Optics: measuring and testing – Photometers – Photoelectric
Reexamination Certificate
2000-11-06
2002-09-24
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Photometers
Photoelectric
C356S229000
Reexamination Certificate
active
06456373
ABSTRACT:
FIELD OF THE INVENTION
The invention refers to a method for monitoring the light emitted from an illumination apparatus, and to an apparatus for carrying out said method.
BACKGROUND OF THE INVENTION
Methods and apparatuses of this kind are used wherever, for reasons of accuracy, the values of the light emitted from the illumination apparatus—for example brightness, brightness fluctuations, spectral properties, and the like—must be kept within narrow parameter ranges. This is the case in particular with optical measuring instruments such as those, for example, for layer thickness determination, in which changes in the measurement light caused by a measured specimen are used to draw conclusions as to the properties and/or dimensional consistency of the measured specimen.
Excellent reliability is important in instruments that are used for dimensional consistency inspection in continuous production lines, for example in the manufacture of wafers in semiconductor production, since the measurement results serve as the basis for obtaining information as to product quality and the stability of the production process. This requires stable accuracy in the measuring instrument technology used.
In instruments that operate on optical principles, measurement accuracy always depends to a considerable degree on consistent parameters of the measurement light that is generated in an illumination apparatus. In the lamps usually used for the purpose, however, the properties of the emitted light change with increasing operating life, so that these lamps become unsuitable for measurement purposes because of their age. For economic reasons, however, it is desirable to use the lamps as long as possible without allowing measurement inaccuracy. For safety reasons as well, it is often not desirable to continue using lamps after a maximum permitted operating life has expired.
All that is known in this regard from the existing art is to sense the failure of a lamp and then to perform a lamp replacement. This is described, for example, in U.S. Pat. No. 3,562,580 A, which refers to a projection apparatus.
From U.S. Pat. No. 4,831,564 A it is also known to estimate the remaining lifetime of a xenon lamp on the basis of the present discharge current. What is utilized here is a predefined relationship between discharge current and lifetime, so that on the basis of the instantaneously sensed discharge current, a theoretical remaining operating life can be determined. Since this method allows absolutely no monitoring of the quality of the light emitted by the lamp, it is unsuitable for use in an illumination apparatus for generating measurement light within narrow quality limits.
U.S. Pat. No. 5,495,329 A furthermore refers to an illumination apparatus for a scanner which, upon startup of the scanner, examines the light emitted by a lamp for the presence of various properties, a high degree of consistency in the luminance over a region being scanned being of paramount importance. In addition, based on the time required for the lamp to warm up, information is obtained concerning the aging status thereof, from which predictions can then be obtained regarding the remaining useful life. In this case as well, however, it is impossible to derive reliable information about the quality of the measurement light or an ideal time at which to exchange the lamps.
SUMMARY OF THE INVENTION
It is one object of the invention to create an economical and effective method of making available a measurement light whose properties remain consistent over long periods of time.
This object is achieved by a method which comprises the following steps:
switching on and off multiple lamps of the illumination apparatus wherein the switching is carried out individually for each lamp or in groups of lamps;
sensing of lamp parameters and/or measurement light parameters;
comparing the sensed parameters with predefined setpoints referred thereto;
signaling a deviation in one or more of the sensed parameters from the predefined setpoints beyond a specific tolerance; and
exchanging the lamp or lamp group thereupon.
A further object of the invention is to provide an apparatus for an optical measuring instrument, in particular a layer thickness measuring instrument wherein the apparatus provides contant illumination properties for a long period of time. Moreover, the downtime of the a layer thickness measuring instrument should be reduced.
The above object is achieved by an apparatus which comprises:
multiple lamps defining a measurement light source, of which at least one is provided for performing a measurement task while the others serve as reserve lamps;
an operating voltage source that can be switched on and off and is connected via contacts to the at least one lamp defining the measurement light source;
an activatable device for selectably conveying at least one lamp to the contacts;
a device for sensing lamp parameters and/or measurement light parameters;
a device for specifying setpoints associated with the respective parameters;
a comparison device that, in the event that one or more of the sensed measurement light parameters deviate from corresponding setpoints, generates a signal representing the deviation and
an activation circuit receiving said signal and the activation circuit is connected to the activatable device.
It is thereby possible to determine the optimum point in time for a lamp exchange that allows a compromise between the maximum lamp lifetime and the measurement light quality necessary for a measuring instrument. The continuous sensing of lamp parameters and/or measurement light parameters, preferably of those parameters that are also read out in the optical instrument, can be performed during a measurement operation itself, so that if necessary a lamp exchange can be authorized immediately, thus guaranteeing high availability of a measurement light within the desired tolerance range.
This is critically significant specifically for production lines with a high throughput, in order to minimize production wastage. In an advantageous embodiment of the invention, the brightness or intensity of the measurement light, the frequency with which brightness or intensity fluctuations occur, and its spectral distribution, are sensed as the measurement light parameters. The method is thus suitable especially for an illumination apparatus that is used in conjunction with spectroscopic measurement methods, for example an optical layer thickness measurement.
The lamp life of lamps used in illumination devices, for example halogen lamps, xenon lamps, or deuterium lamps, is time-limited because of their design. For the aforementioned lamps, lifetimes guaranteed by the manufacturer are in the range of 1000 hours and above. In a further advantageous embodiment of the invention in this context, in order to guarantee a high degree of uniformity in the measurement light, the lamp life of each lamp is added up and the fact that a predefined lamp life has been reached is signaled, whereupon an exchange of the lamp or of a lamp group is performed.
This makes it possible, in particular, to protect against the risk of explosion, which increases toward the end of the lamp's lifetime. Leaving this aside, it is further advantageous also to monitor the illumination apparatus for total failure of a lamp and to signal any such failure, so as thereupon immediately to initiate an exchange of the defective lamp or lamp group.
To simplify the monitoring regime, checking for total failure of a lamp or lamp group, and/or checking the lamp life, can be accomplished with a photodetector close to the lamp, so that malfunction information can be arrived at with particularly high reliability. The monitoring outlay for the aforesaid criteria moreover remains low. Also possible is a process-engineering decoupling of malfunction messages resulting from measurement light parameter deviations. It is also conceivable to monitor the lamp current so that a total failure can be identified.
In a further advantageous embodiment of the method according to the present inventio
Backhaus Kuno
Jaritz Horst-Dieter
Slodowski Matthias
Wienecke Joachim
Wolter Detlef
Foley & Lardner
Leica Microsystems Jena GmbH
Stafira Michael P.
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