Jingfu Bao - Academia.edu (original) (raw)

Conference Presentations by Jingfu Bao

A novel kind of RF MEMS lateral extensional thin­ film piezoelectric-on-silicon(TPoS) resonator w... more A novel kind of RF MEMS lateral extensional thin­ film piezoelectric-on-silicon(TPoS) resonator with narrow tethers is proposed. The conventional three-Iayers supporting beams could be simplified to single material by utilizing different kinds of doping on the bulk structural silicon layer. Within the scope of the fabrication process allowed, the width of tethers could be decreased from 10fLm to 2fLm, which would greatly reduce the dissipation of the energy from anchors. The finite element analysis simulation results show that the quality factor of the resonator is raised from 13800 to 207300. One feasible way to fabricate this novel resonator is demonstrated. To overcome the disadvantage of power capacity limited by the P-N junction, one associative operating circuit is also introduced.

Papers by Jingfu Bao

Microwave and Optical Technology Letters, Jul 27, 2017

A novel continuously tunable lowpass filter (LPF) with compact size, sharp roll-off and low inser... more A novel continuously tunable lowpass filter (LPF) with compact size, sharp roll-off and low insertion loss is presented in this paper. The filter employs two varactor diodes, a pair of open-ended coupled lines and a U-shape step impedance line (SIL) with an open-ended stub loaded at the centre of the SIL to form a very compact layout. The odd-and evenmode analysis and equivalent circuit model are demonstrated for estimation of the transmission characteristics. Tuning the DC voltage applied on the varactor diodes, the varactor capacitance accordingly changes leading to a varying cutoff frequency f c. The measured results show that the achieved 3-dB f c tuning range is 60.6% (1.15 to 2.15 GHz). The measured insertion loss (IL) and roll-off rate are 0.2-0.4 dB and 50-73 dB/GHz, respectively. The overall size of the LPF is only 0.005λ g 2 , which shows a competitive advantage comparing with the state-of-the-art work.

2022 IEEE International Ultrasonics Symposium (IUS), Oct 10, 2022

Japanese Journal of Applied Physics, 2023

This paper proposes the use of a two-dimensional scalar wave model for fast analysis of apodized ... more This paper proposes the use of a two-dimensional scalar wave model for fast analysis of apodized AlN-based radio frequency bulk acoustic wave devices. First, the working principle and model setup is introduced. Only one particular Lamb mode (S1−) is considered, and its behavior is modeled by a simple scalar wave equation looking from the top surface. Multiple layers are added to the side boundaries so that mode conversion to other modes is treated as leakage to surroundings. The wavenumber of field distribution for each resonance mode is calculated, and then its excitation coefficient, quality factor, and resonance frequency are derived. The device input admittance is calculated by applying derived parameters to the equivalent circuit. A series of oval-shaped AlN-based film bulk acoustic resonator devices are simulated and compared with experimental results. The theoretical results agreed well with the experimental ones, and accuracy and effectiveness of the present analysis are verified.

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Nov 1, 2022

Japanese Journal of Applied Physics, Jun 13, 2017

This paper discusses the possibility of realizing multimode filters composed of multiple single-m... more This paper discusses the possibility of realizing multimode filters composed of multiple single-mode resonators by using radio frequency surface and bulk acoustic wave (SAW/BAW) technologies. First, the filter operation and design principle are given. It is shown that excellent filter characteristics are achievable by combining multiple single-mode resonators with identical capacitance ratios provided that their resonance frequencies and clamped capacitances are set properly. Next, the effect of balun performance is investigated. It is shown that the total filter performance is significantly degraded by balun imperfections such as the common-mode rejection. Then, two circuits are proposed to improve the common-mode rejection, and their effectiveness is demonstrated.

International Journal of Rf and Microwave Computer-aided Engineering, Sep 21, 2016

In this letter, a novel compact ultra-wideband (UWB) microstrip bandpass filter (BPF) with a notc... more In this letter, a novel compact ultra-wideband (UWB) microstrip bandpass filter (BPF) with a notched band is presented by using the proposed radial stub loaded resonator (RSLR). The RSLR, on the one hand, is able to control two tunable transmission zeros separately and on the other hand, has the ability to provide a notched band. In addition, the vertical dimension of the UWB BPF can be significantly reduced by using RSLR, as compared with the UWB BPF based on the conventional stub-loaded resonator (SISLR). For validation, two UWB BPFs, one without the notched band and the other with a notched band at 8.0 GHz for rejecting satellite-communication signal, are developed and fabricated. Measured results show two fabricated filters have the merits of low insertion losses, sharp rejections and flat group delays. Moreover, two filters exhibit the simple topologies and compact sizes.

Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, Mar 22, 2012

A novel DMTL capacitive switch with electrostatic actuation metal–air–metal (MAM) capacitors is p... more A novel DMTL capacitive switch with electrostatic actuation metal–air–metal (MAM) capacitors is presented. The top board of MAM capacitors will be pulled down together with the switch bridge. It has higher isolation in down-state than DMTL capacitive switch and has lower insert loss and higher self-actuation RF power comparing with MEMS shunt capacitive switch. Two of the novel DMTL capacitive switches are designed for high isolation and high self-actuation RF power, respectively. The calculated result shows that both of the two novel switches have lower insert loss than the MEMS shunt capacitive switch. The self-actuation RF power of them are 4 and 2.4 times that of MEMS shunt capacitive switch, respectively, at the cost of −6.23 and −3.54 dB reduction in isolation (30 GHz).

Journal of Electromagnetic Waves and Applications, 2011

In this paper, we present a novel compact 3-dB quadrature directional coupler by introducing Hilb... more In this paper, we present a novel compact 3-dB quadrature directional coupler by introducing Hilbert curve to the conventional meander line structure. The proposed coupler uses a modified broadside coupled meander lines featuring Hilbert curves to achieve a smaller size. The structure of the coupler is suitable for multilayer printed circuit board (PCB) implementation. The proposed coupler with an operation

This paper describes complete suppression of transverse modes in thickness shear bulk acoustic re... more This paper describes complete suppression of transverse modes in thickness shear bulk acoustic resonators (TSBARs) using the lithium niobate plate. Previously, the authors conjected from the experiment that cut-off edges with crystal X-plane serve as ideal reflectors for the piston mode operation, and complete suppression is possible by adding the standard piston mode design to the other edges normal to the X-axis. Following to this conjecture, the traditional piston mode structure was designed using two-dimensional finite element method, and the TSBAR was fabricated following to the design. The experiment showed complete suppression of the transverse mode resonances, and the conjecture was verified.

In this article, the leakage from longitudinal ends of finite IDT in I.H.P. SAW resonator is disc... more In this article, the leakage from longitudinal ends of finite IDT in I.H.P. SAW resonator is discussed. Firstly, the finite length model, with end leakage mainly, is built up by Hierarchical Cascading Technology. Then, by the tuning of grating length, two factors that influence end leakage are separated. One is the scattering from end of IDT and another one is from end of reflector. Through the indicator, anti-resonant Q, it is found that the scattering from end of IDT is the dominant impact to end leakage. Comparatively, the Q drop caused by reflector can be improved easily when the number of grating is larger than 30-40. At last, a fitting method is given to evaluate quality factor rapidly with different length of grating in 2.5D model.

Sensors and Actuators A-physical, May 1, 2019

A PZT film based micro vibrating platform fabricated by wafer-layer process to provide high accel... more A PZT film based micro vibrating platform fabricated by wafer-layer process to provide high acceleration and angular velocity for IMU self-calibration. Highlights  Taking advantage of PZT-SOI wafer process to output high acceleration and high angular velocity performances.  Structural optimization of the support structure has been investigated and analyzed to keep the optimal balance between stiffness, elastic coefficient and system damping ratio of the vibrating platform.  A two-layer routing scheme based on silicon dioxide isolation is implemented to achieve IMU of electrical connection on top of the vibratory platform.  The no-load and load properties of the micro vibrating platform with different stuctrues are highly close to the simulation.  The stability and reliability of the micro vibrating platform system are verified.

This paper discusses SAW-BAW-based band reject filters. They are composed of the impedance conver... more This paper discusses SAW-BAW-based band reject filters. They are composed of the impedance converters, where capacitive elements are replaced with SAW/BAW resonators. First, basic properties of the unit cell are studied. It is shown how basic properties of a unit cell change with the design. It is also shown that when two notches caused by the resonators are placed in proximity, two synergy effects occur: (i) an extra matching point appears on one side of the transition band. This make the insertion loss at the point smaller and the transition band steeper, and (ii) the dip level becomes deeper, and the total rejection level becomes better. Then, the wide rejection band filter is designed by cascading multi-stages, and effectiveness of the device configurations is demonstrated.

International Journal of Smart Grid and Clean Energy, 2013

Intermediate frequency (IF) Filters with IC (Integrated circuit) compatibility, narrow bandwidth ... more Intermediate frequency (IF) Filters with IC (Integrated circuit) compatibility, narrow bandwidth and large stop band rejection are in need for wireless communications. This paper presents the design and simulation of a 70 MHz IF Micro-electro-mechanical filter using two identical face shear mode square resonators. By carefully designing the supporting structure of the resonators, a quality factor above 100,000 is achieved in the best designed resonator. With these face shear mode square resonators, three filters are designed and the simulated transmission response shows that these filters can obtain unterminated transmission passbands of about 25 kHz to 35 kHz with the centre frequency of 70 MHz, which represents a 0.036% to 0.05% bandwidth. This ultra narrow bandwidth IF bandpass filter can be an attractive candidate for wireless receivers.

Japanese Journal of Applied Physics, Mar 10, 2022

This paper discusses the influence of displacement and patterning of phase shifters for piston mo... more This paper discusses the influence of displacement and patterning of phase shifters for piston mode operation of the temperature compensated surface acoustic wave (SAW) resonator on SiO2/LiNbO3 structure. As the phase shifters, Cu metals placed on the top surface of SiO2 are considered. First, the conventional Cu stripes are chosen, and their displacement is considered from interdigital transducer (IDT) aperture edges. It is shown that achievable transverse mode suppression is almost identical when the stripe shape is adjusted for each case. Next, Cu dots are considered as patterned phase shifters. It is shown comparable transverse mode suppression is possible also for this case. However, relatively strong SAW lateral leakage occurs when they are placed above IDT fingers. These results indicate that location and pattern can be added as design parameters for the phase shifters on SiO2. It is favorable for further enhancement of total device performances.

This paper describes full three-dimensional (3D) simulation of transverse modes in thickness shea... more This paper describes full three-dimensional (3D) simulation of transverse modes in thickness shear bulk acoustic resonators (TSBARs) on lithium niobate. The calculation is performed by the 3D finite element method (FEM) assisted by the hierarchical cascading technique (HCT). Owing to used rectangular-shaped electrodes, the HCT is applicable to the 3D FEM, and reduces required memory size and computational time significantly. It is shown that without apodization, transverse resonances can be suppressed almost completely by applying free side edges for X planes and standard piston mode design normal to the X direction. The result also indicates that the simple two-dimensional FEM simulation is enough for the standard piston mode design in this case. These conclusions are consistent with those obtained experimentally. The analysis is extended to influence of the edge ratio of the rectangular electrodes to the QQQ factor and spurious responses. The result indicates that the square shape is preferable in total.

Japanese Journal of Applied Physics, Apr 15, 2021

This paper discusses the applicability of free side edges to a thickness shear bulk acoustic reso... more This paper discusses the applicability of free side edges to a thickness shear bulk acoustic resonators (TSBARs) on a rotated Y-cut lithium niobate plate for suppression of lateral energy leakage and the transverse mode resonances. It is shown both theoretically and experimentally that free edges are effective to suppress lateral leakage and transverse mode resonances in free-standing TSBARs. Although this technique is applicable only to the X side boundaries, it is shown that the Y′ side perpendicular to the X side can be designed for the piston mode operation independently by the simple two-dimensional analysis ignoring wave propagation toward the X direction. Then the discussion is extended to the solidly-mounted TSBAR. It is shown that this technique works well also for this case, and removal of the first few Bragg layers is enough for the transverse mode suppression.

Journal of Micromechanics and Microengineering, Aug 30, 2019

A novel kind of RF MEMS lateral extensional thin­ film piezoelectric-on-silicon(TPoS) resonator w... more A novel kind of RF MEMS lateral extensional thin­ film piezoelectric-on-silicon(TPoS) resonator with narrow tethers is proposed. The conventional three-Iayers supporting beams could be simplified to single material by utilizing different kinds of doping on the bulk structural silicon layer. Within the scope of the fabrication process allowed, the width of tethers could be decreased from 10fLm to 2fLm, which would greatly reduce the dissipation of the energy from anchors. The finite element analysis simulation results show that the quality factor of the resonator is raised from 13800 to 207300. One feasible way to fabricate this novel resonator is demonstrated. To overcome the disadvantage of power capacity limited by the P-N junction, one associative operating circuit is also introduced.

Microwave and Optical Technology Letters, Jul 27, 2017

A novel continuously tunable lowpass filter (LPF) with compact size, sharp roll-off and low inser... more A novel continuously tunable lowpass filter (LPF) with compact size, sharp roll-off and low insertion loss is presented in this paper. The filter employs two varactor diodes, a pair of open-ended coupled lines and a U-shape step impedance line (SIL) with an open-ended stub loaded at the centre of the SIL to form a very compact layout. The odd-and evenmode analysis and equivalent circuit model are demonstrated for estimation of the transmission characteristics. Tuning the DC voltage applied on the varactor diodes, the varactor capacitance accordingly changes leading to a varying cutoff frequency f c. The measured results show that the achieved 3-dB f c tuning range is 60.6% (1.15 to 2.15 GHz). The measured insertion loss (IL) and roll-off rate are 0.2-0.4 dB and 50-73 dB/GHz, respectively. The overall size of the LPF is only 0.005λ g 2 , which shows a competitive advantage comparing with the state-of-the-art work.

2022 IEEE International Ultrasonics Symposium (IUS), Oct 10, 2022

Japanese Journal of Applied Physics, 2023

This paper proposes the use of a two-dimensional scalar wave model for fast analysis of apodized ... more This paper proposes the use of a two-dimensional scalar wave model for fast analysis of apodized AlN-based radio frequency bulk acoustic wave devices. First, the working principle and model setup is introduced. Only one particular Lamb mode (S1−) is considered, and its behavior is modeled by a simple scalar wave equation looking from the top surface. Multiple layers are added to the side boundaries so that mode conversion to other modes is treated as leakage to surroundings. The wavenumber of field distribution for each resonance mode is calculated, and then its excitation coefficient, quality factor, and resonance frequency are derived. The device input admittance is calculated by applying derived parameters to the equivalent circuit. A series of oval-shaped AlN-based film bulk acoustic resonator devices are simulated and compared with experimental results. The theoretical results agreed well with the experimental ones, and accuracy and effectiveness of the present analysis are verified.

IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control, Nov 1, 2022

Japanese Journal of Applied Physics, Jun 13, 2017

This paper discusses the possibility of realizing multimode filters composed of multiple single-m... more This paper discusses the possibility of realizing multimode filters composed of multiple single-mode resonators by using radio frequency surface and bulk acoustic wave (SAW/BAW) technologies. First, the filter operation and design principle are given. It is shown that excellent filter characteristics are achievable by combining multiple single-mode resonators with identical capacitance ratios provided that their resonance frequencies and clamped capacitances are set properly. Next, the effect of balun performance is investigated. It is shown that the total filter performance is significantly degraded by balun imperfections such as the common-mode rejection. Then, two circuits are proposed to improve the common-mode rejection, and their effectiveness is demonstrated.

International Journal of Rf and Microwave Computer-aided Engineering, Sep 21, 2016

In this letter, a novel compact ultra-wideband (UWB) microstrip bandpass filter (BPF) with a notc... more In this letter, a novel compact ultra-wideband (UWB) microstrip bandpass filter (BPF) with a notched band is presented by using the proposed radial stub loaded resonator (RSLR). The RSLR, on the one hand, is able to control two tunable transmission zeros separately and on the other hand, has the ability to provide a notched band. In addition, the vertical dimension of the UWB BPF can be significantly reduced by using RSLR, as compared with the UWB BPF based on the conventional stub-loaded resonator (SISLR). For validation, two UWB BPFs, one without the notched band and the other with a notched band at 8.0 GHz for rejecting satellite-communication signal, are developed and fabricated. Measured results show two fabricated filters have the merits of low insertion losses, sharp rejections and flat group delays. Moreover, two filters exhibit the simple topologies and compact sizes.

Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, Mar 22, 2012

A novel DMTL capacitive switch with electrostatic actuation metal–air–metal (MAM) capacitors is p... more A novel DMTL capacitive switch with electrostatic actuation metal–air–metal (MAM) capacitors is presented. The top board of MAM capacitors will be pulled down together with the switch bridge. It has higher isolation in down-state than DMTL capacitive switch and has lower insert loss and higher self-actuation RF power comparing with MEMS shunt capacitive switch. Two of the novel DMTL capacitive switches are designed for high isolation and high self-actuation RF power, respectively. The calculated result shows that both of the two novel switches have lower insert loss than the MEMS shunt capacitive switch. The self-actuation RF power of them are 4 and 2.4 times that of MEMS shunt capacitive switch, respectively, at the cost of −6.23 and −3.54 dB reduction in isolation (30 GHz).

Journal of Electromagnetic Waves and Applications, 2011

In this paper, we present a novel compact 3-dB quadrature directional coupler by introducing Hilb... more In this paper, we present a novel compact 3-dB quadrature directional coupler by introducing Hilbert curve to the conventional meander line structure. The proposed coupler uses a modified broadside coupled meander lines featuring Hilbert curves to achieve a smaller size. The structure of the coupler is suitable for multilayer printed circuit board (PCB) implementation. The proposed coupler with an operation

This paper describes complete suppression of transverse modes in thickness shear bulk acoustic re... more This paper describes complete suppression of transverse modes in thickness shear bulk acoustic resonators (TSBARs) using the lithium niobate plate. Previously, the authors conjected from the experiment that cut-off edges with crystal X-plane serve as ideal reflectors for the piston mode operation, and complete suppression is possible by adding the standard piston mode design to the other edges normal to the X-axis. Following to this conjecture, the traditional piston mode structure was designed using two-dimensional finite element method, and the TSBAR was fabricated following to the design. The experiment showed complete suppression of the transverse mode resonances, and the conjecture was verified.

In this article, the leakage from longitudinal ends of finite IDT in I.H.P. SAW resonator is disc... more In this article, the leakage from longitudinal ends of finite IDT in I.H.P. SAW resonator is discussed. Firstly, the finite length model, with end leakage mainly, is built up by Hierarchical Cascading Technology. Then, by the tuning of grating length, two factors that influence end leakage are separated. One is the scattering from end of IDT and another one is from end of reflector. Through the indicator, anti-resonant Q, it is found that the scattering from end of IDT is the dominant impact to end leakage. Comparatively, the Q drop caused by reflector can be improved easily when the number of grating is larger than 30-40. At last, a fitting method is given to evaluate quality factor rapidly with different length of grating in 2.5D model.

Sensors and Actuators A-physical, May 1, 2019

A PZT film based micro vibrating platform fabricated by wafer-layer process to provide high accel... more A PZT film based micro vibrating platform fabricated by wafer-layer process to provide high acceleration and angular velocity for IMU self-calibration. Highlights  Taking advantage of PZT-SOI wafer process to output high acceleration and high angular velocity performances.  Structural optimization of the support structure has been investigated and analyzed to keep the optimal balance between stiffness, elastic coefficient and system damping ratio of the vibrating platform.  A two-layer routing scheme based on silicon dioxide isolation is implemented to achieve IMU of electrical connection on top of the vibratory platform.  The no-load and load properties of the micro vibrating platform with different stuctrues are highly close to the simulation.  The stability and reliability of the micro vibrating platform system are verified.

This paper discusses SAW-BAW-based band reject filters. They are composed of the impedance conver... more This paper discusses SAW-BAW-based band reject filters. They are composed of the impedance converters, where capacitive elements are replaced with SAW/BAW resonators. First, basic properties of the unit cell are studied. It is shown how basic properties of a unit cell change with the design. It is also shown that when two notches caused by the resonators are placed in proximity, two synergy effects occur: (i) an extra matching point appears on one side of the transition band. This make the insertion loss at the point smaller and the transition band steeper, and (ii) the dip level becomes deeper, and the total rejection level becomes better. Then, the wide rejection band filter is designed by cascading multi-stages, and effectiveness of the device configurations is demonstrated.

International Journal of Smart Grid and Clean Energy, 2013

Intermediate frequency (IF) Filters with IC (Integrated circuit) compatibility, narrow bandwidth ... more Intermediate frequency (IF) Filters with IC (Integrated circuit) compatibility, narrow bandwidth and large stop band rejection are in need for wireless communications. This paper presents the design and simulation of a 70 MHz IF Micro-electro-mechanical filter using two identical face shear mode square resonators. By carefully designing the supporting structure of the resonators, a quality factor above 100,000 is achieved in the best designed resonator. With these face shear mode square resonators, three filters are designed and the simulated transmission response shows that these filters can obtain unterminated transmission passbands of about 25 kHz to 35 kHz with the centre frequency of 70 MHz, which represents a 0.036% to 0.05% bandwidth. This ultra narrow bandwidth IF bandpass filter can be an attractive candidate for wireless receivers.

Japanese Journal of Applied Physics, Mar 10, 2022

This paper discusses the influence of displacement and patterning of phase shifters for piston mo... more This paper discusses the influence of displacement and patterning of phase shifters for piston mode operation of the temperature compensated surface acoustic wave (SAW) resonator on SiO2/LiNbO3 structure. As the phase shifters, Cu metals placed on the top surface of SiO2 are considered. First, the conventional Cu stripes are chosen, and their displacement is considered from interdigital transducer (IDT) aperture edges. It is shown that achievable transverse mode suppression is almost identical when the stripe shape is adjusted for each case. Next, Cu dots are considered as patterned phase shifters. It is shown comparable transverse mode suppression is possible also for this case. However, relatively strong SAW lateral leakage occurs when they are placed above IDT fingers. These results indicate that location and pattern can be added as design parameters for the phase shifters on SiO2. It is favorable for further enhancement of total device performances.

This paper describes full three-dimensional (3D) simulation of transverse modes in thickness shea... more This paper describes full three-dimensional (3D) simulation of transverse modes in thickness shear bulk acoustic resonators (TSBARs) on lithium niobate. The calculation is performed by the 3D finite element method (FEM) assisted by the hierarchical cascading technique (HCT). Owing to used rectangular-shaped electrodes, the HCT is applicable to the 3D FEM, and reduces required memory size and computational time significantly. It is shown that without apodization, transverse resonances can be suppressed almost completely by applying free side edges for X planes and standard piston mode design normal to the X direction. The result also indicates that the simple two-dimensional FEM simulation is enough for the standard piston mode design in this case. These conclusions are consistent with those obtained experimentally. The analysis is extended to influence of the edge ratio of the rectangular electrodes to the QQQ factor and spurious responses. The result indicates that the square shape is preferable in total.

Japanese Journal of Applied Physics, Apr 15, 2021

This paper discusses the applicability of free side edges to a thickness shear bulk acoustic reso... more This paper discusses the applicability of free side edges to a thickness shear bulk acoustic resonators (TSBARs) on a rotated Y-cut lithium niobate plate for suppression of lateral energy leakage and the transverse mode resonances. It is shown both theoretically and experimentally that free edges are effective to suppress lateral leakage and transverse mode resonances in free-standing TSBARs. Although this technique is applicable only to the X side boundaries, it is shown that the Y′ side perpendicular to the X side can be designed for the piston mode operation independently by the simple two-dimensional analysis ignoring wave propagation toward the X direction. Then the discussion is extended to the solidly-mounted TSBAR. It is shown that this technique works well also for this case, and removal of the first few Bragg layers is enough for the transverse mode suppression.

Journal of Micromechanics and Microengineering, Aug 30, 2019

Japanese Journal of Applied Physics, Mar 23, 2021

This paper discusses the impact of etched holes given to the solidly mounted resonator (SMR)-supp... more This paper discusses the impact of etched holes given to the solidly mounted resonator (SMR)-supported A1 Lamb mode resonators. The authors pointed out that the etched holes are effective at suppressing the lateral leakage and the transverse mode resonances for free-standing A1 Lamb mode resonators, and the technique was named the broadband piston mode (BPM). First, the SMR structure is designed for an A1 Lamb mode resonator, and it is shown that the release windows are effective in the suppression of transverse mode resonances even when the SMR is added. Detailed analysis is given to spurious responses that newly appeared by applying the SMR. Then, the discussion is extended to the impact of partial etching of the SMR layers underneath the release windows. The result indicates that the etching of the top SMR layer is enough for recovery of the BPM function.