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

Variable capacitors have a broad usage in radio frequency (RF) circuits. Microelectromechanical systems (MEMS) technology can provide variable capacitors with high quality factor and wide tuning range characteristics. One of the design goals for MEMS varactors has been linear capacitance- voltage (C-V) characteristics. To design a linear C-V varactor, a shaped-finger comb-drive actuator is proposed in this paper. The shaped-finger design method, originally developed to obtain linear wavelength-voltage relationships in a tunable optical filter, is modified in this work for a linear C-V varactor, which involves development of a new governing equation. Moreover, conformal mapping is employed in calculation of capacitances, making the whole design process almost all-analytical with the minimum usage of numerical analysis methods. Variable capacitors with the shaped-finger design show linearity factor (LF)—defined as the maximum deviation from the perfect linear relationship—as low as 0.4%, tremendously improved from that of the conventional constant-finger-gap devices (LF: 49.9%). The characteristics of the designed variable capacitor are further investigated through 3-D numerical analysis, and show LF better than 11.5% for the finger thickness in the range between 1 and 10 micrometers. Versatility of the design method is further demonstrated by design of a varactor with linear resonant frequency-voltage (f-V) characteristics for voltage-controlled oscillator (VCO) applications. The developed analytical design method with shaped fingers can find a wide range of applications where comb-drive actuators are used.

A frequency-tuning negative-conductance (-G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> ) boosted structure and applications for a voltage-controlled oscillator (VCO) is presented in this paper. Analog tuning varactors connected to a -G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> boosted structure is proposed to significantly alleviate the limitation of the tuning range for the -G <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</sub> boosted structure, resulting in a low-voltage low-power wide-tuning-range VCO. Based on the proposed architecture, the fabricated 0.18-μm CMOS VCO exhibits a measured 49.8% tuning range. Operating at 0.65 V low supply voltage, the VCO core consumes 2.37-mW dc power. At this bias condition, the measured average value of phase noise for all frequency ranges is -115.1 dBc/Hz at 1-MHz offset from the carriers. Relative to recently published wide-tuning-range CMOS VCOs, the proposed VCO simultaneously achieves low supply voltage, low dc power dissipation, low phase noise, and a wide tuning range, leading to a good figure-of-merit (FOM) and FOM including the tuning range. Furthermore, formulas of analysis for the proposed frequency-tuning -Gm boosted structure and wide VCO tuning range are presented, and the mechanisms are validated by experiments.

This paper presents the first demonstration of injection molding technology to enable large-scale mass manufacturing of high-performance tunable microwave filters to meet the growing needs of 5G small cell stations. This is the first time that a tunable filter satisfies all four of the following requirements simultaneously: low manufacturing cost, high quality factor, wide tuning range, and high power handling. Exhaustive research exists on the use of polymers for 3D microwave device manufacturing; nonetheless, mass-production technologies, such as injection molding, can provide low costs without compromising performance. The proposed bandpass filter implementation uses a tunable evanescent-mode cavity resonator injection molded with an acrylonitrile-butadiene-styrene thermoplastic polymer. In addition, changing the critical gap size over the resonator’s post using a commercial microactuator provides frequency tuning. The measured filter achieves an 86% tuning range from 2.8 – 5.2 GHz with a state-of-the-art measured unloaded quality factor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q_{u}$ </tex-math></inline-formula> of 1548 – 2573. The filter has a measured insertion loss of 0.06 - 0.1 dB with a fractional bandwidth from 7.6 - 8.4% across the entire tuning range. Moreover, for the first time in this manufacturing technology implementation, a bandpass filter is demonstrated with power handling capabilities beyond 100 W. The manufactured device demonstrates the significant potential of this technology for the scale-up manufacturing of reconfigurable high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> RF filters without compromising the performance.

Miniaturized tunable lasers are important as light sources in optical networks and sensing systems. However, challenges remain in realizing a low-cost on-chip tunable laser with high tuning accuracy and wide tuning range. Here, an external-cavity tunable laser integrated by using a pair of coupled-ring resonators as a reflector onto a single silicon chip is designed and demonstrated. The silicon coupled-ring reflector provides simultaneous tuning of the central reflective wavelength and the free spectral range of the entire cavity to guarantee phase matching. The effective optical length of the coupled-ring reflector is controlled by the complementary thermo-optic and free-carrier dispersion effects, thus enabling high-resolution wavelength fine-tuning while maintaining wide wavelength tuning range. Experimentally, the tunable laser has high external-cavity efficiency (22%) and high wavelength tuning accuracy (2 pm) throughout the 41.6 nm tuning range. Potential applications of the external-cavity tunable laser include optical networks, optical sensing, and on-chip optoelectronic systems.

Actuators for RF MEMS Devices

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.

A ring-shaped on-chip tunable capacitor is proposed and fabricated with CMOS-compatible micromachining processes for radio frequency integrated circuits (RFICs). The rotationally driven variable capacitor features a much higher axial mechanical stiffness compared to its conventional straight-line-driven counterpart, and therefore, can effectively depress the instability caused by environmental vibration when the varactor is used in mobile systems. Meanwhile, the rotationally driven intedigitated capacitor is designed to be very flexible for wide range of electrostatic tuning. Near-room-temperature micromachining techniques are developed for post-CMOS integrating the tunable capacitors into RFIC chips. The fabricated varactor is measured with a tuning ratio of 2.1:1 under an actuation of 12 V. Q-factor is measured as 51.3 at 1 GHz and 35.2 at 2 GHz, while self-resonance frequency is as high as 9.5 GHz. The rotationally driven tunable capacitor shows about two orders of magnitude higher antivibration capability compared to the conventional straight-driven one. Therefore, the high-performance CMOS-compatible tunable capacitors are promising for practical RFIC applications in mobile electronic telecom systems.

On-chip 1-D photonic crystal nanocavities were designed and fabricated in a 300 mm silicon-on-insulator wafer using a CMOS-compatible process with 193 nm immersion lithography and silicon oxide planarization. High quality factors up to 105 were achieved. By changing geometrical parameters of the cavities, we also demonstrated a wide range of wavelength tunability for the cavity mode, a low insertion loss and excellent agreement with simulation results. These on-chip nanocavities with high quality factors and low modal volume, fabricated through a high-resolution and high-volume CMOS compatible platform open up new opportunities for the photonic integration community.

A design method for a frequency-tunable planar bandpass filter is demonstrated. With the method, all reasonators can be designed with identical capacitances, allowing the filter to be tuned with identical varactors using a single bias control. This allows not only low-cost fabrication, but also a simple structure and design procedure. Moreover, the filter is based on miniaturized transmission lines. Therefore, a wide tuning range is achieved with a small size and outstanding out-of-band response. Most importantly, the method facilitates the design of a filter to vary the fractional bandwidth in a predetermined way as the center frequency is tuned. Thus, a tunable filter can be designed not only to maintain the fractional bandwidth, but also to expand or to reduce the bandwidth as the frequency is tuned. The experimental results of a compact third-order bandpass filter based on CPW lines with a 0.11λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ×0.8λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> size show a very wide frequency tuning range of 62.8% with a 3-dB fraction bandwidth that remains within 28.65±1.55% and an outstanding spurious suppression of under -40 dB up to as high as 3f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> .

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically &amp;gt;160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm.

A wide tuning range CMOS VCO based on a switched substrate-shield inductor is presented. The proposed VCO uses a high quality factor, switchable inductor with smaller parasitics for frequency tuning range extension. Inductance is switched by controlling the eddy currents in a modified, floating substrate-shield. Using the proposed design, a 29 % inductance switching is achieved while maintaining a high quality factor of > 15.5. The prototype VCO shows an excellent frequency tuning range of 34.8 % and a phase noise of −120.1 dBc/Hz at 10 MHz offset for 25.3 GHz carrier frequency. This VCO has a FOM T of 192.1±2.5 dBc/Hz across the tuning range with only 4.1 mW power consumption.

In this paper, we propose a tunable absorber to realize the same range of tuning and the same absorption rate with the same DC voltage in two orthogonal directions. The absorber consisting of a square metal with a gap in diagonal and a varactor diode is highly symmetrical and can induct the same current in x and y polarization, which ensures the identical absorption rate under the same DC voltage. In order to augment the tuning range of the absorber, the detailed analysis is carried out. There is an inevitable problem that the wide tuning range is closely related to the reflection coefficient of proposed absorber. Thus, a compromising dimension is decided to achieve relatively wide tuning range and good reflection in the meantime. The tuning range and absorption rate, respectively, can reach 30.8% and over 95% in simulation.

This paper proposes a hybrid class-AB/class-B voltage-controlled oscillator (VCO) with low voltage, low power, and a wide tuning range. The class-AB VCO core is designed to ensure robust start-up of the class-B VCO core. Featuring a high-efficiency class-B VCO core, the combination of class-AB/class-B VCO cores consumes low dc power. Based on the proposed architecture, the fabricated 0.18-μm CMOS VCO exhibits a measured tuning range of 25.6%. Operating at a low supply voltage of 0.75 V, the hybrid class-AB/class-B VCO cores consume a low total dc power of 2.4 mW. In this bias condition, the measured average value of phase noise for all frequency ranges is -101.4 dBc/Hz at 1-MHz offset from the carriers. Compared to recently published wide-tuning range VCOs, the proposed hybrid class-AB/class-B VCO simultaneously achieves a low supply voltage, a low dc power dissipation, and wide tuning range, leading to a good figure-of-merit including the tuning range (FOMT). In addition, the theories for analyzing the VCO are given in detail, and the mechanisms are validated by experiments.

A sixth-order low-pass transconductor-C filter with a very wide tuning range (fc = 100 Hz to 10 MHz) is presented. The wide tuning range has been achieved without using switchable components or programmable building blocks. A single-stage folded cascode transconductor is employed to implement the proposed filter. A modified biquadratic topology is introduced to improve linearity performance of the filter over its tuning range. Power consumption of the filter scales linearly with cutoff frequency (60 pW/Hz/pole). Implemented in 0.18-μm complementary metal-oxide-semiconductor technology, the filter exhibits relatively constant noise and linearity performance over its entire tuning range and occupies a silicon area of 0.16 mm2 (0.027 mm2/pole).

Optical source generation has attracted significant attention recently, especially in fiber optical communications. Today there is a growing a demand for optical source generation beyond conventional telecommunication wavelength bands. However, high quality and versatile optical source is generally not available over those wavelength bands due to the lack of efficient gain medium. Thanks to fiber optical parametric amplifier (FOPA), which is based on the third order nonlinear susceptibility of optical fibers, offers ultrafast response, wide-gain bandwidth, high gain and large frequency detune from the pump, serves as a promising candidate for signal amplification over those wavelength bands. By using the corresponding fiber optical parametric oscillator (FOPO) configuration, widely tunable source from continuous-wave (CW) to sub-picosecond pulses can be potentially generated to serve different applications from communication to biomedical imaging.\n\nIn this thesis, we first demonstrate an all-fiber widely-tunable picosecond FOPO using highly-nonlinear fiber (HNLF). The tuning range is as wide as 250 nm, which is higher than previous picosecond FOPOs reported in the 1550-nm region. Second, time-dispersion tuning of the FOPO is investigated with fixed pump wavelength. It is a relatively simple and economic approach, and there will be no filter induced cavity loss. We then describe using FOPO to generated nearly-transform limited sub-picosecond pulses with a 60-nm tuning range. Another FOPO with a tuning range of 440-nm with dispersion-shifted fiber (DSF) as the gain medium is proposed and demonstrated. Compared with FOPOs demonstrated using HNLF as the gain medium, the use of DSF offers two key advantages: a wider tuning range and a narrower linewidth.\n\nIn addition to picosecond FOPO, CW FOPO is also of great interest in fiber optical communications and biomedical imaging. We also demonstrate an all-fiber CW single-longitudinal-mode (SLM) FOPO with tuning range covers the S and L bands. SLM oscillation with a side-mode suppression ratio greater than 43 dB is achieved, which has been extended to 1-μm region under stable operation.\n\nApart from static tuning, dynamic wavelength tuning of the FOPO is also discussed in this thesis with a cumulative speed exceeds 4,000,000 nm/s, which is higher than previous work reported in wavelength-swept FOPOs. The high-speed swept source would be useful in biomedical imaging and sensing applications.\n\nThe amplification of the sub-picosecond pulses of the FOPO output is also investigated, for the first time to our knowledge, by using a fiber optical parametric chirped pulse amplifier(FOPCPA).The totally fiber-integrated nature of the whole system allows complete self-alignment and further integration to other fiber-based systems.\n\nAll these research effort will show the versatility of FOPO techniques for generating wide range of optical sources for varies applications. These schemes may be useful in generating CW and short pulse for potential optical communication and biomedical imaging in non-conventional wavelength bands.