Radiant energy – Electron energy analysis
Reexamination Certificate
2000-06-05
2003-04-08
Berman, Jack (Department: 2881)
Radiant energy
Electron energy analysis
C250S492200, C378S034000
Reexamination Certificate
active
06545272
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to monitoring apparatus that detect the state of contamination of optical components of an optical system (e.g., microscope, analysis device, or microlithography apparatus) that utilizes electromagnetic radiation such as X-rays or ultraviolet light, or a charged particle beam. The invention also pertains to optical systems (e.g., microscope, analysis device, or microlithography apparatus) including such a monitoring apparatus. The invention also pertains to microlithography methods including contaminant-monitoring of certain optical components.
BACKGROUND OF THE INVENTION
X-rays produced by synchrotron radiation have high brightness and variable wavelength, and hence are used as X-ray sources for X-ray analysis devices, X-ray microscopes, and X-ray microlithography (projection-exposure) apparatus. Another useful X-ray source is a laser-plasma X-ray source (abbreviated “LPX” source). In an LPX source, a pulsed laser light beam is focused onto a target substance contained inside a vacuum chamber. The pulses of laser light impinging on the target substance create a plasma that emits X-rays. The X-rays radiating from the plasma are extracted and formed into an X-ray beam. LPX sources have a brightness comparable to that of synchrotron sources, and have the advantage of compactness. Consequently, LPX sources have been under intensive development recently as the X-ray source of choice for various applications.
Other X-ray sources that are attracting attention utilize a so-called “Z-pinch” plasma, dense plasma focus, or plasma created by a discharge in a capillary. These sources are relatively inexpensive.
In an X-ray microlithography apparatus, a reticle (defining a pattern) is irradiated by an X-ray beam from a source. After irradiating the reticle, the X-ray beam is manipulated and directed to form a corresponding image of the pattern on a suitable substrate (e.g., semiconductor wafer) previously “sensitized” with a coating of an appropriate “resist.” The X-ray beam is manipulated and directed using X-ray optical components (mainly specialized mirrors). The wavelength of X-rays used in conventional X-ray microlithography apparatus is in the extreme ultraviolet region, having a wavelength in the range of a few nanometers to approximately 50 nanometers. These X-rays are termed “soft” X-rays. Since many substances are highly absorptive to radiation in this range of wavelengths, adhesion of even a slight amount of a contaminant substance to an X-ray optical component can cause a conspicuous deterioration in the optical characteristics (e.g., reflectivity and transmissivity) of the X-ray optical component.
In optical devices that use soft X-rays, the optical path typically is evacuated to high vacuum to eliminate attenuation of the X-rays by the atmosphere. Accordingly, the X-ray optical components are enclosed in a vacuum chamber, and the interior of the vacuum chamber is evacuated to high vacuum using a suitable vacuum device such as a rotary-vane pump or diffusion pump, etc. Unfortunately, these vacuum devices tend to produce a slight back-flow of pump-oil vapor into the vacuum chamber during operation, thereby introducing oil molecules into the vacuum chamber. Also, the resist tends to outgas in a vacuum. With extended operation, these introduced oil molecules and resist-outgas molecules tend to accumulate on the X-ray optical components inside the vacuum chamber, causing progressive deterioration of the optical characteristics of the X-ray optical components.
In addition, LPX and discharge-plasma X-ray sources tend to produce particulate debris from the plasma and/or from structures located near the plasma. The debris can adhere to a nearby X-ray optical component, causing the optical characteristics of the optical component to deteriorate. Such deterioration can result in a decline in throughput of the apparatus itself.
In conventional microlithography apparatus, there currently is no practical technique with which to monitor the degree of contamination of the optical components during operation of the apparatus. Rather, whenever an exposure dosage applied to the object of irradiation has degraded to an insufficient level from repeated or prolonged operation of the apparatus, contamination of the optical components is suspected and corrective action taken.
For example, in the case of an X-ray microscope employing an LPX source, ten “shots” normally are required to obtain a clear image. Whenever the number of shots required to obtain a suitably clear image increases to, say, 20 shots, the X-ray optical components of the microscope are deemed to be excessively contaminated. In X-ray microlithography apparatus, the constituent X-ray optical components are deemed contaminated when the time required to achieve transfer of a pattern having a certain minimum linewidth becomes excessively long.
In each of the foregoing methods, the presence or absence of contamination of the optical components is adjudged only after the contamination has begun to exert a large adverse influence on operation of the apparatus. In other words, the presence or absence of contamination of the optical components is unknown until the effects of contamination are manifest to an apparent unacceptable degree.
SUMMARY OF THE INVENTION
In view of the problems of the prior art, as summarized above, an object of the present invention is to provide apparatus and methods with which the state of surficial contamination of an optical component in an optical system is measured. Another object is to provide any of various optical systems, such as an X-ray microlithography apparatus, with which one or more constituent optical components can be monitored for contaminant accumulation so as to allow the need for cleaning or replacement of the optical component to be determined.
To such ends, and according to a first aspect of the invention, apparatus are provided for measuring accumulation of a contaminant substance on a surface of an optical component that, during use, is irradiated with radiation. An embodiment of such an apparatus comprises a contaminant-measuring means situated and configured to perform several tasks. First, the contaminant-measuring means detects electrons emitted by the optical component in response to the optical component being irradiated with the radiation. Of the detected electrons, the contaminant-measuring means selects electrons in a specified energy range, and obtains a measurement of the detected electrons in the specified energy range so as to obtain a measurement of a corresponding amount of accumulated contaminant substance on the optical component. An exemplary measurement is of the quantity of electrons in the specified energy range.
In this apparatus, the radiation can be electromagnetic radiation (e.g., X-rays or ultraviolet radiation) or charged-particle-beam radiation (e.g., an electron beam) sufficient to cause emission of electrons (e.g., photoelectrons and/or Auger electrons) from a surface of the optical component irradiated with the radiation.
The apparatus also can include means for irradiating the optical component. Such means can be, for example, an X-ray optical system situated and configured to direct an X-ray beam from a source to the optical component.
The contaminant-measuring means can comprise detection means for detecting electrons emitted from the optical component, wherein the detection means produces an output signal having a parameter corresponding to a detected parameter of the electrons within the specified energy range. The detected parameter can be, for example, time of flight of the electrons. Generally, the number of electrons emitted from the optical component changes (typically is reduced) whenever a contaminant substance accumulates on the optical component. Hence, the contaminant-measuring means is configured such that the state of contamination of the optical component is detected by measuring the quantity of electrons in the specified energy range.
Typically, such as in an X-ray optical system, the opt
Berman Jack
Klarquist & Sparkman, LLP
Nikon Corporation
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