Geometrical instruments – Gauge – Coordinate movable probe or machine
Reexamination Certificate
1999-12-16
2002-02-19
Bennett, G. Bradley (Department: 2859)
Geometrical instruments
Gauge
Coordinate movable probe or machine
C033S0010MP, C033S573000, C356S500000, C269S902000
Reexamination Certificate
active
06347458
ABSTRACT:
CROSS REFERENCE TO A RELATED APPLICATION
This application claims priority from an earlier filed German Patent Application DE 198 58 428.8-52 filed on Dec. 17, 1998, which German application is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates generally to the field of analytical instruments used for determining a location of an object, such as a substrate in an analytical instrument. More particularly, the invention relates to a displaceable X/Y coordinate measurement table used for interferometrically determining the coordinates of the substrate. The X/Y coordinate measurement table of the present invention is designed in such a way that the substrate positioned on the measurement table is accessible from above and from below.
BACKGROUND OF THE INVENTION
Measurement tables are used in wafer steppers and in high-precision coordinate measuring instruments, such as interferometers. The measurement mirrors mounted on the tables make it possible to determine interferometrically the current position of the table while the table itself supports the analyzed or processed substrates.
A measurement table of that type is described in German Patent Application DE 198 19 492.7-52 as a component of a coordinate measuring instrument used for a highly accurate coordinate determination of structures on substrates, e.g. masks and wafers, but, in particular, of structures on transparent substrates. The measurement table described in that Application is vertically and horizontally displaceable and has a receiving rim for receiving the substrate. In addition, the measurement table is equipped with a frame-shaped opening, so that the received substrate is accessible from both above and below. Flat mirrors attached to two mutually perpendicular sides of the measurement table serve as measurement mirrors for a laser interferometer system that determines the X/Y position of the measurement table.
The described measuring instrument comprises a reflected-light illumination device and a transmitted-light illumination device having a common optical axis through an opening in the measurement table. Also present are an imaging device and a detector device for observing the analyzed structures. The measured coordinates of a particular analyzed structure are derived from the current interferometrically measured coordinates of the measurement table and from the coordinates of the analyzed structure relative to the optical axis of the table.
The article “Maskenmetrologic mit der LEICA LMS IPRO für die Halbleiterproduktion” [Mask metrology using the LEICA LMS IPRO for semiconductor production] by K.-D. Röth and K. Rinn, Mitteilungen für Wissenschaft und Technik Vol. XI, No. 5, pp 130-135, Oct. 1997, describes a coordinate measuring instrument in which a measurement table of the aforesaid type is used to receive and analyze various substrates. The instrument comprises an interferometer and a separate interferometer measurement beam path for each coordinate axis X and Y of the measurement table. Two measurement mirrors located at the ends of the two interferometer measurement beam paths are attached to two mutually perpendicular sides of the measurement table. Such an instrument utilizes the two mirrors to interferometrically determine the coordinates of the measurement table, which coordinates are then used to calculate the coordinates of various structures on the substrates.
However, the above-described measuring instrument does not allow a user to perform high accuracy coordinate measurements of structures and defects at a nanometer scale, because in order to achieve such an accuracy, a number of instrumental error sources has to be taken into account and reduced or eliminated. For example, it would be desirable to eliminate such sources of measurement errors as thermal expansion of the materials of which various components of the instrument are made, changes in ambient temperature, humidity and air pressure affecting the wavelength of light in the interferometer, vibrations of the building where the instruments is located. The need, therefore, exists to provide a measuring instrument in which instrumental errors affecting the precision of the measurements are either minimized or eliminated.
The geometry of the measurement mirrors often also becomes a source of errors, since unidentified changes in the mirror geometry result in errors of the coordinates of the measurement table. This problem occurs generally in measurement tables which employ interferometric position determination, regardless of whether the measurement tables are utilized in coordinate measuring instruments or in steppers. The need, therefore, exists to provide an analytical instrument which is able to compensate for geometrical and other imperfections of the instrument.
SUMMARY OF THE INVENTIQN
The present invention addresses the above-described needs by providing an instrument in which a wide variety of instrument error sources is taken into account instrumentally and by providing various methods for minimizing or eliminating such error sources.
It is, therefore, an object of the present invention to minimize the effect of mechanical, physical, geometrical and other instrumental errors on the accuracy of high precision measurements performed by the instrument. For example, in order to minimize the effect of external building vibrations, the measuring instrument is mounted on a vibration-damped, air-mounted granite block with the air-mounted measurement table coupled to the block. In order to minimize the effects of changes in relative humidity and temperature of the surrounding environment, the measuring instrument of the present invention is housed in a climate-controlled chamber. Because the wavelength of the interferometer measurement light depends on air pressure, temperature, humidity, and the composition of air, the instantaneous value of the wavelength is continuously measured and monitored by taking into account a comparison measurement of a standard test article in calculating the final wavelength measurement result.
It is another object of the present invention to provide a device and method for error-free interferometric determination of the position of the measurement table by using software corrections of the instrumental errors. To achieve an error-free measurement, it is always desirable for the measurement mirrors to be flat, not tilted, and disposed orthogonally to one another. Since these criteria are hard to achieve instrumentally on a nanometer scale, the residual instrumental errors attributed to geometrical imperfections of the measurement mirrors, such as flatness, orthogonality, and tilting are detected and then compensated for by software corrections. In order to compensate for instrumental errors by using software corrections, the once-identified errors attributed to the mirrors should remain the same during subsequent measurements, i.e. even after the analyzed mask has been changed. This is achieved in the present invention by fabricating the measurement mirrors from a material with an extremely low coefficient of thermal expansion in order to minimize the effect of temperature changes on mirror geometry.
However, since the measurement mirrors are attached to the measurement table, the design of the measurement table itself becomes a critical component in measurement error reduction. For example, the flexible air-bearing system causes the measurement table to deform slightly, therefore, adversely affecting the mirror geometry. In addition, the analyzed masks and wafers have different dimensions and weights: the lightest mask, for example, weighs only about 80 g, but the heaviest weighs approximately 1.4 kg, while wafer chunks are even much heavier. If substrates with very different weights are placed on the measurement table one after another, the measurement table deforms differently each time.
The measurement table is also slightly deformed during displacement. The reason is that as the measurement table is displaced, it slides on the air bearings on the granite blo
Bennett G. Bradley
Brown Rudnick Freed & Gesmer
Leica Microsystems Wetzlar GmbH
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