Abstract:
In a sensing system having relatively movable fixture and sample elements in which measured data is corrupted in proportion to the departure of the fixture from its ideal position, as from mechanical intolerances, etc., a reference disc and reference sensor are disclosed. The reference sensor and disc are provided for measuring the non-repeatable departure from ideal so that the measured data can be compensated thereby in real time for the error thereby improving the accuracy of the measured data. This system is operable in combination with a repeatable error elimination system insuring complete error removal.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to the field of measurement and testing, and more particularly, to a novel system for compensating measurement data for error introduced into the measurement data mechanically by the fixture moving the object from which the measurement data is obtained. 
     BACKGROUND OF THE INVENTION 
     The flatness, bow, warp and other qualities of semiconductor wafers must be within precise standards in order for the wafers to be usable. Individual wafers are measured to determine whether and the extent of conformance to such standards. Errors are, however, introduced into the measurements by the mechanical errors of the fixtures holding the wafers, thereby limiting the accuracy of the wafer measurement. One solution in high accuracy scenarios is the use of wafer test fixtures that are so precisely fabricated as to exhibit substantially no error themselves. This solution is disadvantageous, however, due to the limited processing throughput that such devices often entail and due to the expenses that are associated with providing and maintaining such test fixtures. Another solution is to electronically process the measurement data to compensate the measurement data for the fixture induced errors to the extent that they are repeatable. In commonly assigned copending U.S. utility patent application Ser. No. 802,049, incorporated herein by reference, now U.S. Pat. No. 4,750,141 method and apparatus are disclosed for separating fixture errors from measured data, and for compensating the measured data for the error introduced by the fixture error. In a bow and warp station embodiment thereof, an X, θ, and Z movable fixture is calibrated to determine its X and θ fixture induced errors that repeat measurement cycle to measurement cycle. Once the fixture is calibrated, the measurement data is therewith compensated providing an accuracy not limited by the repeatable X, θ errors associated with the mechanical nature of the fixture. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates as its principal object a measurement system that separates non-repeatable fixture associated errors from measured data and that compensates the measured data for the non-repeatable fixture induced errors thereby improving the accuracy of the measured data. A sample to be measured receiving fixture in accordance with the present invention includes a movable fixture to which the sample is releasably mounted. A reference is mounted for movement with the movable fixture. A first sensor provides a measurement signal as the sample is moved thereto by the movable fixture, and a second sensor provides an error signal as the reference is moved thereto by the movable fixture as it moves the sample to the first sensor. In this manner, for each sample point that the first sensor provides a measurement signal for the second sensor provides an error signal from the corresponding point on the reference. The measurement signal and the error signal are combined to compensate the measurement signal for the non-repeatable error signal providing a high accuracy output signal. In a presently preferred embodiment, the output signal is digitized, compensated for non-repeatable error, then processed to provide the one or more desired characteristics associated with the object with a very high degree of accuracy. In combination with the system for the elimination of repeatable error, complete error compensation is achieved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects, aspects and advantages of the present invention will become apparent as the invention becomes better understood by referring to the following solely exemplary and non-limiting detailed description of a preferred embodiment thereof and to the drawings, wherein: 
     The sole FIGURE is a diagram illustrating a presently preferred embodiment of a measurement fixture and non-repeatable error compensation system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention has utility in applications where the sample to be measured is releasably held by a fixture that is cooperative with a sensor to move the sample and sensor relatively to provide measurement of the sample in the form of a sensor output signal from which one or more sample characteristics can be determined but which is subjected to error due to imperfection in the mechanical aspect of the fixture which corrupts the sensor output signal and thereby limits the accuracy of the determination of the desired characteristics. The present invention especially has utility in such situations where the fixture is movable to move the sample to the sensing head but is subject to instantaneous, spontaneous deviations that dislocate the fixture, and therewith the sample, from its intended spacial position thereby inducing error into the measured data provided by the sensing element. Therefore the present invention contemplates means for providing a reference representative of the ideal position of the fixture, means for measuring deviations of the fixture from the reference position, and means for compensating the measured data with the deviations from the ideal. In the preferred embodiment, the present invention is disclosed in the context of a X, θ, and Z movable vacuum chuck and cooperative capacitive sensing elements for measuring proximity to the sample from the probe. From the measured data such wafer characteristics as flatness and bow/warp are able to be readily obtained as will be appreciated by those skilled in the art. The invention in compensating the measurement data for fixture related error thus enables the providing of such characteristics with a very high degree of accuracy. Other applications and configurations as well are contemplated and are not to be limited by the purely exemplary presently preferred embodiment. 
     Referring now to the FIGURE, generally designated at 10 is a block diagram of a presently preferred embodiment of the fixture and non-repeatable error compensation system of the present invention. The system 10 includes a vacuum chuck 12 for removably holding a semiconductor wafer 14 (or other sample), and a sensor generally designated 16 positioned near the chuck 12 that is operative to measure the distance to a preselected surface of any region of the wafer 14 brought into operative proximity with the sensor. The wafer 14 has a characteristic to be determined from the measurements of the points thereof brought into the sensor 16, such as flatness and bow/warp profiles. The sensor 16 preferably consists of a first probe 18 designated &#34;A&#34;, and a second spaced-apart probe 20 designated &#34;B&#34;, defining in combination a measuring head generally designated 22. The probes 18, 20 are preferably capacitive sensors. Probe 18 is fixably mounted to a support 24. Probe 20 is fixably mounted to the same support 24. The probes 18, 20 may be moved relative to each other so that the size of the head 22 can be adjusted. A reference probe 26 is mounted to the same support 24 to which the probes 18, 20 are mounted. In this way, all of the several probes are stationary with respect to each other. Any suitable means for mounting the probes in such a manner can of course be employed. While any suitable probes 18, 20, 26 may be employed, it is preferred that the capacitive gauging system disclosed and claimed in commonly assigned U.S. Pat. No. 3,990,005 entitled Capacitive Thickness Gauging For Ungrounded Elements, incorporated herein by reference, be employed. A reference 28 is mounted for movement with one or more of the movement degrees of freedom to be described of the chuck 12. In one preferred embodiment, the reference 28 can be a metallic disc, mounted for movement in X, θ and Z with the chuck. In another preferred embodiment, the reference 28 can be a bar mounted for movement in X alone with the chuck. The reference probe 26 defines the ideal location of the chuck 12 through reference 28 and, as measured by the reference probe 26 provides an output reflecting the departure from that ideal. Analog signal conditioning and combining electronics illustrated by a dashed block 30 are coupled to the output of the probes 18, 20, 26. The electronics 30 may be in any suitable configuration to condition the analog output signal from the probes 18, 20, 26 in dependence on the type of the wafer characteristic to be determined and to combine them so as to compensate for the non-repeatable mechanical imperfections of the fixture. As will readily be appreciated by those skilled in the art, the combination of the probe 18, 20 signals is selected to conform to the desired characteristics of the wafer to be determined, as, for example, when added, wafer thickness is obtainable, and when differenced, distance to the median centerline of the wafer is obtainable. To provide high accuracy bow/warp profiles, for example, the combining and conditioning circuitry includes linearizers 32, 34, and 36 connected to the output of the probes 18, 20, and 26 respectively, and the outputs of the probes 18, 20 are differenced, as in an analog adder 38. To the output of the analog adder 38, the output of the reference probe is combined so as to compensate the measurement data in real time for the non-repeatable fixture errors thereby providing a compensated output signal. For the position of the reference probe and reference disc below the wafer as illustrated in the Figure, a mixing element 40 such as an analog adder is provided to subtract the output of the probe 26 from a reference level designated &#34;REF&#34; that is representative of nominal fixture position, deviations from which indicate that the vacuum chuck is performing mechanically in a way that departs from its ideal positional specification. The compensated output signal is received by an analog to digital converter 42. 
     An X, θ, and Z assembly 44 is operatively connected to the vacuum chuck 12 for rotating the chuck about its axis θ radians, for moving the vacuum chuck along an X axis, and for moving the chuck along a Z axis. The X, θ, and Z assembly 44 is responsive to a plurality of control signals produced by a processor 46 to controllably manipulate the chuck 12 to successively position different preselected points of the wafer 14 into proximity with the capacitance sensing head 22 in the X, θ, and Z directions. The points are preferably selected for the exemplary bow/warp profile to cover the entire spacial extent of the wafer 14. While any suitable X, θ, and Z assembly can be employed, it is preferred that the X, θ, and Z assembly shown and described in commonly assigned U.S. Pat. No. 4,457,664 entitled Wafer Alignment Station, incorporated herein by reference, be employed. Any suitable movement pattern that manipulates the X, θ, and Z assembly may be employed, and reference may be had to commonly assigned U.S. utility patent applications Ser. Nos. 572,695, abandoned in favor of continuing U.S. application Ser. No. 07/282,877 and 802,049, now U.S. Pat. No. 4,750,141 respectively entitled Wafer Flatness Station and Method and Apparatus For Separating Fixture-Induced Error From Measured Object Characteristics and For Compensating The Measured Object Characteristics With The Error, and A Bow/Warp Station Implementing Same, both incorporated herein by reference, for exemplary movement sequences. The processor 46 may be any suitable processor, and in the exemplary embodiment, is indicated to the fixture and is connected to the analog to digital converter 42 over a data and address bus 48. The processor 46 is operatively connected to the X, θ, and Z assembly 44 over the bus 48 via conventional latched drivers 50. The processor 46 has RAM 52 and PROM 54 associated therewith in the usual manner. A central control processor 56 is preferably connected to the bus 48 via a communication link, preferably an IEEE 488 bus 58 and an IEEE 488 interface 60. The processor 46 is preferably slaved to the central control processor 56, although as will be appreciated, a single processor could be employed as well. 
     In operation, the X, θ, and Z assembly 44 controllably moves the chuck 12 and thereby moves the wafer 14 received on the chuck 12 so as to bring different points of the wafer into the capacitive sensing head 22 for measurement. For each such point, the probes 18, 20 produce signals representative of the distance respectively to the corresponding confronting surface of the wafer at that particular point. The reference probe 26 is simultaneously operative to provide an output signal representative of distance to the corresponding surface of the reference 28 for each such wafer point. If the chuck is where it should be, the signal produced by the reference probe 26 will be equal to the nominal signal, but, if the chuck is spontaneously experiencing a non-repeatable error and is not in Z where it should be, the output of the probe 26 is correspondingly changed, which change is reflected by the adder 40 as an error component output signal. The signal conditioning and combining electronics 30 is operative to subtract the error component signal from the measurement signal in real time and as the signals are produced. The compensated output signal is then digitized via the analog to digital converter 42. As will be appreciated, the data so made available may be manipulated by the processor to provide one or more desired characteristics associated with the wafers, such as flatness and/or bow/warp. Reference may be had to the application Ser. No. 572,695 abandoned in favor of continuing U.S. application Ser. No. 07/282,877, entitled Wafer Flatness Station, for exemplary processing sequences and algorithms for providing exemplary flatness profiles of the wafer, and reference may be had to the application Ser. No. 802,049 now U.S. Pat. No. 4,750,141 entitled Method and Apparatus For Separating Fixture-Induced Error From Measured Object Characteristics and For Compensating The Measured Object Characteristics With The Error, and A Bow/Warp Station Implementing Same, for exemplary processing sequences and algorithms for providing bow/warp profiles of the wafer free of repeatable errors. Other sequences and algorithms for determining flatness, bow/warp and other wafer characteristics can of course be readily implemented by those skilled in the art. 
     The present invention removes the error induced into the measurement signals provided by the probes by non-repeatable deviations of the fixture from its ideal spatial specifications. The errors induced thereinto by repeatable deviations of the fixture from its ideal spatial specifications may be removed by the technology disclosed and claimed in the above incorporated application Ser. No. 802,049 now U.S. Pat. No. 4,750,141. In this way, complete error compensation of both the repeatable and the non-repeatable spatial dislocations in one or more of the degrees of movement freedom of the X, θ, and Z assembly is accomplished and therewith data accuracy is greatly improved notwithstanding any non-idealities that may be associated with the mechanical imperfections and intolerances of the fixture. 
     Many modifications of the present invention will become apparent to those skilled in the art without departing from the scope of the following claims.