Abstract:
A buried transistor particularly suitable for SOI technology, where the transistor is fabricated within a trench in a substrate and the resulting transistor incorporates completely isolated active areas. The resulting substrate has a decreased topography and there is no need for polysilicon (or other) plugs to connect to the transistor, unless desired. With this invention, better control is achieved in processing, particularly of gate length. The substrate having the buried transistor can be silicon oxide bonded to another substrate to form an SOI structure.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention concerns fabrication methods and structures for semiconductor-based integrated circuits, particularly methods for making silicon-on-insulator structures.  
       BACKGROUND  
       [0002]     Integrated circuits are the key components in most modern electronic products and are interconnected micro-networks of semiconductor-based electrical components. Processing of such devices typically utilizes various techniques, such as layering, doping, masking, and etching, to build electrical components on a silicon substrate. The components are then interconnected (wired) together to define specific electric circuits, such as a computer processor or memory device. The main focus of progress for the future of integrated circuits is driven by the goals of reducing size, lowering power consumption, and increasing operating speed.  
         [0003]     The standard technology used in the semiconductor industry for integrated circuitry has been CMOS technology. Silicon-on-insulator (SOI) differs from conventional CMOS fabrication technology by placing a transistor gate channel region over an insulator. The most common insulators used with this technique are silicon nitride and silicon oxide. With SOI technology, a gate area can have minimal capacitance; a measure of ability to store an electrical charge. Any medium that can conduct electricity has some degree of capacitance. Technically, a MOS transistor is regarded as a capacitive circuit. This implies that the MOS circuit must completely charge the capacitance to activate its switching capability. The process of discharging and recharging the transistor requires a relatively long amount of time in contrast to the time required to actually switch the voltage state of the transistor&#39;s conductive layer. SOI technology attempts to eliminate this capacitance, since a lower capacitance circuit allows faster transistor speeds.  
         [0004]     In SOI technology, as with all other semiconductor technologies, there is always a desire to improve processing techniques to make fabrication less expensive, simpler, and faster. Another consistent desire is to increase the level of integration by making the devices smaller, denser, and more easily integrated.  
       SUMMARY  
       [0005]     The invention relates to a buried transistor for SOI technology and a method of fabricating it, where the transistor is fabricated within a trench in a silicon substrate and has completely isolated active areas. The resulting substrate has a decreased topography and provides greater freedom in transistor connection. The invention also allows better control over fabrication processing, particularly as it relates to gate length.  
         [0006]     These and other advantages of the invention will be more clearly recognized from the detailed description below, which is provided in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  shows a fabrication process in accordance with the invention at an early stage of fabrication.  
         [0008]      FIG. 2  shows the same fabrication process as  FIG. 1  at a subsequent stage of processing.  
         [0009]      FIG. 2   a  shows an alternative fabrication process to that shown in  FIG. 2 .  
         [0010]      FIG. 3  shows the same fabrication process as  FIG. 2  at a subsequent stage of processing.  
         [0011]      FIG. 4  shows the same fabrication process as  FIG. 3  at a subsequent stage of processing.  
         [0012]      FIG. 5  shows the same fabrication process as  FIG. 4  at a subsequent stage of processing.  
         [0013]      FIG. 6  shows the same fabrication process as  FIG. 5  at a subsequent stage of processing.  
         [0014]      FIG. 6   a  shows an alternative fabrication process to that shown in  FIG. 6 .  
         [0015]      FIG. 6   b  shows the same fabrication process as  FIG. 6   a  at a subsequent stage of processing.  
         [0016]      FIG. 7  shows the same fabrication process as  FIG. 6  at a subsequent stage of processing.  
         [0017]      FIG. 8  shows a block diagram illustrating use of a transistor device as described herein in a processor system in accordance with the invention.  
         [0018]      FIG. 9  shows an illustrative circuit diagram of a transistor device incorporated into a memory cell. 
     
    
     DETAILED DESCRIPTION  
       [0019]     The invention disclosed below relates most generally SOI semiconductor transistors, which can be used in a variety of integrated circuits, including memory devices such as DRAM, SRAM, FLASH, PCRAM etc. (see, e.g.,  FIG. 9 ), or peripheral circuitry, logic circuitry, and a number of other circuits.  
         [0020]     In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. These embodiments are described with sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the invention.  
         [0021]     Now referring to the figures, where like reference numbers designate like elements,  FIG. 1  shows a preliminary stage of fabrication of a buried transistor in accordance with the invention. Throughout the following description the fabrication of a single transistor is shown for simplicity sake; however, a plurality of like transistors are typically fabricated simultaneously in the same substrate, adjacent to one another or not, as is known in the art.  
         [0022]     As shown, in  FIG. 1 , a trench  12  is formed in a semiconductor substrate  10  by etching as is known in the art. Preferably, the substrate  10  is a silicon substrate; however, the invention also has applicability to other semiconductor-on-insulator structures, in which the core substrate  10  may be formed of other semiconductor materials. Etching can be performed, for example, by photolithographic masking of the substrate followed by wet etching or dry etching through openings in the masking material. The sides of the trench are preferably substantially vertical relative to the trench&#39;s depth, such that anisotropic etching is preferred. The width  11  of the trench will, in part, dictate the size of the resulting transistor.  
         [0023]     After trench  12  is formed, doping is performed as shown in  FIG. 2 . An ion implant  14  is performed to form a doped layer at the bottom of the trench  12 . As an alternative to implantation, ion diffusion can be used. This doped layer will form a lightly doped drain (LDD) region  16  of the ultimate transistor. The implant  14  for the LDD region  16  can be relatively shallow so as not to dope too much of the substrate  10 . At this stage in processing, it is also possible to use an angled implant  14 , as shown in  FIG. 2   a , if a halo-type implantation of dopant is desired. A halo implant may be desirable if, for example, enhancement of isolation between devices by reducing the depletion region is a goal, or if grading of junctions in order to control hot-carrier effects is needed. The trench  12  itself can act to shadow the implant if a halo implant is desired. If a halo implantation is used, the LDD region  16  will be graded with increased concentration of dopant toward the sides of the trench.  
         [0024]      FIG. 3  shows the next stage in processing where sidewall spacers  18  are formed on the interior of the trench  12 . The spacers  18  are sidewall insulators for the transistor gate to be formed later. If the spacers  18  are nitride, a nitride layer is formed within the trench  12  and over substrate  10  and etched to remove the nitride layer from the bottom of the trench and upper surface of substrate  10  to create the spacers  18 . The etching of the nitride layer can be controlled such that the space between the spacers  18  exposing the bottom of the trench  12  can be made to be a specific and desired length  20 . This length  20  will ultimately be the gate length  20  of the resulting transistor. Controlling gate length  20  is highly desirable in any semiconductor transistor because changing the gate length  20  effects the transistor threshold voltage (V t ) needed to activate the transistor. Different transistors across the wafer can be formed with different gate lengths to set various V t  across the wafer. Also, drive current is related to gate length  20  as well, wherein essentially “faster” logic devices can be fabricated by making certain transistor gates shorter.  
         [0025]     Following the spacer  18  formation of  FIG. 3 , a further doping occurs to set V t , as illustrated in  FIG. 4 . A V t  implant  22  is performed to form a dopant region  24  in the substrate  10  between the nitride spacers  18 . The spacers  18  shield the substrate  10  directly beneath so that what will become the transistor LDD regions  16  remain. As an alternative to ion implantation, ion diffusion can be used to form dopant region  24 . As a general rule, for short channel devices, as the gate length  20  is reduced the V t  is reduced as well. If it is desired that the V t  be increased, for instance, to keep the same V t  with a shorter gate length  20 , the wafer&#39;s bulk doping can be increased, the gate oxide thickness can be increased, source/drain junction depth can be decreased, back-bias voltage can be increased, or the drain voltage can be decreased. More easily, however, the V t  implant  22  can be adjusted in this stage of processing to control V t .  
         [0026]     Next, as shown in  FIG. 5 , the transistor gate structure is fabricated. After a preferred cleaning step, a gate oxide  26  can be grown over the substrate  10  along the bottom of the trench  12  between the spacers  16 . Silicon oxide is a standard gate oxide  26  material, but others can be used as is known in the art. Next, a doped polysilicon layer  28  is formed over the gate oxide  26  and between the spacers  16 . This layer  28  may be deposited by CVD, sputtering, or other techniques known in the art. A metal layer may be next deposited over the polysilicon layer  28  and heat annealed to form a silicide layer  30 . Titanium and tantalum are commonly used for this purpose. A nitride cap  31  is then formed over the silicide layer, if desired; though this protective cap can be excluded if other insulating materials are later provided over the transistor structure. The above-described layers  26 ,  28 ,  30 ,  31  make up the gate stack  32  of the transistor. Any excess materials of these layer  26 ,  28 ,  30 ,  31  over the wafer (i.e., not in the trench  12 ) can be removed after deposition by a polishing or etching step. The wafer is polished (by, e.g., CMP) or etched to expose a surface of the substrate  10  below the surface of the dopant implants  14  and  22  on either side of the gate stack  32 .  
         [0027]      FIG. 6  illustrates the next step in the process. A source/drain implant  34  is performed in substrate  10  to form source/drain regions  36  on either side of the gate stack  32  and spacers  18 . The implant  34  can be accomplished using a mask as needed. The implant  34  should be of such a power and concentration so as to penetrate the substrate  10  to a level “below” the gate stack  32  so that a channel region  38  is formed “below” the level of the gate stack  32 . An annealing step can be included to activate the implanted dopant forming the source/drain  36 , if needed. After implanting (and activating) the source/drain regions  36 , the transistor  90  is substantially complete. Next, an insulating layer  40  (which will become a buried insulator) can be formed over the transistor and substrate. This insulating layer  40  can be formed of silicon oxide or other insulating materials.  
         [0028]     In an alternative embodiment shown in  FIG. 6   a , the silicon of the substrate  10  adjacent to the gate stack  32  can be patterned using, e.g., a photomask  35 , and etched prior to the implant  34  to be recessed below the nitride cap  31  towards the level of the gate oxide  26 , if desired. The etch mask  35  would be subsequently removed after the etch and implant  34 . In such an embodiment a self-aligned implant with no critical mask is necessary. Then, the substrate  10  material (e.g., silicon) can be regrown, by e.g., epitaxy, back up to be level with the “top” of the gate stack  32  as is shown in  FIG. 6   b , or the gate stack  32  can be left exposed for further processing as desired. After such regrowth, the processing continues as described above and hereafter.  
         [0029]     Once a substantially complete transistor  90  and the insulating layer  40  are formed, additional processing can be performed as shown in  FIG. 7 . The wafer can be flipped over and a second substrate  42 , preferably comprising a semiconductor material and, particularly silicon when substrate  10  is also silicon, can be bonded to the insulating layer  40 , making it a buried insulating layer  40 . If the insulating layer  40  is an oxide layer, the bonding of two thermally matched substrates can be accomplished by silicon oxide bonding techniques, wherein a chemical reaction occurs between the oxidized surfaces of each substrate  10  and  42 . An annealing step can facilitate the silicon-oxide bond. In this way, the buried oxide insulating layer  40  truly becomes buried, as does the transistor  90 . The new “top” surface of the substrate  10  can be etched or polished to a desired thickness, wherein the source/drain regions  36  can be exposed for subsequent processing.  
         [0030]     Subsequent processing of the wafer can include the deposition of dielectric layers and formation of other semiconductor devices in contact with the buried transistor  90 . As is known in the art, capacitors can be formed in contact with the source/drain regions  26 , or with plugs thereto, as can bit lines or other interconnects, if for instance, a DRAM device is to be formed. A circuit diagram for a DRAM memory cell incorporating the transistor  90  is shown in  FIG. 9 , where the transistor  90  acts as an access transistor between a bit line and a capacitor that provides charge coupling therebetween. Also, interconnects can be formed to the source/drain regions  26  electrically linking the transistor to, e.g., logic circuitry, or sensing devices (e.g., sense amplifiers) if the transistor is to be located in periphery circuitry. There is no limit to the uses of the buried transistor  90  in an integrated circuit and, as discussed above, the functioning of the transistor  90  can be tuned during processing so that it has a gate length  20 , channel length  38 , or V t  as desired or necessary.  
         [0031]      FIG. 8  illustrates an exemplary processor system  900 , which can utilize the transistor device  90  of the present invention, as incorporated into a CPU  901  or memory devices  100 . The processor system  900  can include one or more processors  901  coupled to a local bus  904 , the processor containing transistors  90  fabricated as described above. A memory controller  902  and a primary bus bridge  903  can also be coupled the local bus  904 . The processor system  900  can include multiple memory controllers  902  and/or multiple primary bus bridges  903 . The memory controller  902  and the primary bus bridge  903  may be integrated as a single device  906 .  
         [0032]     The memory controller  902  can also be coupled to one or more memory buses  907 . Each memory bus accepts memory components  908 , which include at least one memory device  100  containing present invention. The memory components  908  may be a memory card or a memory module. Some examples of memory modules include single inline memory modules (SIMMs) and dual inline memory modules (DIMMs). The memory components  908  may include one or more additional devices  909 . For example, in a SIMM or DIMM, the additional device  909  might be a configuration memory, such as a serial presence detect (SPD) memory. The memory controller  902  may also be coupled to a cache memory  905 . The cache memory  905  may be the only cache memory in the processing system. Alternatively, other devices, for example, processors  901  may also include cache memories, which may form a cache hierarchy with cache memory  905 . If the processing system  900  include peripherals or controllers which are bus masters or which support direct memory access (DMA), the memory controller  902  may implement a cache coherency protocol. If the memory controller  902  is coupled to a plurality of memory buses  907 , each memory bus  907  may be operated in parallel, or different address ranges may be mapped to different memory buses  907 .  
         [0033]     The primary bus bridge  903  can be coupled to at least one peripheral bus  910 . Various devices, such as peripherals or additional bus bridges may be coupled to the peripheral bus  910 . These devices may include a storage controller  911 , a miscellaneous I/O device  914 , a secondary bus bridge  915 , a multimedia processor  918 , and a legacy device interface  920 . The primary bus bridge  903  may also coupled to one or more special purpose high speed ports  922 . In a personal computer, for example, the special purpose port might be the Accelerated Graphics Port (AGP), used to couple a high performance video card to the processing system  900 .  
         [0034]     The storage controller  911  can couple one or more storage devices  913 , via a storage bus  912 , to the peripheral bus  910 . For example, the storage controller  911  may be a SCSI controller and storage devices  913  may be SCSI discs. The I/O device  914  may be any sort of peripheral. For example, the I/O device  914  may be a local area network interface, such as an Ethernet card. The secondary bus bridge may be used to interface additional devices via another bus to the processing system. For example, the secondary bus bridge may be an universal serial port (USB) controller used to couple USB devices  917  via to the processing system  900 . The multimedia processor  918  may be a sound card, a video capture card, or any other type of media interface, which may also be coupled to one additional devices such as speakers  919 . The legacy device interface  920  can be used to couple legacy devices; for example, older styled keyboards and mice, to the processing system  900 .  
         [0035]     The processing system  900  illustrated in  FIG. 8  is only an exemplary processing system with which the invention may be used. While  FIG. 8  illustrates a processing architecture especially suitable for a general purpose computer, such as a personal computer or a workstation, it should be recognized that well known modifications can be made to configure the processing system  900  to become more suitable for use in a variety of applications. For example, many electronic devices, which require processing may be implemented using a simpler architecture, which relies on a CPU  901 , coupled to memory components  908  and/or memory devices  100 . These electronic devices may include, but are not limited to audio/video processors and recorders, gaming consoles, digital television sets, wired or wireless telephones, navigation devices (including system based on the global positioning system (GPS) and/or inertial navigation), and digital cameras and/or recorders. The modifications may include, for example, elimination of unnecessary components, addition of specialized devices or circuits, and/or integration of a plurality of devices.  
         [0036]     The above description and accompanying drawings are only illustrative of exemplary embodiments, which can achieve the features and advantages of the present invention. It is not intended that the invention be limited to the embodiments shown and described in detail herein. The invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. The invention is only limited by the scope of the following claims.