Abstract:
A semiconductor device fabrication-use mask pattern inspection apparatus having an optical configuration adaptable for achievement of a Koehler illumination system using a light source high in spatial coherency is disclosed. This apparatus includes a laser light source, a beam expander which is disposed between the laser source and a mask for expanding laser light to form an optical path of collimated light rays, and a beam splitter placed in the collimated light ray optical path for splitting the optical path into two optical paths. In one of these paths, a transmissive illumination optics is placed which irradiates transmission light onto the mask; in the other path, a reflective illumination optics is placed for irradiation of reflected light onto the mask. A pattern image of this mask is detected by a photosensitive device to generate a detected pattern image, which is sent to a comparator for comparison with a fiducial image thereof.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    Priority is claimed to Japanese Patent Application No. 2007-046341, filed Feb. 27, 2007, the disclosure of which is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to pattern inspection technologies. More particularly but not exclusively, this invention relates to a pattern inspection apparatus for testing for defects a circuit pattern of a photolithography mask to be used in the manufacture of highly integrated semiconductor devices. 
       BACKGROUND ART 
       [0003]    In recent years, as semiconductor integrated circuit devices further increase in integration density, a mask pattern for use in the manufacture of such devices is becoming smaller more and more in minimum feature size. To move with this mask pattern miniaturization, many currently available pattern inspection tools are designed to employ a laser light emitting device as a light source thereof. However, the laser light is inherently high in interference and, for this reason, suffers from unintentional occurrence of interference fringes, called the moire. This poses a serious bar to achievement of further miniaturization of semiconductor device products in near feature. 
         [0004]    One proposed approach to reducing such moire is to use an optical system for illumination, which includes a phase plate having a myriad of stair step-like surface differences of less than or equal to the wavelength, which are formed or “carved” in a surface of the plate. This phase plate is driven by an electric motor to rotate at a predetermined speed. An example of this approach is disclosed in Published Unexamined Japanese Patent Application (PUJPA) No. 63-173322. Recently, it is needed to achieve an optical arrangement which is suitably employed to provide a Koehler illumination system using a laser light source. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    It is therefore an object of this invention to provide a new and improved pattern inspection apparatus having an optical configuration adapted for achievement of the Koehler illumination system using a light source which is high in spatial coherency. 
         [0006]    In accordance with one preferred form of the invention, a mask pattern inspection apparatus is provided, which includes a laser generation device for emitting laser light, a movable table structure supporting thereon a mask having a pattern, a beam expander which is disposed in a light path between the laser generation device and the mask for expanding the laser light to thereby form an optical path of collimated light rays, and a beam splitter placed in the optical path of the collimated light rays for dividing the above-noted light path into first and second light paths. A transmissive illumination optics is disposed in the first light path for irradiating transmitted light onto the mask whereas a reflective illumination optics is placed in the second light path for irradiating reflected light onto the mask. An optical pattern image of this mask is received and sensed by a photosensitive device, which issues at its output a sensed image signal. This signal is sent forth toward a comparator unit, which compares the pattern image with a fiducial image thereof. 
         [0007]    In accordance with the invention, it is possible to optimize the layout of the rotatable phase plate and the beam splitter for separation of transmitted and reflected light rays. This in turn makes it possible to provide the intended optical arrangement suitable for achievement of the Koehler illumination system using the spatial coherency-enhanced light source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a diagram showing, in process flowchart form, an entire configuration of a mask pattern inspection apparatus in accordance with one embodiment of this invention. 
           [0009]      FIG. 2  is a diagram showing an optical arrangement of a pattern image creation device as used in the inspection apparatus shown in  FIG. 1 . 
           [0010]      FIG. 3  is a block diagram showing an exemplary hardware configuration of main part of the pattern inspection apparatus of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    Referring to  FIG. 1 , there is shown an overall configuration of a mask pattern inspection apparatus  10  embodying the invention. This pattern inspection apparatus  10  is the one that inspects for defects a circuit pattern which is drawn or “written” on a photolithography mask  40  for use in the manufacture of highly integrated semiconductor circuit devices, such as ultra large-scale integrated (ULSI) circuit chips. This inspection apparatus includes a pattern image creation device  30  and an image comparing unit  20 . The pattern image creator  30  operates to acquire an image  12  of the pattern of an ULSI circuit which is drawn on the mask  40  being tested. The comparator  20  compares this pattern image  12  with a standard or base image  14  which is for use as the fiducial image of the circuit pattern of mask  40 , thereby to detect defects of the pattern, if any. This fiducial image  14  is typically a referencing image, which was obtained from the original or “master” circuit design data, such as computer-aided design (CAD) data, for manufacturing the circuit pattern of mask  40 . Another example of the fiducial image  14  is a pattern image which is the standard or criterion that was obtained by the pattern image creator  30 . 
         [0012]    As shown in  FIG. 1 , the pattern image creator device  30  is generally made up of a laser emitting device  32  such as a laser light source, a beam expander optics  320 , a rotatable phase plate  34 , a beam splitter optics  36 , a pair of integrator optics  38   a  and  38   b , a movable table structure which supports thereon a workpiece under inspection, e.g., mask  40 , and a photosensitive device  44  which receives incoming light rays. 
         [0013]    The beam expander  320  functions to expand the laser light emitted from the laser emission device  32  to thereby convert it into collimated light rays traveling along a prespecified optical path. The beam expander  320  includes an expander lens or else. 
         [0014]    The phase plate  34  is the one that realizes uniform illumination. An example of this phase plate is a transparent round disc-like plate having its surface in which small-size holes, called the pits, of different depths are formed or “carved.” The transparent disc plate may be made of glass or quartz. The pits are formed in an almost entire surface area of the phase plate  34  in such a way as to deviate or offset the phase of light that passes therethrough. 
         [0015]    The beam splitter  36  is the one that divides the incident light path into a plurality of separate outgoing light paths. An example of the beam splitter  36  is a half mirror which is disposed so that it is slanted—typically, at an angle of 45 degrees—with respect to the optical axis of the incoming light path. This half mirror functions to split the incident light path into two optical paths, i.e., a path of light that passes through the mirror, and a path of light that is reflected therefrom. In other words, these optical paths are a transmissive illumination light path which guides transmitted light so that it falls onto the mask  40 , and a reflective light path for irradiating reflected light onto mask  40 . 
         [0016]    The integrator optics  38   a ,  38   b  is the one that guides the collected or “condensed” light to cause it to reach the mask  40  at increased efficiency, while at the same time obtaining the uniformity of light on a top surface of mask  40 . In the illustrative embodiment, an optical integrator is used therefor. As an example, the integrator optics  38   a ,  38   b  is structured from an ensemble of quartz lenses each having a compound-eye lens structure. Respective light rays that are irradiated onto the mask surface are integrated together thereon. Thus, a distribution of inplane brightness or luminance becomes uniform. 
         [0017]    The mask  40  may be either a reticle or a photomask, which has on its surface a circuit pattern or patterns to be inspected. Mask  40  is stably mounted on the table structure, indicated by numeral  400  in  FIG. 3 , which is position-controlled three-dimensionally in three axis directions, i.e., X-direction, Y-direction, and θ-direction. A focusing optics  42  is provided for focusing the optical image of the pattern of mask  40  onto the photosensitive device  44 . Typically, it is structured from a focusing lens or lenses. The photosensitive device  44  is the one that converts its sensed optical pattern image to a corresponding electrical image signal. An example of the device is a charge-coupled device (CCD) image sensor or a photodiode (PD) array. 
         [0018]    See  FIG. 2 , which depicts a pattern image projection device, which is one example of the pattern image creator  30  stated above. The laser light leaving the laser source  32  is expanded by the beam expander  320  into a radiation of collimated light rays. Thereafter, the laser light passes through the rotating phase plate  34  so that this light is reduced thereby in spatial coherency. 
         [0019]    The rotatable phase plate  34  is placed in a light path between the expander lens  320  and the integrator optics  38   a , along which path the collimated light rays of laser light progress or “fly.” With this layout arrangement, sufficient marginal spaces are held before and after the phase plate  34  to thereby minimize influences upon optical elements residing near or around phase plate  34 , such as physical or mechanical vibrations occurring due to the operation of an electrical motor  340  that drives plate  34  for rotation, fluctuation of the ambient air, etc. Additionally, by placing phase plate  34  at a specific optical location that is after having expanded the laser light by the beam expander  320 , the microstructure of phase plate  34  becomes relatively smaller with respect to the beam diameter of laser light, thereby improving disturbance effects. 
         [0020]    After having passed through the phase plate  34 , the laser light is split by the beam splitter  36  into two subbeams of light, i.e., a transmission light component progressing along a transmissive illumination light path, and a reflected light component traveling along a reflective illumination light path. In this embodiment, the transmissive illumination light path refers to an optical path which is formed by the beam splitter  36 , integrator optics  38   a , mirror  322  and condenser lens  324  whereas the reflective illumination light path is an optical path formed by the beam splitter  36 , mirror  326 , integrator optics  38   b , half mirror  330  and objective lens  328 . Letting beam splitter  36  be installed in the above-noted collimated light ray part makes it possible to freely perform the layout arrangement of the transmissive illumination light path and the reflective illumination light path. 
         [0021]    The integrator  38   a , mirror  322  and condenser lens  324  that are disposed to form the transmissive illumination light path function as a transmissive illumination optics. This optics is for irradiating the transmitted light onto the mask  40  being tested. The beam splitter  36 , mirror  326 , integrator optics  38   b , half mirror  330  and objective lens  328  which form the reflective illumination light path function as a reflective illumination optics. This is to irradiate reflected light onto mask  40 . More specifically, the light that is introduced into the transmissive illumination light path is split by integrator  38   a ; the light as introduced into the reflective illumination light path is split by integrator  38   b  via mirror  326 . The light of the transmissive illumination light path is guided by the mirror  322  and condenser lens  324  to fall onto mask  40  to thereby achieve what is called the Koehler illumination. The light of the reflective illumination light path is projected onto mask  40  via half mirror  330  and objective lens  328  to thereby give it Koehler illumination. 
         [0022]    Light rays that have penetrated the mask  40  or were reflected therefrom are collected together by the objective lens  328  and then pass through the half mirror  330  and next focused by the focusing optics  42  so that an image is formed on the photosensitive surface of sensor device  44 . 
         [0023]    In this way, the pattern image creator device  30  operates so that the spatial coherency is reduced by the rotating phase plate  34  after having expanded the laser light emitted from laser source  32  by beam expander  320 . Very importantly, it is after the completion of this coherency reduction that the laser light is split into a couple of light components traveling along two separate optical paths—i.e., the above-stated transmissive illumination light path which is the optical path formed by the optical elements  36 ,  38   a  and  324 , and the reflective illumination light path which is the other optical path formed by optical elements  36 ,  326 ,  38   b ,  330  and  328 . In respective illumination light paths, separate integrators  38   a  and  38   b  are installed for producing area light sources used for Koehler illumination in a way independent of each other. Thereafter, Koehler illumination is given to the mask  40  by a known relay optics. This relay optics may be provided when the need arises. 
         [0024]    It is noted that the pattern inspection tool  10  embodying the invention is arranged so that both the rotatable phase plate  34  and the transmission/reflection beam splitter  36  are disposed in the collimated light path, which is formed by the optical elements  320  and  38   a . With such the phase plate layout design, it is possible to provide extra marginal spaces around the rotating body. This makes it possible to suppress or minimize influences of physical vibrations of the rotator, heat/air fluctuations, etc. Furthermore, by placing the transmission/reflection beam splitter  36  in the collimated light part, it becomes possible to increase the flexibility of free layout of the transmissive illumination light path and reflective illumination light path. 
         [0025]    Turning to  FIG. 3 , there is shown an exemplary configuration of the mask pattern inspection apparatus  10  of  FIG. 1 . As shown herein, this inspection tool  10  is arranged to include a central data processing unit  50 . Upon acquisition of a pattern image through detection of either the transmitted light or reflected light from the mask  40  by the pattern image creator device  30 , the data processor  50  stores in its internal memory the data including design data of pattern image creator  30  and others. Data processor  50  functions to prepare from the design data a referencing image for use as the fiducial image  14  and also serves to perform a variety of kinds of digital arithmetic data processing. 
         [0026]    The pattern image maker  30  operates to acquire a pattern image  12  from the circuit pattern of the mask  40  being tested. Mask  40  is stably situated on X-Y-θ table structure  400 . This XYθ table may typically be a three-axis (X-Y-θ) manipulator, which is movable in X-axis and/or Y-axis direction and also is rotatable in θ direction. This manipulator is driven by electric motor  402 , called the XYθ motor. This motor is drive-controlled by a table control unit  404 , which operates in responding to receipt of a command(s) from the central data processor  52 , in such a way as to move and/or rotate the XYθ table  400  in any one or ones of the X-, Y- and θ-directions. The XYθ motor  402  may be a known servo motor, a stepper motor or like motors. Position coordinates of XYθ table  400  are measured by a known laser-assisted length measurement system (not shown in  FIG. 3 ) on a real time basis, and an output measurement signal of it is sent forth toward a position measurement unit (not shown). This unit generates at its output a data signal indicative of the position coordinate values of XYθ table  400 , which is then fed back to the table controller  404 . 
         [0027]    The mask  40  is automatically loaded and mounted on the XYθ table  400  by an auto-loader (not shown) under the control of an auto-loader control unit (not shown), and is unloaded in an automated way after completion of inspection. At an upper part of the table  400 , the laser emitting device  32  is disposed. Laser light from this device is irradiated onto mask  40  via either one of the transmissive illumination light path or the reflective illumination light path. The focusing optics  42  is disposed beneath mask  40 . When the pattern image  12  of mask  40  is incident on photosensitive device  44 , it is converted to a corresponding electrical image signal. Focusing optics  42  is subjected to automated focusing adjustment with the aid of a focus adjuster device (not shown), such as a piezoelectric element. 
         [0028]    This focus adjuster is controlled by an auto-focus control circuit (not shown), which is connected to the central data processor  52 . This focusing adjustment may alternatively be performed manually by an operator while monitoring using a separately provided observation scope. The photosensitive device  44  is a photodiode (PD) array module as an example. This PD array may be a linear sensor or an area sensor with a plurality of photodetective sensor elements. The PD array senses the pattern of the mask  40  while the XYθ table  400  is driven to move continuously in X-axis direction, thereby generating at its output a corresponding electrical measurement signal. 
         [0029]    This measurement signal is converted by a sensor circuit  46  into digital data, which is input as the data of the sensed pattern image to a buffer memory  56  and then stored therein. This buffer memory  56  may be either a series connection or a parallel combination of more than two semiconductor memory units on a case-by-case basis. An output of buffer memory  56  is transferred via data bus  60  to the image comparator  20 . An example of the pattern image data is 8-bit unsigned data indicative of the brightness or luminance of each picture element or “pixel.” Usually the pattern inspection tool  10  of this type reads these pattern data out of the above-noted PD array in a way synchronized with a clock frequency of about 10 to 30 megahertz (MHz) and deals with the data as raster-scanned two-dimensional (2D) image data through appropriate data sorting. 
         [0030]    A procedure for acquisition of a pattern image is as follows. An optical image of the integrated circuit pattern of the mask  40  is obtainable by causing the pattern image maker  30  to scan the mask  40 . This mask pattern is acquired, for example, as a pattern image of narrow and long strip-like segments, which are cut along the direction of one side (e.g., X direction) of mask  40 . These strips are in the form of a stream. This stream is a pattern image of an ensemble of further elongated strips which are four-divided in the one-side direction, e.g., X direction. The four-divided stream will be called the sub-stream. This substream is cut into multiple portions in another direction at right angles to the X direction, e.g., Y direction. These cut pattern images are called the frame. An example of this frame is a dot pattern image which consists essentially of a matrix of  512  rows of pixels along X-direction and  512  columns of pixels in Y-direction. Additionally each pixel has a grayscale of 256 different gradation or “graytone” levels. 
         [0031]    As shown in  FIG. 3 , the data processor  50  is generally made up of the central processor  52  which executes data processing, auto loader control unit which controls the auto-loader, table control unit  404  for controlling XYθ table  400 , a reference image creation unit  54  which creates from design data a referencing image for use as the fiducial image  14  that resembles mask pattern image  12 , comparator  20  which compares pattern image  12  and fiducial image  14  to inspect for defects the image being tested, buffer memory  56  for temporary storage of the data of pattern image  12 , a position measurement unit which obtains a present position of mask  40  from the position data of XYθ table  400  as measured by the laser-aided length measurement system, external storage device  62  for storing therein software programs and a large amount of data, such as a database of design data, main memory  64  for storage of a software program(s) along with data necessary for execution of arithmetic processing, printer  66 , and a display monitor such as a cathode ray tube (CRT) or liquid crystal display (LCD) panel. These components are operatively connected together via internal data bus  60 . The design data of mask  40  is stored so that an entirety of inspection area is divided into strip-like areas, by way of example. 
         [0032]    The reference image creator  58  expands the design data to form image data and then applies thereto the image processing, such as edge rounding and/or slight fogging of graphic forms or figures, to force it to have maximized similarity in shape to pattern image  12 , thereby preparing the reference image required. In view of the fact that this reference image is created directly from the original circuit design data, the resulting image is free from unwanted deviations otherwise occurrable in the actually operating pattern image creator device  30 , such as distortions, deformations, level fluctuations, graytone variations, etc. 
         [0033]    Although the invention has been disclosed and illustrated with reference to a particular embodiment, the principles involved are susceptible for use in numerous other embodiments, modification and alterations which will be apparent to persons skilled in the art to which the invention pertains. For example, while in the apparatus configuration shown in  FIG. 3  respective devices and/or functional units are arranged by hardware components such as electrical or electronic circuits, similar results are obtainable by replacing them with software programs or firmware modules. Alternatively, these may be combined together to provide a “hybrid” configuration. Additionally, various types of mask pattern testing systems may be established by using constituent parts or components as indicated in the embodiment stated supra in a combined way on a case-by-case basis. When the need arises, one or several functional components may be eliminatable from those shown in  FIGS. 1-3 . The invention is, therefore, to be limited only as indicated by the scope of the appended claims.