Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-03-22
2002-11-12
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06480286
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a method and apparatus for measuring thickness variation of a thin sheet material. More particularly, the present invention relates to measurement of thickness variation of wafers for use in semiconductor devices, which need to meet stringent requirements for little variation in thickness in a surface direction, and further relates to a probe reflector used in the thickness variation measuring apparatus.
2. Description of Related Art
Wafers used for semiconductor devices consist of a thin sheet material of silicone or the like. For fabrication of semiconductor elements and circuits on the surface of a wafer, photoetching techniques, printing techniques, and various micro machining techniques are applied.
In processing a wafer using these techniques, it is essential to achieve high planarity of the wafer surface. Deterioration in surface planarity may blur the patterns of semiconductor elements or circuits formed by photoetching, or make the contour of a material printed in patterns on the wafer surface unclear. With the increase in density and scale of semiconductor elements and circuits, the above problem becomes crucial.
In the fabrication of semiconductor devices, it is the normal practice to hold the entire surface of the wafer on a flat support surface by means of vacuum suction or the like during various processes. If the thickness of the wafer varies in different locations, the planarity of the wafer surface will in turn vary, as the backside of the wafer is held on a flat support surface in tight contact therewith. There is thus the requirement for minimization of variation in thickness of the wafer. Evaluation of wafer thickness variation in a wafer production process entails precise and efficient measurement of variation in wafer thickness.
Japanese Laid-open Patent Application No. 10-70162 discloses an apparatus for measuring thickness variation of a wafer. In this technique, a disk-shaped wafer is rotated as held vertically, and using a capacitance displacement sensor disposed opposite both faces on one side of the wafer, thickness variation of the wafer is calculated from the measured displacement of wafer surface with respect to the sensor. The capacitance displacement sensor is scanned in a radial direction of the wafer, thereby measuring the thickness variation of the entire surface of the wafer.
However, the above described apparatus has a limit to measurement precision, and is not suitable for highly precise thickness variation measurement required for the fabrication of semiconductor devices of high density in recent years.
The capacitance displacement sensor used in the above described apparatus measures displacement of wafer surface with respect to the sensor by electric measurement of capacitance between the wafer surface and the sensor. For that reason, the measurement results are highly dependent on electrical properties of the wafer which may change depending on the material of the wafer or ambient conditions. Such electrical properties may also vary in different locations on a same wafer. Measurement precision of wafer thickness variation by this capacitance sensor is thus apt to deteriorate. Further, precise measurement of capacitance at the peripheral edge of the wafer is practically impossible, and since the thickness variation in this part cannot be estimated, it is normally regarded that such peripheral portion of the wafer to a width of about 3 mm from the outermost edge of the wafer cannot be used for semiconductor devices. Wafer material is accordingly wasted. Moreover, in some cases depending on conductive properties of the wafer, measurement by the above capacitance displacement sensor is technically difficult. Also, as noted above, measurement results are greatly affected by ambient conditions between the sensor and the wafer.
In accordance with the increase in density of the semiconductor elements and circuits in recent years, it is the normal requirement that thickness variation measurement of a wafer should be carried out with the precision lower than 0.01 &mgr;m.
Theoretically, such high precision in thickness variation measurement can hardly be achieved with the above described capacitance displacement sensor, which is said to have the precision of about 0.05 &mgr;m.
Apart from wafers for semiconductor devices, there is a technical field where high precision is required in the measurement of thickness variation of a thin sheet material such as a substrate for magnetic disks.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a method and apparatus for measuring thickness variation of a thin sheet material such as a wafer highly accurately and efficiently.
In accordance with the present invention, using a pair of optical displacement gauges, measuring light is irradiated onto a surface of the thin sheet material, and displacement of surface position of the thin sheet material is measured by receiving said measuring light reflected by the surface of the thin sheet material. Thickness variation of the thin sheet material is thereby obtained from the measured displacement of the surface position of the thin sheet material with the optical displacement gauges.
The present invention can be applied to any type of thin sheet material irrespective of material, configuration, and dimension, as long as measurement of thickness variation thereof with high accuracy is required. Material may be both conductive and non-conductive. Also, the thin sheet material may be constructed such that characteristics or electric properties thereof vary at different locations, or may be constructed multi-layered with a plurality of different materials. Specifically, thin sheet material includes wafers for semiconductor devices made of silicone or the like, metal plates for magnetic disks, ceramic plates, resin plates, and others. The shape of the thin sheet material is mostly disk-like or circular shape particularly in the case of wafers, but it is not limited to the circular shape.
The surface of the thin sheet material should preferably have superior reflectance such as a mirror plate. However, in the case of using the probe reflector of the present invention to be described later, the surface of the thin sheet material need not particularly have superior reflectance.
The apparatus for measuring thickness variation of a thin sheet material according to the present invention comprises a pair of optical displacement gauges disposed opposite each other, with the thin sheet material being arranged between the two optical displacement gauges. The thin sheet material is supported such as to be rotatable, so that measurement by the optical displacement gauges can be carried out at different locations along a circumferential direction. Furthermore, the optical displacement gauges are movable in a direction along radius of rotation of the thin sheet material, so that, combined with the rotation of the thin sheet material, measurement can be made with respect to the entire surface of the thin sheet material. It should be noted that the optical displacement gauges themselves need not be moved, but an optical system may be provided, which changes position of irradiating measuring light to the thin sheet material and position of receiving the reflected measuring light, for achieving the same function. Such scanning measurement is especially suitable for quality control in a production line.
By summing up the amounts of displacement of the surface positions of both faces of the thin sheet material that are measured respectively by the two optical displacement gauges, thickness variation of the thin sheet material can be obtained. An electronic circuit will suffice for performing such calculation for obtaining thickness variation of the thin sheet material. Alternatively, an operation/processing device such as a microcomputer may be used, in which predetermined operating and data processing procedures are programmed in advance.
For
Handa Koji
Imada Yukio
Kubo Keishi
Takeuchi Hiroyuki
Yoshizumi Keiichi
Jordan and Hamburg LLP
Matsushita Electric Inudstrial Co., Ltd.
Turner Samuel A.
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