1. Field of the Invention
The present invention relates to a disk-shaped object detecting system for detecting the presence or absence and position of a disk-shaped object such as a semiconductor wafer or magnetic disk without physical contact with the object.
2. Description of the Prior Art
A disk-shaped object detecting system has conventionally been used to detect positions of a number of semiconductor wafers stacked so as to be spaced from one another for the purpose of counting or ejecting them. The wafer 1 generally has an orientation flat 2 formed by cutting off a part of the circumferential edge thereof straight as shown in FIG. 10. The wafers 1 are loaded in a cassette carrier 3 when transported or processed. The carrier 3 has a number of grooves 4 formed on each of opposite inner side walls thereof so that the wafers 1 are placed over the opposite grooves 4, with the, orientation flats 2 being directed toward an opening 3A of the carrier 3.
The conventional detecting system shown in FIG. 10 comprises a reflection type sensor 5 including a light emitting section (not shown) emitting light onto the orientation flat 2 of the wafer 1 while the sensor 5 is moved across the wafers 1. The sensor 5 further includes a light receiving section 5A receiving light reflected on the orientation flat 2 as shown in FIG. 11A. The light receiving section 5A delivers a light signal S having a magnitude according to an amount of received light as shown in FIG. 11B. The magnitude of the light signal S varies with changes in the position of the wafer 1 relative to the light receiving section SA (see FIG. 11A). The detecting system detects the presence or absence and position of each wafer 1 on the basis of the variations in the magnitude of the light signal S. More specifically, when the light signal S exceeds a reference voltage Vrf, the detecting system determines that the light receiving section 5A confronts the orientation flat 2 of the wafer 1, detecting the position of the wafer 1. Japanese Utility Model Registration Publication Nos. 5-38761 (1993) and 6-70240 (1994) disclose conventional disk-shaped object detecting systems of the above-described type respectively. Japanese Examined Patent Publication No. 1-52897 (1989) and Japanese Unexamined Patent Publication No. 10-116878 (1998) disclose other conventional disk-shaped object detecting systems respectively. Further, U.S. Pat. No. 5,504,345 to Bartunek et al. discloses further another conventional disk-shaped object detecting system.
The light emitting sections of the conventional reflection type sensors 5 include those using light with a small beam width, for example, a laser, and those using light with a large beam width, for example, a light-emitting diode (LED). Since the light has a small beam width in the light emitting section using the laser, a distinct difference in an amount of received light is obtained between a case where the light receiving section 5 confronts the center of the orientation flat 2 with respect to the thickness thereof and a case where the light receiving section 5 is displaced from the center. This results in an advantage that the accuracy in the position detection of the wafer 1 with respect to the thickness thereof can be improved. However, in a case where the orientation flat is cut obliquely with respect to the thickness thereof or rugged, light is obliquely reflected on the orientation flat 2 when the light receiving section 5 confronts the center of the orientation flat 2 with respect to the thickness thereof. As a result, the light is not detected by the light receiving section 5 such that the detection becomes unstable. Further, the laser beam is dangerous to human bodies and the laser is costly.
On the other hand, light emitted from LED is safe for the human bodies and LED is less costly. Moreover, light emitted from LED has a larger beam width and has such an angle as to expand from the light source. As a result, the light receiving section 5 receives a predetermined amount of reflected light irrespective of the shape of the orientation flat 2 when confronting the center of the orientation flat 2 with respect to the thickness thereof, whereupon the detection is rendered stable. However, since the light emitted from LED has a larger beam width, only a small difference in an amount of detected light is obtained between a case where the light receiving section 5 confronts the center of the orientation flat 2 with respect to the thickness thereof and a case where the light receiving section 5 is displaced from the center. Consequently, the light signal S has a trapezoidal waveform with a flat apex as shown in FIG. 11B.
A reference voltage Vrf is set at a value slightly smaller than the one of a voltage at the apex in the conventional disk-shaped object detecting system employing LED as the light source as shown in FIG. 11B. The detecting system detects the wafer 1 when the light signal S exceeds the reference voltage Vrf. However, since the apex of the light signal S becomes flat as described above as shown by reference symbol H in FIG. 11B, the position of the light detecting section 5 covers a wide range when the light signal S exceeds the reference voltage Vrf as shown by reference symbol H. As a result, the position of the wafer cannot be detected accurately. Accordingly, when the wafers 1 loaded in the wafer cassette 3 at small pitches in consideration of the manufacturing efficiency are to be taken out, an adjacent wafer 1 is sometimes taken out erroneously, for example.
Further, the orientation flat 2 is sometimes directed obliquely relative to the light receiving section 5A while the wafer 1 is loaded in the cassette 3. Accordingly, light is reflected on the orientation flat 2 in an unintended direction such that the light receiving section SA fails to receive the light, whereby the wafer 1 cannot be detected in spite of the presence of the light.
Therefore, an object of the present invention is to provide a disk-shaped object detecting system which can perform an accurate detection of the presence or absence and position of the disk-shaped object.
The present invention provides a disk-shaped object detecting system detecting a disk-shaped object having a circumferential edge, the system comprising a light emitting element emitting light toward the circumferential edge of the object while being moved across the object with respect to a direction of thickness of the object. A light receiving element receives the light reflected on the circumferential edge of the object while being moved across the object with respect to the direction of thickness of the object together with the light emitting element, thereby delivering first and second signals each according to the received light. The light receiving element has two opposite ends. The first and second signals have a ratio depending upon at what position between the opposite ends the light receiving element receives the light reflected on the circumferential edge of the object. The first and second signals have a magnitude relation which is reversed when the light reflected on the circumferential edge of the object has been moved from one end side to the other end side of the light receiving element. The system further comprises an AND processing section to which the first and second signals delivered from the light receiving element are supplied, the AND processing section delivering either one of the signals having a smaller magnitude than the other as an output and a determining section determining whether the output of the AND processing section has exceeded a predetermined level.
In the disk-shaped object detecting system of the present invention, a receiving position where the light receiving element receives the light changes as it moves across the object. The magnitudes of the first and second signals delivered from the light receiving element vary according to the receiving position. The AND processing section delivers either one of the signals having a smaller magnitude than the other as an output. A magnitude relation between the first and second signals is reversed when the position where the light receiving element receives the light reflected on the circumferential edge of the object changes. This causes a sudden change in the output of the AND processing section. As a result, a signal having a triangular waveform with a pointed apex is supplied to the determining section. The determining section determines whether the apex of the triangular waveform has exceeded the predetermined level, thereby detecting the disk-shaped object.
According to the present invention, the triangular waveform has a pointed apex, and the detection of the disk-shaped object is based on whether the apex has exceeded the predetermined level. Consequently, since a range in which the triangular waveform is increased above the predetermined level is limited in the present invention as compared with the prior art in which the light signal delivered from the light receiving section has a trapezoidal waveform with a flat apex, a more accurate detection of the position of the object can be performed.
When two light receiving elements are disposed at both sides of the light emitting section respectively, at least one of the light receiving elements receives the reflected light even when the circumferential edge of the object is inclined relative to the light beam. Furthermore, since an optical axis of each of the light receiving elements is inclined toward the other, one of the light receiving elements receives a sufficient amount of light. Further, when one of the signals having a larger magnitude is supplied through the OR processing section to the determining section, an accurate detection can be performed without adverse effects by noise etc.
Some latitude is allowed in an angle of the light relative to the circumferential edge of the object when a condenser lens is provided for condensing the light emitted from the light emitting element to irradiate the condensed light onto the circumferential edge of the object. Moreover, a beam width of the light irradiated onto the circumferential edge of the object can be increased when the condenser lens has on the circumference of the object a larger irradiating area than the thickness of the object. Consequently, even when the object is cut obliquely along the thickness thereof, rounded or rugged, for example, the light receiving element can stably receive the light reflected on the object, thereby ensuring a stable detection.