Patent Publication Number: US-2010117667-A1

Title: Method and means for optical detection of internal-node signals in an integrated circuit device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority benefit of U.S. Provisional Patent Application No. 61/198,547 (Docket # 84-1), entitled “METHOD AND MEANS FOR IMPROVED OPTICAL DETECTION OF INTERNAL-NODE SIGNALS IN AN INTEGRATED CIRCUIT DEVICE,” filed Nov. 7, 2008, by William K. Lo, which is incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to methods and apparatus for probing of an IC (integrated circuit) device with a CW (continuous-wave) light source. 
     BACKGROUND 
     The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions. 
     Laser Voltage Probing (LVP) is an established technique used to extract signals from the internal circuitry of operating silicon integrated circuit (IC) devices for the purposes of design debug, failure analysis, or other diagnostic activities. The technique dates back to the mid-1980&#39;s with the pioneering work of Heinrich and Bloom (U.S. Pat. No. 4,758,092, July 1988, Method and means for optical detection of charge density modulation in a semiconductor) but was not widely used until the late 1990s when the first commercial system, the Schlumberger IDS2000, became available. 
     The Schlumberger IDS2000 used pulses from a mode-locked laser source and used custom data acquisition electronics to make measurements via stroboscopic sampling (also referred to as equivalent-time-sampling). A noise cancellation technique was invented for the IDS2000 to reduce the effect of noise caused by DUT vibrations (U.S. Pat. No. 5,905,577, May 1999, Dual-laser voltage probing of IC&#39;s). 
     As demonstrated by Heinrich, Bloom, and Hemenway (Applied Physics Letters 48(16), 1986, pp 1066-8, Noninvasive sheet charge density probe for integrated silicon devices) LVP can also be performed using a CW (continuous-wave) laser with a real-time oscilloscope for the acquisition electronics. Modern real-time digital storage oscilloscopes use fast analog-to-digital converters to digitize the data. They acquire the waveform data as a series of samples. For the same average laser power, the number of photons captured in each sampling interval in a CW laser based LVP system is much less than the number of photons in a single pulse from the mode-locked laser in a stroboscopic sampling based LVP system. This relative photon deficit increases the photon shot noise, so it is detrimental to the signal-to-noise ratio of the measurements made by the CW laser based LVP system. 
     In principle, it is possible to increase CW laser power to make up for the photon deficit. In practice, however, there is a limit to the amount of laser power that can be delivered to the DUT before the device is damaged and/or other invasive effects occur. For CW lasers, the primary damage mechanism is thought to be thermal (heating) in nature which is related to the average laser power delivered. Therefore, there is a practical upper limit to how much average CW power can be used during a measurement. 
     SUMMARY 
     Although various embodiments of the invention may have been motivated by various deficiencies with the prior art, which may be discussed or alluded to in one or more places in the specification, the embodiments of the invention do not necessarily address any of these deficiencies. In other words, different embodiments of the invention may address different deficiencies that may be discussed in the specification. Some embodiments may only partially address some deficiencies or just one deficiency that may be discussed in the specification, and some embodiments may not address any of these deficiencies. 
     In an embodiment, a continuous-wave laser beam is chopped to form pulses synchronized to the activity of interest of a device under testing (DUT). In another embodiment, a continuous-wave laser beam is chopped to form pulses synchronized to the measurement activity of the acquisition electronics. By chopping the laser beam the duty-cycle of the beam is reduced, which allows the power delivered to the device during the actual measurement time interval to be increased while maintaining a lower average power. By chopping the laser beam the signal-to-noise ratio (SNR) is improved during the continuous-wave laser voltage probing measurements. By chopping the laser beam the performance of the continuous-wave laser based laser voltage probing system is improved, thereby improving the measuring of the internal signals of an operating integrated circuit device. 
     In an embodiment, the DUT is a complex IC exercised with a test pattern that generates an electrical response in at least part of the internal circuitry of the IC. Various methods for applying the test pattern can be used. For example, the test pattern may be a functional test pattern applied to the input connections of the packaged IC by an automatic test equipment (ATE) tester, or the test pattern may be generated by built-in self-test (BIST) circuitry in the IC, activated by applying signals to the IC through a Joint Test Action Group (JTAG) interface, or the test pattern may be one or more software applications running on a computer, exercising the IC in a system-level environment representative of its intended application. There are many other methods of applying test patterns. To improve SNR, LVP measurement data is accumulated and/or averaged multiple times, requiring at least the signal(s) of interest inside the IC be made repetitive. To generate a repetitive signal(s) of interest, the test pattern itself may be caused to be repetitive, or portions within the test pattern corresponding to the signal(s) of interest may be caused to be repetitive, or other means may be used. 
     In an embodiment, the chosen test pattern is such that the signal(s) of interest only span a fraction of the total test pattern period. In an embodiment, the signal(s) of interest indicates whether or not a portion or a function of interest of the IC is behaving properly. In another embodiment, the signal(s) of interest may indicate if a component attached to the IC is behaving properly. In another embodiment, the IC may be probed to facilitate determining whether the test pattern or other input signals are correct. It may be that the entire test pattern is necessary to produce the signal(s) of interest, or it may be that only a portion of the test pattern is necessary to produce the signal(s) of interest. However, although the rest of the DUT&#39;s response to the test pattern may contain information, the rest of the response is not necessary for determining whether the portion or function of interest of the IC is behaving properly. The IC operation is only analyzed over a portion of the total response of the IC to the test pattern. Instead of illuminating the DUT with laser radiation over the total test pattern period, the CW laser beam is chopped to form pulses that are synchronized with, the signal(s) of interest. The total average laser radiation delivered to the DUT is then significantly reduced and/or the power or intensity of the laser radiation during the actual measurement time span is increased. The total test pattern may span 100 microseconds, for example, while the signal(s) of interest may span only 100 ns. In this case, irradiating the DUT only during the span of the signal(s) of interest instead of the whole test pattern allows the average laser power incident on the DUT to be reduced by 1000 times. Alternatively, the power applied during the measurements may be increased by 1000 times while maintaining the same average power (although, in practice, it may be necessary to limit the actual increase in power to a more modest level). 
     In an embodiment, the signal(s) of interest span the whole test pattern period. In this case, synchronizing the laser pulses to the signal(s) of interest requires that the laser irradiate the DUT continuously, and so no advantage is gained over an unchopped CW-LVP measurement. However, due, for example, to inefficiencies in the acquisition electronics used to measure the signal(s) of interest, not all repetitions of the signal(s) of interest may be measured. For example, dead-time of the acquisition electronics may prevent the making of measurements on a subsequent repetition of the signal(s) of interest if the repetition follows soon after a previous measurement. In an embodiment, the laser pulses are synchronized with the measurement activity of the acquisition electronics instead of more directly to the signal(s) of interest. In this way, laser pulses are only generated when the acquisition electronics is capable of making measurements. Dead time, may, for example, only allow every fourth repetition of the signal(s) of interest to be acquired. Irradiating the DUT only during those repetitions when the acquisition electronics is capable of making a measurement then reduces the average laser power delivered to the DUT by four times. Alternatively, the peak laser power used during the measurements can be increased by up to four times. 
     In another embodiment, the signal(s) of interest may repetitively occur at indeterminate times within the test pattern. For example, when testing signals related to memory read operations in a DUT in a systems-level test environment, the memory read operations may be the same each time for a particular memory element, but the read operation may be performed at indeterminate times within the test pattern. Some instances of the signal(s) of interest may occur close in time to other instances, while some instances may be spaced much further apart in time. In this situation, synchronizing the laser pulses to the measurement activity of the acquisition electronics also provides benefits, by allowing less average laser power to be used for probing, for example. 
     In an embodiment, the laser radiation during the measurement time span is increased over what the radiation would have been had the laser radiation been applied during the entire test pattern or during the entire time of test. Increasing the radiation reduces the impact of shot noise thereby improving the measurement SNR, allowing acquisition times to be reduced by reducing the number of times that the same measurement needs to be taken to obtain a final waveform image with sufficiently high SNR and/or, allowing a final waveform with higher SNR to be obtained in the same amount of time (or a combination both). Optionally, transient effects of the laser beam on the reflected laser beam may be compensated for. Some examples of transient effects are thermal effects and the creation of free carriers, such as electron hole pairs, which may affect the index of refraction and/or the absorption of light by the silicon. 
     It is not necessary to reduce the laser beam to zero intensity during the ‘off’ periods. However, the effectiveness of this scheme may be reduced, depending on how much laser radiation ‘leaks’ through to the DUT during the ‘off’ periods. 
     Any of the above embodiments may be used alone or together with one another in any combination. Inventions encompassed within this specification may also include embodiments that are only partially mentioned or alluded to or are not mentioned or alluded to at all in this brief summary or in the abstract. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In the following drawings like reference numbers are used to refer to like elements. Although the following figures depict various examples of the invention, the invention is not limited to the examples depicted in the figures. 
         FIG. 1  shows a schematic illustration of the basic components of a Laser Voltage Probing (LVP) system. 
         FIG. 2A  shows a block diagram of an embodiment of a Laser Source used in Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2B  shows a block diagram of an embodiment of Acquisition Electronics used in Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2C  shows a block diagram of an embodiment of a Acquisition Electronics used in Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2D  shows a block diagram of an embodiment of a DUT Stimulus used in the Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2E  shows a block diagram of an embodiment of a DUT Stimulus used in the Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2F  shows a block diagram of an embodiment of a Photodetector used in the Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2G  shows a block diagram of an embodiment of a Photodetector used in the Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2H  shows a block diagram of an embodiment of a Microscope Optics used in Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2I  shows a block diagram of an embodiment of a workstation used in Laser Voltage Probing system of  FIG. 1 . 
         FIG. 2J  shows a diagram of an embodiment of a DUT being irradiated with a laser beam. 
         FIG. 3  shows timing diagram of an embodiment of Laser Voltage Probing with Chopped CW Laser and with Real-Time Sampling. 
         FIG. 4A  shows a plot illustrating an example of the temperature response of the DUT to an application of laser light. 
         FIG. 4B  shows a plot illustrating the temperature response to the chopped continuous wave laser signal. 
         FIG. 5A  shows a flowchart of an embodiment of a method of setting up the system of  FIG. 1  for waveform acquisition. 
         FIG. 5B  shows a flowchart of an embodiment of a method of using the system of  FIG. 1   
         FIG. 6  is a flowchart of an embodiment of a method of making the system of  FIG. 1 . 
         FIG. 7A  is a flowchart of an embodiment of a method of determining the damage threshold of a DUT irradiated with a CW laser beam. 
         FIG. 7B  is a flowchart of an embodiment of a method of determining the damage threshold of a DUT irradiated with a chopped CW laser beam. 
     
    
    
     DETAILED DESCRIPTION 
     Although various embodiments of the invention may have been motivated by various deficiencies with the prior art, which may be discussed or alluded to in one or more places in the specification, the embodiments of the invention do not necessarily address any of these deficiencies. In other words, different embodiments of the invention may address different deficiencies that may be discussed in the specification. Some embodiments may only partially address some deficiencies or just one deficiency that may be discussed in the specification, and some embodiments may not address any of these deficiencies. In the specification, the terms irradiate and illuminate and their conjugations may be substituted one for the other to obtain different embodiments. 
     In general, at the beginning of the discussion of each of  FIGS. 1-4B  is a brief description of each element, which may have no more than the name of each of the elements in the one of  FIGS. 1-4B  that is being discussed. The brief description is usually given in numerical order to facilitate easily locating a particular element. After the brief description of each element, each element is further discussed. Nonetheless, there is no one location where all of the information of any element of  FIGS. 1-7B  is necessarily located. Unique information about any particular element or any other aspect of any of  FIGS. 1-7B  may be found in, or implied by, any part of the specification. 
     In general, heavy weight lines in  FIG. 1  and  FIG. 2  are used to represent laser radiation paths while lighter weight lines are used to represent electrical signal paths. While electrical paths may be drawn as a single line, this is done for the purposes of clarity only and it is to be understood that these paths may represent multiple signals and multiple physical wires may be used to carry these signals. While laser radiation is generally shown as traversing ‘free-space’ for the purposes of clarity, it is understood that fiber optic cables may be used for beam delivery, where convenient and appropriate, and that using fiber optic cables may also entail the use of fiber coupling and/or fiber collimating optics. 
     For the sake of clarity, components that are not necessary for an understanding of the invention (but that would be understood to be present by one of ordinary skill in the art) are not shown. These include, but are not limited to, components such as a objective lens turret to allow different microscope imaging fields-of-view and different focused laser spot sizes, mechanical stages to allow navigation of the optical components relative to the DUT, Control signals for the microscope optics to allow the laser beam to be raster scanned for imaging and statically pointed at a specific location for laser voltage probing, cooling apparatus to temperature control the DUT, and power supplies to provide power to the various components. 
       FIG. 1  shows a schematic illustration of the basic components of an embodiment of Laser Voltage Probing (LVP) system  100 . In other embodiments LVP system  100  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
       FIG. 1  includes laser source control signals  105 , clock signal  106 , photodetector electrical output signal  107 , laser radiation  110 , laser source  120 , microscope optics  130 , and objective lens  140 , and incident/reflected laser beam  150 , Device Under Test (DUT)  160 , device stimulus  165 , device drive and response signals  167 , reflected laser beam  170 , photodetector focusing lens  180 , photodetector  190 , acquisition electronics  195 , control and data signals  196 , computer workstation  197 , and trigger signal  198 . 
     A beam of NIR (near-infrared) laser radiation  110  from a laser source  120 , which is generated under the control of control signals  105 , are focused into the DUT  160  using microscope optics  130  and objective lens  140 . During laser voltage probing, laser radiation  110  is a chopped continuous wave laser beam. The portion of the laser radiation reflected by the DUT and re-collected by the objective lens retraces the beam path into microscope optics  130  which diverts it to lens  180  which then focuses it into photodetector  190 . Output signal  107  from photodetector  190  is digitized using acquisition electronics  195 . Acquired waveform data is transferred through data signals  196  to workstation  197  for further processing, display, and storage. 
     Control signals  105  are generated by the acquisition electronics  195  to control the laser source. Laser source  120  incorporates one or more sources of laser radiation that allows generating a continuous output and an optionally chopped output to form pulses of laser radiation for laser voltage probing. Control signals  105  and laser source  120  are further discussed, below, and in the context of  FIG. 2A  and  FIG. 2B . 
     Trigger signal  198  is a signal used to synchronize the laser radiation irradiating the DUT and/or to synchronize the acquisition electronics with the DUT signal(s) of interest. Trigger signal  198  is generated by the electronics that stimulates the DUT, or is, alternately, generated by the DUT and is routed through the electronics for the purposes of signal buffering and/or for convenience. Clock signal  106  is a clock signal generated by the electronics that stimulates the DUT, or is, alternately, a clock signal generated by the DUT that is routed through the electronics for the purposes of signal buffering and/or for convenience. The clock signal  106  may be used to aid in the synchronization of the laser radiation irradiating the DUT and/or to aid in the synchronization of the acquisition electronics with the DUT signal(s) of interest. Clock signal  106 , trigger signal  198 , and DUT stimulus  165  are discussed further, below, and with reference to  FIG. 2B ,  FIG. 2C ,  FIG. 2D , and  FIG. 2E . 
     Electrical output signal  107  is the electrical representation of reflected laser beam signal  170  after conversion by the photodetector. Conversion of the laser signal  170  to electrical signal  107  allows the signal to be processed by the acquisition electronics  195 . The conversion process may involve filtering the signal and/or involve splitting the signal into multiple signals for separate acquisition. Splitting the signal may occur optically and/or electrically. The photodetector  190 , the electrical output signal  107 , and the laser signal  170  are discussed further, below, with reference to  FIG. 2F ,  FIG. 2G  and  FIG. 2H . 
     Acquisition electronics  195  contains electronics that digitizes the photodetector output signal  107 . Prior to and after digitization, further processing of the output signal  107  may occur. Prior to digitization, amplification, attenuation, and/or offsetting of the signal may be necessary to match the signal level to the input range of the analog-to-digital converter (ADC), and bandwidth limiting filters may be applied, for example. After digitization, digital filtering, averaging, binning of the data, for example, may be performed. Acquisition electronics is further discussed below, and with reference to  FIG. 2B  and  FIG. 2C . 
     Computer workstation  197  runs the software which controls the laser voltage probing system and/or further processes, displays, and stores the waveform data. Waveform data is transferred from acquisition electronics  185  through data signals  196  to workstation  197 . Computer workstation  197  is further discussed below, with reference to  FIG. 2I . 
     The microscope optics  130  includes optics that is useful for directing laser beam  110  to objective lens  140  in a manner such that objective lens  140  can direct a focused laser spot into DUT  160  for the purposes of imaging and/or for the purposes of laser voltage probing. Microscope optics  130  also contains the optics required to separate the portion of laser beam  150  reflected by the DUT from the portion of laser beam  150  focused into the DUT and from laser beam  110  delivered to microscope optics  130  from laser source  120 . Microscope optics  130  is further discussed below with reference to  FIG. 2H . 
       FIG. 2A  shows a block diagram of an embodiment of the components in a laser source  120 . Laser source  120  includes laser head  120   a , laser controller  120   b , beam modulator  120   c , modulator driver  120   d , laser drive signal  120   f , modulator control pulses  120   g , and modulator drive signal  120   h . Laser head  120   a  generates CW laser beam  120   i  which is chopped to form pulses  120   j  by modulator  120   c  under control of modulator driver  120   d  and modulator control pulses  120   g . In other embodiments laser source  120  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Modulator control pulses  120   g  are generated by acquisition electronics  195  and transferred to laser source  120  through control signals  105 . In an embodiment, modulator driver  120   d  is an analog driver that can be used to generate laser pulses of variable amplitude, and/or of complex shape, according to the amplitude and shape of the modulator control pulses  120   g.    
     The laser, including laser head  120   a  and laser controller  120   b , is used as a component of laser source  120  to generate CW beam  120   i . Any of several types of laser sources may be used for laser source  120 , including laser diodes, diode-pumped solid-state lasers, fiber lasers, q-switched lasers, etc. 
     Although  FIG. 2A  shows an Acousto-optic modulator  120   c  being used to chop the CW laser beam  120   i , chopping a laser beam can be accomplished by a variety of other means: Mechanical, electro-optical, pulsing of the laser drive current, beam deflection (such as occurs in a laser scanning microscope (LSM)) across an aperture, q-switching of the laser, and other means that will be apparent to those skilled in the art. Electro-optical modulators, acousto-optical modulators, and drive-current pulsing can allow some level of pulse shaping (versus simply turning the laser on and/or off). Shaping of the laser pulse may be advantageous, for example, to limit the size of the impulse delivered to the photoreceiver and electronics by purposely degrading the on/off and off/on transition times. The use of electro-optical modulators, acousto-optical modulators, and drive-current pulsing also allow CW laser pulse-widths, duty-cycles, and pulse shapes to be easily and widely varied. The use of electro-optical modulators, acousto-optical modulators, and drive-current pulsing also allow the laser beam to be variably attenuated for laser power control. 
     Although  FIG. 2A  shows only one laser in laser source  120 , multiple lasers can be used to generate multiple beams  110 . A laser primarily intended for laser voltage probing can be combined with a laser primarily intended for laser scanning microscope (LSM) imaging, for example. In one embodiment, the laser intended for LSM imaging can be a broadband source with wide spectrum for reducing the effects of interference, while the laser intended for LVP can be narrowband with narrow spectrum designed especially to have low laser noise. 
       FIG. 2B  shows a block diagram of an embodiment of the components in acquisition electronics  195 . Acquisition electronics  195  includes oscilloscope  195   a  and signal generator  195   b . In other embodiments, acquisition electronics  195  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Two input channels of oscilloscope  195   a  are used to acquire AC signal  195   c  and DC signal  195   d , which are generated by photodetector  190  and transferred to acquisition electronics through output signal  107 . The acquisition of data by oscilloscope  195   a  is synchronized to the DUT signal(s) of interest using trigger events  195   h  and optionally with the aid of clock pulses  195   i , both of which are from DUT stimulus  165  and transferred to acquisition electronics through trigger signal path  198  and clock signal path  106 . Oscilloscope  195   a  is controlled by control signals  195   e , which are generated by workstation  197  and transferred to acquisition electronics through control signals  196 . Status signals and Data from oscilloscope  195   a  are transferred to workstation  197  using data paths  195   e  and  196 . Signal generator  195   b  is controlled by control signals  195   f , which are generated by workstation  197  and transferred to acquisition electronics through control signals  196 . Status signals from signal generator  195   b  are transferred to workstation  197  through data paths  195   f  and  196 . 
     Signal generator  195   b  is used to generate laser chopping pulses  195   g  which are transferred to laser source  120  through control signal path  105 . Chopping signal  195   g  is a series of variable width and delay pulses that are synchronized to the DUT signal(s) of interest, via, in an embodiment, trigger signal  198 , and optionally with the aid of clock signal  106 . In an embodiment, signal generator  195   b  can be programmed to drive modulator driver  120   d  such that laser pulses  120   j  are shaped to tailor the response of the DUT and/or the photodetector  190  to the pulsed laser radiation. 
     CW-LVP systems in general benefit from the use of commercially available real-time digital storage oscilloscopes. Hence, oscilloscope  195   a  can be one of several different real-time oscilloscopes offered by, for example, Tektronix, Agilent, or LeCroy. Programmable signal generator  195   b  is also available commercially from companies such as Stanford Research Systems or Tektronix. While it may be beneficial to use commercially available electronic equipment, it is not a requirement for the application of the CW-LVP described herein. 
     In an embodiment, AC signal  195   c  and DC signal  195   d  are both acquired by oscilloscope  195   a . While it is sufficient to only acquire AC signal  195   c  to capture the necessary waveform information for LVP, capturing DC signal  195   d  is beneficial in a number of ways. Dividing AC signal  195   c  by DC signal  195   d  allows AC signal  195   c  to be normalized to account for varying amounts of incident laser power, varying reflectivity of the probe location in the DUT, and varying losses in the optical path of the LVP system. Capturing and displaying DC signal  195   d  allows DUT drift to be detected while an acquisition is in progress (versus stopping the acquisition and imaging the DUT with the microscope to directly detect drift). 
     Laser voltage probing requires the use of a separate electrical trigger events  195   h  to trigger the waveform acquisition by oscilloscope  195   a  because the SNR of a single waveform measurement is too low for accurate triggering on the measured signal itself. This trigger signal may be supplied directly to the oscilloscope as trigger event  195   h , or may be generated internally by the oscilloscope using a combination of both trigger event  195   h  and clock pulses  195   i  using the advanced triggering capabilities commonly available in modern oscilloscopes. Oscilloscope  195   a  may include triggering capabilities that allow, for example, the triggering circuitry of the oscilloscope to be only ‘armed’ with trigger event  195   h  but actually triggered by (i.e., measure time relative to) a transition of one of the clock pulses  195   i . Other advanced triggering modes may be available in, and used by, oscilloscope  195   i.    
       FIG. 2C  shows a block diagram of another embodiment of acquisition electronics  195 . In other embodiments, acquisition electronics  195  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. The primary difference between the embodiments illustrated in  FIG. 2C  and  FIG. 2B  is that in the embodiment of  FIG. 2C  the signal generator  195   b  is triggered using a programmable output of oscilloscope  195   a  that has been programmed to output trigger signal  195   h . Trigger output  195   h  is synchronized to the actual start of an acquisition by oscilloscope  195   a . Dead time in oscilloscopes prevent the oscilloscope from acquiring data on every trigger event  195   h . By triggering signal generator  195   b  using trigger output  195   h  instead of trigger signal  198  (with or without aid of clock signal  106 ) the DUT will be irradiated only when the oscilloscope is actually acquiring data. 
     Depending on the rate of trigger events  195   h , this may further reduce laser radiation incident on DUT by approximately 2× or more. 
       FIG. 2D  shows a block diagram of an embodiment of DUT stimulus  165 . DUT stimulus  165  includes ATE tester  165   a  and load board  165   b . In other embodiments, DUT stimulus  165  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. ATE tester  165   a  generates and receives test signals  165   c . Test signals  165   c  are routed through load board  165   b  which configures the signals mechanically and electrically for the particular DUT package. The DUT is driven by drive signals  167 . In an embodiment, ATE tester  165   a  also generates trigger signal  197  and clock signal  106 , which are used to synchronize the test pattern portion of interest to the LVP system&#39;s acquisition electronics. In an embodiment, trigger signal  197  is generated by DUT, but routed through ATE tester  165   a  for buffering. In an embodiment, ATE tester  165   a  generates a repetitive test pattern that exercises the DUT in a repetitive manner. In another embodiment, ATE tester  165   a  generates signals to enable and configure built-in self-test (BIST) features in the DUT which then generates the repetitive test pattern internally. 
       FIG. 2E  shows a block diagram of another embodiment of DUT stimulus  165 . In other embodiments, DUT stimulus  165  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. Computer workstation  165   d  interfaces to applications board  165   e  through signal bus  165   f . Signal bus may be a standard personal computer bus such as PCI express, or USB, for example. Applications board may be a personal computer (PC) motherboard or a PC graphics board, for example. Clock signal may be from a crystal oscillator on the applications board, or may be an internal clock signal of the DUT, routed out through the applications board. Trigger signal may be an event programmed to be output from the DUT through a test pin and routed through applications board  165   e , for example. There are many ways to generate trigger  197  and optional clock signal  106  that can be used to synchronize the Laser source and/or LVP acquisition electronics to the DUT signal(s) of interest. 
       FIG. 2F  shows a block diagram of an embodiment of photodetector  190 . Photodetector  190  includes photoreceiver  190   b , RF bias-T  190   c , RF electronic amplifier  190   d , DC amplifier  190   e , and high pass filter  190   m . In other embodiments, photodetector  190  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. Reflected laser beam  170  is converted by photoreceiver  190   b  to a representative electrical signal  190   g . Electrical signal  190   g  is passed through RF Bias-T  190   c  which splits the electrical signal into two components—an AC component  190   h  and a DC component  190   j . AC component  190   h  is further amplified by RF amplifier  190   d  to give amplified AC signal  190   i , while DC component  190   j  is further amplified by DC amplifier  190   e  to give amplified DC signal  190   k . In accordance with an embodiment, amplified AC signal  190   i  is filtered using high-pass filter  190   m  to form filtered AC signal  190   n . High-pass filtering is performed to remove at least a portion of the low frequency modulations in reflected laser beam  170  that is caused by the transient effects of the pulsed laser radiation. Filtered AC signal  190   n  and amplified DC signal  190   k  are transferred through electrical output signal  107  to acquisition electronics  195 . 
     In an embodiment, an optical amplifier may be used to supplement or replace electronic amplifiers. Optical amplifier would be place before photoreceiver  190   b  to amplify the reflected laser beam  170  before conversion to an electrical signal. The need for an optical amplifier depends on the conversion gain of photoreceiver  190   b , the amplitude of the reflect laser power  170 , and the maximum power specification of  190   b , for example. The need for RF amplifier  190   d  depends on the level of AC signal  190   h  and the sensitivity of the front-end of oscilloscope  195   a  in acquisition electronics  195 . In an embodiment, no RF amplifier  190   d  is used. In an embodiment, no DC amplifier  190   e  is used. In an embodiment, only photoreceiver  190   b  is used. As mentioned above, it is advantageous to capture both the RF and DC components of reflected laser beam  170 , but capturing both the RF and DC components of reflected laser beam  170  is optional. In an embodiment, no high-pass filter  190   m  is used. 
     AC frequency range extends to the highest frequency of interest in the LVP measurement (typically up to 1-20 GHz), while the lowest frequency may be about 1-1000 kHz. The DC frequency range extends from DC to typically 1-1000 kHz. The frequency that divides the AC and DC ranges is determined by RF bias-T  190   c , but the AC and DC ranges can be further narrowed by the frequency responses of RF amplifier  190   d  and DC amplifier  190   e , respectively. In accordance with one embodiment of this invention, the lower limit of the AC frequency range may be selected to be above the thermal time constant of the DUT under laser irradiation to filter out some of the transient effects of pulsing the laser beam. In other embodiments, filtering is performed digitally in the computer workstation  197 . 
       FIG. 2G  shows a block diagram of another embodiment of photodetector  190 . In this embodiment, AC and DC components of reflected laser signal  170  are generated by optically splitting amplified laser signal  190   f  using beam splitter  190   m , instead of electrically splitting them (using RF bias-T  190   c ,  FIG. 2E , for example). Since DC signals are easier to amplify with low noise, beam splitter can have a split ratio that diverts 5% of amplified laser beam  190   f  into pick-off beam  190   r  and 95% maintained in main beam  190   q . Main beam  190   q  is converted into AC electrical signal  190   s  using photoreceiver  190   n , while pick-off beam  190   r  is converted separately into DC electrical signal  190   v  by DC photoreceiver  190   p . Additional amplifiers  190   d  and  190   w  may be used to further amplify AC and DC signals  190   s  and  190   v , respectively to give amplified AC and DC signals  190   t  and  190   x , respectively. This embodiment allows the frequency ranges of AC signal  190   s  and DC signal  190   v  to be set independently of each other, which can be useful, for example, if DC electrical signal  190   v  is also used to generate the reflected light signal used for LSM imaging as well as LVP waveform acquisitions. For LSM imaging, ideally, upper limit of frequency response of DC electrical signal  190   v  extends to about 1 MHz to accommodate the fastest LSM scan rate, while it may be advantageous to set lowest frequency limit of AC signal  190   s  to be less than 1 MHz to allow probing of low frequency signals. Other embodiments are possible that do not use one or more of optical amplifier  190   a , RF amplifier  190   d , and DC amplifier  190   w . An embodiment places a high-pass filter after RF amplifier  190   d  to filter at least some of the transient response of reflected laser beam  170  that are caused by pulsing of the laser source. 
       FIG. 2H  shows a block diagram of an embodiment of microscope optics  130  that is based on use of a laser scanning microscope. In other embodiments, microscope optics  130  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Microscope optics  130  includes photon routing optics  130   a  which splits a portion of incoming laser beam  110  into pick-off beam  130   d  which is detected by incident power monitor  130   e . The remainder of laser beam  110  forms the outgoing portion of main beam  130   b , which is directed to beam scanning module  130   c .  FIG. 2H  shows one possible implementation for a chopped CW system. Photon input/output module  130   a  separates reflected laser beam  170  from incident laser beam  110 . Photon input/output module  130   a  may be a beam splitter, or a Polarization Difference Probing (PDP) optics, or some other optical arrangement. Beam scanning module  130 C has x-y beam scanning mechanism (such as a pair of galvo-mirrors commonly used in laser scanning microscopes) to raster scan the laser beam (for imaging) and to ‘vector’ the beam to a fixed position in the DUT for probing. Galvo-mirrors deflect incoming portion of main beam  130   b  into outgoing portion of scanned laser beam  130   f.    
     Beam manipulation optics  130   g  may include optics to reshape the beam into manipulated beam  130   h  to tailor the beam for microscope objective lens  140 , which is used for both imaging and for probing. In one embodiment, beam manipulation optics  130   g  includes a scan lens and a tube lens arranged to form a telescope arrangement. Laser radiation  150  reflected by the DUT and re-collected by objective lens  140  re-enters microscope optics and retraces the input path through beam manipulation optics  130   g , beam scanning module  130   c  and photon input/output optics  130   a  where the laser radiation is diverted from incoming beam path to form reflected laser beam  170 . In an embodiment, diversion of reflected laser beam  150  into reflected laser beam  170  is accomplished though the use of quarter wave plate in beam manipulation optics  130   g  together with polarizing beam splitter in photon input/output optics  130   a . In another embodiment, faraday isolator is used to divert reflected laser beam  150 . Beam manipulation optics  130   g  might be a Scan Lens plus Tube Lens in a ‘telescope’ arrangement with or without a quarter wave plate, or might include a Wollaston prism or a Michelson interferometer. Other optical arrangements may also be used. 
       FIG. 2I  shows a block diagram of an embodiment of a workstation  200  used in Laser Voltage Probing system  100  of  FIG. 1 . Workstation  200  may include output system  202 , input system  204 , memory system  206 , processor system  208 , communications system  212 , and input/output device  214 . In other embodiments, workstation  200  may include additional components and/or may not include all of the components listed above. 
     Work stations  200  is an embodiment of workstation  197 . Output system  202  may include any one of, some of, any combination of, or all of a monitor system, a handheld display system, a printer system, a speaker system, a connection or interface system to a sound system, an interface system to peripheral devices and/or a connection and/or interface system to a computer system, intranet, and/or interne, for example. Output system  202  may send control signals and/or other signals to acquisition electronics  195 . Output system  202  may also send control signals and/or other signals to other components in a LVP system. 
     Input system  204  may include any one of, some of, any combination of, or all of a keyboard system, a mouse system, a track ball system, a track pad system, buttons on a handheld system, a scanner system, a microphone system, a connection to a sound system, and/or a connection and/or interface system to a computer system, intranet, and/or interne (e.g., IrDA, USB), for example. Input system  204  may receive data and/or other signals from acquisition electronics  195 . Input system  204  may also receive data and/or other signal from other components in a LVP system. 
     Memory system  206  may include, for example, any one of, some of, any combination of, or all of a long term storage system, such as a hard drive; a short term storage system, such as random access memory; a removable storage system, such as a floppy drive or a removable drive; and/or flash memory. Memory system  206  may include one or more machine-readable mediums that may store a variety of different types of information. The term machine-readable medium is used to refer to any medium capable of carrying information that is readable by a machine. One example of a machine-readable medium is a computer-readable medium. Memory system  206  stores machine instructions for controlling the process, which may include instructions for chopping the laser signal and determining the intensity of the signal. 
     Processor system  208  may include any one of, some of, any combination of, or all of multiple parallel processors, a single processor, a system of processors having one or more central processors and/or one or more specialized processors dedicated to specific tasks. Processor  208  implements the instructions stored in memory system  206 , which may include instructions to control acquisition electronics  195 , to chop the laser beam, to control the intensity of the laser beam, to analyze data related to the modulations in reflected laser beam  170 , and/or other instructions. 
     Communications system  212  communicatively links output system  202 , input system  204 , memory system  206 , processor system  208 , and/or input/output system  214  to each other. Communications system  212  may include any one of, some of, any combination of, or all of electrical cables, fiber optic cables, and/or means of sending signals through air or water (e.g. wireless communications), or the like. Some examples of means of sending signals through air and/or water include systems for transmitting electromagnetic waves such as infrared and/or radio waves and/or systems for sending sound waves. 
     Input/output system  214  may include devices that have the dual function as input and output devices. For example, input/output system  214  may include one or more touch sensitive screens, which display an image and therefore are an output device and accept input when the screens are pressed by a finger or stylus, for example. The touch sensitive screens may be sensitive to heat and/or pressure. One or more of the input/output devices may be sensitive to a voltage or current produced by a stylus, for example. Input/output system  214  is optional, and may be used in addition to or in place of output system  202  and/or input device  204 . 
       FIG. 2J  shows one example of device under test  160 . Incident laser beam  150  is directed to the NFET (n-type field effect transistor) drain depletion region  160   a  of CMOS (complementary metal oxide semiconductor) inverter  160   b . Electrical switching activity on inverter input  160   c  causes charge density in NFET drain depletion region  160   a  to vary. This produces a modulation in reflected laser beam  150  that can be detected by laser voltage probing. 
       FIG. 3  shows timing diagram  300  of an embodiment of Laser Voltage Probing with Chopped CW Laser and with Real-Time Sampling.  FIG. 3  shows trigger events  310   a, b , and  c , repetitive signal  320 , reflected chopped CW laser pulses  340   a , b, and c, series of electronic sampling pulses  345   a, b , and  c , corresponding series of sampling points  350   a, b , and  c , corresponding series of signal measurement times  360   a , b, c, and extracted waveform data  370   a, b, c  respectively. In other embodiments timing diagram  300  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Trigger events  310   a, b , and  c  are created for synchronizing repetitive signals. Trigger events  310   a, b , and  c  are events that may be part of trigger signal  198  (which was discussed in conjunction with  FIG. 1 ,  FIG. 2D , and  FIG. 2E ). For simplicity, trigger events are depicted as single pulses. Signal  320  has a repetitive portion that is synchronized to a trigger events  310   a, b , and  c . Trigger events  310   a, b, c  may mark the start of the each repetition of the repetitive portion of the signal  320 . In LVP system  100 , laser pulses  340  are a CW laser beam that is chopped, generating pulses that are sent to, and reflected from, DUT  160 . The laser pulses are synchronized to the signal by way of trigger events  310   a, b, c , respectively. Acquisition electronics  195  generate a series of electronic sampling pulses  345   a, b , and  c , to measure the intensity fluctuations in laser pulses  340   a, b , and  c , respectively (after the pulses have been converted to an electrical representation by photodetector  190 ). The series of sampling pulses occurs at a rate sufficiently high to perform real-time sampling of the intensity fluctuations. Series of sampling pulses  345   a, b , and  c , correspond in time to series of sampling points  350   a, b , and  c , and to series of signal measurement times  360   a, b , and  c . Each extracted signal  370   a, b , and  c , are generated as a result of the series of sampling pulses  345   a, b , and  c  causing measurements to be made on laser pulses  340   a, b , and  c , respectively. Signal measurement times  360   a, b , and  c , occur over intervals during which it is desired to take a measurement of the electrical activity occurring in DUT  160 . The generated laser pulses  340   a, b , and  c  overlap the desired measurement intervals,  360   a, b , and  c , respectively. By chopping the CW laser beam, the heating effects of the beam on the DUT  160  can be reduced, allowing more incident CW laser power to be used during the signal measurement intervals. Increasing the incident CW laser power increases the reflected CW laser power, thereby reducing the effects of shot noise (the statistical fluctuations in the number of photons measured). 
     To estimate an upper limit to the improvement that can be realized, take the simple example where the device damage threshold is 10 mW of average laser power, the total test pattern length is 100 microsecond, and the desired measurement time span is 1 microsecond. Then, to maintain the average laser power at the damage threshold using the current art technique of continuously illuminating the DUT, only 10 mW of power can be used during the actual measurement period. The laser power over the measurement time span can be increased by 100 times if the CW beam is chopped to form a pulse of 1 microsecond duration, with the laser beam off for the remaining 99 microseconds (beam ‘duty-cycle’ of 1%). In both cases, the average power delivered to the DUT is the same, but pulsing the laser allows measurements to be made 100 times faster. 
     In practice, it may be desirable and/or necessary to make more modest increases in the laser power. For a damage mechanism based on temperature rise (heating), the damage threshold is expected to depend on factors other than just average power. Damage threshold may also depend on peak power, pulse period, thermal conductivity of the DUT, laser power density, etc, since all these factors may affect the temperature rise within the DUT. An empirically determined model that gives the relationship between parameters of LVP system  100  ( FIG. 1 ) and the amount that the laser power may be increased (compared to were no chopping present) can be used to either automatically set peak power in a system, or to give the user guidance on how much peak power to use when probing under the particular test conditions (measurement time-span of interest, test pattern length) the user has set up. 
     Note that the CW laser pulses, pulses  340   a, b , and  c  ( FIG. 3 ) are purposely shown to extend beyond the signal measurement interval (though not necessarily with actual proportions). It may be desirable in some embodiments of this invention to allow the DUT  160  and/or electronics to stabilize before the measurements are taken. 
       FIG. 4A  shows a plot  400  illustrating an example of the temperature response of DUT  160  ( FIG. 1 ) to pulses  340   a, b , and  c  ( FIG. 3 ). In other embodiments plot  400  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Horizontal axis represents time, t, vertical axis represents the DUT temperature, T(t), at the region of laser illumination. DUT  160  is at some constant, initial temperature, T initial , before laser illumination. Laser illumination starts at time t=0. ΔT is the total temperature change due to laser illumination, assuming equilibrium conditions with constant illumination. The temperature as a function of time is given by the equation T(t)=ΔT(1-e −t/τ ) T initial , where ΔT=T sat −T initial , and where T sat  depends on the power of the laser radiation. The temperature T sat  is the temperature that irradiated portion of DUT  160  asymptotically tends to when the continuous wave laser is irradiating DUT  160 . If the DUT is initially at room temperature and then heated by the laser, T initial  is room temperature and T sat &gt;T initial . If the DUT is already heated to a given temperature by the laser radiation, and then the laser is shut off, then T sat  is room temperature and T sat &lt;T initial . The equation T(t)=ΔT(1-e −t/τ )+T initial  can also be written as T(t)=T sat −ΔTe −t/τ . The relaxation time constant τ=C/hA, where h is the heat transfer constant, A is the cross sectional area through which the heat travels, C is the total heat capacity, and the heat capacity of a system may be further represented by its mass-specific heat capacity c p  multiplied by its mass m, so that the time constant τ is also given by mc p /(hA). While heating the dominant contribution to τ is from the laser, and consequently, A is the cross section of the laser beam. As an approximation, the cooling may be considered to occur during two stages. During a first stage the heat dissipates throughout DUT  160  and during a second stage the heat leaves DUT  160  into the air or a heat sink or the chip carrier. During the first stage, A is the surface area of the volume heated by the laser while the laser was on, and C is the heat capacity of the silicon. During the second stage A is the surface area of DUT  160 , and C is the heat capacity of air, for example. For simplicity, in the discussion that follows, it will be assumed that one of these two cooling stages dominates, and the relaxation constant while heating will be represented by τ 1  and the relaxation constant during cooling will be represented by τ 2 . 
     Plot  400  assumes laser pulse width is many times greater than the thermal time constant, τ, which characterizes the situation.  410 ,  420 ,  430 , and  440  indicate the DUT temperature after 1, 2, 3 and 4 time constants have passed, respectively. After 4 time constants have passed, the temperature reaches 98.2% of its final value. Since the index of refraction varies with DUT temperature, the effects on the LVP measurement caused by the temperature rise may need to be accounted for. 
       FIG. 4B  shows a plot  450  illustrating an example of the temperature response to the chopped continuous wave laser signal. Plot  450  includes trigger signal  310 , clock signal  106 , chopped continuous wave signal  454  having laser pulse of power P 1 , thermal response  456 , chopped continuous wave signal  458  having laser pulse of power P 1  during a first portion and power P 2  during a second portion, and thermal response  460 . In other embodiments plot  450  may not have all of the elements or features listed and/or may have other elements or features instead of or in addition to those listed. 
     Trigger signal  310  and clock signal  106  were discussed in conjunction with  FIGS. 3 and 1 , respectively. Portion of interest  452  is the portion of clock signal  106  that corresponds to the repetitive DUT signal(s) of interest. The first and last clock pulse of portion of interest  452  may be used to indicate when to pulse laser source  120 . Chopped continuous wave signal  454  has laser pulse that delivers power P 1  to the DUT  160 . The pulse begins before portion of interest  452  (e.g., one clock signal earlier) and ends after portion of interest  452  (e.g., one clock signal earlier). Thermal response  456  is the thermal response of DUT  160  to chopped continuous wave signal  454 . During each laser pulse, having power P 1 , the temperature of DUT  160  rises according to equation T=T P1 +(T room −T P1 )e −t/τ1 . At the end of the laser pulse, the temperature has risen to T max . After the laser pulse ends, the temperature drops according to equation T=T room +(T max −T room )e −t/τ2 . 
     Since the index of refraction of the DUT varies with DUT temperature, the effects on the LVP measurement caused by the rising temperature may need to be accounted for In an embodiment, to reduce and possibly minimize the effects caused by the rising temperature, chopped continuous wave  458  is used instead of chopped continuous wave  454 . Chopped continuous wave  458  has laser pulses that each have a first portion in which power P 2  is delivered by the laser beam to DUT  160  ( FIG. 1 ), which is followed by a second portion in which power P 1  is delivered. Thermal response  460  is the thermal response of DUT  160  to chopped continuous wave  458 . While power P 2  is applied, DUT  160  heats up according to equation T=T p2 +(T room −T P2 )e −t/τ1 . Power P 2  is applied until the temperature reaches T max1 ˜T P1 , which is the temperature that DUT  160  approaches and is stable at while power P 1  is applied. In an embodiment, the temperature T max1  is expected, and is intended, to be equal to T P1 . However, if T max1  and T P1  are not equal, while power P 1  is applied, the temperature of DUT  160  is given by T=T P1 +(T max1 −T P1 )e −t/τ1 . After the laser pulse ends, the temperature of DUT  160  is given by T˜T room +(T P1 −T room )e −t/τ2 . 
       FIG. 5A  shows a flowchart of an embodiment of a method  500 , which is a method of configuring a LVP system for probing a DUT with a chopped laser beam. In step  502 , DUT is prepared for probing. This step may include thinning and polishing the DUT and/or applying an anti-reflection coating on the DUT as necessary for use with microscope objective lens  140  ( FIG. 1 ). In step  504 , the DUT is mounted in DUT stimulus  165  ( FIG. 1 ), which may consist of ATE tester  165   a  with load board  165   b  ( FIG. 2D ), or computer workstation  165   d  with applications board  165   e  ( FIG. 2E ), or other means to stimulate DUT for probing. In optional step  506 , the DUT is mounted, configured, and a temperature control apparatus is started. Optional step  506  may be required if the operating DUT dissipates excessive power, which would cause damage to the DUT if left uncooled and/or if testing the DUT requires the DUT to be held at a particular temperature. In step  508 , DUT stimulus  165  ( FIG. 1 ) is programmed to cause the DUT circuitry of interest to be exercised repetitively over the signal(s) of interest. In step  510 , DUT and/or DUT stimulus is programmed to output trigger signal  198  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D , and  FIG. 2E ) that is synchronized with DUT signal(s) of interest. In optional step  512 , DUT and/or DUT stimulus is programmed to output clock signal  106  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D ,  FIG. 2E ) which may be used in conjunction with trigger signal  198  to generate laser chopping pulse  195   g  ( FIG. 2B ,  FIG. 2C ) that is synchronized with DUT signal(s) of interest, and to serve as a synchronized time reference for oscilloscope  195   a  ( FIG. 2B ,  FIG. 2C ). In step  516 , the DUT is interfaced to LVP system  100 . Step  516  may include the steps of mechanical docking, and connecting of electrical trigger signal  198  and optional clock signal  106  to LVP system  100 . In step  518 , the DUT is imaged using microscope optics  130  and objective lens  140  ( FIG. 1 ) to locate the circuitry of interest for probing. Computer aided design (CAD) information of the DUT may be used if required. Once the circuitry of interest is located, the laser spot is focused and positioned at the node of interest in step  520 . The triggering circuitry of oscilloscope  195   a  ( FIG. 2B ,  FIG. 2C ) is configured in step  522 , which involves setting up an advanced triggering mode if necessary, and which involves setting parameters such as the trigger level, impedance, slope, and coupling. After oscilloscope  195   a  is set up to trigger properly, the horizontal scope settings are configured in step  524 . Configuring the horizontal scope settings may involve setting the trigger delay and acquisition time span to capture the DUT signal(s) of interest. The vertical controls of oscilloscope  195   a  are configured in step  526 . Settings generally differ for the AC ( 195   c ,  FIG. 2B ,  FIG. 2C ) and DC ( 195   d ,  FIG. 2B ,  FIG. 2C ) components of the photodetector output signal  107  and depend on their signal levels. Other acquisition parameters are set in step  528 . The acquisition parameters may include parameters such as laser power level, number of waveforms to accumulate and/or average, and waveform filtering. Step  530  may include programming the programmable signal generator  195   b  ( FIG. 2B ,  FIG. 2C ) to output laser chopping pulses  195   g  ( FIG. 2B ,  FIG. 2C ) synchronized to, or otherwise correlated to, the repetitive DUT signal(s) of interest. Timing, duration, amplitude, and shape of the resultant laser pulses  120   j  ( FIG. 2A ) depend on the acquisition time span and trigger delay set in step  524  and the trigger rate of trigger signal  198  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ) and damage threshold of the DUT. In an embodiment, pulse shape is set to minimize the amplitude of the impulse received by DUT  160  and photodetector  190 . In an embodiment, pulse shape is set to accelerate the reaching of thermal equilibrium in the DUT. 
       FIG. 5B  shows a flowchart of an embodiment of a method  550 , which is a method of using a LVP system  100  for probing a DUT with a chopped laser beam. LVP system  100  is configured for use in step  551 . Configuration includes all the steps in method  500 . The acquisition is started in step  552 . Laser light pulses are generated in step  554  by laser source  120  ( FIG. 1 ,  FIG. 2A ) under control of laser control signals  105  (FIG.  1 ,  FIG. 2A ,  FIG. 2B ,  FIG. 2C ). In step  556 , the laser light pulses are directed to DUT via microscope optics  130  ( FIG. 1 ,  FIG. 2H ) and objective lens  140  ( FIG. 1 ). In step  558 , laser light pulses reflected by DUT are re-collected by objective lens  140  and retrace their path into microscope optics  130 . In step  560 , laser light pulses are separated out of main beam path and directed to photodetector  190  ( FIG. 1 ,  FIG. 2F ,  FIG. 2G ). Photodetector  190  outputs AC and DC component of signal in step  562 . Real-time digital storage oscilloscope  195   a  ( FIG. 2B ,  FIG. 2C ) in acquisition electronics  195  ( FIG. 1 ), digitizes the AC and DC components in step  564 . The digitization may be synchronized to repetitive DUT signal(s) of interest via trigger signal  198  and optional clock signal  106  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D ,  FIG. 2E ). Waveform data is generated by the digitization of the information in each laser pulse. In step  566 , waveform data are accumulated and/or averaged over multiple trigger events. In an embodiment, the oscilloscope just accumulates the data in the form of a 2-D array of bins that plot amplitude versus time. Any bin that is ‘hit’ by a waveform causes the number of ‘hits’ to be incremented in that bin. Collecting the data in bins reduces and possibly minimizes the amount of data that has to be transferred and therefore reduces and possibly minimizes the dead time when compared to a method in which bins are not used. Averaging and/or accumulation of the AC with AC data and the DC with DC data over the repeated signal(s) of interest is performed to increase the final waveform&#39;s SNR. In step  568 , the averaged and/or accumulated waveform data are periodically transferred to workstation  197  for further processing, display, and storage. Periodic transfers are performed so that the user of the LVP system  100  can monitor the progress of the acquisition. Processing may involve applying additional filtering, such as boxcar averaging, or may involve more complex operations, such as calculating the weighted average of a two-dimensional histogram of accumulated data. In step  570 , the optional step of taking the ratio of AC to DC waveform amplitude at each time bin is performed to normalize the waveform amplitude. In optional step  572 , waveforms are processed to subtract, divide, or otherwise remove the signal background caused by the transient effects of the laser pulses. In step  574 , intermediate waveforms (e.g., waveforms generated with less than the final number of averages and/or accumulations desired) are displayed on workstation monitor to allow the waveform acquisition process to be monitored. If the LVP user sees that the intermediate waveform results are not as expected, the user can terminate the acquisition and reconfigure the LVP system  100  as necessary to correct any errors. In step  576 , the waveform acquisition proceeds until desired number of averages and/or accumulations is reach. Waveforms are saved on workstation  197  ( FIG. 1 ) in step  578 . 
       FIG. 6A  shows a flowchart of an embodiment of a method  600 , which is a method of making the LVP system  100  of  FIG. 1 . In step  602 , a CW-LVP system is built. In step  604 , the laser source is modified by adding means to generate laser pulses, such as the addition of modulator driver  120   d  and acousto-optic modulator  120   c  ( FIG. 2A ). In step  606 , the acquisition electronics are modified to incorporate programmable signal generator  195   b  ( FIG. 2B ,  FIG. 2C ). In step  608 , software is loaded onto acquisition electronics  195 , workstation  197 , and/or signal generator  195   b  to control programmable signal generator  195   b , and to incorporate additional processing, including, but not limited to, filtering and background subtracting, into signal processing routines in workstation  197  ( FIG. 1 ). 
       FIG. 6B  shows a flowchart of embodiment of method  620 , which is another method of making the LVP system  100  of  FIG. 1 . In step  622 , an LSM based failure analysis system is built. In step  624 , the microscope optics are modified to route reflected laser beam  170  out from the incident laser beam ( FIG. 1 ). In step  626 , laser source is modified to incorporate a low noise laser (which may include laser head  120   a  and laser controller  120   b  of  FIG. 2A ) and acousto-optic modulator  120   c  and modulator driver  120   d  ( FIG. 2A ). In step  628 , a photodetector  190  is added ( FIG. 1 ,  FIG. 2F ,  FIG. 2G ). In step  630 , acquisition electronics  195  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ) is added. In step  632 , software is loaded into acquisition electronics  195 , workstation  197 , and/or signal generator  195   b  that controls data acquisition electronics  195  and that processes, displays, and stores waveforms. 
       FIG. 6C  shows a flowchart of embodiment of method  640 , which is yet another method of making the LVP system  100  of  FIG. 1 . Method  640  is the same as method  620  except that the starting point is to acquire, instead of build an FA system, and the FA system may not be LSM based. If the FA system is not LSM based, step  644  involves incorporation of laser beam path into microscope optics. In step  646 , laser source is modified to incorporate a low noise laser  120   a  and  120   b  ( FIG. 2A ) and acousto-optic modulator and modulator driver  120   c  and  120   d , respectively ( FIG. 2A ). In step  648 , a photodetector  190  is added ( FIG. 1 ,  FIG. 2F ,  FIG. 2G ). In step  650 , acquisition electronics  195  ( FIG. 1 ,  FIG. 2B ,  FIG. 2C ) is added. In step  652 , software is loaded into acquisition electronics  195 , workstation  197 , and/or signal generator  195   b  that controls data acquisition electronics  195  and that processes, displays, and stores waveforms. 
     In an embodiment, each of the steps of method  640  is a distinct step. In another embodiment, although depicted as distinct steps in  FIG. 6 , step  602 - 618  may not be distinct steps. In other embodiments, method  600  may not have all of the above steps and/or may have other steps in addition to or instead of those listed above. The steps of method  600  may be performed in another order. Subsets of the steps listed above as part of method  600  may be used to form their own method. 
       FIG. 7A  is a flowchart of an embodiment of a method  700  of determining the damage threshold of a DUT irradiated with a CW laser beam. Method  700  may involve the LVP probing a node in a DUT with increasingly high CW laser power until node damage is sustained. In step  702 , a DUT, DUT stimulus, and LVP system are set up for probing, as described in method  500 , steps  502 - 528  ( FIG. 5A ). Since the purpose of method  700  is to determine the safe operating power in CW mode, step  530  may be skipped. In step  704  the DUT is monitored to determine that the selected probe node functions correctly before starting. In step  710  the CW laser power is set at an initial, conservative value of 1 mW. In step  712 , probing commences for 10 minutes, a time duration typical for an LVP. In step  714 , a decision is made, depending on whether the DUT has sustained damage during probing or not. If the DUT has sustained no detectable damage, then method  700  proceeds to step  716  where the laser power is increased by 1 mW, steps  712  and  714  are repeated. Returning to step  716 , DUT damage may be determined by monitoring the electrical behavior of the DUT and/or by monitoring the LVP waveform to detect changes in the waveform shape, amplitude, and/or timing. If during step  716  DUT damage is detected, then method  700  continues with step  718  where probing is ceased. In step  720 , to set a conservative probing power level, the power level which caused DUT damage is multiplied by 0.5 in step  720 , which is used as the maximum probing power, and the value of the maximum probing power is recorded as Pmax(CW) in step  722 . 
     In a different embodiment, factor to determine Pmax(CW) may be 0.75, 0.9, 0.25, 0.10, etc depending on the desired trade-off between minimizing the probability of DUT damage during prolonged probing (lower multiplicative values) versus the need to improve waveform SNR (higher multiplicative values). 
     In a different embodiment, probing time will be changed to match the typical probing time required to extract an LVP waveform with sufficient SNR. 
     In different embodiments, initial laser power and power increases may differ from the values given here. 
     In another embodiment, the node will be probed until maximum CW laser power of the LVP system is reached. 
     In yet another embodiment, the node will be probed until the power limit of photodetector  190  ( FIG. 1 ,  FIG. 2F ,  FIG. 2G ) is reached. 
     In an embodiment, each of the steps of method  700  is a distinct step. In another embodiment, although depicted as distinct steps in  FIG. 7A , step  702 - 722  may not be distinct steps. In other embodiments, method  700  may not have all of the above steps and/or may have other steps in addition to or instead of those listed above. The steps of method  700  may be performed in another order. Subsets of the steps listed above as part of method  700  may be used to form their own method. 
       FIG. 7B  is a flowchart of an embodiment of a method  750  of determining the damage threshold of a DUT irradiated with a chopped CW laser beam. Procedure  750  is similar to procedure  700  for a CW laser beam, except that the loop length, acquisition time span, and beam duty cycle is varied along with peak CW laser pulse power. In step  752 , a DUT, DUT stimulus, and LVP system is set up for probing, as described in method  500  ( FIG. 5A ). In step  754 , the DUT is monitored to determine that the selected probe node functions correctly before starting. In step  756 , the loop length, T, and acquisition time span, S, are record. In step  758 , the inverse of the laser duty cycle, D=T/S, is calculated. In step  760  the chopping mode of the LVP system is enabled. In step  762  the peak CW laser power is set at an initial, conservative value equal to Pmax(CW) found in method  700 . In step  764 , probing commences for 10 minutes, a time duration typical for LVP. In step  766 , a decision is made, as to whether the DUT has sustained damage during probing. If the DUT has sustained no detectable damage, then method  750  proceeds to step  768 , where the laser power is increased by an amount equal to 10% of the inverse duty cycle, D, and steps  764  and  766  are repeated. In other embodiments, different increments are used instead of 10% of the duty cycle. Returning to step  768 , DUT damage may be determined by monitoring the electrical behavior of the DUT and/or by monitoring the LVP waveform to detect changes in the waveform shape, amplitude, and/or timing. If in step  768 , DUT damage has been detected, then method  750  proceeds to step  770  where probing is ceased. To set a conservative probing power level, the power level which caused DUT damage is multiplied by 0.5 in step  772 , which is the maximum probing level for a chopped CW laser, and the maximum probing value for the chopped CW laser is recorded as Pmax(S, T) in step  774 . Similar to step  722 , in a different embodiments of step  774 , factor to determine Pmax(CW) may be 0.75, 0.9, 0.25, 0.10, etc depending on the desired trade-off between minimizing the probability of DUT damage during prolonged probing (lower multiplicative values) versus the need to improve waveform SNR (higher multiplicative values). As indicated, Pmax(S, T) is a function of acquisition time span, S, and loop length, T. In step  776 , S and T are varied, and the test is repeated using a new DUT, or a different node in the same DUT. Finally, in step  778 , the values of Pmax(S, T) are tabulated. The table can be used manually to guide the LVP operator in setting the peak CW laser power to use in chopped mode, or the table can be used to generate a look-up table that is used by the LVP software to automatically set the laser power, depending on S and T. 
     In an embodiment, each of the steps of method  750  is a distinct step. In another embodiment, although depicted as distinct steps in  FIG. 7B , step  752 - 778  may not be distinct steps. In other embodiments, method  750  may not have all of the above steps and/or may have other steps in addition to or instead of those listed above. The steps of method  750  may be performed in another order. Subsets of the steps listed above as part of method  750  may be used to form their own method. 
     Since damage threshold may vary depending on node size (for example, large signal buffers may be able to sustain more laser power before suffering detectable damage than smaller buffer) methods  700  and  750  may have to be used to determine maximum safe laser powers for the different types of nodes probed. 
     Since damage threshold may also vary depending on process technology (65 nm geometry devices might tolerate more laser power than 45 nm geometry devices, for example), methods  700  and  750  may have to be used to determine maximum safe laser powers for each process technology. 
     Each embodiment disclosed herein may be used or otherwise combined with any of the other embodiments disclosed. Any element of any embodiment may be used in any embodiment. 
     Although the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, modifications may be made without departing from the essential teachings of the invention.