High-performance magnetostatic wave resonators through deep anisotropic etching of GGG substrates

TL;DR

The paper tackles the coupling bottleneck in magnetostatic-wave resonators based on YIG on GGG substrates, where prior topologies yielded coupling below $3\%$ and limited bandwidth. It introduces a bulk micromachining workflow to anisotropically etch GGG, enabling hairclip-like inductors and through-GGG vias that close ground planes and realize resonant LC coupling, with additional resonant transducers on the same platform. The authors demonstrate resonantly coupled HYG (RHYG) devices achieving coupling up to $k_t^2 \approx 23\%$ at 10.5 GHz and $\sim 17\%$ at 14.7 GHz, and report unprecedented figures of merit $k_t^2 \times Q$ of $191$ at 10.5 GHz and $222$ at 14.7 GHz, along with tunability and potential for tunable and switched filter banks. This work provides a scalable, high-FOM platform for high-frequency, high-Q MSW filters suitable for next-generation mobile front-ends and potential quantum-integrated photonic-magnon systems.

Abstract

Microscale resonators are fundamental and necessary building blocks for modern radio communication filters for mobile devices. The resonator's Q factor ($Q$) determines the insertion loss while coupling ($K_t^2$) governs the fractional bandwidth. The product $k_t^2 \times Q$ is widely recognized as the definitive figure of merit for microresonators. Magnetostatic wave resonators based on Yttrium Iron Garnet (YIG) are a promising technology platform for future communication filters. They have shown considerably better performance in terms of $Q$ when compared to the commercially successful acoustic resonators in the $>$7 GHz range. However, the coupling coefficients of these resonators have been limited to $<$3 %, primarily due to the restricted design space imposed by microfabrication challenges related to the patterning of gadolinium gallium garnet (GGG), the substrate material used for growing single crystal YIG. This paper reports novel resonator designs enabled by breakthrough bulk micromachining technology for anisotropic etching of GGG, leading to coupling >8 % in the 6-20 GHz frequency range. We use the same technology platform to show resonant enhancement of effective coupling, reaching up to 23 \% at 10.5 GHz. The frequency of resonant coupling can be tuned by design during the fabrication process. The resonant coupling results in an unprecedented $k_t^2 \times Q$ figure of merit of 191 at 10.5 GHz and 222 at 14.7 GHz. The technology platform presented in this paper supports both tunable filter architecture and switched filter banks that are currently being used in consumer mobile devices.

Figures (12)

• Figure 1: Design and realization of HYG resonator (a) Three-dimensional rendering images of the HYG resonator device showing different layers and their topologies. The cross-sectional image (seen from the side of the marked eye) shows a 100 $\mu$m thick GGG substrate on which the resonator was fabricated. The GGG layer is made transparent to show different etching features in the device. A third view, where the GGG layer is hidden, clearly shows the ground plane and through GGG vias (b) Chip micrograph of the HYG resonator showing various resonators on a grid. The HYG chip is mounted on a piece of Si wafer for easy handling. (c) Optical image of a hairclip device after successful fabrication showing different features as explained for the 3D schematic. (d) An optical image of a GGG sample 90 $\mu$m deep etching. The image shows the anisotropic etch profile of the GGG substrate.
• Figure 2: Measurement results from the Hairclip-YIG-on-GGG (HYG) resonator at room temperature. (a) A photograph of the fabricated chip showing a grid of resonator devices. (b) A 3D, and (c) a cross-sectional schematic of the HYG resonator showing the formation of the inductive loop around the YIG resonator. (d) The frequency response of the HYG resonator at various bias fields (300 mT to 900 mT). (e) A broadband equivalent circuit model of the HYG resonator showing a Transmission line (representing the transducers) and a parallel RLC branch representing the MSWs (f) Frequency dependence of effective coupling coefficient and quality factor. (f) Circuit fitted response of the HYG resonator at different applied bias fields.
• Figure 3: Fabrication Process Flow for HYG Resonators (a) Starting YoG material stack. (b) Patterning of YIG layer using ion milling. (c) A shallow (12 $\mu$m) etching of GGG from the top side using H3PO4 @ 160 ° C. An electroplated Au layer is used as a masking layer for wet etching of GGG. An optical image of the sample after this step is shown in (C*). (d) Patterned electroplating by using a Ti/Au/Ti/Au (10/150/25/150 nm) seed layer that was deposited using the glancing angle deposition method. (e) PECVD deposition of SiO2 and glancing angle deposition of Ti/Au, which is subsequently used as a seed to electroplate gold as a protection layer for the next processing steps. (f) Backside deep etching of GGG using the same process as (c) to land on the topside electrodes in the shallow etched region. The schematic represents the status of the sample after removing the Au/Ti masking layers. The choice of seed layers in step (d) of the process is important in this step. The first Ti adhesion layer is lost during the GGG etching step, leaving the Au/Ti/Au layer. During the striping of Au/Ti masking layers, the exposed Au and Ti layers are removed in the via region, leaving a visible Au layer as shown in the optical image (f*). (g) Glancing angle deposition of Ti/Au see and subsequent electroplating, which serves as a ground plane.
• Figure 4: Measurement results from a resonantly coupled HYG resonator (a) A photograph of the chip with multiple devices and a marked device, from which the measurement results are plotted in this figure. (b) A 3D schematic of the resonantly coupled hairclip resonator. The 3D schematic is exactly the same as the previously discussed HYG resonator. The difference is only visible in the cross-sectional schematic, as shown in (c). (c) The cross-sectional representation of the resonantly coupled device shows a capacitor formed between the ground place and the topside metal layer. This capacitor results in a self-resonance of the excitation transducer, which is visible in the measured impedance response in the absence of an external bias field. (d) Equivalent circuit and measured impedance response of the device in the absence of any external bias. The measured impedance is fitted to a series LCR circuit, and fitting parameters are fixed for the subsequent fitting of measured MSW resonance. (d) Equivalent circuit and measured response fitted to circuit model. The device response was measured with an external bias of 743 mT.
• Figure 5: Bulk GGG etching applied to modern radio technologies (a) A typical RFFW with a single tunable filter that would employ tunable resonators. (b) Tunable response of a HYG resonator showing octave tunable performance. (c) A typical RFFE with a switched filter bank employing different fixed frequency filters. (d) Measurement results from three different resonantly coupled HYG resonators (C4, D4, and C1) from the fabricated chip. These fixed frequency resonators can be utilized for building switched filter banks.