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Method for detecting particles using illumination with several wavelengths    

A system for the detection of very small particles on patterned or bare surfaces, particularly of semiconductor wafers.

Overview

A process is provided for detecting small particles on wafer surfaces by irradiating the wafer surface with two light beams having a small difference in wavelength, collecting the light scattered by the wafer in at least one direction, separating the collected light into two component beams having the wavelengths of the irradiating beams, and comparing the intensities of the two component beams. The intensities of the component beams are transduced to digital signals which are fed to a comparator. An apparatus is also provided which comprises a stage for supporting a wafer, laser source and optics for generating two laser beams having different wavelengths, superimposing them and scanning the wafer with them, a sensor for sensing the light scattered by the wafer and separating it into two components having the said wavelengths, an A/D converter for generating digital signals corresponding to said components and a comparator for analyzing whether said signals indicate the presence of small particles.


Summary of Technology
The technology 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 (.alpha.1 and .alpha.2, the normalized difference of the intensities is given by (.alpha.1-.alpha.2)/(.alpha.1 .alpha.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%
The method comprises checking all parts of the pixels of the surface under control, and detecting suspected pixels by a) successively scanning the individual pixels by mean of at least one scanning light beam, b) determining the signature of each pixel, representing the way in which the pixels scatters the light of a scanning beam, and c) determining whether such signature has the characteristics of a signature of a defective pixel.
The apparatus is comprises the same means for supporting and rotating the wafer and varying the distance of the spot of the irradiating beams from the wafer center, and optical means for collecting the light scattered by the wafer, though not necessarily in a number of directions, but in at least one direction. It differs from said previous apparatus in that it comprises means for generating at least two irradiating laser beams, equal except for having different wavelengths, and for directing them onto the wafer, preferably at a slant; and means, e.g. filter means, for separating the scattered light collected into two component beams having the wavelengths of the two irradiating laser beams. The means for processing each of the two component beams--optical signals--so separated comprise photoelectric means for transducing the optical signals to electric analog signals, amplifying means, if desired, means for sampling each analog signal at a high sampling frequency, e.g. 20 MHz, and means for comparing the samples representing the two component beams, to determine whether their difference indicates the presence of a small foreign particle.
Therefore, this invention provides an apparatus for the detection of small particles on surfaces, particularly surfaces of patterned, semiconductor wafers, which comprises:
a) a stage for supporting a wafer;
b) laser source and optics for generating at least two laser beams having different wavelengths;
c) optical components for superimposing said beams and directing them onto the wafer to scan the wafer;
c) a sensor for sensing the scattered light reflected by the wafer and separating it into two components having the wavelengths of the said laser beams;
d) A/D converter for generating digital signals corresponding to said two components; and
e) a comparator for analyzing whether said digital signals indicate the presence of small particles.
The apparatus optionally further comprises means for associating to each detected particle its coordinates on the wafer.
The said stage for supporting the wafer is a turntable and said optical components direct the superimposed laser beams across a straight or curved line over the surface of the wafer to scan the same as it is rotated. This manner of scanning can be called "polar scanning" and the coordinates of the detected particles are polar coordinates. Another aspect of the said stage is a slide, viz. a support that displaces the wafer along a first line, and said optical components direct the superimposed laser beams over the surface of the wafer along another line, to scan the wafer surface. If the said two lines are straight lines perpendicular to one another, this manner of scanning can be called "x-y" or "Cartesian scanning" and the coordinates of the detected particles are Cartesian coordinates. Such an x-y scanning is known in the art and is described e.g. in U.S. Pat. No. 5,699,447.
The sensor comprises a collector for collecting the scattered light reflected by the wafer in at least one direction, means for separating the collected light into two components having the wavelengths of the two laser beams and photo-electric means for generating electric analog signals representing said components; and the A/D converter samples said electric analog signals at a predetermined frequency and converts them to digital signals.
The laser source may comprise two laser diodes, which emit the two laser beams having different wavelengths, or it may comprise a single laser diode and a non-linear crystal whereby to obtain two different wavelengths. The two laser beams may have the same parameters, except for the difference in wavelength, or may have different parameters, which can easily be taken into account when the signals representing their reflections from the wafer surface are compared.

Patent Summary

U.S. Patent Classes & Classifications Covered in this Patent:

Class 382: Image Analysis

This is the generic class for apparatus and corresponding methods for the automated analysis of an image or recognition of a pattern. Included herein are systems that transform an image for the purpose of (a) enhancing its visual quality prior to recognition, (b) locating and registering the image relative to a sensor or stored prototype, or reducing the amount of image data by discarding irrelevant data, and (c) measuring significant characteristics of the image.

Subclass 145: Inspection of semiconductor device or printed circuit board

Class 250: Radiant Energy

This class provides for all methods and apparatus for using, generating, controlling or detecting radiant energy, combinations including such methods or apparatus, subcombinations of same and accessories therefore not classifiable elsewhere.

Subclass 559.18: With discrimination of discrete light diffusing region

Class 348: Television

Generating, processing, transmitting or transiently displaying a sequence of images, either locally or remotely, in which the local light variations composing the images may change with time.

Subclass 125: Flaw detector

Class 356: Optics: Measuring And Testing

Methods and apparatus (1) for analyzing light to measure or test its characteristics, such as intensity, color and polarization; (2) for determining the optical or nonoptical properties of materials or articles by noting, as by inspection, measurement, or test the effect produced by the materials or articles on light associated therewith; and (3) for measuring the dimensions of structures or the spatial relationships such as distances or angle bearings of spaced points by comparison of the respective properties (usually direction or spatial position) of the light from these points or by comparison of the properties of these lights with some scale or standard. The light analyzing includes or is for spectroscopy, interference, polarization, beam direction or pattern, focal position of a light source, shade or color, and photometers. The material or article properties determined are or involve crystal or gem examination, material strain analysis, blood analysis, optical pyrometers, egg candling, cutting blade sharpness, oil testing, document verification, flatness, lens or reflector testing, refraction testing, monitoring moving webs or fabrics, light transmission or absorption, light reflection, inspection for flaws or imperfections in materials, and thread counting. The dimensioning and spatial relationship determination includes triangulation by a light beam, contour plotting, range or height finders, motion stopping, velocity or velocity/height measuring, sighting where the optical element or reticle moves with the sighted object, particle size determination, particle light scattering, electrophoresis, angle measuring or axial alignment, mensuration or configuration comparison, alignment in a lateral direction, and fiducial instruments.

Subclass 237.4: On patterned or topographical surface (e.g., wafer, mask, circuit board)
Subclass 239.8: Detection of an object or particle on surface