Temperature mapping system

A method of and system for determining the location of hot spots on the surface of an object which has thermotropic material applied to the surface are disclosed. The system is designed to varying the nominal temperature of the object through a range of nominal temperatures between a first nominal temperature limit and a second nominal temperature limit, wherein one temperature limit is below and the other nominal temperature limit is above the temperature at which the thermotropic material changes phase. A sequence of images of the surface are acquired, each of the images depicting a two dimensional temperature representation of the surface at a predetermined nominal temperature within the range of nominal temperatures, wherein each image in the sequence represents an incremental change in nominal temperature than that of a preceding image. Select ones of the images are processed to define the intensity signature of valid hot spots in the select ones of the images and to determine the location of the hot spots on the surface as a function of the defined intensity signature.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
Not Applicable 
REFERENCE TO MICROFICHE APPENDIX 
Not Applicable 
FIELD OF THE INVENTION 
The present invention relates to temperature measurements, and more 
particularly to systems for and methods of determining and indicating the 
temperature characteristics of an object so as to derive a high 
resolution, two dimensional graphic representation of the surface 
temperature of the object. 
BACKGROUND OF THE INVENTION 
It is important for designers and manufacturers to have accurate knowledge 
of the surface temperature of a monolithic integrated circuit (hereinafter 
referred to as IC). The presence of "hot spots" on the surface of an IC 
may create reliability and performance problems. If the location of such 
hot spots are known with high resolution, designers may be able to modify 
the IC layout to optimize dissipation or mitigate the problem via other 
techniques known to those in the art. 
Prior art methods of mapping the surface temperature of an IC include 
infrared thermography and the use of an array of temperature sensitive 
elements such as thermocouples, thermistors, RTD's, and bipolar junction 
sensors. Such prior art methods typically suffer from poor spatial 
resolution. 
Prior art methods have used nematic liquid crystals (hereinafter referred 
to as NLCs) as a means for locating hot-spots on ICs and mapping the 
surface temperature of an IC. However, these previous computerized 
analytical methods have had limited success due primarily to their 
inability to reliably discriminate between actual hot-spots on the IC, the 
background features of the IC substrate and the potentially spurious 
behavior and visible internal artifacts associated with NLC materials. 
These prior art NLC methods have also experienced poor spatial resolution, 
and non-repeatable results. 
Since modem IC's are typically fabricated on a sub-micron scale, low 
resolution temperature mapping makes it difficult to resolve and isolate 
the occurrence of nearly-adjacent IC hot spots using prior art techniques. 
It is an object of this invention to provide a temperature mapping system 
and method which significantly overcomes the aforementioned problems 
inherent in the prior art. 
It is another object of this invention to provide a temperature mapping 
system which provides spatial resolution sufficient to detect and resolve 
nearly-adjacent hot spots on the surface of an IC. 
SUMMARY OF THE INVENTION 
The method and system of the present invention is for determining the 
location of hot spots on the surface of an object. In one embodiment this 
is achieved by applying a thermotropic material to the surface of the 
object; varying the nominal temperature of the object through a range of 
nominal temperatures between a first nominal temperature limit and a 
second nominal temperature limit, wherein one temperature limit is below 
and the other nominal temperature limit is above the temperature at which 
the thermotropic material changes phase; acquiring a sequence of images of 
the surface, each of the images depicting a two dimensional temperature 
representation of the surface at a predetermined nominal temperature 
within the range of nominal temperatures, wherein each image in the 
sequence represents an incremental change in nominal temperature than that 
of a preceding image; and processing select ones of the images to define 
the intensity signature of valid hot spots in the select ones of the 
images and to determine the location of the hot spots on the surface as a 
function of the defined intensity signature. 
In another embodiment the nominal temperature of the object is varied in 
incremental steps so that the object is stabilized at each of the nominal 
temperatures when each of the images of the surface are acquired. 
In another embodiment the user selects the images he/she wishes to process 
to determine the hot spots. 
In another embodiment, when defining the intensity signature of valid hot 
spots the potential hot spots are detected in each of the select ones of 
the images. 
In yet another embodiment each of the selected images is represented by a 
plurality of pixels, each of the pixels of a determinable intensity, and 
the potential hot spots in each of the select ones of the images are 
detecting by, at least in part, binarizing a plurality of the pixels 
within each of the select images such that each pixel is designated as 
either a hot-spot pixel or a non-hot-spot pixel. 
In still another embodiment, the potential hot spots in each of the select 
ones of the images is defined at least in part by applying a smoothing 
filter to a plurality of regions within each of the selected images prior 
to the step of binarizing the plurality of the pixels. 
In yet another embodiment, a low-pass spatial filter is applied to a 
plurality of regions within each of the images after binarizing the 
plurality of the pixels. 
And in still another embodiment, when binarizing a plurality of pixels each 
of the pixels are compared to a predetermined threshold range and each of 
the pixels are designated as a hot-spot pixel if the pixel falls within 
the threshold range and designating the pixel as a non-hot-spot pixel if 
the pixel does not fall within the threshold range. 
And in yet another embodiment each potential hot-spot is verified. 
In still another embodiment, each potential hot-spot is verified by, at 
least in part, (i) comparing consecutive images of the select ones in the 
sequence in which they are acquired, and (ii) eliminating hot-spot pixels 
not having a hot-spot pixel at a corresponding location in an immediately 
succeeding image. 
Yet another embodiment verifies each potential hot spot by, at least in 
part, adding pixels from corresponding locations of each of the select 
images to produce a resulting pixel corresponding to an integer value 
representative of the number of images having hot-spot pixels at the 
corresponding location so as to produce the two dimensional graphic 
temperature representation. 
In another embodiment, the resulting pixel includes one of a plurality of 
integer values, each of the integer values corresponding to an assigned 
temperature and being represented by a predetermined color. 
Yet in another embodiment the object is maintained at a first nominal 
temperature, and the surface is viewed through a crossed polarizing filter 
and video data representative of a polarized image of the surface is 
produced prior to applying the thermotropic material. 
In still another embodiment the sequence of images are acquired by, at 
least in part, receiving and recording the polarized image at each of 
nominal temperatures so as to produce a time-series of images, and 
generating a thermal map of the surface from the time-series of images. 
In another embodiment the object is secured to a temperature control 
platform, and the temperature of the object is controlled through the 
temperature control platform. 
And in another embodiment the surface is viewed so as to receive and focus 
light received from the surface, so as to form an image, and the image is 
converted into a series of video data elements representative of the 
image. 
And in yet another embodiment the thermotropic material applied to the 
surface is illuminated for each of the images. 
In still another embodiment the sequence of images is acquired, at least in 
part, by receiving video data from an optics unit for each of the images; 
forming the video data into a frame representative of the image; and 
storing the video data of each of the frames so as to establish a data set 
representative of a time-sequence of the images. 
In another embodiment the sequence of images is acquired by receiving each 
of the images and generating map data representative of a thermal map of 
the surface corresponding to variations in consecutive frames of the 
images; and receiving the map data and displaying the two dimensional 
graphic temperature representation from the map data. 
And in another embodiment the images that are selected are determined by 
detecting the first image of the sequence that is determined to have a 
dark spot, and identifying a temperature corresponding to the first image, 
so as to determine the temperature at which the thermotropic material 
changes phase.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is directed to a method of and apparatus for mapping the 
surface temperature of an object, typically an electronic component, 
although the invention can be used with other objects. The invention uses 
optical and physical properties of a thermotropic material, e.g. the phase 
change of a NLC to derive a high resolution, two dimensional graphic 
representation of the surface temperature of the object. 
The NLC is a stable, intermediate, or meso-thermodynamic phase between a 
pure solid and a pure liquid that some substances, usually organic in 
nature, can exhibit under specific environmental conditions. The NLC can 
result in these substances either from heating the solid phase or by 
cooling the liquid phase. Other forces such as mechanical shear and 
pressure, electric and magnetic fields and chemical reactions can also 
cause the phase change to occur, but the characteristic preferably used in 
the embodiment of the invention is the thermally induced or thermotropic 
process. 
The NLC possesses some of the mechanical properties of a liquid (surface 
tension, viscosity and weak intermolecular bonds), and some of the optical 
properties of a crystalline solid (anisotropy to light, dichroism, and 
birefringence), but it is the combination of these properties that make 
the NLC useful to electronic component design engineers and failure 
analysts. Chief among these properties is the strong light polarizing 
capabilities that the NLC possesses. 
A non-polarizing surface will appear dark when viewed through crossed 
polarizers, because such a filter will substantially block the 
non-polarized, reflected light. However, if the same non-polarizing 
surface is coated with a material exhibiting the NLC, the surface will 
appear bright when viewed through crossed polarizers, because the NLC 
polarizes the incident light and causes it to be reflected into the viewer 
`in-phase` with its polarizing filter. The light reflected in-phase with 
the filter passes through the filter to the viewer, thus causing the 
surface to appear bright. When formulated properly, such materials can 
have a very sharp liquid-crystal to pure-liquid phase transition at a 
temperature referred to as the clearing temperature of the material. This 
behavior allows these materials to be used as micron-sized temperature 
indicators or hot-spot detectors, and forms the basis of operation for the 
invention. 
FIG. 1 shows a block diagram of temperature mapping system 100 constructed 
according to one embodiment of the present invention. A device under test 
102 (hereinafter referred to as DUT) is mounted on a temperature control 
platform 104. The DUT, for example can include an integrated circuit die 
or other electrical component. A temperature control unit 106 controls a 
temperature control platform 104 via electrical, thermal, or mechanical 
means, or via other means known to those skilled in the art. The DUT 102 
is also electrically connected to a DUT probe unit 108, which provides 
power and/or control signals to the DUT 102. The surface of the DUT 102 is 
coated with the NLC material 110. An illumination unit may be used to 
illuminate the liquid crystal material 110 on the surface of the DUT 102, 
so as to provide a controlled, uniform source of light to be reflected 
from the NLC material 110. An optics element 114 receives light 116, which 
has been reflected and modified by the NLC material 110 and has passed 
through a crossed polarizer 118, to derive an image representative of the 
NLC coated surface. In one embodiment of the invention, the optics unit 
114 includes a microscope, although those skilled in the art will realize 
that other optical assemblies may be used to receive the light 116 and 
derive an image. An optical/electrical conversion element 115 receives the 
image representative of the NLC coated surface and converts the image into 
an electrical signal. In one embodiment, the optical/electrical conversion 
element 115 may be a CCD camera, which converts the image into a series of 
digital data elements, although other methods of converting the image may 
be used. 
In one embodiment of the invention a computer system, comprising a central 
processing unit 119 (hereinafter CPU), a frame grabber 120, an image 
processor 122 and a display unit 124 receives the digital data elements 
from the optics unit 114. In general, the CPU 119 coordinates processing 
functions within the computer system. One embodiment of the image 
processor 122 includes a software module executing on a commercial 
computer platform by way of a commercial operating system, although those 
skilled in the are will realize that other embodiments, such as an 
all-hardware signal processor or an application-specific hardware/software 
unit may be used. The frame grabber 120 receives the digital data elements 
which collectively represent the image from the microscope 114 and 
assembles the data elements into a frame format compatible with the 
subsequent image processing components. 
The temperature control unit 106 and the temperature control platform 104 
control (i.e., drive and maintain) the temperature of the DUT 102 through 
a range of temperatures which straddle the clearing temperature of the NLC 
material 110. As the temperature increases over time, the frame grabber 
120 acquires and stores the sequence of images of the DUT 102, each at a 
temperature incrementally greater than the previous. The image processor 
122 evaluates the sequence of stored images as described herein to 
generate a thermal map of the surface of the DUT 102. 
In one embodiment, a hot-spot on the surface of the DUT 102 appears as a 
black spot on the surface of the object coated with the NLC material 110 
in an otherwise light, undisturbed NLC background. The temperature of the 
black spot when it initially appears is the clearing temperature of the 
NLC material 110. As the substrate temperature of the surface rises, the 
black spot will grow in size due to the increase in the amount of the 
surface that is at or above the clearing temperature. By continuing to 
raise the substrate temperature, more of the surface can be made to appear 
black. Temperature mapping of the surface is accomplished by starting with 
a small black spot, and then carefully controlling and tracking the growth 
of the spot, coordinated with precise temperature control of the 
substrate. The illustrated form of the invention implements this 
methodology by synchronizing the acquisition of images of the NLC-coated 
surface with precise control of the substrate temperature. 
In one embodiment, the method of generating a two dimensional graphic 
representation of the surface temperature of the object is based on the 
concept of a thermal test. The key component of a thermal test is its 
image sequence. The image sequence contains the NLC-coated surface images 
of the device, organized in an ordered sequence as a function of 
increasing substrate temperature over a precise temperature range. The 
image sequence also includes a priori information regarding the NLC 
clearing temperature of the NLC material 110 and a predetermined substrate 
temperature range. Temperature mapping begins by applying power to the DUT 
102 mounted to the temperature control platform 104 and bringing the DUT 
102 to a substrate temperature corresponding to a surface temperature that 
is just slightly below the level that allows the smallest hot-spot to be 
seen with the NLC material being used. Next, a digital image of the DUT is 
acquired and stored as the background data image. One embodiment of the 
invention includes interactive temperature stepping and image capture 
features to simplify these steps. The CPU 119 then coordinates automatic 
adjustment and control of the temperature of the temperature platform 104 
and DUT 102, and acquires images of the device that show the outline of 
the hot-spot(s) growing as the temperature is raised and stabilized in 
increments of 0.1.degree. C. steps, although lesser or greater temperature 
resolution may be used. When all of the images have been acquired, the CPU 
119 and image processor 122 processes the image sequence via an image 
sequence processing interface and constructs a color-coded thermal map of 
the device's surface temperature distribution. 
The image sequence processing interface is preferably equipped with 
interactive controls that allow a user to select only the images they want 
to process and to fine-tune the `hot-spot` detection algorithm to extract 
the valid `hot-spot` regions of the image sequence data while eliminating 
the background areas. The hot-spot detection algorithm is built around 
state-of-the-art image processing techniques which are specifically 
tailored to the many nuances of NLC based thermal mapping. 
FIG. 2 shows flow diagram illustrating the temperature mapping and hot-spot 
detection algorithm. 
The Process Variable Structures of the temperature mapping and hot-spot 
detection algorithm are given as follows: 
______________________________________ 
Image Sequence: 
T.sub.min .ident. Auto-Acquisition 
START temperature 
T.sub.max .ident. Auto-Acquisition END 
temperature 
T.sub.cp .ident. Clearing temperature 
of the nematic liquid 
crystal (NLC). Temp- 
erature where a phase 
transition occurs 
within the material 
that causes it to 
radically alter the 
way it modulates 
incoming light. 
.DELTA.T .ident. Auto-acquisition STEP 
temperature 
BeginIndex .ident. Image index to start 
processing from 
EndIndex .ident. Image index to end 
processing 
Nimages .ident. Total number of 
images that were 
acquired 
ORIGINAL Image Array [1, NImages] .ident. 
Array of the original 
acquired images 
DATA Image Array [BeginIndex, EndIndex] .ident. 
Array of processed 
images 
Thermal Map Image .ident. 
Image that will 
contain the compo- 
site thermal map of 
the DATA Image 
Array 
Detector: 
Threshold Level .ident. 
User-selectable (point- 
and-click) intensity 
level 
Tolerance .ident. Threshold level 
tolerance 
Threshold Range .ident. 
Intensity range that 
defines the hot-spot's 
signature (Threshold 
Level .+-. Tolerance) 
Subtract Background? .ident. 
Dictates whether to 
subtract a background 
image from the origi- 
nal image array, or 
not 
Neighborhood .ident. Defines how to weigh 
the influence of a 
pixel's neighbors. 
Neighborhood pro- 
cessing may utilize 4 
neighbor model or 8 
neighbor model. 
FIG. 3 illustrates the 
relative differences 
between the 4 neigh- 
bor model and the 8 
neighbor model. 
______________________________________ 
As shown in FIG. 2, one embodiment of the temperature mapping and hot-spot 
detection algorithm preferably comprises a sequence of five Process Steps. 
Each Process Step is described as follows: 
1. Image Selection 
This step allows the user to `window-select` the images they want to 
process. It gives users the flexibility to properly process images that 
may have been acquired over a wider than necessary temperature range. 
Users will typically acquire additional images at higher and lower 
temperatures than they are truly interested in to ensure that the device's 
full thermal profile is acquired automatically. BeginIndex and EndIndex 
define the image indices for the `true` minimum and maximum temperature 
images respectively that require processing. 
2. Detector Setup 
This is an important step of the process and the system is designed to 
provide the user with instant feedback on the settings he/she chooses. The 
main purpose of this step is to assist the user in properly defining the 
intensity `signature` of valid hot-spot(s) that may be present in ORIGINAL 
Image Array. The parameters that make up the Detector process variable are 
used to practically describe and implement this signature for accurate 
processing. 
3. Hot-Spot Detection 
This step uses the Detector settings in combination with the following 
sub-steps to `detect` the hot-spot regions in all of the window-selected 
images in ORIGINAL Image Array and stores the step-by-step results in DATA 
Image Array. 
Sub-Step 1. 
Smooth: This step implements a standard, North-South-East-West neighborhood 
weighted 3.times.3 Gaussian smoothing filter to smooth out the features of 
the images in ORIGINAL Image Array, although other smoothing filters, can 
be used. 
Sub-Step 2. 
Binarize: This step defines the intensity signature of valid hot spots in 
the selected images, preferably by `binarizing` (pixel intensity values of 
1 or 0 only) the images in DATA Image Array by setting all image pixels 
that fall within Threshold Range to a value of 1. Those pixels that fall 
outside of this range are set to a value of 0. 
Sub-Step 3. 
Clean: This step `cleans` (eliminates unwanted and fictitious artifacts) 
the DATA Image Array images by preferably applying a `low-pass spatial 
filter`. To provide even greater images the low-pass spatial filter is 
applied in combination with "n" consecutive `erosions`, wherein "n" is an 
integer, and in one embodiment is equal to three. The preferred process of 
low-pass spatial filtering combined with erosion of binary images allows 
small (in comparison to other structures present in the image) artifacts 
to be removed from the images without disturbing the contours of larger 
structures (i.e. hot-spots). This results in a very effective procedure to 
accurately detect and filter-out hot-spots in the type of images typically 
encountered with NLC-based thermal mapping applications. 
4. Verification 
The verification step is important to forcing the processed images to 
comply with the physics of NLC thermal mapping. In practical terms this 
means that in order for a candidate hot-spot pixel in an image to be 
considered `real` the pixel at the same location in all subsequent images 
(i.e., higher temperature levels) must also have been detected. If not, 
then the pixel is considered to be non-real and should be discarded. This 
step implements this principle by running the images in DATA Image Array 
through a cascaded logical AND `test`. The test checks the value of all 
detected hot-spot pixels (intensity equal to 1 at this stage) in all 
subsequent images. If the pixels pass the test then their intensity level 
of 1 is preserved else it is set to 0. 
5. Thermal Map Construction 
Since only supposed `real` hot-spots pixels are present in DATA Image Array 
at this point and these pixels have unity intensity, construction of the 
Thermal Map is a straightforward procedure. The procedure begins by simply 
adding the images intensities together and storing the result in Thermal 
Map Image. With this, Thermal Map Image will have corresponding pixel 
intensity values that are exactly equal to the number of occurrences that 
the corresponding pixels were found to be valid in DATA Image Array. This 
simple (by design) fact can be exploited to efficiently assign temperature 
and color levels to the Thermal Map Image pixels, for example, using the 
following steps: 
Sub-Step 1. 
Temperature Relationship: Assign a temperature level to each image pixel 
using the following relationship: 
EQU T(Intensity)=T.sub.cp +Intensity*.DELTA.T 
Sub-Step 2. 
Mask Creation: Create a temporary, binary mask image that has pixel 
intensity levels equal to 1 wherever the corresponding Thermal Map Image 
intensity is non-zero and zero everywhere else. 
Sub-Step 3. 
Image Multiplication: Now multiply the images from Sub-Steps 1 and 2 and 
assign this result to Thermal Map Image. This causes Thermal Map Image to 
have the proper temperature levels assigned to the hot-spot pixels and 
zero everywhere else. 
Sub-Step 4. 
Color Assignment: Use the standard rainbow color palette and adjust its 
range to correspond to the temperature range now present in Thermal Map 
Image (i.e. Red.tbd.Maximum and Blue.tbd.Minimum). 
The ThermoMap Hot-Spot Detection Method consists of 4 steps: 
1. Image selection 
2. Detector setup 
3. Hot-spot verification 
4. Thermal map construction 
In summary, the preferred image selection allows the user to 
`window-select` the minimum and maximum temperature images in the acquired 
image sequence to use for processing. In one embodiment the detector setup 
feature provides an interactive means for the user to specify the key 
parameters of the detection algorithm (intensity range, threshold, 
subtract background and neighborhood size) by clicking the mouse in the 
region that represents a `valid` hot-spot. The user's selection is 
immediately shown by the software as a color highlight around the detected 
region of the selected image template. Once the user is satisfied with the 
results of the detector setup, in one embodiment the algorithm then goes 
on to process all the images in the sequence using the selected detector 
setup parameters. This step produces a binary image representation of the 
candidate hot-spot regions in the image sequence. Since the sequence was 
acquired with increasing substrate temperature, the sequence image will 
represent the GROWTH of the hot-spot regions in the images. This means 
that once a candidate hot-spot pixel in any image has been identified, for 
it to become a valid hot-spot pixel, it must remain a hot-spot pixel in 
all the subsequent image. With this in mind,, the detection algorithm 
quickly verifies this behavior in the image sequence by performing a 
cascaded logical AND of each image with those at higher temperature. Once 
this has been completed, the algorithm smooths the edges of the hot-spots 
using, for example, a 3.times.3 Gaussian smoothing filter, `fills-in` 
these regions using the selected neighborhood, and then assigns an 
appropriate color and temperature to each region. The last step in the 
algorithm combines these validated images into a color/temperature coded 
composite thermal map. 
It should be appreciated that various modifications can be made to the 
illustrated embodiments without departing from the scope of the invention. 
For example, while the illustrated embodiments have been described wherein 
the object is heated through a range of nominal temperatures from a 
relatively low nominal temperature limit below which the thermotropic 
material changes phase, to a relatively high nominal temperature limit 
above the temperature at which the thermotropic material changes phase, so 
that images can be acquired at incremental nominal temperatures within the 
range; other embodiments can be used wherein the object is cooled through 
a range of nominal temperatures from a relatively high nominal temperature 
limit above which the thermotropic material changes phase, to a relatively 
low nominal temperature limit below the temperature at which the 
thermotropic material changes phase, so that images can be acquired at 
incremental nominal temperatures within the range. In either event, the 
images can be stored and processed in succession of increasing incremental 
nominal temperatures as previously described; or processed in succession 
of decreasing incremental nominal temperatures wherein the steps of the 
various embodiments are essentially the same as described above, except 
that the step 4 entitled "verification" will be processed in a logically 
opposite manner. 
Additional disclosure related to the invention is included in Appendix A, 
entitled "Users Guide to ThermoMap. Software Version 2.0, Rev A:06/97," 
and Appendix B, LabVIEW block diagrams for the ThermoMap Hot-Spot 
Detector. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in respects as illustrative and 
not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of the equivalency of the claims 
are therefore intended to be embraced therein.