JPH0147892B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for detecting alignment marks that allows alignment marks provided on a chip to be detected easily and quickly.

When transferring a pattern to each chip on a semiconductor wafer by electron beam exposure, in order to precisely align the pattern with the chip or the pattern formed on the chip in the previous step, the chip is coated with an alignment material in advance. Form a mark. FIG. 1 shows an example, in which 10 is a wafer and 11 is a large number of chips formed on the wafer. The MK for positioning is a concave part with a specific shape (for example, + shape, square, etc.), and there are usually 3 to 3 on one chip 11.
Four pieces are formed. Figure 2 shows an example of the detection system for this mark MK, and schematically shows the separate marks.
When the electron beam EB scans the vicinity of MK in the X and Y directions, the backscattered electrons RE are detected by the backscattered electron detector 1.
Detected at 2. Although only one detector 12 is shown in the drawing, in reality, about four detectors 12 are arranged around the electron beam at the front, back, left and right sides of the electron beam, and the sum of each output is determined. The output of the detector 12 is proportional to the amount of reflected electrons, and when it is amplified by an amplifier 13 and then differentiated by a differentiating circuit 14, positive and negative differential waveforms DIF corresponding to the front and rear edges of the mark MK are obtained. Reference numeral 15 denotes an A/D converter that converts the amplitude of each peak of the differential waveform DIF and the time from the scanning start point into digital quantities, and its output is stored in the memory 16. 17
detects the mark MK from the amplitude of each peak stored in the memory 16 and the timing of occurrence of each peak,
This is a CPU that calculates the center position, and an example of its processing method will be explained with reference to FIG.

Marked with electron beam EB as shown in Figure 3a.
When a certain range including the expected location of MK is scanned, the differential circuit 14 outputs a differential waveform corresponding to the area.
Since the DIF is output and stored in the memory 16, the CPU 17 first searches all the contents of the memory 16 and detects the maximum value. If the maximum value L ₁ is detected, this indicates the leading edge position of the mark MK. Next, the entire memory 16 is searched again to detect the minimum value. If the minimum value S ₁ is found, this is a mark
The trailing edge position of MK is shown. Maximum value L ₁ and minimum value
The interval _S1 is the mark width. Perform such operations as X and Y.
direction, and set the reference points for alignment to X and Y.
The center point of the direction mark width. If the center of the mark MK detected as a result of this analysis deviates from the planned position of the chip to be exposed with the electron beam, the stage movement system or beam deflection system is corrected according to the difference before exposure is performed. . This alignment is performed for each chip.

However, the conventional alignment mark detection method described above has the following drawbacks. (1) As shown in FIG. 3a, in order to detect the mark MK, the CPU 17 must search the memory 16 twice ( ), which increases the analysis time. (2) Since the mark MK is detected using the maximum value and minimum value as a set, if there are patterns P ₁ and P ₂ near the mark MK as shown in Figure 3b, their maximum value L ₂ and minimum value S ₀ , S ₂ also appear in the differential waveform DIF, so in such a case it is impossible to detect the mark MK. For this reason, a region of about 30 μm around the mark MK having a width of about 15 μm is usually set as a pattern formation prohibited region, and the degree of integration is reduced accordingly.

The present invention was made to solve these problems, and its feature is that alignment marks formed on the chip surface of a wafer are scanned with an electron beam, and the reflected electrons are used to scan the alignment marks and the chips. In the alignment mark detection method for detecting the accurate position of the wafer, one of the alignment marks is formed in advance at a suitable location on the wafer, and the width of the mark is actually measured by scanning it with an electron beam. When aligning, an electron beam is used to scan the expected position of the alignment mark to detect one end of the alignment mark, and then an electron beam is used to scan a position in the vicinity of the mark width apart from the one end to detect the position. The point is to detect the other end of the alignment mark and thus detect the alignment mark on the chip. This will be explained in detail below with reference to the illustrated embodiments.

In the present invention, a mark detection mark is placed at a suitable location on the wafer 10, for example, around the wafer where no chips are formed.
Set up an MD. This has the same shape and dimensions as the alignment mark MK provided on the chip on the wafer, and is made in the same process as the mark MK. When aligning, first detect the mark MD for mark detection in the manner described above, and read the mark width MW. For example, the mark width is determined to be 15 μm as mentioned above, but depending on the wafer and in each process performed on the wafer, the mark size may be set to a different size from the above-mentioned 15 μm, such as 3 to 5 μm. Often. Furthermore, it is not uncommon for the mark size to change from the intended value due to overexposure. Therefore, it is effective to actually measure a sample of the mark detection mark MD, that is, the chip alignment mark, and find out its exact size. Knowing the mark size in this way makes it extremely easy to detect the alignment marks on the chip. This will be explained next.

FIG. 4 is an explanatory diagram of one embodiment of the present invention, and the same parts as in FIG. 3 are given the same reference numerals. The detection system shown in FIG. 2 is also used in this example, but the processing of the CPU 17 is different from that shown in FIG. 3. In other words, as shown _in FIG _.
Do this only on the isolated part. In this way, the minimum value S ₁ can be easily and quickly detected. Also
The analysis time of the CPU 17 is also shortened. Moreover, if the search is performed only at positions separated by MW from the maximum value, the minimum values S ₀ and S ₂ will not be detected as shown in Figure 4b, so it is impossible to mistakenly recognize the patterns P ₁ and P ₂ as marks MK. do not have. Therefore, since a pattern can be formed even close to the mark MK, the degree of integration can be improved without reducing the reliability of alignment.

As described above, according to the present invention, there is an advantage that high integration is possible without reducing the reliability of alignment in electron beam exposure, and analysis time by the CPU can be shortened.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the alignment mark, Fig. 2 is a schematic configuration diagram showing the detection system of the mark, Fig. 3 is an explanatory diagram of the conventional alignment mark detection method, and Fig. 4 is an explanatory diagram of the alignment mark detection system.
The figure is an explanatory diagram of an embodiment of the present invention. In the figure, 10 is the wafer, MK is the alignment mark, EB is the electron beam, and MW is the mark size.

Claims (1)

[Claims] 1. In an alignment mark detection method in which an alignment mark formed on the surface of a chip of a wafer is scanned with an electron beam and the accurate position of the mark and thus of the chip is detected from the reflected electrons, One of the alignment marks is formed, and the width of the mark is measured by scanning it with an electron beam. During subsequent alignments, the position where the alignment mark is expected to be is scanned with the electron beam and the alignment is performed. One end of the mark is detected, and then the other end of the alignment mark is detected by scanning an area in the vicinity of the mark width away from the one end with an electron beam, thereby detecting the alignment mark of the chip. A method for detecting alignment marks.