Systems and methods for optimizing focus for imaging-based overlay metrology

Abstract

Methods and systems for focusing and measuring by mean of an interferometer device, having an optical coherence tomography (OCT) focusing system, by separately directing an overlapped measurement and reference wavefront towards a focus sensor and towards an imaging sensor; where a predefined focusing illumination spectrum of the overlapped wavefront is directed towards the focus sensor, and where a predefined measurement illumination spectrum of the overlapped wavefront is directed towards the imaging sensor. Methods and systems for maintaining focus of an interferometer device, having an OCT focusing system, during sample's stage moves.

Claims

1. A metrology and inspection system, comprising: an optical metrology tool having an optical column with a first beam splitter optically coupled to a first objective, wherein the optical metrology tool is configured to perform optical metrology within a predetermined measurement spectrum; and an optical coherence tomography (OCT) focusing system comprising: a second objective and a reference surface arranged in an interferometer configuration with the first beam splitter, a second beam splitter optically coupled to the output wavefront of the first beam splitter; and an imaging sensor coupled with the second beam splitter; wherein the reference surface is a dichroic reflector, configured to reflect illumination within a focusing spectrum and attenuate light within the predetermined measurement spectrum.

2. The system according to claim 1, wherein the predetermined focusing spectrum is about 380-430 nm and the predetermined measuring spectrum is about 430-1200 nm.

3. A method for focusing and measuring using an interferometer device having an optical coherence tomography (OCT) focusing system; the method comprising a step of separately directing an overlapped measurement and reference wavefront towards a focus sensor and towards an imaging sensor and reflecting only a predefined focusing illumination spectrum from a reference surface of the OCT focusing system; wherein the predefined focusing illumination spectrum of the overlapped wavefront is directed towards the focus sensor, and wherein a predefined measurement illumination spectrum of the overlapped wavefront is directed towards the imaging sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

(2) FIG. 1 schematically demonstrates a prior art image-based overlay measurement tool, with Linnik focus detection;

(3) FIG. 2A schematically demonstrates another prior art image-based overlay measurement tool, similar to FIG. 1;

(4) FIG. 2B schematically demonstrates a prior art output signal of the focus detector (FD), demonstrated in FIG. 2A;

(5) FIG. 3 schematically demonstrates an inspection apparatus, having an interferometer device, according to some embodiments of the invention;

(6) FIG. 4A schematically demonstrates another inspection apparatus, having an interferometer device, according to some embodiments of the invention;

(7) FIG. 4B schematically demonstrates an output signal of the focus detector (FD), demonstrated in FIG. 4A, according to some embodiments of the invention;

(8) FIG. 5 schematically demonstrates another inspection apparatus, having an interferometer device, according to some embodiments of the invention;

(9) FIG. 6 schematically demonstrates another inspection apparatus, having an interferometer device, according to some embodiments of the invention;

(10) FIG. 7 schematically illustrates an exemplary implementation for the inspection apparatus, according to some embodiments of the invention;

(11) FIG. 8A schematically illustrates another exemplary implementation for the inspection apparatus, according to some embodiments of the invention;

(12) FIG. 8B schematically demonstrates an output signal of the focus detector (FD), demonstrated in FIG. 8A, according to some embodiments of the invention;

(13) FIG. 9 schematically demonstrates a method for optimal focusing and measuring, according to some embodiments of the invention; and

(14) FIG. 10 schematically demonstrates a method for maintaining focus, according to some embodiments of the invention.

(15) It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(16) In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

(17) The present invention relates to the field of metrology, and more particularly, to optimizing focus and measuring for imaging-based overlay metrology; for example, target structures at the surface of an overlay wafer, which may serve as a substrate for microelectronic devices.

(18) Embodiments of the present invention are directed to inspection systems that incorporate an OCT focus system in a configuration with a potential to improve focus performance by maintaining focus during sample's stage moves and/or by providing different illumination spectrums for focusing and measurement tasks.

(19) Some embodiments of the present invention provide a focusing tool, which is configured to provide optimal illumination spectrums for each of the measurement and focusing tasks. As different wavelengths penetrate to different depths in wafer device structures, it is often desirable to control, from which depth in the device the focus signal is derived. Accordingly, the optimal spectrum for focusing may be different than the optimal spectrum for wafer's overlay measurement.

(20) According to some embodiments the present invention provides an inspection apparatus, as schematically demonstrated in FIGS. 3 and 4A, having: an interferometer device 3000,4000 configured to direct a measurement wavefront to reflect from a measurement surface 3060 and subsequently overlap with a reference wavefront to form an interference pattern, the measurement and reference wavefronts being derived from a common source.

(21) According to some embodiments, the interferometer device 3000 comprises: a first beam splitter 3040 positioned to direct the measurement and reference wavefronts along different directions and subsequently recombine and direct them to a second beam splitter 3100; the second beam splitter 3100 positioned to direct the recombined measurement and reference wavefront (also noted as interference wavefront or interference beam) towards an imaging sensor 3120 and towards a focus sensor 3110; and a reference surface (also noted as reference reflector) 3090 positioned and configured to reflect the reference wavefront back towards the first beam splitter 3040, as demonstrated, all arranged in a Linnik-type interferometer configuration.

(22) According to some embodiments, the interferometer 3000 further comprises a measurement objective lens 3050, optically coupled between the first beam splitter 3040 and the sample 3060. The measurement objective 3050 configured to focus and concentrate the measurement illumination beam onto a target on the sample's surface and in order to image the target onto the imaging sensor 3120.

(23) According to some embodiments, the interferometer 3000 further comprises a reference objective lens 3080, optically coupled between the first beam splitter 3040 and the reference surface 3090. The reference objective 3080 configured to focus and concentrate the reference illumination beam onto the reference surface 3090. According to some embodiments, the reference objective lens is identical to the measurement objective lens.

(24) The inspection apparatus as in FIG. 4A, demonstrates an interferometer device 4000 having some features similar to the device as demonstrated in FIG. 3. According to some embodiments of the invention, the interferometer device 4000 comprises: an optical metrology tool having an optical column with a first beam splitter 3040 optically coupled to a first objective 3050; and an optical coherence tomography focusing system comprising: a second objective 3080 and a reference surface 3090 arranged in an interferometer configuration with the first beam splitter (where the reference objective lens has complementary optical properties to the measurement objective lens, such that the optical path length of the measurement and reference arms match); a second beam splitter 3100 optically coupled to an output wavefront of the first beam splitter 3040.

(25) According to some embodiments the first beam splitter 3040 is a 50/50 beam splitter (e.g. a Linnik beam splitter).

(26) According to some embodiments, the second beam splitter 3100 is a dichroic beam splitter, which is configured to direct light/illumination within a predefined focusing spectrum to the focus sensor 3110 (optionally via a focus detector lens 3115), and direct light/illumination within a predefined measurement spectrum to the imaging sensor 3120 (optionally via relay lens/es 3105).

(27) According to some embodiments, the reference surface 3090 is a dichroic reference reflector, which is configured to reflect light/illumination within a predefined focusing spectrum and to attenuate light/illumination within a predefined measurement spectrum.

(28) According to some preferred embodiments, the second beam splitter 3100 is a dichroic beam splitter, configured to direct light/illumination within a predefined focusing spectrum; and the reference surface 3090 is a dichroic reference reflector, configured to reflect light/illumination within the predefined focusing spectrum and to attenuate light/illumination within a predefined measurement spectrum.

(29) A non-limiting example for a preferred focusing spectrum is a wavelength within the range of 380-410 nm or 380-430 nm. A non-limiting example for a preferred measuring spectrum is a wavelength within the range of 430-1,200 nm.

(30) According to some embodiments, the imaging sensor 3120 comprises a field plane array sensor. According to some embodiments, the imaging sensor 3120 comprises Charge-Coupled Device (CCD).

(31) According to some embodiments, the interferometer 3000,4000 further comprises illumination optics 3025, optically coupled between an illumination source 3010 and the first beam splitter 3040. The illumination optics 3025 having angular and/or spatial dynamic control, configured for generating a probe beam of radiation 3310. According to some embodiments, the illumination source 3010 is a fiber optic coupler, which is configured to carry both the focusing and the measurement illumination spectrums.

(32) According to some embodiments, the illumination optics 3025 comprise at least one of: an illumination pupil aperture 3020, configured to define the angular distribution of the illumination at the imaged object; an illumination field stop 3030, configured to limit the spatial extent of the illumination at the imaged object (thereby define the field of view); and at least one optical lens 3021.

(33) According to some embodiments, the interferometer device 3000,4000 further comprises a mechanical shutter 1070 configured to selectively open and close an optical path between the first beam splitter 3040 and the reference objective lens 3080.

(34) According to some embodiments, the interferometer device 3000,4000 further comprises a translating stage (not shown) configured to translate sample 3060 relative to the optical measurement beam.

(35) The interferometer configuration, as demonstrated in FIGS. 3 and 4A, allows the providing of different illumination spectrum for the focusing and the measurement tasks. Accordingly, the focus signal 4200, demonstrated in FIG. 4B, can be detected simultaneously to the capturing of the measurement images, as the provided configuration allows only light within the focusing spectrum to return from the dichroic reference mirror 3090, while the dichroic beam splitter 3100 blocks light within the focusing spectrum from reaching the imaging sensor 3120, and blocks light within the measurement spectrum from reaching the focus sensor 3110.

(36) Furthermore, according to some embodiments, the interferometer 3000,4000 configuration (as demonstrated in FIGS. 3 and 4A) enables optimizing illumination spectrum, used for deriving focus, to minimize influence of the wafer's processing variations; enables optimizing illumination spectrum, used for acquiring overlay target images, to maximize the desired system performance metrics (e.g. accuracy, overlay residuals, tool-induced-shift, and more), and enables a simultaneous optimization for both the acquiring (measuring) and the focusing tasks.

(37) Some embodiments of the present invention provide a focusing tool, which is configured to maintain focus during the moves of the sample's stage, particularly when changing targets. Deviations of wafer and stage travel flatnesses induce unacceptable levels of image defocus. Accordingly, separation between the measurement objective and the wafer must be adjusted after moving to each overlay target and before acquiring the measurement image. Some embodiments of the present invention allow continuous focus maintenance during a stage move, and therefore the time required for re-acquiring focus after the move can be eliminated or at least reduced.

(38) According to some embodiments, the interferometer device 3000,4000 further comprises a translation mechanism (not shown) mechanically coupled to the reference surface 3090, where the translation mechanism is configured to move the reference surface 3090 in a direction 4091 parallel to the optical axis of the reference objective 3080, for facilitating interferometer balancing. FIG. 4B demonstrates a focus sensor's 3110 optional output signal 4200, which can be analyzed in order to detect whether the probe beam 3310 is in focus at the sample 3060.

(39) According to some embodiments, the interferometer device 3000,4000 further comprises a mechanical shutter 3070, configured to selectively open and close the optical path between the first beam splitter 3040 and the second objective 3080.

(40) According to some embodiments, light returning from the reference arm (e.g. light returning from the reference surface 3090) is blocked during measurement (e.g. during the actual image capturing), from reaching to the first beam splitter 3040 (e.g. by a shutter 3070), to prevent the overlapping spectral region from reaching the imaging sensor 3120.

(41) A common practice for an OCT system is generally noted with three steps: move, focus, and measure; where at the “move” step, the sample's stage (not shown) is moved to bring a target structure at the sample's surface into the imaging sensor's field of view, and after steps of “focus” and “measure” the stage is moved to set the measurement illumination onto the next target structure on the sample's surface which is to be imaged.

(42) In order to maintain focus during stage move to each target at the sample's surface, some embodiments of the present invention provide a method for continuous focus monitoring and adjusting of an interferometer device having an optical metrology tool and an optical coherence tomography (OCT) focusing system (for example, a Linnik interferometer); the method comprising the following steps: scanning and measuring a separation between a reference surface (for example 3090) and a reference objective lens (for example 3080); monitoring an interference beam, of a reference beam and a measurement beam, via a focus sensor (for example 3110); shifting the reference surface position, where such that a maximal fringe contrast is observed at the monitored interference beam; and before a measurement, translating the reference surface back to its nominal position and focusing the optical metrology tool, by adjusting the separation between a measurement objective lens (for example 3050) of the optical metrology tool and a sample (for example 3060), such that the monitored interference beam yields the observed maximal fringe contrast.

(43) According to some embodiments, the separation adjustment between the sample and measurement objective lens is according and proportional to the translation of the reference surface back to its nominal position. According to some non-limiting embodiments, the term “proportional” refers to a coefficient of 1.3 or less.

(44) According to some embodiments, the term “nominal”, refers to an accepted condition, which is a goal or an approximation. According to some embodiment, the nominal position of reference surface is about ±10 nm or less of its initial position.

(45) According to some embodiments, the shifting is provided by a translation mechanism, which is configured to move the stage in a direction parallel to the optical axis of the reference objective lens.

(46) According to some embodiments, the method for the focus monitoring and adjusting can be continuous method, by means that the steps of the scanning, monitoring, and adjusting are continuous steps.

(47) According to some embodiments, the method for the focus monitoring of the interferometer device enables a continuous monitoring of the separation between wafer and objective lens, and as a result changes in wafer height can be corrected during and/or after stage moves. Consequently, the time required to measure the overlay metrology can be considerably reduced. More details in FIG. 10.

(48) Reference is now made to FIG. 5, which schematically demonstrates an interferometer device 5000, according to some embodiments of the invention, where some of its features are similar to the features of devices 3000,4000, as demonstrated in FIGS. 3 and 4A. According to some embodiments, the present invention provides an inspection apparatus comprising: an interferometer device 5000 configured to direct a measurement wavefront to reflect from a measurement surface 3060 and subsequently overlap with a reference wavefront to form an interference pattern, the measurement and reference wavefronts being derived from a common source. The interferometer 5000 comprises: an optical metrology tool having an optical column with a first beam splitter 3040 optically coupled to a first objective 3050; and an optical coherence tomography focusing system comprising: a second objective 3080 and a reference surface 3090 arranged in an interferometer configuration with the first beam splitter, and a second beam splitter 3100 optically coupled to an output wavefront of the first beam splitter 3040.

(49) According to some embodiments, the reference surface 5090 is a dichroic reference mirror, which is configured to reflect light within a focusing spectrum and attenuate light within a measurement spectrum.

(50) According to some embodiments, the second beam splitter 5100 is a 50/50 beam splitter.

(51) According to some embodiments, the interferometer device 5000 further comprises a focus spectral filter wheel 5130, optically coupled between second beam splitter 5100 and the focus detector lens 3115, and which is configured to direct light within a focusing spectrum to the focus sensor 3110.

(52) According to some embodiments, the interferometer device 5000 further comprises an imaging spectral filter wheel 5140, optically coupled between the second beam splitter 5100 and the tube lens/es 3105, and which is configured to direct light within the measurement spectrum to the imaging sensor 3120.

(53) The interferometer configuration as demonstrated in FIG. 5 allows the providing of different illumination spectrum for the focusing and the measurement tasks. Accordingly, the focus signal, can be detected simultaneously to the capturing of the measurement images, as the provided configuration allows only light within the focusing spectrum to return from the dichroic reference mirror 3090, while the imaging spectral filter wheel 5130 blocks light within the focusing spectrum from reaching the imaging sensor 3120, and while the focus spectral filter wheel 5130 blocks light within the measurement spectrum from reaching the focus sensor 3110.

(54) The interferometer configuration, as demonstrated in FIG. 5, enables focus and measurement illumination spectra to overlap. According to some embodiments, both the focus and the measurement spectra propagate on the measurement and reference arms. According to some embodiments, both the focus and the measurement spectra are directed toward both the focus and field plane sensors. According to some embodiments, a band pass filter (such as the focus filter wheel 5140) is inserted into the focus sensor arm to isolate the spectrum used for focus sensing. According to some embodiments, a different band pass filter (such as the focus imaging filter wheel 5130) is inserted into the field plane array sensor arm to isolate the spectrum used for overlay measurement. According to some embodiments, focus and measurement filters can be replaced by filters that modify the spectrum that reaches the sensor on each arm.

(55) Reference is now made to FIG. 6, which schematically demonstrates an interferometer device 6000, according to some embodiments of the invention, where some of its features are similar to the features of devices 3000, 4000 and 5000, as demonstrated in FIGS. 3, 4A and 5. According to some embodiments, the present invention provides an inspection apparatus comprising: an interferometer device 6000 configured to direct a measurement wavefront to reflect from a measurement surface 3060 and subsequently overlap with a reference wavefront to form an interference pattern, the measurement and reference wavefronts being derived from a common source. The interferometer 6000 comprises: an optical metrology tool having an optical column with a first beam splitter 3040 optically coupled to a first objective 3050; and an optical coherence tomography focusing system comprising: a second objective 3080 and a reference surface 3090 arranged in an interferometer configuration with the first beam splitter, and a second beam splitter 3100 optically coupled to an output wavefront of the first beam splitter 3040.

(56) According to some embodiments, the reference surface 3090 is a dichroic reference reflector, which is configured to reflect light within a predefined focusing spectrum and attenuate light within a predefined measurement spectrum.

(57) According to some embodiments, the second beam splitter 6100 is a dichroic beam splitter, as detailed in the above description of FIGS. 3 and 4A. According to other embodiments, the second beam splitter 6100 is a 50/50 beam splitter and where the interferometer device 6000 further comprises a focus spectral filter wheel 5130 and an imaging spectral filter wheel 5140, as shown and detailed in the description of FIG. 5 above (wheels 5130,5140 are not demonstrated in FIG. 6).

(58) Accordingly, the interferometer configuration as demonstrated in FIG. 6 allows the providing of different illumination spectrum for the focusing and the measurement tasks.

(59) According to some embodiments, the configuration of the device 6000 allows the use two of separate light sources 6210 and 6310 for supplying different illumination spectrums for the measurement and for the focus monitoring, and a dichroic beam splitter/combiner 6400, which is configured to efficiently combine the light from the two light sources 6210 and 6310, so that they can illuminate the same field of view.

(60) According to some embodiments, the image-based overlay metrology system with focus sensing by Linnik interferometer as demonstrated in FIGS. 3, 4A, 4B, 5 and 6 enables: independent definition for illumination spectra, for each of focus and measurement; simultaneous focus acquisition and measurement by the isolating of the light returning from the wafer and reference mirror, based on their spectral content; monitoring the separation between the wafer and measurement objective lens, by modulating or adapting the separation between reference mirror and Linnik objective lens (reference surface vs. reference objective lens); accordingly, separation between the wafer and measurement objective lens can be monitored continuously.

(61) According to some embodiments, applications and/or implementations of the present invention are configured for inspecting grating beside grating targets on a wafer, with each grating being printed on a different process layer. In some embodiments, applications of the present invention are configured for inspecting one pair of gratings oriented in the X axis for measuring X-overlay, and/or one pair in Y axis for measuring Y-overlay.

(62) In some embodiments, target illumination can have broad or narrow spectral content. In some embodiments, wafer height can be adjusted to maximize contrast of image of target formed onto the imaging sensor. In some embodiments, Linnik interferometer can be used to regain wafer focus, after each stage move to a subsequent overlay target. In some embodiments, overlay measurements are extracted from relative locations of targets within captured images.

(63) Reference is now made to FIG. 7, which schematically illustrates an exemplary implementation for the inspection apparatus, according to some embodiments detailed above, a broadband illuminator 7000 with UV led for focus detection. As shown in FIG. 7 example, a laser driven plasma light source 7010 generates broadband radiation spanning visible and near-IR spectra. The laser driven plasma light source 7010 is optically coupled to a natural density wheel 7020, via at least one optical lens 7321 and an illumination field stop 7020. The neutral density wheel 7020 is configured to enable attenuation of light to avoid sensor saturation. The neutral density wheel 7020 is further coupled to a band pass filter wheel 7040, which is configured to enable selection of different spectral distributions from within the source's broad distribution. A dichroic beam splitter 7060 efficiently combines plasma spectra received via the band pass filter wheel 7040 with illumination received from a UV LED 7050, which is configured to provide fixed spectrum for focus sensing. The combined beam is then coupled into a multi-mode fiber 7070, thereby a broadband illuminating output.

(64) Reference is now made to FIG. 8A, which schematically illustrates an exemplary implementation for the inspection apparatus, according to some embodiments detailed above, an overlay metrology microscope head 8000. Output from a multi-mode fiber 7070 (also shown in FIG. 7) is split into measurement and reference paths by a Linnik beam splitter 3040. Reference mirror 8090 comprises a dichroic coating, such that it reflects the UV spectrum from the LED 7050 (shown in FIG. 7) and attenuates the broad band spectrum from a laser driven plasma source 7010 (shown in FIG. 7). Wafer 3060 reflects both the UV spectrum from the LED and the broad band spectrum from the laser driven plasma source. In particular, the UV spectrum reflects from the top layers of the wafer, so that impacts on focus sensing from wafer processing variations and features on previous layers are minimized. Visible and near-IR spectrums are able to penetrate to lower layers in the wafer as necessary for viewing overlay targets printed on those layers. Dichroic beam splitter 3100 efficiently routes UV light from the reference and measurement paths to the focus sensor. Dichroic beam splitter also efficiently routes visible and near-IR measurement light to the field array sensor. According to some embodiments, reference mirror 8090 can be modulated to provide continuous sensing and correction of wafer height position. FIG. 8B schematically demonstrates an optional output signal 8200 of the focus detector (FD) 3110, as demonstrated in FIG. 8A, according to some embodiments of the invention.

(65) Reference is now made to FIG. 9, which schematically demonstrates the process of focusing and measuring a target on a sample (e.g. wafer), according to some embodiments of the invention, using an interferometer device having an OCT focus system. The process includes: translating 9100 the sample, to set the field of view (FoV) onto a current target on the sample's surface; focusing 9200 the illumination onto the target, by adjusting separation between the sample and the measurement objective lens; and measuring (i.e. imaging) 9300 the current target, while blocking the reference beam path, from returning to the first (Linnik) beam splitter.

(66) According to some embodiments, the focusing 9200 is achieved by: directing the measurement wavefront to reflect from the target, and subsequently overlap with a reference wavefront, to form the interference beam; and by directing the interference beam towards the focus sensor and towards the imaging sensor; and where the adjusting of the separation between the measurement objective lens and the sample is such that a maximal fringe contrast is observed at the scanned interference beam (via a focus sensor).

(67) According to some embodiments, the present invention provides a method for optimal focusing and measuring using an interferometer device having an optical coherence tomography (OCT) focusing system; the method 9500 comprising a step of separately directing 9520 an overlapped measurement and reference wavefront towards a focus sensor and towards an imaging sensor; wherein a predefined focusing illumination spectrum of the overlapped wavefront is directed towards the focus sensor, and wherein a predefined measurement illumination spectrum of the overlapped wavefront is directed towards the imaging sensor.

(68) According to some embodiments, the method 9500 further comprises reflecting 9510 only the predefined focusing illumination spectrum from a reference surface of the OCT focusing system, that is, prior to the overlapping of the measurement and reference wavefront.

(69) FIG. 9 further demonstrates the integration of the method 9500 for optimal focusing and measuring with the process of focusing 9200 and measuring 9300, according to some embodiments of the invention.

(70) Reference is now made to FIG. 10, which schematically demonstrates a method 10000 for maintaining focus of an interferometer device having an optical metrology tool and an optical coherence tomography (OCT) focusing system; for example during sample's stage move to each target at the sample's surface.

(71) According to some embodiments of the present invention; the method 10000 comprising: monitoring and measuring 10100 an interference beam of a reference beam and a measurement beam, via a focus sensor (of the OCT system); translating 10200 a reference reflector surface (of the OCT system), relative to a reference objective lens, to observe a maximal fringe contrast, at the monitored interference beam; responsive to arriving to a measurement location of a sample, translating 10300 the reference reflector surface back to its nominal position in a known amount; and focusing the optical metrology tool, by adjusting 10400 separation between a sample and a measurement objective lens (of the optical metrology tool), in a vertical step, proportional to the known amount, such that the monitored reference beam yields the observed maximal fringe contrast.

(72) According to some embodiments, the method 10000 further comprises scanning and measuring separation 10250 between the reference reflector and the reference objective lens.

(73) According to some embodiments, the steps of monitoring, measuring, scanning, translating and focusing are continuous.

(74) According to some embodiments, the method as described above and in FIG. 10 can be applied on an interferometer device 3000, 4000, 5000, 6000, 7000, 8000 as in the above embodiments.

(75) According to at least some of the provided embodiments, scatterometry-based overlay systems can have the ability to optimize focus at wavelengths different than the measurement wavelength. Imaging-based overlay systems can provide similar levels of robustness to process variations through the implementation of this invention.

(76) Furthermore, the process of re-acquiring focus of the overlay target after each move, constitutes a significant amount of the “move and measure” time budget. Continuous focus tracking can significantly reduce the cost-of-ownership for image-based overlay measurement.

(77) While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.