Patent Publication Number: US-6337497-B1

Title: Common source transistor capacitor stack

Description:
TECHNICAL FIELD 
     The present invention relates to a novel arrangement of an FET, serving as access device, and a capacitor, both together forming a storage cell of a memory, e.g. a Dynamic Random Access Memory (DRAM). 
     BACKGROUND OF THE INVENTION 
     In order to be able to make memory chips, logic devices and other kind of devices of high integration density, one has to find a way to further scale down certain components thereof. 
     Vertical field effect transistors (FETs) play an important role in different such devices. They are of particular importance in DRAMs. Another important building block of memory devices are the capacitors which actually store the information. The FETs usually serve as access devices for these capacitors. By application of appropriate signals to the word lines and bit lines of a storage cell, the respective FET can be switched to allow the charging of the corresponding capacitor. 
     In order to achieve a high packing density necessary for Gbit DRAM memory, it is crucial that a minimum memory cell size be maintained. This minimum cell size typically is on the order of 4F 2 , where F is the minimum lithographic line-width. 
     It is an object of the present invention to provide a way for the high density integration of access devices and capacitors, e.g. for use in a memory device. 
     It is another object of the present invention to provide an improved FET/capacitor arrangement which is suited for high density integration. 
     SUMMARY OF THE INVENTION 
     The present invention concerns a novel arrangement of a vertical transistor and a capacitor (e.g. a stack capacitor), and a method for operation of such an arrangement. This arrangement is well suited as memory cell of a memory device, for example. 
     The novel arrangement comprises an FET with a vertically arranged channel and a capacitor being formed on top of the FET&#39;s uppermost electrode, e.g. the drain. The capacitor is interconnected with this uppermost electrode. The source electrode of the FET is coupled to a common source voltage V DD , for example. The gate electrode is linked to a word line and the capacitor&#39;s upper electrode is connected to the bit line. 
     In a preferred implementation, the gate electrodes (comprising polysilicon, for example) directly connect all devices along a common word line or row line. 
     This new arrangement requires a novel operation scheme for the storing and reading of information, as will be described in the detailed description. 
     In an alternative embodiment of the present invention the doped region serving as drain and the doped region serving as source are interchanged. Such a structure can be referred to as common drain transistor capacitor stack. 
     The integration of a vertical FET serving as access transistor with a storage capacitor stacked on top of it allows to realize very small memory cells. 
     It is an advantage that the inventive cells can be made using processing steps which are compatible with current semiconductor manufacturing processes. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail below with reference to the following schematic drawings. These drawings are not drawn to scale. 
     FIG. 1A Is a schematic cross-section of a first embodiment of the present invention. 
     FIG. 1B Is a block diagram of the new arrangement shown in FIG.  1 A. 
     FIG. 2 Is a block diagram of two memory cells, according to the present invention, the memory cell on the left hand side being selected in order to write a ‘1’ into the respective capacitor. 
     FIG. 3 Is a block diagram of two memory cells, according to the present invention, the memory cell on the right hand side being selected in order to write a ‘0’ into the respective capacitor. The effect on the memory cell on the left hand side is also shown. 
     FIG. 4A Is a block diagram of two memory cells, according to the present invention, the stored information being read of either cell by selecting the appropriate word line. In the present example, the bit line is precharged from 0V to V DD /2 before word line selection. 
     FIG. 4B Is a block diagram of two memory cells, according to the present invention, the stored information being read of either cell by selecting the appropriate word line. In the present example, the bit line is precharged from V DD  to V DD /2 before word line selection. 
     FIG. 5 Is a schematic cross-section of an array of memory cells in the word line direction (similar to those of FIG.  1 A), according to the present invention. 
     FIG. 6A Is a schematic cross-section of another array of memory cells in the word line direction, according to the present invention. Please note that in this embodiment the gate electrodes of adjacent FETs are connected forming a common word line. 
     FIG. 6B Is a schematic cross-section of the array of FIG. 6A in the bit line direction. Please note that in this embodiment the gate electrodes of adjacent FETs are not connected. 
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION 
     In the present context mainly DRAM memories and DRAM memory cells will be addressed. It is to be understood that the novel FET/capacitor arrangement presented herein can also be used in other devices such as logic devices (e.g. shift registers) for example. 
     Polysilicon (also referred to as polycrystalline silicon) can be formed using chemical vapor deposition (CVD), or low pressure chemical vapor deposition (LPCVD), as well as other known processes. Polysilicon is a solid composed of a myriad of small single crystal regions. 
     The expression memory is meant to cover any kind of memory device formed on, or in a substrate. Such a memory may comprise different memory cells and other kind of circuits. 
     When referring to a “sidewall”, any kind of surface of a semiconductor structure is meant which is sloped with respect to the plane of the substrate of said semiconductor structure. Of particular importance are sidewalls which are perpendicular with respect to said substrate&#39;s plane. 
     A vertical FET, as used herein, is a three-dimensional transistor that is formed on the sidewalls of a pillar. The areal dimension of this pillar may be lithographically defined and the pillar may comprise silicon, for example. In a vertical FET, the current flows vertically along the sidewalls of the pillar and is controlled by the polysilicon gate which encircles the pillar. The gate length is adjusted by the pillar height. The vertical FET is a natural choice because of its small area foot-print, as well as its potential for easy scalability, as addressed in co-pending U.S. patent application with application Ser. No. 08/749,748, filed on Nov. 15, 1996. This U.S. patent application is herewith incorporated by reference. The utilization of the vertical dimension for the transistor channel eliminates short-channel effects as a constraint while still allowing areal scaling. Any kind of vertical FET is suited for use in connection with the present invention, and the described FETs are examples only. 
     As will be seen later, the poly gates are deposited uniformly around the pillars of the the vertical FETs. Due to this, the spacing between adjacent devices (memory cells) along the bit lines perpendicular to the word lines must be slightly greater than F (namely F+f, where f preferably is on the order of about 0.1F) to ensure that the gates are not shorted in this direction. It is obvious that f may be bigger, but by choosing a bigger f, the integration density will be reduced. 
     In the following, the uppermost electrode of the vertical transistors is the drain. Likewise, the source may be the uppermost electrode. 
     The present invention is now described in connection with FIGS. 1A and 1B. In FIG. 1A, a first embodiment of the present invention is shown. In FIG. 1B, it is illustrated how the two basic building blocks of the first embodiment are arranged and coupled to each other. 
     In FIG. 1A, a schematic cross-sectional view of a vertical FET  10  is shown on top of which a capacitor  24  is situated. In the following, the combination of an FET with a capacitor is referred to as memory cell  20 , or simply cell. 
     The FET  10  has a pillar-like shape. The pillar may have a circular, square or rectangular cross-section in the plane of the substrate  12 . Substrate  12  may be silicon, for example. 
     The gate channel  11  is the area between the source  13  and drain electrodes  15  of FET  10 . As can be implanted into the substrate  12  and the top of the pillar to form the source  13  and drain  15 , respectively. Well suited for n-type doping are: P, As and Sb, for example. The channel  11  is selectively made conductive or non-conductive to effect the desired operation. The pillar serving as gate channel consists of the same material as the substrate  12  (e.g. silicon). It may be p-doped. Well suited for p-type doping are: B, In and Ga, for example. A well defined gate channel  11  and polysilicon gate layer  14  are crucial for the operation of such an FET. However, as the desired size of such channels and the adjacent polysilicon layer get increasingly smaller, inaccuracies of the manufacturing process can easily cause the small channel  11  to be short-circuited or leaky, rendering the respective FET  10  and eventually the whole memory cell  20  or even the DRAM—of which the memory cell  20  is only one part—non-functional. In addition, the dimensions of the channel  11  and the polysilicon layer  14  control the electrical characteristics of the device. The performance of an FET depends very much on the geometry of the device, the doping profile and quality of the materials proximate the surfaces and so forth. This requires that the dimensions of the polysilicon gate conductor  14  are controlled precisely. As mentioned above, the process described in the co-pending U.S. patent application, with application Ser. No. 08/749,748, is well suited to make a vertical FET. 
     Gate Induced Drain Leakage (GIDL) in a vertical FET may be reduced by employing a lightly doped drain (LDD). Such an LDD provides for a vertical grading of the junction profile in the vertical FET. 
     As shown in FIG. 1A, the polysilicon gate layer  14  is separated from the channel  11  by a gate oxide  17 . Another oxide layer  16  is situated between the source  13  and the polysilicon gate layer  14  providing for an isolation of source and gate. The gate oxide  17  is arranged such that also the drain  15  is separated from the polysilicon gate layer  14 . 
     A stack capacitor  24  is formed on top of the FET  10 . This capacitor comprises an upper electrode  21  and a lower electrode  23 . TaSiN may be used as lower electrode  23 , and TiN and Pt may be used as upper electrode  21 . The upper electrode  21  may likewise comprise a layered structure, e.g. comprising a Pt layer and a TiN layer. A dielectric layer  22  is situated between these two electrodes. BSTO (barium Strontium Titanate) may be used as dielectricum. In the present embodiment, the lower electrode  23  is directly connected to the FET&#39;s drain  15 . It may likewise be connected to the FET by means of a via or stud or any other interconnection. Such a stud may be made of W, for example. 
     The novel arrangement of the FET  10  and capacitor  24  is illustrated in FIG.  1 B. As shown in this Figure, a supply voltage V DD  is applied to the source  13  of the FET  10 . According to the present invention, the sources of adjacent FETs are all common and connected to the V DD  supply voltage. One may either connect all sources to one and the same V DD  power supply, or the FETs may be grouped into smaller arrays to each of which a separate power supply is assigned. 
     The gate  14  of the FET  10  is connected to the word line  26  or a row line. As addressed in connection with FIG. 6, the gates may form the word line if being formed close enough to each other. The FET&#39;s drain  15  is directly coupled to the lower electrode  23  of the capacitor  24 . The point of interconnection is also referred to as cell node. The upper electrode  21  is connected to the cell&#39;s bit line or a column line, or is itself the bit line. 
     The array suitability for very high density DRAMs, for example, is obtained by utilizing the novel and inventive common source configuration shown in FIG. 1B, for example. In this configuration, a new circuit arrangement of a vertical FET and capacitor is made use of. The capacitor and FET are interchanged compared to conventional approaches. This arrangement allows the placement of the stack capacitor on top of the access vertical device (FET) with no increase in array area while facilitating a very simple process for the production of very high density DRAMs, for example. 
     In the following, the operation of an inventive common source memory cell is addressed. Details are illustrated in FIG.  2 . As already mentioned, the supply voltage V DD  is applied to the source nodes. To write a logic ‘1’ in a cell (i.e. a cell voltage of V DD ), the bit line  35  is first driven to 0 volts (V BL =0V). The access transistor  30  is now fully turned on allowing the capacitor  34  to be charged. Only transistor  30  is turned on because this transistor was selected through the respective word line  1 . Transistor  40  is not turned on. By selection of an appropriate word line (word line  1  in the present example) the full voltage V DD  can be stored across the corresponding capacitor  34 . The written level is now stored by turning off the access transistor  30 . Please note that a ‘1’ is stored only in the memory cell on the left hand side which was selected via the respective word line. In the illustrated example, word line  1  was selected and a voltage V DD  is written and stored in capacitor  34 . Since word line  2  was not selected, the transistor  40  is not turned on and no voltage V DD  is written into capacitor  44 . 
     To write a logic ‘0’ in a memory cell (i.e. a cell voltage of 0 Volts), the bit line  35  is driven to V DD  (i.e. V BL =V DD  ), as illustrated in FIG.  3 . The access transistor  40  is turned on allowing the storage of 0 Volts (V DD  −V DD  ) across the capacitor  44 . During this operation, the cell node of an unselected cell (transistor  30  and capacitor  34 , for example) with a voltage V DD  stored, will be booted in voltage to about 2V DD  , as shown on the left hand side of FIG.  3 . Please note that a ‘1’ remains stored in the cell on the left hand side. Please also note that due to the vertical grading nature of the junction profile in a vertical FET (see FET  10  in FIG. 1A, for example), Gate Induced Drain Leakage (GIDL; drain to substrate junction leakage due to gate modulated high field across jot) in an access transistor is minimized allowing proper cell operation in this unique common source scheme, according to the present invention. 
     For the reading of a cell (see FIGS.  4 A and  4 B), there are two different approaches depending on the previous state of the bit line. As shown in FIG. 4A, the bit line  35  may be pre-charged from 0V to a V DD /2 level (i.e. V BL =V DD /2). The access transistor  30  is now turned on allowing the stored cell charge to cause a change in the bit line voltage. This change in the bit line voltage can be sensed. Sensing may be done by means of comparison with a second bit line pre-charged to a V DD /2 voltage, as for example described in “½−V DD  bit-line sensing scheme in CMOS DRAMs” , IEEE JSSC Vol. SC19, No. 4, pp. 451-454, April 1984. 
     If the bit line  35  is at a voltage V DD , it has to be pulled down to V BL =V DD /2, as illustrated in FIG. 4B, in order to be able to read out the stored information. The access transistor  30  is now turned on allowing the stored cell charge to cause a change in the bit line voltage. This change in the bit line voltage can be sensed. Please note, that independent of whether the bit line was previously at 0V or at V DD , the ‘1’ stored in the cell on the left hand side as well as the ‘0’ stored in the cell on the right hand side can be sensed. 
     Now that the basic building block—a memory cell comprising the inventive arrangement of at least one vertical FET and a stack capacitor—has been addressed, two examples of arrays comprising such memory cells (see FIGS. 5 and 6A,  6 B) are described. 
     In FIG. 5, a cross-section of part of a memory array  50  is shown. The memory array is cut parallel to a word line. Several memory cells  64 - 66  are formed in substrate  62  (e.g. consisting of silicon). Adjacent memory cells ( 64  and  65 ,  65  and  66 ) have common sources  53 . In an alternative embodiment adjacent memory cells may have separate sources instead of common sources. As shown in FIG. 5, the space between the memory cells is filled with a conductive material  55  serving as common word line. In order to interconnect adjacent cells along one and the same word line, the conductive material  55  has to be provided. This requires deposition and lithography steps. On top of the conductive material an isolating material  63  is situated. Well suited are oxides, for example. Likewise, the memory cells could be placed closer such that the polysilicon gates of adjacent FETs provide for the interconnection (see FIG.  6 A). In such a case there would be no need for the conductive material  55 . 
     Each memory cell is connected to a bit line. In the present example, memory cell  64  is connected to bit line  76 , cell  65  to line  77  and cell  66  to line  78 . As can be seen, the bit lines are perpendicular with respect to the word lines. These bit lines can be formed directly on top of the upper electrode  57  of the capacitor. Likewise, the upper electrodes may form the bit lines. 
     Two cross-section of another array  80  are illustrated in FIGS. 6A and 6B. The cross-section of FIG. 6A is parallel to the word line, whereas the cross-section of FIG. 6B is parallel to a bit line. As shown, the array  80  comprises several memory cells  87 - 89  and  92 , 93  formed on a substrate  83 . The polysilicon gates  90  of the cells  87 - 89  form the word line (see FIG.  6 A). Three bit lines  84 ,  85 ,  86  are formed on top. In the present embodiment the distance between adjacent memory cells within one word line is F (where F is the minimum lithographic line width) to ensure that adjacent gate electrodes  90  are interconnected (see FIG.  6 A), whereas the distance between two memory cells, e.g  87  and  92 , of adjacent but independent word lines is chosen to be F+f (see FIG.  6 B). f may be about 0.1F, for example. As in case of FIG. 5, there is an insulating material  91  situated on top of the word line  90  between the cell&#39;s capacitors. In the present embodiment, the same insulating material  91  is also used to fill the gap between cells of adjacent word lines, as shown in FIG.  6 B. This insulating material  91  used to fill the gaps between word lines is not really necessary, if the bit line  84  would be arranged and structured differently. Please note that the array of FIGS. 6A and 6B has the advantage, besides other advantages, that no separate deposition and lithography steps for the formation of a word line interconnection are required. It is to be noted that the word lines and bit lines are not in electrical contact with each other. 
     In the following a exemplary manufacturing process is described which is suited to make an array, according to the present invention. 
     After p-well implantation of a substrate, a nitride mask may be used to define the pillars later serving as vertical channels. These pillars are then formed in the substrate by reactive—ion etching using a HCl—Cl 2 —N 2 —plasma, for example. The channel length is defined by the pillar height. A channel length of about 250 nm requires a pillar being about 350 nm high, and a channel of about 450 nm requires a pillar being about 550 nm in height. Following a sacrificial oxidation step to remove plasma related damage and a LOCOS (local oxidation of silicon) or an STI (shallow trench isolation) isolation step, an As implant is used to form the common source and isolated drains for the cells. The dose of As may be 2×10 15  cm −2 . It is an advantage of this process that the source implantation is self aligned to the pillars. In a next step the structure is then annealed. This can be done at 1050° C. for 30 seconds in N 2  atmosphere. After stripping the sacrificial oxide, the gate oxide is thermally grown. This gate oxide may be a few nanometers thick. Preferably, its thickness is between 5 and 7 nm. In-situ As doped polysilicon is deposited using an LPCVD technique. The polysilicon word lines (gates) are now formed by self aligned etching using Cl 2 —O 2  silicon etch, for example. A gate polysilicon thickness (F/2, e.g. 100 nm) is sufficient to fill the F (e.g. 200 nm) gaps between adjacent cells in the word line direction, but insufficient for filling the gaps (F+f, e.g. 300 nm) between pillars of adjacent word lines. The polysilicon gates serving as word lines are reactive-ion etched until the top of the gate electrodes lies about 100 nm below the top of the pillar. By means of the above process, continuous word lines are formed by those pillars whose polysilicon gate electrodes interconnect. Planarization can be achieved by chemical mechanical planarization (CMP) of oxide or a BPSG (Boron-Phosphorous Silicate Glass) deposited layer, for example. A capacitor is formed on each pillar over the drain. Each capacitor has a lower electrode, a dielectricum and an upper electrode. The upper electrode may be connected to a bit line or may form part of the bit line. 
     The capacitor shown and discussed herein can be replaced by any kind of other capacitor suited to be stacked on top of the access transistor underneath. In order to ensure sufficient capacitance of such a capacitor, usually capacitors are employed which are formed in a trench (like the one illustrated in FIG.  1 A), or on a ridge or pillar. By increasing the effective area of the capacitor&#39;s electrodes the capacitance to surface area ratio can be increased. In connection with the present invention planar-type capacitors as well as three dimensional stack-type and trench-type capacitors can be employed. 
     A process suited for making vertical FETs is described in co-pending U.S. patent application, with application Ser. No. 08/749,748, mentioned above. The process described in this US patent application allows to precisely define the size and shape of the polysilicon gate conductor being an essential part of such an FET. There are other ways to make vertical FETs. Conventional semiconductor processing steps can be used to realize the inventive structures. 
     In an alternative embodiment, the drains of adjacent memory cells (instead of the sources) can be interconnected. In such an embodiment, the FET&#39;s source is connected to the lower electrode of the capacitor. Such an alternative embodiment looks the same as the one shown in FIG. 1A, with the difference that source and drain are interchanged. I.e., in the alternative embodiment the source region is on top of the vertical pillar and the drain region is located at the bottom of the pillar. Such a structure can be referred to as common drain transistor capacitor stack.