Optics: measuring and testing – By light interference
Reexamination Certificate
2000-08-02
2002-09-03
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
C359S370000
Reexamination Certificate
active
06445453
ABSTRACT:
BACKGROUND
Scanning microscopy techniques, including near-field and confocal scanning microscopy, conventionally employ a single spatially localized detection or excitation element, sometimes known as the scanning probe [see, e.g., “Near-field Optics: Theory, Instrumentation, and Applications,” M. A. Paesler and P. J. Moyer, (Wiley-New York) (1996); “Confocal Laser Scanning Microscopy,” C. Sheppard,
BIOS
(Scientific-Oxford and Springer-New York) (1997).] The near-field scanning probe is typically a sub-wavelength aperture positioned in close proximity to a sample; in this way, sub-wavelength spatial resolution in the object-plane is obtained. The sub-wavelength aperture is an aperture smaller than the free-space optical wavelength of the optical beam used in the near-field microscopy application. Spatially extended images, e.g., two dimensional images, are acquired by driving the scanning probe in a raster pattern.
SUMMARY OF THE INVENTION
The invention features systems and methods that incorporate interferometric techniques into near-field microscopy. The near-field aspects of the system provide high spatial resolution and the interferometric techniques enhance the signal-to-noise ratio. Moreover, the systems and methods can further incorporate confocal microscopy techniques to further enhance the signal-to-noise ratio. The systems and methods can operate in reflection or transmission mode, and can be used to study surface properties of an unknown sample, to inspect a sample, such as microlithography mask or reticle, and to read information from, and/or write information to, and optical storage medium.
The systems and methods produce a near-field probe beam by illuminating a mask having an aperture with a dimension smaller than the free-space wavelength of the illuminating beam. The near-field probe beam interacts with a sample to produce a near-field signal beam, which is subsequently mixed with a reference beam to produce an interference signal. The properties of the near-field probe beam, such as its electric field and magnetic field multipole expansions, and their resulting interaction with the sample can be controlled by varying the incident angle of the illuminating beam, varying the distance between the mask and the sample, and tailoring the properties of the aperture. Furthermore, in some embodiments, the polarization and wavelength of the near-field probe beam can be varied.
The interference signal produced by interfering the near-field signal beam with a reference beam increases the near-field signal because the resulting signal scales with the amplitude of the near-field signal beam rather than its intensity. Furthermore, the changes in the phase and/or amplitude of the near-field signal beam, and the surface information corresponding to such changes, can be derived from the interference signal as the near-field probe beam scans the sample. Moreover, background contributions to the interference signal can be suppressed by introducing multiple phase shifts between the reference beam and the near-field probe beam and analyzing the interference signals as a function of the phase shifts. The phase shifts can be introduced, e.g., by using a phase-shifter or by introducing a difference frequency in components of input beam used to produce the near-field probe beam and the reference beam. In many embodiments, the phase shifting techniques are used in conjunction with a mask further having a non-transmissive scattering site adjacent the aperture, with the systems and methods producing an interference signal derived from mixing the light scattered from the scattering site with the reference beam to provide information about background signals that may be present in the interference signal derived from the near-field signal beam.
In further embodiments, the mask includes an array of apertures to direct an array of near-field probe beams to different locations on the sample, and the methods and systems produce a corresponding array of interference signals to more rapidly analyze the sample.
One embodiment of the invention can be generally described as follows.
An input beam including a linearly polarized single frequency laser beam is incident on a beam splitter. A first portion of the input beam is transmitted by the beam splitter as a measurement beam. A first portion of the measurement beam is incident on a sub-wavelength aperture in a conducting surface and a first portion thereof is transmitted as a near-field probe beam. The wavelength referenced in the sub-wavelength classification of the size of the sub-wavelength aperture is the wavelength of the input beam. A portion of the near-field probe beam is reflected and/or scattered by an object material back to the sub-wavelength aperture and a portion thereof is transmitted by the sub-wavelength aperture as a near-field return probe beam (i.e., the near-field signal beam). A second portion of the measurement beam incident on the sub-wavelength aperture is scattered by the sub-wavelength aperture as a first background return beam. The near-field return probe beam and the first background return beam comprise a return beam.
A second portion of the measurement beam is incident on a sub-wavelength non-transmitting scattering site located on the conductor at a position laterally displaced from the sub-wavelength aperture in the conductor by a preselected distance. The preselected distance is greater than or of the order of the wavelength of the input beam. A portion of the measurement beam incident on the sub-wavelength scattering site is scattered as a second background return beam.
A second portion of the input beam is reflected by the beam splitter as a reference beam. The reference beam is incident on a reference object and reflected as a reflected reference beam.
The return beam and a portion of the reflected reference beam are incident on the beam splitter and mixed by a polarizer as a first mixed beam. The first mixed beam is then focused on a pinhole in an image plane such that an image of a sub-wavelength aperture is in focus in the plane of the pinhole. The size of the preselected separation of the sub-wavelength aperture and the sub-wavelength scattering site, the size of the pinhole, and the resolution of an imaging system producing the image of the sub-wavelength aperture on the pinhole are selected such that a substantially reduced portion of the second background return beam is transmitted by the pinhole. A portion of the focused first mixed beam is transmitted by the pinhole and detected, preferably by a quantum photon detector [see Section 15.3 in Chapter 15 entitled “Quantum Detectors”,
Handbook of Optics
, 1, 1995 (McGraw-Hill, New York) by P. R. Norton], to generate a first electrical interference signal. The amplitude and phase of the first electrical interference signal is measured.
The second background return beam and a second portion of the reflected reference beam are incident on the beam splitter, mixed by a polarizer, and the second mixed beam is then focused on a second pinhole such that the sub-wavelength scattering site is in focus in the plane of the second pinhole. The size of the preselected separation of the sub-wavelength aperture and the sub-wavelength scattering site, the size of the second pinhole, and the resolution of an imaging system producing the image of the sub-wavelength scattering site on the second pinhole are selected such that a substantially reduced portion the return beam is transmitted by the second pinhole. A portion of the focused second mixed beam is transmitted by the second pinhole and detected, preferably by a quantum photon detector, to generate a second electrical interference signal.
The amplitude and phase of the second interference signal are measured and the measured amplitude and phase of the second interference signal is used to compensate for the effects of the first background return beam on the measured amplitude and phase of the first electrical interference signal.
The measured amplitude and phase of the compensated first electrica
Connolly Patrick
Fish & Richardson P.C.
Turner Samuel A.
Zetetic Institute
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