Ion Beam Etching Applications in RF Filter Processing

Blogs | Mar 18, 2021

Introduction

5G technology focuses on delivering enhanced mobile broadband services to consumers. Advancements in 5G wireless communication come with faster download/upload speeds, reduction of cost per data bit, lower latency, and more. In a world of increased connectivity, 5G is enabling and essential.

Countries and carriers are now preparing the necessary infrastructure. First, 5G carriers will deliver data speeds without requiring a new core buildout. In the next two years, there will be an estimated 20% to 30% increase in 5G radio frequency front-end (RFFE) filter devices, such as solidly mounted resonator (SMR) and film bulk acoustic resonator “bulk acoustic wave” (FBAR BAW). BAW filters can be fabricated as an SM) or a thin-film FBAR. BAW supports frequencies above 1.5GHz, making it an ideal technology for sub-6GHz 5G filters, complementary to surface acoustic wave (SAW) devices.

Ion beam etching

Although reactive etching is possible, standard ion beam etching (IBE) is a non-volatile physical process, and therefore it has a high material compatibility. Compared with other dry etching methods such as reactive ion etching (RIE), there is no risk of chemical corrosion, residue contamination, or under-cut damage that leads to device failure. This is because IBE avoids under-etching, which occurs frequently during RIE due to differences in material reactive selectivity. Table 1 gives more details on the advantages of IBE compared with RIE.

IBE is anisotropic and has a relatively constant etch rate compared with RIE, regardless of the material or its thickness. This is important for many BAW device manufacturers because the material stack that is etched almost always consists of diverse materials of varying thicknesses. These factors, along with improved uniformity, sidewall smoothing, low ion damage, and removal of under-etch, ultimately lead to devices with improved Q factors as well as improved yield and device performance. Figure 1 shows silicon dioxide (SiO2) uniformities for a range of fixture tilt angles for different voltages.

Device manufacturers may require vertical etch profiles, endpoint control, temperature control (to avoid photoresist burning), and minimal re-deposition. Here, we focus on two important etching steps and materials for BAW fabrication: electrode stack and piezoelectric (PZ) crystal etching.

Electrode layer stack

BAW resonance frequency is determined in part by electrode thickness. Furthermore, electrodes and support layers for 5G RFFE devices can consist of various materials. Typically, high-density metals such as molybdenum (Mo) or tungsten (W) are used as electrodes, which allows for increased electromechanical coupling. Additional material layers may be added, for reasons such as electromigration and temperature compensation, among others. A diagram of a common FBAR is shown in Figure 2, illustrating such a stack. Finally, because photoresist burn impairs effective photoresist removal, threatening Q factor values, stacks must be etched in a timely manner with no burning or destruction of resist. Table 3 shows possible etch rates for Mo, W, and other materials.

Table 2 demonstrates the high etch rates and uniformity achieved when etching Mo using our Lancer tunable source platform at two different operating powers. It also demonstrates the angle dependence of these two variables. Similarly, Figure 3 shows the angular dependence of etch rate for SiO2 and W. Unlike chemical etching techniques, IBE etches through a stack of diverse materials reliably, with stability, and with no under-etch shape. Endpoint detection can be used, adding to the reliability of the process.

Piezoelectric (PZ) mini case study

Aluminum nitride (AlN) is widely accepted as a PZ material due to the synchronization of its performance and manufacturability. It has intrinsically low damping, excellent heat conductivity, and a large temperature coefficient of frequency, and it is deposited reliably. Aluminum is often stoichiometrically replaced with scandium, as this increases its PZ response. While aluminum scandium nitride (AlScN) is chemically stable, using reactive gases during the etching process may lead to chemical residue contamination in the PZ layer. As well-textured and low-roughness PZ surfaces result in higher electromechanical coupling, these parameters have become key specifications for BAW device manufacturers. Our standard argon IBE avoids corrosive chemistries that lead to under-etching, damage, and chemical contamination.

Figure 3 shows the etch rate dependence on angle of AlN, while Figure 4 shows uniformity and etch rates for AlScN, in a tighter process angle window.

Conclusion

At Veeco, our IBE equipment allows manufacturers to address challenges they might face for RFFE filter fabrication. These platforms etch single layers and material stacks with no under-etch, at a reliable etch rate with high uniformity, and it’s all done without harmful corrosive chemistries and residue contamination. For the unique challenges faced today by RFFE manufacturers, our IBE platforms are an excellent solution. Learn more here.

About Veeco

Veeco offers multiple platforms that are critical for 5G device manufacturing. Our proven ion beam, laser annealing, lithography, MOCVD, and single wafer etch and clean technologies play an integral role in the fabrication and packaging of advanced semiconductor devices. With equipment designed to optimize performance, yield, and cost of ownership, Veeco holds leading technology positions in the markets we serve. To learn more about Veeco’s systems and service offerings, visit www.veeco.com.

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