Measuring and testing – Liquid level or depth gauge – Hydrostatic pressure type
Reexamination Certificate
2001-04-27
2004-04-06
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Hydrostatic pressure type
Reexamination Certificate
active
06715348
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for detecting the level of a liquid.
2. Related Background Art
In the fabrication of semiconductor devices, various wafer processing systems are utilized to remove surface contamination such as small particles, organic contaminant materials, metallic impurities or the like from a semiconductor wafer (referred hereinafter to as “wafer”) using a processing liquid such as chemicals, pure water and the like. A wet-type processing system, among others, has been widely used wherein the wafers are submerged in a processing tank or bath containing the processing liquid for cleaning.
In such a processing system, it is necessary to fill the processing tank with a selected amount of the processing liquid enough to perform an effective cleaning operation. To this end, a liquid level detector apparatus is used in conjunction with the wafer processing tank for detecting the level of the liquid being supplied in the tank and terminating the supply of the liquid as the liquid level reaches a predetermined position or elevation in the tank.
The liquid level detector apparatus comprises a hollow sensor tube having an open distal bottom end positioned at a predetermined height I the tank and adapted to be immersed in a processing liquid whose level are adapted to be detected, gas supply means for supplying gas having a pressure substantially greater than a pressure prevailing over the liquid upper surface to the sensor tube sufficiently to fill the interior thereof entirely to a depth of the open distal bottom end, a pressure sensor associated with the sensor tube for generating an output signal representative of the gas pressure within the sensor tube and an electrical circuitry for determining the liquid level based on the magnitude of the output signal from the pressure sensor. Thus, the liquid level detecting apparatus is configured for detecting the liquid level based on the gas pressure measured at the predetermined position (i.e., at the predetermined depth) in the processing tank.
More particularly, with the open distal bottom end of the sensor tube immersed in the processing liquid in the tank, the gas supply means supplies the gas such as compressed air or an inert gas having a pressure substantially greater than a pressure (e.g., atmospheric pressure) on the liquid surface into the hollow interior of the sensor tube to a depth level where the open distal bottom end thereof is located, so that the gas pressure within the sensor tube is equal to the liquid pressure being applied to the open distal bottom end of the sensor tube. It is to be understood that the liquid level detector apparatus of the type as outlined above is capable of detecting the liquid level relying on the fact that the gas pressure within the sensor tube is proportional to the distance (depth) measured from the liquid surface to the distal bottom end of the sensor tube. On the other hand, in view of the fact that the distal bottom end of the sensor tube is located at a predetermined height or a known depth in the processing tank, the pressure of the gas being introduced into the hollow sensor tube is equal to the pressure (e.g., atmospheric pressure) on the liquid surface until the level of the processing liquid being supplied comes into contact with the distal bottom end of the sensor tube. During that time period, the pressure sensor will generate an output signal having a constant level or magnitude. Such a pressure-detected output signal generated by the pressure sensor under this condition can be set as a reference signal.
The liquid level will rises as the processing liquid is fed into the processing tank. As can be appreciated by those skilled in the art, the pressure sensor continues to provide the reference pressure signal until the liquid level reaches the level (the predetermined position) where the open distal bottom end of the sensor tube is positioned. When the output signal from the pressure sensor exceeds the reference signal level, this event indicates that the liquid level reaches the predetermined position in the processing tank. The magnitude of the pressure-detected output signal from the pressure sensor rises linearly as the level of the liquid being supplied rises in the tank. The electrical circuitry operates to detect the depth of the distal bottom end and thus the liquid level based on the level of the pressure-detected output signal from the sensor. The pressure-detected output signal of the predetermined level generated by the pressure sensor indicates that the liquid level reaches the predetermined level in the processing tank. This allows an immediate termination of the supply of the processing liquid to the tank.
The pressure sensor may comprise any conventional pressure transducer of the type having a diaphragm displaceable depending on change in the gas pressure and a strain gauge adapted to convert the displacement of the diaphragm into an electrical signal. By way of example, a highly sensitive piezoresistive semiconductor pressure sensor may be used which comprises a diaphragm formed in an N-type silicon substrate and a P-type strain gauge fabricated on a portion of the diaphragm by diffusing a boron layer therein.
The pressure sensor may experience drift under the influence of various environmental conditions (e.g., temperature and humidity) and/or deterioration with age and produce a reference signal drift. For example, in case where the pressure sensor erratically generates a reference signal having a level higher than the normal signal level before the distal bottom end of the sensor becomes immersed in the processing liquid in the cleaning tank, the pressure sensor will generate a signal which is up-shifted relative to a pressure-detected signal level indicative of an actual gas pressure prevailing within the sensor tube at the time when the distal bottom end of the sensor tube becomes immersed in the processing liquid. Under such condition, the pressure sensor will provide prematurely an output signal of the predetermined level to terminate the supply of the processing liquid to the tank before the liquid level reaches the predetermined level in the processing tank, which may lead to reduced cleaning capability due to shortage of the processing fluid. In contrast, when the pressure sensor erroneously generates a reference signal having a level lower than the normal signal at the time the distal bottom end of the sensor tube becomes immersed in the processing liquid, the pressure sensor will produce an output signal having the predetermined level after the liquid level actually reaches the predetermined level or position. As a result, the actual liquid level may ascend past the predetermined level in the tank, which will result in waste of the processing fluid due to excessive supply thereof to the processing tank.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an apparatus and method for precisely detecting the level of a liquid in spite of any sensor output signal drift.
With the object in view, a first embodiment of the present invention relates to an apparatus for detecting the level of a processing liquid in a processing tank, comprising detector means (e.g., detecting sensor) for generating an output signal related to the level of the liquid being supplied to the tank; a determination circuit for determining when the liquid level reaches a first position established in the tank on the basis of the output signal from the detector means; a liquid level detecting circuit for correcting a relationship between a level of the output signal from the detector means and the liquid level based on a signal generated by said detector means from the time the processing liquid is initially supplied to the tank until immediately after the liquid level reaches the first position to thereby detect the actual liquid level in the tank.
In accordance with the first embodiment of the liquid level detecting apparatus according to the invention,
Morrison & Foerster / LLP
Tokyo Electron Limited
Williams Hezron
Wilson Katina
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