KR100363734B1 - An electron-beam exposure system, a mask for electron-beam exposure and a method for electron-beam exposure - Google Patents

Abstract

SUMMARY OF THE INVENTION The present invention provides an electron beam exposure process and an electron beam exposure system, wherein the electron beam exposure system has a scattering angle limitation with a mask comprising a scattering region and a limiting aperture that limits the amount of scattered electrons passing through the mask. A type electron beam exposure system, comprising: a first limiting aperture fixed around a crossover surface, the first limiting aperture having a central opening and a closed extended opening surrounding the central opening, and capable of phase shifting along the optical axis; And a second limiting aperture having an opening and a closed extended opening surrounding the central opening. The present invention also provides a stencil mask suitable for the exposure system and the exposure process. According to the present invention, the proximity effect correction can be adjusted to be excellent in line width accuracy without reducing the output, especially in the patterning step of the process of manufacturing the semiconductor device. The mask according to the present invention may be provided with an accurate mask pattern so that a pattern exposure with good resolution and accuracy can be performed.

Description

Electron beam exposure system, mask for electron beam exposure system and electron beam exposure method {AN ELECTRON-BEAM EXPOSURE SYSTEM, A MASK FOR ELECTRON-BEAM EXPOSURE AND A METHOD FOR ELECTRON-BEAM EXPOSURE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam exposure system, a mask for an electron beam exposure system, and an electron beam exposure method for use in a semiconductor device, and more particularly, to an electron beam exposure system suitable for correction of proximity effect, a mask for an electron beam exposure system, and an electron beam exposure method.

In electron beam exposure, the proximity effect due to scattered electrons in the resist layer and the substrate has a significant effect on the precision of the line width of the pattern. Therefore, correcting the proximity effect is one of the important technical factors.

In cell projection lithography, the most common electron beam exposure process, the dose compensation method is used, which employs an exposure intensity distribution (EID) function or a pattern density method. perfect quantitation by a self-consistent method using the method.

On the other hand, in the case of scattering by controlling the angle in the projection beam lithography, which is attracting attention as the next generation electron beam exposure technology, the proximity effect causes the GHOST method (i.e., SCALPEL) The correction is performed by the compensation method used as the correction beam to which the GHOST method is applied.

The scattering angle limited electron beam exposure process uses a segmented transfer technique, in which a required pattern of the entire chip to be formed or one of several regions is separated into a plurality of segments, and a mask is partially applied to each segment. It consists of a pattern of, and by using a mask, the exposure is made for each segment and finally transfers the desired pattern onto the wafer.

The mask used in the scattering angle limited electron beam exposure process refers to a mask formed on an electron beam transmission film (hereinafter referred to as a scattering film mask) in which a pattern constituting the electron beam scattering body does not scatter electrons so much. The wafer is exposed to an electron beam which is not scattered or scattered at a relatively small angle after passing through the electron beam transmission membrane. Thus, a contrast in shape is formed on the wafer due to the difference in electron beam scattering between the film and the scattering region.

The proximity effect in the scattering angle limited electron beam exposure process selectively selects a portion of the electrons scattered by the scatterer on the scattering mask mask into an annular opening formed in a limiting aperture disposed in the crossover surface. The projection is made by defocusing the projected and scattered electrons into a roughly backscattering range due to spherical and chromatic aberration of the object lens, and then emitting electrons as a correction beam to the wafer. Such proximity effect correction techniques are disclosed in G. P. Watson et al., J. Vac. Sci. Technology., B, 13 (6), 2504-2507 (1995). In the feature of the above method, the proximity effect is corrected by simultaneously performing exposure of the pattern and exposure of correction. On the other hand, with the GHOST method according to the prior art, the wafer is separately exposed to the defocused beam of the inverse pattern with respect to the original exposure pattern. Therefore, the above-mentioned proximity effect correction by the correction exposure performed simultaneously with the pattern exposure may make a significant contribution to the improvement of the yield.

However, the conventional proximity effect correction technique in the scattering angle limited electron beam exposure process has the following problems.

The proximity effect is determined by the shape of the substrate and the mask pattern. Thus, when performing exposure using a substrate made of a heterogeneous material or a mask of a heterogeneous pattern, the correction dose must be readjusted for proximity effect correction suitable for the substrate or mask. In the case of using a mask having electron scatterers of different thicknesses, the limiting aperture should be replaced with a limiting aperture having a different opening size. However, the correction dose is adjusted by changing dimensions such as the size and width of the annular opening formed in the orthopedic opening. Therefore, in order to optimize the correction dose, it is necessary to prepare an electron beam exposure and another limiting aperture which must be arranged after opening the chamber in the air to break the vacuum state. Therefore, there is a problem according to the prior art that attempting the optimum proximity effect correction involves a significant reduction in the yield.

Moreover, the above-described scattering film mask used in the conventional scattering angle limited electron beam exposure process has the following problems.

First, since the transmitted electrons are also scattered in the electron beam transmission film, the energy distribution of the image forming electrons is diffused to cause chromatic aberration, resulting in blur of the beam. In order to minimize the blurring of the beam, the beam convergent semi-angle should be reduced. However, the reduction of the beam convergent semi-angle causes the Coulomb effect to be large, reducing the resolution. The coulombic effect can be minimized by reducing the current in the beam. However, the above results in a longer exposure, resulting in a decrease in yield. Thus, the scatter mask does not provide adequate electron exposure.

Second, the scattering mask is prepared by forming a patterned heavy metal film such as tungsten (about 50 nm) on a thin film (about 100 nm) silicon nitride film. Therefore, the preparation is difficult and the yield is poor.

In addition to the above problems, the above-described proximity effect correction technique has the following problems.

When a lower pattern such as an interconnection wiring composed of a heavy metal such as tungsten is formed on the base film of the resist film on the wafer surface, the image forming electrons are reflected or reverse scattered by the lower pattern. As a result, a difference in proximity effect occurs between a resist region at the top of the region without the lower pattern and a resist region at the top of the region with the pattern. In the conventional proximity effect correction method, it is not easy to adjust the correction dose for each region corresponding to the lower pattern, and furthermore, such attempts have not been made.

The mask used for conventional cell transmission or the system used above (cell projection type electron beam exposure system) will be described below.

Conventional cell transmission (or exposure system) typically employs a mask (hereinafter referred to as a stencil mask) prepared by forming an opening pattern on a substrate that blocks an electron beam, such as a silicon substrate having a thickness of at least 20 μm.

As patterns become more precise in correspondence with more integrated semiconductor devices, stencil masks composed of thick substrates face the following problems. When preparing the mask, it is not easy to accurately form the opening pattern on the silicon substrate with a thickness of at least about 20 μm, so that a change in dimension occurs. Moreover, in electron beam exposure, the mask absorbs the electron beam and is heated accordingly to reduce durability, and thermally expands to improperly change the mask position. In addition, since it is necessary to increase the acceleration voltage for improving the resolution by reducing the aberration of the electro-optical system, the mask substrate becomes thicker and the above problems become more serious.

A relatively thin mask improves the linewidth accuracy of the opening pattern and reduces heat generation while electrons to be blocked pass through the mask substrate area (non-opening area). As a result, areas of the unexposed wafer are exposed, resulting in poor contrast and reduced resolution.

In order to solve the above problems, JP-A 10-97055 includes the steps of forming an electron beam scattering layer on the back of the mask to form an opening pattern on a relatively thin mask substrate and scatter the electron beam passing through the mask. Disclosed is a mask for electron beam exposure manufactured by a process. The electron beam scattering layer may be a polycrystalline layer or a wavy layer, such as polycrystalline silicon, tungsten silicide, molybdenum silicide and titanium silicide. The electron beam scattering layer may be formed to scatter electrons passing through the pattern layer of the mask (non-opening substrate region) so that the electrons do not reach the wafer.

JP-A 6-163371 shows an electron beam drawing device, in which an opening is formed on a substrate of a thickness shorter than the depth through which electrons are transmitted to provide an electron beam shaping aperture used as a mask. And a mechanism for blocking scattered electrons passing through the substrate region of the orthopedic aperture (mask) is provided. In the disclosed invention, a mechanism is provided for blocking scattered electrons through the substrate region of the orthopedic aperture. For example, a small aperture opening plate is provided on the crossover surface to pass only the electron beam passing through the mask opening and remove scattered electrons in the mask substrate region. Another blocking mechanism is also disclosed, in which an energy filter is formed that changes the direction of the slowed electrons that lose some of the energy after passing through the mask substrate that is subsequently removed by the limiting aperture.

JP-A 6-163371 discloses that the above-described shaped apertures (masks) are relatively transparent to masks used for electron beam projection lithography, which are exposed to uniform electron beams to perform exposure to large shapes at once, such as electron beams as support layers. It has been shown that the first problem of the mask to be prepared can be solved by forming a pattern made of heavy metal capable of significantly scattering the electron beam on the (transmissive) film.

It is an object of the present invention to provide a scattering angle limited electron beam exposure system and a scattering angle limited electron beam exposure system in which correction for the proximity effect can be adjusted with excellent line width accuracy without reducing the yield.

It is another object of the present invention to provide a stencil mask in which exposure of a pattern suitable for scattering angle limited electron beam exposure can be performed with correction of proximity effect. It is a further object of the present invention to provide a mask in which an accurate mask pattern can be easily provided and the exposure of the pattern is performed with improved resolution and accuracy. Another object of the present invention is to provide a mask that allows the proximity effect to be optimally corrected according to the lower pattern of the wafer.

It is still another object of the present invention to provide an electron beam exposure system and an electron beam exposure process that performs a very good exposure with a correct pattern and accurate pattern and allows a proximity effect to be corrected along with the pattern exposure to obtain an improved yield. It is another object of the present invention to provide an electron beam exposure process that allows the proximity effect to be optimally corrected according to the lower pattern of the wafer.

The present invention provides a scattering angle limited electron beam exposure system having a mask comprising a scattering region and a limiting aperture for controlling the amount of scattered electrons passing through the mask. The electron beam exposure system;

A first limiting aperture fixed near the crossover surface and including a central opening and a closed extended opening;

It is capable of phase shifting along the optical axis and includes a central opening and a closed elongated opening that surrounds the central opening.

The present invention also provides a scattering angle limited electron beam exposure system having a mask including a scattering region and a limiting aperture for controlling the amount of scattered electrons passing through the mask, wherein the electron beam exposure system comprises;

A limiting aperture changing part in which a plurality of limiting apertures having different sizes of the closed and extended openings are arranged or manufactured;

And a mechanism for actuating said limiting aperture changer to place a desired aperture of said plurality of limiting apertures in an optical system.

SUMMARY OF THE INVENTION The present invention provides a scattering angle limited electron beam exposure system having a mask comprising a scattering region and a limiting aperture for controlling the amount of scattered electrons passing through the mask, wherein the electron beam exposure system comprises;

A first limiting aperture fixed to the optical system with an opening only in the center thereof;

A limiting aperture changing part having a plurality of second limiting apertures having an outer diameter larger than the first limiting aperture, and having a central opening of different sizes;

And a mechanism for actuating said limiting aperture changer to position a desired second limiting aperture on said axis of said first limiting aperture among said plurality of second limiting apertures.

SUMMARY OF THE INVENTION The present invention provides a scattering angle limited electron beam exposure system having a mask comprising a scattering region and a limiting aperture for controlling the amount of scattered electrons passing through the mask, wherein the electron beam exposure system comprises;

A first limiting aperture changing part including a plurality of limiting apertures A having a central opening different from each other only in the outer circumference thereof, instead of the first limiting aperture;

A second limiting aperture changing portion having a plurality of limiting apertures each having a central opening having a different size and having a larger outer diameter than the limiting aperture;

A mechanism for operating the first and second limiting aperture changers to position the desired limiting aperture of the plurality of limiting apertures and the desired limiting aperture of the plurality of limiting apertures on the same axis of the optical system; It includes.

The present invention provides a scattering angle limited electron beam exposure process using a scattering angle limited electron beam exposure system according to the present invention, the exposure process;

In order to control a correction dose and adjust the amount of scattered electrons through the openings of the first and second limiting apertures for the purpose of controlling the proximity effect simultaneously with the exposure of the pattern, the limiting apertures are Phase shifting the second limiting aperture along the optical axis while failing to block the center of the electron beam trajectory forming an image corresponding to the outermost periphery of the pattern on the mask.

According to the present invention described above, in particular, the proximity effect correction can be easily adjusted, the yield is improved, and a very good line width accuracy can be obtained in the pattern process in manufacturing the semiconductor device.

The present invention uses a mask comprising scattering regions to perform pattern exposure by scattering contrast due to differences in electron beam scattering, while using some of the electrons scattered by the mask to correct proximity effects. Also providing an electron beam exposure mask for use in a scattering angle limited electron beam exposure process comprising:

The mask substrate has a scattering region whose thickness is shorter than the depth of electron transmission, and comprises a region corresponding to the backscattering range of the image forming electrons of the wafer,

Patterned openings are formed in the scattering area.

The present invention also provides the above-described electron beam exposure mask, wherein the electron scattering film is formed at least in the scattering region of the mask substrate.

The present invention also provides the above-described electron beam exposure mask in which the thickness of the scattering region varies with backscattering in which the lower pattern of the wafer is affected.

SUMMARY OF THE INVENTION The present invention provides a scattering angle limited electron beam exposure system comprising a mask, wherein a central opening and a closed extended opening surrounding the central opening are provided to control the amount of mask-scattered electrons passing through the mask. The formed limiting aperture may use some of the scattered electrons to correct the proximity effect simultaneously with the pattern exposure.

The present invention includes using a mask having scattering regions to execute an exposure pattern by scattering contrast due to a difference in electron beam scattering, while using some of the electrons scattered by the mask to correct the proximity effect. The mask is prepared by forming a scattering region on a mask having a thickness shorter than an electron transmission depth, and includes a region corresponding to the backscattering range of the image forming electrons of a wafer and patterning the scattering region. Formed openings.

The present invention also provides the above-described electron beam exposure process in which the thickness of the scattering region varies with backscattering in which the underlying pattern of the wafer is affected.

The present invention described above can provide a stencil mask suitable for a scattering angle limited electron beam exposure process and the proximity effect is coincident with the pattern exposure. The mask of the present invention can be easily provided with an accurate mask pattern, so that the resolution of the pattern exposure is high and the accuracy is high. Moreover, the present invention can provide a mask in which the proximity effect can be optimally corrected according to the lower pattern of the wafer.

In addition, the present invention provides an electron beam exposure system and an electron beam exposure process with high output, high resolution, and pattern accuracy, thereby enabling proximity effect simultaneously with exposure of the pattern. In addition, the present invention can provide an electron beam exposure system and an electron beam exposure process in which the proximity effect can be optimally corrected according to the underlying structure of the wafer.

1 is a conceptual diagram of an optical system showing a basic concept of the present invention.

2 is a conceptual diagram showing the basic principle of the proximity effect correction of the present invention.

3 is a conceptual diagram showing the basic principle of the proximity effect correction of the present invention.

4 is a conceptual diagram of an optical system showing a process of proximity effect correction of the present invention.

5 is a conceptual diagram of an optical system showing a process of proximity effect correction of the present invention.

6 is a conceptual diagram illustrating the intensity distribution of scattered electrons at a second limiting aperture of the optical system of FIG.

7 is a graph showing the phase shift of the correction dose versus limiting aperture according to an embodiment of the present invention.

8 illustrates a limiting aperture of an electron beam exposure system of the present invention.

9 illustrates a limiting aperture of an electron beam exposure system of the present invention.

Fig. 10 is a diagram showing the structure of an electron beam exposure mask of the present invention.

Fig. 11 is a diagram showing the structure of an electron beam exposure mask of the present invention.

Fig. 12 is a process cross sectional view showing a step of manufacturing an electron beam exposure mask according to the prior art.

Fig. 13 is a process cross-sectional view showing a step of manufacturing an electron beam exposure mask of the present invention.

Fig. 14 is a process cross sectional view showing a step of manufacturing an electron beam exposure mask of the present invention.

Fig. 15 is a cross-sectional view showing a step of manufacturing the electron beam exposure mask of the present invention.

<Description of the symbols for the main parts of the drawings>

1 mask 2 first projection lens

3: limiting aperture

4 second projection lens 5 wafer

6: resist

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

The basic configuration of the present invention will be described with reference to the conceptual diagram for the optical system of FIG. The electrons for image passing through the mask 1 are focused by the first projection lens 2 and then pass through the central opening of the limiting aperture 3 arranged in the crossover plane, that is, the reverse-focal plane, An image is formed in the resist on the wafer 5 by the second projection lens 4 as the object lens. The resist 6 of FIG. 1 is of a negative type in which an irradiation area remains, and the figure shows the shape after development for the purpose of illustration. Positive resists may also be used in the present invention. The first and second projection lenses constitute doublet optics.

On the other hand, most of the electrons scattered by the mask 1 are blocked by the limiting aperture 3 and some of the electrons pass through the central opening and the closed extending opening that surrounds the central opening. The transmitted scattered electrons are defocused into a roughly backscattered area by spherical and chromatic aberration of the second projection lens 4 (object lens), and impinge on the wafer as a correction beam. The central and closed extending openings may be arranged concentrically and the closed extending openings may be annular or polygonal, such as rectangular and square. It is usually annular but may be a polygon such as a rectangle or a square, taking into account the material of the opening or the manufacturing conditions. Ribs are provided to connect the interior of the opening, which is normally closed with the periphery. While the necessary proximity effect correction is performed, the ribs expand to cause partial closure of the closed and extended openings.

When the limiting aperture is fixed to the crossover surface, the intensity of the correction beam (correction dose proportional to the intensity) is adjusted by changing the area of the closed and extended opening. The range of out of focus is adjusted by changing the distance of the closed and extended opening from the center of the limiting aperture, and by changing its size for the annular opening. Because the area of the opening is larger in the closed opening than in the central opening, the proximity effect correction is almost dependent on the scattered electron passing through the closed opening in view of the angular distribution in the scattered electron.

The mask used in the present invention is preferably a scattering film mask described above in which a pattern composed of an electron beam scattering body is formed on an electron beam transmissive film mask that hardly scatters electrons. The film preferably contains optical elements, such as Sin and SiC, which hardly scatter electrons. The scatterers are preferably composed of heavy metals such as tungsten, tantalum, chromium, molybdenum, titanium, gold and platinum, which tend to scatter electron beams. The film of the mask is in the range of about 0.1 to 0.2 μm in thickness at an acceleration voltage of 100 kV.

The mask used in the present invention may be any scattering stencil mask to be described later.

The basic principle of proximity effect correction will be described with reference to FIGS. 2 and 3.

2 (a) shows a scattering mask, in which the element number of the film is 21 and the sign of the scattering body is 22. FIG. FIG. 2 (b) shows the distribution of energy deposition in the resist on the wafer in the case of using a limiting aperture without a closed, extended opening and without using a correction beam, for example without correction of proximity effect, FIG. 2 (c) shows the distribution of energy deposition in the case of using a limiting aperture with a closed and extended opening and using a correction beam, for example, correction of proximity effect. In the figure, βb is the reverse scattering range. Assuming that the energy of the forward scattering electrons is 1, the reverse electrons have an energy corresponding to the reverse scattering coefficient η. The value η should be compensated by the correction exposure, and strictly speaking, η / (1 + η) of the electrons passing through the scatterer (correction dose rate δ) should hit on the wafer.

The correction beam can be defocused into the coarse scattering range βb, ie L, to apply a constant deposition energy as shown in Fig. 2 (c), which is reduced near the boundary line of Fig. 2 (b). As a result, the linewidth accuracy of the pattern can be improved. As the line width of the elongated closure opening in the limiting aperture increases, i.e., as the opening area increases, the correction dose increases and as the size of the closed opening elongates, the aberration increases and the defocus increases further. Can be large.

In Fig. 3, the scattering mask of Fig. 2A is replaced with a line-space pattern of 1: 1, that is, a pattern having a pattern density of 50% is formed. As is apparent from Fig. 3, even when the pattern density changes, the proximity effect is similarly corrected, and as in the conventional GHOST technique, it is necessary to separately perform inverse scattering correction exposure which requires complicated calculations each time the pattern changes. none.

The basic concept of the present invention and the basic principle of proximity effect correction have been described. The features and principles of the present invention will then be described with reference to FIGS. 4 to 6.

In the prior art, the limiting aperture 3 is not scattered at the crossover plane (inverse focal plane) or only a small amount is scattered and the image forming electrons affecting the image formation are not blocked by the limiting aperture 3. Since it can pass through the central opening of the limiting aperture, it is fixed to the crossover surface. If the limiting aperture is excessively phase shifted along the axis, the image forming electrons are blocked by the limiting aperture so that the periphery of the image is distorted. The phenomenon will be described with reference to FIG. The limiting aperture 3 is phase shifted upward from S ₂ , and the electron beam trajectory forming an image corresponding to the outermost periphery of the pattern is almost completely blocked so that the periphery of the image is distorted. Therefore, in the prior art, it is necessary to fix the limiting aperture to the crossover surface. Moreover, since there is no need to move the limiting aperture in the process of the prior art, it is undoubtedly known technique to fix the limiting aperture.

On the other hand, the inventors consider not only the image forming electrons but the scattered electrons affecting the proximity effect correction, so that the spatial distribution of the scattered electron intensities (amount of scattered electrons) changes along the direction of the optical axis, It has been found that the amount of transmitted scattered electrons can be changed by phase shifting the limiting aperture along the optical axis from the crossover plane.

SUMMARY OF THE INVENTION The present invention provides a scattering angle limited electron beam exposure system having a mask comprising a scattering region and a limiting aperture that limits the amount of scattered electrons passing through the mask.

A first limiting aperture having a central opening and a closed extended opening that is secured proximate to the crossover surface and surrounds the central opening;

It is possible to change the phase according to the optical axis and includes a second limiting aperture having a central opening and a closed extended opening surrounding the central opening.

The second limiting aperture is, for example, the position of the second limiting aperture on the optical axis as long as the second limiting aperture does not block the center of the electron beam trajectory forming an image corresponding to the outermost periphery of the pattern on the mask. By providing a mechanism capable of controlling, the phase shift is possible along the optical axis according to the required correction dose.

In the above invention, a second limiting aperture capable of phase shifting on an optical axis is provided in addition to the first limiting aperture fixed close to the crossover surface. The second limiting aperture may adjust the amount of scattered electrons passing through an opening that is closed at both limiting apertures and phase shifted along the optical axis. That is, the correction dose can be adjusted.

FIG. 5 shows the trajectory of the scattered electrons emitted from the mask 1 and the optical system in which the first and second limiting apertures 3a and 3b are arranged. The closed extended opening of the second limiting aperture 3b has the same shape (annular) and is the same size as the first limiting aperture 3a. The opening in the center of the second opening 3b is slightly larger in size than the first limiting aperture 3a.

Considering the scattered electrons from A of the mask 1 in Fig. 5, the scattered electrons that can pass through the annular opening of the first opening 3a without the second limiting aperture 3b are the second limiting upper. It is clear that the blockage is blocked by the stop 3b. This is due to the relative positional relationship between the annular openings of the first and second limiting apertures 3a and 3b in the spatial intensity distribution for the electrons scattered at the position of the first limiting aperture 3a. .

The phase shift of the relative positional relationship between the annular openings as described above will be described with reference to FIG. Fig. 6 shows the spatial intensity distribution for the scattered electrons at the position of the first limiting aperture 3a fixed to the crossover surface. In Fig. 6 (a), the first and second limiting apertures 3a, 3b overlap at the same position. Hatched regions on both sides are due to scattered electrons passing through the annular opening of the limiting aperture, and centrally hatched regions are due to scattered electrons passing through the limiting aperture. The number of electrons reaching the wafer is calculated by the distribution x pi ² . Thus, the number of scattered electrons passing through the central opening is small, and the deterioration of contrast is not so great. Since the annular openings of the first and second limiting apertures are of the same shape and size, the range of the intensity distribution for the scattered electrons passing through the annular openings of the first and second limiting apertures completely overlaps. With respect to the center opening of the limiting aperture, the center opening of the second limiting aperture is located at the position indicated by the dot line because the center opening of the second limiting aperture is slightly larger than the center opening of the first limiting aperture.

In contrast, in FIG. 6, the second limiting aperture 3b is phase shifted upward along the optical axis from the position of the first limiting aperture 3a by the distance S as shown in FIG. 5. 6B shows the position of the opening of the second limiting aperture 3b. In FIG. 6B, the second limiting aperture 3b is phase shifted along the optical axis so that the annular opening p ₂ of the second limiting aperture 3b is the annular opening of the first limiting aperture 3a. Phase shift to the left with respect to the position of p ₁ ). Thus, the amount (intensity) of scattered electrons passing through both openings of the first and second limiting apertures is represented by the hatched area, wherein the area corresponding to the opening position p ₁ is the opening position p. _It overlaps with the area corresponding to ₂ ) and is reduced compared to Fig. 6 (a). Thus, the correction dose can be controlled by phase shifting the second limiting aperture 3b along the optical axis.

But. As mentioned above, excessive phase shift of the limiting aperture results in deterioration of the linewidth accuracy due to distorting around the image or inadequate image forming electrons on the wafer. Therefore, the limiting aperture is phase shifted along the optical axis or the size of the center opening of the limiting aperture is set long enough that the above problem can be avoided. That is, in the present invention, it is necessary to phase shift the limiting aperture within a range that does not block the center of the trajectory for the image forming electrons corresponding to the outermost periphery of the pattern of the mask yarn. Within this range (S ₀ to S ₁ of FIG. 4), the resist on the wafer is irradiated with suitable image forming electrons for energy deposition and the correction dose due to scattered electrons can be adjusted to the required value. Although the second limiting aperture is phase shifted upward from the first limiting aperture in the embodiment of FIG. 5, the second opening can be shifted downward from the first limiting aperture.

The size of the center opening of the second limiting aperture that is phase shifted is preferably larger than the opening of the center of the first limiting aperture that is fixed to phase shift the second limiting aperture to further avoid blocking of the image forming electrons. The central opening of the second limiting aperture may be set in a range (smaller than the inner diameter for the annular opening) that is physically closed and included in the extended opening. As shown in Fig. 4, increasing the size of the central opening can increase the range through which the image forming electrons can pass, thereby enlarging the range in which the second limiting aperture can be phase shifted. As a result, the correction dose can be changed in a wider range.

Even if the shape and size of the elongated closed mouth of the second limiting aperture are the same as or almost the same as the first limiting aperture, it can be appropriately changed as long as the adjustment to the desired correction dose cannot be prevented.

Although it is described to adjust the correction dose by phase shifting the limiting aperture along the optical axis, the crossover surface additionally blocks the center of the orbit relative to the image forming electron beam where the limiting aperture corresponds to the outermost periphery of the pattern on the mask. While not doing so, it can be phase shifted by changing the excitation of the first protruding lens 1. Phase shifting the crossover surface along the optical axis allows the correction dose to be adjusted accurately after positioning the second limiting aperture.

In the present invention, the intensity of the image forming electron beam corresponding to the outermost periphery of the pattern on the mask within a range in which the limiting aperture does not block the center of the trajectory for the image forming electron beam corresponding to the outermost periphery of the pattern on the mask. It is confirmed that (image forming electron intensity Is) is at least approximately 50% when the limiting aperture is at the crossover surface S ₀ . In the present invention, the imaging electron intensity I _S is preferably at least 70%, more preferably at least 80%, even more preferably at least 90%. In the present invention, it is preferable to keep the correction dose as high as possible the image forming electron intensity. Even when the image forming residual intensity Is is 95% or more, the correction dose can be appropriately adjusted. As described above, when the size of the center opening of the second limiting aperture larger than the first limiting aperture is changed by the image forming electrons blocked by the second limiting aperture, the correction dose can be changed in a wider range. Even if the phase shift with respect to the second limiting aperture is increased, it can be reduced or eliminated to ensure the appropriate intensity of the image forming electrons.

The invention has been described in terms of certain numerical values in connection with FIG. With beam convergent semi-angle d = 5 mrad, F1 = 160 mm, F2 = 40 mm, and mask pattern width W = 1 mm, a suitable range of change for the correction dose without blocking the image forming electrons It is good if 0.7 mm is chosen for the radius of the central opening of the second limiting aperture to ensure that.

FIG. 7 shows the variation of scattered electron intensity (correction dose) through the phase shift annular opening for the second limiting aperture. The ordinate represents the ratio of the correction dose when the correction dose is assumed to be 1 in the crossover surface S ₀ . The first limiting aperture has the size of the central opening as follows, i.e.

(Radius): 0.2mm, outside size of annular opening

(Radius): 7.04mm, inside size of annular opening

(Radius): 2.24mm, width of annular opening: 4.8mm.

On the other hand, for the second limiting aperture, the size (radius) of the center opening with respect to the annular opening is 0.7 mm and the size of the outside (radius) and the size of the inside (radius) are as shown for the first limiting aperture. . For example, when the phase shift S of the second limiting aperture is 95 mm, as shown in FIG. 7, the correction dose is reduced to about 5% (image forming electron intensity is maintained at about 100%).

Proximity effect correction has been described in which the correction dose is adjusted by controlling the amount of scattered electrons passing through the openings of the first and second limiting apertures. A limiting aperture changing mechanism will be described, which is equipped with a number of limiting apertures to facilitate changing the openings without breaking the void. The approach can be implemented alone or in combination. In the case where the above mechanisms are combined, it is preferable to use the limiting aperture changing mechanism described below instead of the first limiting aperture fixed near the crossover surface.

FIG. 8 is a plan view showing an embodiment of the limiting aperture changing unit 62, in which a plurality of limiting apertures 61 having different sizes of the closed and extended openings are arranged or manufactured. Rotate the member in one of the directions indicated by the arrow (rotating type in Fig. 8 (a)) of the required limiting apertures among the plurality of limiting apertures, or the arrow (sliding type in Fig. 8 (b)). The member is arranged in the optical system by phase shifting the member in one of the directions indicated by.

Although ribs are typically provided that connect the interior and the periphery of a closed elongated opening, the rib can be widened to cause the closed elongated opening to be partially closed.

9 illustrates another embodiment, in which FIGS. 9 (a) and 9 (b) are a plan view and a side view, respectively. The first limiting aperture 71 has an opening in the center thereof and is fixed to the optical system. The second limiting aperture 72 has a central opening whose inner diameter is larger than the outer diameter of the first limiting aperture 71. The second limiting apertures having different opening sizes are arranged or manufactured in one limiting aperture changing portion 73. The first and second limiting apertures may be arranged on the same axis to provide a limiting aperture that clearly defines the annular opening and the center opening surrounding the central opening and that can correct the proximity effect. The width of the annular opening can be changed by rotating the limiting aperture changing member 73 to select the required second limiting aperture.

Another embodiment is provided, in which the first limiting aperture changing member including a plurality of first limiting apertures having a plurality of first limiting apertures having an opening only at the center and having a different outer diameter, and having a different center size and having a different center size. A second limiting aperture changing member, in which a plurality of second limiting apertures are arranged or fabricated, the second limiting aperture comprising an opening larger than an outer diameter of the limiting aperture; and the first opening and the desired first of the plurality of first limiting apertures; And a mechanism for operating said first and second limiting aperture changing members for disposing a desired second opening of said second limiting aperture on the same axis in the optical system.

Using the limiting aperture changing mechanism described above, the limiting aperture can be selected from a variety of changes without breaking the space and opening the system to the shape, so that deviation of focus through aberration for correction dose or proximity effect correction is avoided. It can be properly calibrated.

An electron beam exposure mask (hereinafter referred to as a scattered stencil mask) of the present invention will be described.

10 illustrates an embodiment for a scattering stencil mask in accordance with the present invention. Fig. 10A shows the transmitted pattern 31 in line width, and Fig. 10B shows a cross-sectional view of the mask along line A-B in Fig. 10A. In the figure, βb is the backscattering range. The ratio of the mask size to the size of the transmitted pattern is set to 1: 1 to clearly show the relationship between the transmitted pattern and the mask.

The scattering stencil mask according to the present invention is a mask substrate provided with a patterned opening, and the mask substrate has a scattering area having a thickness shorter than the electron transmission depth, and the back scattering range (βb) of the image forming electrons of the wafer. ), The corresponding area is included.

The substrate of such a scattering stencil mask may be made of silicon or materials such as tungsten and molybdenum, but silicon is most preferably used.

An opening pattern of the scattering stencil mask is formed in a scattering region thinner than an electron transmission depth to the mask substrate. Electrons passing through the opening impinge on the wafer as image forming electrons, but electrons passing through a thin region (scattering region) of the substrate are scattered electrons, most of which are blocked by the limiting aperture on the crossover surface. Thus, the contrast of shape is formed on the wafer. On the other hand, some of the scattered electrons passing through the closed and extended opening of the limiting aperture impinge on the wafer as a correction beam.

The thickness of the scattering region of the scattering stencil mask according to the invention is chosen to scatter through the electron beam as appropriate. The upper limit for the thickness of the scattered area should be thinner than the electron beam range (electron penetration depth), preferably about half as thin as the depth of residue, and preferably 25 times thinner, even better. Should be as thin as 15 times, and even better, as much as 10 times the normal free pass. The lower limit for the thickness of the scattering area should be thicker than the normal freepass, preferably at least 1.5 times, more preferably at least 2 times, and even more preferably at least 3 times the normal freepass. . Since the electron penetration depth and the normal free pass depend significantly on the material and acceleration voltage of the mask substrate, the thickness of the scattered region is appropriately selected in consideration of the above factors. The normal free pass can be calculated using the equation described in Jpn. J. Appl. Phys., Vol. 10, p. 678 (1971). The thickness of the acid region of the mask substrate set as described above may preferably be selected to give on the wafer a beam contrast of at least 90%, more preferably at least 95%, more preferably at least 98%. . Moreover, factors related to manufacturing should be considered. For example, for a silicon substrate, the ratio of the shape of the opening is up to about 10, and therefore should be considered when determining the thickness.

In the scattering stencil mask of the present invention, the opening pattern is formed in the thin scattering region, so that the accuracy of the process is improved and most electron beams pass through the scattering region, thereby minimizing the heating of the mask due to the electron beam radiation. However, if the scattering region of the mask substrate is excessively thin, the mask is mechanically weak. Therefore, as shown in FIG. 10 (b), it is preferable that the area except the scattering area is thinner than the scattering area maintaining the mechanical strength of the mask, and in particular, the area is preferably at least twice the thickness of the scattering area. .

In the scattering stencil mask according to the invention, for example, if a silicon substrate is used and the accelerating voltage is 100 kV (electron penetration depth is about 67 μm), the thickness of the silicon substrate is, for example, in the range of 0.2 to 2 μm. The lower limit of the thickness of the scattering region of the silicon substrate is preferably at least 0.2 μm, more preferably at least 0.3 μm, and even more preferably at least 0.6 μm. The upper limit is preferably 5 μm, more preferably 3 μm, and even more preferably 2 μm.

In the scattering stencil mask according to the present invention, as the scattering region of the mask substrate becomes thin, the scattering region of scattered electrons decreases. Thus, the thickness of the scattered area is appropriately selected to prevent deterioration of the contrast due to scattered electrons with smaller scattering angles through the central aperture of the limiting aperture, or the electron beam scattering layer is used to increase the scattering area. It is preferable to be deposited on the scattering area of the mask substrate. It is preferable if the electron beam scattering layer is formed on the front, back, or both sides of the mask. The thickness of the electron beam scattering layer may be appropriately selected to transmit the appropriate amount of electrons and increase the scattering angle of the electrons while ensuring the line width accurately. For example, in the case of using a heavy metal such as tungsten, the thickness is preferably up to about 1 μm from a mechanical point of view, and at least 10 nm from the point of view of electron scattering. The electron beam scattering film may be composed of polycrystalline materials such as tungstem, chromium, molybdenum, titanium, gold and platinum or polycrystalline silicon, tungsten silicide, molybdenum silicide and titanium silicide. By forming the electron beam scattering layer as described above, the mask intensity can be improved. Furthermore, since the scattered electron energy decreases and the scattering angle is widened while the scattered electron density decreases, the electron beam scattering layer can be formed to give a relatively large width of the annular opening of the limiting aperture, thereby illuminating the limiting aperture. The accuracy of the line width of the opening can be ensured.

In addition, in the scattering stencil mask according to the present invention, it is essential that the mask substrate region corresponding to the backscattering range (hereinafter referred to as a backscattering region) for the image forming electrons of the wafer is thinner than the electron transmission depth. In other words, the scattering region of the mask substrate should be formed including the backscattering substrate region. If electrons are not transmitted or improperly scattered electrons are generated in the backscatter substrate region, the proximity effect is not satisfactorily corrected. As described above, since the scattered electrons must be focused out of the roughly backscattered area and radiated with the correction beam, the scattered electrons should be radiated from at least the mask substrate area corresponding to the backscattered area, that is, from the backscattered substrate area.

Referring to FIG. 11, another embodiment of an electron beam exposure mask (scattering stencil mask) will be described. FIG. 11 (a) shows a pattern 31 transferred for lines and widths and a bottom pattern 32 comprising a filling portion of gates and connecting lines and contact holes in a cell, for example made of a material of higher density than wafer material, FIG. 11B shows a cross section of the mask along line AB of FIG. 11A. In the figure, βb _si is the backscattering range due to the silicon substrate, and βb _W is the backscattering range where the lower pattern also affects. The ratio of the size of the transferred pattern to the mask size is set to 1: 1 to clearly indicate the relationship between the transferred pattern and the mask. The heavy metals constituting the bottom pattern may be heavy metals such as tungsten, copper, cobalt, titanium and molybdenum. The transferred pattern 31 of the above-described line and width corresponds to an upper layer pattern such as, for example, bit lines and aluminum connecting lines.

In the above embodiment, considering that the backscattering coefficient η and the backscattering range βb vary depending on the material, the line width for the transferred pattern can be more accurately controlled.

In this embodiment, in consideration of the back scattering not only affecting the lower pattern but also the material of the substrate, regions having different thicknesses of the mask are formed. The thickness of the region where the thickness of the mask is different may be selected according to the backscattering coefficient of the substrate material and the lower pattern. Further, the thickness of the region having a different mask thickness is selected in consideration of the region corresponding to the lower pattern as well as the region corresponding to the backscattering range which the lower pattern affects.

As shown in FIG. 11, the thickness of the mask is thinnest in the region R _W where the lower pattern 32 affects backscattering, _and most in the region R _{si that} is not affected by the lower pattern 32. Thick. The two regions R _W and R _si are thinner than the electron penetration depth to penetrate and scatter the electron beam, and the other region R _SP is thicker than the regions above to maintain the mechanical strength of the mask. The region R _W is thicker than the region R _si that emits more correction dose to the wafer to compensate for the increase in energy deposition due to backscattering that the underlying pattern affects. Therefore, the mask substrate region corresponding to the backscattering region affected by the lower pattern is made thinner to correct the proximity effect of the backscattering caused by the lower pattern, and also to improve the accuracy of the line width of the resist pattern after development. Let's do it.

In the embodiment shown in FIG. 11, in order to adjust the correction beam dose emitted on the wafer, the scattered electrons are partially changed by partially changing the thickness of the scattering region of the mask substrate according to the backscattering influenced by the underlying pattern. Controlled. Instead of changing the thickness of the scattering region of the mask substrate, another method may be used to partially control the correction dose, in which the electron beam scattering layer is partially formed on the scattering region to control some of the correction dose. do. For example, in FIG. 11, the total thickness of the scattering regions R _W and R _si is optimized for the region R _W corresponding to the backscattering region in which the underlying pattern affects, and the electron beam scattering layer is the region R _si . For example, a lower pattern is formed on other scattering regions other than the region R _W corresponding to the backscattering region to which it affects. The electron beam scattering layer allows the scattering angle of the scattered electrons to be increased, so that the electrons passing through the limiting aperture are reduced and thus the conservation dose is partially reduced. The electron beam scattering layer is made of the same material and thickness as the embodiment shown in FIG.

The electron beam exposure mask can be prepared by applying any one of the conventional manufacturing processes such as the manufacturing process for the stencil mask used in the cell-projection type of the electron beam exposure system.

A conventional process of making a stencil mask will be described with reference to FIG. 12, and then an embodiment of the process for an electron beam exposure mask (scattering stencil mask) according to the present invention will be described.

First, a resist layer is formed on the synthetic wafer 44 (Si / SiO ₂ / Si) and then patterned by a lithography process as shown in Fig. 12A. In the above, reference numerals 41 and 43 denote Si layers, and reference numeral 42 denotes SiO ₂ layers.

As shown in Fig. 12B, the Si layer 43 is dry etched using the patterned resist layer 45 as a mask.

Thereafter, after removing the resist layer, a silicon nitride film 46 is formed as a protective layer in preparation for the subsequent wet etching step. Thereafter, as shown in Fig. 12C, on the rear surface, a resist layer is formed and patterned to form a resist layer 47 having an opening window in the center.

Thereafter, as shown in Fig. 12 (d), the exposed silicon nitride film and Si layer in the opening are wet etched with an alkaline solution such as a Potassium hydroxide solution. The formed Si layer 41 having a sharp shape is due to the orientation of the Si layer. Thereafter, the exposed SiO ₂ film is removed by wet etching.

Thereafter, as shown in Fig. 12E, the resist layer 47 and the protective film 46 are removed, and the conductive film 48 made of, for example, gold, platinum or palladium is applied to a suitable technique such as sputtering. Is formed on the surface.

The electron beam exposure mask (scattering stencil mask) according to the present invention can be prepared by the above manufacturing process.

To partially change the thickness of the scattering region of the mask, the Si layer can be selectively removed by radiating the ion beam from the backside after formation of the mask. In addition, after the step of FIG. 12 (d), the resist layer 47 and the protective film 46 are removed, and then the Si layer is selectively selected before the ion beam is radiated from the front surface to form the conductive film 48 on the surface. To remove it.

For example, after the process of Fig. 12B, the resist layer 45 is removed, and then a resist layer is formed and patterned by a lithography process (Fig. 12A). Thereafter, dry etching is performed using the patterned resist layer 49 as a mask to form a portion of a thin region (Fig. 13 (b)). In addition, after the step of FIG. 12 (d), the resist film 47 and the protective film 46 are removed, and then, as described above, one resist layer is formed and patterned by a lithography process, and then, Dry etching is performed using the patterned resist layer 49 as a masco to form part of a thin thin film before forming the conductive film 48 on the surface.

The electron beam scattering film may be partially formed on the scattering area of the mask. That is, it may be formed as follows.

After the step of Fig. 12D, the resist layer 47 and the protective film 46 are removed. Thereafter, one resist layer is formed and patterned by a lithography process (Fig. 14 (a)). The electron beam scattering film 51 is formed on the patterned resist layer 50 (Fig. 15 (b)). Then, the electron beam scattering film 51 on the resist layer is removed together with the resist film 50 to provide a part of the electron beam scattering film before forming the conductive film 48 on the surface (Fig. 14 (c)).

Also, for example, after the step of FIG. 12 (d), the resist layer 47 and the protective film 46 are removed. The electron beam scattering film 51 is formed on the surface (Fig. 15 (a)). One resist layer is formed and patterned by a lithography process (Fig. 15 (b)). Thereafter, using the patterned resist layer 50 as a mask, it is etched to remove the unnecessary electron beam scattering film, and thereafter, the resist layer 50 is removed to form the electron film before the conductive film 48 is formed on the surface. Some scattering films are provided (FIG. 15 (c)).

According to the present invention described above, in particular, the proximity effect correction can be easily adjusted, the yield is improved, and a very good line width accuracy can be obtained in the pattern process in manufacturing the semiconductor device.

The present invention described above can provide a stencil mask suitable for a scattering angle limited electron beam exposure process and the proximity effect is coincident with the pattern exposure. The mask of the present invention can be easily provided with an accurate mask pattern, so that the resolution of the pattern exposure is high and the accuracy is high. Moreover, the present invention can provide a mask in which the proximity effect can be optimally corrected according to the lower pattern of the wafer.

In addition, the present invention provides an electron beam exposure system and an electron beam exposure process with high output, high resolution, and pattern accuracy, thereby enabling proximity effect simultaneously with exposure of the pattern. In addition, the present invention can provide an electron beam exposure system and an electron beam exposure process in which the proximity effect can be optimally corrected according to the underlying structure of the wafer.

Claims (21)

A scattering angle limited electron beam exposure system having a scattering region and a mask comprising a limiting aperture for limiting the amount of scattered electrons passing through the mask, A first limiting aperture fixed around the crossover surface, said first limiting aperture having a central opening and a closed extended opening surrounding said central opening; And a second limiting aperture having phase shifting along the optical axis, the second limiting aperture having a center opening and a closed extended opening surrounding the center opening. The method of claim 1, And the mask comprises a pattern composed of electron beam scatterers formed on the electron beam transmissive film. The method of claim 1, And said mask comprises an aperture pattern comprised of apertures in areas thinner than a depth through which electrons are transmitted. The method of claim 1, A limiting aperture changing portion in which a plurality of limiting apertures different in size from the closed extended opening surrounding the central opening are arranged or manufactured; And a mechanism for operating said limiting aperture changer to place a desired aperture in said engineering system as said first limiting aperture of said plurality of limiting apertures. The method of claim 1, A limiting aperture A having an opening only in the center and fixed to the optical system instead of the first limiting aperture, A limiting aperture changing portion having a plurality of limiting apertures B having a central opening of different sizes and having a larger outer diameter than the limiting aperture A; A scattering angle including a mechanism for operating the limiting aperture changing unit to arrange a desired limiting aperture B of the plurality of limiting apertures B on the axis of the limiting aperture A; Limited electron beam exposure system. The method of claim 1, A first limiting aperture changing part including a plurality of limiting apertures A having a central opening different from each other only in the outer circumference thereof, instead of the first limiting aperture; A second limiting aperture changing part having a plurality of limiting apertures B having a central opening of different sizes and having a larger outer diameter than the limiting aperture A; The first limiting aperture A of the plurality of limiting apertures A and the required limiting aperture B of the plurality of limiting apertures B are arranged on the same axis of the optical system. And a mechanism for operating the first and second limiting aperture modifiers. A scattering region and a mask comprising a limiting aperture for limiting the amount of scattered electrons passing through the mask, the limiting aperture comprising a scattering angle comprising a central opening and a closed extended opening surrounding the central opening; In a limited electron beam exposure system, A limiting aperture changing part in which a plurality of limiting apertures having different sizes of the closed and extended openings are arranged or manufactured; And a mechanism for operating said limiting aperture changer to place a desired aperture of said plurality of limiting apertures in said optical system. A scattering angle limited electron beam exposure system having a scattering region and a mask comprising a limiting aperture that limits the amount of scattered electrons passing through the mask, A first limiting aperture having an opening only in the center and fixed to said optical system, A limiting aperture changer including a plurality of second limiting apertures having a different diameter and having a larger outer diameter than the first limiting aperture; A scattering angle limited electron beam comprising a mechanism for operating said limiting aperture changer to place a desired second limiting aperture on said axis of said first limiting aperture among said plurality of second limiting apertures; Exposure system. A scattering angle limited electron beam exposure system having a scattering region and a mask comprising a limiting aperture that limits the amount of scattered electrons passing through the mask, A first limiting aperture changer including a plurality of first limiting apertures having an opening only in the center and having different outer diameters, A second limiting aperture changing portion including a plurality of second limiting apertures having a different diameter and having a larger outer diameter than the first limiting aperture, The first and second portions for arranging a first limiting aperture among the plurality of first limiting apertures and a second limiting aperture among the plurality of second limiting apertures on the same axis of the optical system; A scattering angle limited electron beam exposure system comprising a mechanism for operating a limiting aperture changer. A scattering angle limited electron beam exposure system using the scattering angle limited electron beam exposure system claimed in claim 1, In order to control a correction dose and adjust the amount of scattered electrons through the openings of the first and second limiting apertures for the purpose of controlling the proximity effect simultaneously with the exposure of the pattern, the limiting apertures are Phase shifting the second limiting aperture along the optical axis while failing to block the center of the electron beam trajectory forming an image corresponding to the outermost periphery of the pattern on the mask. Electron beam exposure process. The method of claim 10, And the correction dose can be further adjusted by changing the excitation state of the projection lens to shift the crossover surface along the optical axis. delete delete delete delete delete delete While using some of the electrons scattered by the mask to correct the proximity effect, using a mask comprising a scattering area to perform pattern exposure by scattering contrast due to the difference in electron beam scattering. A scattering angle limited electron beam exposure system comprising an electron beam exposure mask, Limiting apertures formed with a central opening and a closed elongated opening surrounding the central opening to control the amount of mask-scattered electrons passing through the mask are scattered electrons to correct proximity effects simultaneously with pattern exposure. A scattering angle limited electron beam exposure system, characterized in that using a portion of. In scattering angle limited electron beam exposure process, While using some of the electrons scattered by the mask to correct the proximity effect, using a mask with scattering regions to execute the exposure pattern by scattering contrast due to the difference in electron beam scattering, The mask is prepared by forming a scattering region on the mask having a thickness shorter than the electron transmission depth, and includes a region corresponding to the backscattering range of the image forming electrons of the wafer and forms a patterned opening in the scattering region. Scattering angle limited electron beam exposure step. The method of claim 19, A scattering angle limited electron beam exposure process, characterized in that the thickness of the scattering region changes with backscattering influenced by the lower pattern of the wafer. The method of claim 19, In the scattering region, the electron beam scattering layer is formed on a region other than the region corresponding to the backscattering region to which the lower pattern of the wafer affects.