INTEGRATED CIRCUIT TRENCH CELL

Abstract: A multiple element integrated circuit trench cell (10) having at least one vertical field effect transistor (FET) (46) in a wall (17) of a trench (18) in a semiconductor substrate (12). The cell (10) further comprises a central load device (54) within the trench (18) which is electrically connected to the vertical FET (46). The central load device (54) may be an active load device, such as another field effect transistor, or a passive load device, such as a resistor. Additionally, a further FET (50) may be present in another wall (19) of the trench (18) or in a lateral orientation adjacent the trench (18) in the semiconduc­ tor surface (14). Two of these multiple element trench cells (10) may be interconnected in various configurations to form conventional static random access memory (SRAM) cells (56).

Description: Field of the Invention The invention relates to integrated circuit elements formed in trench structures and particularly related to trench integrated circuits where at least one transistor is formed in the trench. Background of the Invention As the integrated circuit industry has explored ways of packing more circuits onto a given semiconductor substrate, more effort has been devoted to not only orienting the various devices in a planar fashion along the surface of the substrate, but also to orienting the devices vertically either by building the devices up from the substrate surface or by burying the devices in trenches formed within the face of the semiconductor body. For example, a number of schemes for isolating customary planar devices with trench isolation regions are now known. Trench capacitors are also being employed on a commercial basis on the largest memory chips, such as 1-Mbit and 4-Mbit DRAMs. Trench-like transistors have been employed in various forms. One of the earliest forms of a "trench-type" transistor uses various layers of semiconductive material to form a stack of a source/drain region covered by a layer that serves as a channel region and a top layer that serves as the other source/drain region. A groove or trench is cut through this stack. The groove or trench is coated or covered with a thin dielectric material which serves as the gate dielectric, and the remainder of the trench is filled with a conductive material plug which serves as a gate. Another trench-type transistor structure is an MOSFET as disclosed in U.S. Patent 4,683,643. An n-type source-drain region is formed at the surface of a p-type silicon substrate and a trench is etched into the substrate. A gate oxide layer is formed on the sidewalls of the trench and a layer of polysilicon is conformally deposited to cover the gate oxide layer on the sidewalls of the trench. The polysilicon layer is anisotropically etched to form a sidewall gate-electrode on the vertical surfaces of the trench. An oxide layer is formed over the polysilicon gate-electrode and on the bottom of the trench. A second n-type source/drain region is formed in the bottom of the trench and the gate oxide layer is etched to expose the second source/drain region. The trench is then filled with polysilicon which contacts the second source/drain region. The sidewall gate-electrode defines a channel region along the sidewalls of the trench between the source-drain region, at the surface of the substrate, and the second source-drain region at the bottom of the trench. Other configurations employ a shallower trench-type structure or V-shaped groove where the trench or groove not only is coated or covered with a thin dielectric layer which serves as the gate dielectric, but which dielectric is further coated with a thin layer of conductive material to form the gate electrode; at least thin relative to the plug-type gate configurations. Also of interest in relationship to these types of structures are V-MOS devices with self-aligned multiple electrodes. Vertical CMOS transistors that are built up from the surface of the semiconductor wafer rather than buried into the substrate are also known, along with vertical FETs built into trenches where the gate is a thin layer inside the trench separated from another gate within the same trench by some sort of structure. Other proposed vertical FETs include a structure where a trench contains a cylindrical gate and one source/drain is positioned at the bottom of the trench with its contact running through the middle of the cylindrical gate. Also being investigated are trench-type structures which incorporate both transistors and capacitors into the trenches in some manner. From the above brief history, it is noted that more complex devices are being designed for trench manufacture. As noted, the present challenge for trench technology involves placing more than one device in the same trench, such as a transistor and other element to form a dynamic or static random access memory (DRAM or SRAM) memory cell. Nevertheless, as trench technology is still developing, a need exists for additional trench structures, particularly of the newer, multiple device type, so that the best alternatives may be made available and considered in the fabrication of extremely high density integrated circuits. Very few of the proposed trench structures have actually been implemented in commercial integrated circuit chips. There is a need for future SRAM cells employing the new trench technologies, particularly those which provide a low soft error rate (SER). Summary of the Invention Accordingly, it is an object of the present invention to provide a memory cell for a SRAM employing multiple elements, which may be transistors. Another object of the invention is to provide a multiple element SRAM cell capable of having an active load formed therein. It is another object of the present invention to provide a multiple element trench SRAM cell that minimizes substrate surface area and the soft error rate of the memory of which it is a part. Still another object of the present invention is to provide a vertical trench transistor cell requiring no extra isolation space between the elements therein. In carrying out these and other objects of the invention, there is provided, in one form, an integrated circuit cell formed in a substrate with a trench, when the trench has at least one vertical wall, and a bottom floor, where the vertical wall has a top portion. The cell also has at least one field effect transistor (FET) in the vertical wall, wherein the FET has two source/drain regions, one near the top portion of the wall and one in the bottom floor of the trench. The FET also has a gate region along the wall. The cell also contains a central active load device within the trench, where the central load device has top and bottom contacts, wherein the bottom contact of the load device is electrically connected to the FET in the vertical wall. Brief Description of the Drawings FIG. 1 is a cross-section illustration of one embodiment of the SRAM memory cell of this invention employing three transistors in a trench; FIG. 2 is a rough, overhead layout of the SRAM memory cell of FIG. 1; FIG. 3 is a schematic circuit diagram of a conventional six transistor SRAM memory cell for illustrating how the transistors of the SRAM cell embodiment in FIG. 1 would be connected and configured to form the entire cell; FIG. 4 is a cross-section illustration of a trench early in the stages of fabrication of a trench cell of this invention; FIG. 5 is a cross-section illustration of the trench of FIG. 4 after the source/drain regions of the transistors have been formed; FIG. 6 is a cross-section illustration of the trench capacitor of FIG. 5 after application of the second conductive layer; FIG. 7 is a cross-section illustration of the trench cell of this invention in an intermediate stage of construction between that shown in FIG. 6 and that shown in FIG. 1; FIG. 8 is a cross-section illustration of another example of a multiple transistor cell that utilizes two transistors, a resistor and an interconnect layer; FIG. 9 is a schematic circuit diagram of the conventional four transistor and two resistor SRAM cell illustrating how the cell of FIG. 8 may be connected; FIG. 10 is a cross-section illustration of another cell which employs a buried layer to make a two transistor, one resistor unit for an SRAM memory; FIG. 11 is a schematic circuit diagram of the four transistor and two resistor SRAM cell illustrating how the cell of FIG. 10 may be connected; FIG. 12 is a rough, schematic layout view of the SRAM cell of FIG. 11; FIG. 13 is a cross-section illustration of still another embodiment of the invention which employs two vertical transistors and a lateral transistor on a substrate with a buried layer, where the two vertical FETs have common gates; FIG. 14 is a schematic circuit diagram of another conventional six-transistor cell employing the multiple transistor cell of FIG. 13; and FIG. 15 is an approximate, schematic overhead layout of two cells of the embodiment shown in FIG. 13 and interconnected as shown in FIG. 14. It will be appreciated that the cross-section illustrations of the invention are not to scale, and that often the vertical proportions are exaggerated relative to the horizontal proportions for clarity. Detailed Description of the Invention Shown in FIG. 1 is a cross-section of one embodiment of the cell (10) of this invention that may be employed in very large scale integration (VLSI) or ultra large scale integration (ULSI) circuit SRAMs or other circuits. The cell (10) is fabricated in a semiconductor substrate (12), which may be, but is not limited to, monocrystalline silicon or other suitable material. Isolation regions (16) are provided to surround the active regions where the cell (10) resides. Semiconductor substrate (12) will be taken in this description to be p⁻ doped, although it will be appreciated that the substrate may be of the opposite conductivity type, and that the other doped semiconductor material regions may also be reversed. The substrate (12) has a trench (18) therein which has at least two walls (17) and (19) and a bottom floor (21). The cell (10) has at least two field effect transistors (FETs). First FET (46) has a source/drain region (26) near the top portion of wall (17) and a drain/source region (30) in the bottom floor (21) of trench (18), separated by a channel (48). Over channel (48) is thin gate dielectric layer (20) which in turn is covered by gate (22). Second FET (50) has drain/source region (30) in common with FET (46) and another source/drain region (28) separated from drain/source region (30) by channel (52). Channel (52) is covered by thin gate dielectric layer (20′) and the FET (50) gate is (22′). As will be explained during the process flow description, the layers forming gates (22) and (22′) and dielectric layers (20) and (20′) are originally one and are etched apart during processing to provide contact for the third FET (54). Third FET (54) functions as the load on the cell (10) herein and has source/drain region (42) at the top of the cell (10) separated from drain/source region (44) at the bottom (21) of the cell (10), separated by channel (40). The gate dielectric for cylindrically-shaped load FET (54) is thin gate dielectric layer (36) under which is gate electrode (34). Note that drain/source region (44) is directly connected to gate (34) as well as drain/source region (30) of FETs (46) and (50). The construction of the cell (10) in FIG. 1 will now be discussed. Shown in FIG. 4 is an early step in the fabrication of the multiple transistor cell (10) of the invention, particularly the embodiment shown in cross-section in FIG. 1. The fabrication process begins by providing a semiconductor substrate (12) having a surface (14) into which are provided isolation regions (16) surrounding the active areas of the cell (10) to isolate the various cells (10) from each other. The isolation structures (16) may be silicon oxide, silicon nitride, undoped polysilicon or combinations thereof, or other suitable material. Further, the isolation structures (16) may be provided by any conventional process, such as local oxidation of silicon (LOCOS), sealed interface local oxidation (SILO), or by trench isolation, etc. If trench isolation is employed, as is recommended since the cell will also be built into a trench, the trench should be about the same depth, or about 3 microns (1 micron ≙ 1 µm) deep and approximately 0.5 to 1 micron wide. A trench (18) is etched via generally anisotropic etch techniques, such as plasma etching or reactive ion etching (RIE) in the surface (14) of substrate (12). It is recommended that the etch process to form trench (18) be one which rounds both the top and bottom corners of the trench (18) to facilitate conformal layer coverage. The trench should have at least two walls (17) and (19), and a bottom floor (21). Although the trench may be of any dimensions or proportions, one size might be a trench approximately 2 to 3 microns deep by about 2.0 to 3.0 microns wide. The first layer to be formed is a first thin gate dielectric layer (20) which may be silicon dioxide, silicon nitride or a combination of the two, such as an oxide-nitride-oxide (ONO) sandwich. Next, a conductive material layer (22), such as doped polycrystalline silicon (polysilicon or poly), a metal or metal silicide is conformally provided over dielectric layer (20). For the purposes of this example, it will be assumed that this first conductive material layer is in situ doped polysilicon of n⁺ conductivity type. Layers (20) and (22) are patterned and etched to provide openings for forming source/drain regions, and a thin pad dielectric (24) such as silicon dioxide is provided if the source/drain regions are to be incorporated by ion implantation as illustrated in FIG. 5. The ion implanta-tion may be performed conventionally. Since n⁺ source/-drain regions are to be formed, phosphorus or arsenic, in elemental or compound form, may be used. The n⁺ source/drain regions formed include regions (26) and (28) near the top portions of walls (17) and (19) respectively, and drain/source region (30) in the bottom floor (21) of the trench (18). Note that regions (26), (28) and (30) may be self-aligned to gate layer (22). Typically, the implanted impurities should be annealed to fully form source/drain regions (26), (28) and (30). Next, an oxidation is performed to provide thick dielectric isolation layer (32), as shown in FIG. 6. This layer may be formed on top of the thin ion implantation pad (24), or may be provided after the pad (24) is removed by some conventional etching means. Thick dielectric isolation region (32) may again be silicon dioxide, silicon nitride, combinations thereof, etc. This region (32) should be patterned at the bottom of the trench to enable contact to drain/source region (30), and then second conductive material layer (34) is formed to give the structure shown in FIG. 6. Second conductive material layer (34) may be formed of any of the materials suggested for first conductive material layer (22). For the example being discussed, second conductive material layer (34) is considered to be n⁺ polysilicon that is deposited by chemical vapor deposition (CVD). This process is known to give a conformal layer. Next, as shown in FIG. 7, a second thin gate dielectric layer (36), such as silicon dioxide, is placed over the second conductive material layer (34). Second conductive material layer (36) may or may not be etched to a pattern at this point, depending on how the device engineer wishes to isolate it from the rest of the circuit. In any event, both thin gate dielectric layer (36) and second conductive material layer (34) need to be patterned to provide opening (38), as seen in FIG. 7. The remainder of trench (18) is filled by a semiconductive material layer or plug (40), which may be polysilicon. Refer to FIG. 1. However, in this case, it may be desirable for the plug (40) to be in situ doped p⁻ CVD polysilicon since plug (40) will form the channel for the central load device which in this embodiment will be a transistor. A separate layer of CVD in situ doped n⁺ poly (42) is placed on top of polysilicon plug (40) as one of the source/drain regions (42). The connection to the bottom of plug (40) is provided by an anneal which would diffuse enough impurity from drain/source region (30) and second conductive material layer (34) into the bottom region (44) of plug (40) to make the contact. The conductive materials that formed plug (40) and n⁺ source/drain region (42) should be patterned and etched to limit it to the trench area. Alternatively, it may be possible to use a plug fill technique and n⁺ implantation step to form regions (40) and (42) in trench (18) without employing masking and patterning etch steps. Interconnect layers and dielectric layers may be formed and etched at this point and vias or openings to source/drain regions (26) and (28) should be provided. The process up to this point gives the essentially finished cell shown in FIG. 1. Other conventional processing steps, such as providing a passivation layer and interconnection formation, may be performed as needed. Shown in FIG. 2 is an overhead or layout schematic of the cell (10) in FIG. 1. It will be appreciated that this is only one possible layout and is not an optimized layout, nor is it even a practical one. However, it is provided to illustrate how two cells (10) and (10″) might appear and be connected to form a six-transistor SRAM cell (56) where two of the multiple transistor cells (10) and (10″) are interconnected. For example, it may be seen that source/drain regions (26) of FETs (46) and (46″) are connected to bit lines D and inverse bit line D, respectively. The gates (22) of these FETs (46) and (46″) are both connected to the word line. As noted earlier, the other drain/source regions (30) of FETs (46) and (46″) are at the bottom of the trench and thus not visible in FIG. 2. Again, with respect to FETs (50) and (50″), one of the source/drain regions thereof (30) is buried and not visible. The other source/drain regions (28) are both connected to V ss , while the gate (22′) of FET (50) is connected to the gate (34) of vertical FET (54″). Similarly, the gate (22′) of FET (50″) is connected to the gate (34) of vertical FET (54). Finally, the top source/drain regions (42) of both vertical FETs (54) and (54″) are connected to V cc line as shown in FIG. 2. Shown in FIG. 3 is a schematic circuit diagram of the two multiple transistor cells (10) and (10″) in the SRAM cell (56), simplified from FIG. 2. The six transistor SRAM cell (56) is known in the art. However, its particular structural embodiment into two trenches (58) is new. The operation of the six transistor SRAM cell is well known in the art, and not being the subject of the present invention, is omitted herein for brevity. Typically, devices (54) and (54″) are load devices and may be other devices besides transistors. For example, in the embodiment of the invention illustrated in cross-section in FIG. 8, these load devices are resistors and not transistors. Shown in FIG. 8 is another example of a multiple transistor cell (60) in a semiconductor substrate (62) comprising a passive load. Cell (60) has a trench (64) with at least two walls (66) and (68) which in turn bear first FET (70) and second FET (72), respectively. FET (70) has source/drain region (74) in the top portion of wall (66). This n⁺ source/drain region (74) may be a layer in the surface of substrate (62) that has been formed in or added thereto. The other drain/source region (76) may be a buried layer, or other type of layer formed by conventional means. The FET (70) source/drain regions (74) and (76) are separated by channel (78). Channel (78) is covered by thin dielectric layer (80) (analogous to layer (20) in FIG. 1) over which is positioned the gate electrode (82) for FET (70). Similarly, second FET (72) has a source/drain region (84) in the substrate (62) surface near the top portion of wall (68) which is separated from the buried drain/source region (76) by channel (86). The channel (86) is covered by thin gate dielectric (80′), which is in turn covered by gate electrode (82′). The gate electrodes (82) and (82′) are separated from interconnect region (96) by means of thick dielectric layer (88). Source/drain region (84), drain/source region (76) and source/drain region (74) may be formed by impurity introduction in a manner like that used in the embodiment of FIG. 1, for example, by ion implantation or diffusion. Another feature apparent in the cell of FIG. 8 is the presence of another layer of polysilicon (96) somewhat analogous to second conductive material layer (34) in the embodiment of FIG. 1. In the adaptation of this cell (60) to the circuit in FIG. 9 as will be described later, it becomes apparent that there is a need for a means to connect resistor contact (76) with the gate (82) of a transistor (72) of an adjacent cell (60″). This layer (96) facilitates that connection. Thus, it is apparent that dielectric layer (88) should also be present and that there should be a dielectric layer (98) to isolate the bulk of resistor (90) from this second conductive layer (96). Dielectric layer (88) should be at least thick enough so that current flow in conductor (96) does not create sufficient field strength in channels (78) or (86) to induce carrier flow in FETs (70) and (72), respectively. In the cell shown in FIG. 8, load device (90) is a resistor formed by n-doped polysilicon plug (92) between drain/source connection (76) and top n⁺ electrode (94), which is connected to V cc . Source/drain region (74) is connected to bit lines or data lines D and inverse D, while source/drain region (84) is connected to V ss . It will be appreciated by those skilled in the art that the cell of FIG. 8 can be made according to the processes described for the embodiment of FIG. 1. Shown in FIG. 9 is a schematic circuit diagram of how two of the multiple transistor cells (60) and (60″) may be interconnected to form a four-transistor, two-resistor SRAM cell. Again, the particular circuit shown in FIG. 9 is well known in the art, although its structural implementation herein is new. Similarly to the discussion of FIG. 3, source/drain (74) of cell (60) is connected to bit line D while source/drain (74) of cell (60″) is connected to inverse bit line D. The other source/drain regions of the cells are connected to the gates of the other transistors of the opposite cells; drain/source region (76) of cell (60) is connected via conductive layer (96) to gate (82′) of FET (72″), and drain/source region (76) of cell (60″) is connected via conductive layer (96) to gate (88) of FET (72). Source/-drains (76) of both cells (60) and (60″) are, of course, also connected to the bottom contact of resistors (90) and (90″) and serve as one source/drain region for transistors (72) and (72″), respectively. Source/drain regions (84) of transistors (72) and (72″) are both connected to V ss , while the top contacts 94 of resistors (90) and (90″) are both connected to V cc . Shown in FIG. 10 is still another example of a a VLSI or ULSI memory cell (100) using a polysilicon resistor load buried in a trench structure on a p⁻ wafer with a n⁺ buried layer (102). In this example the cell comprises two FETs, a lateral FET (106) and a vertical FET (108) in the vertical wall (110) of trench (112) which also houses the load device (114), which in the case illustrated in FIG. 10 is a passive load device such as a resistor. Lateral FET (106) has source/drain region (116) and drain/source region (118), the latter of which is situated in the surface of p⁻ layer (104) near the top portion of wall (110). Source/drain regions (116) and (118) are spaced apart by channel (120) which is covered by thin gate dielectric (122) in turn covered by gate electrode (124). Vertical FET (108) has drain/source region (118) in common with lateral FET (106), as well as source/drain region (102) which is provided as an n⁺ buried layer. Source/drain regions (118) and (102) are separated by channel (126) in the wall (110) of trench (112). The channel (126) and trench wall (110) are covered by thin gate dielectric layer (128), which is in turn covered by gate electrode (130). Gate dielectric layer (128) and conductive layer (130) are continuous around the shape of the trench and may be cylindrical or box-shaped. Thus, the isolation regions (16) should be spaced apart from the trench (112) on all sides, as shown in FIG. 12, so that current can move around the cell. Further, the central load device (114) is a resistor comprising a lightly doped n⁻ polysilicon plug (132) capped by a n⁺ doped polysilicon contact (134). The bottom of resistor (114) contacts doped polysilicon interconnect layer (36) which in turn contacts n⁺ drain/source region (118) previously discussed. Interconnect layer (136) is insulated from the bulk of resistor (114) resistive polysilicon plug (132) by dielectric layer (138) and from vertical FET (108) gate (130) by dielectric layer (140). The processes and materials used to form the cell (100) in FIG. 10 are similar to those discussed with regard to FIGS. 4 through 7 and 1. However, it should be noted that the source/drain region (116) and drain/source region (118) in the FIG. 10 structure may be formed by self-aligned polysilicon gate processes, rather than by providing a photomask and pattern etching. It will be seen that self-aligned processes may also be used in the cells depicted in FIGS. 13 and 16, subsequently discussed. However, as might be expected, the cell (100) is configured somewhat differently in its layout and circuit diagram. Shown in FIG. 11 is the same four-transistor, two-resistor SRAM cell (142) shown in FIG. 9. However, in FIG. 11, lateral transistors (106) and (106″) serve the functions of transistors (70) and (70″), respectively, in the FIG. 9 circuit. Vertical transistors (108) and (108″) in the FIG. 11 circuit serve the function of vertical FETs (72) and (72″) of the FIG. 9 circuit; while resistors (114) and (114″) of the FIG. 11 circuit serve the function of resistors (90) and (90″) in the FIG. 9 circuit. Since this basic circuit has been discussed previously in detail, the present brief discussion will suffice for FIG. 11. Again the operation of SRAM cell (142) is well known in the art, since the circuit as depicted in FIG. 11 is not new, though its structural implementation is. Shown in FIG. 12 is a rough, schematic overhead layout of two cells (100) and (100″) of the example shown in FIG. 10 and interconnected as shown in FIG. 11. The same reference numerals used in FIGS. 10 and 11 are used in FIG. 12 for consistency and clarity between views. As with the layout shown in FIG. 2, it will be appreciated that the layout of FIG. 12 is not an optimized layout. For example, from FIG. 12 it is apparent that drain/source regions 118 could be positioned closer to trench (112) within the active area; and further that the isolation region 16 on the right side of cells (100) and (100″) may be positioned appreciably closer to trench wall (110). Briefly, the interconnections discussed above with respect to FIG. 11 will be reviewed with reference to FIG. 12. Note that source/drain region (116) of FET (106) is connected to bit line D, while source/drain region (116) of FET (106″) is connected to complementary bit line D. The gates (124) for both FETs (106) and (106″) are integral with the word line (which may, of course, be narrower than depicted). Drain/source region (118) of FET (106) is connected to interconnection conductive layer 136 of cell (100) which is in turn connected to gate electrode (130) of vertical FET (108″) of cell (100″). Similarly, drain/source region 118 of lateral FET (106″), which is also a source/drain region for vertical FET (108″), is connected to interconnection conductive layer (138) of cell (100″), which in turn is connected to the gate electrode (130) of companion FET (108) of cell (100). Interconnection conductive layers (136) of both cells (100) and (100″) are also connected to their respective load devices (114), the other ends of which are connected to common V cc . The other source/drain regions 102 of FETs (108) of both cells (100) and (100″) are the common n⁺ buried layer, which is connected to V ss . Shown in FIG. 13 is multiple transistor cell (144) which is similar to cell (100) depicted in FIG. 10, except that in this case the central load device (114) is a vertical FET (146). Most of the elements of cell (144) are similar to those of cell (100) and the reference numerals used are the same; these elements will not be discussed again for the sake of brevity. Second vertical FET (146) has a bottom source/drain region (148) and a top source/drain region (150), both regions (148) and (150) being n⁺ doped and separated by channel (152) which is p-doped. Elements (148), (150) and (152) may be any suitable semiconductor material, such as poly-crystalline silicon. The channel (152) is surrounded by thin gate dielectric layer (156), which in turn is surrounded by gate electrode (158), which again may be n⁺ doped polysilicon. As is apparent, gate electrode (158) is directly connected to bottom source/drain region (148). Further, gate electrode (158) not only is the gate electrode for second vertical FET (146), but is also the gate electrode for first vertical FET (108). These differences in interconnections of the circuit elements prevent the utilization of cells (144) of FIG. 13 in the six-transistor SRAM cell circuit (56) of FIG. 3. Instead, when two multiple transistor cells (144) and (144″) are interconnected to form the circuit of an SRAM cell (160), the circuit appears as shown in FIG. 14. Again the operation and function of this circuit is well known in the art, although the structure of the cells (144) and (144″) is new. A rough, schematic layout of the six-transistor SRAM cell of FIG. 14 utilizing the multiple-component cells (144) and (144″) of FIG. 13 is shown in FIG. 15. It will again be appreciated that the cells may not be laid out to best advantage with respect to space utilization. Nevertheless, it is expected that FIG. 15 may aid the reader in understanding the cells of FIG. 13, and thus, common reference numerals are used. Thus, the multiple-component trench cells of the invention provide new structures for integrated circuit devices, particularly memories such as SRAMs. These cells require a minimum of lateral silicon area since they are built into the substrate. The soft error rate caused by stray alpha particles erroneously reading and writing wrong information into the cell is minimized since theese trench cell structures offer possibilities of increasing the coupling capacitances of critical cell nodes and decreasing their junction area.

Claims: An integrated circuit trench cell (10, 60, 100, 144) formed in a substrate (12, 62, 104), including a trench (18, 64, 112) having at least one vertical wall (17, 66, 110) and a bottom floor wherein the vertical wall (17, 66, 110) has a top portion, the cell (10, 60, 100, 144) comprising: at least one field effect transistor (FET) (46, 70, 108) in the vertical wall (17, 66, 110), wherein the FET (46, 70, 108) has: two source/drain regions (26, 30; 74, 76; 118, 102), one near the top portion of the wall (17, 66, 110) and one in the bottom floor of the trench (18, 64, 112); and a gate region (22, 82, 130, 158) along the wall (17, 66, 110); and wherein the integrated circuit trench cell is characterized by a central active load device (54, 90, 114, 146) within the trench, the central load device (54, 90, 114, 146) having top and bottom contacts, wherein the bottom contact of the load device (54, 90, 114, 146) is electrically connected to the FET (46, 70, 108) in the vertical wall (17, 66, 110). The integrated circuit trench cell (10, 60, 100, 144) of claim 1 wherein the gate region (22, 82, 130, 158) and the central load device (54, 90, 114, 146) contain doped polycrystalline silicon. The integrated circuit trench cell (10, 144) of claim 1 wherein the active load (54, 146) comprises a central vertical region having a top source/drain region (42, 150), a bottom source/drain region (44, 148), a channel region (40, 152) between the top and bottom source/drain region, a gate dielectric (36, 156) covering the channel region and a gate electrode (34, 158) at least partially covering the gate dielectric (36, 156) and oriented across the gate dielectric (36, 156) from the channel region (40, 152), wherein the bottom source/drain region (44, 148) is in contact with the source/drain region (30, 102'') in the bottom floor of the trench (18, 112''). The integrated circuit trench cell (10, 60, 100, 144) of claim 1 wherein the source/drain region (30, 76, 102 ) in the bottom floor of the trench (10, 60, 100, 144) consists of a semiconductor element selected from the group consisting of a doped semiconductor buried layer or a doped semiconductor wafer. The integrated circuit trench cell (10, 60, 100, 144) of claim 1 wherein a channel (48, 78, 126) is present between the source/drain region (26, 74,118) near the top portion of the wall and the one (30, 76, 102) in the bottom floor of the trench (18, 64, 112), a thin gate dielectric layer (20, 80, 128) is present on the trench wall (17, 66, 110) and the gate region (22, 82, 130, 158) is present on the thin gate dielectric layer (20, 80, 128). The integrated circuit trench cell (100, 144) of claim 1 wherein a lateral FET (106) is present having a source/drain region (118) near the top portion of the trench wall (110). The integrated circuit trench cell (10, 60) of claim 1 wherein the trench (18, 64) has at least two vertical walls (17, 19; 66, 68) and an FET (46, 50; 70,72) is present in at least two of the vertical walls. The integrated circuit trench cell (10, 60) of claim 7 wherein: each FET has two source/drain regions, one (26, 28; 74, 78) near the top portion of the wall and one (30, 76) in the bottom floor of the trench (18, 64) where the source/drain region (30, 76) in the bottom floor of the trench is common to both FETs; and wherein the bottom contact of the load device (54, 90) is in electrical connection with the common source/drain region (30, 76) in the bottom floor of the trench.

Inventors: NGUYEN BICH-YEN, TENG KER-WEN, WANG KARL L, WU WEI, NGUYEN, BICH-YEN, TENG, KER-WEN, WANG, KARL, L., WU, WEI
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1988
Main classification: H01L29
IPC codes: H01L27, H01L29