Electronic interface for use with dual electrode capacitance...

Details Electronic interface for use with dual electrode capacitance... Electronic interface for use with dual electrode capacitance...

2002-06-13

2004-05-11

Tang, Minh N. (Department: 2829)

Electricity: measuring and testing

Measuring, testing, or sensing electricity, per se

Magnetic saturation

C324S118000, C324S12300R, C073S724000

Reexamination Certificate

active

06734659

ABSTRACT:

BACKGROUND OF INVENTION
Field of the Invention
The present invention relates generally to measurement systems and more particularly to systems and methods for improving the performance and reducing the cost of Capacitance Diaphragm Gauges (“CDGs”) which utilize dual electrode Capacitance Diaphragm Sensors (“CDSs”) by improving the electrical interface to the CDS.
Background of the Invention
Capacitance diaphragm gauges (or capacitance diaphragm manometers) are widely used in the semiconductor industry. In part, this is because they are typically well suited to the corrosive services of this industry. They are also favored because of their high accuracy and resistance to contamination. In particular, those CDGs in which the CDS is heated exhibit enhanced resistance to contamination and operate longer without maintenance.
A CDS serves as the vacuum/pressure sensing element within a CDG and may be used to measure and/or control the pressure within a process chamber. A CDS has a housing containing two chambers separated by a circular tensioned diaphragm. The first chamber is in fluid communication with the process chamber or other assembly in which the pressure is to be measured. The second chamber of the CDS is commonly referred to as the reference chamber and is typically (although not necessarily) evacuated and sealed at a pressure which is substantially less than the minimum pressure the sensor will be required to resolve.
The circular, tensioned diaphragm (“the diaphragm”) which separates the two chambers within a CDS housing is essentially a thin metal diaphragm which is mechanically constrained about its periphery. The diaphragm reacts to differential pressures by deforming into a bowed shape with the periphery remaining stationary. The diaphragm thereby serves as a flexing, grounded electrode. The diaphragm deforms as a reaction to the pressure difference across it and also interacts with electrostatic fields such that the deformation of the diaphragm may be resolved through these electrostatic interactions.
In close proximity to the diaphragm lies the electrode assembly. This assembly consists of a stiff platform with a polished, electrically insulating surface, which bears two conductive electrodes. The electrode assembly is mechanically constrained a fixed distance from the plane containing the periphery of the diaphragm so that the electrodes are very close to the diaphragm (<0.005 in) and run parallel to its surface. Flexure of the diaphragm, due to applied pressure, can easily be computed by measuring the capacitance to ground at each electrode and subtracting one measurement from another.
Modern CDSs utilize two electrodes to monitor the flexure of the diaphragm. The capacitance to ground of the two electrodes (“common-mode capacitance”) varies with flexure of the diaphragm, but also changes with movement of the electrode assembly. Such movement occurs with temperature changes, temperature transients, and mechanical loading. Measurements using the difference in capacitance of the two plates (“difference capacitance”) are more stable since they reject motions between the diaphragm and electrodes and instead reflect the deflection of the diaphragm.
Systems that utilize CDGs generally have stringent requirements for the repeatability of pressure readings, with offset drift typically limited to 0.02% of full scale per day. Full-scale deflection typically results in differential capacitance of 0.2 2.0 pF(10
−12
F). 0.02% of this value gives an allowable equivalent change of 0.04-0.4 femtoFarad (10
−15
F) per day, where some of the change is due to electrical errors when measuring and subtracting the capacitance at the CDS.
The measurement of the CDSs capacitances is performed by the Analog Front End (“AFE”) electronics. The AFE is not only responsible for interfacing to the CDS, it also performs the subtraction operation which gives the difference capacitance. Since the full-scale difference capacitance may be as low as 0.2 pF (10{circumflex over ( )}
−12
F) with a common mode capacitance of 68 pF, even at full-scale, the common mode capacitance is 340 times greater than the difference we wish to measure. For a case involving a daily allowable drift of 0.04 femtoFarad, the common mode capacitance is about 1.7 million times the allowable variation in difference capacitance. Thus the subtraction operation must be extremely well balanced and stable to ensure that the AFE maintains a reasonable drift error.
An obvious source of measurement error within a CDG is the accumulation of incidental capacitance due to interactions between circuits, lead wires, and structures within the construction. These effects even occur on the circuit board and within integrated circuits bringing along leakage currents which makes the circuitry sensitive to humidity and contamination. The solution to these leakage elements is guarding. A node which is surrounded by a conducted surface bearing the same voltage (a “guarding surface”) generally will not experience capacitance or leakage current. By surrounding important nodes with guarding, they are free to operate without interference and variations due to shifting, flexing, or changes in humidity. The source and greatest need for guard potential lies, for the most part, in the AFE.
Given the stringent performance requirements of the AFE, few circuit topologies have proven suitable. Three topologies currently dominate the CDG market: the balanced diode bridge; the guarded-secondary transformer bridge; and the matched reference-capacitor bridge.
The balanced diode bridge topology, which is illustrated in
FIG. 1
, utilizes an excitation source, which provides an alternating voltage to drive the electrodes of the CDS. Charge is alternately supplied to and removed from each of the electrodes through a diode bridge to and from capacitors, C
A
and C
B
. Each of the capacitors serves to supply current to one electrode while discharging current from the other. Thus any imbalance in the capacitance to ground of the two electrodes results in a voltage difference between the output pins of C
A
and C
B
.
Diode bridge AFEs are simple and inexpensive, which makes them a suitable choice for less demanding applications, such as 10 Torr unheated sensors. With stabilized temperature and humidity, they have even been used down to 100 mTorr. However, they generally need to be in close proximity to the CDS since they lack an easy means of producing a useful guard potential. Also, they suffer from diode mismatch and boar contamination issues.
Referring to
FIG. 2
, a guarded-secondary transformer-based bridge is shown. This circuit utilizes a center-tapped secondary constructed of coaxial cable to produce the excitation voltage along with proportionally increasing guard voltage. The current induced in the CDS by the excitation voltage flows from one electrode of the sensor to the other. Thus, charge is conserved and any differential capacitance results in a net voltage at the center-tap of the innermost conductor of the secondary. A high-input-impedance, unity-gain amplifier follows the center tap of the inner conductor and places a similar voltage on the shield, allowing for guarding. The output of the unity gain amplifier represents the difference capacitance in the sensor. It is amplified and is sent to a synchronous detector to generate a DC level proportional to the difference in capacitance.
When implemented well, the guarded-secondary transformer-based bridge represents a vast improvement over the balanced diode bridge in stability and accuracy. It allows the CDS to be remotely placed through utilization of the same guarded, coaxial cables, which are wrapped about the transformers core. The principal problems with this technology lie in its implementation. The coaxial cable must be of exceptional consistency and must be free of cracks or holes in the shielding. In addition, the guarding method is somewhat Imperfect, being based upon a less than unity gain follower, and stable construction is essential to achieve a stable CDG. Finally,

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