Abstract:
A semiconductor memory device with a buried drain is provided. The device comprises a semiconductor substrate ( 107 ); one drain region ( 108 ) of a first doping type; two source regions ( 101   a,    101   b ) of a second doping type; and a stacked gate provided on the semiconductor substrate for capturing electrons. A memory array formed by a plurality of semiconductor memory devices and a manufacturing method thereof are also provided. The semiconductor memory device has the advantages of small cell area, simple manufacturing process and the like. The manufacturing cost of the memory device is reduced and the storing density of the memory device is increased.

Description:
FIELD OF INVENTION 
     The present invention refers to a self-aligned vertical non-volatile semiconductor memory device and a contactless array composed by these memory devices. A method of forming the same is also disclosed in the present invention. 
     BACKGROUND OF THE INVENTION 
     Semiconductor memory chips are widely used in many fields such as industrial control and consuming electronics. High integration density, low power consumption, and high speed are essential properties of these memory chips. To enhance the storage capability of a memory chip in the same die area, generally there are two methods. One method is to scale down the feature size of the semiconductor memory cell. The second method is to get a better design by improving the architecture or applying novel devices. Due to the MOSFET-based structure in the electrically erasable programmable read only memory (EEPROM) and nitride read only memory (NROM), the short channel effects of MOSFET become handicaps as the feature size is scaled down in these memory cells. Therefore, a novel device structure with suppressed short channel effects is preferred. According to this requirement, a self-aligned vertical Tunneling-Field-Effect-Transistor Read only memory (TROM) is proposed in the present invention. Because of the suppressed short channel effects in a Tunneling-Field-Effect-Transistor (TFET) structure, the TROM can be scaled down to 20 nm gate length and the leakage current remains low. 
     The density of a memory chip can be improved by using a better design. An example is the application of NROM. The NROM has 2-bit storage in one memory cell. Therefore, the NROM has a higher storage density than that of EEPROM. Similar to NROM, the presently invented self-aligned vertical TROM cell has the 2-bit storage capability. The density of the vertical TROM will be higher than that of EEPROM. 
     The memory cells array is generally arranged in a matrix layout to realize the mass storage. There are two architectures in the EEPROM flash memory matrix. One architecture is the NAND architecture and the other is the NOR architecture, Using the NAND architecture, the memory density is higher than that of NOR architecture because that the source and drain contact pads are not necessary in the NAND architecture. Because of the self-aligned vertical structure of a memory cell, a new architecture that applies both the NOR architecture and the NAND architecture becomes possible in the present invention. The advantages of both NAND and NOR architectures are realized in the combined architecture. 
     BRIEF SUMMARY OF THE INVENTION 
     In the present invention, the inventors intend to enhance the density of the storage memory chip and lower the stand-by and the programming power consumption of a single memory device. The self-aligned vertical TROM and its contactless array are proposed in the present invention. The method of forming the device is also disclosed at the same time, 
     A semiconductor memory device with a buried drain, compromising:
         a semiconductor substrate,   a drain of a first conductivity,   two sources of a second conductivity,   and a gate stack with charge trapping layer to locally trap electrons.   said substrate is intrinsic and said semiconductor substrate is lightly doped.
 
a gate with a stack is composed by:
   a first dielectric layer a width of bandgap covering the channel region,   a second dielectric layer disposed over the said first dielectric layer and with narrower bandgap than that of the said first dielectric layer,   and a third dielectric layer disposed over the said second dielectric layer and with wider bandgap than that of the said second dielectric layer,   a conductive gate material disposed over the said third dielectric layer.
 
said substrate is a part of an Si wafer, an SiGe wafer or a strained Si wafer.
       

     A semiconductor memory string with a buried drain compromising: plural semiconductor memory string configured by the semiconductor memory devices described in claim  1 , and two NMOSFETs located at the both end of the string.
         the source electrodes of the said semiconductor memory devices are connected in series.       

     A TROM memory array, comprising a silicon substrate, and at least one semiconductor memory string.
         the drain is buried into the said semiconductor substrate and works as buried bitline.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  Cross-sectional view of a self-aligned vertical TROM. 
         FIG. 2  Electrical symbol for a self-aligned vertical TROM. 
         FIG. 3  An example for storing and accessing two bits in a TROM. 
         FIG. 4  A 16-bit string composed by eight TROM cells. 
         FIG. 5  The 8×8 memory array configured by TROM strings. This array is able to store 16×8 bit data. 
         FIG. 6  Cross-section of a TROM array. 
         FIG. 7  Top-down view of a TFET array. 
         FIG. 8  Top-down view of a self-aligned vertical TROM storage cell. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A single vertical TROM device is shown in  FIG. 1 , left. This device is built in a semiconductor substrate ( 107 ). The buried n +  doped region ( 108 ) acts as the drain and the p +  doped region acts as the source ( 101   a ,  101   b ). Over the channel region ( 106 ) between two p +  doped source ( 101   a. ,  101   b ), a gate stack with a first dielectric ( 104 ), a second dielectric ( 103 ), a third dielectric ( 102 ), and a gate metal ( 105 ) is formed. 
     To clearly illustrate the working principle, this cell is divided into two memory devices in  FIG. 1 , right. Both the left half cell and the right half cell can store one-bit information. As a positive gate voltage is applied, an electron channel ( 106 ) is created under the said first dielectric ( 104 ). As a result, a p + /n +  Esaki-tunnel junctions is formed at the location ( 109 ). When the left p +  region ( 101   a ) is grounded and the n +  region is positive biased (e.g. 2V), electrons will tunnel from the valence band to the conduction band at the left p + /n +  Esaki-tunnel junction. Depending on the gate voltage, a portion of the hot electrons at the tunneling junction can be injected into the first gate dielectric ( 104 ) and locally trapped in the second gate dielectric ( 103 ) like a ONO stack [6]. The trapped charges will change the threshold voltage and the left part of this memory cell is programmed. In the same way, the right part of the memory device can also be programmed. Therefore, two-bit information can be stored in one memory cell. 
     The electrical symbol for this self-aligned vertical TROM is shown in  FIG. 2 . There are two storage units in a vertical TROM cell. The left TROM storage unit is composed by the left source (S L ), the common drain (D), and the common gate (G). The right TROM unit consists of the right source (S R ), the common drain, and the common gate as well. As the left TROM unit is activated the positive gate voltage, electrons will tunnel from the left source (S L ) to the channel. Then, these electrons are collected by the common drain. As the right TROM unit is activated with the positive gate voltage, the electrons tunnel from the right source (S R ) to the common drain with a positive gate voltage. 
     The erase function is realized by the injection of channel hot holes. The p +  doped source ( 101   a  or  101   b ) is positive biased and the gate ( 105 ) is negative biased during the erasing process. Hot holes will be generated and the information stored can be erased. 
     Several TROM cells are shown in  FIG. 3  to explain how to access two bits from a TROM cell. There are three TROM cells in  FIG. 3 . They are TROM (n−1) , TROM (n) , and TROM (n+1) . 
     The left source ( 301 ) of TROM (n−1)  is grounded during reading. When the channel ( 302 ) of TROM (n−1)  is conductive and other channels ( 305 , 308 ) are not conductive, the left part of TFET (n)  is activated and the right part of TFET (n)  is deactivated. The n +  doped common drain is positive-biased. When the gate ( 306 ) is positive-biased, electrons flow from the p +  source ( 304 ) to the n +  drain by tunneling mechanism. The tunneling current density depends on the density and location of trapped charges in the said second dielectric, named the charge-trapping layer. In the same manner, the right part TROM n  can be activated. Thus, the stored 2-bit information can be recognized separately. 
     Based on the TROM cell shown in  FIG. 2  and  FIG. 3 , an 8-TROM string is composed as an example and shown in  FIG. 4 . There are two NMOSFETs at the ends of the memory string. The source of the left NMOS ( 400   b ) and the source of the right NMOS ( 409   c ) are grounded. The bit-line ( 410 ) is positive-biased. 
     As explained by  FIG. 3 , the 2-bit information stored in a TROM, is accessed in two directions. In  FIG. 4 , when the gate of the left NMOS ( 400   a ) is positive-biased while the gate of the right NMOS ( 409   a ) is grounded, the string is accessed from left to right in sequence, vice verse. For example, the TROM source  401   b  will be grounded when the gate  400   a  is positively-biased. A vertical TROM cell composed by gate  401   a , drain  410 , and source  401   b  is then active and the information stored inside it can be accessed. The stored information can be read-out by sensing the current flowing through the common drain. Then, the gate  401   a  is negative-biased so that the ground-voltage will be transferred to the source of the next vertical TROM ( 402   b ). The TROM cell composed by gate  402   a , drain  410 , and source  402   b  is active and the information can be read or written. Then the gate  402   a  is turned to negative voltage. The similar procedure will continue and the all the left part of each TROM can be accessed. During this process, the right side NMOS  409  is switched off. The current in the bit-line ( 410 ) can be detected and the state of each TROM is able to be recognized. To access the right part of each TROM, the left side NMOS  400  is switched off while the right side NMOS  409  is switch on. Then the TROM cells are accessed from right to left sequentially. 
     The memory string shown in  FIG. 4  realizes 16 bit storage using 8 TROM cells. The similar string with n (n=1, 2, 3 . . . ) TROM cells can realize 2n bit storage. In this string shown in  FIG. 4 , the TROM sources are connected using a NAND architecture. However, the TROM units are connected using a NOR architecture. Because of the vertical structure, this NOR architecture doesn&#39;t cost additional area for contact. The combination of NAND and NOR architecture takes the advantage of. NAND for fast reading and high density. It also takes the advantage of NOR architecture for the fast single bit access. 
     The most particular property of this TROM storage string during operation is that the electrons are injected from the p +  region to the channel region, and then collected by the n +  common drain. The current flows from the n +  doped drain to the p +  doped source. That means the electrons are injected into the reverse-biased p-i-n diode. The detection of this electron injection is similar to the detection of photo-generated electrons. As proved by the high frequency applications as photo detectors, the reverse-biased p-i-n diode has a fast detection of the non-equilibrium carriers. That makes TROM a memory device with very high detection speed. 
     Using the memory string shown in  FIG. 4 , the layout of a TROM array is proposed and shown in  FIG. 5 . The word-line connects the gates in the same column and then connected to the X MUX/DEC. The bit-line and the source of NMOS are connected to the Y MUX/DEC. The peripheral logic circuits are needed to realize the necessary algorithms of X and Y MUX/DEC for reading and writing the core memory array.  FIG. 5  shows an 8×8 TROM array having a 128 bit capability. The n×n TROM array can be configured using the same principle. Because of the low power consumption property of TROM device, the TROM array is able to do parallel read and write. That will enhance the operating speed of this array. 
     The merits of implementing the present invention is summarized as the following: 
     Firstly, as shown in  FIG. 1 , a p-i-n structure is designed in this device, as suppresses the short channel effect of TFET. The simulation study shows that TFET has better scalability than MOSFET. That means that higher density memory can be realized by using TFET device than other types of MOSFET-based memory such as NROM. 
     Secondly, TROM has a low off leakage current, as will enable low standby power consumption. Further, TROM has a high write efficiency. Simulation shows that the TROM device can be programmed with very small drain current. That means that the TROM device has low program power consumption. Thus, TROM is attractive for low power applications. On the contrary, NROM needs higher drain current to program a memory cell and its power consumption is higher than the TROM memory device proposed in the present invention. 
     Thirdly, the proposed TROM can store two bits inside a single memory device, the storage ability is doubled. As a result, the area for storing a single bit is reduced. The drain is buried into the substrate in a TROM device. The TROM cell size is further reduced compared to the conventional EEPROM and NROM memory devices. 
     Finally, TROM combines the structure of NAND and NOR and each bit can be accessed directly and promptly. The tunneling mechanism will also enhance the reading speed. 
     In the following the manufacturing method of forming the disclosed TROM array will be described. 
     The contactless TROM array can be fabricated using the standard CMOS technology. FIG.  6 . a  shows the cross-section along the bit-line of a TROM array. The p +  regions can be formed using the self-aligned process. A buried n +  common drain can be formed by ion implantation. The common drain can be separated by Shallow Trench Isolation (STI). The cross-section along the word-line of a TROM array with separated common drain can be seen in FIG.  6 . b . The common drain can be a plate as well. FIG.  6 . c  shows the cross-section along the word-line of a TROM array with a common drain plate. 
     The top-down views of the TROM array during the fabrication processes are shown in  FIG. 7 . The substrate ( 702 ) is n doped or intrinsic. In step a, the STI is formed and the active area is opened. Then an n +  buried layer ( 701 ) is formed (preferably by ion implantation). The bit-line can be separated by the shallow trench isolation. Next, in step b, the word-line gate stack ( 703 ) is formed and patterned. In step c, a p-type ion implantation will result in the self-aligned p +  pads. To adjust the threshold voltage of the PMOS, an additional threshold voltage adjustment process may be necessary. The following processes such as passivation, metallization, and interconnection are the same to the conventional VLSI processes. 
     As can be seen in  FIG. 8 , a TROM cell realizes 2-bit storage using 4F 2  area. The high density memory array can be fabricated.