUPIA, SINTEF is currently developing a MEMS Strain Gauge that will implement vacuum sealing at wafer level.
• Through MUPIA, Micross expects to increase its UK based capability and technical knowledge of this branch of semiconductor packaging and thus be in a position to introduce the increased capacity necessary to service an increasing market demand for such specialist packaging.
