JPH047091B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for detecting a reference mark position at high speed and with high precision in electron beam direct writing.

In direct writing using an electron beam, it is necessary to accurately overlap and draw patterns at predetermined positions on a sample in each step of manufacturing a microcircuit. Therefore, a step is formed in advance on the sample surface as a positioning mark 2 as shown in FIG. It's summery. In conventional mark position detection, reflected electrons from the mark 2 generated by electron beam scanning are detected by a plurality of reflected electron detectors. The output waveform when the outputs of all the detectors are added is shown in FIG. 1b. An appropriate threshold level L _t is set for this detection signal to generate a binarized signal. Simultaneously with the start of beam scanning, the beam scanning clock pulses are counted by a counter, and the count values C _1l and C _1r at the time when the above-mentioned binary signal is generated are read. C _1l and C _1r are mark edge position data, and (C _1l + C _1r )/2 is the mark position.
However, shot noise of the beam itself, noise due to backscattering of the beam, noise of the detector, etc. are superimposed on the detection signal, and these random noises reduce mark position detection accuracy. Therefore, the mark position is determined by performing beam scanning a plurality of times and averaging the mark position data obtained for each scan to improve mark detection accuracy. It is known that if scanning is performed m times, the detection accuracy will be improved by a factor of √.

In actual drawing, it takes 20~20 seconds to detect one mark.
The beam is scanned about 40 times. Since such electron beam scanning is performed multiple times, there is a problem in that high-speed mark detection is not possible. In addition, erroneous signals are generated due to impulsive external noise.
Incorrect mark position detection may occur.
In conventional mark position detection, in order to prevent the influence of this erroneous signal, a value corresponding to the mark width Δ = |C _1r −C _1l
If Δ is not within the specified tolerance, the detected mark/edge position data C _1l , C _1r
was rejected. However, this method has the problem that even if the section of the detection signal corresponding to the mark edge is normal, the mark edge data is discarded due to an erroneous signal, making it impossible to detect the mark position.

In order to eliminate these drawbacks, the present invention detects the positions and peak levels of a plurality of maximum and minimum peaks of a detection signal, and sets an interval for capturing mark edge data based on the peak positions. and the offset within the detected peak value.
Mark positions are detected by setting multiple thresholds between the maximum peak closest to the level value and the minimum peak value level, thereby eliminating false signals and detecting multiple marks and positions in one electron beam scan. - Provides a mark position detection method and apparatus in electron beam exposure that reduces the number of scans and achieves high-speed and highly accurate mark position detection by obtaining data.

FIG. 2 is an embodiment of the present invention. This example will be explained below. control circuit 6
generates a scanning start signal, and the electron beam 1 scans over the mark 2 according to a prespecified scanning starting point and scanning distance. Simultaneously with the start of scanning, counter 7 begins counting scan clock pulses.
The backscattered electron detector 3 detects backscattered electrons from the sample surface on which the mark 2 is formed, and the detection signal is
The AGC circuit 4 amplifies the signal to a certain level. The peak signal detection circuit 5 is shown in FIG. 3a. The counter values P ₁ , P ₂ , P ₃ , P ₄ of the peak positions of the detection signals, and the respective peak values and offset level values L _p are detected, and these pieces of information are sent to the control circuit 6. The control circuit 6 temporarily stores this peak position information in the register 8, and selects the two peak values Vh, Vh, and _Vh closest to the offset level value _Lp .
V _l (maximum peak value V _h and minimum peak value V _l ) as DA
The voltage is input to circuits 10 and 10a and converted into an analog voltage. Based on this voltage, appropriate threshold voltages V ₁ , V ₂ ...V _o (V _h > V ₁ ,
V ₂ , ...V _o > V _l ) are given. In addition, the mark edge data acquisition section setting circuit 9
Based on the information in the register 8 and the information in _the counter 7, a mark edge data acquisition section _setting signal E _g as _shown in _FIG . Make sure to import mark/edge data only for _w . As mentioned above, in the first electron beam scan, the mark, edge, and data acquisition section T _w is set, and for each comparator 11,
A threshold voltage is set.

Thereafter, the scanning of the electron beam is repeated a predetermined number of times. The comparator 11 compares the detection signal generated by each scan with each of the threshold voltages, and only when they match, causes the monostable multivibrator 12 to generate a binary signal pulse.
The AND gate 13 performs an AND operation on this pulse signal and the mark/edge data acquisition section setting signal E _g to exclude pulse signals generated outside the above section T _w . The output signal of the AND gate causes the latch circuit 14 to latch the count value of the counter 7, and this data is sent to the arithmetic circuit 15.
The arithmetic circuit 15 has n threshold levels V ₁ , V ₂ ,
...The data C _1l , C _2l , ... detected for each V _o
Based on C _ol , C _1r , C _2r , ...C _or, M ₁ = (C _1l + C _2l +
...C _ol + C _1r + C _2r + ... C _or )/2n is performed to find the mark position. This calculation is performed for each scan, and M=(M ₁ +M ₂ +...M _n )/m (where m is the number of scans) is determined as the final mark position.

As explained above, by setting the mark/edge data acquisition section, it is possible to eliminate the negative influence from erroneous signals caused by impulse-like external noise _Np, etc., as shown by the dotted lines in Figures 3a and 4. In addition, it becomes possible to set a large number of threshold values V ₁ , V ₂ , . . . V _o in a section where the detection signal waveform changes sharply, which has the advantage of improving mark detection accuracy. For example, data acquisition interval
If T _w is not set, it is difficult to accurately detect the mark position due to noise even if the threshold value V _i is set close to the offset level L _p . If the number of threshold values to be set increases by k times, even if the number of electron beam scans on the mark is reduced to 1/k, the detection accuracy will not change because the mark position data will be obtained for the number of times. Therefore, the threshold
If V _i can be set to a large value, the number of scans can be reduced in proportion to the increase in the number of threshold values, which has the advantage of enabling high-speed detection of mark positions. Furthermore, due to the characteristics of the backscattered electron detector, the shape of the mark, etc., the actual detection signal will not have symmetrical sections corresponding to the left and right mark edges, as shown in Figure 4.
The peak values of P ₁ and P ₄ and P ₂ and P ₃ rarely coincide. If multiple thresholds are set between the maximum peak value and the minimum peak value in this state, the threshold signal and the detection signal may not cross normally, making it impossible to detect the mark position normally, but the offset level This problem can be advantageously solved by setting a plurality of threshold values V _i between the two peak values (maximum peak and minimum peak) closest to the value L _p .

In the above example, a step mark was used, but marks made of materials with different electron beam reflectances (for example, S i and S _i shown in Figure 5a)
It can also be applied when using a mark consisting of M _p ). The detection signal obtained by scanning this mark with an electron beam is as shown in FIG. 5b. in this case,
Detect the offset level L _p and signal level L _s ,
A plurality of threshold values V _i may be set between these levels.

[Brief explanation of drawings]

Figure 1a is a diagram showing a reference mark, Figure 1b is a diagram illustrating a detection signal obtained by scanning the reference mark with an electron beam and an example of a conventional mark position detection method, and Figure 2 is a diagram illustrating the present invention. A block diagram showing an embodiment of the invention, FIGS. 3a and 3b are waveform diagrams for explaining an embodiment of the mark position detection method according to the invention, and FIG. 4 is a waveform diagram for explaining the effect of the invention. Time Chart FIG. 5 is a time chart for explaining another embodiment of the present invention. 1... Electron beam, 2... Reference mark, 3...
Backscattered electron detector, 4...AGC circuit, 5...Peak signal detection circuit, 6...Control circuit, 7...
... Counter, 8 ... Register, 9 ... Data acquisition section setting circuit, 10, 10a ... DA conversion circuit, 11 ... Comparator, 12 ... Monostable multivibrator, 13 ... AND circuit, 14 ... Latch Circuit, 15... Arithmetic circuit.

Claims (1)

[Claims] 1. An electron beam mark formed on a substrate that serves as a reference for positional information is scanned with an electron beam, and by the scanning, changes in the intensity of reflected electrons from the mark and the substrate are detected as a detection signal of the mark. In a mark position detection method for electron beam exposure, the mark detection signal includes multiple peaks indicating the maximum signal intensity, multiple peaks indicating the minimum signal intensity, and signals from the substrate other than the mark portion. Information about the peak signal level values of the maximum peak and the minimum peak of the mark detection signal, the position information of the peak, and the offset level are obtained during the first scanning of the electron beam as intensity. It specifies and stores a partial section of the detected signal based on the value information, and also selects the maximum peak and minimum peak whose peak value level is closest to the offset level value among the plurality of maximum peaks and minimum peaks. Select one of each, set a plurality of threshold levels between the selected maximum peak value level and the selected minimum peak value level, and set the plurality of threshold levels and the mark detection signal waveform. A pulse signal corresponding to the intersection point is generated only for the intersection point within the specified section, and when scanning the electron beam from the second time onwards, the mark is detected in the memorized specified section. The intersections between the waveform and the plurality of threshold levels are repeatedly detected, the pulse signal corresponding to the intersection point is repeatedly generated, and the mark is created based on the pulse signal during the plurality of electron beam scans. 1. A mark position detection method in electron beam exposure, characterized by detecting the position of a mark. 2. Means for scanning a reference mark formed on a substrate with an electron beam and detecting the reflected electron beam, circuit means for setting an appropriate threshold level for the detection signal, and the threshold signal and the detection signal waveform. In an apparatus for detecting a mark position in electron beam exposure, the apparatus includes circuit means for generating a pulse signal corresponding to an intersection point of the electron beam, and a circuit means for determining a scanning amount of the electron beam using the pulse signal and obtaining mark position information from the scanning amount. , having means for specifying and storing a partial section of the detection signal based on information on peak signal level values of maximum peaks and minimum peaks of the detection signal waveform, position information of the peaks, and information on offset level values; means for selecting one local maximum peak and one local minimum peak whose peak value level is closest to the offset level value from among the plurality of local maximum peaks and local minimum peaks; means for setting a plurality of threshold levels between the minimum peak value levels; and a means for setting a plurality of threshold levels between the plurality of threshold levels and the mark detection signal waveform, and only at the intersections that are within the designated section. 1. A mark position detection apparatus for electron beam exposure, comprising means for generating a pulse signal corresponding to the pulse signal, and means for detecting the position of the mark based on the pulse signal.