Kazuya Kikunaga (Researcher), Kazuhiro Nonaka (Leader), Kazufumi Sakai (Invited Senior Researcher), and Toshihiro Kamohara (AIST Postdoctoral Researcher), Optical Measurement Solution Team, the Measurement Solution Research Center (Director: Michiru Sakamoto) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), have devised a novel method for detecting static electricity by using vibrations and an electromagnetic field. They have developed a non-contact static electricity measuring technology that can be applied flexibly to production sites with various restrictions.

The vibration of an object with static electricity (i.e. a charged body) induces an electromagnetic field. Measuring the changes in the induced electromagnetic field enables the amount of static electricity in the object to be measured. This technology, which has promising applications at production sites with various types of spatial restrictions, can 1) detect the induced electromagnetic field omnidirectionally, 2) vibrate objects by using sound waves, and 3) be installed flexibly in various environments with simple device configuration. In addition, the technology allows flat-surface static electricity distributions to be visualized through the scanning of focused sound waves on the surfaces of objects. This is a promising technology that is capable of visualizing static electricity in moving objects at production sites, such as persons and products, within short periods of time.

Details of this technology were presented on August 30 at the 72nd Autumn Meeting (2011) of the Japan Society of Applied Physics, held at Yamagata University.

Figure 1 : Measurement of static electricity distribution on a polyimide film

The static electricity distribution is visualized by combining positional information on focused sound waves with measured static electricity information.

Static electricity is produced irregularly in various places at production sites and is a source of reduced productivity, causing problems such as damage to electronic devices, film adsorption to objects, and contaminant deposition to products. Especially in semiconductor production, problems with static electricity have become more serious, despite the countermeasures taken. This is because levels of electrostatic discharge resistance in electronic devices have decreased through the use of highly insulating materials and the miniaturization of multilayer semiconductor circuits. To surely and effectively reduce problems of static electricity, production site engineers have been seeking a high-speed technology to measure the static electricity produced during each production process.

Surface electrometers, which have been used widely as static electricity measuring devices, have spatial restrictions, as the sensor must be placed close to the measured object. Moreover, in principle, surface electrometers are easily affected by the measuring environment, including the nearby charged objects and ground. Furthermore, it is difficult to enhance the speed of static electricity distribution measurement on two-dimensional surfaces, because for such measurements the sensor must be moved from one spot to the other.

AIST has been working to develop an on-site measuring technology for static electricity in order to quantify the static electricity produced irregularly at production sites and enable feedback to production processes. The difficulty in static electricity measurement lies in measuring stationary electric charge. The researchers have developed a novel static electricity measuring technology that detects the electromagnetic field generated by vibrating the charged body.

The developed technology generates an electromagnetic field by reciprocally inducing electric and magnetic fields through vibration of the charged body with sonic irradiation. Static electricity is measured with an antenna that detects the changes in the electric field. Schematic illustrations of the technology are shown in Fig. 2. In general, an electromagnetic field is generated when electric and magnetic fields are mutually induced by the temporal and spatial changes that occur with electric current flowing through an electrical conductor. The devised method involves creating spatial changes in the positions of the stationary electric charge (static electricity) by vibrating the charged body. An electromagnetic field is then induced by creating the same effect as an alternating current.

Figure 2 : Schematic illustrations of the developed technology

(1) The charged body (electrically charged object) (2) The charged body is vibrated by sound waves. (3) The stationary electric charge itself is vibrated with the object. (4) The vibrations of the electric charge induce the electric and magnetic fields, thus generating the electromagnetic field. (5) An antenna detects the electric field induced and the signal is converted into information on static electricity.

By using the equipment shown in Fig. 3, a polyimide film was vibrated by sound waves and the changes in the electric field intensity were detected by the antenna. No changes in the electric field intensity were observed when an uncharged polyimide film was vibrated by sound waves, whereas changes in the electric field intensity were observed for the charged polyimide film (Fig. 4). These results revealed that vibrations of objects and the changes in the electric field have close relationships and that an electromagnetic field is generated by physically and periodically vibrating electrically charged objects.

Figure 3 : Configuration of the experimental system

Figure 4 : Change of electric field intensities with sonic irradiation for uncharged and charged films

In the measurement of static electricity, information on the amount of static electricity and on the electrical polarity (positive or negative) is required. To quantitatively evaluate static electricity, the researchers used an electrostatic voltmeter that can measure surface potential. Analysis of the relationships between the surface potential of the charged polyimide film and the electromagnetic field characteristics revealed that the electric field intensity detected was proportional to the amount of surface potential, regardless of the electrical polarity (Fig. 5). In addition, the phases of the induced electric field varied greatly depending on the electrical polarity (Fig. 5). This was due to the fact that the direction of the virtual electrical current was reversed when the positive and negative electric charges moved in the same direction and the phases of the induced electric field are different. As described above, the developed technology can measure the amount of static electricity and the electrical polarity by determining the induced electric field intensity and phases. The technology also enables the detection of the electromagnetic field in only the vibrating area from any 360-degree direction. Therefore the measuring technology is unaffected by the surrounding electrical environment or by spatial restrictions.

Vibrating a small area by using focused sound waves enables the amount of static electricity in only the selected area to be measured. The two-dimensional distribution of static electricity can be measured by scanning with the focused sound waves. As the position targeted by the focused sound waves can be controlled, high-speed measurement of the surface static electricity distribution can be achieved by increasing the scanning speed.

Figure 5 : Relationships between an electric field and static electricity Information on static electricity can be obtained by measuring an electric field.

The researchers will begin the development of a static electricity sensor that can be installed at production sites that are highly spatially restricted. In addition, they plan to develop a super-directive sound system that allows the high-speed scanning of focused sound waves and to develop a system that quickly visualizes static electricity distribution.