JPH0523620B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a foreign matter detection method suitable for inspecting foreign matter on a surface on which a pattern is formed, such as a patterned wafer or a photomask.

[Prior Art] Wafers, photomasks, etc. used in the manufacture of semiconductor devices are extremely sensitive to foreign matter, so
Foreign matter inspection equipment is used to inspect surfaces for foreign matter.

Such foreign matter inspection equipment irradiates the wafer surface (generally the surface to be inspected) with an S-polarized beam, converts the P-polarized light component of the reflected light from the wafer surface into an electrical signal, and detects the wafer based on this electrical signal. A method for detecting foreign matter on a surface is known.

Even if a pattern exists within the irradiation spot of the S-polarized beam, the surface of the pattern is microscopically smooth, so that the reflected light is almost exclusively the S-polarized component. On the other hand, since the surface of a foreign object generally has minute irregularities, if a foreign object exists within the irradiation spot, the irradiated S-polarized beam will be scattered and the plane of polarization will be disturbed.
The reflected light will contain a considerable amount of P-polarized light component. Therefore, by comparing the level of the photoelectric conversion signal of the P-polarized light component with a certain threshold value, and determining that it is a foreign object when the photoelectric conversion signal exceeds the threshold value, the foreign object can be detected by distinguishing it from the pattern.

[Problems to be Solved] However, when such a conventional foreign matter inspection apparatus inspects a patterned wafer or the like for foreign matter, some patterns may be erroneously detected as foreign matter.

[Object of the Invention] An object of the present invention is to provide a foreign object detection method that prevents such erroneous detection of patterns and enables reliable foreign object detection.

[Means for Solving the Problem] According to the inventor's research, patterns are incorrectly detected in conventional foreign object inspection equipment when the irradiation direction of the S-polarized beam is around 45 degrees with respect to the normal direction of the pattern. This often occurs when the angle is .

Focusing on this, it is possible to prevent erroneous detection by adjusting the irradiation direction of the S-polarized beam so that such an angular relationship does not occur in a conventional foreign object inspection apparatus. However, in addition to patterns in the X and Y directions (which are the majority), wafers and masks often have diagonal patterns and minute isolated patterns, and wafer foreign particle inspection equipment In many cases, the wafer is inspected while rotating due to the inspection time and equipment configuration. Therefore, even if the irradiation direction of the S-polarized beam is adjusted, erroneous pattern detection cannot be sufficiently prevented.

Therefore, the inventor conducted a more detailed study regarding the erroneous detection of patterns and found the following. Note that the following results are based on the assumption that the optical system for receiving reflected light of the foreign matter inspection apparatus when irradiated with an S-polarized beam is arranged in a direction perpendicular to the wafer plane, and the wafer is rotated and scanned.

When a pattern is irradiated with an S-polarized beam, the normal direction of the pattern (hereinafter referred to as the normal direction of the pattern) when the relationship between the incident light and the pattern is projected onto the wafer plane is the irradiation of the S-polarized beam. At an angle of around 45° to the direction, the level ratio of the S and P polarized components (S/P or P/S) in the reflected light from that pattern becomes approximately 1.
Moreover, the level of the P-polarized light component at that time is
The level is comparable to that of the P-polarized light component included in the reflected light from a foreign object with a diameter of about 1 μm. For this reason, if the threshold value for comparison with the photoelectric conversion signal of the P-polarized light component is selected to detect a foreign object with a diameter of about 1 μm or more, it becomes impossible to distinguish between the pattern and the foreign object.

At other angles, the level ratio of the S-polarized light component and the P-polarized light component of the reflected light from the pattern deviates significantly from 1.

Regarding foreign matter, the above-mentioned directionality is hardly observed, and S/P is always approximately 1.

Since reflected light from a portion that is neither a pattern nor a foreign object contains almost no P-polarized component, the level ratio of the S-polarized component and the P-polarized component deviates significantly from 1.

Focusing on these points, in the present invention, the same point on the surface to be inspected is irradiated with S-polarized beams from two or more different directions, and the reflected light from the point for each S-polarized beam is calculated. The S-polarized light component and the P-polarized light component are separated and extracted, and the S-polarized light component and the P-polarized light component are
The presence or absence of a foreign object at the point is determined by simultaneously detecting that the level ratio with the polarized light component is approximately 1.

[Operation] In the foreign matter inspection apparatus, the objective lens system is located in a direction perpendicular to the wafer except in special cases. Furthermore, the pattern on the wafer is a linear pattern running in the XY directions. Therefore, when the wafer is rotated and scanned, the normal direction of the pattern on the wafer surface rotates in accordance with the rotation of the wafer. As a result, a situation occurs in which the S-polarized beam is at an angle of about 45 degrees with respect to the normal line of the pattern.

As in the above configuration, if multiple fixed beams are arranged at angles other than integral multiples of 90°, when one is around 45 degrees from the normal to the pattern, the other will It must be around 45 degrees from the normal.

For example, regarding the relationship between the pattern and the S-polarized beams, assume that the angle α between the S-polarized beams is set to about 45 degrees as in the embodiment described later.
When the wafer rotates and the angle β of one beam with the pattern direction becomes around 45 degrees, the angle of the other beam is β + α or 180° − α − β, and when β is about 45° Therefore, it will be around 90 degrees. This almost coincides with the normal direction of the pattern and is not at 45° with respect to the normal direction, so the S-polarized beam and P
The detection level ratio with the polarized beam does not become 1.

In this way, when the P/S level ratio is detected for each S-polarized beam, even if one of the ratios becomes 1 in the pattern, both level ratios will not become 1 at the same time.

In this way, if the relative angle of the irradiation direction of each S-polarized beam is appropriately selected, even if the angle between the irradiation direction of a certain S-polarized beam and the normal direction of the pattern is around 45 degrees, other S-polarized beams can be beam irradiation direction,
The angle with the normal direction of the pattern is not around 45 degrees. Therefore, irradiation S from all directions
The level ratio of the S and P polarization components of the reflected light to the polarized beam becomes approximately 1 at the same time only when a foreign object is present at the irradiation point. However, S, P
If foreign matter is determined based on the fact that the level ratio of the polarized light components is approximately 1 at the same time, patterns in any direction on the surface to be inspected can be reliably distinguished from foreign matter, and erroneous detection of patterns can be prevented.

Furthermore, the P-polarized component in the reflected light is proportional to the size of the foreign object. Therefore, S of the reflected light,
By checking not only that the level ratio of the P-polarized light component is approximately 1 at the same time, but also referring to the level of the P-polarized light component when determining foreign objects, it is possible to prevent false detection of patterns and to detect only foreign objects of a certain size or more. It can detect and determine the size of foreign objects.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing a simplified structure of an optical system and the like of a wafer foreign matter inspection apparatus to which the present invention is applied. In this figure, 10 is an X stage supported by a base 12 so as to be slidable in the X direction. This X stage 10 includes a screw 16 directly connected to the rotating shaft of the stepping motor 14.
are screwed together, and by operating the stepping motor 14, the X stage 10 can be moved forward and backward in the X direction. 18 is a linear encoder that generates a code signal corresponding to the position x of the X stage 10 in the X direction.

A Z stage 20 is attached to the X stage 10 so as to be movable in the Z direction. The driving means is omitted in the figure. Further, the Z stage 20 rotatably supports a rotation stage 22 on which a wafer 30 as an object to be inspected is placed.

This rotation stage 22 is connected to a DC motor 24, and can be rotated by operating this. This DC motor 2
4 has a built-in rotary encoder that outputs a code signal corresponding to the rotational direction position θ of the rotation stage 22.

Note that the wafer 30 is positioned and fixed on the rotation stage 22 by negative pressure suction, but means for this purpose are omitted in the figure.

In this wafer foreign matter inspection apparatus, an S-polarized laser beam is used as the S-polarized beam. To generate this, S-polarized laser oscillators 32A, 32B, 34A, and 34B are provided. The S-polarized laser oscillators 32A and 32B generate S-polarized laser beams with a certain wavelength λ ₁ , and in this embodiment are semiconductor laser oscillators with a wavelength of about 8300 angstroms. The S-polarized laser oscillators 34A and 34B generate S-polarized laser beams with other wavelengths λ ₂ .
It is a 6328 angstrom He-Ne laser oscillator.

The S-polarized laser beam having a wavelength λ ₁ is irradiated onto the upper surface of the wafer 30 from the Y direction at an irradiation angle φ of about 2°. Since the irradiation angle is thus small, when a beam of S-polarized laser light having a circular cross section is irradiated, the spot on the wafer surface is elongated, making it impossible to obtain a sufficient irradiation density. Therefore, cylindrical lenses 36 and 38 are arranged in front of the S-polarized laser oscillators 32A and 32B, and the S-polarized laser beam is focused into a beam with a flat cross-sectional shape crushed in the Z direction, and then irradiated onto the wafer surface. ing.

The S-polarized laser beam of wavelength λ ₂ is irradiated onto the wafer surface at an irradiation angle φ of about 2° from a direction at an angle η with respect to the Y direction. The angle η is approximately
45° is selected. Generally, η is chosen to be an angle other than an integer multiple of 90°. This S-polarized laser beam is also focused in the Z direction by cylindrical lenses 40 and 42, and then irradiated onto the wafer surface.

The laser light reflected in the Z direction from the wafer surface for each S-polarized laser light beam passes through an objective lens 44 and a slit 46, and reaches a dichroic mirror 48 for wavelength separation. The reflected laser beam having a wavelength of λ ₁ is reflected by the dichroic mirror 48 toward the polarizing mirror 50 . The reflected laser beam having a wavelength of λ ₂ passes through the dichroic mirror 48 and enters the polarizing mirror 54 .

The polarizing mirror 50 transmits the P-polarized light component of the incident laser beam and causes it to enter the photomultiplier 52P, and reflects the S-polarized light component and causes it to enter the photomultiplier 52S. Similarly, polarizing mirror 54
makes the P-polarized light component of the incident laser light enter the photomultiplier 56P, and makes the S-polarized light component enter the photomultiplier 56S. Each photomultiplier outputs an electric signal (detection signal) according to the intensity of the incident laser light.

Here, the foreign matter inspection is performed while rotating the wafer and feeding it in the X direction (radial direction) as described above. Following such rotational movement of the wafer 30,
The spot of the S-polarized laser beam moves spirally on the upper surface of the wafer 30 from the outside toward the center. The field of view of the slit 46 is always contained within the spot and moves to follow the spot. That is, the wafer surface is inspected while being spirally scanned.

FIG. 2 is a block diagram of the signal processing section of this foreign matter inspection apparatus. In this figure, the detection signals of the photomultipliers 52S and 52P are transmitted to the amplifier 1.
The signal is amplified by 00S and 100P and input to the level ratio circuit 102. This level ratio circuit 102
outputs a signal proportional to the level ratio of the two input signals. This output signal is transmitted to the next stage comparison circuit 104.
is input. The comparison circuit 104 compares the level of the input signal with two threshold values, determines whether the level ratio of the detection signals of the photomultipliers 52S and 52P is approximately 1, and when the level ratio is approximately 1, the theoretical Outputs a “1” level signal.

Similarly, the detection signals of photomultipliers 56P and 58S are amplified by amplifiers 106P and 106S and inputted to level ratio circuit 108, and the output signal of this level ratio circuit 108 is inputted to comparison circuit 110. This comparator circuit 110 outputs a signal of logic "1" level when the level ratio of the detection signals of the photomultipliers 56P and 56S is approximately 1.

The output signals of comparison circuits 104 and 110 are input to AND gate 112. This and gate 11
The output signal No. 2 indicates the foreign object detection result, and is input to a data processing system (not shown). Note that the output signals of the linear encoder 18 and rotary encoder are also input to the data processing system.

The foreign matter inspection operation of the foreign matter inspection apparatus having the above configuration will be explained. First, under the control of a data processing system, motors 14 and 2 are operated to perform the helical scanning.
4 is driven and the inspection begins.

The reflected laser light from the scanning point at each time is wavelength-separated and separated into an S-polarized component and a P-polarized component, and is sent to photomultipliers 52S, 52P, 56.
S, 56P.

If a foreign object is present at the scanning point, the S and P polarized components in the reflected laser beam will be at almost the same level, so logic "1" level signals are simultaneously output from the comparison circuits 104 and 110, and the AND gate 11
The output signal of No. 2 is at the "1" level. On the data processing system side, “1” is output from the AND gate 112.
When the signal is received, it is determined that a foreign object has been detected at that scanning point, and the scanning position information (x, θ) at that time is stored in the wafer foreign object table in the internal memory.

Assume that no foreign matter exists at the scanning point, but the irradiation direction of the S-polarized laser beam of a certain wavelength forms an angle of approximately 45° with respect to the normal to the pattern existing there. In this case, the comparison circuit (104 or 11) associated with the reflected laser beam of that wavelength
0) outputs a “1” level signal. However, the irradiation direction of the S-polarized laser beam of the other wavelength,
The angle with the normal direction of the same pattern is around 90°,
Since the deviation is large from 45°, the comparison circuit (110 or 10
The output signal of 4) becomes "0" level. As a result, the output signal of the AND gate 112 becomes "0", and the data processing system side determines that there is no foreign object at that scanning point.

When neither a foreign object nor a pattern exists at the scanning point, the level ratio of the S and P polarized components of the reflected laser beam is significantly different from 1, so the outputs of the comparison circuits 104 and 110 are at the "0" level. Of course, and gate 1
The output signal of No. 12 also becomes "0" level.

FIG. 3 shows a modification of the signal processing section of the foreign object inspection device. In this figure, the detection signals of photomultipliers 52P and 56P for P-polarized light components are added and amplified by a summing amplifier 114 and input to a comparison circuit 116. Comparison circuit 116 compares the input signal level with a specific threshold and outputs a "1" level signal when the input signal level exceeds the threshold. This output signal is the comparator circuit 104, 11
It is input to AND gate 118 with an output signal of 0.

The level of the P-polarized light component of the laser beam reflected by a foreign object is proportional to the size of the foreign object. Therefore, if the threshold value of the comparison circuit 116 is set in accordance with the size of the minimum foreign object to be detected, the output signal of the comparison circuit 116 will be at the "1" level only when a foreign object larger than the minimum size exists at the scanning point. . On the other hand, the output signals of the comparator circuits 104 and 110 reach the "1" level only when there is a foreign object at the scanning point. Therefore, according to this modification, only foreign objects larger than the set minimum size can be detected.

Note that with a similar circuit configuration, foreign object detection can also be performed for each size of foreign object. In that case, for example,
Only the output signals of the comparison circuits 104 and 110 are input to the AND gate 118, and the comparison circuit 116
In this step, input signals are compared using a plurality of threshold values, and comparison result signals for each threshold value are output. In this way, on the output processing system side, the presence or absence of a foreign object and the size of the foreign object can be recognized from the output signal of the AND gate 118 and the output signal of the comparison circuit 116.

Another embodiment of the present invention will be explained with reference to FIGS. 4 to 6. Note that in FIGS. 4 and 5, the same parts as in FIGS. 1 and 2 are given the same reference numerals.

In this embodiment, as shown in FIG.
The reflected laser light is incident on the polarizing mirror 54 without being wavelength-separated, and is separated into P and S polarized components, which are then incident on photomultipliers 56P and 56S. Instead, the S-polarized laser beam of wavelength λ ₁ and the S-polarized laser beam of wavelength λ ₂ are intermittent at a sufficiently high speed compared to the scanning speed and are irradiated alternately. This alternate irradiation of the S-polarized laser beams is performed by the laser beam oscillators 32A, 32B, 34.
If high-speed switching is possible for A and 34B, switching control may be used.If high-speed switching is not possible, a selective light shielding means such as a rotating slit may be provided in the irradiation path of the S-polarized laser beam (not shown). It will be held in

Regarding the signal processing section, as shown in FIG. 5, the detection signals of the photomultipliers 56S and 56P are amplified by amplifiers 106P and 106S and then input to the level ratio circuit 108.
The output signal of this level ratio circuit 108 is
10, and its output signal is input to a 1-bit latch circuit 122. This latch circuit 12
2, the output signal of the comparison circuit 110, and the gate signal G are input to the AND gate 124.

The S-polarized laser light beams of λ ₁ and λ ₂ are shown in Figure 6 (a).
The irradiation is performed alternately at the timing shown in (b) of the same figure, and the gate signal is a signal with the timing shown in (b) of the same figure.

As can be easily understood from the timing diagram of FIG. 6, at the "1" time of the gate signal during the irradiation period of the S-polarized laser beam of wavelength λ ₁ , the output signal of the comparator circuit 110 and the output signal of the latch circuit 122 (the output signal of the comparator circuit 110 in the immediately preceding irradiation period of wavelength λ ₂ ) and the AND gate 124 outputs an AND signal. Similarly, at the "1" time of the gate signal during the irradiation period of wavelength λ ₂ , the output signal of the comparison circuit 110 and the output signal of the comparison circuit 110 during the irradiation period of the S-polarized laser beam of wavelength λ ₁ just before that An AND gate 124 outputs an AND signal.

Since the S-polarized laser beams of each wavelength are switched and irradiated at high speed, the output signal of the AND gate 124 indicates the presence or absence of a foreign object at the scanning point, as in the previous embodiment.

In this embodiment, the quantities of the photoelectric conversion system and signal processing section can be reduced to approximately half of those of the previous embodiment.

Note that in this embodiment, it goes without saying that the wavelengths of the S-polarized laser oscillators 32A, 32B, 34A, and 34B may all be the same. Also,
The direction of irradiation of the S-polarized laser beam emitted from the same S-polarized laser oscillator may be switched using, for example, a rotating mirror, and similar switching irradiation from different directions may be realized.

Here, the present invention is not limited to the above-mentioned embodiments, but can be implemented with appropriate modifications.

For example, the functions of the level ratio circuit, comparison circuit, and AND gate of the signal processing section may be realized by software.

The photomultiplier can also be replaced by other photoelectric conversion elements.

Scanning for inspection is not limited to spiral scanning, but may also be linear scanning. However, since linear scanning stops at the scanning edge, the scanning time tends to increase, and when scanning a circular surface such as a wafer, position control at the scanning edge tends to become complicated. . Therefore, when inspecting foreign objects such as wafers,
Spiral scanning is generally advantageous.

Further, the present invention is also applicable to similar wafer foreign matter inspection apparatuses that utilize light beams other than polarized laser light.

Furthermore, the present invention can be applied to objects to be inspected other than wafers,
For example, it can be applied to devices that inspect foreign substances on the surfaces of masks, reticles, pellicle membranes, and the like.

[Effect of the invention] As detailed above, according to the specific invention of this application, the same point on the surface to be inspected is irradiated with S-polarized beams from two or more different directions, and the The S-polarized light component and the P-polarized light component of the reflected light from the point are separated and extracted, and the presence or absence of foreign matter at the point is determined based on the fact that the level ratio of the S-polarized light component and the P-polarized light component is approximately 1 at the same time. Therefore, it is possible to reliably distinguish between foreign objects and patterns in any direction on the surface to be inspected, and to prevent erroneous detection of patterns.

Furthermore, in the related invention of this application, the S-polarized light component and the P-polarized light component of the reflected light with respect to the S-polarized light beam in each irradiation direction are separated and extracted, and the S-polarized light component is separated and extracted.
The polarized light component and the P polarized light component are separated and extracted, and the level ratio of the S polarized light component and the P polarized light component is approximately 1 at the same time.
Since the presence or absence of a foreign object is determined based on this fact and the level of the P-polarized light component, erroneous pattern detection can be prevented and only foreign objects of a specific size or larger can be reliably detected.

[Brief explanation of the drawing]

FIG. 1 is a schematic perspective view showing the optical system portion of an embodiment of the present invention, FIG. 2 is a block diagram of the signal processing section of the embodiment, and FIG. 3 is a modification of the signal processing section of the embodiment. FIG. 4 is a schematic perspective view showing the optical system portion of another embodiment of the present invention.
FIG. 5 is a block diagram of a signal processing section of another embodiment, and FIG. 6 is a timing diagram for explaining the operation of another embodiment. 10...X stage, 14, 24...motor,
22... Rotating stage, 30... Wafer, 32
A, 32B, 34A, 34B...S polarization laser oscillator, 44...Objective lens, 46...Slit,
48...Dichroitsch mirror, 50,54...
Polarizing mirror, 52S, 52P, 56S, 56P...
...Photomultiplier, 102, 108... Level ratio circuit, 104, 110, 116... Comparison circuit, 114... Summing amplifier, 122... Latch circuit.

Claims (1)

[Scope of Claims] 1. For the same point on the surface to be inspected on which a pattern is formed, the angles that they make when projected onto a plane parallel to the surface to be inspected exclude an integral multiple of 90°. 2
irradiate each S-polarized beam with a low irradiation angle of about 2° in the direction perpendicular to the parallel plane.
Separate and extract the S polarization component and the P polarization component of the reflected light from the point with respect to the polarized beam, and
A foreign object detection method characterized in that foreign object detection is performed by simultaneously detecting that the level ratio of a polarized light component and a P-polarized light component is approximately 1. 2 The angle formed by each projection on a plane parallel to the surface to be inspected is about 45 degrees, and the level ratio of each of the S-polarized light components and the P-polarized light components is approximately 1 at the same time, and each of the P-polarized light components 2. The foreign object detection method according to claim 1, wherein the foreign object is detected by detecting that the level of the polarized light component is equal to or higher than a predetermined level.