Volume acoustic component and method for manufacturing a volume acoustic component

Abstract

The present invention provides a volume acoustic component (10) comprising a first electrode contact (120) and a second electrode contact (122); a piezoelectric element (100) with at least one first piezoelectric layer (101) as a semiconductor with a first doping and / or at least one second piezoelectric layer (102) as a semiconductor with a second doping, which are arranged between the first electrode contact (120) and the second electrode contact (122) and are stacked in a stacking arrangement or form the first electrode contact (120) and / or the second electrode contact (122) themselves;and a voltage source (SP) which is connected to the first electrode contact (120) and to the second electrode contact (122) and to which a DC or AC voltage can be applied, such that the thickness of a space charge region (RL) in the first piezoelectric layer (101) and / or in the second piezoelectric layer (102) is controllable, wherein the volume acoustic component (10) further comprises a substrate (SB) and an acoustic reflector element (RE), wherein the acoustic reflector element (RE) is arranged in or on the substrate (SB) and the stack arrangement is arranged on the substrate (SB) and / or on the reflector element (RE).

Description

The present invention relates to a volume acoustic component and a method for producing a volume acoustic component.Prior ArtVolume acoustic components can be used as resonators in filters and oscillators, wherein the operating frequencies can be determined by a layer thickness of the piezoelectric layer and the speed of sound in the piezoelectric material. In order that higher working frequencies can be achieved, the layer thickness thereof can be reduced, tolerances playing an important role. However, a capacitance of the component can increase with a smaller layer thickness. In the event that the electromagnetic wave impedance is to be maintained in this case, it may therefore be necessary to simultaneously reduce the component area, wherein acoustic energy may be lost at the edge of the component, however, so that, with a reduction in size of the component, the edge losses may increase quadratically with the operating frequency. In this case, it may be desirable to increase a suitability of volume acoustic components (BAW) for a range above 10 GHz.An alternative to a reduction in the layer thickness can be the excitation of higher modes, which is summarized under the collective term overtoned bulk acoustic resonator (OBAR). However, this concept is susceptible to the ringing of other modes and inefficient in excitation (low piezoelectric coupling).Furthermore, it may be difficult to deposit high-quality piezoelectric layers with a small thickness, since a high defect density may typically be present at the interface to the underlying material, which defect density heals as the layer grows further, i.e. with increasing layer thickness.Furthermore, there may be a use of thick regions with alternating material polarization, wherein a substantial advantage may be that an exciting electric field can directly excite an overtone here, since the exciting force also changes direction with the alternating material polarization. The production of such layers of alternating polarization is demanding. Since the aforementioned concept aims at being able to use resonance that is as high as possible, the electrode thickness plays a decisive role. If the electrode thickness is not small compared to the acoustic wavelength, the mass of the electrode significantly reduces the resonant frequency. The electrode also degrades the quality of the resonator. One possibility is to remove the electrodes from the piezoelectric material and to couple them to the resonator in a capacitive manner via a gap.US 2018 / 085787 A1 describes a volume acoustic component.Disclosure of the InventionThe present invention provides a volume acoustic device according to claim 1 and a method of manufacturing a volume acoustic device according to claim 12.Preferred refinements are the subject matter of the dependent claims.Advantages of the InventionThe idea underlying the present invention is to specify a volume acoustic component and a method for producing a volume acoustic component, wherein a suitability of the volume acoustic component for operation in high frequency ranges, in particular above 10 GHz, can be improved. Higher operating frequencies can be decided and a simple technological realization can be effected.An alternative technology path for establishing higher frequency ranges can be achieved by the volume acoustic component mentioned.According to the invention, the volume acoustic component comprises a first electrode contact and a second electrode contact; a piezoelectric element having at least one first piezoelectric layer as a semiconductor having at least one first doping and / or at least one second piezoelectric layer as a semiconductor having a second doping, which are arranged between the first electrode contact and the second electrode contact and are stacked in a stack arrangement or form the first electrode contact and / or the second electrode contact itself; a voltage source which is connected to the first electrode contact and to the second electrode contact and to which a direct voltage or alternating voltage can be applied in such a way that the thickness of a space charge zone in the first piezoelectric layer and / or in the second piezoelectric layer can be regulated, wherein the volume acoustic component further comprises a substrate and an acoustic reflector element, wherein the acoustic reflector element is arranged in or on the substrate and the stack arrangement is arranged on the substrate and / or on the reflector element.By the invention, avoiding or reducing inefficient excitation of higher modes can be achieved. Furthermore, avoiding or reducing the lowering of the resonant frequency by the mass load of the electrode on the resonator can be achieved. Furthermore, avoiding or reducing technologically challenging layers with alternating polarization or very thin piezoelectric layers can be achieved. Furthermore, a simplified technological realization can be achieved by growing the structures instead of bonding two different material layers and simply tuning the resonant frequency via a DC voltage.Only one piezoelectric semiconductor layer can also be present. This can then form the n-type and p-type regions by doping, which can lead to the space charge zone between the regions (within the same layer). However, it is also possible to use different piezoelectric semiconductors having two or more layers, which can then comprise within their own different dopings or only a specific doping or no doping or an intrinsic zone (other doping) or a combination of these cases.All the features mentioned and advantages and properties relating to a plurality of piezoelectric layers can likewise also apply to the embodiment with only one piezoelectric layer.According to the invention, it is possible to achieve an explanation of higher operating frequencies above, for example, 10 GHz for volume acoustic components, for example resonators.The volume acoustic component according to the invention can, at least according to one embodiment, represent an acoustic high-frequency resonator made of a piezoelectric semiconductor (AlN, GaN, AlGaN, etc.), which can be embodied by the piezoelectric element, and with electrical excitation for frequencies >10 GHz. The one or more layers of the piezoelectric element can be heavily doped in such a way that a space charge zone can form, for example, adjacent to a transition to a respective adjacent semiconductor or metal layer or other doping of the same layer. This may be the case, for example, between a heavily n- and p-doped region, or between a metal and an n- or p-doped region, which may represent a Schottky contact. No free charge carriers are present in said space charge zone, unlike in the rest of the semiconductor, so that the total applied voltage can drop over exactly this space charge zone. In a piezoelectric semiconductor, this can lead to a mechanical expansion being excited exclusively in the space charge zone by the inverse piezoelectric effect, for example by an electric field.In the case where a thickness of the space charge zone is about half an acoustic wavelength, a resonator can thereby be obtained whose acoustic wave length corresponds to a space charge zone.The resonant frequency is determined by the thickness of the space charge zone and not by the piezoelectric layer thickness(m). It should be noted here that the layer thickness should be very thin for high frequencies and is difficult to produce in this way. In the case of a total thickness of the piezoelectric element, the metal layer can be added in the case of an embodiment as a Schottky contact and should be one or a multiple of half the wavelength for the stated effect. A highly doped region of the semiconductor outside the space charge zone (for example in the first and / or second piezoelectric layer) in each case function as an electrode if the corresponding doping results in sufficiently high conductivity.If a harmonic is formed in the overall layer / stack due to the dimensions of the layer(s), which corresponds to more than half an acoustic wavelength, the piezoelectric coupling can be higher than in the case of a conventional OBAR. This is due to the fact that in the OBAR, the applied voltage drops over the entire layer, thereby making an exciting electric field lower.It is therefore possible to achieve a conclusion of higher working frequencies for volume acoustic resonators, which is possible by a simple technological realization. Furthermore, a tunable capability can be effected by an electrical DC voltage and, by modulating the space charge zone thickness by an AC voltage, nonlinear effects can be used for frequency mixing. With the above-mentioned structure, easy electrode patterning can also be achieved.According to a preferred embodiment of the volume acoustic component, this represents an acoustic resonator.According to a preferred embodiment of the volume acoustic component, the acoustic reflector element is an air cavity in the substrate or is arranged as an acoustic Bragg reflector on the substrate and / or in the stack arrangement.According to a preferred embodiment of the volume acoustic component, the piezoelectric element comprises at least one metal layer which abuts the first piezoelectric layer or the second piezoelectric layer and wherein the space charge zone forms in the first piezoelectric layer or in the second piezoelectric layer and adjacent to the metal layer by a doping difference with respect to the metal layer.According to a preferred embodiment of the volume acoustic component, a thickness of the piezoelectric element corresponds to a multiple of half a wavelength of an operating frequency of the volume acoustic component.According to a preferred embodiment of the volume acoustic component, a thickness of the piezoelectric element corresponds to more than half a wavelength of an operating frequency of the volume acoustic component and a harmonic of the operating frequency.According to a preferred embodiment of the volume acoustic component, the first electrode contact is represented by the first piezoelectric layer or by the metal layer and / or the second electrode contact is represented by the second piezoelectric layer or by the metal layer.According to a preferred embodiment of the volume acoustic component, a DC voltage and / or an AC voltage is applied to the piezoelectric element, with which the thickness of the space charge zone can be modulated.According to a preferred embodiment of the volume acoustic component, the piezoelectric element comprises at least one intrinsic region which abuts or is formed in the first piezoelectric layer or the second piezoelectric layer.According to a preferred embodiment of the volume acoustic component, this is a MEMS component and constitutes an acoustic filter.According to a preferred embodiment of the volume acoustic component, it is designed for operation as a resonator at an acoustic frequency of greater than or equal to 10 GHz.The volume acoustic component can advantageously be used for or as volume acoustic resonator component(s), for example as resonators / oscillators, RF filters in the transmission and reception path of a communication system or radar system, gravimetric sensors or others and can relate to or represent MEMS sensors.According to the invention, in the method for producing a volume acoustic component, a substrate is provided and a first electrode contact and a second electrode contact and / or a piezoelectric element are arranged therebetween with at least one first piezoelectric layer having at least one first doping and / or a second piezoelectric layer having a second doping, which are arranged between the first electrode contact and the second electrode contact and are stacked in a stack arrangement or form the first electrode contact and / or the second electrode contact itself; a voltage source is provided, which is connected to the first electrode contact and to the second electrode contact and a direct voltage or alternating voltage can be applied to said first piezoelectric layer, such that the thickness of a space charge zone in the first piezoelectric layer and / or in the second piezoelectric layer can be regulated; providing an acoustic reflector element, wherein the acoustic reflector element is arranged in or on the substrate and the stack arrangement is arranged on the substrate and / or on the reflector element.For production, it is possible in particular to realize epitaxial and monocrystalline growth, for example of GaN or AlGaN layers, which can be realized on various types of substrates, for example of GaN, sapphire, Si and SiC, wherein at least in the case of Si and SiC, one or more intermediate layers may be necessary for lattice matching.The doping can be introduced, for example, by ion implantation, doping by diffusion or, particularly advantageously since it is possible with very high homogeneity, directly during the (epitaxial) growth of the GaN layer. Possible dopants such as Si, Ge, Ti, Zr, Nb or S are suitable for use for n-doped regions and, for example, Mg, Ca, Zn, Mn or Be for p-doped regions. By introducing the dopants during layer growth, a dopant gradient can be introduced into the layer, wherein a gradient in the growth direction or layer thickness direction can be meant. It may be advantageous to dope the region in which the space charge zone is later formed with a concentration that results in the desired thickness of the space charge zone. Below or outside this range, a significantly higher concentration can be selected in order to increase the conductivity and to reduce series resistance and thus also the losses.In the case of an applied alternating voltage, the material properties inside and outside the space charge zone may deviate from one another, which may lead to different acoustic wavelengths inside and outside the space charge zone. It may therefore be necessary to optimize the layer thicknesses accordingly. BAW resonators according to the invention can be characterized in that their mechanical excitation can take place exclusively in the space charge zone due to the piezoelectric effect.Further features and advantages of embodiments of the invention will become apparent from the following description with reference to the attached drawings.Brief Description of the DrawingsThe present invention is explained in more detail below with reference to the exemplary embodiments indicated in the schematic figures of the drawing.The following are shown: FIG. 1 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to an exemplary embodiment of the present invention; FIG. 2 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 3 is a profile of the thickness of the space charge region with the doping concentration; FIG. 4 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 5 ashows a schematic sectional representation of a volume acoustic component according to an embodiment of the present invention; FIG. 5 bshows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 6 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 7 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 8 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 9 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 10 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 11 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 12 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 13 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 14 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention; FIG. 15 shows a schematic illustration of suppression of the resonant frequency by a DC voltage; and FIG. 16 shows a block diagram of method steps of the method for producing a volume acoustic component according to an exemplary embodiment of the present invention.In the figures, identical reference numerals designate identical or functionally identical elements.FIG. 1 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to an exemplary embodiment of the present invention.The piezo element 100 may comprise at least a first piezoelectric layer 101 as a semiconductor with a first doping (for instance in the n-semiconductor) and a second piezoelectric layer 102 (for instance in the p-semiconductor) as a semiconductor with a second doping, which may form the first electrode contact 120 and the second electrode contact 122 itself. In this case, a space charge zone RL can form in the first piezoelectric layer 101 as a result of the doping and a space charge zone lying against it can form in the second piezoelectric layer 102, it being possible for both lying space charge zones to be counted as a total space charge zone RL.On the other hand, the piezoelectric element 100 can also have only the first piezoelectric layer 101 as a semiconductor, which can then itself have a first doping (for example n-contact) and a further doping (for example p-contact) within the layer itself. As a result, the space charge zone can then also form within this one layer. The remaining examples from FIGS. 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 to 14 can also be realized with only one piezoelectric layer, which can have correspondingly doped subregion.The piezoelectric element 100 can thus represent a pn junction in a piezoelectric semiconductor, electrons being introduced in the n-type region by the donor atoms, which electrons do not enter into covalent bonds with the adjacent atoms, i.e. are available for charge transport. Conversely, holes can be introduced in the p-type region by acceptor atoms, which holes can be occupied by electrons. At the pn junction, the space charge zone is formed because electrons diffuse from the n-type region into the p-type region. The donor and acceptor atoms remain, which are then ionized (electrons for charge neutrality can be diffused away). This is associated with an electric field which can form on account of the donor or acceptor atoms. As a result, the free charge carriers (drift current) in this field move counter to the diffusion direction. The size (thickness) of the space charge zone is reached when drift and diffusion currents balance out.FIG. 2 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to a further exemplary embodiment of the present invention.The illustration of FIG. 2 is similar to that of FIG. 1, wherein the piezoelectric element of FIG. 2 comprises, instead of the second piezoelectric layer, a metal layer MS which bears against the first piezoelectric layer 101 and the space charge zone RL forms in the first piezoelectric layer 101 and bears against the metal layer MS, which represents a Schottky contact. In this case, the free carriers may diffuse into the metal MS because the energy level of the conduction band electrons / valence band holes in the n / p type semiconductor of the first piezo layer 101 is higher / lower than that in the metal MS. The thickness of the space charge region RL is approximate (in the static case, the piezoelectric space charge is neglected) determinable by the electron charge, the permittivity, the acceptor concentration, the donor concentration, the diffusion voltage and the externally applied voltage, where q is the electron charge, - the permittivity, NA the acceptor concentration, ND the donor concentration, ΦD the diffusion voltage and U is the externally applied voltage.Now, when a voltage U=UDC+UACsin(ωt) is applied, if φD-UDC>UAC, the space charge region thickness can be regarded as constant. More precisely, this is then determined by the impurity concentration (ND and NA) and by the DC voltage component UDC. A static deflection can also be formed in the space charge zone because of the diffusion voltage in the piezoelectric semiconductor. Due to the high linearity of the mechanical variables, the static and time-dependent deflection can be regarded as a linear superposition.Fig. 3 shows a profile of the thickness of the space charge region having the doping concentration.In the left graph, the example space charge zone thickness RL-D is plotted against donor concentration (logarithmic x-axis scaling) in the GaN semiconductor-metal contact (Schottky contact). For this case, three different voltages are shown for UDC, it being evident that space charge zone thicknesses of less than 50 nm are possible.The thickness of the space charge region can be determined only by the dopant concentration, the diffusion voltage and the applied voltage, and these parameters can easily be kept constant across a resonator and also across a wafer.According to the right-hand image, it can be seen that with a high doping concentration, a sharper separation can be formed between the free and the free charge carrier zone, which is shown by the steep drop of the curves. The decreasing variable relates to the normalized (to the maximum concentration) concentration of free charge carriers. Where the curve drops to zero, the space charge zone (no free charge carriers) is located. The Schottky contact is located at x=500 nm, for example, and a normal ohmic contact is located at x=0 nm. It can also be seen that, as a result of the doping concentration, the space charge zone d becomes thinner, the higher the doping concentration, the smaller d.FIG. 4 shows a schematic sectional illustration of a piezoelectric element for a volume acoustic component according to a further exemplary embodiment of the present invention.FIG. 4 is similar to FIG. 1 or FIG. 2, wherein, however, the piezoelectric element 100 can comprise at least one intrinsic region 200, which can be formed so as to abut the first piezoelectric layer 101. In the case of FIG. 4, this intrinsic region 200 may be located between the first piezoelectric layer 101 and the metal layer MS, wherein a space charge zone RL may form in the first piezoelectric layer 101 towards the intrinsic region 200.When the space charge zone RL becomes thinner due to the doping concentration, such an intrinsic region can be used. The thickness without free charge carriers, i.e. the thickness in which mechanical excitation takes place, is then obtained from the thickness d of the space charge zone RL and the thickness di of the intrinsic region. In this case, the voltage which is necessary for a current flow can become higher. A linear behavior can already be achieved at lower reverse voltages. In the passive case (UDC=0V), the alternating voltage can have higher amplitudes. The intrinsic region may correspond to a region of the first or second (or further) piezoelectric or semiconductor layer which may have no or a low base doping compared to the residual region of the layer.A high applied negative DC voltage may provide high linearity and a thick space charge zone. If a passive (i.e. UDC=0V) approach is present, the diffusion voltage can be very high (φD>>UAC). A high dopant or acceptor concentration can cause a sharp and thin space charge zone and high conductivity outside it.FIG. 5 ashows a schematic sectional representation of a volume acoustic component according to an embodiment of the present invention.The sectional view shows a schematic section through the layers of the bulk acoustic component 10.The volume acoustic component 10 comprises a first electrode contact 120, which may correspond to the first piezo layer 101 (approximately n-type), and a second electrode contact 122, which may correspond to a metal layer MA and may form a piezo element 100.Furthermore, an ohmic contact EK can be applied laterally to the side and from the stack of the first piezoelectric layer 101 on the first piezoelectric layer 101 and contact the latter. In the first piezoelectric layer 101 and in contact with the metal layer MS, the space charge zone RL can form. The first piezoelectric layer 101 can be arranged on a substrate SB and between these two the first piezoelectric layer 101 can extend over a cavity as (acoustic) reflector element RE on lateral remaining regions of a sacrificial layer SL made of SiO2, for example.The invention can be used advantageously for volume acoustic resonator components such as resonators / oscillators, RF filters or others and relate to MEMS components for RF systems in the mobile radio (filter) or also in the radar (oscillators) range.The space charge region RL may thus be realized by a Schottky contact with the metal layer MS, which may form the upper electrode 122. The metal thickness and the doped semiconductor thickness (space charge zone RL and free charge carrier zone) together form the resonant body and should in total correspond to a half wavelength of the effective acoustic wave. In this case, the metal layer MS and the layer with free charge carriers can become as thin as possible, since the acoustic wave is excited only in the space charge zone RL and the ratio between space charge thickness and half wavelength thus critically determines the excitation (qualitative plot of the mechanical deflection to the right next to the stack in FIG. 5 a). The layers mentioned should not become too thin, since otherwise the electrical conductivity deteriorates.For producing such a structure, growth of an n-doped GaN layer can be used, wherein this can be effected, for example, with Si-doped, on a sacrificial substrate. A layer transfer can then take place onto an Si wafer with a structured passivation layer, for example made of SiO2. The GaN layer can then be patterned, for example by plasma etching, for example by means of Cl chemistry with stop in the GaN layer. The etching depth must correspond at least to the thickness of the space charge zone RL in order to laterally isolate the space charge zone from the electrical contact EK. In this case, the contact point for the ohmic contact EK can be placed lower or only an insulation trench can be introduced between space charge zone and contact zone EK. The isolation trench or the recess need not necessarily be implemented with respect to depth as shown in FIG. 5 a, which means that the etching depth does not have to correlate with the extent of the space charge zone, because it can also be etched deeper or less deeply. Furthermore, a metallization for the ohmic contact EK can be applied and patterned, and optionally another metallization for the Schottky contact (for example illustration of the layer MS). Typical metals for an ohmic contact on n-doped GaN are e.g. Ti, Al, Ti / Al and the local dopant concentration under the ohmic contact can be additionally increased by local ion implantation for a lower contact resistance. Schottky contacts can be realized in n-doped GaN typically via Pt, Ni, Ni / Au.Alternatively, an n-doped GaN layer can be grown on Si by means of intermediate layers, e.g. AlN and AlGaN. Here too, the GaN layer can be patterned, for example by plasma etching, for example by means of CI chemistry with stop in the GaN layer, and a metallization EK for the ohmic contact and optionally another metallization for the Schottky contact can be applied and patterned.In this alternative, the intermediate layers can furthermore be removed under the resonator, for example by etching through the Si wafer with stop on the intermediate layers and subsequent selective removal of the intermediate layers. In this case, the passivation between Si substrate and GaN layer may be, for example, an AlN layer.Other components of the other exemplary embodiments can also be produced analogously.FIG. 5 bshows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.The volume acoustic component 10 of FIG. 5 bcorresponds to that of FIG. 5 a, wherein the side regions of the passivation layer SL and the cavity as a reflector element can be replaced by a layer stack of an acoustic Bragg reflector, which can be located below the first piezo layer 101 (first electrode 120) and on the substrate SB. Such a surface mount resonator SMR can also be used in any of the following examples.FIG. 6 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.The volume acoustic component from FIG. 6 has similarities to the component from FIG. 5 a, but with the difference that instead of the metal layer MS on the upper side a second piezo layer 102 is arranged, which can be connected via a side contact EK and itself can serve as upper electrode 122. In this example, the first piezo layer 101 (also assuming the role of the lower electrode 120) may be an n-type and the second piezo layer may be a p-type. As a result, a space charge zone RL can be formed in both piezoelectric layers 101 and 102. The function as an electrode can be realized with high doping concentrations.FIG. 7 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.In order to avoid too thin a space charge zone due to high doping concentrations, an intrinsic region 200 may be inserted into the stack. This embodiment is shown in FIG. 7 for the example according to FIG. 5 a, wherein a Schottky contact with the metal layer MS is shown as the upper electrode 122, and for the corresponding embodiment according to FIG. 6, an intrinsic region 200 according to FIG. 8 is inserted between the first piezoelectric layer 101 and the second piezoelectric layer 102, wherein the space charge zone RL (also comprising the intrinsic region 200) expands in the region RL- 1 into the first piezoelectric layer 101 and expands in the region RL- 2 into the second piezoelectric layer 102.FIG. 9 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.The resonators of FIGS. 1, 2, 3, 4, 5, 6, 7 to 8 can come to their limit for very high frequencies, since the thicknesses of the electrodes (regardless of whether metallic or realized as doped semiconductor) can become so thin that a technological realization can become difficult and the exciting region (space charge zone) can become too small due to the thicknesses of the electrodes. Furthermore, the entire stack can become so thin that it can be implemented only with difficulty or no longer with technological measures. To remedy this, any desired harmonic of the layer stack can be used, which can be the case for a Schottky contact in the embodiment according to FIG. 9. The construction of the bulk acoustic component can correspond to that of FIG. 5 a, with the difference that a thicker first piezoelectric layer 101 is used, which can correspond to the thickness of a harmonic. FIG. 10 shows this analogously for a pn junction of the structure according to FIG. 6, wherein the first piezoelectric layer 101 and the second piezoelectric layer 102 can be correspondingly thicker for the harmonics. FIGS. 9 and 10 show this thickness by way of example for a third harmonic.The great advantage over a conventional OBAR is that modal electromechanical coupling can improve for higher modes, one reason for this being that the total voltage falls only across the space charge zone and not across the entire stack. For this reason, the electric field cannot act against the deflection.Simplified for a 1D layer stack (in the z direction), the modal electromechanical coupling can be calculated by an integral over the layer thickness l from n of the mode number (n=1 is the fundamental oscillation and n=2 is the first harmonic), e33 of the piezoelectric coupling constant, Sn of the amplitude of the strain and Ez of the electric field (via the sin(nπz / l)). If an alternating voltage V is now applied to the OBAR and also to the resonator, Ez,OBAR=V / l approximately results for the electric field in the first case and Ez,RLZ=V / d / =nV / l results for the latter.The coupling of the OBAR is always zero for even n and remains constant for odd n. In contrast, the coupling of the resonator according to the invention is the coupling, which can be improved with n.FIG. 11 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.As shown in FIGS. 11 and 12, an intrinsic region 200 can also be used when applying the harmonics, wherein FIG. 11 shows this for a Schottky contact and for the embodiment according to FIG. 5 a(only according to FIG. 11 with thicker first piezoelectric layer 101) and FIG. 12 shows this for a pn junction, correspondingly for the embodiment according to FIG. 8.FIG. 13 shows a schematic sectional illustration of a volume acoustic component according to a further exemplary embodiment of the present invention.For high frequencies, the space charge region may become very thin and, as a result, the junction capacitance may increase, which may result in a low impedance level. This can be reduced or avoided in that in the Schottky version according to FIG. 9 of the resonator the lower electrode 120 can likewise be realized with a Schottky contact to the piezoelectric semiconductor 101, in other words a metal layer MS can also be connected to the first piezoelectric layer 101, wherein then the lower metal layer MS can be contacted laterally by an ohmic contact EK. In this way, a lower space charge zone RL may form in the lower region of the first piezo layer 101 and an upper space charge zone RL- 2 may form in the upper region of the first piezo layer 101. According to FIG. 14, it can be seen that, in the case of a pn contact from FIG. 10, even any desired number of pn junctions can be realized, wherein correspondingly a plurality of space charge zones can form therebetween.FIG. 15 is a schematic diagram showing suppression of the resonance frequency by a DC voltage.Figure 15 on the left shows the strain at resonance in a piezoelectric semiconductor (oriented along the z-axis) when no DC voltage is applied. The 5thharmonic is present. The hatched areas (positive direction as shown in the left image) are the areas where space charge zones are present. This means that they are also those regions in which the piezoelectric coupling takes place (the electric field is not zero there). For a DC voltage of zero volts, the ranges are of equal size. The coupling is obtained as an integral over the product of the direct electric field (constant) and the elongation. Since both peaks are positive, the coupling is also positive. In the case of the right image, when a DC voltage is applied which is not zero, namely in such a way that one space charge zone just disappears (starts conducting) and the other becomes twice as large, the coupling becomes zero. This is because the above integral becomes zero (area balance becomes zero because the positive areas with their area equal to the negative side of the hatched areas). However, this means that no resonance can be excited, so that the resonator is "switched off".In both cases of Figs. 13 and FI. In the case of FIG. 14, the applied direct voltage should be zero, since otherwise the space charge zones can become of different thickness and are therefore no longer suitable for exciting individual resonances.This fact can also be advantageously used to switch off the resonator, since a DC voltage UDC=Uofffor which the coupling can become zero, which is shown in FIG. 15.Coupling may not improve because the applied voltage may divide equally across all space charge zones in these cases.The resonators presented can also be used as frequency-generating components (e.g. as mixers). Not only is a direct voltage and the useful alternating voltage applied, but also a further alternating voltage (local oscillator LO).This modulates the space charge thickness (ULO is sufficiently large) as a function of the angular frequency ωLO and leads to strong non-linearities which in turn generates usable non-modulation products (e.g. ω±ωLO).FIG. 16 shows a block diagram of method steps of the method for producing a volume acoustic component according to an exemplary embodiment of the present invention.In the method for producing a volume acoustic component, a provision S 1 of a substrate and arranging a first electrode contact and a second electrode contact and / or a piezoelectric element therebetween with at least one first piezoelectric layer) and / or a second piezoelectric layer, which are arranged between the first electrode contact and the second electrode contact and are stacked in a stack arrangement or form the first electrode contact and / or the second electrode contact itself; a provision S 2 of a voltage source, which is connected to the first electrode contact and to the second electrode contact and to which a direct voltage or alternating voltage can be applied, such that the thickness of a space charge zone in the first piezoelectric layer and / or in the second piezoelectric layer can be regulated; providing S 3 an acoustic reflector element, wherein the acoustic reflector element is arranged in or on the substrate and the stack arrangement is arranged on the substrate and / or on the reflector element.Although the present invention has been fully described above with reference to preferred exemplary embodiments, it is not restricted thereto, but can be modified in a variety of ways.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedUS 2018 / 085787 A1

[0006]

Claims

Volume acoustic component (10) comprising - a first electrode contact (120) and a second electrode contact (122); - a piezo element (100) having at least one first piezoelectric layer (101) as a semiconductor having at least one first doping and / or at least one second piezoelectric layer (102) as a semiconductor having a second doping, which are arranged between the first electrode contact (120) and the second electrode contact (122) and are stacked in a stack arrangement or form the first electrode contact (120) and / or the second electrode contact (122) itself; and - a voltage source (SP) which is connected to the first electrode contact (120) and to the second electrode contact (122) and wherein a direct voltage or alternating voltage can be applied to the latter such that the thickness of a space charge zone (RL) in the first piezoelectric layer (101) and / or in the second piezoelectric layer (102) can be regulated, wherein the volume acoustic component (10) furthermore comprises a substrate (SB) and an acoustic reflector element (RE), wherein the acoustic reflector element (RE) is arranged in or on the substrate (SB) and the stack arrangement is arranged on the substrate (SB) and / or on the reflector element (RE).The volume acoustic device (10) according to claim 1, constituting an acoustic resonator.Volume acoustic component (10) according to claim 1 or 2, wherein the acoustic reflector element (RE) is an air cavity in the substrate (SB) or is arranged as an acoustic Bragg reflector on the substrate (SB) and / or in the stack arrangement.Volume acoustic component (10) according to one of Claims 1 to 3, in which the piezoelectric element (100) comprises at least one metal layer (MS) which bears against the first piezoelectric layer (101) or against the second piezoelectric layer (102), and wherein the space charge zone (RL) forms in the first piezoelectric layer (101) or in the second piezoelectric layer (102) and adjacent to the metal layer (MS) by a doping difference with respect to the metal layer.The volume acoustic component (10) according to any one of claims 1 to 4, wherein a thickness of the piezo element (100) corresponds to a multiple of a half wavelength of an operating frequency of the volume acoustic component (10).The volume acoustic component (10) according to any one of claims 1 to 4, wherein a thickness of the piezo element (100) corresponds to more than half a wavelength of an operating frequency of the volume acoustic component (10) and a harmonic of the operating frequency.Volume acoustic component (10) according to one of Claims 1 to 6, in which the first electrode contact (120) is represented by the first piezoelectric layer (101) or by the metal layer and / or the second electrode contact (122) is represented by the second piezoelectric layer (102) or by the metal layer.Volume acoustic component (10) according to one of Claims 1 to 7, in which a DC voltage and / or an AC voltage is applied to the piezoelectric element (100), with which the thickness of the space charge zone (RL) can be modulated.Volume acoustic component (10) according to one of Claims 1 to 8, in which the piezoelectric element (100) comprises at least one intrinsic region (200) which bears against the first piezoelectric layer (101) or against the second piezoelectric layer (102) or is formed therein.Volume acoustic component (10) according to one of Claims 1 to 9, which is a MEMS component and constitutes an acoustic filter.Volume acoustic component (10) according to one of Claims 1 to 10, which is designed for operation as a resonator at an acoustic frequency of greater than or equal to 10 GHz.Method for producing a volume acoustic component (10), comprising the steps of: - providing (S1) a substrate (130) and arranging (S2) thereon a first electrode contact (120) and a second electrode contact (122) and / or a piezo element (100) therebetween with at least a first piezoelectric layer (101) with at least a first doping and / or a second piezoelectric layer (102) with a second doping, which are arranged between the first electrode contact (120) and the second electrode contact (122) and are stacked in a stack arrangement or form the first electrode contact (120) and / or the second electrode contact (122) itself; providing (S2) a voltage source (SP) which is connected to the first electrode contact (120) and to the second electrode contact (122) and wherein a direct voltage or alternating voltage can be applied to the latter such that the thickness of a space charge zone (RL) in the first piezoelectric layer (101) and / or in the second piezoelectric layer (102) can be regulated; providing (S3) an acoustic reflector element (RE), wherein the acoustic reflector element (RE) is arranged in or on the substrate (SB) and the stack arrangement is arranged on the substrate (SB) and / or on the reflector element (RE).

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