Patent Publication Number: US-2015085291-A1

Title: Compact Self-Contained Holographic and Interferometric Apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional application 61/617,348 which was filed on 29 Mar. 2012, and which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates to an inspection apparatus usable, for example, in the manufacture of devices by lithographic techniques. 
     2. Related Art 
     A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. 
     In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between two layers formed in or on the patterned substrate and critical line width of developed photosensitive resist. This measurement may be performed on a product substrate and/or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. Holography and interferometry are related techniques for making measurements of the microscopic structures. Any apparatus that utilizes holography or interferometry requires the use of a reference beam for interference with the object beam. In either technique, the light from a laser is split into two beams. One beam is for object illumination and the other is for reference beam formation. These two beams follow substantially different object and reference beam paths. 
     One disadvantage of previous holographic and interferometric approaches is that the object beam path and the reference beam path are substantially different, requiring very high dimensional and mechanical stability. Vibrations are not fully eliminated through the use of tables, benches, and other isolation configurations. This often creates serious obstacles in generating quality holograms, especially in microscopic settings. Another way for holographic assemblies to reduce the effects of vibration has been to partially combine the optical and reference beam paths. However, a disadvantage to this approach is that significantly long segments of the optical beam path and the reference beam path remain independent from one another, rendering it incapable of eliminating the vibration problem. 
     SUMMARY 
     Accordingly, there is a need for improved holographic and interferometric inspection apparatuses. 
     In one embodiment, a method of eliminating vibration and dimensional instability includes illuminating an object with a light beam and forming an object beam using an objective lens that is configured to direct the object beam through a tube lens onto an image plane. A reference beam is formed from a portion of the object beam passing through a pupil plane of the objective lens, using a reference beam lens group that is configured to propagate the reference beam along a shared optical path with the object beam. The method further includes combining the reference beam and the object beam to create an interference pattern at the image plane. 
     In another embodiment, an inspection apparatus includes a light source configured to produce a light beam, an objective lens configured to direct an object beam from an object illuminated by the light beam, and a reference beam lens group. The reference beam lens group is configured to form a reference beam from a portion of the object beam passing through a pupil plane of the objective lens, the reference beam being propagated along a shared optical path with the object beam. The inspection apparatus further includes a tube lens configured to direct the object beam and the reference beam onto an image plane. In addition, a processor is configured to determine an interference pattern on the image plane from the object beam and the reference beam. 
     In another embodiment, a method within an optical system includes illuminating an object with a light beam, forming an object beam using a microscope lens arrangement that is configured to direct the object beam through a tube lens onto an image plane along a main axis of the optical system, and forming a reference beam. The reference beam is formed using a reference beam lens group that is positioned at a central portion of a pupil plane of the microscope lens arrangement along the main axis of the optical system, wherein the reference beam is formed from a portion of the object beam passing through the pupil plane of the microscope lens arrangement. The method further includes propagating the reference beam along a shared optical path with the object beam, shifting a phase of the reference beam using a phase plate, and combining the reference beam and the object beam to create an interference pattern at the image plane. 
     In another embodiment, a method for microscopy includes propagating an object beam along an optical path and a longitudinal axis of an optical arrangement, the object beam formed from light scattered by an illuminated object. A reference beam is also propagated along the optical path substantially simultaneously with the object beam, the reference beam being formed from a portion of the light scattered by the illuminated object. The reference beam and the object beam interfere at an image plane to create a hologram image. 
     In another embodiment, a method for microscopy includes providing a first optical arrangement having a longitudinal axis to propagate an object beam in an optical path along the longitudinal axis, the object beam being formed from the light scattered by an illuminated object. A second optical arrangement is integrated with the first optical arrangement to propagate a reference beam substantially simultaneously with the object beam in the object path along the longitudinal axis, the reference beam being formed from a portion of the light scattered by the illuminated object. The reference beam causes interference with the object beam at an image plane. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. 
         FIG. 1  depicts a lithographic apparatus. 
         FIG. 2  depicts a lithographic cell or cluster. 
         FIG. 3  illustrates an optical schematic of an interferometric/holographic apparatus according to an embodiment that utilizes a spherical wave reference beam. 
         FIG. 4  illustrates an optical schematic of an interferometric/holographic apparatus according to another embodiment that utilizes a plane wave reference beam. 
         FIG. 5  illustrates an optical schematic of an interferometric/holographic apparatus according to another embodiment that utilizes a plane wave reference beam in a de-magnification configuration. 
         FIG. 6  depicts an optical schematic of the interferometric/holographic apparatus of  FIG. 3 . 
         FIG. 7  depicts an optical schematic of the interferometric/holographic apparatus of  FIG. 4 . 
         FIG. 8  depicts an optical schematic of an interferometric/holographic apparatus according to another embodiment that utilizes a pixelated phase mask dynamic interferometer. 
         FIG. 9  depicts an optical schematic of an interferometric/holographic apparatus according to another embodiment that utilizes heterodyne interferometry/holography. 
         FIG. 10  illustrates computer system hardware useful in implementing the embodiments shown in  FIGS. 3 through 9 . 
         FIG. 11  is a flow diagram of a method of sharing an optical path to eliminate errors due to vibration. 
     
    
    
     The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the present invention. The scope of the present invention is not limited to the disclosed embodiment(s). The present invention is defined by the claims appended hereto. 
     The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. 
       FIG. 1  schematically depicts a lithographic apparatus. The apparatus comprises an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a support structure (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters, a substrate table (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W. 
     The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. 
     The support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support structure may be a frame or a table, for example, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” 
     The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
     The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix. 
     The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system.” 
     As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask). 
     The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. 
     The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure. 
     Referring again to  FIG. 1 , the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. 
     The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. 
     The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in  FIG. 1 ) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies. 
     The depicted apparatus could be used in at least one of the following modes:
         1. In step mode, the mask table MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.   2. In scan mode, the mask table MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the mask table MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PL. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.   3. In another mode, the mask table MT is kept essentially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.       

     Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
     As shown in  FIG. 2 , the lithographic apparatus LA forms part of a lithographic cell LC, also sometimes referred to a lithocell or cluster, which also includes apparatus to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK. A substrate handler, or robot, RO picks up substrates from input/output ports I/O1, I/O2, moves them between the different process apparatus and delivers then to the loading bay LB of the lithographic apparatus. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency. 
     In order that the substrates that are exposed by the lithographic apparatus are exposed correctly and consistently, it is desirable to inspect exposed substrates to measure properties such as overlay errors between two layers, line thicknesses, critical dimensions (CD), etc. If errors are detected, adjustments may be made to exposures of subsequent substrates, especially if the inspection can be done soon and fast enough that other substrates of the same batch are still to be exposed. Also, already exposed substrates may be stripped and reworked—to improve yield—or discarded, thereby avoiding performing exposures on substrates that are known to be faulty. In a case where only some target portions of a substrate are faulty, further exposures can be performed only on those target portions which are good. 
     An inspection apparatus is used to determine the properties of the substrates, and in particular, how the properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection apparatus may be integrated into the lithographic apparatus LA or the lithocell LC or may be a stand-alone device. 
     Embodiments of the present invention may be used with or independently of scatterometers, or in combination with other tools as part of an in situ reticle inspection system, or other types of systems. For example, embodiments of the present invention may be included with microscope systems, such as electron microscopes, as an inexpensive attachment to the microscope objective. Such systems may include a broadband (white light) radiation projector which projects the radiation to an object under inspection. In such configurations, embodiments of the present invention would be located external to the main lens of the microscope system. The discussion that follows details different potential embodiments that may be applied in these different types of systems. 
       FIG. 3  illustrates an optical schematic of an apparatus  300  according to an embodiment that utilizes a spherical wave reference beam. Apparatus  300  can be an interferometric or holographic measurement device. For brevity, the following discussion will reference a holographic measurement device, though the skilled artisan will appreciate that the discussion will equally apply to an interferometric measurement device. Holographic measurement device  300  can sense one or more properties, such as defects in a patterning device (e.g., mask MA) before patterning. Such defects may be inherent in the patterning device or introduced externally by, for example, particles deposited on the patterning device. In one embodiment, holographic measurement device  500  operates in a wide spectral range from about 200 nm to about 850 nm. 
     Holographic measurement device  300  operates by utilizing the light scattered by an illuminated object  302  to form a reference beam. Illuminated object  302  may be a patterning device (e.g., mask MA) or any other object subject to inspection by a microscope, of which the holographic measurement device  300  may be a part. The light propagating through a central part of the pupil of an imaging lens  304  is used for forming the reference beam. For example, 0 th  order light propagating through a central part of pupil plane  306  of imaging lens  304  is used. This is possible because the optical information of the object  302  under investigation, especially the optical information associated with fine and mid-sized features of the object  302 , is concentrated in the outer part of the pupil plane  306  of imaging lens  304 . 
     Holographic measurement device  300  also includes a reference beam forming lens group  308 , a spatial filter  310 , a phase plate  312 , tube lens  314 , and image plane  316 . As indicated above, the central part of the pupil optical field of imaging lens  304  is used for reference beam formation. The reference beam forming lens group  308  intercepts a central part of the light scattered by the illuminated object  302 , e.g. near an optical axis of the light path, and spatially filters, if desired, the intercepted light along with the spatial filter  310  to form reference beam  352 . Spatial filter  310  is useful to remove from the reference beam any structure of the object beam that may have been intercepted from the central part of the light scattered by the illuminated object  302 , such as rings and side lobes of the light. Spatial filter  310  may be a set of lenses with a pinhole, or may be implemented in other configurations as would be recognized by the skilled artisan. The spatial filter  310  may be positioned such that it is located at a waist of the reference beam  352 . The remaining light scattered by the illuminated object  302  from the outer part of the pupil plane  306  constitutes an object beam  350 . 
     Both beams—object beam  350  and reference beam  352 —propagate along the main axis of the holographic measurement device  300  through the tube lens  314 . In this manner, relative displacement or vibration of the object beam  350  and reference beam  352  are eliminated because each beam traverses a shared optical path. The phase plate  312  changes the phase of the reference beam  352  to enable the creation of holograms at the image plane  316 , when the reference beam  352  is recombined with the object beam  350 . The phase plate  312  may be situated, with reference to the direction of propagation of the object beam  350  and reference beam  352 , before or after the tube lens  314 , depending on the particular configuration of the reference beam forming lens group  308 , as will be discussed in more detail below. 
     The optical configuration of holographic measurement device  300  is such that the reference beam  352  is divergent as it passes through the tube lens  314 , resulting in a divergent spherical wave pattern on the image plane  316 . The tube lens  314  further directs the object beam  350  onto the image  316 , where the beams combine to form an interference pattern. This interference pattern may be recorded at the image plane  316  using a detector, e.g. a CCD, or any other suitable imaging device. The recorded pattern may then be reconstructed using a processor, which is not shown in  FIG. 3 . 
       FIG. 4  illustrates another optical schematic of a holographic measurement device  400  according to another embodiment that utilizes a plane wave reference beam. Holographic measurement device  400  is similar in configuration and operation as device  300  above. Like device  300 , holographic measurement device  400  operates by utilizing the light scattered by an illuminated object  402  to form a reference beam. Illuminated object  402  may be a patterning device (e.g., mask MA) or any other object subject to inspection by a microscope, of which the holographic measurement device  400  may be a part. The light propagating through a central part of the pupil of an imaging lens  404  is used for forming the reference beam. For example, 0 th  order light propagating through a central part of pupil plane  406  of imaging lens  404  is used. 
     Holographic measurement device  400  also includes a reference beam forming lens group  408 , a spatial filter  410 , a tube lens  412 , a phase plate  414 , and an image plane  416 . The central part of the pupil optical field of imaging lens  404  is used for reference beam formation. The reference beam forming lens group  408  intercepts a central part of the light scattered by the illuminated object  402 , e.g. near an optical axis of the light path, and spatially filters, if desired, the intercepted light along with the spatial filter  410  to form reference beam  452 . The remaining light scattered by the illuminated object  402  from the outer part of the pupil plane  406  constitutes an object beam  450 . 
     Both beams—object beam  450  and reference beam  452 —propagate along the main axis of the holographic measurement device  400  through the tube lens  412 . In this manner, relative displacement or vibration of the object beam  450  and reference beam  452  are eliminated because each beam traverses a shared optical path. The phase plate  414  changes the phase of the reference beam  452  to enable the creation of holograms at the image plane  416 , when the reference beam  452  is recombined with the object beam  450 . The phase plate  414  may be situated, with reference to the direction of propagation of the object beam  450  and reference beam  452 , before or after the tube lens  412 , depending on the particular configuration of the reference beam forming lens group  408 , for example after the tube lens  412  as depicted in  FIG. 4 . 
     The optical configuration of holographic measurement device  400  is such that the reference beam  452  is convergent as it passes through the tube lens  412 , resulting in a convergent plane wave pattern on the image plane  416 . The tube lens  412  further directs the object beam  450  onto the image plane  416 , where the beams combine to form an interference pattern. This interference pattern may be recorded at the image plane  416  using a detector, e.g. a CCD, or any other suitable imaging device. The recorded pattern may then be reconstructed using a processor, which is not shown in  FIG. 4 . 
     Embodiments of the present invention may also be applied in de-magnification schemes.  FIG. 5  illustrates another optical schematic of a holographic measurement device  500  according to another embodiment that utilizes a plane wave reference beam in a de-magnification configuration. Holographic measurement device  500  operates by utilizing the light scattered by an illuminated object  502  to form a reference beam. Illuminated object  502  may be a patterning device (e.g., mask MA) or any other object subject to inspection by a microscope, of which the holographic measurement device  500  may be a part. The scattered light propagates through an object lens  504 . 
     After passing a pupil plane  506 , the scattered light is incident upon an imaging lens  508 . A reference beam forming lens group  510 / 511  is situated at a central part of the imaging lens  508 , where it directs a central part of the light propagating along the optical axis of the holographic measurement device  500  to form a reference beam  552 . For example, 0 th  order light propagating through a central part of imaging lens  508  is used. The reference beam forming lens group may include lenses  510  and  511 , with a spatial filter  512 , if desired, situated between the lenses  710  and  711 . A phase plate  514  may be located after imaging lens  508  and reference beam forming lens group  510 / 511 . The remaining light from the outer part of the pupil plane of imaging lens  508  scattered by the illuminated object  502  constitutes an object beam  550 . 
     Both beams—object beam  550  and reference beam  552 —propagate along the main axis of the holographic measurement device  500 . In this manner, relative displacement or vibration of the object beam  550  and reference beam  552  are eliminated because each beam traverses a shared optical path. The phase plate  514  changes the phase of the reference beam  552  to enable the creation of holograms at an image plane  516 , when the reference beam  552  is recombined with the object beam  550 . The phase plate  514  may be situated, with reference to the direction of propagation of the object beam  550  and reference beam  552 , before or after the imaging lens  708 . The location of phase plate  514  depends on the particular configuration of the reference beam forming lens group  510 / 511 , for example after the imaging lens  508  as depicted in  FIG. 5 . 
     The optical configuration of holographic measurement device  500  is such that the reference beam  552  is convergent after it is formed in the reference beam forming lens group  510 / 511 , resulting in a convergent plane wave pattern on the image plane  516 . The imaging lens  508  forms and directs the object beam  550  onto the image plane  516 , where the beams combine to form an interference pattern. This interference pattern may be recorded at the image plane  516  using a detector, e.g. a CCD, or any other suitable imaging device. The recorded pattern may then be reconstructed using a processor, which is not shown in  FIG. 5 . 
     Example Embodiments of a Holographic or Interferometric Measurement Device 
       FIG. 6  depicts an optical schematic of the holographic measurement device of  FIG. 3 . Holographic measurement device  600 , just as device  300  in  FIG. 3 , is a homodyne phase-step holographic arrangement with a spherical wave reference beam. Holographic measurement device  600  operates as indicated above in  FIG. 3 . Because relative displacement or vibration between the object beam and reference beam is eliminated by utilizing a shared optical path, the required temporal coherence of the light upon an object is significantly relaxed. This applies to the other embodiments as well as the present embodiment. In one embodiment, holographic measurement device  800  operates in a wide spectral range from about 200 nm to about 850 nm. 
     The imaging lens  304  of holographic measurement device  600  is depicted in greater detail, including four doublet lenses  602 ,  604 ,  606 , and  608 , for example. As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. In this embodiment, 0 th  order light propagating through a central part of pupil plane  306  of imaging lens  304  is used. This is possible because the optical information of the object  302  under investigation, especially the optical information associated with fine and mid-sized features of the object  302 , is concentrated in the outer part of the pupil plane  306  of imaging lens  304 . 
     Reference beam forming lens group  308  may include lenses  610 ,  612 , and  614 . As will be recognized by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. These lenses operate together to intercept a central part of the light scattered by the illuminated object  302 . In this embodiment, the lenses  610 ,  612 , and  614  that comprise the reference beam forming lens group  308  are all located before the tube lens  314 . By situating the reference beam forming lens group lenses in this manner, the reference beam  352  is divergent as it passes through the tube lens  314 . 
     The phase plate  312  may include a set of three or more phase plates (not shown) to cover a 2π phase range, as required for homodyne holography. In this embodiment, phase plate  312  is situated, with reference to the direction of propagation of the object beam  350  and reference beam  352 , after the tube lens  314 . As indicated above, the object beam  350  and reference beam  352  propagate along a shared optical path, in this example along a central axis of the holographic measurement device  600 , which eliminates relative displacement and/or vibration of the object beam  350  and reference beam  352 . This results in a divergent spherical wave pattern on the image plane  316  that has been phase-shifted. The object beam  350  combines with the reference beam  352  on the image plane  316  to form the interference pattern, which is processed as indicated above with reference to  FIG. 3 . 
       FIG. 7  depicts an optical schematic of the holographic measurement device of  FIG. 4 . Holographic measurement device  700 , just as device  400  in  FIG. 4 , is a homodyne phase-step holographic arrangement with a plane wave reference beam. In addition to the elements introduced in  FIG. 4  and discussed above, the imaging lens  404  of holographic measurement device  700  includes four doublet lenses  702 ,  704 ,  706 , and  708 , for example. As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. In this embodiment, 0 th  order light propagating through a central part of pupil plane  406  of imaging lens  404  is used. This is possible because the optical information of the object  402  under investigation, especially the optical information associated with fine and mid-sized features of the object  402 , is concentrated in the outer part of the pupil plane  406  of imaging lens  404 . 
     Reference beam forming lens group  408  includes lenses  710  and  712 . These lenses operate together to intercept a central part of the light scattered by the illuminated object  402 . As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. In this embodiment, the lenses  710  and  712  that comprise the reference beam forming lens group  408  are located, respectively, before and after the tube lens  412 . For example, lens  710  may be situated after the imaging lens  404  but before the spatial filter  410  and tube lens  412 , as depicted in  FIG. 7 . Lens  712  may be situated after tube lens  412 . By situating the reference beam forming lens group lenses  710  and  712  in this manner, the reference beam  452  is convergent as it is incident upon the imaging plane  416 . 
     As indicated above, the phase plate  414  may include three or more phase plates (not shown) to cover a 2π phase range, as required for homodyne holography. In this embodiment, phase plate  414  is situated, with reference to the direction of propagation of the object beam  450  and reference beam  452 , after the tube lens  412 . As indicated above, the object beam  450  and reference beam  452  propagate along a shared optical path, in this example along a central axis of the holographic measurement device  700 , which eliminates relative displacement and/or vibration of the object beam  450  and reference beam  452 . This results in a convergent plane wave pattern on the image plane  416  that has been phase-shifted. The object beam  450  combines with the reference beam  452  on the image plane  416  to form the interference pattern, which is processed as indicated above with reference to  FIG. 4 . 
       FIG. 8  depicts an optical schematic of holographic measurement device  800  according to another embodiment that utilizes a pixelated phase mask dynamic interferometer. Holographic measurement device  800  can sense one or more properties, such as defects in a patterning device (e.g., mask MA) before patterning. These defects could be inherent in the patterning device or introduced externally by, for example, particles deposited on the patterning device. In one embodiment, holographic measurement device  800  operates in a wide spectral range from about 200 nm to about 850 nm. 
     Holographic measurement device  800  operates by utilizing the light scattered by an illuminated object  802  to form a reference beam. Illuminated object  802  may be a patterning device (e.g., mask MA) or any other object subject to inspection by a microscope, of which the holographic measurement device  800  may be a part. The light propagating through a central part of the pupil of an imaging lens  804  is used for forming the reference beam. Imaging lens  804  may include four doublet lenses  806 ,  808 ,  810 , and  812 , for example. As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. For example, 0 th  order light propagating through a central part of a pupil plane  832  of imaging lens  804  is used. This is possible because the optical information of the object  802  under investigation, especially the optical information associated with fine and mid-sized features of the object  802 , is concentrated in the outer part of the pupil plane  832  of imaging lens  804 . 
     Holographic measurement device  800  may also include a reference beam forming lens group  814 . For example, in this embodiment the reference beam forming lens group  814  includes lenses  816 ,  818 , and  820 , all of which are situated before the tube lens  826 . As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. Holographic measurement device  800  may also include a circular polarizer  822  that circularly polarizes both the object beam  850  and reference beam  852 . The circular polarizer  822  polarizes one beam to left-handed circularly polarized light and the other beam to right-handed circularly polarized light. Thus, for example, the circular polarizer  822  may be configured to polarize the object beam  850  to become left-handed circularly polarized light and the reference beam  852  to become right-handed circularly polarized light. Or, in the alternative, the object beam  850  becomes right-handed circularly polarized light, and the reference beam  852  becomes left-handed circularly polarized light. The object beam  850  and reference beam  852  thus obtain orthogonal circular polarizations to each other. A processor that reconstructs the interference pattern recorded at an image plane  830  may be programmed with the particular configuration of the circular polarizer  822  to establish which beam has which polarization. 
     Holographic measurement device  800  may also include a spatial filter  824 , tube lens  826 , and a pixelated phase mask  828 . As indicated above, the central part of the pupil optical field of imaging lens  804  is used for reference beam formation. The reference beam forming lens group  814  intercepts a central part of the light scattered by the illuminated object  802 , e.g. near an optical axis of the light path, and spatially filters, if desired, the intercepted light with the spatial filter  824  to form the reference beam  852 . The spatial filter  824  is useful to remove from the reference beam  852  any structure of the object beam  850  that may have been intercepted from the central part of the light scattered by the illuminated object  802 , such as rings and side lobes. The spatial filter  824  may be a set of lenses with a pinhole, for example, but other implementations will become apparent to the skilled artisan. The remaining light scattered by the illuminated object  802  from the outer part of the pupil plane  832  constitutes the object beam  850 . 
     Both beams—object beam  850  and reference beam  852 —propagate along the main axis of the holographic measurement device  800  through the tube lens  826 . In this manner, relative displacement or vibration of the object beam and reference beam are eliminated because each beam traverses a shared optical path. In this embodiment, no phase plate is necessary because of the orthogonal circular polarization of the beams  850  and  854 , and the pixelated phase mask  828 . The pixelated phase mask  828  may include a CCD array, where each pixel of the CCD array has its own phase plate. Thus, each pixel has an image with a different phase. In addition, each pixel may have a separate lens. The pixelated phase mask  828  may have pixels arranged in groups of 4, for example, where each pixel in the group of 4 has a phase mask with a different phase shift. This pattern may then be repeated across the entirety of the CCD array. 
     The situation of the lenses  816 ,  818 , and  820  in reference beam forming lens group  814  before the tube lens  826  results in a divergent spherical wave pattern as the reference beam  852  passes through the tube lens  826 . The lenses in the reference beam forming lens group  814  could also be placed to impart a convergent plane wave pattern to the reference beam  852 . Either configuration is possible for this embodiment. The tube lens  826  further directs the object beam  850  onto the pixelated phase mask  828 , where the object and reference beams combine to form an interference pattern. This interference pattern may be recorded at the image plane  830 . The recorded pattern may then be reconstructed using a processor, which is not shown in  FIG. 8 . 
       FIG. 9  depicts an optical schematic of a holographic measurement device  900  according to another embodiment that utilizes heterodyne interferometry or holography. Like the other embodiments discussed above, holographic measurement device  900  can sense one or more properties, such as defects in a patterning device (e.g., mask MA) before patterning. These defects could be inherent in the patterning device or introduced externally by, for example, particles deposited on the patterning device. In one embodiment, holographic measurement device  900  operates in a wide spectral range from about 200 nm to about 850 nm. 
     Holographic measurement device  900  operates by utilizing the light scattered by an illuminated object  902  to form a reference beam. Illuminated object  902  may be a patterning device (e.g., mask MA) or any other object subject to inspection by a microscope, of which the holographic measurement device  900  may be a part. The light propagating through a central part of the pupil of an imaging lens  904  is used for forming the reference beam. Imaging lens  904  may include four doublet lenses  906 ,  908 ,  910 , and  912 , for example. As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. For example, 0 th  order light propagating through a central part of a pupil plane  914  of imaging lens  904  is used. This is possible because the optical information of the object  902  under investigation, especially the optical information associated with fine and mid-sized features of the object  902 , is concentrated in the outer part of the pupil plane  914  of imaging lens  904 . 
     Holographic measurement device  900  may also include a reference beam forming lens group  916 . For example, in this embodiment the reference beam forming lens group  916  includes lenses  918  and  922 , with the lens  918  situated before a tube lens  924  and the lens  922  situated after the tube lens  924 . As will be understood by the skilled artisan, more or fewer lenses, and of other types, may be used to similar effect. As a result of the lens placement, the reference beam  952  is convergent as it passes through the tube lens  924 , resulting in a convergent plane wave pattern. Holographic measurement device  900  may also include a spatial filter  920 , whose operation is as described above regarding the other figures. 
     Once the reference beam  952  has passed through the tube lens  924  and the lens  922 , a first mirror  926  is set in the optical path of the reference beam  952  at an incline. The first mirror  926  is inclined such that the reference beam  952  is directed to a second mirror  928 . The second mirror  928  is inclined such that the reference beam  952  is again directed at the image plane  930 . As will be recognized by the skilled artisan, the two inclined mirrors  926  and  928  are by way of example only. More could also be used, or other in the alternative could be prisms instead of mirrors to achieve the same effects. By placing the reference beam  952  at such an angle, a carrier wave is created at a frequency slightly different from the frequency of the object beam  950 . When the reference beam  952  is combined with the object beam  950  at the image plane  930 , a beating is created which represents the difference between the optical frequencies of the object beam  950  and reference beam  952 . Phase plates are not necessary in this embodiment because the use of the mirrors imparts the necessary phase difference to cause an interference pattern at the image plane  930 . 
     Both beams—object beam  950  and reference beam  952 —propagate along the main axis of the holographic measurement device  900  through the tube lens  924 . Only near the image plane  930  is the reference beam  952  diverted from the shared optical path with the object beam  950  by the inclined mirrors  926  and  928 . In this manner, relative displacement or vibration of the object beam and reference beam are substantially eliminated because each beam traverses a shared optical path up until the reference beam  952  is incident upon the first mirror  926 , near the image plane  930 . 
       FIG. 10  illustrates computer system hardware useful in implementing the embodiments discussed in  FIGS. 3 through 9 . In particular,  FIG. 10  illustrates a computer assembly useful as a processor configured to reconstruct a recorded pattern on an image plane and determine the interference pattern. The computer assembly may be a dedicated computer in the form of a control unit in embodiments of the assembly according to the invention or, alternatively, be a central computer controlling the lithographic projection apparatus. The computer assembly may be arranged for loading a computer program product comprising computer executable code. 
     A memory  1002  connected to a processor  1024  may comprise a number of memory components like a hard disk drive (HDD)  1004 , Read Only Memory (ROM)  1006 , Electrically Erasable Programmable Read Only Memory (EEPROM)  1008  and Random Access Memory (RAM)  1010 . Not all aforementioned memory components need to be present. Furthermore, it is not essential that aforementioned memory components are physically in close proximity to the processor  1024  or to each other. They may be located at a distance away from each other. 
     The processor  1024  may also be connected to some kind of user interface, for instance a keyboard  1012  or a mouse  1014 . A touch screen, track ball, speech converter or other interfaces that are known to persons skilled in the art may also be used. 
     The processor  1024  may be connected to a reading unit  1020 , which is arranged to read data, e.g. in the form of computer executable code, from and under some circumstances store data on a data carrier, like a floppy disc  1018  or an optical disk drive  1016 . DVDs, flash memory, or other data carriers known to persons skilled in the art may also be used. 
     The processor  1024  may also be connected to a printer  1022  to print out output data on paper as well as to a display  1030 , for instance a monitor or LCD (Liquid Crystal Display), of any other type of display known to a person skilled in the art. 
     The processor  1024  may be connected to a communications network  1028 , for instance a public switched telephone network (PSTN), a local area network (LAN), a wide area network (WAN) etc. by means of transmitters/receivers  1026  responsible for input/output (I/O). The processor  1024  may be arranged to communicate with other communication systems via the communications network  1028 . In an embodiment of the invention external computers (not shown), for instance personal computers of operators, can log into the processor  1024  via the communications network  1028 . 
     The processor  1024  may be implemented as an independent system or as a number of processing units that operate in parallel, wherein each processing unit is arranged to execute sub-tasks of a larger program. The processing units may also be divided in one or more main processing units with several sub-processing units. Some processing units of the processor  1024  may even be located a distance away of the other processing units and communicate via communications network  1028 . Connections between modules can be made wired or wireless. 
     The computer system can be any signal processing system with analogue and/or digital and/or software technology arranged to perform the functions discussed here. 
       FIG. 11  is a flow diagram of a method  1100  of sharing an optical path to eliminate errors due to vibration, according to embodiments of the present invention. The method begins at step  1102 , where an object is illuminated with light from a light source. The light source may be, for example, a broadband light source. 
     In step  1104 , an object beam is formed by an objective lens from light that has been scattered by the illuminated object. The objective lens is configured to direct the object beam through a tube lens, for example a tube lens that is set on the same main optical axis of a holographic device as the objective lens. 
     In step  1106 , a reference beam is formed from the object beam as it exits the objective lens by a reference beam lens group. In one example, the reference beam is formed from the central part of a pupil plane of the objective lens, using 0 th  order light of the object beam. 
     In one example, the reference beam lens group is arranged so that the reference beam is divergent as it passes through the tube lens, resulting in a divergent spherical wave pattern on the image plane. In another example, the reference beam lens group is arranged so that the reference beam is convergent as it passes through the tube lens, resulting in a convergent plane wave pattern on the image plane. In a further example, the reference beam and the object beam are each circularly polarized so that they are orthogonally polarized to each other, one having a right-handed circularly polarized beam and the other a left-handed circularly polarized beam. 
     In step  1108 , the object beam and the reference beam are propagated along a shared optical path to an image plane. As indicated above, this configuration eliminates relative displacement or vibration of the object beam and reference beam. 
     In one example, the object beam and the reference beam are propagated along the shared optical path until both are combined at the image plane. In another example, the object beam and the reference beam share an optical path until just before the image plane, where a set of inclined mirrors divert the reference beam from its path and then recombine the diverted reference beam at the image plane. 
     In step  1110 , the reference beam is spatially filtered to remove from the reference beam any structure of the object beam that may have been intercepted from the central part of the light scattered by the illuminated object, such as rings and side lobes of the light. 
     In step  1112 , a phase of the reference beam is shifted as it traverses the shared optical path. In one example, one or more phase plates are placed along the optical path before the tube lens. In another example, the one or more phase plates are placed after the tube lens. In another example that utilizes the set of inclined mirrors, no phase plates are necessary since the phase shifts are imparted by the use of mirrors. In another example, no phase plates are necessary because there is a pixelated phase mask next to the image plane that has individual phase plates associated with each pixel. 
     In step  1114 , the object beam and the reference beam are combined at the image plane. In one example, the object beam and the reference beam combine where the divergent, spherical wave reference beam overlaps the object beam. In another example, the object beam and the reference beam combine at the center of the image plane where the convergent, plane wave reference beam overlaps the object beam. In another example, the object beam and the reference beam combine across the image plane to create a beating, which represents the difference between the optical frequencies of the object beam and reference beam. 
     Once the object beam and the reference beam are combined, an interference pattern is created which may then be recorded at the image plane using a detector, e.g., a CCD, or any other suitable imaging device. The recorded pattern may then be reconstructed using a processor. 
     Although specific reference may be made in this text to the use of methods and apparatus in the manufacture of ICs, it should be understood that the inspection methods and apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. 
     Although specific reference may have been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured. 
     The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm). 
     The term “lens,” where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. 
     While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. For example, the present invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g., semiconductor memory, magnetic or optical disk) having such a computer program stored therein. 
     The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the present invention as described without departing from the scope of the claims set out below. 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way. 
     The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the present invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.