IEC 62575 2 Edition 1 0 2012 07 INTERNATIONAL STANDARD NORME INTERNATIONALE Radio frequency (RF) bulk acoustic wave (BAW) filters of assessed quality – Part 2 Guidelines for the use Filtres radiofréqu[.]
General
RF BAW filters are characterized by their compact size, lightweight design, and high stability, making them adjustment-free and highly reliable These filters enhance the capabilities and applications of surface acoustic wave (SAW) filters and dielectric resonator filters Currently, RF BAW filters with low insertion attenuation are extensively utilized across various applications in the gigahertz frequency range.
RF BAW filters are gaining popularity in mobile communication applications due to their compact size and low insertion attenuation Compared to RF SAW filters with equivalent bandwidth, RF BAW resonator filters offer superior performance in terms of size and attenuation However, their bandwidth is constrained by factors such as the piezoelectric materials used and design methodologies Understanding these limitations is essential for users of RF BAW resonator filters This standard outlines the principles and characteristics of these filters.
RF BAW filters utilize a ladder filter configuration made up of several RF BAW resonators, which are categorized into two types: film bulk acoustic resonators and solidly mounted resonators Figure 2 illustrates the frequency range and relative bandwidth of RF BAW filters in comparison to ceramic, crystal, dielectric, helical, SAW, and stripline filters.
Specified stop-band relative attenuation
Minimum insertion attenuation Nominal insertion attenuation
Figure 2 – Applicable range of frequency and relative bandwidth of the RF BAW filter and the other filters
Fundamentals of RF BAW resonators
When a mechanical impact strikes a solid surface, it generates acoustic waves, with some energy transmitted through the bulk as bulk acoustic waves (BAW) The remaining energy propagates along the surface as surface acoustic waves (SAW).
There are two main types of Bulk Acoustic Waves (BAWs): longitudinal (dilatational) BAWs, which have displacement in the direction of wave propagation, and transverse (shear) BAWs, with displacement perpendicular to the propagation direction In solids, acoustic wave velocities range from several hundred to twenty thousand meters per second, with longitudinal BAWs typically traveling several times faster than shear BAWs for the same material and orientation.
Acoustic wave propagation in parallel plates leads to mechanical resonance, specifically thickness resonance, when the plate thickness \( h \) is a half-integer multiple of the wavelength \( \lambda \) of the acoustic waves This relationship is expressed as \( h = n \frac{\lambda}{2} \), where \( n \) is an integer representing the order of modes The mechanical resonance frequencies \( f_r \) can be derived from this condition.
The acoustic wave velocity, denoted as \( V \), is described by the equation \( 2 r V nV ( h f = \lambda) \) This equation reveals that, alongside the fundamental resonance (n=1), multiple higher-order resonances (n≠1) can be excited These higher-order resonances, known as harmonics or harmonic resonances, occur at integer multiples of the fundamental frequency \( f_r \) When the longitudinal bulk acoustic wave (BAW) induces the thickness resonance, it is referred to as thickness extensional (TE) resonance; conversely, when the shear wave is responsible, it is termed thickness shear (TS) resonance.
R el at iv e bandw idt h ( % )
Surface acoustic waves (SAWs) are acoustic waves that propagate along the top surface of a plate, where the wave energy is well confined and the influence of the back surface is minimal In contrast, when the wave energy penetrates into the plate and the back surface's influence becomes significant, these waves are referred to as plate waves or Lamb waves Additionally, piezoelectric excitation and detection play a crucial role in the generation and measurement of these acoustic waves.
In the case where a piezoelectric plate is sandwiched between two parallel electrodes (see
When an electrical voltage \( E \) is applied between two electrodes, it generates mechanical force through piezoelectricity, inducing acoustic motion Conversely, electric fields associated with propagating acoustic waves induce electrical charges on the electrodes.
Figure 3 – Basic BAW resonator structure
The electromechanical equivalent circuit, illustrated in Figure 4, is derived from the relationships between various components In this circuit, \(C_0\) represents the clumped capacitance due to electrostatic coupling between two electrodes, while \(C_1\), \(L_1\), and \(R_1\) denote the motional capacitance, inductance, and resistance, respectively, which arise from mechanical reactions such as elasticity, inertia, and damping This configuration is known as the Butterworth-Van Dyke (BVD) model.
Figure 4 demonstrates that the mechanical resonances can be electrically excited and detected via electrodes, indicating that this device functions as an electrical resonator known as a BAW resonator The selection of an appropriate piezoelectric material minimizes acoustic attenuation, leading to prolonged mechanical vibrations.
This mechanical property influences the electrical one as large quality (Q) factor of the electrical resonance circuit
Figure 5 illustrates the typical resonance characteristics derived from the BVD model, highlighting that a series resonance occurs at the frequency \( f_r \) At this frequency, the electrical impedance \( Z \) between the two electrodes becomes purely resistive and significantly low According to the BVD model, the frequency \( f_r \) is defined as follows:
On the other hand, at a frequency f a slightly above f r , a parallel resonance occurs where Z becomes pure resistive and very large From the BVD model, f a is given by
These frequencies are called the resonance and the anti-resonance frequencies, respectively 2
The capacitance ratio r is often used as a measure of the resonator performance, and is defined by
From the BVD model, and r is given by
In the filter design discussed later, r limits achievable fractional frequency bandwidth for filter applications
At frequencies significantly below the resonant frequency \( f_r \), the resonator behaves like a capacitor with a total capacitance of \( C_0 + C_1 = C_0 (1 + r - 1) \) This capacitance is expressed as \( \frac{\varepsilon S}{h} \), where \( \varepsilon \) represents the dielectric constant and \( S \) denotes the electrode area Consequently, \( C_0 \) can only be adjusted through changes in the electrode area \( S \), as the height \( h \) is primarily influenced by the frequency setting.
Equation 5 demonstrates that the parameter \( r \) signifies the weakness of piezoelectricity Moreover, a comprehensive wave analysis establishes a relationship between the ratio \( \frac{f_r}{f_a} \) and the electromechanical coupling factor \( k_t^2 \) for the thickness-longitudinal vibration of the piezoelectric material.
The frequencies \( f_m \) and \( f_n \) correspond to the minimum and maximum values of impedance, which also indicate the maximum and minimum admittance When the quality factor \( Q \) is high, these frequencies closely align with the resonant frequency \( f_r \) and the anti-resonant frequency \( f_a \).
This indicates three important facts:
1) achievable r is limited by k t 2 of employed piezoelectric material;
2) even-order overtones cannot be excited electrically; and
3) γ increases rapidly with an increase in n
Equations (6) and (7) apply exclusively to a uniform piezoelectric layer situated between two infinitesimally thin electrodes with infinite conductance However, the impact of the electrodes cannot be overlooked, as will be elaborated later Consequently, the piezoelectric strength of the resonator structure is typically represented by the effective electromechanical coupling factor.
From the BVD model, the Q factor at f r is given by
The resonance Q, denoted as Q = π (9), plays a crucial role in filter design, influencing the steepness of the pass-band edges Additionally, the Q factor can be assessed at the anti-resonance frequency, resulting in the anti-resonance Q, or Q a Both Q r and Q a are essential for optimizing filter performance in various applications.
For resonator characterization, the figure of merit, M is defined as r Q
In the filter design, M determines achievable minimum insertion attenuation
It is interesting to note that the BVD model indicates that min r max a C Z f C Z f
M ≅ 2 π 0 ≅ 1 2 π 0 (11) where Z max and Z min are electrical impedances of the resonator at f n (≈f a ) and f m (≈f r ), respectively Thus Z max /Z min called the impedance ratio is also used for the resonator characterization
NOTE 1 This approximated form is valid only when Q r and r are large c) Secondary effects
The basic operation of BAW resonators is effectively simulated using the BVD model However, real devices experience various secondary effects that must be carefully managed during design and production Key secondary effects significantly influence device performance.
RF resonator structures
For applications below a few tens of MHz, BAW resonators can be efficiently mass-produced by thinning and polishing piezoelectric materials However, for higher frequencies, where the required thickness is reduced to the micrometer range, thin film technologies become more suitable than traditional mechanical processing While utilizing overtones with \( n \neq 1 \) is an alternative, it leads to a significant increase in resistance.
Aluminium nitride (AlN) is a preferred choice for the piezoelectric layer in RF BAW resonators due to its unique properties, including low propagation loss, high electrical resistivity, and the ability to grow high-quality films on metal electrodes In contrast, other materials like zinc oxide (ZnO) and lead zirconate titanate (PZT) have been studied but demonstrate significantly lower performance compared to AlN, making them less viable for practical applications.
Lack of material choice limits applicability of RF BAW filters That is, r limiting the filter bandwidth is mostly determined by the piezoelectric material as indicated in 4.2 Molybdenum,
Ruthenium and Tungsten are utilized for electrodes due to their high acoustic impedance, which results in a minimal reduction in the parameter \( r \) or an increase in the effective electromechanical coupling coefficient \( k_{t}^{2} \) Additionally, these materials serve as an excellent seed layer for the growth of Aluminum Nitride (AlN).
RF BAW resonators are categorized into two types
The first type is called film bulk acoustic wave resonator (FBAR), which employs a free standing membrane supported at side edges Three kinds of FBAR structures were proposed:
Figure 8 illustrates two methods of creating air cavities in resonator structures Part a) depicts an air cavity formed by back-side etching of the supporting substrate, while parts b) and c) demonstrate air cavities created by etching a layer beneath the resonator structure after its fabrication is complete.
The solidly mounted resonator (SMR) utilizes an acoustic mirror to achieve acoustic isolation from the substrate while maintaining tight physical contact This mirror consists of multiple layers with varying acoustic impedances, such as a combination of tungsten (W) and silicon dioxide (SiO₂), which provides adequate reflection with just a few layers Each layer is engineered to approximately a quarter wave thickness to ensure optimal reflection at the desired operational frequency.
Lower electrode Supporting substrate Air gap
Ladder filters
Basic structure
Ladder type filters consist of a combination of series and parallel BAW resonators arranged in a ladder configuration They can be effectively modeled using the BVD model in and around the pass-band with resonant circuits While several types of RF BAW resonator filters have been suggested, the ladder type is the only one that has been successfully implemented.
Figure 10 shows an example of a filter structure and Figure 11 shows an example of an equivalent circuit of a half-section of a ladder filter assuming that the resistance is negligible
The filter's half-section is composed of a series-arm resonator (R1) and a parallel-arm resonator (R2), with the series-arm resonator exhibiting a slightly higher resonance frequency than the parallel-arm resonator Additionally, the synthesized resonators R1′ and R2′ are introduced, where R1′ possesses half the static capacitance of R1, while R2′ has double the static capacitance of R2.
Figure 10 – Structure of ladder filter
Figure 11 – Equivalent circuit of basic section of ladder filter
Principle of operation
Figure 12 illustrates the relationship between the reactance of R1 (X s) and the susceptance of R2 (B p) as a function of frequency Notably, the anti-resonance frequency (f ap) of the parallel-arm resonator closely aligns with the resonance frequency (f rs) of the series-arm resonator The image transfer constant γ is defined in terms of X s and B p through a specific equation.
The theory of image-parameter filters indicates that a filter exhibits a pass-band characteristic when γ is an imaginary number, while it demonstrates a stop-band characteristic when γ is a real number Consequently, the condition \(0 < B_p X_s < 1\) defines the pass-band.
B p X s > 1 or B p X s < 0 gives the stop-band shown in Figure 12
Figure 12 – Basic concept of ladder filter
Characteristics of ladder filters
The pass-band width of a ladder filter is affected by the employed piezoelectric material
Ideally it is effective to use an appropriate piezoelectric material having a high electromechanical coupling coefficient in order to obtain a filter with a wide pass-band
Currently, aluminum nitride (AlN) remains the preferred choice for piezoelectric materials due to its superior performance compared to alternatives The steepness of the pass-band edges in resonators is influenced by their Q factor, while the insertion attenuation of a filter is governed by the resonators' figure of merit (M) Additionally, the stop-band attenuation is primarily determined by the static capacitance ratio between parallel-arm and series-arm resonators, as well as the number of stages in the resonators' connection.
The frequency characteristics of a 1.9 GHz band-pass filter are illustrated in Figure 13, demonstrating a minimum insertion attenuation of under 2 dB and a return attenuation exceeding 10 dB, achieved without the need for an external matching circuit.
This filter is engineered to improve rejection at approximately 2.14 GHz, albeit with a trade-off in performance at frequencies below the pass-band This design feature is achieved through the incorporation of small parasitic inductances within the filter package.
Figure 13 – Typical characteristics of a 1,9 GHz range ladder filter
Ins er tion at tenuat ion (dB )
R et ur n at tenuat ion (dB )
Application to electronics circuits
RF BAW filter characteristics are also influenced by electrical characteristics of peripheral circuits In order to obtain a satisfactory performance, certain precautions are required
Insertion attenuation for RF BAW filters is mainly caused by ohmic loss of metal electrodes, acoustic propagation loss due to scattering and/or viscosity, leakage loss from reflectors
(SMR case), and lateral leakage loss to surroundings It should be noted that AlN films are poly-crystalline, the propagation attenuation is significantly dependent on the film quality.
Availability and limitations
An RF BAW filter features a complex mechanical structure that can lead to various unwanted responses, potentially affecting its performance It is essential to suppress or minimize these unwanted responses to maintain the filter's characteristics Additionally, long-term stability is a crucial factor to consider in practical applications, particularly regarding feed-through signals.
Feed-through signals directly connect input and output circuits through electrostatic or electromagnetic coupling, resulting in an immediate appearance at the output terminal upon applying input voltage These signals induce ripple in the pass-band, with the frequency period (\$ \delta f = \frac{1}{t} \$) determined by the delay of the main signals Additionally, they can occupy frequency traps in the stop-band, negatively impacting its characteristics.
High Q factors can lead to the excitation of unwanted acoustic waves, resulting in spurious resonances that create ripples in the pass-band and satellite peaks in the rejection band A common example of this phenomenon is the inharmonics produced by lateral wave propagation Additionally, the ageing performance of these systems is a critical consideration.
RF BAW filters, like SAW filters, demonstrate remarkable long-term stability The aging rate of RF BAW filters is influenced by factors such as the input level, substrate mounting method, and the surrounding atmosphere.
Input levels
Drive level performance is limited by: a) Electrode damage
Excessive drive levels can lead to irrecoverable damage, often resulting in flashovers and physical erosion of the electrodes This damage causes a shift in the center frequency, distortion in the pass-band, and degradation of insertion attenuation.
The RF signal drive level should be agreed upon with the manufacturer b) Frequency and/or response change
RF acoustic power is confined in a small volume Therefore, RF BAW devices may exhibit non-linear characteristics at lower drive levels more easily than conventional bulk-wave devices
General
The incorrect usage of a RF BAW filter may at times result in its unsatisfactory performance
It is necessary to take care of direct feed-through, impedance matching conditions, etc.
Feed-through signals
Feed-through signals are caused mainly by the electrostatic and electromagnetic couplings between the input and output circuits
To effectively reduce feed-through, utilizing a balanced (differential) circuit is the most efficient approach, as it cancels out unwanted coupling signals caused by stray capacitance (electrostatic) or current loops (electromagnetic) Integrated circuits (ICs) can seamlessly incorporate balanced input and output circuits, enhancing performance in RF applications.
BAW filter connected with a balanced input (output) IC is effective to reduce the feed-through
However, it is not effective to use a balun transformer to connect an unbalanced RF BAW filter with a balanced IC
To minimize electrostatic feed-through, implementing a shield between the input and output circuits on the printed circuit board (PCB) is an effective strategy Additionally, modifying the circuit pattern, particularly the ground configuration, often yields significant improvements.
To minimize electromagnetic feed-through, it is crucial to design the input and output circuit patterns in a way that completely cancels the electromagnetic coupling caused by the current loop in the input circuit at the output Therefore, the circuit design must focus on reducing or eliminating both electrostatic and electromagnetic couplings.
To prevent the adverse effects of feed-through signals caused by common residual impedance in high-frequency applications with low terminating impedance, it is essential to design input and output ground patterns separately on the PCB This approach helps to avoid the issues associated with ground loops.
Load and source impedance conditions
The load and source impedances affect the pass-band characteristics The specified terminating (load) impedances have to be used to obtain the specified performance A RF
BAW filter is designed for specific impedance Impedance mismatching increases the amplitude ripple and the insertion attenuation of the RF BAW filter
Soldering conditions
Improper soldering techniques or conditions can harm RF BAW filters and negatively impact their performance To avoid such issues, it is essential to use approved soldering methods and adhere to specified temperature and time limits Additionally, when soldering is performed multiple times, the total cumulative soldering duration must remain within the permissible range.
Static electricity
High static electricity can lead to the degradation or destruction of RF BAW filters Therefore, it is crucial to avoid applying static electricity or excessive voltage, such as electrostatic discharge (ESD), during transportation, assembly, and measurement processes.
When the requirements can be met by a standard item, it will be specified in the corresponding detail specification
If existing detail specifications do not fully meet the requirements, they should be referenced alongside a deviation sheet In exceptional cases where the discrepancies are significant enough to render an existing specification unsuitable, a new specification must be created, following the format of the standard detail specifications.
The following checklist will be useful when ordering a RF BAW filter and should be considered in drawing up a specification a) Application b) Description c) Electrical requirements:
– Test fixture(s) and test circuit(s)
• Minimum/nominal/maximum insertion attenuation
• Cut-off frequency (if necessary)
– Transition-band characteristics (if necessary)
• Guaranteed relative insertion attenuation ( _ MHz to _ MHz)
– Time/maximum temperature/signal waveform/signal frequency range (pass-band, stop- band) for power durability
– Other factors (for example, electrostatic damage, etc.) e) Physical requirements:
– Packaging form (for example, bulk, taping, magazine, etc.)
– Other factors (for example, weight, colour, etc.) f) Inspection requirements:
IEC 60368-2-1:1988, Piezoelectric filters Part 2: Guide to the use of piezoelectric filters –
Section One: Quartz crystal filters
IEC 60862-1:2003, Surface acoustic wave (SAW) filters of assessed quality – Part 1: Generic specification
IEC/TS 61994-1:2007, Piezoelectric and dielectric devices for frequency control and selection
– Glossary – Part 1: Piezoelectric and dielectric resonators
IEC/TS 61994-2:2011, Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control, selection and detection – Glossary – Part 2: Piezoelectric and dielectric filters
IEC 62047-7:2011, Semiconductor devices – Micro-electromechanical devices – Part 7:
MEMS BAW filter and duplexer for radio frequency control and selection
IEC 62575-1, Radio frequency (RF) bulk acoustic wave (BAW) filters of assessed quality –
4 Aspects fondamentaux des filtres RF à OAV 29
4.2 Aspects fondamentaux des résonateurs RF à OAV 30
4.4.3 Caractéristiques des filtres en échelle 40
6.3 Conditions sur les impédances de la charge et de la source 43
Figure 1 – Réponse en fréquence d'un filtre RF à OAV 29
Figure 2 – Gamme de fréquences et largeur de bande relative applicables des filtres
RF à OAV par rapport à d'autres filtres 30
Figure 3 – Structure de base d'un résonateur OAV 31
Figure 6 – Caractéristiques typiques d'impédance des dispositifs RF à OAV 35
Figure 10 – Structure d’un filtre en échelle 38
Figure 11 – Circuit équivalent de la section de base d'un filtre en échelle 39
Figure 12 – Concept de base d’un filtre en échelle 40
Figure 13 – Caractéristiques typiques d'un filtre en échelle dans la gamme de 1,9 GHz 41
FILTRES RADIOFRÉQUENCES (RF) À ONDES ACOUSTIQUES
DE VOLUME (OAV) SOUS ASSURANCE DE LA QUALITÉ –
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La Norme internationale CEI 62575-2 a été établie par le comité d’études 49 de la CEI:
Dispositifs piézoélectriques, diélectriques et électrostatiques et matériaux associés pour la détection, le choix et la commande de la fréquence
Le texte de cette norme est issu des documents suivants:
Le rapport de vote indiqué dans le tableau ci-dessus donne toute information sur le vote ayant abouti à l’approbation de cette norme
Cette publication a été rédigée selon les Directives ISO/CEI, Partie 2
Une liste de toutes les parties de la série de normes CEI 62575, publiées sous le titre général
Filtres radiofréquences (RF) à onde acoustiques de volume (OAV) de qualité reconnue, est disponible sur le site web de la CEI
The committee has determined that the content of this publication will remain unchanged until the stability date specified on the IEC website at http://webstore.iec.ch On that date, the publication will be updated accordingly.
• remplacée par une édition révisée, ou
IMPORTANT – The "colour inside" logo on the cover of this publication signifies that it contains colors deemed essential for a better understanding of its content Users are therefore encouraged to print this publication using a color printer.
RF filters with OAV technology are now widely utilized in mobile communications While these RF filters come with various specifications, many can be categorized into a few fundamental types.
The standardized specifications outlined in IEC 62575, along with national and manufacturer-specific guidelines, define the available combinations of nominal frequencies, bandwidth, ripple, form factor, and terminal impedance for RF filters These specifications are compiled to encompass a wide range of RF filters with standardized performance Users are strongly advised to select RF filters based on these specifications whenever possible, even if it requires minor circuit modifications to accommodate standardized filters, particularly regarding the selection of nominal frequency.
This standard has been developed in response to frequent requests for guidance from users and manufacturers regarding the use of RF filters with OAV It aims to optimize the utilization of these filters by explaining their general and fundamental characteristics in this section of IEC 62575.
This standard is not intended to explain the theory or cover all possible practical situations It highlights key aspects that users should consider before ordering RF filters for a new application, helping them avoid unsatisfactory performance.
FILTRES RADIOFRÉQUENCES (RF) À ONDES ACOUSTIQUES
DE VOLUME (OAV) SOUS ASSURANCEDE LA QUALITÉ –