Chromatic optical ranging sensor

Abstract: A chromatic optical ranging sensor, comprising: means for focusing a source beam (30) of optical energy onto a target (6) from a known location, whereby different wavelengths of said source beam are focused at different distances from said known location; means for collecting a reflected beam from said target (6); means for detecting and interpolating said reflected beam to determine a distance of said target from said known location, means for separating said reflected beam into a focused portion and a unfocused portion; and means for determining a ratio of an amplitude of said focused portion to an amplitude of said unfocused portion.

Description: BACKGROUND OF THE INVENTION This invention relates to an apparatus for three dimensional (3-D) vision sensing and measurement using chromatic range discrimination. Continual advances in circuit complexity and feature miniaturization in the semiconductor industry have resulted in a requirement for substantially increased densities of input/output (I/O) leads. Surface mount technology (SMT) packaging has largely replaced the traditional through-hole integrated circuit (IC) package. SMT leads, such as those on quad flat packs (QFP's), are soldered directly to connection pads on a surface of a circuit board, thereby permitting increased density of leads and a smaller device footprint. Recently, the new ball grid array (BGA) package has expanded the SMT class of devices. BGA replaces the peripheral SMT leads with a matrix of solder balls on the underside of the device. Future trends will extend the SMT concept to direct-attach methods such as flip chips, in which the connection bumps are integral to the wafer. These new packaging technologies present significant challenges to device manufacturers. Any lead deformation can result in an inability to solder correctly the device to the board. For BGA and flip chip devices, the solder balls and connection bumps must be precisely sized and located in order to mate properly with the corresponding pads on the printed circuit (PC) board. A device's ensemble of SMT leads, BGA solder balls, or flip chip connection bumps is a 3-D structure which may exhibit monostable or bistable seating on a board. The ensemble may or may not be within acceptable tolerances for successful soldering during board assembly. Typical inspections of SMT devices with peripheral leads include checks of coplanarity, pin spread, pin width, gap width, true position spread, true position span, and lead angle. A typical inspection of a BGA device includes checks of coplanarity, true position error, pitch, ball diameter, and board warpage. The evolving requirements for flip chip devices are expected to include similar geometric requirements with substantially higher accuracies required than are needed for today's BGA's. A number of commercial lead inspections systems use 2-D machine vision technology. Rather than measure 3-D lead geometry directly, 2-D systems infer 2-D shape from 2-D shadows and are, therefor, limited in their ability to characterize adequately the critical parameters of SMT leads. In addition, "-D systems often seek to achieve adequate throughput by using multiple cameras, which are, however, susceptible to consequential calibration drift and accuracy degradation. Finally, all 2-D systems require extensive device handling that reduces throughput and damages the devices. Other commercial systems use 3-D machine vision such as disclosed in U.S. Patents 4,238,147, 4,590,367, 4,529,316, 4,594,001 and 4,991,968. Typical 3-D machine vision sensors are based on the well-established principle of structured-light optical triangulation. The object (e.g., SMT lead or BGA ball) is illuminated by a scanned, focused laser source: The reflected energy is captured by an offset camera. The known spatial relationship between the illumination source and the camera enable an accurate 3-D measurement to be computed. The combination of camera and structured light projector is a 3-D sensor. It is desirable to enhance the familiar optical triangulation method to accommodate the impending high-resolution, high-throughput requirements. One technique for improving the resolution of optical triangulation is to increase the angular separation between transmitter and receiver. However, this improvement is incremental at best, only applies to one dimension (range resolution), and is achieved at the penalty of increased susceptibility to occlusion of the viewed surface by neighboring surfaces. Another potential enhancement is to improve both the range resolution and lateral resolution by reducing a focused spot size of a transmitted laser beam. The spot size is directly proportional to the product of the f-number of the projection optics (as viewed from the target) and the laser wavelength. The projection optics may be improved incrementally, but not by an order of magnitude while maintaining depth-of-field. Achieving shorter wavelengths and higher resolution is possible using higher frequency illumination sources. Current optical triangulation systems operate in the near infrared (800-900 nm wavelength). The shorter wavelengths of interest are in the deep ultraviolet (<300 nm). Unfortunately, the ultraviolet (UV) region is characterized by a lack of low-cost, solid-state illumination sources, very low photodetector sensitivity, and the degradation of optical components with use.UV wavelengths present major practical problems for a commercial, high duty-cycle inspection system. A recently popular technique for high-accuracy optical measurement on semiconductor wafer surfaces, known as laser confocal microscopy, utilizes a diffraction-limited, monochromatic spot of light projected on a target surface. A pinhole in the focal plane of the receiver optics is mechanically translated in range. The returned energy through the pinhole is maximum when the focal plane coincides with the target surface image, thus enabling measurement of target range. This technique is capable of submicron range resolution. However, it is a very slow process due to the mechanical servoing of a minimal depth-of-focus across the total depth-of-filed where the target is to be found. Microscope calibration is difficult. The moving parts can wear with use, causing maintenance downtime. Another recent technique uses the dispersion characteristics of a diffractive micro-Fresnel lens (MLF) in a spectrometer as outlined in "Spectrometer Employing a Micro-Fresnel Lens", Optical Engineering Vol. 34, No. 2, pp. 584-588 (February 1995). MFL's generally have large chromatic aberration. MFL's therefore transform the incident light spectrum into an intensity distribution along the optical axis. Different colors (wavelengths) have different focal lengths. A movable pinhole is positioned between a photodetector and a MFL. The lght that passes through the pinhole is detected by the photodetector, which outputs a current corresponding to the light intensity of the light focused at the pinhole. An intensity distribution is obtained by moving the pinhole. The distances between the MFL and the pinhole passing focused light directly correspond to the wavelengths of the incident light. The spectrum of the incident light is thus obtained. Conversely, if the wavelength of the incident light (and its attendant focal length) is known, the distance between the pinhole and the MFL is readily determined. An adaptation of this principle is used in the present invention. The above technique of dispersing a beam (i.e. with a Fresnel lens) and measuring range based upon a detection of a peak in the returned light is also discussed in a publication entitled "Longitudinally Dispersive Profilometer", Pure and Applied Optics, Vol. 4, No. 3, pp. 219-228 (May 1995). A similar optical surface profiler is disclosed in "Optical Surface Profiler Transducer", Optical Engineering, Vol. 27, No. 2, pp. 135-142 (Feb. 1998). Another technique uses three chromatic filters in order to determine distance as outlined in "Three-dimensional image sensing by chromatic confocal microscopy", Applied Optics, Vol. 33, No. 10, pp. 1838-1843 (April 1994). Light is reflected off of a target through a chromatically aberrated objective lens. Multiple wavelengths of light are simultaneously reflected off the targets surface and directed to three different chromatic filters which function similar to the human eye. Each chromatic filter is used to discriminate a different color. The distance of the target from a known location is deduced from the relative amount of light received through each of three chromatic filters. Another way of determining a range calculation is outlined in U.S. Patent No. 3,815,409. According to the disclosure, a chirped frequency modulated sound wave can be passed through a dispersive lens that has varying focal lengths with frequency. Each frequency ios focused at a different depth to the object. A time delay is measured for the transmission of the sound wave until a reception of a reflected sound wave into a transducer. The time delay corresponds to a particular depth to the object Co-pending European Patent Application No. 96 111 721 discloses a sensor comprising a source beam from a polychromatic electromagnetic radiation source; first means for processing said source beam to produce an incident beam including means for modulating said source beam by wavelength ver time, to produce said incident beam, which is projected along an incident beam axis; said incident beam being projected on a target; a reflected beam reflected from a surface of said target; and means for collecting said reflected beam; whereby said incident beam is a time-modulated beam such that said incident beam has a first wave length at a first time and a second wavelength at a second time; whereby there are provided means for focusing said incident beam onto said target, whereby said first wavelength and said second wavelength are wavelengths which are focused at different distances along said incident beam axis from a known location; whereby the reflected beam has a focused and a unfocused portion and whereby means are provided for processing said reflected beam to determine a distance of said target from said known location by comparing ratios of said focused to unfocused portions at each said time. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to overcome the drawbacks and limitations of the prior art. It is another object of the present invention to provide a non-triangulation approach to optical 3-D measurement. It is a further object of the present invention to prove a fast, efficient method and apparatus for sensing and measuring ranges on the order of a micron. It is a still further object of the present invention to provide an optical ranging sensor that compensates for wavelength dependent reflectivity of a target. The above and other objectives are realized by a chromatic optical ranging sensor according to the features of claim 1. According to the present invention a chromatic optical ranging sensor includes means for focusing a source beam of optical energy onto a target from a known location, whereby different wavelengths of the source beam are focused at different distances from the known location, means for collecting a reflected beam from the target, and means for detecting and interpolation the reflected beam to determine a distance of the target from the known location. The sensor comprises further means for separating the reflected beam into a focused portion and an unfocused portion and means for determining a ration of an amplitude of said focused portion to an amplitude of said unfocused portion. According to an embodiment of the present invention there is provided a chromatic optical ranging sensor, further comprising: said optical energy; being broadband high intensity optical energy including a plurality of individual wavelength components; means for polarizing said optical energy; means for tuning said polarized optical energy so that said range of polarization of said optical energy corresponds to a range of optical wavelengths; and means for selectively transmitting said optical energy according to a particular oriented polarization. According to another embodiment of the invention there is provided a chromatic optical ranging sensor according to claim 1; further comprising: said optical energy being a polychromatic source of light; means for applying dispersion to said polychromatic light, whereby different wavelengths of said polychromatic light are focusable at different distances; and means for detecting a wavelength at which said reflected beam has a maximum amplitude, wherein said maximum amplitude is related to a distance between said target and said means for applying dispersion. The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded to view of a 3_D sensor according to the present invention. Fig. 2 is a schematic alternative of the embodiment of Fig. 1 for detecting and interpolating an output reflected from a target being sensed. Fig. 3 is an exploded top view of an embodiment for projecting optical energy onto a target being sensed. Fig. 4A is an exploded top view of an embodiment for detecting an output from a target being sensed that removes wavelength dependent reflectivity from the depth calculation. Fig. 4B is an exploded top view of an alternative embodiment for detecting an output from a target being sensed that removes wavelength dependent reflectivity from depth calculation. Fig. 4C is an exploded top view of an alternative embodiment for detecting an output from a target being sensed that removes wavelength dependent reflectivity from depth calculation. Fig. 5A is a perspective view of a fiberoptic bundle for detecting an output from a target being sensed that removes wavelength dependent refelctivity from the depth calculation. Fig. 5B is an exploded top view of an embodiment for detecting an output from a target being sensed that removes wavelength dependent reflectivity from the depth calculation by incorporating the fiberoptic bundle of Fig. 3A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The sensor configuration of Fig. 1 uses a range of simultaneously transmitted wavelengths to illuminate the entire depth of field. This is in contrast to the sensor configurations of Figs. 1- 3 of co-pending European Patent Application No. 96 111 721, each of which uses sequentially transmitted wavelengths to scan the depth of field as the transmitted wavelength is changed. The collimated output of lens 2 is shaped by a lens 12 and a lens 13. The shaped beam is focused onto a pinhole 14 by a lens 23. The beam that emerges from pinhole 14 contains the full optical spectrum of broadband illumination source 1. The broadband beam is then collimated by a lens 9. The collimated beam is incident on a beamsplitter 15. A first portion of the beam is transmitted through beamsplitter 15 to objective lens 22. A second portion of the beam is reflected by beamsplitter 15, redirected to optical axis 17, where it is incident on a detector 8. Objective lens 22 projects the first portion of the beam onto a target 6, with a different focal plane 100 for each wavelength. As previously described in the first embodiment, longer wavelengths are focused closer to objective lens 22 and shorter wavelengths are focused farther away. More light of the wavelength that is mist closely focused on target 6 is reflected than is light of other wavelengths. The light reflected from target 6 is collected by objective lens 22 and reflected by beamsplitter 15 along an optical axis 21. The reflected light is focused onto a pinhole 27 by a lens 26. The light emerging from pinhole 27 is collimated by lens 37. The collimated beam emerging from lens 37 is incident on a diffraction grating 41. Diffraction grating 41 causes spatial dispersion of the broadband optical wavelengths reflected by target 6. A wavelength-dispersing prism optionally may be used for this purpose. This spatial wavelength dispersion substitutes for the temporal wavelength dispersion in the sensors disclosed in the co-pending application. The spatially-dispersed wavelengths merging from diffraction grating 41 are focused by a lens 38 onto array detector 39. Different wavelengths are incident simultaneously on different cells of array detector 39 due to the spatial dispersion. Array detector 39 is an array of any suitable detectors such as high gain avalanche photodiodes (APD's), silicon PIN diodes, charge coupled device (CCD) photodetectors, or a microchannel plate (MCP) image intensifier which contains a CCD or other array. Each distinct resolution cell of array detector 39 detects energy in a distinct narrow wavelength band of illumination source 1. Each distinct resolution cell of array detector 39 detects energy corresponding to a distinct narrow range interval contained within the large depth of field of objective lens 22. Signal 40, the electrical output of detector 39, includes either a sequential readout of the detected signal for all resolution cells of detector 39, or a simultaneous parallel readout of the detected signals from all of the resolution cells of detector 39. Signal 40 is analog or digital depending on the particular device used as detector 39. Using standard interpolation techniques, signal processing electronics 20 resolve the location of the target to a small fraction of a resolution cell of detector 39. Referring to Fig. 2, an alternative to the array detector of the previous embodiment is a linear position sensor PSD which outputs currents I 1 and I 2 proportional to a location of the focused light along a length L of the PSD. A conventional OP AMP arrangement outputs a sum I 1 + I 2 and a difference I 1 - I 2 which are used to determine an offset x of the focused light according to the equation 0.5L(I 1 - I 2 ) = x (I 1 + I 2 ). Lens 38 is preferably a binary optic lens such as is manufactured by Teledyne Brown Engineering. A binary optic lens with grating (not shown) optionally is used in place of diffraction grating 41 and lens 38. Other detectors that perform the same function, such as dual detectors used with spatially linearly varying optical attenuators, optionally are used to perform the same function and are well known in the art. Referring to Fig. 3, an alternative to objective lens 22 includes a lens element 22a which is a Fresnel or diffractive mirror which changes ist focal length depending on wavelength. In such a case, the light beam is deflected according to an angle between the illuminating beam and the normal to the mirror surface of lens element 22a. A positioner 25 moves along a normal to a target 6 to establish a coarse range location of the scanned depth-of-field by mechanically positioning lens element 22a. In an alternative embodiment, lens element 22a is rotatable along axis 30, thereby causing the sampling beam to scan across the surface of target 6. Thus, lens element 22a is rotated instead of moving the sensor or the target. Referring to Fig. 4A, an arrangement for compensating for wavelength sensitive reflectivity of a target is shown. After the light reflected from the target is focused by lens 26 and is, through pinhole 27, incident on detector 5, a beamsplitter 102 diverts some of the optical energy, preferably half, to a central stop 101. Central stop 101 acts as an inverse pinhole, blocking that portion of the optical energy that is precisely focused on it. The optical energy blocked by central stop 101 equals the optical energy passed through pinhole 27. A detector 105 detects that optical energy that passes central stop 101, which equals the optical energy not passed by pinhole 27. Detector 105 outputs a signal 128 to signal processing electronics 20. Signal processing electronics 20 finds an instant of time within the sweep when a ratio of the amplitude of the output of detector 5 to the amplitude of the output of detector 105 is at its maximum. At this instant of time, the energy passed through pinhole 27 is a result of the light being reflected off target 6 due to the target being at the focal point of objective lens 22 rather than being a result of the light reflected off the target due to a particular wavelength sensitive reflectivity. A calibrated depth for the wavelength transmitted at this instant of time therefore corresponds to the depth of the surface of the target since the ratio depends only upon the focus to pinhole 27 and is independent of the reflectivity of the target. The sizes of detectors 5 and 105 optionally are reduced by adding field lenses (not shown) between pinhole 27 and detector 5 and between central stop 101 and detector 105. Referring to Fig. 4B, an alternative embodiment for removing the wavelength reflectivity from the depth calculation replaces pinhole 27 with a detector 115 with a pinhole in it. This embodiment avoids the energy lost by beamsplitter 102 shown in Fig. 4A. Detector 115 directly detects and outputs signal 128 representing the energy not passing through the pinhole. Signal 128 is used in this embodiment the same as previously described for the embodiment of Fig. 4A. Referring to Fig. 7C, an alternative embodiment is shown which also avoids beamsplitter losses while removing the wavelength reflectivity from the depth calculation. Pinhole 27 of Fig. 4A is repklaced by a mirrored pinhole 127 which reflects that portion of optical energy not passing through the pinhole. Detector 105 and signal 128 are used as previously described in the embodiment of Fig. 4A. In each embodiment shown in Figs. 4A-4C, the peaking of the ratio of the signal from detector 5 divided by the signal from detectors 105 or 115, as appropriate, corresponds to the range of the surface of the target as indicated by the best focus of the light incident on the target. In other words, a ratio of focused color light to unfocused color light is obtained. The unfocused color light is detected by detectors 105 or 115. Comparing the ratio of the focused light to the unfocused light prevents obtaining false readings from some color sensitive characteristic of the target. The principle embodied in the separation method of the embodiments of Figs. 4A-4C is used to remove sensitivity to target color vs. reflectivity variations from the embodiment of Fig. I by separating the light leaving lens 26 into two paths. One path includes only light passing through pinhole 27. The other path includes only light passing around pinhole 27. However, there are problems associated with using any of the embodiments of Figs. 4A-4C with the embodiment of Fig. 1 because of the need to spatially disperse the broadband optical wavelengths reflected by target 6. Referring to Figs. 5A-5B, a fiberoptic bundle 130 accomplishes a separation of paths. An end of fiberoptic bundle 130 is positioned at a distance from lens 26. An inner portion 131 is sized the same as pinhole 27. An outer portion 132 surrounds inner portion 131. Inner portion 131 receives the same light as pinhole 27 would have, while outer portion 132 receives the light that would have been blocked. That is, inner portion 131 receives primarily focused light while outer portion 132 receives unfocused light. Thus the separation of the light reflected from target 6 into focused and unfocused light is accomplished. The light from inner portion 131 passes through lens 37, diffraction grating 41, lens 38, and onto array detector 39 as described in the Fig. 1 embodiment. Different wavelengths are incident simultaneously on different cells of array detector 39 due to the spatial dispersion. Signal 40 therefore is a measure of the focused light reflected from target 6. The light from outer portion 132 similarly passes through a lens 137, a diffraction grating 141, a lens 138, and onto an array detector 139. Signal 140 is a measure of the unfocused light reflected from target 6. A ratio is calculated indicative of best focus independently of target reflectivity vs. wavelength. The ratio of energy passing through inner portion 131 to energy passing through outer portion 132 is at a maximum for the color (wavelength) at which focus is best Care must be taken to ensure that enough signal is present to avoid dividing by zero when calculating the ratio. It is preferable to have the same number of fibers receiving energy in inner portion 131 as in outer portion 132 to simplify the calculations. Then, if only unfocused light is reflected from target 6, the ratio of light received in inner portion 131 to light received in outer portion 132 is 1 : 1. The ratio changes as more and more focused light is received in inner portion 131. It is further contemplated that the invention may be practiced with varying wavelengths of electromagnetic energy ranging from below the deep ultraviolet to the long infrared.

Claims: A chromatic optical ranging sensor, comprising: means for focusing a source beam (30) of optical energy onto a target (6) from a known location, whereby different wavelengths of said source beam are focused at different distances from said known location; means for collecting a reflected beam from said target (6); means for detecting and interpolating said reflected beam to determine a distance of said target from said known location, characterized in that the sensor further comprises: means for separating said reflected beam into a focused portion and a unfocused portion; and means for determining a ratio of an amplitude of said focused portion to an amplitude of said unfocused portion. A chromatic optical ranging sensor according to claim 1, wherein said means for separating includes a fiberoptic bundle (130) having an inner portion (131) and an outer portion (132). A chromatic optical ranging sensor according to claim 1, further comprising means for determining an amplitude of said source beam as a function of time. A chromatic optical sensor according to claim 1, wherein said means for detecting and interpolating said reflected beam includes means for dispersing said reflected beam. A chromatic optical sensor according to claim 1, wherein said means for detecting and interpolating said reflected beam includes one of a high-gain avalanche photodiode, a silicon PIN diode, a charge coupled device, a microchannel plate image intensifier, and a linear position sensing detector. A chromatic optical ranging sensor according to claim 1, further comprising: said optical energy being broadband high intensity optical energy including a plurality of individual wavelength components; means for polarizing said optical energy; means for tuning said polarized optical energy so that a range of polarization of said optical energy corresponds to a range of optical wavelengths; and means for selectively transmitting said optical energy according to a particular oriented polarization. A chromatic optical ranging sensor according to claim 6, further comprising: means for establishing a precise time base of said selectively transmitted optical energy; and means for determining an amplitude of said selectively transmitted optical energy as a function of time. A chromatic optical ranging sensor according to claim 1, further comprising: said optical energy being a polychromatic source of light; means for applying dispersion to said polychromatic light, whereby different wavelengths of said polychromatic light are focusable at different distances; and means for detecting a wavelength at which said reflected beam has a maximum amplitude, wherein said maximum amplitude is related to a distance between said target and said means for applying dispersion. A chromatic optical ranging sensor according to claim 8, wherein said means for detecting a wavelength includes: means for simultaneously reflecting at least a plurality of wavelengths of said polychromatic light from said spot to produce said reflected beam; means for applying dispersion to said reflected beam; means for transversely deflecting said reflected beam an amount related to each of said plurality of wavelengths; and means for detecting a transverse position of a maximum amplitude of the transversely deflected reflected beam, whereby said transverse position is related to a distance between said target and said means for applying dispersion. A chromatic optical ranging sensor according to claim 8, wherein said means for detecting a wavelength includes an array detector (39). A chromatic optical ranging sensor according to claim 8, wherein said means for detecting a wavelength includes a linear position sensing detector. A chromatic optical sensor according to one of the preceding claims, wherein said means for focusing includes one of a diffraction-limited lens, an objective lens (22), a diffractive mirror, and a micro-Fresnel-lens.

Inventors: METZGER ROBERT J, STERN HOWARD, METZGER, ROBERT J., STERN, HOWARD
Applicants: ROBOTIC VISION SYSTEMS, ROBOTIC VISION SYSTEMS INC.
Application year: 1996
Main classification: G01S17
IPC codes: G01S7, G01S17