BAW Application of Aluminum Scandium Material in 5G RF Filters
By: Zhi Yan, Applications Engineer and David P. VanHeerden, Applications Engineer Manager
5G is here and is fundamentally changing the way we interact with the world around us. In practical terms, 5G is the new wireless standard that has superseded the current 4G LTE standard for wireless communication. The new standard is designed to offer much faster wireless speeds and larger bandwidths than the current technology. This high data rate transmission is not just about speeding up smartphone uploads or streaming connections; it will enable the internet of things (IoT), self-driving cars, the factories of tomorrow, wearable health monitors, and so much more.
Bulk Acoustic Wave (BAW) Technology & 5G RF Filters
The higher frequencies and wider bandwidths associated with 5G are necessitating the development of a new generation of radio frequency filters (RF filters) using bulk acoustic wave (BAW) technology. BAW filters are piezoelectric-based acoustic resonators in which the frequency and bandwidth of the filter are determined by the properties of a thin film piezoelectric material. The bandpass frequency of the RF filter in BAW devices is primarily determined by the thickness of the film, but also by the geometry of the piezoelectric element and the geometry and properties of the associated electrodes. This dependence of bandpass frequency on film thickness allows BAW devices to filter frequencies higher than traditional surface acoustic wave (SAW) RF filters. Consequently, BAW filters are extensively used for frequencies from 2.2 to 6.0 GHz where SAW filter technology is severely challenged.
Until recently, BAW filters typically utilized an Aluminum Nitride (AlN) piezoelectric resonant element. AlN has the advantage relative to the other candidate materials such as Zinc Oxide (ZnO) or Lead Zirconate Titanate (PZT) of being CMOS (complementary metal–oxide–semiconductor) compatible. However, AlN is a relatively poor piezoelectric material with low coupling. Doping AlN with Scandium (Sc) has been shown to significantly improve piezoelectric performance. The upper limit for the incorporation of Sc in AlN has been shown to be 43 at%, as at higher concentrations, the lattice of AlScN changes from the hexagonal wurtzite structure of AlN to the cubic rock salt structure of ScN and loses its piezoelectric properties. Consequently, AlScN with a Sc content approaching 43 at% exhibits the largest piezoelectric response. Published research has shown that doping AlN with 35 at% Sc improves piezoelectric performance (Keff2) to 15.5%, 2.6 times higher than that of pure AlN. This improved coupling is at the heart of the next generation RF filters as it allows designers to create BAW devices that require less power to operate than AlN filters (extending your phone’s or tablet’s battery life). It also facilitates the design of devices with smaller form factors (facilitating the fabrication of thinner, lighter devices), and high “out of band attenuation” (minimizing crosstalk). In addition, doping with 35 at% Sc increases the maximum relative bandwidth from 2.4% to 7.0%. The increase in relative bandwidth associated with Scandium doping of the AlN will allow the effective utilization of the wider bandwidths that are being opened up for use in 5G.
Producing AlScN Films
AlScN thin films are produced by reactive sputtering of an AlSc material in a nitrogen-containing atmosphere. There are two methods for the preparation of AlScN films:
1. Reactive Co-Sputtering from Separate Al and Sc Targets
AlScN films with scandium doping levels are prepared by adjusting the relative sputtering powers of each target. However, the chemical ratio and phase composition in the deposited film is challenging to control across the wafer and over the lifetime of the target. Consequently, this fabrication technique is most commonly used in laboratory studies.
2. Reactive Sputtering from an AlSc Metallic Alloy Target
Sputtering from a single uniform alloy target enables significantly better on-wafer chemical and phase composition uniformity across the wafer and over the life of the target. This stability is essential in a production environment, and it is the preferred route in industry for manufacturing BAW devices.
In practice, fabricating the AlSc alloy sputter targets required for mass production is challenging as Scandium exhibits a low solubility in aluminum, and the alloy system exhibits a number of brittle intermetallic alloys. This makes the system prone to both segregation and to cracking when processed via traditional manufacturing methods.
Application of Materion’s AlSc Material in BAW-based 5G RF Filters
Through more than seven years of collaboration with research institutes, equipment manufacturers and our customers, Materion has developed and commercialized AlSc alloy sputter targets with a wide range of compositions. Through this work we have created a unique technology to manufacture AlSc alloy targets with high purity, highly controlled Sc contents, and low impurity levels. Our material is widely used in BAW fabs around the world and is the material of record for several sputter tool manufacturers.
Materion Electronic Materials has seven sputter target global manufacturing facilities, including locations in the United States, Germany, China, and Singapore. To learn more about our Aluminum Scandium sputter targets and how we are contributing to the evolution of 5G wireless communications, contact our team today.
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