METHOD AND APPARATUS USING AN INFRARED LASER PROBE FOR MEASURING VOLTAGES DIRECTLY IN AN INTEGRATED CIRCUIT

Abstract: A method and an apparatus for detecting an electric field in the active regions (403) of an integrated circuit (405) disposed in a semiconductor. In one embodiment, a laser beam (407) is operated at a wavelength greater than approximately 0.9 microns. The laser beam is focused onto a P-N junction, such as for example the drain of a MOS transistor, through the back side of the semiconductor substrate. As a result of free carrier absorption, the laser beam is partially absorbed near the P-N junction. When a signal is impressed on the P-N junction, the degree of free carrier absorption will be modulated in accordance with the modulation of the depletion region near the P-N junction. The laser beam passes through the P-N junction region, reflects off the oxide interface and the metal behind the junction, and returns back through the P-N junction and back out of the silicon surface. Amplitude modulation in this reflected laser beam is detected with an optical detection system.

Description: METHOD AND APPARATUS USING AN INFRARED LASER PROBE FOR MEASURING VOLTAGES DIRECTLY IN AN INTEGRATED CIRCUIT RELATED APPLICATION This application is related to co-pending application serial no. 08/766,149, filed December 12, 1996, entitled "Method And Apparatus Using An Infrared Laser Based Optical Probe For Measuring Electric Fields Directly From Active Regions In An Integrated Circuit," and assigned to the Assignee of the present application. BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to integrated circuit testing and, more specifically, the present invention relates to optical-based probing of integrated circuits. Description of the Related Art Within the integrated circuit industry there is a continuing effort to increase integrated circuit speed as well as device density. As a result of these efforts, there is a trend towards using flip chip technology when packaging complex high speed integrated circuits. Fiip chip technology is also known as controlled collapse chip connection (C4) or flip chip packaging. In flip chip packaging technology, the integrated circuit die is flipped upside down. This is opposite to how integrated circuits are packaged today using wire bond technology. By flipping the die upside down, ball bonds may be used to provide direct electrical connections from the bond pads directly to the pins of the flip chip package. Figure 1A illustrates integrated circuit packaging 101 which utilizes wire bonds 103 instead of ball bonds to electrically connect integrated circuit connections in integrated circuit die 105 through metal interconnects 109 to the pins 107 of package substrate 11 1. With the trend towards high speed integrated circuits, the inductance generated in the wire bonds 103 of the typical integrated circuit packaging 101 becomes an increasingly significant problem. Figure 1 B illustrates flip chip packaging 151 with the integrated circuit die 155 flipped upside down. In comparison with the wire bonds 103 of Figure 1A, the ball bonds 153 of flip chip packaging 151 provide more direct connections between the integrated circuit die 155 and the pins 157 of package substrate 161 through metal interconnects 159. As a result, the inductance problems associated with typical integrated circuit packaging technologies that use wire bonds are minimized. Unlike wire bond technology, which only allows bonding along the periphery of the integrated circuit die, flip chip technology allows connections to be placed anywhere on the integrated circuit die surface. This leads to a very low inductance power distribution to the integrated circuit which is another major advantage of flip chip. A consequence of the integrated circuit die 155 being flipped upside down in flip chip packaging 151 is that access to internal nodes of the integrated circuit die 155 for testing purposes has become a considerable challenge. In particular, during the silicon debug phase of a new product that is designed to be packaged into flip chip, it is often necessary to probe electrical signals from internal nodes of the chip, insitu, while the chip is packaged in its native flip chip packaging environment. During the debug process it is often necessary to probe certain internal nodes in order to obtain important electrical data from the integrated circuit. Important data include measuring device parameters such as, but are not limited to, voltage levels, timing information, current levels and thermal information. Present day debug process for wire bond technology is based on directly probing the metal interconnects on the chip front side with an electron beam (E-beam) or mechanical prober. Typical integrated circuit devices have multiple layers of metal interconnects and it is often difficult to access nodes that are buried deep in the chip. Usually other tools such as plasma etchers and focused ion beam systems must be used to mill away the dielectric and or metal above the node to expose nodes for probing . With flip chip packaging technology, however, this front side methodology is not feasible since the integrated circuit die is flipped upside down. As illustrated in Figure 1 B, access to the metal interconnects 159 for the purpose of conventional probing is obstructed by the package substrate 161. Instead, the P-N junctions forming the active and passive regions 163 of the integrated circuit are accessible through the back side of the silicon substrate of integrated circuit die 155. Figure 2 illustrates a prior art method used to probe active doped regions in integrated circuits. In the setup shown in Figure 2, a device under test (DUT) 231 includes an active region 239 and non active region (metal) 241. A laser 221 is positioned to focus a laser beam 223 through a beam splitter 225, a birefringent beam splitter 227 and an objective lens 229 through the back side of the silicon of DUT 231 on the doped region 239 and metal 241. As shown in Figure 2, birefringent beam splitter 227 separates the laser beam 223 into two separate laser beams, a probe laser beam 235 and reference laser beam 237. Both probe laser beam 235 and reference laser beam 237 are reflected from active region 239 and metal 241 , respectively, back through objective lens 229 into birefringent beam splitter 227. Probe laser beam 235 and reference laser beam 237 are then recombined in birefringent beam splitter 227 and are guided into detector 233 through beam splitter 225. By operating the DUT 231 while focusing probe laser beam 235 on active region 239 and reference laser beam 237 on metal 241 , waveforms may be detected with detector 233 through the silicon substrate of DUT 231. Detection is possible due to the plasma-optical effect in which the refractive index of a region of free charge is different to a region with no charge. The application of a bias causes the free charge, and hence the refractive index, in the probed region to be modulated whereas the refractive index of the region under the reference beam is unaltered. This results in phase shift between probe beam 235 and reference beam 237. Accordingly, by measuring the phase difference between the reflected reference beam 237 and probe laser beam 235, detector 233 is able to generate an output signal 241 that is proportional to the charge modulation caused by operation of the P-N junction region under the probe. This optical measurement can then be combined with conventional stroboscopic techniques to measure high frequency charge and hence voltage waveforms from the P-N junction region 239. There are a number disadvantages with the prior art technique shown in Figure 2. First, being a phase detection scheme, two beams are required (a reference beam 237 and a probe beam 235) to interfere with each other to generate the phase signal. These beams are created with birefringent beam splitter 227. Therefore, use of both probe laser beam 235 and reference laser beam 237 is limited in that they must be separated by a distance of x as shown in Figure 2. Consequently, the layout of the doped regions 239 and metals 241 in DUT 231 must be such that a metal 241 is located at a distance x from doped region 239. It is appreciated that many modern integrated circuit layouts may not lend themselves to be probed with a setup as shown in Figure 2 since the prior art technique shown requires a reflective surface 241 with no charge modulation ahead of it for the reference beam to be in close proximity (a distance x) to the probed region 239 undergoing charge modulation. It is difficult to meet these requirements in today's advanced technologies. In addition, it is noted that the technique shown in Figure 2 has only been applied to bipolar junction transistor technology and that no successful applications of the prior art technique shown in Figure 2 have been used with complementary metal oxide semiconductor (CMOS) technology. This is because the charge modulation in the depletion region of a reverse biased P-N junction (e.g. the drain of an metal oxide semiconductor (MOS) transistor) is much smaller than the charge modulation in the same junction when it is forward biased (e.g. the base region of a bipolar junction transistor). Furthermore, since the channels of MOS devices are lateral whereas the base- emitter junctions of bipolar devices are vertical, direct measurement of charge modulation in the channel of an MOS device is not possible due to prohibitively small laser spot size required which is far below that of the wavelength of light in silicon. Another prior art optical technique for probing integrated circuits utilizes the electro-optic effect or Pockels effect. This electro-optic effect involves measuring the change in the optical index of refraction that occurs in an asymmetrical crystal when an electric field is applied. The refractive index of electro-optic materials changes when an electric field is applied. As a result, the polarization of an optical beam passing through the electro-optic material then changes according to the strength of the electric field or voltage impressed across the electro-optic material. Figure 3 illustrates an application of the electro-optic effect to an integrated circuit utilizing an asymmetrical crystal such as a gallium arsenide substrate 301. In the illustration shown in Figure 3, a fringing electric field 307 exists between electrodes 305. A probe beam 303 enters from the back side of substrate 301 , passes through the fringing electric field 307 and is reflected from an electrode 305. By measuring the changes in polarization of probe beam 303 and thus changes in the refraction index of the substrate 301 , the electric field 307 can be measured. It is appreciated, however, that although this technique may be utilized in gallium arsenide based integrated circuits, electro-optic sampling in silicon is not possible because silicon is a symmetrical crystal and hence does not exhibit the electro-optic or Pockels effect. Therefore, what is desired is a method and an apparatus for probing active regions in CMOS integrated circuits through the back side of silicon. Such a method should be able to probe active regions of a CMOS integrated circuit through the back side of silicon without the need for reference laser beam to be reflected off a metal near the doped region to be probed. In addition, this method should be compatible with present day CMOS integrated circuit technology. SUMMARY OF THE INVENTION A method and an apparatus for detecting a signal in an integrated circuit is disclosed. In one embodiment, the method includes the steps of directing an infrared optical beam through a back side of a semiconductor substrate of the integrated circuit through a depletion region of the integrated circuit. The depletion region is modulated in response to the signal. The method also includes the step of detecting an amplitude modulation of a reflected infrared optical beam passed through the depletion region through the back side of the semiconductor substrate. Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example and not limitation in the accompanying figures. Figure 1 A is an illustration showing present day wire bond technology. Figure 1 B is an illustration showing flip chip or flip chip packaging technology. Figure 2 is a diagram of a prior art method of probing an active region through the back side of silicon bipolar junction transistor using two laser beams and measuring the phase shift due to charge density modulation of one of the laser beams with respect to the other laser beam. Figure 3 is a diagram of a prior art method of optical probing integrated circuits using the electro-optic effect used in gallium arsenide substrate. Figure 4 is a diagram of an active region being probed using a single laser beam in accordance with the teachings of the present invention. Figure 5 is an illustration showing various waveforms which may be generated using the present invention. Figure 6 is an illustration of a depletion region in a P-N junction being probed by a single laser beam in accordance with the teachings of the present invention. Figure 7 is a graph illustrating plots of the measured electro-absorption as a function of wavelength in a highly doped silicon substrate with a high voltages applied across the substrate and measured at various temperatures. Figure 8 is a diagram of another embodiment of the present invention that probes and monitors an active region in accordance with the teachings of the present invention. Figure 9A is a diagram illustrating free carriers near a P-N junction during the absence of a nearby depletion region. Figure 9B is a diagram illustrating the absence of free carriers in a depletion region near a P-N junction. DETAILED DESCRIPTION A method and an apparatus for detecting a voltage in an active region of an integrated circuit disposed in a semiconductor is disclosed. In the following description numerous specific details are set forth, such as for example wavelengths and energy values, in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention. One embodiment of the present invention provides a method and apparatus for measuring electric fields, and hence voltages, directly from an active region of an integrated circuit with a focused infrared (IR) laser beam through the back side of a semiconductor such as silicon. One embodiment of the present invention directly probes integrated circuit active regions through the silicon back side of the chip using infrared laser based techniques. Since silicon is partially transparent to infrared light, one can focus the infrared laser beam through a partially thinned silicon integrated circuit to directly reach the active regions. Figure 4 shows an embodiment of the present invention which allows an integrated circuit active region 403 to be probed through the back side of a semiconductor in accordance with the teachings of the present invention. An optical source or laser 407 is positioned to provide an optical beam or laser beam 413 which is focused on an active region 403. Laser beam 413 passes through a beam splitter 409 and an objective lens 411 which focuses the laser beam 413 on active region 403. Laser beam 413 passes through the substrate and active region 403, reflects off the contact/metal behind the active region and passes back through the active region 403 and the substrate. The reflected laser beam 415 returns back through objective lens 411 and is guided into detector 417 through beam splitter 409. Detector 417 generates an output signal 419 which corresponds to the voltage in the active region 403. As will be discussed in greater detail below, detector 417 is included in an optical detection system which detects amplitude modulations in reflected laser beam 415 and attributes the modulations in amplitude to the voltage in the active region 403. As shown in Figure 4, active region 403 is included in device under test (DUT) 405. In one embodiment, DUT 405 is a CMOS integrated circuit disposed in silicon accessible from the back side of the substrate. Thus, in one embodiment, DUT 405 is a flip chip packaged product. Since the presently described technique detects amplitude modulation as opposed to phase modulation in the reflected beam, the need for a reference beam for the interferometric phase detection is eliminated. In one embodiment of the present invention, DUT 405 is partially thinned to a thickness of approximately 100 to 200 μm before probing, to allow increased transmission of laser beam 413 through the highly doped silicon substrates, as used in modern VLSI integrated circuits, and correspondingly increases the reflected laser beam 415 returning through the back side of the silicon of DUT 405. In one embodiment of the present invention, laser 407 is a mode locked laser operating at a wavelength of approximately 1.064 μm. Operation of the present invention is as follows. Assume a focused laser beam 413 of power P impinges on a P-N junction 403. If the energy of the photons of laser beam 413 is greater than or equal to the band gap energy of the silicon of DUT 405, there will be some photo-absorption ΔP of the laser beam in the P-N junction. ΔP and P are related by the fundamental absorption coefficient α as given by: ΔP = αP (Equation 1 ) When an electric field E is then applied to the P-N junction, a mechanism known as electro-absorption modulates α--the photo-absorption. The power absorbed in the P-N junction then becomes: T(E)-ΔP (Equation 2) where r(E) is the functional relationship between electro-absorption and the electric field. When the laser 413 reflects off a non-active region, such as for example a metal pad, the laser power detected by detector 417 is P 0 . If the same laser beam 413 is moved onto an active region 403 the laser power then seen by the detector is reduced by ΔP 0 to P-ΔP 0 due to photo-absorption in the P-N junction. When an AC electric field E(t) is applied to the P-N junction, the laser power seen by the detector 417 will now be: P 0 - r[E(t)]-ΔP 0 (Equation 3) where as before, r is the functional relationship between electro-absorption and the electric field E(t). Thus, by measuring, with detector 417 the AC component of the detected signal, the electric field, and hence the voltage, modulation can be recovered. The unknown function r is determined through calibration. Since the P-N junction region is separated from the metal region 406 behind it with an oxide film 404, the probing laser beam will be reflected both from the metal 406 and the oxide-active region interface 408. The latter arises because the refractive index of the silicon forming the P-N junction and oxide are different. In addition to electro-absorption, there is also electro-refraction, which leads to a modulation in the refractive index in the P-N junction as a function of the voltage. Thus, the portion of the reflected laser beam that originated at the oxide-P-N junction interface will also be modulated due to this effect. This effect is smaller than the electro-absorption effect. The two effects combine to give an overall amplitude modulation in the reflected laser power as seen by the detector 417. Figure 6 is an illustration having increased detail of an active region 603 in a DUT 605. In one embodiment of the present invention, active region 603 is an N-doped region in a P-doped silicon substrate. Conversely, it is appreciated that another embodiment of the present invention may utilize a P- doped active region in an N-doped substrate. Laser beam 609 is shown passing through the back side of the silicon of DUT 605 into active region 603 through oxide film 604 and reflecting from metal 607 back out of the back side of the silicon of DUT 605. In the embodiment shown in Figure 6, laser beam 609 is an infrared laser beam and is therefore able to pass through the silicon of DUT 605 since silicon is partially transparent to infrared light. When a bias is applied to active region 603, a depletion region 61 1 is increased at the P-N junction interface between active region 603 and the silicon substrate of DUT 605. With the high doping densities of the active areas, region 603 and substrate of DUT 605, the depletion region 611 at the P- N junction interface is very narrow. In one embodiment of the present invention, depletion region 611 is only about 70 nm thick. Therefore, when a typical integrated circuit operating voltage is applied across such a narrow 5 depletion region 61 1 , a high voltage, such as for example approximately 1 x10 volts per centimeter, is produced across the depletion region 611 as a result. This high voltage increases the tunneling probability, which in turn leads to an increase in the fundamental absorption coefficient. This effect is also known as electro-absorption or the Franz-Keldysh effect. The modulation of the photo-absorption of the laser beam 609 depends on the modulation of the voltage applied at the junction. This modulation in the absorption of the laser beam is the signal of interest since it is related to the voltage applied to the junction. In addition, electro-absorption also leads to electro-refraction. That is, the effects of electro-absorption and electro-refraction are linked. Electro- refraction leads to change in refractive index. This means that at the same time, the refractive index change leads to a modulation in the reflection coefficient for light reflected from the junction/oxide interface. This modulation is superimposed upon the modulation due to electro-absorption and the photodetector actually measures the total amplitude modulation due to both effects. The present invention utilizes detector 417 of Figure 4 to simultaneously detect both the effects of electro-absorption and electro refraction caused by the applied voltage. Both these effects cause power modulation in the reflected laser beam 415. Detector 417 converts these modulations into output signal 419. The degree of the amplitude modulation is related to the applied electric field (i.e. voltage) across the P-N junction, by the function T of Equation 2. r is determined through calibrating the detector output to known applied voltages. By operating the DUT 405 while focusing laser beam 413 on an active region 403, electrical waveforms similar to waveforms 501 , 503 and 505 of Figure 5 may be detected with detector 417 through the substrate of DUT 405 which correspond to varying voltages applied to active region 403. This optical measurement can then be combined with conventional stroboscopic techniques to measure high frequency voltage waveforms from the active region 403. Figure 7 is a graph 701 illustrating plots of the measured electro- absorption as a function of wavelength in a highly doped silicon substrate with a high voltage applied across the substrate and measured at various temperatures. Plots 703, 705 and 707 of graph 701 show the measured change in transmission over the total transmission of the photons of an infrared beam passing through a doped silicon substrate as a function of photon wavelength. Specifically, plot 703 shows the change in transmission over total transmission through a thinned silicon substrate versus wavelength with an externally applied voltage of approximately 1 x 10 5 volts per centimeter applied to the silicon substrate at room temperature. As can be seen from Figure 7, there is a peak electro-absorption at approximately 1.064 μm. Plot 705 shows the relationship of change in transmission over total transmission of photons versus wavelength through the same thinned silicon substrate with no voltage applied at room temperature. As shown in Figure 7, there is no corresponding electro-absorption peak in plot 705 at 1.064 μm. Figure 7 demonstrates the effect that a high voltage has on the electro-absorption characteristics in silicon. The magnitude of the electro-absorption is directly related to the magnitude of the voltage in the silicon. By calibrating this functional relationship one can extract the applied voltage applied to the P-N junction. With the doping levels of the silicon sample utilized in plots 703 and 705, one embodiment of the present invention measures electro-absorption by implementing an infrared, high frequency, mode locked laser operating at a wavelength which corresponds with the peak in plot 703. The optics of the embodiment of the present invention focus the mode locked laser pulses onto the active regions to be probed. The probing system consists of a single beam that is focused through the back side of the silicon of a fully flip chip packaged integrated circuit into the N doped regions located in a P substrate. It is appreciated that the present invention may also be employed with integrated circuits having P doped regions located in an N substrate. The laser beam passes through the P-N junction interface, reflects off the front side metal, and returns back through the P-N junction region and back out of the silicon surface. When a bias is applied across the P-N junction, a voltage results across the P-N junction interface. Plot 703 illustrates how some of the photons will be absorbed due to electro-absorption. The change in transmission over total transmission will be approximately 2 x 10 -5 at a wavelength of 1.064 μm. If no voltage is applied, the photon absorption will be reduced. By operating the DUT while focusing probe laser beam on the active region to be probed, electrical waveforms which correspond to varying voltages applied to the doped region may be detected and generated using a photo detector in accordance with the teachings of the present invention. It is appreciated that the band gap of silicon decreases with both increasing doping concentration and with increasing temperature. For instance, it is noted that the band gap decreases by approximately 30 meV at 100 degrees Celsius for highly doped silicon. Accordingly, corresponding plot 707 shows the change in transmission over total transmission through the silicon substrate versus wavelength with an externally applied voltage of approximately 1 x 10 5 volts per centimeter at 100 degrees Celsius. It can be seen by plot 707 that the electro-absorption peak has accordingly moved to approximately 1.085 μm due to the increase in temperature. If the integrated circuit being probed operates at an elevated temperature, the peak in the measured photo-absorption moves to longer wavelengths as shown by comparing plots 703 and 707 of Figure 7. As a result, one embodiment of the present invention is not limited to using a mode locked laser having a fixed wavelength of 1.064 μm. Instead, the embodiment of the present invention employs a tunable mode locked laser that can be varied or adjusted with temperature or other conditions which may change the band gap of the semiconductor material. By enabling the wavelength of the mode locked laser to follow and match the peaks of plots 703, 707 or the like, the signal to noise ratio of one embodiment of the present invention is continuously optimized. It is noted that the transmission of an infrared laser in silicon decreases as a function of temperature. It is also appreciated that other techniques for compensating for the decrease in transmission of an infrared laser in silicon due to temperature may be employed to optimize the signal to noise ratio. In one embodiment of the present invention, DUT 405 is run and operated in a tester environment on a tester apparatus (not shown) when the output waveforms are generated. In another embodiment of the present invention, DUT 405 may be run and operated in a system environment, such as for example on a computer motherboard, when the output waveforms are generated. That is, laser beam 413 is focused directly onto doped region 403 and waveforms such as waveforms 501 , 503 and 505 may be obtained while DUT 405 is mounted and operating in an actual system. As a result, one embodiment of present invention enables the debugging of flip chip packaged integrated circuits while the parts are operating on a tester apparatus or in a system environment. It is appreciated that the present invention may obtain waveforms from DUT 405 in other types of test environments so long as laser beam 413 is focused onto active region 403 through the back side of the semiconductor substrate while DUT 405 is operated. All this is possible because the DUT is not limited to being operated in a vacuum as opposed to an E-Beam probing environment, which requires the DUT to operate in vacuum. It is noted that it is impractical to probe a DUT operating in a system environment such as on a large PC board using a conventional E-beam probe because the vacuum chamber of the E-beam probe would be required to be prohibitively large to accommodate the large PC board. However, since the present invention does not require the DUT to be in a vacuum chamber, waveforms can now be obtained in a variety of operating environments. Figure 8 is a diagram of another embodiment 801 of the present invention. A mode locked laser 803 operating at 1.064 μm produces a laser beam 805 which passes through optical isolator 807, through λ/2 half wave plate 808, through polarizing beam splitter 809, through λ/4 quarter wave plate 810, through dichroic beam splitter 81 1 and through objective lens 813 is focused onto doped region 817 through the back side of the silicon of DUT 815. The laser beam 805 is reflected off the front side metal 819 back through objective lens 813 and is guided to aperture 821 with dichroic beam splitter 811 , λ 4 quarter wave plate 810 and polarizing beam splitter 809. The reflected laser beam 805 is directed by aperture 821 through focusing lens 823 into photo diode 825. Photo diode 825 is coupled to electronics 827 to detect the amplitude modulation of photons in the reflected laser beam from DUT 815 to determine the presence of a voltage at doped region 817. Optical isolator 807 is used to minimize the number of photons reflected back into laser 803. The λ/2 half wave plate 808 and λ/4 quarter wave plate 810 are used in conjunction with polarizing beam splitter 809 to increase the percentage transmission of laser beam 805 to DUT 815 as well as increase. the percentage transmission of laser beam 805 reflected from the DUT 815 to aperture 821 to optimize the signal to noise ratio. The embodiment 801 shown in Figure 8 also includes imaging elements for monitoring, including a tungsten illumination source 829 which generates light 831 which is directed through infrared filter 833, through collimating lenses 835, through beam splitter 837, through dichroic beam splitter 811 , and objective lens 813 onto doped region 817 through the back side of the silicon of DUT 815. The light 831 is reflected from DUT 815 back through the objective lens 813, through dichroic beam splitter 811 into infrared camera 843 through beam splitter 837 and focusing lens 841. The reflected light 839 allows imaging of the back side of the DUT 815 with infrared camera 843. A video monitor 845 is coupled to infrared camera 843 to observe the images of the back side of the silicon of DUT 815. This enables direct observation of doped regions insitu while focusing the laser into doped regions needed to be probed. In yet another embodiment of the present invention, laser 803 may also be configured to act as a scanning laser light source. In this embodiment, a laser spot is scanned or rastered across the back side of DUT 815 for imaging purposes. In the embodiment, laser light reflected from DUT 815 is transmitted to infrared camera 843 so that an image of DUT 815 may be observed on video monitor 845. In the embodiment, the optical detection system is a confocal based optical system. In still another embodiment of the present invention, the optical source or laser 407 is operated at a wavelength that is transmissible through the semiconductor substrate of the DUT. In one embodiment, laser 407 is operated at a wavelength greater than approximately 0.9 μm through a silicon substrate. Referring briefly back to Figure 4, laser 407 is positioned to provide an optical beam or laser beam 413 having a wavelength greater than approximately 0.9 μm, which is transmissible through a silicon substrate. As shown in Figure 4, laser 407 is focused on the active region 403. In one embodiment, optical source or laser 407 is an infra red continuous wave laser or a pulsed laser, such as for example a Q switched laser, a mode locked laser, or the like. In one embodiment, laser beam 413 is focused through a thinned silicon substrate to active region 403. In another embodiment, laser 407 is operated at a wavelength of approximately 1.3 μm and laser beam 413 is focused through an unthinned silicon substrate. Laser beam 413 passes through the beam splitter 409 and the objective lens 411 , which focuses the laser beam 413 on active region 403. Laser beam 413 passes through the substrate and active region 403, reflects off the contact/metal 406 behind the active region 403 and passes back through the active region 403 and the substrate. The reflected laser beam 415 returns back through objective lens 41 1 and is guided into detector 417 through beam splitter 409. Detector 417 generates an output signal 419, which corresponds to the signal at active region 403. In this embodiment, reflected laser beam 415 is modulated in response to the free carrier absorption occurring as a result of a modulated depletion region near the active region 403. For example, Figure 9A shows free carriers 911 in the substrate of DUT 905. In one embodiment, the substrate of DUT 905 includes silicon. Laser beam 909 is shown passing through the back side of the of the silicon of DUT 905 into active region 903 through insulator 904 and reflecting from metal 907 back out of the back side of the silicon of DUT 905. In one embodiment, active region 903 includes a P-N junction, which may in one embodiment be the drain of an MOS transistor or the like. In the embodiment shown in Figure 9A, laser beam 909 is an infrared laser beam having a wavelength that is transmissible through the silicon of DUT 905 since silicon is partially transparent to infrared light. As a result of free carrier absorption, the laser beam 909 is partially absorbed near the P-N junction. Figure 9A illustrates that free carriers 911 may be distributed nearby the active region 903 during the absence of a depletion region near the active region 903. However, Figure 9B illustrates that when a bias is applied to active region 903, a depletion region 913 is formed near active region 903 and free carriers 91 1 are thereby swept away from active region 903. By modulating the bias applied to active region 903, depletion region 913 is modulated accordingly. By modulating depletion region 913, the free carriers 91 1 near active region 903 are modulated. By modulating the free carriers 911 near active region 903 in response to a signal applied to active region 903, the free carrier absorption of laser beam 909 is modulated accordingly. This can be explained mathematically as follows: By taking a standard P-N junction model, the space charge distribution profile of the P-N junction can be approximated as a one-sided abrupt junction. In this case, the electric field E, and the depletion layer width w, are given by: Electric Field E m = ~ sΥ hl +V ω ) (1 ) Depletion width w = (2) where q is the electronic charge, ε s is the permittivity of silicon, V RB is the kT reverse bias voltage (or signal), and V. = — In A is the built-in potential of the P-N junction. As an initial approximation, it can be assumed that the modulation of laser beam 909 passing through depletion region 913 is of the form Modulation ~ (1 - e "a(w) ) (3) where a(w) is the resulting contribution due to free carrier absorption in depletion region 913. Knowing that free carrier effects are very small (on the order of parts per million), hence the exponential in Equation 3 can be approximated through a power series expansion. Since for small values of x, e " ~ (1 - x), equation (3) can be approximated as: Modulation ~ a(w) (4) It can be observed from Equation 4 that the optical modulation of laser beam 909 is then proportional to the width of depletion region 913. From Equation 2 the optical signal is then directly proportional to the electrical signal (V RB ) applied to the P-N junction. It is noted that the free carrier absorption of light is proportional to the square of the wavelength of the light. Thus, the free carrier induced modulation of laser beam 909 is proportional to the square of the wavelength of laser beam 909. Therefore, in one embodiment, the modulation of laser beam 909 in response to free carrier absorption increases proportional to the square of the wavelength of laser beam 909. Thus it is possible to use longer wavelengths (>0.9 μm) to increase the magnitude of the signal. However, it is noted that as the wavelength of laser beam 909 increases, there is a corresponding increase in absorption in the substrate, which may limit the maximum wavelength that can be used. A benefit of using wavelengths greater than approximately 1.1 μm, the band gap wavelength of silicon, for laser beam 909 is that the longer wavelengths have lower photon energies than the silicon bandgap of DUT 905. Thus, at 1.3 μm there will be very little photocurrent generated by laser beam 909. This allows one to use a larger incident laser power for laser beam 909, which results in a larger return signal to the photo diode. Thus, what has been described is a method and an apparatus for detecting an electric field at a doped region of an integrated circuit disposed in a semiconductor without having to mill the substrate to expose a doped region or supply a second laser beam to be used as a reference beam. With the presently described infrared laser based optical probe, a technique for measuring voltages and electric fields directly from doped regions through the back side of silicon on a flip chip mounted integrated circuit is provided. In the foregoing detailed description, the method and apparatus of the present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.

Claims: CLAIMS What is claimed is: 1. A method for detecting a signal ... an integrated circuit, the method comprising the steps of: directing an infrared optical beam through a back side of a semiconductor substrate of the integrated circuit through a depletion region of the integrated circuit, the depletion region modulated in response to the signal; and detecting an amplitude modulation of a reflected infrared optical beam passed through the depletion region through the back side of the semiconductor substrate. 2. The method of claim 1 wherein the step of directing the infrared optical beam through the back side of the semiconductor substrate includes the step of operating a laser at a wavelength that is transmissible through the semiconductor substrate. 3. The method of claim 2 when the step of operating the laser at the wavelength that is transmissible through the semiconductor substrate includes the step of operating the laser at a wavelength greater than approximately 0.9 μm. 4. The method of claim 2 when the step of operating the laser at the wavelength that is transmissible through the semiconductor substrate includes the step of operating the laser at a wavelength of approximately 1.3 μm. 5. The method of claim 1 wherein the depletion region is proximate to an active region of the integrated circuit. 6. The method of claim 1 wherein the semiconductor substrate comprises silicon. 7. The method of claim 5 wherein the active region comprises an N doped region. 8. The method of claim 7 wherein the N doped region is disposed in a P substrate. 9. The method of claim 5 wherein the active region comprises a P doped region. 10. The method of claim 9 wherein the P doped region is disposed in an N substrate. 1 1. The method of claim 1 wherein the semiconductor substrate is included in a fully packaged integrated circuit. 12. The method described in claim 11 wherein the fully packaged integrated circuit is included in a flip chip packaged integrated circuit. 13. The method described in claim 11 including the additional step of operating the fully packaged integrated circuit using a tester apparatus. 14. The method described in claim 11 including the additional step of operating the fully packaged integrated circuit on a printed circuit board. 15. The method described in claim 5 wherein the step of detecting the amplitude modulation of the reflected infrared optical beam includes the step of reflecting the optical beam off a front side metal disposed near the active region. 16. The method described in claim 1 including the additional step of thinning the semiconductor substrate to approximately 100 to 200 μm before the step of directing the infrared optical beam through the back side of the semiconductor substrate. 17. A device for detecting a signal in an integrated circuit, comprising: an optical source configured to provide an infrared optical beam; an objective lens positioned between the optical source and the integrated circuit to direct the optical beam through a depletion region of the integrated circuit through a back side of a semiconductor substrate, the depletion region modulated in response to the signal; a beam splitter positioned between the optical source and the objective lens; and, a detector positioned to detect an amplitude modulation in a reflected optical beam passed through the depletion region, through the back side of the semiconductor substrate, through the objective lens and through the beam splitter. 18. The device described in claim 17 wherein the depletion region is proximate to an active region of the integrated circuit. 19. The device described in claim 17 wherein the amplitude modulation is caused by free carrier absorption of the optical beam. 20. The device described in claim 17 further comprising an optical isolator positioned between the optical source and the beam splitter. 21. The device described in claim 20 wherein the beam splitter comprises a polarizing beam splitter, the device further comprising a λ/2 half wave plate and λ/4 quarter wave plate, the polarizing beam splitter positioned between the λ/2 half wave plate and the λ/4 quarter wave plate. 22. The device described in claim 17 wherein the optical source comprises a continuous wave laser. 23. The device described in claim 17 wherein the optical source comprises a pulsed laser. 24. The device described in claim 17 further comprising an illumination source configured to illuminate the active region. 25. The device described in claim 17 wherein the optical source is an illumination source configured to illuminate the active region. 26. The device described in claim 24 wherein the illumination source illuminates the active region through an infrared filter, a collimating lens, another beam splitter, a dichroic beam splitter and the objective lens. 27. The device described in claim 26 further comprising an infrared camera configured to monitor the active region through the objective lens, the dichroic beam splitter, the other beam splitter and a first focusing lens. 28. The device described in claim 17 wherein the detector comprises an aperture configured to guide the reflected optical beam onto a second focusing lens, the second focusing lens configured to focus the reflected optical beam onto a photo diode, the photo diode coupled to detector electronics to detect the reflected optical beam. 29. A method for detecting an electric field in an integrated circuit, the method comprising the steps of: operating a laser beam at a wavelength near a band gap of a semiconductor of the integrated circuit; focusing the laser beam through a back side of the semiconductor on an active region of the semiconductor; and detecting an amplitude modulation of a reflected laser beam passed through the active region through the back side of the semiconductor. 30. The method described in claim 29 including the additional step of adjusting the wavelength in response to a change in the band gap of the semiconductor. 31. The method described in claim 29 including the additional step of generating an output waveform in response to the amplitude modulation of the reflected laser beam. 32. The method described in claim 29 wherein the semiconductor comprises silicon. 33. The method described in claim 29 wherein the active region comprises an N doped region. 34. The method described in claim 33 wherein the N doped region is disposed in a P substrate. 35. The method described in claim 29 wherein the active region comprises a P doped region. 36. The method described in claim 35 wherein the P doped region is disposed within an N substrate. 37. The method described in claim 29 wherein the semiconductor is included in a fully packaged integrated circuit. 38. The method described in claim 37 wherein the fully packaged integrated circuit is a flip chip packaged integrated circuit. 39. The method described in claim 37 including the additional step of operating the fully packaged integrated circuit using a tester apparatus. 40. The method described in claim 37 including the additional step of operating the fully packaged integrated circuit on a printed circuit board. 41. The method described in claim 29 wherein the detecting step includes the step of reflecting the laser beam off a front side metal disposed near the active region. 42. The method described in claim 29 including the additional step of thinning the semiconductor to approximately 100 to 200 μm before the step of focusing the laser beam on the active region through the back side of the semiconductor. 43. The method described in claim 29 wherein the detecting step includes the step of applying a bias across the active region of the integrated circuit. 44. The method described in claim 29 wherein the laser beam is a continuous wave laser beam. 45. The method described in claim 29 wherein the laser beam is a pulsed laser beam. 46. The method described in claim 29 wherein the detecting step includes the step of guiding the reflected laser beam into an optical detection system. 47. The method described in claim 29 including the additional step of illuminating the active region through the back side of the semiconductor with a light source. 48. The method described in claim 47 wherein the light source is the laser beam, wherein the step of illuminating the active region includes the step of rastering the laser beam across the active region. 49. The method described in claim 48 wherein the optical detection system is a confocal based optical system. 50. The method described in claim 29 including the additional step of monitoring the active region through the back side of the semiconductor with an infrared camera coupled to a video monitor.

Inventors: PANICCIA MARIO I, RAO VALLURI R M, YEE WAI MUN, PANICCIA, MARIO, I., RAO, VALLURI, R., M., YEE, WAI, MUN
Applicants: INTEL CORP, PANICCIA MARIO I, RAO VALLURI R M, YEE WAI MUN, INTEL CORPORATION, PANICCIA, MARIO, I., RAO, VALLURI, R., M., YEE, WAI, MUN
Application year: 1999
Main classification: G01R31
IPC codes: G01N21, G01R1, G01R19, G01R31