Suspended Inductors Research Articles

Origami-Inspired Fabrication of High-Performance Wafer-Level Packaged Three-Dimensional Radio-Frequency Inductors

Planar radio-frequency (RF) on-chip inductors suffer from large electromagnetic losses, stemming from the low resistivity silicon substrate. Though it is well-known that 3-dimensional (3D) MEMS-based RF inductors offer significant performance enhancement by reducing the substrate proximity effects, they are prone to mechanical failures and often come without any appropriate packaging, leading to reliability issues. Herein, we report an approach to fabricate highly robust packaged 3D RF inductors, while relying on the concept of stress-induced self-assembly. By leveraging the large residual stresses in evaporated chromium thin films at low thicknesses (below 50 nm), we assemble the planar copper coils coated with chromium nano-layers into out-of-plane folded RF inductors, directly improving their <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> -factor. Through measurements, we show more than three-fold ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 300%) improvement in the inductor <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> -factor after the bending. Considering the inherent fragility of suspended inductors, we then add a dedicated wafer-level polymer packaging to our inductors, to improve their mechanical robustness. Specifically, we coat a thick SU-8 layer using a quasi-static volumetric dispensing process, to embed the inductors without deforming them. Upon baking, the polymer layer solidifies, making the buried inductors extremely rigid. We show that though our inductors have <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> -factors similar to existing MEMS inductors, their low design complexity and high strength gained via polymer packaging make them stand out among the others. This packaging technique essentially paves the way for the widespread commercialization of suspended inductors, which has been hindered in the past by their poor mechanical reliability. 2023-0018

Analysis of the Failure and Performance Variation Mechanism of MEMS Suspended Inductors with Auxiliary Pillars under High-g Shock.

Microelectromechanical systems (MEMS) suspended inductors have excellent radio frequency (RF) performance and they are compatible with integrated circuit (IC). They will be shocked during manufacturing, transportation, and operation; in some applications, the shock amplitude can be as high as tens of thousands of gravitational acceleration (g, 9.8 m/s2). High-g shock will lead to the inductor deformation which affects its performance or even failure of the inductor structure. However, few studies have been carried out on the inductors under high-g shock. In this study, a kind of MEMS suspended inductor with excellent RF and mechanical performance is designed and fabricated. The failure and performance variation mechanism of the inductor under high-g shock is analyzed by measuring and comparing the performance measurement results and the π model parameters extraction results of the inductors before and after air cannon shock test. The results show that the increase of energy loss caused by substrate parasitic effect and the properties variation of the coil material affected by high-g shock are the main reasons for the decrease of RF performance parameters, and the critical stress exceeding the interlayer adhesion is the main reason for the failure of the inductor.

A Suspended Thick-Winding Inductor for Integrated Voltage Regulator Applications

In this letter, a suspended thick-winding inductor is demonstrated for integrated voltage regulator applications. A suspended thick-winding inductor with an area of 2.9 mm2 and an inductance of 25 nH is designed and fabricated. The inductor has a winding thickness of $65~\mu \text{m}$ and a DC resistance of 121 $\text{m}\Omega $ . A quality factor >30 is maintained at 100–200 MHz. A peak inductor efficiency of 97.8% is analytically calculated for voltage conversion from 1.8 V to 1.1 V at 100 MHz.

Analysis of the Performance Variation Mechanism of MEMS Suspended Inductors under Mechanical Shock.

Micro-electromechanical system (MEMS) suspended inductors have been widely studied in recent decades because of their excellent radio frequency performance. However, the deformation of the inductor coil and the performance variation usually occur to the MEMS suspended inductors when the inductors are under mechanical shock. Few studies have been carried out on the performance variation of MEMS suspended inductors under shock. In this study, the mechanism of the performance variation of MEMS suspended inductors under mechanical shock is analyzed by combining theoretical analysis and experiments. A theoretical analysis based on the lumped-element equivalent model is presented and shock tests are carried out. The shock tests show that the main reason of the MEMS suspended inductor performance variation after mechanical shock is the variation of the substrate parasitic effect, which is caused by the variation of the suspension height of the inductor after shock. The test results agree with the theoretical analysis.

Method of Calculating the Inductance Value of MEMS Suspended Inductors with Silicon Substrates.

Microelectromechanical system (MEMS) suspended inductors have excellent radio-frequency (RF) performance. The inductance value is one of the main features that characterizes the performance of inductors. It is important to consider the influence of the substrate and the suspension height in calculating the inductance value accurately. In this paper, a method is proposed to calculate the inductance value of the MEMS suspended inductor wire with a silicon substrate, as the wire is the basic component of the inductor coil. Then the method is extended to the suspended inductors consisting of a single turn coil. The calculation results obtained by this proposed method were verified by finite-element analysis (HFSS) and they were found to agree well with the results of the HFSS simulation.

Failure mode analysis of MEMS suspended inductors under mechanical shock

Micro-electromechanical system (MEMS) suspended inductors have been widely studied in recent decades because of their excellent radio frequency performance. However, few studies have been performed on the failure analysis of MEMS suspended inductors under mechanical shock. In this study, the failure of MEMS suspended inductors with a planar spiral coil is investigated through analytical and experimental methods. We present a stress and deformation analysis to study the failure mode of the suspended inductors under shock. To verify the theoretical analysis, MEMS inductors are designed and fabricated, and shock tests are carried out. The shock tests show that the failure mode of the MEMS suspended inductors is a fracture that occurs at the ends of the inductor coil, and the test results agree with the theoretical analysis.

Mechanical Response of MEMS Inductor with Auxiliary Pillar under High-g Shock.

Micro-electromechanical system (MEMS) suspended inductors have excellent radio-frequency (RF) performance, but poor mechanical properties. To improve their reliability, auxiliary pillars have been used. However, few studies have been carried out on the response of a suspended inductor with auxiliary pillars under high mechanical shock. In this paper, a theoretical method is proposed that combines a single-degree-of-freedom (SDOF) model and a method for solving statically indeterminate structures. The calculated results obtained by this proposed method were verified by finite-element analysis (ANSYS). The calculated results obtained by the proposed method were found to agree well with the results of ANSYS simulation. Finally, this method was extended to a suspended inductor with double auxiliary pillars. The method proposed in this paper provides a theoretical reference for mechanical performance evaluation and reliability optimization design for MEMS suspended inductors with auxiliary pillars.

Design and Analysis of a Micromachined LC Low Pass Filter For 2.4GHz Application

This paper reports design and analysis of a passive low pass filter with cut-off frequency of 2.4 GHz using MEMS (Micro Electro-Mechanical Systems) technology. The passive components such as suspended spiral inductors and metal-insulator-metal (MIM) capacitor are arranged in T network form to implement LC low pass filter design. This design employs a simple approach of suspension thereby reducing parasitic losses to eliminate the performance degrading effects caused by integrating an off-chip inductor in the filter circuit proposed to be developed on a low cost silicon substrate using RF-MEMS components. The filter occupies only 2.1 mm x 0.66 mm die area and is designed using micro-strip transmission line placed on a silicon substrate. The design is implemented in High Frequency Structural Simulator (HFSS) software and fabrication flow is proposed for its implementation. The simulated results show that the design has an insertion loss of -4.98 dB and return loss of -2.60dB.

Electromechanics of suspended spiral capacitors and inductors

Most electromechanical devices are in two-dimensional metallic drums under high tensile stress, which causes increased mechanical frequency and quality factor. However, high mechanical frequencies lead to small zero-point displacements xzp, which limits the single-photon interaction rate g0. For applications which demand large g0, any design with increased xzp is desirable. It is shown that a patterned drum by a spiral shape can resolve this difficulty, which is obtained by a reduction of mechanical frequency while the motion mass is kept almost constant. An order of magnitude increase in g0 and agreement between simulations and interferometric measurements is observed.

High performance suspended spiral inductor and band-pass filter by wafer level packaging technology

Suspended inductors and 2.45 GHz BPF with patterned ground shields on the lossy silicon substrate by using Cu/BCB based wafer level packaging and bulk Si etching technologies were fabricated. Thick BCB interlayer is used as the supporting dielectric and the backside cavity is easily removed by using a two-step back-etching process. The fabricated 2.7 nH inductor has a maximum Q factor 49 at 8.2 GHz and high Q factors more than 22 in the broadband frequency range from 1 to 10 GHz. And the realized 2.45 GHz BPF has the insertion loss of 3.0 dB and the return loss of more than 14 dB at the pass band. It is also featured by more than 48 and 25 dB attenuation at 0.9 and 1.8 GHz respectively, with the second harmonic rejection being 33 dB.

A NOVEL WL-INTEGRATED LOW-INSERTION-LOSS FILTER WITH SUSPENDED HIGH-Q SPIRAL INDUCTOR AND PATTERNED GROUND SHIELDS

A novel wafer level integrated low-insertion-loss filter working at 1.8 GHz (DCS LPF) with suspended inductors and patterned ground shields on the lossy silicon substrate is fabricated. Thick BCB interlayer is used as the supporting dielectric, and the backside cavity on Si wafer is formed by using a two-step back-etching process. The influence of patterned ground shields on the Q factor of the suspended inductors and the influence of low-resistivity silicon on the insertion loss of filters are analyzed by EM simulation. The fabricated 2.7 nH inductor has a maximum Q factor of 49 at 8.2 GHz and high Q factors more than 22 in the broadband frequency range from 1 GHz to 10 GHz. And the realized LPF in DCS band has the insertion loss of 0.35 dB and return loss of more than 15.5 dB at the pass band, with the second harmonic rejection being 23 dB and the third harmonic rejection being 38 dB respectively.

RF Performances of MEMS Suspended Inductor under High Overload Environments

The radio frequency (RF) performances of MEMS suspended spiral inductor under high overload environments are studied. Firstly, a suspended spiral inductor and its MEMS surface micromachining process which is compatible with CMOS process are developed. Then, the mechanical responses and RF performances of the inductor are simulated by ANSYS and HFSS, respectively. The simulation results show that, as the overload increases, the inductance and quality factor decrease significantly when the frequency band is closed to the resonant frequency but have no significant change when the frequency band is much lower than resonant frequency; the resonant frequency of the suspended inductor decreases monotonically with the increase of overload. A modified lumped parameter model is utilized to illustrate the simulation results, which theoretically indicates that the substrate loss is more severe than the ohmic loss as the overload increases.

ANALYTICAL OPTIMIZATION OF HIGH PERFORMANCE AND HIGH QUALITY FACTOR MEMS SPIRAL INDUCTOR

In this paper, design and simulation of optimized MEMS spiral inductor are presented. The efiects of design parameters on characteristics of inductor have been considered. The suspended spiral inductor was designed on silicon substrate using MEMS technology to reduce the metal and substrate losses of inductor. The results show that the quality factor of the inductor is 27 at 5.23GHz and that the maximum Q-factor is 42 at 26.56GHz. The dimension of the inductor is 185 £ 200m 2 , which occupies less area on chip than other works. In this work, the high quality factor inductor with small size is obtained.

Suspended high Q integrated inductor by wafer level packaging technology

A novel integrated passive devices fabrication technique which combines wafer level packaging and bulk Si etching technologies to prepare silicon based suspended inductors with high quality factor (Q) is presented. Inductors based on copper/benzocyclobutene and copper/polyimide structures are fabricated with two-step back-etching technology on low resistive silicon wafer. The characteristics of 3-turn spiral inductors are investigated. The measured Q of 6.8 nH suspended inductors are 29.3 at 2.4 GHz and 20.6 at 5 GHz while the Q of the others without suspended are 11.0 at 2.4 GHz and 4.4 at 5 GHz, respectively. This technique can significantly improve the performance in high frequency range and has a wide application in hand-held RF module.

Development and Characterization of a CMOS-MEMS Accelerometer With Differential LC-Tank Oscillators

This paper presents a CMOS-MEMS accelerometer based on differential LC-tank oscillators. Two LC-tank oscillators composed of differential MEMS capacitors and suspended inductors were designed and fabricated by standard TSMC 0.18 μm CMOS processes and post-CMOS dry etching. The outputs of the differential oscillators are mixed and the output frequency of the mixer is proportional to the capacitance change caused by the external acceleration. The device has been characterized and has a mechanical resonance frequency of 7 kHz, absolute sensitivity of 3.62 MHz/g, relative sensitivity of 1.9×10-3 Δf/f0/g, in-axis nonlinearity of 1.2%FS, bias stability of 2.1 mg, and average noise floor of 0.205-0.219 mg/√Hz in the frequency range of 15-250 Hz.

Performance Improvement of a 0.18-$\mu{\rm m}$ CMOS Microwave Amplifier Using Micromachined Suspended Inductors: Theory and Experiment

This paper presents the quality factor improvement of the micromachined suspended inductors and their usage in a fully integrated 0.18-μm CMOS microwave amplifier. Removing the lossy silicon substrate underneath the spiral inductors by adopting post-CMOS compatible microelectromechanical system (MEMS) technology allows construction of suspended structures with a high Q-factor. Although micromachining technology has had a remarkable impact on industries in recent years, it is still challenging to design fully integrated CMOS-MEMS active circuits at microwave frequencies. To evaluate the performance improvement of 0.18-μm CMOS microwave amplifiers by using micromachined suspended inductors, two amplifiers in the 24-GHz industrial, scientific, and medical band were designed and fabricated with and without the MEMS process for comparison. At 0.85-V low supply voltage, the measured peak gain of the microwave amplifier can be significantly increased from 12.3 to 13.7dB due to the high Q-factor suspended inductors. In addition, the measured noise figure of the 24-GHz microwave amplifier is effectively improved from 5.8 to 5.0dB. Theories for predicting the gain and noise figure improvement are also presented, and the mechanisms are validated by experiments. Also, a proposed simplified double-π micromachined inductor model considering the skin effect is introduced and characterized.

Manufacture and Characterization of High Q-Factor Inductors Based on CMOS-MEMS Techniques

A high Q-factor (quality-factor) spiral inductor fabricated by the CMOS (complementary metal oxide semiconductor) process and a post-process was investigated. The spiral inductor is manufactured on a silicon substrate. A post-process is used to remove the underlying silicon substrate in order to reduce the substrate loss and to enhance the Q-factor of the inductor. The post-process adopts RIE (reactive ion etching) to etch the sacrificial oxide layer, and then TMAH (tetramethylammonium hydroxide) is employed to remove the silicon substrate for obtaining the suspended spiral inductor. The advantage of this post-processing method is its compatibility with the CMOS process. The performance of the spiral inductor is measured by an Agilent 8510C network analyzer and a Cascade probe station. Experimental results show that the Q-factor and inductance of the spiral inductor are 15 at 15 GHz and 1.8 nH at 1 GHz, respectively.

Fast release process of metal structure using chemical dry etching of sacrificial Si layer

An ultra-fast removal process of a silicon sacrificial layer for the selective release of a metal structure on a Si substrate was studied, which uses a chemical dry etching method. The chemical dry etching of a Si layer was performed in an NF 3 remote plasma with the direct injection of additive nitric oxide (NO) gas. When the NO gas was injected into the chamber into which F radicals were supplied from a remote plasma source using NF 3 input gas, the silicon layer was removed selectively and the metal structure could be released easily. It was found that the etch rate on the sidewall (up to ≅ 18.7 μm/min for an opening width of 100 μm) and the bottom (up to ≅ 24.5 μm/min for an opening width of 100 μm) depends on the NO/(NO + Ar) gas flow ratio, time duration, and opening width. The developed dry etching process could be used to release a Ni structure with near infinite selectivity in a very short time. The process is well suited for fabricating various devices which require a suspended structure, such as in radio-frequency microelectromechanical system switches, tunable capacitors, high-Q suspended inductors and suspended-gate metal-oxide semiconductor field-effect transistors.

Capacitive RF switches manufactured by the CMOS–MEMS technique

This work investigates the fabrication and characterization of a capacitive radio frequency (RF) switch using the complementary metal oxide semiconductor (CMOS)–microelectromechanical system (MEMS) technique. The micromechanical RF switch had a high isolation and a low driving voltage. The structure of the micromechanical RF switch contains a coplanar waveguide (CPW), a suspended membrane, eight springs and four suspended inductors in series. The suspended inductors are used to enhance the characteristic of the RF switch. The finite element method software, CoventorWare, is employed to simulate the mechanical behaviors of the RF switch. After completion of the CMOS process, the RF switch requires a post-process to release the suspended structures. The post-process utilizes a dry etching and a wet etching to etch the sacrificial layer, and to obtain the suspended structures of the RF switch. Experimental results reveal that the RF switch has an insertion loss of 0.85dB at 35GHz and an isolation of 25dB at 35GHz. The driving voltage of the switch is about 11V.

Suspended Spiral Inductor and Band-Pass Filter on Thick Anodized Aluminum Oxide

We make suspended inductors and 2.45 GHz BPFs on the selectively anodized aluminum. The thick anodized alumina is used as the supporting dielectric and it is easily removed by using chemical wet-etching. The thickness of anodized alumina is 80 mum and the depth of the total air-cavity is 200 mum. The fabricated inductor over an air cavity has a 25% greater Q factor and 21% higher inductance at 2 GHz compared to an inductor on anodized aluminum. The 2.45 GHz BPF adjusting the suspended inductors has 2.8 dB of insertion loss with a narrow bandwidth.