Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
2000-04-21
2002-08-13
Sherry, Michael J. (Department: 2829)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S455000, C361S231000
Reexamination Certificate
active
06433552
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates the art of electrical measurements, and more particularly to an electrical measuring method and apparatus which provides for the voltage measurement and pre-charging of an isolated and/or remote surface or structure such as the ion collecting plate element of a floating plate ion monitoring system.
Floating plate ion monitoring systems are typically used to measure the effectiveness and efficiency of room air ionization systems employed in the semiconductor manufacturing process industry to reduce or eliminate charge accumulation on charge sensitive semiconductor or LCD (liquid crystal devices) during the manufacturing process.
As conventionally known, charge accumulation on charge sensitive semiconductor elements such as mosfet gates arrays, digital memory or logic elements, or LCD devices using TFT device (thin film transistors), if not controlled or eliminated will cause immediate destruction or early life field failure of the semiconductor junctions of these devices.
Air ions, both positive and negative, are typically used to flood areas where semiconductor devices are being processed to provide a pool of mobile air ion charges which can be attracted by the undesirable charges associated with the semiconductor devices under process to effectively cancel them to zero net charge.
The typical floating plate ion monitoring system plays an important role in the air ion control system by providing a means of measuring the quantity of mobile air ions of each polarity being produced by an air ionization system, as well as providing a measure of the ability of the produced air ion field to hold various spatial areas associated with the semiconductor process line at a zero or near zero voltage level.
In use, the typical floating plate monitoring system provides two modes of operation, a “decay” mode and a “float” mode. In the decay mode the ion collecting plate or surface associated with the monitor is pre-charged to a positive or negative voltage level of typically 1000 volts or more. The time required for the collecting plate to be discharged to a level of 10% of the starting value, i.e. 100 volts, by ion impingement from the ion field is measured by a timer in the monitor to indicate the quantity of either polarity of air ion associated with the ion field. This measurement is done with a pre-charge level of positive 1000 volts to indicate the quantity of negative air ions or with a pre-charge level of negative 1000 volts to indicate the quantity of positive air ions. In either case the time required for the plate to be discharged to a 100 volt level by the attraction of the oppositely charged air ion to the ion collecting plate is measured and used to indicate the “richness” of the particular ion in the field.
In the float mode the ion collecting plate is initially pre-set to a zero voltage level and then allowed to “float” to a voltage level dictated by the impingement of incident ions from the ion field. The float measurement indicates the effectiveness of the field in reducing the net charge on the semiconductor devices to a low value while also indicating to what voltage level all devices, even devices which were not initially charged, will be driven to by the ion field.
Floating plate monitoring systems heretofore available suffer from many disadvantages. One results from the fact that the ion collecting plate element of the monitor system, in order to be isolated from ground by a high impedance, is typically monitored by use of a non-contacting electrostatic voltmeter probe device to indicate the voltage level of the plate element in reference to ground. These electrostatic voltmeter probes, typically of the field “mill” or tuning fork chopper types, are expensive and require a large mounting space at the ion collecting plate assembly to effectively read the plate voltage.
Another disadvantage is that to pre-charge the ion collecting plate for decay mode measurements, a relay or solenoid is typically used to momentarily connect the plate to a pre-charge level voltage source. This requires a low leakage relay scheme to maintain the plate at a high impedance level relative to ground thus typically requiring the relay to be positioned at the ion collecting plate structure itself.
A further disadvantage arises from the fact that as dictated by the measurement standards, the capacitance between the ion collecting plate and its associated ground referenced structures must be held at a specified capacitance of typically 20 pf+/−10%. This requires a specific plate geometry configuration that dictates a minimum volume ion collecting plate structure. This structure is typically large, in the order of 15 cm×15 cm with a thickness of 6 to 7 cm minimum. Using this type structure, it is not possible to position the ion collecting plate structure directly on line with the semiconductor devices for direct measurement of the effectiveness of the ion field at the location of the semiconductor devices themselves, therefore reducing the accuracy of the measurement.
Another disadvantage is that to operate the ion collection plate assembly a large diameter connecting cable to the monitor electronic assembly is typically used to accommodate the wiring for the electrostatic probe device electronics, the HV relay device actuation wiring, the high voltage wiring for the pre-charge supply and the ground reference connection for the ground reference element. This results in a bulky ion collecting plate assembly cable which limits assembly position flexibility and easy placement of the assembly.
A further disadvantage is that whereas it is desirable to construct an ion collection plate which is of the same relative size and thickness as the semiconductor devices which are placed within the ion field, the current art ion collecting elements, due to their required size to accommodate the required relay, electrostatic probe, and required measurement capacitance, cannot be constructed in the size desired.
Another disadvantage is that to produce the required ion collecting plate capacitance relative to the ground referenced element precise manufacturing of the ion collecting plate is required while other mechanical components such as spacers to support the ion collecting plate element from the ground reference element must be of high precision, low leakage and high cost construction.
A still further disadvantage is that the bandwidth (frequency response) of current art floating plate monitoring systems is limited to approximately 40 to 50 Hz due to the use of the typical electrostatic probe system used to monitor the ion collecting plate. This is a serious limitation particularly where A.C. type air ion production equipment is employed. The A.C. type ion production equipment can induce damaging A.C. fields and voltages at the location of the semiconductor devices and cannot be accurately measured using current art type floating plate monitors.
SUMMARY OF THE INVENTION
It is, therefore, a primary object of this invention to eliminate the aforementioned disadvantages found with floating plate monitoring systems heretofore available while reducing the cost of such systems.
It is a further object of this invention to eliminate the necessity and use of the typical electrostatic voltage probe in the ion plate assembly construction.
It is a further object of this invention to eliminate the necessity and use of a high voltage relay or solenoid in the ion plate assembly construction.
It is a further object of this invention to eliminate the requirement of providing an ion collecting plate structure which is dependent upon mechanical considerations such as plate area and plate spacing, both relative to the ground reference structure, to establish the specified ion plate capacitance to the ground reference, but to establish the specified capacitance using electrical means.
It is a further object of this invention to reduce the connecting cable to the ion collecting plate structure to a highly flexible small diameter cable to allow easy positioning of the
Hodges Russ LLP
Nguyen Trung
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