Patent Publication Number: US-11657492-B2

Title: Reticle backside inspection method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. non-provisional application Ser. No. 16/115,699 (now U.S. Pat. No. 10,997,706) filed Aug. 29, 2018, which claims priority to U.S. provisional application No. 62/566,126, filed on Sep. 29, 2017, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to a method and apparatus for lithography, particularly to inspecting a reticle used in the manufacture of semiconductor integrated circuits, and more particularly to a method of inspecting the backside of the reticle. 
     BACKGROUND 
     As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, the wavelength of radiation used for photolithography has decreased from ultraviolet (UV) to deep ultraviolet (DUV) and, more recently to extreme ultraviolet (EUV). Further decreases in component size require further improvements in resolution of photolithography which are achievable using extreme ultraviolet lithography (EUVL). Because transmittance of most materials is substantially reduced at EUV wavelengths, EUVL uses reflective reticles rather than the traditional masks which use a patterned transparent plate for creating patterns in the photoresist layer. 
     Because patterns created by EUV lithography have a feature size in the range of several nanometers, maintaining the flatness of the reflective surface of the EUV reticle is important to prevent distortions in the pattern. These requirements have resulted in the development of apparatuses and methods for reticle inspection and detecting contaminants on the front and back of EUV reticles, which continue to evolve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1 A  is a schematic view of an EUV lithography system. 
         FIG.  1 B  is a simplified schematic diagram of a detail of an extreme ultraviolet lithography tool. 
         FIG.  2    schematically illustrates a mask holding mechanism relating to an embodiment of the present disclosure. 
         FIG.  3 A  illustrates distorted topography of a reticle resulting from contaminants and other factors. 
         FIG.  3 B  illustrates the effect of distorted topography of a reticle resulting from contaminants and other factors. 
         FIG.  4 A  schematically illustrates an apparatus for inspecting a backside of a reticle in accordance with an embodiment of the present disclosure. 
         FIG.  4 B  schematically illustrates an illuminator for the apparatus for inspecting a backside of a reticle in accordance with an embodiment of the present disclosure. 
         FIG.  4 C  schematically illustrates an imaging device for the apparatus for inspecting a backside of a reticle in accordance with an embodiment of the present disclosure. 
         FIG.  5    is a flowchart illustrating a method of inspecting a reticle in accordance with embodiments of the present disclosure. 
         FIG.  6    schematically illustrates the process of obtaining a surface profile map of the reticle surface. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific embodiments or examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, dimensions of elements are not limited to the disclosed range or values, but may depend upon process conditions and/or desired properties of the device. Moreover, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the first and second features, such that the first and second features may not be in direct contact. Various features may be arbitrarily drawn in different scales for simplicity and clarity. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. In addition, the term “being made of” may mean either “comprising” or “consisting of.” In the present disclosure, a phrase “one of A, B and C” means “A, B and/or C” (A, B, C, A and B, A and C, B and C, or A, B and C), and does not mean one element from A, one element from B and one element from C, unless otherwise described. 
     The present disclosure generally relates to inspection of reticle (mask) used in lithography for manufacturing semiconductor devices. More particularly, embodiments of the present disclosure are related to methods and apparatuses for inspecting a backside of a reticle (mask) used for EUV lithography. Backside of the reticle refers to the side of the reticle opposite the side having the mask pattern. Because the feature size of patterns made by EUV lithography is in the range of several nanometers, even minute particulate contamination on the backside of the reticle can sufficiently distort the reflective face of the reticle to cause a distortion in the pattern of EUV reflected on the photoresist layer. Thus, maintaining the cleanliness and flatness of the backside of the reticle is as important as maintaining the cleanliness of the front (reflective) side of the reticle. In some embodiments of the present disclosure, the reticle backside is inspected via 3-dimensional (3D) images of the reticle backside without taking the reticle offline. Such in-line inspection can avoid delays caused by taking the reticle offline, which typically requires shutting the production and removing the reticle from the vacuum chamber. Further, such in-line inspection also avoids potentially exposing the reticle to other contamination while being inspected offline. 
       FIG.  1    is a schematic view of an EUV lithography system with a laser production plasma (LPP) based EUV radiation source, in accordance with some embodiments of the present disclosure. The EUV lithography system includes an EUV radiation source  100  to generate EUV radiation, an exposure tool  200 , such as a scanner, and an excitation laser source  300 . As shown in  FIG.  1   , in some embodiments, the EUV radiation source  100  and the exposure tool  200  are installed on a main floor MF of a clean room, while the excitation laser source  300  is installed in a base floor BF located under the main floor. Each of the EUV radiation source  100  and the exposure tool  200  are placed over pedestal plates PP 1  and PP 2  via dampers DP 1  and DP 2 , respectively. The EUV radiation source  100  and the exposure tool  200  are coupled to each other by a coupling mechanism, which may include a focusing unit. 
     The lithography system is an EUV lithography system designed to expose a resist layer by EUV light (also interchangeably referred to herein as EUV radiation). The resist layer is a material sensitive to the EUV light. The EUV lithography system employs the EUV radiation source  100  to generate EUV light, such as EUV light having a wavelength in a range from about 1 nm to about 100 nm. In one particular example, the EUV radiation source  100  generates an EUV light with a wavelength centered at about 13.5 nm. 
     The exposure tool  200  includes various reflective optical components, such as convex/concave/flat mirrors, a mask holding mechanism including a mask stage, and wafer holding mechanism. The exposure tool  200  further includes an exposure chamber  205  that encloses all of the optical components, mask holding mechanism and wafer holding mechanism of the exposure tool  200 . The exposure chamber  205  provides a low-pressure of vacuum environment for the exposure tool  200  to avoid loss of intensity of the EUV radiation because of absorption from gases. 
     The lithography system may further include other modules or be integrated with (or be coupled with) other modules. 
     As shown in  FIG.  1   , the EUV radiation source  100  includes a target droplet generator  115  and a LPP collector  110 , enclosed by a chamber  105 . The target droplet generator  115  generates a plurality of target droplets DP, which are supplied into the chamber  105  through a nozzle  117 . In some embodiments, the target droplets DP are tin (Sn), lithium (Li), or an alloy of Sn and Li. 
     The excitation laser LR 2  generated by the excitation laser source  300  is a pulse laser. The laser pulses LR 2  are generated by the excitation laser source  300 . The excitation laser source  300  may include a laser generator  310 , laser guide optics  320  and a focusing apparatus  330 . In some embodiments, the laser source  310  includes a carbon dioxide (CO 2 ) or a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source with a wavelength in the infrared region of the electromagnetic spectrum. For example, the laser source  310  has a wavelength of 9.4 μm or 10.6 μm, in an embodiment. The laser light LR 1  generated by the laser generator  300  is guided by the laser guide optics  320  and focused into the excitation laser LR 2  by the focusing apparatus  330 , and then introduced into the EUV radiation source  100 . 
     In some embodiments, the excitation laser LR 2  includes a pre-heat laser and a main laser. In such embodiments, the pre-heat laser pulse (interchangeably referred to herein as the “pre-pulse) is used to heat (or pre-heat) a given target droplet to create a low-density target plume with multiple smaller droplets, which is subsequently heated (or reheated) by a pulse from the main laser, generating increased emission of EUV light. 
       FIG.  1 B  is a simplified schematic diagram of a detail of an extreme ultraviolet lithography tool according to an embodiment of the disclosure showing the exposure of photoresist coated substrate  410  with a patterned beam of EUV light. The exposure device  200  is an integrated circuit lithography tool such as a stepper, scanner, step and scan system, direct write system, device using a contact and/or proximity mask, etc., provided with one or more optics  250   a ,  250   b , for example, to illuminate a patterning optic  250 , such as a reticle, with a beam of EUV light, to produce a patterned beam, and one or more reduction projection optics  250   c ,  250   d , for projecting the patterned beam onto the substrate  410 . A mechanical assembly (not shown) may be provided for generating a controlled relative movement between the substrate  410  and patterning optic  250 . As further shown in  FIG.  1 B , the EUVL tool includes an EUV light source  100  including an EUV light radiator ZE emitting EUV light in a chamber  105  that is reflected by a collector  110  along a path into the exposure device  200  to irradiate the substrate  410 . 
     As used herein, the term “optic” is meant to be broadly construed to include, and not necessarily be limited to, one or more components which reflect and/or transmit and/or operate on incident light, and includes, but is not limited to, one or more lenses, windows, filters, wedges, prisms, grisms, gradings, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, the term “optic”, as used herein, is not meant to be limited to components which operate solely within one or more specific wavelength range(s), such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or any other specific wavelength. 
       FIG.  2 A  schematically illustrates a mask holding mechanism in accordance with an embodiment of the present disclosure. The following description refers to  FIG.  1 B  and  FIG.  2   . The EUV radiation EUV generated by the EUV radiation source  100  is guided by the reflective optical components onto a mask  250  secured on the mask stage  210 . In some embodiments, the mask stage  210  includes an electrostatic reticle holder  220  (interchangeably referred to herein as an electrostatic chuck or e-chuck) to secure the mask  250 . 
     In the present disclosure, the terms mask, photomask, and reticle are used interchangeably. In the present embodiments, the reticle  250  is a reflective mask. In an embodiment, the reticle  250  includes a substrate  252  formed of a suitable material, such as a low thermal expansion material or fused quartz. In various examples, the material includes TiO 2  doped SiO 2 , or other suitable materials with low thermal expansion. The mask  250  includes multiple reflective multiple layers (ML)  253  deposited on the substrate  252 . The ML  253  includes a plurality of film pairs, such as molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the ML  253  may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configurable to highly reflect the EUV light. The mask  250  may further include a capping layer  254 , such as ruthenium (Ru), disposed on the ML for protection. The mask  250  further includes an absorption layer  255 , such as a tantalum boron nitride (TaBN) layer, deposited over the ML. The absorption layer  255  is patterned to define a layer of an integrated circuit (IC). Alternatively, another reflective layer  256  may be deposited over the ML and is patterned to define a layer of an integrated circuit, thereby forming an EUV phase shift mask. 
     The reticle  250  and electrostatic chuck  220  of  FIG.  2    are positioned such that EUV radiation supplied from the EUV radiation source  100  is in focus when it arrives at the surface of the semiconductor workpiece. 
     In the present embodiments, the reticle  250  is secured by the electrostatic chuck  220  by an electrostatic potential. The reticle  250  abuts burls  225 , which reside on a surface of the electrostatic chuck  220 , and have a width (w) in a range from about 100 μm to about 500 are spaced apart in a range from about 1.0 mm to about 5.0 mm, and a height from the bottom surface of the reticle  220  in a range from about 1.0 μm to about 100 μm. Some industry-standard electrostatic chucks utilize greater than 2,000 of such burls comprising roughly 1.4% of the surface area of the electrostatic chuck  220 , to support the reticle  250 . 
     Electrostatic chucking can secure and release the reticles in times of less than 1 second with an equal or greater force than conventional vacuum chucks. In addition to overlay error due to reticle  250  misalignment, as shown in  FIG.  3 A , distorted topography between the reticle  250  and the electrostatic chuck  220  can result in overlay error. Distorted topography can result from burl wear or contaminants  290  between a surface of the electrostatic chuck  220  and the back surface  251  of reticle  250 . For instance, burl wear is observed on one of the burls, where corners have rounded due to repeated use, which can result in non-planar clamping. Additionally, if another burl has a flattened contaminant  290  (e.g., gold, etc.) on its top surface, reticle topography is further distorted. And while small contaminants  290  (e.g., with a diameter less than h) resting on portions between burls may not impact reticle topography, larger contaminants  290  (e.g., with a diameter greater than h) will impact reticle topography. 
     In various cases, the contaminant  290  is a metal particle, a resist particle or a particle of any material generally used in semiconductor manufacturing process such as, for example, silicon dioxide, silicon nitride, etc. 
       FIG.  3 B  further illustrates the effect of distorted topography of the reticle  250  resulting from the contaminants and other factors, which causes an out of plane distortion (OPD) of the reticle surface, resulting in a lateral displacement or in plane distortion (IPD) of the radiation reflected by the reticle. For example, if a particulate contaminant  290  having a diameter larger than the burl height h is adhered to the back surface  251  of the reticle  250  between neighboring burls, the contaminant  290  pushes against the back surface  251  of the reticle  250  when the reticle  250  is secured to the reticle holder  220 . Depending on material properties (e.g., elasticity, malleability, etc.) of the reticle  250  and the material properties of the contaminant  290 , the distortion in the back surface of the reticle  250  may be transmitted to the front (i.e., reflecting) surface  255  of the reticle  250 . Such distortion in local topology of the reflecting surface of the reticle  250  is illustrated in  FIG.  3 B  and is represented by the OPD. While such OPD may not significantly affect the focus of the EUV radiation at the wafer surface  410 , the OPD shifts the optical path of the EUV radiation incident on the curved portion—resulting in IPD. For example, for a pattern with an absorption region laterally adjacent to a reflecting region on the front surface  255  of reticle  250 , IPD may cause EUV radiation that is supposed to be reflected to get absorbed, and EUV radiation that is supposed to get absorbed to be reflected. In other words, IPD results in loss of pattern fidelity. In other instances, IPD may additionally result in loss of overlay control for the reticle  250  and degradation in pattern uniformity such as critical dimension (CD) variation, shape biasing, etc. 
     It is, therefore, desirable to ensure that the back surface  251  of the reticle  250  is contaminant free in some embodiments. Periodic inspection of the back surface  251  of the reticle  250  is generally performed to scan the back surface  251  and obtain a 2D map of the reticle back surface  251  to monitor the amount and size of contaminants on the back surface  251 . 
       FIG.  4 A  illustrates an apparatus for inspecting a backside of a reticle in accordance with an embodiment of the present disclosure. In an embodiment, the apparatus includes a vacuum housing  510  having transparent windows  530 . The vacuum housing  510  includes a stage  505  on which the reticle  250  rests while being inspected. Illuminators  520   a  and  520   b  illuminate the backside  251  of the reticle  250  through windows  530 . In other words,  FIG.  4 A  shows the reticle  250  is placed facing down such that the pattern surface  255  faces the stage  505 . An imaging device  540  detects the light scattered and/or reflected from the backside  251  of the reticle  250 . A scanner  550  moves the stage  505  in a direction perpendicular to the surface  251  of the reticle  250 . A focus controller  547  moves the focal plane of the imaging device  540  in the direction perpendicular to the surface  251  of the reticle  250 . An image acquisition controller  590  controls the illuminators  520   a  and  520   b , the imaging device  540  and the scanner  550  to obtain images of the backside surface  251  of the reticle  250  at a plurality of different heights. In an embodiment, the image acquisition controller  590  additionally includes an image processor programmed to combine the acquired images to obtain a 3-dimensional surface profile map of the backside surface  251  of the reticle  250 . In some embodiments, the image processor is additionally programmed to analyze the surface profile map to determine whether the reticle  250  is acceptable for lithography process. 
     The vacuum housing  510  is sized to hold the stage  505  on which the reticle  250  rests, and is made of a suitable material such as, for example, metals like stainless steel or aluminum in some embodiments. For convenience of reference, the plane defined by the backside  251  of the reticle  250  defines the X and Y directions as illustrated in  FIG.  4 A . The direction perpendicular to the surface  251  of the reticle  250  is the Z direction. In  FIG.  4 A , the positive Z direction is upwards extending from the reticle  250  to the imaging device  540 . 
     In an embodiment, the stage  505  can be moved in X, Y and/or Z directions using the scanner  550 . In some embodiments, the scanner  550  includes one or more actuators that can move the stage  505  in a prescribed direction by a prescribed distance. In some embodiments, the actuators include stepper motors, piezoelectric actuators or a combination of stepper motors and piezoelectric actuators. In some embodiments, the scanner  550  additionally includes a controller to control the one or more stepper motors and/or piezo actuators such that a desired movement of the stage  505  can be achieved with precision at a prescribed time, or repeat the movement of the stage  505  over a prescribed period of time. In various embodiments, the scanner  550  moves the stage  505  in one or more of X, Y and Z directions. For example, in an embodiment, the scanner  550  moves the stage  505  only in Z direction (i.e., up and down). 
       FIG.  4 B  illustrates an example of an illuminator for the apparatus for inspecting a backside of a reticle in accordance with an embodiment of the present disclosure. The illuminators  520   a  and  520   b  are lasers  535  in some embodiments. In an embodiment, the illuminators  520   a  and  520   b  have a wavelength in the visible region of electromagnetic spectrum. In other embodiments, the illuminators  520   a  and  520   b  have wavelengths in the infrared region or ultraviolet region. 
     In some embodiments, the illuminators  520   a  and  520   b  each are provided with a suitable mechanism to move the light radiated by the respective illuminator to scan the backside surface  251  of the reticle  250 . For example, in an embodiment, the illuminators  520   a  and  520   b  are coupled to actuators that change the angle of the light beam radiated by the respective illuminator with respect to the backside surface  251  of the reticle  250 . One example of such an actuator is a steerable mirror  532  placed at a suitable position. Another example of such an actuator is a stepper motor (not shown) attached to one end of the respective illuminator. In an embodiment, the light from the illuminators  520   a  and  520   b  scan the backside surface  251  of the reticle  250  in different directions, e.g., perpendicular to each other. For example, in an embodiment, the light from illuminator  520   a  scans the backside surface  251  of the reticle  250  in the X direction and the light from the illuminator  520   b  scans the backside surface of the reticle  250  in Y direction. In various embodiments, the light beams from respective illuminators are rastered in respective directions to scan the backside surface of the reticle  250 . Other paths of scanning are also contemplated within the scope of the present disclosure. In embodiments where the illuminators  520   a  and  520   b  scan the backside  251  of the reticle  250  simultaneously, the illuminators  520   a  and  520   b  have different parameters for the light beam to avoid interference from the two sources. For example, in an embodiment, the illuminators  520   a  and  520   b  emit light at different wavelengths. 
     In an embodiment, the illuminators  520   a  and  520   b  produce a planar “curtain” of light extending in a particular direction, e.g., the light curtain from the illuminator  520   a  extends in Y direction and the light curtain from the illuminator  520   b  extends in X direction. The “curtain” of light is achieved, in some embodiments, by using line sources for illuminators  520   a  and  520   b . In other embodiments, each of the illuminators  520   a  and  520   b  are provided with a slit through which the light emitted by the illuminators  520   a  and  520   b  passes to form the “curtain” of light. In such an embodiment, the illuminators  520   a  and  520   b  are moved such that the light curtain from the illuminator  520   a  traverses in X direction and the light curtain from the illuminator  520   b  traverses in Y direction, to scan substantially the entire backside surface  251  of the reticle  250 . 
     The windows  530  are positioned to allow light from the illuminators  520   a  and  520   b  to pass through to be incident on the backside  251  of the reticle  250  and also to allow light scattered/reflected from the backside  251  of the reticle  250  to be received at the imaging device  540 . The windows  530  are made of a material that is transparent to the wavelength of the illuminators  520   a  and  520   b , and sufficiently mechanically strong to withstand the vacuum in the vacuum housing  510 . The windows  530  are made of fused quartz in some embodiments. In other embodiments, the windows  530  are made of glass or sapphire. 
       FIG.  4 C  schematically illustrates an example of an imaging device for the apparatus for inspecting a backside  25  of a reticle in accordance with an embodiment of the present disclosure. The imaging device  540  is positioned to detect light scattered and/or reflected from the backside  251  of the reticle  250  as it is illuminated by the light from illuminators  520   a  and  520   b . In an embodiment, as shown in  FIG.  4 A , the imaging device  540  is placed outside the vacuum housing  510  and the light scattered and/or reflected from the backside  251  of the reticle  250  reaches the imaging device  540  through one of the windows  530 . However, the placement of the imaging device  540  is not restricted in this manner for the purposes of the present disclosure. For example, in an embodiment, the imaging device  540  is placed inside the vacuum housing  510  at a suitable position (not shown in the figures) to detect the light R scattered and/or reflected from the backside  251  of the reticle  250 . In an embodiment, the imaging device  540  includes a camera  545  such as, e.g., a charge-coupled device (CCD). In an embodiment, the imaging device  540  further includes one or more lenses and/or prisms  542  configured to focus the light scattered R and/or reflected from the backside of the reticle  250  at the imaging plane of the imaging device  540 . In such an embodiment, the lenses and/or prisms  542  as well as the imaging device  540  are arranged in a suitable housing (not shown). 
     In an embodiment, using the imaging acquisition controller  590 , the light from the illuminators  520   a  and  520   b  is scanned across the backside surface  251  of the reticle  250  while keeping the stage  505  at a given height. The light scattered and/or reflected from the backside surface  251  of the reticle  250  is detected by the imaging device  540  to render a two dimensional (2-D) map of the backside surface  251  of the reticle  250 . The image acquisition controller  590  then causes this process to be repeated by moving the stage  505  in Z direction to positions at different heights, e.g., via the scanner  550 , producing a plurality of 2-D maps of the backside surface  251  of the reticle  250 . 
     In another embodiment, using the image acquisition controller  590 , the stage  505  is kept at a fixed position, and the light from the illuminators  520   a  and  520   b  is repeatedly scanned across the backside surface  251  of the reticle  250  while changing the plane of focus of the imaging device  540  using, for example, the focus controller  547 . The focus controller  547 , in some embodiments, includes a stepper motor to move the lens assembly  542  (or individual components within the lens assembly) in the Z direction. In other embodiments, lenses and/or prisms are adjustable (i.e., can have variable foci) and the focus controller  547  modulates the focus (or foci) of the lenses and/or prisms. By this process, a plurality of 2-D maps of the backside surface  251  of the reticle  250  is produced at various planes including and above the backside surface  251  of the reticle  250 . 
     The plurality of 2-D maps is then processed using the image acquisition controller  590  to compile a three dimensional (3-D) map of the backside surface  251  of the reticle  250 . Such a 3-D map provides detailed information about the position, size and shape of particulate contaminants resting on the backside surface  251  of the reticle  250 . This information is used to determine whether the reticle  250  can be used as is in further lithography processes, needs a repair or needs to be discarded. 
     In some embodiments, the image acquisition controller  590  provides the surface profile of the backside surface  251  of the reticle  250  to a surface profile analyzer  595  to analyze the surface profile of the backside surface  251  of the reticle  250  and determine whether the reticle is to be accepted as is, rejected or repaired before further use. The determination of whether the reticle is to be accepted, rejected or repaired is made based on values of one or more parameters of the backside surface  251  of the reticle  250 . In various embodiments, the parameters include, without limitation, average roughness of the backside surface  251  of the reticle  250 , number of particles on the backside surface  251 , average size of the particles on the backside surface  251 , size of the largest particle on the backside surface  251 , area density of particles on the backside surface  251 , etc. 
     In various embodiments, the image acquisition controller  590  and the surface profile analyzer  595  include one or more processors configured to variously perform processes for acquiring images of the backside surface  251 , constructing a 3D map of the backside surface  251  based on the acquired images, and determine, based on the 3D map of the backside surface  251 , whether the reticle  250  is to be accepted, rejected or repaired. In some embodiments, the image acquisition controller  590  and the surface profile analyzer  595  are implemented using a single processor. In other embodiments, each of the image acquisition controller  590  and the surface profile analyzer  595  are implemented using more than one processor. Other configurations are also contemplated. 
       FIG.  5    is a flowchart illustrating a method of inspecting a reticle in accordance with embodiments of the present disclosure.  FIG.  6    schematically illustrates the process of obtaining a surface profile map of the reticle surface. The method of inspecting the reticle will be described in reference to  FIGS.  5  and  6   . 
     In an embodiment, the method includes, at S 601 , obtaining a first image, e.g., P 1 , of a reticle surface at a first height by scanning the surface of the reticle with a light source at the first height, e.g., H 1 , of the reticle surface relative to a reference surface height of the reticle surface. The reference surface height, in an embodiment, corresponds to the backside surface of the reticle. 
     In various embodiments, the light source includes one or more illuminators such as, for example, lasers, to illuminate the surface of the reticle. In an embodiment, the one or more illuminators have a wavelength in the visible region of electromagnetic spectrum. In other embodiments, the one or more illuminators have wavelengths in the infrared region or ultraviolet region. 
     In some embodiments, the one or more illuminators are each provided with a suitable mechanism to move the light radiated by the respective illuminator to scan the backside surface of the reticle. For example, in an embodiment, the one or more illuminators are coupled to actuators that change the angle of the light beam radiated by the respective illuminator with respect to the backside surface of the reticle. One example of such an actuator is a mirror placed at a suitable position. Another example of such an actuator is a stepper motor attached to one end of the respective illuminator. 
     In some embodiments, the one or more illuminators provide a beam of light that creates a point-spot on the surface of the reticle. In such embodiments, the illuminators are moved (or the light from the illuminators is directed) such that the point-spot is rastered across the surface of the reticle. In other embodiments, the one or more illuminators provide a light beam that creates a line across the surface of the reticle. In one such embodiment, two illuminators create orthogonal lines across the surface of the reticle, and the two illuminators are moved (or the light from the two illuminators is directed) such that the line traverses the surface of the reticle in a direction perpendicular to the line. For example, if one of the illuminators creates a line along the X direction, the line is moved along the Y direction to cover substantially the entire surface of the reticle. 
     Any particulate contaminant, e.g.,  690 , resting on the surface of the reticle reflects and/or scatters the light from the illuminators, and is registered in the image, e.g., P 1  detected by an imaging device. The imaging device includes a camera and a set of lenses and/or prisms in an embodiment. The camera, in some embodiments, is a charge-coupled device (CCD) camera. In some embodiments, the set of lenses and/or prisms is configured to create a plane of focus, at which the camera is focused, at the surface of the reticle in an embodiment. In some embodiments, the set of lenses and/or prisms can be moved to change the plane of focus of the imaging device in a direction perpendicular to the surface of the reticle. In some embodiments, the plane of focus is fixed, and a movable stage on which the reticle rests is provided to move the reticle in a direction perpendicular to the surface of the reticle. 
     At S 602 , a second image, e.g., P 2 , of the reticle surface at a second height, e.g., H 2 , is obtained by scanning the surface of the reticle at a second height of the reticle surface relative to the reference surface height of the reticle surface. The second height H 2  is different from the first height H 1 . In an embodiment, the second height H 2  is achieved by moving the reticle in a direction perpendicular to the surface of the reticle, e.g., in the Z direction. In another embodiment, the second height H 2  is achieved by moving the plane of focus of the camera. 
     At S 603 , a plurality of images, e.g., P 1 , P 2 , P 3 , P 4 , etc., is obtained by repeatedly scanning of the surface of the reticle a plurality of times at a plurality of different heights, e.g., H 1 , H 2 , H 3 , H 4 , etc. As discussed elsewhere herein, the different heights can either be achieved by changing the plane of focus of the camera or by moving the reticle in a direction perpendicular to the surface of the reticle. In various embodiments, the distance between the first height and the last height in the Z direction, i.e., the scanning distance, is determined by the potential contaminants. For example, in some embodiments, potential contaminants that may be present on the reticle backside have an average size in a range from about 1 nm to about 100 nm. In such embodiments, the scanning distance is at least about 100 nm from the backside surface of the reticle. In other embodiments, the potential contaminants have an average size in a range from about 10 nm to about 500 nm. In such embodiments, the scanning distance is at least 500 nm from the backside surface of the reticle. 
     In an embodiment, the process of obtaining the images at different heights is automated by programming a reticle stage controller to move the reticle in predetermined steps beginning at the backside surface of the reticle and stopped when no reflection and/or scattering is detected at the camera. In another embodiment, the process is automated by programming an imaging device to change a plane of focus of the camera in predetermined steps beginning at the backside surface of the reticle and stop when no reflection and/or scattering is detected at the camera. In various embodiments, the predetermined steps change the height at which the image of the reticle is obtained in increments of, for example, about 10 nm, about 50 nm, about 100 nm or about 500 nm (or any other increments between these values), at a time. In other words, planes at which images of the reticle surface are obtained are separated by 10 nm, 50 nm, 100 nm, 500 nm, etc., depending on the increment by which the height is changed. 
     At S 604 , the plurality of images is combined to obtain a surface profile map, e.g.,  650 , of the reticle. Such a surface profile map provides information relating to the size, shape and position (i.e., location) of the contaminant particle  690  resting on the backside surface of the reticle. 
     In an embodiment, the surface profile map provides a volume rendering for the reticle surface by combining the 2D images obtained by the imaging device using volume rendering techniques. In an embodiment, each of the 2D images (at different heights of the reticle surface) is preprocessed to determine portions of the image that are of interest (e.g., portions where a contaminant particle is detected. For each image, the x and y coordinates of each pixel in the region of interest are transferred to an empty space, the z coordinate being the height at which the image is obtained. The pixels between slices are the interpolated to obtain a volume contour for the region of interest. In some embodiments, the interpolation is simple linear interpolation based on the numerical values of the coordinates alone. In other embodiments, the characteristics of the individual pixels are used to obtain the interpolation function. For example, in an embodiment, the brightness of the individual pixel and the difference between the brightness of neighboring pixels within each 2D image is used to obtain the interpolation function to join corresponding pixels from different 2D images obtained at different heights. 
     It will be understood that the techniques described herein are merely some of the techniques that can be used for obtaining surface profile maps using a stack of 2D images. Other techniques for obtaining surface profile maps using stacks of 2D images can be used depending on availability of computational resources and time. 
     In some embodiments, the information about the contaminant particles provided by the surface profile map  650  is analyzed to determine whether to accept the reticle, repair the reticle, or reject the reticle. For example, in some embodiments, an average roughness of the backside surface of the reticle is determined based on the surface profile map  650 . In some embodiments, one or more of the number, size, and area density of particles on the backside surface of the reticle is determined based on the surface profile map  650 . 
     Upon analysis of the surface profile map  650 , it is determined whether the reticle is acceptable as is, or whether it needs to be repaired or rejected based on, for example, whether a particular parameter such as surface roughness, particle size, particle density or number of particles is above or below a threshold. For example, in an embodiment, the reticle is rejected if it is determined that the average particle size is more than, for example, 100 nm regardless of number or density of particles. In another embodiment, the reticle is rejected if the surface roughness is more than 50 nm/μm 2  and if a particle size is greater than, for example, 75 nm. In yet another embodiment, for example, the reticle is rejected if the number of particles is greater than 100 particles/mm 2  and an average particle size is greater than 50 nm. In some embodiments, it is determined that the reticle needs to be repaired before further use. In such embodiments, remedial measures such as, for example, ultrasonic cleaning, gas jet cleaning, liquid jet cleaning and/or any combination thereof, of the backside surface of the reticle, are taken before further use of the reticle. In various embodiments, the threshold values of parameters to determine whether the reticle needs to be repaired are in a range between those used for determining whether the reticle is acceptable and those used for determining that the reticle is to be rejected. For example, in an embodiment, a reticle with a surface roughness of 10 nm/μm 2  or less is acceptable, and a reticle with a surface roughness of 50 nm/μm 2  or more is rejected. In such embodiments, a reticle with a surface roughness between 10 and 50 nm/μm 2  would be repaired before further use. 
     In some embodiments, the determination whether to accept the reticle, repair the reticle, or reject the reticle is automatically made by a controller programmed to make the determination. In certain embodiments, the reticle is used in an extreme ultraviolet lithography (EUVL) system. 
     Thus, embodiments of the present disclosure provide a more rigorous and exacting inspection of reticles, particularly reticles with very tight tolerances, such as those used in EUVL systems. 
     The present disclosure is not limited to the inspection of reticles used in EUVL systems. Rather the present disclosure is applicable to inspecting reticles used in a variety of exposure systems including mercury lamps, and KrF and ArF lasers. 
     It will be understood that not all advantages have been necessarily discussed herein, no particular advantage is required for all embodiments or examples, and other embodiments or examples may offer different advantages. 
     According to one aspect of the present disclosure, a method of inspecting a reticle includes obtaining a first image of a surface of the reticle at a first height by scanning the reticle surface with a light source at the first height of the reticle surface relative to a reference surface height of the reticle surface and obtaining a second image of the reticle surface at a second height by scanning the reticle surface with the light source at the second height of the reticle surface relative to the reference surface height of the reticle surface are obtained. The second height is different from the first height. The distance between the first height and the second height is smaller than a thickness of the reticle. The first and second images are combined to obtain a surface profile image of the reticle. In some embodiments, the method further includes analyzing the surface profile image to determine whether to accept the reticle, repair the reticle, or reject the reticle. In some embodiments, the determination whether to accept the reticle, repair the reticle, or reject the reticle is automatically made by a controller programmed to make the determination. In some embodiments, following a determination to accept the reticle, the reticle is used for patterning a wafer in a lithography process. In some embodiments, following a determination to repair the reticle, the surface of the reticle is cleaned using a process selected from the group consisting of ultrasound cleaning, gas jet cleaning, liquid jet cleaning or a combination thereof. In some embodiments, the method further includes, prior to obtaining the first image of the surface of the reticle, patterning a front surface of the reticle, the front surface being opposite the surface of which the first image is obtained. In some embodiments, the reticle is a reflective reticle. In some embodiments, the scanning of the reticle surface is repeated by moving the reticle in a direction perpendicular to the reticle surface by prescribed increments and scanning the reticle surface at each increment. In some embodiments, a plurality of images, including the first image and the second image, is obtained by detecting light reflected and/or scattered by the reticle surface at the plurality of different height, including the first height and the second height, by an imaging device. In some embodiments, the scanning of the reticle surface is repeated by moving a plane of focus of an imaging device used to detect light reflected and/or scattered by the reticle surface by prescribed increments and capturing the light reflected and/or scattered by the reticle surface at each increment. 
     According to another aspect of the present disclosure, a reticle inspection system includes a reticle stage on which a reticle to be scanned is located, a scanning light source to direct radiation at the reticle, and a light detector to detect radiation reflected and/or scattered from a surface of the reticle. The system includes a controller programmed to control the reticle inspection system to repeat scanning of the surface of the reticle a plurality of times at a plurality of different heights to obtain a plurality of images. A processor is included in the system, and is programmed to combine the plurality of images to obtain a surface profile image of the reticle, and analyze the surface profile image to determine whether to accept the reticle, repair the reticle, or reject the reticle. In some embodiments, the light detector includes a camera and a set of lenses and/or prisms. In some embodiments, obtaining a plurality of images includes detecting light reflected and/or scattered by the reticle surface at each of the plurality of different heights. In some embodiments, the scanning light source includes a laser. In some embodiments, the determination of whether to accept the reticle, repair the reticle, or reject the reticle is based on whether a contaminant particle is detected upon analysis of the surface profile image, and if detected, whether a size of the contaminant particle exceeds a threshold. 
     According to yet another aspect of the present disclosure, an apparatus for obtaining a surface profile map of a reticle surface includes a scanner including a light source and a beam steering mechanism configured to scan light provided by the light source across the reticle surface at a prescribed height of the reticle surface and an imaging device configured to detect light reflected and/or scattered by the reticle surface. The apparatus includes a controller programmed to cause the imaging device to obtain a plurality of images of the reticle surface at a plurality of heights, and a processor programmed to obtain the surface profile map of the reticle surface by combining the plurality of images of the reticle surface at the plurality of heights. In some embodiments, the beam steering mechanism includes a mirror. In some embodiments, causing the imaging device to obtain a plurality of images includes moving the reticle in a direction perpendicular to the reticle surface by prescribed increments and scanning the reticle surface at each increment. In some embodiments, the causing the imaging device to obtain a plurality of images includes moving a plane of focus of the light detector by prescribed increments and detecting the light reflected and/or scattered by the reticle surface at each of the prescribed increments. In some embodiments, the processor is programmed to analyze the surface profile map to determine whether to accept the reticle, repair the reticle, or reject the reticle. 
     The foregoing outlines features of several embodiments or examples so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments or examples introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.