SAW device designed for high frequencies

The passband frequency of a typical SAW filters is primarily dependent on the pitch (finger-finger distance) of its built- in IDT's (IDT = interdigital transducer) and hence is limited to not exceed a maximum frequency due to photo-lithography limitations while fabricating such IDT's. A typical maximum frequency is around 3-4 GHz. Up to now higher frequencies can only be operated with devices other than SAW that is with BAW devices. With SAW devices it is not possible to build up pass band at frequencies that substantially exceed a limit around 5 GHz

It is hence an object to provide a SAW device that is not bound to this upper frequency limit and can operate at higher frequencies .

This and other objects are met by a SAW device according to claim 1. Specific further features of the new SAW device as well as advantageous embodiments can be taken from the dependent claims.

The general idea is to use a higher mode like the plate mode PM of a SAW resonator to build a passband/rej ect-band filter, functioning at higher frequencies, which may not be

realizable with SAW devices that are using only the main mode MM of the resonator.

The passband range of a typical SAW filters is dominantly dependent on the pitch or finger distance of its built-in IDT's and hence is bounded at the higher frequency side due to photo-lithography limitations on fabrication of such

IDT's, typically around 4-5 GHz.

However, resonators using certain layer stacks containing metal, SiCy and SiN on LiNbCy Substrates possess, beside the main mode, also a second mode, called z-mode or plate-mode. The resonance frequency and the amplitude of the PM depends on geometric parameters of the resonator like pitch and metallization ratio as well as on the heights of single layers in the stack. In particular, the resonance frequency of the PM is mostly much higher than that of MM, typically around 20-30% above the main mode. The amplitude of PM mainly depends on the height of SiCy and SiN layer. Depending on the underlying substrate and layer stack, the resonance frequency of PM may have even a higher Q factor than that of MM.

On the other hand, filters utilizing the plate mode PM of a resonator are expected to have a higher power-durability, since PM is mainly located within the dielectric layer above the IDT. Hence, wave energy does not affect the IDT as much as the main mode that propagates mainly within the piezo layer and directly in the plane of the IDT.

The new SAW device has a layer stack comprising a substrate with a piezoelectric layer on top or consists of a

piezoelectric bulk material. An interdigital transducer - IDT - is arranged on top of the substrate and a SiCy layer covers the surface of the IDT and the connecting pads. The height of the SiCy layer and optionally the height of further layers that are part of the stack like the above mentioned SiN layer is/are set to a value that facilitates exciting of a

substantial amount of a respective plate mode that can propagate in the layer stack system. The SAW device is construed to make use of the resonance frequency of the plate mode for filtering purpose.

The substrate may be a monocrystalline piezoelectric chip of a suitable cut angle. Alternatively, the substrate may comprise a carrier and a thin film of a piezoelectric applied on the carrier.

An SiCy layer is often used for compensating the temperature drift of frequency or the TCF. Such TCF compensated SAW filter are usually formed on a LiNbCy substrate - LN - due to the better coupling of this piezo material. In a SAW device formed on a LN material with a cut angle of e.g. ROT 128 a plate mode is excited and can propagate.

Starting from a given SAW device on such a piezo material design parameter like metallization ratio, pitch, and heights of any or all dielectric layers that are present in the layer stack can be varied to shift the frequencies of main mode MM, plate mode PM and the relative signal amplitudes thereof. By doing such design variations it is possible to make the plate mode become a dominant mode. Any stages between excitation of only a main mode, a main mode and a plate mode at the same time and only plate mode are possible and be set dependent on the desired use of plate mode and/or main mode.

According to an embodiment of the SAW device the resonance frequency of the plate mode is used for forming a pass band of a filter. Hence, the SAW device comprises at least one resonator that is designed such that the plate mode has substantial amount, is dominating or is even the mode that can mainly propagate in the layer stack. In another embodiment the SAW device is construed as a ladder type filter comprising resonators that are designed to allow excitation of main mode and plate mode at the same time. The resonance frequencies of both existing modes that is plate mode and main mode are used for forming a dual pass band filter. Such a band pass filter works optimally if the amplitudes of both modes are set to be comparable or equal. Shifting one or both resonance frequencies of the two modes as mentioned above can provide a SAW filter with two pass bands of a wide range of possible distances in frequencies.

The new SAW device is not restricted to band pass filter but can also be embodied as a band reject filter. Such a filter can be construed as a ladder type filter comprising series and or parallel resonators that are designed to allow

excitation of a main mode and a plate mode at the same time. In this embodiment the resonance frequencies of plate mode and main mode are used for forming a dual reject band filter.

But even if the plate mode is not the dominant mode of a filter nevertheless it can be used along with the main mode. According to an embodiment the SAW device is a ladder type filter construed out of SAW resonators comprising series and parallel resonators. At least one of the resonators is designed to allow excitation of the plate mode. While the resonance frequency of the main mode is used for forming the pass band of the filter the resonance frequency of the plate mode is used for improving the selectivity of the filter. Setting the main mode at the pass band frequency and shifting the resonance of the plate mode of at least one parallel resonator that enables exciting a plate mode to a frequency where additional attenuation is required results in a filter with improved selectivity above the passband. Alternatively the selectivity below the pass band may be improved. According to an embodiment the SAW device is a ladder type filter construed out of SAW resonators comprising series and parallel resonators. At least one of the series resonators is designed to allow excitation of the plate mode the resonance frequency of the plate mode is used for forming the pass band. At the same time the resonance frequency of the main mode is used for improving the selectivity of the filter at the resonance frequency of the main mode that is usually below the pass band frequencies.

The SAW filter may be construed as a band reject filter. In such a filter SAW resonators are arranged in a ladder type arrangement of series and parallel resonators thereby

respecting a suitable resonance frequency position of series and parallel resonators.

Further it is possible use a resonator showing a main mode and a plate mode as well. The resonance frequency of the plate mode of this resonator is used to create a notch by circuiting the resonator as a parallel resonator in in a common ladder type filter circuit. By suitably shifting the resonance frequency of the plate mode the plate mode can be set just at or near a frequency where a higher suppression of a parasitic signal is required.

The main advantage that can be achieved with most of the embodiments is that the resonance frequency of the plate mode is much higher than that of the main mode. When a SAW device is construed with an IDT of minimum pitch that is a pitch that can still safely reproduced in a manufacturing process, the plate mode excited with this minimal pitch lies at a frequency well above the main mode that is usually excited in such a filter. Such high operating frequencies have up to now not been achieved with known filters. The proposed SAW resonator can hence be used for construing a pass band filter or a band reject filter with a passband/rej ect band frequency higher than a maximum frequency up to now achieved with a SAW device. The maximum frequency is the resonance frequency of a resonator using its main mode which is optimized for the highest possible frequency that can be safely reproduced with a given structuring method.

According to a further embodiment the plate mode producing SAW device can be used not only for construing complete filters but can only be used to improve the selectivity of any other filter circuit. Hence, a SAW resonator with a substantial amount of plate mode can be used in a circuit comprising a DMS filter as a main filter component. The plate mode of the resonator is used to improve the selectivity of the DMS filter at a frequency above the pass band.

In the following the invention will be explained in more detail with respect to embodiment and the accompanied

figures .

Figures 1A to 1C show the admittance of a SAW resonator comprising a layer stack depicted threefold as real part, imaginary part and magnitude wherein the height of a layer of the layer stack is varied.

Figures 2A to 2C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW filter with two

comparable passbands. Figures 3A to 3C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW band reject filter with two comparable reject-bands.

Figures 4A to 4C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW passband with improved selectivity below the passband.

Figures 1A to 1C show the admittance of a SAW resonator with a similar layer structure comprising a layer stack with a piezo layer, an IDT, a SiCy layer and a SiN layer as trimming and/or passivation layer depicted threefold as real part, imaginary part and magnitude wherein the height of a layer of the layer stack is varied. In the upper part - Figure 1A - the real part of the admittance of this resonator is

depicted. Figures IB and 1C show respective imaginary part and magnitude thereof. Each diagram shows three curves a to c that are measured with a different height of the Si02 layer. The height is varied between 460 and 520 nm. The figures clearly show that signal amplitude and frequency of the plate mode can be set by suitably setting the height of the Si02 layer. For each curve a to c two admittance maxima are present. The peak on the low frequency side at about 2700 MHz is assigned to the respective main mode while the plate mode arises at frequencies between about 3200 and 3400MHz.

Figures 2A to 2C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW bandpass filter showing two comparable passbands. The filter is construed in a common manner as a ladder type arrangement of series and parallel SAW resonators. However, the used resonators are configured according to the proposed idea and show two comparable signal amplitudes that are assigned to a main mode and a plate mode. Accordingly the resulting ladder type filter shows two passbands of nearly the same amplitude. Such dual passband filters can be used as duplexer or multiplexer having a much simpler topology as no separate resonators for Tx and Rx are required .

Figures 3A to 3C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW band reject filter showing two comparable reject bands. The filter is construed in a common manner as a ladder type arrangement of series and parallel SAW resonators. However, the used resonators are configured according to the proposed idea and respectively show two comparable signal amplitudes that are assigned to a main mode and a plate mode.

On the other hand, filters utilizing PM are expected to have a higher power-durability, since PM is mainly located within the dielectric layer above the IDT.

Besides the main application, the PM can also be combined with the main-mode simultaneously, to create a dual passband filter or a dual notch-filter, or to improve the selectivity on the right or left side of the passband/rej ect-band of a filter .

Figures 4A to 4C show the P-matrix elements S21, Sll and S22 and respective Smith charts of a SAW bandpass filter with improved selectivity below the passband. Here the plate mode of the SAW resonators is used to form the passband. The main mode of some resonators is used to further improve the lower stop band by using the resonance frequency of the respective main mode of some resonators that are mainly circuited as parallel resonators in respective shunt arms of the ladder type filter circuit.

Deviating from the above examples it is possible to use the general idea for construing a bandpass filter of the DMS type by using the plate mode of the DMS transducers to achieve a pass band at a frequency higher that up to now possible with a SAW filter. Alternatively, the main mode of a DMS filter's resonators can be used for creating the passband and the plate mode of the resonators for improving the selectivity.

On the other hand, matching elements such as inductors and capacitors can be connected to the resonators (in series or parallel) to shift the resonance or anti-resonance of the main mode to set it to the desired frequencies. The invention has been shown by reference of a limited number of embodiments but is not restricted to specific features of any embodiment. Additionally the invention can generally be applied to BAW filters as well. The embodiments are only illustrative and the scope of the invention is only defined by the claims.