Radiant energy – Inspection of solids or liquids by charged particles – Electron probe type
Reexamination Certificate
2001-12-20
2004-02-17
Lee, John R. (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Electron probe type
C250S306000, C250S307000, C250S309000, C250S311000, C359S368000, C382S147000, C382S149000
Reexamination Certificate
active
06693278
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit under 35 USC 119 of earlier filed EP Pat. App. 00204808.0, filed on Dec. 22, 2000.
STATEMENT REGARDING FEDERALLY PROMISED RESEARCH OR DEVELOPMENT
Not Applicable
INCORPORATION BY REFERENCE OF MATERIALS SUBMITTED ON A COMPACT DIS
Not Applicable
BACKGROUND OF THE INVENTION
The invention relates to a device for the inspection of patterns on objects, which device is provided with:
a carrier unit for carrying the objects during the inspection,
for each object an inspection unit that includes at least one particle-optical column for scanning the pattern to be inspected on the associated object,
a comparison circuit for comparing the scan signals that are produced by the particle-optical column in a first inspection unit and by the particle-optical column in a second inspection unit,
which device is arranged for the simultaneous inspection of corresponding patterns on a plurality of objects.
A device of the kind set forth is known from U.S. Pat. No. 5,641,960.
In the semiconductor industry there is a need for equipment that is suitable for the inspection of patterns written on wafers, for example, for the detection of defects arising during the manufacturing process. Such defects may be the cause of malfunctioning of the manufactured integrated circuits (ICs) so that they have to be rejected. As the smallest critical dimensions of the ICs become smaller and smaller, it is necessary to have inspection equipment available that is still capable of suitably discriminating such small details. The contemporary dimensions of the order of magnitude of 200 nm of the smallest details necessitate a resolution of the inspection equipment of approximately 50 nm; it is generally expected that this resolution will have to be improved even further in the near future.
Optical equipment that operates in the field of the visible light is no longer adequate to achieve such a high resolution. The use of particle-optical equipment, notably scanning electron microscopes (SEMs), however, enables suitable observation of such small details.
The cited United States patent describes a device for the inspection of patterns on objects, said device being arranged to inspect circuits on semiconductor wafers. The known device is provided with a carrier unit with a carrier or stage for the wafers that rotates during the acquisition of the signals that are required for the inspection. For each wafer there is provided an inspection unit in the form of an electron optical SEM column that carries out the wafer inspection. The entire wafer surface can be accessed for inspection by the SEM columns in that the carrier rotates relative to the columns and in that during the inspection the columns are displaced in the radial direction relative to the axis of rotation of the carrier. Because the described known device is arranged for the simultaneous inspection of corresponding patterns on different wafers, the scan signals produced by the particle-optical columns can be compared by a comparison circuit, so that the existence of a defect can be decided upon when a difference between these signals is detected. This comparison can be performed in “real time”, so that it is not necessary to form and maintain very large data bases with which the signals have to be compared and that it is not necessary either to carry out time-consuming digital comparison operations between the instantaneous scan signals and the stored data.
Because the known device is provided with only one column for each wafer, inspection in this device can take place at a comparatively low feed-through rate only. This can be demonstrated as follows. As the details of the patterns to be inspected become smaller, the number of details to be inspected for each wafer increases, that is, by the square of the detail reduction (the wafer dimensions remaining the same). As a result, inspection of the entire wafer surface is dispensed with and only the areas within the patterns in which the smallest detail size occurs are inspected, that is, the so-called Care Areas. The fraction of the pattern in which such Care Areas are situated is known as the Care Area Fraction (CAF) that typically has a value of the order of magnitude of 1%. The feed-through rate during inspection can be considerably increased by inspecting only the CAF; however, in order to make wafer inspection in future keep pace with the production rate of the wafers (which is necessary for on-line inspection of the wafers), the CAF must be strongly reduced, so that parts of the patterns that are prone to defects would have to be skipped; of course, such a development is undesirable.
The inspection of the wafers in the known device is performed by making the wafer rotate relative to the inspecting column. During this rotational displacement the passing wafer is irradiated by an electron beam that is produced by the column and is stationary relative to the column during the execution of the inspection scan. The area of the wafer that is inspected per revolution of the carrier is thus shaped as a circular path; if a larger area is to be inspected, for example, an area having a rectangular shape, such an area will have to be composed from a number of adjoining circular paths. This means that it is still possible to select a number of care areas for inspection, but also that the surface area of the circular paths that are situated between the care areas must also be covered. Consequently, the scan time cannot be reduced by skipping such intermediate areas.
This method of scanning also has the drawback that the factor limiting the speed of rotation (for example, the processing speed of the electronic circuitry used for the data comparison) is based on the highest speed that occurs, that is, the speed of the areas at the outer periphery of the circle of rotation, that is, the location where the maximum radial position of the columns is situated. All areas to be inspected within this maximum circle of rotation, therefore, have a non-optimum speed, so that the device is overproportioned for the vast majority of the areas to be inspected.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide a solution to the problem of maintaining the feed-through rate during on-line inspection of semiconductor wafers with increasingly greater circuit densities. To this end, the device in accordance with the invention is characterized in that the particle-optical columns are arranged to carry out an x-y scan of the pattern on the objects by x-y deflection of the particle beam that is produced by the relevant particle-optical column, and that the carrier unit is arranged to realize a rectilinear translatory feed-through direction for the objects. In the device in accordance with the invention the objects to be inspected (the wafers) are arranged underneath the inspection column in such a manner that they are oriented on the carrier in the direction perpendicular to the feed-through direction of the carrier (for example, to be referred to hereinafter as the x direction) in such a manner that they occupy the desired position relative to the column. In the (rectilinear translatory) feed-through direction (for example, referred to as the y direction) wafers are arranged in the desired location underneath the inspection column in such a manner that they occupy the desired position relative to the column in said direction as a result of adjustment of the feed-through distance. The scanning of the desired care areas takes place after such positioning in that the electron beam that is produced by the column is scanned across the area to be inspected in the customary manner. Because this scan is executed under the control of an electron beam, such scanning requires a substantially smaller amount of time than the physical rotary displacement of the objects relative to the column.
An embodiment of the device in accordance with the invention is arranged to superpose on at least one of the scan signals a periodic signal that varies linearly in time and w
Krans Jan Martijn
Maas Diederik Jan
El-Shammaa Mary
FEI Company
Lee John R.
Nordstrom Erik
Scheinberg Michael O.
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