JPS582450B2 - Pattern defect inspection equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pattern defect inspection apparatus, and more particularly to a pattern defect inspection apparatus for photomasks used in the manufacture of semiconductor integrated circuits.

One of the current processes for manufacturing integrated circuits is the process of photo-etching silicon wafers.

That is, a mask is placed on the silicon wafer in close contact with the silicon wafer, and visible light or ultraviolet rays are irradiated from above to perform photoetching.

If there is a defect in the mask on which this pattern is printed, the yield of integrated circuit manufacturing will be significantly adversely affected.

This mask is formed by depositing a metal film such as chromium on a glass plate that has been polished to a perfectly flat surface, and then baking a predetermined pattern on it. Defects may occur if

The applicant has already proposed a device that automatically detects such pinholes with high precision.

However, even if there are no pinholes in the metal vapor deposited film, there may be defects in the mask after the pattern is baked.

The present invention relates to a method for inspecting a mask for defects after a pattern has been printed.

FIG. 1 shows a mask 1 for manufacturing integrated circuits as described above.

This mask 1 has a large number of patterns 3 defined by a large number of scribe lines 2 extending in the vertical and horizontal directions.

The patterns of these patterns 3 are the same.

FIG. 2 shows a microscopic image of a portion of the pattern of the mask 1 on which the pattern has been printed.

This mask part has no defects and is perfect.

The pattern of the mask 1 is composed of transparent parts 4 and opaque parts 5.

FIG. 3 shows a microscopic image of the same pattern portion having defects.

Portions A and B show the state in which the deposited film remains.

In part A, lands that should originally be separated are connected.

On the other hand, since the remaining deposited film in portion B is in space, it may not cause a defect in the integrated circuit depending on the case.

In part C, part of the land is missing.

However, since the land has not reached the point where it breaks, it may not cause a defect in the integrated circuit.

In part D, the land is completely cut off,
This may cause defects.

Conventionally, there are the following methods for inspecting defects in mask patterns as described above.

(1) Find defects by visual inspection using a microscope.

Generally, a pattern is drawn with a right-angled outline, but since defects often have an irregular shape as shown in FIG. 3, defects can be detected.

However, this method is time-consuming and error-prone, making it extremely unsuitable for inspecting the large number of patterns on a mask.

(2) As shown in FIG. 4, in addition to the mask 6 to be inspected, a sample mask 7 having a complete pattern is prepared, and the images of these two masks 6 and 7 are superimposed and observed using a microscope 8.

At this time, the image of the mask 6 to be inspected is a red image, and the image of the sample mask 7 is a green image having a complementary color relationship therewith.

For this purpose, a red light source 9 is provided, and the mask 6 to be inspected is irradiated with red light rays emitted from the red light source 9.
2 and the eyepiece 13 to enter the observing eye 14 .

On the other hand, the sample mask 7 is illuminated by an edge-colored light source 15, and the light transmitted through this mask is made to enter the observation eye 14 via the objective lens 16, mirror 17, half mirror 18, and eyepiece 13.

When observing, for example, a sample mask 7 without defects as shown in FIG. 1 and a defective mask 6 to be inspected as shown in FIG. It looks green because only green light comes from it.

On the other hand, parts C and D only receive red light from the mask 6 to be inspected, so they appear red.

In addition, the transparent part 4 other than these parts A, B, C, and D receives both the green light from the sample mask 7 and the red light from the mask to be inspected 6, so it appears white, and the opaque part 5 naturally Looks black.

That is, defective parts appear green or red, and non-defective parts appear white or black, making it relatively easy to find defects.

As shown in FIG. 1, a large number of identical patterns are formed on a mask for manufacturing integrated circuits, and in order to sequentially inspect these patterns, the mask to be inspected 6 and the sample mask 7 are placed on the same moving table 19. It is necessary to carry out the inspection one by one while moving the moving table 19 slightly.

In this way, when using the sample mask 7 and observing the image of the sample mask 7 by superimposing it on the image of the mask to be inspected 6, it is necessary to superimpose the two images extremely precisely, and there is no error in this superposition. If this happens, it will be impossible to perform an accurate test.

In particular, when two masks 6 and 7 are placed on one moving table 19, it is necessary to precisely align these masks 6 and 7 in the X and Y movement directions of the moving table 19, and if there is an error in this alignment, As the movable stage 19 moves, an error occurs in the superposition of the two mask images.

Furthermore, the backlash of the movable table 19 also has a large effect on the overlay error.

Furthermore, since this method uses visual inspection, fatigue of the observer is unavoidable, human error is introduced, and the inspection requires a long time.

(3) An electric signal corresponding to the pattern of the defect-free sample mask is stored in advance in a recording medium such as a storage element or a magnetic tape using a computer or the like.

Next, using a microscope television camera or the like, the mask to be inspected is imaged to extract a video signal, and this video signal is compared with a pre-stored information signal to find defects.

Although this method has the advantage of being able to automatically discover defects without using the human eye, it requires an extremely large and complicated device, and is extremely expensive.

In order to eliminate such defects, the images of the same part of the two patterns are superimposed on the light-receiving surface of one image pickup tube, and defects are detected by detecting the amplitude of the image signal output from the image pickup tube. The applicant considered a method to do this, but in this method, the defective part appears as a halftone in the video signal, and this halftone is detected using an amplitude limiter, etc., but the amplitude of the video signal fluctuates greatly. Therefore, the accuracy of this detection is not very high.

In order to avoid this drawback, two image pickup tubes are used, images of the same portion of the two patterns are focused on the light receiving surface of each image pickup tube, and the output signals of the two image pickup tubes are adjusted by changing the phase of one signal. We devised a method to selectively extract only the signal corresponding to the defect by inverting and then mixing.

With this method, the accuracy of defect detection processing in the electric circuit portion can be made much higher than when a single image pickup tube is used.

However, it is extremely difficult to completely match the operations of the two image pickup tubes.

Furthermore, when an image pickup tube is used as described above, it is necessary to intermittently move the movable stage on which the mask is placed due to the afterimage effect, which makes the drive mechanism for the movable stage extremely complicated.

Furthermore, since the operating speed of the image pickup tube is slow, it takes about 70 to 100 mS to inspect one field of view, and it takes a relatively long time to inspect a large number of patterns.

The inventor further devised a pattern defect inspection apparatus as shown in FIG. 5 as an apparatus capable of eliminating the above-mentioned drawbacks and inspecting pattern defects accurately and at high speed with a simple configuration.

In FIG. 5, an inspection mask 20 and a complete sample mask 21 without defects are placed on one moving table 22. In FIG.

Furthermore, a flying spot cathode ray tube 23 is provided, and a scanning raster image formed by the flying spot is formed on the mask 20 to be inspected via a lens 24, a half mirror 25, a mirror 26, and a lens 27. 28, a mirror 29, and a lens 30 to form an image on the sample mask 21.

The light transmitted through the mask 21 to be inspected is received by the phototube 31,
The light transmitted through the sample mask 21 is received by a phototube 32.

In this case, the portion onto which the rask image of the pattern of the mask to be inspected 20 is projected is made to coincide with the portion onto which the rask image of the pattern of the sample mask 21 is projected.

Therefore, if there is no defect in the mask 20 to be inspected, the video signals supplied from both phototubes 31 and 32 will be completely consistent, but if there is a defect in the mask 20 to be inspected, the two signals will be different. Become.

Therefore, if the phase of the output signal of one phototube is reversed and mixed with the output signal of the other phototube, the signal level will be zero at the matching part of the pattern, but the corresponding electrical signal will be at the defective part. remains, and this signal can be used to detect the size, type, etc. of the defect.

In such a pattern defect inspection apparatus, the mask to be inspected 20
and the sample mask 21 are placed on one moving table 22 and moved in the X and Y directions, so it is necessary to precisely align both masks in the moving direction. If there is any play, move the moving platform 2.
2, the scanned portion of the pattern of the mask to be inspected 20 and the scanned portion of the pattern of the sample mask 21 become different, making it impossible to accurately detect defects.

Another drawback is that a complete sample mask without any defects must be prepared.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate all of the above-mentioned drawbacks and to provide a pattern defect inspection apparatus that can detect pattern defects with high precision without using a sample mask.

The present invention is an apparatus that generates a scanning light spot in order to inspect for defects in patterns of a sample in which a large number of identical patterns are formed adjacent to each other at a constant pitch, such as a photomask used in the manufacture of semiconductor integrated circuits. an optical system that simultaneously irradiates the scanning light spot onto the same portion of two identical patterns that are close to each other in the same sample; and two light receivers that separately receive the transmitted light or reflected light of these two patterns. a circuit that inverts the phase of an output signal from one of these two light receiving devices, and a circuit that mixes one output signal with the phase inverted and an output signal from the other light receiving device whose phase is not inverted. It is characterized by having the following.

The present invention will be described in detail below with reference to the drawings.

FIG. 6 shows another example of the pattern defect inspection apparatus according to the present invention.

In the elements of this example, the same elements as those shown in FIG. 5 are denoted by the same reference numerals.

In this example, only the mask 20 to be inspected is placed on the movable table 22.

A raster image of the flying spot cathode ray tube 23 is focused on a certain pattern 20A of the mask 20 to be inspected via lenses 24 and 27, and the lenses 24 and 3
0 to the same portion of the pattern 20B located near the pattern 20A.

At this time, adjustment handles 33 and 34 are adjusted so that the same portions of both patterns 20A and 20B can be scanned.
is provided so that the distance between the optical axes of both lenses 27 and 30 can be adjusted.

According to this example, since the patterns 20A and 20B observed at the same time are very close to each other, the moving stage 22
It is not affected by the looseness of the

An example of a method for processing the output signals of the phototubes 31 and 32 shown in FIG. 6 is shown in FIG. 7, and its operation will be explained with reference to the signal waveforms in FIG. 8.

FIG. 8a shows the output signal waveform of one phototube 31 receiving light from pattern 20A, and FIG. 8b shows the output signal waveform of pattern 20A.
The output signal waveform of the other phototube 32 receiving light from 0B is shown.

Now, it is assumed that one pattern 20A has no defects and is perfect, but the other pattern 20B has a defect.

Pulse B in the signal shown in Figure 8b is part B of the signal shown in Figure 3.
Pulse D shows a defect like part D.

The signal amplified by the output signal amplifier 35 of one phototube 31 (FIG. 8C) and the signal amplified by the amplifier 36 and phase inverted by the phase inverter 37 of the output signal of the other phototube 32 (FIG. 8D) Both are supplied to the mixer 38.

The output of the mixer 38 is as shown in FIG. 8e.

That is, the signal level is O in the matched portions of both patterns 20A and 20B, and the pulse B' and pulse appear only in the defective portion.

Furthermore, since these defects appear as pulses of opposite polarity, the pulses are passed through the slicer 39 and then supplied to the defect detection processing circuit 40 for processing.

Furthermore, the output signal of the mixer 38 is also supplied to the monitor 42 via the switch 41, and the two patterns 20A and 20
Observe how B overlaps.

That is, the user adjusts the handles 33 and 34 while observing the overlap between the patterns 20A and 20B displayed on the monitor 42, so that the two patterns perfectly match.

In addition, in order to observe the image of one pattern in detail, the output signal of the amplifier 35 or 36 is supplied to the monitor 42 via the switch 41, and the image of one pattern is displayed on the monitor 42.
2. Make it appear on the screen.

The reason for comparing two patterns in the same mask to be inspected as in this example is because there are many patterns with the same pattern on the mask, and the probability that these patterns will have exactly the same defect is extremely small. This is based on the fact that defects can be detected with sufficient accuracy even without the use of a mask.

However, with this method, the number of comparisons of the patterns around the mask 1 shown in FIG. Since this is only the pattern surrounded by the broken line circle 43 in FIG. 1, it does not pose much of a problem.

The present invention is not limited to the above-mentioned example, but can be modified in many ways.

For example, in the above-mentioned example, a defect in the pattern of a transparent photomask made of an integrated circuit was detected, but it is of course possible to detect defects in other patterns.

Furthermore, although a flying spot cathode ray tube is used in the above-mentioned example, other types of scanning light generating devices may also be used, such as a device using a vibrating reflector.

Further, in the example shown in FIG. 6, every second pattern is observed at the same time, but it is also possible to inspect any pattern, such as adjacent patterns or every other pattern.

However, if the distance between the two patterns becomes too long, the influence of play in the moving table will appear, so it is preferable to select patterns that are as close as possible, taking into consideration the arrangement of the optical system.

[Brief explanation of the drawing]

FIG. 1 is a plan view showing the structure of a photomask for integrated circuit manufacturing suitable for inspection by the pattern defect inspection apparatus of the present invention, FIG. 2 is a view showing a microscopic image of a portion of a pattern without defects, and FIG. The figure shows a microscopic observation image of a circumferential portion of a defective pattern, FIG. 4 is a diagram showing the configuration of an example of a conventional photomask pattern defect inspection apparatus, and FIG. 5 is not included in the scope of the present invention. However, in the process of arriving at the present invention, the present inventors prototyped a diagram showing the configuration of a pattern defect inspection device, and FIG. Similarly, FIG. 8 is a circuit diagram showing the configuration of an example of the signal processing circuit portion, and FIG. 8 is a signal waveform diagram for explaining the operation thereof. 20, 21...Mask, 20A, 20B...
...Pattern, 22...Moving table, 23...
...Flying spot cathode ray tube, 24. 27,30...Lens, 3L32...
Phototube, 35, 36...Amplifier, 37...
...Phase inverter, 38...Mixer, 39...
... Slicer, 40 ... Signal processing circuit,
41...Switch, 42...Monitor.

Claims (1)

[Claims] 1. In order to inspect for defects in the pattern of a sample in which many identical patterns are formed adjacent to each other at a constant pitch, such as a photomask used in the manufacture of semiconductor integrated circuits,
A device that generates a scanning light spot; an optical system that simultaneously irradiates the scanning light spot onto the same portion of two identical patterns close to each other in the same sample; Two light receiving devices that each receive light separately, a circuit that inverts the phase of the output signal from one of these two light receiving devices, and one output signal whose phase is inverted and the phase of the output signal from the other light receiving device is not inverted. A pattern defect inspection device characterized by comprising a circuit for mixing an output signal.