JPH0359413A - Position measuring instrument - Google Patents

Abstract

PURPOSE:To measure the position of a sample surface with high accuracy by projecting a vibration grating on the sample surface, and detecting the spatial phase shift of the image of the vibration grating reflected by the sample surface. CONSTITUTION:A light source 2 slantingly above the sample surface 3 projects the image of the vibration grating 1 on the sample surface 3 and the image of the grating is formed by a lens 4. The image of the grating 1 reflected by the sample surface 3 is formed by a lens 5 and a fixed grating 6 is arranged on the image formation plane of the lens 5 to detect the quantity of transmitted light of the grating 6 by a photodetector 7. At such a time, an exciter 9 vibrates the grating 1 finely at constant frequency and the spatial phase of the image of the grating 1 projected on the grating 6 is modulated. Therefore, the modulat ed signal from the detector 7 is detected and amplified by an amplifier 8. Then the output of an oscillator 10 which drives the exciter 9 is used as a reference signal to detect the phase of the modulated signal by a lock-in amplifier 11, and consequently a high-accuracy position shift signal is obtained, so the position of the sample surface 3 is measured with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a position measuring device, and more particularly to a non-contact type position measuring device that can be applied to position measurement of a wafer surface in a semiconductor exposure device or inspection device.

[Conventional technology]

As a conventional technique, for example, as shown in Japanese Unexamined Patent Publication No. 56-2632, there is a method in which the wafer surface is irradiated with light from an oblique direction and the position of the wafer surface is measured from the position of the reflected light. Can be mentioned.

A conventional position measuring device using this method includes means for condensing light emitted from a light source onto a sample surface and means for focusing reflected light from the sample surface on a detector.

Next, a conventional position measuring device will be explained with reference to the drawings.

FIG. 4 is a block diagram showing an example of a conventional position measuring device.

The position measuring device shown in FIG. 4 includes a light source 41 and a lens 43 that focuses the light emitted from the light source 41 onto a sample surface 42.
and a lens '4 that forms an image of the reflected light from the sample surface 42.
4 and a detector 45 disposed on the imaging plane of the lens 44. As this detector 45, a two-split detector, a semiconductor device detection element (hereinafter referred to as PSD), or the like is used, and the position of the beam focused by the lens 44 can be detected.

When the height of the sample surface 42 changes, the spot position on the sample surface 42 moves, and the beam position on the detector 45 changes accordingly. Therefore, by detecting the beam position on the detector 45, the height of the sample surface 42 can be measured.

If the incident angle of the beam is θ, then the beam position shift ΔX on the detector 45 when the sample surface changes by Δh screen is
is expressed by the following equation.

Δx = 2 cosθ0Δh0n n: Magnification of lens 44 Two-split detector or PSD used as detector 45
etc. outputs two signals a and b, and by calculating the difference between these two signals, the position of the beam irradiating the detector 45 can be determined.

However, if the intensity of the beam incident on the detector 45 fluctuates due to changes in the reflectance of the sample surface 42, etc., a position measurement error occurs, making it impossible to accurately measure the height of the sample surface 42.

Therefore, in the conventional position measuring device shown in FIG. 4, the outputs a and b of the detector 45 are subtracted by a subtracter 46 and added by an adder 47. Then, the output (a-b) of the subtracter 46 is divided by the divider 48 into the output (a-b) of the adder 47 (
By dividing and normalizing by a+b), a positional deviation signal that is not affected by changes in the intensity of the beam incident on the detector 45 is obtained.

Conventional position measuring devices such as those described above are often used to measure the height of the wafer surface in order to focus the projection lens on the wafer surface in reduction projection type semiconductor exposure equipment (Stellanori). The exposure area is usually about 15 ma + x 15 mm, and in order to accurately focus, it is necessary to know the average height of the wafer surface within the exposure area.This is because the wafer has warpage and unevenness due to processing. This is because the height within the exposure area is not necessarily constant.

In the conventional position measuring device shown in FIG. 4, only the height at the spot position on the sample surface 42 can be measured in order to collect light into a spot and detect a change in the spot position. If the spot diameter on the sample surface 42 is increased, the average height can be determined accordingly, but the spot diameter formed on the detector 45 also becomes larger, and the resolution of position detection decreases.

Therefore, in order to find out the average height of the wafer surface more accurately, a method can be considered in which multiple position measuring devices shown in Figure 4 are installed to determine the height of the wafer surface at multiple points. However, the optical system increases and the overall device becomes complicated.

[Problem to be solved by the invention]

The conventional position measuring device described above focuses the beam into a spot on the sample surface, so it can only measure the height at one point on the sample surface. Therefore, if the sample surface is warped or uneven, It has the disadvantage of not being able to accurately detect the position.

Furthermore, in order to accurately detect a position, conventional position measuring devices require multiple position measuring devices with the same configuration.
This has the disadvantage that the entire device becomes complicated and expensive.

[Means to solve the problem]

The position measuring device of the present invention includes a projection optical system that projects a first grating image formed by a first grating from an oblique direction onto a sample surface, and a projection optical system that re-images the first grating image reflected on the sample surface. a light-receiving optical system that forms a second grating image, a second grating disposed on the imaging plane of the light-receiving optical system, a detector that detects the amount of light that has passed through the second grating, and a detector that detects the amount of light that has passed through the second grating; means for vibrating a first grating image projected on the first grating;
and means for phase-detecting the output of the detector modulated by the means for vibrating the grating image of the detector.

〔Example〕

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the position measuring device of the present invention.

The position measuring device shown in FIG. 1 includes a vibrating grating l as a first grating, a light source 2 for projecting the vibrating grating l, and a lens 4 for forming an image of the vibrating grating l onto a sample surface 3. , a lens 5 that re-images the image of the vibrating grating l reflected on the sample surface 3, a fixed grating 6 which is a second grating placed on the imaging plane of the lens 5, and an amount of light transmitted through the fixed grating 6. a photodetector 7, which is a detector for detecting the
and a lock-in amplifier II that performs phase detection on the output of the amplifier 8 using the output of the oscillator IO as a reference signal.

Note that the pitch of the image of the vibrating grating l projected onto the fixed grating 6 is designed to be equal to the pitch of the fixed grating 6.

Assuming that the illumination angle at which the vibrating grating l is projected onto the sample surface 3 is θ, the relationship between the change Δh in the height of the sample surface 3 and the displacement ΔX of the image of the vibrating grating l on the fixed grating 6 is given by If n is the optical magnification, then Δ is expressed as =2 cos θ·Δh−n.

Therefore, the phase change Δφ of the image of the vibrating grating l is Δx=
P・Δφ/2yr=2cosθ・Δh−n,°, Δφ=
4xn cos θ・Δh/P [rad:] However, P is the pitch of the fixed grating 6, and as the height of the sample surface 3 changes Δh, the vibration grating l
The spatial phase of the image will change.

This indicates that the amount of light I transmitted through the fixed grating 6 having the same pitch as the vibrating grating 1 changes periodically.

FIG. 2 is a graph showing an example of the relationship between the change Δh in the height of the sample surface 3 and the amount of light I transmitted through the fixed grating 6 in FIG. As shown in FIG. 2, the signal becomes a periodic signal whose period is P/2n cos θ in terms of change Δh in the height of the sample surface 3.

In this embodiment, in order to measure the position of the sample surface 3 with high precision, the vibrating grating l is minutely vibrated at a constant frequency by the vibrator 9, and the image of the vibrating grating l projected onto the fixed grating 6 is Modulates the phase.

Therefore, a modulated signal is detected by the photodetector 7 and amplified by the amplifier 8, and the phase of the modulated signal is detected by the lock-in amplifier 11 using the output of the oscillator lO that drives the exciter 9 as a reference signal. , it is possible to obtain a highly accurate positional deviation signal.

FIG. 3 is a graph showing an example of the relationship between the change in height Δh of the sample surface 3 in FIG. 1 and the output of the lock-in amplifier 11. The positional deviation signal shown in FIG. 3 is a differential curve of the curve of change in the amount of light I shown in FIG. 2, and the maximum and minimum values of the amount of light I are zero-crossing points.

The linearity is good in the vicinity of this zero cross point, and when the position measuring device of this embodiment is applied to a focusing mechanism, it is adjusted so that the focused point is at the point where the output of the lock-in amplifier 11 becomes zero.

This zero-crossing point does not change even if the amount of light l changes due to a change in the reflectance of the sample surface 3, so that highly accurate focusing that is not affected by the reflectance of the sample surface 3 can be achieved.

〔Effect of the invention〕

In order to measure the height of the sample surface by projecting the image of the first grating having a certain area onto the sample surface and detecting the spatial phase change of the image of the first grating reflected from the sample surface. This has the effect that the average height within a certain area of the sample surface can be measured with high precision.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the position measuring device of the present invention, and FIG. 2 is an example of the relationship between the change Δh in the height of the sample surface 3 in FIG. 1 and the amount of light I transmitted through the fixed grating 6. 3 is a graph showing an example of the relationship between the change in height Δh of the sample surface 3 in FIG. 1 and the output of the lock-in amplifier 11, and FIG. 4 is an example of a conventional position measuring device. It is a block diagram. 1... Vibration grating, 2... Light source, 3...
...Sample surface, 4.5...Lens, 6...
...Fixed grid, 7...Photodetector,
8... Amplifier, 9... Exciter, lO・
...m generator, 11 ... Rofin amplifier,
41... Light source, 42... Sample surface, 4
3, 44...Lens, 45...Detector, 46...Subtractor, 47...Adder,
48...Divider.

Claims (1)

[Claims] a projection optical system that projects a first grating image formed by a first grating from an oblique direction onto a sample surface; and a projection optical system that refocuses the first grating image reflected on the sample surface to form a second grating image. a second grating arranged on an imaging plane of the light receiving optical system, a detector that detects the amount of light passing through the second grating, and a first grating projected onto the sample surface. A position measuring device comprising: means for vibrating an image; and means for phase-detecting the output of the detector modulated by the means for vibrating the first grating image.