WO2008071324A1

WO2008071324A1 - Apparatus for measurement of structures on photolithographic masks

Info

Publication number: WO2008071324A1
Application number: PCT/EP2007/010532
Authority: WO
Inventors: Ulrich Stroessner
Original assignee: Carl Zeiss Sms Gmbh
Priority date: 2006-12-15
Filing date: 2007-12-05
Publication date: 2008-06-19

Abstract

The invention relates to an apparatus for measurement of structures on photolithographic masks (1), said apparatus comprising at least one coherent-light emitting source of illumination (3, 7), which illuminates the photolithographic mask (1) via an illumination beam path (4, 8); a spatially displaceable stage (2) receiving the photolithographic mask (1) whose position is controlled by means of laser interferometry; an imaging device, which images light coming from the photolithographic mask (1) onto a detecting device (13); as well as an evaluating device (14) coupled to the detecting device, said evaluating device (14) evaluating the detected signals and determining the positions of the structures. In such an apparatus, at least one field stop (15, 16) is provided in the illumination beam path (4, 8), said field stop (15, 16) having a size which corresponds to an area around the structure during imaging onto the photolithographic mask (1), which area appears identical for all structures.

Description

Title

Apparatus for measurement of structures on photolithographic masks

Background of invention The invention relates to an apparatus for measurement of structures on photolithographic masks. Such apparatus comprises at least one coherent-light emitting source of illumination, which illuminates the photolithographic mask via an illumination beam path; a spatially displaceable stage receiving the photolithographic mask whose position is controlled by means of laser interferometry; an imaging device, which images light coming from the photolithographic mask onto a detecting device; as well as an evaluating device coupled to the detecting device, said evaluating device evaluating the detected signals and determining the positions of the structures.

State of the art

In the production of computer chips, there has been a trend to generate increasingly small structures on the same surface area. Such chips presently consist of approximately 30 different, superimposed layers. The size of the functional structures, the so-called features, is approximately 45 nm. The photolithographic masks used for producing these features have to be produced with correspondingly high precision. For this purpose, a wafer is exposed up to 30 times, but each layer requires a different mask. Therefore, very precise production of the masks is required on the one hand, and very precise positioning such that the layers are exactly aligned with each other is required on the other hand. With respect to superimposed layers, a precision of 4.8 nm has to be achieved for the most recent applications. This is the precision with which the masks, which usually have a side length of 150 mm, have to be aligned with each other. Thus, it is essential that the mask structures in the substrate which is to form the mask be generated at the correct positions with respect to a reference coordinate, for example one of the corners of the mask. For quality control, structures called marks, for example crosses with dimensions of 10μm ^* 10μm and a stroke width of 1 μm on the mask, are applied onto the mask. A so-called registration tool then analyzes whether these structures are located at the right positions within the admissible tolerance. Such a device is, for example, IPRO3 from the Vistec corporation. This device operates at a wavelength of 365 nm. However, the precision achievable thereby is not high enough for future structures.

The structures or marks, respectively, to be measured in the mask are usually of like types so as to ensure that comparisons and reproductions are possible. Therefore, systematic errors should also have like effects on the structures. For example, the structures used may be crosses having bars with a length of 10μm and a width of 1 μm. Between 100 and 400 of these crosses are then generated on a mask. The crosses are usually located in areas having no structures required for wafer exposure, so that the vicinity of each of these structures usually appears identical.

However, the applications demanded in future will permit a precision of 4.8 nm at the most for positioning and aligning the masks relative to each other. Not only the masks, but also the wafer to be exposed, have to be suitably positioned. However, during both writing of the mask and measuring of the masks and the control marks, effects may appear which result in observed deviations of up to 10 nm in the structures' positions; this is outside the allowed tolerance range, however. These effects are caused, on the one hand, by so-called charging effects of the mask which may occur during writing of the mask using an electron beam writer. The influences of this charging effect on the structure to be measured for quality control depend strongly on the neighborhood of the structure on the mask. These charging effects cause the structure to be written, not at the location provided for this purpose, but at a position deviating from it by several nanometers.

Moreover, imaging of the structures may be influenced by scattered light effects which may occur in the registration tool. The deviations in detection are of a similar order of magnitude as for the charging effects. However, in contrast to the latter, the scattered light effects only cause an offset of the structure's image, but not of the structure itself. Thus, the structure is only seemingly offset.

In the prior art, these two effects cannot be distinguished from each other, which has played no role so far, because a precision of 10 nm has been sufficient. However, future applications will require a higher precision. The methods known in the prior art, however, do not allow to determine whether a seeming offset due to scattered light effects or an actual offset caused by charging effects is present. In the latter case, the settings of the electron beam writer would have to be changed. Description of the invention

Therefore, it is an object of the invention to improve an apparatus for measurement of structures on photolithographic masks such that the above-described effects can be distinguished.

This object is achieved in an apparatus for measurement of structures on photolithographic masks of the above-described type by providing at least one field stop in the illumination beam path. This field stop has a size corresponding to an area around a structure during imaging on the mask, which area appears identical for all structures. Accordingly, only this area around the mask is illuminated. In this manner, the scattered light effects, which may be different due to differing neighborhoods, are suppressed. In the boundary case, the area around a structure corresponds to the smallest possible area still containing this structure. For a cross-shaped mark, this area may be, for example, a square, a circle or also a cross. Blocking out the scattered light effects allows to distinguish whether a seeming offset or an actual offset is present: If an offset of the structure occurs during measurement with the field stop, a charging effect will be present. This offset will then appear also without using the field stop, and the electron beam writer will have to be suitably adapted. However, if an offset appears only in cases where no field stop is used, it may be assumed that the offset was caused by scattered light effects. In this case, there will be no need to change the setting of the electron beam writer.

The at least one field stop is preferably exchangeable, allowing an adaptation to different structures, different structure sizes as well as different image ratios. This may also be achieved by the size of the illuminated field being variably adjustable for the at least one field stop.

Brief description of the figure

The invention will be explained in more detail below by way of an exemplary embodiment. In the respective drawing, Fig. 1 shows the basic structure of an apparatus according to the invention.

Detailed description of the figure

In the apparatus shown in Fig. 1 , a photolithographic mask 1 is supported on a carrier on a stage 2. The stage 2 can be moved in all three spatial directions. In order to ensure high precision, the actual position or the path-length difference, respectively, is monitored by means of laser-interferometric measuring devices not shown. The photolithographic mask 1 and the stage 2 are arranged horizontally, i.e. perpendicular to the action of gravity. A first source of illumination 3, e.g. a laser emitting light at a wavelength of 193 nm, is arranged above the stage 2 with the photolithographic mask 1. The light is directed onto the photolithographic mask 1 via a first illumination beam path 4, which includes only lenses and/or mirrors. The first source of illumination 3 and the first illumination beam path 4 serve to illuminate the photolithographic mask 1 by transmitted light. The illumination beam path 4 may be provided as a completely free beam path, as symbolized by lenses 5 and 6. On the other side of the stage 2, there is a second source of illumination 7 comprising a second illumination beam path 8, which may also be provided as a completely free beam path, as symbolized by the lenses 9 and 10. The second source of illumination and the second illumination beam path serve to examine the photolithographic mask 1 by incident light. Light coming from the photolithographic mask 1 , which is light either transmitted through or reflected by the photolithographic mask 1 , is imaged, via imaging optics 11 and a semi- transmitting mirror 12, onto a spatially resolving detecting device 13, which may be designed as a CCD camera. In the latter, the detected intensities are converted to electrical signals and transmitted to an evaluating unit 14.

The structures which are located on the photolithographic mask 1 and serve the purpose of quality control are imaged onto the detector 13 by this arrangement. The position of the photolithographic mask 1 as determined by means of interferometry allows to derive the position of the structure on the photolithographic mask 1. The beam paths for both illumination and imaging are preferably parallel to the force of gravitation, thus exposing the lenses and their mounts to the force of gravitation only in an axial direction, which increases the precision of beam guidance considerably.

In addition, a field stop 15 or 16, respectively, is also provided in the illumination beam path 4 and/or in the illumination beam path 8. The field stops 15 and 16 serve to adjust the size of the field illuminated on the photolithographic mask 1 such that only the smallest possible area, which appears identical for all of the - usually similar - structures during imaging, is illuminated. In this manner, scattered light effects caused by the neighborhood of the structures used for marking can be suppressed. This allows to determine whether an offset which is visible in the image of the photolithographic mask 1 or of the structure applied for quality control, respectively, and which relates to a desired position has been caused by scattered light effects or by charging effects. The field stop may be exchangeable and/or variably adjustable, which enables adaptation to different structural sizes and/or image ratios. List of reference numerals

I photolithographic mask 5 2 stage

3 first source of illumination

4 first illumination beam path 5, 6 lens

7 second source of illumination

10 8 second illumination beam path

9, 10 lens

I I imaging optics

12 semitransmitting mirror

13 detecting device 15 14 evaluating unit

15, 16 field stop

Claims

1. An apparatus for measurement of structures on photolithographic masks (1 ), said apparatus comprising at least one coherent-light emitting source of illumination (3, 7), which illuminates the photolithographic mask (1 ) via an illumination beam path (4, 8); a spatially displaceable stage (2) receiving the photolithographic mask (1) whose position is controlled by means of laser interferometry; an imaging device, which images light coming from the photolithographic mask (1 ) onto a detecting device (13), as well as an evaluating device (14) coupled to the detecting device (13), said evaluating device (14) evaluating the detected signals and determining the positions of the structures, characterized in that at least one field stop (15, 16) is provided in the illumination beam path (4, 8), and the at least one field stop (15, 16) has a size corresponding to an area around a structure during imaging onto the photolithographic mask (1 ), which area appears identical for all structures.

2. The apparatus as claimed in claim 1 , characterized in that the at least one field stop (15, 16) is exchangeable.

3. The apparatus as claimed in claim 1 or 2, characterized in that the size of the illuminated field is variably adjustable for the at least one field stop (15, 16).