Abstract:
A system and method are disclosed for optically measuring the temperature of a packaged integrated circuit (IC). A light beam is focused inside the IC package onto a region of the integrated circuit whose temperature is to be measured. The through-substrate reflectivity of the IC, which is a function of temperature and material characteristics (e.g., doping) of the region, determines how much of the incident light is reflected to a photo-detector. The photo-detector measures the intensity of the reflected light and generates a corresponding through-substrate reflectivity signal R. The reflectivity signal R is correlated with transmission curves for that region or materials with similar characteristics and the temperature of the region determined therefrom. Confocal techniques can be applied to substantially reduce measurement signal noise due to light reflected from the IC package which interferes with the light reflected from the IC. Measurements can be made at one specific position within the IC or a temperature map can be produced for the entire IC. Because different IC layers with different material characteristics have different transmission properties at the same temperature, the temperature measurement of a multi-layer region can be localized to a particular layer by selecting a measurement wavelength at which that region&#39;s transmission properties are most sensitive to temperature variations.

Description:
The present invention relates generally to systems for optically determining the temperature of a test object and, particularly, to systems for optically determining the temperature of a semiconductor device or integrated circuit. 
    
    
     BACKGROUND OF THE INVENTION 
     Integrated circuits comprising millions of transistors generate significant amounts of heat during operation due to the power dissipated by the transistors as they switch on and off. If this heat is not removed, the chip&#39;s operating temperature will increase, sometimes to the point of causing chip failure. Chip heating can be global, affecting the entire chip, or local, affecting just a few regions of the chip. Consequently, it is extremely important to determine how much heat all or part of an integrated circuit design generates during operation. 
     The heat generated by a circuit for a range of operating voltages and frequencies can be estimated using circuit simulations wherein the power dissipated by the circuit while performing various functions is determined and heating estimated therefrom. However, due to the variation in integrated circuit fabrication and operating conditions from the simulated values and inaccuracies inherent in circuit simulation, there has long been recognized a need to determine physically the temperature of all or part of an integrated circuit during actual operation. 
     Another reason for physically measuring integrated circuit (IC) temperature is that, often, IC defects are indicated by local hot spots that occur during operation. Such defects cannot be predicted with simulation. Consequently, it is common in IC fabrication for engineers to locate defects by searching for hot spots while exercising the IC with test patterns. There are two widely-used, optical methods for accomplishing such temperature measurements of ICs. 
     Typically, an IC is selected for temperature measurement randomly or because it failed previous operational testing. In both methods, the IC is decapped so that only a thin layer of packaging material covers the active regions of the IC. Test signals are then applied to the IC&#39;s circuit pins and the resulting temperature of one or more regions of the IC is sensed using one of the two prior art methods. 
     The first method employs a charge-coupled device (CCD) sensitive to infrared radiation to image the surface of the IC during operation. The heat generated by an IC, apart from a few very hot spots, is not intense. Thus, to generate a temperature map of the IC, the CCD must integrate over time to collect the relatively few photons emitted from regions that are less than very hot. CCD cameras with this capability/sensitivity are quite expensive. Another disadvantage of this method is that it only measures temperature at the surface of the IC. Because an IC is a multi-level device, each level of which can be at a different temperature, each temperature measurement represents a weighted average of the temperatures of the various layers in the area of the measurement. This method is therefore unable to indicate the temperatures of each layer in the measurement area. 
     The second method involves coating the surface of the IC with a temperature-sensitive fluorescent dye and then illuminating the surface with ultraviolet light while operating the IC. The temperature map of the IC surface is then determined by observing the color or amount of fluorescence. This method is cumbersome and, like the first method, cannot look inside the IC to determine the respective temperatures of the IC&#39;s multiple layers. Moreover, this method is difficult to calibrate given variances in the fluorescent dye employed and imprecision in correlating the degree of fluorescence with temperature. Also, the fluorescent dyes require special handling (e.g., they must be used with adequate venting), which makes this technique even more cumbersome. 
     In addition to the problems mentioned above, these methods cannot be used successfully with ICs packaged with the new “flip-chip” technology. Flip-chip technology enables large numbers of IC inputs and outputs to be coupled to external pins without conventional and space-consuming lateral leads. Referring to FIG. 1, in a flip-chip  50  the IC  52  is formed on a semiconductor substrate  54  in the conventional manner. Conductive nubs  56  connected to IC inputs and outputs mate with conductive regions  58  of a carrier  60 . The conductive regions  58  connect to respective external contacts  62 . Because the IC  52  is sandwiched between the carrier and the substrate its temperature and hot spots are very difficult or impossible to evaluate from surface measurements. 
     One problem with using conventional methods to look at hot spots from the backside of a chip is measurement smearing due to heating of the chip substrate between the hot spot and the measurement point. For example, when measured from the backside of a 500 micron-thick wafer, smearing causes a 10 micron hot spot to appear to be 500 microns when measured. 
     Therefore, there is a need for an optical temperature measurement system that is precise and relatively simple to operate and that can be used to look inside the IC being tested to determine the respective temperatures of the IC&#39;s layers. 
     There is also a need for an optical temperature measurement system that addresses the problem of measurement smearing associated with using conventional measurement techniques from the backside of an integrated circuit. 
     SUMMARY OF THE INVENTION 
     In summary, the present invention is a laser temperature measurement system and method that can be used to determine the temperature of different layers of an integrated circuit while the circuit is operating. 
     In particular, the present invention is a system and method for determining the operating temperature of an IC as a function of the absorptivity of the IC&#39;s semiconductor substrate to selected wavelengths of light. In the present invention, an IC whose temperature is to be measured is illuminated through the backside by a focused light beam (e.g., a laser beam), which is then reflected from one or more of the IC&#39;s reflecting internal structures. The intensity of the reflected beam is measured by an optical sensor. The absorption coefficient of the illuminated region of the IC substrate through which the light beam traveled is determined as a function of the intensities of the incident and reflected light and other factors, such as the surface reflectivity of the IC. 
     The absorption coefficient of a material at a particular illumination wavelength is a known function of temperature and impurity characteristics (i.e., dopant type and concentration). Therefore, assuming the average doping characteristics of the region are known, it is possible using the present invention to determine the average temperature of the illuminated region from the known material characteristics and the absorption coefficient determined for that region. 
     For situations where the doping characteristics are not precisely known, the present invention provides a calibration procedure wherein a set of absorption curves is generated for one or more regions of the IC substrate, each of the curves representing the absorption coefficient of one region as a function of temperature at one illumination wavelength. Thus, the temperature of a region can be determined by comparing the measured absorption of that region to the absorption curves generated during the calibration procedure. 
     Typically, the adjacent IC and substrate layers through which the light beam travels during a single measurement have very different doping characteristics and commensurately different absorption coefficients. The present invention recognizes that, for such adjacent layers, there is a defined set of wavelengths at which the absorption values of one set of absorption curves are relatively constant and at which the absorption values of the other set of absorption curves are changing rapidly. The present invention acts on this observation by making absorption measurements at the defined set of wavelengths so as to be able to independently measure the temperature of one or the other of the layers. 
     A preferred embodiment of the present invention improves the sensitivity and spatial resolution of the temperature measurements using a lock-in technique wherein the temperature of the IC being tested is modulated at a specific frequency. The present invention then measures the temperature changes of the circuit at the modulation frequency using a lock-in amplifier. This measurement method enables the present invention to detect weak, normally undetectable signals in the presence of a strong noise background. 
     A basic preferred embodiment of the present invention includes a laser, a beam expander, a beam splitter, an objective lens, a focusing lens and a photo-detector. The laser generates a laser beam, which is expanded by the beam expander into an expanded beam that is diverted by the beam splitter towards the objective lens. The objective lens focuses the beam onto a region of the IC under measurement. The beam reflects from the IC, is collimated by the objective lens, passes through the beam splitter and is focused by the focusing lens onto the photo-detector. The photo-detector measures the intensity of the received light, from which the absorption coefficient of the measured region is determined. 
     Other preferred embodiments employ confocal measurement systems and techniques that eliminate the effect of reflections from the top surface of the IC. These confocal systems and techniques provide higher signal to noise ratios than non-confocal systems and techniques. 
     In any of the embodiments, a temperature map of the entire IC under measurement can be produced by mechanically scanning the IC through the incident light or by optically scanning the light across the IC, which remains stationary. At each scan point the absorption coefficient and temperature are determined. 
     Any of the embodiments can be employed to determine IC operating temperatures at a resolution suited to locating possible IC defects or to evaluate IC performance based on the internal operating temperatures for different regions of the IC. The preferred embodiments are well-adapted to measure operating temperatures of ICs packaged as flip-chips due to the ability of the embodiments to focus light through the backside of the substrate onto the features of the IC whose heating is to be evaluated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional objects and features of the invention will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which: 
     FIG. 1 is a diagram illustrating the construction of an IC packaged with conventional flip-chip  50  technology; 
     FIG. 2 is a plot of light absorption coefficient versus temperature at an unspecified wavelength for a semiconductor material; 
     FIG. 3 is a block diagram of a basic embodiment of the present invention for measuring the internal temperature of an integrated circuit; 
     FIG. 4 is a depiction of a substrate  54  showing transmissions t i , t s  and reflection r that influence the measurement of through-substrate reflectivity; 
     FIG. 5 is a plot of the signal generated by the photo-detector of FIG. 2 versus IC temperature; 
     FIG.6A is a block diagram of a preferred embodiment of the present invention employing confocal techniques to reduce the effect of undesirable reflections from the IC; 
     FIG. 6B is a block diagram of another preferred embodiment of the present invention employing confocal techniques to reduce the effect of undesirable reflections from the IC; 
     FIG. 7 is a block diagram of a preferred embodiment wherein the IC is scanned past the probe beam to allow a temperature map to be made of the IC; 
     FIG. 8 is a block diagram of a preferred embodiment wherein the beam is scanned across the IC to allow a temperature map to be made of the IC; 
     FIG. 9 illustrates a cross-section of a typical IC; 
     FIG. 10 is a plot of light transmission versus wavelength for a plurality of temperatures for two IC layers with different doping characteristics; and 
     FIG. 11 is a block diagram of a preferred embodiment employing a lock-in amplifier. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, there is shown a plot  90  of the light absorption coefficient of a semiconductor as a function of temperature. The light absorption coefficient of a material is a fraction that defines the relative transparency of the material to light at a particular wavelength. For example, for wavelengths to which a material is transparent the material&#39;s absorption coefficient is near zero and for wavelengths to which the material is opaque its absorption coefficient is a positive value. The present invention recognizes that the absorption coefficient of a semiconductor varies with the temperature of the semiconductor and employs this recognition to determine the semiconductor&#39;s temperature by measuring its absorption coefficient or, equivalently, its light transmission. Specifically, the present invention first determines the absorption coefficient of a semiconductor and then derives the temperature from an appropriate absorption curve of the type shown in FIG.  2 . 
     Referring to FIG. 3, there is shown a block diagram of a basic embodiment  100  of the present invention for measuring the internal temperature of an integrated circuit. The embodiment  100  includes a laser light source  102 , a beam expander  104 , a beam splitter  106 , an objective lens  108 , an integrated circuit (IC)  110  whose temperature is to be measured, a focusing lens  112  and a photo-detector  114 . 
     The laser  102  generates a laser beam  103 , which is expanded by the beam expander  104  and then partially reflected by the beam splitter  106  towards the objective lens  108 . The objective lens  108  focuses the incident light  109  onto a selected region of the circuit  110  through the backside of the IC. The light  111  reflected from the circuit  110  is collected by the objective lens  108 , passes through the beam splitter  106  and is re-focused onto the photo-detector  114  by the focusing lens  112 . The photo-detector  114  generates an electrical signal R that represents the strength of the detected light  111 ′. The signal R is then digitized and recorded by a computer (not shown), which determines the absorption coefficient/transmission of the IC and derives its temperature therefrom. 
     The photo-detector  114  operates as a linear device. Its output signal R is proportional to the strength of the detected light  111 ′, which is in turn proportional to the amount of light reflected from the circuit through the IC substrate. Thus, the signal R serves as a direct measurement of the through-substrate reflectivity of the circuit. 
     The through-substrate reflectivity (TSR) can be represented as follows:                TSR   =         t   i     ×     t   s     ×   r   ×     t   s     ×     t   i       =     r   ×     t   s   2     ×     t   i   2           ,           (   1   )                                
     where r is the optical reflectivity of the circuit measured at the substrate, t i  is the optical transmission of the air-substrate interface and t s  is the single path optical transmission through the substrate, which can be expressed as:                  t   s     =          -       ∫   0   d            α        (       T   ′          (   z   )       )               z               ,           (   2   )                                
     where d is the thickness of the IC substrate, T′(z) is the temperature of the substrate at depth z measured from the back surface of the IC, α(T′(z)) is the optical absorption coefficient of the substrate at depth z, which is a function of the temperature at that point. Referring to FIG. 4, there is shown a diagram of a semiconductor substrate  54  on which the quantities used in the previous equations are depicted. As in FIG. 1, the IC features  52  are illuminated through the backside of the substrate  54 . For clarity, the quantities t i , t s  and r are shown for two different features  52 , but each measurement of the reflectivity TSR, indicated by the signal R, is affected by all of these quantities. In FIG. 4 the incident light is represented by the variable l o  and an internal version of the incident light is represented by the variable l 1 , where l 1 =l o ×t i ×t s . 
     Referring again to FIG. 3, given a typically thin IC substrate, the temperature of the substrate is assumed to be almost constant along a line perpendicular to its surface and approximately the same as the temperature of the circuit  110  underneath. Thus, Eq. (2) can be simplified as: 
     
       
           t   s   =e   −α(T)d ,  (3) 
       
     
     In view of Eq. (3), Eq. (1) can be rewritten as follows:              TSR   =       t   i   2        r                            -   2          α        (   T   )          d       .               (   4   )                                
     Referring to FIG. 5, there is shown a plot  130  of the through-substrate reflectivity (corresponding to the signal R generated by the photo-detector  114  of FIG. 3) for a typical silicon-based IC. It is evident from the plot  130  that each measured value R of the through-substrate reflectivity TSR corresponds to a specific temperature T. Therefore, by measuring the circuit&#39;s through-substrate reflectivity, the present invention is able to determine the temperature of the circuit. However, before a measured through-substrate reflectivity can be correlated to an unknown temperature, it is first necessary to calibrate the measurement system by measuring the through-substrate reflectivity of the circuit at several known temperatures. For example, referring to FIG. 5, during calibration the reflectivities R0 and R1 might be measured at the known temperatures T0 and T1, respectively. Then, using linear or non-linear curve-fitting, the temperature T of the circuit can be extrapolated from the calibration measurements. 
     For example, using linear curve fitting as applied to the measurements shown in FIG. 5, the IC temperature can be approximately represented as:                T        (   R   )       =           (       T   1     -     T   0       )          (     R   -     R   0       )           R   1     -     R   0         =           (       T   1     -     T   0       )     ×     R   0           R   1     -     R   0         ×     (       R     R   0       -   1     )                 (   5   )                                
     For wider measurement range and higher accuracy, the laser temperature measurement probe is preferably calibrated at multiple temperatures distributed over an extended temperature range. The circuit temperature can then be determined from a particular measurement by performing interpolation on the two neighboring calibration measurements. 
     The laser measurement probe  100  of FIG. 3 has one drawback: the reflection from the back surface of the IC mixes with the reflection from the circuit itself, reducing the signal to noise ratio of the through-substrate reflectivity measurement R. A confocal setup reduces the effects of these undesirable reflections and therefore increases the measurement signal to noise ratio. Two preferred confocal laser temperature probe configurations are now described in reference to FIGS. 6A and 6B. 
     Referring to FIG. 6A, there is shown a first preferred confocal setup including a laser light source  162 , a first pinhole  164 , a collimating lens  166 , a beam splitter  168 , an objective lens  170 , a focusing lens  176 , a second pinhole  178  and a photo-detector  180 . The light from the light source  162  is intercepted by the first pinhole  164 , which acts as a point source for the incident light  165 . The incident light  165  is collimated by the collimating lens  166  and diverted towards the objective lens  170  by the beam splitter  168 . The objective lens  170  focuses the light onto the circuit  172  through the backside  174  of the IC substrate. Light  173  reflected from the circuit  172  and the backside  174  of the substrate is transmitted by the beam splitter  168  to the focusing lens  176 , which focuses the light  173  onto the second pinhole  178 . The second pinhole  178  is placed so that it transmits to the photo-detector  180  only that portion of the light  173  reflected from the circuit  172 . That is, the pinhole  178  eliminates a substantial amount of “unfocused light”, including the light reflected from the package back  174 . Due to the elimination of unfocused light, the system  160  operates at a higher signal to noise ratio than the embodiment of FIG.  3 . 
     Referring to FIG. 6B, there is shown a second preferred confocal temperature measurement system  190  including a light source  192 , a beam splitter  194 , a pinhole  196 , a tube lens  198 , an objective lens  200 , a relay lens  206  and a photo-detector  208 . In contrast to the confocal measurement system shown in FIG. 6A, this embodiment includes a single pinhole  196 , which acts as a point source for the incident light  197  and removes the unfocused light (including light reflected from the back  204  of the IC  202 ) from the reflected light  205 . The tube lens  198  and the objective lens  200  focus the incident light  197  onto the circuit  202  and the reflected light  205  onto the pinhole  196 . The reflected light  195  that passes through the pinhole/point source  196  is transmitted by the beam splitter  194  to the relay lens  206 , which focuses the light  195  onto the photo-detector  208 . Because the light  195  predominantly includes only that part of the light  205  reflected from the IC  202 , the signal R generated by the photo-detector  208  represents with high SNR the through-substrate reflectivity of the circuit  202 . 
     Thus far, embodiments have been described for measuring the temperature of an IC at a single point. It is also useful to determine a temperature map for an IC under measurement. Such a map can be produced by combining the teachings of the present invention with conventional scanning confocal microscopic techniques. Scanning can be accomplished in two ways: (1) the sample can be scanned across a stationary beam, or (2) the beam can be scanned across a stationary sample. Preferred embodiments using these respective techniques are described in reference to FIGS. 7 and 8. 
     Referring to FIG. 7, there is shown an embodiment  220  based on the confocal measurement system of FIG. 6A wherein the IC to be measured  222  is moved across the light beam. In addition to the elements described in reference to FIG. 6A, the embodiment  220  includes a temperature control device  224  that supports the IC under measurement  222 , an X-Y mechanical stage  226  that supports the temperature control device  224 , and a computer  228 . The computer  228  controls the temperature control device  224  and the mechanical stage  226  and processes measurements R from the photo-detector. In this embodiment, the computer  228  first performs a calibration step wherein it issues signals  229   a  to the temperature control device  222  to establish the IC  222  at a known temperature and then issues a series of signals  229   b  to the mechanical stage  226  that cause the face of the IC  222  to be scanned across the beam. A set of measurements R is made by the computer  228  for a set of the scan positions for each temperature at which the calibration step is performed. Once calibration has been completed, the computer  228  initiates a measurement step while the chip is operating wherein through-substrate reflectivity measurements are made at the same set of scan positions. The through-substrate reflectivity measurements can be derived for each scan pixel using linear or nonlinear curve-fitting. In this way, a temperature map can be obtained of the IC during operation. 
     Referring to FIG. 8, there is shown a block diagram of a preferred embodiment  240  wherein the probe beam is scanned across an IC  242  to allow a temperature map to be made of the IC. This embodiment  240  is similar to that shown in FIG. 7 except that it also includes an X-Y optical scanner  248  and a scan lens  250 . The X-Y optical scanner  248  intercepts the incident beam  243  transmitted by the beam splitter and, in response to control signals  247   a  from the computer  246 , scans the beam  243 ′ across the face of the scan lens  250  in a regular X-Y pattern. The scan lens  250  forms an incident beam  251  that converges on the objective lens with an angle of incidence that depends on the region of the scan lens  250  forming the beam  251 . The objective lens in turn focuses the incident beam on the IC at scan positions that depend on the angle of incidence. In this way, the beam is scanned across a stationary sample  242 . The temperature of the sample  242  is set by the temperature control device  244  in response to control signals  247   b  from the computer  246 . Calibration and operating measurements are made as described in reference to FIG.  7 . 
     As mentioned above, ICs are formed from multiple layers having different electrical characteristics. For example, referring to FIG. 9, there is shown a cross-section of a typical IC (back side up) including doping regions  302 ,  304 , at least one oxide layer  306  and a reflective surface  308 , such as a metal contact. Typically, the regions  302 ,  304 ,  306  have different respective light absorption properties at a given temperature and illumination wavelength. Consequently, assuming that the IC is illuminated by an incident beam  310 , the through-substrate reflectivity indicated by the intensity of the reflected beam  312  is an accumulation of the absorptivities and therefore a function of temperatures of the doping regions  302  and  304 . However, depending on the types of layers through which the beams  310 ,  312  travel, the wavelength of illumination can be selected so that the temperature of one or the other of the regions can be individually determined. This wavelength selection technique is now described in reference to FIG.  10 . 
     Referring to FIG. 10, there are shown plots  350 ,  352  of light transmission versus wavelength for a plurality of temperatures T1, T2, T3 for the first and second doped layers  302 ,  304  (FIG.  9 ). It is apparent from FIG. 10 that at the wavelength λ 1  the optical transmission  350  of the first doped region changes much more rapidly with temperature than that 352 of the second doped region. In other words, at the wavelength λ 1  the optical transmission of the first doped region is much more sensitive to temperature variation than that of the second doped region. Thus, in any of the described embodiments, probing the IC at the wavelength λ 1  predominantly restricts the temperature measurement to the first doping region. Similarly, by probing the IC at the wavelength λ 2  the temperature measurement is predominantly restricted to the first doping region. 
     To further increase the measurement sensitivity and spatial resolution, one may use a lock-in technique. The lock-in technique requires modulating, at a specific frequency, the temperature of the circuit under test. By measuring the temperature changes of the circuit at the modulation frequency using a lock-in amplifier, weak, normally undetectable signal can be detected from a strong noise background. The lock-in technique provides another benefit: it can enhance the spatial resolution of the measurement. A preferred embodiment employing a lock-in amplifier is now described in reference to FIG.  11 . 
     Referring to FIG. 11, unique components of a measurement system  400  include an IC power supply and control electronics block  402 , a lock in amplifier  406 , a function generator  404  and a preamplifier  408 . The remaining components  410 , which include an IC under measurement, a photo-detector and optical components, are interchangeable with similar components of any of the embodiments described in reference to FIGS. 2,  6 A,  6 B,  7  and  8 . That is, the lock-in technique described in reference to FIG. 11 can be employed in any system embodying teachings of the present invention. 
     The lock-in amplifier  406  has reference and signal inputs  412 ,  414  that respectively receive a modulation signal  418  from the function generator  404  and a preamplified signal  420  from the preamplifier  408 . The lock-in amplifier  406  provides a temperature signal  424  on an output  422  that is coupled to a computer (not shown). As described above, the lock-in amplifier  406  measures the temperature changes of the circuit (represented by the preamplified signal  420 ) at the frequency of the modulation signal  418 , which is also coupled to the IC power supply and control electronics  402 . The IC power supply and control electronics block  402  can respond to the modulation signal  418  in many ways, the object of each way being to modulate the temperature of the IC under measurement via control signals  403 , which are coupled to the IC under measurement via power and signal wires. For example, the block  402  can change the test patterns on the IC signal wires and/or the level of the power on the power wires. The basis of the lock-in technique and a few preferred measurement methods employing the lock-in technique are now described. These methods are only exemplary and are not intended to limit the scope of the present invention; the present invention applies to these and all similar methods. 
     Due to the heat capacitance and limited thermal conductivity of the IC substrate and circuits, the temperature modulation caused by a modulated heat source is limited to the region close to the heat source. The size of the region is usually comparable to the wavelength of the heat wave, which is inversely proportional to the modulation frequency. The higher the modulation frequency, the smaller the temperature modulation region is. Thus, by measuring at a high frequency, the spatial resolution of the measurement is greatly increased. 
     The easiest way to achieve temperature modulation of the circuit is to modulate its heat generation. There are several methods to modulate the heat generation. The first and the simplest method is to turn on and off the power to the circuit under test. When the power to the IC is on, heat is generated and the temperature of the circuit increases. However, when the power to the IC is off, no heat is generated and the temperature of the circuit decreases due to heat dissipation. This method is simple and straightforward, but its application is very limited because it disrupts the normal operation of the circuit under test. 
     The second method is to modulate the supply voltage or current of the IC around its nominal value. The modulation amplitude should be limited to a level so that the IC still functions properly. Since variation in supply voltage or current changes the amount of heat generated by the circuit, the temperature near the circuit is modulated. This method is relatively simple and is capable of probing circuits under normal operating condition, but has limited modulation amplitude. 
     The third method involves controlling the test pattern to the circuits. It is a common knowledge that circuits at different states generate different amount of heat and circuits switching from one state to another generate more heat than those at static states. By choosing a periodic test pattern in such a way that one part of the pattern exercises the circuits to generate more heat than the remaining part of the pattern, the heat generation of the circuits is thus modulated. The heat modulation frequency is naturally the repetition rate of the test pattern. A simple example of this method employs such a test pattern that one part of the pattern runs the circuits at a high clock rate, causing more heat generation while the remaining part of the test pattern slows down or freezes the clock, resulting in less heat generation. Other examples require careful selection of test pattern. 
     The fourth method involves switching on and off the power to the circuits in synchronization with the test pattern. The circuits need to be initialized to the same state each time when the power is applied. Obviously the heat generation of the circuit is modulated. 
     The fifth method is to turn on and off the power to the IC with a random test pattern. Since the clock frequency is much higher (in hundreds of MHz) than the on/off modulation frequency of the power supply (in tens of kHz), the heat generation during the power-on period is averaged over several thousands of clock cycles of random test pattern, therefore the amount of the heat generated is almost the same from one modulation cycle to another. Again, since no heat is generated during power-off period, heat generation is modulated as the power to the circuits is switched on and off. 
     With conventional fluorescence method, due to heating of IC substrate, the hot spot spreads or smears as it propagates from its origination point to the surface of the backside substrate, where the measurement is carried out. The smearing of the hot spot at the surface is comparable in size to the distance from the heat source to the measurement point, i.e., the IC Substrate thickness, typically 500 μm. Since the laser temperature measurement technique measures the temperature right at the heat source, its measurement resolution is limited only by its laser spot size, which can be approximately expressed as:        d   =       1.22      λ     NA                            
     where d is the diameter of laser spot, λ is the optical wavelength of the laser. NA is the numerical aperture of the objective. For a typical setup, λ=1.064 μm and NA=0.85, the measurement resolution is d=1.5 μm. 
     In any of the above-described system configurations the backside of the IC substrate may be polished and coated with an anti-reflection coating (ARC). The resulting system will have better measurement sensitivity than an equivalent system without these improvements. 
     Any of the embodiments can be employed to determine IC operating temperatures at a resolution suited to locating possible IC defects or to evaluate IC performance based on the internal operating temperatures for different regions of the IC. The preferred embodiments are well-adapted to measure operating temperatures of ICs packaged as flip-chips due to the ability of the embodiments to focus light through the backside of the substrate onto the features of the IC whose heating is to be evaluated. 
     The principles, methods and techniques we described above can be used for measuring any types of semiconductor circuits, including but not limited to silicon, germanium and GaAs ICs. 
     While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.