Abstract:
A nonvolatile flash memory array having silicon device islands isolated from the substrate by an insulator. Each island comprises a split-gate transistor with a control gate and floating gate formed in the upper portion of the island, and source, drain and channel regions formed in a lower portion of the island. High array density is achieved by forming source and drain interconnects in the space between the islands. Also disclosed are processes for forming and programming such arrays.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an improved semiconductor structure for high density device arrays, such as flash memory arrays. In particular, this invention relates to a Silicon-On-Insulator (SOI) flash memory array of fully isolated device islands that can be implemented with a small feature size, and a process for its formation. 
     BACKGROUND OF THE INVENTION 
     SOI technology provides many advantages when used in Complementary Metal-Oxide-Semiconductor (CMOS) Integrated Circuits (ICs). One primary advantage is a significant reduction of parasitic capacitance between a source/drain and a substrate. Other major advantages include elimination of latch-up, reduction of chip size and/or increased device density, and increased circuit speed. SOI devices also have lower power requirements and higher speeds compared to non-SOI devices, making SOI technology popular for use in battery-operated equipment. 
     The advantages of SOI structures result from the total isolation of the device islands from the substrate. Total isolation has been achieved using a sapphire substrate instead of a semiconductor substrate, but the resultant product is expensive and the quality of the crystalline silicon grown on sapphire is usually poor. Processes to achieve device-substrate isolation using a silicon substrate are known, but such processes may not achieve total isolation, and may cause defects in the resultant structures. 
     SOI structures may be formed through a variety of processes, including SIMOX, wafer bonding, FIPOS and etch and oxidation processes. Separation by Implanted Oxygen (SIMOX) involves oxygen ion implantation into a silicon substrate to form a buried oxide insulating layer. This method is expensive and has the disadvantage of damaging the crystalline structure of the silicon above the insulating layer due to the passage of high energy oxygen ions. 
     Wafer bonding is another technique for forming an isolation layer in a substrate. It involves the fusing together of two oxidized silicon wafers in a high-temperature furnace. However, wafer bonding is an undesirable technique because it increases the substrate thickness, and has low production yields due to voids and particles interfering with adequate bonding between the wafers. 
     The Full Isolation by Porous Oxidized Silicon (FIPOS) process forms an insulating layer through the initial formation of a doped layer in the silicon substrate, covered with a layer of undoped epitaxial silicon. The substrate is then anodized to create a porous layer of silicon under islands of undoped silicon in the substrate. The FIPOS process is slow and expensive, and produces a substrate with a tendency to warp or curl due to the thermal stresses it has undergone. 
     Another known technique used to form an isolation layer is a series of etch and oxidation steps used to create silicon islands, as described in U.S. Pat. No. 4,604,162. Islands are formed by the etching and subsequent partial undercutting of the islands. Silicon filaments maintain the connection of the islands to the substrate during the undercutting step, and the subsequent thermal oxidation step. During thermal oxidation, which creates an isolation layer under the islands, expansion of the oxide subjects the islands to substantial mechanical stress and crystal damage. The silicon filaments connecting the islands to the substrate are also under tensile stress that creates dislocations of the islands, resulting in high junction leakage and low carrier mobility. 
     In addition to the isolation of the devices from the substrate, it is also important to isolate the devices from each other. The use of silicon device islands with isolation trenches is known, but such devices typically do not have a high functional density. As the minimum feature size decreases, the number of devices in a chip area (active device density) increases, but the area occupied by interconnection lines on the chip surface minimizes the number of interconnected devices in a chip area (functional density). To be effective, a high density device array should maximize both active device density and functional density. 
     Nonvolatile semiconductor memory devices based on Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) are well-known in the art. There are currently three types of MOSFET nonvolatile memory devices in use: EPROMs, EEPROMs, and Flash EEPROMs. A Flash EEPROM is comprised of an array of non-volatile storage cells from which data may be read any number of times without disturbing the state of the stored data. Each cell is an individual Field Effect Transistor (FET) that stores a bit of information as the presence or absence of an electrical charge on a floating gate. 
     Data is written to a cell by hot electron injection which occurs when a high positive voltage is applied to both the control gate and the drain line. Some of the electrons in the device channel will acquire sufficient energy to jump the energy barrier at the interface of the device channel and the tunneling oxide. Once they are in the tunneling oxide, the electrons are pulled toward the floating gate by the positive voltage on the control gate. This results in charge collection on the floating gate, which in turn affects the threshold voltage of the control gate. 
     The cells are read by addressing the control gate and drain line of a cell with a positive voltage (e.g., 3 to 5 volts). If the floating gate is negatively charged (logical state “1”), the threshold voltage will be high and the cell device will not turn on when addressed. If the floating gate is uncharged (logical state “0”), the threshold voltage will be low, and the device channel will invert when addressed, causing a resulting current in the drain line that can be sensed by current sensing methods known in the art. 
     Erasure is accomplished by Fowler-Nordheim tunneling, also called “cold electron” tunneling. Cold electron tunneling is a quantum-mechanical effect allowing electrons to pass through, instead of over, the energy barrier at the interface of the device channel and the tunneling oxide. Because the electrons are passing through the barrier, this process requires less energy than hot electron injection, and can occur at a lower current density. A high voltage (e.g. 10 volts) is applied between the control gate and the source, causing electrons to leave the floating gate and tunnel through the tunneling oxide to the drain. Any individual cell or all cells may be simultaneously erased by applying an electrical pulse to any or all cells. 
     Typically EEPROMs are comprised of an array of paired transistors: a select or access transistor and a storage transistor. Many flash EEPROMs combine these two transistors into one device—a split gate transistor with two gates sharing a single device channel. The control gate serves the function of the select or access transistor, and the floating gate serves as a storage device. The split gate configuration alleviates the over-erase problem caused by Fowler-Nordheim tunneling. Over-erase occurs because Fowler-Nordheim tunneling is not a self-limiting process, and the floating gate not only loses negative charge when erased, but becomes positively charged. The layered gate structure eliminates this over-erase problem, but results in a larger cell size. 
     The easy reprogrammability, inherent short access time and non-volatility of the stored data make flash memory very attractive for many computer applications. However, high density flash memory cells have been difficult to produce so far. Although densities as high as one cell per F 2  (lithographic feature square) are known, they are typically fabricated by processes that are not easily integratable with the technology needed to produce support circuits and other logic function devices, and the flash cell structure itself may not be integratable. Conventional memory cells which are directly compatible with existing VLSI technologies require an area of 8F 2  or more. 
     There is a need for a high density fully isolated semiconductor structure suitable for use in flash memory arrays. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high density fully isolated semiconductor structure suitable for a flash memory array. The present invention permits effective isolation between devices and reduces the area occupied by interconnection lines on the device surface. 
     In addition, the present invention provides a process for fabricating an isolated structure that minimizes damage to the structure and avoids dislocations during the isolation process. 
     Advantages and features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a device array constructed in accordance with a preferred embodiment of the invention; 
     FIG. 2 is a perspective view of the device array after the formation of the silicon dioxide bars; 
     FIG. 3 is a perspective view of the device array after the definition of the second set of trenches; 
     FIG. 4 is a cross-sectional view of the device array illustrating silicon islands; 
     FIG. 5 is a cross-sectional view of the device array illustrating evacuated regions after the silicon islands have been undercut; 
     FIG. 6 is a cross-sectional view of the device array after the formation of the insulation layer; 
     FIG. 7 is a cross-sectional view of the device array illustrating the location of the source trenches; 
     FIG. 8 is a cross-sectional view of the device array illustrating the electrical contact plug; and 
     FIG. 9 is a cross-sectional view of the device array illustrating the polysilicon drain lines. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is 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, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. 
     The terms wafer or substrate used in the following description include any semiconductor-based structure having an exposed silicon surface in which to form the silicon-on-insulator structure of this invention. Wafer and substrate are to be understood as including silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a wafer or substrate in the following description, previous process steps may have been utilized to form regions/junctions in the base semiconductor structure or foundation. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims. 
     Referring now to the drawings, where like elements are designated by like reference numerals, an embodiment of the device array  20  of the present invention is shown in FIG.  1 . The device array  20  is comprised of a plurality of silicon islands  22  resting on an insulation layer  24  of silicon dioxide on top of a silicon substrate  26 , where the silicon islands  22  are separated from each other by isolation trenches  28 . Each silicon island  22  has dimensions of one F by one F, and each isolation trench  28  is preferably one F wide, where F is the feature size, e.g., 0.25 μm. Each silicon island  22  is a device, such as a transistor, capacitor or other single-function component, but preferably the islands  22  are split-gate transistors. Thus, with the inclusion of island to island isolation, the area per programmed device cell is 4F 2  (2F×2F) The silicon islands  22  have doped regions diffused into their sides, such as the source regions  30  and drain regions  32  shown in FIG.  1 . Polysilicon drain lines  34  run within the isolation trenches  28  on the drain side of the silicon islands  22 . The source regions  30  of the silicon islands  22  form source contacts to the silicon substrate  26  through a contact plug  40  made from a refractory metal and located within alternate parallel isolation trenches  28 . Gate conductor lines  42  connecting individual silicon islands  22  are metal wires running orthogonal to the polysilicon drain lines  34 . 
     There are three sets of interconnects in the device array  20 . The contact plug  40  serves as a source interconnect, the polysilicon drain line  34  serves as a drain interconnect, and the gate conductor lines  42  serve as gate interconnects. A high density array is achieved by locating one or more of these interconnects in the isolation trenches  28 , thereby utilizing this normally empty space. Even though the preferred embodiment of the device array  20  places the source and drain interconnects in the isolation trenches  28 , island-to-island isolation is maintained by the insulation layer  24  and by careful design of the device array  20 . 
     The structure of each island may best be seen by reference to FIGS. 1 and 9. Each island  22  is a split-gate transistor with two gates: a floating gate  44  and a control gate  46 . The floating gate  44  and control gate  46  are completely self-aligned with the device channel  48 . The device channel  48  comprises doped silicon, preferably p− silicon, and is overlaid by a tunneling oxide layer  50  comprised of thermal oxide. On top of the tunneling oxide  50  is the floating gate  44 , which is formed of doped polysilicon, preferably n+ polysilicon. A gate oxide layer  52  is formed of thermal oxide on top of the floating gate  44 , and serves to insulate the floating gate  44  from the control gate  46 . The control gate  46  lies on top of the gate oxide layer  52 , forming the top layer of the island  22 , and comprises doped polysilicon. Because the island is an NMOS device island  22 , the control gate  46  will be n+ polysilicon. 
     As seen in FIG. 2 the device island  22  has an n+ doped source region  30  and an n+ doped drain region  32  flanking a p− doped device channel  48 . The control gate  46  of the device island  22  is n+ doped. 
     Data is written to a cell  22  by hot electron injection which occurs when a high positive voltage is applied to both the control gate  46  and the drain line  34 . The cells  22  are read by addressing the control gate  46  and drain line  34  of a cell  22  with a positive voltage (e.g., 3 to 5 volts). Erasure is accomplished by Fowler-Nordheim tunneling, which occurs when a high voltage (e.g. 10 volts) is applied between the control gate  46  and the source  30 , causing electrons to leave the floating gate  44  and tunnel through the tunneling oxide  50  to the drain  32 . 
     The device array  20  is manufactured through a process described as following, and illustrated by FIGS. 2 through 9. First, a silicon substrate  26  is selected as the base for the device array  20 . The silicon substrate  26  may be doped or undoped, but a p− type doped wafer is preferred. Second, photolithography is used to define areas where PMOS devices are to be formed and n-wells (not shown) are implanted. The level of doping in the n-wells may vary but should be of comparable strength to the doping level of the silicon substrate  26 . After any resist and mask are removed, a tunneling oxide layer  50  of approximately 3.5 to 6 nm in thickness is grown on top of the silicon substrate  26  by means of thermal oxidation. N+ doped polysilicon is then deposited, preferably by Chemical Vapor Deposition (CVD), on top of the tunneling oxide layer  50  to form a floating gate layer  44  preferably 40 to 90 nm thick. 
     Photolithography and subsequent etching are used to remove the floating gate layer  44  and the tunneling oxide layer  50  in non-array areas. A gate oxide layer  52  of approximately 10 nm thickness is formed over the entire device array  20  by thermal oxidation. The gate oxide layer  52  serves as a control gate oxide in array areas, and as a device gate oxide in non-array areas. P+ doped polysilicon is then deposited, preferably by CVD, on top of the gate oxide layer  52  to form a control gate layer  46  preferably about 100 nm thick. In array areas the control gate layer  46  serves as a control gate, and in non-array areas it serves as a device gate. A photoresist and mask are then applied to cover PMOS device areas, and n-type dopant is implanted into the control gate layer  46  of NMOS devices  22  to counterdope the control gate layer  46 . The photoresist and mask are then stripped. 
     A nitride pad  64  is then formed by depositing a layer of silicon nitride (Si 3  N 4 ) (“nitride”) by CVD or other means, on top of the control gate layer  46 . The nitride pad  64  is preferably about 60-100 nm thick. Taken together, the tunneling oxide layer  50 , the floating gate layer  44 , the gate oxide layer  52 , the control gate layer  46 , and the nitride pad  64  form the device layer  66 . 
     The next step is to define trenches in the silicon substrate  26 . A resist (not shown) and mask (not shown) are applied, and photolithographic techniques are used to define the area to be etched-out. A directional etching process such as Reactive Ion Etching (RIE) is used to etch through the device layer  66  and into the silicon substrate  26  to a depth of preferably about 600 nm to form a first set of trenches  70 . The first set of trenches  70  is defined by laterally-isolated bars of silicon  72  in the silicon substrate  26 . After removal of the resist, the first set of trenches  70  is filled with silicon dioxide by CVD or other suitable process, to form silicon dioxide bars  74 . The device array  20  is then planarized by any suitable means, such as Chemical-Mechanical Polishing (CMP), stopping on the nitride pad  64 . The structure now appears as shown in FIG.  2 . 
     A nitride masking layer  76  is applied, preferably by CVD, to a thickness of about 60-100 nm. Photolithography is used to define a second set of trenches  80  orthogonal to the first set of trenches  70 . The nitride masking layer  76  and the device layer  66  are etched out by a directional etching process such as RIE to define the sides of the second set of trenches  80 . Etching is continued down to the level of the silicon substrate  26 , and then the resist is removed. The device array  20  now appears as shown in FIG.  3 . 
     Next, a thermal oxide layer  82  of approximately 10 nm thickness is grown on the edges of the exposed floating gate layer  44  and the control gate layer  46 , and directionally etched to remove thermal oxide produced on horizontal surfaces of the silicon dioxide bars  74 , while retaining the thermal oxide layer  82  on the edges of the exposed gate layers  44 ,  46 . The thermal oxide layer  82  serves to protect the gate layers  44 ,  46  from contact with the source and drain interconnects  40 ,  34  during the subsequent steps of the fabrication process. 
     The exposed silicon substrate  26  is then etched with a preferential anisotropic etchant such as a RIE to a depth of preferably about 0.3 μm+½ the island width creating sidewalls  84 . The depth should be sufficient to ensure that the silicon island  22  will not be consumed by the thermal oxidation process in the isolation step. Now that the silicon islands  22  have been created, the next step is to isolate them from the silicon substrate  26 . A nitride film  86  approximately 20 nm thick is formed on the sidewalls  84  by depositing a layer of CVD nitride and directionally etching to remove excess nitride from horizontal surfaces of the silicon dioxide bars  74  and the nitride masking layer  76 . The nitride film  86  acts as an oxidation and etch barrier during subsequent steps. The structure is now as shown in FIG.  4 . 
     The silicon islands  22  are then fully undercut by a timed isotropic etching process that extends the second set of trenches  80  downward and laterally until the silicon islands  22  are completely undercut as shown in FIG.  5 . Any suitable isotropic etchant may be used, such as a commercial etchant sold under the name CP4 (a mixture of approximately 1 part of 46% HF: 1 part CH 3 COOH: 3 parts HNO 3 ). An isotropic etch is employed in this step to compensate for the volume of oxide to be formed in the next step. The time of the etching process is calculated to undercut the islands completely while not excessively consuming the silicon comprising the silicon islands  22 . As is known, thermal oxidation produces a volume of oxide approximately twice that of the silicon consumed. Although completely undercut, the silicon islands  22  are supported by contact (bonding) with the silicon dioxide bars  74 . Each silicon island  22  retains a quantity of silicon attached to the bottom of the device layer  66  that forms a device base  90 , and the space between the device base  90  of each silicon island  22  and the silicon substrate  26  defines an evacuated region  88 . 
     The evacuated regions  88  are then filled with an insulation layer  24  by thermally oxidizing the exposed silicon substrate  26  and the device bases  90 . The insulation layer  24  horizontally isolates the silicon islands  22  from the silicon substrate  26 , as seen in FIG. 6. A suitable oxidation process can be performed in a standard silicon processing furnace at a temperature of approximately 900 to 1100 degrees Celsius. A wet, oxidizing ambient is used in the furnace chamber to oxidize the exposed silicon substrate  26  and the device bases  90  in a parallel direction to the surface of the silicon substrate  26 . The oxidation time is selected to cause the evacuated regions  88  to be filled. As the desired width of the silicon islands  22  decreases, so does the required oxidation time. For example, for sub-micron technology the oxidation time is approximately 3 to 4 hours. For sub-0.25 micron technology, the oxidation time is approximately 1 hour. 
     Because no silicon filaments connect the silicon islands  22  to the silicon substrate  26  during thermal oxidation, tensile stress-induced crystal damage is minimized and preferably eliminated at the silicon/oxide interface. Furthermore, dislocation of the silicon islands  22  due to tensile stress is also minimized or eliminated because the silicon islands  22  are bonded into position by the silicon dioxide bars  74 . The second set of trenches  80  are then filled with polysilicon, preferably intrinsic polysilicon, i.e., purified and undoped polysilicon, by CVD or other suitable process, and subsequently the device array  20  is planarized, preferably by CMP, to form intrinsic polysilicon blocks  100 , as shown in FIG.  6 . 
     Source trenches  102  are then defined by photolithography, and subsequent etching of the intrinsic polysilicon blocks  100 , as shown in FIG.  7 . The source trenches  102  are formed by etching out alternate trenches of the second set of trenches  80 , and are defined by sidewalls  84 , as shown in FIG.  4 . The source trenches  102  extend through the intrinsic polysilicon blocks  100  to expose the insulation layer  24  and the nitride film  86 . After the resist is removed, the nitride film  86  is removed from the exposed trench sidewalls  84 , preferably by isotropic etching with a nitride etchant such as phosphoric acid. 
     With reference to FIG. 8, a polysilicon liner  104  abutting the sidewalls  84 , approximately 50 nm thick, is then formed by deposition of doped polysilicon, preferably by CVD. The nature of the dopant in the deposited polysilicon will vary depending on the nature of the device  22  to be formed. For example, an NMOS device  22  will have n+ doped polysilicon. A short thermal anneal sufficient to drive dopant from the polysilicon liner  104  into the sides of the exposed device bases  90  is then performed, forming source regions  30  of the silicon islands  22 . 
     The source trenches  102  are then cleaned out by directionally etching to remove the polysilicon liner  104  from all horizontal surfaces of the device array  20 . A timed directional etch of the insulation layer  24  is then performed to expose the silicon substrate  26  at the bottom of the source trench  102 . Overetch should be minimized because the etching process can also attack the exposed silicon dioxide bars  74 . 
     A contact plug  40  is then formed between the silicon substrate  26  and the polysilicon liner  104 , preferably by chemical vapor deposition of refractory metal (not shown) to fill the source trenches  102 . A timed selective etch such as RIE is then performed to recess the electrical contact plug  40  and to clear off any metal which was deposited on the top of the silicon dioxide bars  74 . 
     In addition, to prevent undesirable rectification which may result from inadvertent formation of a Schottky diode, the contact plug  40  is preferably comprised of refractory metal. Any refractory metal such as tungsten, titanium, tantalum or molybdenum is suitable for forming the contact plug  40 , but tungsten is preferred. A cross-section of the device array  20  at this point in the process is shown in FIG.  8 . 
     The next step is to remove excess polysilicon by isotropic etching of exposed portions of the polysilicon liners  104  so that they are recessed below the level of the floating gate layer  44 . This etch will have the side effect of partially recessing the intrinsic polysilicon blocks  100  across the device array  20 . Next, silicon dioxide is deposited to form oxide filler  106 , and subsequently planarized. Any suitable deposition method may be used, such as CVD, and any effective planarization method such as CMP is suitable for use. 
     Drain trenches  108  are next defined by photolithography to mask the remaining alternate trenches of the second set of trenches  80 . Directional etching is used to remove the oxide filler  106 , and to clean out the drain trench  108  to the approximate level of the bottom of the silicon islands  22 . A selective etch is then used to remove any remaining polysilicon from the intrinsic polysilicon blocks  100 . 
     Isotropic etching is then performed to remove any remaining nitride film  86  from the sidewalls  84 . Doped polysilicon (not shown) is deposited by chemical vapor deposition, and directional etching, such as RIE, is performed to remove polysilicon from horizontal surfaces of the device array  20 , and to recess the polysilicon to the approximate level of the floating gate layer  44 . The remaining doped polysilicon forms polysilicon drain lines  34  abutting the sidewalls  84 . An annealing process is then performed to drive dopant from the polysilicon drain lines  34  into the sides of the exposed device bases  90 , forming drain regions  32 . The device array at this stage is shown in FIG.  9 . 
     The device array  20  then undergoes a finishing process. The drain trenches  108  are filled with silicon dioxide and the surface of the device array is planarized, by CVD and CMP, respectively, or other suitable processes. Selective etching with hot phosphoric acid or another suitable etchant is then performed to remove the nitride masking layer  76  and the nitride pad  64  from the top surface of the silicon islands  22 . Conventional processing methods may then be used to form contact holes and metal wiring to connect gate lines and to equip the device array  20  for peripheral circuits. The final structure of the device array  20  is as shown in FIG.  1 . 
     The process sequence described and illustrated above provides for the formation of minimum dimension programmable devices of one carrier type. It follows that devices of either carrier type may be made on the same chip by substitution of the appropriately doped materials and addition of appropriate masks. In addition, implanted source/drain devices may be fabricated on the same chip by process integration with common process steps. 
     The above description and drawings illustrate preferred embodiments which achieve the objects, features and advantages of the present invention. It is not intended that the present invention be limited to the illustrated embodiments. Any modification of the present invention which comes within the spirit and scope of the following claims should be considered part of the present invention.