Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
1998-07-07
2001-03-27
Mehta, Bhavesh (Department: 2721)
Image analysis
Applications
Manufacturing or product inspection
C250S559180, C348S125000, C356S237400, C356S239800
Reexamination Certificate
active
06208750
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and apparatus for the detection of very small particles on patterned or bare surfaces, particularly of semiconductor wafers.
BACKGROUND OF THE INVENTION
Bare, as well as patterned, semiconductor wafers are checked for the presence of defects, particularly for the presence of particles. Prior art methods and devices are used in order to detect foreign substances in patterned wafers.
One method comprises scanning the wafer surface with a laser beam and analyzing the number and direction of diffraction lights, produced by the pattern edges, by means of a plurality of light detection cells arranged cylindrically. Other methods involve using polarized light, or comparing an inspected pattern with an ideal pattern, or analyzing the light reflected from a wafer surface by distinguishing between normal directions and abnormal directions due to reflection from particles, or employing a planar array of individually addressable light valves for use as a spatial filter in an imaged Fourier plane of a diffraction pattern.
U.S. Pat. No. 4,731,855 includes in its Background of the Invention a list of various methods for performing semiconductor wafer inspections, and said list is incorporated herein by reference.
The methods and apparatus of the prior art have several drawbacks, partly discussed in the cited references, such as errors due to faulty registration and other causes, false alarms consisting in the detection of defects that are only apparent, and so on. All of them, further, have the common defect of requiring complex apparatus, with high mechanical precision, and requiring long operation times and having therefore a low throughput.
In order to overcome the disadvantages of the prior art, the applicant has invented a method and an apparatus that are described and claimed in a copending patent application, Ser. No. 09/110,870 filed concurrently with this application, the contents of which are entirely incorporated herein by reference. The prior art methods do not permit the identification of particles having extremely small dimensions, particularly smaller than the width of the pattern lines, e.g. sub-micron particles. Such identification is difficult even with the method and apparatus of the aforesaid copending application. Modern wafers have pattern widths in the order of 0.18-0.5 &mgr;m. Particles having smaller sizes, e.g. a size of about 0.1-0.2 &mgr;m, are particularly difficult to detect. Their detection is difficult on uniform surfaces, such as those of unpatterned wafers or memory areas of patterned wafers, as well.
It is therefore a purpose of this invention to permit the detection of such particles.
It is another a purpose of this invention to permit the detection of such particles in each pixel of the controlled surface, without reference to its pattern and without comparing patterns.
It is a further purpose of this invention to provide a high speed detection method which has a very high throughput and permits the on-line detection of sub-micron particles.
It is a still further purpose of this invention to provide a detection method that is compatible with the method that forms an object of the aforesaid copending application and can be carried out concurrently with it.
It is a still further purpose of this invention to provide an apparatus for the detection of sub-micron particles.
It is a still further purpose of this invention to provide an apparatus that can be combined with the apparatus that forms an object of the aforesaid copending application, to form a single unit.
Other purposes and advantages of the invention will appear as the description proceeds.
SUMMARY OF THE INVENTION
This invention comprises a process for detecting small particles of foreign matter on a surface, particularly a patterned or bare surface of a semiconductor wafer, which comprises:
I—irradiating the surface with two light, generally laser, beams, that are identical in all their parameters, but have a small difference in wavelength;
II—collecting the light that is scattered by the surface in at least one direction, preferably by means of optical fibers,
III—separating the collected light by filtering it in two component beams, each of which has the wavelengths of one of the irradiating beams; and
IV—comparing the intensities of said two component beams.
Preferably, the scattered light is collected in a plurality of directions and the intensities of said two component beams are compared in each of them. However, one direction is chosen for processing the signals generated in it, as hereinafter described, and the signals generated in other directions are neglected. The direction chosen is one in which the intensity of the two component beams is not too high.
By “small particles” is meant herein particles that have dimensions much smaller than the wavelengths of the illuminating laser beams, especially particles having dimensions in the order of a few tenths of a micron (hereinafter “sub-micron” particles). The “parameters” of the laser beams, as this term is used herein, comprise every physical and geometric features of the laser beams, such as polarization, size and shape of their footprints or light spots on the irradiated surface, angle of illumination, etc. The laser beams are preferably, though not necessarily, at a slant with respect to the wafer surface. Slanting the beams causes the wafer surface to be seen as flatter than it would if the beam were perpendicular to it, which is advantageous from the processing viewpoint. They may have superimposed or adjacent footprints, as will be better explained hereinafter.
While pattern lines may scatter light with different intensity depending on the wavelengths, or in other words produce scatter signals having different intensities when they are irradiated with light beams having different wavelengths, such intensity differences are much smaller than those of the light scattered by small particles. Therefore a marked difference in the intensities of the aforesaid two component beams is the index of the existence of the small particle. Said intensity difference is preferably determined by directing each component onto a photodetector, thereby producing an optical signal, transducing the optical signal into an analog electric signal, sampling the two electric signals, and feeding the resulting digital signal to hardware or software comparing means.
Before comparing the intensities of the two component beams, they will be preferably amplified, and, if desired, shaped. A threshold intensity may be established, below which the signals will be considered as noise and therefore irrelevant, and will not be further processed.
In order to obtain two laser beams, having a similar geometry and the same intensity but a small difference in wavelength, two laser beams of the desired wavelength can be generated, and then imparted the desired similar geometry and focused on the surface under examination by optical means, which may include mirrors, beam splitters, lenses and the like.
The difference in the wavelengths of the two irradiating beams should be small, preferably between 1 and 5% of the average of the two wavelengths. The average wavelength of the two beams is not critical. By way of example, the two beams may have wavelengths of 630 nm and 670 nm, respectively.
The intensity difference of the collected light components, having the two wavelengths of the irradiating beam, is normalized by means of the sum of the intensities of the two components. If the two components have intensities (&agr;1 and &agr;2, the normalized difference of the intensities is given by (&agr;1−&agr;2)/(&agr;1+&agr;2). To be considered significant, for the purpose of this invention, the normalized difference of intensities must be above a threshold, which is between 5 and 100%
In the aforementioned copending application, a method and an apparatus for the analysis of surfaces, particularly for the detection of particles of patterned semiconductor wafers, are described and claimed. The method comprises checkin
Applied Materials Inc.
Mehta Bhavesh
Sughrue Mion Zinn Macpeak & Seas, PLLC
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