Qualcomm announced its ultraSAW filters in February 2020, claiming them to be “groundbreaking.”  Frankly, without any details from Qualcomm, the claims did not impress me, and I thought the filters were probably no better than filters from competitors.

A few days ago, Qualcomm provided new details about ultraSAW, and it may indeed represent a new, better technology for RF filters:

• The filters use piezo-on-insulator (POI) substrates provided by Soitec. The company produces these using its Smartcut buried oxide technology starting with a donor silicon wafer. We don’t know which SAW piezo material is used; it could be lithium niobate, lithium tantalate, aluminum nitride, or even something less commonly used for acoustic filters.
• This is basically the same process used to produce Soitec’s silicon-on-insulator substrates.

Starting in 2003, SOI revolutionized RF switches in smartphones, allowing CMOS on SOI switches to completely replace GaAs p-HEMT switches, which require separate CMOS logic and control die.  CMOS on SoI, pioneered by Peregrine Semiconductor (now pSemi, a Murata company), allows design of a single die CMOS switch for smaller, less expensive RF switches without sacrificing performance.  As a result, CMOS on SOI quickly took over the RF switch functions in mobile phones. 

Will history repeat itself in filters?

Advantages of ultraSAW using POI:

• The use of POI for acoustic RF filters reduces the temperature coefficient of frequency (TCF), cutting undesirable frequency shifts from variations in temperature.
• The high resistivity of POI reduces parasitic capacitance and signal loss; POI reduces insertion loss by up to 1 dB, and increases the upper frequency limit of SAW filters built on POI to about 2.7 GHz, according to Qualcomm.  This would make them competitive with many BAW and FBAR filters on the market.
• POI helps improve Q and filter cutoff sharpness, and allows design of wider bandwidth filters.

According to Qualcomm, these benefits are inherent to using POI, allowing a relatively simple manufacturing process with very uniform piezo thickness.  The material and manufacturing also allow multiple filters stacked on the same die.

Qualcomm and Soitec announced an agreement to bring POI wafer production to high volume manufacturing for Qualcomm’s RF filters for discrete filters and RF front end modules for smartphones.  As part of the deal, Soitec will work to scale POI technology up from the present 150 mm diameter wafers to larger sizes.

The agreement does not exclude Soitec from selling POI to other filter companies, but for now Qualcomm is taking all of Soitec's capacity.  The material specifications for Qualcomm such as cut angle relative to the main crystal axes and thickness are proprietary, and of course the ultraSAW filter designs are Qualcomm proprietary.  Qualcomm's Christian Block, SVP & GM, stated on July 16, 2020, that Qualcomm has qualified a second wafer supplier to use mainly for the high bands (2.5 - 2.7 GHz), and may seek a third supplier to mitigate risk.

OEMs have expressed great interest in ultraSAW, and Qualcomm expects design wins later in 2020.

It appears that POI and ultraSAW will provide high-performance, low-cost alternatives to SAW, TC-SAW, IHP-SAW, BAW and FBAR from suppliers Murata, Broadcom, Qorvo, Skyworks, Taiyo Yuden, especially at higher frequencies, and should ultimately help make 5G phones more affordable.

Updated on July 16, 2020.