JPH11219892A - Scanning type exposure system and measuring method of visual field stop position - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent an irregular exposure from occurring in a scanning type exposure system by a method wherein a means which measures the position of a visual field stop is provided, and projection optical systems which project an image of a mask pattern joining exposure regions together for exposure are corrected on positional deviation of a visual field stop. SOLUTION: The shape of a slit 20a provided to a slit plate 20 is scanned keeping the visual field stops 45 of projection optical systems stationary to measure the position of the visual field stops 45. That is, the edge position of the opening 45a of a visual field stop 45 is obtained resting on the position of a slit 20a obtained by a laser interferometer 57, a position where a photodetector sensor 50 varies in output and an ideal edge position of a visual field stop are compared with each other, whereby the deviation of the edge position of the opening 45a of the visual field stop 45 is detected, and the visual field stop 45 is corrected on a positional deviation by a visual field stop drive mechanism 56. By this setup, irregular exposure can be prevented from occurring.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a scanning exposure apparatus used for manufacturing a semiconductor element, a liquid crystal display element and the like, and a method of measuring a field stop position of the scanning exposure apparatus.

[0002]

2. Description of the Related Art A scanning exposure apparatus is used to manufacture a large-sized liquid crystal panel, a large-area semiconductor element, and the like at a high throughput. This scanning exposure apparatus illuminates a mask or a reticle (hereinafter, referred to as a mask) with a slit-shaped illumination area, scans the mask in the short side direction with respect to the illumination area, and scans the exposure area conjugate with the illumination area. The pattern on the mask is sequentially exposed on the substrate by synchronously scanning a photosensitive substrate (hereinafter simply referred to as a substrate) such as a glass plate or a semiconductor wafer coated with a photoresist.

In the case of a scanning type exposure apparatus, when the size of a processing substrate is increased, a method of increasing the exposure area of the projection optical system can be considered to increase the throughput.
As a method of expanding the exposure area without increasing the size of the projection optical system, a method in which the projection optical system is configured by a plurality of small projection optical systems, and the exposure areas of the plurality of small projection optical systems are joined and exposed is considered. ing.

[0004]

When a mask pattern is exposed on a substrate by splicing exposure areas by a plurality of projection optical systems, each projection optical system must be exposed in order to splice the exposure areas by the projection optical systems accurately. A field stop is provided in the system. FIG. 14 shows an exposure area 39 on a substrate determined by trapezoidal field stops provided in a plurality of projection optical systems.
, 39b, 39c,..., but when the positions of the field stops provided in the respective projection optical systems are shifted, the areas exposed so as to overlap with the edge portions of the adjacent field stops. A splice error occurs, which leads to uneven exposure.

FIG. 15 is an explanatory view showing an exposure state at a joint portion when a position of a field stop of an adjacent projection optical system is shifted from a normal position. FIG. 15A shows a case where the field stop of the adjacent projection optical system is at a normal position,
The exposure amount on the substrate is also constant at the joint between the exposure area 39a and the exposure area 39b. On the other hand, FIG. 15B shows a case where the position of the field stop of the adjacent projection optical system is shifted.
The exposure amount on the substrate changes (increases in the case of the figure) at the joint between the exposure region 39a and the exposure region 39b, and the exposure amount unevenness occurs. SUMMARY OF THE INVENTION The present invention has been made in view of such a problem, and corrects a position shift of a field stop of a plurality of projection optical systems that perform exposure of a mask pattern by splicing exposure areas, thereby reducing exposure unevenness. It is an object of the present invention to provide a scanning exposure apparatus that can prevent the occurrence of a scan and a method of measuring the position of the field stop.

[0006]

According to the present invention, there is provided a scanning exposure apparatus comprising: a mask stage for holding a mask; a substrate stage for holding a substrate; an illumination optical system for illuminating a plurality of illumination areas on the mask; A plurality of projection optical systems for projecting the image of the illumination area on the substrate, scanning means for scanning the mask stage and the substrate stage in synchronization with the projection optical system, and disposed in each projection optical system. A scanning exposure apparatus that includes a field stop and that exposes images of a plurality of illumination regions on a substrate by jointing the plurality of images is provided with a measuring unit that measures the position of the field stop.

In this scanning type exposure apparatus, the position of the field stop is measured by the field stop position measuring means.
Since the position of the field stop can be grasped, by correcting the position of the field stop to an appropriate position, a joint error due to an edge portion of the opening of the field stop is reduced, and exposure unevenness is eliminated. The field stop position measuring means can be constituted by a slit held on the mask stage and a photodetector installed on the substrate stage. In this case, it is sufficient that the slit held by the mask stage is shaped to match the shape of the field stop, specifically, the slit shape is made parallel to the edge of the exposure area by the field stop. It can be measured with an appropriate S / N ratio.

The method for measuring the position of the field stop according to the present invention is as follows.
In a method of measuring a position of a field stop of an exposure apparatus that performs exposure by joining a plurality of illumination regions on a mask on a substrate through a plurality of projection optical systems each having a field stop, a slit arranged at a position of a mask is provided. The position of the field stop is detected based on the relationship between the amount of light detected on the substrate and the position of the slit by moving the field stop relative to the field stop.
In this measuring method, the position of the field stop can be measured from the relationship between the amount of light detected on the substrate and the position of the slit by moving the slit disposed at the position of the mask with respect to the field stop. The position of the field stop can also be detected by stopping the slit at a plurality of positions and detecting the amount of light.

Further, it is also possible to dispose a slit at the position of the mask and move the field stop to detect the position of the field stop from the relationship between the amount of light detected on the substrate and the position of the field stop. At this time, by setting the position of the slit to the design position of the field stop edge, the position of the field stop can be set simultaneously with the detection of the position of the field stop. Furthermore, the position of the field stop can also be detected by stopping the field stop at a plurality of positions and performing light amount detection. Further, by setting the shape of the slit to the shape of the field stop, a sufficient S / N can be obtained.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing a scanning exposure apparatus according to the present invention, FIG. 2 is a perspective view showing the projection optical system shown in FIG. 1, and FIG. 3 is a lens configuration diagram of the projection optical system. FIG.
In, the exposure light emitted from a light source 2 such as an ultra-high pressure mercury lamp is condensed by an elliptical mirror 3, reflected by a dichroic mirror 4, and guided to a wavelength selection filter 5. Light having a wavelength necessary for exposure transmitted through the wavelength selection filter 5 is converted into a light beam having a uniform illuminance distribution by a fly-eye integrator 6 and shaped into a slit by a blind 7 having a slit-shaped opening. The slit-shaped light beam that has passed through the blind 7 is reflected toward the condenser lens 9 via the reflection mirror 8, and the condenser lens 9 forms an image of the light beam on the mask 10 so that the mask 10 forms the slit-shaped illumination area 38 a. Illuminate at ~ 38g. The pattern image on the mask 10 is
Is projected and exposed on the substrate 12 held on the substrate stage 14 through the substrate.

At this time, what is projected onto the substrate 12 is a slit-shaped pattern image (exposure areas 39a to 39g), which is an image of a part of the mask pattern formed on the mask 10. Therefore, in the scanning exposure apparatus, the mask 10
The carriage 15 in which the mask stage 13 holding the substrate 12 and the substrate stage 14 holding the substrate 12 are integrated is driven and controlled by the carriage control unit 17 and moved with respect to the illumination areas 38 a to 38 g on the mask 10 and the projection optical system 11. By scanning, all of the mask patterns formed on the mask 10 are transferred onto the substrate 12. The carriage speed measurement unit 16 measures the speed of the carriage 15, and the carriage control unit 17 controls the carriage 15 so that the speed of the carriage 15 measured by the carriage speed measurement unit 16 becomes equal to the speed given by the speed command signal from the host computer. The drive of the carriage 15 is controlled.

In FIG. 1, the mask 10 is illuminated with a light beam from the light source 2 using the reflection mirror 8 or the like. Alternatively, a plurality of slit-like illumination regions 38a to 38g are formed using an optical fiber or the like. May be illuminated. For example, as shown in FIG. 2, the projection optical system 11 has a configuration in which a plurality of small projection optical systems 32a to 32g are spliced together, and is an erect real image system composed of a plurality of projection optical systems 32a to 32g. An optical system is used. Here, the X axis indicates the moving direction of the carriage 15, the Y axis indicates a direction orthogonal to the X axis in the plane of the mask 10, and the Z axis indicates the normal direction (focus direction) of the mask 10. ing.

Each of these projection optical systems 32a to 32g has a configuration in which two sets of Dyson type optical systems are combined as shown in FIG. 3, for example.
It comprises a field stop 45 and second partial optical systems 46 to 49. The first partial optical systems 41 to 44 include the mask 1
A right-angle prism 41 having a reflection surface arranged at a 45 ° inclination facing 0 and a plano-convex lens having an optical axis along the in-plane direction of the mask 10 and having a convex surface facing the reflection surface of the right-angle prism 41 A component 42, a lens component 43 having a meniscus-shaped overall reflecting surface with a concave surface facing the plano-convex lens component 42 side, and a 45 ° And a right-angle prism 44 having a reflecting surface arranged at an inclination.

Exposure light from the illumination optical system through the mask 10 has its optical path deflected by 90 ° by the right-angle prism 41 and is connected to the plano-convex lens component 42 joined to the right-angle prism 41.
Incident on. A lens component 43 made of a glass material different from the plano-convex lens component 42 is joined to the lens component 42, and light from the right-angle prism 41 is refracted at a joint surface 42 a of the lens components 42 and 43. The light reaches the reflection surface 43a on which the reflection film is deposited. The light reflected by the reflection surface 43a is refracted by the joint surface 42a and reaches the right-angle prism 44 joined to the lens component 42. The light from the lens component 42 is deflected by 90 ° in the optical path by the right-angle prism 44 to form a primary image of the mask 10 on the exit surface side of the right-angle prism 44, that is, at the position of the field stop 45.

The light passing through the opening 45a of the field stop 45
Light from the next image forms a secondary image of the mask 10 on the surface of the substrate 12 via the second partial optical systems 46 to 49. The configuration of the second partial optical system is the same as that of the first partial optical system. The second partial optical systems 46 to 49 form the same magnification image in which the X direction is positive and the Y direction is negative, as in the first partial optical system. Therefore, the secondary image formed on the surface of the substrate 12 is an equal-size erect image of the mask 10 (an image having a positive horizontal magnification in the vertical and horizontal directions). Projection optical system 32a (first
And the second partial optical system) are double-sided telecentric optical systems.

The field stop 45 is, for example, an exposure area 39 shown in FIG.
a, 39b, 39c, trapezoidal opening 4 corresponding to ‥‥
5a, 45b, 45c, ‥‥, and the trapezoidal openings 45a, 45b, 45c, ‥‥ define the exposure area on the substrate 12 in a trapezoidal shape. That is, the projection optical system 3
2a to 32g have visual field regions 38a to 38g defined by the field stop 45 in the projection optical system. The images of the visual field regions 38a to 38g are formed as the same-size erect images on the exposure regions 39a to 39g on the substrate 12. Here, the projection optical systems 32a to 32d
8a to 38d are provided so as to be arranged along the Y direction in the figure. Further, the projection optical systems 32e to 32g are located at positions different from the visual field regions 38a to 38d in the X direction in the drawing.
The viewing areas 38e to 38g are provided so as to be arranged along the Y direction. The projection optical systems 32a to 32d and the projection optical systems 32e to 32g are arranged such that the right-angle prisms are located very close to each other.

On the substrate 12, projection optical systems 32a to 32
d, exposure areas 3 arranged along the Y direction in the figure
9a to 39d are formed, and exposure regions 39e to 39g arranged along the Y direction at positions different from the exposure regions 39a to 39d are formed by the projection optical systems 32e to 32g. These exposure areas 39a to 39g are
It is an erect image at the same magnification of a to 38 g.

FIG. 4 is a schematic view showing an example of a projection optical system provided with a field stop position measuring means according to the present invention, and corresponds to the projection optical system 11 of FIG. Field stop 4
When measuring the device of No. 5, the slit plate 20 provided with the slit 20a is set on a mask table. The mask table on which the slit plate 20 is placed is moved and scanned by a mask table drive mechanism 55 driven by a drive control unit 54 under the control of the controller 52. The position of the slit 20a of the slit plate 20 is monitored by a laser interferometer 57 that receives reflected light from a movable mirror 57a fixed to a mask table, and the monitor output is given to a controller 52.

The field stop 45 is provided with a controller 52
Can be moved by a field stop drive mechanism 56 driven by a drive control unit 54 under the control of. The position of the field stop 45 is monitored by a laser interferometer 58 which receives reflected light from a movable mirror 58 a fixed to a table on which the field stop 45 is mounted, and the monitor output is given to a controller 52. The illumination light from the light source 2 is
Field stop 45 through slit 20a of slit plate 20
To the projection optical system 11 having On the emission side of the projection optical system 11, an SPD (silicon photodiode)
Light receiving sensor 50 is disposed, and the slit 20a
The illumination light passing through the opening 45 a of the field stop 45 in the projection optical system 11 is received by the light receiving sensor 50. The light receiving signal from the light receiving sensor 50 is converted into an electric signal by the photoelectric converter 51 and then output to the signal processing unit 53 controlled by the controller 52.

Next, a specific example of a method for measuring the position of the field stop 45 will be described. (First method) In the first method, the shape of the slit 20a of the slit plate 20 is regarded as a sufficiently small point, and the slit 20a is scanned with the field stop 45 side fixed.
The position of the field stop 45 is measured. The position of the slit 20a of the slit plate 20 is monitored by the laser interferometer 57. The slit 20a is scanned in the X direction (scan direction at the time of exposure) by driving the mask table driving mechanism 55, and a position where the output of the light receiving sensor 50 changes is detected as shown in FIG. The output change of the light receiving sensor 50 is caused by the light transmitted through the slit 20a crossing the edge of the opening 45a of the field stop 45. At this time, the position of the field stop is determined from the position of the slit 20a obtained by the laser interferometer 57. The edge position of the opening 45a is obtained. Then, by comparing the detected position with the ideal field stop edge position shown in FIG. 5, the shift amount of the edge position of the opening 45a of the field stop 45 is detected.

Here, the opening 45a of the field stop 45 is
Considering the case of a trapezoidal shape as shown in FIG.
The amount of deviation between the X direction, which is the scanning direction during scanning exposure, and the θ direction, which is the rotation direction on the X, Y plane, is determined as follows. Consider a case in which point A and point B are separated by a distance L in the Y direction with respect to the aperture 45a of the field stop shown in FIG. At this time, as shown in FIG. 7, the measured value of the shift amount in the X direction at the point A is Xa,
Assuming that the measured value of the shift amount in the direction is Xb, the shift amount X in the X direction of the field stop 45 and the shift amount θ in the rotation direction can be obtained by the following expressions under the condition that θ is sufficiently small. it can.

X = (Xa + Xb) / 2 θ = (Xa−Xb) / L The shift amount in the Y direction is the shift amount of the measured value in the Y direction at the point C shown in FIG. Based on such measurement results,
In order to correct the position of the field stop 45, for example, as shown in FIG.
X, y from the field stop drive mechanism 56 against the urging force of 0
A force of 1, y2 may be applied.

Next, a method of measuring the shift amount of the field stop in the focus direction (Z direction) will be described. First, the entire field stop 45 is moved in the focus direction (Z direction),
At some points in the focus direction, for example, the X-direction scan of the slit 20a at the point A in FIG. 6 is performed. At this time, if there is a shift in the position of the field stop 45 in the focus direction (Z direction), the opening 4
Since light bleeding occurs at the edge portion 5a due to the diffraction phenomenon, if the amount of bleeding is measured at several points in the focus direction (Z direction), the focus direction (Z direction)
Can be known.

When the field stop 45 is stepped with Z1, Z2 and Z3 at appropriate intervals in the focus direction (Z direction), and the slit 20a is scanned in the X direction at the point A in FIG. 6, as shown in FIG. It is assumed that an appropriate light amount distribution is obtained. If the field stop 45 is at the ideal Z-direction position, FIG.
As shown in (d), when the slit 20a crosses the opening edge of the field stop 45, the amount of light received by the light receiving sensor 50 shows a rapid rise, and the amount of bleeding in the light amount distribution becomes almost zero.

On the other hand, the measurement result at the position Z1 shown in FIG. 8A indicates that the bleeding amount X1 is large and the position of the field stop in the Z direction is shifted from the ideal position. FIG.
The measurement result at the Z2 position shown in (b) shows that there is bleeding, but the bleeding amount X2 is smaller than the bleeding amount X1 in the case of FIG. 8A, and that Z2 is closer to the ideal Z position than Z1. Recognize. The measurement result at the Z3 position shown in FIG.
The bleeding amount X3 is larger than the bleeding amount X2, and when the Z position of the field stop 45 is stepped from Z2 to Z3, it exceeds the ideal Z position, that is, the ideal Z position is between Z2 and Z3. It is understood that there is.

From the above measurement results, the relationship between the ideal positions Za and Z1, Z3 of the point A of the opening 45 of the field stop 45 can be expressed as shown in FIG. 9 using the bleeding amounts X1, X3. The ideal position Za can be represented by the following equation.

X1 / (Za−Z1) = X3 / (Z3−Za) ∴Za = (X1 · Z3 + X3 · Z1) / (X1 + X3) Such a measurement is performed at points B and C in FIG. 6, respectively. This makes it possible to measure Zb and Zc, which are ideal positions in the respective focus directions, and also to measure the deviation of the tilt (the rotation component about the X axis and the rotation component about the Y axis).

When correcting the position of the field stop 45 in the focus direction based on the measurement result, for example, as shown in FIG.
As shown in FIG. 5, a driving element such as a piezo element 61 is provided on a support plate 62 having an opening 63, and the piezo element 61 is moved to a point A, By driving in the directions in which the shift amounts in B and C become zero, the position shift of the field stop 45 in the focus direction can be corrected.

(Second Method) The first method is to scan the slit 20a of the slit plate 20 and measure the edge position of the aperture of the fixed field stop. And the field stop driving mechanism 5 of FIG.
6, the edge position of the opening 45a of the field stop 45 can be measured by scanning the opening 45a side of the field stop 45. At this time, the light beam transmitted through the slit 20a of the slit plate 20 is positioned at the field stop 45,
By setting so as to pass through the designed edge position of the aperture 45a of the field stop 45, the displacement of the field stop 45 is measured and the position of the field stop 45 is corrected in one step. The time required for the position correction can be reduced as compared with the method of (1).

(Third Method) In the first and second methods,
Although the case where the shape of the slit 20a of the slit plate 20 or the opening 45a of the field stop 45 is regarded as a sufficiently small point has been described, a sufficient amount of light is obtained at the position of the light receiving sensor 50, and electric noise and stray light are suppressed. In order to avoid the influence, as shown in FIG. 10, it is preferable that the shape of the slit 20 a provided in the slit plate 20 is made to match the shape of the opening 45 a of the field stop 45. FIG. 10 (a)
Is an example in which the opening 45a of the field stop 45 is trapezoidal. In this case, the slit 20a of the slit plate 20 is used.
The X-direction measuring slit 20ax is connected to the field stop opening 45.
It is an elongated shape parallel to the X-direction edge 45ax of a. In addition, Y of the slit 20a of the slit plate 20
The direction measuring slit 20ay is provided with a field stop opening 45a.
In a slender shape parallel to the Y-direction edge 45ay.

As described above, the X-direction measuring slit 20a
By forming the slits 20ay for measuring in the x and Y directions in an elongated shape parallel to the edges 45ax and 45ay to be measured, the amount of light transmitted through the slits can be increased, and a sufficient S / N ratio can be obtained. Can be obtained. FIG. 10B shows the aperture 4 of the field stop 45.
5a is an example of a hexagonal shape. In this case as well, the slit 2 for the X-direction
0ax is the edge 45ax in the X direction of the field stop opening 45a.
And the slit 20ay for measuring the Y direction of the slit plate 20
By forming the elongated shape parallel to the direction edge 45ay, the amount of light transmitted through the slit is increased, and a sufficient S / N
Ratio can be obtained.

(Fourth Method) In the first to third methods, the case where the slit 20a of the slit plate 20 or the field stop 45 is scanned to measure the shift amount of the field stop 45 has been described. The amount of deviation can be obtained without performing scanning. That is, the slit plate 20
Alternatively, the displacement of the field stop 45 is measured by moving the position of the field stop 45 in several steps in a stepwise manner and measuring (sampling) the X direction, the Y direction, and the focus direction at each position. Can be. According to this method, it is not necessary to consider the speed unevenness due to the above-described scan, the response time of the sensor, the sampling time at the time of measurement, and the like, and it is a simple method.

[0033]

According to the present invention, since the position of the field stop can be grasped by measuring the position of the field stop of the projection optical system by the measuring means, the position of the field stop is corrected to an appropriate position. By doing so, the exposure amount at the joint between the plurality of projection optical systems during scanning exposure can be made constant, and the occurrence of exposure unevenness can be prevented.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a scanning exposure apparatus according to the present invention.

FIG. 2 is a perspective view showing a projection optical system in which a plurality of small projection optical systems are joined.

FIG. 3 is a lens configuration diagram of a projection optical system.

FIG. 4 is a schematic diagram showing a projection optical system including a field stop position measuring unit according to the present invention.

FIG. 5 is a view for explaining position measurement of a field stop.

FIG. 6 is an explanatory diagram showing a positional relationship between an opening of a field stop and a measurement slit.

FIG. 7 is an explanatory diagram showing a method for measuring the position of a field stop.

FIG. 8 is a view for explaining a method of measuring the position of the field stop in the Z direction.

FIG. 9 is a view for explaining a method of measuring the position of the field stop in the Z direction.

FIG. 10 is a view showing a slit shape for position measurement of a field stop.

FIG. 11 is a perspective view showing an example of a field stop driving mechanism.

FIG. 12 is a sectional view showing an example of a field stop driving mechanism.

FIG. 13 is an explanatory view showing an exposure area formed by a hexagonal field stop.

FIG. 14 is an explanatory diagram showing an exposure area formed by a trapezoidal field stop.

FIG. 15 is an explanatory diagram showing an exposure state where the field stop is displaced from a normal state.

[Explanation of symbols]

2 light source, 10 mask, 11 projection optical system, 20 slit plate, 20a slit, 45 field stop, 45a
... Opening part, 50 ... Light receiving sensor, 51 ... Photoelectric converter, 52
... Controller, 53 ... Signal processing unit, 54 ... Drive control unit, 55 ... Mask table drive mechanism, 56 ... Field stop drive mechanism, 57,58 ... Laser interferometer, 57a, 58a ...
Moving mirror, 61 ... Piezo element

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI G03F 7/20 G03F 7/20 H01L 21/30 502G

Claims (8)

[Claims] 1. A mask stage for holding a mask, a substrate stage for holding a substrate, an illumination optical system for illuminating a plurality of illumination areas on the mask, and images of the plurality of illumination areas on the substrate. A plurality of projection optical systems for projecting, scanning means for scanning the mask stage and the substrate stage in synchronization with the projection optical system, and a field stop disposed in each of the projection optical systems, A scanning exposure apparatus for exposing the images of the plurality of illumination areas on a substrate by jointly exposing, comprising a measuring unit for measuring a position of the field stop. 2. The scanning exposure apparatus according to claim 1, wherein said measuring means includes a slit held by said mask stage, and a photodetector provided on said substrate stage. 3. The scanning exposure apparatus according to claim 2, wherein the slit has a shape corresponding to the shape of the field stop. 4. A method for measuring a position of a field stop of an exposure apparatus for exposing a plurality of illumination regions on a mask to a substrate through a plurality of projection optical systems each having a field stop and exposing the same, the mask comprising: Moving the slit disposed at a position relative to the field stop, detecting the position of the field stop from the relationship between the amount of light detected on the substrate and the position of the slit. Measuring method. 5. A method according to claim 4, wherein the slit is stopped at a plurality of positions to detect the amount of light. 6. A method for measuring a position of a field stop of an exposure apparatus for exposing a plurality of illumination regions on a mask by joining them on a substrate through a plurality of projection optical systems each having a field stop, the mask comprising: Moving the field stop by disposing a slit at the position of the field stop position, wherein the position of the field stop is detected from the relationship between the amount of light detected on the substrate and the position of the field stop. Measuring method. 7. The method according to claim 6, wherein the field stop is stopped at a plurality of positions to detect the amount of light. 8. The method for measuring a field stop position according to claim 4, wherein the shape of the slit is adapted to the shape of the field stop.