Abstract:
A method of programming and erasing an electrically erasable programmable read-only memory (EEPROM)device includes performing a band-to-band tunneling induced hot-electrons program and performing a Fowler-Nordheim tunneling erase. The EEPEOM device includes a P-type transistor, an N-type transistor, and a double gate P-type transistor. A source of the P-type transistor and the N-type transistor are respectively electrically connected to a program bit-line and an erase bit-line. A drain of the double gate P-type transistor is electrically connected to a drain of the P-type transistor and a drain of the N-type transistor.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention provides a non-volatile memory and a method of programming and erasing, and more particularly, the present invention relates to an electrically erasable programmable read-only memory (EEPROM) and a method of programming and erasing. 
   2. Description of the Prior Art 
   An EEPROM device is a kind of non-volatile memory. The structure of EEPROMs is similar to that of erasable programmable read-only memories (EPROMS) since both of them have a floating gate for storing charges and a control gate for controlling data access. In a large number of memory cells, each memory cell comprises a floating gate for storing charges which represent data. After the floating gate of the memory cell is charged, the threshold voltage of the memory cell is lifted, so the charged memory cell will not be in a conductive state during addressing in reading. A state of not conducting is regarded as a “0” state or a “1” state by detecting circuits utilizing a binary system. Uncharged memory cells will be regarded as being in a “1” state or in a “0” state correspondingly. In comparison with an EPROM which is programmed and erased as a whole, EEPROM has the advantage of erasing and programming data bit by bit. Therefore, the EEPROM is a byte addressable device. 
   Since the flash memory is erased block by block rather than bit by bit, EEPROM is superior to the flash memory in partial data revision when compared with the flash memory product, which is rapidly growing in market. The EEPROM is very suitable to be used in an embedded function, such as an address book in cell phones, because of its byte program/erase feature. In addition, EEPROM products usually have good high reliability performance, which increases applicability in application fields requiring repetitive programming, reading, and erasing. 
   Please refer to  FIG. 1 ,  FIG. 1  is a cross-sectional schematic diagram illustrating a prior art EEPROM device  10 . As shown in  FIG. 1 , the prior art EEPROM device  10  is disposed on a semiconductor wafer  11 . The semiconductor wafer  11  comprises a P-type silicon substrate  12 . The EEPROM device  10  comprises a memory cell  14 . The memory cell  14  comprises a source region  16  and a drain region  18  disposed on a surface of the P-type silicon substrate  12 , and a channel region  22  between the source region  16  and the drain region  18 . Both the source region  16  and the drain region  18  are N-type heavy doped regions. The memory cell  14  further comprises a tunnel oxide layer  24 , a floating gate  26 , a dielectric layer  28 , and a control gate  32 . The tunnel oxide layer  24  is disposed on a top surface  25  of the P-type silicon substrate  12 , and the tunnel oxide layer  24  covers the channel region  22 . The floating gate  26  is disposed on a surface of the tunnel oxide layer  24 . The dielectric layer  28  covers the floating gate  26 . The control gate  32  is disposed on a surface of the dielectric layer  28  and the surface of the tunnel oxide layer  24 . 
   The EEPROM device  10  further comprises an N-type select gate transistor  34 . The select gate transistor  34  comprises a source region  36 , a drain region, and a gate  38 . Since the drain region of the N-type select gate transistor  34  is overlapped with the source region  16  of the memory cell  14 , it is not specially marked. In addition, the source region  36  of the N-type select gate transistor  34  is electrically connected to a bit line (BL). 
   When the memory cell  14  is selected to perform a program operation, a high positive potential (such as +12 V) is supplied to the control gate  32 . At this time, the N-type select gate transistor  34  is turned on to pass a program potential (such as 2.5 V) supplied to the bit line to the source region  16  of the memory cell  14 . In addition, a terminal  42  electrically connected to the P-type silicon substrate  12  is grounded. Since the program potential and the potential of the P-type silicon substrate  12  are obviously lower than the positive potential supplied to the control gate  32 , high potential differences exist to produce an electric field that transverses the tunnel oxide layer  24 . Therefore, electrons flowing from the source region  16  to the drain region  18  will acquire kinetic energy due to the existence of the electric field, and change their acceleration direction and transverse the tunnel oxide layer  24  by Fowler-Nordheim (FN) tunneling mechanism. The electrons then are injected into the floating gate  26  and are trapped in the floating gate  26  to complete the program operation. The threshold voltage of the N-type memory cell  14  is thus lifted. 
   When the memory cell  14  is selected to perform an erase operation, a high negative potential (such as −12 V) is supplied to the control gate  32 . At this time, the N-type select gate transistor  34  is turned on to pass an erase potential (such as +2.5 V) supplied to the bit line to the source region  16  of the memory cell  14 . In addition, the terminal  42  electrically connected to the P-type silicon substrate  12  is grounded. Since the erase potential and the potential of the P-type silicon substrate  12  are obviously higher than the potential supplied to the control gate  32 , not only is the channel region  22  of the memory cell  14  not conducted, but also high potential differences exist to produce an electric field that transverses the tunnel oxide layer  24  (opposite to the direction of the electric field when programming). Therefore, electrons stored in the floating gate  26  will be driven to move toward the channel region  22 , and are sucked out to the channel region  22  from the floating gate  26  by Fowler-Nordheim tunneling mechanism. The erase operation is thus completed. 
   In the prior art, both the program operation and the erase operation which charge or discharge the floating gate utilizes Fowler-Nordheim tunneling mechanism. This method has its native limitation in that the Fowler-Nordheim tunneling behavior of electrons does not happen under the electric field produced by a low potential difference. That means, in order to make this behavior happen, a high potential difference must exist to result in a very slow program speed and erase speed. Furthermore, electrons enter and leave the floating gate  26  through a tunnel window  44 , as shown in  FIG. 1 , in the prior art. Because the tunnel oxide layer  24  inside the tunnel window  44  is very thin, the tunneling behavior of electrons is benefited to improve the performance of the memory device. However, a process problem is encountered. 
   In the prior art memory cell  14 , a buried implant region  46  is respectively formed in portions of the channel region  22  near the source region  16  and the drain region  18 , as shown in  FIG. 1 , before forming the tunnel window  44 . The buried implant region  46  is an N-type lightly doped region. The objective of forming the buried implant regions  46  is to lift tunneling efficiency so as to improve program speed and hot electrons injection. Since the tunnel window  44  is within the range of the buried implant region  46 , it thus becomes very difficult when aligning the tunnel window  44 . In consideration of this problem, the size of the tunnel window  44  cannot be shrunk to avoid the misalignment problem, leading to a barrier to memory cell  14  shrinkage. In some of the prior arts, the methods for forming tiny tunnel windows are taught. However, process steps are complex in these methods to increase processing complexity and cost of products. 
   Therefore, it is very important to develop a new EEPROM structure. This EEPROM structure should perform program and erase under low operation voltages to improve operation speed. In addition, the need of the tunnel window is eliminated in this EEPROM structure such that the memory cell is shrunk without increasing processing complexity and product costs. 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the present invention to provide an EEPROM structure and a method of programming and erasing to resolve the above-mentioned problems. 
   The present invention provides a method of programming and erasing an EEPROM device. The method comprises performing a band-to-band tunneling induced hot-electrons program via a program bit-line and performing a Fowler-Nordheim tunneling erase via an erase bit-line. The EEPEOM device comprises a P-type transisitor, an N-type transistor, and a double gate P-type transistor. A source of the P-type transistor is electrically connected to the program bit-line. A source of the N-type transistor is electrically connected to the erase bit-line. A drain of the double gate P-type transistor is electrically connected to a drain of the P-type transistor and a drain of the N-type transistor. 
   It is an advantage of the present invention that the present invention EEPROM device utilizes the P-type EEPROM cell to replace the N-type EEPROM cell. Furthermore, a P-type select gate transistor electrically connected to the program bit-line is utilized to perform the band-to-band tunneling induced hot-electrons program, and an N-type select gate transistor electrically connected to the erase bit-line is utilized to perform the Fowler-Nordheim tunneling erase. Owing to the considerable current generated by the band-to-band tunneling induced hot-electrons phenomenon, the injection of hot electrons caused by band-to-band tunneling mechanism is faster than that caused by Fowler-Nordheim tunneling mechanism. Therefore, program speed is greatly improved. Because of the obviously lifted program efficiency, the tunnel window, adapted in the prior art EEPROM device structure, can be replaced by a common tunnel oxide layer in the present invention EEPROM device structure. As a result, the problems of complex processing and raised cost incurred from misalignment, which usually occurs in the prior art, are avoided. The barrier to device shrinkage is not encountered. In addition, operation voltage is obviously lowered to expand the range of application under the industry stream of lightweight and small size. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional schematic diagram illustrating a prior art EEPROM device. 
       FIG. 2  is a layout diagram of the present invention EEPROM device. 
       FIG. 3  is a cross-sectional schematic diagram of the EEPROM device shown in FIG.  2 . 
       FIG. 4  is a cross-sectional schematic diagram along line  4 — 4 ″ of the EEPROM device shown in FIG.  2 . 
       FIG. 5  is a circuit diagram of the present invention EEPROM device. 
       FIG. 6  is an example table illustrating operation voltages of the present invention EEPROM device. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2  to  FIG. 4 ,  FIG. 2  is a layout diagram of the present invention EEPROM device  100 .  FIG. 3  is a cross-sectional schematic diagram of the EEPROM device  100  shown in FIG.  2 .  FIG. 4  is a cross-sectional schematic diagram along line  4 — 4 ″ of the EEPROM device  100  shown in FIG.  2 . As shown in FIG.  2  and  FIG. 3 , the present invention EEPROM device  100  is disposed on a semiconductor wafer  101 . The semiconductor wafer  101  comprises a P-type silicon substrate  102 , a deep N-well (DNW)  103  disposed in the P-type silicon substrate  102 , and a P-well (PW)  104  disposed in the deep N-well  103 . The EEPROM device  100  comprises a P-type memory cell  106 , an N-type select gate transistor  108 , and a P-type select gate transistor  112 . 
   The memory cell  106  comprises a source region  114  and a drain region  116  disposed on a surface of the deep N-well well  103 , and a channel region  118  between the source region  114  and the drain region  116 . Both the source region  114  and the drain region  116  are P-type heavy doped regions, and the source region  114  is electrically connected to a source line (SL). The memory cell  106  further comprises a tunnel oxide layer  122 , a floating gate  124 , a dielectric layer  126 , and a control gate  128 . The tunnel oxide layer  122  is disposed on a top surface  123  of the deep N-well  103 , and the tunnel oxide layer  122  covers the channel region  118 . The floating gate  124  is disposed on a surface of the tunnel oxide layer  122 . The dielectric layer  126  covers the floating gate  124 . The control gate  128  is disposed on a surface of the dielectric layer  126  and the surface of the tunnel oxide layer  122 . 
   The N-type select gate transistor  108  comprises a source region  132 , a drain region  134 , and a select gate (SG) 136 . The source region  132  of the N-type select gate transistor  108  is electrically connected to an erase bit line (Eb 1 ). The P-type select gate transistor  112  comprises a source region  138 , a drain region, and a select gate  142 . Since the drain region of the P-type select gate transistor  112  is overlapped with the drain region  116  of the memory cell  106 , it is not specially marked. The source region  138  of the P-type select gate transistor  112  is electrically connected to a program bit line (Pb 1 ). Because the select gates  136 ,  142  and the floating gate  124  in the memory cell  106  are formed by etching a same polysilicon layer, a polysilicon layer  143  is shown on top of each of the select gates  136 ,  142  in FIG.  3 . 
   When viewing along line  4 — 4 ″, the deep N-well  103  is disposed in the P-type silicon substrate  102 , and the P-well  104  is disposed in the deep N-well  103 . The tunnel oxide layer  122  is disposed on the P-type silicon substrate  102 . A polysilicon layer  125  used as the floating gate  124  is disposed on the tunnel oxide layer  122 . The dielectric layer  126  covers the polysilicon layer  125  used as the floating gate  124 . Another polysilicon layer  129  used as the control gate  128  is disposed on the dielectric layer  126  and the tunnel oxide layer  122 , as shown in FIG.  4 . In addition, the P-well  104  and the deep N-well  103  are isolated from each other by a shallow trench isolation  144 . 
   By cross-referring  FIG. 2 ,  FIG. 3 , and  FIG. 4 , it is very clear to see the polysilicon layers  146  disposed in pairs and in parallel, the heavy doped region used as the source region  132  of the N-type select gate transistor  108  disposed in the P-well  104 , the heavy doped region used as the source region  138  of the P-type select gate transistor  112  disposed in the deep N-well  103 , and the shallow trench isolations  144  used for isolating the P-well  104  and the deep N-well  103  in FIG.  2 . It is worth noting that the shallow trench isolations  144  are not shown in  FIG. 3  in order to prepare the drawing more conveniently. 
   Please refer to  FIG. 5 ,  FIG. 5  is a circuit diagram of the present invention EEPROM device  100 . As shown in  FIG. 5 , the present invention EEPROM device  100  comprises the P-type select gate transistor  112 , the N-type select gate transistor  108 , and the P-type memory cell  106 . The source region  138  of the P-type select gate transistor  138  is electrically connected to the program bit-line, and the source region  132  of the N-type select gate transistor  108  is electrically connected to the erase bit-line. The drain region  116  of the P-type memory cell  106  is electrically connected to the drain of the P-type select gate transistor  112  (overlapping with the drain region  116  of the memory cell  106 ) and the drain region  134  of the N-type select gate transistor. The P-type select gate transistor  112  and the N-type select gate transistor  108  are electrically connected through the select gates  136 ,  142  (please refer to FIG.  3 ), and the P-type memory cell  106  is simultaneously electrically connected to the P-type select gate transistor  112  and the N-type select gate transistor  108  due to the special layout shown in FIG.  2 . 
   Please refer to  FIG. 6 ,  FIG. 6  is an example table illustrating operation voltages of the present invention EEPROM device  100 . As shown from  FIG. 3  to  FIG. 6 , a first positive potential (such as +8 V) is supplied to the control gate  128  such that the positive voltage is capacitively coupled to the floating gate  124  to build an electric field that transverses the tunnel oxide layer  122 , when the present invention EEPROM device  100  performs programming. Then a negative potential (such as 8 V) is supplied to the select gate  142  of the P-type select gate transistor  112  to turn on the P-type select gate transistor  112 . When a negative program potential (such as 6 V) is supplied to the program bit-line, the program potential is therefore passed to the drain region  116  of the P-type memory cell  106  through the turned-on P-type select gate transistor  112 . Since a high positive potential difference exists between the control gate  128  and the drain region  116 , band-to-band tunneling (BTBT) phenomenon thus occurs to generate electron-hole pairs at a junction of the drain region  116  of the P-type memory cell  106 . Electrons in the electron-hole pairs are accelerated by the electric field in the depletion region to acquire sufficient energy to become hot electrons. The hot electrons then inject into the floating gate  124  to complete the band-to-band tunneling induced hot-electrons (BTBTIHE) program. 
   When the present invention EEPROM device  100  performs erasing, a second negative potential (such as 8 V) is supplied to the control gate  128  first. Then a second positive potential (such as +10 V) is supplied to the select gate  136  of the N-type select gate transistor  108  to turn on the N-type select gate transistor  108 . When a positive erase potential (such as +8 V) is supplied to the erase bit-line, the erase potential is passed to the drain region  116  of the P-type memory cell  106  through the turned on N-type select gate transistor  108 . Since a high negative potential difference exists between the control gate  128  and the drain region  116 , and-another high negative potential exists between the control gate  128  and the deep N-well  103  (the deep N-well  103  is grounded through a terminal), electrons stored in the floating gate  124  are affected by the electric field that transverses the tunnel oxide layer  122 . The electrons thus transverse the tunnel oxide layer  122  by Fowler-Nordheim tunneling mechanism to complete the Fowler-Nordheim erase. 
   Furthermore, when the present invention EEPROM device  100  performs reading, a third positive potential (such as +3.3 V) is supplied to the source line electrically connected to the source region  114  of the P-type memory cell  106 . Then a potential lower than the third positive potential (such as +1 V) is supplied to the program bit-line. At this time, since a potential difference exists between the source line and the program bit-line, electrons stored in the floating gate  124  will flow out to cause a current measurable at the terminal of the source line. Oppositely, if there are no electrons stored in the floating gate  124 , the current higher than a specific value cannot be measured at the terminal of the source line 
   The EEPROM device according to the present invention utilizes the P-type EEPROM cell to replace the prior art N-type EEPROM cell. Therefore, a P-type select gate transistor electrically connected to the program bit-line is utilized to perform the band-to-band tunneling induced hot-electrons program, and an N-type select gate transistor electrically connected to the erase bit-line is utilized to perform the Fowler-Nordheim tunneling erase. Since the band-to-band tunneling induced hot-electrons phenomenon can generate a considerable current, the injection of hot electrons caused by band-to-band tunneling mechanism is faster than that caused by Fowler-Nordheim tunneling mechanism. Program speed and program efficiency are thus greatly improved to eliminate the need of the tunnel window utilized in the prior art EEPROM structure. When applying the present invention structure to a practical production line, byte-addressable EEPROM products having high programming speed, low operation voltage, high reliability, and small size are fabricated once the high gate coupling ratio and the high quality of the tunnel oxide are maintained. 
   Compared to the prior art EEPROM device and structure and method of operation, the present invention EEPROM device utilizes the P-type EEPROM cell to replace the N-type EEPROM cell. In addition, a P-type select gate transistor electrically connected to the program bit-line is utilized to perform the band-to-band tunneling induced hot-electrons program, and an N-type select gate transistor electrically connected to the erase bit-line is utilized to perform the Fowler-Nordheim tunneling erase. Due to the considerable current generated by the band-to-band tunneling induced hot-electrons phenomenon, the injection of hot electrons caused by band-to-band tunneling mechanism is faster than that caused by Fowler-Nordheim tunneling mechanism. Program speed is therefore greatly improved. Because of the obviously lifted program efficiency, the tunnel window, adapted in the prior art EEPROM device structure, can be replaced by a common tunnel oxide layer in the present invention EEPROM device structure. As a result, the problems of complex processing and raised cost incurred from misalignment, which usually occurs in the prior art, are avoided. The barrier to device shrinkage is not encountered. In addition, operation voltage is obviously lowered to expand the range of applicability under the industry stream of lightweight and small size. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.