US-12620961-B2 - Patterned cavity walls for transversely-excited film bulk acoustic resonator frontside membrane release

US12620961B2US 12620961 B2US12620961 B2US 12620961B2US-12620961-B2

Abstract

Acoustic resonator devices and methods are disclosed. An acoustic resonator device includes a substrate having a front surface and an intervening oxide layer on the front surface and having a cavity. A thickness of the intervening oxide layer defines a depth of the cavity, and the substrate has vertical etch-stop material for etching the intervening oxide layer. Lateral fences formed in the intervening oxide layer define a perimeter of the cavity. The lateral fences has a lateral etch-stop material for etching the intervening oxide layer. A single-crystal piezoelectric plate has a back surface attached to the front surface of the intervening oxide layer except for a portion of the piezoelectric plate forming a diaphragm that spans the cavity. An interdigital transducer is formed on the front surface of the single-crystal piezoelectric plate such that interleaved fingers of the IDT are disposed on the diaphragm.

Inventors

Albert Cardona
Chris O'Brien
Akanksha Saha
Andrew Kay

Assignees

MURATA MANUFACTURING CO., LTD.

Dates

Publication Date: 20260505
Application Date: 20220602

Claims (6)

1 . An acoustic resonator device comprising: a substrate having a surface, the substrate comprising a substrate material; an intervening oxide layer on the surface of the substrate and having a cavity, a thickness of the intervening oxide layer defining a depth of the cavity, the substrate comprising a vertical etch-stop material that includes silicon; a piezoelectric layer having opposing front and back surfaces, the back surface attached to the intervening oxide layer and the piezoelectric layer having a portion that forms a diaphragm that is over the cavity in the intervening oxide layer; an interdigital transducer (IDT) on the front surface of the piezoelectric layer and having interleaved fingers on the diaphragm; and lateral fences in the piezoelectric layer and intervening oxide layer that define a perimeter of the cavity, the lateral fences comprising a lateral etch-stop material that includes at least one of silicon oxide, silicon nitride, aluminum oxide, oxynitride, chromium, aluminum and platinum, wherein the piezoelectric layer includes one or more holes extending therethrough and that do not overlap the lateral fences in a direction perpendicular to the surface of the substrate.
2 . The device of claim 1 wherein the intervening oxide layer comprises a device layer thickness of silicon oxide material on the silicon substrate.
3 . The device of claim 1 wherein: the piezoelectric layer and the IDT are configured such that a radio frequency signal applied to the IDT excites a primary shear acoustic mode in the diaphragm, the primary shear acoustic mode is a bulk shear mode where acoustic energy propagates along a direction substantially orthogonal to the front and back surfaces of piezoelectric layer, which is also transverse to a direction of an electric field generated by the interleaved fingers of the IDT, and the piezoelectric layer comprising one of lithium niobate and lithium tantalate.
4 . The device of claim 1 wherein the substrate comprises: a device portion having the intervening oxide layer, wherein the cavity extends from the back surface of the piezoelectric layer through the device portion to the substrate.
5 . The device of claim 4 wherein: the substrate is silicon; and the intervening oxide layer is silicon dioxide.
6 . The device of claim 1 wherein: the cavity is formed by a frontside membrane release etch process that etches the intervening oxide layer; and the lateral fences and substrate are substantially impervious to the etch process.

Description

RELATED APPLICATION INFORMATION This patent claims priority from and incorporates by reference Provisional Application No. 63/228,058, filed Jul. 31, 2021, entitled PATTERNED CAVITY WALLS FOR XBAR FRONTSIDE MEMBRANE RELEASE. NOTICE OF COPYRIGHTS AND TRADE DRESS A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. BACKGROUND Field This disclosure relates to radio frequency filters using acoustic wave resonators and to filters for use in communications equipment. Description of the Related Art A radio frequency (RF) filter is a two-port device configured to pass some frequencies and to stop other frequencies, where “pass” means transmit with relatively low signal loss and “stop” means block or substantially attenuate. The range of frequencies passed by a filter is referred to as the “pass-band” of the filter. The range of frequencies stopped by such a filter is referred to as the “stop-band” of the filter. A typical RF filter has at least one pass-band and at least one stop-band. Specific requirements on a passband or stop-band depend on the specific application. For example, a “pass-band” may be defined as a frequency range where the insertion loss of a filter is better than a defined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be defined as a frequency range where the rejection of a filter is greater than a defined value such as 20 dB, 30 dB, 40 dB, or greater depending on application. RF filters are used in communications systems where information is transmitted over wireless links. For example, RF filters may be found in the RF front-ends of cellular base stations, mobile telephone and computing devices, satellite transceivers and ground stations, IoT (Internet of Things) devices, laptop computers and tablets, fixed point radio links, and other communications systems. RF filters are also used in radar and electronic and information warfare systems. RF filters typically require many design trade-offs to achieve, for each specific application, the best compromise between performance parameters such as insertion loss, rejection, isolation, power handling, linearity, size and cost. Specific design and manufacturing methods and enhancements can benefit simultaneously one or several of these requirements. Performance enhancements to the RF filters in a wireless system can have broad impact to system performance. Improvements in RF filters can be leveraged to provide system performance improvements such as larger cell size, longer battery life, higher data rates, greater network capacity, lower cost, enhanced security, higher reliability, etc. These improvements can be realized at many levels of the wireless system both separately and in combination, for example at the RF module, RF transceiver, mobile or fixed sub-system, or network levels. High performance RF filters for present communication systems commonly incorporate acoustic wave resonators including surface acoustic wave (SAW) resonators, bulk acoustic wave (BAW) resonators, film bulk acoustic wave resonators (FBAR), and other types of acoustic resonators. However, these existing technologies are not well-suited for use at the higher frequencies and bandwidths proposed for future communications networks. The desire for wider communication channel bandwidths will inevitably lead to the use of higher frequency communications bands. Radio access technology for mobile telephone networks has been standardized by the 3GPP (3rd Generation Partnership Project). Radio access technology for 5th generation mobile networks is defined in the 5G NR (new radio) standard. The 5G NR standard defines several new communications bands. Two of these new communications bands are N77, which uses the frequency range from 3300 MHz to 4200 MHz, and N79, which uses the frequency range from 4400 MHz to 5000 MHz. Both band N77 and band N79 use time-division duplexing (TDD), such that a communications device operating in band N77 and/or band N79 use the same frequencies for both uplink and downlink transmissions. Bandpass filters for bands N77 and N79 must be capable of handling the transmit power of the communications device. The 5G NR standard also defines millimeter wave communication bands with frequencies between 24.25 GHz and 40 GHz. The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is an acoustic resonator structure for use in microwave filters. The XBAR is described in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILM BULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigital transducer (IDT) formed on a thin floating la