Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/15—Constructional features of resonators consisting of piezoelectric or electrostrictive material
  • H03H9/17—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator
  • H03H9/171—Constructional features of resonators consisting of piezoelectric or electrostrictive material having a single resonator implemented with thin-film techniques, i.e. of the film bulk acoustic resonator [FBAR] type
  • H03H9/172—Means for mounting on a substrate, i.e. means constituting the material interface confining the waves to a volume
  • H03H9/175—Acoustic mirrors
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/02—Details
  • H03H9/02007—Details of bulk acoustic wave devices
  • H03H9/02086—Means for compensation or elimination of undesirable effects
  • H03H9/02149—Means for compensation or elimination of undesirable effects of ageing changes of characteristics, e.g. electro-acousto-migration
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/46—Filters
  • H03H9/54—Filters comprising resonators of piezoelectric or electrostrictive material
  • H03H9/58—Multiple crystal filters
  • H03H9/582—Multiple crystal filters implemented with thin-film techniques
  • H03H9/583—Multiple crystal filters implemented with thin-film techniques comprising a plurality of piezoelectric layers acoustically coupled
  • H03H9/585—Stacked Crystal Filters [SCF]
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03H—IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS
  • H03H9/00—Networks comprising electromechanical or electro-acoustic elements; Electromechanical resonators
  • H03H9/46—Filters
  • H03H9/54—Filters comprising resonators of piezoelectric or electrostrictive material
  • H03H9/58—Multiple crystal filters
  • H03H9/582—Multiple crystal filters implemented with thin-film techniques
  • H03H9/586—Means for mounting to a substrate, i.e. means constituting the material interface confining the waves to a volume
  • H03H9/589—Acoustic mirrors

Definitions

• a membrane can also be arranged as a thin carrier layer between the lower electrode and the substrate. This arrangement is also called a bridge-type resonator.
• Another disadvantage of a BAW resonator of the mirror type lies in the complexity of the processes for the deposition and structuring of the multilayer structure required for this.
• Each ⁇ / 4 layer increases the complexity and thus the cost of the manufacturing process.
• the errors also increase, so that a considerable spread of the resonance frequencies of the resonators and thus the center frequency of filters can be accepted over an entire wafer.
• Layers with a high dielectric constant can lead to a coupling of electrical signals to the substrate, which leads, for example, to undesired crosstalk and to an increase in insertion loss.
• the acoustic impedance Z is according to the formula calculated, these two low values c and p result in extremely low acoustic impedances which, in combination with materials of high impedance Z, result in the highly reflective acoustic mirror according to the invention.
• the reflectivity of a single ⁇ / 4-layer pair is sufficient to produce a good acoustic mirror for a BAW resonator.
• layer pairs with layer thicknesses of odd-numbered Chen of ⁇ / 4 are understood, as well as layers that differ slightly from this value.
• the acoustic reflectivity at the interface between the lower electrode and the low-k dielectric alone is over 90% if Au is selected as the material for the lower electrode.
• the acoustic reflectivity at the Au / low-k interface is only 40%.
• the acoustic refectivity R at the interface between two layers 1 and 2 is calculated using the formula
• Low-k dielectrics preferred for the invention are polyaromatic polymers which are obtained, for example, by polymerizing monomers substituted with cyclopentadienone and acetylene. Ren of the polyphenylenes or bisbenzocyclobutene monomers are derived.
• the low-k dielectric SiLK has a glass transition temperature (maximum temperature stability) of more than 490 ° C, an elastic constant of about 2.45 GPa, an average density of 1.2 g / cm A 3 and thus an acoustic impedance of only 1.7 x 10 A 6 kg / s / m A 2.
• the high thermal stability of this material is ideally suited to enable subsequent layer deposition of further functional layers, such as, for example, another high-impedance layer, piezoelectric layers, electrodes, diffusion barriers or passivations, even at the high temperatures required for this. It can be applied in a very homogeneous layer structure using the spin-on technique, and a desired layer thickness can be set with high accuracy.
• the coupling in the BAW resonator is increased with a pair of mirror layers which comprises such a polyaromatic layer. This also changes the bandwidth in the BAW filter increased by an average of 14% compared to conventional acoustic mirrors. In this way, a BAW resonator with an acoustic mirror becomes a fully fledged alternative to a BAW resonator with a bridge structure.
• the scattering of the center frequency over all resonators produced on a single wafer is also reduced.
• the interfering electrical coupling to the substrate caused in particular when using metallic high-impedance layers in the acoustic mirror, is reduced by reducing the number of layers in the mirror and by reducing the relative dielectric constants of the low-k dielectric.
• BCB Another material with low dielectric constant, low density and a low elastic constant c besides SiLK ® is benzocyclobutene.
• This material known as BCB for short, is known from microelectronics as a dielectric, insulation and cover layer. It is also outstandingly suitable for acoustic mirror layers according to the invention with low acoustic impedance, since it also ensures high layer thickness homogeneity during application and can be applied in a desired and reproducible manner to a desired thickness on a substrate, for example by means of spin-on technology.
• the layer thicknesses for the layer pairs of the acoustic mirror are adapted to the desired resonance frequency of the resonators and thus to the center frequency of the resulting filters. Because of the high bandwidth, a given pair of layers of ⁇ / 4 or 3 ⁇ / 4 layers can be used not only for a given resonance frequency, but also for other resonance frequencies within the bandwidth of the mirror or for resonators with such resonance frequencies.
• a material with a high coupling constant is preferably selected which is homogeneous in the desired and dependent on the center frequency
• Zinc oxide or aluminum nitride are particularly suitable for BAW resonators. In principle, however, other piezoelectric materials are also suitable, provided that they meet the specified boundary conditions. In principle, the same selection applies to the upper electrode layer as to the lower electrode layer.
• Each of the "lower" layers of a BAW resonator layer structure serves as a "substrate" for the layer applied above it and must accordingly withstand at least the deposition conditions of the layer above it undamaged, as well as have low surface roughness so that no acoustic scattering at the interfaces , Losses and growth disturbances occur which reduce the dynamics of the resonators and thus increase the insertion loss of the resulting filters.
• reactance filters made of BAW resonators have required resonators that have a separate resonance frequency for each resonance frequency. fitted acoustic mirrors required.
• the acoustic mirrors according to the invention it is now possible to use a single acoustic mirror consisting of only one pair of layers for the two filters of a duplexer and yet to meet the high requirements for the slope, bandwidth and near-selection.
• the aid of the invention it is thus possible to implement the resonators for two filters operating in closely adjacent frequency ranges on only one substrate with only one acoustic mirror according to the invention applied over the entire surface. This simplifies production and reduces costs.
• FIG. 3 shows the ad itance curves of a BAW resonator according to the invention compared to the admittance curve of a conventional resonator.
• FIG. 4a shows an example of a possible structure of a reactance filter in a ladder-type structure.
• FIG. 4b shows an example of a further possible structure of a reactance filter in a ladder-type structure.
• FIG. 5 shows an example of the transmission curve of a reactance filter constructed from BAW resonators according to the invention, compared to the transmission curve of a reactance filter constructed from conventional BAW resonators.
• FIG. 7 shows the transmission curves of a duplexer constructed from BAW resonators according to the invention as a function of layer thickness deviations of the low-k dielectric.
• BAW resonators constructed duplexers as a function of layer thickness deviations of the high impedance layer of the acoustic mirror according to the invention.
• FIG. 1 a shows a schematic cross section of a BAW resonator with an acoustic mirror according to the invention, whose mirror layers can be designed independently of one another as ⁇ / 4 layers or 3 ⁇ / 4 layers. Slight deviations from these conditions serve to optimize the resonator properties.
• FIG. 1 a shows a schematic cross section of a BAW resonator with an acoustic mirror according to the invention, whose mirror layers can be designed independently of one another as ⁇ / 4 layers or 3 ⁇ / 4 layers. Slight deviations from these conditions serve to optimize the resonator properties.
• Below the schematic cross section is an electronic equivalent circuit diagram of the BAW
• Resonators specified This is built up on a substrate S, which serves only as a mechanically solid support.
• the choice of suitable materials is correspondingly broad.
• a first layer HZ with high acoustic impedance is applied directly to the substrate, for example a tungsten layer. This has an impedance of approximately 105 x 10 e kg / sm 2 .
• the thickness of the layer HZ is selected so that it has a thickness of ⁇ / 4 at the desired resonance frequency of the BAW resonator and the propagation speed of the acoustic wave given in the material (tungsten). At a resonance frequency of 2.1 GHz, for example, this is a thickness of 611 nm.
• a low-k dielectric is arranged above the layer HZ as a further layer with a thickness of ⁇ / 4 or a thickness of 3 ⁇ / 4, for example a material which is marketed as a dielectric by the Dow Chemical Corporation under the name SiLK®.
• the SiLK® material system consists of cross-linked polyphenylenes, which can be obtained by polymerizing monomers substituted with cyclopentadienone and acetylene.
• the acoustically very similar material, BCB consists of cross-linked bisbenzocyclobutenes.
• the mirror layer thickness of a ⁇ / 4 layer is selected to be approximately 165 nm
• the mirror layer thickness of a 3 ⁇ / 4 layer is selected to be approximately 500 nm.
• a broadband acoustic mirror A can be realized, which has more than 95% reflectivity for the acoustic energy of a wave of the mentioned center frequency.
• a second electrode layer E2 is arranged above the piezoelectric layer P, not necessarily made of the same material as the first electrode layer E1.
• the upper electrode El is additionally provided with tuning, trimming or passivation layers.
• An SCF as described in the previous paragraphs relating to FIG. 1 a, is initially constructed like a BAW resonator. The materials, manufacturing techniques and processes used are similar.
• a second piezoelectric layer P2 is applied over the electrode layer E2, which is deposited on a piezoelectric layer P1, for example by means of sputtering technology. This in turn is covered by a top electrode E3.
• one or more acoustic mirror layers are inserted between electrode E2a and a further electrode E2b deposited thereon, which change the electrical and acoustic coupling between the resonator El-Pl-E2a and the resonator E2b-P2-E3.
• these mirror layers which can be designed as ⁇ / 4 or 3 ⁇ / 4 layers, low-k dielectrics such as SiLK or BCB can be used as low-impedance layers.
• An electronic equivalent circuit diagram of the simple embodiment of the SCF is given below the schematic cross section.
• Low-k dielectrics such as BCB or SiLK ® , which are applied to surfaces using spin-on technology, additionally smooth out the roughness of the layers below thanks to their flow properties.
• the interface roughness which can accumulate upwards in conventional mirrors, is not shown further up in these low-k dielectrics. Measures such as polishing surfaces that are conventional
• FIG. 3 shows the admittance curve of a BAW resonator provided with an acoustic mirror A according to the invention. Its curve 2 is compared with a curve 1, which is determined using a conventional BAW resonator with a conventional acoustic mirror. This known acoustic mirror is made up of two ⁇ / 4 pairs of layers Si0 2 / W. The figure shows that a higher bandwidth is obtained with the resonator according to the invention than with the resonator with a conventional mirror. The bandwidth results from the distance between the resonance frequency f r and the anti-resonance frequency f a . The broadband is the best.
• FIG. 3 shows an increase in the bandwidth of 9.4 MHz. Compared to the bandwidth of a BAW resonator with a conventional mirror of 57.5 MHz, this means a bandwidth increase of 16%.
• the admittance curves shown correspond to BAW resonators with gold and aluminum as the electrode material. Silicon serves as the substrate.
• the improved properties of the resonator according to the invention are in particular attributed to the favorable properties of the low-k dielectric layer LK, which is improved in essential parameters compared to the silicon dioxide previously used for this.
• the following table provides an overview of the most important properties of the known material (Si0 2 ) and the polyaromatic SiLK ® used in accordance with the invention:
• FIG. 4 shows three exemplary possibilities of how a reactance filter can be constructed from several BAW resonators.
• at least one resonator R s is connected in series between the filter input and the filter output.
• at least one further resonator R p is connected to ground.
• the ladder-type structure can be expanded by any further serial and parallel resonators, wherein each parallel resonator R p can be composed of two parallel resonators connected in parallel and each serial resonator R s can be composed of two resonators connected in series.
• the well-known design rule for ladder-type filters can be applied.
• the bandwidth here also increases by 13% for TX filters and by 14% for the RX filters.
• the TX filter has a ladder-type structure as shown in FIG. 4a, the RX filter has a further parallel resonator Rp at the input.
• the layer thickness of the first high-impedance mirror layer (here: tungsten) is varied by +/- 300 nm in deviation from the optimal value (here: ⁇ llnm).
• the two transmission curves, each with non-optimal layer thicknesses are almost indistinguishable, although the layer thickness deviation for the tungsten ⁇ / 4 layer is almost +/- 50%.
• the pass curves lead to bandpass filters that meet the specified specifications.
• acoustic mirrors according to the invention provide a higher degree of freedom in the selection of the desired materials, which enable further optimization of BAW resonators and filters made therefrom.
• the properties of conventional BAW resonators are at least achieved, but, as shown, considerably exceeded for optimal layer combinations.

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• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Chemical & Material Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Piezo-Electric Or Mechanical Vibrators, Or Delay Or Filter Circuits (AREA)

Priority Applications (2)

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Cited By (38)

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• 2001
  • 2001-12-11 DE DE10160617A patent/DE10160617A1/de not_active Withdrawn
• 2002
  • 2002-12-06 WO PCT/DE2002/004498 patent/WO2003050950A1/de not_active Ceased
  • 2002-12-06 JP JP2003551898A patent/JP2005512442A/ja active Pending
  • 2002-12-06 CN CNB02824737XA patent/CN100517965C/zh not_active Expired - Fee Related
  • 2002-12-06 US US10/498,203 patent/US7230509B2/en not_active Expired - Fee Related

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