System and method for laser voltage imaging state mapping

The present invention relates to devices and methods for investigating integrated circuits using laser irradiation.

This application claims the priority benefit of US Provisional Patent Application No. 61/174962, filed May 1, 2009, the entire disclosure of which is reliable and by reference herein. It is built into.

Probing systems are used in techniques for inspecting and debugging integrated circuit (IC) designs and layouts. Various laser-based systems for probing ICs are known in the art. Although some prior art descriptions are provided herein, it is also encouraging readers to consider Patent Documents 1 to 3, which are incorporated herein by reference in their entirety.

Further relevant information can be found in Non-Patent Documents 1-12 and 13 which are incorporated herein by reference in their entirety.

As is known, commercial test platforms, also known as automated test evaluation (ATE) testers, such as automated test equipment, are tested during IC debugging and testing of IC devices (DUTs). ) Is used to generate a test pattern (also called a test vector). Various systems and methods can then be used to test the response of the DUT to the test vector. One such method is commonly referred to as laser voltage probing (LVP). When a laser-based system such as LVP is used for probing, the DUT is irradiated by the laser and the light reflected from the DUT is collected by the probing system. When the laser beam hits the DUT, the laser beam is modulated by the response of various elements of the DUT to the test vector. This is due to the electrical modulation of the free charge density and the combined disturbance of the refractive index and absorption coefficient of the IC material, most commonly silicon. Therefore, the analysis of reflected light provides information about the operation of various devices in the DUT.

FIG. 1 is a schematic diagram depicting the main components 100 of the architecture of a laser-based voltage probe system according to the prior art. In FIG. 1, the dashed arrow represents the path of light, while the solid arrow represents the path of the electrical signal. The curved light path is generally made using fiber optic cables. The probe system 100 includes a laser light source DLS110, which is a dual laser light source in this particular example, an optical mount 112, and a data acquisition analyzer 114. The optical mount 112 includes equipment for mounting the DUT 160. A normal ATE tester 140 provides a stimulus signal to the DUT 160, receives a response signal 142 from the DUT 160, and further provides a trigger and clock signal 144 to the Time base. board) 155 can be provided. The signal from the tester is generally transmitted to the DUT via a test board, a DUT board (adapter plate), and various cables and interfaces connecting all these components. The time base board 155 synchronizes signal acquisition with DUT stimulation and laser pulses. The workstation 170 controls, receives, processes, and displays data from the signal acquisition board 150, the time base board 155, and the optical mount 112.

Next, various elements of the probe system 100 will be described in more detail. Since the time resolution is very important in the DUT test, the conventional pulsed laser is used in the embodiment of FIG. The pulse width of this laser determines the time resolution of the system. The dual laser light source 110 has two laser light sources. That is, the dual laser light source 110 is used to generate a pulse with a width of 10-35 picoseconds, the light source of the pulse mode lock laser MML104, and from the outside to generate a pulse with a width of about 1 microsecond. It is composed of a continuous wave laser light source CWL106 that can be gate-controlled. The MLL104 light source operates at a fixed frequency, typically 100 MHz, and via the phase-locked loop (PLL) of the time baseboard 155, the stimulus 142 provided to the DUT 160 and the trigger and trigger provided by the ATE tester. It must be synchronized with the clock signal 144. The output of the DLS 110 is transmitted to the optical mount 112 using the fiber optic cable 115. The laser beam is then processed by the beam optics 125, which guides the laser beam to illuminate the selected portion of the DUT 160. The beam optics 125 has a laser scanning microscope (LSM130) and beam manipulation optics (BMO135). Specific elements with conventional optical configurations, such as objective lenses, are not shown. In general, the BMO 135 has the optics needed to process the beam into the required shape, focus, polarization, etc. The LSM 130, on the other hand, has the elements needed to scan the beam over a particular region of the DUT. In addition to scanning the beam, the LSM 130 is a vector-pointing mode for directing the laser beam to any point in the field of view of the LSM and the objective lens. mode). The XYZ stages 120 move the beam optics 125 relative to the stationary DUT 160. By using the vector indication modes of stages 120 and LSM 130, any point of interest in the DUT 160 can be illuminated and investigated.

To probe the DUT 160, the ATE 140 transmits a stimulus signal 142 to the DUT in synchronization with the trigger and clock signals provided in the phase-locked loop of the timebase board 155. The phase-locked loop controls the MLL 104 so that its output pulse is synchronized with the stimulus signal 142 to the DUT. The MLL 104 emits a laser pulse that illuminates a particular device of interest in the stimulated DUT. The reflected light from the DUT is collected by the beam optics 125 and transmitted to the photodetector 138 via the optical fiber cable 134. The reflected beam changes its properties depending on the response of the device to the stimulus signal. In order to monitor the incident laser output for the purpose of compensating for fluctuations in the laser output, for example, the optical mount 112 diverts part of the MLL104 incident pulse to the photodetector 136 via the fiber optic cable 132. Have the means. The output signals of the optical sensors 136 and 138 are transmitted to the signal acquisition board 150, which in turn transmits the signal to the controller 170. By processing the phase-locked loop of the time baseboard 155, the controller 170 controls the exact time position of the MLL104 pulse with respect to the stimulus signal 142 of the DUT 160. By changing this time position and monitoring the optical sensor signal, the controller 170 can analyze the temporal response of the DUT to the stimulus signal 142. The time resolution of the analysis depends on the pulse width of the MLL 104.

Techniques for performing with continuous wave LVP are also known. In that technique, a DUT device is illuminated with a continuous wave laser and continuously reflected light is collected. The continuously reflected light contains timing information regarding the response of the active device to various stimulus signals, that is, switching. The reflected light signal is continuously converted into an electrical signal and amplified by a photodetector, such as an avalanche photodiode (APD). The timing information is included in the electrical signal and represents the detected modulation of the device. Timing information can be displayed in the time domain using an oscilloscope or in the frequency domain using a spectrum analyzer.

Recently, laser voltage imaging (laser voltage) Imaging) techniques have evolved to provide two-dimensional grayscale images that correspond to voltages at different points within the region of the DUT. More specifically, the LSM is used to raster scan an area of the DUT, at each point within that area, a reflected light signal is collected to provide a single data value. That is, at each point, the amplitude of the signal at a particular frequency spectrum is obtained by a spectrum analyzer, rather than providing a spectrum of the entire frequency band. In practice, the spectrum analyzer is configured to extract a single frequency of interest (called zero span) and provide an output value that is directly proportional to the strength of the signal received at that frequency. Thus, as the LSM scans the selected area of the DUT, if the frequency of interest is inactive, the spectrum analyzer provides a low output or is inactive, while the frequency is active. Spectrum analyzers provide high output. That is, it provides an output signal with an amplitude proportional to the strength of the signal at the selected frequency of interest. This output can be used to generate a map of the scan area that shows the grayscale level corresponding to the device activity at each point in the scan area.

<p><patcit num="1"><text>U.S. Pat. No. 5,208648</text></patcit><patcit num="2"><text>U.S. Pat. No. 5,220,403</text></patcit><patcit num="3"><text>U.S. Pat. No. 5,940,545</text></patcit></p>

<p><nplcit num="1"><text>Yee, WM et al., Laser Voltage Probe (LVP): A Novel Optical Probing Technology for Flip-Chip Packaged Microprocessors, International Symposium for Testing and Failure Analysis (ISTFA), 2000, p.3-8</text></nplcit><nplcit num="2"><text>Bruce, M. et al., Waveform Acquisition from the Backside of Silicon Using Electro-Optic Probing, International Symposium for Testing and Failure Analysis (ISTFA), 1999, p. 19-25</text></nplcit><nplcit num="3"><text>Kolachina, S. et al., Optical Waveform Probing --Strategies for Non-Flipchip Devices and Other Applications, International Symposium for Testing and Failure Analysis (ISTFA), 2001, p. 51-57</text></nplcit><nplcit num="4"><text>Soref, RA and BR Bennett, Electrooptical Effects in Silicon, IEEE Journal of Quantum Electronics, 1987, QE-23 (1), p. 123-9</text></nplcit><nplcit num="5"><text>Kasapi, S. et al., Laser Beam Backside Probing of CMOS Integrated Circuits, Microelectronics Reliability, 1999, 39, p. 957</text></nplcit><nplcit num="6"><text>Wilsher, K. et al., Integrated Circuit Waveform Probing Using Optical Phase Shift Detection, International Symposium for Testing and Failure Analysis (ISTFA), 2000, p. 479-85</text></nplcit><nplcit num="7"><text>Heinrich HK, Picosecond Noninvasive Optical Detection of Internal Electrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits, IBM Journal of Research and Development, 1990, 34 (2/3), p. 162-72</text></nplcit><nplcit num="8"><text>Heinrich HK, DM Bloom and BR Hemenway, Noninvasive sheet charge density probe for integrated silicon devices, Applied Physics Letters, 1986, 48 (16), p. 1066-1068</text></nplcit><nplcit num="9"><text>Heinrich HK, DM Bloom and BR Hemenway, Erratum to Noninvasive sheet charge density probe for integrated silicon devices, Applied Physics Letters, 1986, 48 (26), p. 1811</text></nplcit><nplcit num="10"><text>Heinrich HK et al., Measurement of real-time digital signals in a silicon bipolar junction transistor using a noninvasive optical probe, IEEE Electron Device Letters, 1986, 22 (12), p. 650-652</text></nplcit><nplcit num="11"><text>Hemenway, BR et al., Optical detection of charge modulation in silicon integrated circuits using a multimode laser-diode probe, IEEE Electron Device Letters, 1987, 8 (8), p. 344-346</text></nplcit><nplcit num="12"><text>A.Black, C.Courville, G Schultheis, H.Heinrich, Optical Sampling of GHz Charge Density Modulation in Silicon Bipolar Junction Transistors, Electronics Letters, 1987, Vol. 23, No. 15, p. 783-784</text></nplcit><nplcit num="13"><text>Kindereit U, Boit C, Kerst U, Kasapi S, Ispasoiu R, Ng R, Lo W, Comparison of Laser Voltage Probing and Mapping Results in Oversized and Minimum Size Devices of 120nm and 65nm Technology, 19th European Symposium on Reliability of Electron Devices, Failure Physics and Analysis (ESREF 2008), 2008, 48, 1322-1326</text></nplcit></p>

<p num="0013"> While the systems and methods provide useful information about the functionality of the DUT, it is desirable to obtain additional information about the responses of various active devices within the DUT non-invasively.</p>

<p num="0014"> The following abstracts are provided to provide a basic understanding of some aspects and features of the invention. This abstract is not broader than the outline of the invention and is not intended to clarify or delineate the scope of the invention in particular. Its sole purpose is to present some concepts of the invention in simple form as a prelude to the more detailed description presented below.</p><p num="0015"> Various embodiments of the present invention provide devices and methods for laser voltage imaging state mapping of DUTs.</p><p num="0016"> Devices and methods for laser probing of DUTs are disclosed. The system enables laser voltage imaging state mapping of devices in the DUT. While the DUT is receiving a test signal to switch an active device, the selected area of the DUT is illuminated. The light reflected from the DUT is collected and converted into an electrical signal. Phase information is extracted from the electrical signal, and a two-dimensional image spatially corresponding to the selected region is generated from the phase information.</p><p num="0017"> Other aspects and features of the invention will become apparent from the description of the various embodiments described herein, the aspects and features of the invention as claimed in the appended claims. Included in the scope of intention and spirit.</p>

<figref num="1">FIG. 1 is a schematic diagram depicting the main components of a laser-based voltage probe system according to the prior art.</figref><figref num="2">FIG. 2 is a diagram showing the main components of the system according to the embodiment of the present invention.</figref><figref num="3">FIG. 3 is a diagram showing waveforms of signals at selected points A and B.</figref><figref num="4">FIG. 4 is a diagram showing the waveforms of the selected points A and B signals and the interference with the added interference signal.</figref><figref num="5">FIG. 5 shows an embodiment of the invention in which an "RF interference" signal is applied to a "tuned" signal from an APD and applied to a spectrum analyzer. Here, "tuned" means amplified, shifted, current-to-voltage and voltage-to-current conversion, and the like.</figref><figref num="6">FIG. 6 shows an embodiment of the present invention according to a modified example of the embodiment of FIG.</figref><figref num="7">FIG. 7 shows another embodiment of the invention in which an "RF interference" signal is added to the "tuned" signal from the APD and fed to a spectrum analyzer.</figref><figref num="8">FIG. 8 shows yet another embodiment of the invention in which an "RF interference" signal is added to the "tuned" signal from the APD and fed to a spectrum analyzer.</figref>

The present invention is described herein with reference to the particular embodiments illustrated in the figure. However, the various embodiments shown in the figures are merely examples and do not limit the present invention as defined in the appended claims.

Various embodiments of the present invention are non-invasive and non-contact methods for distinguishing the relative polarities of active transistors within a selection region of a DUT, excluding conventional IC design knowledge. A device and a method for this are provided. These systems and methods are referred to herein as laser voltage imaging (LVI) state mapping of the DUT. The methodology augments the prior art system by providing phase information for various active devices within the DUT. The phase information may be provided in the form of a map of the scanning area of the DUT. In the map, grayscale is used to show the phase information of the active device, that is, the transistor, arranged in the scanning region. This allows IC testing and debugging even when circuit design is not available.

According to one embodiment of the invention, the lock-in amplifier is used to perform LVI state mapping of the region of interest within the DUT. This embodiment provides the ability to observe the relative logical states of various active transistors by extracting phase information from the reflected laser beam. According to one embodiment, the lock-in amplifier is used to measure the phase of a reflected signal with respect to a reference signal internally generated by the lock-in amplifier or externally supplied to the lock-in amplifier. According to one implementation example, this is achieved by replacing the conventional LVI spectrum analyzer with a lock-in amplifier.

FIG. 2 is a schematic diagram of a system according to an embodiment of the present invention for performing phase detection and mapping. In FIG. 2, the lock-in amplifier is used in place of the spectrum analyzer used in systems known in the art. The laser light source 210 provides a laser beam (indicated by a solid arrow) delivered to the input optical fiber 215. The optical I / O module 214 shapes the beam and provides the tuned beam to the LSM 230. The LSM 230 scans the tuned beam over the selected area of the DUT 260. In this particular example, the path from LSM230 to DUT260 includes a scanning lens, a reflector, a tube lens, a waveplate, and an objective lens. These devices are provided to properly scan the laser beam over a selected area of the DUT, but other devices may be used as needed for a particular design.

As the laser beam scans over the selected region of the DUT 260, the stimulus signal 240 is applied to the DUT 260 so that the active elements in the DUT 260 modulate the beam, i.e. the transistors in the DUT switch. The stimulus signal 240 can be generated by a function generator, ATE, or the like. When the active device switches, the active device changes the absorption coefficient and index of refraction of the material that makes the device, such as silicon, and the amplitude of the reflected laser beam (indicated by the dashed arrow) corresponds accordingly. It is modulated. The reflected beam is focused by an optical element and guided to the output fiber 232. The output fiber 232 guides the beam to the sensor. In this particular example, an avalanche photodiode APD236 is used, but other optical sensors such as PIN sensors may be used. The output signal of APD is a trans-impedance amplifier (trans-impedance). It is input to amplifier) 237, and the output of this TIA is input to a signal separator such as Biasty (Diplexer) 250 which outputs DC component and AC component. The DC component is amplified by the video amplifier 252 and sent to the frame grabber 254 for generating an image of the scanning region of the DUT. The AC component (of RF frequency) is tuned by RF amplifier 273 and then transmitted to lock-in amplifier 270. The output of the lock-in amplifier 270 is also amplified by the video amplifier 256 and used to generate a phase image of the scanning region. As more fully described below, the X / Y or R / Θ output of the lock-in amplifier is converted into a grayscale image of the scanning area. Here, the grayscale value represents the phase of the active device in the scanning region of the DUT.

Next, the operation of the embodiment of the present invention using the lock-in amplifier will be described. The X and Y values of the lock-in amplifier are proportional to the amplitude and relative phase of the signal, i.e. XV_sigcosΘ YV_sigsin Θ Is. Here, V_sigIs the amplitude of the signal of interest (reflected laser beam) and Θ is the phase difference between the signal of interest and the reference signal (eg, the reference clock signal), ie Θ = Θ_sig-<sub />Θ_refIs. For a pair of transistors adjusted to opposite states or polarities, the X or Y output values will have opposite polarities, regardless of the phase of the input reference frequency. For example, transistor A is Θ_１Transistor B is Θ when modulated by_２ = Θ_１ +/- Modulated at 180 ° (asynchronous). Therefore, the X value for transistor A is cosΘ._１On the other hand, the X value for transistor B is cos Θ._１It is proportional to +/- 180 °. In other words X_AV_sigcosΘ_１ X_BV_sigcos (Θ₁ +/- 180 °) = -V_sigcosΘ_１ = --X_ASimilarly Y_AV_sigsin Θ_１ Y_BV_sigsin (Θ₁ +/- 180 °) = -V_sigsin Θ_１ = --Y_AIs. Therefore, the relative logic state can be extracted from the X or Y output of the lock-in amplifier. However, it should be noted that this logic system is not limited to synchronous and asynchronous detection. Rather, as long as the phase difference between the two transistors is greater than 90 °, the X and Y values of these two transistors will have opposite polarities, even if the absolute amplitudes are different. The X or Y output of the lock-in amplifier is converted into a grayscale image. Here, the value of each pixel corresponds to the phase at its spatial position.

According to another embodiment, a combination of R and Θ values of a lock-in amplifier is used. According to this embodiment R = V_sig = (X^２+ Y^２) Θ = tan^-1(Y / X) Θ is the phase difference between the signal of interest and the reference signal. However, when the laser beam scans the region without transistors in the IC, there is no reflected RF electrical signal, and the Θ value becomes messy. As a result, the Θ output voltage of the lock-in amplifier is messy and this voltage is considered as noise. This causes the transistor-derived Θ value to be obscured by Θ "noise". Therefore, according to one embodiment, the R output is monitored to determine if the Θ output voltage value should be used, i.e., if the Θ value is messy. When the R of the reflected RF electric signal becomes a non-zero value, the Θ value can be used for a specific pixel in the scanning region of the IC. On the other hand, in the absence of the reflected RF electrical signal, R is near zero and the Θ value cannot be used for a particular pixel. In one example, a threshold is set for the magnitude of the R value that allows / disallows the use of the Θ value.

According to the above embodiment, in a pair of transistors that modulate in opposite states, the difference in Θ values is constant 180 ° (ΔΘ = Θ) regardless of the phase of the input reference frequency._A --Θ_B = 180 °). Lock-in amplifiers typically output an analog voltage of +/- V to correspond to the measured +/- 180 ° phase difference. Since the phase difference is 180 °, the amplitude difference of the analog voltage is V (ΔV = V)._A --V_B = V). And the relative polarity between the two transistors is V, using various methods._AAnd V_BIt can be extracted by setting a threshold between.

According to various other embodiments of the present invention, the ability to observe the relative logical state of LVI is apparent by introducing "RF interference" into the acquisition system and feeding the resulting signal to a spectrum analyzer. Will be done. The term acquisition system is meant to include any one or combination of APD, TIA, biasy, RF amplifier, and spectrum analyzer, ie, "RF interference" is any one of these or their connection. Can be combined with points. As used herein, the frequency spectrum of this "RF interference" is referred to as the "interference" spectrum, which performs a function somewhat similar to the reference signal in the embodiment of FIG. In the following embodiments, examples of using a sweep-tuned superheterodyne spectrum analyzer are illustrated, but similar results can be obtained with other real-time spectrum analyzers (also called FFT spectrum analyzers), vector signal analyzers, and the like. It can also be achieved by means.

For effective results, "RF interference" should be at the same frequency as the internal signal under analysis and synchronized. If these requirements are met, this "RF interference" will constructively or destructively interfere with the detected modulation of the transistor carrying the internal signal under analysis (the modulation converted from light to electricity by the acquisition system). To do. If the amplitude of the electrical signal is smaller than the amplitude of the "RF interference" -only electrical signal due to destructive interference, the resulting spectrum will have less energy than the "interference" spectrum. Let's go. For optimal / maximum constructive or destructive interference, a phase shift of the "RF interference" signal is performed to ensure that the "RF interference" signal is synchronized and asynchronous with the signal of interest. You may.

Here, we will refer to FIG. 3, which shows the waveforms of the signals at the selected points A and B. In this example, consider the case where point A and point B are the same inverter and are connected in series. This means that the signals at points A and B are in asynchronous or opposite logical states with respect to each other. In reality, the signal levels are very low, from sub-microvolts to hundreds of microvolts, and averaging is required to achieve the desired SNR, but the modulation detected by the acquisition system is shown in FIG. Has been done. In this figure, the term "signal" refers to the detected optically modulated electrical signal. In a typical spectrum analyzer, both waveforms have the same amplitude, so the power spectrum of the frequency of interest is the same. That is, there is no difference between point A and point B.

Using an embodiment of the invention, the modulated electrical signal detected at points A and B when an "RF interference" electrical signal that is at the same frequency as the modulated signal and is also synchronized is introduced into the acquisition system. Interferes with this introduced signal. Such a situation is shown in FIG. f_intIf the "RF interference" signal shown as and has the amplitude x au interferes with the signal at point A (having the magnitude x au), the resulting electrical signal is the waveform Σ._{f + A}As shown by, it has a total amplitude, i.e. 2x au. On the other hand, the interference signal f_intWhen is interfering with the signal at point B, the resulting electrical signal is the waveform Σ._{f + B}As shown by, it becomes zero, that is, 0 au. Therefore, the spectrum of interest measured by the spectrum analyzer will take three different amplitudes. -2x au at point A -X au in an inactive location (only RF interference signals are measured) -0 au at point B When this is standardized to a grayscale level, point A looks like a white pixel, inactive points look like gray pixels (background level), and point B looks like a black pixel, which is relative between points A and B. Provides logical state mapping.

As mentioned above, the introduction of the interfering signal can be performed at different points in the acquisition system. When a certain amount of "RF interference" electromagnetic waves emitted from a test cell (stimulus, DUT, etc.) are present, the "RF interference" signal can be received, for example, through an electrical connector or by an antenna. Can be collected by any method. The "RF interference" signal is then delivered in various ways, for example using a summing amplifier / voltage adder, through the deliberate transmission of "RF interference" electromagnetic waves, or through simple electrical T-wiring. , Can be input to the acquirer.

Regardless of the mode of collection and input of "RF interference", the collected "RF interference" signal needs to be gain adjusted (simply based on the above embodiment). Programmable RF amplifiers are required to amplify or attenuate the amplitude of the collected "RF interference" signal, depending on how the signal was collected. The "RF interference" signal may be phase adjusted to allow maximum interference. If the gain adjustment does not achieve sufficient constructive or destructive interference due to the slight phase shift of the "RF interference" signal with respect to the particular signal under analysis, then "RF" It may be necessary to phase shift the "interference" signal.

FIG. 5 shows an embodiment of the invention in which an "RF interference" signal is collected through a cable (electrical signal) or an antenna (RF electromagnetic wave). RF interference can be collected from a DUT, a tester (eg, ATE), a tester board, a DUT board, or a cable that interfaces these components. The "RF interference" signal is tuned (amplified / attenuated and phase shifted) after the RF amplifier and before being added by the summing amplifier or voltage adder. The portion of the embodiment of FIG. 5 that collects the irradiation and the reflection of the beam is similar to that of FIG. 2 and is therefore not described again here. Subsequently, elements different from the embodiment of FIG. 2 will be described. Most notably, in FIG. 5, the lock-in amplifier of FIG. 2 is replaced by a spectrum analyzer 572. However, the following elements have been added so that the spectrum analyzer can detect and generate phase signals. That is, the interference signal is collected from the antenna 580 or the cable 582 (both the antenna and the cable are shown in this embodiment, but this is for illustration purposes only and only one or the other or both. May be included). The interfering signal is tuned, i.e. amplified or attenuated by the signal regulator 571 and then phase shifted by the phase shifter 570. The tuned interference signal is then input to a summing amplifier or voltage adder 574 and added to the APD tuned signal. The output is then supplied to a spectrum analyzer 572. The output of the spectrum analyzer is supplied to the video amplifier. The video amplifier supplies the signal to the data acquisition module. In this example, the frame grabber is used to generate a grayscale image mapping that shows the phase of the active element within the scanning region of the DUT. Of course, other data acquisition cards and modules may be used.

FIG. 6 shows an embodiment of the present invention, which is a modification of the embodiment of FIG. In particular, in FIG. 6, the addition of the "RF interference" signal is performed before the RF amplifier 273. That is, the tuned interference signal is added to the RF signal from the bias tee 250 by the summing amplifier or voltage adder 674. The output of adder 674 is then amplified by RF amplifier 273 and then input to a spectrum analyzer.

FIG. 7 shows an embodiment of the present invention, which is a modification of the embodiment of FIG. In particular, in FIG. 7, the "RF interference" signal is added by radiating the interference signal into the electrical path of the APD signal. That is, the tuned interference signal is applied to the antenna 700 by the RF gain / attenuator 570 and / or the phase shifter 571. The antenna 700 is arranged so that its radiation goes into the electrical path of the signal from the APD, is detected by the signal of TIA237, bias 250 and / or amplifier 273, and interferes with the signal. In this way, the interference signal is added to the signal input to the spectrum analyzer 572.

FIG. 8 shows yet another embodiment of the present invention, which is a modification of the embodiment of FIG. In particular, in FIG. 8, the addition of the "RF interference" signal is performed by connecting the interference signal to the adjusted APD signal using a T-connection coupler. That is, the adjusted interference signal is applied to the T connection that also receives the adjusted signal from the amplifier 273. In this way, the interference signal is added to the signal input to the spectrum analyzer 572.

The present invention has been described with respect to its particular embodiments, but is not limited to those embodiments. In particular, various modifications and improvements may be performed by those skilled in the art as long as they do not deviate from the spirit and intent of the invention as defined by the appended claims. In addition, all of the prior art references described above are incorporated herein by reference.