JPH10302696A - Electron beam projection lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of the natural aberration characteristic of a projection lens by controlling a magnetic field on a mask and/or a sample surface to reduce the imaging aberration, and forming a crossover in a point where the electron beam incident on a first projection lens internally divides the distance between the mask and the sample in a prescribed ratio. SOLUTION: A magnetic field on a mask and/or a sample surface is controlled by a projection lens system for contracting and transferring the pattern of the mask to the sample surface in 1/N by use of two stages of projection lenses, or a first lens 3 and a second lens 4 to reduce the imaging aberration. A crossover is formed in a point where the distance between the mask and the sample is internally divided in N:1 by the electron beam incident on the first lens 3. Even when the main plane of the projection lens is moved by an additional magnetic field, for example, the magnetic field by the third lens 1 and the fourth lens 7, the crossover is formed in a prescribed position on the basis of the moving quantity, whereby the aberration can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron optical system used in a lithography apparatus for forming a high-density pattern having a fine line width which cannot be formed by an optical stepper with a high throughput. In particular, high performance imaging characteristics by immersing the mask and sample in the magnetic field of the projection optical system,
Further, the present invention relates to an electron beam optical system for obtaining high throughput characteristics.

[0002] In this specification, as a description of a positional relationship between element parts of an electron optical system, a portion closer to an electron beam source (for example, an electron gun) is referred to as a front stage and a portion closer to a sample (for example, a wafer) is referred to as a rear stage. Is used. In order to specifically represent these, the Z coordinate axis is taken as the mechanical center axis of the lens, and
The origin of = 0 was taken as the sample surface, and the electron beam source direction was set to a positive value. Furthermore, the direction of the main field of view described later was defined as the X axis, and the direction perpendicular thereto was defined as the Y axis.

[0003]

2. Description of the Related Art As a conventional technique for forming a high-definition pattern of this kind with a high throughput, a symmetric magnetic doublet type lens (for example, MBHeritage "Electro
n-projection microfabrication system "J.Vac.Sci.T
echol. Vol. 12, No. 6; 1975 P. 1135), PREVAIL
Method (HC Pfeiffer "Projection exposure with Vari
able Axis Immersion Lenses: A High-Throughput Elec
tron Beam Approach to “Suboptical” Lithography ”, J
pn., J.Appl.Phys.Vol. 34, Pt.1, No.12B 1995; P.66
85-6662) are known.

In the symmetric magnetic doublet system, two lenses which form a pair between a mask and a sample (generally, a wafer) and satisfy specific conditions (described later as symmetric magnetic doublet conditions) -a front lens and a rear lens The crossover of the system is formed at a position determined by the reduction ratio 1 / N, the main surface of the front lens is located at the midpoint between the mask and the crossover, and the main surface of the rear lens is It is located at the midpoint of the crossover and the sample. In the symmetric magnetic doublet type lens designed in this way, the aberration on the optical axis is reduced over a considerably wide image field.

[0005] On the other hand, the PREVAIL system is based on the following concept. Since it is impractical to project an entire memory chip within an allowable aberration in an electron optical imaging system, the pattern of the mask is divided into fields having a size within the allowable aberration range of the imaging system. (This is referred to as a sub-field of view), and the images of the sub-field of view are joined to form an overall image. As for the connection, a region where the selection of the sub-field of view can be mainly performed by deflecting the electron beam in one direction (this is the main field of view) is scanned by the deflector, and the connection between the main fields of view is maximally performed. This is performed by mechanical scanning of the wafer. Therefore, in order to obtain better device characteristics, a projection optical system having a sub-field of view as wide as possible and a main field of view as wide as possible is required. In response to this request, in Document A, in order to broaden the main field of view,
In other words, in order to improve off-axis imaging characteristics far from the optical axis, a deflector that generates an auxiliary magnetic field so that the magnetic field involved in off-axis imaging satisfies the paraxial magnetic field condition is provided. I have. That is, by applying an auxiliary magnetic field satisfying specific conditions, the 'optical axis' having less aberration is shifted off-axis from the original optical axis (mechanical center axis of the lens), and the off-axis aberration characteristic is changed. The aberration is set to the same degree as above. Furthermore, both the mask and the sample are immersed in a magnetic field to improve the paraxial imaging characteristics. An implementation example of this electron optical system is described in FIG. 4 of Document A, and AXIS SHIFTING YOKE is this auxiliary deflector.

The symmetric magnetic doublet condition (hereinafter abbreviated as SMD condition) referred to in the present invention means that the main plane of the front lens is located at the midpoint between the mask and the crossover, and the main plane of the rear lens is used. Is the sample and cross-over
It is at the midpoint of the bar. An N-fold similar shape of the subsequent lens centered on the crossover is point-symmetric with the lens of the previous stage centered on the crossover. The imaging field excitation condition means that the absolute values of the AT numbers are equal to each other and the current directions are opposite to each other.

[0007] The terms "main field of view" and "sub-field of view" are described. For this concept, see, for example, Japanese Patent Application No. 07-338372 filed by the present inventor.
The coordinate system is slightly different.

[0008]

However, when the PREVAIL system--that is, the operation of shifting the optical axis of the lens and the immersion of the mask and the sample in the magnetic field--is applied to the symmetric magnetic doublet system lens, the following problems occur. I found something. The Z-dependent on-axis magnetic field distribution of the on-axis magnetic field distribution Bz of the PREVAIL type lens is shown by a curve 30 on the right side of FIG. Bz is 0 at a position near the predetermined crossover 12, whereas a finite value that is not 0 at the mask position 2 and the sample position 5 to reduce aberrations--the mask side of the lens at the front stage or the rear stage. Has almost the same value on the sample side as the lens main surface. Therefore, the main surface of the lens is SM as shown by dotted lines 10 and 11.
It is shifted from the principal planes 8 and 9 satisfying the condition D to the mask side and the sample side. Values Hu and Hd on the Z-axis of the shifted main surface
Hu is obtained by making the value obtained by integrating Bz from a predetermined crossover point to Hu equal to the value obtained by integrating Hu to the mask, and the value obtained by integrating Bz from the sample to Hd and Hd by Hd is determined by making the value integrated up to a predetermined crossover point the same. Although the deviation is not so large, when the center of the sub-field of view is on the optical axis, the electron beam emitted from the mask in parallel to the optical axis does not pass through the crossover 12, and therefore satisfies the predetermined SMD condition. It was also found that the aberration was large. Next, when transferring the sub-field of view away from the optical axis, when the main field of view becomes 10 mm x 0.5 mm, the incident angle at the end of the main field of view becomes 5 mrad or more, and the pattern when the sample surface moves up and down Errors can no longer be ignored. That is,
The problem is that the landing angle is not perpendicular to the sample, and the position in the image plane changes due to the deviation of the height of the sample surface.

The present invention has been made in view of such a conventional problem. When the aberration is to be reduced by controlling the magnetic field on the mask surface or the sample surface, the original aberration characteristic of the projection lens is reduced. A method for preventing deterioration--that is, a method for optimizing aberrations and landing angles when the main lens surface is not positioned between the crossover and the mask surface or the sample surface, for example, as described above. -To provide

[0010]

Means for Solving the Problems To solve the above problems, the present invention uses the following means. As a first means, the pattern of the mask is divided by a projection lens of two stages--a first projection lens and a second projection lens--to the surface of the sample.
N. A projection lens system for reducing transfer to N, comprising a mask and / or
Alternatively, the imaging aberration is reduced by controlling the magnetic field on the sample surface, and a crossover is formed at a point where the electron beam incident on the first projection lens internally divides the mask and the sample into N: 1. I tried to do it.

As a second means, in the first means,
A third lens is disposed in front of the mask to generate an on-axis magnetic field in the same direction as the first projection lens, and a fourth lens is disposed downstream of the sample surface in the same direction as the second projection lens. An on-axis magnetic field distribution is generated in a direction opposite to that of the first and third lenses, thereby controlling the magnetic field on the mask or the sample surface.

As a third means, in the second means,
Instead of the fourth lens, a ferromagnetic plate was provided. As a fourth means, in the first to third means, an electron beam projection lens for dividing one field of view into a plurality of sub-fields and performing transfer while correcting the optical system for each sub-field. In the above, two stages of at least an X deflector are provided at the subsequent stage of the mask in order to transfer a sub-field of view away from the optical axis, and the chief ray vertically emitted from the mask is corrected so as to pass through the crossover; At least the X deflector in the stage is provided in front of the sample, and correction is performed so that the principal ray passing through the crossover is perpendicularly incident on the sample.

As a fifth means, in the fourth means,
A plurality of deflectors are provided between the mask and the crossover, and a plurality of deflectors are further provided between the crossover and the sample. In order to minimize the aberration, the arrangement of the plurality of deflectors, the deflection intensity, and the deflection angle in the rotational direction were optimized.

As a sixth means, in the first to fifth means, the N of the subsequent lens centered on the crossover is used.
The electron beam projection lens is characterized in that the double-like shape is point-symmetric with respect to the preceding lens and the crossover as a center. Irradiation was performed with a divergent electron beam.

As an eighth means, in the seventh means,
An electron beam projection lens, wherein at least two stages of X deflectors are provided in front of a mask so that a principal ray is perpendicularly incident on the mask. As a ninth means, a mask pattern
A two-stage projection lens--a first projection lens and a second projection lens--to transfer a 1 / N reduction image onto a sample surface, wherein the two-stage lens is a symmetric magnetic lens. A symmetric magnetic doublet-type lens that satisfies the doublet condition. In an electron beam projection lens that reduces imaging aberration by controlling a magnetic field on a mask and / or a sample surface, the sample surface is separated from a predetermined position. Aberration is reduced.

As a tenth means, in the ninth means, at least two stages of X-deflection between the crossover and the sample are performed in order to correct the vertical incident condition on the sample of the sub-field image outside the optical axis. The vessel was equipped.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is based on the fact that even if the main plane of the projection lens moves due to an additional magnetic field--for example, a magnetic field generated by the third lens 1 and the fourth lens 7--the amount of movement is determined. The aberration can be reduced by forming a crossover at a predetermined position, and the landing angle is adjusted so that the chief ray is perpendicularly incident on the sample while keeping the crossover point at the predetermined position by the deflector. It is based on finding out what can be done. FIG. 1 is a sectional view of the optical system of the embodiment when the reduction ratio is 1/2 (N = 2). As described above, the right side is the on-axis magnetic field distribution Bz
Shows the Z dependence of. Bz has a finite value other than 0 at the mask position 2 and the sample position 5 while it is 0 at the position of the crossover 12. Therefore, the main surface of the lens is SMD as shown by dotted lines 10 and 11.
From the principal surfaces 8 and 9 under the conditions, the surface is shifted to the mask side and the sample side. Now, consider the case where the center of the sub-field to be transferred is on the optical axis. If the electron beam is emitted from the mask in parallel with the optical axis, the electron beam does not pass through the predetermined crossover 12 and S
The MD condition is not satisfied, and the aberration is large. However, the condition for illuminating the sub-field is not a parallel beam but a slightly diverging beam, so that the beam can pass through the crossover 12 and the aberration can be reduced. Was. In this case, the angle of incidence on the sample at the end of the sub-field of view was 0.5 mrad or less (when the sub-field of view was 0.5 mm square on the sample).

Next, a case where a sub-field of view located at a position distant from the optical axis is transferred will be described. Also in this case, it was found that when the mask was irradiated with a divergent beam instead of a parallel beam, a crossover was formed at the crossover position, and the aberration was small. However, if the main field of view is 10 mm x 0.5 mm, the incident angle at the end of the main field of view becomes 5 mrad or more,
The pattern error when the sample surface moves up and down cannot be ignored. Therefore, in order to improve the landing condition, a divergent beam is formed by the lens 1, the chief rays are deflected by the deflectors 13 and 14 so as to be parallel to the optical axis, and passed through the crossover by the deflectors 15 and 16. Deflect. Further, the principal rays having passed through the crossover were deflected by the deflectors 17 and 18 so as to be perpendicularly incident on the sample 5. Therefore, the angle of incidence on the sample could be made as small as in the case of the sub-field on the optical axis.

Further, by optimizing the position or the deflection intensity ratio or the deflection direction of a plurality of deflectors 19 in addition to the deflectors 15 and 16 and in addition to the deflectors 19 and 17 and 18, the sample surface can be optimized. Was also able to minimize aberrations. Further, it has been found that, when the position of the sample is adjusted in the Z-axis direction with respect to the displacement of the main surface, the aberration is reduced, and an adjustment mechanism for the sample stage is provided.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 collectively shows the components of the above-mentioned solution of the present invention. The operation will be described below based on this and FIG. 600 mm between mask 2 and sample 5
, The positions of the dotted lines 10 and 11 on the main surface of the lens and SM
The difference between the positions 8 and 9 of the solid line satisfying the condition D was 10 mm and 5 mm for the mask-side lens and the sample-side lens, respectively. However, the reduction ratio was 1/2. Therefore, originally Z = 40
The main plane should be arranged at points 0 and Z = 100, and the crossover should be arranged at the point Z = 200, but Z = 410 and Z = 95.
The main plane is shifted. FIG. 2 shows the imaging conditions in this case.

The focal length of the lens 3 is f = 190 mm, and the focal length of the lens 4 is f = 95 mm. The imaging point of the beam passing through the crossover is 1/105 + 1 / b = 1/95 1 / b = 1 / 95-1 / 105 b = 997.5
The incident angle at the sub field end of 0.5 mm square is 0.25√2 / 997.5 = 0.35 mrad, and the vertical fluctuation of the sample surface of ± 5 μm is ± 1.75 nm.
Only the position error of On the other hand, the incident angle at the end of the main visual field of 20 mm is 10 / 997.5 = 10.03 mrad, and a vertical error of ± 5 μm causes a positional error of ± 50 nm.

The following equation holds for the incident beam. 1 / a + 1/210 = 1/190 ∴a = 1995 mm That is, there is a crossover at a position of Z = 1995 + 410 = 2405 mm, and it is understood that the mask should be irradiated with an electron beam diverging therefrom. In the above description,
For simplicity, the description has been made using the thin lens formula, but in practice, the values of a and b at which the aberration is minimized are obtained by computer simulation. The above is an example using the first, second, and seventh solving means. When the third solution is used, a ferromagnetic plate 6 is used instead of the second lens 7 in FIG.
With The lens system that satisfies the SMD condition is used. This is an example using the solution 6. next,
When transferring the sub-field of view away from the optical axis, the deflectors 13 and 14 are used.
The beam is bent so that the principal ray is incident on the mask 2 at a right angle (portion 22 of the principal ray 21 in FIG. 2), and the beams emitted from the mask in parallel to the optical axis are deflected by the deflectors 15 and 16 to Z = Adjust in the direction coming from 2405mm. Adjustment is also performed by the deflectors 17 and 18 so that the chief ray is perpendicularly incident near the sample surface (portion 23 of the chief ray 21 in FIG. 2). Here, the reason why the deflectors are provided in two stages is to eliminate beam position fluctuation on the mask and the sample surface even when the angle is changed. Naturally, the situation is the same in the θ direction. Therefore, the deflectors 13 to 18 have x and y, and the θ direction satisfies the condition of passing through the crossover and the condition of vertical incidence on the mask and the sample surface. This is an example using the fourth, eighth, and tenth solving means. In addition to these, a deflector 19 is provided between the mask and the crossover, and a deflector 20 is provided between the sample and the crossover, so that the deflectors 15, 16, 19, and 20, 17, 17,
The position, strength, or rotation direction of 18 was optimized by computer simulation to minimize the aberration of the image of the sub-field away from the optical axis. This is an example using the fifth solution.

Further, using a lens system that satisfies the SMD condition, placing the sample surface and the mask surface in a magnetic field, changing the position of the sample in various ways, and calculating the aberrations by simulation, the result is that the sample surface is shifted from the Gaussian surface. The minimum aberration was obtained when the lens was moved to the 30 μ crossover side. This is an example using the ninth means.

[0024]

As described above, according to the present invention, when the projection lens system of the electron beam with less aberration is used, and the magnetic field of the sample and / or the mask surface is added and controlled to increase the low aberration, Also, an electron beam projection lens system having good transfer characteristics can be obtained by preventing the characteristics of the original lens from being degraded by aberrations due to the influence of the added magnetic field.

[Brief description of the drawings]

FIG. 1 is a sectional view (center) and an axial magnetic field distribution (right side) of an electron beam projection lens according to an embodiment of the present invention.

FIG. 2 is an image diagram of an electron beam projection lens according to an embodiment of the present invention.

[Description of Signs of Main Parts]

1 ... third lens 2 ...
Mask 3 ... first lens 4 ...
Second lens 5 ... Sample 6 ...
Ferromagnetic plate 7 Fourth lens 8
Predetermined main plane 9 ... predetermined main plane 10 ...
Shifted main plane 11 ... shifted main plane 12 ...
Predetermined crossover 13, 14, 15, 16 ... Deflector 17, 18 ... Deflector 19, 20 ... Deflector 21 ... Chief ray of the sub-field of view at a position away from the optical axis Orbits 22... 21 deflected to satisfy the vertical incidence condition on the mask surface 23... 21 orbits satisfying the vertical incidence condition on the sample surface. 30 ... axial magnetic field distribution

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H01L 21/027 H01L 21/30 541B

Claims (10)

[Claims] A mask pattern is formed by a two-stage projection lens.
A projection lens system for performing 1 / N reduction transfer to a sample surface using a first projection lens and a second projection lens, and forms an image by controlling a magnetic field on a mask and / or a sample surface. An electron beam projection lens, wherein aberration is reduced and a crossover is formed at a point where an electron beam incident on the first projection lens internally divides the mask and the sample into N: 1. . A third lens disposed in front of the mask to generate an on-axis magnetic field in the same direction as the first projection lens;
Is disposed downstream of the sample surface to generate an axial magnetic field distribution in the same direction as the second projection lens and in the opposite direction to the first and third lenses, thereby reducing the magnetic field on the mask or the sample surface. 2. The electron beam projection lens according to claim 1, wherein the lens is controlled. 3. The electron beam projection lens according to claim 2, wherein a ferromagnetic plate is provided instead of the fourth lens. 4. An electron beam projection lens according to claim 1, wherein one field is divided into a plurality of sub-fields, and an electron beam is transferred while correcting an optical system for each sub-field. In the projection lens, at least two stages of X deflectors are provided at the subsequent stage of the mask to transfer the sub-field of view away from the optical axis, so that the principal ray emitted vertically from the mask passes through the crossover, and 2. An electron beam projection lens, wherein at least two stages of X deflectors are provided in front of a sample, and correction is performed so that a chief ray passing through the crossover is perpendicularly incident on the sample. 5. The apparatus according to claim 4, wherein a plurality of deflectors are provided between the mask and the crossover, and a plurality of deflectors are provided between the crossover and the sample. An electron beam projection lens, characterized by optimizing the arrangement of the plurality of deflectors, the deflection intensity, or the deflection angle in the rotational direction so that the image of the sub-field thus obtained has the minimum aberration on the sample surface. 6. A lens according to claim 1, wherein the similar shape of N times of the subsequent lens centered on the crossover is point-symmetric with the lens of the previous stage centered on the crossover. Electron beam projection lens 7. The electron beam projection lens according to claim 1, wherein the mask is irradiated with a divergent electron beam. 8. The electron beam projection lens according to claim 7, wherein at least two stages of X deflectors are provided in front of the mask so that the chief ray is perpendicularly incident on the mask. 9. The pattern of a mask is formed by a two-stage projection lens.
A projection lens system for performing a 1 / N reduction transfer onto a sample surface using a first projection lens and a second projection lens, wherein the two-stage lens is a symmetric magnetic doublet satisfying a symmetric magnetic doublet condition. An electron beam projection lens that reduces imaging aberration by controlling a magnetic field on a mask and / or a sample surface, wherein the electron beam projection lens reduces the aberration by moving the sample surface away from a predetermined position. 10. The electron beam according to claim 9, further comprising at least two stages of X deflectors between the crossover and the sample in order to correct the vertical field condition of the subfield image outside the optical axis on the sample. Projection lens.