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Pixel Based Machine for Patterned Wafers    

A method is provided for the detection of defects on a semiconductor wafer by checking individual pixels on the wafer...


A method is provided for the detection of defects on a semiconductor wafer by checking individual pixels on the wafer, collecting the signature of each pixel, defined by the way in which it responds to the light of a scanning beam, and determining whether the signature is that of a faultless pixel or of a pixel that is defective or suspect to be defective. An apparatus is also provided for the determination of such defects, which comprises a stage for supporting a wafer, a laser source generating a beam that is directed onto the wafer, collecting optics and photoelectric sensors for collecting the laser light scattered by the wafer in a number of directions and generating corresponding analog signals, an A/D converter deriving from said signals digital components defining pixel signatures, and selection systems for identifying the signatures of suspect pixels and verifying whether the suspect pixels are indeed defective.

The technology, both as to method and apparatus, is based on the principle of inspecting all or part of the individual pixels of the patterned wafers under control, without comparing patterns or needing specific information about the patterns. In other words, the technology is based on the principle of detecting suspected pixels, viz. pixels that show signs of having a defect, particularly the presence of foreign particles, without reference to the pattern to which the pixel belongs or to the position of the pixel on the wafer and without comparison between patterns. This inventive inspection method is termed herein "design rule check". Although reference will be made herein to patterned semiconductor wafers, the analysis of which is the primary purpose of the technology, it will be apparent that the technology can be applied in general to the analysis of different surfaces, particularly of any surfaces not patterned or having patterns the dimensions of which are similar to those of wafer patterns, e.g. in the order of microns or fractions of microns.
The method and apparatus of the technology can be used "inline", viz. is suitable to be integrated with the production process tool, using the same wafer handling and interface system, and can operate as an integrated particle monitor to provide a constant check of the wafers produced, and in this way will detect any irregularities or defects that may arise in the production line. Sometimes unforeseen phenomena may occur in the production line that are so far-reaching as to render its further operation impossible or useless. It is important to detect such phenomena, which may be termed "catastrophic", as soon as possible, and this invention permits to do so. These inline checks are rendered possible for the first time in the art by the high speed of the pixel-based inspection method and the moderate cost and footprint of the apparatus.
According to an aspect of the technology, the same comprises a method for the determination of defects, particularly the presence of foreign particles, in patterned, semiconductor wafers, which comprises successively scanning the individual pixels, defining the signature of each pixel, and determining whether said signature has the characteristics of a signature of a faultless or of a defective, or suspected to be defective, pixel.
In some embodiments of the technology, the determination of the characteristics of the pixel signatures is preceded by preliminary steps of evaluation of the characteristics of the individual signals that make up the signature, which permit to conclude that certain signatures cannot belong to defective pixels, and therefore require no further processing, whereby to reduce the amount of data that must be processed. Therefore the method of the technology may comprise defining the signature of each pixel, evaluating each signal of each signature, and, based on said evaluation, excluding a number of signatures from further processing. Preferably, the pixels are optically scanned by means of an illuminating beam and their signature is defined by their optical reaction to the illuminating light. In this case, the various embodiments of the method of the technology are characterized by the following features:
I--The type of light being used;
II--The physical and geometric parameters of the illumination;
III--The property and/or parameters by which the optical reaction of the pixels, and therefore their signature, is characterized;
IV--The physical and geometric parameters of the detection of said optical reaction.
I--The Type of Light Being Used
According to the technology, one can use laser beams or light produced by other sources, such as flash lamps, fluorescence lamps, mercury lamps, etc. Laser beams can be produced e.g. by diode lasers and have any wavelength, e.g. 400 to 1300 nm. The choice of the appropriate wavelength can be carried out by skilled persons in any case, so as to produce optimization for a given material or pattern. Relatively long wavelength (e.g. 600-810 nm) are generally preferred because of the high energy fluence achievable. Short wavelengths can be preferred for detecting small particles and for finer design rules. Laser beams can also be produced by non-diode generators, of any wavelength from IR to deep UV. The illuminating radiation may be narrow band or wide band (important for spectral analysis). It can be coherent or non-coherent, polarized or non-polarized. As to fluence, it can be CW, pulsed or quasi-CW. One or a plurality of light beams can be used.
II--The Physical and Geometric Parameters of the Illumination
1. The number of the illumination sources can be changed.
2. The geometric placement of the illumination sources can be changed.
3. The size and form of the light source and of the illuminated spot can be changed.
4. The way in which the illumination light is delivered can be changed.
Important changes can arise from changing the size of the illuminated spot with respect to a given pattern. A spot of 5 square microns will provide a completely different set of signatures than a spot of 75 square microns, and different discrimination capability. Some useful light source forms are a point source, a ring source, a large aperture source, and a line source. It may sometimes be beneficial to illuminate through the wafer (or through another article, when such is being inspected, such as a reticle or some other transparent article) with a relatively large wavelength (more than 1 micron). Thus, one could illuminate from beneath the wafer and collect the received radiation from above.
The illumination light can be delivered by optical trains, fiber optics, or other directing elements.
III--The Property and/or Parameters by Which the Pixel Signatures are Characterized
1. In this system, the energy of the scattered light is the main property that is being measured.
2. Another property is the height of the surface. This is measured by the height measurement system.
3. Other properties can also be used successfully for creation of a signature. These are:
3.1. The polarization of the received radiation, in P and S planes. This is important, since there are many geometric locations at which the pattern on the wafer induces a well determined polarization, so that a correctly aligned polarizer would sense only particles.
3.2. The phase of the received radiation.
3.3. The wavelength of the received radiation, which can be tested in various ways, e.g. by testing for fluorescence or by testing the spectral response of a pixel.
IV--The Physical and Geometric Parameters of the Detection of the Optical Reaction of the Pixels
The optical reaction, and therefore the signature of the pixels is defined by the light scattered by the pixels. The way in which it is detected can vary widely. It is detected in a plurality of directions, which will be called, for descriptive purposes, "fixed directions". Each direction is defined by a line from the pixel to a point of light collection. Therefore, the geometry of the scattered light detection is defined by the disposition of the points of light collection. Said points may be disposed e.g. in azimuthal symmetry on horizontal concentric circles ("horizontal" meaning herein parallel to the wafer surface), or in elevational symmetry on vertical or slanted semi-circles, or in a flat grid parallel to the wafer surface, or in a semi-spherical or other vault-like arrangement above the wafer.

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 147: Inspecting printed circuit boards
Subclass 149: Fault or defect detection

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 126: Of electronic circuit chip or board