Ashwin Samarao

Temperature compensation of silicon micromechanical resonators via degenerate doping

We report on the degenerate doping of a silicon resonator as a new method for reducing its temperature coefficient of frequency (TCF). This is the first TCF reduction technique reported till date that takes advantage of free charge carrier effects on the elastic constants of silicon. The TCF of silicon bulk acoustic resonators (SiBAR) are reduced from −29 ppm/°C to −1.5 ppm/°C on 5 µm thick devices using degenerate boron doping and to −2.72 ppm/°C on 20 µm thick devices using boron-assisted aluminum doping while maintaining a high quality factor (Q) of 28000 in vacuum.

Passive TCF compensation in high Q silicon micromechanical resonators

This paper reports on passive temperature compensation techniques for high quality factor (Q) silicon microresonators based on engineering the geometry of the resonator and its material properties. A 105 MHz concave silicon bulk acoustic resonator (CBAR) fabricated on a boron-doped substrate with a resistivity of 10−3 Ω-cm manifests a linear temperature coefficient of frequency (TCF) of −6.3 ppm/°C while exhibiting a Q of 101,550 (fQ = 1.06×1013). The TCF is further reduced by engineering the material property via a wafer-level aluminum thermomigration process to −3.6 ppm/°C while maintaining an fQ of over 4×1012. Such high fQ products with low TCF values are being reported for the first time in silicon and are critical for successful insertion of these devices into low-power low-phase noise frequency references and high performance resonant sensors.

Temperature Compensation of Silicon Resonators via Degenerate Doping

This paper demonstrates the dependence of temperature coefficient of frequency (TCF) of silicon micromechanical resonators on charge carrier concentration. TCF compensation is demonstrated by degenerate doping of silicon bulk acoustic resonators (SiBARs) using both boron and aluminum dopants. The native TCF of -33×ppm/^°C for silicon resistivity of >; 10³ Ω · is shown to reduce to -1.5×;ppm/^°C at ultralow resistivity of ~10×^-4;Ω·cm using relatively slow diffusion-based boron doping. However, the faster thermomigration-based aluminum doping offers TCF reduction to as low as -2.7×;ppm/^°C with much reduced processing time. A very high <i>Q</i> of 28 000 at 100 MHz is measured for a temperature-compensated SiBAR.

A 145MHz low phase-noise capacitive silicon micromechanical oscillator

This paper reports on the implementation and characterization of a low phase-noise oscillator based on a very high quality factor (Q) 145MHz capacitive silicon micromechanical resonator. The utilized resonator is a silicon bulk acoustic resonator (SiBAR) operating in its first width-extensional mode with a maximum Qunloaded~74,000 that is specifically optimized for low motional impedance. The sustaining circuitry is a 3.6mW CMOS transimpedance amplifier (TIA) that uses common source topology with local shunt-shunt feedback. The measured phase-noise of the oscillator at 1kHz offset from the carrier is -111dBc/Hz with phase-noise floor reaching below -133dBc/Hz.

Post-Fabrication Electrical Trimming of Silicon Bulk Acoustic Resonators using Joule Heating

This paper presents a new method to electrically trim the resonance frequency of a Silicon Bulk Acoustic Resonator (SiBAR) post fabrication. Width-extensional mode silicon resonators are heated by passing a current through their resonating elements. This causes a mass loading gold pattern to diffuse into the bulk of the resonator. Upon cooling, the gold diffusion increases the stiffness of the resonating structure slightly, which reflects as an upward shift in resonance frequency. Thus, silicon resonators can be permanently trimmed to a desired frequency value by an electrical calibration step. As a proof of concept, an upward frequency shift of 240 kHz is demonstrated for a 40% mass loaded 100 MHz SiBAR after one hour of Joule heating with 30 mA of DC current.

Intrinsic temperature compensation of highly resistive high-Q silicon microresonators via charge carrier depletion

We report on a novel temperature compensation technique that exploits the dependence of TCF on the free charge carriers in silicon bulk acoustic resonators (SiBARs). The free charge carriers are considerably minimized by creating single and multiple pn-junction based depletion regions in the body of the resonator. The TCF of a highly resistive (>1000 Ω-cm) conventional rectangular SiBAR has been reduced from −32 ppm/°C to −3 ppm/°C. We previously exploited the dependence of TCF on silicon resonator geometry for TCF compensation. However, at large charge carrier depletion levels achieved in this work, the TCF is found to become independent of silicon resonator geometry.

Combined capacitive and piezoelectric transduction for high performance silicon microresonators

This paper introduces the Aluminum Nitride - High Aspect-Ratio Polysilicon and Single-crystal Silicon (AlN - HARPSS) process technology that for the first time enables combined capacitive (via air-gaps) and piezoelectric (via Mo/AlN/Mo piezo-stack) transduction in silicon micromechanical resonators. Lateral air-gaps as small as 150 nm have been realized for a 20 µm thick microresonator (air-gap aspect-ratio = 133∶1) while simultaneously improving the c-axis orientation of aluminum nitride sputtered on its top surface. Such a combined transduction has been demonstrated to efficiently harvest the individual advantages of both the technologies. A 100 MHz silicon microresonator under combined capacitive and piezoelectric transduction measures a ∼25 dB reduction in feedthrough compared to a capacitive-only transduction while measuring a 106% improvement in quality factor (Q), 10 dB reduction in insertion loss (I.L.) and a substantial suppression of spurious modes compared to a piezoelectric-only transduction.

Postfabrication Electrical Trimming of Silicon Micromechanical Resonators via Joule Heating

This paper presents a method to electrically trim the resonance frequency of a silicon bulk acoustic resonator (SiBAR) after its fabrication is completed. The small volume of the microresonator can be Joule heated to a sufficiently high temperature to allow for diffusion of deposited metals from its surface onto its bulk. Such high temperatures also facilitate the formation of silicon-metal bonds which, depending on the metal, are either stronger or weaker compared to the existing silicon-silicon bonds. The former leads to an overall increased stiffness of the resonating element thereby trimming up its resonance frequency, while the latter does the opposite. Both trimming-up and trimming-down by ~400 kHz have been demonstrated at a resonance frequency of 100 MHz (i.e., trimming range of 4000 ppm) using gold and aluminum, respectively. The possibility of increasing the trimming range to ~4 MHz (i.e., 40 000 ppm) by engineering the resonator geometry is also discussed and demonstrated.

Rapid Fabrication of a Nano Interdigitated Array Electrode and its Amperometric Characterization as an Electrochemical Sensor

Nano interdigitated array (IDA) electrodes (electrode finger width = 100 nm; finger spacing = 200 nm; surface area = 0.2 mm2) have been fabricated and characterized amperometrically for the electrochemical detection of the concentrations of reversible redox species. Using p-aminophenol as the redox species, a detection limit of 10 pM of the species concentration has been achieved. This detection limit is three orders of magnitude lower than the micro IDA counterpart that has been reported to date, proving the enhanced redox cycling at the nano IDA electrodes. Using a higher electron dose, the proximity effect of the electron beam in the e-beam lithography process has been utilized to reduce the duration of the nano IDA pattern transfer step to less than 30 minutes. This makes it possible to fabricate the entire sensor within a day, including the electrode metal evaporation, metal lift-off and electroplating of reference electrode.

Self-polarized capacitive silicon micromechanical resonators via charge trapping

We present for the first time a charge trapping technique as a viable passive biasing mechanism for capacitive silicon micromechanical resonators. Potential wells are created on the surface of the microresonator to trap charges for mimicking a polarization voltage (Vp) of 8 V. With no externally applied Vp, the resonance peak of a 20 µm thick silicon bulk acoustic resonator (SiBAR) with 50 nm transduction air-gaps comes up by ∼25 dB from the noise floor. An insertion loss (I.L.) of 30.7 dB and a quality factor (Q) of 59,000 has been measured in vacuum at a resonance frequency of 104.81 MHz.