Abstract:
Both the 1 st  and 0 th  diffraction orders are detected in a scatterometer. The 1 st  diffraction orders are used to detect the overlay error. The 0 th  diffraction order is then used to flag if this is a false overlay error calculation of magnitude greater than the bias but smaller than the pitch of the grating.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques. 
         [0003]    2. Description of the Related Art 
         [0004]    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 steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and 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. 
         [0005]    In order to determine features of the substrate, such as its alignment, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on a camera of the reflected beam. By comparing the properties of the beam before and after it has been reflected off the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. 
         [0006]    Such a system of illuminating a target and collecting data from the reflected radiation is often used to illuminate a plurality of superimposed patterns, for example gratings. The second pattern has a predetermined bias compared to the first pattern. By analysing the characteristics of the reflected radiation it is possible to determine the overlay error OV, between the patterns. 
         [0007]    Analyzing the characteristics of the higher diffraction orders of the reflected radiation can be used to determine small overlay errors, for example of the order of ±20 nm. Similarly, errors of the order of the pitch of the grating can be detected using a method described in U.S. Application 11/455,942. However, overlay errors which are smaller than the pitch (usually 400 nm-1 μm) but larger than the bias (usually 5-20 nm) between the gratings can sometimes go undetected. An example of this is shown in  FIG. 4  in which the top left hand figure shows illumination with TM polarized light (solid line) and TE polarized light (dashed lines). In this example there is a predetermined bias between the gratings of ±15 nm. The asymmetry resulting from a first pair of overlayed gratings, shown by open circles, indicates a (correct) overlay error of 0. However, the asymmetry resulting from a second pair of overlayed gratings (shown by solid dots) also indicates an overlay error of 0 which is incorrect as the correct overlay error is 70 nm. 
         [0008]    Such overlay errors which are greater than the bias but smaller than the pitch of the grating can therefore sometimes go undetected. 
       SUMMARY OF THE INVENTION 
       [0009]    It is desirable to provide a method for identifying false low overlay calculations. 
         [0010]    According to one embodiment of the invention, a method of measuring a property of a substrate comprises projecting radiation onto a substrate using a radiation projector and; detecting the 0th order and ±nth orders of the reflected radiation, wherein n&gt;1 and the reflected radiation is indicative of the property to be measured. 
         [0011]    According to a further embodiment of the invention, a scatterometer configured to measure a property of a substrate comprises a radiation projector configured to project radiation onto a plurality of superimposed patterns on the substrate; a detector configured to detect the 0th and ±nth diffraction orders of the radiation beam reflected from a surface of the substrate, wherein n&gt;1; and a data handling unit configured to calculate overlay error between the plurality of superimposed patterns based on the zero and ±nth diffraction orders. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which: 
           [0013]      FIG. 1   a  depicts a lithographic apparatus; 
           [0014]      FIG. 1   b  depicts a lithographic cell or cluster; 
           [0015]      FIG. 2  depicts a scatterometer; 
           [0016]      FIG. 3  depicts the general operating principle of measuring an angle resolved spectrum in the pupil plane of a high-NA lens; and 
           [0017]      FIG. 4  is a series of graphs showing the intensity of the ±first diffraction orders and the zero diffraction order for various overlay errors. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1   a  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 EUV radiation). A support (e.g. a mask table) MT is configured to support a patterning device (e.g. a mask) MA and is 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 is configured to hold a substrate (e.g. a resist-coated wafer). W and is connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters. A projection system (e.g. a refractive projection lens system) PL is 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. 
         [0019]    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, to direct, shape, and/or control radiation. 
         [0020]    The support supports, e.g. 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 can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The support may be a frame or a table, for example, which may be fixed or movable as required. The support 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.” 
         [0021]    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. 
         [0022]    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. 
         [0023]    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”. 
         [0024]    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). 
         [0025]    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. 
         [0026]    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. 
         [0027]    Referring to  FIG. 1   a , the illuminator IL receives radiation 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 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. 
         [0028]    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. 
         [0029]    The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support (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 PS, 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 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   a ) 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 M 1 , M 2  and substrate alignment marks P 1 , P 2 . 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. 
         [0030]    The depicted apparatus could be used in at least one of the following modes: 
         [0031]    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. 
         [0032]    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 PS. 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. 
         [0033]    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. 
         [0034]    Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
         [0035]    As shown in  FIG. 1   b , the lithographic apparatus LA forms part of a lithographic cell LC, also sometimes referred to a lithocell (lithographic cell) or cluster, which also includes apparatus to perform pre- and post-exposure processes on a substrate. The cluster includes 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/O 1 , I/O 2 , 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. Thus, the different apparatus can be operated to maximize throughput and processing efficiency. 
         [0036]    In order that the substrates that are exposed by the lithographic apparatus are exposed consistently for each layer of resist, there are certain properties of the substrate that need to be measured to determine whether there are changes in alignment, rotation, etc., that must be compensated for by the lithographic apparatus. A separate 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. 
         [0037]    The properties of the surface of a substrate W may be determined using a sensor such as a scatterometer such as that depicted in  FIG. 2 . The scatterometer comprises a broadband (white light) radiation projector  2  which projects radiation onto a substrate W. The reflected radiation is passed to a spectrometer detector  4 , which measures a spectrum  10  (intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconfigured, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom of  FIG. 2 . In general, for the reconstruction the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. 
         [0038]    The scatterometer may be a normal-incidence scatterometer or an oblique-incidence scatterometer. Variants of scatterometry may also be used in which the reflection is measured at a range of angles of a single wavelength (or a limited range of wavelengths), rather than the reflection at a single angle of a range of wavelengths. 
         [0039]    Scatterometers for measuring properties of a substrate may measure, in the pupil plane  11  of a high numerical aperture lens, the properties of an angle-resolved spectrum reflected from the substrate surface W at a plurality of angles and wavelengths as shown in  FIG. 3 . Such a scatterometer may comprise a radiation projector  2  for projecting radiation onto the substrate and a detector  14  for detecting the reflected spectra. The pupil plane is the plane in which the radial position of radiation defines the angle of incidence and the angular position defines azimuth angle of the radiation, and any substantially conjugate plane. The detector  18  is placed in the pupil plane of the high numerical aperture lens. The numerical aperture may be high and preferably at least 0.9 and more preferably at least 0.95. Immersion scatterometers may even have lenses with numerical apertures over 1. 
         [0040]    Some angle-resolved scatterometers only measure the intensity of scattered light. However, more recent scatterometers allow several wavelengths to be measured simultaneously at a range of angles. The properties measured by the scatterometer for different wavelengths and angles may be the intensity of transverse magnetic and transverse electric-polarized light and the phase difference between the transverse magnetic and transverse electric-polarized light. 
         [0041]    Using a broadband light source (i.e. one with a wide range of light frequencies or wavelengths, and therefore of colors) is possible, which gives a large etendue, allowing the mixing of multiple wavelengths. The plurality of wavelengths in the broadband preferably each has a bandwidth of, say, M and a spacing, therefore, of at least 2λδ (i.e. twice the wavelength). Several “sources” of radiation can be different portions of an extended radiation source which have been split using, say, fiber bundles. In this way, angle resolved scatter spectra can be measured at multiple wavelengths in parallel. A 3-D spectrum (wavelength and two different angles) is measured, which contains more information than a 2-D spectrum. This allows more information to be measured which increases metrology process robustness. This is described in more detail in U.S. Patent Application Publications 2006/0066855 A1 and 2006/0033921 A1. 
         [0042]    A scatterometer that may be used with the present invention is shown in  FIG. 3  . . . . The radiation of the radiation projector  2  is focused using lens system  12  through interference filter  13  and polarizer  17 , reflected by partially reflected surface  16  and is focused onto substrate W via a microscope objective lens  15 . The radiation is then transmitted through partially reflective surface  16  into a detector  18 . (e.g. a CCD) in the back projected pupil plane  11  in order to have the scatter spectrum detected. The pupil plane  11  is at the focal length of the lens system  15 . A detector and high aperture lens are placed at the pupil plane. The pupil plane may be re-imaged with auxiliary optics since the pupil plane of a high-NA lens is usually located inside the lens. 
         [0043]    A reference beam is often used for example to measure the intensity of the incident radiation. When the radiation beam is incident on the beam splitter  16  part of it is transmitted through the beam splitter towards a reference mirror  14 . The reference beam is then projected onto a different part of the same detector  18 . 
         [0044]    The pupil plane of the reflected radiation is imaged on the detector  118  with an integration time of, for example, 40 milliseconds per frame. In this way, a two-dimensional angular scatter spectrum of the substrate targets is imaged on the detector. The detector may be, for example, an array of CCD or CMOS sensors. 
         [0045]    A set of interference filters  13  is available to select a wavelength of interest in the range of, say, 405-790 nm or even lower, such as 200-300 nm. The interference filter may be tunable rather than comprising a set of different filters. A grating could be used instead of interference filters. 
         [0046]    The substrate W may be a grating which is printed such that after development, the bars are formed of solid resist lines. The bars may alternatively be etched into the substrate. This pattern is sensitive to chromatic aberrations in the lithographic projection apparatus, particularly the projection system PL, and illumination symmetry and the presence of such aberrations will manifest themselves in a variation in the printed grating. Accordingly, the scatterometry data of the printed gratings is used to reconstruct the gratings. The parameters of the grating, such as line widths and shapes, may be input to the reconstruction process from knowledge of the printing step and/or other scatterometry processes. 
         [0047]      FIG. 4  depicts the asymmetry, the ±1st diffraction order and the 0th diffraction order for various overlay errors. As can be seen from this figure the intensity of the 0 th  order varies symmetrically as a function of the overlay. Therefore if the zero order is detected it is possible to identify a difference between two asymmetries that otherwise have the same value. For example, the two open circles shown in the top left hand graph of  FIG. 4  show the asymmetry with zero overlay error but a bias of ±15 nm. The solid dots on the same graph show the asymmetry when the overlay error is 70 nm but the bias is the same, 15 nm. However, the detected asymmetry in both these situations (i.e. the overlay error being 0 nm and 70 nm) would be the same. If the intensity of the 0 th  order was detected a different intensity between the overlay error of 0 nm and 70 nm would be detected. As shown in the bottom right-hand graph of  FIG. 4  the detected intensity (shown by two solid dots) for an overlay error of 70 nm would be greater than the detected intensity (shown by open circles) with an overlay error of 0. It is thus possible to differentiate between two different overlay which result in the same asymmetry. It is therefore possible to identify such false low overlay error calculations. An alternative to using the absolute value of the intensity of the 0 th  order to identify large overlay errors is to use the difference between the intensities of the 0 th  order for the gratings with a positive bias and the 0 th  order for the gratings with a negative bias. If the overlay error is small the difference in intensities will be small due to the symmetric nature of the 0 th  order (as shown in  FIG. 4 ). 
         [0048]    The apparatus measures the intensity of the +1, −1 and 0 diffraction order for 2 sets of overlapping gratings, a first having a bias of +d, the second having a bias of −d. The asymmetry is calculated as: 
         [0000]    
       
         
           
             
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         [0000]    where I 1  is the intensity of the +1 order and L 1  is the intensity of the −1 order and OV is the overlay error. 
         [0049]    The overlay is then determined by: 
         [0000]    
       
         
           
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       Further details about this can be found in EP1628164A. 
       [0050]    The difference in intensity between the two sets of gratings is given as: 
         [0000]      Δ I   0   =I   0+   −I   0 . 
       Where I 0+  is the intensity of the 0 th  order of the overlapping gratings having a bias of +d and I 0−  is the intensity of the 0 th  order of the overlapping gratings having a bias of −d. 
       [0051]    If ΔI 0  is below a certain threshold, I thr , then the overlay measurement is valid. However, if ΔI 0  is above I thr  then it indicates a large overlay error and the calculated overlay error is unreliable. The value of I thr  can be determined using modelling or calibration using a calibration substrate. 
         [0052]    Although this invention has been described as relating to the first diffraction orders together with the 0 th  order, higher orders (for example the second, third or fourth diffraction orders) could instead be used together with the 0 th  order.