The present invention relates to a method for determining the end of useful life of a gas purifier and an apparatus therefor.
It is known that gases are used in many industrial processes. Some of these processes, such as the manufacture of semi-conductor devices, require that these gases have an extremely high level of purity. Any traces of gas present as an impurity must be held at levels of the order of several parts per billion with respect to the working gas (that is several parts of impurity per 10.sup.9 parts of working gas).
In order to reach these low levels of impurities there are employed "gas purifiers". In the semiconductor industry there are widely used "inert gas purifiers". By this is intended a purifier of the rare gases He, Ne, Ar, Kr and Xe, as well as N.sub.2. Such purifiers of inert gas are described, for example, in United Kingdom patents N.sup.o 2,177,079 and N.sup.o 2,177,080. See also European patent application publication N.sup.o 0,365,490 A 1 in the name of the present assignee.
Although these inert gas purifiers are very efficient in maintaining the levels of impurities of the output gas at the extremely low levels required, at a certain moment they begin to loose their ability to remove the gaseous impurities. The process gas thus becomes less pure and the semiconductor devices produced begin to demonstrate an excessive number of defects and must therefore be rejected. These rejects can be extremely expensive, especially when it is considered that the manufacture of the semiconductor devices requires the employment of techniques utilizing micron or sub-micron technologies.
It is therefore essential to guarantee that the levels of impurities of the process gases employed are below the levels specified.
There are presently known several methods to measure the impurity levels of these working gases. One method is to continually monitor the level of each gaseous impurity present. However this requires extremely specialized equipment and highly qualified personnel. Another method is to measure the quantity of gas which has flown through the purifier and, assuming a known impurity content, to calculate when the gas purifier should start to loose its purification efficiency. Unfortunately the level of impurities of the gas to be purified can vary in an unknown manner thus leading to errors in calculation with the consequence that the purifier may continue to be used even though it has already reached the end of its useful life.
Other methods have been proposed which are based on changes in physical properties of the material which sorbs the impurity gases such as, for example a change of colour.
On the other hand, when the gas purification takes place by means of metallic gettering material, there may be no indication of a colour change.
Italian patent application N.sup.o 19012 A/90 filed by the present assignee describes a method for the determination of the end of useful life of an inert gas purifier based on the changes in pressure difference across the purifier, and particularly on an increase of the pressure drop when the purifier nears the end of its useful life, that is to say the purification efficiency is beginning to reach an unacceptable level. Unfortunately this method, based on differences in pressure requires the employment of costly pressure measuring instruments and electronic circuitry, whose use can only be justified on large scale gas purification plants.
When smaller scale gas purification plants having a lower cost are required, whose output of purified gas is less than about 10 1/min, it is necessary to have an equally reliable indication of the approach of end of life but also at a reduced relatively low cost.