Optics: measuring and testing – By alignment in lateral direction – With light detector
Reexamination Certificate
1999-09-20
2001-12-04
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With light detector
C356S401000, C355S053000
Reexamination Certificate
active
06327034
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention relates to an improved apparatus for automatically and precisely aligning two superimposed objects (i.e., aligning a semiconductor wafer or other substrate coated with photoresist material with a mask), and to a method of aligning objects using the apparatus.
BACKGROUND ART
In the present day manufacture of integrated circuits, complex circuit patterns are formed on a silicon wafer by photoresist techniques employing a series of contact printings on the wafer. These contacts printings are made from several transparent masks used in succession and in a preselected order. Each successive mask must be accurately aligned with the previous print or prints made on the wafer from the prior masks so that the completed pattern is accurate within a few microns.
The alignment of each mask with the wafer may be accomplished manually by manipulation of the mask over the wafer while the operator observes the mask and wafer through a high power microscope. Alignment may be aided by the use of a pair of spaced-apart detection marks or alignment patterns, for example, crosses, bull's-eyes, or pinpoint alignment holes, formed on each of the wafer and mask, the pair of alignment patterns on the mask being arranged so that they are superimposed over and aligned with the pair of alignment patterns on the wafer when the wafer and mask are properly aligned
Apparatus has been proposed for producing alignment of the mask and wafer automatically, thus relieving the operator of this tedious task One form of such automatic apparatus is described in U.S. Pat. No. 3,497,705 issued Feb. 24, 1970, to A. J. Adler and entitled “Mask Alignment System Using Radial Patterns And Flying Spot Scanning”. In that system a pair of spaced-apart radial alignment patterns on the wafer is adapted for alignment with a superimposed pair of spaced-apart radial alignment patterns on the transparent mask. The radials of the alignment patterns on the mask are uniformly angularly displaced relative to the radials of the alignment patterns on the wafer when the mask and wafer are properly aligned. A scanning system employing two flying spot scanners scans each of the two pairs of alignment patterns in a circular manner about a center point, measuring the angular distance between the successive radials encountered by the scanning beam. Error signals derived from misalignment of the radials of the alignment patterns on the mask relative to the radials of the alignment patterns on the wafer are utilized to produce relative movement in X, Y, and rotational directions to bring the two pairs of alignment into proper alignment.
Karlson et al., in U.S. Pat. No. 4,052,603, published Oct. 4, 1977, discloses a system for aligning a pattern mask and a photoresist-coated substrate which are separated by an optical element which involves a system for correcting misalignment in the x- or y-directions between the mask and the substrate by using stepper motors to move the substrate in the x-direction or in the y-direction relative to the mask. Rotational misalignment is corrected using a separate mechanism involving gears and levers to rotate the mask relative to the substrate. This apparatus involves use of two separate systems simultaneously to achieve successful alignment, and is rather complicated It would be desirable to have a system which moves the mask relative to a fixed substrate, rather than moving both the mask and the substrate relative to each other. It would also be desirable to achieve rotational motion and motion in the x-direction or in the y-direction relative to the substrate with a single mechanism, rather than with two separate mechanisms.
SUMMARY OF THE INVENTION
The objects of the invention are to provide an improved apparatus for bringing two products into alignment with high precision, and to provide a method for using the apparatus to successfully align two objects.
The invention provides a system for aligning a product, such as a semiconductor wafer or other substrate coated with a photoresist material, and a mask with high precision. The mask has a plurality of first detection marks, and the product has a plurality of second detection marks. When the mask and the product are properly aligned, each first detection mark is precisely aligned with a second detection mark. The system features a product securing surface, where the product securing surface is made from a porous plate adapted to support the product, a means for securing the product to the porous plate by applying vacuum to the product through the porous plate, and at least two registration pins adapted to fit through registration holes in the product. The invention also features a surface for positioning a mask. Normally, the mask is surrounded by a rectangular mask frame having at least two holes therethrough. The mask frame is then positioned on the mask positioning surface. The mask positioning surface features a holding plate adapted to support the mask frame and at least two alignment pins adapted to fit through alignment holes in the mask frame. Preferably, the porous plate is a recessed portion of the holding plate supporting the mask frame.
When the mask frame is positioned on the mask positioning surface, a movable first plunger presses against a first edge of the mask frame, and two movable second plungers press against a second edge of the mask frame, where the second edge of the mask frame is perpendicular to the first edge. The plungers provide resistance to accidental movement of the mask frame. Purposeful movement of the mask frame is generated by a series of computer-controlled push plates. A first computer-controlled movable push plate is designed to move the mask frame in a first direction by pushing the mask frame against the movable first plunger, while two second movable computer-controlled push plates are designed to move the mask frame in a second direction by pushing the mask frame against said two movable second plungers. The two second movable computer-controlled push plates can also cause rotation of the mask frame by moving one of the second movable push plates toward one of the movable second plungers, while either holding the other of the second movable push plates in a stationary position; or by simultaneously moving one of the second movable push plates toward one of the movable second plungers moving the other of the second movable push plates away from the other of the movable second plungers.
The system additionally features an imaging means comprising a video camera and an optical microscope adapted to focus an image of the mask and/or the product onto the imaging plane of the video camera. The video camera records an image of each second detection mark on said product and an image of each first detection mark on said mask, and transmits each image recorded on the imaging plane to a computer.
The computer records the position of each second detection mark on the product; and also records the position of each first detection mark on said mask. The computer then compares the positions of the first detection marks to the positions of the second detection marks, and determines that to bring each first detection mark into alignment with a second detection mark, the mask must be moved in the first direction by a first distance, moved in the second direction by a second distance, and rotated by a defined amount.
Then computer then:
a) signals the stepper motor controlling the first computer-controlled movable push plate to move the mask frame toward the movable first plunger (hereinafter defined as the x-direction) by the computer-determined first distance;
b) signals the stepper motors controlling the two second computer-controlled movable push plates to move the mask frame toward the movable second plungers plunger (hereinafter defined as the y-direction) by the computer-determined second distance; and
c) signals the stepper motors controlling the two second computer-controlled movable push plates to rotate the mask frame by the defined amount.
REFERENCES:
patent: 5324012
Hoover Rex
Hoover Robert G.
Devnani, Esq. Papan
Evans F. L.
Shroff C. C.
Smith Zandra
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