WO2001055671A1

WO2001055671A1 - Fiber-filtered laser system for use in measuring thin film thicknesses

Info

Publication number: WO2001055671A1
Application number: PCT/US2000/041610
Authority: WO
Inventors: Alan George; William Yang; Joanne Law
Original assignee: Therma-Wave, Inc.
Priority date: 2000-01-31
Filing date: 2000-10-25
Publication date: 2001-08-02

Abstract

The present invention relates to a fiber-filtered, semiconductor diode laser configuration that yields a light source having optical properties for Beam Profile Reflectometry (BPR) and Beam Profile Ellipsometry (BPE) applications that are superior to those achieved in the prior art. The present invention achieves these superior optical properties by coupling the semiconductor laser diode to a single-mode polarization-preserving fiber of sufficient length to effectively extinguish all but the dominant transverse mode. The fiber output is mechanically mounted with a high degree of stability relative to a simple collimating lens. A polarization cube may be inserted into the optical path either before or after the collimating lens in order to assure a high degree of polarization purity and stability.

Description

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE

APPLICATION FOR PATENT

TITLE: FIBER-FILTERED LASER SYSTEM FOR USE IN MEASURING THIN FILM THICKNESSES

Field Of The Invention

Beam profile reflectometry (BPR) and beam profile ellipsometry (BPE) are known optical techniques for measuring the thicknesses of thin films in semiconductor manufacturing and other applications. The BPR and BPE techniques require a laser light source having certain stringent optical properties. The present invention relates to a fiber- filtered, semiconductor diode laser configuration that yields a light source having optical properties for BPR and BPE applications that are superior to those achieved in the prior art.

Background Of The Invention

There is considerable need to accurately measure the thicknesses of thin films, particularly in the semiconductor manufacturing industry. Two examples of techniques for making such measurements are BPR and BPE. A suitable BPR technique is described in U.S. Patent No. 4,999,014, which issued on March 12, 1991 , and is hereby incorporated by reference in its entirety. Suitable BPE techniques are described in U.S. Patent Nos. 5,042,951 and 5,181 ,080, which patents issued on August 27, 1991 and January 19, 1993 respectively and are hereby incorporated by reference in their entireties. Implementation of BPR or BPE techniques requires very precise measurements of light beam profiles by photodetectors. It is critical for these techniques that the light beam parameters be stable over time because any change in the beam parameters will cause drift in the measurements made by the photodetector. Accordingly, laser pointing stability, beam profile smoothness and stability, and beam polarization purity and stability are all of utmost importance. In addition, it is also desirable to have a beam power output that is stable and to have a beam delivery system that is not sensitive to feedback. It is highly desirable to have a smooth transverse profile for the collimated laser beam because such a profile increases the tolerance for small variations in the pointing of the beam. In other words, the BPR and BPE measurements are more tolerant of beam pointing variations if the laser profile is smoother. With BPR, for example, typically a reference reflectance profile is taken on bare silicon and this profile is then used to normalize subsequent measurements. Because of the normalization step, drift in the pointing of the light beam will tend to introduce error into subsequent measurements. However, a smooth Gaussian profile for the light beam tends to minimize the measurement error resulting from pointing drift. Since drift in the pointing of the beam cannot be completely eliminated, a smooth Gaussian beam profile is highly desirable.

In prior art BPR and BPE systems, the laser source requirements have been met by the use of a semiconductor diode laser whose output beam was collimated by an expensive, high quality, and high numerical aperture (NA) lens. The output beam, typically elliptical in cross-section, was then circularized by a pair of prisms. Beam power and mode stability were achieved by careful control of diode drive current and the temperature of the laser resonant cavity. The prior art system as described above has several disadvantages. The high NA collimating lens needed for the prior art system is expensive. In addition, the degree of alignment required for the collimating optics and the circularization prisms is such as to make difficult both the initial assembly of the system and any re-alignment needed when parts are replaced during maintenance. Another significant problem is that laser beam profiles had significant structure that interacted with beam pointing variations to cause measurement errors. Stress in the circularization prisms could also cause degradation of the beam polarization.

Summary of the Invention

It is an object of the present invention to permit beam collimation to be achieved by a lower NA and more inexpensive lens.

It is a further object of the invention that alignment of the collimation optics is much simpler and easier to reproduce than in the prior art.

It is a further object of the invention that the need for circularization prisms is eliminated.

It is a further object of the invention to achieve a smoother, more Gaussian transverse beam profile for the light beam, making BPR and BPE measurements more tolerant to variations in beam pointing.

It is a further object of the invention to reduce sensitivity to feedback, particularly with respect to beam profile variations.

It is yet another object of the present invention that the laser diode may be replaced without the need to re-align the collimation optics.

The present invention achieves this and other objects by coupling the semiconductor laser diode to a single-mode polarization-preserving fiber of sufficient length to effectively extinguish all but the dominant transverse mode. The fiber output is mechanically mounted with a high degree of stability relative to a simple collimating lens. A polarizer may be inserted into the optical path either before or after the collimating lens in order to assure a high degree of polarization purity and stability.

Brief Description of the Figures Fig. 1 shows a prior art configuration for providing a light beam source for performing BPR or BPE techniques. Fig. 2 shows a preferred embodiment of the present invention for providing a light beam source for performing BPR or BPE techniques.

Fig. 3 shows a high-level block diagram of an apparatus suitable for performing BPR. Fig. 4 shows a high-level block diagram of an apparatus suitable for performing BPE.

Detailed Description Of The Invention.

Fig. 1 depicts an optical path used in the prior art to supply a light beam for BPR and BPE applications. A suitable power supply circuit 3 is used to drive a semiconductor diode laser 5. The laser beam is collimated by an expensive, high NA lens 11. Two prisms 14 and 16 are used to circularize the laser beam, which would otherwise typically have an elliptical cross-section. The laser 5, collimating lens 11 , and circularization prisms 14 and 16 must all be maintained in a state of precise alignment. Although the prior art system shown in Fig. 1 provides a workable laser light source for BPR and BPE applications, the laser beam profile is not ideal and still has significant structure which causes measurement error.

Fig. 2 shows an optical path according to the present invention for use in supplying a light beam for measuring thin films, particularly through BPR and BPE applications. A power supply circuit 2 is used to drive the semiconductor diode laser 4. Alternatively, lasers other than semiconductor diode lasers could be also used, such as gas discharge lasers. A lens 6 is used to focus the laser beam in order to help launch the laser beam into a fiber optic cable 8. Preferably lens 6 is a gradient index lens having a refractive index which varies with the radial position within the body of the lens. Fiber optic cable 8 is preferably made from a single mode, polarization maintaining optical fiber. Such an optical fiber maintains the linear polarization state of the light that it transmits and supports only one mode of propagation at a selected wavelength. In the context of the present invention, the term "single mode" should be understood to describe an optical fiber as supporting only one transverse mode of propagation at a range of frequencies that includes those characterizing the beam of light being transmitted by the fiber. Suitable polarization maintaining optical fibers may be obtained from the vendor Wave Optics. A preferred optical fiber is a polarization maintaining optical fiber supporting only a single mode of propagation at wavelengths above 633 nm and obtained from Wave Optics. Such optical fibers are sold by Wave Optics under the designations "PM: Panda", "PM: Bow-Tie", and "PM: Oval-Inner Clad". These preferred Wave Optics fibers support only one mode of propagation at the 670 nm wavelength characterizing a preferred type of semiconductor laser diode 4.

It is also preferable to use a length of this optical fiber sufficient to insure that modal noise is extinguished and that only a single mode propagates forward. In a preferred embodiment of the invention, two meters of the Panda cable are used, but a longer or shorter optical cable could also be used. The filtering effect of the optical fiber on the laser beam profile is an important aspect of the present invention because this filtering effect produces a smoother, more Gaussian beam profile. The output beam from the fiber optic cable 8 is captured by a collimating lens 10. Multiple collimating lenses may also be used rather than a single lens. Typically the fiber optic cable has a numerical aperture of about .11. Because the output of the fiber optic cable has a lower divergence than diode laser 5, the collimating lens 10 does not need to have as high an NA as the prior art lens 11 (which typically has an NA of about .6), and consequently may be much less expensive. In order to insure a single polarization state for the output light beam, a polarization cube 12 is included in the optical path after the collimating lens 10. Although the output of the polarization maintaining fiber optic cable is already highly polarized, the polarization cube 12 permits an even higher polarization purity to be achieved. The transmission plane of the polarization cube 12 must be aligned with the polarization axis of the optical fiber selected for transmission. In turn, it is also necessary for the selected polarization axis of the optical fiber to be aligned with the polarization axis of the laser light beam emitted by the laser 4. The polarization cube 12 functions to create an output beam with a single polarization plane to a very high level or precision. Extinction ratios for the polarization state of 200:1 and even 500:1 are readily achievable with the use of such polarization cubes. A different kind of polarizer such as a film polarizer could be inserted into the optical path either before or after the collimating lens 10 in lieu of the polarizing cube 12. However, the polarizing cube 12 is preferred because film polarizers are subject to interference effects that would be detrimental to the smoothness of the output beam profile. As is known in the art, the polarizing cube 12 is composed of two prisms bonded together with an index-matching cement. The hypotenuse face of one of the prisms is coated with a multi-layer dielectric coating such that the reflection from each layer is partially polarized. Because of the cumulative effect of the multi-layer coating, the polarizing cube produces a transmitted beam and a reflected beam, both of which are highly polarized. Placement of the polarizing cube 12 after the collimating lens 10 as shown is preferred.

The purpose of the optical configuration described above is to provide an improved light source for measuring thin film thicknesses, particularly through BPR and BPE techniques. A suitable BPR technique is described in U.S. Patent No. 4,999,014 and shown generally in Fig. 3. The optical configuration described above and shown in Fig. 2 serves well in place of the prior art as the laser 20 and delivery optics 22 of Fig. 3 for generating the polarized probe beam of light 24. In the preferred BPR technique as shown in Fig. 3, the polarized probe beam of light 24 is focused substantially normal to the surface of a sample using a high numerical aperture lens 30. The high numerical aperture lens 30 provides a large spread of angles of incidence for the rays within the incident focused beam. Light reflected from the sample 28 passes back up through the focusing optics and is directed to a detector. The position of the rays within the reflected beam correspond to specific angles of incidence with respect to the surface of the sample 28. The detector 50 measures the intensity of the various rays of the reflected probe beam, then, as a function of the angle of incidence with respect to the surface of the sample 28. A processor 52 functions to calculate the thickness or other characteristics of the thin film layer 32 based on these angle dependent intensity measurements using variations of the Fresnel equations.

While BPR techniques for measuring thin film thicknesses rely on various interferometric measurements, ellipsometry techniques rely on measuring changes in polarization. In an ellipsometer, a probe beam of light having a known polarization state is reflected from the sample. The thin film layer on the sample will effect the polarization state in a way that depends on its thickness. By measuring the polarization state of the reflected beam and comparing this state to the known polarization state of the incident beam, the thickness or other characteristics of the thin film can be calculated. Fig. 4 shows a preferred BPE technique as described in U.S. Patent No. 5,042,951. The optical configuration described herein and shown in Fig. 2 serves better than the prior art as the laser 32 and delivery optics 36 shown in Fig. 4 for generating the polarized beam of light 34. In the preferred BPE technique as shown in Fig. 4, the polarized probe beam of light 34 is focused substantially normal to the surface of a sample using a high numerical aperture lens 46. As with the BPR technique, the high numerical aperture lens 46 provides a large spread of angles of incidence for the rays within the incident focused beam. Light reflected from the sample 40 passes back up through the focusing optics and is directed to an analyzing section 48 for analyzing the change in the polarization state of the probe beam. Such an analyzing section 48 may include linear or circular polarizers and linear or circular analyzers. In one example of the BPE technique a rotating linear polarizer was used for this purpose. A detector 50 is placed in an optical path behind the analyzing device. Because the position of the rays within the reflected beam correspond to specific angles of incidence with respect to the surface of the sample 40, the detector 50 may measure the intensity of the various rays of the reflected probe beam as a function of the angle of incidence. A processor 52 functions to calculate the thickness or other characteristics of the thin film layer 42 based on these angle dependent intensity measurements using known mathematical relationships respecting the changes in the polarization state of the reflected beam.

The scope of the present invention is meant to be that set forth in the claims that follow and equivalents thereof, and is not limited to any of the specific embodiments described above.

Claims

What is claimed is:

1. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light; directing said probe beam to the surface of said sample; reflecting said probe beam from said sample; measuring optical properties of the reflected probe beam; and filtering said probe beam prior to directing it to said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

2. A method as recited in claim 1 wherein said filtering step includes using a fiber optic cable.

3. A method as recited in claim 2 wherein said fiber optic cable is of a single mode, polarization maintaining type.

4. A method as recited in claim 1 further including the step of polarizing said probe beam prior to directing it to said sample.

5. A method as recited in claim 4 wherein said polarizing step includes using a polarizing cube.

6. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light; means for directing said probe beam to the surface of said sample; means for measuring optical properties of the probe beam as reflected from said sample; and means for filtering said probe beam prior to directing it to said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

7. An apparatus as recited in claim 6 wherein said means for filtering includes a fiber optic cable.

8. An apparatus as recited in claim 7 wherein said fiber optic cable is of a single mode, polarization maintaining type.

9. An apparatus as recited in claim 6 further including means for polarizing said probe beam prior to directing it to said sample.

10. An apparatus as recited in claim 9 wherein said means for polarizing includes a polarizing cube.

11. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light; focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; measuring the intensity of various rays as a function of position within the probe beam as reflected with the position of the rays within said reflected probe beam corresponding to specific angles of incidence with respect to said surface; determining the characteristics of said thin film layer based upon the intensity measurements; and filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

12. A method as recited in claim 11 further wherein said incident probe beam is focused in a manner such that it includes at least one ray that is substantially normal to said surface of said sample.

13. A method as recited in claim 11 or claim 12 wherein said filtering step includes using a fiber optic cable.

14. A method as recited in claim 11 or claim 12 wherein said filtering step includes using a single mode, polarization maintaining fiber optic cable.

15. A method as recited in claim 11 or claim 12 further including the step of polarizing said probe beam prior to focusing it on said sample.

16. A method as recited in claim 11 or claim 12 further including the step of polarizing said probe beam prior to focusing it on said sample by using a polarizing cube.

17. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light; means for focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; detector means for measuring the intensity of various rays as a function of position within the probe beam as reflected with the position of the rays within said reflected probe beam corresponding to specific angles of incidence with respect to said surface; processor means for determining the characteristics of said thin film layer based upon the intensity measurements made by the detector means; and means for filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

18. An apparatus as recited in claim 17 further wherein said incident focused probe beam includes at least one ray that is substantially normal to said surface of said sample.

19. An apparatus as recited in claim 17 or claim 18 wherein said means for filtering includes a fiber optic cable.

20. An apparatus as recited in claim 17 or claim 18 wherein said means for filtering includes a single mode, polarization maintaining fiber optic cable.

21. An apparatus as recited in claim 17 or claim 18 further including means for polarizing said probe beam prior to focusing it on said sample.

22. An apparatus as recited in claim 17 or claim 18 further including means for polarizing said probe beam prior to focusing it on said sample, said means for polarizing including a polarizing cube.

23. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light having a known polarization state; focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; analyzing the polarization state of one or more rays in said probe beam after interaction with said sample and determining the angle of incidence with respect to said surface of said sample of said one or more rays based on the position of said one or more rays within said probe beam; calculating the characteristics of said thin film layer based upon the polarization states and angles of incidence; and filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

24. A method as recited in claim 23 further wherein said incident probe beam is focused in a manner such that it includes at least one ray that is substantially normal to said surface of said sample.

25. A method as recited in claim 23 or claim 24 wherein said filtering step includes using a fiber optic cable.

26. A method as recited in claim 23 or claim 24 wherein said filtering step includes using a single mode, polarization maintaining fiber optic cable.

27. A method as recited in claim 23 or claim 24 further including the step of polarizing said probe beam prior to focusing it on said sample by using a polarizing cube.

28. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light having a known polarization state; means for focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; means for analyzing the polarization state of one or more rays in said probe beam after interaction with said sample; means for determining the angle of incidence with respect to said surface of said sample of said one or more rays based on the position of said one or more rays within said probe beam; processor means for calculating the characteristics of said thin film layer based upon the polarization states and angles of incidence; and means for filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam.

29. An apparatus as recited in claim 28 further wherein said incident focused probe beam includes at least one ray that is substantially normal to said surface of said sample.

30. An apparatus as recited in claim 28 or claim 29 wherein said means for filtering includes a fiber optic cable.

31. An apparatus as recited in claim 28 or claim 29 wherein said means for filtering includes a single mode, polarization maintaining fiber optic cable.

32. An apparatus as recited in claim 28 or claim 29 further including means for polarizing said probe beam prior to focusing it on said sample, said means for polarizing including a polarizing cube.

33. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light; directing said probe beam to the surface of said sample; reflecting said probe beam from said sample; measuring optical properties of the reflected probe beam; and filtering said probe beam prior to directing it to said sample using a single mode, polarization maintaining fiber optic cable so as to enhance the smoothness of the cross-sectional profile of said probe beam.

34. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light; means for directing said probe beam to the surface of said sample; means for measuring optical properties of the probe beam as reflected from said sample; and: means for filtering said probe beam prior to directing it to said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam, said means for filtering including a single mode, polarization maintaining fiber optic cable.

35. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light; focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; measuring the intensity of various rays as a function of position within the probe beam as reflected with the position of the rays within said reflected probe beam corresponding to specific angles of incidence with respect to said surface; determining the characteristics of said thin film layer based upon the intensity measurements; and filtering said probe beam prior to focusing it on said sample using a single mode, polarization maintaining fiber optic cable so as to enhance the smoothness of the cross-sectional profile of said probe beam.

36. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light; means for focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; detector means for measuring the intensity of various rays as a function of position within the probe beam as reflected with the position of the rays within said reflected probe beam corresponding to specific angles of incidence with respect to said surface; processor means for determining the characteristics of said thin film layer based upon the intensity measurements made by the detector means; and means for filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam, said means for filtering including a single mode, polarization maintaining fiber optic cable.

37. A method of measuring the characteristics of a thin film layer on the surface of a sample comprising the steps of: generating a probe beam of light having a known polarization state; focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; analyzing the polarization state of one or more rays in said probe beam after interaction with said sample and determining the angle of incidence with respect to said surface of said sample of said one or more rays based on the position of said one or more rays within said probe beam; calculating the characteristics of said thin film layer based upon the polarization states and angles of incidence; and filtering said probe beam prior to focusing it on said sample using a single mode, polarization maintaining fiber optic cable so as to enhance the smoothness of the cross-sectional profile of said probe beam.

38. An apparatus for measuring the characteristics of a thin film layer on the surface of a sample comprising: means for generating a probe beam of light having a known polarization state; means for focusing said probe beam on the surface of said sample such that various rays within the focused probe beam create a range of angles of incidence with respect to said surface; means for analyzing the polarization state of one or more rays in said probe beam after interaction with said sample; means for determining the angle of incidence with respect to said surface of said sample of said one or more rays based on the position of said one or more rays within said probe beam; processor means for calculating the characteristics of said thin film layer based upon the polarization states and angles of incidence; and means for filtering said probe beam prior to focusing it on said sample so as to enhance the smoothness of the cross-sectional profile of said probe beam, said means for filtering including a single mode, polarization maintaining fiber optic cable.

39. An apparatus for evaluating the characteristics of a sample comprising: a laser for generating a probe beam; a single mode optical fiber having an entrance face and an exit face; a first lens for focusing the probe beam into the entrance face of the fiber, and wherein the length of the fiber is sufficient so that the output thereof is limited substantially to a single transverse mode; a second lens for focusing the probe beam exiting the fiber onto the sample surface to create a range of angles of incidence; an array detector for measuring the intensity of various rays as a function of radial position within the probe beam after the beam has reflected from the sample and generating output signals in response thereto; and a processor for evaluating the sample based on the output signals from the detector.

40. An apparatus as recited in claim 39 wherein said laser source is a semiconductor laser.

41. An apparatus as recited in claim 39 further including a polarizer located between the exit face of the fiber and the second lens for polarizing the beam.

42. An apparatus as recited in claim 41 wherein the polarizer includes a polarizing cube.

43. An apparatus as recited in claim 41 further including a third lens for collimating the beam prior to entering the polarizer.

44. An apparatus as recited in claim 39 further including optical elements which permit detection of the change in polarization state of the probe beam upon reflection from the sample and wherein said processor evaluates the sample characteristics using an ellipsometric analysis.