Bulk acoustic wave resonator

A bulk acoustic wave resonator includes: a support part disposed on a substrate; a layer disposed on the support part, wherein an air cavity is formed between the support part, the substrate and the layer; and a frame extending along the layer, within the air cavity, and spaced apart from the support part.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Korean Patent Application Nos. 10-2016-0104946 and 10-2016-0159469 filed on Aug. 18, 2016 and Nov. 28, 2016, respectively, in the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference for all purposes.

BACKGROUND

The following description relates to a bulk acoustic wave resonator.

2. Description of Related Art

Recently, in accordance with the rapid development of mobile communications devices, and chemical and biological devices, for example, demand for a small and light filter, oscillator, resonant element, and acoustic resonant mass sensor used in these devices has increased.

A bulk acoustic wave resonator is a known device for implementing such a small and light filter, oscillator, resonant element, and acoustic resonant mass sensor. The bulk acoustic wave resonator may be mass-produced at minimal cost, and may be implemented to have a subminiature size. In addition, the bulk acoustic resonator may implement a high quality factor (Q) value, which is a main characteristic of a filter, may be used in the microwave frequency band, and, particularly, may also implement bands of a personal communications system (PCS) and a digital cordless system (DCS).

The bulk acoustic wave resonator includes a resonant part implemented by sequentially stacking a lower electrode, a piezoelectric layer, and an upper electrode on a substrate. When electrical energy is applied to the upper and lower electrodes to induce an electric field in the piezoelectric layer, the electric field generates a piezoelectric phenomenon in the piezoelectric layer to allow the resonant part to vibrate in a predetermined direction. As a result, acoustic waves are generated in the same direction as the direction in which the resonant part vibrates, resulting in resonance.

SUMMARY

In one general aspect, a bulk acoustic wave resonator includes: a support part disposed on a substrate; a layer disposed on the support part, wherein an air cavity is formed between the support part, the substrate and the layer; and a frame extending along a boundary surface of the layer, within the air cavity, and spaced apart from the support part.

The layer may include a resonant part including a first electrode, a piezoelectric layer, and a second electrode.

The frame may extend along the boundary surface, within the air cavity, in an inactive region of the resonant part.

The frame may extend along the boundary surface, within the air cavity, at an edge of an active region of the resonant part.

The frame may be formed of a material including any one or both of compressive stress and tensile stress.

The frame may include a temperature coefficient of elasticity having a polarity that is different from a polarity of a temperature coefficient of elasticity of the resonant part.

The bulk acoustic wave resonator may further include an auxiliary electrode disposed on the second electrode at an edge of an active region of the resonant part.

The second electrode may have an acoustic impedance that is different than an acoustic impedance of the auxiliary electrode.

In another general aspect, a bulk acoustic wave resonator includes: a support part disposed on a substrate; a layer disposed on the support part, wherein an air cavity is formed between the support part, the substrate and the layer; and a frame extending along a boundary surface of the layer, within the air cavity, wherein a width of an upper surface of the frame is greater than a width of a lower surface of the frame, and wherein side surfaces of the frame connecting the upper surface and the lower surface to each other are inclined.

The layer may include a resonant part including a first electrode, a piezoelectric layer, and a second electrode.

The frame may extend along the boundary surface, within the air cavity, in contact with the support part in an inactive region of the resonant part.

The frame may extend along the boundary surface, within the air cavity, in contact with the support part at an edge of an active region of the resonant part.

The layer may further include a membrane disposed between the resonant part and the air cavity.

The support part may include a first support part and a second support part sequentially disposed in a direction away from the air cavity. The membrane may be further disposed at a boundary between the air cavity and the frame, a boundary between the air cavity and the first support part, a boundary between the first support part and the second support part, a boundary between the frame and the second support part, and an upper surface of the second support part.

The support part may include a first support part and a second support part sequentially disposed in a direction away from the air cavity, wherein the first support part is formed of a material that is different than a material of the second support part.

The frame may extend along the boundary surface, within the air cavity, and may penetrate through portions of the first support part and the second support part.

A thickness of the first support part and a thickness of the second support part may be the same as a thickness of the air cavity.

An angle formed by the lower surface and the side surface may be 110° to 160°.

DETAILED DESCRIPTION

FIG. 1is a cross-sectional view illustrating an example of a bulk acoustic wave resonator1, andFIGS. 2A and 2Bare partially enlarged views of the bulk acoustic wave resonator1.

Referring toFIG. 1, the bulk acoustic wave resonator1includes a substrate11, an insulating layer12A electrically isolating the substrate11, an etch stop layer12B protecting the insulating layer12A from an etching process, an air cavity13A formed on the etch stop layer12B, a membrane13B covering the air cavity13A, and a lower electrode14, a piezoelectric layer15, and an upper electrode16sequentially stacked on the membrane13B. In addition, the bulk acoustic wave resonator1includes a protective layer17preventing the upper electrode16from being externally exposed and an electrode pad18configured to apply electrical signals to the lower electrode14and the upper electrode16.

The air cavity13A is positioned below a resonant part including the lower electrode14, the piezoelectric layer15, and the upper electrode16so that the resonant part vibrates in a predetermined direction. The air cavity13A may be formed by forming a sacrificial layer on the etch stop layer12B, forming the membrane13B on the sacrificial layer, and then removing the sacrificial layer by an etching process.

The air cavity13A may have an approximately trapezoidal shape. Referring toFIGS. 2A and 2B, cracks may form in the lower electrode14, the piezoelectric layer15, the upper electrode16, and the electrode pad18stacked on the membrane13B due to a height of the air cavity13A and an angle of a side surface of the air cavity13A. In addition, crystals of the piezoelectric layer15stacked on the membrane13B may grow abnormally. Due to the formation of the cracks and the abnormal growth of the crystals, insertion loss characteristics and attenuation characteristics of the bulk acoustic wave resonator1may be deteriorated.

FIGS. 3 through 10are cross-sectional views illustrating respective bulk acoustic wave resonators10,10a,10b,10c,10d,10e,10f, and10g, according to various embodiments. The example bulk acoustic wave resonators10illustrated inFIGS. 3 through 10are similar to one another, and, thus, the bulk acoustic wave resonator10illustrated inFIG. 3will be mainly described. In the bulk acoustic wave resonators according to the embodiments illustrated inFIGS. 4 through 10, a description of contents that are the same as or overlap those of the bulk acoustic wave resonator10will be omitted, and contents different from those of the bulk acoustic wave resonator10will be primarily described.

Referring toFIG. 3, the bulk acoustic wave resonator10is a film bulk acoustic resonator (FBAR). The bulk acoustic wave resonator10includes a substrate110, an insulating layer115, an etch stop layer123, an air cavity133, a first support part134, a second support part135, a frame136, and a resonant part155including a first electrode140, a piezoelectric layer150, a second electrode160, a protective layer170, and an electrode pad180.

The substrate110may be a silicon substrate, and the insulating layer115is provided on an upper surface of the substrate110to electrically isolate the resonant part155from the substrate110. The insulating layer115may be formed on the substrate110by performing chemical vapor deposition, radio frequency (RF) magnetron sputtering, or evaporation of silicon dioxide (SiO2) or aluminum oxide (Al2O3).

The etch stop layer123is formed on the insulating layer115. The etch stop layer123protects the substrate110and the insulating layer115from an etching process, and is a base for depositing layers or films on the etch stop layer123.

The air cavity133, the first support part134, and the second support part135are formed on the etch stop layer123. The air cavity133, the first support part134, and the second support part135may be formed at the same height, such that one surface provided by the air cavity133, the first support part134, and the second support part135is approximately flat. The insulating layer115and the etch stop layer123are separated from each other inFIG. 3, but the insulating layer115and the etch stop layer123may be integrated with each other as a single layer. The insulating layer115and the etch stop layer123may be integrated with each other using an oxide layer.

The resonant part155includes the first electrode140, the piezoelectric body150, and the second electrode160. A common region in which the first electrode140, the piezoelectric layer150, and the second electrode160overlap one another in a vertical direction is positioned above the air cavity133. The first electrode140and the second electrode160may be formed of any one of gold (Au), titanium (Ti), tantalum (Ta), molybdenum (Mo), ruthenium (Ru), platinum (Pt), tungsten (W), aluminum (Al), iridium (Ir), and nickel (Ni), or alloys thereof. The piezoelectric layer150, which generates a piezoelectric effect that electrical energy is converted into mechanical energy having an elastic wave form, may be formed of any one of aluminum nitride (AlN), zinc oxide (ZnO), and lead zirconium titanium oxide (PZT; PbZrTiO). In addition, the piezoelectric layer150may further include a rare earth metal. As an example, the rare earth metal includes at least one of scandium (Sc), erbium (Er), yttrium (Y), and lanthanum (La). The piezoelectric layer150may include 1 to 20 at % of rare earth metal. Although not illustrated in detail, a seed layer for improving crystal alignment of the piezoelectric layer150may be additionally disposed below the first electrode140. The seed layer may be formed of any one of aluminum nitride (AlN), zinc oxide (ZnO), and lead zirconium titanium oxide (PZT; PbZrTiO) having the same crystallinity as that of the piezoelectric layer150.

The resonant part155includes an active region and an inactive region. The active region of the resonant part155, which is a region that vibrates and resonates in a predetermined direction due to a piezoelectric phenomenon generated in the piezoelectric layer150when electrical energy such as a radio frequency signal is applied to the first electrode140and the second electrode160, is a region in which the first electrode140, the piezoelectric layer150, and the second electrode160overlap with one another in the vertical direction above the air cavity133. The inactive region of the resonant part155, which is a region that does not resonate by the piezoelectric phenomenon even when the electrical energy is applied to the first and second electrodes140and160, is a region outside the active region.

The resonant part155outputs a radio frequency signal having a specific frequency using the piezoelectric phenomenon. In detail, the resonant part155outputs a radio frequency signal having a resonant frequency corresponding to vibrations depending on the piezoelectric phenomenon of the piezoelectric layer150.

When the electrical energy is applied to the first electrode140and the second electrode160, an acoustic wave is generated by the piezoelectric phenomenon generated in the piezoelectric layer150. In this case, a lateral wave is collaterally generated from the generated acoustic wave. When the collaterally generated lateral wave is not trapped, loss of the acoustic wave may occur to deteriorate a quality factor of the resonator.

Referring toFIG. 4, in a bulk acoustic wave resonator10a, according to an embodiment, an auxiliary electrode165is provided along an edge of the active region of the resonant part155. The edge of the active region refers to a portion in the active region adjacent to a boundary between the active region and the inactive region. The auxiliary electrode165may be formed of the same material as that of the second electrode160.

Alternatively, the auxiliary electrode165may be formed of a material different from that of the second electrode160. In detail, the auxiliary electrode165and the second electrode160may have different acoustic impedances. For example, the auxiliary electrode165is formed of a material having an acoustic impedance higher than that of the second electrode160. As an example, when the second electrode160is formed of ruthenium (Ru), the auxiliary electrode165is formed of one of iridium (Ir) and tungsten (W) having an acoustic impedance higher than that of ruthenium (Ru). Alternatively, the auxiliary electrode165may be formed of a material having an acoustic impedance lower than that of the second electrode160. As an example, when the second electrode160is formed of ruthenium (Ru), the auxiliary electrode165is formed of one of aluminum (Al), gold (Au), nickel (Ni), copper (Cu), titanium (Ti), chromium (Cr), cobalt (Co), manganese (Zn), and magnesium (Mg) having an impedance lower than that of ruthenium (Ru), and is formed of one of aluminum (Al), gold (Au), nickel (Ni), copper (Cu), cobalt (Co), manganese (Zn), and magnesium (Mg) when considering electrical resistivity.

According to the embodiment illustrated inFIG. 4, the auxiliary electrode165is additionally provided along the edge of the active region of the resonant part155to trap a lateral wave. Therefore, a quality factor of the bulk acoustic wave resonator10ais improved, and loss of an acoustic wave may be reduced. A configuration of the auxiliary electrode165of the bulk acoustic wave resonator10aillustrated inFIG. 4may be variously applied to other embodiments disclosed herein.

Again referring toFIG. 3, the air cavity133is positioned below the resonant part155including the first electrode140, the piezoelectric layer150, and the second electrode160so that the resonant part155may vibrate in a predetermined direction. The air cavity133is formed by forming a sacrificial layer on the etch stop layer123, stacking the first electrode140, the piezoelectric layer150, the second electrode160, and the like, on the sacrificial layer, and then etching and removing the sacrificial layer by an etching process. In an example, the sacrificial layer includes poly-silicon (poly-Si).

Referring toFIGS. 5 and 6, in bulk acoustic wave resonators10band10c, a membrane128is additionally provided between the air cavity133and the resonant part155. The membrane128adjusts stress of the bulk acoustic wave resonator10b/10c, and is a stop layer of a planarization process of an etching stop material to be described below. As an example, the membrane128includes silicon nitride (SiN).

Referring toFIG. 5, in the bulk acoustic wave resonator10b, the membrane128is formed on an approximately flat surface formed by the air cavity133, the first support part134, and the second support part135to adjust stress of the bulk acoustic wave resonator10b. Referring toFIG. 6, in the bulk acoustic wave resonator10c, the membrane128is formed at a boundary between the air cavity133and the resonant part155, a boundary between the air cavity133and the frame136, a boundary between the air cavity133and the first support part134, a boundary between the first support part134and the etch stop layer123, a boundary between the first support part134and the second support part135, a boundary between the frame136and the second support part135, and an upper surface of the second support part135.

Again referring toFIG. 3, the first and second support parts134and135are formed laterally outside the air cavity133. A thickness of each of the first and second support parts134and135may be the same as a thickness of the air cavity133. An upper surface formed by the air cavity133and the first and second support parts134and135is flat. According to the disclosed embodiments, the resonant part155is disposed on a flat surface from which a step is removed, resulting in improvement of insertion loss characteristics and attenuation characteristics of the bulk acoustic wave resonator10/10a/10b/10c/10d/10e/10f/10g.

The first support part134and the second support part135are sequentially disposed in a lateral direction away from the air cavity133. The first support part134and the second support part135may be formed of different materials. For example, the first support part134is formed of a material that is not etched in an etching process for removing the sacrificial layer. The first support layer134may be formed of the same material as that of the etch stop layer123. As an example, the first support part134is formed of one of silicon dioxide (SiO2) and silicon nitride Si3N4. The second support part135corresponds to a portion of the sacrificial layer formed on the etch stop layer123that remains after the etching process. A cross section of the first support part134may have an approximately trapezoidal shape. For example, a width of an upper surface of the first support part134is greater than a width of a lower surface of the first support part134, and side surfaces of the first support part134connecting the upper surface and the lower surface of the first support part134to each other are inclined.

The frame136extends in the air cavity133along a boundary surface of the one or more layers (e.g., the first electrode140, the piezoelectric layer150, the second electrode160, and the protective layer170) disposed on the air cavity133. As described herein, the boundary surface is an innermost surface of the one or more layers that bounds the air cavity133. The frame136may have a thickness that is less than a thickness of the first support part134. A cross section of the frame136may have an approximately trapezoidal shape. For example, a width of an upper surface of the frame136is greater than a width of a lower surface of the frame136, and side surfaces of the frame136connecting the upper surface of the frame136to the lower surface of the frame136to each other are inclined.

The frame136is disposed in at least one of the active region and the inactive region of the resonant part155. When the frame136is disposed in the active region, the frame136may be disposed along the edge of the active region.

Referring toFIG. 3, in the bulk acoustic wave resonator10, the frame136is spaced apart from the first support part134, and extends in the air cavity133along the boundary surface of the resonant part155in the inactive region. Alternatively, in the bulk acoustic wave resonator10dofFIG. 7, the frame136is spaced apart from the first support part134, and extends in the air cavity133along the boundary surface between of the resonant part155at the edge of the active region.

Referring toFIG. 8, in the bulk acoustic wave resonator10e, the frame136extends in the cavity133along the boundary surface of the resonant part155in the inactive region, and is in contact with the first support part134. Referring toFIG. 9, in the bulk acoustic wave resonator10f, the frame136extends in the cavity133along the boundary surface of the resonant part155at the edge of the active region, and is in contact with the first support part134.

Alternatively, referring toFIG. 10, in the bulk acoustic wave resonator10g, the frame136extends in the cavity133along the boundary surface of the resonant part155, penetrates through the first support part134, and protrudes toward the second support part135. The frame136, having a thickness that is less than a thickness of the first support part134, protrudes from side surfaces of the first support part134toward the air cavity133and the second support part135, which is stepped.

The frame136may be formed by the same process as the process of forming the first support part134, and may be formed of the same material as the material of the first support part134. As an example, the frame136is formed of any one of silicon dioxide (SiO2) and silicon nitride (Si3N4).

According to the embodiments disclosed herein, the frame136is formed of a material having compressive stress and tensile stress, and the bulk acoustic wave resonator may thus be robust against stress acting in an upward direction and a downward direction of the bulk acoustic wave resonator10/10a/10b/10c/10d/10e/10f/10g.

As an example, when the frame136is formed of a material having compressive stress, a phenomenon in which the resonant part155including the first electrode140, the piezoelectric layer150, and the second electrode160sags to a lower portion of the air cavity133may be prevented. In this case, the frame136may be formed of silicon dioxide (SiO2). In addition, as another example, when the frame136is formed of a material having tensile stress, a phenomenon in which the resonant part155is bent toward a portion opposing the air cavity133may be prevented. In this case, the frame136may be formed of silicon nitride (Si3N4).

According to the disclosed embodiments, the frame136is formed of a material having an elastic coefficient that varies depending on a temperature (hereinafter, referred to as a temperature coefficient of elasticity) to reduce a variation in a frequency depending on a temperature change. As an example, a temperature coefficient of elasticity of the frame136and a temperature coefficient of elasticity of the resonant part155including the first electrode140, the piezoelectric layer150, and the second electrode160have different polarities. When the resonant part155has a negative temperature coefficient of elasticity of −30 to −80 ppm/K, in a case in which a temperature of the resonant part155rises by 1K, elasticity is reduced by 30 to 80 ppm. The reduction in the elasticity of the resonant part155causes a resonant frequency of the bulk acoustic wave resonator10/10a/10b/10c/10d/10e/10f/10gto be lowered, which causes deterioration of performance of a filter including the bulk acoustic wave resonator10/10a/10b/10c/10d/10e/10f/10g. In this case, when the frame136is formed of silicon dioxide (SiO2) having a positive temperature coefficient of elasticity of 130 ppm/K, a reduction in elasticity due to an influence of the resonant part155is compensated for, and elasticity of the bulk acoustic wave resonator10/10a/10b/10c/10d/10e/10f/10gincreases depending on a rise in a temperature.

A case in which the frame136has a positive temperature coefficient of elasticity and the resonant part155has a negative temperature coefficient of elasticity is described above. However, the resonant part155may have a positive temperature coefficient of elasticity and the frame136may have a negative temperature coefficient of elasticity.

The protective layer170is formed on the second electrode160to prevent the second electrode160from being externally exposed. The protective layer170may be formed of any one of a silicon oxide based insulating material, a silicon nitride-based insulating material, and an aluminum nitride-based insulating material. The electrode pad180for applying the electrical signals to the first electrode140and the second electrode160is formed on the first electrode140and the second electrode160, and is externally exposed.

FIGS. 11A through 11Eare views illustrating processes of a method of manufacturing the bulk acoustic wave resonator10.

Referring toFIGS. 11A through 11E, the method of manufacturing the bulk acoustic wave resonator10begins with forming the substrate110, the insulating layer115, the etch stop layer123, and the sacrificial layer131, and forming a first pattern P1and a second pattern P2on the sacrificial layer131(seeFIG. 11A). Portions of the etch stop layer123are exposed by the first pattern P1, which is thicker than the sacrificial layer131, and the etch stop layer123is not exposed by the second pattern P2, which is thinner than the sacrificial layer131.

Referring toFIG. 11B, after the first pattern P1and the second pattern P2are formed on the sacrificial layer131, an etching stop material120covering the sacrificial layer131and the etch stop layer123externally exposed by the first pattern P1are formed. The first and second patterns P1and P2are filled by the etching stop material120, and the etch stop layer123and the etching stop material120may be formed of the same material.

After the etching stop material120is formed, one surface of the etching stop material120is planarized so that the sacrificial layer131is externally exposed. In a process of planarizing the one surface of the etching stop material120, portions of the etching stop material120are removed, and the first support part134and the frame136are formed by the etching stop material120remaining in the first pattern P1and the second pattern P2after portions of the etching stop material120are removed. As a result of the process of planarizing the one surface of the etching stop material120, a surface formed by the sacrificial layer131, the first support part134, and the frame136is approximately flat.

FIG. 12is an enlarged view of part A ofFIG. 11C. Referring toFIG. 12, a dishing phenomenon in which upper surfaces of the first support part134and the frame136remaining after portions of the etching stop material120are removed are indented concavely by a step between the first pattern P1and the second pattern P2may occur. For example, a thickness of the center of the upper surfaces of the first support part134and the frame136is less than a thickness of an edge of the upper surfaces of the first support part134and the frame136.

According to the disclosed embodiments, side surfaces of the first pattern P1and the second pattern P2in which the first support part134and the frame136are provided, respectively, are inclined to prevent an abrupt step from being formed on boundaries between the first support part134and the frame136, and the sacrificial layer131, and the first support part134may be formed so that a lower surface of the first support part134has a width that is less than a width of the upper surface of the first support part134to prevent occurrence of the dishing phenomenon. As an example, angles formed by the side surfaces and lower surfaces of the first pattern P1and the second pattern P2are 110° to 160°.

Again referring toFIGS. 11A through 11E, after portions of the etching stop material120are removed, the first electrode140, the piezoelectric layer150, and the second electrode160are sequentially stacked on an approximately flat surface formed by the first support part134, the frame136, and the sacrificial layer131, the protective layer170is disposed on the second electrode160, and the electrode pad180for applying the electrical signals to the first electrode140and the second electrode160is formed on the first electrode140and the second electrode160so as to be externally exposed (seeFIG. 11D). Then, the portion of the sacrificial layer131that is disposed inside the first support part134is removed by an etching process, such that the air cavity133is formed, and the sacrificial layer131disposed outside the first support part134remains as the second support part135(seeFIG. 11E).

FIGS. 13 and 14are schematic circuit diagrams of filters1000and2000, respectively, according to embodiments. Bulk acoustic wave resonators1100,1200,2100,2200,2300and2400used in the filters ofFIGS. 13 and 14may correspond to the bulk acoustic wave resonators10through10gaccording to the various embodiments disclosed herein.

Referring toFIG. 13, the filter1000is formed in a ladder type filter structure. In detail, the filter1000includes first and second bulk acoustic wave resonators1100and1200.

The first bulk acoustic wave resonator1100is connected between a signal input terminal to which an input signal RFin is input and a signal output terminal from which an output signal RFout is output, in series, and the second bulk acoustic wave resonator1200is connected between the signal output terminal and a ground.

Referring toFIG. 14, a filter2000according to another embodiment is formed in a lattice type filter structure. In detail, the filter2000includes bulk acoustic wave resonators2100,2200,2300, and2400. The filter2000filters balanced input signals RFin+ and RFin−, and outputs balanced output signals RFout+ and RFout−.

As set forth above, in the bulk acoustic wave resonators according to the disclosed embodiments, formation of a crack in a film or a layer stacked on a substrate may be prevented, and normal growth of crystals may be induced.

In addition, in the bulk acoustic wave resonators according to the disclosed embodiments, a variation in a frequency depending on a temperature change is reduced, and the bulk acoustic wave resonator is robust with respect to stress acting in an upward direction and a downward direction.