Patent Publication Number: US-2005138497-A1

Title: Apparatus and method for testing a flash memory unit

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to integrated circuit devices and, more particularly, to the testing of memory systems in integrated circuit devices. The testing technique is especially applicable to programmable non-volatile memory units such as Flash memory units.  
      2. Background of the Invention  
      In the present state of integrated circuit technology, programmable, non-volatile memory units, such as Flash memory units have become increasingly important. These memory units have been in integrated circuits devices. Referring to  FIG. 1 , an integrated circuit device  10  can include a processing core  11  and at least one memory unit  151 - 153 . The processing core  11 , for purposes of this discussion includes the central processing unit, random access memory, peripheral units, buses, etc. for the integrated circuit device  10 . Each of the memory unit(s)  151 - 153  exchanges signal groups with the processing core.  11 . For purposes of this discussion, each of the memory units, for example memory unit  151 , includes a storage array  1511  and a charge pump  1512 . The storage cell array  1511  includes a multiplicity storage cells, each cell storing a bit of binary information, and a detector for determining the logic state of the storage cells during read operation. Because the operation of the typical programmable, non-volatile memory cells requires voltage levels in excess of the voltage levels normally available for an integrated circuit device, a charge pump  1512  is provided. The charge pump  1512  is designed to take the voltage levels available for the integrate circuit device  10  and increase (i.e., “pump”) voltage levels to a magnitude that can permit the operation of the storage cells.  
      Referring to  FIG. 2 , the principal components of a Flash memory storage cell are illustrated. The storage cell  20  has a source terminal  21 , bit line terminal  22 , and a word line terminal  23 . Located between the terminal coupled to the word line terminal  23  and the region coupling the bit line terminal  22  and the source terminal  21  is a floating gate  24 . The operation of the memory cell  20  can be understood as follows. By proper biasing of the terminals, a charge can be stored on the floating gate  24 . The amount of charge stored on the floating gate  24  of a properly biased bit cell  20  can determine the amount of charge flowing between the source terminal  21  and the bit line terminal  22  during a read operation. The amount of the charge flowing between the terminals can represent a logic state. The bit line terminal is coupled to a charge (logic state) detector  29 . In a read operation, the detector  29  determines the logic state by the amount of charge transferred through the storage cell  20  under read operation conditions. Table 1 illustrates the conditions for the various operations on the storage cell according to one implementation of Flash memory storage cell.  
                                   TABLE 1                                   OPERATION   SOURCE   WORD LINE   BIT LINE                          READ     0 volts     5 volts     1 volt               WRITE     0 volts   11.4 volts   5.8 volts           ERASE   5.8 volts   -8.2 volts   FLOAT                      
 
      The successful operation of the storage bit cell  20  depends on the ability of the floating gate  24  to retain the charge without excessive decay. Excessive decay of the stored charge can result in the improper identification of the logic state that has been written into the storage bit cell  20 . The testing of the programmable, non-volatile bit cells is therefore elaborate, requiring the simulation of conditions that would result in erroneous operation. One of the causes of storage cell failure is the mobility of charge to or from the floating gate. Instead of waiting a period of time for movement of the charge, an over-voltage or stress voltage is applied bit cell to expedite the testing. The stress voltage provides enhanced mobility of the stored charge without the need to wait for a long period of time. After the stress voltage has been applied the storage cell is tested to see if the increased mobility of the charge on the floating gate has resulted in a change of the stored logic state. A change in the stored logic state indicates that the storage cell is defective and should not be used by the processing core  11 . To test the storage cell, the source line, the bit line  32 , the word line  23 , and the source line  21  have stress voltage individually applied thereto in the test procedures to determine the effect on charge stored or erased from the floating gate. The change in the charge stored on the floating gate can be determined by a read of the logic state represented by the charge. In addition, the word line voltage can be varied to determine the change in the threshold voltage wherein the incorrect logic state is identified during a read operation.  
      In the past, the storage cells of the memory units have been tested by external test apparatus. For example, in  FIG. 1 , the external connections to test apparatus are labeled  1515 ,  1525 , and  1535 , respectively. By means of the external connections, the operation of the programmable, non-volatile memory units could not be tested independently of the core processor  11  without additional device pins. The testing could be controlled and the proper voltages could be applied to the leads of the storage cell by external testing apparatus, and the results of the procedures applied to the external apparatus for testing. This technique did not permit the testing to be performed in parallel without additional device pins. In addition, the several additional voltage levels needed to adequately test the storage cells have been provided by the external testing apparatus. However, the exchange of signals needed to configure the memory unit for several test procedures and to report the result of the test procedures has proven to be time-consuming. Because the length of the test procedures impacts the cost of the circuit board, the testing procedures described above have proven too expensive. In addition, the number of pins to provide the exchange of signals and the voltage levels for testing purposes has proven to be excessive. It will be clear that, while the foregoing description for testing configurations has been described with respect to only one storage cell, the typical testing configuration will typically include a plurality of storage cells such a the storage cells required to store, for example, a word of data. When one storage cell is found to be defective, the whole plurality of storage cells will be considered defective.  
      A need has therefore been felt for apparatus and an associated method having the feature that the time to test to a memory unit would be reduced. It would be another feature of the apparatus and associated method to provide a tested integrated circuit device, the integrated circuit device having a memory unit, at a reduced cost. It would be yet another feature of the apparatus and associated method to reduce the number of pins on the circuit board required to test the memory unit. It would be still another feature of the apparatus and associated method to provide for testing apparatus for each memory unit of the integrated circuit device. It would be yet a further feature of the apparatus and associated method to provide a controllable charge pump in the memory unit responsive to the provided testing apparatus. It would be yet another feature of the apparatus and associated method to provide, as part of the memory unit, a processing unit with an associated memory unit that controls the testing of the memory in response to external control signals. It would be a still further feature of the apparatus and associated method to provide the results of the testing of the memory unit to external apparatus.  
     SUMMARY OF THE INVENTION  
      The aforementioned and other features are provided, according to the present invention, by the incorporation in the memory unit of testing apparatus needed to test the memory. The testing apparatus includes a memory unit for the storage of software procedures necessary to test the memory unit. A processing unit controls the selection and sequencing of the software procedures in response to external control signals. The local processing unit also provides the control of the addressing apparatus to exercise the storage cells in a predetermined and systematic manner. Because memory units, such as Flash memory units, typically require voltages different from the voltage levels available on the circuit board, a programmable charge pump is used to raise the voltage to a level compatible with the operation of the memory storage cells and to provide the voltages necessary for testing the memory unit. The charge pump, in response to control signals from the processing unit, provides the correct voltages to the terminals of the plurality of storage cells under test. The control of the charge pump by the processing unit includes the ability to provide a step function for determining threshold voltage. The testing logic includes apparatus for the storage of test results, the test results being transferred to external apparatus in response to control signals.  
      Other features and advantages of present invention will be more clearly understood upon reading of the following description and the accompanying drawings and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of an integrated circuit device having at least one programmable, non-volatile memory unit according to the prior art.  
       FIG. 2  illustrates the components of Flash memory cell according to the prior art.  
       FIG. 3  is a block diagram of the components of the storage unit according to the present invention.  
       FIG. 4  is a block diagram illustrating the coupling of the charge pump to a storage cell during test and debug procedures according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      1. Detailed Description of the Figures  
       FIG. 1  and  FIG. 2  have been described with respect to the related art.  
      Referring next to  FIG. 3 , the apparatus for the storage and retrieval of signal groups along with apparatus for testing the memory unit  30 , according to the present invention, is shown. The memory unit  30  has a storage cell array  31  in which the logic signals are received from the processing core  11  and stored, for example, as charges on the floating gates of the cells of the array. In response to signals from the processing core  11 , the addressing unit  82  accesses sub-set of the storage cells, e.g., a sub-set storing a group of logic signals. In response to the access of the sub-set, the logic signals from the processing core are stored in the addressed locations of the storage cell array  31  in response to write control signals from the processing core  11 . In response to read control signals from the processing core, logic signals stored in the addressed sub-array are transferred to the processing core  11 . The voltage levels needed for the operation of the memory unit are higher than the voltage levels generally available for the integrated circuit device within which the memory  30  has been fabricated. A charge pump  33  is coupled to a power source on the circuit board and provides a voltage appropriate to the operation of the storage cell array in response to control signals from the processing core  11 . In the testing operations, the local processing unit  34  receives control signals from the test and debug apparatus  5 . In response to the control signals from the test and debug apparatus  5 , the local processing unit  34  executes procedures stored in the local memory unit.  35 . In executing these procedures, the local processing unit  34  applies control signals to the addressing unit  32  and to the charge pump  33 . The results of the test procedures are applied to the local processing unit  34  and are stored in the results storage unit  36 . The test results stored in the results storage unit  36  can be transferred to the test and debug apparatus  5  for further analysis.  
      The charge pump  33  of the present invention is implemented to provide all the voltages required for the operation of the storage cells including the additional voltages needed for the testing operations. In this manner, no external voltages need to be introduced testing of the memory further reducing the number of leads. The charge pump  33  is designed so that, in response to control signals, the voltage level applied to the word line can have a step function configuration for the threshold tests.  
      Referring to  FIG. 4 , the storage cell of  FIG. 2  is shown. However, in place of the external voltage leads needed to activate the terminals of the storage cell, the charge pump  33  is designed to provide, in response to control signals from the local processing core  34 , the proper bias and stress voltages for the test procedures. In the present invention, the control signals original with the local processing unit during the test and debug procedures.  
      2. Operation of the Preferred Embodiment  
      It has been found that when the length of time for testing an memory unit is taken into account, the provision of a tested integrated circuit device having a Flash memory unit is less expensive when the testing facilities are included as part of the memory unit. In addition, because processing core  11  shares pins with the memory test function, parallel testing can not be performed. This invention eliminates the shared pin requirement. Therefore, the processing core and memory unit can be tested in parallel. By incorporating the testing apparatus in the template for each memory, the number of memory units associated with each processing core can be varied with a minimum of design change for each memory configuration.  
      The parameters, such as the voltage levels shown in Table 1, are provided for purposes of illustration. As will be clear to those skilled in the art, these parameters depend on the processes and materials that are used in fabricating the devices. Thus, other parameters can be used without departing from the present invention.  
      In the preferred embodiment, the external test and debug apparatus is used to load the test procedures into the local memory unit. The execution of the test procedures is controlled by the external test and debug apparatus. The external test and debug apparatus can also analyze and respond to the results of the testing procedures. For example, the failure of at least one of group of memory bits can be responded to by rendering that group of storage cells inaccessible to the core processor and/or replacing access to the faulty group of memory cells with access to a group of functional cells.  
      Because of the limited set of signals that must be exchanged between the test apparatus in the memory unit and external test and debug apparatus in the present invention, the number of pins for coupling the memory unit to the external test apparatus is reduced as compared to the number of pins required when no test apparatus is included in the memory unit.  
      In the foregoing description, the testing of the Flash memory unit has been described. However, the techniques of this invention are applicable to other memory implementations.  
      While the invention has been described with respect to the embodiments set forth above, the invention is not necessarily limited to these embodiments. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention, the scope of the invention being defined by the following claims.