Abstract:
A device and method for probing high-speed local supply voltage fluctuations in VLSI circuits. The device includes a voltage probe coupled to a source of the local supply voltage, the voltage probe detectably emitting infrared radiation having an intensity that is related to a magnitude of the local supply voltage. The method includes emitting infrared radiation having an intensity that is related to the magnitude of the local supply voltage, taking initial measurements of the emitted radiation intensity for a range of supply voltages while digital activity is suspended in a vicinity of the local voltage probe, and compiling a calibration table matching measured intensity values with a magnitude of the supply voltage. Thereafter, digital activity is initiated by running a repetitive pattern through circuitry in the vicinity of the local voltage probe, where the repetitive pattern stimulates local supply voltage fluctuation events. Samples of emitted radiation intensity are taken using a time correlated photon counting or equivalent time sampling process and local supply voltage fluctuations are determined from the detected radiation.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to very large scale integrated (VLSI) circuitry, and in particular relates to a device and method for probing instantaneous high-speed local supply voltage fluctuation in VLSI circuits using infrared emission  
         BACKGROUND INFORMATION  
         [0002]    As VLSI system clock frequencies increase, cycle times become smaller. Fluctuations in gate delay due to such factors as MOSFET strength variations, temperature fluctuations and local supply voltage variation account for an ever greater proportion of cycle time. These variations, which are accounted for by including extra margins in cycle time during the design process, increasingly limit the design options (“design space”) available to engineers.  
           [0003]    In particular, gate delay variation due to fluctuations in supply voltage is thought to account, currently, for more than 20 percent of the margin required for maximum delay. While estimates of the magnitude of the instantaneous supply voltage at the drains of MOSFET gates have been deduced based upon knowledge of the physical layout of a typical VLSI circuit, an accurate determination of how supply voltage varies over very short durations has not been made owing to the lack of viable techniques for taking real-time high-speed measurements of supply-voltage variation. An accurate determination may reveal that the margins that have been incorporated as parameters into the design process overestimate the actual supply voltage variation. Consequently, such a determination can lead to a reduction in the budgeted margin and a corresponding widening of the design space. A larger design space, in turn, translates directly into shorter product development cycles and better-targeted product frequencies of operation.  
           [0004]    It has been experimentally determined that certain integrated circuit components emit infrared radiation, the intensity of which is functionally related to the supply voltage powering such components. Recent technological advancements in time resolved emission (“TRE”) enables time resolution of emission events on a picosecond (10 −12  s) time scale. These newly developed measurement techniques rely on the dramatic improvement in methods for temporally resolving extremely faint optical signals on extremely short time scales. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 shows a schematic diagram of an exemplary time resolved emission (TRE) testing system.  
         [0006]    [0006]FIG. 2 shows the MOSFET circuit embodiment of the infrared emission voltage probe according to the present invention.  
         [0007]    [0007]FIG. 3 shows a graph illustrating the relationship between emission intensity and supply voltage.  
         [0008]    [0008]FIG. 4 shows the decoupling capacitor embodiment of the infrared emission voltage probe according to the present invention.  
         [0009]    [0009]FIGS. 5 and 5A show a flow chart of the probing method using the MOSFET circuit according to the present invention.  
         [0010]    [0010]FIG. 6 illustrates an exemplary histogram showing the number of photons detected in each of a set of time intervals using a time correlating photon counting technique. 
     
    
     DETAILED DESCRIPTION  
       [0011]    The present invention provides a device and method to probe instantaneous high-speed local variation in supply voltage at the drains of VLSI gates with a timing resolution on the order of 10 picoseconds. In a first embodiment of the voltage probe according to the present invention, a dedicated, spatially isolated n-channel MOSFET (n-FET) device is placed at the localized area of interest. The device is switched on permanently during a test, and radiation emitted from the voltage probe is captured by a detector and the timing of the emission is recorded via a histogram according to a time correlated, photon counting technique (TCPC). The histogram indicates the number of photons detected during individual time intervals. A large number of photons are detected to improve the signal-to-noise ratio of the detected radiation intensity. The record of emitted light intensity versus time is correlated to values of the supply voltage at the drain of the probe using a calibration table which is assembled using predetermined supply voltages.  
         [0012]    In a second embodiment of the voltage probe a decoupling capacitor is connected between the supply voltage and ground. In this embodiment, weaker intensity of emissions by the capacitive elements are compensated for by their comparatively greater surface areas.  
         [0013]    [0013]FIG. 1 shows a schematic diagram of a TRE testing system  1 . In FIG. 1, an exemplary flip-chip packaged integrated circuit IC  10  including multiple layers of metalization is shown. The IC  10  is mounted front-side down on a chip carrier  21 . Over the chip carrier  21  lies an epoxy underfill layer  17  that includes solder balls  16  that connect the upper metalized layers  14  to the chip carrier via I/O  18  and substrate pads  19 . A silicon substrate layer  12  is situated on the back side of the IC  10  (at the top of the IC shown in FIG. 1). Since the flip-chip packaging (including chip carrier  21 ) encloses all but the back side of the IC  10 , external physical access to the IC can only occur via the silicon substrate layer  12 . A local voltage probe  20  is embedded in the silicon substrate layer  12 . Although only a single voltage probe  20  is depicted in FIG. 1, it is to be appreciated that numerous voltage probes are arranged throughout the silicon substrate layer  12 , and as will be discussed below, each probe indicates supply voltage variation in a particular localized region of the chip.  
         [0014]    Because silicon is transparent to light in the infrared spectrum, infrared emissions arising from the voltage probes  20  incorporated in the silicon substrate  12  can penetrate the silicon layer and radiate from the back side of the IC  10 . However, due to the fact that when silicon is doped its infrared absorptivity increases, the silicon substrate layer  12  is thinned to approximately 50-200 μm to decrease absorption and thereby to enhance infrared radiation intensity from the IC  10 .  
         [0015]    A test pattern generator  50  transmits a digital test pattern to the IC. The transmitted signal is synchronized to the clock cycle of the IC  10  being tested. Depending upon the implementation of the voltage probe, as described below, the local voltage probe  20  may be stimulated to emit radiation. The radiation emitted from the localized voltage probe  20  is incident upon an collection element  28 , such as a lens or a mirror, which focuses the radiation onto a photo-detector  30  which may be, for example, an avalanche photomultiplier device sensitive to single photons. To prevent spurious radiation arising from other sources from reaching the detector  30  during testing, various protective shields (schematically shown as a single shield  35 ) may be used or the system  1  may be situated in a dark chamber. To synchronize the detected events with the IC clock cycle and the test signal, the test pattern generator  50  also transmits a synchronizing signal to the picosecond resolution infrared analyzer  40 . The synchronizing signal may include, for example, the IC clock signal and the generated test signal.  
         [0016]    [0016]FIG. 2 shows a first embodiment of an infrared emission voltage probe  100 . As shown in the figure, a multiply-legged n-channel MOSFET  120  is fed from the voltage supply (V cc ) intended to be probed. The MOSFET  120  is placed in a layout clearance area  125  and is separated from other circuit components a distance corresponding to the smallest area that can be resolved by the TRE detector  30 . Currently, clearance spacing on the order of several microns is possible, but improvements in optical resolution may allow even smaller clearance dimensions.  
         [0017]    A test access port shift register element  110  is placed in the vicinity of the MOSFET  120 , and its output is used to drive the MOSFET transistor&#39;s gate. The shift register element  110  has a clock input  112  and a tap (test access port) pattern input  113 . To turn the MOSFET  120  ON, a 0 or 1 (as appropriate) is scanned into the shift register element  110  via the tap pattern input  113 . The shift register element  110  also includes outputs  117  and  118  which can supply further shift register elements with the clock signal and the tap pattern, respectively. In this regard, it is to be noted that the numerous shift register elements  110  situated at various separate locations on the IC can be arranged in a daisy-chain relationship in which a single test access port shift register element can drive all or a number of the voltage probes  100  located on the IC chip. Equally, one may use the shift register elements from other pre-existing test access port chains intended for other purposes that may be included in the IC. A switching signal from output  114  of the shift register element  110  is fed to a buffering component  115 , and then transmitted to the gate of the MOSFET  120 .  
         [0018]    The MOSFET  120  can be used as a supply voltage probe because under certain conditions it emits infrared radiation of an intensity that is related to its supply voltage. Within the conduction channel of the MOSFET, a portion of the electrons, termed “hot” electrons, are given higher kinetic energies due to the presence of an applied electric field when the FET is “ON” or in saturation. Accordingly, the energy distribution of the hot electrons widens and intensifies when a greater electric field is applied. When the hot electrons scatter in contact with other electrons or the semiconductor lattice or defects, the electrons ‘relax’ into lower energy states and photons in the infrared region of the spectrum are simultaneously emitted. The scattering rates are sufficiently high that the relaxation occurs on subpicosecond time scales, allowing the distribution of hot electrons and the associated light emission to respond essentially instantaneously to changes in the electric fields and currents that occur on the picosecond time scales characteristic of modern integrated circuits.  
         [0019]    The intensity of light, which is proportional to the number of hot electrons generated in the MOSFET  120 , is a function of both the source-to-drain voltage, i.e., the supply voltage V cc  and the gate voltage supplied from the shift register element  110 . If the gate voltage is kept constant during an ON state, the intensity of the emission is approximately related to V cc  according to the equation  
         Intensity= K exp (− c/Vcc )  (1)  
         [0020]    where K and c are constants. If K and c are determined experimentally, the supply voltage V cc  can be deduced from measurements of emission intensity. Since the black body temperature equivalent of a V cc  of 1V is on the order of 2000° K, the emission intensity is largely independent of the Silicon lattice temperature of the MOSFET channel. A graph illustrating the relationship (on a logarithmic scale) between emission intensity and V cc  is shown in FIG. 3.  
         [0021]    In a second embodiment of the voltage probe, a decoupling capacitor is used as the emission source. The typical density of hot electrons in the decoupling capacitor is lower than can be achieved using the MOSFET embodiment  100 , however, the area of the decoupling capacitor, which is orders of magnitude larger than the area of a MOSFET channel, compensates for the lower carrier density. A schematic illustration of the decoupling capacitor voltage probe  200  is shown in FIG. 4. As shown, a polysilicon layer  210  built from gate polysilicon is layered on top of a gate oxide layer  220 . The gate oxide layer is in turn arranged upon an N diffusion layer  230  composed of doped silicon within silicon substrate  240 . Contact pins  234 ,  238  are arranged on either side of the gate oxide layer  220  and are coupled to the N diffusion layer. The supply voltage V cc  is coupled to the polysilicon layer  210 .  
         [0022]    Both the upper polysilicon layer  210  and the N diffusion layer can be considered as the plates of a capacitor which are separated from each other by a dielectric oxide layer  220 . Such capacitive arrangements are used in integrated circuits as decouplers to protect circuit components from sudden spikes in voltage and current supply. During operation, after the polysilicon layer  210  charges up to V cc , a tunneling or leakage current begins to flow through the oxide layer  220 . Within the oxide layer, tunneling electrons accelerate in the presence of the electric field between the plate layers  210 ,  230  and emit infrared radiation as a result of their acceleration and subsequent deceleration. The intensity of the emission is a function of the strength of the voltage V cc  supplied across the capacitor  200 .  
         [0023]    The temporal behavior of the emission from the decoupling capacitor  200  is as fast that of hot electron emission in the MOSFET embodiment  100 . It therefore tracks the supply voltage on a time scale in the range of approximately a few picoseconds. Thus, an accurate time domain waveform may be obtained at a decoupling capacitor  200  without the need for a special circuit.  
         [0024]    A method of probing local high-speed supply voltage fluctuations is described with reference to FIG. 5 and FIG. 5A. Initially, in order to accurately probe the supply voltage, the emission intensity is pre-calibrated to the supply voltage by constructing a calibration table. In step  300  the probing method begins. In step  302 , the digital activity in the vicinity of the voltage probe is turned off by stopping the chip&#39;s clock distribution in the area. If the first embodiment of the voltage probe is used, the MOSFET is turned ON into a saturated state using an appropriate tap pattern. In step  304 , the optical light collection components of the TRE testing system are adjusted to focus on the voltage probe of interest and to obtain infrared intensity measurements. If there have been no previous measurements at the probe location (step  305 ), then in step  306 , the supply voltage V cc  is coupled to a direct current voltage source through which the supply voltage can be controlled and gradually swept in defined increments. According to an implementation, the supply voltage is swept from approximately 70 percent up to 120 percent of the nominal voltage value. At each increment, the infrared emission from the voltage probe is recorded via measurements of photon emission rates (step  308 ) and a table is constructed (step  310 ) that establishes a correspondence between a particular value of the supply voltage and an emission intensity value.  
         [0025]    After completion of the calibration table, the digital activity of the IC is turned on (step  312 ) and a repetitive pattern is run through the probed circuitry (step  314 ) in order to stimulate equivalent supply voltage fluctuation events numerous times in a regular temporal pattern. A time correlated photon counting technique is used to determine the timing of emission events associated with the supply voltage fluctuations. For illustrative purposes it is assumed that the repetitive pattern causes a supply voltage fluctuation event that occurs over approximately 200 picoseconds. Although this time scale is typical for such events, the method described applies equally to events of shorter and longer duration. Since the time resolution of present TRE analyzer systems is on the order of 10 picoseconds, the 200 picosecond event can be divided into twenty (20) time intervals of 10 ps in duration, numbered 1 through 20. Because the TRE system is synchronized to the chip clock, the timing of emission detection can be resolved precisely relative to the occurrence of the fluctuation events. For example, after a timer at time t=0 starts running (step  316 ), a first stimulated event an initial infrared emission may be detected (step  318 ) approximately 75 picoseconds later during time interval #8. Because photons are not normally detected for every voltage supply fluctuation event, the stimulating pattern is repeated a predetermined number of times (up to a threshold number) to allow a sufficient number of emission photons to be detected. If, during a pattern execution, a photon emission is detected, the time interval during which the detection occurs is recorded (step  320 ). The remaining number of photons to be detected or repetitions to be executed is debited by one (step  322 ). If the number of remaining detections or repetitions is greater than zero (step  324 ), the event is repeated (step  314 ) and the timer is reset (step  316 ). A successive emission detection may be made during, for example, time interval #3. After a series of detections, the number of photons detected in each time interval is totaled (step  326 ) and can be displayed graphically as a histogram (shown in FIG. 6). An average intensity value can be calculated (step  330 ) from the number of photons detected in each increment, and this intensity value can then be matched with voltage supply values (step  332 ) using the calibration table. Accordingly as FIG. 6, which shows both supply voltage versus time (top) and the histogram of the number of photons detected (bottom), the photon emission pattern displayed in the histogram accurately indicates the timing and strength of voltage supply fluctuation events.  
         [0026]    In an alternative implementation, an equivalent time sampling (ETS) technique may be used in place of or in addition to TCPC. In ETS, a photon-emission sample is taken in a single time interval for each repetition of the voltage fluctuation pattern. With each successive repetition the sample is taken at a later time interval in the pattern. In this manner, samples are taken at each time-interval of the repetitive pattern, and a relationship of emission intensity versus time interval is determined.  
         [0027]    In the foregoing description, the method and system of the invention have been described with reference to a number of examples that are not to be considered limiting. Rather, it is to be understood and expected that variations in the principles of the method and apparatus herein disclosed may be made by one skilled in the art and it is intended that such modifications, changes, and/or substitutions are to be included within the scope of the present invention as set forth in the appended claims. For example, it is to be appreciated that in a given implementation of the first embodiment of the voltage probe, p-channel MOSFETs may be used, however, the intensity of infrared radiation emitted by p-channel MOSFETs is generally lower than that of n-channel MOSFETs.