Abstract:
An XRF (XRF=x-ray fluorescence) measurement apparatus ( 1 ) has an x-ray source ( 2 ) for generating x-rays ( 4 ), x-ray optics ( 3 ) for directing x-rays ( 4 ) from the x-ray source ( 2 ) to a sample ( 5 ) and an EDS (EDS=energy dispersive spectroscopy) detector ( 7 ) for detecting fluorescent x-rays ( 14 ) from the sample ( 5 ). The apparatus is characterized in that the sample ( 5 ) is a wafer ( 6 ), in particular a Si wafer, wherein the x-ray optics ( 3 ) is positioned to direct the x-rays ( 4 ) onto the bevel ( 12 ) of the wafer ( 6 ). The x-ray source ( 2 ) plus the x-ray optics ( 3 ) has a brilliance of at least 5*10 7  counts/sec mm 2 , preferably at least 1*10 8 counts/sec mm 2 . The apparatus allows an improved contamination control of wafers, in particular silicon wafers.

Description:
[0001]    This application claims Paris convention priority from EP 13 153 344.0 filed Jan. 30, 2013, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to an XRF (XRF=x-ray fluorescence) measurement apparatus, comprising
       an x-ray source for generating x-rays,   x-ray optics for directing x-rays from the x-ray source to a sample,   the sample,   and an EDS (EDS=energy dispersive spectroscopy) detector for detecting fluorescent x-rays from the sample.       
 
         [0007]    Such an XRF measurement apparatus is known from U.S. Pat. No. 5,778,039 A. 
         [0008]    Wafers, in particular silicon wafers, are a basic component in the production of semiconductor electronics. These semiconductor electronics are based on pn-transitions, in particular in diodes and transistors. Semiconductor material of p-type and n-type is produced by carefully controlling the chemical composition of a basic material (such as silicon). More specifically, dopant materials having a number of valence electrons different from the basic material are deliberately added to the basic material. 
         [0009]    However, contaminations may act similar to dopant materials, changing the properties of the semiconductor material in an unintended way. Accordingly, semiconductor production is performed under clean room conditions, and the contamination levels are monitored closely. 
         [0010]    For silicon wafers, it has been proposed to examine the flat side surfaces of the wafer by means of TXRF (total reflection x-ray fluorescence) spectroscopy. In TXRF, a typically monochromatic x-ray beam is directed to a sample surface, and characteristic x-rays resulting from the refilling of depleted deep electron shells of the sample material are detected. Contaminations of the sample surface result in x-ray peaks at additional wavelengths, as compared to the sample material alone. XRF spectra may be evaluated quantitatively, for determining the amount of contaminations. The flat side surface may be completely scanned with the x-ray beam (“wafer mapping”), if desired. 
         [0011]    During production processes, wafers have to be transported at numerous occasions. For this purpose, grippers typically act on the bevel of the wafer; the bevel is also sometimes called “grip edge”. Thus contaminations of the flat side surfaces of the wafer shall be avoided. 
         [0012]    However, contaminations of the bevel may be passed on to the flat surfaces later on, for example by surface diffusion, in particular at elevated temperatures. Therefore, bevel contaminations should be avoided, too, and accordingly, bevel contaminations should be monitored for this purpose. 
         [0013]    For monitoring bevel contaminations, it is possible to wipe the edge of the wafer with a receptive carrier (such as a cotton bud), and to analyze the receptive carrier, for example with ICP-MS (inductively coupled plasma mass spectrometry). However, this is a complex and time-consuming procedure, and the receptive carrier itself may contaminate the wafer. 
         [0014]    It is the object of the invention to allow an improved contamination control of wafers, in particular silicon wafers. 
       SUMMARY OF THE INVENTION 
       [0015]    This object is achieved, in accordance with the invention, by an XRF measurement apparatus as introduced in the beginning, characterized in that the sample is a wafer, in particular a Si wafer, wherein the x-ray optics is positioned to direct the x-rays onto the bevel of the wafer, and that the x-ray source plus the x-ray optics has a brilliance of at least 5*10 7  counts/sec mm 2 , preferably at least 1*10 8  counts/sec mm 2 . 
         [0016]    The invention proposes to use XRF on the bevel (edge) of a wafer, such as a silicon wafer, and to direct x-rays onto the bevel accordingly. Preferably, the (primary) x-ray hits only the bevel of the wafer, and not the flat side surface of the wafer when the bevel is analyzed. Further, the invention proposes to apply an x-ray source, in particular of micro-source type, with a high brilliance. This ensures that a sufficient signal level is achieved from possible contaminations, so contaminations can reliably be detected. The XRF measurement can be evaluated immediately, without delays for, for example, transporting a receptive carrier to a mass spectrometer. The inventive method is non-destructive and not likely to introduce new contaminations. 
         [0017]    Note that typical wafers used as samples, in accordance with the invention, are basically circular disc shaped, often with a cut-out part along a secant. Generally, the surface area of the flat side of a wafer is at least 10 cm 2 , often 100 cm 2  or more, and the thickness is 750 μm or less, often 375 μm or less. Typical wafer materials are silicon or germanium; however other materials such as aluminum oxide or steel are also possible. 
         [0018]    In a preferred embodiment of the inventive apparatus, the x-ray optics and the wafer are positioned such that the x-rays hit the surface of the wafer at the bevel at an angle of between 0.05° and 6°. This geometry results in larger signal levels from contaminations, as compared to incident primary beams closer to a perpendicular orientation. More contamination material can be illuminated at the same time, and total reflection may occur at the wafer&#39;s surface what keeps the signal from wafer material low. 
         [0019]    Preferred is also an embodiment, wherein the x-ray optics and the wafer are positioned such that the x-rays directed to the sample propagate essentially in a plane parallel to a flat side of the wafer. This geometry also leads to larger signal levels from contaminations, as compared to incident primary beams closer to perpendicular orientation, for typical wafer designs, using the x-ray beam basically tangentially. Again, more contamination material can be illuminated at the same time. 
         [0020]    Further preferred is an embodiment wherein the wafer is oriented with the surface normal of a flat side of the wafer being oriented horizontally. This saves space, and in some situations may allow a quick change of the investigated wafer by moving a row of wafers horizontally. 
         [0021]    Also preferred is an embodiment wherein the x-rays directed to the sample propagate in an essentially horizontal direction. This offers a good access to the equipment and samples in practice. 
         [0022]    In an advantageous embodiment, the x-ray source is of metal jet target type. Metal jet target type x-ray sources allow a particularly high brilliance. Heat in the target material is easily dissipated; 
         [0023]    further, the target area hit by an electron beam can be chosen small, according to the diameter of the jet. Note that source spot diameters of 100 μm or less (qualifying as micro-source) are preferred, in accordance with the invention. 
         [0024]    In a preferred embodiment, the x-ray optics include a Montel mirror or a Göbel mirror or a double curved multilayer mirror. These parts have shown high efficiency in focusing or collimating x-ray beams. In particular, a multilayer mirror having a single reflective surface curved with respect to both a sagittal and a meridional direction of incident x-rays (see U.S. Pat. No. 7,248,670 B2), referred to as a double curved multilayer mirror, may be used, in accordance with the invention. Note that the x-ray optics may comprise further parts, alternatively or in addition, such as capillary optics or apertures. 
         [0025]    Particularly preferred is an embodiment wherein the bevel of the wafer is located in a focus of the x-ray optics. Then the flux of primary x-rays can be used efficiently for XRF analysis of the wafer bevel, and influences from areas away from the bevel may be excluded or at least minimized . Alternatively, a parallel x-ray beam may be used. Further alternatively or in addition, areas next to the bevel may be shadowed, for example using a mask or an aperture. 
         [0026]    Advantageous is further an embodiment wherein at a position at the surface of the sample, the width of the x-rays directed to the sample matches the width of the wafer. This makes sure that basically all contaminations may be detected in a single revolution of the wafer, and influences from areas away from the bevel may be excluded. Further, the primary x-rays may be used efficiently. Note that the wafer typically has a thickness of 750 μm or less, such as 450 μm or 375 μm. 
         [0027]    Particularly preferred is an embodiment providing that the apparatus further comprises an auxiliary x-ray optics for directing x-rays from the x-ray source to the sample and switching means for switching the apparatus between a first operation mode and a second operation mode, 
         [0000]    wherein in the first operation mode, the x-ray optics are positioned to direct x-rays form the x-ray source onto the bevel of the wafer, and wherein in the second operation mode, the auxiliary x-ray optics are positioned to direct x-rays form the x-ray source onto a flat side of the wafer. Such an apparatus allows an investigation of the complete wafer surface, including the flat side surface (at least the front surface, or even both flat side surfaces of back and front) and the bevel, no contaminations can be missed then. 
         [0028]    In a preferred further development of this embodiment, the switching means comprise a first moving stage for exchanging the x-ray optics with the auxiliary x-ray optics in the path of the x-rays. The first moving stage is typically motorized and allows a quick and simple change of the x-ray optics. 
         [0029]    Another preferred further development provides that the switching means comprise a second moving stage for pivoting and/or shifting the wafer relative to the path of the x-rays. The second moving stage is typically motorized and simplifies the change of the area of the sample illuminated with the primary x-ray beam. 
         [0030]    A preferred embodiment is characterized in that the apparatus further comprises
       a further EDS detector for detecting fluorescent x-rays from the sample, and   a handling stage for shifting the wafer relative to the path of the x-rays directed to the sample in two independent, in particular orthogonal, directions transverse to the x-rays directed to the sample, and for rotating the wafer with respect to a rotation axis perpendicular to a flat side of the wafer,
 
in particular wherein the EDS detector and the further EDS detector view the sample at basically right angles with respect to the x-rays directed to the sample and at a basically right angle with respect to each other. This embodiment allows a very simple switching between an investigation of the bevel and the flat side of the wafer, with only requiring a minimum of moving parts, namely the handling stage.
       
 
         [0033]    Also within the scope of the present invention is the use of an inventive apparatus as described above, for detecting contaminations on the bevel of a wafer, in particular a silicon wafer, by means of XRF. The XRF analysis is non-destructive and can give immediate results on the contamination level. Note that typical contaminations looked for by means of the invention include Al (from grippers) and Na (from salt contained in human sweat),In a preferred variant of the inventive use, a gallium L line is used for x-ray generation in the x-ray source. This has shown good results in practice; gallium can well be used in a metal jet, since gallium has a relatively low melting point of about 30° C. and therefore needs only a minimum of heating. 
         [0034]    Further within the scope of the present invention is a method for investigating the surface of the bevel of a wafer, in particular a Si wafer, wherein an x-ray beam is directed onto the bevel of the wafer and fluorescent x-rays emitted by the wafer are detected by EDS (EDS=energy dispersive spectroscopy). The spectra of contaminations will make them immediately observable. 
         [0035]    Further advantages can be extracted from the description and the enclosed drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any combination. The embodiments mentioned are not to be understood as exhaustive enumeration but rather have exemplary character for the description of the invention. 
         [0036]    The invention is shown in the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0037]      FIG. 1   a  shows an inventive XRF measurement apparatus, in a schematic side view, in a first operation mode wherein x-ray optics are positioned to direct x-rays on the bevel of a wafer; 
           [0038]      FIG. 1   b  shows the apparatus of  FIG. 1   a,  in the first operation mode, in a schematic top view; 
           [0039]      FIG. 2   a  shows the apparatus of  FIG. 1   a,  in a schematic side view, in a second operation mode wherein auxiliary x-ray optics are positioned to direct x-rays on the flat side of the wafer; 
           [0040]      FIG. 2   b  shows the apparatus of  FIG. 2   a , in the second operation mode, in a schematic top view 
           [0041]      FIG. 3   a  shows a rear part of an inventive measurement apparatus, in a schematic top view, with a handling stage position allowing investigating the bevel of a wafer; 
           [0042]      FIG. 3   b  shows the rear part of  FIG. 3   a , in a schematic side view; 
           [0043]      FIG. 3   c  shows the rear part of  FIG. 3   a , in a schematic top view, with a handling stage position allowing investigating the flat side of the wafer; 
           [0044]      FIG. 3   d  shows the rear part of  FIG. 3   c , in a schematic side view. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0045]      FIGS. 1   a  and  1   b  illustrate an embodiment of an inventive XRF measurement apparatus  1  by way of example, in a side view ( FIG. 1   a ) and a top view ( FIG. 1   b ). 
         [0046]    The apparatus  1  comprises an x-ray source  2 , x-ray optics  3  directing x-rays  4  from the x-ray source  2  to a sample  5 , which is a disc shaped wafer  6 , and an EDS detector  7 . 
         [0047]    The x-ray source  2  is, in the illustrated embodiment, of metal jet type, with a jet of liquid metal  8 , for example slightly heated gallium, being hit by an electron beam  9  at a focal spot  9   b.  The electron beam  9  is generated by an electron beam source  9   a;  note that the electron beam  9  and metal jet  8  preferably propagate in vacuum. At the focal spot  9   b  of the electron beam  9 , characteristic x-rays  10  and Bremsstrahlung are emitted. A fraction of the generated x-rays which passes an aperture  11  and is used as x-rays  4  (or primary beam) in the subsequent experimental setup. The brilliance of the x-ray source  2  together with the x-ray optics  3  is here at about 10 8  counts/(sec mm 2 ). 
         [0048]    The x-rays  4  are directed towards the sample  5  by means of x-ray optics  3 , here a double curved multilayer mirror, mounted on a first stage  20 . In the example shown, the x-rays  4  are focused in two dimensions onto the bevel  12  of the wafer  6  by means of the x-ray optics  3 , with a matching (equal) width W of the x-rays  4  and the wafer at a focal spot  13 . If desired, the x-ray optics  3  may be chosen such that the focal spot  13  is a 1:1 image of the focal spot  9   b.  The multilayer mirror also causes a monochromatization of the x-rays  4 . The x-rays  4  hit the bevel  12  at an angle α with respect to the tangent of the bevel  12  of the wafer  6  at the focal spot  13 ; the tangent (see dashed line in  FIG. 1   a ) represents the wafer surface at the focal spot  13  here. The angle α is typically between 0.05° and 6°, so total reflection occurs at the wafer surface (not shown in detail). Note that the figures exaggerate some angles and proportions in order to make them better visible. Further note that the angle α is here measured against the farther outer part of the x-ray beam; the beam size may be determined by the half maximum lines of the photon flux. 
         [0049]    At the focal spot  13 , fluorescent (characteristic) x-rays  14  are emitted, which may originate from the material of the wafer  6 , and from contaminations on the surface of the wafer  6 . By means of the EDS detector  7 , the fluorescent x-rays  14  are detected in an energy resolved manner. The EDS detector  7  is located directly above the focal spot  13  in order to receive a maximum fraction of the fluorescent x-rays  14 . 
         [0050]    The wafer  6  is mounted on a second stage  15 , which grabs the wafer  6  from its back side  16  by means of a vacuum gripper  17 . The vacuum gripper  17  is rotatable with respect to a rotation axis  18  perpendicular to the flat side  19  of the wafer  6 , in order to subsequently expose the complete bevel  12  to the x-rays  4 . 
         [0051]    In the embodiment shown, the x-rays  4  propagate in  FIGS. 1   a,    1   b  basically parallel to the vertical xz plane, and mostly horizontally in x; the tangent of the bevel  12  at the focal spot  13  runs horizontally (in x). The flat side  19  of the wafer  6  is oriented vertically, in parallel to the xz plane, too, with the surface normal SN of the flat side  19  and the rotation axis  18  running horizontally (in y direction). 
         [0052]    The apparatus  1  can be switched from a first operation mode, which is illustrated in  FIGS. 1   a,    1   b  and has been explained above, to a second operation mode, which is illustrated in  FIG. 2   a  (side view) and  FIG. 2   b  (top view). In this second operation mode, the flat side  19  of the wafer  6  may be investigated by means of XRF. In  FIGS. 2   a  and  2   b , only the major differences to the setup of  FIGS. 1   a  and  1   b  are explained in detail, and for simplification, the x-ray source  2  is not shown in detail. 
         [0053]    For being able to switch between the operation modes, the first stage  20  is built as a first moving stage  20 . By means of a motor (not shown), the first moving stage  20  can be moved in a vertical direction (z direction). In a lower position (see also  FIG. 1   a ), x-ray optics  3  are in the path of the x-rays  4 , whereas in an upper position (shown in  FIG. 2   a ), auxiliary optics  21  are in the path of the x-rays  4 . The auxiliary optics  21  comprise a double curved multilayer mirror again, which is oriented to deflect the x-rays  4  in the horizontal plane (yx-plane) and to focus the x-rays in two dimensions onto a focal spot  22  on the flat side  19  of the wafer  6 . Note that the auxiliary x-ray optics  21  are placed on a wedge  23  to ensure a proper position, since the first moving stage  20  typically cannot be pivoted. 
         [0054]    Further for switching between the operation modes, the second stage  15  for the wafer  6  is built as a second moving stage  15 . By means of one or several motors (not shown), the second moving stage  15  can be moved in all translative directions x, y, z, and rotated with respect to a vertical axis  24 . This allows the wafer  6  to be placed as shown in  FIGS. 2   a ,  2   b , and to scan the surface of the flat side  19  with the stationary focal spot  22 . The x-rays  4  hit the flat side  19  at an angle β of typically between 0.05° and 6°, again measured against the farther outer part (outer edge) of the incoming x-ray beam. 
         [0055]    Further, in the embodiment shown, the EDS detector  7  can also be moved, preferably with a motorized stage (not shown), so the EDS detector  7  can be placed directly above the focal spot  22  in the second operation mode, too. 
         [0056]      FIGS. 3   a  through  3   d  illustrate another inventive apparatus, showing only the rear part (i.e. omitting the x-ray source and the x-ray optics, compare  FIG. 1   a,    1   b  for these components), which can be switched between a first operation mode in which the bevel of the wafer is investigated (see  FIGS. 3   a ,  3   b ), and a second operation mode in which the flat side of the wafer is investigated (see  FIGS. 3   c ,  3   d ). 
         [0057]    In the first operation mode, compare  FIGS. 3   a  (top view) and  FIG. 3   b  (side view, perpendicular to the propagation direction of the x-rays  4 ), the x-rays  4  hit the bevel  12  of the wafer  6 , compare focal spot  13 . The x-rays  4  hit the bevel  12  at a small angle, such as about 1° against the tangent of the wafer  6  in the bevel region, so they are totally reflected. At the focal spot  13 , characteristic x-rays  14  are emitted, which can be detected by an EDS detector  7 . The EDS detector  7  is fixed at the height (sideways) of the wafer  6 , receiving the characteristic x-rays  14  at a basically right angle with respect to the incoming x-rays  4 , for XRF analysis. 
         [0058]    The wafer  6  is held on a handling stage  25 , which has been positioned at the correct height (z position) and traverse position (y position) such that the x-rays  4  hit the wafer  6  at the bevel  12  at said small angle, i.e. almost tangentially. During measurement, the wafer  6  is slowly rotated (typically in an incremented way) to check the complete circumference. Typically, the handling stage  25  is motorized for z and y position adjustments, and for rotation about the rotation axis  18 . 
         [0059]    Before or after measurement of the bevel, the flat side  19  of the wafer  6  may also be investigated by XRF with the apparatus, compare  FIG. 3   c  (top view) and  FIG. 3   d  (side view, in a direction perpendicular to the propagation direction of the x-rays  4 ), in a second operation mode. As compared to  FIGS. 3   a  and  3   b , the handling stage  25  has been moved slightly downward and to the left for this purpose. In this movement position, the x-rays  4  hit the wafer  6  at the flat side  19  under a small angle, such as 1°, against the plane of the flat side  19 , and are totally reflected. In order to scan the flat side surface of the wafer  6  completely, the handling stage  25  is moved in an incremented way in y direction, and at each y position, the wafer  6  is rotated about rotational axis  18  over a full turn (typically in an incremented way). Characteristic x-rays  14  emitted at the focal spot  13  are detected with a further EDS detector  26 , fixed above the wafer  6 . The further EDS detector  26  is positioned to receive the characteristic x-rays  14  at a basically right angle with respect to the x-rays  4  again. 
         [0060]    Note that the EDS detector  7  and the further EDS detector  26  are oriented at right angles with respect to their field of view, with only one of them operating at a time, depending on the operation mode. For switching between the modes here, it is not necessary to move or exchange the x-ray optics or the EDS detectors  7 ,  26 , but only movement of the handling stage  25  or the wafer  6 , respectively, is required.