Rf filter POI substrate

01 What is POI Substrate?

Christophe Didier, Manager of Soitec's filter business, explained the technology and market prospects of POI substrates to industry media, including the electronics enthusiast network, in a recent media briefing.

The top layer of the POI substrate is a single crystal piezoelectric layer about a few hundred nanometers thick, and under it is a layer of buried oxide layer, about a few hundred nanometers thick, and the bottom part of the base is a high-resistivity silicon material.

The piezoelectric material used in POI is the same as the material currently commonly used in SAW (surface acoustic wave) filters, which mainly includes two materials, namely lithium tantalate and lithium niobate. The buried oxide layer here is equivalent to the temperature compensation layer, the role is to inhibit the piezoelectric material, because the piezoelectric material may expand or contract when the temperature changes, thus affecting the frequency, so the buried oxide layer is needed to inhibit the piezoelectric material, namely the lithium tantalate or lithium niobate of the device layer.

The role of the underlying high-resistivity silicon is to reduce losses. Capable of limiting wave energy equivalence in extremely thin piezoelectric layers. Because the whole piezoelectric layer can be well controlled, better equivalent wave propagation can be obtained in the piezoelectric layer.

As a result, major RF front-end manufacturers have started to produce POI-based filters, mainly with a diameter of 150 mm POI, which is the first optimized substrate for non-silicon materials using Smart CutTM technology, and is currently in mass production at Soitec's Benin Plant 3 in France.

From the perspective of future technological development, Soitec plans to use lithium niobate instead of lithium tantalate to better control the bandwidth of the filter, and is also developing a POI substrate with a diameter of 200 mm to reduce the overall cost.

02 POI substrate has significant SAW advantage

There are different types of filters, including SAW, BAW, XBAR, etc., and POI substrate SAW can replace TC-SAW (temperature compensated SAW) and BAW on 5G architecture with its unique performance advantages.

Christophe Didier analyzed that SAW is too sensitive to temperature, and temperature change will affect frequency, but this temperature sensitivity can be improved. The change brought by temperature is called TCF, measured in ppm.K, and the improvement method isto add a layer of temperature compensation through TC-SAW (temperature-compensated SAW).

Another filter technology, BAW (bulk acoustic wave), is particularly complex, requiring many steps to build an acoustically desirable "cavity."

By omparison, we can see that the advantage of SAW technology based on POI substrate is higher energy efficiency and less energy consumption than TC-SAW. Therefore, compared with SAW and TC-SAW, it can obtain higher frequency and wider bandwidth. Compared to BAW, this POI substrate based SAW process is simpler and less costly. Because many filters can be integrated on the same chip, the area is also smaller. From a production point of view, its production process is similar to SAW, but much simpler than TC-SAW and BAW.

Based on several sets of test data to see the performance and advantages of POI substrate SAW:

The first coefficient is the quality coefficient, using Bode Q to measure the parameters of the resonator in the middle frequency band, and the measured parameters are greater than 4000.

The second parameter is the coupling coefficient, which relates to the filtering function that can achieve a wider bandwidth, where the coupling coefficient k2 is measured to be greater than 8%.

The third parameter is TCF (frequency temperature coefficient), which is a coefficient related to temperature that causes frequency changes, and the TCF we measured is less than 10 ppm.

Compared with TC-SAW, which is generally used for low-band and mid-band filtering technology, BAW is generally used for mid-band and high-band, and POI substrate SAW can solve the technical problems of the two bands, and become a better solution to replace TC-SAW and BAW in 5G filters.

03 Market Prospect

The filter is a very critical component on 5G smartphones, and in view of the use of 5G MIMO, carrier aggregation and other technologies, it is expected that the filter area of the front-end module of smartphones in the 5G era will increase by 50%.

According to Yole Development, the future market size of RF filters is expected to rise from $3.1 billion in 2018 to about $5.1 billion in 2025. In terms of the value of POI substrates, Christophe Didier believes that POI substrates will become an industry standard in the coming years. By 2024, the serviceable market size of POI substrates is expected to reach 1 million wafers.

BAW filters have their advantages, but their advantages are gradually being replaced by TF-SAW, especially on the cost side, where the cost of TF-SAW can be reduced by more than half compared to BAW. Avargo and Qorvo have dominated the BAW space for so long that they have high bargaining power, and BAW's gross margins may reach more than 70%. And the domestic can not break through the FBAR bridge patent wall and can not scale production. It's like a company is always debugging the film, constantly debugging the film, always debugging the film. From the point of view of copying patents, a company recruiting two "experts" with a little knowledge can basically copy out BAW, but the copied things can never be mass-produced, and no customers dare to use the copied products.