Wafer-fetching sensing device for wafer storage apparatus

A wafer-fetching sensing device for wafer storage apparatus uses a planar detection means to replace a conventional linear detection means to detect if a wafer is properly positioned at a desirable access level before being moved out of an access opening of the wafer storage apparatus by a robot. Wafer tilting and damage incident that might otherwise happen thus may be avoided. Only a correctly positioned wafer will be fetched. A warning signal will be generated when the wafer is not properly positioned so that preventive action may be taken to avoid wafer damage. Production yield thus may be increased.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a wafer-fetching sensing device for wafer storage
 apparatus, and more particularly to an apparatus that upgrades the wafer
 positioning detection from a linear sensing level to a plane sensing level
 for increasing the production yield.
 2. Description of the Prior Art
 In semiconductor industry, transportation and storage of wafers in the IC
 fabrication process usually involve a wafer storage apparatus for storing
 a plural number of wafers and a robot for fetching the wafer. The wafers
 are arranged, at a predetermined order, into the wafer storage apparatus
 before an automation process can begin. In a fabrication site, the wafer
 storage apparatus usually locates beside a workstation or a group of
 workstations, and includes a wafer access opening for the robot to access
 and fetch the wafer thereinside.
 FIG. 1 illustrates a conventional wafer storage apparatus 1 which includes
 a housing 100 providing an interior accommodation space 11 for orderly
 storing a plurality of wafers 3 therein; for example, to store wafers 3
 upon a plurality of movable racks 13. By providing the movable racks 13,
 the wafers 3 then can be sequentially fetched out by the robot, through an
 access opening 50 of the housing 100, where the access opening 50 is
 arranged parallel to a normal storage state of wafer 3.
 In order to make sure that each wafer 3 in the wafer storage apparatus 1
 can be fetched out through the access opening 50 safely and right at an
 acceptable access level 10 formed parallel to the normal storage state of
 wafers 3, a first sensor unit 5 is installed at the access opening 50 for
 sensing whether or not the wafer 3 is posed by the normal storage state at
 the access level 10. Generally, the access opening 50 and the access level
 10 is coplanar.
 Referring now to FIG. 2, the first sensor unit 5 includes a first emitter
 17 located at the access opening 50 for sending a signal to a first
 receiver 15 along a first measuring line 101 on the access level 10. The
 first measuring line 101 may coincide with the center axis of the access
 opening 50. When the first sensor unit 5 detects the wafer 3 is properly
 positioned and leveled, the robot may proceed to start the fetching
 operation to move out the wafer 3 through the access opening 50.
 However, there are some occasions in which a wafer 3 may be improperly
 positioned and may deviate from its normal state upon the rack 13. Upon
 occurring such an occasion, a wafer-fetching problem that the first sensor
 unit 5 fails to detect the misplacement may arise. FIG. 3 illustrates one
 of such occasions. Due to any possible operation error (e.g. by putting
 the wafer 3a with two lateral sides anchored at different levels of racks
 13, the wafer 3a is tilted about the first measuring line 101 (also
 symbolized by the position of the first receiver 15) in the center of the
 access level 10, and forms an angle with the access level 10, but still
 keeps the center line of the tilted wafer 3a coinciding with the first
 measuring line 101. Under such a situation shown in FIG. 3, the first
 sensor unit 5 can still decide that the wafer 3a is posed at the normal
 storage state, and then proceeds to issue a normal signal for the robot to
 fetch out the wafer 3a. However, both lateral sides of the wafer 3a have
 deviated from the access level 10 and thus may result in the wafer 3a
 hitting the storage wall of the housing 10 or dropping to the floor, while
 being fetched out. It may cause damage (either minor or major) to the
 wafer 3a or even sequentially shut down whole production operation.
 SUMMARY OF THE INVENTION
 In view of aforesaid disadvantages, it is therefore an object of this
 invention to provide a wafer-fetching sensing device for wafer storage
 apparatus capable of detecting if a wafer is correctly positioned for
 fetch out from the wafer storage apparatus, so that wafer damage incident
 may be avoided.
 It is another object of this invention to provide a wafer-fetching sensing
 device for wafer storage apparatus that enhances the wafer level detection
 from conventional linear mode to a planar mode, so that the wafer level
 detection accuracy can be greatly improved, the wafer damage problem can
 be greatly reduced, and the total production yield can be increased.
 The apparatus according to this invention uses a planar detection means at
 the access level of the access opening of the wafer storage apparatus to
 make sure that a wafer is precisely positioned by a normal storage state
 before moving the robot to fetch the wafer. The planar detection means
 implements a second sensor unit besides the first sensor unit of a
 conventional one set forth above. There are two pairs of emitters and
 receivers for measuring two spaced (and preferably parallel) measuring
 lines on the access level of the wafer. Thus, the wafer-tilting problem
 may be easily detected and alarmed before any further damage occurs. Wafer
 damage and production interruption problem thus may be reduced, and
 production yield may be enhanced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 The wafer-fetching sensing device for wafer storage apparatus according to
 this invention uses a planar detection means to measure and detect if a
 wafer is positioned correctly for safely moving through an access opening
 to replace a conventional linear detection means. Referring to FIGS. 1 and
 4, the wafer storage apparatus 1 is constructed largely like a
 conventional one shown in FIG. 1 except that a planar detection means 200
 is located at the access opening 50 for measuring purpose on the access
 level 10.
 The plane detection means 200 includes a first sensor unit 5 which
 comprises a first emitter 17 and a first receiver 15 for measuring along a
 first measuring line 101, and a second sensor unit 7 which comprises a
 second emitter 27 and a second receiver 25 for measuring along a second
 measuring line 103, in which the second measuring line 103 is spaced from
 and preferably parallel with the first measuring line 101. Both the first
 and second measuring lines 101 and 103 are located on the access level 10.
 By providing the first and the second sensor units 5 and 7, only a wafer
 aligned with the access level 10 can induce the planar detection means 200
 to generate correct signals for a robot to fetch out the wafer from the
 wafer storage apparatus without alarming.
 According to the present invention, the sensor units 5 and 7 can be
 embodied as a combination of optical emitters and receivers as described
 above. However, they can also apply any precise measuring means with plane
 locating capability; such as a probe with its sensing tip on the access
 level 10.
 According to the present invention, the arrangement of the first and the
 second measuring lines 101 and 103 can be two parallel lines as shown in
 FIG. 4, yet they can also be embodied as two non-intersecting or
 intersecting lines on the access level 10 by which a plane can be
 determined.
 Referring now to FIG. 5, when a wafer 3a tilts about the access level 10,
 the first measuring line 101 may still decide that the wafer coincides
 with the access level 10, but the second sensor unit 7 can detect that the
 wafer 3a has already deviated from the access level 10. Thus, by combining
 the detection results from the first and the second sensor units 5 and 7,
 an abnormal signal can be generated for the robot to stop the fetching
 operation.
 In another aspect of this invention, a warning means (not shown in figures)
 may be included to alert the abnormal situation set forth above so that
 operator may take prompt action to fix the problem.
 It may thus be seen that the objects of the present invention set forth
 herein, as well as those made apparent from the foregoing description, are
 efficiently attained. While the preferred embodiments of the invention has
 been set forth for purpose of disclosure, modifications of the disclosed
 embodiments of the invention as well as other embodiments thereof may
 occur to those skilled in the art. For instance, the first measuring line
 may be located at other located of the access level rather than at the
 center line of the access opening. The sensor unit may consist of other
 types sensing device for detecting a plane rather than a pair of emitter
 and receiver. Accordingly, the appended claims are intended to cover all
 embodiments which do not depart from the spirit and scope of the
 invention.