Abstract:
A method and arrangement for calibrating a laser processing machine for processing workpieces. The present invention ensures a fast and reproducible method and arrangement for calibrating a laser processing machine by measuring a position of a predetermined test pattern on a test plate while taking into account any imaging errors caused by the deflection unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method and an arrangement for calibrating a laser processing machine for processing workpieces. 
     2. Description of the Related Art 
     The processing of workpieces using lasers is acquiring increasing importance principally in the field of micromachining, for example in laser direct structuring, in laser drilling, in laser fusion, in laser soldering, in laser inscription and in laser cutting. In laser drilling and in the structuring of wiring components (printed circuit boards and multichip modules), it is possible to produce holes and conductor structures having structure dimensions of less than 50 μm. Prototypes having conductor track structures of 20 μm have already been displayed. Accuracy is extremely important with these structure finenesses. The goal is to produce structure dimensions of less 10 μm. Inaccuracy in successive processing steps, for example, drilling and structuring on preformed injection-molding parts or substrates, needs to be alleviated. This is because this inaccuracy to a large extent, determines the structure&#39;s size; for example, for reliable isolation of conductor tracks that run parallel, it would be necessary to take into account large tolerances. This leads to large spacings between the conductor tracks and thus enlarge the structure&#39;s size. 
     German document DE 44 37 284 discloses a method for calibrating a controller for deflecting a laser beam, in which a light-sensitive medium is irradiated with a laser beam at predetermined positions for the purpose of generating a test image and afterwards, in a separate apparatus, digitized individual images of image excerpts from the test image are generated and from them correction data are determined for the controller for deflecting the laser beam. The separate measurement of the test image is time-consuming and a separate apparatus is required for the measurement; moreover, only the entire laser beam arrangement is calibrated by means of this method, a detailed calibration of individual components is not possible. 
     U.S. Pat. No. 4,584,455 discloses a machine for processing workpieces by means of laser beams, which provides, in addition to a processing laser beam, a laser beam superposed therewith in the visible region, the position of which on a workpiece is measured with the aid of a camera. This measured position is compared with a desired position and the processing laser beam is controlled in such a way that the difference between the desired position and the measured position is taken into account. In this case, the camera only has a field of view which encompasses part of the workpiece, with the result that deviations which are mirrored in other parts of the workpiece cannot be taken into account. 
     SUMMARY OF THE INVENTION 
     The object of the invention, therefore, is to specify a fast and reproducible method and an arrangement for calibrating a laser processing machine which ensures highly accurate processing of workpieces in the entire workpiece region. 
     This object is achieved according to the present invention in an arrangement for a laser processing calibration, including a laser source, a device for shaping the laser beam, a deflection device for the laser beam, a camera for visually observing the workpiece and an objective for focusing a deflected laser beam on to the workpiece. 
     This arrangement is operated by first positioning in front of the objective a calibration plate and recording its image. This image is generated by the deflection unit moving the objective and the field of view of the camera over the calibration plate. This image is supplied to a control unit which determines any imaging errors that are caused by the deflection unit. 
     Subsequently, the calibration plate is removed and a test plate is placed in front of the objective. The laser beam then writes attest pattern onto the test plate with a predetermined drive of the deflection unit. The position of the test pattern is measured and the optical offset of the laser source is determined based on the measured position of the test pattern, taking in to account any imaging errors that were previously determined with patent claim  6 . 
     The optical imaging according to the present invention, is characterized by two mutually separate, successive method steps, first the deflection device for the laser beam and the objective are measured by a calibration plate, instead of the workpiece. The calibration plate is positioned in front of the objective and the image of the calibration plate, is being generated by the objective and the deflection device is measured by a camera. The field of view of the camera is moved over the calibration plate under the control of the deflection device. Accordingly, the entire region which can be occupied by a workpiece is calibrated. Because the deflection device (unit) and the objective always remain together in the beam path, the imaging error caused by these two units can then be analyzed in the camera, and stored, then taken into account during the subsequent processing of workpieces. In the second step, the components which generate the laser beam are characterized. As such, the laser beam, which is generated by a laser source and is shaped by a device for shaping the laser beam, writes a predetermined pattern (test marking) onto a test plate via the deflection device. This test marking is subsequently measured and by taking into account the previously determined imaging errors of the deflection device and the objective, the optical offset that is caused by the laser source and the device for shaping the laser beam, is determined. Here, the optical offset is the difference between a predetermined desired position and the actual position of the test marking. This optical offset is also stored and taken into account during subsequent processing of workpieces. The method operates without a special device for measuring the test markings if the camera measures the test plate via the deflection device and the objective. 
     Another aspect of the invention is the optical construction is configured particularly advantageously if a mirror, transparent to the laser light is placed in the beam path between the device for shaping the laser beam and the deflection unit the such mirror that it reflects the light for the measurement of the calibration plate and/or the test plate into the camera. 
     The influence of workpieces on the optical imaging is advantageously taken into account by the camera that records the reference points arranged on the workpiece. Later the measured influences are stored and taken into account during subsequent processing. 
     Thus, particularly in the case of structuring of non-planar areas of workpieces, the influence of the lack of planarity is taken into account by the data provided beforehand, with the result that even curved substrates, for example, can be written on by the laser processing machine. 
     Another aspect of the invention provides for processing curved workpieces, in which the focal point of the imaging by the objective is adapted to the corresponding curvature of the workpiece by a device for modulating the optical path. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the schematic construction of a laser processing machine for the structuring and/or measurement of a workpiece or of a test plate, 
     FIG. 2 shows the schematic construction of a laser processing machine for the measurement of a calibration plate, and 
     FIG. 3 shows the schematic construction of a laser processing machine for the structuring of a workpiece. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is explained in more detail using exemplary embodiments as shown in FIG. 1, FIG.  2  and FIG.  3 . 
     A laser processing machine  1  is illustrated in which a structure is written into a workpiece  14  by means of a laser beam  2 . The structure may be produced by drilling, fusion, soldering or cutting. The laser beam  2  is generated by a laser source  4  and expanded by a device  5  for expanding the laser beam  2 . In order to modulate the optical path of the laser beam  2 , a device for modulation  6  is subsequently arranged in the beam path before the laser beam is imaged onto the workpiece  14 —curved in this example—by a deflection device  7  and an objective  8 . In one embodiment, two mirrors (not illustrated), which are arranged perpendicularly to one another are provided in the deflection device  7 . The mirrors move the laser beam over the area of the workpiece  14 , so that the entire area of the workpiece  14  can be processed by the deflected laser beam  9 . Here, the device  5  expands the laser beam and the device  6  modulates the optical path of the laser beam  2 , and together they form a device for shaping the laser beam  2 . The device  6  for modulating the optical path of the laser beam  2  is controlled by a control device  13 . Consequently, the focal point of the laser beam  2 , downstream of the objective  8 , is shifted in the Z-direction, i.e. in the propagation direction of the laser beam  2 . Through this control, the focal point of the laser beam  2  can be adapted to a curved structure of a workpiece  14 . In order to measure the imaging errors of the deflection device  7  and of the objective  8 , a partially transparent mirror  10  is arranged in the beam path between the device  6  for modulating the optical path and the deflection device  7 . In one embodiment, this mirror  10  is transparent to the laser beam  2  and reflects the light coming through the objective  8  and the deflection device  7  from the direction of the workpiece  14  into a camera  11 , which is likewise connected to the control device  13  for evaluation purposes. Another embodiment which is also possible but is not illustrated is one in which the mirror  10  is transparent to the light from the direction of the workpiece  14  and reflects the laser beam  2  in the direction of the workpiece  14 . 
     The method for calibrating the laser processing machine  1  proceeds, as follows. As is illustrated in FIG. 2, first a calibration plate  3  with patterns (for example a grid with a predetermined grid spacing) situated on the calibration plate  3  is imaged onto the camera  11  through the objective  8  and the deflection device  7  via the mirror  10 . In this case, the control unit  13  controls the deflection device  7  in such a way that the beam path  12  of the image of the calibration plate  3  is moved over the entire calibration plate  3 . Using the markings of the calibration plate  3 , it is possible to characterize the beam path and thus the imaging errors of the objective  8 , of the deflection device  7 , of the mirror  10  and of the camera  11 . The imaging errors that have been identified are stored in the control device  13  and are compensated for during the subsequent processing of workpieces  3 . 
     In the second step of the method, instead of the calibration plate  3 , a test plate  15 , for example made of aluminum, is positioned in front of the objective. The laser beam  2  then writes a predetermined pattern (test marking) onto the test plate  15  by means of a predetermined control of the deflection device  7 . The position of this test marking is then imaged via the mirror  10  onto the camera  11  via the beam path in accordance with the image of the calibration plate. In the control device  13 , taking account of the imaging errors previously determined, the optical offset which occurred during the structuring of the test marking is determined, stored and taken into account during the processing of workpieces. 
     In order to process curved workpieces  14 , as illustrated in FIG. 3, the influence of the geometrical shape and/or of the optical imaging properties of the workpieces  14  is additionally being determined. As such, reference points which are already present on the workpiece  14  are imaged onto the camera  11  via the objective  8  and the deflection device  7  and also the mirror  10 . In the control device  13 . The workpiece influence is then determined A from the imaging errors previously determined and the optical offset. The method can then subsequently be used for identical workpieces in order to control the device for modulating the optical path  6  such that that the focal point of the laser beam  2  is oriented optimally to the surface of the workpiece  14 . 
     This method and the arrangement also take into account, among other things the errors caused by the driving of the motors for the mirrors of deflection device  7 . 
     Although other modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.