Abstract:
A time temperature monitoring system and method for use with a microchip or similar structure. A disclosed system includes: a substrate having an active region; a dopant source located proximate the active region; an activation system for activating a diffusion of the dopant source into the active region; and a set of spatially distributed electrodes embedded in the active region of the substrate, wherein the electrodes are configured to detect the diffusion in the active region at varying distances from the dopant source to provide time temperature information.

Description:
BACKGROUND 
       [0001]    The disclosure relates generally to time temperature monitoring of a microchip or package. More specifically, embodiments of the present disclosure include a system and method that measures the diffusion of a dopant to determine the time temperature history of a microchip or package. 
         [0002]    In general, time temperature indicators provide a summary of the exposure history of a product to time and temperature. In products that are sensitive to time and temperature, a time temperature indicator may for example indicate the remaining shelf-life of a product, or the rate at which the product is deteriorating. 
         [0003]    Although most prominently used with perishable food items, time temperature indicators have potential applications with any device that is subject to high temperatures that could negatively impact performance of the device. For example, devices such as microchips are potentially subject to failures when operating outside temperature specifications for prolonged periods of time. Unfortunately, implementing time temperature indicators for use with microchips tend to involve complex solutions that limit their efficacy. 
       BRIEF SUMMARY 
       [0004]    A first aspect of the disclosure provides a time temperature monitoring system, comprising: a substrate having an active region; a dopant source located proximate the active region; an activation system for activating a diffusion of the dopant source into the active region; and a set of spatially distributed electrodes embedded in the active region of the substrate, wherein the electrodes are configured to detect the diffusion in the active region at varying distances from the dopant source to provide time temperature information. 
         [0005]    A second aspect of the disclosure provides a method for generating time temperature information with a substrate having an active region, a dopant source located proximate the active region, and a set of spatially distributed electrodes embedded in the active region of the substrate, the method comprising: periodically detecting a diffusion in the active region at varying distances from the dopant source; and analyzing the diffusion to provide time temperature information. 
         [0006]    A third aspect of the disclosure provides a microchip having an integrated time temperature monitoring system, comprising: a substrate having an active region; a dopant source located proximate the active region; an activation system for activating a diffusion of the dopant source into the active region; a set of spatially distributed electrodes embedded in the active region of the substrate, wherein the electrodes are configured to detect the diffusion in the active region at varying distances from the dopant source; and an interrogation system that periodically monitors the electrodes. 
         [0007]    The illustrative aspects of the present disclosure are designed to solve the problems herein described and/or other problems not discussed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    These and other features of this disclosure will be more readily understood from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings that depict various embodiments of the disclosure, in which: 
           [0009]      FIG. 1  shows a schematic of a microchip having a time-temperature monitoring system according to embodiments of the present disclosure. 
           [0010]      FIG. 2  shows a top schematic view of a sensor comprising a FET array according to embodiments of the present disclosure. 
           [0011]      FIG. 3  shows a cross-sectional view of the FET array of  FIG. 2  according to embodiments of the present disclosure. 
           [0012]      FIG. 4  shows a graph of copper diffusion profiles in silicon according to embodiments of the present disclosure. 
           [0013]      FIG. 5  shows illustrative time temperature information according to embodiments of the present disclosure. 
           [0014]      FIG. 6  shows illustrative time temperature according to embodiments of the present disclosure. 
           [0015]      FIG. 7  shows a flow diagram of a method for collecting time temperature information. 
       
    
    
       [0016]    It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements among the drawings. 
       DETAILED DESCRIPTION 
       [0017]    Embodiments of the present disclosure provide a time temperature monitoring system that is integrated with a device such as a microchip to provide localized time temperature information. Aspects of the time temperature monitoring system may be fabricated directly on a device or on a dummy package that can be adhered to a device. Accordingly, for the purposes of this disclosure, the term “device” refers to any package that can incorporate micro-electronic features and an active dopant region. The time temperature information may be used for any purpose including, e.g., to provide real-time data for system diagnosis of a device, to evaluate packaging processes, to establish reliability standards, etc. 
         [0018]    In general, time temperature history is captured by periodically monitoring changes in the resistance of a highly doped resistive (“active”) region. More particularly, a structure is provided to track resistivity changes that occur due to diffusion of a dopant source, such as copper, through an active region of a silicon substrate, which is time and temperature dependent. As the dopant source diffuses through the active region, resistivity of the active region breaks down, which is monitored and analyzed to provide time temperature information. 
         [0019]    The structure incorporates a set of spatially distributed electrodes that can be periodically monitored to detect a distance that the diffusion has travelled. In the examples that follow, an array of field effect transistors (FETs) is utilized to provide electrodes that can sense resistivity breakdown beyond a predetermined contamination limit in a substrate in which the FET is imbedded. Namely, resistivity breakdown will result in a detectable short circuit between selected ones of the FET&#39;s gate, source and drain nodes. It is recognized that other electrical structures (e.g., a fuse) for sensing resistivity, or lack thereof, could likewise be utilized. 
         [0020]      FIG. 1  depicts a generalized schematic of a microchip  10  that incorporates a time temperature monitoring system. In this example, a set of localized sensors  12  are placed throughout the microchip  10 . Each sensor  12  includes an array of FETs that are probed periodically via an interrogation system  14  to collect, analyze and output time temperature information  20 . As described in more detail herein, FETs within each sensor  12  are strategically designed to fail in response to known time and temperature exposure profiles. Interface logic  15  includes the necessary circuitry and logic to allow interrogation system  14  to interface with each sensor  12 . 
         [0021]    In this embodiment, interrogation system  14  includes a data collector  16  to periodically check to see which FETs within each sensor  12  remain operational, and which FETs are inoperable. Based on the collected data, a time temperature analyzer  18  will determine and output time temperature information  20  for each sensor  12 . Also included in interrogation system  14  is an initialization system  22  that triggers the interrogation process to begin. Depending on the application, interrogation system  14  may be integrated into the microchip  10 , be externally implemented, or be implemented as a combination of internal and external components. 
         [0022]      FIGS. 2 and 3  depict a top view and a cross-sectional view, respectively, of an illustrative sensor  12 . As shown in  FIG. 2 , each FET in an array of FETs  30  are placed at predetermined distances from a copper substrate contact  32 , or ground terminal, which acts a dopant source. Each FET  30  resides within a nitride spacer  36  and sits on top of active layer  38 , as shown in  FIG. 3 . A silicon based substrate  40  (e.g., SiCoH) resides below the active layer  38 . An activation system for the sensor  12  is provided with an application of current between copper substrate contact  32  and V-terminal  34 . Once activated, the copper (Cu) will begin to diffuse away from the copper substrate contact  32 . The rate of diffusion will depend on time and temperature, and as the copper diffuses, FETs  30  will be short circuited (“shorted”) rendering them inoperable. Accordingly, FET1 will be the first FET to fail at some point after copper is diffused into the channel of the FET1 device, followed by FET2 and so on. 
         [0023]    Accordingly, by strategically placing FETs  30  at known distances away from the dopant source, a time temperature history of the sensor  12  and surrounding circuits can be assessed by probing electrical readouts of the FETs  30 . Sensors  12  may be fabricated with standard FEOL (front end of the line) processes, thus incurring no additional costs. A copper BEOL (back end of the line) contact can be placed near each sensor  12 , with no additional cost. The interrogation process is initialized by bias-stressing the copper into the silicon contact  35 . This initialization can be done at any time, e.g., at chip burn-in, or at a later time. Although this embodiment uses copper, any controllable diffusive source may be utilized, e.g., aluminum, chromium, gallium, etc. 
         [0024]    Once initialized, each sensor  12  is periodically tested to see which FETs  30  have failed, i.e., are shorted. As noted, a short would indicate that the copper has diffused into the channel of the device. Because the diffusion process is known to depend on time and temperature, time temperature information can be ascertained based on the location of the different FETs, and which FETs have failed. 
         [0025]    Diffusion rates in silicon have been studied and behave in predictable manners. For example,  FIG. 4  shows a graph of copper diffusion in silicon for the FETs  30  shown in  FIGS. 2 and 3 , based on diffused distance. Each solid line curve in the graph depicts a copper diffusion profile for a given time. For instance, it can be seen that, based on a Cu profile at one month  41 , the location of FET1 will be at the contamination limit and therefore likely fail; based on the Cu profile at one year  42 , the locations of FET1 and FET2 will be at or above the contamination limit and likely fail; based on the Cu profile at five years  44 , the locations of FET1, FET2, and FET3 will be at or above the contamination limit and likely fail; and based on the Cu profile at 10 years  46 , the locations of FET1, FET2, FET3 and FET4 will be at or above the contamination limit and likely fail. 
         [0026]    Also shown as a dashed line is the Cu profile at one month at an elevated temperature  48  (i.e., above a predefined operating temperature threshold). In this case, with the elevated temperature, FET1 and FET2 will be at or above the contamination limit and likely fail. Similar elevated temperature curves for the other time periods, although not shown, can likewise be ascertained. Furthermore, although only a single elevated temperature curve is shown at one month, different curves could be utilized based on an amount or range of the elevated temperature for each time period. For instance, a first set of curves could be utilized for an elevated temperature at a range of 10-30 degrees Celsius, and a second set of curves could be utilized for an elevated temperature at a range of 31-60 degrees Celsius, and so on. Regardless, the time temperature analyzer  18  ( FIG. 1 ) is implemented to take advantage of these known diffusion rate behaviors. 
         [0027]      FIG. 5  depicts illustrative expected time temperature information based on a readout of the sensor  12  operating under normal conditions. Namely, as shown, FET1 is expected to fail at one month, FET2 is expected to fail at one year, etc.  FIG. 6  depicts an illustrative expected read out at one month under both normal and elevated temperatures. As shown, FET2 is expected to fail at one month under elevated temperatures. Accordingly, by probing the FETs periodically, and comparing the results to expected results, time temperature information can be readily determined to indicate whether a chip, or an area on a chip, has experienced elevated temperatures over a period of time. 
         [0028]      FIG. 7  depicts a flow diagram of the process. At S 1 , a microchip is provided with one or more sensors  12  (as shown in  FIGS. 1-3 ). At S 2 , each sensor  12  is initialized to enable the migration/injection of the copper from a dopant source into an active silicon region. Initialization is for example done by placing a current between the copper substrate contact  32  and V-terminal  34  shown in  FIG. 1 . Next, at S 3 , each sensor  12  is periodically probed to determine which FETs are operational and which have failed. At S 4 , the probing results are analyzed relative to known or expected diffusion behaviors to provide time temperature information, and at S 5 , the time temperature information is outputted. 
         [0029]    The time temperature monitoring system described herein may be employed for any number of purposes. For example, such a system could be employed to gauge wafer finishing and module building in which thermal processes can vary, thus detecting potential reliability issues. In other cases, such a system could be used to monitor an application environment, e.g., to determine whether a device has been operated inside or outside temperature specs. Along the same lines, the actual “stress age” of devices in the field can be calculated in order to schedule service calls. In cases where a device has been overly stressed, the device may for example be set to a “limp-along” mode in which clocks are slowed using standard techniques to allow continued operation. Such as system could also be used for calibration purposes, e.g., to calibrate or correlate on chip thermal sensors and digital thermal sensor structures by comparing such sensors to the time temperature information generated by the present system. 
         [0030]    Further features include the fact that such a system does not require the use of log files or processing elements on the package die to function. It is also noted that the distal range of diffusion is generally immobile in SiO2, i.e., it is limited to about 220 nm, under extremely accelerated thermal aging. Accordingly, the described process will not impact any surrounding circuitry on a chip. 
         [0031]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.