Electret Film Research Articles

Silicon Electret Condenser Microphone with Inorganic Electret

We developed a silicon electret condenser microphone (ECM) fabricated with the micro-electro-mechanical systems (MEMS) process using single-crystalline silicon. A silicon ECM has a SiO2/Si3N4 inorganic electret film (permanently charged dielectric) within microphone element, and it can remove 48-V bias voltage. We confirmed that a prototype microphone operated with 3-V or 1.5-V power supply had excellent acoustic characteristics. Experimental results also show that developed silicon-based electret had high heat and humidity resistance. We conclude that it is feasible to use such low-voltage high-performance MEMS microphones for a wide range of applications such as broadcasting, hearing-aids, and mobile communications.

G030061 シリカ凝集体を用いた耐熱エレクトレットフィルムの作製と特性評価

G030061 シリカ凝集体を用いた耐熱エレクトレットフィルムの作製と特性評価

In situ wafer-level polarization of electret films in MEMS acoustic sensor arrays

Polymer electrets, polymers with a quasi-permanent electrostatic polarization, are commonly employed in a macro-scale form factor within transducers such as the electret condenser microphone. In addition, MEMS-based electret transduction has been reported for both acoustic sensors and energy harvesters. In these micro-devices, the polymer film is typically polarized prior to assembly into the device. Common techniques employed in MEMS electret polarization include corona discharge and backlighted thyratron, with wafer-bonding and simple stacked assemblies being employed to perform the actual assembly and integration. In contrast, this paper reports a method for post-release in situ polarization of electret films within a MEMS device. The method utilizes microplasma discharges with self-aligned charging grids integrated within the device to charge fluoropolymer films in a fashion similar to the corona discharge technique. This in situ approach enables the integration of uncharged electret films into MEMS and subsequent post-fabrication charging, simultaneously enabling the formation of buried or encapsulated electrets as well as eliminating the need to restrict fabrication processes that might otherwise discharge pre-charged electret materials. The method is applied to a single-chip array of ultrasonic sensors designed to capture and analyze waveforms from impacts.

The Investigation of Electret Film Durability in Polyethylene Terephthalate (PET) from a Certain Angle of their Application as Pressure Sensors

Article presents research methods and their results for 100μm polyethylene terephthalate PET film, from a certain angle of its application as pressure sensors. Film was polarized at 150V/μm electric field intensity in dwo different temperatures: 60°C and 90°C. Direction of polarization was measured by thermally stimulated depolarization current TSDC. Results shows presence of homo-and heterocharge. Relaxation was calculated by two methods: from relationship ln J (T) = const – W/kT and from relationship (T) = P (T) / J (T). Activation energy for heterocharge calculated by first relationship is 6.14 eV to 6.77 eV and its relaxation time in 24°C is ~1020 years. For second one relationship, relaxation time in 24°C is ~1035 years. Piezoelectric voltage in 33 direction is 74V for termination resistance 1014Ω and 30V for termination resistance 109Ωfor applied stress 10N/cm2 from 10 cm2 surface. Obtained results for studied PET film, encourage to use it in practice as pressure sensors.

Effect of Polarization Temperature on the Chain Segment Motion and Space Charge Detrapping in Polyamide 610 Film Electrets

The effect of polarization temperature on the chain segment motion and charge trapping and detrapping in polyamide 610 films has been investigated by means of thermally stimulated depolarization current (TSDC) and wide-angle X-ray diffraction (WAXD). A small part of the amorphous phase of quenched polyamide 610 changes into the crystalline state with increasing polarization temperature. There are three current peaks (named α, ρ1, and ρ2 peak, respectively) in the TSDC spectra. The α peak corresponds to the glass transition, the ρ 1 peak is attributed to space charge trapped in the amorphous phase, and interphase between crystalline and amorphous phases, and the ρ 2 peak originates from space charge trapped in the crystalline phase. By analyzing the characteristic parameters of these peaks, it was found that the increase of polarization temperature induced a decrease of the chain segment mobility and promoted the creation of structural traps in polyamide 610. The decrease of the chain segment mobility in the amorphous phase made the intensity of the α peak weak and the activation energy increased. The higher the polarization temperature, the higher the degree of crystallinity and the more charge carriers trapped in the crystalline phase. So, the increase of polarization temperature made the intensity of the ρ 2 peak strong and increased the stability of trapped charge in the crystalline phase. The increase of polarization temperature also made the intensity of the ρ 1 peak strong and decreased the stability of trapped charge in the amorphous phase and interphase.

Effect of Dispersing Ag Micro Particles on Enhancing Surface Potential of CYTOP Electret Film

Polymer CYTOP is known as a good electret material that traps electrons semipermanently. It realizes a high surface potential, making it possible to be used for a micro power generator recently besides a microphone. We note that electrons are liable to be trapped at the interface between different materials. Ag micro particles are dispersed inside a CYTOP film for making interfaces. As a result, a surface potential that is two times higher than that of a pure film was experimentally achieved. The film preparation, the electron charging method, and the charge holding property are also reported.

Effect of Dispersing Ag Micro Particles on Enhancing Surface Potential of CYTOP Electret Film

Effect of Dispersing Ag Micro Particles on Enhancing Surface Potential of CYTOP Electret Film

Electret accelerometers: Physics and dynamic characterization

Electret microphones are produced in numbers that significantly exceed those for all other microphone types. This is due to the fact that air-borne electret sensors are of simple and low-cost design but have very good acoustical properties. In contrast, most of the discrete structure-borne sound sensors (or accelerometers) are based on the piezoelectric effect. In the present work, capacitive accelerometers utilizing the electret principle were constructed, built, and characterized. These electret accelerometers comprise a metallic seismic mass, covered by an electret film, a ring of a soft cellular polymer supplying the restoring force, and a metallic backplate. These components replace membrane, spacer, and back electrode, respectively, of the electret microphone. An adjustable static pressure to the seismic mass is generated by two metal springs. The dynamic characterization of the accelerometers was carried out by using an electrodynamic shaker and an external charge or voltage amplifier. Sensors with various seismic masses, air gap distances, and electret voltages were investigated. Charge sensitivities from 10 to 40 pC/g, voltage sensitivities from 600 to 2000 mV/g, and resonance frequencies from 3 to 1.5 kHz were measured. A model describing both the charge and the voltage sensitivity is presented. Good agreement of experimental and calculated values is found. The experimental results show that sensitive, lightweight, and inexpensive electret accelerometers can be built.

Formation and Biodegradation of Polyethylene-based Electret Films

Abstract Intensive development of packing industry in response to the needs of modern society has aggravated the global situation with recovery of polymer wastes. One of the trends that may alleviate this problem is manufacture of ecologically friendly biodegradable polymeric films. Such films undergo accelerated destruction after burial under the action of soil microorganisms. The factor that speeds up biodestruction of the films in soil is the electric field of their own. The available experimental data have proved that the kinetics of biodestruction of polyethylene films, including starch-filled and inorganic salt-filled ones, alters significantly after such film transfer to electret state.

Stabilization of positive charge on polytetrafluoroethylene electret films treated with titanium-tetrachloride vapor

The surface of polytetrafluoroethylene films was treated with titanium-tetrachloride vapor. The treatment was carried out in a flow reactor by means of molecular-layer deposition, a method from the arsenal of chemical nanotechnology. X-ray photoelectron spectroscopy reveals that such a treatment results in considerable changes in the chemical composition at and near the surface of the fluoropolymer film. Both, defluorination and oxidation of the surface were observed. At the same time, samples treated with titanium tetrachloride show a significant enhancement in the thermal stability of the positive homocharge. The thermally stimulated surface-potential-decay curves were observed to shift to higher temperatures by more than 100 °C

High-speed electret charging using vacuum UV photoionization

We propose a high-speed charging method of electrets using vacuum ultraviolet irradiation. Due to a large amount of the ionization current at reduced pressure, it takes only a few seconds to charge 15-μm-thick polymer electret film to the surface potential of −900 V. This charging rate is two orders of magnitudes larger than corona/soft-x-ray charging methods. The purity of N2 gas depends on the charging rate since the O2 quenching mechanisms of exited N2 molecule would exist. No charge decay is observed for 3000 h, which indicates charged electrets are as stable as those by other charging methods.

In-Situ Wafer-Level Polarization of Electret Films in MEMS Acoustic Sensor Arrays

MEMS-based electret and polymer piezoelectric transduction techniques have been reported for both acoustic sensors and energy harvesters. Common techniques employed in MEMS polymer polarization include corona discharge and backlighted thyratron. This paper reports a method for post-fabrication in-situ polarization of polymer films embedded within the MEMS device itself. The method utilizes microplasma discharges with self-aligned charging grids integrated within the device to charge fluoropolymer films in a fashion similar to the common corona discharge technique. This in-situ approach enables the integration of uncharged polymer films into MEMS and subsequent post-fabrication and post-packaging polarization, simultaneously enabling the formation of buried or encapsulated electrets as well as eliminating the need to restrict fabrication and packaging processes that might otherwise discharge pre-charged materials. CYTOP, a thermoplastic fluoropolymer encapsulant for electronics, is used as a polymer electret in the current process because it can be spin-cast, has a high resistivity, and is easily etched in oxygen plasma. A microscale charging grid structure is then fabricated and suspended a short distance above the polymer film. After fabrication of the charging grid, standard microfabrication steps are performed to build a single-chip array of MEMS capacitive acoustic sensors designed to capture and analyze waveforms from impacts. After completing the entire fabrication and packaging flow, the polarization process is performed. When energized by a high voltage, the sharp metal edges of the charging grid lead to high dielectric fields that ionize the air in the gap and force electric charge onto the polymer surface. Final sensor arrays have been demonstrated and applied in the classification of acoustic stress pulses generated during impacts of various materials.

Patterning of inorganic electrets

A yearlong experiment has been conducted to prove that the field emission and atmospheric ions are responsible for discharging of an electret at its edge. Based on the obtained results, a technology for patterning electret layers has been developed for technical applications. It allows fabrication of electret film structures with a feature size of 20 μm and probably less. Patterned inorganic electret lines fabricated using photolithography show no dependence of charge stability on line width based on monitoring the charge retention during one year. A general approach to electret patterning is discussed.

Applications of tension and nip load profile measurement using a novel roll mounted electret film sensor

An intelligent roll is a mechatronic system consisting of a roll in a web handling machine that also is used as a transducer for sensing cross-machine tension or linear load. The intelligent roll has electret film force sensors mounted on it in a helical arrangement. The sensors measure the force applied by the material being produced, such as a paper web, and thus provide information about the behavior and quality of the product. In addition to the force sensors, the intelligent roll system has an electronic signal processing module on the roll end and a digital radio link to transmit the data from the rotating roll. The receiver is connected to an automation network. An intelligent roll can be used to measure the tension profile online without separate scanning devices. An intelligent roll used as a reel or winder drum enables online measurement of linear load profiles during reeling and winding. Intelligent roll technology also enables temporary process and runnability analysis measurements by using tape-mounted sensors. Problems such as loose web edges, off-machine coater web shifting, winding problems, and reeling defects have been solved with these measurements. This paper describes the technology and presents three examples of its application to improve process runnability.

Electret condenser microphones in hearing aids.

As sensors small, high-gain devices, hearing aid microphones must be tiny, have high signal-to-noise ratios, be relatively insensitive to vibration from the nearby speaker, and operate at low hearing aid battery voltages. The subminiature backplate-electret condenser microphone was developed in the early 1970s specifically for hearing aids, with a very lightweight mechanical system for lower vibration sensitivity and somewhat improved signal-to-noise compared to previous designs. Although the design has been refined over several generations, the basic technology of electret films has proven to be very robust over the last 40 years, enabling continual size reductions while maintaining high signal-to-noise ratios. Noise generation and vibration sensitivity continue to be competitive dimensions for hearing aid microphones. Some of the mechanisms for noise generation and vibration pickup are intrinsic to the design of these microphones. Other mechanisms may be caused by interaction between the microphones and the hearing aid system of which they are a part. In this presentation, we will discuss both types of mechanisms and some design guidelines for minimal noise application of microphones in hearing aids.

Integration of Electroactive Polymers in MEMS Ultrasonic Sensors

The U.S. Army AMRDEC, Stanley Associates, and the Georgia Institute of Technology are developing spectrally sensitive acoustic emission sensors for the detection of various phenomena of interest in Army missiles and assets. Multiple systems would benefit from monitoring the acoustic spectrum and identifying signatures of interest. These would include monitoring sounds external to Unattended Ground Sensors, looking for specific sounds associated with machine failure in a condition-based maintenance scenario, and identifying items that are impacting each other. Potential device designs employ electroactive polymers to convert acoustic waves into electrical impulses. Many electroactive polymers are, however, not compatible with standard MEMS processing, particularly at elevated temperatures. Piezoelectric films, such as PVDF, require stretching and poling processes to orient the crystal structure. Electret films also require poling to create a permanent polarization, plus discharge occurs at relatively low-temperatures. These types of films are difficult to integrate directly into MEMS devices because of these incompatible processes. The films are therefore added using post-fabrication bonding and assembly processes, thus reducing design flexibility and increasing cost. This paper will present techniques being developed to integrate electroactive polymers directly in MEMS sensors as opposed to performing post-fabrication assembly of electroactive films into sensor structures.

Properties of SiO2 electret films charged by ion implantation for MEMS-based energy harvesting systems

This paper presents the results of charging SiO2 thin film electret by ion implantation. A maximum charge density of 16 mC m−2 has been shown using 500 nm thermal oxide. Charge is reproducible and stable in time. Two types of ions were used for ion implantation: phosphorus (P+) and boron (B+). Directly after implantation, it was found that charging with B+ resulted in more stable surface potential compared to charging with P+, but after 50 days the difference is negligible. The extrapolated long-term stability of SiO2 electrets charged by ion implantation is around 1 year. SiO2 is a promising electret for energy harvesting, which can be charged in a reproducible and stable way by ion implantation. The results are compared to corona charging of SiO2. The developed process can be used to realize vibration-based capacitive energy harvester systems using CMOS-compatible processes.

Charge storage stability of electret film made of polypropylene/porous polytetrafluoroethylene/polypropylene composite

Objective:To study charge storage stability of electret film made of polypropylene(PP)/porous polytetrafluoroethylene(PTFE)/polypropylene composite.Methods:The PP/porous PTFE/PP composite electret film was prepared with porous PTFE,polyethylene(PE) and PP by heat melting technique.The charge storage stability of the electret film and transportation were evaluated by using the methods of isothermal surface potential attenuation and open-circuit thermally stimulated discharge(TSD) current spectra.Results:(1) The charge storage stability of PP/porous PTFE/PP composite electret was superior to that of PP electret made under ordinary temperature,because the space charge of PP/porous PTFE/PP electret was transferred from deep trap to flat trap during the heat melting process.(2) Satisfactory charge storage stability was observed in PP/porous PTFE/PP electret when under the condition of high humidity.Conclusion:PP/porous PTFE/PP electret film has satisfactory charge storage stability and can be used as a novel enhancer in transdermal delivery of drugs.

M5-2 シリコンエレクトレットコンデンサマイクロホンの実現に向けた無機エレクトレット膜および着電技術の開発(M5 アクチュエータ/フィジカルセンサ)

Towards low-voltage and high-performance subminiature microphone, we are studying silicon electret condenser microphone. Recently, we developed a novel charging method for electret films using soft X-ray irradiation. In this paper, the principle of the charging method and the surface potential of a charged inorganic electret film are reported.

Trap capture properties of space charge and void breakdown charge in a cellular polypropylene electret film

The following aspects have been indicated by systematic studies on the cellular polypropylene (PP) electret film. Trap capture properties of space charges changed with their injected amount or surface potential of the sample. The space charges were captured largely in the deep traps of the sample surface and the second-deep traps of the near-surface region at small amount of charge injection. They would fill these traps in the surface layer (surface and near-surface region) and the shallow traps in bulk at the same time in the case of greater amount of charge injection. The charge detrapping temperatures of the three kinds of trap centers are about 160℃，138℃ and 92℃，respectively. The charges formed due to void breakdown (called the void breakdown charges) were captured not only in the surface layers of voids which have similar traps as the surface layer of the sample，but also there were numerous void breakdown charges which penetrated the void surface layer，entered into the bulk，and were captured by the shallow bulk traps.