IL94405A - System for high throughput non-contact measurements of electrical circuits - Google Patents

Description

94405/2 System for high throughput non-contact measurements of electrical circuits Orbotech Limited C. 80659 - 1 - 94405/3 FIELD OF THE INVENTION This invention is related to a method and system for measuring the height of a feature relative to a substrate. It finds particular application in the inspection of printed circuit boards.

BACKGROUND OF THE INVENTION The field of automatic optical inspection of printed circuit boards has become in recent years a well established discipline. Most of the techniques employed today in the high speed automatic inspection machines currently available provide only two-dimensional information associated with an area of a substrate containing a plurality of conductive traps. Typically, prior art systems employ reflective or fluorescent light to differentiate between the substrate and the current-carrying conductors. Whilst this permits deficiencies in the surface of a conductive track to be observed, it provides no indication of the height of the conductive track relative to the substrate. However, with ever increasing circuit complexity and data rate speed which require tightly controlled impedances of the conductive tracks, defects in the thickness of the tracks must be detected as well. - 2 - 94405/3 U.S. Patent No. 4,650,333 (Crabb et al.) discloses a non-contact system for detecting printed circuit wiring defects and for measuring circuit feature height relative to a substrate. The system employs a laser for illuminating the substrate and circuit features and a scanner for both instantaneously receiving energy reflected from the substrate and circuit features and for generating a signal in response to the reflected energy, the signal being adapted to vary with the intensity of the reflected energy. An analyzer connected to the scanner correlates the generated signal to a measurement representative of the height of the circuit features relative to the substrate. The line CCD imaging device is employed for imaging a slit of light generated by the laser through a suitable optical lens assembly. If required, the line CCD may be replaced by an array of CCDs polled so that each CCD device is individually accessed and the data therefrom is appropriately organized for subse-quent analysis.

In either case, the scan data stored in either the line CCD or in the CCD array permits height measurement of a circuit feature to be determined by correlating the CCD data with the spatial location of the circuit feature. Thus, the actual location of a circuit feature must be determined by processing the CCD data and, since this is relatively time-consuming, the process is slow and not suited for high speed on-line height measurement.

U.S. Patent No. 4,796,997 (Svetkoff et al.) discloses a method and system for high-speed, three-dimensional imaging of an object at a vision station including a flying spot laser scanner in an optical depth sensing system. The system includes an elaborate optical arrangement which splits the image signal into two beams, one of which is further transmitted through a variable transmission filter which is used to encode position which, in turn, is proportional to the height of the object. The second or reference split beam is provided to compensate for changes in the reflectance of the object and the power of the laser scanner. Although such a system is capable of high-speed scanning it is limited by dynamic range. It cannot tolerate large changes in reflectance of the scanned circuit board simultaneously with substrate warpage.

U.S. Patent No. 4,677,302 (Chiu et al.) discloses an apparatus for inspecting, the profile of a printed circuit board having components present on at least one side. A beam of collimated light is directed to one side of the printed circuit board and variations in the overall profile as a function of incidence illumination are detected above the surface plane of the circuit board. European Patent No. 342 864 (corresponding to U.S. Patent No. 5,011,960 in the name of Ando et al.) discloses a method of a detection of a wiring pattern using a triangulation method to detect a height thereof. The principle of height measurement disclosed by Ando et al. is that by illuminating a PCB feature at an oblique angle, light will be reflected by the PCB feature at an angle which varies as a function of the PCB feature's height.

To this end, Ando et al. show (in Fig. 7, for example) a substrate 1 having a wiring pattern 2 thereon whose height is measured by directing a laser beam at an oblique angle thereto such that the reflected beam passes through an anamorphic lens 31. A beam splitter 6 intercepts the reflected, magnified beam passing through the anamorphic lens 31 so as to reflect a first component thereof to a first optical sensor 7A and to pass a transmitted component thereof to a second optical sensor 7B. The magnitudes of the respective signals generated by the two optical sensors 7A and 7B depends on where the reflected beam strikes the beam splitter 6 and this, in turn, is a function of the height of the wiring pattern 2. Consequently, by determining the difference between the respective signals generated by the optical sensors 7A and 7B, it is possible to determine the height of the wiring feature 2.

A principal drawback of prior art systems such as that to which the Ando et al. patent is directed concerns the indirectness with which the height of the PCB feature or wiring pattern is determined. Specifically, the relationship of the feature height to a predetermined threshold value is determined as a function of the difference between the two signals - a - corresponding to the reflected and transmitted components of the beam reaching the beam splitter. It would be preferable to measure the feature height directly using a position sensitive sensor. As is well known in the art, such sensors also provide an output signal which is proportional to the difference between two signals fed to corresponding inputs of the device. However, the difference signal provided by such position sensitive detectors is by no means similar to that associated with prior art systems. The requirement to determine height as the difference of two signals is based on the understanding that height itself is a relative concept: in the present context actually being the difference in height between an upper surface of the PCB substrate and an upper surf ace of the PCB feature thereof.

In Ando et al. a base threshold is provided only over a very small area of the beam splitter 6 proximate its center whilst it would clearly be desirable to obtain a base threshold over the complete width of the position sensitive detector.

Israel Patent No. 72183 (N.V. Optische Industrie) discloses a measuring system for determining the distance between a reference level and a spot on the surface area of an object when the spot is within a measuring range extending from the reference level using the principle of triangulation. This method also is similar to that employed by Ando et al. described in detail above and consequently suffers from the same drawbacks so far as its implementation is concerned in the height measurement of a PCB feature.

In none of the prior art references discussed above is a two-dimensional position sensitive detector employed for determining the height of a printed circuit board feature whereby the width of the scan line and the maximum height resolution are matched to the position sensitive detector by anamorphically imaging the scanned line on to the position sensitive detector. Furthermore, none of the prior art references teaches the use of a position sensitive detector for generating an electrical signal having a base level corresponding to a nominal height of a - - predetermined base line and an instantaneous magnitude with respect to the base level proportional to the height of the point relative to the base line, so that an overall width of the scan line is matched to a corresponding width of the position sensitive detector and such that a maximum height resolution may be measurable by the position sensitive detector regardless of limited fluctuations in the height of the base line.

SUMMARY OF THE INVENTION It is an object of the invention to provide a method and system for measuring a height of a feature relative to a substrate in which the drawbacks associated with hitherto proposed systems are substantially reduced or eliminated.

According to a broad aspect there is provided a method for measuring a height of a feature relative to a substrate, comprising the steps of: scanning along a scan line on a surface of the substrate with a light source directed towards the substrate in a first direction so as to illuminate at each instant of time a respective point on said feature, anamorphically imaging the scanned line on to a position sensitive detector so as to generate an electrical signal having a base level corresponding to a nominal height of a predetermined baseline and an instantaneous magnitude with respect to the base level proportional to the height of the point relative to the baseline, said electrical signal having an overall width matched to a corresponding width of the position sensitive detector and a maximum height pre-calibrated to be measurable by the position sensitive detector regardless of limited fluctuations in the height of the baseline, and processing the electrical signal so as to determine the height therefrom.

According to a further aspect of the invention there is provided a system for measuring a height of a feature relative to a substrate, the system comprising: - - scanning means for scanning a light source along a scan line on a surface of the substrate in a first direction so as to illuminate at each instant of time a respective point on said feature with a flying spot of light, - 4 - 94405/3 collecting optics for focusing each light spot along a second direction not collinear with the first direction, a position sensitive detector for intercepting the focused light spot at a point thereon whose location along a predetermined axis is proportional to the height of the light spot relative to a precalibrated baseline and generating a corresponding electrical signal having a base level corresponding to a nominal height of a predetermined baseline and an instantaneous magnitude with respect to the base level proportional to the height of the point relative to the baseline, said electrical signal having an overall width matched to a corresponding width of the position sensitive detector and a maximum height pre-calibrated to be measurable by the position sensitive detector regardless of limited fluctuations in the height of the baseline, and processing means coupled to the position sensitive detector and responsive to the electrical signal for determining the height of the light spot relative to the substrate.

When used for height measurement of features on a printed circuit board (PCB), the system according to the invention uses a laser flying spot to scan a line on the surface of the PCB. The reflected light is collected at an angle different to the direction of the scan beam and, after suitable magnification, is imaged on an area image sensor. Preferably, the image is magnified using an anamorphic optical element which enlarges the image impinging on the area image sensor in the height measuring direction perpendicular to the scanning direction and reduces the image to a more confined spot in the scan direction.

The area image sensor is preferably constituted by a position-sensitive photomultiplier which produces a signal proportional to the height of the impinging laser spot, immaterial of the intensity of the optical image impinging thereon.

A processing means coupled to the position-sensitive photomultiplier is responsive to the signal produced thereby for determining the height of the features relative to the substrate. 94405/3 In such a system, the scanning means provides immediate feedback relating to the location of the circuit feature whilst the area image sensor virtually instantaneously provides the corresponding height data. Such a system is inherently faster than hitherto proposed systems.

BRIEF DESCRIPTION OF THE DRAWINGS For a clearer understanding of the invention and to see how the same may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: Fig. 1 is a pictorial representation showing, in perspective, an optical system according to the invention for determining the height of a conductive track on a PCB substrate; Fig. 2 shows a detail of a conductive track seen in Fig. 1; Fig. 3 is a block diagram showing the principal functional elements in a system according to the invention; Fig. 4 is a schematic representation of a position sensitive photomultiplier tube for use with the system shown in Fig. 3; Fig. 5 shows schematically a detail of the system of Fig. 1 wherein the photomultiplier tube is tilted so as to magnify beam spread; Fig. 6 shows schematically an optical grating for use with the system of Fig. 1 for magnifying beam spread; and , , Fig. 7 shows schematically an optical fiber bundle for use with the system of Fig. 1 for magnifying beam spread.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT As shown in Fig. 1, there is provided an optical system depicted generally as 10 for scanning a printed circuit board (PCB) 11 having associated therewith a plurality of electrically conductive tracks 12.

The optical system 10 includes a laser source 14 (Fig. 3) which produces a laser beam 15 which is reflected by a reflector 16 on to 94405/3 a facet of a rotating polygonal mirror 17. The laser beam 15 is reflected by the rotating facet of the rotating polygonal mirror 17 through a tele- acentric scanning lens 18 which reshapes the laser beam 15 and transmits it as a scanning beam 19 through a first cylindrical lens 20 which focuses it as a spot of light on the PCB 11. Thus, as the rotary polygonal mirror 17 rotates, the scan beam 19 scans along the line 25 on the surface of the PCB substrate 11.

A second cylindrical lens 26 (constituting a collecting optics) is provided for collecting light scattered by the substrate 11 and the conductive tracks 12 along the line 25 in a direction which is not collinear with the direction of the scanning beam 19. The output from the second cylindrical lens 26 is an expanded beam 27 which is focused by a telecentric lens 28 which, together with the second cylindrical lens 26, performs anamorphic imaging of the scan line 25 on to a position sensitive photomultiplier 29. A processing means 30 coupled to the position sensitive photomultiplier 29 is responsive to an electrical signal generated thereby for determining the height of the conductive track 12 with respect to the substrate 11.

Preferably, the focusing lens 28 is of identical construction to the telecentric scanning lens 18 and, together with the second cylindrical lens 26, it constitutes a collecting optics for collecting light scattered from the surface of the substrate 11 and the conductive tracks 12 thereon and focusing the scattered light on to the position sensitive photomultiplier 29. The second cylindrical lens 26 is so designed that, in combination with the telecentric lens 28, the cross-scan magnification is approximately 10. Thus, for example, the second cylindrical lens 26 may be provided with a focal length of 20 mm, the telecentric lens 28 having a focal length of 200 mm.

Fig. 2 shows a detail of one of the conductive tracks 12 scanned by the scanning optics along the scan line 25. From Fig. 2 it will clearly be seen that, providing the scan line 25 is viewed in a direction which is not normal to the surface of the substrate 11, there exists a - 7 - 94405/3 parallax error between the scan line 25 striking the substrate 11, on the one hand, and striking the conductive track 12, on the other hand. The detection and measurement of the parallax error thus produced forms the basis for height determination according to the invention.

Referring now to Fig. 3 of the drawings, there is shown schematically the principal functional elements associated with the system 10 described above with reference to Fig. 1 of the drawings.

Thus, there is shown a laser 14 together with a rotary polygonal mirror 17 (together constituting a scanning means) for scanning a line along a printed circuit board 11 rigidly secured to an inspection table 31. Rotation of the rotary polygonal mirror 17 about its axis is synchronized by means of a CONTROL & CLOCK circuit 32 which likewise synchronizes lateral movement of the table 31 in a direction perpendicular to the scan line 25 along the surface of the printed circuit board 11.

The reflected beam 27 is collected by the second cylindrical lens 26 and focused by the telecentric flat field lens 28 on to the surface of the position sensitive photomultiplier tube 29 which, as shown in Fig. 3, is disposed at an angle of 45° to the scanning optics.

Power is supplied to the photomultiplier tube 29 by a power supply unit (PSU) 35 and the photomultiplier tube 29 produces two analog electrical signals which are amplified by respective amplifiers 36 and 37 and then fed to analog-to-digital (A/D) converters 38 and 39 which produce respective digital output signals which are fed to a DIGITAL SIGNAL PROCESSOR 40.

The A/D converters 38 and 39 are synchronized by the clock in the CONTROL & CLOCK circuit 32. The amplifiers 36 and 37 together with the two A/D converters 38 and 39 and the DIGITAL SIGNAL PROCESSOR 40 constitute the processing means 30 shown in Fig. 1 of the drawings. The output from the DIGITAL SIGNAL PROCESSOR 40 is fed to a POST PROCESSOR 41 for analyzing the data so as to determine the heights of the conductive tracks 12 relative to the 94405/3 substrate 11 and for determining the boundaries between adjacent conductive tracks and the substrate.

Fig. 4 shows schematically the position sensitive photo-multiplier 29 having two electrodes 45 and 46 for producing respective signals YC and YD whose magnitudes are proportional to the displacement from a predetermined origin point along the Y-axis of an illuminated pixel on the surface of the photomultiplier tube 29. It will be understood that photomultiplier tubes of the type described are also provided with corresponding electrodes for determining the relative displacement of the illuminated pixel with respect to a predetermined origin along the X-axis but this is of no interest so far as the calculation of height is concerned and so the other two electrodes are not shown in Fig. 4.

In effect, the pixels on the surface of the photomultiplier tube 29 are connected in a voltage divider network whereby the axial displacement along the Y-axis of the illuminated pixel relative to a predetermined origin (and being proportional to the height of the conductive track) is calculated to be proportional to: TT Usually the intensity of the reflected signal depends on the material being measured but the above ratio is not affected by variations in reflected intensity and is directly proportional to the height of the reflecting spot.

In an actual example of the invention reduced to practice, a photomultiplier tube 29 having a 40 mm face plate with a 0.3 mm resolution was employed so as to accommodate a height gauging dynamic range of 133. Thus, if the required resolution for measuring the thickness of the conductive tracks 12 is equal to 2 pm, then a substrate deformation of 0.3 mm can be tolerated. In practice, such tolerances can be achieved by securing the substrate 11 to the inspection table 31 by good vacuum 94405/3 suction as is known in the art.

For each pixel in the scan direction 9 bits of height information are stored and, at any given time, the pixel data associated with three sequential scan lines are stored. The height data in each pixel of the middle line is compared to all its neighbors in both directions in order to allow for any direction of circuit feature relative to the direction of the scan line 25. If the difference in the calculated height exceeds a predetermined threshold, this is interpreted as a transition from substrate to conductive track. The system is initialized by specifying whether the calculated height corresponds to substrate or conductive track.

It may be shown that with a moderately powered laser of a few milliwatts in conjunction with the optical system 10 described above, 4000 photo-electrons can be produced at a frequency bandwidth of 50 Mhz. In this case, the expected resolution is 0.3 mm. Since the required height resolution is of the order of a few microns (1 to 4 pm), additional magnification is required in the cross scan direction only in order to convert the minute deviations in the height data to coarser deviations on the face of the photomultiplier 29.

Fig. 5 shows pictorially how the additional magnification can be realized, in practice, by tilting the photomultiplier 29 so that a front surface 50 thereof is inclined at an angle of 5 ° to the line of sight. The light beam 51 strikes the front surface 50 over a greater area than would a normally incident light beam, thereby effectively magnifying the beam displacement due to the circuit feature. In this case, only a small amount of the light beam 51 impinging thereon will be absorbed, the rest of the light being reflected. The unrequired reflection can be prevented by applying a suitable anti-reflective coating 52 directly to the glass face of the photomultiplier 29 or, alternatively, by disposing a suitably treated optical element adjacent to the face of the photomultiplier 29.

Fig. 6 shows an alternative approach employing an ordered fiberoptics bundle 53 for directing the light on to the front surface 50 of the position sensitive photomultiplier tube 29. - 10 - 94405/2 As shown in Fig. 7, it is likewise possible to use an optical grating 54, such as a hologram, inclined at an angle of 5 ° relative to the optical path. The incoming light 55 is scattered by the grating in a direction perpendicular to its plane so that the remainder of the light 56 passes therethrough at right angles to the surface of the photomultiplier 29.

It will be understood that the figures given above are merely illustrative and that modifications to the preferred embodiment will be readily apparent to those skilled in the art without departing from the spirit of the invention.

Claims (12)

- 11 - 94405/2 CLAIMS:

1. A method for measuring a height of a feature relative to a substrate, comprising the steps of: , scanning along a scan line on a surface of the substrate with a light source directed towards the substrate in a first direction so as to illuminate at each instant of time a respective point on said feature, anamorphically imaging the scanned line on to a position sensitive detector so as to generate an electrical signal having a base level corresponding to a nominal height of a predetermined baseline and an instantaneous magnitude with respect to the base level proportional to the height of the point relative to the baseline, said electrical signal having an overall width matched to a corresponding width of the position sensitive detector and a maximum height pre-calibrated to be measurable Ϊ -by the position sensitive detector regardless of limited fluctuations in the height of the baseline, and processing the electrical signal so as to determine the height therefrom.

2. The method according to Claim 1, wherein the step of anamorphically imaging comprises: tilting the position sensitive detector relative to said second direction, and disposing an anti-reflecting element adjacent a surface of the position sensitive detector.

3. The method according to Claim 1, further including the steps of: storing height data relating to successive heights along the scan line, and - comparing successive height data for identifying boundaries of said feature with the substrate. - 12 - 94405/2

4. A system for measuring a height of a feature relative to a substrate, the system .comprising: , scanning means'for scanning a light source along a scan line on a surface of the substrate in a first direction so as to illuminate at each instant of time a respective point on said feature with a flying spot of light, collecting optics for focusing each light spot along a second direction not collinear with the first direction, a position sensitive detector for intercepting the focused light spot at a point thereon whose location along a predetermined axis is proportional to the height of the light spot relative to a precalibrated baseline and generating a corresponding electrical signal having a base level corresponding to a nominal height of a predetermined baseline and an instantaneous magnitude with respect to the base level proportional to the height of the point relative to the baseline, said electrical signal having an overall width matched to a corresponding width of the position sensitive detector and a maximum height pre-calibrated to be measurable by the position sensitive detector regardless of limited fluctuations in the height of the baseline, and. processing means coupled to the position sensitive detector and responsive to the electrical signal for determining the height of the light spot relative to the substrate.

5. The system according to Claim 4, wherein the collecting optics includes an anamorphic optical element for anamorphically magnifying the light spot.

6. The system according to Claim 4, wherein the scanning means is a laser scanner.

7. The system according to Claim 4, wherein the position sensitive detector is a position sensitive photomultiplier. - 13 - 94405/2

8. The system according to Claim 5, wherein: the position sensitive detector is disposed non-perpendicularly to said second direction, and an anti-reflecting element is disposed between the collecting optics and the position sensitive detector adjacent the position sensitive detector; whereby the focused light spot is anamorphically magnified prior to striking the position sensitive detector.

9. The system according to Claim 8, wherein the anti-reflecting element is constituted by an anti-reflecting coating coated on a surface of the position sensitive detector.

10. The system according to Claim 4, further including an ordered fiberoptics bundle disposed between the collecting optics and the position sensitive detector for directing light on to an increased area of the position sensitive detector.

11. The system according to Claim 4, further including an optical grating disposed between the collecting optics and the position sensitive detector at an angle to the second direction for directing light an increased area of the position sensitive detector.

12. The system according to Claim 4, further including: securing means for securing the substrate in a substantially flat disposition, and traversing means for effecting relative movement between the substrate and the scanning means in a third direction perpendicular to both the scan lines in the plane of the substrate. For the Applicants, DR. REINHOLD COHN AND PARTNERS By: 80659spc/3