JPS61223603A - Mark detecting machine - Google Patents

Abstract

PURPOSE:To obtain high detecting accuracy by simple circuit constitution by measuring a time difference between output of a zero-cross comparator photodetecting the reflected light from a mark and output of a mark center detecting means. CONSTITUTION:The zero-cross comparator is formed of a position sensor 4, electric current-voltage conversion circuits 11 and 12 and a comparator 17. When a control signal is given to an optical scanner and an optical beam is scanned, a reference signal A indicating the relation between the beam position and the output voltage is outputted to the output of the comparator 17. Further, a mark signal B indicating an amount of error from the mark reference position to the actual mark position is outputted from an electric current-voltage conversion circuit 18 inputting the output of an optical detector 5. The signal B is inputted to a peak comparator 19 and transformed into the shape of a mark position signal C and inputted to a time difference signal generating circuit 20 with the signal A. The circuit 20 outputs a pulse D of a time difference between the signals A and C and outputs a flag 24 corresponding to the polarity of a mark position error. The pulse D is inputted to a counter 23 to count the position error.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Description of the field to which the invention pertains The present invention relates to a mark detector for alignment of wafer masks, etc. in semiconductor manufacturing equipment.

(2) Description of the Prior Art FIG. 3 shows an outline of the mark detection device according to the present invention, which is an optical scanner using a galvanometer mirror.

3 is a beam splitter and 24 is a position sensor.

5 is a light detector, 6 is a wafer, and 7 is a diffraction grating mark processed on the wafer.

In this configuration, when the optical scanner 2 is rotated in the direction of the arrow A, the light beam moves over the wafer 6 in the direction of the arrow a in the figure. When the light beam moves and hits the mark 7, diffracted light (L) as shown in the figure is generated and hits the light detector 5, from which a mark signal is output. Since there is a certain relationship between the control input voltage of the optical scanner 2 and the rotation angle, conventionally, mark position information is obtained from the control input voltage of the optical scanner 2 at the time when this mark signal is obtained. By periodically applying a reference control input voltage to the optical scanner 2 and reading the position of the light beam at that time with a position sensor 4, position detection errors due to drift of the optical scanner 2 are corrected. When such a drift correction method is used, the positive control sensor 4 must detect the absolute position with high precision over the entire range in which the light beam drifts. A complex circuit as shown in FIG. 4 must be used. In Fig. 4, 4 is a two-dimensional position sensor, 11.12 is a current-voltage conversion circuit, 13 is a subtracter,
14 is an adder, and 15 is a divider. Here, a high-precision subtractor/2 divider has the disadvantage that it requires matching resistors and a large number of trimmers, the elements are expensive, and it takes time to adjust.

Note that even in a position sensor circuit with such a configuration, if the ambient temperature changes and the dark current of the position sensor 4 increases, only the denominator input (D) of the 9-9 divider 15 will change. The gain changes, resulting in a position detection error and poor correction.

Further, there is an inconvenience in that the amount of drift of the optical scanner periodically measured by the position sensor must be stored in a memory and read out and corrected each time a mark is detected.

(3) Object of the Invention The object of the present invention is to solve these drawbacks and provide a mark detector with high detection accuracy using a simple circuit configuration.

(4) Description of structure and operation of the invention The following description assumes that the workpiece is a wafer and the mark is a diffraction grating mark. In semiconductor manufacturing equipment, a wafer must be placed on a sample stage so that the mark processed on the wafer is at a fixed position in the equipment's coordinate system (hereinafter referred to as the mark reference position). When the wafer is placed, the mark is located at a location slightly deviated from the mark reference position due to dimensional errors or shakes during transportation. Therefore, the mark detector detects the coordinate position where the mark actually exists, using the mark reference position as the origin.

Before explaining the use of the position sensor as a position detection zero cross comparator, the position sensor used in the present invention will be briefly explained. The element used here is a non-divided type semiconductor device detector, such as 51544 or 51545 manufactured by Hamamatsu Photonics or PI manufactured by UDT.
Such as N-LSC15D. In these detectors, when the photocurrent generated at the point hit by the light flows to the two electrodes, it branches in inverse proportion to the distance from the point hit by the light to both electrodes, so the difference in the current flowing from the two electrodes The position where the light hit can be determined from this difference value. However, as the amount of light hitting the detector changes, the difference in photocurrent with respect to position also changes, so as shown in FIG. Unless the circuit is configured to divide by

Furthermore, even in such a circuit configuration, if the dark current changes, errors will occur as described above.

In the present invention, we focus on the fact that light hits the center of both electrodes of such a position sensor (hereinafter referred to as the 0 point), and we use a circuit as shown in Figure 5 to calculate the difference between the currents flowing from the two electrodes. A position sensor is used as a zero-cross comparator that outputs only the polarity of the In FIG. 5, 4 degrees 11 and 12 are the same as in FIG. 4, and 17 is a voltage comparator. In addition, Figures 4 and 5 are shown in Figures 6a and 6b.
The relationship between the position of the light beam and the output voltage 16.18 in each position sensor circuit shown in the figure is shown. As can be seen from Fig. 6(a), in the circuit of Fig. 4, there is a linear relationship between the position hit by the light beam and the output voltage 16, and the position hit by the light beam can be directly read from the value of the output voltage. When the dark current of the position sensor 4 increases, the input/output relationship changes as shown by the dotted line, resulting in a detected position error.

On the other hand, if the circuit shown in Fig. 5 is used (in the case of Fig. 6 (b))
Although it is not possible to determine the position where the light beam hits from the output voltage, the moment when the light beam moves on the position sensor and passes the 0 point can be determined by the change in the output voltage.The position of this 0 point can be determined by the amount of incident light or Dark electric slag! It is possible to have a car that feeds accurately without being sidecared. Therefore, if the position sensor is installed so that the light beam hits the zero point of the position sensor when the light beam is at the mark reference position, the light beam will stay at the mark reference position no matter how the optical scanner drifts when scanning the light beam. The moment when the object passes through the position can always be accurately detected by detecting the zero point of the position sensor.

According to the present invention, the distance from the mark reference position to the actual mark position is determined by scanning the light beam at a constant speed using the zero cross comparator and measuring the time difference between the O point detection time of the zero cross comparator and the mark signal detection time. An embodiment of the mark detector is shown in FIG. In FIG. 1, blocks 4, 11, and 12 are the same as in FIG. 5, and 5 is the photodetector shown in FIG.
20 is a time difference signal generation circuit, 21 is a reference clock generation circuit, 22 is an AND gate, and 23 is a counter.

FIG. 2 is a waveform diagram showing signal waveforms at various locations in the block diagram shown in FIG. 1. In Figure 2 (A) to (F)
All signal waveforms are shown with the horizontal axis representing the beam position (~rotation axis of the optical scanner).

When a control signal is given to the optical scanner to scan the beam, a signal like that shown in FIG. 2 (A) appears at the output of the voltage comparator 17, as shown in FIG. 6 (b). From the output of 18, a signal as shown in Fig. 2 (B) is obtained depending on the position of the mark, and Er
The length marked r is the amount of error from the mark reference position to the actual mark position. The mark signal (B) enters the peak comparator 19, is transformed into a mark position signal (FIG. 2(C)), and is then input to the time difference signal generation circuit 20 together with the reference position signal (A).

The time difference signal generation circuit 20 receives two input signals (A) and (C).
At the same time, it outputs a flag (24) indicating which signal was input first. This corresponds to the polarity of the mark position error.

The reference clock output from the reference clock generation circuit 21 is for converting the time difference signal (D) into distance information.
The pulse train (F) is output by the D gate 22 only during the time difference signal (D), and enters the counter 23, where it is counted as a position error.

When this circuit (Fig. 1) is used, at the end of one scan, the absolute value of the error amount from the reference position to the actual mark position is sent to the counter 23, and the polarity of the error is sent to the flag 24 of the time difference signal generation circuit 20. ing.

Although FIG. 3 is an example of one-dimensional mark detection, a two-dimensional mark detector can be realized by adding another set of optical scanners and replacing the position sensor with a two-dimensional one. In this case, the linearity of the position sensor itself becomes poor compared to one-dimensional one, and twice as many peripheral circuits are required, so that the merits of implementing the present invention are further expanded.

Further, in the present invention, the optical scanner must be driven at a constant speed, but since optical scanners are generally driven, this does not present any disadvantage.

(5) Description of Effects As explained above, according to the present invention, it is possible to eliminate all of the main error causes of the conventional technology: non-linear errors of the position sensor, errors due to changes in dark current, and errors of the divider. This not only greatly improves the position detection accuracy but also simplifies the circuit.

Furthermore, since the position of the mark detected by the present invention is not affected by the drift of the optical scanner, there is no need to perform correction calculations after measurement, which is also very advantageous.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention. 2 is a diagram of signal waveforms at various points in the block diagram shown in FIG. 1, FIG. 3 is a schematic diagram of a mark position detection device according to the present invention, FIG. 4 is a conventional mark position detection circuit, and FIG. It is a characteristic diagram which shows the relationship between a position sensor and a comparator-person position, and an output voltage. (Explanation of symbols of main parts) l...Laser 2...Optical scanner 4...Position sensor 5...Optical detector 7...Diffraction grating mark 11.12...Current-voltage conversion circuit 17 ... Comparator 21 ... Reference clock generation circuit 23 ... Counter

Claims (1)

[Claims] It consists of a scanner that sweeps a marked workpiece at a constant speed with a light beam, a position sensor, and a comparator, a zero-cross comparator arranged to receive reflected light from the mark, a mark center detection means, and a zero-cross A mark detector comprising: a timer for measuring the time difference between the output of the comparator and the output of the mark center detection means.