Abstract:
An embodiment of a floating-gate memory cell has a tunnel dielectric layer formed overlying a semiconductor substrate; a drain region formed in a semiconductor substrate adjacent a first side of the tunnel dielectric layer, a source region formed in a semiconductor substrate adjacent a second side of the tunnel dielectric layer, a floating-gate layer formed overlying the tunnel dielectric layer, a control-gate layer formed overlying the floating-gate layer, and an intergate dielectric layer formed interposed between the floating-gate layer and the control gate layer. The control-gate layer includes a silicide layer in contact with an underlying polysilicon layer. There is no interposing dielectric layer between the control-gate layer and an overlying bulk insulator layer, and a width of the silicide layer is substantially equal to a width of the polysilicon layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional application of application Ser. No. 10/602,324, Pat. 6,939,764, titled METHODS OF FORMING MEMORY CELLS HAVING SELF-ALIGNED SILICIDE, filed Jun. 24, 2003 (allowed), which application is assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to integrated circuit devices and, in particular, to the formation of self-aligned silicides for word lines and contacts of a semiconductor memory device. 
     BACKGROUND OF THE INVENTION 
     Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address. 
     One type of memory is a non-volatile memory known as Flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features. 
     A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate. 
     Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a bit line. In NAND flash architecture, a column of memory cells are coupled in series with only the first memory cell of the column coupled to a bit line. 
     Memory device fabricators are continuously seeking to reduce the size of the devices. Smaller devices facilitate higher productivity and reduced power consumption. However, as device sizes become smaller, resistance of the various conductors becomes an ever-increasing problem. High resistance can lead to slower performance. One solution is to utilize materials having higher conductivity. 
     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternate methods and circuits for providing reduced resistance to portions of a semiconductor memory device. 
     SUMMARY OF THE INVENTION 
     The above-mentioned problems with memory devices and other problems are addressed by the present invention and will be understood by reading and studying the following specification. 
     Various embodiments of the invention facilitate reduced resistance and/or reduced device sizing by providing a process to concurrently form self-aligned silicides on word lines and contacts of a memory device. The word-line silicide is formed at a stage significantly later than in standard processing, decreasing concerns of thermal stability of the silicide, thus allowing the use of lower-resistance suicides. In addition, by forming contacts to drain and source regions prior to forming the silicide for the word lines, aspect ratios for the contact holes or trenches are reduced, thus improving effectiveness of processing to remove material from these holes and trenches or allowing the use of a smaller pitch, i.e., a smaller spacing between adjacent word lines. By providing a process for the application of a silicide in array source interconnects, a single array source interconnect can couple an entire row of memory cells, thereby reducing the number of contacts made to an array ground. 
     For one embodiment, the invention provides a floating-gate memory cell. The memory cell includes a control-gate layer having a first silicide layer in contact with an underlying first polysilicon layer and a first contact having a second silicide layer in contact with an underling second polysilicon layer, the second polysilicon layer in contact with a source region of the memory cell. The first and second silicide layers are formed concurrently. For a further embodiment, the memory cell further includes a second contact to the drain region having a third silicide layer in contact with a third underlying polysilicon layer, the third underlying polysilicon layer in contact with the drain region. The third silicide layer is formed concurrently with the first and second silicide layers. 
     The invention further provides methods and apparatus of varying scope. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1H  are cross-sectional views of a portion of a memory array during various stages of fabrication in accordance with an embodiment of the invention. 
         FIG. 2  is a schematic of a portion of a non-volatile memory array in accordance with an embodiment of the invention. 
         FIG. 3  is a functional block diagram of a basic flash memory device in accordance with an embodiment of the invention coupled to a processor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the inventions 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 process or mechanical changes may be made without departing from the scope of the present invention. The terms wafer and substrate used previously and in the following description include any base semiconductor structure. Both are to be understood as including silicon-on-sapphire (SOS) technology, silicon-on-insulator (SOI) technology, thin film transistor (TFT) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor, as well as other semiconductor structures well known to one skilled in the art. 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. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and their equivalents. 
       FIGS. 1A-1H  generally depict a method of forming a portion of a memory array in accordance with an embodiment of the invention.  FIG. 1A  depicts a portion of the memory array after several processing steps have occurred. Formation of the structure depicted in  FIG. 1A  is well known and will not be detailed herein. In general,  FIG. 1A  depicts several stacks of layers that will form word lines of the memory array. The stacks include a tunnel dielectric  110  is formed on a substrate  105 . Tunnel dielectric  110  is generally a silicon oxide, but may be any dielectric material. Some specific examples include silicon oxides (SiO/SiO 2 ), silicon nitrides (SiN/Si 2 N/Si 3 N 4 ) and silicon oxynitrides (SiO x N y ). For one embodiment, substrate  105  is a P-type silicon substrate. A first polysilicon layer  115  is formed over the tunnel dielectric  110 . First polysilicon layer  115  will become the floating gate for this embodiment. First polysilicon layer  115  may be conductively doped. An example would be an n-type polysilicon layer. 
     An intergate dielectric layer  120  is formed over the first polysilicon layer  115 . Dielectric layer  120  contains any dielectric layer. For one embodiment, the dielectric layer  120  contains the dielectric ONO (oxide-nitride-oxide). Other dielectric materials may be substituted for the ONO, such as tantalum oxide, barium strontium titanate, silicon nitride and other materials providing dielectric properties. 
     A second polysilicon layer  125  is formed overlying the dielectric layer  120 . The second or control gate polysilicon layer  125  may also be conductively doped. A cap layer  130  is formed overlying the control gate polysilicon layer  125 . For one embodiment, cap layer  130  is silicon nitride. However, cap layer  130  is a sacrificial layer and may be any material that can function as an etch or planarization stop layer and that is selective to removal over surrounding layers. The layers are patterned into stacks, as shown in  FIG. 1A , and drain regions  135  and source regions  140  are formed in the substrate  105 , such as by implantation or diffusion of dopant materials. The drain regions  135  and source regions  140  will have the same conductivity type and be different from the conductivity type of the substrate  105 . The source/drain regions are also generally more heavily doped than the substrate  105 . For one embodiment, the substrate  105  has a p-type conductivity while the drain regions  135  and source regions  140  have an n + -type conductivity. While the drain regions  135  and source regions  140  were formed after formation of the word line stack for this embodiment, they could also be formed earlier. 
     In a typical word line stack, a metal or metal silicide layer would be formed on the control gate polysilicon layer  125  prior to formation of the cap layer  130 . By forming the cap layer  130  without the intervening metal or metal silicide layer, the stack for the future word lines can be shortened relative to a typical word line, thereby reducing the aspect ratio of the space between adjacent word lines. This reduced aspect ratio can provide advantages during subsequent fabrication as will be described below. 
     Following formation of the stacks as depicted in  FIG. 1A , dielectric spacers  145  are formed on the sidewalls of the stacks as shown in  FIG. 1B . The dielectric spacers  145  should contain a material different from the cap layer  130 . In formation of the dielectric spacers  145 , for one embodiment, a layer of dielectric material, such as TEOS (tetraethylorthosilicate), is formed by a blanket deposition process and the layer is anisotropically etched to leave behind the spacers  145 . By selecting a material different than the cap layer  130 , the cap layer  130  can act as an etch stop in the formation of the spacers  145 . 
     An insulator layer  150  is then formed between the future word lines. The insulator layer  150  should contain a dielectric material that is selective over the materials of the substrate  105 , the cap layer  130  and the spacers  145 . This will permit subsequent removal of the insulator layer  150  where desired. One example for the insulator layer  150  would be a doped silicate glass. Examples of doped silicate glasses include as BSG (borosilicate glass), PSG (phosphosilicate glass) and BPSG (borophosphosilicate glass). 
     In  FIG. 1C , a mask layer  155  is formed overlying the structure of  FIG. 1B  to define areas for removal of the insulator layer  150 . As one example, the mask layer  155  is a patterned photoresist layer as is commonly used in semiconductor fabrication. While each portion of the insulator layer  150  that is visible in  FIG. 1C  is depicted to be exposed, it is noted that some areas of the insulator layer  150  will generally be left unexposed by the mask layer  155 . The exposed areas of the insulator layer  150  are then removed in  FIG. 1D  such as by dry etching or other removal process. This exposes portions of the substrate  105  at one or more of the drain regions  135 , forming contact holes  136 , and one or more source regions  140 , forming trenches  141 . Alternatively, the contact holes  136  and trenches  141  may be formed separately using separate mask and etch steps. Because of the lower aspect ratio of the spaces containing the insulator layer  150  relative to a typical word line construction of the same pitch, clean removal of portions of the insulator layer  150  is relatively easier. Alternatively, a smaller pitch could be used to achieve the same aspect ratio. 
     The holes  136  and trenches  141  exposed in the removal of portions of the insulator layer  150  are then filled with a conductively-doped polysilicon contact layer  160  as contacts to the drain regions  135  and source regions  140  as depicted in  FIG. 1E . The mask layer  155  would generally be removed prior to filling the holes  136  and trenches  141 . For one embodiment, a blanket layer of polysilicon is formed over the structure of  FIG. 1D  followed by a removal of excess polysilicon such as by CMP. The sacrificial cap layer  130  is then removed to leave the structure depicted in  FIG. 1F . For one embodiment, removal of the cap layer  130  includes selective etching of the cap layer  130 . Upon removal of the cap layer  130 , portions of the contact layer  160  and the control gate layer  125  are exposed. 
     A self-aligned silicide (i.e., salicide) is formed on the exposed portions of the control gate layer  125  and the contact layer  160 . In a salicide process, generally a layer of refractory metal, e.g., titanium (Ti), is formed overlying the entire structure and followed by an anneal. The metals of chromium (Cr), cobalt (Co), hafnium (Hf), molybdenum (Mo), niobium (Nb), tantalum (Ta), tungsten (W), vanadium (V) and zirconium (Zr) are generally recognized as other refractory metals. Where the refractory metal is in contact with a silicon layer, such as monocrystalline silicon or polysilicon, the refractory metal will react with the silicon to form a refractory metal silicide. Where the refractory metal is in contact with a layer not containing free silicon, e.g., silicon oxide, silicon nitride, TEOS, etc., the refractory metal will tend to remain unreacted during the anneal process. The unreacted refractory metal may then be selectively removed such as by a wet strip, leaving behind the refractory metal silicide portions. 
     For the embodiment depicted in  FIG. 1F , a silicide layer  165  would be formed on the exposed portions of the control gate layer  125  and contact layer  160  as depicted in  FIG. 1G . Thus, the refractory metal overlying word lines, contacts to the drain regions and interconnects to the source regions will react to form silicide while the refractory metal overlying interposing structures, e.g., spacers, will remain unreacted. Because the silicide layer  165  is formed so much later in the processing than with typical word line fabrication, the silicide layer  165  will be exposed to less processing at elevated temperatures. This will permit the use of silicides having lower thermal budgets, but higher conductivity. For example, titanium silicide and cobalt silicide would be preferred over tungsten silicide for their reduced resistance, but tungsten silicide may be required in a typical word line, i.e., silicide layer formed prior to formation of the cap layer, where the expected heat load for subsequent processing exceeds the thermal budget of lower resistivity silicides. Similarly, regardless of the refractory metal chosen, by forming the silicide layer later in processing, the silicide layer will be less thermally stressed than a silicide layer formed prior to forming the cap layer  130 . 
     Following formation of the silicide layer  165 , a bulk insulator layer  170  is formed overlying the structure and connections are made between drain contacts  175  and bit lines  180  as shown in  FIG. 1H . Connections (not shown) will also be made to the word lines  185  and array source interconnects  190 . However, while a drain contact  175  will generally be a single contact shared between two adjacent memory cells, word lines  185  and array source interconnects  190  will generally extend across multiple memory cells in a direction that is perpendicular to a face plane of the figure. For one embodiment, the array source interconnects  190  extend to the same number of columns as their associated word lines  185 . 
     It is not uncommon for word lines  185  to be shared by hundreds of memory cells. In a typical flash memory device, an array source may be formed as a deep heavily-doped junction in the substrate shared by 32 memory cells, i.e., extending only 16 columns before coupling to the nearest array ground. Note that this could mean a span of 32 columns with a coupling to an array ground at each end of the span. In embodiments of the invention, by utilizing a shared silicide layer for reduced resistance, the source interconnect  190  can extend over a much larger group of memory cells. This configuration can facilitate array source interconnects extending 32 columns or more without coupling to an array ground, i.e., a span of 64 columns, and may extend as deep as the adjacent word lines  185 , i.e., an entire row of memory cells. In addition, by utilizing a shallow junction for the source region  140 , a smaller channel can be utilized in the memory array, thus facilitating a reduction in device size and a reduction in pitch. Connections to the word lines  185  and source interconnects  190  can thus be made once per row of memory cells. As described herein, a memory cell is a single floating-gate transistor formed of a word line  185 , drain region  135 , source region  140  and a channel region defined by the area interposed between the drain region  135  and source region  140 . 
       FIG. 2  is a schematic of a portion of a non-volatile memory array  200  as a portion of a memory device in accordance with one embodiment of the invention. The detail of memory array  200  is provided to better understand the various embodiments of the invention. The memory array  200  includes word lines and contacts formed in accordance with an embodiment of the invention. 
     The layout of  FIG. 2  corresponds to one example of a NOR flash architecture. However, other types of memory arrays can benefit from embodiments of the invention. As one example, word lines, drain contacts and source interconnects can be fabricated in accordance with the invention for NAND flash architectures as well, although only one drain contact is required per string and only one source interconnect is required per block. Accordingly, the invention is not limited to the specific layout described with reference to  FIG. 2 . 
     As shown in  FIG. 2 , the memory block  200  includes word lines  202  and intersecting local bit lines  204 . For ease of addressing in the digital environment, the number of word lines  202  and the number of bit lines  204  are each some power of two, e.g., 256 word lines  202  by 4,096 bit lines  204 . The local bit lines  204  may be selectively coupled to global bit lines (not shown) for coupling to sense amplifiers (not shown in  FIG. 2 ). 
     Floating-gate transistors  206  are located at each intersection of a word line  202  and a local bit line  204 . The floating-gate transistors  206  represent the non-volatile memory cells for storage of data. Typical construction of such floating-gate transistors  206  include a source  208  and a drain  210  constructed from an n + -type material of high impurity concentration formed in a P-type semiconductor substrate of low impurity concentration, a channel region formed between the source and drain, a floating gate  212 , and a control gate  214 . Floating gate  212  is isolated from the channel region by a tunneling dielectric and from the control gate  214  by an intergate dielectric. Floating-gate transistors  206  having their control gates  214  coupled to a word line  202  typically share a common source  208  depicted as array source interconnect  216 . As shown in  FIG. 2 , floating-gate transistors  206  coupled to two adjacent word lines  202  may share the same array source interconnect  216 . Floating-gate transistors  206  have their drains coupled to a local bit line  204 . A column of the floating-gate transistors  206  are those transistors commonly coupled to a given local bit line  204 . A row of the floating-gate transistors  206  are those transistors commonly coupled to a given word line  202 . 
     The array source interconnects  216  may be coupled to a metal or other highly conductive line to provide a shared path to a ground potential node. The array ground  218  serves as this shared path. For one embodiment, a connection between an array source interconnect  216  and the array ground  218  occurs only once for each row of memory cells  206 . Typical memory devices may make require a contact between an array source interconnect and an array ground every 16 columns. 
       FIG. 3  is a functional block diagram of a basic flash memory device  300  that is coupled to a processor  301 . The memory device  300  and the processor  301  may form part of an electronic system. The memory device  300  has been simplified to focus on features of the memory that are helpful in understanding the present invention. The memory device  300  includes an array of non-volatile memory cells  302 . The memory array  302  includes word lines and contacts formed in accordance with an embodiment of the invention. 
     Each memory cell is located at an intersection of a word line and a local bit line. The memory array  302  is arranged in rows and columns, with the rows arranged in blocks. A memory block is some discrete portion of the memory array  302 . Individual word lines generally extend to only one memory block while bit lines may extend to multiple memory blocks. The memory cells generally can be erased in blocks. Data, however, may be stored in the memory array  302  separate from the block structure. 
     The memory array  302  is arranged in a plurality of addressable banks. In one embodiment, the memory contains four memory banks  304 ,  306 ,  308  and  310 . Each memory bank contains addressable sectors of memory cells. The data stored in the memory can be accessed using externally provided location addresses received by address register  312  from processor  301  on address lines  313 . The addresses are decoded using row address multiplexer circuitry  314 . The addresses are also decoded using bank control logic  316  and row address latch and decode circuitry  318 . 
     To access an appropriate column of the memory, column address counter and latch circuitry  320  couples the received addresses to column decode circuitry  322 . Circuit  324  provides input/output gating, data mask logic, read data latch circuitry and write driver circuitry. Data is input through data input registers  326  and output through data output registers  328 . This bi-directional data flow occurs over data (DQ) lines  343 . 
     Command execution logic  330  is provided to control the basic operations of the memory device including memory read operations. A state machine  332  is also provided to control specific operations performed on the memory arrays and cells. A high voltage switch and pump circuit  345  is provided to supply higher voltages during erase and write operations. A status register  334  and an identification register  336  can also be provided to output data. 
     The memory device  300  can be coupled to an external memory controller, or processor  301 , to receive access commands such as read, write and erase command. Other memory commands can be provided, but are not necessary to understand the present invention and are therefore not outlined herein. The memory device  300  includes power supply inputs Vss and Vcc to receive lower and upper voltage supply potentials. 
     As stated above, the flash memory device  301  has been simplified to facilitate a basic understanding of the features of the memory device. A more detailed understanding of flash memories is known to those skilled in the art. As is well known, such memory devices  301  may be fabricated as integrated circuits on a semiconductor substrate. 
     CONCLUSION 
     Method and apparatus have been described to facilitate reduced resistance and/or reduced device sizing by concurrently forming self-aligned suicides on word lines and contacts of a memory device. The word-line silicide is formed at a stage significantly later than in standard processing, decreasing concerns of thermal stability of the silicide, thus allowing the use of lower-resistance suicides. In addition, by forming contacts to drain and source regions prior to forming the silicide for the word lines, aspect ratios for the contact holes or trenches are reduced, thus improving effectiveness of processing to remove material from these holes and trenches or allowing the use of a smaller pitch. By providing a process for the application of a silicide in array source interconnects, a single array source interconnect can couple an entire row of memory cells, thereby reducing the number of contacts made to an array ground. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.