JPH041494B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improvement in a mark position detection device that detects the center position of a mark by scanning the alignment mark with an electron beam.

[Technical background of the invention and its problems]

Recently, various types of electron beam exposure equipment have been used to form fine patterns on specimens such as semiconductor wafers and mask substrates, but these equipment require highly accurate positioning of the specimen to enable precise exposure. Is required. For this purpose, alignment marks are provided on the sample, cassette, sample stage, etc., and the mark position is detected based on reflected electrons, secondary electrons, or absorbed electrons obtained by scanning this mark with an electron beam. I try to align the sample.

By the way, as a mark position detection device used for detecting the mark position, one that utilizes the following methods (1) and (2) has recently been devised.

(1) From the peak of the mark signal obtained by detecting the reflected electrons from the mark and the reference value, the mark signal is binarized at a level such that the mark part obtains a binary signal of "1", and a binary signal is generated. A method of determining the center position of the mark by calculating the first moment from ``1'' and the scanning position of the electron beam.

(2) The mark signal is A/D converted and stored in a storage circuit, and the electron beam is scanned a plurality of times to average the A/D converted values of the mark signal and remove noise components. The slice level is determined from the P·P value (Poak to Peak) of the mark waveform thus obtained, and the first moment is calculated from the A/D conversion value of the mark waveform above the slice level and the scanning position of the electron beam. A method to find the center position of a mark.

However, the mark position detection device using this type of method has the following problems.
That is, in the first method, since only a part of the mark signal is used, the detection accuracy of the mark center position is poor. Furthermore, in the second method, A/
It takes a lot of time to average and add the D-converted mark waveforms, and a storage circuit is required for this purpose, which causes problems such as increased costs.

Therefore, the present inventors used an electron detector to detect reflected electrons, secondary electrons, or absorbed electrons obtained by scanning alignment marks with an electron beam, and set the differential waveform of this detection signal to a predetermined amplitude threshold X _O The place where the time series of the differential waveform of the threshold value X _O or more is continuous for a predetermined continuous time series threshold G _O or more is determined to be the mark end, and the amplitude value of the above detection signal at this time is determined as the mark end.
We propose a mark position detection device that stores X _A , calculates the first moment from the amplitude value of a detection signal larger than the amplitude value X _A and the scanning position of the electron beam, and detects this calculation result as the center position of the mark. did.

According to this proposal, the information of the entire mark signal can be used without waste, and the mark center position can be detected with high accuracy. Further, there is no need to perform average addition, no memory is required for this purpose, and furthermore, no time is required for average addition. Furthermore, since the calculation is performed simultaneously with the electron beam scanning, there are advantages such as the time required to detect the mark center position can be shortened.

However, in such a device, a complicated circuit is required to determine the calculation range of the first moment, resulting in a complicated device configuration. In addition, it is susceptible to the influence of noise, leading to problems such as a decrease in mark position detection accuracy due to noise.

[Purpose of the invention]

An object of the present invention is to provide a mark position detection device that can reduce the influence of noise and improve mark position detection accuracy.

[Summary of the invention]

The gist of the present invention is to cumulatively add A/D converted mark signals for a certain period of time to reduce the influence of noise,
The purpose is to calculate the first moment from the signal distribution of cumulatively added values.

That is, the present invention provides a mark position detection device for detecting the center position of an alignment mark, in which the mark is scanned with an electron beam and reflected electrons due to the scanning are detected.
Secondary electrons or absorbed electrons are detected to obtain a mark signal, this mark signal is A/D converted, and this A/D
The converted mark signals are cumulatively added for a certain period of time, and the calculation area is shifted one by one, and the first moment is calculated from a predetermined value or more of the signal distribution obtained by the cumulative addition and the scanning position of the electron beam. This is what I did.

[Effects of the Invention] According to the present invention, since A/D converted mark signals are cumulatively added over a certain period of time, the influence of noise can be significantly reduced. Therefore, detection errors due to noise can be eliminated and mark position detection accuracy can be improved. Furthermore, the center position of the mark can be detected by using various arithmetic units without requiring a complicated circuit to determine the calculation range of the first moment. For this reason,
This has effects such as simplifying the device configuration.

[Embodiments of the invention]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention. In the figure, reference numeral 1 denotes a positioning mark, and reflected electrons from the mark 1 obtained when the mark 1 is scanned by an electron beam 2 are detected by a reflected electron detector 3. Here, the detection signal (mark signal) 4 of the backscattered electron detector 3 has a large amplitude when the electron beam 2 crosses the mark 1 as shown in FIG. The mark signal 4 is supplied to an analog-to-digital converter (hereinafter abbreviated as A/D) 5 and converted into a digital signal. This conversion output is directly supplied to the subtracter 6, and is also supplied to a delay circuit (hereinafter abbreviated as DL).
7, the signal is delayed by a certain period of time d _O and is supplied to the subtracter 6. The subtracter 6 subtracts the output of the DL7 from the output of the A/D5, and this subtracted output is supplied to the cumulative adder 8. The cumulative adder 8 cumulatively adds the subtracted output, and this cumulative addition output is supplied to the first-order moment calculation circuit 9 and the first-order moment setter 10. The first-order moment calculation circuit 9 calculates, based on the signal distribution of the cumulative addition output equal to or higher than the value set in the first-order moment setter 10 and the signal X indicating the scanning position of the electron beam,
The first moment of the signal distribution is determined, and its output is supplied to the subtracter 11. The subtracter 11 subtracts the beam scanning distance d/2 corresponding to 1/2 of the delay period d _O of the DL 7 from the above output (first moment value), and this subtraction result is output as the center position information of the mark signal 4. It has become something that is done.

The first-order moment calculation circuit 9 is composed of a multiplier 21, a divider 22, and cumulative adders 23 and 24, as shown in FIG. 3, for example. Then, when the output signal of the cumulative adder 8 is a, (Σa·x/Σa) is calculated, and the weighted center position of the mark signal 4 is determined.

The operation of the apparatus thus constructed will be explained with reference to FIG. 4.

First, if the output of the A/D 5 is A[i] and the beam position after a time corresponding to the delay period d _O of the DL 7 has elapsed from the initial beam position is d, then the cumulative adder 8 outputs the above period d _O Addition output B[i] at
is calculated as follows. However, i=1,
2,...,N (i: electron beam position).

A [1] + A [2] + ... + A [d] → B [d] A[2]+A[3]+...+A[d+1] =B[d]-A[1]+A[d+1]→B[d+1] ~ A[n+1]+A[n+2]+...+A[d+n] =B[d+n-1]-A[n] +A [d+n] → B [d+n] However, n=0, 1, 2, . . . .

The subtracter 6 calculates A[i]-A[i-d] from i=d+1st. If i<d+1, the cumulative adder 8
A[i] is cumulatively added. In the case of i≧d+1, the output of the subtractor 6 becomes A[i]-A[i-d], and by adding this to the previously calculated cumulative addition value, the cumulative addition value B[ of the period d _O i] is calculated continuously with the beam scanning. Primary moment setter 1
If the value set to 0 is C, the first moment calculation circuit 9 calculates the first moment based on the electron beam position X and the signal distribution of the cumulative addition value B[i] under the condition of B[i]≧C. calculated. Here, the first cumulative addition value B[d] is performed after taking in the data of A[d]. Cumulative addition value B [d]
When is calculated, the position of the electron beam is at d.
The cumulative addition value B [d] is when the electron beam position is d/2
Therefore, the value obtained by subtracting the output M of the first-order moment calculation circuit 9 by d/2 by the subtracter 11 becomes the center position of the mark signal.

In this way, the center position of the mark signal is detected from the output of the subtracter 11. In addition, mark 1
To find the center position of mark 1, scan the mark 1 with the beam at least twice in the X direction to find the center line of mark 1 in the Y direction, and then scan the mark 1 at least twice in the Y direction to find the center line in the X direction. , the coordinates of the intersection of these center lines can be found.

In this way, according to this device, the mark signal digitized by the A/D 5 is cumulatively added by the subtracter 6, the DL 7, and the cumulative adder 8 for a certain period of time d, so that the noise component of the mark signal is reduced. can be done. Then, a predetermined value C of the signal distribution obtained from the cumulative addition value with few noise components.
Since the center position of the mark signal is determined by calculating the above-mentioned first order moment, it is possible to improve the mark position detection accuracy. Furthermore, the mark center position can be detected simply by using various arithmetic units without requiring a complicated circuit for determining the calculation range of the first moment, so that the apparatus configuration can be simplified.

Note that the present invention is not limited to the embodiments described above. For example, instead of the reflected electron detector, one that detects secondary electrons or absorbed electrons obtained when a mark is scanned with an electron beam may be used. Furthermore, it is also possible to simplify the peripheral circuit by dividing the cumulative addition value B[i] by the cumulative number of times (=d) corresponding to the delay period d _O to reduce the number of digits in calculating the first moment. It is. Furthermore, if the number of accumulations is set to the nth power of 2, the division may be performed by shifting the cumulative addition output by the nth power. Further, the delay period d _O of the delay circuit 7 may be determined as appropriate depending on conditions such as the width of the mark and the beam scanning speed. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention;
FIG. 2 is a schematic diagram showing the relationship between beam scanning and mark signal waveforms according to the above embodiment, FIG. 3 is a block diagram showing the main structure of the above embodiment, and FIG. 4 explains the operation of the above embodiment. FIG. 3 is a schematic diagram showing the relationship between a mark signal waveform and a signal distribution obtained by cumulative addition. 1... Mark, 2... Electron beam, 3... Backscattered electron detector, 4... Mark signal, 5... Analog-to-digital converter (A/D), 6... Subtractor, 7... Delay circuit (D/L), 8 ...cumulative adder, 9...first-order moment calculation circuit, 10...first-order moment setter, 11...subtractor.

Claims (1)

[Claims] 1. means for scanning a mark used for position detection with an electron beam and detecting reflected electrons, secondary electrons, or absorbed electrons resulting from the scanning to obtain a mark signal; and means for converting the mark signal from analog to digital;
means for accumulatively adding the analog-to-digital converted mark signals for a certain period of time and shifting the calculation area one by one; and based on the signal distribution obtained by the means exceeding a predetermined value and the scanning position of the electron beam. 1. A mark position detecting device comprising: means for determining a first moment.