JPS59188502A - Optical axis adjusting method - Google Patents

Abstract

PURPOSE:To improve the precision of the adjustment of the optical axis of illumination light by forming a pattern which consists of grooves or projections in a symmetrical step shape on a sample surface, detecting the symmetry of the light quantity distribution of an image formed by the reduced or enlarged projection of an objective, and adjusting the optical axis of the illumination light so that the symmetry is maximum. CONSTITUTION:Light from a lamp 1 illuminates the pattern 12 for optical axis adjustment through a contactor lens 2, light guide 10, intermediate lens 11, half- mirror 3, and objective 4. Its reflected light forms an image on a linear image sensor 14 through the objective 4, half-mirror 3, and a cylindrical lens 13. Light intensity signals regarding respective parts of the sensor 14 are stored in a memory 18 through an AD converter 17 and further processed by an arithmetic circuit 19 to adjust the optical axis of the illumination system so that the symmetry of an image on a monitor 20 corresponding to the symmetry that the pattern 1 has is maximum. Thus, the adjustment precision of the illumination optical axis is improved.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a microscope that uses epi-illumination, and more particularly to an optical axis adjustment method suitable for a mask alignment device and a pattern dimension measuring device that measure the position and dimensions of a pattern. It is something.

[Background of the invention]

A conventional method for adjusting the illumination optical axis in an epi-illumination microscope will be described with reference to FIGS. 1 and 2.

Figure 1 shows the configuration of a microscope for epi-illumination. A portion of the light emitted from the lamp 1 passes through the collector lens 2, is reflected by the Zinoff mirror 3, passes through the objective lens 4, and epi-illuminates the sample 5. In such an illumination system, when replacing the lamp, the light passing through the half mirror 3 is used to place the light source image of a high-brightness part of the lamp 1, such as the filament or arc part, on the crosshair plate 6 with a cross image. After forming an image, the lamp 1 was moved so that the center of the light source image coincided with the center of the crosshair, and the illumination optical axis was adjusted 1-.

Another method for adjusting the optical axis is to use an object with a flat surface and relatively high light reflectance as the sample 5, and to pass the light reflected from the sample 5 and bent by the prism 7 through the eyepiece lens 8. This is a method of removing and observing. That is,
In this method, the lamp 1 is adjusted so that the center of the light source image can be seen at the center of the tube by looking into the microscope cylinder from which the eyepiece 8 has been removed in the direction of the arrow. FIG. 2 shows an example of the field of view when observing a circuit pattern on a semiconductor wafer using a microscope whose illumination optical axis has been adjusted using the conventional method described above. For a pattern having a symmetrical cross-sectional shape as shown in (a), the light intensity distribution of pattern 9 detected as shown in (b) K (the light intensity distribution in the A-A section of (a) is In such a state, even if the dimensions and position of the pattern 9 are measured visually, the edges of the pattern are unclear and the accuracy of the measurement is reduced. this is,
A similar problem arises when pattern recognition is performed automatically using photoelectric detection means.

[Purpose of the invention]

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art,
In order to correctly recognize the shape of the pattern on the sample surface,
An object of the present invention is to provide an optical axis adjustment method that allows adjusting the illumination optical axis of a microscope with epi-illumination.

[Summary of the invention]

That is, in the present invention, a pattern consisting of grooves or protrusions with bilaterally symmetrical step shapes is provided on a sample surface, and the light intensity distribution of an image formed by enlarging and projecting the pattern with an objective lens is measured. Calculate the symmetry of the light intensity distribution of the
This method is characterized in that the illumination optical axis is adjusted based on the degree of symmetry so that it is maximized. - [Embodiments of the Invention] The present invention will be specifically described below based on embodiments shown in the drawings. FIG. 6 shows the structure of an epi-illumination microscope and image intensity distribution measuring means of the present invention. Light emitted from a lamp 1 is collected by a collector lens 2 and projected onto the end face of a light guide 10. Since the light guide 10 is made of real buttical fibers in which each fiber is irregularly arranged, it becomes a light source with a uniform brightness distribution at the light emitting end. Of course, the emitted light passes through the intermediate lens 11, the nozzle 3, and the objective lens 4, and epi-illuminates the pattern 12 for adjusting the optical axis. 52 reflected from pattern 12. .

・3 The light passes through the objective lens 4. Half Mira 3 Interchangeable Cylindrical Jinzu 1
3, an image is formed on the linear image sensor 14. The light intensity distribution of the image in the X direction is as shown in FIG. Figure 4 (Car b) Samples at positions Xl, 'X2...
...1.15 sample holes to obtain yn.

The code circuit is controlled by a sampling signal generation circuit 16,
The above-mentioned light intensity outputs y1, y2, . . . yn are passed through a hole P. Thereafter, the AD converter 17 converts the hold value into a digital signal and stores it in the memory 18. 19 has Xl as the symmetric turning point.

This is a circuit that calculates the symmetry function z (x,) shown in the following equation (1) in relation to the sampling positions of ±j with the point as the center.

In the above equation (1), m is a value determined in consideration of the size of the pattern 12, and is determined, for example, as shown in FIG. 4(b). Furthermore, the arithmetic circuit 19 sequentially changes the symmetric turning point X1 to X1, X2, ...... Symmetrical turning point X indicating the value. This is what we seek. 5 This position X. is the point at which folding pattern matching is the best, and z(xo) is an index indicating the degree of symmetry. Monitor this index z (xo) and z (x+).

20, and calculation/output processing is performed every time the linear image sensor 14 is scanned. This monitor 2 apparent index z
The illumination optical axis is adjusted so that (Xo) is minimized.

In this embodiment, an image sensor 14 is used as an image sensor.
However, the present invention is not limited to this, and it is also possible to use a device in which a slit is arranged to scan the front surface of the photoelectric conversion element. Furthermore, 2
If you use a dimensional image sensor, you can scan not only in one direction, but also in one direction.
The optical axis can also be adjusted in the perpendicular direction.

〔Effect of the invention〕

As explained above, according to the present invention, what is achieved by adjusting the illumination optical axis by aligning the light source image with the reference position by visual inspection is compared to the evaluation index of the symmetry of the light intensity variation distribution of the pattern. This has the effect of improving the accuracy of adjusting the illumination optical axis and increasing the accuracy of measuring the dimensions and position of the pattern.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of a microscope explaining the method of adjusting the illumination optical axis of a conventional microscope, and Figure 2 (a) shows a circuit pattern on a semiconductor wafer observed using a microscope with the illumination optical axis adjusted using the conventional method. Figure 2 (b) is a diagram showing the field of view at the time of Figure 2 (al
The pattern shown in Figure 3 is a diagram showing the light intensity distribution in the A-A section of the book. FIG. 4(a) is a configuration diagram showing one embodiment of the invention. FIG. 4(b) is a diagram showing the output signal waveform of the imaging device. is a diagram showing the optical intensity signal y when the signal in Figure 4 (a) is sampled, and Figure 4 (c) is the symmetry numerical aperture z (xυ
It is a figure showing. 1... Lamp, 4... Objective lens, 12... Optical axis adjustment pattern, 15... Cylindrical lens, 14... Linear image sensor, 15...
... Sample hold circuit, 17 ... AD converter, 19 ... Arithmetic circuit, 5
20...Monitor. 0 1st rfJ 9- 22nd Wharf 3 Figure JP-A-59-188502 (4) Figure 4

Claims (1)

[Claims] 1. In a method of adjusting the optical axis of a microscope that illuminates the object plane through an objective lens, a pattern consisting of grooves or protrusions formed on the object plane and having symmetrical step shapes is projected through the objective lens. An optical axis adjustment method comprising: measuring a light intensity distribution of an imaged image, calculating an index indicating the symmetry of the measured light intensity distribution, and adjusting the above-mentioned optical axis based on the calculated index.