EP0642676A1

EP0642676A1 - Scanning object lens

Info

Publication number: EP0642676A1
Application number: EP92922848A
Authority: EP
Inventors: Anton Schick
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1992-05-25
Filing date: 1992-11-03
Publication date: 1995-03-15
Also published as: JPH063587A; KR950701741A; WO1993024854A1; US5608564A; JPH07117647B2; SG44693A1; KR0177848B1

Abstract

Scanning object lenses for the line or pointwise three-dimensional scanning of surfaces of objects are required to have a high resolution together with high examination rates. In order to be able to display the largest possible number of image points with high resolution, the scanning object lens must have a correspondingly high numeric aperture with, at the same time, a large image field. To this end it consists of three groups of lenses (1; 2; 3). The first and second groups (1; 2) provide both a reduction in the scanning angle and an increase in the pupil. The entrance pupil (61) is imaged in the entrance pupil (63) of the third group of lenses (3), which has a high numeric aperture of, for example, 0.6. In addition, the real intermediate image (5) is imaged into infinity by the second group of lenses (2) with positive refractive capacity and a greater focal distance (f) than the first group (1). A working distance of, for instance, 5 mm can be attained with a beam diameter (D) at the inlet of the scanning object lens of 7.5 mm, a scanning angle ( THETA ) of +/-16 DEG and a scanning distance of 3.5 mm.

Description

Scanob ective

The invention relates to a scan lens for line-by-line or point-by-point three-dimensional scanning of object surfaces with high resolution and high test speed.

The increasing packing density on an electronic assembly requires a correspondingly adapted test. This check is usually carried out by means of a high-resolution optical inspection system. In principle, the triangulation principle can be used in conjunction with a system for quickly scanning a surface. In the case of reflecting surfaces of structured objects, however, the confocal principle is generally more suitable. A point-shaped light source, which is usually defined by a pinhole, is imaged on the object surface and the backscattered light is imaged on an almost point-shaped detector. For this, reference is made, for example, to the European patent application with the official file number 91 120 863.5. The depth of field of a confocal optical configuration is a measure of the height resolution of the system. The depth of field is inversely proportional to the square of the numerical aperture. A scanning lens with a high numerical aperture and a very fast one. Beam deflection unit, for example a polygon mirror rotating at high speed, meets the requirements for high resolution for an automatic test system. However, a scanning objective with a sufficiently high numerical aperture is currently not available. A sufficiently high numerical aperture should have a value of at least 0.15. The essential variable in the design of a diffraction-limited scan lens, which reflects the theoretically achievable imaging performance, is the Lagrangian variant L. It is formed by the product of half the beam diameter D at the location of a beam deflection unit and the scan angle or deflection angle kel θ. This is equivalent to the product of the numerical aperture NA and half the scan length S of the scan lens.

L = ^* D - Θ ^* = - S - NA

Since the resolution capability is predetermined by the numerical aperture, L is proportional to the number of scanned points per scan line. A high scanning rate is thus achieved by a high scanning speed and the largest possible Lagrangeinversion L of the scanning system. In accordance with the above-mentioned dependencies, both the beam deflection unit and the scanning objective have to be matched to one another.

With regard to the beam deflection unit, the result is that for rotating polygon mirrors in relation to other beam deflectors, the feasible angular velocity with a large beam diameter and thus the pixel data rate is very high. Other beam deflectors are understood to mean, for example, acousto-optical deflectors, resonance scanners or galvanometer mirrors. The data rate is limited upwards by the increasing rotational inertia with increasing beam cross-section or increasing mirror facet diameter of the mirror. Depending on the material used and the mechanical design of the polygon mirror, this results in an optimal poly gon mirror dimensioning.

The basic design of a scan lens must take the following dependencies into account:

The larger the Lagrangian variant (high resolution with a large image field), the more difficult it is to minimize geometric aberrations and to design a scan lens to be suitable for production.

If the requirements for the resolution increase, which is equivalent to the enlargement of the numerical aperture, the scan length is reduced with unchanged Lagrangein variants.

If the numerical aperture is increased with a constant focal length for a system with predetermined Lagrangein variants, the beam diameter increases with a reduction in the scanning angle. However, increasing the beam diameter with a small scanning angle increases the number and size of the facets of the polygon mirror, which in turn reduces the rotational speed that can be achieved.

- If the numerical aperture is increased for a system with predefined Lagrangeinvarianten while maintaining the scanning angle, the objective focal length is reduced. This means that, in the case of conventional scanning objectives, the front and rear focal planes come very close to the lens mount, since otherwise the correction of the aberrations will be made considerably more difficult. This in turn limits the space required for positioning polygon mirrors.

The listed relationships make it clear that a A scanning lens with a large numerical aperture and at the same time a large image field cannot be constructed using conventional means.

Scanning lenses currently available on the market have numerical apertures of approximately 0.1 and have a scanning angle of approximately 13 ° to 25 ° with focal lengths of at least 20 mm. These lenses can be adapted very well to polygon mirrors rotating at high speed. The Lagrangein variant is about 1 mm. However, lenses of this type are not suitable for high resolution confocal scanners.

Confocal laser scanning microscopes also exist. These are mostly constructed on the basis of ordinary light microscopes or light microscope objectives and can therefore also have a high numerical aperture. With a Lagrangein variant of approx. 0.15 mm, however, the overall imaging performance is relatively low due to the small image field or the short scan length. Acoustic-optical beam deflectors are frequently used here, as a result of which the imaging power is reduced due to the small deflection angle and the small aperture which is not circular. In addition, the beam deflection is not aberration-free (astigmatism).

The aim of the invention is the construction of a scan lens, by means of which a large image field is scanned at a high resolution, corresponding to a high numerical aperture, with a simultaneously high scanning rate. ^•

This object is achieved by the features according to claim 1. In principle, a high resolution of an objective is guaranteed by a high value of the numerical aperture. The scan or deflection angle determines the size of the image field in connection with the focal length of the objective.

The invention is based on the knowledge that the required quality features can be achieved by means of a scan lens consisting of three lens groups with an appropriate design. A first lens group, viewed in the direction of the scanning beam, with a positive refractive power and a relatively low numerical aperture can be regarded as an ideal coupling element to the beam deflection unit with high imaging power. The structure of this optic corresponds to a typical scan lens from the prior art with a relatively large numerical aperture. It can be manufactured with less than 6 individual lenses with limited diffraction. A real intermediate image is generated by this lens group. The sufficient vertex distance between the entrance pupil and the surface of the first lens is of great advantage.

A third lens group, which is arranged on the object side, receives a high numerical aperture with a large one

Focal length and at a relatively small deflection angle of approx. 2.5 °. In order to remain within the diffraction limit, care must be taken that geometric

Aberrations are optimally corrected. If necessary, a reduction in the deflection angle must be accepted. Due to the similarity of the design with collimator optics, essentially only that is

Spherical aberration correction. Due to the described configuration of the third lens group, the imaging performance of a scanning system or a scanning lens is high on the object side numerical aperture possible.

A second lens group with positive refractive power, which images the real intermediate image of the first lens group to infinity, must be constructed so that the exit pupil, formed by the first and second lens groups, coincides with the entrance pupil of the third lens group. The angle of view of the second lens group must coincide with the deflection angle of the third lens group. This second lens group can be an inverted scan lens, for example. This means that a previously known scanning lens is used in the opposite direction. Attention must be paid to the appropriate adjustment of the scan length (image height of the intermediate image) and the numerical aperture in relation to the first lens group. Furthermore, the scan angle (image angle) and the diameter of the entrance pupil of the third lens group must be adjusted. The ratio of the focal lengths of the first lens group to the second lens group corresponds here to the ratio between the scanning angle (image angle) of the first lens group and the third lens group. All three lens groups are diffraction limited.

A particularly advantageous embodiment of the invention includes the design of the scan lens as an F-Θ lens. The necessary condition for an F-Θ lens is the proportionality between the scan or deflection angle and the current deflection or the current image height. This reduces the control expenditure in the case of automatic test systems.

It is advantageous to parallel the main rays between the first and the second lens group parallel to the optical see axis run. This construction, which corresponds to a telecentric design, can apply on the one hand to the first and second lens groups and on the other hand to the entire scanning objective. The resulting advantages are that individual telecentric lens groups can be easily connected in series. The telecentric design of the entire scan lens means in particular an advantageous application for measurement purposes. There is no need for complex electronic (hardware or software) corrections.

In the event that the total focal length of the scan lens is smaller than the distance between the first lens surface of the first lens group and the entrance pupil of the scan lens in front of it, the coupling between the scan lens and the beam deflection unit can be carried out in a simple manner.

If the numerical aperture of the first lens group is at least 0.15 (the numerical aperture of the entire scanning objective can be, for example, 0.6), this is advantageous insofar as the design of the downstream lens groups offers greater freedom of design with regard to an overall high level numerical aperture can be achieved.

Another advantageous embodiment of the invention provides that the scanning angle is approximately 16 ° and the entrance pupil has a diameter of approximately 7.5 mm. The definition of these two parameters of the scan objective to the numerical values mentioned gives particular advantages with regard to the design of a polygon mirror rotating at high speed (e.g. number of facets 12, mirror diameter approx. 70 mm, facet length approx. 19 mm). If the numerical aperture of the third lens group is at least 0.25 in combination with a scan angle of approx. 2.5 ° and a scan length of approx. 8 mm, a working distance between the lens and the object of approx. 30 mm can already be set.

An exemplary embodiment is described below with the aid of schematic figures.

Figure 1 shows a schematic diagram of a typical scan lens with a numerical aperture of 0.1.

FIG. 2 shows the beam path of a scan lens when the numerical aperture is enlarged compared to the illustration in FIG. 1.

FIG. 3 shows the detailed structure of a scan lens with a high numerical aperture.

FIG. 4 shows the basic illustration of the scan lens corresponding to FIG. 3.

In FIG. 1, the beam path of an objective 11 shown only by a line is sketched. The objective 11 is positioned with respect to an optical axis 10. The focal length f is indicated on both sides of the objective 11. Starting from the beam diameter D at the entrance pupil 61, the main beam 7 and edge beams 8 run in accordance with the scanning angle θ. The object plane 9 is indicated in the right part of the image. The scan length S corresponds to the image size.

Known scan lenses as shown in FIG. 1 are commercially sold by several manufacturers.

They usually have a numerical aperture of <= 0.1 and focal lengths of more than 20 mm. The scan angles are in the range of +/- 13 ⁰ to 25 °. Since the Lagrangin variant represents a measure of the imaging performance of an optical component, the numerical aperture determines the size of the pixel and the imaging performance is based on the number of resolved pixels, it should always be dimensioned such that the desired aperture is present. If one assumes that the Lagrangian variant remains constant as a measure of the imaging power and that the focal length of the entire system does not change, then when the numerical aperture is enlarged, a beam path corresponds to that shown in FIG. 2. The entrance pupil 61 and the object plane are also located here 9 each in focus. The scan length S is sketched correspondingly symmetrically to the optical axis 10 and the beam diameter D at the entrance pupil 61, which is considerably larger than in FIG. 1, is entered. It is clearly evident that the scan angle θ has decreased significantly in relation to the scan angle θ of FIG. 1. The same applies to the scan length S. Main ray 7 and marginal rays 8 are also shown. However, a construction according to FIG. 2 is not practical for solving the task, since it is generally difficult to accommodate a beam deflection unit. The design itself corresponds approximately to collimator optics, which usually only represent one point. In this way, a high numerical aperture can be achieved in a simple manner, since the image angle may be small.

If, based on the representation in FIG. 1, the Lagrangin variant and the scan angle θ are kept constant and the numerical aperture is enlarged, the focal length f becomes smaller. This also gives rise to difficulties with regard to the accommodation of a Beam deflection unit in front of the scan lens.

The use of a pure microscope objective, for example 50x / 0.6, is also difficult to achieve the task, since the deflection angle is too low. In addition, the entrance pupil is usually very close to the lens system of the microscope objective.

FIG. 3 shows a scan objective with an upstream beam deflection unit 4. The scan lens, which is constructed from the lens groups 1 to 3 described, has a numerical aperture of 0.6. The first lens group 1 and the second lens group 2 each consist of 5 individual lenses. The first lens group has a numerical aperture of 0.15 and a focal length of 25 mm. The second lens group collimates the rays emanating from the real intermediate image 5 at a focal length of 160 mm. In this case, the beam is expanded and the angle is reduced. The third lens group 3, which consists of a double element (doublet), three directly adjoining positive refractive single lenses and a lens triplet, has a focal length of 40 mm and a numerical aperture of 0.6. The total length of the objective is approximately 650 mm.

A scan objective according to FIG. 3 has a high numerical aperture (0.6 and a large scan length S). The first lens group 1 having a still low nume¬ step aperture of 0.15 has a large scan angle of about +/- 16 ^0th The first lens group 1 ″ represents the coupling element to the beam deflection unit 4. The high numerical aperture of 0.6 of the third lens group 3 with positive refractive power is achieved by a small image angle of 2.5 °. for the third lens group 3, a pupil diameter of 48 mm, a focal length f of 40 mm and an image field of 3.5 mm. In terms of its construction, this group of lenses is similar to a microscope objective that has been enlarged to scale and magnified 50 times.

In principle, the first two lens groups 1, 2 thus cause a scan angle reduction and a pupil enlargement (seen from left to right in the figure). The entrance pupil 61 of the first lens group 1 is imaged into the entrance pupil 63 of the third lens group 3, or the exit pupil 62 of the first and second lens groups 1, 2 coincides with the entrance pupil 63 of the third lens group 3. This has the advantage that the distance between the entrance pupil 63 and the glass surface of the first lens of the third lens group 3 is released as an additional degree of freedom in the construction and for the optimization of the third lens group 3. The observation of pupils of individual lens groups is only used for the construction of a scanning objective. Such a scanning objective has only one entrance pupil and one exit pupil. An additional, rather telecentric, construction of the scan lens simplifies the measurement data evaluation of an automatic 3D inspection system.

According to FIG. 3 or 4, the intermediate image 5 is generated by the lens group 1. The course of the main ray 7 and the marginal rays 8 with respect to the lens groups 1-3 and with respect to the optical axis 10 is indicated. The second lens group 2 images the intermediate image 5 to infinity. The pupils are imaged by the first and second lens groups 1, 2, the entrance pupil 61 of the first Lens group 1 is imaged in the entrance pupil 63 of the third lens group 3. This can also be called a scan angle transformation. The entrance pupil 63 of the third lens group 3 lies between the second and the third lens group 2, 3. Parallel rays are guided through the pupil at maximum diameter. The first lens group 1 taken on its own can be viewed as a scan lens with low resolution. The same applies to the second lens group 2, but this has a larger focal length f.

With a described scan lens, for example, it can be resolved in the range of 0.5 μm.

FIG. 4 shows a basic illustration of a scan lens with a high numerical aperture.

Possible technical data:

Example 1:

Numerical aperture (total): 0.6

Focal length f: 6.25 mm beam diameter D, at the entrance: 7.5 mm

Working distance: 5 mm

Scan angle θ: +/- 16 °

Distance a: 15 mm

Scan length S: 3.5 mm

Example 2

Beam diameter and scan angle are constant Numerical aperture: 0.28 scan length S: 7.5 mm

Working distance: 27 mm Focal length f: approx. 14 mm

The basic structure of a scan lens according to the invention includes, in accordance with the schematic illustration in FIG. 4, a first, second and third lens group 1; 2; 3. The surface of the object to be checked or the object plane 9 is sketched on the right edge of the picture. The entrance pupil 61 of the entire scanning objective is shown on the left edge of the image. The entire arrangement has an optical axis 10. The characteristic course of the main beam 7 and the edge beams 8 is outlined accordingly.

The entrance pupil 61 shown as a diaphragm in FIG. 4 can be replaced by a rotating polygon mirror. The exit pupil 62 of the first and second lens groups 1, 2 coincides with the entrance pupil 63 of the third lens group 3, the second lens group 2 depicting the intermediate image 5 generated by the first lens group 1 to infinity.

Claims

1. Scan objective for guiding an illuminating beam or an illuminating beam and a measuring beam, in particular for a scanning system for the surface inspection of electronic flat modules, consisting of three lens groups (1; 2; 3), with coupling to a beam deflection unit (4) has a first lens group (1) with a positive refractive power, which generates an intermediate image (5) with a numerical aperture that is small in relation to the entire scanning objective and at the same time has a large scanning angle (θ) - a third on the object side There is a lens group (3) with a larger numerical aperture relative to the first lens group (1), an interposed second lens group (2) with positive refractive power and a focal length (f) greater than the first lens group (1), which is one of the intermediate lens (5) produced in the infinite of the first lens group (1), the exit pupil (62), which is defined by the first and second lens size uppe (1; 2) is formed, comes to rest in front of the third lens group (3) and through the first and second lens group (1; 2) a scan angle reduction is achieved, with the effect that the third lens group (3) is similar in terms of the structure of a collimator lens.

2. Scan lens according to claim 1, characterized in that the scan lens is an F-Θ object.

3. Scan objective according to one of the preceding claims, that the main beams (7) between the first and second lens groups (1; 2) run parallel to the optical axis (10).

4. Scanning lens according to one of the preceding claims, that the scanning lens as a whole has a telecentric beam path.

5. Scan lens according to one of the preceding claims, characterized in that the distance (a) between the apex of the first lens surface of the first lens group (1) and the entrance pupil (61) of the scan lens in front of it is greater than the total focal length te (f) of the scan lens.

6. Scanning objective according to one of the preceding claims, that the numerical aperture of the first lens group is d a d u r c h g e k e n n z e i c h n e t

(1) is at least 0.15.

7. Scan lens according to one of the preceding claims, characterized in that the scan angle (θ) is approximately 16 ° and the beam diameter (D) at the entrance pupil (61) of the scan lens is approximately 7.5 mm. Scanning objective according to one of the preceding claims, characterized in that the numerical aperture of the third lens group (3) is at least 0.25 and the scanning angle (θ) is approximately 2.5 °.