Patent Publication Number: US-2012040504-A1

Title: Method for integrating dram and nvm

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for fabricating a memory, particularly to a method for integrating DRAM and NVM. 
     2. Description of the Related Art 
     A system usually needs RAM (Random Access Memory), which can be read and written rapidly, and ROM (Read Only Memory), which can keep data when power is removed. RAM includes DRAM (Dynamic RAM) and SRAM (Static RAM). ROM includes Flash Memory and EEPROM (Electrically Erasable Programmable Read Only Memory). Both Flash Memory and EEPROM are nonvolatile memories (NVM), which are electrically erasable and programmable and able to keep data when power is removed. Therefore, Flash Memory and EEPROM are widely used in various electronic products. 
     Recently, the memory used in a system is required to have a greater capacity with a lower cost. In such a requirement, various fabrication processes are developed to integrate a high-capacity DRAM and a flash memory/or EEPROM in a chip. However, the fabrication process of integrating DRAM and NVM are very complicated and expensive. Further, too much time and money is usually spent in developing the abovementioned process. For example, in the MCP (Multiple Chip Package) technology of mobile phones, a flash chip and a DRAM chip are packaged in an encapsulation with two sets of I/O pads equipped therein. Such a technology has higher complexity and higher cost. Further, two independent IC chips consume more power. Besides, MCP booting takes too much time because data must be transferred from ROM to DRAM. 
     Accordingly, the present invention proposes a method for integrating DRAM and NVM to overcome the abovementioned problems. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a method for integrating DRAM (Dynamic Random Access Memory) and NVM (Non-Volatile Memory), which is based on the DRAM process, whereby are reduced the fabrication cost, package cost and power consumption, and whereby is increased the transmission speed. 
     To achieve the abovementioned objective, the present invention proposes a method for integrating DRAM and NVM, which comprises steps: providing a DRAM semiconductor substrate; sequentially forming on a portion of the surface of the DRAM semiconductor substrate a first gate insulation layer and a first gate layer functioning as a floating gate; implanting ion into the regions of the semiconductor substrate, which are at two sides of the first gate insulation layer, to form two heavily-doped areas, which are adjacent to the first gate insulation layer and respectively function as the drain and the source; sequentially forming over the first gate layer a second gate insulation layer and a second gate layer functioning as a control gate. 
     Below, the embodiments are described in detail in cooperation with the drawings to make easily understood the technical contents, characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a )- 1 ( d ) are sectional views schematically showing the steps of a method for integrating DRAM and NVM according to a first embodiment of the present invention; 
         FIG. 2  is a sectional view schematically showing the integration of a stack-type capacitor structure and NVM according to the first embodiment of the present invention; 
         FIG. 3  is a sectional view schematically showing the integration of a trench-type capacitor structure and NVM according to the first embodiment of the present invention; 
         FIG. 4  is a sectional view schematically showing the operation of NVM according to the first embodiment of the present invention; 
         FIGS. 5(   a )- 5 ( c ) are sectional views schematically showing the steps of a method for integrating DRAM and NVM according to a second embodiment of the present invention; 
         FIG. 6  is a sectional view schematically showing the integration of a stack-type capacitor structure and NVM according to the second embodiment of the present invention; 
         FIG. 7  is a sectional view schematically showing the integration of a trench-type capacitor structure and NVM according to the first embodiment of the present invention; and 
         FIG. 8  is a sectional view schematically showing the operation of NVM according to the second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally to speak, DRAM and NVM are hard to be integrated into the same chip because the fabrication processes thereof are different. However, many applications use DRAM and NVM (EEPROM or FLASH) simultaneously. Especially in a handheld electronic product, DRAM and FLASH are enveloped in the same package, which increases the area and package cost but decreases the transmission speed. 
     To overcome the abovementioned problems, the present invention proposes a method for integrating DRAM and NVM. Refer to  FIGS. 1(   a )- 1 ( d ) for a first embodiment of the present invention. Firstly, provide an n-type semiconductor substrate  10  functioning as a DRAM semiconductor substrate, as shown in  FIG. 1(   a ). Next, form a p-type well  12  in the n-type semiconductor substrate  10 , as shown in  FIG. 1(   b ). Next, sequentially form a first gate insulation layer  14  and a first gate layer  16  on the surface of the p-type well  12 , as shown in  FIG. 1(   c ). The first gate insulation layer  14  is made of silicon dioxide. The first gate layer  16  is made of a polysilicon material. Next, implant n-type ion into the regions of the p-type well  12 , which are at two sides of the first gate insulation layer  14 , to form two n-type heavily-doped areas  18  and  20  that are adjacent to the first gate insulation layer  14  and respectively function as the source and the drain, as shown in  FIG. 1(   d ). Next, sequentially form over the first gate layer  16  a second gate insulation layer  22  and a second gate layer  24  functioning as a control gate. The second gate insulation layer  22  is thicker than the first gate insulation layer  14 . The second gate insulation layer  22  is an ONO (Oxide-Nitride-Oxide) layer or a TEOS (tetraethyl-ortho-silicate) layer. The second gate  24  is made of a polysilicon material. 
     In  FIG. 1(   d ), the two heavily-doped n-type areas  18  and  20  may be firstly formed in the p-type well  12 , and then the second gate insulation layer  22  and the second gate layer  24  are sequentially formed over the first gate layer  16 . Alternatively, the second gate insulation layer  22  and the second gate layer  24  are sequentially formed over the first gate layer  16  before the two heavily-doped n-type areas  18  and  20  are formed in the p-type well  12 . In the two methods, the floating gate and the control gate are exempted from line-to-line alignment. Therefore, the layers of the photomasks, the complexity of fabrication and the cost of fabrication are greatly reduced. 
     The fabrication of NVM according to the first embodiment of the present invention has been completed in  FIG. 1(   d ). Refer to  FIG. 2  for the integration of NVM and DRAM, wherein a stack-type capacitor structure  26  is formed on the n-type semiconductor substrate  10 . If the DRAM has a trench-type capacitor structure, form a trench-type capacitor structure  28  in the n-type semiconductor substrate  10  after the step of  FIG. 1(   a ). Then, sequentially undertake the steps of from  FIG. 1(   b ) to  FIG. 1(   d ) to form the structure shown in  FIG. 3 . The present invention embeds EEPROM into DRAM, whereby is reduced the package cost, and whereby is saved one set of I/O pad. Thus, data lines can be widened to increase the transmission speed and decrease the power consumption. 
     Below is described the operation of a nonvolatile memory fabricated according to the first embodiment of the present invention. Refer to  FIG. 4 . In operation, a drain voltage V D , a source voltage V S , a gate voltage V G  and a well voltage V well  are respectively applied to the above-mentioned drain, source, control gate and the p-type well  12 . In a write activity, the abovementioned voltages satisfy the following conditions: V well  is grounded; V D &gt;V S &gt;0, and V G &gt;V S &gt;0. The present invention adopts a hot electron program method and needn&#39;t use a voltage higher than 8V. Thus, the layers of photomasks are greatly decreased. In an erase activity, the abovementioned voltages satisfy the following conditions: V well  is grounded; V D &gt;&gt;V S ≧0, and V G ≧V S ≧0. The present invention adopts a hot hole erase method and needn&#39;t use a voltage higher than 8V. Thus, the layers of photomasks are greatly decreased. When data is transmitted from EEPROM to DRAM, signals needn&#39;t pass through the I/O pad, whereby is accelerated the transmission speed, and whereby the data lines can be widened. 
     In the above description of the first embodiment, an n-type transistor is used to exemplify the first embodiment of the present invention. The present invention also applies to a p-type transistor, wherein the n-type semiconductor substrate  10 , the p-type well  12  and the n-type heavily-doped areas  18  and  20  are respectively replaced by a p-type semiconductor substrate, an n-type well and two p-type heavily-doped areas. 
     In the operation of a p-type transistor of the first embodiment, a drain voltage V D , a source voltage V S , a gate voltage V G  and a well voltage V well  are respectively applied to the drain, source, control gate and the n-type well. In a write activity, the abovementioned voltages satisfy the following conditions: V well &gt;V S &gt;V D , and V well &gt;V S &gt;V G . In an erase activity, the abovementioned voltages satisfy the following conditions: 
         V   well   =V   S   ≧V   G   &gt;V   D . 
     Refer to  FIGS. 5(   a )- 5 ( c ) for a second embodiment of the present invention. The second embodiment is different from the first embodiment in that no well is used in the second embodiment. Firstly, provide a p-type semiconductor substrate  30 , as shown in  FIG. 5(   a ). Next, form a first gate insulation layer  14  and a first gate layer  16  on the p-type semiconductor substrate  30 , as shown in  FIG. 5(   b ). The first gate insulation layer  14  is made of silicon dioxide. The gate layer  16  is made of a polysilicon material. Next, implant n-type ion into the regions of the p-type semiconductor substrate  30 , which are at two sides of the first gate insulation layer  14 , to form two n-type heavily-doped areas  18  and  20  that are adjacent to the first gate insulation layer  14  and respectively function as the source and the drain, as shown in  FIG. 5(   c ). Next, sequentially form over the first gate layer  16  a second gate insulation layer  22  and a second gate layer  24  functioning as a control gate. The second gate insulation layer  22  is thicker than the first gate insulation layer  14 . The second gate insulation layer  22  is an ONO (Oxide-Nitride-Oxide) layer or a TEOS (tetraethyl-ortho-silicate) layer. The second gate layer  24  is made of a polysilicon material. 
     In  FIG. 5(   c ), the two heavily-doped n-type areas  18  and  20  may be firstly formed in the p-type semiconductor substrate  30 , and then the second gate insulation layer  22  and the second gate layer  24  are sequentially formed over the first gate layer  16 . Alternatively, the second gate insulation layer  22  and the second gate layer  24  are sequentially formed over the first gate layer  16  before the two heavily-doped n-type areas  18  and  20  are formed in the p-type semiconductor substrate  30 . The objectives and efficacies of the abovementioned two steps are similar to those described in the first embodiments and will not repeat here. 
     The fabrication of NVM according to the second embodiment of the present invention has been completed in  FIG. 5(   c ). Refer to  FIG. 6  for the integration of NVM and DRAM, wherein a stack-type capacitor structure  26  is formed on the p-type semiconductor substrate  30 . If the DRAM has a trench-type capacitor structure, form a trench-type capacitor structure  28  in the p-type semiconductor substrate  30  after the step of  FIG. 5(   a ). Then, sequentially undertake the steps of  FIG. 4(   b ) and  FIG. 4(   c ) to form the structure shown in  FIG. 7 . The objectives and efficacies of integrating NVM and DRAM in the second embodiment are the same as those described in the first embodiment and will not repeat here. 
     The second embodiment is different from the first embodiment only in that no well is formed in the second embodiment. In the operation of the second embodiment, a substrate voltage V sub  applying to the p-type semiconductor substrate  30  replaces the well voltage V well  in the first embodiment to achieve the same function. Refer to  FIG. 8 . In the operation of a nonvolatile memory fabricated according to the second embodiment of the present invention, a drain voltage V D , a source voltage V S , a gate voltage V G  and a substrate voltage V sub  are respectively applied to the abovementioned drain, source, control gate and p-type semiconductor substrate  30 . In a write activity, the abovementioned voltages satisfy the following conditions: V sub  is grounded; V D &gt;V S &gt;0, and V G &gt;V S &gt;0. In an erase activity, the abovementioned voltages satisfy the following conditions: V sub  is grounded; V D &gt;&gt;V S &gt;0, and V G  V S &gt;0. 
     In the above description of the second embodiment, an n-type transistor is used to exemplify the second embodiment of the present invention. The second embodiment of the present invention also applies to a p-type transistor, wherein the p-type semiconductor substrate  30  and the n-type heavily-doped areas  18  and  20  are respectively replaced by an n-type semiconductor substrate and two p-type heavily-doped areas. 
     In the operation of a p-type transistor of the second embodiment, a drain voltage V D , a source voltage V S , a gate voltage V G  and a substrate voltage V sub  are respectively applied to the drain, source, control gate and n-type semiconductor substrate. In a write activity, the abovementioned voltages satisfy the following conditions: V sub &gt;V S &gt;V D , and V sub &gt;V S &gt;V G . In an erase activity, the abovementioned voltages satisfy the following conditions: V sub =V S ≧V G &gt;V D . 
     In conclusion, the present invention not only reduces the costs of fabrication and package but also increases the transmission speed of signals. 
     The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the structures, characteristics or spirit disclosed in the specification is to be also included within the scope of the present invention, which is based on the claims stated below.