Analytical design of linear variable capacitors with shaped-finger comb-drive actuators

MEMS (micro electro mechanical systems) products have significant growth potential, and are being used in an increasing number of applications. One issue with MEMS devices is manufacture, as current MEMS manufacturing processes not standardized. Design techniques were developed to allow a single MEMS manufacturing flow to meet a wide range of accelerometer specifications. One technique is Metal on proof this is a design method that adds thermally dissimilar materials (such as metal) to the accelerometer proof mass, and isolates the proof mass from resulting thermal stresses. Another technique is variable gap sense capacitance, which uses two different sensing gaps in capacitive transducers. This allows z-axis transducers to provide increased output and improved linearity.

This study was devoted to oscillator noise analysis and simulation in order to apply it on VCOs (Voltage Controlled Oscillator) for Intermediate Frequency integrated circuits, especially for TV receivers. An original period jitter transient simulation method, thanks to the transient noise simulator in ELDO and to a period extractor (ELDO-FAS based language software) has been worked out, making it is possible to deduce the VCO phase noise.With this kind of simulation, it is possible to evaluate the effect of the various noise sources in the VCO on the period jitter. Two VCOs, with optimized phase noise, have been designed for Intermediate Frequency integrated circuits. The first VCO, using a LC (inductance - capacitance) resonant architecture(the inductance is located outside the integrated circuit) has been designed and implemented. The frequency variation is produced by a variable capacitance. One of the major defects of the variable capacitor (parasitic phase displacement of the capacitive current) has been originally compensated. This VCO phase noise has been measured, simulated in many ways, with a good correlation. The period jitter simulation methodology has been so validated. A second VCO, of relaxation kind, completely integrated (it does not need any external element), dedicated to replace the first one, has been designed. Its architecture is based on RC (resistance - capacitance) elements in differential mode. It is able to operate with a low power supply (5V). Its phase noise has been optimized thanks to the previous methodology.

Variable capacitors are fabricated in standard silicon substrate by using a CMOS-compatible metal MEMS process. A low temperature process is implemented with two layers of mask. The suspended structure of the variable capacitor effectively depresses substrate loss, thereby, achieving both high Q-factor and high self-resonance frequency. The tunable-capacitor can be flexibly connected into an RFIC, and a rotationally driven structure is proposed and formed with anti-vibration capacity for mobile applications. By integrating the variable capacitor with a solenoid inductor, a parallel tunable LC-tank is fabricated and tested, resulting in a broad tuning-range of resonant frequency. With the low temperature post-CMOS process, the high-performance variable capacitors and LC-tanks are promising in RF ICs.

Development of a wide-tuning range and high Q variable capacitor using metal-based surface micromachining process

To determine a micro electro mechanical systems (MEMS) inductor configuration that gives large inductance variations and high Q-factors, air-suspended MEMS inductor configurations are studied: (a) an inductor with two angularly meandered lines, (b) a solenoid inductor with a pair of movable shields, (c) a planar spiral inductor with a patterned shield of different areas, and (d) a planar spiral inductor with a metallic frame of different sizes. The configuration of (a) is shown to give a large inductance variation of 115%. However, its Q-factor is only about 10, and difficulties in fabrication are also expected. The configuration of (b) could be a reasonable engineering solution. The inductance variation is 67%, and the maximum Q-factor is over 22. The solenoidal inductor configuration is thus considered suitable for realizing an RF MEMS variable inductor with large inductance variations and high Q-factors.

Low-power CMOS LC QVCO using zero-biased transistor coupling of MWCNT network-based VCO structure

This paper presents a low-power, wide tuning range CMOS voltage-controlled oscillator with MCML (MOS current mode logic) differential delay cell. Voltage controlled oscillator (VCO) circuit is designed in TSMC 0.25 μm CMOS process. To achieve the broad frequency range concept of variable capacitance is employed in the proposed VCO circuit. Source/drain tuning voltage (Vtune) and body bias voltage (Vb) of I-MOS varactor are used to achieve variable capacitance at different I-MOS varactor widths (W). The dual control voltage of I-MOS varactor results in a tuning range from 0.528 GHz to 2.014 GHz. VCO's figure of merit (FoM) is 152.13 dBc/Hz with phase noise of −93.77 dBc/Hz at 1 MHz offset from the oscillation frequency. The proposed VCO dissipates maximum power of 3.127 mW.

Microelectromechanical system (MEMS) RF components for millimeterwave reconfigurable transceivers are investigated. The components include reconfigurable Vee-antennas, sliding planar impedance tuners, microswitches and variable capacitors. These different components are fabricated using the same three-layer-polysilicon surface micromachining techniques. In this work, we focus on the demonstration of component architectures and the integration issues. The reconfigurability of the Vee-antenna is demonstrated with beam-steering and beam-shaping functionality. The antenna characteristics are also studied. The planar feature of the Vee-antennas allows integration of planar impedance tuners. The antenna impedance can be matched dynamically by two shunt sliding planar backshort impedance tuners. The tuning ranges of planar backshort impedance tuners on different transmission lines are studied. The mechanical architectures for microswitches, parallel-plate variable capacitors and circular variable capacitors are also investigated.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper presents the design and characterization of a negative resistance type 1.9-GHz oscillator using high quality factor (Q) embedded lumped-element LC passives in an organic-based substrate with liquid crystalline polymer (LCP) dielectric material. A design strategy using analytical models is implemented to determine the value of the base inductance subject to the constraints set by power dissipation. Additionally, the effect of component Qs on the phase noise is qualitatively discussed. This paper also addresses the effects of the parasitics of the surface-mount active devices on the noise spectrum of a negative resistance type voltage-controlled oscillator (VCO). The designed VCO is fully embedded in the LCP substrate and uses high Q on-package passive components. The VCO was measured to operate at 1.92 GHz dissipating 14 mW of dc power and measured a phase noise of -118dBc/Hz and -133 dBc/Hz at 600 KHz and 3-MHz offset, respectively. The high Q of the LC tank circuit was utilized to optimize the VCO to operate from a 2-V supply at bias current of 0.9 mA. Finally, the design and implementation issues in a 2.25-GHz Colpitt's oscillator on LCP substrate are shown. The effects of scaling capacitance ratio on VCO phase noise and on power consumption are verified for the Colpitt's oscillator

There exists a growing demand on high performance variable filters for multi-function, multi-band wireless systems used in wide variety of applications. A variable capacitor with high capacitance ratio (CR) is one of the key devices to realize such controllable/reconfigurable filters. This paper reports the device structure, fabrication process, and the performance of a very high CR variable capacitor using pure water as dielectric material. The device holds two reservoir cells and was fabricated by microelectromechanical systems (MEMS) technology. It exhibited three digitally-stepped capacitances; varied from 0.93 pF to 32.9 pF (CR=35), then to 74.4 pF (CR=80) at 1 MHz by filling each of the cells with pure water. In order to confirm the electrical behavior for such a fluid MEMS capacitor, its microwave performance was evaluated. An equivalent RF circuit model with 9 elements has been introduced and the device performance was successfully simulated up to 4 GHz. It is also confirmed that the 3 step capacitance values stay effective in the same frequency range.

MEMS (Micro-Electro-Mechanical Systems) based components such as inductors, variable capacitors, transmission line, filter, switches, phase-shifter, and resonators are superior in performances in term of quality factor, noise, linearity, power consumption, size, and cost, which cannot be achieved by conventional approach. This work discusses the MEMS enabled RF (Radio Frequency) signal source that can be prime candidate for ubiquitous connectivity, and is also amenable for IC (integrated circuit) fabrication.

Future microwave networks require miniature high-performance tunable elements such as switches, inductors, and capacitors. In this paper, high performance variable capacitor was fabricated by simple microelectromechanical systems (MEMS) technology. The capacitance and quality (Q) factor at 1 GHz are 0.792 pF and 51.6. The pull-in voltage is 13.5 V and the tuning ratio of the capacitor is more than 1.31:1. A reduced-order model for the dynamic characteristics of the capacitor is established based on the equilibrium among the plate flexibility.

This paper presents a balanced-bridge method for the characterization of dielectric charging effect in RF MEMS (radio frequency micro-electromechanical system) cantilever switches. The method measures the relative change of up-state capacitance caused by dielectric charging. Compared with previous methods, this approach employs simple and inexpensive test equipment. The test system consists of two symmetric capacitive switches, two adjustable resistors, a function generator, and a voltmeter. Measurement results indicate that it can achieve a resolution of 0.1 fF/Q. By this test system, the slight variation of up-state capacitance can be converted to the significant change of the resistance. Analysis of the experimental data shows that dielectric charging under the DC-voltage stress mainly occurs in the first 1200 seconds. The mechanisms resulting in capacitance variation can be divided into two groups: dielectric polarization and charge injection. Moreover, in contrast to the DC voltage, the RF signal does not have a significant influence on the charging process.

Differential capacitance sensor with movable electrodes is usually applied for accelerometers, pressure gauges, etc. because of its high sensitivity and resolution. Two axis inclination sensor with differential capacitance structure using MEMS (Micro Electro Mechanical Systems) technology has recently been designed for industrial and environmental purposes. The initial capacitance of the sensor should be very low, as few pF, therefore, a C-V converter circuit, which is able to detect very small capacitance variations is required. In this study, the particular principle of a “diode-bridge type differential capacitance detection circuit” has been analyzed. After the mathematical simulations performed by using PSpice simulator, the detection of the capacitance variations in fF order resolution are considered.

A new Ka-band voltage controlled oscillator (VCO) using a planar ring resonator (RR) and a GaAs PHEMT microwave monolithic integrated circuit (MMIC) is presented. The resonator operates at harmonic frequencies and is manufactured on a temperature stable calcium magnesium titanate substrate using photolithographic thin-film processes. An innovative fully automated active laser-trimming procedure is used to adjust the frequency of the free-running oscillator as well as the varactor tuning sensitivity. In addition, automated laser-trimming is used to equalize the electrical length between the ring resonator and the MMIC. High unloaded quality factors of more than 350 have been obtained for the harmonic ring resonator. With these high quality factors a single side-band phase noise of better than -110 dBc/Hz is achieved at an offset frequency of 1 MHz. In addition, the frequency tuning range of the laser-trimming procedure is larger than 10% without any significant change of the VCO's characteristics such as phase noise and varactor tuning sensitivity. The typical temperature drift of the oscillator frequency is less than 4 ppm/K. We use a RR-VCO with an electrical length of four wavelengths, which is phase-locked to an external reference, as a local oscillator in our latest microwave Point-to-Multipoint (PMP) transceivers.