CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. § 119(a) of Korean Patent Application Nos. 10-2017-0008171 and 10-2017-0062807 filed on Jan. 17, 2017 and May 22, 2017, respectively, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference for all purposes.

BACKGROUND

1. Field

The following description relates to a bulk acoustic wave resonator.

2. Description of Related Art

In general, a bulk acoustic wave (BAW) filter is a core element that passes signals within a desired frequency band among radio frequency (RF) signals and blocks signals within an unwanted frequency band among the RF signals in a front end module of a smartphone, a tablet PC, or other mobile device. Demand for BAW filters has increased in accordance with an increase in the market for mobile devices.

A BAW filter may include a plurality of bulk acoustic wave (BAW) resonators, and, if a quality coefficient (Q performance) of the bulk acoustic wave resonator is good, the capability to select only a desired band in the BAW filter are improved, while insertion loss and attenuation performance are also improved.

In addition, in order to improve the quality coefficient of the BAW resonator, resonance energy should be confined in an active area of the BAW resonator by forming a frame around the resonator and reflecting lateral waves generated at the time of a resonance occurring in the resonator.

In general, the frame is formed to be thicker than the active area using the same material as an upper electrode. In a case in which the frame is formed, however, performances of other elements may be degraded due to the active area occupied by the frame. Further, noise may occur in a wide band region due to frame resonance.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In one general aspect, a bulk acoustic wave resonator includes: a substrate; a membrane layer forming a cavity together with the substrate; a lower electrode disposed on the membrane layer; a piezoelectric layer disposed on a flat surface of the lower electrode; and an upper electrode covering a portion of the piezoelectric layer and exposing a side of the piezoelectric layer to air, wherein the piezoelectric layer includes a step portion extended from the side of the piezoelectric layer and disposed on the flat surface of the lower electrode.

The thickness of the step portion may be less than half of an overall thickness of the piezoelectric layer.

A width w by which the step portion protrudes from the side of the piezoelectric layer may satisfy w=n×λ/4(n=1, 3, 5, . . . ), wherein λ is a wavelength of a lateral wave occurring in an active area of the bulk acoustic wave resonator.

A first inclined surface may be formed on the side of the piezoelectric layer, and may be disposed on the step portion.

The first inclined surface may have an angle of inclination of 60° to 90° with respect to the membrane layer.

A second inclined surface may be formed on a side of the step portion.

An edge portion of the lower electrode may protrude from the step portion of the piezoelectric layer or may be disposed inwardly from the step portion.

The bulk acoustic wave resonator may further include a residual sacrificial layer disposed in a space formed by the membrane layer, the piezoelectric layer, and the upper electrode.

The residual sacrificial layer may be formed to embed the step portion of the piezoelectric layer.

The bulk acoustic wave resonator may further include a frequency adjusting layer disposed on the upper electrode.

The bulk acoustic wave resonator of claim may further include a conductive layer partially formed on either one or both of a region of an edge of a top surface of the lower electrode and a region of an edge of a top surface of the upper electrode.

The conductive layer may cover a portion of the piezoelectric layer.

A frequency adjusting layer may be disposed on the top surface of the upper electrode, over the cavity. The conductive layer may cover a portion of an edge of the frequency adjusting layer.

The upper electrode may include a support member disposed on the membrane layer so as to be spaced apart from the lower electrode, an extending part extended from the support member and spaced apart from the piezoelectric layer, and a connecting part extended from the extending part and covering a top surface of the piezoelectric layer.

In another general aspect, a bulk acoustic wave resonator includes: a substrate; a membrane layer including a convex part forming a cavity together with the substrate; a lower electrode disposed on the membrane layer; a piezoelectric layer disposed on a flat surface of the lower electrode; an upper electrode including a support member disposed on the membrane layer so as to be spaced apart from the lower electrode, an extending part extended from the support member and spaced apart from the piezoelectric layer, and a connecting part extended from the extending part and disposed on a top surface of the piezoelectric layer, wherein the piezoelectric layer includes a step portion extended from a side of the piezoelectric layer and disposed on the flat surface of the lower electrode.

The extending part may be inclined, and an edge of the convex part of the membrane layer may be inclined to have a gradient corresponding to a gradient of the extending part.

A thickness of the step portion may be less than half of an overall thickness of the piezoelectric layer. A width w by which the step portion protrudes from the side of the piezoelectric layer may satisfy w=n×λ/4(n=1, 3, 5, . . . ), wherein λ is a wavelength of a lateral wave occurring in an active area of the bulk acoustic wave resonator.

The support member, the extending part, and the connecting part may form a step shape.

Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

