Patent Publication Number: US-6906354-B2

Title: T-RAM cell having a buried vertical thyristor and a pseudo-TFT transfer gate and method for fabricating the same

Description:
FIELD OF THE INVENTION 
   This invention relates to the field of integrated circuit (IC) design. Specifically, the invention relates to a Thyristor Random Access Memory (T-RAM) cell and method for fabricating the same. More specifically, the invention relates to a T-RAM cell having a buried vertical thyristor, a stacked pseudo-TFT transfer gate and a planar cell structure, and method for fabricating the same. 
   BACKGROUND OF THE INVENTION 
   A low-power, high-speed and high-density negative differential resistance (NDR) based (NDR-based) SRAM cell which can provide DRAM-like densities at SRAM-like speeds has been proposed by Farid Nemati and James D. Plummer in “A Novel High Density, Low Voltage SRAM Cell with a Vertical NDR Device,” 1998 Symposium on VLSI Technology Digest of Technical Papers, IEEE, pages 66-67, 1998. 
   The memory device structure is shown by FIG.  1  and is designated by reference numeral  10 ; the memory device structure is called a Thyristor-based Random Access Memory (T-RAM) cell. The T-RAM device or memory cell  10  consists of a thin vertical pnpn thyristor  12  with a surrounding nMOS gate  14  as the bistable element and a planar nMOSFET as the access transistor  16 . The circuit schematic of the T-RAM cell  10  is shown by FIG.  2 . 
   To access the T-RAM cell  10 , two wordlines are necessary. The first wordline WL 1  is used to control an access gate of the transfer nMOSFET device  16 , while the second wordline WL 2  is the surrounding nMOS gate  14  which is used to control the switch of the vertical pnpn thyristor  12 . The thyristor  12  is connected to a reference voltage Vref. The second wordline WL 2  improves the switching speed of the thyristor  12  from 40 ns to 4 ns with a switching voltage. A bitline BL connects the T-RAM cell  10  to a sense amplifier for reading and writing data from and to the T-RAM cell  10 . The T-RAM cell  10  exhibits a very low standby current in the range of 10 pA. 
   When writing a “high”, the bitline BL is set at low, and both wordlines WL 1 , WL 2  are switched on. At this moment, the thyristor  12  behaves like a forward biased pn diode. After a write operation, both gates are shut off, and a “high” state is stored in the thyristor  12 . In a read operation, only the first wordline WL 1  is activated, a large “on” current will read on the bitline BL through the access gate. When writing a “low”, the bitline BL is set at “high” state, and both wordlines WL 1 , WL 2  are switched on. At this moment, the thyristor  12  behaves like a reverse biased diode. After the write operation, both gates are shut off, and a “low” state is stored in the thyristor  12 . Similarly, in a consequence read, a very low current will be detected on the bitline BL. Further details of the operation of the T-RAM cell  10  and its gate-assisted switching are described in Nemati et al.; the contents of which are incorporated herein by reference. 
   A T-RAM array having a plurality of T-RAM cells  10  has demonstrated a density equivalent to that of DRAM arrays and a speed equivalent to that of SRAM arrays. Hence, the T-RAM array provides advantages afforded by both SRAM and DRAM arrays. These advantages make T-RAM an attractive choice for future generations of high speed, low-voltage, and high-density memories and ASICs. 
   However, there are several drawbacks of the T-RAM cell  10 . First, there is the requirement of forming the thyristor  12  having a vertical pillar on a substrate during a fabrication process. Difficulties arise in controlling the dimensions of the vertical pillar and reproducing these dimensions for each T-RAM cell  10  in the T-RAM array. Second, due to the existence of a vertical thyristor  12  in each T-RAM cell  10 , each T-RAM cell  10  is not planar and therefore difficult to scale. Third, it is difficult to control the dimension while forming the surrounding base gate around the base of each vertical thyristor  12 . Fourth, each T-RAM cell is fabricated prior to or after fabricating any other devices, such as p-MOS and n-MOS support devices (i.e., sense amplifiers, wordline drivers, column and row decoders, etc.), which results in extra fabrication steps, thereby increasing thermal budget and manufacturing cost. Finally, due to these drawbacks, the resulting T-RAM cell  10  cannot be smaller than 8F 2  and the cost of fabricating a T-RAM array is high. 
   SUMMARY 
   An aspect of the present invention is to provide a T-RAM array having a planar cell structure for overcoming the disadvantages of the prior art. 
   Another aspect of the present invention is to provide a T-RAM array having a plurality of T-RAM cells, wherein each of the plurality of T-RAM cells has a planar cell structure, a buried vertical thyristor and a horizontally stacked pseudo-TFT (Thin-Film Transistor) transfer gate. 
   Also, another aspect of the present invention is to provide a memory system having a plurality of T-RAM cells arranged in an array, wherein each of the plurality of T-RAM cells has a planar cell structure, a buried vertical thyristor and a horizontally stacked pseudo-TFT transfer gate. 
   Further, another aspect of the present invention is to provide a method for fabricating a high-density, high-yield and low-cost T-RAM array having a plurality of T-RAM cells and a planar cell structure on a SOI substrate. Each of the plurality of T-RAM cells has a planar cell structure, a buried vertical thyristor and a horizontally stacked pseudo transfer gate. 
   Another aspect of the present invention is to provide a method for fabricating a T-RAM array which improves performance and yield, and reduces cost and thermal budget. 
   Other aspects of the present invention is a method for fabricating a surrounded base gate of a buried vertical thyristor of a T-RAM cell and a method of making contact with a horizontally stacked pseudo-TFT transfer gate. Another aspect of the present invention is a method of forming a high-quality, self-aligned pseudo-TFT transfer gate of a T-RAM cell. 
   Accordingly, in an embodiment of the present invention, a T-RAM array is presented having a planar cell structure and a plurality of T-RAM cells where each T-RAM cell is planar and has a buried vertical thyristor and a horizontally stacked pseudo-TFT transfer gate. The inventive structure of each T-RAM cell results in higher performance at low voltage, e.g., Vdd=1V. 
   In another embodiment of the present invention, a memory system is presented having a plurality of T-RAM cells arranged in an array. Each of the T-RAM cells in the array has a planar cell structure, a buried vertical thyristor and a horizontally stacked pseudo-TFT transfer gate. 
   Further still, in another embodiment of the present invention, a method is presented for fabricating a T-RAM array having a planar cell structure. Each of the T-RAM cells in the T-RAM array has a buried vertical thyristor and a horizontally stacked pseudo-TFT transfer gate. The method entails forming a surrounded base gate of the buried vertical thyristor and making contact with the horizontally stacked pseudo-TFT transfer gate. The method also entails forming a high-quality, self-aligned stacked pseudo-TFT transfer gate for each of the T-RAM cells. Preferably, the T-RAM array is built on a semiconductor silicon-on-insulator (SOI) wafer to reduce junction capacitance and improve scalability. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  illustrates the device structure of a prior art T-RAM cell; 
       FIG. 2  is a circuit diagram of the prior art T-RAM cell; 
       FIG. 3  is a cross-sectional view of a portion of a semiconductor silicon-on-insulator (SOI) wafer having a buried oxide layer and a series of layers for fabricating T-RAM cells according to the present invention; 
       FIGS. 4-11  are cross-sectional views illustrating a preferred process for fabricating a T-RAM cell according to the present invention; 
       FIG. 12  is a top phantom view of a portion of a T-RAM array showing the T-RAM cell fabricated according to the present invention; 
       FIG. 13  is a cross-sectional view of a T-RAM cell according to the present invention; and 
       FIG. 14  is a top phantom view of a portion of the T-RAM array showing a plurality of T-RAM cells fabricated according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a T-RAM array having a planar cell structure and a plurality of T-RAM cells. Each of the T-RAM cells of the inventive T-RAM array is planar and includes a buried vertical thyristor and a horizontally stacked pseudo-TFT transfer gate. Hence, the T-RAM array of the present invention provides for less control during manufacturing, and is planar and more scalable than prior art T-RAM arrays. The present invention also provides a preferred method for fabricating the T-RAM array to reduce cost and thermal budget, while increasing performance, density and yield. 
   Unlike a DRAM cell, the state of a T-RAM cell does not depend on the amount of charge stored in a capacitor, rather it depends on the internal programming state of the thyristor. In order to maintain the internal programming state of the thyristor, a low DC current is permitted to constantly flow through the thyristor. Similar to an SRAM cell, when the state of the T-RAM cell is stored and latched, a small leakage current flows from Vdd to ground via off-state devices. For an SRAM cell with a resistive load or TFT load, the DC leakage current is required in order to maintain the state of the SRAM cell. 
   In the present invention, in order to save area, a pseudo-TFT is used as the transfer gate for the inventive T-RAM cell. The pseudo-TFT transfer gate can be fabricated on top of a thyristor of the inventive T-RAM cell which causes the resulting cell area to be very small. The off-state leakage current of the pseudo-TFT transfer gate may be higher than that of a conventional device. However, it is acceptable to be implemented for the inventive T-RAM array disclosed herein, if performance of the T-RAM array is increased and size of the TRAM array is decreased, for a slightly higher standby power. Nonetheless, such a standby current exists in the thyristor of a T-RAM cell whether or not the TFT is used as the transfer gate. 
   The stacked pseudo-TFT transfer gate of the inventive T-RAM is fabricated using a lateral overgrowth epi technique via a single crystal seed area. The quality and performance of the stacked pseudo-TFT transfer gate is significantly better than that of a conventional TFT device. 
     FIG. 3  is a cross-sectional view of a portion of a semiconductor silicon-on-insulator (SOI) wafer having a buried oxide layer and a series of layers for fabricating T-RAM cells according to the present invention. The wafer is designated by reference numeral  100 . It is contemplated that other types of semiconductor wafers besides semiconductor SOI wafers, such as semiconductor bulk wafers, can be used for fabricating T-RAM cells according to the present invention. 
   With reference to  FIG. 3 , a semiconductor SOI wafer  100  having a buried oxide layer  102  and a p+ layer  104  is used as the substrate to form the T-RAM cells. The p+ layer  104  is preferably a p+ silicon layer doped with a dosage of between 2E14 cm 2  to 8E14/cm 2  of boron. The p+ layer  104  will be used to form the p terminal of the thyristor which will be tied to a reference voltage (about one volt) during operation. The p+ layer  104  will also be used as the seed for epitaxial growth as described below. 
   Over the SOI wafer  100 , a predetermined thickness, approximately 4000 Å, of nitride and oxide are deposited to form a nitride-oxide layer  106  having a nitride layer  106   a , approximately 1000 Å, and an oxide layer  106   b , approximately 3000 Å. A predetermined thickness, approximately 100 to 300 Å, of nitride is deposited over the oxide layer  106   b  to form a nitride layer  108 . The total thickness of the nitride-oxide layer  106  is approximately equal to the height of the thyristor (see FIG.  11 ). 
   A description of the preferred method for fabricating a T-RAM cell of the T-RAM array will now be provided. The same fabrication method is used for simultaneously fabricating all of the T-RAM cells of the T-RAM array. With reference to  FIGS. 4-11  there are shown cross-sectional views of the semiconductor wafer  100  for fabricating the T-RAM cell. 
   With reference to  FIG. 4 , a first mask  110  is positioned over the wafer  100 . The nitride layer  108  and the nitride-oxide layer  106  are etched to define a thyristor region  112 . The height of the thyristor region  112  determines the height of the gate surrounding the base region of the thyristor (see FIG.  11 ). 
   With reference to  FIG. 5 , an in-situ n+ doped gate polysilicon layer  114  is formed. The dimension on the left of the polysilicon layer  114  is made wider than the dimension on the right of the polysilicon layer  114  because the wider region will be used for gate contact as shown by FIG.  12 . 
   With reference to  FIG. 6 , a spacer etching process is performed to form spacer gates  116   a ,  116   b . A gate dielectric formation process is then performed on the outer surface of the spacer gates  116   a ,  116   b  to form a dielectric layer  118  thereon. The gate dielectric formation process may entail performing an oxidation process followed by a thin oxy-nitride deposition process as known in the art. 
   With reference to  FIG. 7 , an etching process is performed to remove the exposed nitride layer  106   a  to the surface of the p+ layer  104  to form a thyristor region  119 . 
   With reference to  FIG. 8 , an epitaxial growth procedure is performed to form an n-p-n layer  120  having an n layer  120   a , a p layer  120   b  and an n+ layer  120   c . The n-p-n layer  120  is formed using a conventional selective epi technique to precisely control the thickness and the doping concentration of each layer. 
   The n layer  120   a  is preferably formed by epi with an n-type arsenic impurity with doping concentration of between 2E13/cm 2  and 8E14/cm 2 . The p layer  120   b  is preferably formed by epi with a p-type boron impurity with doping concentration of between 4E13/cm 2  and 1E14/cm 2 . The n+ layer  120   c  is preferably formed by epi of an n-type arsenic impurity with doping concentration of between 8E14/cm 2  and 3E15/cm 2 . 
   With reference to  FIG. 9 , an epitaxial lateral overgrowth procedure with a p-doping is performed to form an epi layer  122  for forming the body of the horizontally stacked pseudo-TFT transfer gate. The epi layer  122  is mostly single crystalline, since it uses the n+ layer  120  as the seed material. The epi layer  122  is capped with a dielectric film  124 , such as oxide and/or nitride. 
   With reference to  FIG. 10 , the stacked pseudo-TFT transfer gate region is defined by a conventional masking process. A sacrificial insulating film  126 , such as CVD oxide, glass, or polymer is deposited over the structure. A Ddamascene gate process is used to form two gates  128 ,  130 . After a chem-mech polish, the upper surfaces of the two gates  128 ,  130  are co-planar. 
   With reference to  FIG. 11 , the sacrificial insulating film  126  is stripped. An n+ dopant is implanted to create n+ regions  132   a ,  132   b , thereby forming the source and drain regions of the stacked pseudo-TFT transfer gate. The n+ dopant is preferably formed by using an n-type arsenic implant at an energy in the range of 2 to 15 KeV and a dosage of between 8E14/cm 2  and 3E15/cm 2 . Spacers  136  are then formed to fully fabricate two wordline gates WLL 1 , WLL 2  of a T-RAM memory cell  200 . 
   With reference to  FIG. 12  there is shown a top phantom view of the T-RAM cell  200  fabricated according to the present invention and showing the connections to the two wordlines WLL 1 , WLL 2  and a bitline BL. The T-RAM cell  200  has a size measurement of 8F 2 , where F is the minimal printable feature size. It is appreciated that one skilled in the art can modify the process described herein to realize similarly structured T-RAM cells having a size measurement of less than 8F 2 . 
   With continued reference to  FIG. 12 , one of the two wordlines, i.e., wordline WLL 2 , contacts the bitline BL via a metallic bitline contact  138 . The metallic bitline contact  138  is provided in order for the wordline WLL 2  to contact the bitline BL and for maintaining a distance “D” from the wordlines WLL 1 , WLL 2  (see FIG.  13 ). 
   The heavy line box which is identified by the letters “TR” in  FIG. 12  designates the thyristor region of a buried p-n-p vertical thyristor  210 . The thyristor region TR contacts wordline WLL 1  and is surrounded by a surrounded gate  140 . The surrounded gate  140  contacts the wordline WLL 2  by a contact stud  138  and is formed by spacer gates  116   a ,  116   b . The box identified by the letters “TGR” designates the transfer gate region of the horizontally stacked pseudo-TFT transfer gate  220 . The bitline BL contacts n+ region  132   b  of the stacked pseudo-TFT transfer gate  220  via contact stud  138 . 
     FIG. 14  is a top phantom view of a portion of a T-RAM array designated by reference numeral  300  showing a plurality of T-RAM cells  200   a-d  fabricated according to the present invention.  FIG. 14  also shows the location of the first and second left wordlines WLL 1 , WLL 2 , the location of first and second right wordlines WLR 1 , WLR 2 , and the location of first and second bitlines BL 1 , BL 2 . A first pair of adjacent cells  200   a ,  200   b  of the array  300  share the same bitline contact  138   b . A second pair of adjacent cells  200   c ,  200   d  of the array  300  share the same bitline contact  138   e.    
   With the method of the present invention, minimal process steps and mask levels are required to fabricate planar T-RAM cells  200  of a T-RAM array on bulk or SOI which results in a significant saving in manufacturing cost and provides a better thermal budget over prior art methods. The buried p-n-p vertical thyristor  210  and the horizontally stacked pseudo-TFT transfer gate  220  of each T-RAM cell  200  are simultaneously fabricated in a very compact dimension to reduce manufacturing cost and time, while providing reliable T-RAM cells  200 . Additionally, the planar T-RAM array structure is easy to fabricate, since the depth of focus for the lithographic tools and metal interconnects is easier to handle than the three-dimensional vertical device of the prior art T-RAM structure (see FIG.  1 ). 
   By sharing etching and diffusion steps, thermal budget is tightly controlled. This leads to better device quality and reliability. Otherwise, extra thermal steps needed to form T-RAM cells will cause the junction depth of the normal device deeper which leads to higher junction capacitance and poorer performance. Additionally, the T-RAM cells having the surrounded gate structure which is fabricated according to the inventive method are suitable for future scaling and metallization than the prior art T-RAM cells which have the vertical thyristor. Further, the structure of the present invention is fully planar and results in higher performance at low voltage (e.g., Vdd=1V). 
   Further, the method of the present invention provides twin-tower T-RAM cells having a size of less than or equal to 8F 2 . Accordingly, the density of a T-RAM array is small, while the yield and memory performance is high. 
   A T-RAM array having a plurality of T-RAM cells  200  according to the present invention can be provided within a central processing unit or other type of processor to provide a reliable and high performance memory system. 
   What has been described herein is merely illustrative of the application of the principles of the present invention. Other arrangements and methods, such as using different substrates, may be implemented by those skilled in the art without departing from the scope and spirit of this invention.