JPH0670955B2 - Observation device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an observing apparatus for observing two objects, and in particular, for aligning a mask and a wafer in a semiconductor exposure apparatus, observing an alignment mark on the mask and the wafer. The present invention relates to an observation device that is useful when doing.

[Prior art]

Conventionally, VSP4, 406, 5
Devices such as those disclosed in 46 are well known.

When observing a wafer through a mask, the influence of reflected light from the glass surface of the mask poses a problem. When the anti-reflection treatment is not applied to the glass surface of the mask, about 4% of the light is reflected from one surface of the glass, so about 8% of the light is reflected by the mask as flare light. It will be mixed with the reflected light from the wafer.

In the above-mentioned USP 4,406,546, an apparatus which solves such a problem and removes the reflected light from the mask when observing the mask and the wafer with the reflected light from the wafer is shown.

FIG. 6 shows a schematic diagram of this device. In the device of FIG. 6, the reflected light from the mask is cut during observation by using polarized light. Therefore, the pattern of the mask is illuminated by the reflected light from the wafer. Light from a light source not shown
L1 passes through the polarized beam splitter 5 and becomes P-polarized light, which illuminates the mask 1. At this time, the light L1 may be light whose polarization direction matches the polarization azimuth of the polarization beam splitter, or may be unpolarized light. Since the reflected light L2 from the mask 1 is incident on the polarized beam splitter 15 again, it remains as P-polarized light that is transmitted to the polarized beam splitter, and therefore passes through the polarized beam splitter 15 and comes back. I will return to the direction. In addition, the projection optical system 22
A λ / 4 plate 23 is disposed inside the light source, and the light L3 that has passed through the λ / 4 plate 23 becomes circularly polarized light and illuminates the wafer 2. The light L4 reflected by the wafer 2 again passes through the λ / 4 plate 23 and becomes linearly polarized light (S-polarized light) whose polarization direction is 90 ° orthogonal to that at the time of incidence.
It is incident on the polarized beam splitter 15 via. Therefore, as a result, the light reaching the wafer is reflected by the beam splitter 15 and is received as the observation light L5 for detecting the relative positional relationship between the mask 1 and the wafer 2 alignment mark by the naked eye 53 or the image sensor. Therefore, the mark on the mask 1 is illuminated by the reflected light from the wafer. In the case of a device that does not use polarized light, 1/4 in Fig. 6
This corresponds to the fact that the polarization beam splitter 5 does not have the wave plate 23 and is simply a beam splitter having no polarization characteristic.

There are three problems in this case.

The first problem is the amount of light. When the polarized beam splitter is not used, the light utilization efficiency is highest when the transmittance and the reflectance are both 50%, and the light utilization efficiency of the observation light at this time is 25%. On the other hand, if a polarized beam splitter is used, the efficiency after passing through the polarized beam splitter is 100% if the setting of the λ / 4 plate is the best. The efficiency of the first transmission through the polarized beam splitter is 50% if the incident light is unpolarized, and when using a polarization type laser, the polarization direction of the laser beam and the polarization direction of the polarized beam splitter are properly matched. If you do, it is 100%. That is, if the polarized light is not used, the light use efficiency becomes 1/2 to 1/4 or less. This is a disadvantage for photoelectric detection.

The second problem is that the light reflected from the glass surface of the mask is flared with respect to the light reflected from the wafer.

The third problem is a signal processing problem relating to the contrast of the wafer image and the contrast of the mask image. That is, when the reflectance of the wafer 2 is low, the reflected light of the chrome of the reticle 1 is strongly detected, which makes it difficult to detect the wafer. FIG. 7 shows the signal output corresponding to the mask image and the wafer image with and without the use of polarized light. Fig. 7 (A)
The chrome part 7 (alignment mark) of the mask as shown in
If there is a wafer alignment mark 9 between
Signal outputs along the scanning line 10 when these are imaged by the CCD camera are shown in FIGS. 7 (B) and 7 (C). When the polarized light described in FIG. 6 is utilized in FIG. 7B, the signal output 7'corresponding to the chrome portion 7 of the mask becomes lower than the signal output 107 'corresponding to the unmarked portion of the wafer. . This is because the mask illuminates with the reflected light from the wafer without directly receiving the reflected light from the mask. On the other hand, FIG. 7 (C) shows a case where the polarized light is not used, and since the strong reflected light from the chrome portion 7 of the mask is received as it is, the signal output 7 ″ corresponding to the chrome portion 7 becomes high. The signal corresponding to the mark 9 has a lower reflectance of the wafer and a lower contrast due to the reflected light from the glass surface of the mask, and therefore the difference in the signal output between the two is too large if polarization is not used. Considering that the light quantity of the incident light is adjusted so that the signal from the image pickup device has an optimum output, when the polarized light is used, the wafer shown in FIG. The signal output 107 'corresponding to the light output may be dimmed, but in the output of FIG. 3C when the polarization is not used, the output of the chrome portion of the reticle is 7 "
It is not suitable for detecting the alignment mark on the wafer because the light will be dimmed.

Observing the mask or the wafer using the polarized light in this way has a great advantage as compared with the case where it is not used, but since the mask is illuminated by the reflected light from the wafer, the output 107 'corresponding to the wafer is obtained. If is low, the mask cannot be detected.

FIG. 8 shows a state where the photoresist 114 is applied on the wafer surface 113. In the vicinity of the wafer alignment mark 116 for alignment, the thickness of the resist 117 changes under the influence of the step of the mark. Generally, the reflectance of the wafer changes depending on the thickness of the resist 117, and the resist thickness T _PR
If the thickness is equal to λ / 4 of the incident light, the reflected light 118 on the resist surface interferes with the reflected light 119 from the wafer surface, and the intensity of the reflected light from the wafer becomes low. When the wavelength of the incident light is λ and the refractive index of the resist 117 is N, the intensity of the reflected light becomes low when T _PR = (m ± 1/4) λ / N (where m is an integer). In the worst case, the intensity of the reflected light 118 from the resist is equal to the intensity of the reflected light 119 from the wafer surface, and the output of the reflected light is lost. If it happens that the wafer portion corresponding to the mask pattern satisfies such a condition, the signal corresponding to the mask cannot be detected. On the other hand, the resist 117 on the alignment mark 116 on the wafer changes in interference condition because the thickness changes near the step of the mark. Therefore, the reflected light also changes, and the scattering from the mark edge is coupled, so that the mask cannot be detected, but only the wafer can be detected. As described above, there is a problem that the mask may not be detected even if the wafer can be detected. In particular, the wafer is greatly affected by the process and there is a big problem in detection accuracy, but the mask is a detection target on which the chrome pattern is almost ideally patterned on the glass substrate, and this mask cannot be detected. Is a serious drawback.

[Outline of Invention]

In view of the above conventional problems, an object of the present invention is to provide an observation device that can always observe two objects.

In order to achieve the above object, the exposure apparatus according to the present invention,
A light source and a polarization splitting means provided between the light source and the first object;
A first phase conversion element provided between the second object onto which the pattern of the first object is projected and the first object; and An apparatus for selectively extracting reflected light from the second object toward a second object via the first phase conversion element and the polarization splitting means, and observing the first and second objects by the reflected light. Therefore, a second phase conversion element is provided in the optical path between the polarization splitting means and the first object.

Further features and specific forms of the present invention will be described in detail in the embodiments described later.

〔Example〕

FIG. 1 is an explanatory diagram showing the basic principle of the present observation apparatus. The difference from the conventional apparatus described in FIG. 6 is that the polarization beam splitter 5 (polarization splitting means), the mask 1 (first object) The point is that a λ / 4 plate 54 (second phase conversion element) is further provided between them.

When the light state is followed in the same manner as in FIG. 2, the P-polarized incident light L1 from the light source 57 is incident on the λ / 4 plate 54 as linearly polarized light (P-polarized light) even after passing through the polarization beam splitter 5. .
The set angle of the λ / 4 plate 54 (the polarization direction of the incident light and λ
If the angle formed by the crystal axis of the / 4 plate 54) is 0 °, 54 does not have any effect and is the same as the conventional case. On the other hand, when the set angle is shifted from 0 °, the light L2 'reflected by the mask 1 has not only the light (L5 ") transmitted through the polarized beam splitter 5 but also the reflected light component (L5'). λ / 4
Assuming that the set angle of the plate 54 is θ, the light that has passed through the λ / 4 plate 54 twice becomes linearly polarized light whose polarization direction differs by 2θ with respect to the incident direction. Therefore, when θ = 0 °, λ / 4 plate 54
Has no effect, but when θ has a certain value, it also has a polarization component of the light component reflected by the polarization beam splitter 5. When the set angle is 45 °, the transmitted light L5 ″ disappears and all the light becomes reflected light L5 ′. This situation will be described with reference to FIG.

FIG. 2 shows the relationship between the polarization direction of the incident light L1, the crystal axis OA of the λ / 4 plate and the reflected light L5 '. FIG. 5 (A) shows an example in which the set angle θ is θ = 45 °, and when it passes through the λ / 4 plate 54 twice, the set angle θ is doubled, that is, 2θ = 90 °, the polarization direction changes. That is, the light beam is incident on the polarization beam splitter 5 in a state in which the plane of polarization is rotated by 90 ° from the incident light L1 (S polarization).

The polarized beam splitter 5 has a property of transmitting the light in the polarization direction (P-polarized light) of the incident light L1, but in the future, on the contrary, it reflects the light reflected directly from the mask 1 (S-polarized light). Have the effect of Therefore, all the reflected light from the mask 1 is received. On the other hand, at this time, the light passing through the mask 1 is circularly polarized light and has a wavelength of λ / 4 in the projection optical system 22.
Pass plate 23. Here, the set angle θ of the λ / 4 plate 23 with respect to the incident light L1 is θ = 45 °. Therefore, in the case of FIG. 2 (A), the two λ / 4 plates 23 and 54 both have the crystal axis OA oriented in the same direction, and the wafer 2 (second object)
Is the same as the incident light L1 (P-polarized) or 90 ° when illuminating
It becomes polarized light (S-polarized light) whose polarization plane is rotated. The reflected light reflected by the wafer 2 passes through the two λ / 4 plates 23 and 54 again, and finally becomes light having the same polarization direction as the incident light L1. As a result, the light reaching the wafer 2 becomes a light L6 ″ which is transmitted through the polarized beam splitter 5 in terms of its polarization direction, and the light (L6 ′) reflected by the polarized beam splitter 5 becomes zero. Reference numeral 53 receives only the reflected light L5 'from the mask 1 and does not receive the reflected light L6' from the wafer.On the other hand, as shown in FIG.
When the angle is 22.5 °, the reflected light L2 ′ from the mask 1 is 2θ.
= 45 ° Change the polarization direction. Of the reflected light from the mask 1, the intensity of the light L5 ′ reflected by the polarized beam splitter 5 is
The vector indicating the reflected light L2 'in the figure is the light L5' projected on the x axis, and the intensity of the transmitted light L5 "is the light projected on the y axis in the same manner. The set angle of 54 is θ
Then, the reflected light from the mask 1 has its polarization plane rotated by 2θ and is transmitted through the polarized beam splitter 5 as light L.
5 ″ is the light amount of “cos ^{2 2} θ” when the light amount of the incident light L1 is 1. Similarly, the light L reflected by the polarized beam splitter 5
The light quantity of 5'is "sin ² 2θ".

Therefore, the intensity of the light component L5 'of the reflected light from the mask 1 can be optimally adjusted while rotating the λ / 4 plate 54 about the optical axis.

The reflectivity of the mask 1 is stable, but the reflectivity of the wafer 2 is not stable because it varies depending on the process at the time of IC manufacturing. Therefore, in order to obtain a stable light quantity from the mask 1 and the wafer 2, when the wafer 2 has a low reflectance, strong light,
When the wafer 2 has a high reflectance, dimming such that weak light is emitted is required.

In the case where the polarized light is not used like the signal example of FIG. 7 (C) described above, the wafer 2 is not affected adversely to the mask signal.
However, it is impossible to dimm well, but if a λ / 4 plate 54 that is a phase conversion element is provided as in the present invention and dimming is performed by changing the set angle θ, it depends on the incident light intensity. It is possible to obtain light of a certain intensity from the mask without doing so, and it is possible to perform dimming in the original sense of increasing the level of the wafer signal.

FIG. 3 shows an example of signals when the marks of the mask and the wafer are picked up by the apparatus of FIG.

Similar to FIG. 7A, FIG. 3A shows a state of images of the alignment marks 7 and 9 on the mask and the wafer. Third
FIG. 3B shows the case where the wafer 2 has a high reflectance and the output 7'corresponding to the mask mark 7 has a sufficient contrast. FIG. 3C shows an example of the worst case where the reflectance of the wafer 2 is low. Although the edge of the signal of the wafer mark can be detected, the contrast of the mask mark signal is low because the reflectance of the wafer 2 is low. At this time, when the λ / 4 plate 54 in FIG. 1 is rotated by a predetermined amount, the mask mark signal can be detected with good contrast as shown in FIG. 3 (d). When the λ / 4 plate 54 is further rotated, the mask mark signal is further increased as shown in FIG. At this time, the wafer mark signal becomes relatively low, but the signal as shown in FIG. 3 (F) can be obtained by increasing the intensity of the incident light on the wafer.

As described above, both the reticle mark signal and the wafer mark signal are suitable by controlling the state of the wafer mark signal and the mask mark signal by the monitor 1 and the state of the λ / 4 plate 54 (the direction of the crystal axis) as appropriate. It is a major feature of the present invention that a signal having a level that can be processed into a signal can be processed.

Here, the state of the λ / 4 plate is a comprehensive expression of the rotation or removal of the λ / 4 plate 54, and various modifications are allowed in the present invention.

For example, when the absorption resist is applied to the wafer 2 and the reflectance of the wafer 2 is low, the mask mark signal may be chronically difficult to detect. At this time, the wafer mark signal can detect a slight change in the film thickness near the alignment mark as shown in FIG. 8, but the mask is often difficult to detect due to the combination of absorption of light energy and interference. In such a case, it is particularly effective to use the λ / 4 plate 54 in the optical path between the polarization beam splitter and the mask 1, and if the process is stable, the λ / 4 plate 54 is fixed under the optical path. A sufficient effect can be obtained just by inserting it inside. Of course, in the case of a reduced projection optical path device such as a stepper, the angle of the λ / 4 plate 54 may be controlled for each shot based on the mark detection signal.

In order to control the signals of the wafer and reticle in this way, the device shown in FIG.
A drive system 56 for the plate 54 is provided, and the light source 57 is provided with a light quantity controller.

As shown in the figure, the CPU 55, the drive system 56, the image sensor 53, and the light source 5
7 is electrically connected by a signal line, and the image sensor 53
Depending on the level and contrast of the image signal from the CPU55
Controls the light amount controller of the drive system 56 and the light source 57, the rotation angle of the polarizing plate 54 is adjusted by the drive system 56, and the intensity of the light from the light source 57 is adjusted by the light amount controller.

Particularly in the case of a stepper, the position of the wafer 2 is sent in advance to the predetermined position with an accuracy of ± several μm by another pre-alignment system. Therefore, if the signal processing is performed by limiting the area for the image pickup device 53, the output control as shown in FIG. 3 can be performed.

As described above, since the present invention uses the property of polarization, in the case of a reflection optical system, a reflection film (phase conversion element) having a phase characteristic is used instead of the λ / 4 plate 23 in the projection optical system 22. This idea can be applied when used. Further, it can be applied to various exposure apparatuses such as a proxy type semiconductor exposure apparatus.

[Other Examples]

FIG. 4 shows an example in which the apparatus shown in FIG. 1 is shared with a laser scanning optical system. The polarized laser light emitted from the laser 11 is polarized by the polygon mirror 13, and the f-θ lens is used.
The beam is scanned at a constant speed via 14, and the beam is focused on the reticle by the polarized beam splitter 15, the field lens 16, the beam splitter 17, the relay lens 18, and the objective lens 21 to be scanned. Beam Splitter 17 at this time
Is a polarized beam splitter, not a laser light
It is a half mirror prism that transmits 90%.

The laser light transmitted through the reticle 1 is λ in the projection lens 22.
After passing through the / 4 plate 23 and reflected by the wafer 2, it again passes through the λ / 4 plate 23 and changes its polarization direction by 90 °. As a result, the reflected light from the wafer 2 is reflected by the polarized beam splitter 15 and the projection lens
22 A dark field signal is received by a light receiving element 34 by a spatial frequency filter 32 for dark field observation arranged at a position conjugate with the pupil plane.

The laser light directly reflected by the reticle 1 includes light reflected by the prism 17 and light transmitted therethrough. The reflected light is transmitted through the polarized beam splitter 15 ′ and is a space arranged at a position conjugate with the pupil plane of the projection lens 22. The frequency filter 32 'receives the light as a dark field signal by the light receiving element 34'.

Next, the case where the image of the alignment mark on the reticle 1 and the wafer 2 is picked up by the image pickup device 53 will be described. Unlike the above-described laser beam scanning, a diffusion plate 63 is placed in front of the polygon mirror 13 in order to irradiate the reticle 1 and the wafer 2 with light having a large spot diameter as illumination light. Of course, during the above-mentioned laser beam scanning, the optical path is retracted so as not to obstruct the optical path. By rotating the diffuser plate 65 when the alignment mark image is picked up, the influence of the spectrum pattern generated in the direction perpendicular to the scanning direction on the reticle 1 by the polygon mirror 13 is eliminated. When the λ / 4 plate 54 is not inserted between the polarized beam splitter 15 and the reticle 1, only the reflected light from the wafer 2 is reflected by the polarized beam splitter 15 as shown in FIG. The light reflected by the splitter 15 enters the beam splitter 17 and is then reflected towards the image sensor 53.

Since the rotating diffusion plate 63 is included when the image sensor 53 is used, the laser beam scanning method for dark field detection cannot be used.
The light utilization efficiency is better when the beam splitter 17 is switched to a mirror. The light reflected by the beam splitter 17 forms an image of the alignment mark between the mask and the wafer on the image sensor 53 via the erector 52. Here, in addition to the conventional optical system, a λ / 4 plate 54 is further installed, and the λ / 4 plate is set so as to obtain an optimum set angle according to the reflectance of the wafer 2.
Turn 54 to stop. At this time, when it is necessary to change the illumination light amount by the laser 11, the light amount is changed by the light control system 55 provided in the optical path of the laser light emitted from the laser 11. Also, there are various other methods of adjusting the amount of illumination light. If a semiconductor laser is used as a light source, the operating current of the laser may be changed, and several kinds of NO filters should be prepared and these should be used in the optical path. It is also possible to replace with each other, to rotate a polarizing plate provided in the optical path, and to modulate the light intensity by an AO element, an EO element, or the like.

As shown in FIG. 4, the present observation apparatus can be implemented in various optical systems that observe two objects using polarized light.

FIG. 5 is a further embodiment of the present invention and shows an example of the configuration when a normal light source is used instead of a laser. In addition, this figure is a bird's-eye view of the optical system as seen from diagonally above.
Only the upper part is shown. Reticle 1 in this example
The left and right alignment marks are viewed, and the visual field combining prism 40 combines the visual fields corresponding to the two marks. A mirror M is shown, but the mirror M is for folding the optical system, and various other modifications are possible. The light source is a fine bar 42, and 41 is a condenser lens for the illumination system. Light is guided from a light source (not shown) to the other end surface of the fiber 42. The action of the polarized beam splitter 5 is shown in FIGS.
Similar to the figure, the rotation angle θ of the λ / 4 plate 54 is controlled based on the output of the image sensor 53.

〔The invention's effect〕

As described above, in the present invention, by adding a phase conversion element such as a λ / 4 plate to the conventional optical system and controlling the set angle θ, signals corresponding to the first and second objects such as a mask, a reticle, and a wafer can be obtained. It was possible to detect both with a good S / N ratio.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the basic principle of the present invention. FIGS. 2A and 2B are explanatory views showing the relationship between the crystal axis direction of the λ / 4 plate and the polarization direction of incident light. FIGS. 3A to 3F are explanatory views showing changes in output signals from the image pickup device in the observation device shown in FIG. FIG. 4 is a schematic configuration diagram showing an example of an alignment mark detection optical system equipped with the observation apparatus of the present invention. FIG. 5 is a bird's-eye view showing another example of the alignment mark detection optical system equipped with the observation device of the present invention. FIG. 6 is a schematic view showing a conventional observation device. 7 (A) to 7 (C) are explanatory diagrams showing changes in the output signal from the image sensor in the device shown in FIG. 6 and the device not utilizing polarization. FIG. 8 is an explanatory view showing the influence of interference by the photoresist applied on the wafer. 1 ...... Mask 2 ...... Wafer 5 ...... Polarized beam splitter 22 ...... Projection optical system 23 ...... λ / 4 plate (first phase conversion element) 53 …… Image sensor 54 …… λ / 4 plate (second Phase conversion element) 55 …… CPU 56 …… λ / 4 plate drive system 57 …… Light source

Claims (2)

[Claims] 1. A light source, a polarization splitting means provided between the light source and a first object, and a first phase provided between a second object and a first object onto which a pattern of the first object is projected. A conversion element, directing light from the light source to the first and second objects via the polarization splitting means, and reflecting light from the second object to the first phase conversion element and the polarization splitting means. A device for selectively observing the first and second objects with the reflected light, wherein a second phase conversion element is provided in an optical path between the polarization splitting means and the first object. An observation device characterized by the above. 2. The observation apparatus according to claim 1, wherein the second phase conversion element is rotatable around the optical axis.