Abstract:
Charge-trapping field effect transistors may be formed into an array on a wafer suitable to be a NAND memory device. A thick oxide layer is applied over the gates to ensure that the gap between the gates is filled. The filled gap substantially prevents nitride from being trapped, which could otherwise decrease the yield of the devices. This technique, and its variations, are useful for a range of semiconductor devices.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates generally to spacer fabrication, and is well suited to forming spacers in a NAND flash memory array. 
     2. Background Art 
     The semiconductor market has been undergoing extensive growth over the past few decades. This trend is expected to continue for the foreseeable future since a large portion of this market is the memory segment. The memory segment can be broadly categorized into two classes, namely volatile memory and non-volatile memory. Volatile memory such as SRAM and DRAM lose their data content when the power supply is removed. In contrast, non-volatile memories such as EEPROM and flash memories maintain their data content after the power supply has been removed. 
     Non-volatile memories offer particular advantages, and thereby support a wide range of applications including computer, automotive and consumer electronic devices. Flash memory is a non-volatile memory that can be electrically erased and reprogrammed. In fact, flash memory has undergone an explosive market growth that has in particular been driven by cellular telephones, memory cards, flash drives and other types of portable data storage devices. Indeed, with the need to support persistent data storage in portable devices, it is clear that the flash memory will continue to grow at an ever increasing rate. Further, the market place will demand flash memory designs that support lower cost and higher performance, including higher densities of storage. 
     An ongoing barrier to reducing cost, improving performance and storage density is yield of the devices. Semiconductor devices are manufactured on wafers (currently 300 mm discs of silicon, which are expected to increase to 450 mm discs in the near future). The wafers are divided into a rectangular grid, and semiconductor devices are fabricated in the rectangles. After fabrication, the devices are tested. If a device fails, it will not be sold to consumers, thus reducing profit for the manufacturer or increasing cost to the consumer. However, if more and more devices are manufactured without errors or defects, more aggressive designs may be pursued, which can provide customers with devices that have higher performance and higher densities. 
     Improving yield remains an elusive goal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. 
         FIG. 1  is a schematic illustration of a memory array. 
         FIG. 2  is a side view of a wafer undergoing fabrication. 
         FIG. 3  is a side view of the wafer of  FIG. 2  at a later stage of fabrication. 
         FIG. 4  is a side view of the wafer of  FIG. 3  at a later stage of fabrication. 
         FIG. 5  is a flow chart of a method for fabricating a spacer. 
     
    
    
     The features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION 
     The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     Currently, intense global competition in the semiconductor manufacturing space, particularly for NAND flash memory, means that improvements in yield can be quite valuable. To a lesser extent, reducing the number of operations required to manufacture a wafer of NAND memories can also lead to a competitive advantage. 
     An issue can arise when nitride gets trapped in oxide spacers. Trapped nitride can lead to defects in volume production because the trapped nitride changes the conductivity of the spacer oxide, which is designed to serve as an insulator. In particular, nitride in a spacer could trap electrons during erase and cause string current reduction. Either of these defects could cause a NAND memory to fail a quality control check, leading to a need to dispose of that NAND memory and possibly the entire wafer. 
     Therefore, what is needed is new techniques to manufacture semiconductors to prevent trapped nitride. 
     According to an embodiment of the present invention, there is provided a semiconductor device comprising first, second and third gates on a substrate. A first portion of oxide fills a region between the first gate and the second gate, such that the oxide reaches from the substrate up to at least almost a top of at least one of the first gate and the second gate. A second portion of oxide forms a drain adjacent to the third gate. The second portion of oxide and the third gate have a width large enough to meet a minimum feature size and the first portion of oxide is narrower than the minimum feature size. 
     According to another embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, by forming first, second and third gates on a substrate, depositing oxide over the first, second and third gates, a first etching of the oxide that does not substantially etch oxide below tops of the first and second gates, a second etching of a region of oxide adjacent to the third gate that etches to the substrate, and forming a drain in the region of oxide adjacent to the third gate. The oxide between the first gate and the second gate is narrower than a critical dimension. The region of oxide adjacent to the third gate and the third gate are wider than a critical dimension. 
     Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
       FIG. 1  depicts a transistor level schematic of a memory array  100 . Memory array  100  comprises an array of Field Effect Transistors (FET)  102 . FET  102  is capable of storing charge corresponding to a ‘1’ or ‘0’, and thus an array of FETs  102  may be used to form an array of memory cells, such as a NAND flash memory. NAND memories at sizes such as 8 GB, 16 GB or 32 GB are commercially available. GB stands for gigabyte, which for mass storage is generally understood to be one billion bytes, with eight bits to the byte. In some cases, GB refers to 1024^3 bytes, or about 1.07 billion bytes. An array of FETs  102  may therefore be a two by four array, or a multibillion device array. A memory array  100  may be other sizes in addition to these. Each column  104  of FETs  102  has a drain select transistor  106  at one end and a source select transistor  108  at the other end. 
     In one embodiment, drain select transistor  106 , source select transistor  108 , or both may be a Metal Oxide Semiconductor Field Effect Transistor (MOSFET). Operation of such MOSFETs is well known and not discussed herein. 
     In one embodiment, FETs  102  between drain select (DS) transistor  106  and source select (SS) transistor  108  may be Charge-Trapping (CT) FETs. In this example, NAND flash memory  100  may also be referred to as a CT-NAND flash memory. A CT-FET  102  is formed with two gates, with one of the gates on top of the other (which could also be described as one gate with two layers), as opposed to standard FETs that have a single gate. The first or top gate  110  on CT-FET  102  is a control gate, which is the same type of gate as is found on selecting transistors  106  and  108 . The second or bottom gate  112  is a floating gate, which is often surrounded by an insulating layer. A floating gate type allows a CT-FET  102  to store charge after the device has been powered off due to the insulation. Therefore, CT-FETs  102  may be used to build non-volatile memory. 
     In an embodiment, NAND flash memory  100  also includes Word-Lines (WL)  114 . WL  114  addresses FETs  102  in a given row. For example, WL  114  can activate FETs  102  when NAND technology addresses groups of FETs  102 . The column of CT-FETs  102  is also connected to a bit line (BL)  116 . 
     In some instances, spacer nitride can become trapped in spacer oxide between CT-FETs  102  during manufacturing. Trapped nitride can lead to defects in a NAND memory  100  because nitride adjacent to a word-line gate, such as WL 0 , can trap electrons during an erase cycle and cause string current reduction. These issues may cause a NAND memory device  100  to fail inspection, thus leading to problems in volume production. Embodiments of the present invention fill gaps between CT-FETs  102  with silicon dioxide, which substantially eliminates the ability for nitride to become trapped, thus alleviating the concern over production defects. It is to be appreciated that the gap filling can be used for other devices aside from NAND memory  100 . 
       FIGS. 2 ,  3 , and  4  depict side-views of a substrate  210  during various stages of fabrication, according to an embodiment of the present invention. Reference is made through the rest of the description interchangeably between  FIGS. 2 ,  3 , and  4 . For example, substrate  210  represents a portion of a NAND logic device, e.g., NAND  100  in  FIG. 1 , during fabrication to substantially eliminate gaps between CT-FETs  102  and thus eliminate trapped nitride. 
       FIG. 2  depicts substrate  210  with gates  212  during a first stage of fabrication. In one example, substrate  210  may be part of a 300 mm or 450 mm wafer, which may be 0.75 mm thick. As shown, substrate  210  may form a layer onto which electronic devices (e.g., gates  212 ) may be created. Substrate  210  may be manufactured from silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide (GaAs), an alloy of silicon and germanium, or indium phosphide (InP) or other suitable material. In some cases (not shown), a substrate  210  may comprise an insulator with a layer of semiconductor on top of the insulator. 
     In one example, deposition of gate  212  may occur through physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) or atomic layer deposition (ALD), or other techniques for creating semiconductor features on a substrate. 
     In one example, gates  212  may be manufactured into FETs  102  shown in  FIG. 1 . In another example, gates  212   a - c  may become word-lines  114  and gate  212   d  may become a peripheral gate, such as drain select transistor  106  or source select transistor  108 , or both. 
     In one example, gates  212  may be manufactured from a variety of materials, e.g., good conductors. Candidates include, but are not limited to, highly doped polycrystalline silicon, silicide (e.g., the highly doped polycrystalline silicon&#39;s top layers are alloyed with at least one of tungsten, titanium, cobalt, and nickel). For very scaled down designs, a metal gate, such as one made from tantalum, tungsten, tantalum nitride, or titanium nitride may be used, possibly with a high-k dielectric. 
       FIG. 3  depicts a subsequent stage of fabrication of substrate  210 . For example, in this stage a first layer  320  is formed over substrate  210  and gates  212  and a second layer  322  is formed over first layer  320 . In one example, first layer  320  and substrate  210  entirely surround gate  212 , such that when second layer  322  is deposited, there is very little possibility that second layer  322  will come into contact with the gate  212  or become trapped in a space occupied by the first layer  320 . 
     First layer  320  may be a gate dielectric, for example silicon dioxide (sometimes referred to as a “gate oxide” or “oxide”). In certain embodiments, alternative materials with a higher dielectric constant may be used. One way to produce a layer of silicon dioxide is thermal oxidation, where an oxidizing agent is diffused into a wafer containing silicon at a high temperature to form silicon dioxide through the resulting reaction. 
     First layer  320  may be thick enough to fill in any gaps between word-line gates  212   a - c , i.e., first layer  320  does not leave behind a valley as exists between gates  212   c  and  212   d . In one example, first layer  320  may be about 150-500 or 300-500 angstroms thick. 
     The term “portion of oxide” is intended to be a broad term. A portion of oxide might be a region of a larger oxide, such as the oxide  320  between gate  212   a  and gate  212   b , or as between gate  212   b  and gate  212   c . A portion of oxide might be an entire layer of oxide, such as first layer  320 . A portion of oxide might be a discrete amount of oxide, as depicted in  FIG. 4 . 
     In one example, second layer  322  may be a nitride layer. In one embodiment, second layer  322  may be a silicon-rich nitride having an atomic ratio of silicon to nitrogen that is about 3:4 or greater. A nitride layer may function as a dielectric, and may work in conjunction with first layer  320  in this capacity. A nitride layer may be deposited at high temperatures in a LPCVD reactor or be plasma-enhanced in a PECVD system. In some embodiments, certain combinations of silicon, nitrogen and hydrogen (i.e., SiN x H y ) may be included in a second layer  322 . In some embodiments, a nitride layer may consist of a layer of nitride above a layer of silicon-rich nitride. 
     In certain embodiments, second layer  322  of silicon nitride may be used to form a lightly doped drain (LDD) spacer in conjunction with a silicon dioxide layer as a first layer  320 . One example of a LDD is a drain region with reduced doping in FETs  102  ( FIG. 1 ) with very small geometries. A LDD may be used to control breakdown between the drain and substrate because a lowered doping gradient between drain and channel will lower the electric field in the channel in the vicinity of the drain. One way to implement a LDD is to deposit a first, moderate implant, followed by deposition and etching of a LDD spacer, then deposition of a second, heavy implant. 
     In such embodiments, second layer  322  may be thick enough that, after etching, the LDD spacer will be at least as wide as a Critical Dimension (CD), also referred to as a target CD requirement. A critical dimension may also be referred to as a minimum feature size or target design rule. Presently, all types of electromagnetic radiation can be used, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. In one example, deep ultra-violet light can be used to produce wavelengths of about 248 and 193 nm, which provides for a critical dimension of about 50 nm. 
     In one embodiment, as shown in  FIG. 3  there may be a distance X between sides of adjacent gates  212 . Additionally, in one example X is about 86 nanometers. There may be a distance Y between gates  212   a  and  212   b , and similarly between gates  212   b  and  212   c . Additionally, distance Y may be between about 30 to 40 nanometers. 
       FIG. 4  shows a further subsequent stage of fabrication, for example a stage after the embodiment shown in  FIG. 3  and after at least one etching step. For example, in a wet etch process, silicon dioxide may be etched with hydrofluoric acid (HF), or with a buffered oxide etch, such as ammonium fluoride (NH 4 F) and hydrofluoric acid (HF). A wet etch process for silicon nitride may use 85% Phosphoric acid (H 3 PO 4 ). In another example, during a dry etch process, also known as plasma etching, either silicon dioxide or silicon nitride may be etched with CF 4 , SF 6  and NF 3 . 
     In one embodiment, a top nitride layer  322  is etched down to an oxide layer  320 . An oxide spacer (i.e., the oxide layer between a periphery gate  212   d  and the nearest word-line gate  212   c ) may then be etched, such that the etching stops at the silicon substrate  212 . In this manner, the core spacer (e.g., the oxide  320  between gates  212   a  and  212   b ) will remain as all oxide, and periphery gate  212   d  will have the desired critical dimension width for a lightly doped drain implant. In one example, this approach may prevent nitride from becoming trapped in the space between the word-line gates  212   a - c.    
       FIG. 5  is a flowchart depicting a method  500  for manufacturing a semiconductor  500 , according to one embodiment of the present invention. In one example, method  500  may be used to from the fabrication stages shown in  FIGS. 2 ,  3 , and  4  to produce a NAND memory show in  FIG. 1 . It is to be appreciated that not all steps discussed below may be required, or be required to be performed in the order described. 
     In step  502 , various initial fabrication processes are performed. Such processes  502  may include creating a substrate on a silicon wafer. 
     In step  504 , first, second, third and fourth gates are deposited or formed on a substrate. The first second and third gates may be used to form a NAND memory array, e.g., gates  212   a - c  of array  100 . The fourth gate may be a peripheral gate (e.g., gate  212   d ) which may eventually become a selection transistor (e.g., drain transistor  106  or source transistor  108 ). All four gates may be made from polysilicon. In one embodiment, the gates may be placed periodically, e.g., one every 86 nanometers, such that there are about 30-40 nanometers between the gates. 
     In step  506 , an oxide is deposited over the first, second, third and fourth gates. In one embodiment, a thick layer of oxide, e.g., about 150-500 or 300-500 angstroms thick, may be deposited. In another embodiment, the oxide layer may be as thick as the gap between consecutive CT-FETs  102  is wide (e.g., the gap between gates  212   a  and  212   b ). In another embodiment, the oxide layer may be as thick as half the period of gate placement, e.g., about 86 nm/2, which is about 430 angstroms. 
     In step  508 , the oxide layer is etched, but the etching does not substantially etch oxide below tops of the first, second and third gates. As described with reference to  FIG. 4 , this may occur with a dry etching of a top nitride spacer  322  down to an oxide spacer  320 . 
     In step  510 , a region of oxide adjacent to the fourth gate is etched to the substrate. As described with reference to  FIG. 4 , this may be a dry etching that etches an oxide spacer  320  and stops on a silicon substrate  212 . 
     In step  512 , various subsequent fabrication processes are performed. These may include creating a drain from oxide next to the fourth gate (e.g. gate  212   d ), lightly doping the drain, and fabricating a selection transistor (e.g., drain transistor  106  or source transistor  108 ). Similarly, the various subsequent fabrication processes may include fabricating the first, second and third gates (e.g., gate  212   a - c ) into CT-FETs  102  and fabricating a NAND memory array  100  from the CT-FETs  102 . Additional examples may be found in co-owned U.S. patent application Ser. No. 12/973,631, filed Dec. 10, 2010, which is hereby incorporated by reference in its entirety for its teachings regarding fabrication, but not necessarily for definitional purposes. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the current invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. For example, the specification uses nanoscale measurements and one of ordinary skill in the art will appreciate that these should be interpreted in light of the relevant manufacturing or measuring tolerances. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections may set forth one or more but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way. 
     The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
     The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
     The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.