Providing image data

Embodiments of the present invention provide a method of providing image data for constructing an image of at least a region of a target object, comprising the steps of simultaneously recording, at a detector, a plurality of separable diffraction patterns formed by a respective portion of radiation scattered by the target object; and providing the image data via an iterative process responsive to the detected intensity of radiation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage application of PCT/GB2013/051168 filed May 3, 2013, which claims priority to Great Britain Application 1207800.2 filed May 3, 2012, the entire disclosures of which are hereby incorporated herein by reference in their entireties.

BACKGROUND

The present invention relates to methods and apparatus for providing image data from which an image of at least a portion of a target object may be generated. In particular, embodiments of the invention relate to methods and apparatus which reduce a time taken to record data for producing the image data.

WO 2005/106531, which is incorporated herein by reference for all purposes, discloses a method and apparatus of providing image data for constructing an image of a region of a target object. Incident radiation is provided from a radiation source at the target object. An intensity of radiation scattered by the target object is detected using at least one detector. The image data is provided responsive to the detected radiation. A method for providing such image data via an iterative process using a moveable softly varying or bandwidth limited probe function such as a transmittance function or illumination function is also disclosed. The methods and techniques disclosed in WO 2005/106531 are referred to as a ptychographical iterative engine (PIE).

PIE provides for the recovery of image data relating to at least an area of a target object from a set of diffraction pattern measurements. Several diffraction patterns are recorded at a measurement plane using one or more detectors, such as a CCD or the like. A probe function, which might be a transmittance function associated with a post-target object aperture or an illumination function, must be known or estimated.

WO 2010/064051, which is incorporated herein by reference for all purposes, discloses an enhanced PIE (ePIE) method wherein it is not necessary to know or estimate the probe function. Instead a process is disclosed in which the probe function is iteratively calculated step by step with a running estimate of the probe function being utilised to determine running estimates of an object function associated with a target object.

Other methods of providing image data based on measurement of scattered radiation are also known.

FIG. 1illustrates an apparatus100suitable for use in the PIE and ePIE methods referred to above, and other coherent diffractive imagine techniques. The apparatus100is suitable to provide image data of an object which may, although not exclusively, be used to produce an image of at least a region of the object.

A radiation source, which although not shown inFIG. 1, is a source of radiation10which falls upon a focusing arrangement20, such as one or more lenses, and is caused to illuminate a region of a target object30. It is to be understood that the term radiation is to be broadly construed. The term radiation includes various wave fronts. Radiation includes energy from a radiation source. This will include electromagnetic radiation including X-rays, emitted particles such as electrons. Other types of radiation include acoustic radiation, such as sound waves. Such radiation may be represented by a wave function Ψ(r). This wave function includes a real part and an imaginary part as will be understood by those skilled in the art. This may be represented by the wave function's modulus and phase. Ψ(r)* is the complex conjugate of Ψ(r) and Ψ(r)Ψ(r)*=|Ψ(r)|2 where |Ψ(r)|2 is an intensity which may be measured for the wave function.

The lens20forms a probe function P(r) which is arranged to select a region of the target object30for investigation. The probe function selects part of an object exit wave for analysis. P(r) is the complex stationary value of this wave field calculated at the plane of the object30.

It will be understood that rather than weakly (or indeed strongly) focusing illumination on the target object30, unfocused radiation can be used with a post target aperture. An aperture is located post target object to thereby select a region of the target30for investigation. The aperture is formed in a mask so that the aperture defines a “support”. A support is an area of a function where that function is not zero. In other words, outside the support, the function is zero. Outside the support the mask blocks the transmittance of radiation. The term aperture describes a localised transmission function of radiation. This may be represented by a complex variable in two dimensions having a modulus value between 0 and 1. An example is a mask having a physical aperture region of varying transmittance.

Incident radiation10thus falls upon the up-stream side of the target object30and is scattered by the target object30as it is transmitted. The target object30should be at least partially transparent to incident radiation. The target object30may or may not have some repetitive structure. Alternatively the target object30may be wholly or partially reflective in which case a scattering pattern is measured based on reflected radiation.

A specimen wave O(r) is thus formed as an exit wave function of radiation after interaction with the object30. In this way O(r) represents a two-dimensional complex function so that each point in O(r), where r is a two-dimensional coordinate, has associated with it a complex number. O(r) will physically represent an exit wave that would emanate from the object which is illuminated by a plane wave. For example, in the case of electron scattering, O(r) would represent the phase and amplitude alteration introduced into an incident wave as a result of passing through the object30of interest. The probe function P(r) (or transmission function) selects a part of the object exit wave function for analysis. It will be understood that rather than selecting an aperture a transmission grating or other such filtering function may be located downstream of the object function. The probe function P(r-R) is an aperture transmission function where an aperture is at a position R. The probe function can be represented as a complex function with its complex value given by a modulus and phase which represent the modulus and phase alterations introduced by the probe into a perfect plane wave incident up it.

An exit wave function ψ(r,R) is an exit wave function of radiation35as it exits the object30. This exit wave ψ(r,R) forms a diffraction pattern Ψ(u) at a diffraction plane. Here r is a vector coordinate in real space and u is a vector coordinate in diffraction space.

In order to select the region of the target object30to be illuminated or probed, the lens(es)20or aperture may be mounted upon an x/y translation stage which enables movement of the probe function with respect to the object30. It will also be realised that the object30may be moved with respect to the lens(es) or aperture.

A detector40is a suitable recording device such as a CCD camera or the like which allows the diffraction pattern to be recorded. The detector40allows the detection of the diffraction pattern in the diffraction plane. The detector40may comprise an array of detector elements, such as in a CCD.

As will be appreciated, in order to produce image data corresponding to the target object, such as the object function O(r), a plurality of diffraction patterns are recorded at corresponding, partly overlapping, probe positions.

It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.

SUMMARY OF THE INVENTION

According to an aspect of the present invention there is provided method of providing image data for constructing an image of at least a region of a target object, comprising the steps of simultaneously recording, at a detector, a plurality of separable diffraction patterns formed by a respective portion of radiation scattered by the target object; and providing the image data via an iterative process responsive to the detected intensity of radiation.

Radiation incident upon the target object may be substantially coherent. In some embodiments a plurality of beams of radiation incident upon the target object are substantially mutually coherent. In embodiments of the invention the iterative method is based upon the plurality of diffraction patterns, wherein each diffraction pattern corresponds to a respective probe position. The method may be a coherent diffractive imaging method. In some embodiments the image data is based on more than one plurality of diffraction patterns wherein the group of all diffraction patterns comprises partially overlapping adjacent diffraction patterns. That is, adjacent diffraction patterns in the group partially overlap. The overlap may be in range 50-90%. The overlap may be an overlap in radiation responsible for each respective diffraction pattern upon the object. The diffraction patterns recorded simultaneously may not overlap, but may be substantially separate. However a totality of all recorded diffraction patterns recorded using a plurality of different positions of an aperture array may comprise overlapping diffraction patterns.

It will be appreciated that the image data does not necessarily have to be used for generating an image. The image data may merely be indicative of the target object and used for another purpose rather than generating an image, for example as a process control input, or to output one or more values derived from some or all of the image data.

According to an aspect of the present invention there is provided a method of providing image data for constructing an image of a region of a target object, comprising the steps of:simultaneously recording at a detector a plurality of diffraction patterns formed by radiation scattered by the target object;providing the image data via a process responsive to the detected intensity of radiation.

In some embodiments, radiation incident upon the target object comprises a plurality of portions each forming a respective diffraction pattern at the detector. The wavefront comprising the plurality of portions may be formed by being incident upon an aperture array. A lens may be provided to cause each of the plurality of portions to be divergent.

In some embodiments, an exit wave emanating from the target object may be incident upon an aperture array to select a plurality of regions of the exit wave to each form one of the diffraction patterns at the detector. In some embodiments, the method may comprise moving the aperture array amongst a plurality of positions.

The aperture array may be arranged such that zero order regions of each diffraction pattern are spatially separated at the plane of the detector.

Each of the diffraction patterns may correspond to a probe function.

According to an aspect of the present invention there is provided an apparatus for determining image data for constructing an image of a region of a target object, comprising:a detector for recording an intensity of radiation falling thereon;a processor for determining the image data based upon data received from the detector, wherein the processor is arranged to determine the image data based upon a plurality of diffraction patterns simultaneously recorded by the detector.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In embodiments of the invention a plurality of diffraction patterns are simultaneously recorded by a detector. Image data is determined based, at least in part, upon the plurality of diffraction patterns. Advantageously, this reduces a time required to record data sufficient for the determination of the image data.

FIG. 2illustrates a wavefront at a sample plane inFIG. 2(a)and at a detector plane inFIG. 2(b)according to an embodiment of the invention.

Referring toFIG. 2(a)the wavefront at the sample plane (the plane of the object) is arranged to simultaneously form a plurality of probe functions at different respective positions. In the example shown inFIG. 2(a)the wavefront is arranged to form four probe functions, however it will be realised that this number is merely an example and that other numbers of probe functions may be chosen. The wavefront illustrated inFIG. 2(a)is arranged to form first210, second220, third230and fourth240probe functions which select corresponding regions of a target object.

FIG. 2(b)illustrates diffraction patterns recorded by a detector corresponding to the wavefront shown inFIG. 2(a). In some embodiments of the invention at the plane of the target object, the portions of the wavefront corresponding to each probe function are arranged to be partially overlapping and divergent from one-another. The divergent nature of the portions of the wavefront causes the formation of diffraction patterns at the plane of the detector which have a further degree of separation than at the plane of the object. In particular, the portions of the wavefronts have diverged sufficiently that zero-orders (the bright spots shown inFIG. 2(b)) of each diffraction pattern are spatially separated. It will be realised that embodiments of the invention may be envisaged in which the portions of the wavefront corresponding to the plurality of probe functions are not divergent at the object plane.

The detector is able to record the plurality of diffraction patterns in a single exposure. In some embodiments, the image data may be determined based upon the intensity of radiation measured in a single exposure i.e. all diffraction patterns are recorded in the single exposure, whilst in other embodiments the plurality of diffraction patterns recorded in one exposure may be combined with a further plurality of diffraction patterns recorded in one or more subsequent exposures.

FIG. 3illustrates an apparatus300according to an embodiment of the invention. The apparatus comprises an aperture array310, a lens and a detector350. Also illustrated inFIG. 3is a focus plane330and an object340.

The aperture array310is a mask having a plurality of apertures formed therein. The mask is formed from material capable of substantially blocking incident radiation. The plurality of apertures are arranged such that radiation incident on the mask passes through the apertures to form a wavefront having corresponding probe function portions. The wavefront emanating from the aperture array310is directed toward a lens320to focus radiation toward the object340. As shown inFIG. 3, a focal plane330of the lens320is arranged to be upstream (prior to) of the object340. This causes the portions of the wavefront corresponding to each probe function to diverge downstream of the focal plane330. The wavefront interacts with the object340and is at least partly transmitted through the object340. In other embodiments, the object may be reflective and the wavefront may be at least partially reflected by the object340. The detector350is arranged to simultaneously record an exit wave from the object which includes portions corresponding to the plurality of probe functions. Based on these portions a process may be performed as described in the PIE and ePIE references, as well as other process which are known to the skilled person, to provide image data based on the measured intensity of the diffraction patterns.

Embodiments of the invention may also be envisaged in which a post target aperture array310is used. In these embodiments radiation impinging upon the target object340forms an exit wave and the aperture array310is arranged downstream of the target object340such that portions of the exit wave corresponding to each aperture form respective probe functions. As the radiation incident upon the target object340is not divergent, the apertures in the aperture array310do not overlap. In order to provide a degree of overlap of probe functions necessary for a phase retrieval algorithm, the aperture array is moved between a plurality of positions to achieve overlap between probe positions. The aperture array310may be moved by, for example, an x, y translation stage.

FIG. 4illustrates probe positions according to an embodiment of the invention. A first plurality of probe positions410corresponds to those in the aperture array310with the array310at a single location. The aperture array310may be moved in one or both of x and y directions such that the detector350records diffraction patterns at one or more further locations such that probe positions overlap. Some of the further probe positions420are indicated inFIG. 4. It will be realised that the number of aperture array locations and number of apertures present in the array310is merely illustrative.

A process may be performed as described in the PIE and ePIE references, as well as other process which are known to the skilled person, to provide image data based on the measured intensity of the plurality of diffraction patterns formed with the aperture array310at each of the plurality of locations.

FIGS. 5 and 6illustrate apparatus according to further embodiments of the invention.

FIG. 5shows an apparatus500comprising a fibre optic connector510which is arranged to receive radiation in the form of light from a fibre optic cable. The connector510terminates a fibre optic cable to couple the cable to a laser output. It will be realised that the apparatus500may be used with other types of radiation other than light received from a fibre optic. A first lens520is arranged a distance L1from the connector510to collimate received light. The distance L1may be chosen to provide suitable a radiation beam width, such as 10 mm FWHM. This beam width may be equal to a spacing distance of apertures in an aperture array530so that each aperture receives substantially the same radiation intensity. The aperture array530is arranged a distance L2from the first lens520. In some embodiments L2is equal to L1. In the embodiment shown, the aperture array530is a 2×2 array providing four portions of radiation, although it will be realised that other numbers of aperture may be present in either dimension. A diameter of each aperture in the exemplary embodiment is 150 μm, although it will be realised that other diameters may be used. A second lens540may be arranged in an 2-f configuration with the aperture array530and a mask aperture550. The second lens540is arranged a focal length L3from the aperture array530and a further focal length L4from the mask aperture550in some embodiments. The focal length in one exemplary embodiment is 50 mm, although it will be realised that other focal lengths may be used. The mask aperture comprises an aperture which may central in the mask. The aperture has a size which is less than an airy disc formed by the apertures in the aperture array at a plane of the mask aperture. The mask aperture may be 600 μm, although it will be realised that this is merely exemplary. A target object560is arranged downstream of the mask aperture550and is preceded, in some embodiments, by a diffuser which may be made of a plastic material. The presence and material of the diffuser570may be chosen appropriately. The diffuser570acts to reduce a dynamic range of signal at a detector590. The apparatus further comprises a reflection mask580. The reflection mask580is provided to prevent or reduce multiple reflections, such as between a glass slide supporting the object and the CCD. The reflection mask is arranged a distance L6from the sample. The distance L6is selected depending upon a diameter of the reflection mask580. A larger diameter mask580enables a larger distance L6, but is less effective at removing reflections. In one embodiment the reflection mask580has a diameter of 1 mm and L6is 2× the mask diameter i.e. 2 mm. In an exemplary embodiment the detector590is placed a distance L7away from the sample560, which may be, for example, 33 mm. This distance ensures that the scattered light from each probe is isolated spatially at the detector590. The target560is located a distance L5away from a focal plane of the second lens540. This distance L5ensures that the multiple probes have a sufficient overlap at the plane of the target. The focal plane of the second lens540may coincide with the plane of the mask aperture550.

FIG. 6illustrates an apparatus600according to an embodiment of the invention. The apparatus600comprises like parts to that described with reference toFIG. 5and, except where necessary, a repetition thereof will be omitted.

The apparatus comprises an aperture array having 4×4 (16) apertures. Due to the greater number of apertures, the outer-most apertures in opposing corners have a greater spacing from each other than in the embodiment described with reference toFIG. 5. Therefore a spherical aberration of the radiation may be important to focussing the radiation. The second lens640is selected to have a greater focal length of, for example, 100 mm. The mask aperture650may be formed from an iris. The iris may have a minimum closed diameter of approximately 800 μm, although other diameters may be used. A third lens651is provided with a microscope objective lens652arranged in-between the mask aperture650and the target object660, upstream of the target object660. The third lens651may have a focal length of, for example, 35 mm and is arranged to form a de-magnified image of the aperture array530. A collimated image is formed when the third lens651is arranged spaced apart from a focal plane of the second lens640by its focal length e.g. 35 mm. In some arrangements the third lens651may be arranged closer than its focal length to the focal plane of the second lens640which results in a reduced numerical aperture of the microscope objective lens652, for example due to space restrictions. The microscope objective lens652is arranged to focus the plurality of beams of radiation. The microscope objective lens may have a magnification ratio of 10× or 20×, although other ratios may be used. The target object660is arranged a distance L6from the focal plane of the objective lens652. The distance L6determines a degree of overlap between probe positions. At L6=0 the probe positions have 100% overlap. A desired degree of overlap is around 70%, although may be in the range of 80-60% or 90-50%. L6is therefore selected to obtain a desired overlap of probe positions. The distance may be, for example, 400 μm although other distances may be used. The detector690is located a distance L7from the target object which may be 20-30 mm to ensure sufficient spatial resolution of each of the 16 diffraction patterns. It will be realised that other distances may be used.

Some embodiments of the invention include a step of segmenting the detector590,690. The detector is segmented to isolate each diffraction pattern such that the simultaneously recorded diffraction patterns may be separately processed to determine the image data. *. A method according to an embodiment of the invention will be described with reference toFIG. 7.

In step710the detector is divided into a plurality of regions or segments. It will be realised that the division of the detector may be performed in software i.e. with no physical division of the detector. Instead the response of the detector to the impinging radiation is allocated amongst a plurality of “virtual” detectors, such as by being independently stored in memory in separate data structures. The segments may be equal sized. In an exemplary embodiment a detector having 1024 by 1024 pixels is divided into segments of 256 pixels by 256 pixels when the detector is arranged to simultaneously receive 16 diffraction patterns in a 4×4 arrangement.

In step720a centre of each diffraction pattern within the respective segment is determined. The centre of the diffraction pattern may be found by means of thresholding, edge detection and fitting the measured pattern to a circle. In this sense, thresholding is understood to mean that any pixel of the detector response below a threshold value is set to a predetermined value, for example 0.

In step730an average centre position for each diffraction pattern within the segments is determined based upon the plurality of centres determined in step720. The average position may be a mean position determined from amongst the plurality of centres from step720.

In step740each diffraction pattern is shifted, if necessary, so as to centre the pattern at the average centre position determined in step730. The diffraction pattern may be padded with a predetermined value, such as 0, to allow for the movement of the measured data.

In step750each segment is cropped to its original size, if necessary. In step760any cross-talk between neighbouring segments may be removed. The removal of cross-talk may be performed by the application of a hamming or Gaussian window to each segment.

FIG. 8illustrates data800output by a detector indicative of measured diffraction patterns810(only two of which are indicated) using a 4×4 aperture array which produces 16 simultaneously measured diffraction patterns.

FIG. 9illustrates a diffraction pattern910based on a portion of the data800shown inFIG. 8following application of an embodiment of the method700described with reference toFIG. 7. As can be appreciated from a comparison ofFIGS. 8 & 9, the detector response inFIG. 8is 1024×1024 pixels, whereas the diffraction pattern for one diffraction patterns shown inFIG. 9is 256×256 pixels. A border exists on the right and lower sides of the diffraction pattern920. The border is as a result of centering the diffraction pattern before applying a Gaussian window. In this case the Gaussian window is generated by first forming an aperture of ‘1s’ of a predetermined diameter, such as 210 pixels, and then smoothing the aperture edge with a Gaussian drop off with a predetermined width, such as 20 pixels. In another embodiment, the window is applied before shifting the diffraction pattern as in step740, but whilst ensuring the centre of the window corresponds to the centre of the diffraction pattern.

In prior art methods, a position of a respective probe function corresponding to a measured diffraction pattern is known from movement of, for example, an aperture, focussing arrangement or the object. Embodiments of the invention comprise a method of determining a location of the probe position corresponding to a diffraction pattern. In a first step a distance between the plurality of diffraction patterns is determined. The distance may be determined based upon the average centre position of each diffraction pattern determined in step730ofFIG. 7. The distance between diffraction patterns, at the plane of the detector, is determined based on the average centre location of the diffraction patterns. The distance may be converted from pixels to meters based on the dimensions of the detector pixels, as will be appreciated. The distance at the detector plane may be scaled using a ratio of the focal distance (distance between the object and the point of focus) to the camera length. The focal point may be determined using a back propagation of the probe from a prior reconstruction using, for example, ePIE.

FIG. 10illustrates a representation of image data determined according to an embodiment of the invention using a plurality of simultaneously measured diffraction patterns. In the present case, the image data is based on use of a 2×2 aperture array i.e. 4 simultaneously recorded diffraction patterns. Embodiments of the invention allow image data to be determined based on a single exposure of the object to the radiation, a so called “single shot” method, wherein the method of determining the image data uses a plurality of diffraction patterns.