John D. Larson

Modified Butterworth-Van Dyke circuit for FBAR resonators and automated measurement system

Microwave Film Bulk Acoustic Resonators (FBARs) may be characterized by means of the Mason transmission line model, but for parameter extraction and design studies, the lumped Butterworth-Van Dyke (BVD) model is more useful. We propose a modification to the standard five element BVD model, in which a second resistor is added in series with the plate capacitance C/sub 0/. This improves the model predictions as compared to the data obtained from a network analyzer (NWA). Here, the modified model will be developed in terms of the resonant frequencies, effective coupling constant k/sub t//sup 2/, and the quality factor Q, as determined from the S parameters of an FBAR measured by the NWA. To evaluate the FBAR resonators on a routine basis, an automated data acquisition and parameter extraction method based on the Modified Butterworth-Van Dyke model (MBVD) is described. An Agilent Technologies 8753ES NWA operating under Personal Computer control is used to acquire and process FBAR data by means of a custom HPVEE/sup TM/ program, which transfers data from the NWA, and extracts the six MBVD circuit parameters. Excellent agreement is obtained between the measured data for a typical FBAR resonator and calculated postdictions obtained from the MBVD circuit. Coupled with the automated method, which takes about 10 seconds per resonator to perform a complete extraction cycle, a computer controlled probing station is used to acquire data from several hundred resonators on the wafer upon which the FBARS were fabricated. With this speed and probing capability, it is feasible to wafer map the FBARs for uniformity. Contour plots of the measured resonant frequency and coupling constant k/sub t//sup 2/ will be presented to illustrate the capability.

Thin film bulk wave acoustic resonators (FBAR) for wireless applications

For some time, FBAR technology has lagged behind ceramic technology and surface acoustic wave resonator (SAW) technology for commercial applications. There were several technologies that had to be developed before FBAR technology became viable for rf filters. First, a process is needed that can make the resonators manufacturable, robust and repeatable. Second, maximizing the coupling coefficient, k/sub t//sup 2/ and the Q of the resonator (k/sub t//sup 2/ Q product) is necessary. Another technology needed is a method to eliminate ripple (or suck out) associated with lateral mode excitation. Lastly, a method is needed for maintaining a uniform thickness (for frequency control and a means to target frequency to within +-0.03%). If one overcomes these sets of hurdles, the rewards are high. The Quality factor, Q, inherent in these structures is impressive (over 2500) and the intrinsic k/sub t//sup 2/ has been inferred to be close to the theoretical maximum of 6.5%. The k/sub t//sup 2/ Q product (Figure of Merit for FBAR filters) have been as high as 100 for our devices. These two properties can be combined in a filter to achieve low pass band insertion loss and extremely sharp skirts. One intrinsic advantage of FBAR over SAW technology is the ability to handle input power in excess of 4 Watts. Resistance to Electrical Static Discharge (ESD) is another desirable property of FBAR devices. Finally, FBAR technology is intrinsically a low temperature process technology-compatible with semiconductor technology. This implies future integration of FBARs with semiconductor circuits.

Ultra-miniature high-Q filters and duplexers using FBAR technology

An ultra-miniature PCS duplexer uses thin-film bulk wave acoustic resonator (FBAR) technology. FBAR resonators are made using aluminum nitride for the piezoelectric material and silicon as the substrate. It has better than -52 dB rejection for the receive (Rx) filter in the transmit (Tx) band and pass-band insertion losses are on the order of 2 dB (Tx) and 3 dB (Rx). Performance is comparable to that of much larger ceramic duplexers.

Method of making an acoustic wave resonator

A method of fabricating an acoustic resonator includes forming a ferromagnetic compensator, such as one comprised of a nickel-iron alloy, which at least partially offsets temperature-induced effects introduced by an electrode-piezoelectric stack. The compensator has a positive temperature coefficient of frequency, while the stack has a negative temperature coefficient of frequency. By properly selecting the thickness of the compensator, temperature-induced effects on resonance may be neutralized. Alternatively, the thickness can be selected to provide a target positive or negative composite temperature coefficient of frequency. In order to prevent undue electromagnetic losses in the ferromagnetic compensator, a metallic flashing layer may be added to at least partially enclose the compensator.

Power handling and temperature coefficient studies in FBAR duplexers for the 1900 MHz PCS band

Duplexers for 1900 MHz PCS handsets based on FBARS have been realized by micro-machined thin film AlN devices. A major advantage of the FBAR duplexer is a 10-fold reduction in physical volume compared to that of dielectric types. However, since the RF input power to the transmit (Tx) filter remains at levels up to +29 dBm input, the Poynting power density is /spl sim/1 kWatts/cm/sup 2/ with concomitantly large RF strain RF levels. The in-band insertion loss for the entire multi-element Tx ladder filter is/spl sim/3 dB (343 mW dissipated maximum), which results in a volume power dissipation per FBAR/spl sim/1 MWatts/cm/sup 3/. Power densities of this order can lead to 1) frequency shifts due to heating, 2) long term degradation, 3) strain levels approaching the fracture limit of the thin films comprising the FBAR, and 4) thermal destruction of the Tx filter FBARS. We discuss two methods to measure the temperature coefficient of frequency: 1) probing individual FBAR resonators on a hot chuck, or 2) heating packaged duplexers in an oven. The measured resonator frequency temperature coefficient is/spl sim/27 ppm//spl deg/C, while the duplexer Tx response shows a somewhat lower value. Self-heating temperatures can be estimated from this and the observed frequency shift, and were also measured by infrared microscopy. Next, we present preliminary results on the duplexer power handling capabilities, based on a small sample of parts. The Tx devices will withstand up to +36 dBm (4 Watts) input power without destruction. Above this, catastrophic failures can be observed. A scanning electron micrograph example illustrating a catastrophic failure in the duplexer Tx filter will be presented. Finally, we will discuss the effects of high power (+30 to +36 dBm) on duplexer performance. At present we do NOT observe any long term, cumulative effects which could lead to catastrophic failures. Our observations support a model in which device characteristics shift slightly with time.

A film bulk acoustic resonator (FBAR) duplexer for USPCS handset applications

We will describe the design and measured performance of a duplexer based on film bulk acoustic resonators (FBARs) for the 1900 MHz PCS cellular phone market. Typical specifications for the duplexer require the Tx filter to attenuate frequencies in the Rx band by >40 dB while maintaining a worst-case insertion loss of 3.5 dB over the Tx band. VSWR must be better than 2.2 in the pass-band (8.5 dB return loss). The Rx filter must attenuate the Tx frequencies by >50 dB while maintaining a worst-case insertion loss of 4.2 dB in the Rx band. Again, VSWR must be better than 2.2.

A Temperature-Stable Film Bulk Acoustic Wave Oscillator

This letter reports a passively temperature-compensated CMOS oscillator utilizing a film bulk acoustic resonator. The resonator exhibiting an f ldr Q product of 2-4 X 1012 s-1 is composed of molybdenum, aluminum nitride, and a compensation material that has a positive temperature coefficient of Youngs modulus. The 604-MHz oscillator consumes 5.3 mW from a 3.3-V supply and achieves excellent phase noise performances of -102, -130, and -149 dBc/Hz at 1, 10, and 100 kHz carrier offsets, respectively. The oscillators temperature-dependent frequency drift is less than 80 ppm over a temperature range of -35degC to +85degC.

Coupled resonator filter with single-layer acoustic coupler

We discuss the operation of novel coupled-resonator filters with single-layer acoustic couplers. Our analysis employs the physical Mason model for acoustic resonators. Their simpler fabrication process is counterbalanced by the high acoustic attenuation of suitable coupler materials. At high levels of attenuation, both the phase and the acoustic impedance must be treated as complex quantities to accurately predict the filter insertion loss. We demonstrate that the typically poor near-band rejection of coupled resonator filters can be improved at the die level by connecting a small capacitance between the input and output of the filter to produce a pair of tunable transmission minima. We make use of these theoretical findings to fabricate coupled resonators filters operating at 2.45 GHz.

Non-Ideal Radiators in Phased Array Transducers

One dimensional arrays for medical acoustic imaging are a group of point radiators. The individual r adiators (transducer elements) are modeled as single mode, piston v ibrators w ith negligible cross-coupling. It is straightforward to calculate the acoustic beam response -- steering, focusing, and resolution -- from such a model. In real world arrays with high element density the situation becomes more complex. Cross-coupling between elements and multi-moding of the individual elements become the dominant problems. The beam response then departs from the ideal. This paper will discuss the sources of cross-coupling and explore suppression techniques. The mass-spring mode will be reviewed, and novel suppression techniques will be presented. Vibration modes in a single element will be explored, in particular, the relation between the non-ideal behavior of the array elements and the resulting beam response deficiencies.

PZT material properties at UHF and microwave frequencies derived from FBAR measurements

Lead zirconate titanate (PZT) is a high dielectric constant, /spl epsiv//sub r//sup S/, high coupling, k/sub t//sup 2/, piezoelectric material with possible application to film bulk acoustic resonators (FBAR) at low microwave frequencies. PZT is a widely used material at audio and low MHz range frequencies, but the high acoustic attenuation of PZT tends to preclude its use above UHF. Likewise, the bulk dielectric/piezoelectric/elastic (DPE) properties are well known at low frequencies, but the thin film properties have not been as well measured into the microwave range. To obtain better data, the material properties of thin film PZT deposited by several methods were measured at GHz frequencies from air- or silicon-backed FBARs fabricated on silicon wafers. PZT thin films, deposited by metal-organic chemical vapor deposition (MOCVD), sol-gel deposition, RF sputtering, or jet vapor deposition, were obtained from various sources. The films examined ranged from 0.4 to 2 /spl mu/m in thickness, and were deposited on thin (/spl sim/0.1 /spl mu/m) Pt or Ir bottom electrodes. The samples had compositions within the tetragonal phase (Zr/Ti = 30/70, 40/60) or near the morphotropic phase boundary (Zr/Ti = 50/50). Scattering parameter, S/sub 11/, vs frequency measurements were made. Using the Mason equivalent circuit model, the DPE coefficients were extracted by varying parameters in the Mason model until the predicted S/sub 11/ vs frequency fitted the measured data. The devices exhibited /spl epsiv//sub r//sup S/ in the range of 300 to 500, k/sub t//sup 2/ between 5% and 35%, and acoustic attenuation from 400 to 2100 dB/cm at 1 GHz.