JPH04102019A - Double focus apparatus using reference reticle - Google Patents

Abstract

PURPOSE:To make it possible to detect with sufficient resolution an X-ray mask and a wafer being apart from each other to a large extent by a method wherein a reference reticle on the opposite surfaces of which marks necessary for detection of positions of the two objects are formed is provided at two imaging positions of an objective lens. CONSTITUTION:In order to detect an X-ray mask 1 and a wafer 2 simultaneously by an objective lens 14, a half mirror 12 is provided and an illuminating light from a white light source 11 such as a halogen lamp is reflected thereby so as to conduct vertical illumination. Images of a mask mark 3 on the X-ray mask 1 and of a wafer mark 2 are transmitted through the half mirror 12 and formed separately at a point A and a point B on the optical axis. A reference reticle 16 being a parallel-plane plate is inserted at this position and a wafer reference mark 15 is formed of a reticle of chrome or the like on the optical axis in the center at the point A, while mask reference marks 17 are formed of this reticle above and below the point B. When a double focus device thus prepared is applied to an alignment device for measuring the relative positions of the X-ray mask and the wafer accurately, the relative positions of the two can be detected highly precisely without being affected at all by a wafer process.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention accurately determines the relative position between a first object and a second object, such as a mask and a wafer, which are arranged at a minute interval in the optical axis direction. The present invention relates to a position detection device for detecting.

[Prior Art] If the present invention is applied to an alignment device for an X-ray exposure device, its position detection methods can be broadly classified into the following three types.

■Pattern measurement method ■Fresnel diffraction method ■Diffraction grating method In this category, this invention relates to pattern measurement method (■). Therefore, the conventional technology regarding this pattern measurement method will be briefly explained.

First, I will list the known literature.

(1) Research on alignment method for rX-ray exposure” 1980, 8
1985 Precision Machinery Vol. 46 No. 8 P, P, 83 - 88 (2) Evaluation of alignment performance of rX-ray exposure device 5R-1" 1985 Precision Machinery Vol. 51 No. 5 P, P, 1
56-161 (3) "Alignment pattern detection method for xmn optical device using oblique imaging optical system" September 1989 Precision Engineering Journal P, P, 139-1
45(4) "High-precision position detection using a chromatic aberration bifocal device (first report)" 1990 Japan Society for Precision Engineering Autumn Conference P, P, 667-668 In the above document (1), a vibrating photoelectron microscope is used as an alignment device. is used in the detection system. That is, in FIG. 8, in order to align the focal plane of the photoelectric microscope 30 with the X-ray mask 1 and the wafer 2, the objective lens 31 is moved in the optical axis direction by the optical axis moving mechanism of the photoelectric microscope 30 (not shown). The alignment marks on the line mask 1 and the wafer 2 are detected separately. Specifically, first, the optical axis is moved to change the focal plane of the objective lens 31 to the Xla mask l.
and the reference slit 32 in the photoelectron microscope 30 and
After detecting the relative position of the enlarged image of the alignment mark on the line mask 1 (10x), the optical axis is moved to match the focal plane of the lens with the wafer 2, and the relative position of the enlarged image of the alignment mark on the reference slit 32 and the wafer 2 is detected. I am trying to detect the position.

Since the relative position between the X-ray mask 1 and the wafer 2 from the reference slit 32 is calculated by this sequence, the relative position between the X-ray mask 1 and the wafer 2 is determined.

A disadvantage of this device is that since the optical axis of the detection system is moved, the amount of movement of the optical axis in a plane perpendicular to the optical axis results in a detection error. In other words, the position of the optical axis when detecting the wafer 2 with respect to the position of the optical axis when detecting the X-ray mask l is within a plane perpendicular to the optical axis (△X,
If the movement is △y) μm, the detection error will be (ΔX, Δy
) will be added with that value.

The problem is the amount of (ΔX, Δy), but since the target detection resolution of the X-ray exposure device is 0. (ΔX, Δy)≦001 μm must be achieved.

However, if the optical axis of the moving objective lens is set to 0.01μ
It is clear that it is extremely difficult to move below m.

As in this example, it can be said that having a mechanical or electrical drive part in the detection part poses a problem that must be avoided at least in a device whose purpose is to detect a position on the order of 0.01 μm. .

Next, the apparatus of the above-mentioned document (2) is shown in FIG.

In this example, a bifocal lens 33 that utilizes the principle of birefringence is used as the objective lens of the alignment scope, and enlarged images of the alignment marks on the X-ray mask 1 and the wafer 2 are detected simultaneously.

The problems with this device are listed below.

■In order to obtain a high-contrast image with the same resolution (resolving power) as a normal lens, it is necessary to
Double the numerical aperture (N, A,) is required.

(b) By using an objective lens having a numerical aperture twice as large as (2) above, the depth of focus becomes extremely shallow, making it difficult to detect the X-ray mask 1 and the wafer 2 within the two focal planes.

■The gap between the X-ray mask 1 and the wafer 2 cannot be set large; that is, considering the refractive power of the birefringent surface and the aberration of the lens, the distance between the bifocals must be wide (
It is extremely difficult to separate them, and in FIG. 9, the distance between the X-ray mask 1 and the wafer 2 is set to 10 μm.

■Production requires advanced technology and skills, which increases costs.

In this way, since this device does not have a driving part compared to the device (1) described above, the accuracy is improved accordingly, but in order to further increase the accuracy, it is necessary to increase the lens resolution. However, since the principle of birefringence is used, the lens resolution is less than half of normal, and the design and
It can be said that this method is extremely difficult from a manufacturing point of view.

Next, the apparatus of (3) above is shown in FIG. 10. In this apparatus, an objective lens with a low numerical aperture is used to detect X-rays from an oblique direction with respect to the X-ray mask 1 and the wafer 2.
The line mask 1 and the wafer 2 can be observed simultaneously. By performing oblique detection, it is possible to simultaneously observe the X-ray mask 1 and the wafer 2, which are separated by a distance greater than the depth of focus of the objective lens 34.

The problems with this device are listed below.

■To obtain sufficient resolution, the numerical aperture (N
, A, ), but due to oblique imaging, in order to obtain a sufficiently wide gap corresponding to the wafer 2 and mask 1, the numerical aperture (N, A,) of the objective lens 34 must be lowered. Therefore, resolution and gap cannot be satisfied at the same time.

- Due to oblique imaging, the focal plane is extremely narrow and the image is blurred, making it impossible to detect marks that are imaged over a sufficiently wide range.

■Due to oblique imaging, the imaging pattern is tilted, leading to measurement errors.

■The tilt error between the illumination optical axis and the detection optical axis directly becomes the detection error.

To summarize these points, by tilting one focal plane of the lens defined by Rayleigh's equation, the X-ray mask and wafer, which are object planes that are far apart, are simultaneously imaged, so the resolution and gap settings can be adjusted to each other. They interfere and it is impossible to satisfy both at the same time.

Next, the optical path diagram showing the principle of the device (4) above is shown in the 11th section.
As shown in the figure, this device utilizes the axial chromatic aberration of the objective lens 35 to create two focal planes M, M' and two different focal planes on the same optical axis.
This realizes an optical system with one imaging plane B that forms images at two wavelengths. In these two focal planes M and M', Rayleigh's definition equation holds true independently, so the necessary lens resolution can be freely selected completely independent of the gap δ between the X-ray mask 1 and the wafer 2. Therefore, it is ideal as a detection optical system.

In addition, the inventor of the present invention has proposed "bicolor illumination method and band light illumination method in a bifocal detection device using chromatic aberration" (see Japanese Patent Application No. 63-261379), and according to this, two wafers It is a very effective means because it can be detected using band light and is not affected by wafer processes such as resist film as an alignment device.

The disadvantage of this device is that although there are no technical problems with design and manufacturing, it requires a certain amount of development cost. If extremely inexpensive common optical components are sufficient for measurement,
If an optical system with heavy focus can be realized, this would be the best method.

[Problems to be Solved by the Invention] As described above, various improvements have been made to alignment devices for X-ray exposure devices. Both use an optical system as the detection system. In a system that uses a lens in this detection system, as shown in FIG. , A, = 0.4, and the in-focus range, that is, the depth of focus, is 35 μm from Rayleigh's equation, and it is impossible to simultaneously detect the X-ray mask surface 1 and wafer surface 2, which are 40 μm apart. be. - The characteristics of a lens are expressed in terms of resolution and depth of focus, which are defined by Rayleigh's equation, i.e.

As is clear from these equations (1) and (2), the depth of focus and resolution are determined by the numerical aperture (N, A,) of the lens if the wavelength λ is constant.

FIG. 7 shows a graph of the relationship between depth of focus and numerical aperture (N, A,). From this graph, the numerical aperture (N.

A, ) and depth of focus are inversely proportional; increasing the numerical aperture to increase resolution will shallow the depth of focus, and conversely, increasing the depth of focus will decrease the numerical aperture (N, A,). It can be seen that the lens resolution becomes lower.

In other words, in order to simultaneously detect the X-ray mask surface 1 and the wafer surface 2, which are far apart in FIG. However, the lens resolution will be greatly reduced.

Incidentally, if the depth of focus is set to 40LLm in order to simultaneously detect the X-ray mask surface 1 and the wafer surface 2, then
According to the above formula (1), N, A, = 008, which is extremely small, and the resolution is 0.0 when N, A, ≧0.3, according to the experiment of the inventor of the present invention.
Since a detection resolution of 1 μm has been obtained, the X-ray mask and wafer can be detected simultaneously with sufficient resolution using a general lens defined by Rayleigh's equations (1) and (2) above. That is impossible.

The invention of this application was made in view of the above points, and satisfies the basic properties of lenses, and
The object of the present invention is to extremely easily realize a bifocal optical system that is sufficient for detecting a line mask and a wafer with sufficient resolution and measuring the relative positions of the two.

[Means for Solving the Problems] In the present invention, in a position detection device that detects the relative position of a first object and a second object that are separated by a minute distance in the optical axis direction in a direction perpendicular to the optical axis, corresponds to the minute distance between the object and the second object, has a thickness determined from the magnification of the objective lens, and has marks necessary for detecting the positions of the first object and the second object formed on both sides. This is a bifocal device using a reference reticle, characterized in that reference reticles are provided at two imaging positions of an objective lens.

In addition, in this invention, in order to further enlarge the images of the first object and the second object formed on both sides of the reference reticle, two relay lenses are used to branch the optical paths so that the focus is on both sides of the reference reticle. This is a double focusing device using a reference reticle, which is characterized in that it is placed as a reference reticle.

[Example] Hereinafter, the present invention will be described in detail based on the drawings. 1st
The figure is an optical path diagram showing the basic configuration of a double focusing device using a reference reticle. In order to simultaneously detect the X-ray mask 1 and the wafer 2, which are separated by a gap of δ μm, by the objective lens 14, a half mirror 12 is installed on the right side of the objective lens 14 at an angle of 45 degrees in the optical path, and a halogen lamp is inserted from above. Reflecting light from a white light source 11 such as the like is reflected, and epi-illumination is performed via an objective lens 14. The illuminated images of the mask mark 3 and wafer mark 2 on the X-ray mask 1 are transmitted through the half mirror 12 and are reflected by the objective lens 14 at A on the optical axis.
The images are formed separately at point and point B. The interval between points A and B is δ·N2, where N is the magnification of the objective lens 14. At this position, a reference reticle 16 of a parallel plane plate with a thickness of δ·N2 is placed.
Insert the wafer reference mark 15 on the central optical axis of point A on the left side, and the mask reference mark 1 above and below point B on the right side.
7 is formed by a reticle made of chrome or the like.

Therefore, the wafer mark 2 forms an image enlarged N times by the objective lens 14 at point A, and this image, together with the wafer reference mark 15, is directed downward by the half mirror 18 provided at an angle of 45 degrees on the optical axis. It is reflected and imaged onto the line sensor 20 by the first relay lens 19 . on the other hand,
The image of the mask mark 3 on the X-ray mask l is captured by the objective lens 14.
The image is formed at point B, and this image passes through the half mirror 18 together with the mask reference mark 17, and passes through the second relay lens 21.
As a result, an image is formed on the line sensor 22.

The pattern 30 imaged on the line sensor 20 is
As shown in Figure (A), the image of the wafer mark 2 is projected in parallel with the image of the central wafer reference mark 15 sandwiched therebetween. In addition, the pattern 3 imaged on the line sensor 22
2 is the mask mark 3 in the center as shown in FIG. 3(A).
The image of the mask reference mark 17 is projected in parallel with the image of . Therefore, these line sensors 2
The waveform detected by 0 and 22 becomes the waveform 3133 shown in FIG. 2(B) and FIG. 3(B), respectively.

In the wafer imaging pattern 30, the relative position α between the wafer reference mark 15 and the wafer mark 2 can be measured. Furthermore, in the mask imaging pattern 32, the relative position β between the mask mark 3 and the mask reference mark 17 can be measured.

Since the wafer reference mark 15 and the mask reference mark 17 are formed on both sides of the reference reticle 16, the positional relationship between the two marks is constant.
The relative position of the wafer mark L and the wafer mark L is determined by (α-β). This relative position (α-β) is the reference reticle 1
6 in the optical axis direction. When the reference reticle 16 moves by △X in the direction perpendicular to the optical axis, the relative position is α-β=(a+△X)-(β+△X) =α-β (3), which is is not affected at all.

From the above explanation, it is clear that the reference reticle 16 is
The wafer mark l and mask mark 3 are imaged at different positions A and H on the optical axis 19.
It can be seen that it is possible to measure the relative position of.

In other words, the optical system shown in FIG. 1 has focal points for the X-ray mask 1 and the wafer mark 2, and is capable of measuring the relative position between the focal plane and the object in a plane perpendicular to the optical axis. It is possible. In this optical system, Rayleigh's equations (1), i3, and (2) are satisfied at each focal plane, so the necessary lens resolution, that is, the numerical aperture (N, A,) can be freely set regardless of the gap δ. It is possible to do so.

Furthermore, since images of each focal plane are formed using white illumination light, the wafer process is not affected at all and is ideal. Furthermore, since it is white lighting,
There is no multiple reflection interference between the X-ray mask 1 and the wafer mark 2, and there is no influence from variations in the gap δ.

Furthermore, x! Ji! Since a white image is formed on the mask, there are no restrictions on the thickness, refractive index, spectral transmittance, etc. of the mask, which is extremely convenient.

Therefore, this optical system can be said to be an optical system having two focal points necessary for detecting the relative positions in the direction orthogonal to the optical axis of two objects separated by a minute path lIi in the optical axis direction. Furthermore, in terms of cost, it does not require any special lenses or optical parts, and can be constructed entirely from commercially available products, making it extremely inexpensive.

The reference reticle 16 is also not a special component, and can be easily formed at a very low cost by depositing one or two layers of chromium or the like on a parallel plane glass plate.

Next, a supplementary explanation will be given of the detailed points in the above embodiment. First, the relationship between the half mirror 18 and the 1° second relay lens 19.21. The half mirror 18 divides the optical path from the reference reticle 16 into two, and it is desirable that its reflectance and transmittance divide the amount of light into two equal parts with a reflectance and transmittance of 5050. Therefore, the amount of illumination light from the white light source 11 needs to be twice that of a normal epi-reflection microscope, but this increase in the amount of light can be easily accommodated by increasing the output of the white light source 11. Further, this optical path branching has no effect on the numerical apertures (N, A,) of the first and second relay lenses.

Next, the reference reticle 16 will be explained.

Both sides A and B of the reference reticle 16 are a wafer mark 2 formed by the objective lens 14 and a mask mark 3 on the X-ray mask 1.
It is desirable to coat the image forming surface with an anti-reflection film.

In addition, the wafer reference mark 15 and mask reference mark 17 formed on both sides are one-layer chrome like -M or two-layer chromium.
A layer of chrome is sufficient.

Next, magnification error will be explained. Although very slight, a magnification error occurs between the wafer mark 1 and the mask mark 3 imaged by the objective lens 14. However, this magnification error causes the final imaged wafer pattern 3
0 and a line sensor 20 that detects the mask pattern 32,
22 can be easily corrected. Specifically, after quantizing the electrical signals obtained by the line sensors 20 and 22, it is only necessary to adjust the scaling coefficient (equivalent to optical magnification) when performing digital processing. This method has already been applied in document (4).

Next, the resolution of the lens will be explained. In the optical system shown in FIG. 1, wafer marks 2 and XIJ are detected.
The optical resolution of the mask mark 3 on the il mask 1 is
Numerical aperture (N) of objective lens 14.

A). This numerical aperture is set completely independent of the gap δ between the wafer mark 2 and the mask mark 3.

The set value is set within the range of 0.3≦NA, ≦0.5 (4) based on the relationship between the depth of focus of the lens determined by the numerical aperture and the step of the wafer mark 2, and the control accuracy of the gap δ. Ru.

Also, the wafer mark 2 which is the object plane of the objective lens 14
Although the resolution of the mask mark 3 located at a position shifted by the gap δ from the wafer mark 3 decreases by a very small amount due to the design, compared to the resolution of the wafer mark 2, this is an amount that can be completely ignored.

Next, signal processing for relative position detection will be explained. In the optical system shown in FIG.
The images detected at step 2 are converted into electrical signals, and become a wafer signal waveform 31 and a mask signal waveform 33. By quantizing these signal waveforms and subjecting them to digital signal processing, the relative positions of the wafer mark 1 and the mask mark 3 are determined. Here, the flow of digital signal processing is shown in FIGS. 4 and 5.

FIG. 4 corresponds to a wafer signal waveform 31 and a mask signal waveform 33 which have been made into plates, and this digital waveform is subjected to first-order differentiation processing to become a digital waveform as shown in FIG. The relative position is given by the first differential data shown in Figure 5 as A (W, P)=ΣV (P+j)XV
(P0W+j) (5) It is determined by a correlation equation called similarity pattern matching defined as follows.

The similarity pattern matching is a function that is extremely sensitive to the width W of the left and right patterns, and the correlation value A (W,
The width W of the left and right patterns where P) is the maximum value is taken as the distance between the marks. Note that this process has already been patented as a "method for detecting alignment mark position" by the inventor of the present invention.

(See Japanese Patent Application No. 63-242598.) Next, the thickness and resolution of the reference reticle will be explained. Wafer mark 2 generated on both sides of reference reticle 16
The image of the mask mark 3 is an intermediate image formed by the objective lens 14, and the thickness δ·N2 of the reference reticle 16 affects the resolution of the first relay lens 19 and the second relay lens 21. There is no.

Next, a specific design example is shown in FIG.

■Gap δ=30μm ■Objective lens 14 N, A, = 0. 35 Working distance (W, D,) = 20mm Magnification = 10× ■Reference reticle 16 Thickness δ・N2=3mm Double-sided anti-reflection coating Wafer reference mark 15 and square Reference mark 17 = double layer chrome.

Thickness = 03μmt Mark width = 7LLm (7/10 = 0.7μm on the wafer) ■First. Second relay lens 1921 N, A, = 0.2 Working distance (W, D,) = 20 mm Magnification = 10 x ■ Line sensor 20, 22 Number of elements = 2048 Element spacing = 14 μm Element width = 200 μm Clock frequency = 12 M Hz (maximum) ■ White light source 11 50W halogen lamp ■ Half mirror 12.18 Reflectance and transmittance = 50 + 50 In addition to the above ■ ~ ■, be careful to place an aperture at an appropriate position in the detection optical path. It is necessary to prevent the reflected light from the inner surface of the lens barrel from becoming flare, which overlaps the imaging light flux and reduces the contrast.

Finally, the features of the above embodiments will be summarized.

(1) Using a normal lens, bifocal detection using white light was made possible by using a reference reticle in the optical path.

(2) The detection accuracy of the relative position in the direction perpendicular to the focal plane is
It is completely unaffected by the displacement of the reference reticle.

(3) The distance between the dual focal points can be freely changed by changing the thickness of the reference reticle.

(4) Since Rayleigh's definition equation for white light holds independently for each bifocal plane, the required lens resolution can be freely set regardless of the distance between the bifocals.

(5) White light illumination is completely unaffected by wafer process gap variations and X-ray mask membrane thickness variations.

(6) Since no special optical parts are used, the device can be constructed at an extremely low cost.

(7) Since no special optical technology is used, there are no technical problems.

[Effects of the Invention] As explained above, the bifocal device using the reference reticle of the present invention can be used as an X-ray mask in an X-ray stepper that is considered promising for exposure of next-generation LSIs with a minimum pattern line width of 05 μm or more. It is applied to an alignment device that accurately measures the relative position of a wafer, and can measure the relative position of a wafer and an X-ray mask in a direction perpendicular to the optical axis, which are separated by approximately 30 μm, without being affected by the wafer process. It becomes possible to detect with accuracy (on the order of 0.01 LLm).

The position signal from the alignment device is sent to the drive device as a feedback signal for the ultra-precision stage that moves the X-ray mask and wafer, and accurately controls the relative position of the X-ray mask and wafer to a predetermined positional relationship. becomes possible.

[Brief explanation of drawings]

FIG. 1 is an optical path diagram showing the configuration of a bifocal device using the reference reticle of the present invention, FIG. 2(A) and FIG. 3(A) are line segments showing the image formation pattern on the line sensor, Figures 2 (B) and 3 (B)
) is a waveform diagram, Figures 4 and 5 are waveform diagrams showing signal processing, Figure 6 is an objective lens, wafer, and mask, and Figure 7 is a graph showing the relationship between depth of focus and numerical aperture. 8 to 10 are a side view and a perspective view of a conventional alignment device, and FIG. 11 is an optical path diagram of a bifocal lens. 1... X-ray mask 2... Wafer mark 3... Mask mark... White light source 2118... Half mirror 5... Wafer reference mark 6... Reference reticle 7... Mark Sansho mark 9.21... Relay 0.22... Line sensor lens

Claims (2)

[Claims] (1) In a position detection device that detects the relative position of a first object and a second object in a direction perpendicular to the optical axis, which are separated by a minute distance in the optical axis direction, A reference reticle, which has a thickness corresponding to the distance and determined from the magnification of the objective lens, and which has marks necessary for detecting the positions of the first object and the second object on both sides, is placed between the two of the objective lenses. A bifocal device using a reference reticle, characterized in that it is provided at an imaging position. (2) In order to further magnify the images of the first object and the second object formed on both sides of the reference reticle, two lenses are arranged with optical paths branched so that the focal points match on both sides of the reference reticle. A bifocal device using the reference reticle according to claim 1.