Thermal measuring and testing – Temperature measurement – Nonelectrical – nonmagnetic – or nonmechanical temperature...
Utility Patent
1998-10-13
2001-01-02
Bennett, G. Bradley (Department: 2859)
Thermal measuring and testing
Temperature measurement
Nonelectrical, nonmagnetic, or nonmechanical temperature...
C250S227180, C250S342000, C374S121000
Utility Patent
active
06168311
ABSTRACT:
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'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'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'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'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'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'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 tech
Paniccia Mario John
Xiao Guoqing
Bennett G. Bradley
Checkpoint Technologies LLC
Flehr Hohbach Test Albritton & Herbert LLP
Verbitsky Gail
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