Aluminum Nitride Reconfigurable RF-MEMS Front-Ends

Transcription

1 From the SelectedWorks of Chengjie Zuo October 2011 Aluminum Nitride Reconfigurable RF-MEMS Front-Ends Augusto Tazzoli University of Pennsylvania Matteo Rinaldi University of Pennsylvania Chengjie Zuo University of Pennsylvania Nipun Sinha University of Pennsylvania Jan Van der Spiegel University of Pennsylvania et al. Available at:

2 Aluminum Nitride Reconfigurable RF -MEMS Front-Ends Augusto Tazzoli* Matteo Rinaldi* Chengjie Zuo Nipun Sinha Jan Van Der Spiegel Gianluca Piazza * Department of Electrical and Systems Engineering University of Pennsylvania 200 S. 33r d Street Philadelphia P A USA * {tazzoli rinaldim piazza}@seas.upenn.edu Abstract - Aluminum Nitride based piezoelectric microelectromechanical systems (MEMS) technology has the potential to develop a fully integrated radio frequency (RF) platform that satisfies the requirements of next-generation communication standards: reconfigurability miniaturization and low power consumption. Here we report on the recent developments of this AlN thin-film based technology namely resonators filters oscillators and switches. These examples highlight how MEMS will enable the mass manufacturing of reconfigurable RF front-ends. 1. ntroduction n recent years the use of Micro and Nano-Electro-Mechanical systems (MNEMS) has been extensively explored for many applications given their ability of-overcoming the intrinsic limits of current solid state technologies [1]. n particular the Radio Frequency (RF) community is continuously looking for high-performance single-chip multiband and reconfigurable RF solutions for next-generation wired and wireless communication systems. One of the best approaches to achieve this goal and reduce production costs is to develop a technology platform that is capable of implementing all of the required components on a single chip. The aluminum nitride (A N) piezoelectric contour-mode resonator (CMR) technology offers the ability to span a broad frequency range (from few MHz to GHz) on the same silicon substrate with 50 Q matched impedances and high power handling. n addition the ability of integrating these laterally vibrating AlN microstructures with AlN switches and tunable components not only provides the advantages of compact size low power consumption and compatibility with high yield mass producible components but also enables paradigm-shifting solutions for reconfigurable RF front-ends and simpler frequency synthesizers. Compared to other piezoelectric materials (e.g. PZT and ZnO) AlN is the only material that has proved post-cmos compatibility and it is widely used in the wireless communication industry (e.g. FBAR filters as duplexers for cell phones [2]). We are currently developing monolithically integrated MNEMS-based RF front-ends formed by piezoelectric Aluminum Nitride (AlN) contour-mode resonators filters oscillators switches and tunable capacitors. Here we report some of the most recent advancements in the synthesis of the main building blocks of these reconfigurable RF front-ends (Fig. 1 ) ". AN Switch l AN Front-End Filter LO Baseband Electronics Resonator Figure 1. Schematic of an envisioned reconfigurable RF front-end realized with AlN-based resonators filters oscillators and switches. 2. AN-based Resonators Resonators are the basic building blocks for oscillators and filters. Currently available technologies are based on fixed and single frequency devices that are individually packaged and assembled on a printed circuit board (PCB). MEMS resonators have instead emerged as the best candidate for the implementation of compact and multi-frequency banks of high quality factor (Q) mechanical elements on the same silicon chip. Different micro-machined resonator technologies based on electrostatic [3] or piezoelectric [4] transduction have been investigated. Among these the AlN CMR technology [5] has emerged as one of the most promising solutions in enabling the fabrication of multiple frequencies (100 MHz - 10 GHz) and high-performance resonators on the same silicon chip thanks to the low values of achievable motional resistance (tens of ohms) and the ability to be integrated with conventional electronics. A CMR is composed of an AlN film sandwiched between two metal electrodes (see inset of Fig. 2). When an alternating-current signal is applied across the thickness T of the AlN film a contour extensional mode of vibration is excited through the equivalent d /11/$ EEE

3 piezoelectric coefficient of AN. Given the equivalent mass density Pe q and Young's modulus Ee q of the material stack that forms the resonator the center frequency /0 of the laterally vibrating mechanical structure is set by the period W of the metal electrode patterned on the AN plate and can be approximately expressed as: i" (1) - 2W...JP;;; The other three geometrical parameters i.e. thickness T length L and number of electrodes n set the equivalent electrical impedance of the resonator [6] and can be designed independently of the desired resonance frequency. Figure 2. Schematic view and micrograph of the reconfigurable CMOS oscillator based on AN MEMS CMRs. nset: schematic representation and SEM picture of one of the CMRs. 800 MHz W was designed to vary from 15 to 6!!m. L and n were respectively adjusted to change from 200 to 100!!m and 3 to 9 to set the device impedance between 35 and 150 Q (Table 1). Table 1. Contour Mode Resonators characteristics MHz C!k CtP % llk ff n Further details on the design and manufacturing process of the four AN CMRs on the same chip can be found in [6]. Manufactured devices were characterized at the wafer level using an RF probe station and a calibrated Agilent N5230A Vector Network Analyzer. A Pierce-like design with CMOS switches was then employed to synthesize a reconfigurable oscillator based on the 4 CMR resonators. The oscillator circuit topology is shown in Fig. 3. Transistors Ml and M2 form the CMOS inverting amplifier while transistor M3 acts as a large resistor to provide the biasing of Ml and M2 in the active region. Four CMOS switches operating in a time-multiplexed mode and addressed by a digital decoder are used to simultaneously connect the four AN resonators to the oscillator circuit. 3. Reconfigurable Oscillators Single-frequency high-precision oscillators are typically implemented by connecting a high-q mechanical resonator in the feedback network of a self-sustained oscillator circuit. The frequency of oscillation of the circuit is then determined by the natural resonance frequency of the mechanical resonator. Traditional high-stability oscillators are based on quartz crystals or surface-acoustic-wave (SAW) resonators. Despite their high Q these technologies have the limit of providing a single output frequency (some tens of megahertz for quartz crystals and hundreds of megahertz for SAWs) and generally resort to a phased-locked-loop (PLL) frequency synthesizer for multiple or higher frequencies of operation. The introduction of a PLL affects the system phase noise and increases the size of the circuit and its overall power consumption [7]. As an example of the potentials of the AN CMR technology for the making of a new class of oscillators here we show a reconfigurable four frequency ( and 785 MHz) CMOS oscillator (see Fig. 2). n this design the resonators were formed by an AN film of thickness T equals to 1.2!!m. n order to achieve frequencies of operation ranging from about 250 to Figure 3. Micrograph and circuit schematic of the multiplexed CMOS oscillator chip (area: 1.05 mm 2 ). This first prototype of this reconfigurable oscillator occupies a fraction of the area (30x smaller) typically taken by Voltage Controlled Oscillators (VCOs). n addition the recorded phase noise values (Table 2) are in line with the specifications required by different RF applications (Table 2). Table 2 Reconfigurable oscillator performance. r!l!t:!] fmj m:ti -. MHz dbm dbc/hz (ts - rms) dbc/hz @30 KHz @40 KHz @40 KHz @400 KHz

4 4. VHF Band-Pass Filters Band-Pass filters are a key element in RF reconfigurable front ends. By simply cascading self-coupled piezoelectric AN contour-mode MEMS resonators it is possible to realize very efficient band-pass filters. n [8] we connected two-port AN contour-mode resonators in series in order to realize multi-frequency ( MHz) narrow bandwidth (0.2%) low insertion loss (2.3 db) high off-band rejection (60 db) and high linearity (P3 100 dbm V) channel-select filters on the same chip. A two-port AN contour-mode resonator consists of two sets of one-port sub-resonators: one for input actuation and the other for output sensing which are modeled to be coupled at infinitely stiff locations and have perfect mechanical energy coupling. n our filter design several two-port AN resonators were connected in series and coupled by their intrinsic capacitance Co in order to realize higher-order filtering. This solution offers the possibility to realize filters with good shape factors and off-band rejection without the need for different frequency devices. The transmission response and photomicrograph of the 3 rd order filter at 271 MHz are shown in Fig. 4. (3) _+----'H-_+--+_-i m :!! _+-#_+-_+--+_-i Ul... o.... ' ' -30 H'H--4ffTt '''tW-lt"++'''t.. :;; r--+-1-T-;=1 (b) Frequency [MHz) MHz L = 4.2 db F1JJYJ.d8 = ;. SF... = 3.76 SF"". = 6.61 R T Q= 2100 k"_1.5% R" = 1880 CM= A7fF LJ/ 235 ph C = 239 ff Ctc 0.2 ff Rp' =3.7 ko Cpl =97 ff s-0.3 Figure 4. (a) Transmission response and (b) SEM picture of the 271 MHz 3 rd order filter. The parameters in (a) refer to: (top-box) the filter performance and (bottom-box) the individual resonator forming the filter. A detailed discussion on substrate parasitics and their impact on the performance of miniaturized filters at the microscale can be found in [8] together with the results on the characterization of the third-order nonlinearity of the developed channel-select filters. 5. Switches Another core element of the envisioned AN RF front-end platform is the MEMS switch which is introduced to enable dynamic system reconfigurability. Micromachined AN mechanical switches were integrated with the resonator technology with the aim of overcoming the limitations of current solid-state devices and guarantying large isolation low loss and power consumption. n [9] we demonstrated that all these envisioned characteristics are obtained with the AN switch technology. We were also able to show the monolithic integration of CMR-based filters with an AN based three-finger dual-beam actuation switch (Fig. 5). Figure 5. SEM of an AN based three-finger dual-beam actuation switch integrated with an AN CMR filter. nset: 3-D view of one of the switch beams. The three-finger dual-beam (two AN layers moving in opposite directions) configuration was selected with the aim of reducing the switch susceptibility to residual stresses and improving the contact reliability. The newly introduced switch topology was also designed with the goal of attaining reduced coupling between DC and RF lines. The central finger is used for carrying the RF signal whereas the outer two fingers are used for actuation (see inset of Fig. 5) therefore avoiding any overlap and keeping the two signals completely separated. The actuating beams are made with two layers of AN sandwiched between three layers of Pt and the electrodes were routed in order to actuate the two bimorphs in opposite directions (reversed electric field in the two AN layers). Despite the increase in the number of steps in the fabrication process with respect to the manufacturing of the sole resonator measurements of the integrated switch and filter network (see Fig. 5) showed RF performances in line with what was achieved without the switch (see the previous section on VHF band pass filters). The presence of the switch permitted to tum the filter on and off. This demonstration lays the foundations for the envisioned fully-mechanical reconfigurable filter bank. ndividual switches were also characterized. t is worth mentioning that these devices exhibited very fast switching time in the range of hundreds of nanoseconds (Fig. 6). The fast switching speed compared to other MEMS implementations

5 opens the possibility of using AN switches in other RF systems that require fast reconfiguration times. 0.8 'r'" - ; nS : i-----i : a: - - al : : :----- E o z : : j!!! ? Time (l1s) Figure 6. Switching time of a 300x200!lm switch biased with a square-wave form signal (±27.5V). 6. Super High Frequency Resonators n the previous sections we have shown some of the capabilities of the AN CMR technology for applications requiring devices operating in the hundreds of MHz. A unique feature of the AN CMR technology is its scalability in frequency as a function of the lithographically defined dimensions. This is an extremely important characteristic since it follows the same trends of the integrated circuit (C) industry which is driven by the continuous miniaturization of lithographically set features. By scaling the CMR dimensions to the nano realm both in the lateral W and vertical T directions it is possible to increase the frequency of operation of the device while keeping relatively small values of the motional resistance. By optimizing the deposition of a very thin film of AN ( nm) directly on top of a Si substrate and adopting an electrode configuration that is known as lateral field excitation (LFE) we demonstrated AN resonators working in the 5-10 GHz range [10]. n this configuration only a top electrode is required to set contour-extensional vibrations in each of the nano-strips leading to an overall higher Q. Figure 7. SEM picture of a fabricated 9.9 GHz NEMS resonator with zoomed view of the nano-strip array (nano-strip width is 500 nm electrode width is 300 nm). For example in the device of Fig. 7 an alternating electric field is applied across the thickness of each AN nano-strip by coplanar signal and ground electrodes. Since all the AN nano-strips are mechanically coupled a lamb-wave like mode is excited in the nano-plate. The admittance curve of an 8.05 GHz resonator (after de-embedding) is shown in Figure 8. 5 iii' CD "C :::J :!:: c D as g L= 17!1m W= 0.6!1m E -60 n= Frequency [GHz] Figure 8. Experimental and MBVD fitted admittance curves of the fabricated 8.05 GHz nano-cmr. 7. Summary The requirements of next generation RF applications are very stringent and one of the best ways to implement high performance low power consumption high speed and reconfigurable systems is to develop an integrated platform that resorts to high-q MNEMS passives. Thanks to the advantages given by the AN technology we demonstrated that it is possible to integrate multi-frequency resonators band pass filters oscillators and switches in a CMOS compatible process. We strongly believe that the presented work opens the space to the development of next generation all-integrated and micromachined reconfigurable RF front-ends. References [1] G. Rebeiz et a. EEE Microwave Magazine Dec pp [2] V. Kaajakari et a. EEE Electron Device Letter Vol. 25 N. 4 Apr pp [3] C.T.-C. Nguyen EEE Trans. Ultrason. Ferroelectr. Freq. Control Vol. 54 N. 2 Feb pp [4] R. Abdolvand et a. Proc. of EEE MEMS Kobe Japan Jan. 2007pp [5] G. Piazza et a. Journal of Microelectromechanical Systems Vol. 15 N. 6 Dec pp [6] M. Rinaldi et a. EEE Trans. on Electron Devices Vol. 58 N. 5 May 2011 pp [7] G. B. Razavi Monolithic Phase-Locked Loops and Clock Recovery Circuits NJ: EEE Press [8] C. Zuo et a. Sensors and Actuators A Vol. 160 N. 1-2 May 2010 pp [9] N. Sinha et a. Proc. of EEE Frequency Control Symposium April 2009 pp [10] M. Rinaldi et a. Proc. of EEE MEMS Sorrento taly Jan pp