1.  Field of the Invention
The present invention relates to an apparatus for measuring an electrostatic potential or charge and particularly to an apparatus for measuring an electrostatic charge borne on a semiconductor device. 
2.  Description of the Related Art
A semiconductor device is commonly produced by forming a plurality of semiconductor devices on a substrate of Si (silicon) or GaAs (gallium arsenide); sawing or machining the substrate into individual semiconductor devices (each referred to as a die) by the dicing process; mounting the die in a package and connecting electrodes formed on the die to external leads using gold wires; and finally completing the manufacturing procedure by sealing the package in resin, for example. 
Quality deficiencies in semiconductor devices have been caused by failures in the processes of forming semiconductor devices on a substrate: for example, failure in the photolithography to form a pattern of a designed size; and penetration fault of a contact hole to be penetrated through an insulator layer to connect upper and lower wiring layers. 
Recently, the tendency toward decreasing a size of a semiconductor device has carried about a deteriorated immunity from static electricity of a semiconductor device.  As a result, quality deficiencies caused by the static electricity have increased in addition to those caused by faulty processes as described above. 
In order to track the sources of the deficiencies, the present inventor et al.  have disclosed an apparatus for measuring an electrostatic charge using a coulomb meter in Japanese Patent Laid-open No.  H07-325119. 
The measuring apparatus will be set forth with reference to the drawings below. 
FIG.  1 is a schematic diagram of the apparatus described in the quoted literature.  A dielectric substance 4 of a known dielectric constant surrounds a metal rod 14.  Grounded metal plate 15 covers dielectric substance 14. 
The wiring arranged between an object to be measured and dielectric substance 4 is positionally fixed with respect to dielectric substance 4 to keep the relative position unchanged, thereby preventing measured values from being varied by a deformation of the wiring.  Since metal plate 15 shields metal rod 14 from an external electromagnetic wave, the measurement of the electrostatic charge is not affected by an electromagnetic induction.  Furthermore, dielectric material 4 is configured so as to establish a distributed capacity along the path through which the charge to be measured moves.  This configuration of dielectric substance 4 allows the distributed capacitor to act as a delay line.  Thus, charging of the distributed capacitor propagates successively from the proximity to the object concerned.  Consequently, voltmeter 5 is protected from being instantaneously applied with a high voltage of the object 1.  In this way, the electrostatic charge in question can be preserved from the leakage that will otherwise occur through voltmeter 5 to the ground. 
FIG.  2 represents an example in which the above-described measuring apparatus is applied to the manufacturing process of an LSI. 
In this example, an excess mobile electric charge is measured at a lead terminal.  Such an electric charge is often generated when LSI 16 is slid on a sloped metal rail 17 and carried from the top to the bottom, as is shown in FIG.  2a). 
Such a carrying system has been often employed in a process step of manufacturing an LSI.  The measurement is carried out by two procedures: the preliminary procedure and the measurement procedure.  The preliminary procedure is performed, as is shown in FIG.  1, by connecting metal plate 15 arranged outermost of the measuring apparatus to the ground potential; connecting voltmeter 5 between the top end of inner metal rod 14 and the ground potential; and sufficiently discharging the capacitor made up of metal rod 14 and metal plate 15 interposed by dielectric substance 4 by short-circuiting metal rod 14 and metal plate 15. 
Next, the measurement procedure is carried out, as is shown in enlarged exploded FIG.  2b), by bringing the pointed bottom tip of metal rod 14 into contact with lead terminal 18 of LSI 16 that has slid down on metal rail 17; observing the value indicated in voltmeter 5 ; and calculating excess mobile electric charge from the equation Q=CV. 
Lately, the tendency toward small-sizing a semiconductor device such as an MR head (magneto-resistive head) for reading magnetic data in semiconductor ICs (a DRAM, a processor and a CCD) and a hard-disk has been rapidly advanced. 
Small-sizing of semiconductor devices entails high susceptibility to an electrostatic charge created on a semiconductor device while it is manufactured, assembled and practically used.  For this reason, it is an urgent need to improve a technique for measuring an electrostatic charge borne on a semiconductor device to protect the device from troubles caused by the electrostatic charge. 
In electronic devices since 1997, front-end products have suffered from significant deterioration in electrostatic-charge immunity. 
For example, the wiring rule of a large-scale integrated circuit is now reducing to less than 0.25 xcexc m and will presumably reach 0.18 xcexcm in 2000.  Furthermore, in a field of a semiconductor device as well, the area of each pixel in a picture CCD has been small-sized in order to improve fineness of a picture. 
A storing density of a hard disk, on the other hand, has been increased to as high as 5-15 GBit/inch2.  This has enabled to realize a small-sized head for read and/or write. 
In the field of the liquid crystal display (LCD) as well, an improvement of the resolution has been advanced through small-sizing of a display pixel.  As the digital broadcast becomes full-fledged and the high-vision TV becomes widespread in the near future, the pixels in the picture display such as a plasma display will be rendered small-sized.  The small-sizing entails susceptibleness to an electrostatic charge. 
The minimum quantity of the static electricity that adversely affects the small-sized semiconductor device does not fall within a measurable range of the above-described traditional measuring apparatus.  Thus, an improvement of the measuring apparatus is required. 
In Southeast Asia where manufacture sites have shifted to as well as in Japan, U.S.A. and Europe, troubles caused by static electricity in manufacture lines and markets have been occurring in rapid succession.  Thus, it is an urgent need to dissolve the troubles.  For this reason, it is necessary to provide for measurement of an electrostatic charge with improved precision and simplicity. 
It is an object of the present invention to provide measuring apparatuses capable of measuring an electrostatic potential of a human body and an electrostatic potential of a metal workbench, neither of which can be measured by the traditional measuring apparatus using a coulomb meter. 
It is another object of the present invention to provide a measuring apparatus capable of detecting a state of the ions in the ambient air.  It is a further object to obviate troubles caused by an electrostatic charge in a manufacture line comprehensively allowing for the results of the measurements by means of above-described measuring apparatuses. 
In order to achieve the object of the present invention, a first electrometer of the present invention comprises an external electrical conductor, a surface electrometer, an inner electrical conductor, an amplifier circuit and a control section. 
The external electrical conductor has two sections: a first section probes an electric potential of an object of interest and a second section is configured to adapt for contacting the object. 
The surface electrometer detects an electric potential of the first section. 
The inner electrical conductor is isolated from the external electrical conductor and serves for supporting the surface electrometer and shielding it from an external electric field. 
The amplifier circuit converts the output of the surface electrometer into a low-impedance signal and amplifies the low-impedance signal.  The control section computes the electrical potential of the object based on the output of the amplifier circuit. 
The first section is configured to adapt to a measurement by the surface electrometer. 
Preferably, the surface electrometer is a noncontact vibrating-capacitor electrometer and the first section is configured as a probe plate to make up a capacitor with a measuring electrode of the noncontact vibrating-capacitor electrometer. 
In the case that the object of interest is a human body, the second section of the external electrical conductor is formed to adaptively contact a predetermined portion of a human body, such as a wristband. 
In the case that the object of interest is a semiconductor devise under processing in a manufacture line, the second section is preferably formed thin to adaptively contact the object. 
The above-described first electrometer of the present invention is directed to detecting an electrostatic potential of an object of interest.  For this end, the second section of the external electrical conductor is placed in contact with the object.  Since the second section is an electrical conductor, it becomes equipotential to the object when it is brought into contact with the object.  Thus, the electrostatic potential of the object is detected by measuring the electrostatic potential of the first section by means of the surface electrometer. 
A second electrometer of the present invention is directed to monitoring the concentrations of positive and negative ions in the air. 
It comprises: an electrically conductive probe plate supplied with a bias voltage for probing a surface electrostatic charge caused by ambient air ions attracted thereto; an external electrical conductor for shielding the entirety of the electrometer from an external electric field, electrically isolated from the probe plate; a surface electrometer for detecting a surface potential of the probe plate; an inner electrical conductor for supporting the surface electrometer; an amplifier circuit for converting the output of the surface electrometer into a low-impedance signal and amplifying it; and a control section for computing the ion concentration in the ambient air based on the output of the amplifier circuit, wherein the probe plate is configured to have a capacitance with respect to the ground as well as to adapt to a measurement by said surface electrometer, and wherein the ion concentration is determined by detecting the electrostatic potential of the probe plate caused by the surface electrostatic charge. 
The procedure to determine the ion concentration comprises the steps of: first applying a high voltage with respect to the ground potential to the probe plate as the bias voltage in order to charge the to-ground capacitance of the probe plate, next cutting off a supply of the high voltage to the probe plate, detecting the values of a decaying voltage in the probe plate by the surface electrometer, computing a decay time of the voltage in the probe plate and identifying the concentration of the ambient air ions based on the decay time by the control section. 
When a high voltage, say a positive high voltage, is applied to the probe plate, negative ions are attracted to the probe plate.  Because of the potential barrier in the boundary between the probe plate and the air, no charge transfer takes place between the probe plate and the air ions.  As a result, electric double layer is formed at the boundary. 
This circumstances correspond to the air capacitor made up of the probe plate and the ground interposed with the ambient air as a dielectric which is electrically polarizable under the electric field produced between the probe plate and the ground.  This capacitor is referred to, in the present Specification, as a to-ground capacitor with a probe-plate electrode or a to-ground capacitor of the probe-plate, and the capacitance of the to-ground capacitor is referred to as the to-ground capacitance of the probe plate. 
Since the number of ions attracted to the probe plate increased as the ion concentration increases, the polarlizability of the dielectric in the to-ground capacitor (the ambient air) increases as the ion concentration increases. 
Accordingly, an increase in the ion concentration results in an increase in a dielectric constant of the ambient air, and the increase in the dielectric constant in turn causes an increase in the to-ground capacitance of the probe plate.  The increase in the to-ground capacitance further in turn results in an increase in a decay time for a discharge of the to-ground capacitance. 
In this way, the measurement of the decay time in the to-ground capacitor enables the identification of the ion concentration in the ambient air. 
In order to identify a plus ion concentration and a minus ion concentration individually, it is necessary that a plus high voltage and a minus high voltage are individually applied to the probe plate as a bias voltage.  In order to identify a surface charge of an object, the probe plate preferably has a to-ground capacitance substantially the same as that of the object concerned. 
A manufacture line preferably includes at least two of the three electrometers in combination: the electrometer for measuring an electric potential of a human body; the electrometer for measuring an electric potential of a semiconductor device; and the electrometer for monitoring an ion concentration in the ambient air (cf.  claims 3, 4 and 5). 
The above-described at least two electrometer can share the control section.  The shared control section can receive data supplied from the individual amplifier circuits, judge comprehensively the received data, and operate to execute a predetermined control based on the comprehensive judgement. 
In this way, the present invention enables to eliminate the deficiencies caused by static electricity in a manufacture line even if the manufacture line is for manufacturing and assembling the products susceptible to static electricity. 
By taking counter-measures against static electricity at a start-up of the line, the static-electricity-susceptible products thereafter entered into the line can be properly processed solely by revising a predetermined reference value. 
By applying the present invention to an assemble line of a hard disk, the deficiencies caused by static electricity, which amount to 65% to 70% of the products, are nearly completely eliminated. 
The above and other objects, features and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate examples of preferred embodiments of the present invention.