Thin-film SAW devices

Abstract

The thin-film SAW device includes a carrier substrate (CA), a TCF compensation layer (CL), a piezoelectric layer (PL), and an IDT electrode (EL) on top of the piezoelectric layer. A functional layer (FL) is arranged between the piezoelectric layer and the TCF compensation layer to further reduce the TCF. Taking into account the speed of sound, density, and stiffness, the material properties of the functional layer are matched to those of the piezoelectric layer, so that the material properties of the functional layer and the piezoelectric layer do not deviate from each other by more than 10% in the absence of the piezoelectric effect. The functional layer can have the same crystal structure as the useful piezoelectric layer without piezoelectric properties.

Description

Background Art

[0001] Next generation mobile communication devices require excellent performance and characteristics, such as high electromechanical coupling coefficient k 2 and low frequency temperature coefficient TCF.

[0002] The SAW filter device presented on the sandwich substrate system inherently provides a high coupling coefficient due to the sagittal waveguide effect provided. One of the layers of the conventional multilayer substrate system is a SiO2 layer that provides a reduction in TCF. Therefore, this SiO2 layer is used as a TCF compensation layer, and its thickness can be adjusted to achieve the desired TCF reduction. However, the compensation effect is usually too small. Moreover, a higher thickness of the SiO2 layer, which only has poor acoustic performance, leads to more spurious modes (e.g., bulk acoustic modes), which is undesirable for generating interfering resonances in adjacent frequency bands of higher frequencies. In addition, for certain frequency bands that require smaller bandwidths, high coupling in the sandwich substrate system may be disadvantageous. For these narrow frequency bands, external circuit devices (e.g., capacitors) are required to reduce the bandwidth again. This in turn leads to higher area consumption, which is contrary to the common miniaturization requirements.

[0003] Therefore, there is a need for a SAW device with a compensated TCF without increasing the layer thickness of the SiO2 layer. Summary of the Invention

[0004] This and other objects are solved by a thin film SAW device and a manufacturing method according to the independent claims.Particular features and advantageous embodiments are subject matter of the dependent claims.

[0005] The general idea is to provide a thin-film SAW device that includes an additional functional layer. This can be an additional layer near or immediately adjacent to the piezoelectric layer of the thin-film SAW device and has mechanical properties very similar to those of the piezoelectric layer. However, this functional layer is not piezoelectric.

[0006] Such a layer then has outstanding acoustic properties and the acoustic waves propagate at least partially in this functional layer. As a result, due to the lack of the piezoelectric effect, the coupling is reduced and thus the electromechanical coupling coefficient k 2 This is advantageous for SAW filters, which are designed to operate in a narrow bandwidth in a frequency band having a narrow bandwidth. Otherwise, the bandwidth must be reduced with the help of circuit elements such as external capacitors, which would require additional space and / or chip area. In addition, such external elements reduce the quality factor Q of the entire device due to their low quality factor Q. With the proposed functional layer and the resulting reduced coupling factor, such circuit devices are not required when designing narrowband SAW filters.

[0007] The temperature dependence of the piezoelectricity is one of the main contributions to the negative TCF of the device. The lack of piezoelectricity of the additional functional layer leads to an improvement in the TCF of the combined layer stack. As a result, the previously highly negative TCF is converted into a more positive value compared to the layer systems commonly used to date. Therefore, even in layer systems without sufficient TCF compensation, the functional layer can allow improved TCF compensation to provide very low final TCFs. In addition, the functional layer allows the thickness of conventional TCF compensation layers, which are SiO2 layers, to be reduced. By reducing the thickness of SiO2 layers with poor acoustic properties through the insertion of the functional layer, the acoustic properties of the entire layer system of the thin-film SAW device can be improved. As an additional advantage, the occurrence of spurious modes can be reduced due to the lower overall layer thickness.

[0008] If necessary, the coupling coefficient k can be compensated by reducing the thickness of the piezoelectric layer 2 As a result, the occurrence of stray plate and bulk modes is further reduced.

[0009] This new thin-film SAW device consists of a carrier substrate, a TCF compensation layer, a piezoelectric layer, and an electrode structure on top of the piezoelectric layer. The functional layer is arranged between the piezoelectric and TCF compensation layers. Compared to conventional thin-film SAW devices, the thickness of the piezoelectric and TCF compensation layers can be reduced, while achieving at least the same TCF compensation.

[0010] Advantageously, the material properties of the functional layer are matched to the material properties of the piezoelectric layer with respect to the speed of sound, density and stiffness, with a deviation of less than 10%.

[0011] More preferably, the functional layer comprises the same material as the piezoelectric layer but does not exhibit a piezoelectric effect, for example due to a special thermal treatment, mechanical treatment, electrical treatment or ion bombardment.

[0012] This functional layer can be formed by destroying the structure and thereby the piezoelectric effect in a damaged region of the single-crystal piezoelectric layer. The damaged region can be formed by implanting ions from the top of the piezoelectric layer to the desired depth. However, it is preferred that the damaged region be formed in the piezoelectric wafer before bonding the piezoelectric wafer to the surface of the multilayer substrate system. Preferably, the damaged region is in close proximity to the surface to which it is bonded.

[0013] According to an embodiment, the piezoelectric layer is a single crystal layer of LT or LN and has a thickness dP. The functional layer is a crystalline layer of the same material but does not have a piezoelectric effect. For the thickness dD of the damage layer, the following relationship holds:

[0014] 0.005dP <dD<0.5dP

[0015] In a specific embodiment, the piezoelectric layer is a single-crystalline layer of lithium tantalate (LT) with a thickness dP of 400 nm to 700 nm. The functional layer is then a crystalline LT layer of the same material but without the piezoelectric effect. Based on the above relationship, the thickness dD of the damaged layer is:

[0016] 2nm≤dD<350nm

[0017] Due to the TCF-reducing properties of the functional layer, the thickness of the TCF compensation layer can be reduced compared to known thin-film SAW devices having such a layer, which is typically made of SiO2. An exemplary thickness dC of the TCF compensation layer / SiO2 layer conforms to the following relationship:

[0018] 50nm≤dC<500nm

[0019] Hereinafter, a thin film SAW device will be explained in more detail with reference to specific embodiments and accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic cross section through a thin film SAW device is shown.

[0021] Figure 2 The curve showing the variation of the TCF of a thin film SAW device depending on the thickness of the functional layer is shown.

[0022] Figure 3 The coupling coefficient k of the thin film SAW device is shown to depend on the thickness of the functional layer. 2 's change curve.

[0023] Figure 4 The actual part of the admittance of a thin film SAW device is shown.

[0024] Figure 5 The magnitude of the admittance of the thin film SAW device is shown. DETAILED DESCRIPTION

[0025] Figure 1 A thin film SAW device is shown in cross-section. The figure is schematic only and is not drawn to scale. For better understanding, some details are depicted in an exaggerated form so that both absolute and relative dimensions cannot be obtained from the figure.

[0026] The carrier substrate CA is preferably a wafer made of a mechanically stable, rigid material. Silicon is a preferred material for this purpose. An optional layer having a higher acoustic velocity than SiO2 can be placed on top of the carrier substrate CA. For example, the optional layer can be made of a hard material such as AlN, polysilicon, or amorphous silicon.

[0027] A TCF compensation layer CL (e.g., of SiO2) is applied to this optional layer HV or directly to the carrier substrate CA. This can be accomplished using conventional PVD or CVD processes. However, any other deposition method is also possible. Subsequently, the functional layer FL and the piezoelectric layer PL are deposited as the next layers. According to a preferred embodiment, the deposition of these two layers involves wafer bonding a piezoelectric wafer having an integrally formed functional layer FL on its top surface to the underlying TCF compensation layer CL.

[0028] After reducing the thickness of the piezoelectric layer PL to the desired value dP, an electrode EL is formed on top of the piezoelectric layer PL. The electrode structure EL implements the function of the thin film SAW device and can include an interdigital transducer - IDT - reflector, resonator or any other structure required for the operation of the electroacoustic SAW device (such as a filter function).

[0029] Figure 2 The calculation process of the TCF of a thin-film SAW device with the above-mentioned structure is shown. The TCF is plotted depending on various thicknesses dF of the functional layer FL. The first value corresponds to a device without a functional layer FL and with a thickness dF of zero. Here, the SAW device still shows a negative TCF, which is due to the influence of the piezoelectric layer PL with a strong negative TCF on the TCF of the SAW device. At a thickness dD of 20 nm, it can be assumed that the TCF is greatly reduced. A functional layer with a thickness of about 50 nm can perfectly compensate for the initial negative TCF of the device. With higher thicknesses dF, the TCF becomes positive. The overall variation curve is almost linear and shows that the TCF compensation effect is proportional to the thickness of the functional layer if all other geometric parameters of the other layers of the SAW device remain constant. For the sake of completeness only, the actual value depends on the layer system and the layer thickness. However, the dependence of the TCF on the functional layer thickness dF remains unchanged.

[0030] Since the mechanical properties that control acoustic behavior are the same in the functional layer FL, as in the piezoelectric layer, a large amount of wave energy is concentrated in the functional layer without piezoelectric effect. Therefore, the coupling factor is reduced compared to SAW devices without the functional layer FL.

[0031] Figure 3 The coupling coefficient k of the thin film SAW device is shown as a function of the thickness dF of the functional layer FL. 2 Obviously, the highest value is observed at zero thickness. At higher values ​​of thickness dF, the observed k 2 Since this dependence is also linear, it can be used for applications that require smaller bandwidth and therefore lower k 2 The device setting required k 2If the desired TCF is obtained according to the plotted curve with a thickness dF of the functional layer FL that results in a too high coupling coefficient k, then other structural parameters of the SAW device need to be changed. 2 Then, the thickness dF of the functional layer FL must be increased to first set the desired k 2 Then, the overly positive TCF can be compensated, thereby reducing the thickness dC of the TCF-compensating SiO2 layer CL, and vice versa.

[0032] The proposed SAW device allows the substrate layer system to be optimized according to the desired parameters, without having to look at the desired low TCF or compensated TCF. First, after such optimization, the TCF can be compensated by choosing a suitable thickness dF for the functional layer. As a result, all other design features and the corresponding physical parameters associated with them can be kept constant and unchanged, because they are the results of the optimization. For example, the geometric parameters can be set to values ​​that are less sensitive to the inevitable tolerances in the manufacturing process. In known thin-film SAW devices, in order to achieve a low TCF, a high thickness dC of the SiO2 layer would be required. However, this would excite a large number of acoustic body and plate modes and cause the occurrence of undesirable resonances.

[0033] Next, the admittances of the three devices are compared to show the positive effect of the proposed thin film SAW device behaving as a resonator. Figure 4 shows the actual part of the admittance of the thin film SAW device, while Figure 5 The corresponding magnitudes are shown. According to the prior art, the first curve without complete TCF compensation Figure 1 is assigned to the reference SAW device. The second curve Figure 2 In line with the proposed SAW device, the SAW device has been designed to achieve complete TCF compensation. Finally, the curve Figure 3 In accordance with the SAW device without the functional layer FL, the functional layer FL has been designed to achieve complete TCF compensation by increasing the thickness dC of the TCF compensation SiO2 layer accordingly.

[0034] For this example, the layer systems chosen are:

[0035] -Silicon carrier substrate

[0036] -200nm SiCy

[0037] -50nm damage layer based on non-piezoelectric LT

[0038] -600nm piezoelectric LT

[0039] -Al electrode

[0040] curve Figure 3The layer system described above is consistent with the difference that there is no damage layer and the thickness dC of the SiO2 layer is increased to 400nm, which is 200nm greater than the above example. Therefore, a 50nm damage layer has approximately the same TCF compensation effect as a 200nm SiO2 layer.

[0041] When compared with the curve according to the present invention Figure 2 and the curves of a SAW device with conventional TCF compensation Figure 3 The most significant effect can be seen when . The example chosen is a thin film SAW resonator designed for a resonant frequency of 1900 MHz. All three graphs show a maximum at this frequency. Figure 3 The figure shows additional resonance peaks due to the occurrence of unwanted spurious modes at approximately 2150 MHz and a lower spurious mode at 2210 MHz. Due to the local energy distribution of the wave, the spurious modes are caused by the layer structure of the substrate with a relatively thick SiO2 layer and thus cause spurious modes that are mainly bulk modes to occur. Figure 5 As can be best seen in the Figure 2 These peaks are not shown.

[0042] An additional positive effect can be seen at frequencies around 2800 MHz. Figure 1 and curves Figure 3 Here too, resonances due to further spurious modes in conventional devices are shown. However, the curve Figure 2 The peak value and the curve shown Figure 1 and curves Figure 3 It has been greatly reduced in comparison.

[0043] When compared Figure 5 The curve in Figure 1 and curves Figure 2 The resonant peak and anti-resonant peak values ​​show that, according to the new design, the curve Figure 2 The pole-zero distance PZD and the coupling factor in the MOSFET are reduced by approximately 10 MHz.

[0044] Therefore, the new thin film SAW device provides a higher Q factor, complete TCF compensation and a significant reduction in spurious modes. In addition, coupling is reduced or can be reduced.

[0045] The present invention is not limited by the embodiments but only by the claims. Therefore, further variations of the examples shown are considered to be within the scope of the present invention as long as they are covered by the claims.

[0046] List of reference symbols:

[0047] 1, 2, 3 Examples and Reference Admittance Curves

[0048] CA carrier substrate (silicon wafer)

[0049] CL TCF compensation layer (SiO2 layer)

[0050] dC TCF compensation layer thickness

[0051] Thickness of dF functional layer

[0052] dP thickness of the piezoelectric layer

[0053] EL IDT electrode

[0054] FL functional layer

[0055] PL piezoelectric layer

Claims

1. A thin film SAW device comprising: Carrier substrate CA; a temperature coefficient of frequency (TCF) compensation layer CL arranged on top of the carrier substrate; Piezoelectric layer PL; as well as an IDT electrode EL arranged on top of the piezoelectric layer, wherein the functional layer FL is arranged between the piezoelectric layer and the TCF compensation layer, and wherein the functional layer does not provide a piezoelectric effect, In which, the functional layer is a crystalline layer and includes the same material as the piezoelectric layer, and taking into account the speed of sound, density and stiffness, the material properties of the functional layer match the material properties of the piezoelectric layer, so that the deviation between the material properties of the functional layer and the material properties of the piezoelectric layer does not exceed 10%.

2. The thin film SAW device according to claim 1, The piezoelectric layer is a single crystal layer of lithium tantalate LT or lithium niobate LN with a thickness of dP, For the thickness dF of the functional layer, the following is valid: 0.005dP≤dF≤0.50dP. 3 . The thin film SAW device according to claim 1 , wherein one or more additional layers having a higher acoustic velocity than the TCF compensation layer CL are added between the carrier substrate CA and the TCF compensation layer CL.

4. The thin film SAW device according to claim 1, wherein The piezoelectric layer is a single crystal layer of lithium tantalate LT with a thickness dP of 400nm-700nm, wherein the functional layer is a crystalline LT layer of the same material but without piezoelectric effect, wherein the thickness dF of the functional layer conforms to 2nm≤dF≤350nm.

5. The thin film SAW device according to claim 1 , wherein the TCF compensation layer CL comprises a SiO 2 layer having a thickness dC satisfying the following conditions: 50nm≤dC≤500nm.

6. A method for manufacturing a thin film SAW device, the thin film SAW device comprising: Carrier substrate CA, TCF compensation layer CL, Functional layer FL, piezoelectric layer PL, and The IDT electrode EL on top of the piezoelectric layer Therein, the method comprises wafer bonding of a piezoelectric wafer having an integrally formed functional layer FL on its top surface to an underlying TCF compensation layer CL arranged on the carrier substrate, and The functional layer FL is a damaged area formed by injecting ions from the top of the piezoelectric layer until a desired depth is reached before wafer bonding, so that the material properties of the functional layer match the material properties of the piezoelectric layer taking into account the speed of sound, density and stiffness, so that the material properties of the functional layer and the material properties of the piezoelectric layer deviate from each other by no more than 10%, and the functional layer does not provide a piezoelectric effect.

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