Abstract:
A method and device for identifying leaks in or a leakage rate of an integrated circuit package. The method and device include integrating a micromachined-thermal-convection accelerometer in the integrated circuit package and evaluating the initial and subsequent sensitivities of the accelerometer. A change in sensitivity with time provides indicia of a leak and a measure of leakage rate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The present invention is related to the field of detecting leaks from small, enclosed volumes and, more particularly, to means and methods for detecting leaks in an integrated circuit package by sealing a micromachined-thermal-convection accelerometer in the integrated circuit package. 
     Sealed integrated circuit (IC) packages are routinely inspected or testing for leakage. A common test for fine leaks is the helium mass spectrometer test (MIL-STD-883) by which an IC package is stored for a pre-determined period of time (t b ) under a pre-determined positive pressure (p w ) of helium (He) gas. If there are any leaks, the positive pressure will force the He gas into the “sealed” IC package. Subsequently, the IC package is placed in a He mass spectrometer. If any He gas were forced into the IC package, it will leak out into the He mass spectrometer absent any positive pressure. The He mass spectrometer can generally detect “fine” leakage rates in a range between 10 −9  and 10 −5  atm-cc/sec. 
     For “gross” leaks, which by definition are leaks having leakage rates that exceed 10 −5  atm-cc/sec, a “bubble test” is common. In conventional “bubble testing”, initially, efforts are made to force a first fluid into any leaks in a “sealed” IC package. Subsequently, the IC package is submerged into a second fluid such as a hot liquid, e.g., fluorocarbon. Leakage from the IC package manifests visually as the first fluid will form gaseous bubbles that out gas from any leaks in the “sealed” IC package. 
     Alternatively, for “gross” leaks, a residual gas analysis can be performed on the IC package. However, such analyses are destructive and not favored. 
     Although the above techniques known to the art have their applications, they are unsuitable for testing for leaks or for the leakage rate of an enclosed volume that is smaller than approximately 0.1 cc. Accordingly, it would be desirable to provide means and methods for detecting leaks and/or a leakage rate of an enclosed volume of an IC package that is smaller than approximately 0.1 cc. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a method and device for identifying leaks in or a leakage rate of a “sealed” integrated circuit package. The method and device include integrating a micromachined-thermal-convection accelerometer in the integrated circuit package and evaluating the initial and subsequent sensitivities of the accelerometer. The difference in sensitivity with time provides indicia of a leak and a measure of leakage rate. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be better understood by reference to the following more detailed description and accompanying drawing in which: 
         FIG. 1  shows a diagram of a micromachined-thermal-convection accelerometer in accordance with the prior art; 
         FIG. 2  shows temperature profiles with and without acceleration in accordance with the prior art; 
         FIG. 3  shows a hermetically sealed integrated circuit package in accordance with the prior art; 
         FIG. 4  shows chip scale packaging in accordance with the prior art; and 
         FIG. 5  shows a flow chart of a method of detecting leaks in and the leakage rate of a sealed integrated circuit package. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Means and methods of leak detection by integrating a micromachined-thermal-convection accelerometer into a hermetically-sealed, integrated circuit (IC) package are disclosed. Because micromachined-thermal-convection accelerometers are very small and non-intrusive and can be manufactured as small as 1 mm square, they can be integrated with other devices in a hermetical package to great effect, to detect and continuously monitor leaks in a non-vacuum hermetic sealed package. More specifically, the micromachined-thermal-convection accelerometer can be sealed with a cap as a part of the chip scale package ( FIG. 4 ) or included in a small size hermetical IC package ( FIG. 3 ). 
     The operating principle of a micromachined-thermal-convective accelerometer  10  will be described. Referring to FIG.  1 , the accelerometer  10  includes a central heating device  12  and opposing pairs of temperature sensitive elements  14  and  16  that are disposed on a substrate  18 . The temperature sensitive elements  14  and  16  extend symmetrically over a cavity  15  in the substrate  18 , at or along two opposing sides  13  and  17 . Preferably, the central heating device  12  is disposed between and equidistant from each of the temperature sensitive elements  14  and  16 . 
     The operating principle is based on free-convection heat transfer of a hot air bubble in an enclosed chamber. As the central heating device  12  heats up, the density of the fluid surrounding it in the cavity  15  decreases and a symmetrical temperature gradient in the fluid is produced. The symmetrical temperature gradient extends outwardly from the heating device  12  to each pair of temperature sensitive elements  14  and  16 . 
     As long as the temperature profile produced by the heating device  12  is symmetrical, there is no output from the pair of temperature sensitive elements  14  and  16 . This situation is reflected in the solid lines  22  on the temperature profiles  20  shown in  FIG. 2 , in which the temperature difference (σT) between the pair of temperature sensitive elements  14  and  16  is zero or substantially zero. 
     When the accelerometer  10  is accelerated, the symmetry of the temperature gradient is disturbed, which is illustrated by dashed lines  24  in  FIG. 2 . Indeed, when accelerated, the resulting temperature difference (σT) between the pair of temperature sensitive elements  14  and  16 , whose positions are fixed, produces an output signal proportional to the applied acceleration. 
     Instrument sensitivity is one means of leak detection. The absolute output signal of a micro-pressure sensing device, in which offset is included, is proportional to the pressure of the fluid within the sealed package. However, micro-pressure sensing devices, e.g., micro-capacitor pressure sensors, micro-Pirani pressure sensors, micro-resonator pressure sensors, and the like, are affected by offset shift with time. In contrast, with a micromachined-thermal-convection accelerometer  10 , the issue is eliminated. 
     More particularly, using the Earth&#39;s gravity is the most convenient way to measure the sensitivity of an accelerometer  10 . Sensitivity is understood herein to refer to an output signal produced by the pair of temperature sensitive elements  14  and  16  at one gravity acceleration or “1 g”. 
     Sensitivity can be measured using the Earth&#39;s gravity, by changing the orientation of the sensitive axis of the package in which the accelerometer  10  is sealed. For example, if the sensitive axis of the accelerometer  10  is horizontal with respect to the Earth&#39;s gravity vector, there is no acceleration applied to the accelerometer  10  and the output signal is offset. If the accelerometer&#39;s  10  sensitive axis is rotated vertically to align with the Earth&#39;s gravity, output signals for the +1 g and the −1 g cases can be measured. 
     For example, if the sensitive axis and the Earth&#39;s gravity are in the same direction, let the output signal for the +1 g case be T 1 , in which T 1  is the maximum positive magnitude. If the sensitive axis and the Earth&#39;s gravity are in opposite directions, let the output signal for the −1 g case be T 2 , in which T 2  is the maximum negative magnitude. 
     The maximum positive magnitude, T 1 , is equal to the sensitivity (S) plus the offset (T 1 =S+offset) while the maximum negative magnitude, T 2 , is also equal to the offset plus the sensitivity, which has a negative sign (T 2 =offset+(−S)). Recognizing that sensitivity is the difference between the maximum positive and the maximum negative,
 
 T 1 −T 2=(offset+ S )−(offset+(− S )) or
 
 T 1 −T 2=2 S.  
 
The offsets cancel each other, leaving just the sensitivity (S) Rearranging the above equation, sensitivity is defined by the equation:
 
 S =( T 1− T 2)/2,
 
     Knowing this, by measuring the sensitivity of the accelerometer  10  with time, the leakage rate of the package can be determined as shown in the flow chart of  FIG. 5  and as described below. First, the initial pressure (P 1 ) of the cavity can be measured and the sensitivity (S 1 ) can be measured when the IC package is first sealed (STEP  1 ). After a time period (t), the sensitivity (S 2 ) having the same environment temperature with the same heating device power can be measured (S 2 ) (STEP  2 ). 
     Subsequently, sensitivity is proportional to pressure squared, the inner pressure (P 2 ) of the package can be calculated (STEP  3 ) using the equation:
 
 P 2/ P 1=( S 2/ S 1) 1/2  or
 
 P 2= P 1*( S 2/ S 1) 1/2 .
 
Finally, the average leak rate can be calculated (STEP  4 ) using the equation:
 
 R =( P 2− P 1)/ t.  
 
     In short, micromachined-thermal-convection accelerometers  10  can be packaged together with other devices  32  in an IC package  30  ( FIG. 3 ) or can be sealed alone with a cap  45  as a part of a chip scale package  40  ( FIG. 4 ) to detect leaks and leakage rate. 
     It will be apparent to those skilled in the art that modifications to and variations of the disclosed methods and apparatus are possible without departing from the inventive concepts disclosed herein, and therefore the invention should not be viewed as limited except to the full scope and spirit of the appended claims.