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Acoustic Resonators for RF: MEMS with No Moving Parts

Acoustic Resonators for RF: MEMS with No Moving Parts
by Paul McLellan on 08-05-2015 at 7:00 am

 There is an annual conference known officially as the Sensors and Actuators Workshopand informally as Hilton Head since it is held on Hilton Head island in South Carolina. Coventor talked to some of the top researchers last year about RF filters and decided to develop a simulation solution that would better serve both the researchers and commercial designers. Recently they delivered with the recent CoventorWare 10 release which includes a new (and unique in the industry) fast analysis capability for acoustic resonators.

Market pressure for RF filters to be compact and inexpensive, yet meet the higher performance requirements of the 4G standards, has spurred great interest in novel filter design concepts that further miniaturize the transmit/receive chain of RF frontends The first successful filters were surface acoustic wave (SAW) filters, but in the past few years the number of bulk acoustic wave (BAW or FBAR) filters within a handset has grown rapidly to fill the market demand above 1 GHz where SAW device performance degrades. Most recently, bulk mode resonators that vibrate in plane to allow multi-frequency filters on one substrate have garnered significant interest within the research community. These in-plane bulk-mode resonators come by various names such as contour-mode resonators (CMR) and laterally-vibrating resonators (LVR).

These acoustic resonators have become more popular as the number of filters in a typical mobile device has increased to around 30 per phone. The best known success story in the space has been Avago’s FBAR which proved the higher performance of bulk-mode devices compared to conventional SAW filters.

Acoustic resonators are designed using MEMS techniques even though they do not have any moving parts. It is one thing to be able to design them but there has been a major missing piece to the jigsaw, computationally efficient algorithms to perform frequency sweep simulations in a reasonable time. Current solutions take days (literally) and that is a 2D simulation, which is not good enough. 3D takes so long as to be impractical. This obviously limits both the amount of exploration that can be done and the accuracy with which the performance can be predicted.

Matt Kamon of Coventor told Professor Songbin Gong at the University of Illinois about the proprietary fast frequency-sweep algorithms specifically for MEMS piezoelectric resonators. He claimed Coventor could simulate his 3D designs in minutes where his current tool took hours, and do in hours what previously took days. Since the Hilton Head 2014 conference, Professor Gong and his group have worked with Coventor to polish this offering specifically for rapid design of cutting-edge acoustic resonators.

Coventor have named the offering “FastPZE” and it is part of CoventorWare 10. The diagram above shows the accuracy. The red line shows conventional simulation (actually the red circles are the only points simulated and then the points were just joined up). The black line, which shows a lot more detail, is the result of the FastPZE algorithm. Both simulations took the same amount of time.

 Another point to emphasize is that designs are not necessarily constructed with Manhattan geometries. A design may consist of arbitrary polygons or curves. In some simulation tools, this would require a mesh made of tetrahedrons. Unfortunately, since these are thin-film devices, this can lead to an extremely large number of mesh elements and consequently very long simulation times. CoventorWare, in contrast, provides very efficient meshing for thin-film devices of non-Manhattan shape with straight or curved edges as shown in the above diagram.

Download Matt’s white paper Fast Acoustic Resonator Analysis for the Rapidly Growing Premium RF Filter Market here.

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