A Compact LTCC Dual-Band WLAN Filter using Two Notch Resonators

Transcription

1 J Electr Eng Technol Vol. 8, No. : 68-75, ISSN(Print) ISSN(Online) A Compact LTCC Dual-Band WLAN Filter using Two Notch Resonators Jun-Hwan Park**, Seong-Jong Cheon* and Jae-Yeong Park Abstract This paper presents compact dual-band WLAN filter and filter module. They were developed by embedding all of the passive lumped elements into a LTCC substrate. In order to reduce the size/volume of the filter and avoid EM parasitic couplings between the passive elements, the proposed filter was designed using a 3rd order Chebyshev circuit topology and J-inverter transformation technology. The 3rd order Chebyshev bandpass filter was firstly designed for the bandselection of the 802.b and was then transformed using finite transmission zeros technologies. Finally, the dual-band filter was realized by adding two notch resonators to the 802.b filter circuit for the band-selection of the 802.a/g. The maximum insertion losses in the lower and higher passbands were better than 2.0 and.3 db with minimum return losses of 5 and 4 db, respectively. Furthermore, the filter was integrated with a diplexer to clearly split the signals between 2 and 5 GHz. The maximum insertion and minimum return losses of the fabricated module were 2.2 and 4 db at GHz, and.6 and 9 db at GHz, respectively. The overall volume of the fabricated filter was mm3. Keywords: Dual-band filter, Low-temperature co-fired ceramic(ltcc), Notch resonator, Passive embedding, Wireless LAN. Introduction Recent developments in wireless communication with dual-band functions have created more potential for worldwide mobile communications and digital multimedia broadcasting [, 2]. In particular, the explosive expansion of wireless local area network (WLAN) markets has been enhanced by the introduction of dual-band wireless systems which utilize the frequency bands of IEEE 802. b/g from 2.4 to 2.5GHz and IEEE 802.a from 5.5 to 5.85 GHz. These dual-band WLAN systems enable the user to access their preferred frequencies, such as IEEE 802.b/g for convenient access and IEEE 802.a for high speed data rates. To realize compact dual-band WLAN systems, dual-band filters have been developed [3-7]. In the beginning, planar dual-band filters were developed by combining two single bandpass filters that were individually designed for each frequency band into one circuit [6, 7]. However, these filters were too bulky and an additional matching circuit was required between the two filters. Thus, band-rejection circuits such as bandstop resonators or notch filters have been applied to reduce the size and cost of the filter. Another approach was to combine a broadband bandpass filter and a bandstop filter, instead of employing Corresponding Author: Dept. of Electrical and Electronic Engineering, Kwangwoon University, Seoul, Korea. (jaepark@kw.ac.kr) * Dept. of Research & Division, Amkor technologies, Ltd., Seoul, Korea. ** Dept. of Electrical and Electronic Engineering, Kwangwoon University, Seoul, Korea. Received: March 2, 202; Accepted: July 2, 202 the simple cascade connection of two bandpass filters [8-0]. To fabricate these dual-band filters, microstrip lines were commonly used. These dual-band filters were designed with reduced-length parallel coupled lines and stub-loaded open loop resonators in order to have good performance. However, their size was still bulky [, 2]. In recent years, parallel-coupled microstrip filters using stepped-impedance resonators (SIR) were reported to reduce their size [3-5]. However, these dual-band filters still have some limitations in terms of their sizes and insertion loss. Therefore, passive embedding into multi-layered substrates has been widely investigated for the purpose of mini-aturizing the passive filters. In this study, a compact dual-band WLAN filter was newly developed using J-inverter transformation technology, notch resonators, and low-temperature co-fired ceramic (LTCC) fabrication technology. Since the commonly used T-type 3 rd order Chebyshev filter was comprised of several inductors with relatively large inductance values, the filter performances were degraded by the parasitic electromagnetic (EM) coupling that occurred between the 3-dimensional geometrical structures of the filter. To eliminate these parasitic effects, a J-inverter was adopted to change the series LC resonators into shunt parallel ones. The J- inverter transformed filter circuit had relatively small inductance and large capacitance values. Moreover, to improve the rejection characteristics of the filter, a shunt capacitor and two shunt inductors were used to form 68

2 Jun-Hwan Park, Seong-Jong Cheon and Jae-Yeong Park (a) Front-end modules with conventional configurations Fig.. Circuit diagram of the proposed dual-band WLAN filter to be embedded into the LTCC substrate. independent finite transmission zeros. These transmission zeros were independent of each other and controllable without moving the central frequency of the lower passband. In order to form the higher passband, two notch resonators were symmetrically inserted between each resonator of the bandpass filter, as shown in Fig.. These resonators were effective in keeping the insertion loss of the higher passband in the zero db state. Therefore, the size of the proposed dual-band filter could be reduced by about 20 % without any degradation of the performance in comparison with that of conventional dual-band filters using integrated lumped elements. In addition, the proposed dual-band filter was integrated with a diplexer circuit to split the 2GHz and 5GHz signals for the purpose of evaluating its practical applicability for commercial products. 2. Design Theory 2. Dual-Band WLAN front-end module A new architecture for RF front-end modules has been widely researched for the purpose of developing compact dual-band WLAN systems with high performance and a low production cost. Fig. 2 (a) shows a conventional block diagram of the dual-band WLAN front end module. Since it was comprised of a dual-band power amplifier (PA), dual-band low noise amplifier (LNA), four discrete filters, two diplexers, and double-pole double-throw (DPDT) antenna switch, it occupied too large an area. Fig. 2 (b) shows a new block diagram. As shown in Fig. 2 (b), the number of discrete components and the size of the frontend module can be reduced by using the dual-band filters. When these dual-band filters are also integrated with the diplexers to split the 2GHz and 5GHz frequency signals, the performance of the dual-band WLAN front-end module is highly improved. (b) Front-end modules with new configurations Fig. 2. Block diagrams of dual-band WLAN front-end modules. 2.2 Dual-Band wlan filter To reduce its size, the proposed dual-band filter was designed using a single bandpass filter and two notch resonators, as shown in Fig.. The proposed dual-band filter was firstly designed using a T-type 3 rd order Chebyshev lumped element filter circuit topology [6]. For improving the insertion losses in the passband ranging from 2.4 to 2.5 GHz, the T-type filter circuit was firstly designed with wide-passband ranging from 2.2 to 2.6 GHz. However, the designed T-type filter required large inductance and small capacitance values. Since inductors with large values were difficult to embed into the LTCC substrate and caused EM coupling problems between the inductors due to their 3D geometrical structures, they resulted in the degradation of the filter performance. To avoid the EM coupling problem, J-inverter transformation technology was adopted to convert the serial type resonant circuit into a parallel shunt type one [7]. The J-inverter transformed Chebyshev filter circuit was comprised of shunt parallel LC resonant circuits and admittance J-inverters. The inductance L i, capacitance C i (i =, 2,, n), and characteristic admittance J i,i+ (i = 0,,, n) of the transformed Chebyshev filter are given by the following equations, where the inductance L i (or capacitance C i ) and input/output impedances G 0, G n,n+ may take arbitrary values, ω o is the central angular frequency of the bandpass filter, FBW is the fractional bandwidth, and g i (i=0,,, n+) is the lowpass 69

3 A Compact LTCC Dual-Band WLAN Filter using Two Notch Resonators normalized parameters. J L, = 2 ω Ci ( i= to n, Ω / rad / sec) i= c o G FBWω C Ω g g =, c 0 J n, n + = Gn + FBWω0Cn Ω g g c n n+ FBWω0 CiCi + Ji, i + =, ( i= to n ) Ω g g c i i+ Since the J-inverter transformed filter circuit was designed to have relatively small inductance and large capacitance values compared with the T-type Chebyshev filter circuits, it provided much lower parasitic EM coupling than the T type ones. Moreover, the J-inverter transformed filter provides better rejection characteristics in the PCS band at.9 GHz than the conventional T type one. In addition, an independent transmission zeros technology was applied by adding shunt inductors to the two transformed parallel LC resonant circuits for the purpose of () (2) (3) obtaining high attenuation characteristics between the lower and higher passbands ranging from 3.5 to 3.9 GHz. A shunt capacitor was connected to the non-transformed parallel LC resonant circuit to improve the attenuation performance at.9 GHz [8]. In order to control the rejection characteristics independently in the frequency bands lower than.9 GHz and ranging from 3.5 to 3.9 GHz, a shunt capacitor and two shunt inductors were added to each LC resonant circuit, respectively. Fig. 3 shows a comparison of the insertion losses and return losses of the T-type 3 rd order Chebyshev filter, J-inverter transformed filter, and J-inverter transformed filter with three independent transmission zeros. The inductance values of the two shunt inductors, L Ti, used for forming the independent transmission zeros in the higher stopband were calculated from the input admittance (i=, 3) 2 2 ω ωo BL( ω) = 2 2 ω( Li+ LTi) ω ω p / / where the series and parallel resonant frequencies, ω o and ω p, were defined by (4) = / L C, ω = / L C ω o i i p s i (5) and the total inductance, L s, was defined by L s = LiLTi /( Li + LTi) (6) (a) Insertion losses In the case where ω p > ω o, the two independent transmission zeros were formed in the higher stopband. On the other hand, the capacitance value of the shunt capacitor, C T, used for forming the independent transmission zeros in the lower stopband was calculated as 2 2 ω / ωo ω ) = ω 2 (7) ω / ω BC( CT 2 p where the series and parallel resonant frequencies, ω o and ω p, were defined by ωo p + 2 = / L2C 2, ω = / L2( CT C ) (8) (b) Return losses Fig. 3. Circuit simulated frequency responses of T type 3 rd order Chebyshev filter, J-inverter transformed filter, and transformed filter with three independent transmission zeros. Consequently, the independent transmission zero was formed in the lower stopband (ω p < ω o ). Moreover, the inductance values of these two shunt inductors were made to be similar for the purpose of enhancing the roll-off characteristics in the higher sub-band, as shown in Fig. 3. The insertion loss at frequencies higher than 5 GHz became near zero-db due to the roll-off characteristics of these independent transmission zeros. Independent notch resonators were then inserted between each J-inverter 70

4 Jun-Hwan Park, Seong-Jong Cheon and Jae-Yeong Park transformed LC shunt resonator to form the higher passband of the dual-band filter ranging from 5.5 to 5.85 GHz. The independent transmission zeros formed at.9 GHz and 3.9 GHz were unaffected by these inserted notch resonators. The component values of the two notch resonators were determined using their resonant frequencies, f bs0. f bs 0 = (9) 2π LbsCbs These two notch resonators were also effective in rejecting the unwanted signals at frequencies higher than 8 GHz, as shown in Fig. 4. The resonant frequency of the notch resonator was placed at higher than 8 GHz. Since the resonant frequency was formed at frequencies below 8 GHz, the insertion loss in the higher band would be degraded. Fig. 4 shows the frequency responses of the proposed dual-band filter with three independent transmission zeros through a circuit simulation. Table shows the numerically calculated and optimized components values of the proposed dual-band filter circuit. The components values of the T-type Chebyshev filter and J-inverter transformed filters were first calculated by the method described in [6] and they were then optimized through an ADS circuit simulation. The calculated and J- inverter transformed values of the inductors and capacitors were optimized to form the independent transmission zeros through the circuit simulation. The optimized circuit parameters were transformed into the structural geometries by using a 3D EM full-wave simulator. At this stage, the mutual parasitic coupling of the filter components embedded as the multi-layered structures would result in slightly different responses from those obtained through the circuit simulation. Therefore, the LTCC layout was finally fine tuned to minimize the differences between the circuit and 3D EM full-wave simulated results. 2.3 Diplexer A diplexer is an effective component for frequency separation [9]. To develop the compact WLAN dual-band filter module, the diplexer was combined with the dualband filter to clearly split the GHz and GHz frequency bands. As shown in Fig. 5, the diplexer was comprised of an LC lowpass filter, highpass filter, and additional inductors and capacitors. These additional passive components were highly effective in improving the attenuation characteristics at.9 GHz and 5 GHz. Fig. 6 shows the EM simulated and measured frequency responses Fig. 5. Schematic drawing of proposed diplexer circuit for the dual-band WLAN front-end module application. Fig. 4. Circuit simulated frequency responses of the proposed dual-band WLAN filter with three independent transmission zeros and notch resonators. Table. Design circuit parameters of the proposed dualband filter J-inverter transformed filter with 3 independent transmission zeros Notch resonator L C L T 0.28 nh 3.9 pf.3 nh L 2 C 2 C T.4 nh 0.4 pf 2.9 pf L 3 C 3 L T nh 3.9 pf.0 nh C J2 C J23. pf. pf L bs C bs 0.2 nh 0.65 pf Fig. 6. 3D EM simulated and measured insertion loss, return loss, and isolation characteristics of the fabricated LTCC diplexer. 7

5 A Compact LTCC Dual-Band WLAN Filter using Two Notch Resonators of the fabricated diplexer. The measured insertion losses were 0.6 db and 0.4 db in the lower band and higher frequency bands, respectively. The isolation between the two channels was better than 2 db. The measured insertion loss and isolation were slightly worse than the simulated ones, due to the fabrication tolerance. In the near future, the diplexer will be further optimized to improve its performance characteristics and reduce its size. 3. Fabrication Fig. 0. Photograph of the fabricated LTCC filter module comprised of dual-band WLAN filter and diplexer mounted on a PCB evaluation board. Figs. 7 and 8 show the three dimensional layout and photograph of the fabricated LTCC dual-band WLAN filter, respectively. Figs. 9 and 0 show the three dimensional layout and photograph of the fabricated filter module, respectively, which was comprised of the dual-band WLAN filter and diplexer. All of the components of the filter and module were embedded into the LTCC multilayered substrate (CMK-B6) [20]. The relative constant and loss tangent of the ceramic substrate were 6.64 and at GHz, respectively. The LTCC substrate was comprised of 9 layers and fabricated at a sintering temperature of 870 oc. The co-fired ceramic and silver conductor layers had thicknesses of 54 µm and 0 µm, respectively. The total thickness of the LTCC substrate was 590 µm. The minimum width and space between the conductor lines were 65 µm and 50 µm, respectively, and via holes between the layers were formed with a diameter of 80 µm. The embedded capacitors were fabricated with a multi-layered metal-insulator- metal (MIM) structure and a spiral geometry was utilized for the embedded inductors. The sizes and volumes of the fabricated dual-band filter and front-end module were (H) mm3 and (H) mm3, respectively. Fig. 7. 3D layout of the proposed dual-band WLAN filter with three independent transmission zeros and two bandstop resonators. 4. Experimental Results and Discussion The fabricated dual-band filter and front-end module were measured and characterized using an Agilent E507C network analyzer after being mounted onto the PCB evaluation board. To ensure the non-sensitivity of their lengths on the PCB evaluation board, the length of the feed-lines was designed to be less than a quarter of the wavelength (λ/4) and a short-open-load-through (SOLT) was adopted to calibrate the circuit before measuring them. The measured frequencies ranged from GHz to 8.5 GHz for commercial RF system applications. Fig. shows the 3D EM simulated and measured frequencies of the dual-band WLAN filter. The measured insertion losses in the lower band ranging from 2.4 to 2.5 GHz and those in the higher band ranging from 5.5 to 5.85 GHz were better than 2.0 and.4 db, respectively. The return losses in the passband were higher than 5 db Fig. 8. Photograph of the fabricated LTCC dual-band WLAN filter mounted on a PCB evaluation board. Fig. 9. 3D layout of the proposed filter module comprised of dual-band WLAN filter and diplexer. 72

6 Jun-Hwan Park, Seong-Jong Cheon and Jae-Yeong Park and db, respectively. The measured three transmission zeros occurred at.89, 3.52 and 3.93 GHz, respectively, and provided suppressions of 9.7 db at.9 GHz. While the measured suppression of the lower out-of band ranging from to 2 GHz was higher than 8 db, the suppression at 3 GHz and 4.5 GHz between the lower and higher bands was higher than 5 db, as shown in Fig.. The simulated Fig.. Measured and 3D EM simulated frequency responses of the fabricated LTCC dual-band WLAN filter. (a) Frequency responses in lower frequency band (b) Frequency responses in higher frequency band Fig. 2. Measured and 3D EM simulated frequency responses of the fabricated LTCC dual-band WLAN filter. performance characteristics of the dual-band filter were well matched with the measured ones except for the higher out-of band ranging from 6 to 8.5 GHz. This discrepancy occurred because the measured inductances of the grounding vias were smaller than the simulated ones. Therefore, the stopband of the notch resonators was formed at a higher frequency and the bandwidth was expanded and the distance between return poles in higher passband were widen, as shown in Fig. 2(b). The 3D EM simulated and measured frequency responses of the WLAN filter module are presented in Fig. 3. The measured insertion losses in the lower and higher passbands ranging from 2.4 to 2.5 GHz and from 5.5 to 5.85 GHz were better than 2.2 and.6 db, respectively, and the return losses were higher than 4 db and 9 db in the lower and higher passbands. The suppression characteristic at.9 GHz was 2 db and isolation characteristics were better than 22 db and 9 db in the lower and higher passbands ranged from 2.4 GHz to 2.5 GHz and 5.5 GHz to 5.85 GHz, respectively. The fabricated filter exhibited excellent performance and a smaller size than the previously reported ones. 5. Conclusion A highly miniaturized dual-band filter and filter module were newly developed using a lumped element filter topology and fully embedding all of the components into a multi-layered LTCC substrate for compact dual-band WLAN( GHz and GHz) system applications. The dual-band filter was first designed using a Chebyshev lumped element filter circuit topology. To reduce its size and improve its performance, the designed Chebyshev bandpass filter was then modified using J-inverter transformation technology. To improve its attenuation characteristics, inductors and a capacitor were combined with each of the three shunt LC resonator circuits. The formed transmission zeros were independent of each other and were highly effective in suppressing the undesired frequency bands of.9, 3.52 and 3.93 GHz. In addition, the notch resonators were inserted between each of the resonators of the J-inverter transformed Chebyshev filter. They were highly effective in forming the higher passband and the bandwidth of the higher passband was controlled by the capacitors and inductors. To realize a compact dualband WLAN front-end module, an LTCC diplexer was also developed and fully integrated with the dual-band filter. It helped to separate the signals of the GHz and GHz frequency bands. Although the fabricated dualband filter module was simply realized by connecting the dual-band filter and diplexer, it exhibited excellent performance characteristics and demonstrated practical applicability. In the near future, it will be optimally designed to reduce the size of the dual-band WLAN frontend module. 73

7 A Compact LTCC Dual-Band WLAN Filter using Two Notch Resonators Acknowledgements This research was partially supported by the Intelligent RF Engineering Research Center(ERC) of the Korea Ministry(Grant No. R ) of Science and Technology and the New Product Development Project by Conditional Purchase of the Korea Ministry of Knowledge Economy. The fabrication of the LTCC was carried out at SGR Technology Co. Ltd., Korea. The authors are grateful to the MiNDaP group members for their technical support and discussions. References [] J.T. Juo, T.H. Yeh, and C.C. Yeh, Design of Microstrip Bandpass Filters With a Dual-Passband Response, IEEE Trans. Microwave Theory and Techniques, Vol. 53, No. 4, pp , Apr [2] M.H. Weng, H.W. Wu, and Y.K. Su, Compact and Low Loss Dual-Band Bandpass Filter Using Pseudo- Interdigital Stepped Impedance Resonators for WLANs, IEEE Microwave and Wireless Components Letters, Vol. 7, No. 3, pp , Mar [3] G.A. Lee, Mohamed A. Megahed, and Franco De Flaviis, Low-Cost Compact Spiral Inductor Resonator Filters for System-In-a-Package, IEEE Trans. Advanced Packaging, Vol. 28, No. 4, pp , Nov [4] C. W. Tang, S. F. You, and I. C. Liu, Design of a Dual-Band Bandpass Filter With Low-Temperature Co-Fired Ceramic Technology, IEEE Trans. Microwave Theory and Techniques, Vol. 54, No. 8, pp , Aug [5] E. E. Djoumessi and K. Wu, Multilayer Dual-Mode Dual-Bandpass Filter, IEEE Microwave Wireless Components Letter, Vol. 9, No., pp. 2-23, [6] Miyake. H., Kitazawa, S. Ishizaki, T. Yamada, T. and Nagatomi, Y. A miniaturized monolithic dual band filter using ceramic lamination technique for dual mode portable telephones, IEEE MTT-S International Microwave Symposium Digest, Vol. 2, pp , 997. [7] C. W. Tang and S. F. You, Using the technology of low temperature co-fired ceramic to design the dualband bandpass filter, IEEE Microwave Wireless Components. Letter, Vol. 6, No. 7, pp , [8] Y. X. Guo, L.C. Ong, M.Y.W. Chia and B. Luo, Dual-Band Bandpass Filter in LTCC, IEEE MTT-S International Microwave Symposium Digest, pp , Jun [9] A. Bavisi, M. Swaminathan and E. Mina, Liquid Crystal Polymer-Based Planar Lumped Component Dual-Band Filters For Dual-Band WLAN Systems, IEEE Radio and Wireless Symposium, pp , [0] L. C. Tsai and C. W. Hsue, Dual-band bandpass filters using equal-length coupled-serial-shunted lines and Z-transform technique, IEEE Trans. Microwave Theory and Techniques, Vol. 52, No. 4, pp. -7, [] S. Lee and Y. Lee, A planar dual-band filter based on reduced length parallel coupled lines, IEEE Microwave Wireless Components Letter, Vol. 20, No., pp. 6-8, Jan [2] P.Mondal and M. K. Mandal Design of Dual-Band Bandpass Filters Using Stub-Loaded Open-Loop Resonators, IEEE Trans. Microwave Theory and Techniques, Vol. 56, No., pp , Jan [3] L. S. Wu, J. F. Mao, W. Y. Yin and Y. X. Guo, A dual-band filter using stepped-impedance resonator (SIR) embedded into substrate integrated waveguide (SIW), IEEE EDAPS., pp. -4, 200. [4] D. Puttadilok, D. Eungdamrong, and S. Amornsaensak, A microstrip diplexer filter using stepped-impedance resonators, SICE Annual Conference 2008, pp , Aug [5] C. F. Chen, T. Y. Huang, C. P. Chou, and R. B. Wu, Microstrip diplexers design with common resonator sections for compact size but high isolation, IEEE Trans. Microwave Theory and Techniques, Vol. 54, No. 5, pp , May [6] J.S. Hong and M.J. Lancaster, Microstrip filters for RF/Microwave applications. New York: John Wiley Sons, Inc. [7] Jayaseelan, M. and Mazlina E. Equivalent J-Inverter Network Parameters Analysis and Cancellation of Spurious Response of Parallel Coupled Microstrip Line, Proc. of the RFM conference, pp , [8] J. S. Lim and D. C. Park, A modified Chebyshev bandpass filter with attenuation poles in the stopband, IEEE Trans. Microwave Theory and Techniques, Vol. 45, pp , Jun [9] Martin Fritz and Werner Wiesbeck, A Diplexer Based on Transmission Lines, Implemented in LTCC, IEEE Trans. Advanced Packaging, Vol. 29, No. 3, pp , Aug [20] Albert Sutono, Anh-Vu H. Pham, Joy Laskar, and William R. Smith, RF/Microwave Characterization of Multilayer Ceramic-Based MCM Technology, IEEE Trans. Advanced Packaging, Vol. 22, No. 3, pp , Aug

8 Jun-Hwan Park, Seong-Jong Cheon and Jae-Yeong Park Jun-Hwan Park He received his B.S. degree in electronics and communications engineering from Kwangwoon University, Seoul, Korea, in He also received his M. S. degree in electronic engineering from Kwangwoon Uni-versity in 20. He is currently working with the Research and Development Division, Amkor Technology, Korea. His research interests include RF components and integrated front-ends. Seong-Jong Cheon He received his B.S. and M.S. degrees in electronic engineering from Kwangwoon University, Seoul, Korea, in 2006 and 2008, respectively. He is currently working toward a Ph. D. degree in electronic engineering from Kwangwoon University. His current research interests are in the areas of microelectronic devices, RF front-end modules, embedded passive devices (EPDs), and system on packaging (SOP). Jae-Yeong Park He received his M. S. E. E. and Ph. D. degrees in electrical and computer engineering from the Georgia Institute of Tech-nology, Atlanta, in 995 and 997, respectively. He worked at the Packaging Research Center, Georgia Institute of Technology, as a Research Engineering for two years. He also worked at the Micro-System Group in the LG Electronics Institute of Technology as a Team Leader of RF MEMS Research for six years. In September 2004, he joined the faculty of the Department of Electronics Engineering, Kwangwoon Uni-versity, Seoul, Korea. He has published more than 70 journal articles and conference proceedings and filed more than 95 patents. His current research interests include micro-electromechanical systems (MEMS, micro-actuators for optical and RF applications), micro-system packaging (wafer level packaging, SOP, SOC, multichip modules), micro/nano electro-chemical sensors, smart dust, and RF MEMS components. 75