In order to predict the thermal behavior of bulk acoustic wave resonator (BAWR) and evaluate the power handling capacity, a BAWR thermal behavior simulation method is proposed. The conductor surface loss in the BAWR electromagnetic model is extracted and used as the heat source for thermal simulation to obtain the temperature distribution of the resonator. Then the signal feeding edge, active area shape and power handling capacity are researched. The simulation results show that in order to moderate the self-heating effect of BAWR, the signal feeding should follow the principle of “feeding in from long edge, feeding out from long edge”; active area shape has little to do with selfheating effect; the power handling capacity of the designed BAWR can reach to 3 W.
Microstrip interdigital filter requires use of grounding via-holes, but the grounding via-holes will influence performance of interdigital filter. The traditional design method does not consider grounding via-holes effect in the initial design process, which will lead to some uncertainties of the grounding via-holes, such as the number, the size and the position. This will make the subsequent optimization face multi-factor and multi-level problems. In addition, when the substrate thickness and the center frequency increase to a certain extent, the external quality factor obtained by using the traditional 50 Ω tapped-line cannot reach the theoretical value. Therefore, the traditional design method is modified, and the grounding via-holes effect is considered in the initial design process by using 3D electromagnetic field simulation software. When the traditional 50 Ω tapped-line do not meet the design requirements, it is improved to a gradual tappedline with the combination of a 50 Ω microstrip line and a narrower microstrip line. Taking a Ka-band filter as an example, 100 μm and 300 μm thick high resistance silicon are used as substrate respectively. The simulation results indicate that the initial design for filter with the modified method is closer to the specifications, which can reduce subsequent iterations. Besides, the insertion loss of filter with gradual tapped-line is 1.91 dB, and return loss is 18.06 dB. What’s more, the stop rejection at 27.00 GHz and 33.40 GHz are 46.91 dB and 59.58 dB, respectively.
Defects in efficiency or usability have come into being when using the maximum stress on FBAR structure as the reference stress or using the so called “Calculus-like analysis method” to calculate the sensitivity of FBAR transducer. The former does not consider the influence of the strain in the thickness direction, which overestimates the sensitivity. The latter learns from FEM, but it is too complicate and heavy-workload. In order to eliminate these deficiencies, the improved sensitivity prediction method for FBAR transducer is put forward. The whole calculation process is completed with COMSOL FEM software to avoid the complex data processing. The energy weighted average method is used to calculate the average strain of every single layer in FBAR and the average internal pressure of piezoelectric layer. The average strain is used to modify the thickness of FBAR, and the internal pressure is used to modify the elastic constant of piezoelectric material. Then the eigenfrequency solving method is carried out to solve the eigenfrequency of FBAR. The former improves the speed of solving, and the latter enhance the accuracy of the calculation results. The sensitivity of both circular membrane type FBAR transducer and FBAR micro-accelerometer can be calculated through the improved method. The expected sensitivity of the membrane type FBAR transducer is 46.5 MHz/N, which is close to the experimental result, 50 MHz/N. However, due to unawareness of the actual structure parameters, the expected sensitivity of the micro-accelerometer is 27 kHz/g, which is different from the experimental result, 100 kHz/g. The two calculation cases indicate that the improved sensitivity prediction method for FBAR transducer is both effective and available.
In order to determine the order of BAW ladder filter, the relationship among out-of-band rejection, area ratio of parallel and series FBAR, and the filter order N are studied by simulating. Based on FBAR Mason model, firstly, one-order to six-order BAW ladder filters are constructed; next, the area ratio of parallel and series FBAR (Cps) is set from 1 to 6, and the six BAW ladder filters are simulated in ADS; finally, the left out-of-band rejections are extracted from the simulation result, and plotted as a graph. In addition, as the filter orders can be set to a half order, the filters with order N (N=1.5… 5.5) are simulated in the same way. Simulation results show that the out-of-band rejection has an equal increase with the filter order in number when Cps remains constant, and that the out-of-band rejection increases as Cps increases when filter order keeps constant. When optimizing the design, the area ratio of parallel and series FBAR (Cps) is usually set from1 to 4, within which the out-of-band rejection will increase by about 10 dB when the filter increases by one order. And when the filter increases half order, the out-of-band rejection will increase by 5 dB, which is about half of the value by increasing an integer order. In addition, the structure of ladder filter is discussed, and the influence of the filter order and capacitance ratio on the passband performance is studied.
In order to reduce the volume of the filter and increase the number of chips on the wafer, while ensure the filter performance, a design method of the bulk acoustic wave (BAW) ladder filter is proposed. This layout design method consists of 11 design criteria and a 6-step flow. The 11 design criteria limit the shape and position of the BAW resonators (BAWRs), the distance between the BAWRs, the distance between the BAWRs and the pads and the interconnecting wire. The layout design flow has 6 steps. 1) Preset the shape of each BAWR (square/pentagon) according to its active area values. 2) Add an auxiliary circumcircle for each BAWR, tightly align all the series resonator circumcircles along a central line in order, and mate the corresponding electronically neighboring parallel resonator circumcircles one by one at a position above/below the center line. This makes an initial 3-row and n-column 2D arrangement, and the column number N is determined by the filter order. 3) Fix the very first series resonator circumcircle position and incrementally “compress” the initially self-assembled 3-row structure along the row width direction until the row height for row width bargain is no more cost effective. 4) Apodize the square series resonators and fine-tune each resonator’s shape and rotation according to above-mentioned related design criteria. 5) Wiring BAWRs and pads together. 6) A combined acoustic-electromagnetic BAW filter simulation method is used to validate the layout result. In a 5-order BAW ladder filter layout demo case, a layout fill ratio over 44% is obtained. An auto-layout program “BAW-filter Auto-layout Tool (BAT®)” based on the presented method is also presented.
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