Abstract:
In view of the miniaturization of semiconductor components, the prevention of particles and other defects on the wafer surface during production is of great importance. The inspection should proceed as process-oriented as possible. For this purpose, devices are needed which on the one hand, are of very compact construction, while on the other hand, they still are equipped with measuring systems meeting the highest requirements.  
     The device according to the invention for measuring surface defects, comprising a sample holder, a rotation drive for the sample holder, wherein the rotational axis runs perpendicular to the sample surface to be measured, an optical measuring system ( 10 ) for measuring scattered light, as well as at least one linear drive ( 23 ) for the measuring system, wherein the rotational direction is radial to the rotational axis of the sample holder, is capable of scanning the entire sample surface ( 16 ). By moving not only the sample, but also the measuring system, the need for space as a whole is reduced and the total device can be constructed with a more compact design.

Description:
CROSS REFERENCE  
       [0001]    This application is a Continuation-In-Part of U.S. Ser. No. 10/120,641, filed Apr. 11, 2002, for Measuring Module, herein fully incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to a device for measuring surface defects.  
           [0003]    In view of the miniaturization of semiconductor components, the prevention of particles and other defects on the wafer surface during production is of great importance. Since a particle grain can by now be larger than an entire circuitry, even the smallest defects lead to a large amount of waste.  
         BACKGROUND OF THE INVENTION  
         [0004]    In general, scattered light is used for measuring defects on smooth surfaces. For this purpose, the surface of, for example, a wafer is illuminated with a laser beam. The scatter characteristics of different particle sizes are shown in FIG. 1. A small particle  2  as well as a large particle  3  are illuminated with a laser beam  1 . ‘Small particle&#39; means that the particle diameter is much smaller than the wavelength used. &#39;Large particle’ means that the particle diameter is approximately of the same size as the wavelength or a little larger. As shown in FIG. 1, small particles scatter in isotropic manner into the space. Large particles, on the other hand, generate strong backscattering; see also scatter ellipse  4 .  
           [0005]    All methods of detecting surface defects are based on the detection of the scattered radiation, while blanking out the primary light reflected by a mirror. The intensity of the scattered light usually lies several levels below that of the reflected primary light.  
           [0006]    Patent application U.S. Pat. No. 5,377,002 describes an apparatus for measuring scattered light, wherein the scattered light is focused through a converging lens onto a photo detector. In this process, the converging lens defines the acceptance angle before the scattered light. Directly reflected light is blanked out.  
           [0007]    Patent application RU 2064670 proposes to collect the scattered light by means of an elliptical mirror disposed in rotational symmetry around the incident beam. Due to the very large acceptance angle, this apparatus is particularly sensitive to small particles.  
           [0008]    Patent application WO 00/33055, as well, uses either ellipsoid or paraboloid, rotationally symmetrical mirrors, in order to collect the scattered light. However, the reflected primary light is not blanked out, but the scattered light is deflected via a deflection mirror. The incident and the reflected primary beams enter through an opening in the deflecting mirror. This apparatus, as well, is particularly sensitive to small particles.  
           [0009]    Patent application EP 0624787 A1 proposes, in order to enhance the measuring sensitivity for large particles as well, to place two converging lenses in the path of the primary light beam within the ellipsoid rotationally symmetrical mirror, in order to be able to also detect the strong backscattering occurring with large particles. The primary light is blanked out again in front of the detector. This apparatus however has the disadvantage that scattered light is generated also at both converging lenses, which as a whole corrupts the measuring results. Furthermore, the focus is always on measuring the scattered light in its entirety, rather than on differentiating it based on particle size.  
           [0010]    Since with all detection methods described the light spot can always only illuminate a fraction of the sample surface, the entire sample surface has to be scanned. This may be achieved with a rectangular grid, for example. With circular samples, such as wafers, a spiral-shaped scanning path as described in the patent application U.S. Pat. No. 4.314.763 is best suited. With this method, the sample is either rotated around its axis while simultaneously undergoing a translational movement in radial direction and while the light beam remains stationary, or the sample remains immobile and the spiral-form movement is executed by the light beam, which in very sensitive optical systems impairs the measuring accuracy.  
           [0011]    Patent application WO, 00/33055 further develops the spiral scan principle into the so-called record player principle. With this method, the sample surface is rotated around a first rotational axis. The light beam, meanwhile, travels on an arc around a second rotational axis. It swings over the sample surface just like the sensing head of a record player. This record player principle has prevailed on the market and is already used in the process-oriented quality surveillance for semiconductor components (e.g. device “Reflex 300” of the “Reflex” company, Moscow).  
           [0012]    The apparatus already on the market has the disadvantage that the measuring head has to be kept very small, in order to allow for the swinging motion. This leaves very little space for the optical system so that it has to be very simple. For example, only very simple miniature diode lasers can be used as they work within the red spectral range. Since blue lasers require more space, the resolution cannot be further improved in these devices.  
         SUMMARY OF THE INVENTION  
         [0013]    Based on the preceding explanations, the present invention has the object of providing a device for measuring surface defects, which device can be used for process-oriented quality surveillance while still meeting the highest technical measuring requirements.  
           [0014]    This object is achieved by a device according to the claims.  
           [0015]    The device according to the invention for measuring surface defects, comprising a sample holder, a rotation drive for the sample holder with its rotational axis perpendicular to the sample surface to be measured, an optical measuring system for scattered light measurements, as well as at least one linear drive for the measuring system with its rotational direction radial to the rotational axis of the sample holder, is capable of scanning the entire sample surface. By not only moving the sample, but also the measuring system, less space is needed as a whole and the entire device can be constructed in a more compact form. This makes the device particularly suitable for process-oriented quality surveillance, since it can simply be integrated into existing process facilities.  
           [0016]    In the device according to the invention, a compact construction is achieved less by the excessive miniaturization of the measuring system, as for example in devices operating on the basis of the “record player principle”, than by the fact that—as already explained—the sample to be measured is just rotated and the measuring system is moved radially to the rotational axis across the sample surface with the aid of a linear drive.  
           [0017]    In a preferred embodiment, the device is to be provided with a second measuring system for measuring any additional physical property, wherein the two measuring systems are disposed at a fixed distance from one another and one behind the other in translational direction. Particularly preferred in this context is the combination of a scattered light measurement for large particles with the scattered light measurement for small particles.  
           [0018]    In a refinement of the optical measuring system, use of a diaphragm before the scattered light deflecting unit optimizes the collection of light at smaller angles to the emerging beam, which leads to a high measuring sensitivity for large particles. The diaphragm is dimensioned in such a way that light is projected onto the deflecting unit at small angles and detected. By dimensioning the diaphragm in this way it becomes possible to define the dihedral angle measured by the detector. Furthermore, by using a diaphragm a smaller degree of parasitic scattered light can be achieved than is possible with the conventional refractive optical systems.  
           [0019]    Preferably, the scattered light deflecting unit is constructed as a mirror which has an opening for the primary light and only reflects the scattered radiation. With this method, only the scattered light at the smallest angle is not detected by the scattered light signal. The use of a reflective element as scattered light deflecting unit, just like the diaphragm, has the advantage of providing a good signal-to-noise ratio.  
           [0020]    For optimally measuring the scattered radiation, it has proven advantageous to have the light beam of the light source fall essentially perpendicular onto the surface to be measured. The measuring system can be dimensioned in a way to achieve a compact construction and is easy to adjust. In some applications, however, it may be advantageous to select a more grazing angle of incidence, in order, for example, to better be able to detect scratches.  
           [0021]    In context with the detection of particles, the question of the nature of the particles is often asked. One measuring system alone is not capable, for example, of differentiating between external particles and embedded particles or indentations. A second measuring system using a different type of surface illumination can offer additional information.  
           [0022]    This second measuring system could, for example, be a dark field microscope. Equally, the second measuring system can be used to measure a completely different physical property, such as the distribution of layer thickness with the aid of a spectrometer. The information of the two measuring systems is provided one after the other. Subsequently, the information of the two different measuring systems has to be related to the same spot on the surface. Calibration measurements on known objects or samples may be performed to serve this purpose.  
           [0023]    Preferably, a control and evaluation unit will be provided. This unit is, however, spatially removed from the optical measuring system. This permits the device according to the invention to be kept as compact as possible. Furthermore, it is particularly important for the process-oriented quality surveillance to reduce to a minimum all factors which could lead to contamination.  
           [0024]    The provision of two linear motors disposed on opposite sides of the sample holder has proven advantageous for stable operation. It is particularly preferred to connect these two linear motors by a carrier plate stretching across the sample holder and serving as support for the optical measuring system. This can give the optical measuring system the capability to move particularly gently.  
           [0025]    In an additional preferred embodiment of the device according to the invention, a mechanism for the automatic adjustment of the distance of the optical measuring system from the sample surface to be measured is provided. This mechanism comprises an adjusting light source whose beam is directed toward the sample surface, a position-sensitive photo detector for detecting the reflected adjustment beam, as well as a control unit, and a distance adjustment unit for the adjustment of the distance between measuring head and sample surface.  
           [0026]    An apparatus of this kind is proposed by the patent application U.S. Pat. No.  6 , 094 , 199  for ellipsometers. The measurement is performed according to the triangulation principle. Thanks to the automatic correction of the operating distance it becomes possible to measure even arched samples, although the depth of focus of the scattered light measuring device is approximately 100 micrometers.  
           [0027]    An additional preferred embodiment of the device according to the invention comprises a mechanism for recognizing a notch in the sample to be measured. For this purpose, the mechanism is equipped with a laser with a band-shaped laser beam which is directed toward the sample surface, as well as with a detector for measuring the reflected beam.  
           [0028]    Preferably, an alignment mechanism is also provided which is equipped with a vertically movable, driven sample table which is disposed in the rotational axis of the sample holder and can be rotated. It is advantageous for the drive mechanism of the sample table to be linked to a control mechanism which is connected to the notch detection mechanism. This makes it possible to align the sample to be measured accurately in each case by hand or automatically.  
           [0029]    Preferably, a light deflecting unit is placed downstream from the light source. If this light deflecting unit is disposed at the same angle as the scattered light deflecting unit, light source and photo detector can be placed side by side, which makes a very compact construction of the optical measuring system possible  
           [0030]    It may also be advantageous to dispose a light wave guide downstream from the light source. This allows the light source as such to be spatially removed from the other components of the optical measuring system. In this way, the optical measuring system can be miniaturized and/or be designed in compact construction without becoming subject to excessive limitation in the selection of the light source. The use as light wave guide also makes it easer to set up any kind of incidence geometry in the proximity of the sample. Downstream from the light wave, as well, a light deflecting unit may be provided.  
           [0031]    In order to show the scattered light signal on the photo detector, a focusing unit, e.g. a lens, may be disposed between the scattered light deflecting unit and the photo detector. Due to the low intensity of the scattered radiation in this area, the probability of parasitic scattered light radiation is relatively small. Otherwise, the scattered light deflecting unit itself could also be constructed as focusing mechanism, e.g. by arching it.  
           [0032]    In a preferred embodiment, additional optical elements are provided for additional measurements. These need not be complete optical measuring systems. By arranging the additional elements, for example, in the individual channels of a turret-like housing part, the already existing light source and photo detector can be used for additional measurements. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]    The invention is described in detail with the aid of the following drawings:  
         [0034]    [0034]FIG. 1 shows the different scatter characteristics of particles;  
         [0035]    [0035]FIG. 2 shows an optical measuring system for the detection of large particles;  
         [0036]    [0036]FIG. 3 a,b  show an embodiment of the device according to the invention;  
         [0037]    [0037]FIG. 4 a,b  show an additional embodiment of the device according to the invention;  
         [0038]    [0038]FIG. 5 a  shows the operation of the device according to the invention;  
         [0039]    [0039]FIG. 5 b  shows the operation as based on the state of the art;  
         [0040]    [0040]FIG. 6 a,b  show a particularly preferred embodiment of the device according to the invention;  
         [0041]    [0041]FIG. 7 shows an additional measuring system;  
         [0042]    [0042]FIG. 8 shows an additional measuring system;  
         [0043]    [0043]FIG. 9 a,b  show an additional measuring system;  
         [0044]    [0044]FIG. 10 shows a device for the automatic distance adjustment;  
         [0045]    [0045]FIG. 11 a,b  show a notch detection unit; and  
         [0046]    [0046]FIG. 12 a,b  show an alignment unit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]    [0047]FIG. 1 shows the differing scatter characteristics of particles of differing sizes. Incident light  1  falls on a small particle  2  as well as on a large particle  3 . A particle is considered small when its size is much smaller than the wavelength of the incident light  1 . A particle is considered large when its size is comparable to the wavelength of the incident light  1  or even greater. The scatter characteristics of small particles  2  and large particles  3  differ in so far as the small particle  2  scatters in isotropic manner into the space, while the large particle  3  generates strong backscattering.  
         [0048]    [0048]FIG. 2 shows an optical measuring system for the detection of large particles in the form of a measurement block  10  for measuring samples such as wafers  16 . The wafer  16  is supported by a sample holder (wafer chuck)  17  which is caused by a rotation motor  18  to rotate around the axis  27  perpendicularly to the wafer surface  16 . The arrows indicate the possible directions of movement of the optical measurement block  10 . Due to the overlap of the translational movement of the measurement block  10  and the rotational movement of the wafer surface  16 , each spot on the wafer surface  16  can be measured.  
         [0049]    A laser  11  serves as light source. The light beam first traverses a beam-shaping optical system  12  and a diaphragm  13 , before hitting the laser deflection mirror  14  and being reflected onto the sample surface. Not only the incident beam, but also the reflected beam pass the signal deflection mirror  19  and the diaphragm  15  by traversing the respective openings. The isotropic scattered light generated by any possible small particles is blanked out by the diaphragm  15 . Only the back scatter caused by large particles traverses the diaphragm  15  and is deflected at the signal deflection mirror  19  onto the photo multiplier  22 . In order to focus the scattered light and to improve the signal-to-noise ratio, the scattered light traverses the lens  20  and the third diaphragm  21 . The incident beam is not projected onto the sample surface exactly vertically, but rather at a very small angle to the surface normal line so that the reflected beam, as well, can be monitored with the aid of the photo detector  53 . Furthermore, this allows the reflectivity of the sample surface also to be measured.  
         [0050]    Thanks to a particular optical system, especially the diaphragm  15 , the optical measurement block  10  shown in FIG. 2 ensures that predominantly the backscatter of large particles is detected. By using only mirrors and diaphragms upstream from the sample surface and in the proximity of the surface, it can be ensured that parasitic scattering is minimized and that the signal-to-noise ratio is optimized. Another advantage is represented by the compact construction of the measurement block  10  as made possible by the deflection mirrors  14  and  19 .  
         [0051]    [0051]FIG. 3 a  shows a top view of the device according to the invention, while FIG. 3 b  shows a side view. The measurement block  10  is supported by a linear motor  23  and thereby is moved radially outward from the center of the wafer  16 , while the wafer  16  is caused to enter into a quick rotation. The movements of the two drives are balanced in such a way that the entire surface is scanned on a spiral path. The translational movement of the measurement block  10  radially to the wafer edge is particularly important when the measuring spot (in FIG. 3, indicated by the cross) itself is anisotropic, as for example, the beam profile of a diode laser. In this case, the long axis of the measuring spot should be radially aligned.  
         [0052]    [0052]FIGS. 4 a  and  4   b  show a variation of the device according to the invention, wherein the measurement block  10  is supported by a bridge, which in turn is supported by two linear motors  23 . Here again, the measurement block  10  is moved radially outward from the center of the wafer  16  with the aid of the linear motors  23 , while the wafer  16  is caused to enter into a quick rotation.  
         [0053]    [0053]FIG. 5 a  shows once more the operation of the device according to the invention. The measurement block  25  which generates a measuring spot  26  is supported by the linear motor  23  and is moved radially to the rotational axis  27  of the wafer  16 . Only an insignificant amount of more space is needed than for the device schematically shown in FIG. 5 b  which operates on the basis of the record player principle. The measuring head  28 , which is very small, is attached to one end of an arm and is guided over the rotating wafer around the rotational axis  29 . In spite of the comparable spatial needs, the measurement block  25  according to FIG. 5 a  can accommodate a much more complex optical system and/or larger components, such as a blue light laser.  
         [0054]    In a preferred embodiment shown in FIG. 6 a  in a first variation and in FIG. 6 b , in a second variation, it becomes clear how the new space can be used effectively. Two measurement blocks  30  and  32  are disposed next to one another in a fixed relation. The two measurement blocks  30 ,  32  generate elliptical measuring spots  31 ,  33  which are disposed radially to the wafer surface  16  and are moved radially outward with the aid of the two linear motors in FIG. 6 a , and with the aid of the one linear motor  23  in FIG. 6 b.    
         [0055]    This modular model makes the combination of two or more detection channels on one and the same apparatus possible. The results for the two channels for one measuring spot are received one after the other. The information of the two channels has to be brought into the appropriate relation by means of a calibration measurement on a known object or pattern. The second measurement block  32  could be a measurement block for measuring scattered light for small particles, as described below in FIG. 7. It could also, for example, be a dark field microscope. This would bring an additional manner of illuminating the surface into play which would provide additional information, e.g. with regard to the question whether external particles or embedded particles or indentations are being targeted. Furthermore, the second measurement block  32  may also be a spectrometer for measuring layer thickness. Any conceivable combination of measurement blocks may be used in the construction of the devices shown in FIGS. 6 a,b.    
         [0056]    [0056]FIG. 7 shows an example of a second measurement block  52  in the form of a measuring apparatus for measuring the scattered light component of small particles. It differs from the first measuring configuration  10  (FIG. 2) only in so far as an elliptical mirror  34  replaces the diaphragm  15  in the proximity of the surface and collects the light scattered in isotropic manner, projects it onto the deflection mirror  19 , and from there, through the diaphragm  21  onto the photo multiplier  22 . Since the mirror  34  itself is focused already, an additional focusing element, such as the lens  20  in FIG. 2, is not needed. The fact that no refractive optical systems at all are used in the area of the ellipsoid mirror ensures a low degree of parasitic scattered light. This leads to a very good signal-to-noise ratio.  
         [0057]    [0057]FIG. 8 shows an optical measuring system, wherein the laser  11  is located somewhat removed from the actual optical measuring system and its light is projected to the desired area of the measuring system via a light wave guide  54 . The example shown here, therefore, can do without the laser deflection mirror. This permits a slight improvement of the signal-to-noise ratio and/or a slight increase in the intensity of the scattered light signal. Furthermore it is possible that the laser  11  as well as the photo multiplier  22  have to be cooled during operation. This can occur simultaneously without negatively influencing the actual measurement.  
         [0058]    In FIG. 9 a , the optical measuring system is placed in a turret  55 . The turret  55  is schematically shown in a top view in FIG. 9 b . It comprises four channels  56 ,  57 ,  58 ,  59 . Channels  57  and  56  contain measuring systems for a second and a first scattered light measurement. These are shown in cross section in FIG. 9 a . Channel  58  contains a camera. Channel  59  is still unassigned. The optical systems for deflection and collection  14 ,  14 ′,  19 ,  19 ′,  34 ,  34 ′ as well as the diaphragms  13  and  13 ′ of the two channels  57  and  56  all use, as needed, the light source  11  with a beam-shaping optical system  12  of the measurement block  10  as well as the photo multiplier  22  and the diaphragm  21 . In the present case, the two ellipsoid mirrors  34  and  34 ′ have differing dimensions so that differing portions of the scattered light are collected. Instead of the ellipsoid mirror  34 , paraboloid mirrors could also be used.  
         [0059]    [0059]FIG. 10 shows an automated system for distance adjustment. A laser triangulation sensor, consisting of an adjustment laser  35  and a position-sensitive detector  36 , is placed at the exit of the optical apparatus of the measurement block. The adjustment laser beam  37  is projected onto the wafer surface at a slant, e.g. less than 45 degrees. After reflection, the beam hits the position-sensitive detector  36 . The position of the laser beam  37  on the position-sensitive detector  36  is a measure for the distance between the beam spot and the measurement block. The height of the measurement block can be adjusted with the aid of a linear motor  38 . When adjusting the working spot of the scattered light detector, the optimal working distance is found by optimizing the scattered light signal. Subsequently, the associated incidence point of the adjusting laser beam  37  on the position-sensitive detector  36  is determined and provided to a computer or an electronic control system  41  as reference point. If the working distance changes during the translational movement of the measurement block, this is detected by the position-sensitive detector and a corresponding control signal is generated. Based on this control signal, the distance of the measurement block is automatically corrected with the aid of the height adjusting motor  38  and the motor driver  40 .  
         [0060]    [0060]FIG. 11  a  schematically represents the notch detection system  42 . The wafer  45  has a notch  48  toward which a laser  43  is directed. The laser beam  44  used here is band-shaped and directed toward the edge of the wafer  45 . The reflected beam is marked as  47  and the associated detector in the form of a photo diode, as  48 . When during the rotation of the wafer  45 , one part of the band-shaped laser beam  44  is reflected from the wafer surface, the intensity of the reflected beam  47  drops precipitously as soon as the beam reaches the notch  45 , as shown in FIG. 11 b . A signal is received which is periodic over 360 degrees and whose period and phase length provide information about the position of the center of the laser in relation to the rotational axis of the wafer table. A deep cut generated by scanning the notch  46  is superimposed on the periodic signal. From this signal and the associated signal of an angle sensor which is not shown, the position of the notch and thus also the orientation of the wafer can be deduced.  
         [0061]    [0061]FIGS. 12 a  and  12   b  show the wafer alignment mechanism  49  (notch aligner) associated with the notch detector system  42 . The alignment mechanism  49  is centered within the cup-shaped sample holder  17 . The wafer is resting thereon in an elevated state. By rotating the wafer alignment mechanism based on the information provided by the notch detection system, the wafer  16  can be aligned correspondingly and subsequently again be placed on the supporting edge  51 . The wafer lifter  50  can at any time consist of a simple lifting system and comprise, for example, a plunger-type coil drive or a stepping motor. The alignment mechanism and the mechanism for detecting notches are actively connected with one another by the fact that the drive mechanism of the sample lifting table is connected to a control mechanism which in turn is connected to the notch detection mechanism.  
         [0062]    List Reference Characters  
         [0063]    [0063] 1  incident light  
         [0064]    [0064] 2  small particle  
         [0065]    [0065] 3  large particle  
         [0066]    [0066] 4  scatter ellipse  
         [0067]    [0067] 10  optical measurement block  
         [0068]    [0068] 11  laser  
         [0069]    [0069] 12  optical system for shaping the beam  
         [0070]    [0070] 13   13 ′ diaphragm 1  
         [0071]    [0071] 14   14 ′ laser deflection mirror  
         [0072]    [0072] 15  diaphragm 2  
         [0073]    [0073] 16  sample  
         [0074]    [0074] 17  sample holder  
         [0075]    [0075] 18  rotation motor  
         [0076]    [0076] 19   19 ′ signal deflection mirror  
         [0077]    [0077] 20  lens  
         [0078]    [0078] 21  diaphragm 3  
         [0079]    [0079] 22  photo multiplier  
         [0080]    [0080] 23   a,b  linear motor  
         [0081]    [0081] 24 carrier plate  
         [0082]    [0082] 25  measurement block  
         [0083]    [0083] 26  measuring spot  
         [0084]    [0084] 27  rotational axis  
         [0085]    [0085] 28  measuring head  
         [0086]    [0086] 29  rotational axis measuring head  
         [0087]    [0087] 30  measurement block 1  
         [0088]    [0088] 31  measuring spot 1  
         [0089]    [0089] 32  measurement block 2  
         [0090]    [0090] 33  measuring spot 2  
         [0091]    [0091] 34   34 ′ ellipsoid mirror  
         [0092]    [0092] 35  adjustment laser  
         [0093]    [0093] 36  position-sensitive detector  
         [0094]    [0094] 37  adjustment laser beam  
         [0095]    [0095] 38  adjustment motor  
         [0096]    [0096] 39  linear motor  
         [0097]    [0097] 40  motor driver  
         [0098]    [0098] 41  electronic control system  
         [0099]    [0099] 42  notch detection mechanism  
         [0100]    [0100] 43  laser  
         [0101]    [0101] 44  band-shaped laser beam  
         [0102]    [0102] 45  wafer  
         [0103]    [0103] 46  notch  
         [0104]    [0104] 47  reflected beam  
         [0105]    [0105] 48  photo diode  
         [0106]    [0106] 49  alignment mechanism  
         [0107]    [0107] 50  sample lifting table  
         [0108]    [0108] 51  ring-shaped supporting edge  
         [0109]    [0109] 52  optical measurement block  
         [0110]    [0110] 53  photo detector  
         [0111]    [0111] 54  light wave guide  
         [0112]    [0112] 55  turret  
         [0113]    [0113] 56  scattered light channel No. 1  
         [0114]    [0114] 57  scattered light channel No. 2  
         [0115]    [0115] 58  camera channel  
         [0116]    [0116] 59  free channel