Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
2002-07-12
2004-03-30
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S559220
Reexamination Certificate
active
06713746
ABSTRACT:
FIELD OF THE INVENTION
The invention refers to an arrangement for illuminating a specimen field in an optical instrument that serves for specimen viewing, as well as a corresponding method.
BACKGROUND OF THE INVENTION
Uniform and reproducible illumination of a specimen that is to be examined is very important for the functionality of complex optical instruments, for example measurement and inspection systems for the examination of semiconductor wafers. Even slight changes in an illumination device used for this purpose can considerably degrade the performance of the optical instrument.
In particular when high-intensity light sources are used in combination with extremely high-magnification objectives, conventional illumination devices in many cases can no longer meet present-day requirements for high resolution. This applies in particular to demanding inspection and measurement tasks in the UV (ultraviolet) and DUV (deep ultraviolet) regions. The reasons for this are usually inadequate mechanical and thermal stability of the illumination device, and hitherto insufficient alignment capabilities. For example, the changes in illumination caused by the instabilities lower the optical resolution limit for microscopic examinations.
External influences such as vibration and shock thus result in a misalignment between the light source and the illuminating optical system of the illumination device.
As a result of high thermal loading and the effect of strong UV light, changes occur on the internal surfaces of the illumination device housing, in particular on the illuminating optical system and the light source, which negatively affect the intensity of the emitted light over time.
In illumination devices that are used in instruments under clean-room conditions, a sealed housing must be provided in order to prevent contamination of the specimens being examined. This results in the additional problem of a laborious process of replacing the light source, since the housing must be opened in order to make the replacement. In addition, the need to realign the illumination device usually also arises in conjunction with a replacement of the light source. The alignment capabilities available in conventional illumination devices are, however, insufficient in terms of achieving a high illumination quality, or at any rate are very complex with regard to manipulation. If reproducible conditions are to be ensured following a light source replacement, specially trained persons must therefore be used for the purpose.
U.S. Pat. No. 5,925,887 discloses, in conjunction with a projection exposure apparatus, the problem of monitoring the optical exposure parameters when the intention is to work at the limits of technology. In this, the pattern of a mask is aligned with respect to a substrate by means of a projection objective. A device for determining the change in the optical properties of the projection objective is also provided. These changes can result, for example, from heating of the projection objective due to intensive illumination irradiation, but they also depend (inter alia) on the mask, and cause a change in the imaging properties of the projection objective. To compensate for these changes, it is proposed in U.S. Pat. No. 5,925,887 to perform an adjustment of the lens elements of the projection objective that compensates for the deviations. No intervention in the actual illumination device is performed in this context.
It is also already known from U.S. Pat. No. 5,761,336, in order to improve defect detection on semiconductor wafers, to adjust a diaphragm on a microscope in such a way that a maximum detection probability is achieved for specific defect types. Since evaluation of the defects is performed ultimately by an operator, however, the visual effort for defect detection is very great. The risk furthermore exists that changes in the illumination parameters will not be recognized by the operator. Consistent evaluation conditions are thus very difficult to guarantee. Especially in the context of a replacement of the light source, continuity of examination parameters is almost impossible to maintain for examinations at the limit of resolution.
Lastly, U.S. Pat. No. 4,163,150 discloses an arrangement and a method intended to achieve automatic implementation of the Köhler illumination principle in a microscope. This is done by measuring an illumination intensity with a photosensor, and using the measured value to control a diaphragm and/or the focal length of a lens arrangement within the illuminating optical system. The purpose of the illumination principle proposed here is to optimize the illumination conditions, in a context of variable image magnification, in terms of achievable resolution and contrast. Changes in the alignment state of the light source are not, however, taken into consideration here.
SUMMARY OF THE INVENTION
Proceeding therefrom, it is the object of the invention to create an arrangement for illumination of a specimen field in an optical instrument that makes possible reproducible illumination of the specimen field with highly consistent illumination quality for a specific illumination task.
According to the present invention, this object is achieved with an arrangement of the kind described above which comprises: an illumination device having a light source and an illuminating optical system, the position of the light source and/or of the illuminating optical system being adjustable within the illumination device; a setting device having at least one motorized drive system for positional adjustment of the light source and/or the illuminating optical system; at least one measurement device for sensing parameters of the light generated by the illumination device; and a control device that is configured for the generation of positioning commands for positional adjustment of the light source and/or the illuminating optical system by means of the motorized drive system as a function of the sensed parameters.
By way of the automatic positional setting or alignment of the light source, and optionally also of the illuminating optical system present in the illumination device, the characteristics of the illumination of the specimen field can be adapted to different observation specimens and/or illumination tasks. It is possible to reproduce a specimen—or situation—dependent illumination with little effort in terms of manipulation. Automation of the procedure ensures rapid and easy setting of the corresponding subassemblies, with no need to employ specially trained or instructed operating personnel for the purpose. At the same time, a high illumination quality is achieved so that the best utilization conditions can be consistently achieved with the optical instrument.
In addition, replacement of the light source can also be considerably simplified, since the operator no longer needs to perform settings and alignments in this context. Instead, the entire setting operation after an exchange of the light source can proceed automatically, thereby rapidly re-establishing the performance of the optical instrument after a replacement of the light source.
The arrangement according to the present invention can moreover also be used during operation of the optical instrument, by way of a continuous or quasi-continuous monitoring function, to ensure a consistently high illumination quality. If changes in illumination that negatively affect performance occur during operation of the optical instrument, they can be very quickly corrected. By stipulating an appropriate monitoring regime it is possible, for example in the context of the inspection of semiconductor wafers, to achieve optical defect detection at a consistently high level.
Incandescent lamps, halogen lamps, discharge lamps, or laser light sources are possible, for example, as the light source for the optical instrument. The aforementioned “illuminating optical system” is understood here to be all the optical elements and subassemblies that serve to define the illuminating light. These include, in particular, reflectors, lens eleme
Graf Uwe
Veith Michael
Wienecke Joachim
Foley & Lardner
Le Que T.
Leica Microsystems Jena GmbH
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