Abstract:
A practical method for greatly enhancing the strength of the modulated signal from laser probing of IC&#39;s is described. An IC device under test (DUT) is scanned with two spatially separated laser beams. The output from a single laser source is split into two separate components with each focused on different areas of the DUT. The separation between the beams and their intensity is adjustable to maximize the strength of the modulated return signal. Typically a NIR laser is used with flip-chip IC devices to account for the band-gap (transmission) characteristics of the substrate material. Upon reflection from the DUT, the reflected beams are recombined to interfere with one another. The phase difference of the two beams is adjustable to gain maximum interference. This signal is then processed to obtain the waveforms that correspond to the actions of the active gates and nodes as the chip is electronically cycled through its prescribed test loop. This method significantly improves the signal to noise ratio and reduces the time it takes to acquire a useful voltage waveform.

Description:
CLAIM OF PRIORITY 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 62/057,854, filed Sep. 30, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     Aspects of the present disclosure relate generally to optical probing of semiconductor devices and more particularly to split beam optical probing. 
     BACKGROUND 
     Obtaining voltage, switching and timing measurements from currently manufactured CMOS and various IC&#39;s is now a standard procedure in debug and failure analysis of these complex devices. Since the introduction of flip-chip packaging technology, where access to the internal device structures is provided through the backside of the DUT, two optical methods, both non-destructive in nature, are typically used for measuring the electrical activity in IC&#39;s. 
     The first is known as Picosecond Imaging Circuit Analysis (PICA). Here a NIR sensitive camera or optical detectors such as avalanche photo diode is used in conjunction with the appropriate imaging optics to capture photons emitted by a circuit element as it switches logic states. The time-dependent light emission is used to obtain time resolved images of the switching events. 
     The second prevalent methodology is Laser Voltage Probing (LVP). LVP appears the currently preferred method for acquiring voltage and timing data from IC&#39;s. There are several improvements to LVP method that have been made. One such scheme employs two separate laser pulses which are focused to the same spot in DUT. One called the probe beam is used when the electrical circuit is active, and the other, called the reference pulse, is applied when the circuit is off. The two beams are displaced in time with respect to each other, but use a common optical path and sample the same physical location on the DUT. This cycle is repeated and the ratio of these two measurements taken again. By averaging multiple sets of ratios, the noise is reduced as compared to fluctuations inherent in a single measurement taken alone.  FIG. 1  is a schematic illustration of the principles of one embodiment this technique. A laser light source  100  generates an incident beam  101 . The incident beam passes through beam steering optics  150  that include a beam splitter cube  105  and a beam scanning module  115 . The beam splitter cube  105 , e.g., a polarizing beam splitter, diverts a portion of the incident beam to be sensed by an incident beam detector  110 . The main component of the beam passes through the beam splitter cube  105  and into the beam scanning module  115 . The beam scanning module may be programmed to raster scan or vector point the beam over a region of interest  121  on the device under test DUT  120 . Probe beam optics  125  typically serves to focus the incident beam on an active area of the DUT, e.g., a switching gate of an IC. The incident beam, which in this case is also the probe beam, interacts with the electrically active region of the DUT and is modulated in amplitude as the voltage across the junction changes. Upon reflection, the returning laser light which now carries information encoded by the DUT, is captured by the probe beam optics  125  and relayed back along the incoming path. Upon arrival at a signal detector  130 , the optical signal is converted to an electrical output, e.g., by an appropriately chosen fast photodiode and in conjunction with analog to digital conversion electronics, the signal is further processed by an oscilloscope  135  or similar signal processing electronics displaying an averaged voltage waveform. A synchronization circuit  140  handles various timing functions between the oscilloscope, the laser source and a test pattern generator  145 . Further details on this system are described in U.S. Pat. No. 5,905,577, which is incorporated herein by reference. 
     A further refinement of the preceding technique is shown in  FIG. 2 . Since the incident laser beam at the DUT not only undergoes amplitude changes but phase modulation as well, a Michelson type interferometer  200  is used to capture this additional phase information as a change in amplitude that can be measured. This scheme is sometimes referred to as Phase Interferometric Detection (PID) mode. In this mode, a portion of the incident laser beam is picked off by a beam splitter  210 . This portion is referred to as the reference beam. The interferometer  200  further includes a reference arm containing a lens  220  and mirror  230 . The remaining portion of the incident beam is directed to a specific area of interest on the DUT. This portion is sometimes referred to as the probe beam. On reflection from the DUT the probe beam is modulated by the response of the DUT. The light beam  250  reflected by the DUT and the light beam  260  reflected by the reference arm mirror  230  are then spatially combined into the return beam  270  that now contains interference effects. The interference effects convert relative phase differences between the reflected beam from the DUT  240  and the reference arm beam  260  into amplitude differences in the combined return beam  270  which can be detected by a photo detector. Further details of the technique illustrated in  FIG. 2  may be found in U.S. Pat. No. 6,496,261, which is incorporated herein by reference. 
     Another variation on the LVP method is called Polarization Differential Probing (PDP). Here the incident laser beam is divided into two beams each having orthogonal polarization with respect to the other. One of the polarized beams is used as a reference, while the other is designated the probe beam. Both beams are superimposed on each other, and follow a common path to be simultaneously focused onto the same location on a DUT. As shown in  FIG. 3 , a linearly polarized laser beam  300  is incident upon a polarization rotator  310  that rotates the polarization of the beam through some chosen angle to enter beam dividing and recombining optics  320  to provide two orthogonally polarized, but superimposed beams  325  and  330 . Both beams follow a common path through beam pointing optics  335  where they are directed to be simultaneously incident on the same spot on the DUT  340 . The interaction of the DUT with the laser beams is somewhat polarization dependent, and the phase of each is modulated differently according to the DUT test signals. The reflected light which contains this modulated component then retraces its incoming path and is made to interfere where the difference in phases converted to amplitude and sensed by detectors. Two separate detectors are provided to collect the orthogonal components from the two polarized beams. The signals are then passed on to collection electronics and a signal analysis system to extract the desired data. Further details of this technique may be found in U.S. Pat. No. 7,659,981 B2, which is incorporated herein by reference. 
     Yet another conventional technique used for phase detection, sometimes called Spatial Differential Probing (SDP) is illustrated in  FIG. 4 . A laser beam  405  is split into two component beams  420 ,  430  having mutually orthogonal polarization, e.g., by a Wollaston prism, which is located within beam manipulation optics  410 . The two beams  420  and  430  have orthogonal, linear polarization states shown by dots in beam  420  and arrows in beam  430 . One beam, e.g.  420 , is directed to a first region  421  of the DUT, e.g., an active device region, while the other beam, e.g.,  430  can be directed to a second region  422 . Upon reflection of the two beams, beam manipulation optics  410  recombines to two reflected beams and converts them to the same polarization state so that they may interfere with each other to generate amplitude modulated resultant beam  440 . The beam manipulation optics  410  may also include elements to provide phase offset and/or recombination of the returning beams. The phase noise due to DUT vibrations is reduced in this scheme because both beams are modulated similarly. 
     In this arrangement the separation between the beams is fixed. Since the geometry of various DUTs is not standard and depends upon its internal design and by the manufacturers&#39; choices, a practical system must offer adjustability for separation of the beams. Also since the reflectivity of the area where the beams are placed can differ, a practical system must have adjustability of power for both beams to obtain best results. Examples of such systems are described, e.g., in U.S. Pat. Nos. 5,872,360, 7,616,312 B2, and 7,659,981 which are incorporated herein by reference. 
     A major difficulty remains with all laser based probing systems in that the signal is weak and needs separation from residual noise. The typical modulated intensity lies in the range of 100 to 200 parts per million (˜0.01%), requiring considerable time and instrumental capacity for signal averaging. To acquire a waveform with good edge definition, in practice takes from several minutes to an hour or more depending on the DUT design. Tying up equipment for such a long time places a considerable constraint on the output capacity of a semiconductor test facility. 
     It is within this context that aspects of the present disclosure arise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1  is a schematic illustration of a prior art LVP system. 
         FIG. 2  is a schematic illustration of another prior art PID version of system illustrated in  FIG. 1   
         FIG. 3  is a schematic illustration of a prior art based Polarization Differential Probing (PDP) system. 
         FIG. 4  is a schematic illustration of a prior art Spatial Differential Probing (SDP) system. 
         FIG. 5  is a schematic illustration of embodiment split beam optical probing system according to an aspect of the present disclosure. 
         FIG. 6A  is a three-dimensional schematic diagram of a split beam optical probing system according to another aspect of the present disclosure. 
         FIG. 6B  is a schematic diagram illustrating a map of polarization transformations of probe and reference beams in incident and return paths in a split beam optical probing system according to an aspect of the present disclosure. 
         FIG. 6C  is a three dimensional diagram illustrating probe and reference beams retracing their paths from a DUT in a telecentric imaging system. 
         FIG. 7  is a graph illustrating typical signal modulation as a function of phase change induced by the split beam separation on DUT. The figure highlights the importance of the adjustability of the split beam separation to capture π/2 or its integral multiple equivalent phase differences that can provide maximum sensitivity to signal modulations. 
         FIGS. 8A-8C  are graphs depicting three example cases in schematic, the influence of the externally induced phase on the ideal modulation plot. Any such deviations could provide a potential diagnostic tool in applications such as IC fault analysis. 
     
    
    
     DETAILED DESCRIPTION 
     Introduction 
     The technique presented here is novel in reducing signal acquisition time during the laser probing process of a DUT. The method scores some important advantages over prior art in improved throughput by reducing the amount of signal averaging needed, besides providing reliability and simplicity in architecture. A laser beam from a single laser source is split into two beams by optical means, each resultant beam is linearly polarized but in orthogonal directions. One of the beams, called a probe beam, travels along the optical systems axis and is focused onto an active IC structure. The other beam, called the reference beam, is laterally displaced by a small amount from the probing beam so that its focus falls on an adjacent, typically inactive area of the IC. Upon reflection from the device, both beams retrace the reverse path through the optical system and through the split beam generating section where both beams are recombined. Upon further beam conditioning, a detector then measures the intensity fluctuations due to interference of the recombined beams and stores that data as a time-varying signal. 
     Unlike prior art, this suggested technique is capable of controlling the separation between the probe and reference beam focal spots on the DUT. This important feature is highly desirable in applications such as failure analysis of ICs in which the device spacing varies. Additional key features include the ability to control the relative phase of the probe and reference beams and the ability to control the relative intensity of the input probe and reference beams for optimum signal modulation, and simultaneously prevent potential laser damage to the DUT from overexposure. The contemporary literature does not talk about these features in the context of SDP. 
     Variable separation of the probe and reference beams allows for optimal placement to obtain a modulated signal. Since the device structures vary in size according to the type of DUT, controlling the placement of the probe and reference beams becomes important. If, for example, the mirrors in the beam splitting apparatus are aligned in the null position, both beams are aligned and coincident with each other and both will come to focus at the same spot in the DUT. As the reference mirror is tilted with respect to the optics axis, the focal point of the reference beam will be laterally displaced from the probe beam. This unique feature enables the ability to adjust the separation between the probe and reference beam foci on DUT to acquire optimum signal modulation even across different layouts and dimensions of ICs. Also, vibration induced noise is also mitigated by parking the reference beam nearby on the DUT structure. At the DUT probe regions, a portion of both incident beams are reflected and the energy collected and precisely retraced through the telecentric optical system. Upon passing through the beam separating optics in the reverse direction, the beams are recombined and focused on a detector that senses the modulation signal. 
     According to another aspect of this disclosure, the optics may be configured such that the amplitudes of the return beams are the same, which is often the condition for best signal modulation. Since the energy of the reflected beams is dependent on the DUT&#39;s composition at the respective focal points and on its electrical state, their amplitudes may not optimally match. This can be corrected with an optical modulator configured to change the ratio of amplitudes in both beams. In some implementations, the optical modulator may be a ½ wave plate through which both beams pass. Rotating the ½ wave plate changes the ratio of the amplitudes of the two beams. Alternatives to a ½ wave plate include a Faraday rotator, an electro-optic (EO) rotator, two rotatable polarizing elements (e.g., two polarizing beam splitter elements). An EO rotator has the advantage of being faster. As this is done, the polarization angle of the incoming beam changes, and more energy can be diverted into the probe beam and less to the reference, or the reverse, more energy can be launched into the reference beam and less so into the probe beam. As further explanation, if the polarization vector of the incoming beam is rotated so that it strikes the hypotenuse of a polarizing beam splitter (PBS) in the P-polarized state only, all the laser beam energy will pass into the probe beam. If the polarization vector is rotated so that it will strike the hypotenuse in the S-polarization state, all the energy will go into the reference beam. These are the extreme states and the most likely adjustment will probably lie somewhere in between. 
     Further according to another aspect of the disclosure, the intensity of the incident laser beams should be controlled to prevent laser damage to the DUT. In this arrangement it is achieved by varying the laser input power before the beam enters the beam conditioning optics as described in the later part of detailed description. As the laser beam power is increased, the probe and reference beams both increase in equal proportion. Same holds when the laser power is decreased. 
     As can be seen from still another aspect of the disclosure, the phase difference between the probe and reference beams contributes to maximum modulation. Since the probe beam is phase shifted from its interaction with the active structures of the DUT, the optimum phase difference can be set and maintained by adjusting the mirror position along the optical axis within the beam separating apparatus. 
     Adjustable Split-Beam Optical Probing (ASOP) System 
     An adjustable split beam optical probing system according to certain aspects of the present disclosure will now be described, with reference to  FIG. 5 , in a manner that highlights the differences from prior art. Most particularly, a split beam module (SBM)  500  is used to generate two temporally coherent but spatially separated beams, which may be independently placed at separated locations anywhere on the DUT  505  within the field of view (FOV) of the imaging system. Other unique aspects of this invention will be noted in the description provided below. 
     In one aspect of the invention, as shown in the block diagram of  FIG. 5 , a light source control module LSC  570  having a narrow band light source  510  such as a single mode laser and controls for the output power BPC  515  provides a primary beam of radiation, which may be pulsed or continuous wave (CW). The wavelength of the radiation in the primary beam may vary, e.g., in a range from 900 nm to 1400 nm, with the choice of wavelength largely depending upon the material makeup and test parameters of the DUT. The light source  510  may also include two or more separate sources (e.g., two or more lasers) that produce beams of different wavelength that are optically coupled into a single output beam. Such an arrangement can provide the basis for automated control over wavelength selection. By way of example, and not by way of limitation, the light source  510  may be a single mode laser (SML) that provides a single mode, linearly polarized laser beam as the primary beam. Laser sources have been used in LVP techniques to obtain the interference phenomenon, due to the mutual coherence of phase information between the polarized split beams. On the other hand, mutually incoherent sources such as LEDs and white light suffer from random phase in time and do not interfere with each other. However, the split-beam concept described herein may be extended to other divisional properties of light sources such as amplitude, wavelength, or frequency of these incoherent sources and still make use of their associated modulation phenomena. 
     The primary beam from the light source  510  passes through the beam power control module (BPC)  515 . The BPC controls and regulates the total power into the system and thus the probe beam to limit DUT exposure to below its damage threshold. In practice, the power level may be set by an operator via system controls. The BPC output then passes through a polarization maintaining fiber optic cable  520  that is coupled into the I/O module  525 . Fiber optic coupling is used over direct beam coupling to provide more freedom in placing component and assemblies while incurring only minimal coupling losses. Within the I/O module, after further beam conditioning takes place, a small portion of the beam directed to the power monitor assembly (PMA)  530  and the main portion is passed into the SBM  500 . Here the incoming beam is split into two orthogonally polarized components, one of which, called the ‘probe beam’, is directed along the optics axis to a scanner module (SM)  535  and optics module (OM)  540  to focus onto an active region of the DUT. The other, called the ‘reference beam’, undergoes a controlled and selectable small angular displacement with respect to the probe beam and follows a closely adjacent path through the SM and OM to focus onto the DUT  505  at a different location than the probe beam. Typically the separation of the focus locations for the probe and reference beams depends on the objective magnification, FOV and the level of telecentricity of the optics module  540 . The separation could be anywhere between zero to a few tens of microns for high magnification objectives, such as 100× and solid immersion lens (SIL) objectives. 
     At the DUT  505 , typically the probe beam undergoes reflectance and phase change in response to electrical interaction of the structure being probed. Portions of both incoming laser beams are then reflected back through the OM (also called collection optics) and the SM to retrace their path into the SBM to be spatially recombined. The beams exiting the SBM now overlap and have linear but orthogonal polarization states. Upon entering the I/O module, the polarization states are rotated so that components of each can optically interfere. The interfering components are transferred via fiber optic cable to the optical conversion module OCM  545  where the optical signal is transformed to an electrical voltage. An oscilloscope (OSC)  550  then acquires this signal and displays the corresponding voltage waveform. A synchronization circuit (SCM)  560  then coordinates the various timing functions of the test pattern generator  565 , the GSM and OSC. For clarity the computer control connections are not shown. 
     Although  FIG. 5  depicts optical fiber  520  to couple light from the beam power control module  515  to the I/O module  525 , and from the I/O module  525  to the optical conversion module  545  aspects of the present disclosure are not limited to such implementations. Alternatively, these optical connections may be implemented in whole or in part, e.g., using free space optic components. Optical fiber connections are often advantageous, however, in terms of better component placement and portability. 
     A major distinction of aspects of the present disclosure from prior art, is the adjustable separation between probe and reference beams coming from the same light source and focused onto the DUT. This allows probing of ICs that usually have wide range of geometries of active and inactive nodes. The method of achieving adjustable separation of the beam is explained in the detailed description. By strategically choosing one of the components as a reference beam on an inactive node, the other component may be used for active probing the entire FOV. As an alternative technique, both beams can be made to scan together the entire the FOV with or without separation. The two orthogonally polarized reflected components are made to retrace their paths back in to split beam module  500  where they are recombined for enhanced modulation to obtain timing signal waveforms on detector. The same components used to split the laser beams for the illumination beam effectively works for their recombination in return. Besides, such separation adjustability between the reference and probe beams gives additional control to optimize and automate the signal acquisition process, as and when new IC architecture emerges for probing applications. 
     DETAILED DESCRIPTION 
     A more detailed example of optics  600  compatible with the system of  FIG. 5  is depicted in  FIGS. 6A-6C . A single linearly polarized beam B, e.g., from the LSC module  570 , may enter an I/O module  601 . In this module, the beam B may be appropriately shaped by lens  602  and a small portion of the beam is picked-off by a first PBS  603  and sent to a power detector sensor and circuit  604 . The major portion of the beam B passes through the first PBS  603  and enters a split beam module  606  containing a polarization rotator  607  and a second PBS  608 . The polarization rotator  607  re-orients the linear polarization of the beam with respect to an incident angle upon the hypotenuse of the second PBS  608 , which transmits a portion of the beam and reflects the remaining beam. The second PBS  608  separates the S and P polarization components of the beam B. One component, which makes up the probe beam  620 , passes through a ¼ wave plate  609 , is retro-reflected by an adjustable mirror  610 , passes through the ¼ wave plate again, now having its polarization state rotated by 90 degrees, re-enters the second PBS  608  to pass straight through and then on to the imaging optics  640 . The other component which makes the reference beam  630 , also passes through a ¼ wave plate  611 , is then retro reflected by a fixed mirror  612 , passing through the ¼ wave plate again. Now having its polarization rotated by 90 degrees, the reference beam  630  is reflected by the second PBS  608  and follows a path more or less parallel to the optical axis through an imaging section  640 . The imaging section includes a tube lens  617  and microscopic objective  618  that focuses the probe beam  620  and reference beam  630  onto the DUT at selected spatially separated test points. The probe and reference beam spots may be diffraction-limited at the target. Lateral separation between the probe and reference beam spots may be of order a few microns with simplified optics, but in theory the separation may be unlimited. A portion of the incident probe and reference beams are reflected at the focal position, the energy collected by the microscope optics and retraced through the microscope. Upon passing in the return trip through the split beam module  606 , the beams are recombined and focused on an optical signal detector  605  that senses the modulation signal. 
       FIGS. 6A-6C  schematically illustrate an example of optical components of a laser scanning microscope with built-in split beam module  606  that helps to execute the novel technique to split and recombine the laser components. While the description is focused on the illustrated layout, the concept can easily be extended to any other type of microscopic configuration used for laser signal acquisition, with minor changes. In brief, the illumination laser beam is spatially split by the split beam module  606  and used for scanning the DUT  619  as required. The figure illustrates an implementation that may be used in fault analysis of an integrated circuit (IC) on DUT with the help of a scanner and a microscope. The reflected beams from a DUT in a well configured telecentric system with altered amplitude and phase due to DUT structure retrace their paths and interfere to cause an amplified modulated signal in the signal channel. 
     As discussed above, the input beam B may be generated by a light source that may include a linearly polarized laser, an optical isolator, power control mechanism to prevent laser damage to the DUT, and a fiber delivery mechanism. The output of the light source with a slightly tilted horizontal polarization is shaped by lens  602  and is made to go through the first polarizing beam splitter (PBS)  603  oriented for suitable laser power distribution in both orthogonal directions on output. A small s-polarized component may be directed for power calibration to detector  604  while a significantly larger p-polarized component enters the split beam generator module  606 . 
     The adjustable polarization rotator  607  rotates the polarization of the horizontally polarized component of the input beam B is by 45°. In the illustrated example, the second PBS  608  may split the beam B equally in power and send the two polarization components to two corresponding arms of the split beam module  610 . Each beam encounters a quarter wave plate ( 609 ,  611 ) and a mirror ( 610 ,  612 ), flips its polarization and reenters the second PBS  608 . As a result, the PBS  608  directs the orthogonally polarized probe and reference beams towards the scanner  615 , which may include one or more adjustable tilt mirrors. In order to facilitate spatially adjustable reference and probe beams on DUT, the mirror ( 610  in this case) facing the scanner  615  may be piezo-controlled in both axial position and angle. A small amount of rotation of the adjustable mirror  610  generates the ‘probe’ beam  620  (dashed line) that undergoes an angular shift from the optical axis on exit from the PBS  608 . The other beam component is usually kept confined to the optical axis is considered the ‘reference’ beam  630  (solid line). In some implementations, flip-in and flip-out physical stops  613  and  614  may be provided in each arm of the split beam module  606  to selectively work with either probe or reference beam for ease in their identification in assembly and special DUT investigations. As mentioned earlier, at a given instant of time, the probe and reference beams created by the split beam module  606  travels through the scanner  615 , scan lens  616 , tube lens  617  and the microscopic objective  618  and finally focus at two strategically identified positions on DUT with their spatial separation controlled by the piezo-guided mirror  610 . 
     Such split beam can be used for probing IC devices may be understood by noting the transformation of beam polarization at various locations along the optical track for both illumination and return split beam.  FIG. 6B  illustrates the top view of same embodiment with details on the polarization status of the illumination and return split beam across the I/O module  601 , split beam generator module  606 , and scanner  615  and imaging optics  640 . As discussed earlier, input polarization direction may be so selected to output most of the laser power to the split beam generator module  606 , redirecting a small fraction towards power calibration with the help of a PBS  603 . The polarization rotator  607 , and second PBS  608  of the split beam generator module  606  together generate the probe and reference beams of orthogonal polarization with adjustable separation using quarter wave plates ( 609 ,  611 ) and mirrors ( 610 ,  612 ), at least one of which is adjustable. For the sake of simplicity, the discussion that follows describes an example in which one mirror ( 610 ) is adjustable and the other ( 612 ) is not. However, aspects of the present disclosure are not limited to such implementations. In alternative implementations, both mirrors  610 ,  612  may be independently adjustable in terms of axial position and tilt with respect to one or two dimensions. The split beams  620 ,  630  then travel through the scanner and imaging optics  640  to the DUT, where they are reflected by the DUT, follow a reverse path through the imaging optics and scanner and reenter the interferometer block with their respective same polarization states with or without some phase separation induced by DUT feature topography. 
     As may be noted, for the case when there is no tilt on the adjustable mirror  610  the split beam module  606  simply sends a beam of two superposed orthogonally polarized beams towards the scanner module. With a small tilt on the adjustable mirror  610 , the reference beam separates out from the probe beam but travels along with a tilt that focuses spatially separated on DUT, after traversing the scanner  615  and imaging optics  640 . Upon reflection from the DUT, the return probe and reference beams retrace their path back into the interferometer block with respective amplitude and phase modulations introduced by DUT features. The return probe and reference beams upon retracing their paths of the interferometer are closely realigned by PBS  608  and travel towards the I/O section  601 . The beams are again rotated on passing through polarization rotator  607  before entering the first PBS in the I/O module  601 . As a result, two beams of the same polarization, one from the probe beam and the other from the reference beam reflect off the hypotenuse surface of PBS  603  toward the optical signal detector  605  with relative amplitude and phase differences. The output of the optical signal detector  605  depends on these relative amplitude and phase differences. 
       FIG. 6C  illustrates an example in which the imaging optics are telecentric. In this example, the split beam components, i.e., probe beam  620  and reference beam  630  from the imaging objective  618 , fall normally on a first region  654  and second region  655  of a DUT  619 . By way of example, and not by way of limitation, the first region  654  may be an inactive device or region of the DUT  619  and the second region  655  may be an active device or region of the DUT. Alternatively, the first and second regions may be selected for maximum sensitivity for modulation. For example, a phase difference may be induced due to the optical path difference between the probe and reference beams in the split beam generator module  606 . In addition to this induced phase difference, the first and second regions may be selected, e.g., so that probing of an active device by the probe beam at the first region  654  with reference to probing another active device by reference the beam at the second region  654  introduces an additional phase difference. The induced phase difference may be adjusted so that the total relative phase difference is π/2 or an odd integral multiple of π/2, which would meet the condition for maximum sensitivity for modulation sensed by the optical signal detector  605 . 
     The advantages of the split beam technique can be much appreciated by studying some theoretical aspects of the interfering return signal beam modulation characteristics as a function of the split beam separation. In the following, some theoretical facts supported by experimental findings are discussed for completion. 
     The resultant intensity of the recombined interfering beams in the signal module can be written as
 
 I=A   p   2   +A   r   2 +2 A   p   A   r  cos(α)  (1)
 
where α=∝r−∝ p  is the phase difference between reference and probe beams, A p  and A r  are the amplitudes of the probe and reference beams respectively and ∝ p  and ∝ r  being their phase components.
 
Equ.(1) can be rewritten as
 
 I=I   avg (1+ m  cos(α))  (2)
 
Where I avg =A p   2 +A r   2 =I p +I r  is the average intensity and
 
             m   =       2   ⁢         I   p     ⁢     I   r             (       I   p     +     I   r       )             
is the contrast.
 
When I p =I r =I 0 , I avg =2I 0  and m=1, and thus
 
 I= 2 I   o (1+cos(α))  (3)
 
Equ.(3) illustrates the fact that the intensity can be as high as 4I 0  (when α=0, 2π, 4π, . . . ) or as low as 0 (when α=π, 3π, 5π, . . . ).
 
Such cases of perfect contrast is also often represented in interferometry as
 
                   I   =     4   ⁢           ⁢     I   0     ⁢       cos   2     ⁡     (     a   2     )                 (   4   )               
with high and low values of the interference intensity as 4I 0  and 0, respectively. For incoherent sources ∝ p  and ∝ r  are random in time and the average cos(α) term vanishes, causing no modulation. Similarly, single laser sources alone have only amplitude with no relative phase manipulation possible. In both cases, I=I avg =constant with no modulation possible. Subjecting I to its first derivative with respect to the phase difference α in Eq.(2), gives us the option for maximum sensitivity for modulation.
 
δ I=−I   avg   m  sin(α)δα  (5)
 
       FIG. 7  illustrates the scale of such intensity modulation as a function of phase difference between probe and reference beams. The dotted line  701  represents the ideal intensity pattern given by Equ.4 as a function of optical path difference (OPD) induced phase difference between the probe and reference beams. For the embodiment described in this invention, the probe beam is aligned along the optical axis of the imaging system while the reference beam can be moved from superimposed axial position to a few tens of microns away from the probe beam on DUT by turning the adjustable mirror  610  in the split beam module  606 . The controlled angular beam deviations and the resultant split beam separation on DUT for different microscopic objectives in the optical system can be calibrated offline beforehand. In the case of, e.g., a PZT controlled mirror  610 , the PZT controller output in volts can be used, instead, as a means to change phase, oscilloscope  550  together with a spectrum analyzer can be used to obtain the signal waveform amplitude and intensity profiles. The resultant signal as a function of split beam separation in terms of phase or PZT voltage is plotted that resembles the dotted sinusoidal curve  701  shown in  FIG. 7 . 
     Such an outcome of the technique should find some interesting applications where intensity modulations play an important criteria for decision making. It is evident that the phase parameter α is contributed by optical path difference between the split beams as well as voltage driven DUT features. Equ.(5) reveals the maximum sensitivity to phase variations when the phase difference (a) between the probe and reference beam is statically set to π/2 or its odd integral multiple and when the amplitudes of the two beams are the same or very close to the same value (i.e., m≅1). In  FIG. 7 , this is vividly illustrated for two separate split beam separations whose induced phase differences α are 7π/2 and 7π. The split beam separation corresponding to 7π/2 (an odd π/2 multiple) produces significantly enhanced signal modulation  702  for a small induced phase  703  compared to the modulation  704  produced by 7π(a π multiple) phase difference equivalent beam separation. So applications requiring detection of intensity variations may benefit from having such adjustable separation between the split beams. On a similar note, weakly activated nodes in an IC may be probed for maximum signal by adjusting the separation between the split beams for maximum intensity as given by Equ (4). This technique for maximum sensitivity in signal modulation offered by a split beam is of interest in many practical applications in IC fault analysis for efficiently probing even small electronic device nodes that are otherwise difficult to capture during signal mapping process. An interesting part of the waveform acquisition by oscilloscopes is the amplitude reversal with changes in phase induced by split beam separation that result in baseline intensity modulation as predicted by Equ. 4 and the dotted sinusoidal curve  701  in  FIG. 7 . Any additional phase contributions coming out of DUT induced phase from an activated node or device would result in altering the base waveform. This could be an additional debug or diagnostic tool in such applications. 
     It is useful to mention the various forms of phase that can influence the sinusoidal response expected in  FIG. 7 . Three examples of such cases, illustrated in  FIGS. 8A-8C , are worthy of mention. In these illustrations the dotted line curve  801  represents the ideal sinusoidal curve solely accounted by the OPD induced phase difference of the split beam as per Equ 4, and the solid line curve  802  is the real performance curve due to the additional phase term influencing the ‘cos’ term. In reality, this additional phase term could be linear or non-linear depending on the application.  FIG. 8A  represents a simple addition or subtraction of a linear phase from DUT causing simple shift of the sinusoidal function. Non-linear phase terms, on the other hand, cause contraction or elongation of the split beam performance function along the phase axis as shown in  FIG. 8B . Both linear and non-linear phase induction is common to probing applications.  FIG. 8C  illustrates phase aberrations induced by the optical design and assembly errors, causing the amplitude, intensity and thus contrast roll off for large split beam separations. It is important to note that the embodiment and the technique of the present disclosure provide a potentially valuable debugging tool for various applications where the deviation from the expected result is an important diagnostic piece of information. 
     The examples described in this disclosure may operate in a way that optimizes the signal beam modulation. For example, the split beam generator module  606  may be initially coarse adjusted and set for equal path lengths for probe and reference beams until an interference fringe pattern with maximum extinction ratio is obtained as scanned image of a plane mirror in place of the DUT  619 . The adjustable mirror  610  can then be set to adjust the spatial and axial position of the probe and reference beam foci on DUT. Subsequent fine axial adjustments between probe and reference beams aid in maximizing the return power from the split beam on a voltage activated DUT. Furthermore, the flip-in and flip-out physical stops  613  and  614  offer additional control on optimizing the signal beam characteristics by working individually on each component of the split beam. The optimal phase difference α could be met by the axial translation settings on adjustable mirror  610 . The final interfering probe and reference beams entering signal collection I/O module  601  are from the laser with same polarization, and hence can be adjusted for maximum extinction ratio suitable for maximizing the return signal. The condition for near equal amplitudes can be achieved by optimally dividing the power between the input beam components with polarization rotator  607 . 
     Aspects of the present disclosure allow for split beam probing with flexible probe and/or reference beam placement, enhanced signal to noise ratio and decreased sampling time. 
     While the above is a complete description of the preferred embodiment of the present invention, it is possible to use various alternatives, modifications and equivalents. Therefore, the scope of the present invention should be determined not with reference to the above description but should, instead, be determined with reference to the appended claims, along with their full scope of equivalents. Any feature described herein, whether preferred or not, may be combined with any other feature described herein, whether preferred or not. In the claims that follow, the indefinite article “A”, or “An” refers to a quantity of one or more of the item following the article, except where expressly stated otherwise. The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase “means for.”