DEVICE FOR MEASURING A SUBSTRATE AND METHOD FOR CORRECTING CYCLIC ERROR COMPONENTS OF AN INTERFEROMETER

Abstract

The invention relates to a device for measuring a substrate for semiconductor lithography with a reference interferometer for ascertaining the change in the ambient conditions, wherein the reference interferometer comprises a means for changing the optical path length of a measurement section of the reference interferometer, wherein the means is configured to bring about a change in the refractive index.

Furthermore, the invention relates to a method for correcting cyclic error components of a reference interferometer, wherein the reference interferometer comprises a means for changing the optical path length of a measurement section of the reference interferometer, comprising the following method steps: starting up the reference interferometer, continuously detecting measurement values of the reference interferometer, changing the optical path length of the measurement section of the reference interferometer until a path length change of at least one quarter of the wavelength of the reference interferometer is detected, determining the cyclic errors on the basis of the continuously detected measurement values of the reference interferometer, and correcting the current measurement values ascertained by the reference interferometer on the basis of the cyclic errors ascertained.

Claims

1. A device for measuring a substrate for semiconductor lithography with a reference interferometer for ascertaining the change in the ambient conditions, wherein the reference interferometer comprises a means for changing the optical path length of a measurement section of the reference interferometer, wherein the means is configured to bring about a change in the refractive index.

2. The device of claim 1, wherein the means is configured to bring about the change in the refractive index by a change in the pressure and/or the moisture and/or the temperature of a purge gas in the reference interferometer.

3. The device of claim 1, wherein the means is configured to bring about the change in the refractive index by a change in the composition of the purge gas in the reference interferometer.

4. The device of claim 1, wherein the means comprises a purge device.

5. The device of claim 1, wherein the reference interferometer is configured to bring about the change in the optical path length by the movement of a mirror of the reference interferometer.

6. The device of claim 5, comprising an actuator for moving the mirror.

7. The device of claim 6, wherein the actuator is embodied as a piezoactuator.

8. The device of claim 1, wherein an open-loop control and/or a closed-loop control for manual open-loop control and/or closed-loop control of the means are/is present.

9. The device of claim 1, wherein an open-loop control and/or a closed-loop control for electronic open-loop control and/or closed-loop control of the means are/is present.

10. A method for correcting cyclic error components of a reference interferometer, wherein the reference interferometer comprises a means for changing the optical path length of a measurement section of the reference interferometer, comprising the following method steps: starting up the reference interferometer, continuously detecting measurement values of the reference interferometer, changing the optical path length of the measurement section of the reference interferometer until a path length change of at least one quarter of the wavelength of the reference interferometer is detected, determining the cyclic errors on the basis of the continuously detected measurement values of the reference interferometer, and correcting the measurement values ascertained by the reference interferometer on the basis of the cyclic errors ascertained.

11. The method of claim 10, wherein the change in the optical path length is brought about by the change in the pressure and/or the moisture and/or the temperature of the purge gas situated in the reference interferometer.

12. The method of claim 10, wherein the change in the optical path length of the reference interferometer is brought about by a change in the composition of the purge gas in the reference interferometer.

13. The method of claim 12, wherein the composition of the purge gas in the reference interferometer during operation corresponds to the composition of the purge gas of an interferometer for detecting the position of an object stage and/or of an imaging optical unit of the device.

14. The method of claim 12, wherein the composition of the purge gas comprises one or more of the following gases: air, helium, hydrogen, nitrogen, carbon dioxide, sulfur hexafluoride, neon.

15. The method of claim 10, wherein the optical path length is brought about by displacing a mirror of the reference interferometer.

16. The method of claim 12, wherein the optical path length change amounts to at least one quarter of the wavelength of the reference interferometer, in particular to said wavelength, in particular to double said wavelength.

17. The method of claim 10, wherein the reference interferometer corrected on the basis of the cyclic errors ascertained is used for determining changes in ambient conditions for the correction of an interferometer for detecting the position of an object stage and/or of an imaging optical unit of the device.

18. The device of claim 2, wherein the reference interferometer is configured to bring about the change in the optical path length by the movement of a mirror of the reference interferometer.

19. The device of claim 3, wherein the reference interferometer is configured to bring about the change in the optical path length by the movement of a mirror of the reference interferometer.

20. The method of claim 11, wherein the optical path length is brought about by displacing a mirror of the reference interferometer.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0028] Exemplary embodiments and variants of the invention are explained in more detail below with reference to the drawings. In the figures:

[0029] FIG. 1 shows a basic illustration of a registration tool in which the invention is realized,

[0030] FIG. 2 shows a detailed view of the invention, and

[0031] FIG. 3 shows a flow diagram concerning a method according to the invention.

DETAILED DESCRIPTION

[0032] FIG. 1 shows a basic illustration of a registration measuring instrument 1 for photomasks for semiconductor lithography, illustrating an enclosure 2 with an imaging device 4 and a measuring device 10. The imaging device 4 comprises an image capturing device 7, an imaging optical unit 5 and an object stage 6. The object 3, which can be embodied as a photomask or a wafer for semiconductor lithography, is illuminated by an illumination device (not illustrated) either in a reflected light method or in a transmitted light method and is imaged onto the image capturing device 7 by the imaging optical unit 5. Said image capturing device 7 comprises a camera (not illustrated) that captures the image of the object 3. The object 3 is arranged on an object stage 6, which can move the object 3 relative to the imaging optical unit 5, whereby structures at different locations on the object 3 are imaged onto the image capturing device 7. The position of the object stage 6 is detected by an interferometer 13 in the range of a few nanometres, such that the position of the structure on the object 3 can be deduced therefrom. The interferometer 13 is part of a measuring device 10 which, besides the interferometer 13, also comprises a reference interferometer 20, a purge device 18, a purge chamber 15, a light source 11 and an open-loop control 19. The light source 11 generates light having a wavelength, which light is guided via optical waveguides 12 to the interferometer 13 and the reference interferometer 20. The interferometers 13, 20 are operated as heterodyne interferometers, that is to say with light having different wavelengths. The reference interferometer 20 ascertains the change in the wavelength on account of the change in ambient conditions, as a result of which the position ascertained by the interferometer 13 can be corrected. The ambient conditions in the enclosure 2 and in the purge chamber 15 of the reference interferometer 20 are identical during operation of the registration measuring instrument 1, air being used as purge gas. The purge gas is conditioned by the purge device 18 and introduced into the enclosure 2 and the purge chamber 15. The ambient conditions in the enclosure 2 and in the purge chamber 15 change equally as a result during normal operation of the device, as a result of which the changes in the reference interferometer 20 which are attributable to the alteration of the optical path length can be used as correction for the detected path length of the interferometer 13.

[0033] FIG. 2 shows a detail of the invention illustrating the reference interferometer 20, the purge chamber 15 and the feed 16 and the return 17 to the purge device 18. The purge gas is fed to the purge chamber 15 by the feed 16 and returned by the return 17 to the purge device 18 again, where said purge gas is conditioned and fed to the purge chamber 15 again. The reference interferometer 20 is embodied as a Fabry-Perot interferometer comprising an input coupling 23, a polarizing beam splitter 26, a first mirror 24, a second mirror 25 and a housing 22. The light 21 emitted by the light source (not illustrated) is guided via an optical waveguide 12 to the housing 22 and is coupled into the housing 22 of the reference interferometer 20 at the input coupling 23. The polarizing beam splitter 26 splits the light 21 into a measurement beam 32 and a reference beam 33. The reference beam 33 is deflected by 90° in the beam splitter 26 and, downstream of the beam splitter 26, passes through a λ/4 plate 30.1 in order subsequently to be deflected by a further 90° at a deflection mirror 29, such that the reference beam 33 once again runs parallel to the measurement beam 32. The reference beam 33 is reflected back on itself at the first mirror 24 at a highly reflective coating 34.1 arranged in the lower region of the mirror 24.1. Downstream of the beam splitter 26, the measurement beam 32 likewise passes firstly through a λ/4 plate 30.2 and then through the first mirror 24, which has no reflective coating in its upper region 24.2. The measurement beam 32, in its further course, is likewise reflected at a second mirror 25 having a highly reflective coating 34.2. The distance between the first mirror 24 and the second mirror 25 is the measurement section 35, with the aid of which the alterations of the ambient conditions are determined. The reflected reference beam 33 and the reflected measurement beam 32 return on the same path after reflection, are recombined in the polarizing beam splitter 26 and are fed to a detector 31 for evaluation. In the purge chamber 15, the temperature, the moisture and optionally the pressure of the purge gas, which comprises air during operation, that is to say when the registration measuring instrument measures substrates, can be closed-loop controlled in a comparatively simple manner. For the case where the cyclic errors of the reference interferometer 20 are intended to be determined, the optical path length in the reference interferometer 20 is altered. This is brought about either by the change in the pressure, the moisture or the temperature of the purge gas or by a change in the composition of the purge gas. The purge device 18 is embodied such that besides air as purge gas it can also feed helium or some other gas as purge gas to the purge chamber 15. As a result of the refractive index of helium deviating from that of air, the optical path length in the reference interferometer 20 is altered. The path length change is detected continuously during the change in the refractive index and the cyclic errors are determined on the basis of the detected data. Besides the change in the purge gas, the optical path length of the reference interferometer can also be brought about by the movement of one of the two mirrors 24, 25. For this purpose, an actuator 27 and a sensor 28 are arranged at the second mirror 25, whereby the second mirror 25 can be positioned in a controlled manner. The position of the mirror 25 is kept constant during the operation of the registration measuring instrument. During the determination of the cyclic errors, the mirror 25 is moved along the geometric length of the reference interferometer 20 by the actuator and in the process shortens or lengthens the geometric and thus the optical path length of the resonator and thus of the reference interferometer 20. Besides the arrangement in which the reference interferometer 20 is surrounded by a purge chamber 15 as shown in FIG. 1 and FIG. 2, an arrangement in which the reference interferometer 20 does not comprise a dedicated purge chamber 15 is also conceivable. In this case, the purge device 18 closed-loop controls the ambient conditions in the entire enclosure 2. This solution is suitable in particular for retrofitting registration measuring machines 1, since the existing infrastructure can be used.

[0034] FIG. 3 describes one possible method for correcting cyclic error components of a reference interferometer 20, wherein the reference interferometer 20 comprises a means 18, 27 for changing the optical path length of a measurement section of the reference interferometer 20.

[0035] In a first method step 41, the reference interferometer 20 is started up.

[0036] In a second method step 42, the measurement values of the reference interferometer 20 are continuously detected.

[0037] In a third method step 43, the optical path length of the measurement section of the reference interferometer 20 is altered until a path length change of at least one quarter of the wavelength of the reference interferometer 20 is detected.

[0038] In a fourth method step 44, the cyclic error is determined on the basis of the continuously detected measurement values of the reference interferometer 20.

[0039] In a fifth method step 45, the measurement values ascertained by the reference interferometer are corrected on the basis of the cyclic errors ascertained.

LIST OF REFERENCE SIGNS

[0040] 1 Registration measuring instrument, device [0041] 2 Enclosure [0042] 3 Object [0043] 4 Imaging device [0044] 5 Imaging optical unit [0045] 6 Object stage [0046] 7 Image capturing device [0047] 10 Measuring device [0048] 11 Light source [0049] 12 Optical waveguide [0050] 13 Interferometer [0051] 14 Wave normal [0052] 15 Purge chamber [0053] 16 Feed [0054] 17 Return [0055] 18 Purge device [0056] 19 Open-loop control/closed-loop control [0057] 20 Reference interferometer [0058] 21 Light [0059] 22 Housing [0060] 23 Input coupling [0061] 24, 24.1, 24.2 First mirror [0062] 25 Second mirror [0063] 26 Polarizing beam splitter [0064] 27 Actuator [0065] 28 Sensor [0066] 29 Deflection mirror [0067] 30.1, 30.2 λ/4 plate [0068] 31 Detector [0069] 32 Measurement beam [0070] 33 Reference beam [0071] 34.1, 34.2 Coating [0072] 35 Measurement section [0073] 41 Method step 1 [0074] 42 Method step 2 [0075] 43 Method step 3 [0076] 44 Method step 4 [0077] 45 Method step 5