Klaus Reimann

On the Trade-Off Between Quality Factor and Tuning Ratio in Tunable High-Frequency Capacitors

A benchmark of tunable and switchable devices at microwave frequencies is presented on the basis of physical limitations to show their potential for reconfigurable cellular applications. Performance limitations are outlined for each given technology focusing on the quality factor (Q) and tuning ratio (eta) as figures of merit. The state of the art in terms of these figures of merit of several tunable and switchable technologies is visualized and discussed. If the performance of these criteria is not met, the application will not be feasible. The quality factor can typically be traded off for tuning ratio. The benchmark of tunable capacitor technologies shows that transistor-switched capacitors, varactor diodes, and ferroelectric varactors perform well at 2 GHz for tuning ratios below 3, with an advantage for GaAs varactor diodes. Planar microelectromechanical capacitive switches have the potential to outperform all other technologies at tuning ratios higher than 8. Capacitors based on tunable dielectrics have the highest miniaturization potential, whereas semiconductor devices benefit from the existing manufacturing infrastructure.

Small, low-ohmic RF MEMS switches with thin-film package

We report on small, low-ohmic RF MEMS switches with a circular membrane actuator design. A low temperature process is used to manufacture both the MEMS switch as well as its hermetic, thin-film package resulting in a very small footprint device. The hermetic seal of the package significantly increases the switch lifetime and reliability. The switches demonstrate good RF performance and high switching speeds. A comparison with other MEMS switches reveals that these MEMS switches possess a low on-resistance while occupying only a very small area on the wafer.

Comparison of high-resistivity silicon surface passivation methods

This paper describes low-frequency measurements and comparative analysis of methods used for surface passivation of high-resistivity silicon (HR-Si). A number of substrates are evaluated; n-type and p-type HR-Si, with and without surface passivation by means of polysilicon or Ar-ion implantation. Additionally, a selection of samples is prepared with a layer of ferroelectric material. Substrate characteristics are extracted from measurements of the samples, allowing comparison of passivation methods and evaluation of the influence of the ferroelectric film. The study shows all passivation methods successful in removing any bias-dependence of substrate properties. Further, the high-temperature processing of the ferroelectric film is observed increasing the extracted substrate conductivity by about 70% for the Ar-ion implanted samples, and about 50% for the p-type samples passivated by poly-Si. The effective substrate conductivity of the n-type samples passivated by RTA-crystallised poly-Si appears unaffected.

Performance of Coplanar Waveguides on Surface Passivated Highly Resistive Silicon Covered by Ferroelectric Thin Film

Coplanar waveguides (CPWs) on surface passivated highly resistive Si (HRS) covered by ferroelectric film are studied. The focus is on passivation techniques reducing losses caused by surface charge accumulation at the Si/SiO2 interface. Different passivation methods are considered, Ar-ion bombardment into the silicon substrate and deposition of poly-Si below the oxide layer. Both methods remove any bias-dependence and decrease the losses associated with surface charge accumulation. It is also concluded that the effect of passivation remains after pulsed laser deposition of a ferroelectric thin film.

A 0.25mm 2 AC-biased MEMS microphone interface with 58dBA SNR

Capacitive MEMS microphone roadmaps are mainly driven by increasing SNR and reducing size/cost. This requires smaller microphones, ASICs with lower noise and smaller area, and cheaper packaging. Because of fundamental limitations, traditional DC-biased microphones will have difficulty following these trends. This paper proposes an AC-biasing scheme, which leads to a significant reduction in ASIC size and module packaging cost.

Comparison of electrical techniques for temperature evaluation in power MOS transistors

Three electrical techniques (pulsed-gate, AC-conductance and sense-diode) for temperature evaluation in power MOS transistors have been experimentally compared on the same device. The device under test is a silicon-on-insulator (SOI) laterally-diffused MOSFET (LDMOS) design with embedded sense-diodes in the center and at the edge of the device for providing local temperature information. On-wafer measurements have been performed on a thermal chuck in the temperature range 25-200°C to extract self-heating information and predict the junction temperature for different biasing conditions. Good agreement (within 10%) between the different techniques is achieved, evidencing that reliable temperature estimations can be made using each of the proposed electrical techniques. As a result, factors other than experimental accuracy will play a role in the choice of the most adequate technique for the application of interest. Guidelines for this choice are provided in a benchmarking analysis accounting for ease of application, temperature calibration and accuracy of the results.

Integrated ferroelectric stacked mim capacitors with 100 nF/mm 2 and 90 V breakdown as replacement for discretes

This paper shows for the first time integrated thin film ferroelectric metal-insulator-metal capacitors on silicon with a record high capacitance density above 100 nF/mm2 combined with a breakdown voltage of 90 V and a lifetime exceeding 10 years at 85degC and 5 V. The high capacitance density was obtained by a combination of material optimizations resulting in a dielectric constant of 1600, and stacking of capacitors. The reliability of these ferroelectric capacitors was studied in detail with accelerated lifetime testing. The high performance of the integrated capacitors in this paper shows great potential for applications demanding high capacitance densities combined with electrostatic discharge protection.

MEMS oscillating squeeze-film pressure sensor with optoelectronic feedback

This work reports on an oscillating pressure sensor that converts pressure into frequency using the squeeze-film effect. A new aspect is the laser Doppler vibrometer (LDV) in the optoelectronic feedback loop that is used to bring the sensor element into sustained mechanical oscillation. A phase shifter and automatic gain control circuit stabilize the oscillation. The frequency stability of the pressure sensor is investigated by measuring its Allan deviation and is compared to the performance of a quartz oscillating pressure sensor. Finally, the pressure resolution of this oscillating sensor is compared to conventional pressure sensors.

MI004 miniaturised, high performance ferroelectric and piezoelectric thin film devices

Thin film ferroelectric capacitors have been integrated with resistors and active functions such as ESD protection into small, miniaturized modules, which enable a board space saving of up to 80%. With the optimum materials and processes, integrated capacitors with capacitance densities of up to 100 nF/mm2 and breakdown voltages of exceeding 90 V have been achieved. The integration of these high density capacitors with extremely high breakdown voltage is a revolution in the world of passive components and has not yet been achieved in any other passive integration technology. Furthermore, thin film tunable capacitors based on barium strontium titanate with high tuning range and high quality factor at 1 GHz have been demonstrated. Finally, piezoelectric thin films for piezoelectric switches with high switching speed have been realized.

Thin Film Piezoelectric MEMs Devices

Thin film piezoelectric devices, processed in Si‐related processes, are attractive for ultrasound transducers and piezoelectric switches. Thin film ultrasound transducer enable large bandwidth (> 100%), high frequency operation. In piezoelectric micromachined ultrasonic transducers (PMUTs) the ultrasonic waves are generated by flexural motion of the membrane, which is coupled to strain in the piezoelectric film. We have investigated the piezoelectric properties of thin films for ultrasound transducers and piezoelectric switches. Thin film piezoelectric ultrasound transducers as well as piezoelectric switches have been designed, processed and characterized. Thin film piezoelectric devices with excellent quality and reliability have been realized.