Abstract:
Memory devices and methods of making memory devices are shown. Methods and configurations as shown provide folded and vertical memory devices for increased memory density. Methods provided reduce a need for manufacturing methods such as deep dopant implants.

Description:
BACKGROUND 
       [0001]    Thyristor random access memory (TRAM) provides a memory structure that does not need storage capacitors to store a memory state. However device configurations to date use a considerable amount of surface area. Improvements in device configuration are needed to further improve memory density. Further, it is desirable to form devices using manufacturing methods that are reliable and efficient. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  shows a flow diagram of an example method according to an embodiment of the invention. 
           [0003]      FIG. 2A  shows a semiconductor memory device according to an embodiment of the invention. 
           [0004]      FIG. 2B  shows a number of semiconductor memory devices according to an embodiment of the invention. 
           [0005]      FIG. 3A  shows schematic configuration of memory devices according to an embodiment of the invention. 
           [0006]      FIG. 3B  shows another schematic configuration of memory devices according to an embodiment of the invention. 
           [0007]      FIG. 3C  shows another schematic configuration of memory devices according to an embodiment of the invention. 
           [0008]      FIG. 4  shows a semiconductor memory device according to an embodiment of the invention. 
           [0009]      FIG. 5A  shows a manufacturing stage in forming a memory device according to an embodiment of the invention. 
           [0010]      FIG. 5B  shows another manufacturing stage in forming a memory device according to an embodiment of the invention. 
           [0011]      FIG. 5C  shows another manufacturing stage in forming a memory device according to an embodiment of the invention. 
           [0012]      FIG. 6  shows a manufacturing stage in forming a memory device according to an embodiment of the invention. 
           [0013]      FIG. 7  shows an example control line configuration of a semiconductor memory device according to an embodiment of the invention. 
           [0014]      FIG. 8  shows another example configuration of a semiconductor memory device according to an embodiment of the invention. 
           [0015]      FIG. 9  shows another example configuration of a semiconductor memory device according to an embodiment of the invention. 
           [0016]      FIG. 10  shows another example configuration of a semiconductor memory device according to an embodiment of the invention. 
           [0017]      FIG. 11  shows another example configuration of a semiconductor memory device according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof and in which are shown, by way of illustration, specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and chemical, structural, logical, electrical changes, etc. may be made. 
         [0019]    The terms wafer and substrate used in the following description include any structure having an exposed surface with which to form a device or integrated circuit (IC) structure. The term substrate is understood to include semiconductor wafers. The term substrate is also used to refer to semiconductor structures during processing, and may include other layers, such as silicon-on-insulator (SOI), etc. that have been fabricated thereupon. Both wafer and substrate include doped and undoped semiconductors, epitaxial semiconductor layers supported by a base semiconductor or insulator, as well as other semiconductor structures well known to one skilled in the art. The term conductor is understood to include semiconductors, and the term insulator or dielectric is defined to include any material that is less electrically conductive than the materials referred to as conductors. 
         [0020]    The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of a wafer or substrate, regardless of the orientation of the wafer or substrate. The term “vertical” refers to a direction perpendicular to the horizontal as defined above. Prepositions, such as “on”, “side” (as in “sidewall”), “higher”, “lower”, “over” and “under” are defined with respect to the conventional plane or surface being on the top surface of the wafer or substrate, regardless of the orientation of the wafer or substrate. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
         [0021]    It is desirable to provide memory cells that are scalable to increasing demand for high memory density. It is also desirable that such methods are efficient in production, and low in cost. 
         [0022]      FIG. 1  shows an example method of forming a memory cell according to an embodiment of the invention. Specific cell configurations formed using this and other methods are shown in subsequent figures and described in more detail below. In operation  10 , a channel is formed in a first type semiconductor portion to form a “U” shaped portion. In operation  20 , a dielectric material is formed within the channel, and in operation  30 , a control line is formed over the dielectric material. In operation  30 , a second type semiconductor is implanted into top portions of the “U” shaped portion to form a pair of implanted regions. Operation  50  recites forming an upper first type semiconductor portion over one of the implanted regions. 
         [0023]    Implanting in accordance with embodiments of the invention does not require deep implants. The deeper an implant operation goes into a substrate, the more chance there is for damage to the crystalline semiconductor lattice. As a result, deeper implant regions may not operate as efficiently as shallow implant regions with less lattice damage. Shallow implants are also typically easier to produce. 
         [0024]    In one example, the first type dopant is P and the second type dopant is N. Other configurations include the first type dopant as N type, and the second type dopant is P type. In one example, the first type semiconductor portion formed into the “U” shaped portion is a P-type semiconductor. In one example the P-type semiconductor is a top portion of a silicon-on-insulator substrate. In one example the first type semiconductor portion is a native doped portion. When using a native P-doped portion of a substrate, an undamaged crystalline lattice is available, which can provide better performance than an implanted semiconductor portion. Methods described in the present disclosure are used to form devices without deep implant steps. These devices are easier to form, and are more reliable due to a lower amount of lattice damage from deep dopant implants. 
         [0025]      FIG. 2A  shows an example memory device  100  according to an embodiment of the invention. The device  100  includes a thyristor memory device. Thyristor devices have small physical size compared to other memory devices. Embodiments of thyristor devices described herein do not require a storage capacitor to store a memory state, which allows for extremely small individual memory cell dimensions. This allows higher memory density in memory arrays. 
         [0026]    The thyristor configuration in  FIG. 2A  includes a first P-N junction  130 , a second P-N junction  132 , and a third P-N junction  134  that are coupled in series. A control line  116  is shown between two of the P-N junctions. In operation, when activated by the control line  116 , a signal travels from a first transmission line  122 , through the series of P-N junctions, and out to a second transmission line  126 . 
         [0027]    In one example, a channel is formed in a P-type semiconductor portion to form a “U” shaped semiconductor portion  110 . An N-type dopant is implanted into an exposed surface of the top portions of the “U” shaped portion  110  to form first N-region  112  and second N-region  114 . In one example the first N-region  112  is lightly doped, and the second N-region  114  is heavily doped (N+). Because the first N-region  112  and the second N-region  114  are both formed on a surface of the P-type semiconductor portion  110 , before subsequent depositions processes, no deep implant operations are necessary. 
         [0028]    A dielectric material  118  is shown separating the control line  116  from the “U” shaped P-type semiconductor portion  110 . By placing the control line  116  within the channel of the “U” shaped P-type semiconductor portion  110  a large surface area is adjacent to the control line  116 . This provides increased control over activation of the “U” shaped P-type semiconductor portion  110  in contrast to configurations where a control line is only adjacent to one side surface of a semiconductor region. 
         [0029]    An upper first type semiconductor portion  121  is then formed over the first N-region  112 . In the example shown, the upper first type semiconductor portion  121  includes a lightly doped P-type region that is implanted in the first N-region  112 . This method of manufacture allows the upper first type semiconductor portion  121  to be surface implanted, and damage to the lattice is reduced in contrast to deep implants. 
         [0030]    In one example, a heavily doped P+ portion  120  is formed over the upper first type semiconductor portion  121 . In one example, the heavily doped P+ portion  120  includes a physical deposition of P+ polysilicon. A first transmission line  122  is shown formed over the second N-region  114  and a second transmission line  126  is shown formed over the heavily doped P+ portion  120 . In one example, the first transmission line  122  and the second transmission line  126  are substantially orthogonal to one another, and form a row and column memory layout as shown in more detail in subsequent examples. 
         [0031]      FIG. 2B  shows multiple memory devices  100  from  FIG. 2A  in a portion of a memory array  200 .  FIG. 2B  shows a base oxide material  102  that is part of a substrate. In the example of  FIG. 2B , a semiconductor material of an SOI substrate is patterned and etched, leaving behind semiconductor material used to form the U″ shaped P-type semiconductor portion  110  and the first N-region  112  and the second N-region  114 . A dielectric portion  104  is shown formed around the semiconductor structures formed from the SOI substrate. 
         [0032]    In the example of  FIG. 2B , the first transmission line  122  includes a metal conductor  123  that is electrically isolated using a nitride cap  124 .  FIG. 2B  also illustrates the heavily doped P+ portion  120  configured as a continuous structure that contacts the second transmission line  126  along a length of the transmission line  126 . In one example the continuity of heavily doped P+ portion  120  helps to provide a conduction path for the second transmission line  126  and improves performance of the memory array  200 . In one example the second transmission line  126  is formed from a metal, or conductive metallic compound, and serves as a metal cap over the heavily doped P+ portion  120  to enhance conduction in the second transmission line  126 . 
         [0033]    In the example memory array  200 , adjacent memory devices  100  share a common first transmission line  122 . Example configurations of the memory array  200  are further shown in  FIGS. 3A-3C . 
         [0034]      FIG. 3A  shows a schematic diagram of a memory array similar to the array  200  from  FIG. 2B .  FIG. 3A  shows a first memory device  310  and a second adjacent memory device  312  sharing a common transmission line  320 . Similarly,  FIG. 3B  shows a first memory device  310  and a second adjacent memory device  312  sharing a common transmission line  322 . In  FIG. 3B , the common transmission line  322  is routed to one side of the array to sensing circuitry  330 . A second common transmission line  324  is shown routed to an opposite side of the array to sensing circuitry  332 . The configuration of  FIG. 3B  is shown alternating common transmission lines between opposite sides of the array. This configuration provides more room for circuitry on sides of the array because each side need only interface with half of the memory cells in the array. 
         [0035]      FIG. 3C  shows an alternating arrangement of transmission lines  326  and  328 , that is similar to the alternating arrangement of  FIG. 3B . However in  FIG. 3C , transmission lines  326  and  328  are not shared between adjacent memory cells  310  and  312 . 
         [0036]      FIG. 4  shows a memory device  400  according to an embodiment of the invention. Similar to the memory device  100  of  FIG. 2A , the memory device  400  includes a thyristor device with a first P-N junction  430 , a second P-N junction  432 , and a third P-N junction  434  that are coupled in series. A control line  416  is shown between two of the P-N junctions with a dielectric material separating the control line  416  from the adjacent semiconductor body. 
         [0037]    In  FIG. 4 , a channel is formed in a P-type semiconductor portion to form a “U” shaped semiconductor portion  410 . An N-type dopant is implanted into an exposed surface of the top portions of the “U” shaped portion to form first N-region  412  and second N-region  414 . In one example the first N-region  412  is lightly doped, and the second N-region  414  is heavily doped (N+). Because the first N-region  412  and the second N-region  414  are both formed on a surface of the P-type semiconductor portion  410 , no deep implant operations are necessary. 
         [0038]    Similar to memory device  100 , by placing the control line  416  within the channel of the “U” shaped P-type semiconductor portion  410  a large surface area is adjacent to the control line  416 . This provides increased control over activation of the “U” shaped P-type semiconductor portion  410  in contrast to configurations where a control line is only adjacent to one side surface of a semiconductor region. 
         [0039]    An upper first type semiconductor portion  420  is then formed over the first N-region  412 . In one example the heavily doped P+ portion  420  includes a physical deposition of P+ polysilicon. A first transmission line  422  is shown formed over the second N-region  414  and a second transmission line  426  is shown formed over the heavily doped P+ portion  420 . In one example, the first transmission line  422  and the second transmission line  426  are substantially orthogonal to one another, and form a row and column memory layout. 
         [0040]      FIG. 4  further illustrates a back gate  440  formed from a conductor region. Examples of conductor regions include metal regions such as titanium or tungsten, or alloys thereof. The back gate  440  is separated from the “U” shaped P-type semiconductor portion  410  by a dielectric material  442 . The example memory device  400  of  FIG. 4  operates as a hybrid between a thyristor memory cell, and a floating body cell. In operation, the back gate  440  is used to facilitate charge storage beneath the memory device  400 . 
         [0041]    In one embodiment, an amorphous silicon material  444  is further included between the back gate  440  and a base oxide material  402  of the substrate. Inclusion of the amorphous silicon material  444  is included in selected embodiments, and is useful in fabrication of the memory device  400 . 
         [0042]      FIGS. 5A-5C  show steps in fabrication of a material stack  590  used to make memory device  400  from  FIG. 4 . In  FIG. 5A  a dielectric material  552  is formed over a first bulk semiconductor  558 . In one example the first bulk semiconductor  558  includes bulk P-type doped silicon. A conductor region  554  is then formed over the dielectric material  552 . A bonding material  556  is then formed over the conductor region  554  to form a first layered substrate  550 . In one example, the bonding material  556  includes an amorphous silicon material, however the invention is not so limited. Other semiconductor layers, or non amorphous layers can also be used to bond depending on the choice of second substrate as discussed below. 
         [0043]      FIG. 5B  shows a second substrate  500 . In one example, the second substrate  500  includes a bulk silicon substrate  502  with a dielectric material  504 . In one example, the dielectric material  504  includes silicon oxide that is formed by oxidizing the bulk silicon  502 . One of ordinary skill in the art, having the benefit of the present disclosure, will recognize that other dielectric configurations and substrates are also possible. 
         [0044]      FIG. 5C  shows the first layered substrate  550  from  FIG. 5A  flipped over and the bonding material  556  is bonded to the dielectric material  504  of the second substrate  500 . In one example a marker material  560 , as illustrated in  FIG. 5A , is used in configuring the bulk P-type doped silicon  558  to the configuration shown in  FIG. 5C . In one example, a hydrogen implant is placed as the marker material  560  at a desired depth in the bulk P-type doped silicon  558  to define a thickness for subsequent memory device fabrication. After the first layered substrate  550  is flipped and bonded to the second substrate  500 , the backside bulk P-type doped silicon  558  is thinned until the marker material  560  is detected. Although a hydrogen implant marker is described as an example, one of ordinary skill in the art, having the benefit of the present disclosure, will recognize that other techniques of separating the dielectric material  552  and conductor region  554  from the bulk P-type doped silicon  558  are within the scope of the invention. For example, other techniques may not use marker materials. Other examples of separating the dielectric material  552  and conductor region  554  from the bulk P-type doped silicon  558  may include cutting, without a thinning process. 
         [0045]    Once the material stack  590  is formed, the bulk P-type doped silicon  558  can be processed as described above to form memory devices such as memory device  400  from  FIG. 4 . Processing two substrates separately and bonding them as described in  FIG. 5A-5C  simplifies formation of buried structures such as the back gate  440  from  FIG. 4 . Other methods of processing on a single substrate may involve more complicated operations such as deep trench deposition or implantation. Methods as shown in  FIG. 5A-5C  simplify device fabrication and provide more reliable buried structures such as the back gate  440  and dielectric material  552 . 
         [0046]      FIG. 6  describes a material stack  600  used for forming memory devices according to embodiments of the invention. In one example, the material stack  600  is formed from two substrates that are bonded, similar to embodiments described in  FIG. 5A-5C . In one example a first substrate  650 , including a P-type bulk semiconductor  620  is implanted to form an N-type region  618 . A P+ region  616  is then formed over the N-type region  618 . In one example the P+ region  616  is physically deposited over the implanted N-type region  618 . In other embodiments, the P+ region  616  is further implanted into the implanted N-type region  618 . A conductor region  614  is then deposited over the P+ region  616 , and a bonding material  612  is formed over the conductor region  614 . The first substrate  650  is then bonded to a second substrate  610  at interface  602 . In one example, the second substrate  610  includes a silicon oxide material over a bulk silicon substrate, although the invention is not so limited. The material stack  600  can then be used to fabricate memory devices such as those described below. 
         [0047]      FIG. 7  describes a memory device  700  according to an embodiment of the invention, formed from the material stack  600  from  FIG. 6 . A first P-N junction  710 , a second P-N junction  712 , and a third P-N junction  714  are shown coupled in series. The first P-N junction  710 , the second P-N junction  712 , and the third P-N junction  714  of  FIG. 7  are vertically coupled, in contrast to the coupling shown in memory device  100  of  FIG. 1 , which uses a “U” shaped portion to fold the memory device  100 . Vertical coupling the three P-N junctions, as in  FIG. 7 , provides a reduced areal footprint, thus enabling higher memory array density. 
         [0048]    In the memory device  700 , the first P-N junction  710  and the second P-N junction  712  are formed from the material stack  600 . In one example the third P-N junction  714  is formed by implanting region  720  over the material stack  600 . Although implanting is used to form region  720 , alternative embodiments can use physical material deposition or other suitable methods. 
         [0049]    A control line  730  is shown formed laterally between adjacent memory devices, and vertically between two of the vertically coupled P-N junctions. A dielectric material  716  separates the control line  730  from the vertical stack of alternating semiconductor material in the memory device  700 . A buried transmission line  732  is shown, formed from the conductor region  614  of the material stack  600 . Buried transmission line  732  provides space savings in a memory array and increased memory density. A second transmission line  734  is shown coupled to a top of the region  720 . In operation, the control line  730  activates the memory device  700 , and a signal is detected flowing from one transmission line, vertically through the device  700  and into the other transmission line. 
         [0050]      FIG. 8  shows a memory device  800  according to an embodiment of the invention. In one embodiment, the memory device  800  is formed from a material stack, such as the material stack  600  from  FIG. 6 . The memory device  800  includes a pair of control lines  810  and  812  formed in a trench between adjacent memory devices. An isolation trench  820  is shown separating the pair of control lines  810  and  812 . In the example of  FIG. 8 , the isolation trench  820  separates N-regions  801  between vertical pillars  802  of alternating conductivity type semiconductor material. A heavily doped (P+) region  803  is left at least partially continuous along a line parallel with a buried transmission line  804 . In one embodiment, the heavily doped (P+) region  803  aids in conduction along the buried transmission line  804 . 
         [0051]      FIG. 9  shows a memory device  900  according to an embodiment of the invention. Similar to memory device  800  of  FIG. 8 , the memory device  900  includes a pair of control lines  910  and  912  formed in a trench between adjacent memory devices. An isolation region  920  is shown separating vertical pillars  902  of alternating conductivity type semiconductor material. The configuration of  FIG. 9  completely separates adjacent N-regions  901  and heavily doped (P+) regions  904 . 
         [0052]      FIG. 10  shows a memory device  1000  according to an embodiment of the invention. Similar to memory devices described above, the memory device  1000  includes a pair of control lines  1010  and  1012  formed in a trench between adjacent memory devices. An isolation region  1020  is shown separating vertical pillars  1002  of alternating conductivity type semiconductor material. A heavily doped (P+) region  1003  is left at least partially continuous along a line parallel with a buried transmission line  1004 . In one embodiment, the heavily doped (P+) region  1003  aids in conduction along the buried transmission line  1004 . In contrast to memory device  800  of  FIG. 8 , the isolation region  1020  of  FIG. 10  etches back N-region  1001  to further isolate the vertical pillars  1002 , and remove portions of the N-region  1001  adjacent to the control lines  1010  and  1012 . 
         [0053]      FIG. 11  shows a memory device  1100  according to an embodiment of the invention. Similar to memory devices described above, the memory device  1100  includes a pair of control lines  1110  and  1112  formed in a trench between adjacent memory devices. An isolation region  1120  is shown separating vertical pillars of alternating conductivity type semiconductor material  1102 . A heavily doped (P+) region  1103  is left at least partially continuous along a line parallel with a buried transmission line  1104 . In one embodiment, the heavily doped (P+) region  1103  aids in conduction along the buried transmission line  1104 . Similar to memory device  1000  of  FIG. 10 , the isolation region  1120  of  FIG. 11  etches back N-region  1101  to further isolate the vertical pillars  1102 , and remove portions of the N-region  1101  adjacent to the control lines  1110  and  1112 . In addition, memory device  1100  provides an overlap distance  1114 , where the control lines  1110  and  1112  extend downward over a portion of the N-region  1101 . 
         [0054]    While a number of embodiments of the invention are described, the above lists are not intended to be exhaustive. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. It is to be understood that the above description is intended to be illustrative and not restrictive. Combinations of the above embodiments, and other embodiments, will be apparent to those of skill in the art upon studying the above description.