Abstract:
An optical metrological system having a heat-generating light source coaxially mounted near a heat-sensitive lens. The system uses a temperature sensor to monitor the lens temperature and a heating element to heat the lens such that the lens operating temperature is greater than a maximum operating temperature of the light source in order to stabilize the focal length of the lens.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/185,632, filed Aug. 4, 2008, now U.S. Pat. No. 7,602,563, which is a continuation of U.S. patent application Ser. No. 11/633,303, filed Dec. 4, 2006, now U.S. Pat. No. 7,408,728. 
    
    
     BACKGROUND AND SUMMARY 
     This invention is related to metrological methods and systems, and more particularly, to methods and systems for focal length stabilization of metrological systems using active temperature control. 
     Automated metrology systems are used for the optical inspection of an object. Such inspections are performed in order to obtain precise dimensional and other measurements of the object. The object is placed on a stage (with precision movements for X-Y measurements) and the image of the object undergoes computerized image analysis. The Z-axis is also measurable using the auto-focus routine of the software resulting in height measurements of the object. A precise three-dimensional reproduction of the object is obtained using the measurements. The optical assemblies used in such systems are composed of a main imaging path, a calibration system and a surface illumination system for illumination of the object to be inspected. 
     The main imaging system of an optical system used for metrology, as described, for example, in U.S. Pat. No. 6,292,306, is comprised of a front lens, and either a fixed or zoom system behind, with a camera in the focal plane of the system. The system is designed to have collimated space between the front lens and the zoom or fixed lens portion of the imaging system. Z-axis measurements are taken with auto-focus, a computerized image analysis, to find the best focus of the system. Because of the collimated space behind the front lens, the front focus of the system is ultimately the front focus of the front lens. Any environmental changes to the front lens that may cause the physical front focus to change will add error to the Z-axis measurement of the object. 
     The calibration system, as described, for example, in U.S. Pat. No. 5,389,774, allows a user to perform calibration and return to a previously saved magnification. This is done by saving a reticle image at a selected magnification, calling up that image when that magnification is desired again, and waiting for the zoom lens to adjust until the present reticle image matches the saved reticle image. 
     The surface illumination system uses a variety of techniques to illuminate the object to be measured in order to enhance the precision of the measurements. One technique uses an LED ring surface illuminator, as described, for example, in U.S. Pat. No. 5,690,417, that allows for contours, ledges, edges, and other generalized surface height variations to be imaged. In such a system, the light source may surround the front lens of the main imaging system, creating a large amount of heat adjacent to the lens. When heated, the properties of the glass change, thus causing a change in front focal length directly affecting the front working distance/front focus (Z-axis measurement/height) of the system. The problem this creates is inaccurate Z-axis position measurements of the object to be measured. The Z-axis position measurement of the object will change once the light source is turned on, and will continue to change over time, as the light source heats up, until the light source reaches its maximum operating temperature. Any time the light is turned off and the temperature of the optics is allowed to change, the Z-axis position measurement of the object will also change. The fluctuation of lens focal length with temperature also results in repeatability errors as confirmed by repeated measurements of the same object over a period of time. Typically, Z-axis position measurements may fluctuate by 10-20 microns due to a temperature change, depending on the optical system and the amount of heat generated by the light source. An advantage of embodiments is the reduction of Z-axis position measurement variations in an object measured by a metrology system by reducing the temperature fluctuation in a heat-sensitive lens. 
     Embodiments stabilize the lens temperature of an optical system having a heat-sensitive lens in proximity to a heat generating device such as a light source. Embodiments also insulate the lens from environmental temperature variations that can affect Z-axis position measurements. Embodiments can be used in any application in which a stable lens temperature can optimize system performance. Embodiments provide an advantage in that they enable accurate Z-axis position measurements of objects and ensure that such measurements do not vary over time. Maintaining the lens at a constant temperature can also protect the lens from damage such as de-cementing which is caused by sudden and frequent lens temperature variations. 
     An optical system with focal length stabilization in accordance with embodiments includes a housing supporting a heat-sensitive lens within the housing, a light source secured to the housing, a heating element connected to the housing to heat the lens, at least one temperature sensor connected to the housing, and a controller in electrical communication with the at least one temperature sensor and the heating element such that the controller monitors a temperature of the lens and adjusts a current in the heating element to maintain the temperature of the lens within a pre-selected range. 
     A method for stabilizing focal length in a heat-sensitive lens supported in a housing having a light source secured thereto in accordance with embodiments includes the steps of monitoring a lens temperature using at least one temperature sensor connected to the housing, and maintaining the lens temperature within a pre-selected range by controlling a heating element attached to the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of an optical system having a light source coaxially mounted on one end of its objective lens system and using a temperature sensor and heating element for active focal length stabilization in accordance with embodiments. 
         FIG. 2  is a bottom view of an optical system having a light source coaxially mounted on one end of its objective lens system in accordance with embodiments. 
         FIG. 3  is a schematic block diagram of a temperature control system for stabilizing focal length in an objective lens system in accordance with embodiments. 
         FIG. 4  is a side elevation view of an optical system having a light source coaxially mounted on one end of its objective lens system and using multiple temperature sensors, a heating element, and an indicator for active focal length stabilization in accordance with embodiments. 
         FIG. 5  is a schematic flow diagram of a method for stabilizing focal length in a heat-sensitive lens in accordance with embodiments. 
         FIG. 6  is a bottom view of an optical system having a light source coaxially mounted on one end of its objective lens system in accordance with another embodiment of the invention. 
         FIG. 7  is a schematic block diagram of a temperature control system for stabilizing focal length in an objective lens system in accordance with another embodiment of the invention. 
         FIG. 8  is a schematic block diagram of a temperature control system for stabilizing focal length in an objective lens system in accordance with another embodiment of the invention. 
     
    
    
     DESCRIPTION 
     A system  10  and method for stabilizing focal lengths in a heat sensitive lens in accordance with embodiments is illustrated in  FIGS. 1-5 . The system  10  includes a cylindrical lens housing  12  containing a conventional objective lens system  30 , a lamp supporting housing  14 , a temperature sensor  16 , a heating element  18 , a controller  20 , and a light source  22 , although the system  10  can comprise other numbers and types of components in other configurations. The method describes strategically heating the housing and maintaining the temperature of a heat-sensitive lens within a pre-selected range in order to stabilize the focal length of the lens. Embodiments provide a number of advantages including the reduction of inaccurate measurements and repeatability errors caused by heat-induced variations in lens focal length. 
     Referring to  FIGS. 1 and 2 , the system  10  includes a cylindrical lens housing  12  containing a conventional objective lens system  30 , a disc-shaped lamp supporting housing  14 , a temperature sensor  16 , a heating element  18 , and a controller  20  that adjusts current to the heating element to increase and decrease temperature measured at the temperature sensor. In the exemplary embodiment shown in  FIGS. 1 and 2 , an annular, generally disc-shaped lamp supporting housing  14  is secured to and surrounds the lower end of the lens housing  12 . A light source  22  is mounted in the lamp supporting housing  14 . Preferably, the light source comprises a plurality of lamps L, although any suitable type of light source can be used. The lamps L are secured or mounted at their inner ends in the lamp supporting housing  14 , and the lamps L project at their outer, light emitting ends downwardly from the housing  14  in the direction of an object  24 , which object  24  rests on a work table  26 . As shown more clearly in  FIG. 2 , in embodiments the lamps L are mounted in the lamp supporting housing  14  in five circular arrays or rings disposed coaxially of the axial centerline of the housings  12  and  14 . The lamps L are located proximate to and typically surround the lens  30  and can create large amounts of heat in the area around the lens  30 . The lens  30 , which is corrected for color aberrations, is sensitive to heat. When heated, the focal length of the lens  30  changes with temperature, directly affecting the front working distance and front focus of the system, which can result in distortion of the perceived Z-axis distance. 
     Referring to  FIG. 3 , a temperature control system preferably includes the controller  20 , heating element  18 , and temperature sensor  16 . The controller  20  includes a memory  80 , a processor  82 , an input/output unit  84 , and an indicator  86 , which are connected together by a bus  88  or other link, although other suitable types and numbers of components in other configurations and other types of processing systems can be used for the controller. In alternative embodiments, all of the components are placed on a single microchip or semiconductor device. The memory  80  can store instructions and data for performing one or more aspects of embodiments, including the methods described with references to  FIGS. 1-5 , although some or all of these instructions and data can be stored elsewhere. A variety of different types of memory storage techniques, such as a random access memory (RAM), a read only memory (ROM), flash ROM, EEPROM, and the like, and even hard disk drives, can be used by the memory  80  to store the instructions and data. 
     Referring to  FIGS. 1 ,  2  and  3 , the heating element  18  comprises heat tape, preferably of the OMEGA® KAPTON type of insulated flexible heaters (catalog no. khlv 0502/5-p), although other suitable types of heating elements can be used. The heating element  18  is preferably wrapped around the circumference of the cylindrical lens housing  12  as closely as possible to the lens  30 , although other locations and techniques for attaching the heating element  18  can be used. The temperature sensor  16  comprises a thermocouple, although other types of temperature sensors can be used, and is preferably mounted on the cylindrical lens housing  12  as closely as possible to the lens  30 , although other locations for attaching the temperature sensor  16  can be used. The temperature sensor  16  is preferably attached with adhesive, though other attachment techniques can be used. 
     Referring again to  FIG. 3 , the controller  20  in embodiments is operatively connected to the heating element  18  and temperature sensor  16  by wire, although other techniques for connecting the devices may be used, such as wireless communications techniques. The temperature sensor  16  preferably transmits a temperature measured proximately to lens  30  or signal representative thereof to the controller  20  which, in turn, increases or decreases current to the heating element  18  as required to maintain the temperature of the lens  30  within a target temperature range in accordance with methods disclosed herein. 
     Referring to  FIG. 4 , other embodiments for stabilizing focal lengths in a heat sensitive lens will now be described. The system  50  of embodiments includes a cylindrical lens housing  52  containing a conventional objective lens system  54 , a disc-shaped lamp supporting housing  56 , a light source  58  in the lamp supporting housing  56 , temperature sensors  60 ,  62 , and  64 , a heating element  66 , an indicator  86 , and a controller  70 . In such embodiments, the controller  70  preferably calculates a weighted average temperature of the lens system  54  based on inputs from temperature sensors  60 ,  62 , and  64 , although other calculating methods can be used. The controller  70  of embodiments then adjusts current to the heating element  66  as described herein until the temperature measured at the lens system  54  by temperature sensors  60 ,  62 , and  64  falls within the target temperature range. The entire target temperature range is preferably greater than the maximum operating temperature of the light source  58  to minimize temperature fluctuations, and hence minimize focal length-drift in the lens system  54 . The indicator  86  is preferably illuminated when the temperature measured at the lens system  54  is within the target temperature range in order to advise a user that the focal length of the lens system  54  has achieved the desired stability. 
     Referring to  FIGS. 1 ,  3 , and  5 , these figures illustrate an example of a method for stabilizing focal length in a heat-sensitive lens by heating the lens to a temperature that is greater than the maximum operating temperature of a light source surrounding the lens in accordance with embodiments. The method preferably comprises monitoring the temperature of the lens  30  using the temperature sensor  16  and maintaining the temperature of the lens  30  within a pre-selected range that is greater than the maximum operating temperature of a light source  22  surrounding the lens by controlling a heating element  18  attached to a cylindrical housing  12  surrounding the lens  30 . The pre-selected range in embodiments comprises a minimum and maximum temperature that can be stored in the internal memory of the controller  20 , entered into the controller  20  by a user, or provided to the controller by an external sensor such as a temperature sensor. The operating temperature range for the lens  30  will preferably be established based on the accuracy desired for the Z-axis measurements. Heating the lens  30  to the target temperature range and maintaining the lens temperature within that range will ensure the focus will remain constant whether the light source  22  has recently been turned on, remains on for a long period, or is turned off, so long as the entire range is set to be greater than the maximum operating temperature of the light source  22 . 
     In  FIG. 5 , at block  100 , the temperature sensor  16  of embodiments transmits a temperature measured proximately to lens  30  or signal representative thereof to the controller  20 . At block  110 , the controller  20  preferably compares the temperature or signal received from the sensor  16  to a pre-determined minimum temperature that is stored in the internal memory  80  of the controller  20 . If the temperature received from the sensor  16  is less than the range minimum, then at block  120  the controller  20  of embodiments increases current to the heating element  18 , after which the system returns to block  100 . If the temperature received from the sensor  16  is greater than the range minimum, then at block  130  the controller  20  of embodiments compares the temperature received from sensor  16  to a pre-determined maximum temperature that is also stored in the internal memory  80  of the controller  20 . If the temperature received from the sensor  16  is greater than the range maximum, then at block  140  the controller  20  of embodiments decreases current to the heating element  18 , after which the system returns to block  100  to perform another temperature measurement. If the temperature received from the sensor  16  is less than the range maximum, then at block  150  the controller  20  of embodiments maintains the current to the heating element  18  at its present level, and the controller  20  returns to block  100  to receive another temperature measurement. 
     In accordance with another aspect of this invention, the illuminator  22  comprises a plurality of individual light sources are arranged in segments as shown by the dotted lines  23  in  FIG. 6 . In some situations, for example to reduce or eliminate shadows, it is desirable to illuminate some but not all of the segments of light emitting diodes while leaving the other segments dark. This partial illumination enhances the ability to inspect the object under test but results in uneven heating of the lens and its supporting structure. In accordance with another aspect of this invention, the heater is provided that is also divided into segments preferably corresponding to the segments of the illuminator. 
     As shown in  FIG. 7 , a control system is provided for energizing the heater segments  66 ,  68 ,  72  corresponding to the temperature sensors  64 ,  62 , and  60  respectively of the illuminator. Only four segments are shown for simplicity, it being understood that a greater number of segments may be provided. In this way, the lens is heated relatively uniformly either by the heater or by the illuminator and various combinations of illuminated and dark segments may be employed without unevenly heating the lens and supporting structure. 
     The segmented heater of this embodiment of the invention may be controlled by a plurality of sensors  60 ,  62 ,  64  responsive to the temperature of a plurality of segments of the lens and/or the supporting structure to control the operation of the heaters to maintain a uniform temperature. Alternatively, in accordance with a somewhat simpler embodiment of the invention, the heating elements are designed to provide a heat output corresponding essentially to the heat output of the illuminator segments to which they correspond and a control system is provided that energizes heating element when it&#39;s corresponding illuminator segment is dark and the deenergizes the heating element when it&#39;s corresponding illuminator segment is on. 
     Turning for example now to  FIGS. 6 and 7 , four light emitting diode segments are provided each comprising two of the segments delimited by dotted lines  23  and each occupying approximately 90° of the total area of the illuminator  56 , and a four sensors  60 ,  62 ,  64 , and  65  (not shown) are utilized, each of the sensors can measure the temperature of a segment of the lens corresponding to a segment of the illuminator. The heating element is likewise divided into four segments,  66 ,  68 ,  72 , and  73 , not visible in the drawing, each segment corresponding to and generally aligned with one segment of the illuminator. 
     The controller  70  is connected to each of the sensors and each of the heaters and applies power to the heater segments corresponding to the dark sections of the illuminator and the sensors operate in a feedback relationship to maintain the temperature of each segment of the lens and a predetermined range. 
     While a variety of heating elements may be employed in connection with this invention, an arrangement in which the heating elements are resistors or other elements that generate heat when power is applied to them, arranged in the illuminator body is preferred. If resistors are employed, the value is selected so that the heat produced by the resistor is approximately the same as the heat produced by the corresponding segment of the illuminator when the LEDs are activated. 
     In accordance with another aspect of this invention, while a closed loop for static control system may be employed, if the heating element is selected to accurately match the heat output of the LED taking the thermal resistance of the path between the LED and the lens as well as the thermal resistance of the path between the heating element and the lens into account, a constant temperature may be employed by simply ensuring that one of but not both of the LED and heating elements is always illuminated, that is, during the time that the LEDs, or at least one of the LEDs is on, the heating element corresponding to each segment of LEDs that is not on is activated. This will ensure that a relatively constant amount of heat is coupled to the lens thereby making the temperature of the lens substantially uniform. 
     Referring again to  FIG. 8 , the controller  20  in this embodiments is operatively connected to the heating elements  66 ,  68 ,  72  and  73  and temperature sensors  60 ,  62 ,  64 , and  65  by wire, although other techniques for connecting the devices may be used, such as wireless communications techniques. The temperature sensors  60 ,  62 ,  64  and  65  preferably transmit a temperature measured proximately to lens  30  or signal representative thereof to the controller  20  which, in turn, increases or decreases current to the heating elements  66 ,  68 ,  72  and  73  as required to maintain the temperature of the lens  30  within a target temperature range in accordance with methods disclosed herein. 
     We have found that in addition to stabilizing the refractive index of the lens by maintaining a constant temperature, the heater of this invention also minimizes any focus effects due to expansion or contraction of the housing for the lens because of uneven heating. 
     While particular embodiments have been described, alternatives, modifications, variations, improvements, and substantial equivalents that are or may be presently unforeseen may arise to applicants or others skilled in the art. Accordingly, the appended claims as filed, and as they may be amended, are intended to embrace all such alternatives, modifications, variations, improvements, and substantial equivalents. Further, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims.