Abstract:
A protection device and a method for manufacturing integrated circuit devices protect against plasma charge damage, and related charge damage during manufacture. The protection device comprises a dynamic threshold, NMOS/PMOS pair having their respective gate terminals coupled to the semiconductor bulk in which the channel regions are formed. With proper metal connection, the structure protects against plasma charge damage on the integrated circuit device during manufacture, and can also be operated to protect against abnormal voltages during operation of the circuit.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to integrated circuit devices and integrated circuit device manufacturing, including manufacturing of non-volatile memory devices, and more particularly to devices and processes for protection of integrated circuitry from plasma damage during manufacture. 
   2. Description of Related Art 
   In the manufacturing of integrated circuits, the processes are utilized which utilize activated ions. For example, backend processes including metal etching, photoresist stripping, and deposition of inter-metal dielectrics, involve plasmas which induce charge on the structures being treated. These plasma induced charges may damage underlying structures in the device, including structures critical to device performance. For example, tunnel dielectrics used in flash memory devices, gate dielectrics, and inter-polysilicon dielectrics, can be damaged by plasma induced charge. Furthermore, the charge storage structure utilized in SONOS cells is particularly susceptible to damage by plasma based processes. SONOS memory devices are described in U.S. Pat. No. 6,011,725. 
   One characteristic of the plasma induced charge is that it may be either positive or negative, and different types of damage can occur based on the type of induced charge. 
   In conventional semiconductor devices, protection from plasma induced charge is provided by forming a protection diode as shown in  FIGS. 1A and 1B . A protection diode between a p-type region and an n-type well is formed to release “positive” charge from a node coupled to the p-type region as illustrated in  FIG. 1A . A protection diode between an n-type region and the p-type well is formed to release “negative” charge from a node coupled to the n-type region, as shown in  FIG. 1B . However, the structure of  FIG. 1A  does not discharge negative charge until the charge buildup is negative enough for junction breakdown. Likewise, the structure of  FIG. 1B  does not discharge positive charge until the charge buildup is positive enough for junction breakdown. These protection devices are useful and easy to implement. However, if the plasma exposure is too high, damage continues to happen with these prior art structures. 
   In floating gate memory devices as shown in  FIG. 2 , or other stacked gate structures, plasma charge builds up in the floating gate structure, and results in the cell threshold shifting up during the manufacturing process. Typically, the device is exposed to ultraviolet radiation after processing. The radiation causes the built up charge to be discharged to near normal conditions. However, damage to the cell structure caused by the plasma processes is not repaired. 
   For SONOS devices as illustrated in  FIG. 3 , plasma damage is more difficult to repair. The charge buildup caused by plasma based processes cannot be neutralized by ultraviolet exposure, as used in floating gate nonvolatile memory cells. The ultraviolet radiation injects additional electrons into the nitride film shifting the threshold voltage of the memory cell. Furthermore, because the threshold voltage is increased in SONOS devices for either positive or negative charge stress, the typical junction breakdown protection device is not sufficient. Back-to-back junction diodes can be used. However, the combination does not provide sufficient protection because the cell will be damaged before the junction breakdown levels are reached. 
   Accordingly, it is desirable to provide a protection circuit for use in the manufacturing of integrated circuits that protect both positive and negative charge damage. Furthermore, the protection circuit should not affect device operation after manufacturing. Finally, the protection circuit should be easily manufacturable. 
   SUMMARY OF THE INVENTION 
   The present invention provides a protection device, and a method for manufacturing integrated circuit devices, for protecting against plasma charge damage, and related charge damage during manufacture. The protection device comprises a dynamic threshold, NMOS/PMOS pair having their respective gate terminals coupled to the semiconductor bulk in which the channel regions are formed. With proper metal connection, the structure protects against plasma charge damage on the integrated circuit device during manufacture, and can also be operated to protect against abnormal voltages during operation of the circuit. 
   In one aspect, the present invention provides an integrated circuit device, which includes a device substrate. Integrated circuitry is formed on the device substrate. The integrated circuitry is coupled to a node having voltages applied thereto during operation of integrated circuitry. A PMOS device having a gate, a source and drain, and an NMOS device having a gate, a source and drain are formed on the device substrate. One of the source and drain of the PMOS device is coupled to said node. The other of the source and drain of the PMOS device is coupled to a ground reference. One of the source and drain of the NMOS device is coupled to said node. The other of the source and drain of the NMOS device is coupled to a ground reference. The gate of the PMOS device is coupled to the bulk semiconductor in which the channel of the device is formed. Likewise, the gate of the NMOS device is coupled to the bulk semiconductor in which the channel of the device is formed. During manufacturing, the gate of the PMOS device and the gate of the NMOS device are left floating. 
   Thus, during manufacture, dynamic threshold MOS devices are coupled to a node in the integrated circuit, which is to be protected from plasma charge damage. The dynamic threshold MOS devices are conductive with relatively low charge on the gate. Both positive and negative charges are discharged through the devices for protecting the node of the integrated circuit from plasma charge damage. During operation of the integrated circuit, voltages are applied to the gates of the dynamic threshold MOS devices, which turn off the dynamic threshold MOS devices during normal operation. Thus, the gate of the PMOS device is connected during operation to a voltage, such as a high positive voltage greater than the supply potential on the integrated circuit, which is high enough to turn off the PMOS device when the highest operating potential is applied to the node. The gate of the NMOS device is connected to a voltage during operation, such as a negative voltage, which is low enough to turn off the NMOS device when the lowest operating potential is applied to the node. 
   According to another aspect of the invention, the integrated circuitry includes memory array having a word line coupled to a row of memory cells in the array. The word line is coupled to said node in the protection device to protect memory cells along the word line from plasma charge damage. In one embodiment, the memory cells comprise SONOS cells. In another embodiment, the memory cells comprise floating gate memory cells. 
   According to yet another embodiment, the integrated circuit device includes a non-volatile memory circuit to be protected from plasma damage during manufacture. The memory circuit has a plurality of operating modes during which operating voltages are applied to word lines in the memory array. The plurality of operating modes includes a read mode, a program mode and an erase mode. During the program mode, a programming voltage is applied to the word lines using a charge pump or other voltage generator, which generates a voltage higher than the supply potential provided to the device from an external power supply. The voltage applied to the gate of the PMOS device during the program mode is at least as high as said programming voltage. During the erase mode, an erasing voltage is applied to the word line, which is a negative voltage. The voltage applied to the gate of the NMOS device during the erase mode is at least as low as said negative voltage. 
   In one embodiment of the invention, the PMOS device comprises an n-type well in said substrate, which acts as the semiconductor bulk in which the channel of the PMOS device is formed, and p-type source and drain regions in the n-type well. The gate of the PMOS device is coupled to said n-type well. Also in some embodiments, the NMOS device comprises a deep n-type well in said substrate, with the p-type well within the n-type well. The p-type well acts as the semiconductor bulk in which the channel of the NMOS device is formed. N-type source and drain regions are formed within the p-type well, and the gate of the NMOS device is coupled to the p-type well. 
   The present invention also provides a method for manufacturing an integrated circuit device. The method includes forming integrated circuitry on a substrate, the circuitry having a node to be protected from plasma charge damage. Also, a dynamic threshold PMOS device and a dynamic threshold NMOS device are formed on the substrate. The PMOS device and the NMOS device have characteristics such as those described above, and provide protection for the integrated circuitry against plasma charge damage. Thus, the method also includes coupling one of the source and drain of the PMOS device to said node, and other of the source and drain of the PMOS device to a ground reference. Also, the method includes coupling one of the source and drain of the NMOS device to said node, and other of the source and the drain of the NMOS device to a ground reference. The method also includes floating the gate of the PMOS device and the gate of the NMOS device during manufacturing steps that exposed the integrated circuitry to plasma or other processes that may cause charge damage. Circuitry is provided on the substrate to bias the gate of the PMOS device during operation to a voltage at least as high as operating voltages applied to the node by the integrated circuitry during operation. Likewise, circuitry is provided on the substrate to bias the gate of the NMOS device during operation to a voltage at least as low as operating voltages applied to the node by the integrated circuitry during operation. 
   The method of manufacturing according to the present invention is applied to all kinds of integrated circuits, and particularly to non-volatile memory circuits including memory circuits based on SONOS memory cells. 
   Accordingly, the present invention provides a PMOS transistor and an NMOS transistor. The gates of the PMOS and NMOS transistors are floating during manufacturing, and coupled to a first voltage and a second voltage, respectively, during operation. The PMOS device protects against positive charge and the NMOS device protects against negative charge. During manufacturing, because the gates of the PMOS and NMOS devices are floating, the PMOS transistor can discharge positive charge, and the NMOS transistor can discharge negative charge. In addition, during operation, the PMOS transistor can discharge positive charge higher than operation voltages, and the NMOS transistor can discharge negative charge lower than the operation voltages. 
   In embodiments for which protection is desired for only one of positive and negative charge, then only one of the NMOS and PMOS transistors may be used. 
   Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description and the claims, which follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  and  FIG. 1B  illustrate prior art junction based protection devices. 
       FIG. 2  shows exposure of a floating gate device to ultraviolet radiation as used in prior art techniques. 
       FIG. 3  shows exposure of a SONOS device to ultraviolet radiation. 
       FIG. 4  is a simplified block diagram of an integrated circuit including the protection circuit of the present invention. 
       FIG. 5  illustrates a cross-sectional view of one embodiment of the protection circuit according to the present invention. 
       FIG. 6  shows a simplified diagram of an integrated circuit memory device, including protection circuits according to the present invention. 
   

   DETAILED DESCRIPTION 
   A detailed description of embodiments of the present invention is provided with reference to  FIGS. 4–6 .  FIG. 4  illustrates an integrated circuit device  10  including a protection circuit according to the present invention. The integrated circuit device includes a device substrate  11 . Integrated circuitry  12  on the device substrate performs the “mission” operations for the device  10 . The “mission” operations may include providing memory, logic functions, processor functions, or any of a wide variety of functions provided by integrated circuitry. 
   A power supply provides a supply voltage VCC and a ground reference for the device substrate  11 . Voltage generator circuitry  13  on the device substrate  11  provides operational voltages for the integrated circuit  12 , including a lower operating voltage and a higher operating voltage for a node  14 . 
   The protection device on the device substrate includes a PMOS transistor  15  and an NMOS transistor  16 . One of the source/drain terminals of the PMOS transistor  15  is coupled to ground. The other of the source/drain terminals of the PMOS transistor  15  is coupled to the node  14  via a conductive line. The gate of the PMOS transistor  15  is coupled to the semiconductor bulk in which the channel of the transistor is formed. Also, the gate of the PMOS transistor  15  is coupled to line  17  across which the voltage generator  13  supplies during operation of the device a voltage at least as high as the higher operating voltage applied during operation to node  14 . During manufacture, the line  17  is floating. 
   One of the source/drain terminals of the NMOS transistor  16  is coupled to ground. The other of the source/drain terminals of the NMOS transistor  16  is coupled to the node  14  via a conductive line. The gate of the NMOS transistor  16  is coupled to the semiconductor bulk in which the channel of the transistor is formed. Also, the gate of the NMOS transistor  16  is coupled to line  18  across which the voltage generator supplies during operation of the device a voltage at least as low as the lower operating voltage applied during operation to node  14 . During manufacture, the line  18  is floating. 
   The structure of the protection device, according to the present invention, can be understood with reference to the example shown in  FIG. 5 .  FIG. 5  illustrates a semiconductor substrate  20  (PW) having an intrinsic p-type doping. A first deep n-type well  21  (NWD) and a second deep n-type well  22  (NWD) are formed by diffusion of n-type dopants into the substrate  20 . The PMOS transistor has a p-type source  23  and a p-type drain  24  formed within the first deep n-type well  21 . An n-type contact  25  is formed on the surface of the first deep n-type well  21 . The p-type contact region  26  is formed in the surface of the substrate  20  (PW), preferably adjacent to the first deep n-type well  21 . A gate  27  is formed over an insulator (not shown) between the source  23  and the drain  24  over the channel region. The first deep n-type well  21  acts as the semiconductor bulk within which the channel region is formed. The gate  27  is coupled to the first deep n-type well  21  via the contact  25 . The source  23  is coupled to the substrate  20  via the contact  26 , and to a ground reference. The drain  24  is coupled via a conductive line to a node  30  to be protected in integrated circuitry on the device. The gate  27  is also coupled to a circuit on the device which supplies the highest voltage VPCP 11  available during operation, in one embodiment. The voltage on the gate  27  is at least as high as the highest operating voltage applied to the node  30  during operation, and is high enough to bias the PMOS transistor in a normally off position during operation of the device. During manufacture, node  30  is left floating. 
   Within the second deep n-type well  22 , a deep p-type well  31  (PWI) is formed. The NMOS transistor has a source  32  and a drain  33  formed within the p-type well  31  (PWI). A p-type contact  34  is formed by diffusion in the surface of the p-type well  31 . Also, a p-type contact  35  is formed in the surface of the substrate  20 , preferably adjacent the second deep n-type well  22 . A gate  36  is formed over an insulator (not shown) over the channel region between the source  32  and a drain  33  of the NMOS transistor. The gate  36  is coupled to the contact  34 , so that the gate of the NMOS device is coupled to the semiconductor bulk in which the channel of the NMOS device is formed. The source  32  of the NMOS transistor is coupled to the terminal  35  and to a ground reference. The drain  33  of the NMOS transistor is coupled to the node  30 . A contact  37  is formed in the surface of the second deep n-type well  22 . The contact  37  is coupled to the highest voltage VPCP 11  generated on that chip during operation, or to another voltage level sufficient to maintain isolation of the p-type well  31 . The gate  36  of the NMOS transistor is coupled to a circuit which supplies the lowest voltage NVPP provided on the chip, at least as low as the lowest voltage applied at the node  30  during operation, or to a circuit which provides a voltage low enough to turn off the NMOS device during operation of the circuitry. During manufacturing, the gate  36  is left floating. 
   The gate insulator between the gate and channel of the NMOS transistor and of the PMOS transistor should be strong enough to withstand the high or low voltages applied during operation of the device. For example, the gate insulator comprises a relatively thick oxide, compared to gate oxide thicknesses for logic transistors, in one embodiment of the device. 
     FIG. 6  illustrates a semiconductor memory device including a protection circuit according to the present invention. The device includes a substrate  100 , a power supply terminal  101 , and a ground terminal  102 . Integrated circuitry on the device includes a memory structure  111  including an array  110  of memory cells, such as SONOS cells. The memory array  110  in various embodiments comprises DRAM cells, SRAM cells, mask ROM cells, floating gate memory cells, and other types of memory device structures. The memory structure includes the word line decoder  120  and a bit line decoder  121  and other supporting circuitry known in the art for memory devices. Supporting circuitry on the device in this example includes a read, erase and program mode control logic  122 , charge pumps  123  for supplying high positive and negative voltages supporting the operation modes of the memory array, and other supporting circuitry not shown. Within the array  110 , a word line, such as word line WL 1 , is coupled to a row of memory cells to be protected from damage by the protection circuit of the present invention. 
   The protection circuit on the substrate  110  includes a PMOS transistor  112 , and an NMOS transistor  113  constructed as discussed above with reference to  FIG. 5 . The drains of the PMOS transistor  112  and NMOS transistor  113  are coupled to the node  115 , which is connected via a line  114  to the word line WL 1 . The sources of the PMOS transistor  112  and the NMOS transistor  113  are coupled to ground reference terminals. The gate of the PMOS transistor  112  is coupled via the line  124  to a voltage V 1 , supplied in this example by the charge pump circuitry  123 . The gate of the NMOS transistor  113  is coupled via the line  125  to the voltage V 2 , supplied in this example by the charge pump circuitry  123 . 
   In this embodiment, during the manufacturing process for the device  100 , the first voltage V 1  and the second voltage V 2  are floating, so that the gates as the PMOS transistor  112  and the NMOS transistor  113  are floating. Therefore, the device  100  can be protected from plasma charge having both positive and negative polarities. Negative charges are well protected by the NMOS transistor  113  by the current path L 1 . Positive charges are well protected by the PMOS transistor  112  via the current path L 2 . 
   During operation of the device  100 , the first voltage V 1  and the second voltage V 2  are supplied with values that depend on the operating process. In the flash memory example, different voltages are applied for the various operating modes (read, erase, program) of the memory. In the memory embodiment described, the voltages applied to word line WL 1 , the NMOS transistor  113  and the PMOS transistor  112  are shown in the following table (refer to  FIG. 5  for NMOS and PMOS terminals). 
   
     
       
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               PROGRAM 
               ERASE 
               READ 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               WORD LINE: 
               11.5 V 
               −3 V 
               2.6 V 
             
             
                 
               NMOS: 
             
             
                 
               GATE/PWI 
               GND 
               NVPP 
               GND 
             
             
                 
               NWD 
               VPCP11 
               VCC 
               VCC 
             
             
                 
               SOURCE/PW 
               GND 
               GND 
               GND 
             
             
                 
               PMOS: 
             
             
                 
               GATE/NWD 
               VPCP11 
               VCC 
               VCC 
             
             
                 
               SOURCE/PW 
               GND 
               GND 
               GND 
             
             
                 
                 
             
           
        
       
     
   
   Within the table, the parameter VPCP 11  corresponds to the highest voltage available on the chip, or to a voltage at least as high as the highest voltage applied to the word line, and high enough to turn off the PMOS transistor during operation of the device. Also, the parameter NVPP corresponds to the lowest negative voltage available on the chip, or to a voltage at least as low as the lowest voltage applied to the word line, and low enough to turn off the NMOS transistor during operation of the device. 
   As can be seen, the operating voltages for the integrated circuit in the memory example are different in the different operating modes. During the programming process, the operating voltage on the word line may be, for example, 11.5 V. During erasing in reading, the operating voltages on the word line are −3 V and +2.6 V, respectively. Thus, during the programming mode, the normal operating voltage on the word line will not turn on the PMOS transistor  112 . Thus, the normal operating voltage of the word line is not disturbed. However, when an abnormal voltage, such as a voltage higher than the normal operating voltages or lower than the normal operating voltages occurs, the PMOS transistor  112  and the NMOS transistor  113  will turn on to discharge the abnormal voltage so as to protect the memory array. In this case for the programming mode, an abnormal voltage higher than 11.5 V or lower than ground can be discharged. Likewise, by setting the voltage on the gates of the protection devices properly, during erasing and reading modes, normal operating voltages are not discharged by the protection device, while abnormal voltages may be discharged to protect the device. 
   Thus, the present invention provides a protection circuit based on a dynamic threshold MOS pair of transistors. Positive charge is conducted to ground via the PMOS transistor, and negative charge is conducted to ground via the NMOS transistor at very low voltages. For example, the NMOS transistor will conduct at a voltage close to the junction forward turn on voltage of, for example, 0.6 V. Likewise, the PMOS transistor will conduct at a voltage close to the junction forward turn on voltage of, for example, −0.6 V. For a discussion of the operation of dynamic threshold MOS devices, see, IEEE ELECTRON DEVICES, Vol. 38, No. 11, November, 1991. 
   The gate oxide for the MOS pair is preferably thick enough to sustain high-voltage operation for flash memory devices or other high-voltage integrated circuits. The thick gate oxide can be easily manufactured in flash memory devices, by using the same processing step as is used to produce the thick oxides for charge pump transistors. 
   During manufacturing processes, plasma charge may accumulate and be conducted through the word line to the protection device. The number of protection devices used in a particular integrated circuit will depend on the manufacturing circumstances, the space available, and the needs of the particular product. There may be one protection device per word line, in some example products. In other example products, one protection device may be shared among a plurality of word lines. Other nodes in the integrated circuitry on the device can be protected as well. 
   While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims.