Abstract:
A modulated reflectance measurement system includes two diode-based lasers for generating a probe beam and an intensity modulated pump beam. The pump and probe beams are joined into a collinear beam using a laser diode power combiner. One or more optical fibers are used to transport the beams either before and/or after they are combined. The collinear beam is focused through one or more lenses or other optical components for collimation. The collinear beam is then focused by an objective lens onto a sample. Reflected energy returns through an objective and is redirected by a beam splitter to a detector. A lock-in amplifier converts the output of the detector to produce quadrature (Q) and in-phase (I) signals for analysis. A processor uses the Q and/or I signals to analyze the sample.

Description:
PRIORITY CLAIM  
       [0001]    The present application claims priority to U.S. Provisional Patent Application Serial No. 60/390,752, filed Jun. 21, 2002, the disclosure of which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The subject invention relates generally to optical methods for inspecting and analyzing semiconductor wafers and other samples. In particular, the subject invention relates to methods for increasing the accuracy and flexibility of systems that use modulated optical reflectivity to analyze semiconductor wafers.  
         BACKGROUND OF THE INVENTION  
         [0003]    There is a great need in the semiconductor industry for metrology equipment that can provide high resolution, nondestructive evaluation of product wafers as they pass through various fabrication stages. In recent years, a number of products have been developed for the nondestructive evaluation of semiconductor samples. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe. This device incorporates technology described in the following U.S. Pat. Nos.: 4,634,290; 4,646,088; 5,854,710; 5,074,669 and 5,978,074. Each of these patents is incorporated herein by reference.  
           [0004]    In the basic device described in the patents, an intensity modulated pump laser beam is focused on the sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and plasma waves are generated in the sample that spread out from the pump beam s pot. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or plasma from the pump beam spot.  
           [0005]    The presence of the thermal and plasma waves has a direct effect on the reflectivity at the surface of the sample. As a result, subsurface features that alter the passage of the thermal and plasma waves have a direct effect on the optical reflective patterns at the surface of the sample. By monitoring the changes in reflectivity of the sample at the surface, information about characteristics below the surface can be investigated.  
           [0006]    In the basic device, a second laser is provided for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample. A photodetector is provided for monitoring the power of reflected probe beam. The photodetector generates an output signal that is proportional to the reflected power of the probe beam and is therefore indicative of the varying optical reflectivity of the sample surface.  
           [0007]    The output signal from the photodetector is filtered to isolate the changes that are synchronous with the pump beam modulation frequency. In the preferred embodiment, a lock-in detector is used to monitor the magnitude and phase of the periodic reflectivity signal. This output signal is conventionally referred to as the modulated optical reflectivity (MOR) of the sample.  
           [0008]    It can be proven theoretically that the overlap and pointing stability of each individual laser is important for accurate results. Therefore, these devices can be quite sensitive to the laser pointing stability. This problem is particularly acute when dealing with semiconductor lasers.  
           [0009]    As shown in FIG. 1, and as described in U.S. Pat. No. 5,978,074, cited above, the current Therma-Probe system  100  uses a tracker mechanism to correct for the poor pointing stability of diode lasers. The tracker is a device used to optimize the overlap between the pump and probe lasers in the focusing plane (i.e., the sample). The tracker consists of a long focal length plano-convex lens with two motors to allow movement in both the x and y direction. The tracker is placed in front of the collimated pump laser and adjusts the pump laser output to match the output of the probe laser in the focusing plane.  
           [0010]    The following optimization method is then used to correct for the pointing instability. The tracker is scanned first in x-direction and then in y-direction. At each incremental movement the thermal wave signal is recorded on a reference sample. A software routine finds the maximum value of the thermal wave signal at which time the tracker moves to the corresponding position. The tracker scanning function is repeated frequently during operation of the system to improve the pointing stability of the pump laser relative to the probe laser.  
           [0011]    A drawback of this methodology is that pump laser effectively chases the probe beam. If the location of the probe beam drifts in a constant direction both lasers may be clipped at the focusing objective. Also, any “structure” in the pump or probe beam profile (which is typical of diode lasers), can lead to false maximums in the thermal wave signal, resulting in an error in pump/probe overlap. Another drawback of the methodology is that the tracker scanning is not performed in a 2-D plane (map) and thus prevents correction of non-symmetrical beams, which is usually the case of diode lasers.  
           [0012]    To improve the quality of the probe beam, U. S. Pat. 6,049,220 and 6,489,801 (both to P. Borden et al.) describe a photothermal system that uses a fiber-coupled infrared probe laser. However, in these patents only one laser (probe) is shown to have a fiber connection that only partially improves the overall system performance.  
           [0013]    For these reasons and others there is a need for a system that better optimizes the overlap between pump and probe lasers in modulated reflectance measurement systems. This is particularly important as semiconductor geometries continue to shrink and accurate measurements become increasing difficult to achieve.  
         SUMMARY  
         [0014]    The present invention provides a modulated reflectance measurement system that reduces alignment errors between pump and probe beams. The measurement system includes a probe laser and a pump laser, each producing monochromatic light at a different spectrum. A modulator is used to cause the pump laser to have an intensity modulated output, referred to as the pump beam. The probe laser produces an output that is typically non-modulated (i.e., constant intensity). This output is referred to as the probe beam.  
           [0015]    The output of the probe laser and the output of the pump laser are joined into a collinear beam using a laser diode power combiner. An optical fiber transports the now collinear probe and pump beams from the laser diode power combiner to a lens or other optical device for collimation. Once collimated, the collinear beam is focused on a sample by an objective lens.  
           [0016]    A reflected portion of the collinear probe and pump beams is redirected by a beam splitter towards a detector. The detector measures the energy reflected by the sample and forwards a corresponding signal to a filter. The filter typically includes a lock-in amplifier that uses the output of the detector, along with the output of the modulator to produce quadrature (Q) and in-phase (I) signals for analysis. A processor typically converts the Q and I signals to amplitude and/or phase values to analyze the sample. In other cases, the Q and I signals are used directly.  
           [0017]    For another implementation of the measurement system, separate optical fibers are used to collect the pump and probe beams from their laser sources. The optical fibers transport the pump and probe beams to an optical combiner. The optical combiner joins the pump and probe beams into a collinear beam that is transported by another optical fiber to a lens or other optical device for collimation. As before, the collinear beam is reflected by the sample and analyzed using a detector, filter and processor.  
           [0018]    In another implementation of the measurement system, separate optical fibers are used to collect the pump and probe beams from their laser sources. One of these fibers transports the pump beam to a tracking mechanism. The second optical fiber transports the probe beam to a dichroic mirror. The dichoric mirror also collects the pump beam as it leaves the tracking mechanism. The two beams are joined into a collinear beam and focused onto a sample by an objective lens. As before, the collinear beam is reflected by the sample and analyzed using a detector, filter and processor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a block diagram of a prior art modulated reflectance measurement system.  
         [0020]    [0020]FIG. 2 is a block diagram of a modulated reflectance measurement system that uses optical fibers to transport pump and probe laser outputs.  
         [0021]    [0021]FIG. 3 is a block diagram of a modulated reflectance measurement system that uses optical fibers in combination with an optical combiner to transport pump and probe laser outputs.  
         [0022]    [0022]FIG. 4 is a block diagram of a modulated reflectance measurement system that uses a laser diode power combiner and an optical fiber to transport pump and probe laser outputs.  
         [0023]    [0023]FIG. 5A is cross-sectional diagram of the combined pump and probe beam produced by the prior art modulated reflectance measurement system of FIG. 1.  
         [0024]    [0024]FIG. 5B is graph showing signal strength as a function of position for the cross-section of FIG. 5A.  
         [0025]    [0025]FIG. 6A is cross-sectional diagram of the combined pump and probe beam produced by the modulated reflectance measurement systems of FIG. 2.  
         [0026]    [0026]FIG. 6B is graph showing signal strength as a function of position for the cross-section of FIG. 6A.  
         [0027]    [0027]FIG. 7 is graph showing laser pointing stability as a function of time of the prior art modulated reflectance measurement system of FIG. 1.  
         [0028]    [0028]FIG. 8 is graph showing laser pointing stability as a function of time of the modulated reflectance measurement systems of FIGS. 2, 3 and  4 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]    The present invention provides a modulated reflectance measurement system that reduces alignment errors between pump and probe beams. In FIG. 2, a first possible implementation for the modulated reflectance measurement system is shown and generally designated  200 . As shown, modulated reflectance measurement system  200  includes a probe laser  202  and a pump laser  204 . Each laser  202 ,  204  is typically monochromatic and each laser  202 ,  204  typically operates at a different spectrum. Lasers  202 ,  204  are generally diode-based or diode-pumped semiconductor lasers. Solid state laser diodes are available that have outputs throughout the entire visible spectrum as well as in the infrared and near UV. Lasers  202 ,  204  are controlled by a processor  206  and a modulator  208 . Modulator  208  causes pump laser  204  to have an intensity modulated output, referred to as the pump beam. Probe laser  202  produces an output that is typically non-modulated (i.e., constant intensity). This output is referred to as the probe beam.  
         [0030]    As the probe beam leaves probe laser  202 , it is collected by an optical fiber  210 . Optical fiber  210  is typically single mode and directs the probe beam through a dichroic mirror  212  towards a sample  214 . Sample  214  is positioned on an X-Y stage  216  allowing sample to be moved in translation relative to the probe beam. As the pump beam leaves pump laser  204 , it is collected by a second optical fiber  218 . Optical fiber  218  is typically single mode and directs the pump beam to a tracking mechanism  220 . After leaving tracking mechanism  220 , the pump beam is redirected by dichroic mirror  212 . The redirection aligns the pump beam to be collinear with the probe beam as the probe beam travels towards sample  214 .  
         [0031]    After striking sample  214 , the reflected probe beam is redirected by a beam splitter  222  towards a detector  224 . Detector  224  measures the energy reflected by sample  214  and forwards a corresponding signal to a filter  226 . Filter  226  typically includes a lock-in amplifier that uses the output of detector  224 , along with the output of modulator  208  to produce quadrature (Q) and in-phase (I) signals for analysis. Processor  206  typically converts the Q and I signals to amplitude and/or phase values to analyze the sample. In other cases, the Q and I signals are used directly.  
         [0032]    The use of optical fiber to deliver light from both probe laser  202  and pump laser  204  improves the individual pointing stability of each laser as well as the beam structure and circularity (symmetry) of the probe and pump beams. In practice, the pointing stability can be less than 1 μrad/° C.  
         [0033]    The inventors herein have implemented a system as illustrated in FIG. 2. In this system the diode lasers of FIG. 1 are replaced with fiber-coupled diode lasers as seen in FIG. 2. In FIG. 2, two laser diodes  202  and  204  are each fiber-coupled to a polarization-maintaining and single mode fiber,  210  and  218 . The assembly of the laser diodes  202  or  204  with coupling fiber  210  or  218 , respectively, is usually referred to as a “pig-tailed laser fiber” and is provided as a complete assembly from Point-Source, Southhampton, UK. The diode pin-outs and the driving current requirements of the new pig-tailed laser fibers are identical to the old diode lasers of FIG. 1. This served as the basis to a retro-fit design enabling the new fiber-coupled diode lasers to be interfaced to the old PCB boards and to thus maintain the same communication interface as the old system. In the old system the diode laser package included the PCB board, diodes and collimation optics in one assembly which then directed each output beam to a dichoric splitter (FIG. 1). In the retro-fit design the PCB boards with the pigtail diodes are placed in one assembly  1 m away from the usual optical configuration. A  1 m flexible stainless steel jacket is strain-reliefed into a  1 m coiled bare fiber which then guides the beam through an FC connector onto the collimating optics. The  1 m coiled bare fiber and collimating optics are now placed in the same optical path of the old diode lasers and the beam output from each collimating optics is directed to the dichroic splitter,  212 .  
         [0034]    In FIG. 3, a second possible implementation for the modulated reflectance measurement system is shown and generally designated  300 . As shown, modulated reflectance measurement system  300  includes a probe laser  302  and a pump laser  304 . Each laser  302 ,  304  is typically monochromatic and each laser  302 ,  304  typically operates at a different spectrum. Lasers  302 ,  304  are generally diode-based or diode-pumped semiconductor lasers. Solid state laser diodes are available that have outputs throughout the entire visible spectrum as well as in the infrared and near UV. Lasers  302 ,  304  are controlled by a processor  306  and a modulator  308 . Modulator  308  causes pump laser  304  to have an intensity modulated output, referred to as the pump beam. Probe laser  302  produces an output that is typically non-modulated (i.e., constant intensity). This output is referred to as the probe beam.  
         [0035]    The probe beam output of probe laser  302  and pump beam output of pump laser  304  are collected by optical fibers  310  and  312 , respectively. The beams from fibers  310  and  312  are collimated and direct the probe and pump beams to a combiner  314 . The beam combiner typically includes a dichroic element. The now collinear probe and pump beams leave combiner  314  and are focused into fiber  316 . One suitable fiber optic beam combiner is manufactured by OZ Optics of Canada, part number FOBS-12P. Fiber  316  directs the collinear beams through collimating optics  318  to sample  320 . Sample  320  is positioned on an X-Y stage  322  allowing sample to be moved in translation relative to the collinear beams.  
         [0036]    After striking sample  320 , a reflected portion of the collinear probe and pump beams is redirected by a beam splitter  324  towards a detector  326 . Detector  326  measures the energy reflected by sample  320  and forwards a corresponding signal to a filter  328 . Filter  328  typically includes a lock-in amplifier that uses the output of detector  326 , along with the output of modulator  308  to produce quadrature (Q) and in-phase (I) signals for analysis. Processor  306  typically converts the Q and I signals to amplitude and/or phase values to analyze the sample. In other cases, the Q and I signals are used directly.  
         [0037]    In general, modulated reflectance measurement system  300  provides the same combination of pointing stability, beam structure and beam circularity (symmetry) described for the implementation of FIG. 2. In this case, however, the use of combiner  314  means that there is no need for the tracking mechanism used in the implementation of FIG. 2 and other systems.  
         [0038]    In FIG. 4, a third possible implementation for the modulated reflectance measurement system is shown and generally designated  400 . As shown, modulated reflectance measurement system  400  includes a probe laser  402  and a pump laser  404 . Each laser  402 ,  404  is typically monochromatic and each laser  402 ,  404  typically operates at a different spectrum. Lasers  402 ,  404  are generally diode-based or diode-pumped semiconductor lasers. Solid state laser diodes are available that have outputs throughout the entire visible spectrum as well as in the infrared and near UV. Lasers  402 ,  404  are controlled by a processor  406  and a modulator  408 . Modulator  408  causes pump laser  404  to have an intensity modulated output, referred to as the pump beam. Probe laser  402  produces an output that is typically non-modulated (i.e., constant intensity). This output is referred to as the probe beam.  
         [0039]    The probe beam output of probe laser  402  and pump beam output of pump laser  404  are collimated into a collinear beam using a laser diode power combiner  410 . The now collinear probe and pump beams leave combiner  410  and are focused into fiber  412 . One suitable laser power combiner is manufactured by OZ Optics of Canada, part number ULBS-11P. The fiber  412  directs the collinear beams through collimating optics  414  to sample  416 . Sample  416  is positioned on an X-Y stage  418  allowing sample to be moved in translation relative to the collinear beams.  
         [0040]    After striking sample  416 , a reflected portion of the collinear probe and pump beams is redirected by a beam splitter  420  towards a detector  422 . Detector  422  measures the energy reflected by sample  416  and forwards a corresponding signal to a filter  424 . Filter  424  typically includes a lock-in amplifier that uses the output of detector  422 , along with the output of modulator  408  to produce quadrature (Q) and in-phase (I) signals for analysis. Processor  406  typically converts the Q and I signals to amplitude and/or phase values to analyze the sample. In other cases, the Q and I signals are used directly.  
         [0041]    In general, modulated reflectance measurement system  400  provides the same combination of pointing stability, beam structure and beam circularity (symmetry) described for the implementations of FIG. 2 and  3 . In this case, however, the use of laser diode power combiner  410  reduces the number of optical fibers required.  
         [0042]    For the purposes of comparison, FIG. 5A shows a typical combined pump and probe beam as produced by the modulated reflectance measurement system of FIG. 1. As demonstrated in that figure, the combined beam is elliptical and astigmatic. This non-Gaussian output is shown graphically in FIG. 5B. As may be appreciated, there are numerous deviations from the ideal output. FIG. 6A and 6B repeat the same demonstration for the combined output beams produced by modulated reflectance measurement system  200 . As shown in FIG. 6A, the combined beam produced by these devices is substantially circular and, as shown in FIG. 6B, substantially Gaussian.  
         [0043]    [0043]FIG. 7 continues the comparison by showing the pointing stability of the combined pump and probe beam as produced by the modulated reflectance measurement system of FIG. 1. As demonstrated in that figure, the alignment between the pump and probe beams varies over time. This is due, in part to thermal cavity effects in the laser diodes that create the pump and probe beam. The thermal cavity effects result in small deviations in beam direction. As shown in FIG. 8, the fiber configuration used by modulated reflectance measurement system  200  greatly decreases this time varying quality. The result is a system in which the laser output is relative stable, greatly reducing the need for active correction of beam position or other compensating measures.