One-dimensional phase contrast microscopy with a traveling lens generated by a step function change

A method for imaging a surface includes generating a traveling lens in an acousto-optic material and incorporating a traveling mask into a portion of the traveling lens so as to produce a composite traveling lens. The method further includes irradiating the composite traveling lens so as to produce a composite focused beam having a spatial variation across the composite focused beam. The composite focused beam is directed onto a region of the surface so as to generate radiation characteristic of the region from the region. The radiation is imaged onto a detector so as to generate a signal characteristic of the region, responsively to the spatial variation.

FIELD OF THE INVENTION

The present invention relates generally to microscopy, and specifically to phase contrast microscopy.

BACKGROUND

Phase contrast microscopy was invented in 1935 by Zernike, and enables objects which typically only change the phase of radiation, compared to the surroundings of the object, to be seen. The phase contrast microscope devised by Zernike converts object phase changes into amplitude changes in a final image of the object, thus allowing phase details of the object to be seen. Since its initial invention, there have been a number of systems which claim to have improved microscopy in general, as well as phase contrast microscopy.

U.S. Pat. No. 6,937,343 to Feldman, whose disclosure is incorporated herein by reference, describes a method for optical evaluation of a sample by scanning a beam of coherent radiation over the sample. A frequency shifted reference beam is generated from the scanning beam, and the reference beam is combined with scattered radiation from the sample to generate an optical heterodyne signal characteristic of the sample.

U.S. Pat. No. 7,002,695 to Feldman, whose disclosure is incorporated herein by reference, describes apparatus which includes a source generating a coherent radiation beam, and traveling lens optics which focus the beam to two spots on the surface of a sample. Interference fringes are generated from radiation collected from the two spots, and changes in the fringes may be used to assess optical characteristics of the sample.

U.S. Pat. No. 3,851,951 to Eveleth, whose disclosure is incorporated herein by reference, describes a laser system in which light is focused upon an image plane by interaction with frequency modulated acoustic pulses in a Bragg cell. The light is scanned across the image plane in accordance with the movement of the pulses along the cell.

Published International Patent Application WO 2004/042433 to Rietman et al., whose disclosure is incorporated herein by reference, describes programmable lenses and gratings which are claimed to be of use in a variety of optical applications.

U.S. Patent Application 2006/0256350 to Nolte et al., whose disclosure is incorporated herein by reference, describes apparatus for assessing topology of a surface of a target. The apparatus includes an optical source for generating a probe laser beam, and also includes means for scanning the probe laser beam across at least a portion of the surface of the target.

U.S. Patent Application 2005/0057727 to Troyer, whose disclosure is incorporated herein by reference, describes a laser projection system. The laser beam is modulated using a reflective liquid-crystal light valve.

SUMMARY OF THE INVENTION

In embodiments of the present invention, a scanning microscope includes an acousto-optic (AO) element in which a traveling lens, typically a one-dimensional (1D) traveling lens, is generated. The traveling lens is formed by inputting a radio-frequency (RF) signal to a transducer coupled to the AO element. The RF signal has a varying frequency, and is also termed a chirp. A traveling phase and/or amplitude mask is incorporated into the traveling lens by inserting a corresponding step-function phase change and/or step-function amplitude change into the chirp, and the resulting lens is herein also termed a composite traveling lens. Typically a series of generally similar chirps are input to the transducer, so as to generate a series of generally similar composite traveling lenses in the AO element.

A coherent beam, typically generated with a laser, irradiates each composite traveling lens. The irradiation produces a composite beam having a stepped spatial phase and/or amplitude variation across the beam. The beam thus comprises a phase and/or amplitude changed component (generated by the mask) and an unchanged component. Scanning and illumination optics focus the composite beam onto a region of a surface that is to be imaged, and scan the focused beam across the region. Collection optics collect resulting radiation from the region and convey the radiation, via an exit pupil of the optics, to a detector. An amplitude mask, which has a spatial amplitude variation which is stepped to correspond to the stepped spatial phase and/or amplitude variation of the beam, is positioned at the exit pupil. Radiation from the scanned composite beam that has traversed the amplitude mask combines at the detector to produce amplitude changes corresponding to phase and/or amplitude objects at the region of the surface. A signal generated by the detector in response to the received radiation is thus characteristic of phase and/or amplitude objects at the region.

By using embodiments of the present invention a number of advantages accrue, including:a. Virtually arbitrary phase shifts and/or amplitude attenuations may be introduced into the composite beam, via corresponding changes in the chirp generating the composite traveling lens.b. The phase shifts and/or amplitude attenuations may be changed in real-time by changing the RF signal, thereby simplifying optimization of system performance.c. In cases where the AO element supports more than one traveling lens at the same time, chirps having different phases and/or amplitudes may be applied sequentially so as to cause different composite traveling lenses to be present in the AO element simultaneously.

The scanning microscope may be either a transmission or a reflection microscope. In the latter case some of the optic elements of the microscope may function both as illumination and as collection optics.

The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings, a brief description of which follows.

DETAILED DESCRIPTION

Reference is now made toFIG. 1, which is a schematic diagram of a transmission scanning microscope20, according to an embodiment of the present invention. The following description assumes for clarity that microscope20has an optical axis parallel to a z-axis, and that elements of the microscope are typically parallel to an x-y plane. However, microscope20, as well as its elements, may be configured and/or operated in substantially any orientation. Microscope20comprises a radiation source21, typically a laser, which directs an expanded beam of collimated coherent radiation30generally normally to an acousto-optic (AO) element34. Element34is herein assumed to be in the form of a generally rectangular plate oriented in an x-y plane. The expanded beam of radiation is typically produced using a beam expander, not shown in the figure. Typically, source21is selected to emit optical radiation at a wavelength in a region of the electromagnetic spectrum between and including far infra-red and deep ultra-violet (DUV), although it will be understood that the principles of the present invention apply equally to other wavelengths of the spectrum. A processor29, typically incorporating a memory storing operating instructions, drives elements of scanning microscope20.

Processor29generates a radio-frequency (RF) signal, with which it excites a piezoelectric transducer35coupled to AO element34. The RF signal causes transducer35to produce groups32of substantially similar acoustic waves in element34, the groups of waves traveling in the y-direction. Hereinbelow, by way of example, transducer35is assumed to be a transducer which generates groups32as plane waves which have a substantially uniform density in the transverse direction. Three groups32are shown inFIG. 1. Each group32of traveling waves produces a respective converging composite beam36, from incident beam30, two of which composite beams are illustrated inFIG. 1. The RF signal input to transducer35, and its effect on AO element34so as to produce converging focused composite beams36from the AO element, are described in more detail below with respect toFIG. 2andFIG. 3.

FIG. 2shows schematic graphs of the RF signal generated by processor29, according to an embodiment of the present invention. The RF signal generated by the processor and input to transducer35is in the form of pulses106, or “chirps,” each chirp having a variable frequency. Graphs100,102, and104, are respective schematic frequency vs. time, phase vs. time, and amplitude vs. time, graphs of the RF signal. As shown in the graphs, each chirp106has a generally linear change of frequency with time. Each chirp also has a stepped phase shift of

+π2
applied to approximately half of the chirp at a temporal midpoint107of the chirp, i.e., between the approximate center frequency of the chirp and the final chirp frequency. In the initial half of the chirp, there is no phase shift.

For each chirp there is a stepped change from a first substantially constant attenuated amplitude level to a second substantially constant non-attenuated amplitude level, at substantially the same time as the phase change. The attenuated amplitude level is approximately 50% of the non-attenuated level. It will be understood that the particular relationships and values given for graphs100,102, and104are by way of illustration, and that the scope of the present invention comprises other relationships and values, such as a non-linear variation of the RF frequency with time, a phase shift that is different from a phase shift of

+π2,
and an amplitude change other than a change from a 50% reduction in amplitude. Typically, as for the example shown inFIG. 2, positions of changes in phase and/or amplitude of the chirp are approximately symmetrically disposed with respect to temporal midpoint107of chirps106.

A graph108is a schematic voltage vs. time graph of the RF signal, corresponding to a central section110of graphs100,102, and104. Graph108illustrates the effect of the phase and amplitude changes for a given chirp106, showing that there is an amplitude change and a phase change at center107.

FIG. 3is a schematic diagram170showing the effect of one chirp106on AO element34, according to an embodiment of the present invention. Diagram170illustrates one group32of plane acoustic waves produced by one chirp106, as would be seen at an instant in time. The plane acoustic waves are one-dimensional (1D), having a density variation in the y-direction but substantially no density variation in the x-z plane. Other chirps106produce groups of plane acoustic waves which are substantially similar to group32. The acoustic waves travel parallel to the y-direction in AO element34with a speed v, which has a characteristic value for the medium constituting the AO element.

As a consequence of the change of RF frequency of the chirp, the traveling waves produced have a corresponding change of acoustic wavelength along the AO element. Thus, each group32of traveling waves acts as a traveling diffraction grating having a variable spacing for incoming beam30. The variable spacing causes the gratings to deflect sections of the incoming beam by respective different angular values so that each group32operates as a cylindrical optical tens, which focuses collimated incoming beam30to a convergent beam36. Furthermore, the phase change within group32causes the cylindrical lens to add a corresponding phase change to beam36, and the amplitude change within the group causes the cylindrical lens to add a corresponding amplitude change to the beam. Thus, each group32acts as a traveling cylindrical lens with a traveling amplitude and phase mask, and is herein also termed a 1D composite traveling lens32.

It will be understood that beams36have 1D amplitude and phase changes that correspond to the amplitude and phase changes of the RF signal, since sections of the beams deflect by different angles relative to the y direction, but do not have a characteristic change in either the x or z direction.

Graphs172and174schematically show the respective amplitude and phase changes across beam36, measured along a line176parallel to the direction in which group32is traveling. The amplitude and phase changes across beam36effectively divide the beam into two sections, a second section39which is phase shifted with respect to a first section37. In addition second section39is attenuated with respect to the first section. 1D beams36thus comprise a composition of phase and amplitude changes across the beams, and are also herein termed composite 1D beams36.

Returning toFIG. 1, the division of composite beams36into the two sections37and39described above is illustrated by shading part of each converging beam36, the non-shaded part corresponding to attenuated section39, the shaded part corresponding to phase shifted section37. As beams36traverse elements of microscope20, phase shift of the beam is indicated by shading, as for section37, and beam attenuation is indicated by non-shading, as for section39.

A cylindrical lens23focuses converging composite beams36from element34to a series of focused spots38, which are approximately collinear, and which travel in the y-direction. A wedge-shaped prism25, positioned at spots38, receives the focused beams, and diverts the beams to respective diverging conical beams27, the axes of each of the conical beams being approximately parallel to axis33of microscope20. A scanning lens38converts diverging beams27to a series of collimated beams40which transfer through a generally circular pupil41to an objective44. Pupil41is in an x-y plane, and is fixed relative to beams40. Optionally, a stop43is positioned at pupil41, the pupil acting as an exit pupil for lens38, and as an entrance pupil for objective44.

An inset41I shows a cross-section of beams40at pupil41. Each beam40is divided into two approximate semicircles. An upper semicircle45corresponds to sections of collimated beams40which are phase shifted, a lower semicircle47corresponds to sections of beams40which are attenuated. As is shown in the inset, the phase shift and the attenuation both occur in the one-dimensional y direction.

Objective44focuses its incident collimated beams to a series of traveling spots46on a surface48, the traveling spots typically traveling along an approximate straight line in the surface. Surface48is typically a surface of a transparent element49, such as a reticle, which is being inspected by transmission microscope20. Typically, the number of spots46on surface48at any one time is from approximately three to approximately ten. However, the number of spots may vary from this range; it will be appreciated that the actual number on the surface at any one time is set, inter alia, by the rate of repetition of the RF chirps and the speed of the traveling waves v in AO element34.

Consideration ofFIG. 1, and of the description above, shows that optic elements23,25,38, and44act as an optical beam scanning and illumination system51, and that pupil41may be considered as a fixed illumination pupil within the system.

Radiation from spots46, after traversal of element49, is collected by collection optics54, which form the radiation received from the spots into respective collimated beams56. Beams56traverse a fixed exit pupil58, which is at the focal plane of optics54.

An amplitude mask60is placed at the exit pupil. Mask60is configured to have a spatial amplitude variation that corresponds to the spatial phase variation of beams40, and thus to the spatial phase variation at pupil41. Both spatial variations are step functions, the steps occurring at the same y-position for the phase variation and for the amplitude variation. An inset601, illustrating mask60in more detail, shows that the mask comprises an upper transparent semicircle62and a lower opaque semicircle64, so that the mask has a change of amplitude in the one-dimensional y direction.

After traversing mask60, focusing optics66focus the received collimated beams onto spots69on a detector68. Detector68typically comprises an array of charged coupled detectors (CCDs) or photo-multiplier tubes (PMTs), which generate respective signals according to the intensity of the received radiation. The phase mask introduced into traveling lenses32, together with the amplitude mask that has also been incorporated into the lenses, when combined with the effect of amplitude mask60, converts phase changes caused by phase objects on surface48to amplitude changes in focused spots69. Detector68generates a signal in response to the amplitude changes, and processor29processes the signal to form an image of the phase objects on surface48.

FIG. 4is a schematic diagram of a reflection scanning microscope220, according to an embodiment of the present invention. Apart from the differences described below, the operation of microscope220is generally similar to that of microscope20(FIGS. 1,2, and3), and elements indicated by the same reference numerals in both microscopes20and220are generally similar in construction and in operation.

In microscope220a beamsplitter222, typically a 50-50 beamsplitter, partially transmits beams40after they have traversed pupil41. The partially transmitted beams are received by an objective224. Objective224performs substantially the same functions as objective44by focusing beams40to a series of spots226on a surface230of an object228being inspected. Object228typically comprises a wafer. For clarity, inFIG. 4only shaded sections37and unshaded sections39of transmitted beams40are illustrated. Returning radiation from surface230is collected by objective224, which, together with beamsplitter222, performs substantially the same functions as collection optics54, forming collimated beams234. Beamsplitter222reflects the returning collimated beams234so that the reflected collimated beams traverse an exit pupil232. Exit pupil232corresponds to exit pupil58, and is at the focal plane of objective224.

Amplitude mask60is placed at exit pupil232. Thus, as described above for collimated beams56, focusing optics66form collimated beams234into focused spots69having amplitude changes corresponding to phase objects on surface230. As is also described above, detector68and processor29generate an image of the phase objects in response to the amplitude changes.

FIG. 5is a schematic diagram of a transmission scanning microscope240, andFIG. 6is a schematic diagram of a reflection scanning microscope260, according to embodiments of the present invention. Apart from the differences described below, the operation of microscope240is generally similar to that of microscope20(FIG. 1), and elements indicated by the same reference numerals in both microscopes20and240are generally similar in construction and in operation. Also, apart from the differences described below, the operation of microscope260is generally similar to that of microscope220(FIG. 4), and elements indicated by the same reference numerals in both microscopes220and260are generally similar in construction and in operation.

Unlike microscopes20and220, the RF chirps input to AO element34of microscopes240and260do not have a phase change, but have one or more amplitude changes. Exemplary chirps, and the effect of the chirps on element34, are illustrated inFIG. 7.

FIG. 7shows schematic graphs of the RF signal generated for microscopes240and260by processor29, according to an embodiment of the present invention. The RF signal input to transducer35is in the form of chirps272, which are generally similar to chirps106(FIG. 2). Graphs270and274are respective schematic frequency vs. time and amplitude vs. time graphs of the RF signal. As shown in the graphs, each chirp272has a generally linear change of frequency with time. Each chirp also has a temporal midpoint273.

As shown by graph274, for each chirp there is a change from a first substantially constant attenuated amplitude level to a second substantially constant non-attenuated amplitude level, at approximately midpoint273of the chirp. The attenuated amplitude level is approximately 50% of the non-attenuated level. It will be understood that the particular relationships and values given for graphs270and274are by way of illustration, and that the scope of the present invention comprises other relationships and values for chirps272, such as a non-linear variation of the RF frequency with time, and an amplitude change other than a 50% reduction in amplitude.

A graph278is a schematic voltage vs. time graph of the RF signal, corresponding to a central section280of graphs270and274. Graph278illustrates the effect of the amplitude changes for a given chirp272, showing that there is an amplitude change but no phase change at midpoint273.

FIGS. 8A and 8Bare schematic graphs illustrating results produced by microscope220, according to an embodiment of the present invention. A test structure300was used as object228(FIG. 4). Structure300was a silicon wafer upon which a series of parallel lines were formed by removal of silicon from regions between the lines. The parallel lines had a height of approximately 10 nm and a period of approximately 800 nm. A graph302plots height vs. distance along the y-direction of test structure300.

Structure300was imaged using dark field illumination and bright field illumination, by methods known in the art. In addition, four images of structure300were formed by microscope220using four different types of chirps. For one image the chirps were substantially as described above with reference toFIGS. 2 and 3. The other three images were formed using chirps having phase changes as shown inFIGS. 2 and 3, but instead of the 50% amplitude change described, the amplitude changes were 0%, 25%, and 80%. There were thus a total of six images.

Each image has lines with high gray levels and lines with low gray levels. To evaluate the images the high gray level lines were scanned in the x-direction, and an average high gray level value,GLmax, was calculated for each image. Similarly, for each image an average low gray level value,GLmin, was calculated by scanning the low gray level lines.

Graph304is illustrative of the bright field image results. The dark field image gave no measurable differences between high and low gray level lines, so no illustrative graph is given inFIG. 8Aor8B. Graphs314,312,310, and308are illustrative of the four images generated using chirps respectively having 0%, 25%, 50%, and 80% amplitude change. The values ofGLmaxandGLminare given in each graph as well as in Table I below. In addition, an image contrast, illustrative of the quality of the image, is given in each graph and in Table I. The image contrast is calculated according to expression (1):

Inspection of the graphs ofFIGS. 8A and 8B, and of Table I, shows that the parallel lines of test structure300are substantially invisible using bright field non-phase contrast systems. However, graphs308,310,312and314all illustrate that using a composite one-dimensional traveling wave incorporating a phase or phase and amplitude mask makes the phase object lines of the test structure visible, and produces a very good record of the lines. The inventors have also found that using composite one-dimensional traveling waves incorporating only amplitude masks, such as are used in microscopes240and260, may significantly enhance the visibility of phase and/or amplitude objects being inspected.

The characteristics of a specific 1D composite traveling lens32(FIGS. 1,4,5,6) are generated by the RF chirp input to AO element34. The characteristics include a focal length of the lens, an amplitude attenuation incorporated into the beam traversing the composite lens, and a phase shift incorporated into the beam. An example of a 50% amplitude attenuation and a phase shift of

+π2,
each applied to half the lens, has been described above with respect toFIGS. 2 and 3. An example of a lens having no phase shift and a 50% amplitude attenuation applied to half the lens has been described with respect toFIG. 7. Since the characteristics of the lens are dependent on the characteristics of the RF chirp, both the amplitude attenuation and/or the phase shift for each composite lens may be altered, substantially at will, by processor29, typically under direction of an operator of microscopes20,220,240, or260. Examples of further alternative lenses are described below with reference toFIG. 9.

FIG. 9is a schematic diagram illustrating an alternative 1D composite traveling lens in AO element34, according to an embodiment of the present invention. Graphs400,402, and404are respective schematic frequency vs. time, phase vs. time, and amplitude vs. time, graphs of an RF signal producing chirps406. Graph400is generally similar to graph100(FIG. 2), chirps406having temporal midpoints408. However, graph402shows that there is a phase shift of

+π2
applied to a central portion of the traveling lenses produced by chirps406, between times410and412that are approximately symmetrical with respect to midpoints408. Also there is an attenuation of approximately 75% in amplitude produced on either side of a non-attenuated central portion, before time410and after time412. As illustrated schematically in a diagram414, each chirp406produces a 1D composite traveling lens432in AO element34, the traveling lens having two phase shifts and two amplitude changes at positions434and436along the lens. Positions434and436correspond with times410and412, and are typically approximately symmetrically disposed about the midpoint of lens432.

In microscopes20and220(FIGS. 1 and 4), each traveling lens432produces a composite 1D beam440, which at pupil41has a cross-section shown by inset441I. Each beam440is divided into three sections: a central section442where the beam is phase shifted and is un-attenuated, and two generally similar sections444where the beam is attenuated with no phase shift.

An amplitude mask460, shown in inset460I, is placed at exit pupils58and232. Amplitude mask has three amplitude sections corresponding with the three phase sections of beams440: an opaque central section462, and two transparent sections464.

As is explained above with respect toFIGS. 1-5, composite beams substantially similar to composite 1D beam440generate amplitude changes for phase objects on either a substantially transparent object (microscope20) or on a substantially opaque object (microscope220).

Chirps having only amplitude changes, or chirps having only phase changes, rather than chirps406which have a combination of phase and amplitude changes, may be generated by forming chirps which have no phase change or chirps which have no amplitude change. For example, processor29may generate no-phase-change chirps by forming an RF signal which has frequency vs. time and amplitude vs. time graphs similar to graphs400and404, but which instead of the phase vs. time graph402, the phase vs. time graph of the RF signal is a horizontal straight line, showing no change of phase during the chirp.

Such no-phase-change chirps may be used in microscopes240and260. For no-phase-change chirps having frequency vs. time and amplitude vs. time graphs similar to graphs400and404, amplitude mask460may be used.

As is illustrated by the examples described above, by generating 1D composite traveling lenses using chirps, composite beams may be generated which have substantially any amplitude change across the beam, as well as any phase change across the beam. The different composite beams may be generated by processor29incorporating corresponding amplitude and/or phase changes into the chirp. In addition, since a specific composite beam is formed from a specific composite lens, more than one type of composite beam may be generated sequentially and effectively simultaneously in AO element34. For example, for microscope20(FIG. 1), AO element34may have three composite traveling lenses present simultaneously in the element, a first lens as described above for the microscope, a second lens with a phase shift of

+π4,
and a third lens similar to the first lens but with an amplitude attenuation of 80%.

FIG. 10is a flowchart500of a method for optimizing an image of a surface, according to an embodiment of the present invention. By way of example, the method of flowchart500is assumed to be applied iteratively for microscope20(FIG. 1) or for microscope260. Typically the method is implemented by processor29, under overall control of an operator of the microscopes.

In a first step502the operator provides values of the phase shift and/or amplitude attenuation to be applied to chirps106(FIG. 2) by processor29. In a second step504the processor applies the values to produce the chirps, and excites transducer35with the chirps. The processor also operates source21so as to produce beams36. In a third step506surface48or surface230is irradiated, substantially as described above with reference toFIGS. 1 and 6.

In a fourth step508processor29analyzes the signal generated by detector68, and determines a visibility of an object on a surface being inspected by calculating a numerical visibility value from the signal. The numerical value may typically be formed by measurements of contrast, such as is described above with respect to Table I. Alternatively, any other convenient measurement of visibility may be used, such as a function using standard deviation and/or noise of the measured gray levels, a modulation transfer function (MTF), and/or derivatives of these parameters.

In a comparison step510the processor compares the numerical visibility value with a predetermined value that has been provided to the processor by the microscope operator. Alternatively or additionally the predetermined value is a value that the processor has calculated for a prior iteration of flowchart500.

π4
or less, within a range—π≦θ≦π, and an amplitude A may be changed by 10% or less, within a range 0%≦A≦100%. If the comparison in step510returns a true value, flowchart500ends.

The embodiments described above have assumed that amplitude masks60and460, positioned at exit pupils58and232, are substantially static. Thus mask60comprises a low transmission semicircle64and a high transmission semicircle62, which are spatially fixed and which have fixed transmission properties: 0% for the low transmission semicircle, 100% for the high transmission semicircle. Mask460comprises three spatially fixed sections of a circle: a central low transmission section462which has 0% transmission, and two substantially similar high transmission sections464which have 100% transmission. However, it will be understood that amplitude mask60and460may be constructed so that the sections from which they are formed are dynamic in spatial properties and/or in transmission properties, and that the transmission may not necessarily be 0% or 100%.

Such a dynamic amplitude mask, for example, may be constructed from an LCD (liquid crystal display) array, using an appropriate choice of operating wavelength for source21, the LCD display advantageously being operated by processor29. Processor29may adjust the spatial distribution of low transmission and high transmission elements of the array, as well as the percentage transmission of the elements. It will be understood that the spatial distribution of the low and high transmission elements, and the spatial distribution of the phase variation at pupil43, should correspond. Other methods for producing dynamic amplitude masks, such as using MEMS (micro electromechanical systems), may be applied, changing elements of microscopes20,220,240, or260as necessary.