Abstract:
A position measuring device includes a measuring standard and a scanning unit, which are arranged in a manner that allows them to move relative to each other in a measuring direction. The measuring standard includes a graduation, which is scannable by the scanning unit in order to generate positional signals. The scanning unit includes an illumination unit and a detector unit for generating positional signals, the illumination unit being able to emit light in the direction of the graduation and the detector unit being able to detect light modulated by the graduation. The detector unit includes a circuit board and a sensor unit, which is arranged as a semiconductor chip. At least two photodetectors are provided on a front side of the sensor unit facing the graduation, and the electrical connections of the sensor unit are routed to contact surfaces on its rear side by metallic vias. The sensor unit is connected via the contact surfaces to corresponding contact surfaces on the circuit board.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority to Application No. 15179014.4, filed in the European Patent Office on Jul. 30, 2015, which is expressly incorporated herein in its entirety by reference thereto. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a position measuring device, for example, of the type used in automation systems for the purpose of measuring positions and/or angles. 
       BACKGROUND INFORMATION 
       [0003]    Position measuring devices are widely used in tool machines and automation systems, in particular. They are employed for determining the relative position of two objects that can move relative to each other. A basic distinction is made between linear and angle measuring devices. Linear measuring devices, for example, are used to determine the relative position of two machine components of a tool machine that are able to move with respect to each other. A measuring standard, e.g., in the form of a straight scale on which a graduation is provided, is connected to one of the two objects for this purpose, and a scanning unit is connected to the other, so that position-dependent scanning signals, which can be used to determine the degree of movement of the two objects with respect to each other along the movement direction, can be obtained by scanning the graduation. 
         [0004]    Angle-measuring devices, also referred to as rotary encoders, are designed according to the same principle. Instead of the scale, however, a circular disk is used as a measuring standard in such a case, on which the graduation is applied concentrically with the pivot point. The disk is connected in rotationally fixed manner to a shaft to be measured, while the scanning unit is fixedly mounted with respect thereto. 
         [0005]    The graduation may include one or more graduation track(s), a distinction being made between incremental and absolute encoded graduation tracks. Incremental graduation tracks are made up of evenly spaced graduation elements, which, given a uniform relative movement between measuring standard and scanning unit, provide substantially sinusoidal scanning signals when scanned. In this case, the travel is ascertained by counting the signal periods or fractions of signal periods. An absolute travel determination requires the specification of a reference point, for which a reference mark is provided, for example, which is likewise detected by the scanning unit. The scanning signals from absolute encoded graduation tracks, on the other hand, are able to be used for a direct detection of the absolute position, inasmuch as they are encoded in multiple adjacently located graduation tracks (e.g., Gray code, BCD code, etc.) or are serially encoded in one graduation track (e.g., chain code, PRC). In addition, there are also graduations that include both incremental and absolute graduation tracks. 
         [0006]    One widely used operating principle in the case of position measuring devices is optical scanning. In this case, directed light that is emitted by a light source is used for imaging the graduation mounted on a measuring standard on a number of photodetectors. The measuring standard is disposed in the optical path of the light in a manner that allows it to move, and modulates the light when the graduation is moved in relation to the light source and the photodetectors. The position, or the change in position, is ascertained by analyzing the output signals of the photodetectors. The graduation may consist of regions having different optical characteristics such as transparent/opaque. 
         [0007]    The photodetectors required for the scanning are usually situated together on a semiconductor chip, which is mounted directly on a circuit board using chip-on-board technology (COB). In the process, the rear side of the chip is first glued to the circuit board and the contact surfaces on its topside are then connected to the circuit board using wire bonding. In addition to the photodetectors, at least parts of the evaluation electronics for the scanning signals are frequently situated on the semiconductor chip as well. 
         [0008]      FIG. 3  schematically illustrates the components that are relevant in connection with the position measurement of a conventional position measuring device, i.e., a scanning unit  10  and a measuring standard  40 , which includes a graduation  41 . 
         [0009]    Scanning unit  10  and measuring standard  40  are situated in a manner that allows them to move relative to each other in a measuring direction X. For example, if the position measuring device is a device for a linear measurement (linear position measuring device), then measuring standard  40  is provided in the form of a straight scale, which is mounted on a first machine component and on which a graduation  41  is present in measuring direction X. Scanning unit  10  is fastened to a second machine component, such that when the first machine component executes a movement in measuring direction X in relation to the second machine component, scanning unit  10  is moved along the scale and scans graduation  41  in the process. The scanning results in scanning signals, from which position values are generated in the course of further processing. 
         [0010]    In the case of rotary position measuring devices (rotary transducers or angle-measuring devices), measuring standard  40 , for example, is a circular disk, which is connected to a shaft whose angular position is to be measured in a rotatably fixed manner. Graduation  41  is disposed concentrically with the rotational axis of the shaft. The scanning unit, on the other hand, is firmly mounted such that the graduation can be scanned when the shaft to be measured executes a rotation, and position values, in this case, angle values, are in turn able to be generated from the scanning signals. 
         [0011]    Scanning unit  10  includes an illumination unit  20  and a detector unit  530 . The operating principle on which the position measuring device is based is optical transmitted-light scanning. This means that graduation  41  on measuring standard  40  is positioned between illumination unit  20  and detector unit  530 . Illumination unit  20  emits light in the direction of graduation  41 . Graduation  41  includes light-transmitting (transparent) and opaque regions, which modulate the light of illumination unit  20 . The modulated light is imaged on detector unit  530 , which then generates positional signals from it. 
         [0012]    Optical transmitted-light scanning uses light that is collimated. To generate collimated light, illumination unit  20 , for instance, may include a light source  21  which radiates divergent light that is collimated by a collimator  22 . 
         [0013]    Measuring standard  40  may be made from a transparent material, such as glass. In this case, graduation  41  is formed by opaque regions, for instance, made of metal that is applied on the measuring standard. Chromium, for example, is especially suitable for this purpose. However, there are also measuring standards  40  which are made from an opaque material, such as metal. Graduation  41  may include a sequence of metal webs and openings in measuring standard  40 . 
         [0014]    Detector unit  530  includes a circuit board  531 , a sensor unit  532 , and, in particular when small graduation periods of graduation  41  are to be scanned, a scanning plate  533 . 
         [0015]    Circuit board  531  functions, for example, as a carrier for sensor unit  532 , which constitutes the central component of detector unit  530 . In addition to supplementary electronic circuits for sensor unit  532 , input/output interfaces in the form of plug-and-socket connectors, for instance, are provided on circuit board  531 . 
         [0016]    Sensor unit  532  is arranged as a semiconductor chip. It has a front side and a rear side, the side facing circuit board  531  forming the rear side, and the side facing graduation  41  forming the front side of sensor unit  532 . Situated on the front side of sensor unit  532  are a number of photodetectors  535 , which are used for generating positional signals by detecting the light of illumination unit  20 , which is modulated by graduation  41  and possibly scanning plate  533 . The front side of sensor unit  532  is preferably aligned in parallel with the plane in which graduation  41  is situated. In addition to photodetectors  535 , sensor unit  532  includes additional components for processing the positional signals, possibly even including the generation of position values from the positional signals. Moreover, the sensor unit may include a multitude of additional circuit blocks, such as fault correction, circuits for ensuring functional safety, and a digital interface for communicating with sequential electronics. A semiconductor chip which includes both photodetectors and components for signal processing is referred to as an opto-ASIC. 
         [0017]    The rear side of sensor unit  532  is connected to circuit board  531 , the connection being created by bonding, for example. For the electrical connection of the circuit components of sensor unit  532  to circuit board  531 , the front side of sensor unit  532  and the side of circuit board  531  facing sensor unit  532  are provided with corresponding contact surfaces, which are connected to each other in a conventional manner by wire bonding using bonding wires  537 . An encapsulating material  538  protects bonding wires  537  from mechanical effects. 
         [0018]    Wire bonding is a complex manufacturing process, both in terms of the purchasing costs of the required manufacturing systems (wire bonding) and the additional time required for the wire bonding process, the encapsulation of the bonding wires and curing of the encapsulation mass. 
         [0019]    The result of this packaging is a dam around the photodetectors, which projects by a height c in relation to the front side of sensor unit  532  (i.e., the surface of the semiconductor chip on which the photodetectors are situated). Height c amounts at least to 0.5 mm in conventional production methods. Taking into account manufacturing tolerances attributable in particular to fluctuations in the thickness of the encapsulation mass  538 , a scanning distance d of between 0.55 mm and 0.7 mm must be maintained in order to be able to ensure a safe operation, this scanning distance d being defined by the clearance between the front side of sensor unit  32  (the surface of the semiconductor chip on which the photodetectors are situated) and the surface of measuring standard  40  (which is to be equated with the surface of graduation  41  in the example illustrated). 
         [0020]    Scanning plate  533  is required in particular when scanning small graduation periods. It is situated between photodetectors  535  on the front side of sensor unit  532  and graduation  41 . Similar to measuring standard  40 , it has a graduation structure that optimizes the imaging of graduation  41  to photodetectors  535 . 
         [0021]    German Published Patent Application No. 198 55 307 describes a scanning unit in which the sensor unit is contacted with the aid of wire bonding. A cover element, the thickness of which is even greater than that of the contacting region, is additionally situated above the radiation-sensitive region. 
         [0022]    Any dirt that accumulates in the optical path of the light has a detrimental effect on the function in the optical scanning principle. This applies to liquid contaminants, in particular. It occurs in the form of fluid droplets, which consist of lubricants, condensed water, coolant, etc., for example. Not only does this reduce the analyzable light quantity that impinges upon the photodetectors, but it also disperses the light, so that it is distributed to multiple photodetectors situated next to each other. 
         [0023]    The dam of encapsulating material  538  has negative effects with regard to dirt deposits, since it promotes the collection of dirt deposits within the dam, depending on the position. 
         [0024]    To prevent problems caused by dirt deposits, special attention is paid during the construction process of position measuring devices featuring optical scanning to ensure that the introduction of dirt particles, whether in solid or liquid form, is avoided. For example, this may be accomplished through the material selection or special constructive measures. 
         [0025]    However, it is difficult to avoid contamination altogether. Therefore, an attempt is made to compensate for the effect of dirt particles by creating redundancy in the scanning or by complex signal processing of the scanning signals. 
         [0026]    Despite all of these measures, dirt particles, especially in liquid form, are a potential source of failure in optical position measuring devices. 
       SUMMARY 
       [0027]    Example embodiments of the present invention provide a position measuring device which is easy to manufacture and features low sensitivity with regard to soiling. 
         [0028]    According to an example embodiment of the present invention, a position measuring device includes a measuring standard and a scanning unit, which are situated in a manner that allows them to move relative to each other in a measuring direction. The measuring standard includes a graduation, which can be scanned by the scanning unit in order to generate positional signals. The scanning unit includes an illumination unit and a detector unit, the illumination unit being able to emit light in the direction of the graduation and the detector unit being able to detect light modulated by the graduation, the detector unit including a circuit board and a sensor unit, which is arranged as a semiconductor chip. At least two photodetectors are provided on a front side of the sensor unit facing the graduation, and the electrical connections of the sensor unit are routed to contact surfaces on its rear side via metallic vias. The sensor unit is connected to corresponding contact surfaces on the circuit board via the contact surfaces. 
         [0029]    Further features and aspects of example embodiments of the present invention are described in more detail below with reference to the appended Figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  schematically illustrates a position measuring device according to an example embodiment of the present invention. 
           [0031]      FIG. 2 a    schematically illustrates the dispersion of the light caused by a fluid droplet at a large scanning distance. 
           [0032]      FIG. 2 b    schematically illustrates the dispersion of the light caused by a fluid droplet at a short scanning distance. 
           [0033]      FIG. 3  schematically illustrates a conventional position measuring device. 
           [0034]      FIG. 4 a    schematically illustrates a modification of the detector unit illustrated in  FIG. 1 . 
           [0035]      FIG. 4 b    schematically illustrates a variant of the detector unit illustrated in  FIG. 4   a.    
       
    
    
     DETAILED DESCRIPTION 
       [0036]      FIG. 1  schematically illustrates the components of a position measuring device according to an example embodiment of the present invention that are relevant in the context of the position measurement. The basic configuration corresponds to the position measuring device described above with reference to  FIG. 3 . Components that were already described in connection with  FIG. 3  bear the same reference numerals in  FIG. 1 . 
         [0037]    The position measuring device includes a scanning unit  10  and a measuring standard  40  on which a graduation  41  is provided. Scanning unit  10  includes an illumination unit  20 , which may be conventional, and a detector unit  30 . 
         [0038]    In detector unit  30 , a sensor unit  32 , which is arranged as a semiconductor chip, is electrically connected to circuit board  31  not by wire bonding, but rather by routing the connections of the electric circuit in the semiconductor chip via through-connections, or vias,  36  through the semiconductor substrate from which the semiconductor chip is made, to contact surfaces  33  on the rear side of the chip, and by establishing the electrical connection to corresponding contact surfaces  34  of circuit board  31  via a soldering method. It is possible, for example, to first apply small metal balls  37  to contact surfaces  33  of the semiconductor chip, which are then connected to corresponding contact surfaces  34  on circuit board  31  by reflow soldering. The labor-intensive wire bonding process is therefore replaced by a standard soldering process. This makes it possible to place sensor unit  32  on circuit board  31  together with the other components at the time when the circuit board  31  is fitted with components, whereupon the soldering is performed. 
         [0039]    The front side of a sensor unit  32  arranged in this manner has a planar top surface, which makes it more difficult for dirt particles to adhere than in the case of a conventional sensor unit  532 . 
         [0040]    Provided on the front side of sensor unit  32  are at least two photodetectors  35 , which detect the light of illumination unit  20  modulated by graduation  41 . The actual number of photodetectors  35  depends on the configuration of graduation  41 . If graduation  41  has more than one graduation track (for instance an incremental one and one or more absolute encoded graduation track(s) in parallel therewith), then a suitable number of photodetectors  35  is situated at the positions assigned to the graduation tracks transversely to measuring direction X. The positional signals obtained in this manner are processed by a signal processing circuit, which, for example, is partially or also completely integrated in sensor unit  32 , and are forwarded to sequential electronics. As mentioned above, a semiconductor chip that includes both photodetectors and a signal processing electronics system is an opto-ASIC. 
         [0041]    Circuit board  31  may be made from fiber glass-reinforced epoxy resin (epoxy), but it is also possible to use ceramic materials such as aluminum oxide, for example. 
         [0042]    Vias  36  are placed vertically and made from metal, so that they have similar electrical characteristics as the bonding wires. Contact surfaces  33  on the semiconductor chip of sensor unit  32  need not necessarily be situated directly above vias  36 . Instead, they may be placed at an offset from each other and be interconnected via a circuit track  38  (redistribution layer). 
         [0043]    If the semiconductor chip is based on a silicon substrate, as is currently often the case, silicon vias or through-silicon vias (TSV) may be provided. Semiconductor chips provided with silicon vias are currently used predominantly in image sensors for digital photography because the usable light-sensitive surface of the image points (pixels) can be enlarged since the contact surfaces are shifted to the rear side of the chip. In addition, semiconductor chips having silicon vias are employed to produce 3D-integrated circuits (3D-IC), since this packaging technology is suitable for stacking and connecting multiple chips on top of one another. 
         [0044]    Shifting contact surfaces  33  from the front side of sensor unit  32  to the rear side of sensor unit  32  makes it possible to considerably reduce scanning distance d in comparison with conventional systems, scanning distance d being defined as the distance between the front side of sensor unit  32  (i.e., the surface of the semiconductor chip on which the photodetectors are situated) and the surface of graduation  41  (which in the example illustrated is to be equated with the surface of measuring standard  40  due to the limited thickness of the graduation markings). Because bonding wires  537  and encapsulation mass  538  are omitted (that is to say, by completely eliminating the dam having height c), scanning distance c depends solely on structure tolerances (e.g., the parallelism of the surface of sensor unit  32  in relation to the surface of measuring standard  40 ) and the mechanical guidance of scanning unit  10  or sensor unit  32  in relation to measuring standard  40 . Values of less than 0.55 mm can be selected for scanning distance d in all cases, and given a mechanical design of sufficient precision, it is even possible to achieve values of less than 0.1 mm and down to 0.03 mm for scanning distance d. 
         [0045]    Further electronic components that are required in addition to sensor unit  32 , as well as plug-and-socket connectors, etc., may be provided on the rear side of circuit board  31  or in areas in which circuit board  31  and measuring standard  40  do not overlap in the assembled state. 
         [0046]    Another advantage that results when using a sensor unit  32  described herein is that no scanning plate will be necessary at a small scanning distance d. This reduces the number of surfaces in the optical path of the light to be detected whose optical characteristics can be adversely affected by contamination. 
         [0047]      FIGS. 2 a  and 2 b    illustrate the influence of scanning distance d on the contamination sensitivity of optical scanning in the presence of contamination in the form of fluid droplets. Here, it is assumed that a fluid droplet  60  is situated on graduation  41 . Fluid droplets have a dispersing effect on light beams. This causes a diffuseness in the imaging of graduation  41  on photodetectors  35  (a central photodetector  35 . 1  and two adjacent photodetectors  35 . 2 ,  35 . 3  are illustrated in each case), the diffuseness becoming more pronounced as scanning distance d grows larger. 
         [0048]    As illustrated in  FIG. 2 a   , a large scanning distance d causes a considerable portion of the dispersed light to impinge upon adjacent photodetectors  35 . 2 ,  35 . 3 . This reduces a positional signal detected by central photodetector  35 . 1 , while adjacent photodetectors  35 . 2 ,  35 . 3  detect faulty positional signals. On the whole, the reliability of the position measurement is affected in an adverse manner. 
         [0049]    Reducing scanning distance d, as illustrated in  FIG. 2 b   , has the result that a large portion of the light to be detected impinges upon central photodetector  35 . 1  despite the dispersion of the light, and that only a negligible portion of the dispersed light reaches adjacent photodetectors  35 . 2 ,  35 . 3 . This increases the reliability of the position measurement significantly. 
         [0050]    Moreover, a small scanning distance d causes larger fluid droplets to be compressed between the front side of sensor unit  32  and the surface of measuring standard  40  and to thereby form a uniform fluid film, which considerably reduces a dispersion of the light. This effect is noticeable when scanning distance d is reduced to less than 0.3 mm. 
         [0051]    As illustrated in  FIG. 1 , the cavity underneath sensor unit  32  that remains after the solder process is able to be filled with an elastic, temperature-stable plastic material (underfiller  50 ) in an underfill process. Apart from increasing the mechanical stability of the structure and compensating for different coefficients of thermal expansion of the employed materials, this also effectively prevents contamination, in particular fluid droplets  60 , from reaching the region of the solder connections between sensor unit  32  and circuit board  31 . Electrical malfunctions, caused by leakage currents due to an ohmic connection of contacts with conductive fluids, and damage to the solder points by corrosion are able to be prevented in this manner. 
         [0052]    It is furthermore advantageous to provide a side guard  51  of a plastic material around sensor unit  32 , which protects the lateral surfaces of sensor unit  32  from mechanical damage, for example, caused by larger dirt particles. Side guard  51  may end flush with the edge of sensor unit  32  that is associated with the front side. 
         [0053]    The same material may be used for side guard  51  and underfiller  50 . 
         [0054]      FIG. 4 a    shows a modification of detector unit  30  in relation to  FIG. 1 , in which the front side of sensor unit  32  is protected from damage by a protective window  52  made of a suitable transparent material (glass, etc.). The connection between sensor unit  32  and protective window  52  may be created by surface bonding or by optical contact bonding. 
         [0055]    This configuration is particularly easy to produce if in the production of a wafer having a multitude of sensor units  32 , the wafer is already connected to a disk of the material of protective window  52  before individual sensor units  32  are sawed out. By being sawed out jointly, sensor unit  32  and protective window  52  have the same surface area. In this particular case, side guard  51  may be extended to a height such that it projects beyond the side surfaces of sensor unit  32  and at least partially covers the side surfaces of protective window  52 . In this manner, sensor unit  32  will be protected from mechanical damage on all sides in the fully assembled state. 
         [0056]    An even more robust configuration of detector unit  30  is obtained if the surface area of protective window  52  is larger than the surface area of sensor unit  32 , as illustrated in  FIG. 4 b   . In this case, side guard  51  connects circuit board  31  to the rear side of protective window  52  facing sensor unit  32 . This arrangement, as well, is easy to produce if the wafer with sensor units  32  and the disk of the material of protective window  52  are already connected to each other and the regions between sensor units  32  are exposed down to the disk of the material of protective window  52  by etching in an etching step before sensor units  32  are cut out. 
         [0057]    Because of protective window  52 , the minimally achievable scanning distance d is enlarged. Regardless, the thickness of the protective window may be selected such that it is less than height c specified by the wire bonding (approximately 0.5 mm), so that a smaller scanning distance d is achievable than the value of 0.55 mm specified in conventional systems. The thickness of the protective window may be in a range between 0.2 mm and 0.4 mm, the lower limit being able to be reduced even further by greater effort in the production. 
         [0058]    In the event that a scanning plate is required because of greater scanning distance d in conjunction with fine graduation periods of graduation  41 , then a graduation structure that fulfills the function of a scanning plate may be mounted on the front side of protective window  52  facing graduation  41 , without any increase in the number of surfaces exposed to contamination. 
         [0059]    As an alternative, the front side of sensor unit  32  may be protected from mechanical damage by a clear lacquer or a transparent encapsulation compound.