Abstract:
A method and circuit for integrated circuit analysis. The apparatus includes an irradiation component configured to irradiate an integrated circuit, a constant voltage source, one or more current chokes placed between the constant voltage source and one or more connections to the integrated circuit, and one or more voltage amplifiers. The irradiation component introduces changes in the electrical state of the integrated circuit. The one or more voltage amplifiers produces a state signal relating to the changes introduced by the irradiation component in the integrated circuit. The current choke separates the function, a DC function, of supplying a voltage bias to the integrated circuit from the function, an AC function, of signal generation. This allows separate optimization of the two functions—the ability to correctly bias the integrated circuit and obtain high signal-to-noise ratios for the resulting signal. This optimization improves the sensing abilities of the circuit.

Description:
PRIORITY CLAIM 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 60/270,829 filed Feb. 22, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods and apparatus for analyzing failures in semiconductor circuits. 
     BACKGROUND OF THE INVENTION 
     LIVA (Light Induced Voltage Alterations), U.S. Pat. No. 5,430,305, and TIVA (Thermally Induced Voltage Alterations), U.S. Pat. No. 6,078,183, have demonstrated significant capability to for fault isolation in semiconductor circuits. A difficulty with both techniques is their use of a constant current bias, whereas integrated circuits operate on constant voltage bias. OBIRCH (Optical Beam Induced Resistance Changes), U.S. Pat. No. 5,952,837, utilizes constant voltage bias, however, tests have shown that LIVA/TIVA is as much as two orders of magnitude more sensitive (“Backside Localization of Open and Shorted Interconnections” E. I. Cole, Jr. et al., International Reliability Physics Symposium, pp. 129-136, 1998). 
     The current invention utilizes a new technique for fault isolation in semiconductors that makes use of the constant current sensing of LIVA/TIVA, while allowing for use of constant voltage bias on the integrated circuit. The operational advantages include: correct biasing of the integrated circuit, ability to change the state of the integrated circuit without also having to change the current bias conditions, and ability to utilize the technique on lines other than power lines. In addition, the technique is significantly more sensitive (at least one order of magnitude) than the standard LIVA/TIVA approach. 
     LIVA, TIVA, and OBIRCH all utilize a scanned optical beam from a laser scanning microscope to stimulate a device under test (DUT). Nominally, the DUT consists of a semiconductor integrated circuit, which consists of passive (resistors, capacitors) and active (transistors) elements on a semiconductor substrate. Modern integrated circuits can also contain electro-mechanical and sensor elements. In the case of LIVA, the stimulation is due to carrier production by the optical beam in the semiconductor. For TIVA and OBIRCH, the stimulation is thermal only (e.g. the wavelength of the laser is selected such that no carrier production occurs). In each case, the response of the device under test is recorded versus scanner position to produce an image. Nominally, the recording and display utilize an analog to digital (A/D) conversion, a general-purpose computer and a monitor. The response image corresponds to the reflected light image that can also be recorded by the microscope. The response image can indicate the presence and character of a number of failures as described in the referenced patents and papers. 
     The distinction between the LIVA/TIVA and OBIRCH techniques lies in the use of a constant current source versus constant voltage source for the DUT bias. The constant voltage approach is consistent with normal operation of an integrated circuit; however, the constant current approach has several orders of magnitude more sensitivity. The current invention seeks to maintain the optimum bias approach (constant voltage) while producing signal strengths associated with the constant current approach. This desire can be realized by recognizing that the DUT bias requirement lies in low frequency or DC regime, where as the signals lie at higher, AC, frequencies and can thereby be separated by suitable circuitry. An additional benefit is to separately optimize the bias circuitry and the signal circuitry, which produces increased signal-to-noise. 
     One method for performing this separation applies constant voltage bias to the DUT through a choke coil. The coil does not affect the DC bias of the DUT. However, when the DUT is stimulated by the scanned optical beam it momentarily attempts to draw more (or less) current from the bias circuit. The choke coil temporarily suppresses this change in current, i.e. it acts as an AC constant current source. The voltage across the DUT (or coil) will change just as for the LIVA/TIVA systems. An amplifier can be used to increase the signal strength. 
     Additionally, optimization of the bias circuitry and the sensing circuitry is now separated. In both LIVA/TIVA and OBIRCH, the bias circuits (power supplies) are producing the voltage or current (respectively) signal that is then amplified. Power supplies are not known to be optimized for their signal generation properties. 
     Clearly other circuits (more complex, with feedback, etc.) can also be utilized to obtain the same separation of circuit bias and signal sensing, however, the basic operational principles of separation in the frequency domain would be identical. Scanned electron beams and ion beams are also utilized for stimulation of semiconductor circuits in order to locate failures. The invented sensor arrangement can also be utilized with these forms of stimulation of the circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for integrated circuit analysis. The apparatus includes an irradiation component configured to irradiate an integrated circuit, a constant voltage source, one or more current chokes placed between the constant voltage source and one or more connections to the integrated circuit, and one or more voltage amplifiers. The irradiation component introduces changes in the electrical state of the integrated circuit. The one or more voltage amplifiers produces a state signal relating to the changes introduced by the irradiation component in the integrated circuit. The current choke separates the function, a DC function, of supplying a voltage bias to the integrated circuit from the function, an AC function, of signal generation. This allows separate optimization of the two functions—the ability to correctly bias the high signal-to-noise ratios for the resulting signal. This optimization improves the sensing abilities of the circuit. 
     In accordance with further aspects of the invention, the apparatus further includes a component for focusing and scanning the irradiation over the integrated circuit, wherein the scanning acts to modulate the irradiation. 
     In accordance with other aspects of the invention, the irradiation is from one of a laser source, an electron beam or an ion beam. 
     In accordance with still further aspects of the invention, the apparatus further includes a component for recording and displaying an image of the state signal as a function of scan position. 
     In accordance with yet other aspects of the invention, the apparatus further includes a component for producing an image of the integrated circuit. The produced image is utilized to correlate the state signal to corresponding positions on the integrated circuit. 
     As will be readily appreciated from the foregoing summary, the invention provides an integrated circuit analysis device with improved sensing capabilities. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
     FIG. 1 is a block diagram of a complete measurement system; 
     FIG. 2 shows details of an OBIRCH approach to integrated circuit sensing electronics; 
     FIG. 3 shows details of LIVA/TIVA approach to integrated circuit sensing electronics; 
     FIG. 4 shows a block diagram of integrated circuit sensing electronics for the current invention; 
     FIG. 5 is a first embodiment of integrated circuit sensing electronics for the current invention; 
     FIG. 6 is a second embodiment of integrated circuit sensing electronics for the current invention; and 
     FIG. 7 is a third embodiment of the integrated circuit sensing electronics for the current invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 sets forth a preferred embodiment of a complete Externally Induced Voltage Alteration (XIVA) system as applied to analyzing an integrated circuit  2  (device under test or DUT). The XIVA system comprises three major components, a modulated irradiation source  4 , an electronic sensor  6 , and a display means  8 . 
     The integrated circuit  2  nominally comprises a set of active (transistors and diodes) and passive (resistors and capacitors) electronic components on a semiconductor substrate (e.g. silicon, gallium arsenide, and plastic semiconductors). Recent technological advances necessitate the extension of this nominal set of components to include integrated circuits with electromechanical components, so called micro-electromechanical systems (MEMS). An additional extension is to integrated circuits that contain sensing elements such as chemical sensors. 
     In the preferred embodiment, the modulated irradiation source  4 , comprises an irradiation source  20 , a scanner  22 , and a detector  24 . The irradiation source can be an optical source, an electron-beam source, an ion-beam source or any source of irradiation that can affect the operating condition of the integrated circuit  2 . Changes in the operating condition of the integrated circuit can come about due to introduction of free carriers in the integrated circuit  2  or to thermal heating of components in the integrated circuit. When the irradiation source  20  is an optical source, the operating condition or the integrated circuit  2  can be affected when carriers are introduced via photovoltaic and photoconductive effects. Optical sources can also introduce heating effects due to temperature dependence of metal resistance or thermal electric effects. Generally, introduction of free carriers causes stronger changes in the integrated circuit  2  operating condition than heating. Use of optical wavelengths below the semiconductor bandgap allows heating without carrier generation, producing one approach to separating the two effects. Electron-beam and ion-beam irradiation sources can also introduce free carriers and cause heating in the integrated circuit  2 . 
     The scanner  22  directs (focuses) the irradiation from the irradiation source  20  onto the integrated circuit  2 . The scanner  22  further scans the focused irradiation over the integrated circuit. Scanning performs two functions 1) it localizes the effect of the irradiation source  20 , and 2) it effectively modulates the irradiation source  20 . Modulation occurs as only a portion of the integrated circuit is irradiated at any particular time, that portion changing with time as the irradiation source  20  is scanned. Thus, the operating condition of the integrated circuit  2  is a function of time (and therefore position) and is thereby modulated. Other modulation formats (e.g. chopping of the irradiation source  20 ) are also possible. 
     The irradiation source  4 , also contains a detector  24 . The detector  24  collects irradiation returned from the integrated circuit  2  through the scanner and converts it into an image signal  26 . For the case of optical irradiation, the detector  24  could be a photodetector and an amplifier that produces an electronic image signal  26  proportional to the amount of optical irradiation reflected from the integrated circuit  2 . 
     Use of the scanner  22  implies generation of images. The display means  8  allows for display of these images. The preferred embodiment of the display means includes an analog-to-digital (A/D) converter  30 , a general-purpose computer  32 , and a monitor  34 . The A/D converter  30  takes in electrical signals as a function of scanner position so as to produce a digital array of data. The general-purpose computer  32 , controls and coordinates the scanner  22  and the A/D converter  30 . The general-purpose computer also conditions the data array produced by the A/D converter  30  for display as an image on the monitor  34 . As shown in FIG. 1, the two signals taken in by the A/D converter are the image signal  26  and a state signal  14 . 
     A clear understanding of the distinction of the preferred embodiment of the electronic sensor  6  over the approaches utilized in OBIRCH and LIVA/TIVA requires a brief review of their general operating principles. Note is made of the fact that the integrated circuit  2  is an integral part of the electronic sensor  6  in all approaches. 
     FIG. 2 is a schematic of the electronic sensor  6  utilized for OBIRCH. A constant voltage source  10  is connected to a current sensor  40 , which is in turn connected to the integrated circuit  2 . U.S. Pat. No. 5,952,837 does not describe a specific embodiment for the current sensor  40 . One standard approach to current sensing is to place a sense resistor  42  between the constant voltage source  10  and the integrated circuit  2 . A differential voltage amplifier  44  produces a state signal  14  that is proportional to the current flow in the integrated circuit  2 . 
     FIG. 3 is a schematic of the electronic sensor  6  utilized for LIVA/TIVA. A constant current source  50  is connected directly to the integrated circuit  2 . An AC amplifier  52  produces a state signal  14  proportional to changes in the voltage across the integrated circuit  2 . Clearly, an AC amplifier could also be used for OBIRCH, although none is specified in U.S. Pat. No. 5,952,837. 
     OBIRCH offers the advantage of constant voltage bias, which is generally the correct form of power bias and signal line bias for an integrated circuit. Experimentally, LIVA/TIVA is found to be significantly more sensitive to state changes in the integrated circuit  2  and therefore more successful at analyzing the circuit conditions. One potential explanation is that the value of the sense resistor  42  must be chosen to be small compared to the resistance of the integrated circuit  2  so as to not adversely affect circuit operation. For LIVA/TIVA the resistance of the integrated circuit  2  is directly utilized to obtain the state signal  14 . The two circuits will have similar noise properties, however, the state signal  14  for the two approaches will be proportional to the ratio of the value of the sense resistor  42  to the resistance of the integrated circuit  2 . Thus, OBIRCH could be expected to have a significantly lower signal-to-noise ratio than LIVA/TIVA. 
     An additional issue for both approaches is that the state signal  14  is the result of the reaction of the constant voltage source  10  or the constant current source  50  to changes in the state of the integrated circuit  2 . Specifically, the state signal  14  that occurs in OBIRCH is a result of a change in current from the constant voltage source  10  in response to a change in the integrated circuit  2  drive requirements induced by the modulated irradiation source  4 . Similarly, the state signal  14  that occurs in LIVA/TIVA results from a change in voltage from the constant current source  50  in response to a change in the integrated circuit  2  drive requirements induced by the modulated irradiation source  4 . The constant voltage source  10  and the constant current source  50  are typically power supplies or signal line drivers and are not optimized for signal generation. 
     The details of the electronic sensor  6  for the current invention shown in FIG. 1 are reproduced in detail in FIG.  4 . The electronic sensor  6  comprises the constant voltage source  10  connected to a current choke  18  that is then connected to the integrated circuit  2 . The integrated circuit  2  is also connected to a signal filter  16  followed by an amplifier  12  to produce the state signal  14 . The current choke  18  acts as an inductive element. It is designed to have a low voltage drop, so as to allow constant voltage bias of the integrated circuit  2  independent of the static current flow in the circuit. However, when the modulated irradiation source  4  induces changes in the integrated circuit  2  power requirements, the current choke  18  will act to temporarily hold off changes in the current flow to the integrated circuit  2 . Thus, the current choke  18  acts as a temporary constant current source, while the integrated circuit  2  average voltage bias remains constant. In this fashion, the state signal  14  is the same as for constant current bias, while actually utilizing a constant voltage bias  10 . Moreover, the current choke  18 , not the constant voltage source  10  is now performing a signal generation function and can be independently optimized. 
     Stated differently, the current choke  18  separates the DC function of supplying voltage bias to the integrated circuit  2  from the AC function of signal generation. This separation into the two frequency domains allows separate optimization of the two functions; the ability to correctly bias the integrated circuit  2  and obtain high signal-to-noise ratios for the state signal  14  result. 
     An additional change from the prior art for the electronic sensor  6  is the inclusion of the signal filter  16 . Changes induced in the integrated circuit  2  by the modulated irradiation source  4  will have characteristic time signals. For example, a photoconductive response to a pulsed optical irradiation can be a fast rise while the irradiation is present followed by a slow exponential decay. Use of appropriate filters can enhance this particular signal over others with a different time signature and over noise present in the electrical sensor  6 . Although shown placed between the integrated circuit  2  and the amplifier  12 , the signal filter  16  can be placed or even distributed throughout the electronic sensor  6 . In fact, the current choke  18  acts to filter noise present on the constant voltage source and can be considered part of the signal filter  16 . 
     A final feature of the electronic sensor  6  in the present invention is a bypass switch  19 . At times, it will be desirable to externally change the state of the integrated circuit  2 . For example, the integrated circuit  2  may be a digital processor. A program may be run in the processor to set its state prior to examination with the XIVA system. During program execution, the integrated circuit current draw can change dramatically. The current choke will act to suppress these changes and may cause failure of the program and state change of the integrated circuit  2 . The bypass switch  19  is utilized to temporarily bypass the current choke and allow external changes in the integrated circuit  2  state. 
     FIGS. 5-7 show specific embodiments of the electronic sensor  6 . FIG. 5 shows a very simple embodiment wherein the current choke  18  is a single choke coil (inductor)  60 . The signal filter  16  is a simple high-pass filter consisting of a capacitor  62  and a resistor  64 . As described above, the choke coil  60  is also acting as part of the signal filter  16  as it blocks high frequency noise from the constant voltage source  10 . 
     Pick up noise on the lines connecting a circuit is often a predominate noise source in an electronic sensor system. FIG. 6 shows an embodiment of the electronic sensor  6  that reduces much on this noise. This embodiment uses a floating (ungrounded) biasing arrangement. A pair of choke coils  60  are placed on both lines between the constant voltage source  10  and the integrated circuit  2  (now floating), which suppresses high frequency pick-noise on both lines. Similarly, two lines connect between the integrated circuit  2  and the signal filter  16 , which utilizes a balanced filter arrangement. These lines are then passed to the differential amplifier  36  to produce the state signal  14 . Generally, the line pairs are twisted which causes the same pickup noise to be present on both lines. The differential amplifier  36  subtracts out this common noise. Note that the differential amplifier  36  is now acting as part of the signal filter  16 . 
     Noise and drift of the constant voltage source  10  can also reduce the ability of the electronic sensor to produce high quality state signals  14 . FIG. 7 shows a Wheatstone bridge  70  arrangement for the electronic sensor  6 , which nulls out variations in the constant voltage supply. A variable impedance  72  is utilized to match the impedance of the integrated circuit  2 . When correctly matched, the Wheatstone bridge  70  is balanced and variations in the constant voltage source  10  are rejected.