Two dimensional beam deflector

Abstract

A two dimensional beam deflector is disclosed which deflects beams from multiple optical assemblies. The input of beams of the multiple optical assemblies follow parallel optical paths until deflection to a wafer. An ellipsometer using a two-dimensional beam deflector is also disclosed.

Description

CROSS REFERENCE TO RELATED APPLICATION

where Θ is the angle of incidence and Θ′ is the tilt angle of the image plane.

In the monitor of FIG. 5, the grating 167, tilted by the angle Θ relative to axis 188, is the imaging plane for the imaging system of lenses 152 and 164 and is the object plane (or intermediate image plane) for the imaging system of lens 180.

Since the magnification M of the monitor is 1, the object plane (sample surface) and the image plane are tilted at the same angle Θ relative to their optical axes. Unfortunately, when the image plane is thus tilted, only a portion of it can be focussed on the CCD 186.

In order to force the image plane to be perpendicular to the optical axis, thereby ensuring that the entire image plane is focused on the CCD 186, the grating 167 is aligned along the intermediate image plane. The grating 167 is chosen so that only the first diffraction maximum is utilized for imaging to the CCD 186. For example, for lambda_3=780 nm. Θ=70° and a grating frequency 1200 cycles/mm, the image plane of a diffracted beam 190 is almost perpendicular to the optical axis. Therefore the image plane may be enlarged by the magnifying lens 180 without the above-described Scheimpflug problem.

The magnifying lens 180 is placed in the direction of the first diffraction maximum for the wavelength lambda_3 of the incoherent illuminator. The lens 180 provides magnification, of at least 5×, which is strong enough for recognition of the test areas on the wafer.

Mirror 182 directs the diffracted beam 190 towards the CCD 186 which is located at the image plane of magnifying lens 180. The high resolution CCD 186, such as the TM-6CN CCD manufactured by Pulnix America Inc. of Sunnyvale. Calif., U.S.A.. transforms the image to a video signal.

If lasers 130 and 132 operate at the same time as the incoherent light source 134, the optional band pass filter 184 ensures that only light from the incoherent illuminator is used for imaging.

If the incoherent light is not monochromatic, the diffraction of grating 167 will produce spectral dispersion. In this case, the numerical aperture of lens 180 has to be high enough to collect most, if not all, of the diffraction beams within the entire spectral range. In order to achieve high image quality, the lens 180 should be corrected for the chromatic aberrations in the spectral range of the incoherent light.

The quality of the obtained image depends strongly on the F-stops (F#) of the objective and imaging lenses 152 and 164 and the aberrations that they produce. For a focal length for lenses 152 and 164 of about 30 mm and a beam diameter of about 5 mm, the F# is about 6, a value which is used for high quality photography and other applications.

The lens 180 is a high quality microscope grade objective lens and therefore, does not significantly affect the final image quality. The grating 167 also does not significantly affect the image quality because its spatial frequency in the image plane (1200/5×=240 cyc/mm) is significantly greater than the Nyquist frequency of the CCD (about 50 cyc/mm). Therefore the above two-dimensional image acquisition system provides the high spatial resolution needed for accurate pattern recognition.

For the two-dimensional image acquisition system, the pinhole 166 serves to locate the measurement spot in the image of the sample 57. Since the pinhole 166 is located at the intermediate image plane (grating 167) and since it allows light to pass through it, rather than being reflected toward the CCD 186, the pinhole 166 appears as a sharp dark point in the image produced by the CCD 186. Thus, when viewing the CCD image, the location of the measurement spot is immediately known, it being the location of the dark spot.

It is noted that the pinhole 166 performs three functions: 1) it reduces the scattered light from the mirrors 154 and 156; 2) it provides high spatial resolution; and 3) it provides an indication of the location of the measurement spot in the image of the area being measured.

The angle of incidence measurement unit provides feedback about the actual angle of incidence Θ for situations when the surface of the sample 57 is not absolutely flat.

The angle of incidence measurement unit typically comprises the objective lens 152, a beam-splitter 194, a spectral filter 196 and a position-sensitive detector (PSD) 198, such as the S2044 detector manufactured by Hamamatsu Photonics U.U. Beam splitter 194 provides a portion of reflected beam 159 to the PSD 198 via the spectral filter 196.

Since the measurement point on the sample 57 is in the focal plane of objective lens 152 and since the PSD 198 is in the far field of this lens, any change in the angle of reflection (equal to the angle of incidence on the non-flat surface of sample 57) displaces the light spot on the PSD 198 and may be accurately measured. If the light spot is centered on the PSD 198, the angle of incidence is expected angle of incidence Θ, typically of 70°. If the spot is below or above the central location, the actual angle of incidence is larger or smaller, respectively, than Θ. The relationship describing the extent of the skew with the change in the actual angle of incidence is calibrated prior to operating the monitor. The ellipsometric measurements are then interpreted, by a data processor (not shown), in light of the actual angle of incidence.

It is noted that most supports typically comprise means (not shown) for keeping the top surface of sample 57 in a fixed position, such as by vacuum or electrostatic clamping, an auto-focus mechanism. Once such systems have been actuated, the only reason for beam displacement on the PSD 198 is the deviation in the angle of incidence.

Reference is now made to FIG. 6 which illustrates a spectrophotometer designed in accordance with the principles of the present invention. Thus, the spectrophotometer comprises a light source 200, a beam deflector 202, a support 204 holding a sample 208, a detector unit 206.

The beam deflector 202 typically comprises an objective lens 210, a beam splitter 212 and a mirror 214. From the light source 200, the beam splitter receives a light beam 209, parallel to the surface of the sample 208, and deflects it toward the sample 208, via lens 210. The sample 208 is located in the focal plane of lens 210. The reflected light beam (not labeled) is collimated by lens 210, passes through beam splitter 212 and is deflected by mirror 214 along an axis parallel to the surface of sample 208.

Since light beams 209 and 211 are parallel to each other and to the surface of sample 208, the movement of the beam deflector 202 in the X direction does not affect the measurements of the detector unit 206.

As in the embodiments of FIGS. 2 and 3, detector unit 206 typically comprises an imaging lens 218, a diaphragm 220 having a pinhole 222 and a condenser 224. Detector unit 206 additionally comprises a spectrophotometric detector 226. Elements 218-224 reduce the scatter due to mirror 214 and provide a useful beam to the spectrophotometric detector 226.

Reference is now made to FIG. 7 which illustrates a further embodiment of the present invention. In this embodiment, the beam deflector, labeled 300, deflects beams for multiple optical assemblies towards a wafer 301. FIG. 7 illustrates two optical assemblies, an ellipsometric assembly 302 and a spectrophotometric assembly 304, one above the other. It will be appreciated that the order of the two assemblies is not of interest.

The spectrophotometric assembly 304 comprises, shown by a box labeled 306, the entirety of optical and controlling elements needed for spectrophotometry, except those elements which deflect the light beam towards the wafer 380. The details of the elements of box 306 have been provided hereinabove with respect to FIG. 6 and therefore, will not be described in detail herein. Spectrophotometric assembly 304 also comprises a mirror 308 for deflecting a light beam, labeled 305, from assembly 304 to the beam deflector 300 and vice versa.

The ellipsometric assembly 302 comprises an illuminator 310 and an analyzer 312, similar in function to the illuminator 100 and analyzer 102 of FIG. 4, and two mirrors 314 and 316 for respectively deflecting an input light beam 311 from illuminator 310 to the beam deflector 300 and an output light beam 313 from the beam deflector 300 to the analyzer 312.

Below both assemblies 302 and 304 is a fixed input/output surface 320 on which are mounted an input mirror 330 and an output mirror 342. The input and output mirrors 330 and 342, respectively, deflect light beams into and out from the optical assemblies 302 and 304.

Mechanically, the beam deflector 300 is similar to the beam deflector of FIG. 3 and comprises a translatable Y-axis stage 322 (shown in two sections 322a and 322b) and a translatable X-axis stage 324 which translates along the Y-axis stage 322.

Optically, the beam deflector 300 comprises a Y-to-X mirror 332, a spectrophotometry mirror 334, an input ellipsometry mirror 336, an output ellipsometry mirror 338 and an X-to-Y mirror 340. The Y-to-X mirror 332 and X-to-Y mirror 334 are mounted on the separate sections of the Y-axis stage 322, where the Y-to-X mirror 332 deflects input light beams from the Y to the X axis and the X-to-Y mirror 334 deflects output light beams from the X to the Y axis. The remaining elements are mounted on the X-axis stage and perform the actual optical measurement.

For the spectrophotometric measurements, light beam 305 is deflected by input mirror 330 in the Y direction along a first Y axis (not illustrated for clarity) towards the Y-to-X mirror 332 which, in turn, deflects the light beam, now labeled 350, towards the spectrophotometric mirror 334. Mirror 334 deflects the light beam 350 towards the wafer 301 at an angle perpendicular to the wafer 301. The reflected light returns to mirror 334 and is reflected back through mirrors 332 and 330 to the spectrophotometric assembly 304.

For the ellipsometric measurements, light beam 311 also is reflected by mirrors 330 and 332 towards the X-axis stage 324. However, as shown in FIG. 7, light beam 311 is reflected along second set of X and Y-axis (also not illustrated) close but parallel to the first set of axes along which the light beam 305 is reflected. Thus, the two light beams are deflected along parallel paths until reaching the X-axis stage 324.

The input ellipsometry mirror 336 is positioned on the X-axis stage at the appropriate height above the wafer so as to receive “ellipsometric” light beam, labeled 351, from the Y-to-X mirror 332. Mirror 336 deflects beam 351 towards the wafer 301 so as to impinge upon wafer 301 at the appropriate angle α. The angle α, as shown in FIG. 7, is the angle from the normal to the wafer 301. The actual angle of incidence at the time of performing the measurements must be measured for proper ellipsometric measurements. The method described hereinabove with respect to FIG. 5C can be utilized

The reflected beam, labeled 353, impinges upon mirror 338 and is deflected thereby towards X-to-Y mirror 340. From mirror 340, the output beam travels along the Y direction and is deflected by mirror 342 into the ellipsometric assembly 302. Mirror 316, in turn, directs the output beam into the analyzer 312 for processing.

It will be appreciated that the beam deflector 300 provides translation for the measuring optics of a multiplicity of optical measuring units as a single unit. The type of measuring unit does not matter; rather, the optical paths at the point of measurement should not cross. In the embodiment of FIG. 7, the spectrophotometric measuring optics view the wafer perpendicularly to the wafer while the ellipsometric measuring optics view the wafer at an angle. The deflecting mirrors of the two measuring units can thus be combined together into a single, translatable unit.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the claims which follow:

Claims

1. A two-dimensional beam deflector for a thickness measuring device for measuring the thickness of films on a sample with a plurality of different optical systems each performing a different measurement technique, the beam deflector comprising:

two-dimensional translation means for translating said beam deflector along a first scanning axis and along a second scanning axis perpendicular to said first scanning axis;

first deflection means for receiving a plurality of parallel input beams along parallel input axes, said input axes being close to each other and parallel to said first scanning axis, and for deflecting said input beams along a plurality of parallel second axes, said second axes being close to each other and parallel to said second scanning axis;

second deflection means for receiving a plurality of parallel output beams along parallel third axes, said third axes being close to each other and parallel to said second axes, and for deflecting said output beams along a plurality of parallel fourth axes, said fourth axes being close to each other and parallel to said first scanning axis; and

a plurality of optical assemblies, one per input beam, wherein each optical assembly provides its input beam towards said sample, receives its output beam from said sample, processes its input and output beams in accordance with its measurement technique, and provides its output beams along said parallel third axes.

2. A beam deflector according to claim 1 and wherein said optical assemblies comprises at least an ellipsometric assembly and a spectrophotometric assembly.

3. A thickness measuring device for measuring the thickness of thin films on a sample with two measurement devices, the device comprising:

first and second stationary illuminators, one for each of said two measurement devices, for providing first and second collimated input light beams along first and second parallel input axes;

a beam deflector for directing said first and second input light beams toward said sample and for directing and collimating corresponding first and second output light beams from said sample, said beam deflector including two-dimensional translation means for translating said beam deflector along a first scanning axis parallel to said input axis, and along a second scanning axis perpendicular to said first scanning axis; and

first and second stationary receivers, one for each of said two measurement devices, for respectively receiving said first and second output light beams along output axes parallel to said input axes.

4. An ellipsometer for measuring the thickness of thin films on a sample comprising:

a stationary illuminator for providing a collimated input light beam along an input axis:

a beam deflector translatable at least along a first scanning axis parallel to said input axis including:

a first beam deflecting element for deflecting said input light beam at a first angle of deflection towards said sample;

a second beam deflecting element, different from said first beam deflecting element for deflecting an output light beam reflected at a second angle from said sample along an output axis; and

a collimating lens for receiving at least said output light beam from said second beam deflecting element and for collimating at least said output light beam; and

a stationary receiver for receiving said collimated output light beam along an output axis parallel to said input axis.

5. A device according to claim 4 and wherein said beam deflector comprises two-dimensional translation means for translating said beam deflector along said first scanning axis and along a second scanning axis perpendicular to said first scanning axis.

6. A device according to claim 5 where said beam deflector additionally comprises a first mirror for deflecting said input light beam from said input axis to said second scanning axis, a second mirror for deflecting said input light beam from said second scanning axis to said sample, a third mirror for deflecting a reflected light beam reflected from said sample to said second scanning axis, and a fourth mirror for deflecting said reflected light beam from said second scanning axis to said output axis.

7. An ellipsometer for measuring the thickness of thin films on a sample comprising:

a stationary illuminator for providing a collimated input light beam along an input axis;

a beam deflector translatable at least along a first scanning axis parallel to said input axis including:

a first beam deflecting element for deflecting said input light beam at a first angle of deflection towards said sample;

a second beam deflecting element, different from said first beam deflecting element for deflecting an output light beam reflected at a second angle from said sample along an output axis; and

a collimating lens for receiving at least said output light beam from said second beam deflecting element and for collimating at least said output light beam; and

a stationary receiver for receiving said collimated output light beam along an output axis parallel to said input axis,

wherein said beam deflector comprises one-dimensional translation means for translation along said scanning axis.

8. An ellipsometer for measuring the thickness of thin films on a sample comprising:

a stationary illuminator for providing a collimated input light beam along an input axis;

a beam deflector translatable at least along a first scanning axis parallel to said input axis including:

a first beam deflecting element for deflecting said input light beam at a first angle of deflection towards said sample;

a second beam deflecting element, different from said first beam deflecting element, for deflecting an output light beam reflected at a second angle from said sample along an output axis; and

a collimating lens for receiving at least said output light beam from said second beam deflecting element and for collimating at least said output light beam; and

a stationary receiver for receiving said collimated output light beam along an output axis parallel to said input axis,

wherein said first and second beam deflecting elements are mirrors.

9. An ellipsometer for measuring the thickness of thin films on a sample comprising:

a stationary illuminator for providing a collimated input light beam along an input axis;

a beam deflector translatable at least along a first scanning axis parallel to said input axis including:

a first beam deflecting element for deflecting said input light beam at a first angle of deflection towards said sample;

a second beam deflecting element, different from said first beam deflecting element, for deflecting an output light beam reflected at a second angle from said sample along an output axis; and

a collimating lens far receiving at least said output light beam from said second beam deflecting element and for collimating at least said output light beam; and

a stationary receiver for receiving said collimated output light beam along an output axis parallel to said input axis,

wherein said first beam deflecting element is a beam splitter and said second beam deflecting element is a mirror.

10. An ellipsometer for measuring the thickness of thin films on a sample comprising:

a stationary illuminator for providing a collimated input light beam along an input axis;

a beam deflector translatable at least along a first scanning axis parallel to said input axis including:

a first beam deflecting element for deflecting said input light beam at a first angle of deflection towards said sample;

a second beam deflecting element, different from said first beam deflecting element, for deflecting an output light beam reflected at a second angle from said sample along an output axis; and

a collimating lens for receiving at least said output light beam from said second beam deflecting element and for collimating at least said output light beam; and

a stationary receiver for receiving said collimated output light beam along an output axis parallel to said input axis,

and also including means for measuring an actual angle of incidence which may vary from said second angle of deflection, wherein said means for measuring utilizes optical elements forming part of said stationary illuminator and stationary receiver.

11. A device according to claim 10 and wherein said means for measuring comprises a position sensing device for measuring the angle of said output light beam with respect to a desired position.

12. An optical measurement system for monitoring at least one desired parameter of a sample, the system comprising:

a first optical path for directing a polychromatic probe beam substantially normal to the sample surface and collecting returned light therefrom;

a second optical path for directing a polarized probe beam at an oblique angle to the sample surface and collecting returned light therefrom;

at least one common light directing element forming said first and second optical paths.

13. An optical measurement system according to claim 12, wherein said first optical path forms a spectrophotometric measurement channel.

14. An optical measurement system according to claim 13, wherein said second optical path forms an ellipsometeric measurement channel.

15. An optical measurement system according to claim 13, wherein said ellipsometeric measurement channel is capable of carrying out an ellipsometric measurement at a plurality of wavelengths.

16. An optical measurement system according to claim 13, wherein said spectrophotometric measurement channel includes a broadband illuminator.

17. An optical measurement system according to claim 13, wherein said first optical path comprises an objective lens arrangement.

18. An optical measurement system according to claim 12, wherein said second optical path forms an ellipsometeric measurement channel.

19. An optical measurement system according to claim 12, comprising an imaging unit operable for performing a pattern recognition technique for determining a measurement site location.

20. An optical measurement system according to claim 12, wherein said at least one desired parameter is a thickness of at least a top layer of the sample.

21. An optical measurement system according to claim 12, wherein said at least one desired parameter is a refraction index of at least a top layer of the sample.

22. An optical measurement system according to claim 12, comprising an optical monitoring unit configured for defining said first and second optical paths and operable to scan at least a portion of the sample.

23. An optical measurement system according to claim 12, wherein said sample is a semiconductor wafer.

24. A cluster tool comprising several chambers for processing of a semiconductor sample within a working area of the tool, wherein said chamber contains a monitoring area comprising:

an optical measurement system for monitoring at least one desired parameter of a sample, the monitoring system comprising:

a first optical path for directing a polychromatic probe beam substantially normal to the sample surface and collecting returned light therefrom;

a second optical path for directing a polarized probe beam at an oblique angle to the sample surface and collecting returned light therefrom;

at least one common light directing element forming said first and second optical paths.

25. The cluster tool of claim 24 comprising a robot for transferring the sample from said working area to said monitoring area.

26. The cluster tool of claim 25 operable under certain vacuum conditions, said robot transferring the sample from the working area to the monitoring area without breaking said vacuum conditions.