Abstract:
A self-aligned double gate transistor, comprising: a first silicon portion on an isolation layer, the silicon portion having formed therein a source region and a drain region separated by a channel region, and having a first side and a second side, the first side and the second side having a first gate oxide and a second gate oxide, respectively, formed thereon; a first silicon gate abutting said first side of said channel region on said insulator; and a second silicon gate abutting said second side of said channel on said insulator. 
     A method for manufacturing a double gate transistor device, comprising: providing a substrate having a buried oxide region; depositing a first nitride mask layer having a pattern overlying a silicon region; forming a trench in said substrate with a depth to said buried oxide; forming a gate oxide in said trench; depositing polysilicon in said trench; depositing a second nitride mask layer having a pattern formed perpendicular to said first nitride mask; etching the portion of said polysilicon not underlying said first or second nitride layers; removing said second nitride layer; and implanting an impurity into exposed portions of polysilicon in said trench and of said silicon-on-insulator substrate underlying said second nitride layer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a structure and a method for constructing a double-gate transistor device, particularly a double-gate transistor. 
     2. Description of the Related Art 
     Double-gate transistors have a number of different uses. However, construction of vertical double-gate devices has always been somewhat difficult, given alignment problems between the two gates used in the device. 
     FIG. 1 shows a cross-sectional view of a double-gate device formed on a silicon substrate  10  having a top gate  12  and a bottom gate  14  formed in a buried oxide layer  16 . Construction of gates  12  and  14  in this stacked type of device leads to alignment problems between the vertically separated top and bottom gates  12  and  14 , respectively. In addition, care must be taken to provide ample connectivity for gate  14 . 
     Simplification and self-alignment in semiconductor processing methods is constantly advantageous. Hence a method which provides a simplified method for forming a double-gate device would be advantageous. 
     SUMMARY OF THE INVENTION 
     The invention, roughly described, comprises a self-aligned double gate transistor. The transistor includes a first silicon portion on an isolation layer, the silicon portion having formed therein a source region and a drain region separated by a channel region, and having a first side and a second side, the first side and the second side having a first gate oxide and a second gate oxide, respectively, formed thereon; a first silicon gate abutting said first side of said channel region on said insulator; and a second silicon gate abutting said second side of said channel on said insulator. 
     In a further embodiment, the invention comprises a method for manufacturing a double gate transistor device. The method for manufacturing includes the steps of: providing a substrate having a buried oxide region; depositing a first nitride mask layer having a pattern overlying a silicon region; forming a trench in said substrate with a depth to said buried oxide; forming a gate oxide in said trench; depositing polysilicon in said trench; depositing a second nitride mask layer having a pattern formed perpendicular to said first nitride mask; etching the portion of said polysilicon not underlying said first or second nitride layers; removing said second nitride layer; and implanting an impurity into exposed portions of polysilicon in said trench and of said silicon-on-insulator substrate underlying said second nitride layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with respect to the particular embodiments thereof. Other objects, features, and advantages of the invention will become apparent with reference to the specification and drawings in which: 
     FIG. 1 is a cross-section diagram of a stacked gate double-gate device. 
     FIGS. 2-10 show cross-sections of a first embodiment of a semiconductor device formed in accordance with the present invention at various points in the manufacturing process, wherein: 
     FIGS. 2A-10A show a plan view of a series of steps utilized in constructing the device; 
     FIGS. 2B-10B show a first cross-section along lines X-X′ (represented in FIG. 2A) in each of the corresponding FIGS. 2A-10A and  2 C- 10 C; and 
     FIGS. 2C-10C show a cross-section along line Y-Y′ (represented in FIG. 2A) in corresponding FIGS. 2A-10A and  2 C- 10 C. 
     FIG. 11 is a cross-section of a second embodiment of the device of the present invention. 
     FIGS. 12-16 show cross-sections of a second embodiment of a semiconductor device formed in accordance with the present invention at various points in the manufacturing process, wherein: 
     FIGS. 12A-16A show a plan view of a series of steps used in constructing a second embodiment of the device; 
     FIGS. 12B-16B show a first cross-section along lines X-X′ (represented in FIG. 12A) in each of the corresponding FIGS. 12A-16A and  12 C- 16 C; and 
     FIGS. 12C-16C show a cross-section along line Y-Y′ (represented in FIG. 12A) in corresponding FIGS. 12A-16A and  12 C- 16 C. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention provides in one embodiment a unique structure for a double-gate transistor device, and specifically a double-gate silicon-on-insulator device. Moreover, a novel method of constructing the device is provided. In an alternative embodiment, a three-sided gate device, and a method for making the device, is disclosed. 
     It should be recognized that while the devices are described herein with respect to construction as a silicon-on-insulator device, the silicon-on-insulator devices may be formed on any number of methods including bonding a handle wafer to a device wafer, implanting an oxide layer deep within a bulk silicon substrate, or deposited polysilicon on an oxide layer, or any of a number of well-known silicon-on-insulator construction techniques. It should be further recognized that while a silicon-on-insulator substrate is described, other types of substrates including germanium-on-insulator are equally contemplated as being within the scope of the constructed techniques and the structure taught herein. 
     Each of FIGS. 2-10 shows a plan view (FIGS.  2 A- 10 A), a first cross-section  2 B- 10 B along the perspective X-X′ (illustrated in FIG.  2 A), and a second cross-section  2 C- 10 C along line Y-Y′ (illustrated in FIG.  2 A), of the device under construction in accordance with a first embodiment of the method of the present invention. Each FIGS. 2-10 shows the device in that Figure at a single point in time during construction such that, for example, each FIGS. 2A,  2 B,  2 C are three perspectives at the same point in time. 
     FIG. 2A is a plan view of a silicon-on-insulator substrate  100  having, for example, a device wafer  102 , a buried oxide layer  104 , and a handle wafer  106  bonded to the device wafer  102  via oxide  104 . The particular method of construction of the silicon-on-insulator substrate is not germane to the scope of the present invention. Nevertheless, for convention, the terms “device layer,” “buried oxide” and “handle wafer” will be used without intending to limit the nature of the substrate&#39;s construction. 
     As shown in FIGS. 2B and 2C, initially, an oxide layer  112  having a thickness of about 5-15 nanometers is formed on the surface of substrate  100 . Next, a nitride mask layer  110  having a thickness of approximately 100-190 nanometers is deposited on the surface of device substrate  102 . Nitride mask layer  110  will be utilized as a mask for a silicon etch performed in device layer  102 . Nitride layer  110  may be formed by any number of known processes including, for example, low pressure chemical vapor deposition (LPCVD), which forms a blanket layer of nitride across the surface of the wafer, which is then subsequently patterned to form the nitride mask shape shown in FIG. 2A by applying a resist mask (not shown), patterning the mask, and etching the nitride layer. Nitride layer  110  is formed to a thickness which is sufficient to protect the underlying polysilicon and oxide layers during the subsequent processing described herein. In one embodiment, the first nitride will be twice as thick as a second nitride shown as applied in FIG.  7 . In accordance with the invention, the nitride layers are etched twice during silicon processing and must withstand this etching to protect the underlying device areas as discussed below. 
     As shown in FIGS. 3A-3C, a silicon etch removes selected portions of the device layer  102  down to buried oxide layer  104 . The silicon etch may be performed by well-known techniques including reactive ion etching or any directional etch technique, to generate a high aspect ratio trench  120  surrounding silicon  122 . 
     In accordance with the invention, the width W of the silicon  122  as shown in FIG. 3B is defined based on the width W of the nitride layer  110 , and the dimensions of the device which are ultimately to be achieved. As will be understood from the following descriptions, the width of the transistor device formed in accordance with the invention will be determined by the vertical thickness of the silicon  122  (e.g. the thickness of device layer  102 ), the length of the device by the length of a second nitride layer deposited as discussed below with respect to FIG. 7 in the Y-Y′ direction, and the width “W” of the nitride  110  which ultimately controls the distance between the first and second gates. 
     As shown in FIGS. 4A,  4 B,  4 C, following the trench etch, a sacrificial oxide  125  is provided on silicon  122  to clean the sidewalls  120   a ,  12   b  of the silicon  122  so that the subsequent gate oxide will have a conformal and aligned growth, and thereby provide a high quality oxide for the transistor device subsequently formed. This sacrificial oxide will consume aportion of silicon  122  and will narrow the effective distance between the first and second gates. Because of this oxidation step and the gate oxidation step hereinafter described, the width W of silicon  122  cane formed to a minimum of approximately 20 nanometers (following oxidation), at which point oxidation stress will negatively impact the functioning of the device. Thus, for example, if the desired final silicon width W S  in the silicon  122  is to be 30 nanometers, the width W N  of the nitride  110  must be 50 nanometers to accommodate both the sacrificial and the gate oxide; for an initial width W of 70 nanometers, the second nitride  155  will have to be larger than 80 nanometers; for a width of 120 nanometers, the second nitride  155  will have to be larger than 170 nanometers. It should be understood that the delinition of width W limits the channel length (in the Y-Y′ direction) required in order to fully deplete the channel. In addition, buried oxide  104  must be thick enough to withstand subsequent oxide removals in the channel  120 . 
     FIGS. 5A-5C show the device following the sacrificial oxide cleaning step. Subsequent to the oxide cleaning step, a gate oxide will be thermally grown in the trench along the silicon walls  120   a ,  120   b . While conformal oxide formation processes may be used to form gate oxide  130 , thermal oxidation generally provides better gate oxides than deposited oxide processes. Gate oxide  130  can be grown by immersing the substrate  100  as shown in FIGS. 5A-5C in an oxygen ambient and heating the substrate for a period of approximately two minutes to grow the gate oxide to a thickness of approximately 1.2-1.7 nanometers. It should be recognized that the thickness of the gate oxide will depend upon the dimensions of the double gate transistor being formed, and particularly the width W of the trench. Alternatively, a nitrided oxide can be used for layer  130 , formed by immersing the substrate in a nitrogen and oxygen ambient. 
     As further shown in FIGS. 6A-6C, following formation of the gate oxide  130 , the trench is back-filled with a deposited polysilicon  140 . The process of depositing polysilicon  140  to fill the trench and subsequently heating polysilicon will result in an oxide layer  145  covering polysilicon  140 . As discussed further below, polysilicon  140  will serve as the channel region and source and drain regions for the double-gate transistor of the present invention. 
     Following the polysilicon fill step, as shown in FIGS. 7A-7C, a second nitride layer  150  is deposited on the surface of the device wafer  102  to cover the trench area and the polysilicon in the trench. 
     As shown in FIGS. 7A-7C, a second nitride layer  155  is deposited and formed to a perpendicular strip (relative to the first nitride layer) as shown in FIG.  7 A. Nitride  155  may be formed in accordance with the previously-discussed deposition and etch method, or any alternative method suitable for forming the nitride  155  in a pattern as shown in FIGS. 7A-7C. 
     For example, a first nitride layer  110  has a thickness of approximately 50-190 nanometers, while a second nitride layer  155  has a thickness of approximately 25-95 nanometers. 
     As shown in FIGS. 8A-8C, subsequent to the deposition of the second nitride layer  155 , polysilicon regions not underlying the first or second nitride are etched away to isolate the double transistor structure of the present invention. This leaves polysilicon region  140  remaining in the trench surrounded by gate oxide  120   a  and  120   b  and a first  102   a  and second  102   b  regions of device wafer  102  which will form the self-aligned double gates of the device of the present invention. 
     As shown in FIG. 9A, a portion of the first nitride layer  110   a  remains over what will become the channel region of the device of the present invention. At this point, in one embodiment, nitride spacers  142 ,  144  may optimally be provided. In accordance with conventional techniques, a nitride layer may be deposited and etched to form spacers  142 ,  144  prior to the source/drain implant illustrated in FIG.  9 C. 
     As shown in FIG. 9, a source/drain/gate impurity implant is then performed to form the source and drain regions of the device of the present invention. The implant is such that the nitride  150  covering the channel area prevents implantation of the impurity into the channel region. Depending on the device size and strength of the impurity implant, the nitride  110  may be supplanted by an additional layer of oxide formed between nitride  110  and second nitride  155 . 
     For an N-channel device, arsenic or phosphorous may be deposited to a concentration of approximately 2-4×10 15 /cm 2  at energies of approximately 15-20 KeV (arsenic) or 7-10 KeV (phosphorous). For a P-channel device, boron may be implanted into a concentration of 2-3×10 15 /cm 2  at energies of approximately 1.5-2.5 KeV. Zero degree tilt implants are used for these embodiments. The implants are then subsequently annealed in a rapid thermal anneal for approximately 5-10 minutes at a temperature of 1000-1025° C. 
     Finally, as shown in FIG. 10, a TEOS oxide may be deposited into the area  160  which has been etched away by the polysilicon etch described with respect to FIG.  8  and polished back to the nitride layer  150 . The resulting device has gate impurity which is the same species as the source and drain. 
     It should be recognized that it is possible to fabricate the device so as to have an asymmetric work function (one gate with an N+ doping, the other gate with a P+ doping). One manner of forming asymmetric gates is to in-situ dope the device wafer with, for example, a P+ dopant, and then counter dope only one gate using a mask layer over area  102  or  102   b.    
     Yet another embodiment of the invention is shown in FIG.  11 . The device in FIG. 11 has a gate on three sides of the silicon island  140  which forms the channel of the device shown in FIGS. 2-10 and in FIGS. 11,  12 - 16 . The device in FIG. 11 is processed in accordance with the method described below, which varies from the first embodiment in that, instead of depositing a second nitride layer, a gate mask is use to form the gate regions as follows. 
     This process for fabricating the device of FIG. 11 is shown in FIGS. 12A-C to FIGS. 16A-C. The device in FIG. 12 represents the device processed up to the point shown in FIG. 3 (following the device layer etch of FIG.  3 ), but with the exposed (trench) areas filled with TEOS  210 . TEOS  210  may be deposited in accordance with well-known techniques such that an upper surface of the TEOS layer reaches the upper surface of nitride  110 . 
     Next, as shown in FIGS. 13A-13C, a gate photoresist  220  mask is deposited over the surface of the structure. Photoresist  220  may be any positive or negative reacting photoresist layer. Following deposition, as shown in FIGS. 14A-14C, an opening  230  in the photoresist is made of a shape corresponding to that of the second nitride layer shown in the previous embodiment. Processing of the photoresist layer to form opening  230  may be performed in accordance with well-known techniques depending on the type of photoresist layer used. 
     Next, as shown in FIGS. 15A-15C, a directional etch of the TEOS  220  and the first nitride layer opens two vias  240 , 245  in the substrate, as well as removing the first nitride over the polysilicon  122 , exposing the underlying silicon  140 . Following via formation, a gate oxide  242  maybe grown by immersing the structure in an oxygen-containing atmosphere for two minutes. Next, as shown in FIGS. 16A-C, a polysilicon fill  250  forms the three-sided gate region  200  out of silicon over lying the silicon island  122 . The polysilicon deposition maybe in-situ doped with a desired impurity concentration to form the three sided gate structure  200  surrounding island  122 . 
     In one embodiment, where a plurality of transistors are formed on the substrate, all n+ type gates are formed together, and p+ type gates are formed together. First, a p+ device gate mask (such as mask  220 ) is applied and all vias (both  240  and  245 ) for p+ gate devices are opened using an etch as described above. The p+ device gate mask is then removed. Next, gate oxide ( 242 ) is formed in all p+ gate devices and in-situ doped p+ polysilicon ( 200 ) is deposited, followed by a chemical mechanical polish of the polysilicon to remove excess polysilicon from the surface of region  200 . Next, an n+ gate mask ( 220 ) and etch are used to open vias ( 240 , 245 ) for all n+ gate devices, the n+ device gate mask is removed, gate oxide ( 242 ) for the n+ gate devices is formed, and in-situ doped n+ type polysilicon is deposited, followed by a polishing step of the n+ type polysilicon to remove excess polysilicon from the surface of region  200 . Alternatively, a direct implant and diffusion of a desired impurity may be used in each case. Next, an implantation of impurities, as set forth above with respect to the first embodiment, may be made to complete the source and drain regions. 
     In another embodiment, if in-situ doped polysilicon is used, the channel area may be opened twice for each device, as opposed to once. First, to form a p-channel device, an etch to open a first side (for example, via  240 ) may be opened (using a first mask layer  220 ); a first gate oxide grown, and a first deposition of in-situ p+ impurity doped polysilicon deposited in via  240 . This polysilicon is then etched back. Next, a second via (for example, via  245 ) may be formed and a second gate oxide grown (which will oxidize the p+ doped polysilicon to prevent counter doping), followed by a deposition of an n+ impurity doped polysilicon. This n+ polysilicon is then polished back. The center gate region may be opened with either via  240  or  245 . 
     The many features and advantages of the present invention will be apparent to one of average skill in the art. The invention forms both gates of a double gate transistor in the same plane as the silicon substrate. Both gates are fully self-aligned. The channel area can be made to a very thin 20-nanometer thickness by trench oxidation prior to the trench fill. In contrast, the vertically-aligned double-gate transistor has to obtain a very thin silicon layer by selective epitaxial silicon growth with a lack of thickness control. The thickness of the initial silicon island determines the minimum device width. For larger widths, a device has to be divided into multiple fingers. All such features and advantages are intended to be within the scope of the invention as defined herein by the attached claims.