A new space charge balance sensor was developed in order to control ion generation in an air ionizer. Conventionally, a charged plate monitor is widely used in order to detect the space charge balance of the air ionizer. However, the charged plate monitor is not suitable for the feedback system for several reasons. The proposed sensor, in spite of its very simple structure, can detect the space charge balance instantly with enough sensitivity, with a shorter response time, and has a possibility of higher spatial resolution compared to a charge plate monitor. An air ionizer control system that can generate a specific quantity of ions at desired locations is considered to be easily implemented by using feedback of the sensed signal to the air ionizer. 

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Static electricity may cause destruction and/or operation errors of semiconductor devices and magnetic recording devices in their manufacturing and inspection processes, which consequently leads to yield reduction. The effect of static electricity becomes more serious with size reduction of those devices.Several types of ionizers, e.g. a corona discharge ionizer or an X-ray ionizer, have been used in the manufacturing and inspection processes in order to eliminate static electricity. These ionizers generate both positive and negative ions that neutralize the static electricity on the device surface and eliminate the charge. However, the imbalance between positive and negative ions may produce new charges, which means that proper use of these ionizers is required in order to perform charge elimination safely. Therefore, it is generally required to control the voltage due to the imbalance of space charge near the device surface within plus or minus 5 V.Consequently, the measurement of the space charge balance is essentially important, and it is desirable to measure the space charge balance as frequently as possible. Conventionally, the measurement of the space charge balance is performed by using a charged plate monitor (CPM), which is a standard method decided by the Electro Static Discharge Association, by which the measurement is supposed to be the most accurate generally. However, this method has several problems:1. The initial setting of the plate voltage requires manual operation. 2. It takes time to measure the plate voltage because of the slow response of the CPM (several seconds).3. Spatial resolution is poor because of the plate size of the CPM (15 cm ×15 cm = 225 cm 2 ).Therefore, the CPM is not suitable for measuring the space charge balance frequently and instantly, with high spatial resolution.Considering the above, we developed a space charge balance sensor with a simple structure and fast response time. It is easier to handle and has the possibility of higher spatial resolution, compared to a conventional CPM. The proposed sensor consists of three blocks: an antenna, a MOSFET (2SK241) and an operational amplifier (741). Figure 1 shows the diagram of the proposed sensor. A copper disk with a diameter of 50 mm and thickness of 1 mm was used as an antenna in order to capture nearby ions. The captured ions flow to the ground through the gate resistor R, which causes the voltage drop, i.e. the gate voltage V G of the MOSFET. The MOSFET used in this experiment is of a depletion (normally on) type, which allows detection of both positive and negative ions. The capacitor C that − + V OUT C I IN antenna R Fig. 1 Diagram of assembled sensor