Abstract:
A process for manufacturing a non-volatile memory cell including a floating gate MOS transistor, including the steps of: forming a gate dielectric over a surface of a semiconductor material layer; forming a conductive floating gate electrode insulated from the semiconductor material layer by the gate dielectric; forming at least one isolation region laterally to the floating gate electrode; excavating the at least one isolation region; filling the excavated isolation region with a conductive material; and forming a conductive control gate electrode of the floating gate MOS transistor insulatively over the floating gate, wherein the step of forming the floating gate electrode includes: laterally aligning the floating gate electrode to the at least one isolation region; and the step of excavating includes: lowering an isolation region exposed surface below a floating gate electrode exposed surface, the lowering exposing walls of the floating gate electrode; forming a protective layer on exposed walls of the floating gate electrode; and etching the at least one isolation region essentially down to the gate dielectric, the protective layer protecting against etching a portion of the at least one isolation region near the gate dielectric.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to semiconductor devices and to methods for their manufacturing. In particular, the invention relates to floating gate non-volatile MOS memory devices and to methods for the manufacturing thereof.  
       BACKGROUND ART  
       [0002]     The past approaches described in the following could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in the following are not to be considered prior art to the claims in this application merely due to the presence of these approaches in the following background description.  
         [0003]     In the last years, the demand of increasing semiconductor device integration density has brought a reduction in the size of the elements used in integrated circuits.  
         [0004]     For example, a semiconductor memory device which is commonly used in a number of applications to store information (either temporarily or permanently) should be able to store as many data as possible. Since semiconductor memory devices include large matrices of memory cells, a high memory cell density is required in order to increase the storage capacity of the semiconductor memory device and, at the same time, keep the die size from increasing. the smaller the size of the memory cells, the higher the memory cell density achievable.  
         [0005]     On the other hand, the storage capacity of semiconductor memory devices can also be increased by providing memory cells each capable to storage more than just one bit of data, for example two or more data bits.  
         [0006]     In particular, Flash semiconductor memory devices include matrices of floating gate MOS transistors. The floating gate MOS transistor used to form a Flash memory cell essentially is a MOSFET (acronym for Metal-Oxide-Semiconductor Field Effect Transistor) having a gate electrode consisting of a conductive control gate, a dielectric layer, a conductive floating gate and a tunnel oxide layer which are stacked over an active area (i.e., a channel region) of the MOS transistor.  
         [0007]     Floating gate MOS transistors may store a logic value defined by their threshold voltage. The threshold voltage may be set to different levels, each one representing corresponding logical values stored in the memory cell. Particularly, in a two level flash memory the generic memory cell has a threshold voltage that can be set in either one of two different levels, thus enabling storage of one bit of data; in a multi-level (e.g., four-level) flash memory the threshold voltage of the generic cell may be set in more than two (for example, four) different levels, thus allowing storage of a plurality of (e.g., two) bits of data.  
         [0008]     The threshold voltage depends on the electric charge on the floating gate, and it may be modified by injecting charge carriers, particularly electric charges like electrons into, or removing them from the floating gate. In more detail, a programming operation consists in the injection of electrons in the floating gate of the Flash memory cell, through a mechanism of tunneling across tunnel oxide (Fowler-Nordheim tunneling effect) or through a mechanism of injection of channel electrons “heated” by a suitable biasing voltage applied across the source and drain terminals (hot electron programming). An erasing operation consists in the removal of electrons from the floating gate of the Flash memory cell, through a mechanism of tunneling across tunnel oxide (Fowler-Nordheim tunneling effect) using a reverse voltage with respect to programming.  
         [0009]     In a Flash semiconductor memory device, the memory cells may be arranged according to either a NAND or a NOR architecture.  
         [0010]     In  FIGS. 1A and 1B  there are, by way of example, described a matrix portion  110  of a NAND flash memory, and a cross section along a word line of the matrix portion  110  according to the prior art.  
         [0011]     In the memory matrix, the memory cells, labelled MC, are conventionally arranged by rows and columns, with the memory cells belonging to the same row sharing a control gate, formed by a conductive word line strip WL.  
         [0012]     In the exemplary case considered of a NAND architecture, the memory cells of the same column are also grouped in a plurality of strings  111 . Within each string  111 , the memory cells MC (for example, 32 in number) are connected in series to each other between two select transistors ST 1  and ST 2 , selectively enabling the connection of the string to a respective bit line BL, respectively to a source line SL. As visible in  FIG. 1B , each memory cell is a floating gate MOS transistor having a gate electrode  121  completely self-aligned with an active region  122 , formed in a (e.g., P type) substrate region  123 , laterally delimited and separated from the active region of the adjacent memory cell(s) by STI (Shallow Trench Isolation) isolation regions  124 . The generic isolation region  124  is formed by a corresponding trench  125 , extending from a main surface  126  of the substrate region  123  to a trench depth, filled by one or more layers of silicon oxide.  
         [0013]     The gate electrode  121  consists of a thin silicon oxide layer  127 , forming the tunnel oxide, a polysilicon floating gate  128 , an interpoly dielectric layer  129  and a polysilicon control gate  130 , which are stacked on the active region  122  and self-aligned thereto.  
         [0014]     The use of the STI isolation regions allows reducing the memory matrix area compared to other isolation techniques. A further reduction of the memory matrix area is made possible by the adoption of the NAND architecture, which substantially reduces the number of contacts.  
         [0015]     With reference to  FIG. 1A , during the reading operation, the select transistors ST 1  and ST 2  of the generic selected string  111  are turned on, the word line WL of the matrix row including the selected memory cell MC to be read is brought to a reading voltage while the other word lines are brought to a voltage sufficiently high to ensure that the corresponding memory cells MC are conductive irrespective of their threshold voltage. The selected memory cell MC is conductive if its threshold voltage is lower than the reading voltage, otherwise it is not conductive.  
         [0016]     Due to the coupling capacitances Cd (shown in  FIG. 1B ) between the floating gates of adjacent cells, the actual potential which is applied to the floating gate of the selected cell through the coupling capacitance Cc with the control gate may be different from the expected one. As a consequence, the reading of the selected memory cell MC may be erroneous (for example, a memory cell that should be read as programmed may erroneously be considered erased). In other words, the capacitive coupling between the floating gates of adjacent cells makes the threshold voltage of the selected memory cell depend not only on the electric charge stored in its floating gate, but also on the electric charges stored in the floating gates of the adjacent cells. Such effect modifies the threshold voltage of the cells when the adjacent cells are programmed.  
         [0017]     The above mentioned problem is also experienced during programming of the memory cells: for example, if a generic memory cell is verified as programmed at a certain stage of a program sequence, it may then be read as non-programmed when the program sequence is completed and the adjacent memory cells have been programmed as well.  
         [0018]     The disturbing effect described above increases as the ratio between the coupling capacitance Cd with the floating gates of adjacent cells and the coupling capacitance Cc with the control gate increases.  
         [0019]     The disturbances caused by the floating gate of the adjacent cells are in particular dangerous in multi-level memories, since the margins available for discriminating the different stored logic values are smaller.  
         [0020]     The problem is exacerbated by the reduction of the width of the STI trenches, because this increases the coupling capacitances Cd between the floating gates of adjacent cells.  
         [0021]     The above discussed problem has been addressed in the United States patent application US 2004/0012998, which discloses a NAND flash memory structure wherein, thanks to the fact that the word lines extend down between floating gates into isolation trenches until, within or past the level of gate oxide layer, thereby the word lines provide shielding from potentials in adjacent strings undergoing programming.  
       SUMMARY OF THE INVENTION  
       [0022]     The Applicant has addressed at least the problem of disturbances occurring during the reading operation of the memory device.  
         [0023]     According to an aspect of the present invention, a solution is provided for manufacturing a self-aligned floating gate flash memory cell that, when inserted in a memory cell matrix, is less affected by disturbances induced by adjacent memory cells.  
         [0024]     Particularly, an aspect of the present invention proposes a process for manufacturing a non-volatile memory cell including a floating gate MOS transistor, comprising the steps of: forming a gate dielectric over a surface of a semiconductor material layer; forming a conductive floating gate electrode insulated from the semiconductor material layer by the gate dielectric; forming at least one isolation region laterally to said floating gate electrode; excavating the at least one isolation region; filling the excavated isolation region with a conductive material, and forming a conductive control gate electrode of the floating gate MOS transistor insulatively over the floating gate. The step of forming the floating gate electrode includes laterally aligning said floating gate electrode to the at least one isolation region. The step of excavating includes: lowering an isolation region exposed surface below a floating gate exposed surface, said lowering exposing walls of the floating gate electrode; forming a protective layer on exposed walls of the floating gate electrode; and etching the at least one isolation region essentially down to the gate dielectric, the protective layer protecting against etching a portion of the isolation region near the gate dielectric. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The present invention, as well as further features and the advantages thereof will be best understood by reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings, wherein:  
         [0026]      FIG. 1A  schematically shows exemplary representations of a portion of a memory device according to the prior art;  
         [0027]      FIG. 1B  shows a cross-sectional view along a word line of a matrix portion according to the prior art;  
         [0028]      FIGS. 2A through 2L  are cross-sectional views illustrating the main steps of a manufacturing process of a floating gate MOS transistor according to a first embodiment of the present invention; and  
         [0029]      FIGS. 3A through 3B  are cross-sectional views illustrating the main steps of a manufacturing process of a floating gate MOS transistor according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]     In the following description, it should be noted that the drawings are not to scale: relative dimensions and proportions of parts of the drawings may have been increased or reduced in size for the sake of clarity.  
         [0031]     It is pointed out that although in the drawings and in the following description the particular case of a NAND memory cell matrix is considered, this is not to be construed as a limitation of the invention, which can, for example, be straightforwardly applied to NOR memory cell matrices.  
         [0032]     Referring to  FIGS. 2A through 2L , a floating-gate MOS transistor memory cell manufacturing process according to a first embodiment of the present invention is described herein below; in particular the drawings are cross-sectional views of a portion of a memory cell matrix made along a matrix row, i.e. along a generic word line.  
         [0033]     Considering in particular  FIG. 2A , the starting material is a semiconductor substrate  200 , for example, it may be a silicon wafer substrate of the P conductivity type, or a doped well, formed (possibly by means of a dedicated dopant implantation) inside a semiconductor layer, having for example a surface dopant concentration in the memory cell matrix ranging from approximately 5*10 17  ions/cm 3  to approximately 5*10 19  ions/cm 3 .  
         [0034]     Successively, a tunnel oxide layer  205 , for example with a thickness ranging from 6 nm to 10 nm is formed on top of a main surface  210  of the substrate  200 . Preferably, the tunnel oxide layer  205  includes a thermally grown silicon oxide layer; alternatively, it may be a silicon oxide layer which is deposited, for example, by means of a CVD (acronym for Chemical Vapor Deposition) process.  
         [0035]     Moving to  FIG. 2B , a polysilicon layer  215  and a nitride layer  220  are deposited on the tunnel oxide layer  205 , for example by means of a CVD process. The polisilicon layer  215  (possibly doped) is adapted for forming the floating gates of the floating gate MOS transistors, while the silicon nitride layer  220  is used as hard mask for the subsequent definition of the isolation regions and/or as a stopping layer for the subsequent CMP (acronym for Chemical Mechanical Polishing) processes.  
         [0036]     Moving the  FIG. 2C , trenches  225 , extending from the main surface  210  of the substrate region  200  down to an isolation depth d 1  (for example, ranging from 100 nm to 300 nm), are excavated by selectively etching the layers  220 ,  215 ,  205  and  200 . In particular, in order to form the trenches  225 , a photoresist mask (not shown in the figure) is provided on the silicon nitride layer  220 , so as to leave exposed areas of the layer  220  where the trenches  225  are to be formed. The nitride layer  220  is then selectively removed from such exposed areas, and the photoresist mask is stripped off; the remaining portions of nitride layer  220  form the hard mask for the subsequent etching. Using suitable etching techniques, the layers  215 ,  205  and  200  are selectively removed, down to the desired isolation depth d 1 , leaving polysilicon portions  280  and tunnel oxide portions  290 . In particular, an anisotropic etching is performed, thereby the etch rate is much higher vertically than laterally.  
         [0037]     Then, the trenches  225  are filled with an insulator, for example, albeit not limitatively field silicon dioxide  230 . In such a way, isolation regions  270  are formed, adapted to isolate from each other active areas  275  in the substrate  200 , which active areas  275  will form the channel regions of the memory cells. The etching steps leading to the formation of the isolation regions  270  also define (in the direction of the word lines) the polysilicon portions  280  (i.e., the floating gates of the memory cells); the floating gates  280  as a result being self-aligned to the isolation regions  270 .  
         [0038]     As shown in  FIG. 2D , the field silicon dioxide layer  230  is then planarized down to the nitride silicon layer  220 , for example by means of a CMP (acronym for Chemical Mechanical Polishing) process; the silicon nitride layer  220  is used as a stopping layer for stopping the planarizationprocess. Then, the remaining portions of the silicon nitride layer  220  are etched away.  
         [0039]     Moving to  FIG. 2E , the field oxide layer  230  is selectively etched using, as an etching mask, the polysilicon floating gates  280 . In particular, the etching process is selective against polysilicon and it can be both isotropic or anisotropic with respect to two directions X (lateral) and Y (vertical).  
         [0040]     Specifically, the field oxide layer  230  corresponding to each isolation region  270  is etched to a depth such as to protrude a distance d 2  from the main surface  210 . The distance d 2  is chosen so as to ensure that the tunnel oxide portions  290  are not affected during the etching of the field oxide layer  230 . In particular, the distance d 2  may range from approximately 30 nm to approximately 80 nm.  
         [0041]     As shown in  FIG. 2F , a relatively thin, conforming layer  235  of dielectric is deposited, for example by means of a CVD process, over the surface of the wafer; the conforming layer has a thickness such as to substantially follow the profile of the underlying layers. For example, the conforming layer  235  comprises a silicon nitride layer.  
         [0042]     Moving to  FIG. 2G , the conforming layer  235  is then selectively etched by means of an anisotropic etching process, so that the conforming layer  235  is essentially only removed from the horizontal exposed surfaces, thus leaving exposed field oxide portions  240  of the field oxide layer  230  and the surface of the floating gates  280 . In such a way, silicon nitride spacers  245  are formed adjacent the vertical walls of the floating gates  280 .  
         [0043]     As shown in  FIG. 2H , the exposed field oxide portions  240  are then etched by means of an etching process highly selective for silicon dioxide, that uses as a mask the silicon nitride spacers  245  and the floating gates  280 . The etching process must be anisotropic such that the exposed field oxide portions  240  are etched preferably along the vertical direction, down to a depth d 3  past the level of tunnel oxide portions  290  (i.e., a surface of the field oxide portions after the etching is recessed the depth d 3  from the main surface  210 ). In particular, and by way of example, the depth d 3  ranges from approximately 10 nm to approximately 30 nm.  
         [0044]     Moving to  FIG. 21 , the silicon nitride spacers  245  are then removed by a suitable selective etching process, leaving between adjacent floating gates  280  recessed windows  250  adapted for accommodating subsequent material layers.  
         [0045]     Then, as shown in  FIG. 2L , a relatively thin, conforming interpoly dielectric layer  255  is deposited, for example by means of a CVD process, over the surface of the wafer, thus covering the walls of the recessed windows  250 . The interpoly dielectric layer  255  may, for example, comprise a stack of layers SiO 2 /Si 3 N 4 /SiO 2 , referred to as ONO (acronym for Oxide/Nitride/Oxide) layer. The ONO layer  255  is relatively thin (for example, the thickness of the ONO layer  255  ranges from 10 nm to 18 nm). Afterwards, a polysilicon layer  260  is deposited over the whole surface, such as to substantially completely fill the recessed windows  250 . The polysilicon layer  260  is then patterned to define word lines, each of which forms a common control gate for the memory cells of the word line. In particular, according to a conventional scheme, the polysilicon layer  260  and the ONO stack of layers  255  are etched, and at the same time the polysilicon floating gates  280  are defined in the direction orthogonal to the plane of the drawings. The complex of known operations follow that lead to the finished memory device.  
         [0046]     Thanks to the fact that the polysilicon layer  260  fills the recessed windows  250 , the coupling capacitances between the floating gates of adjacent memory cells are significantly reduced. In fact, the polysilicon layer  260  filling the recessed windows  250 , being conductive, shields the floating gate of the generic selected cell from effects due to the charges stored on the floating gates of the adjacent cells.  
         [0047]     An alternative to the sequence of process phases just described, according to a second embodiment of the invention, comprises replacing the formation phase of the silicon nitride spacers  245  with the following process phases, shown in  FIGS. 3A-3B .  
         [0048]     In detail, the process proceeds similarly to the one described above up to the etching of the portions of field oxide layer  230  within the trenches  225  ( FIG. 2E ). Then, as shown in  FIG. 3A , a relatively thin, conforming silicon oxide layer  310  is deposited, for example by means of CVD process, over the surface of the wafer. The thickness of the silicon oxide layer  310  is such that the layer  310  substantially follows the profile of the underlying layers (for example, the thickness may range from about 10 nm to about 30 nm).  
         [0049]     Referring to  FIG. 3B , the silicon oxide layer  310  and the portions of the field oxide layer  230  within the trenches  225  are etched by means of an anisotropic etching process down to a depth d 4  past the level of the tunnel oxide portions  290 , that is, the exposed surface of the field oxide filling the trenches is recessed from the main surface  210  a depth d 4 , which in particular may be equal to the depth d 3  of the previous embodiment. The etching has an isotropic degree such that the layer  310  and the portions of field oxide filling the trenches  225  are etched preferably along the vertical direction. Thanks to the presence of the silicon oxide layer  310 , as well as to the anisotropy of the etching process, it is avoided that the tunnel oxide portions  290  are etched during this phase.  
         [0050]     As a result of the etching, recessed windows  315  are formed between the adjacent floating gates  280 , which are adapted for accommodating the subsequent ONO layer  255  and polysilicon layer  260 . From now on, the process proceeds following a known process scheme, particularly the patterning and definition of the word lines, that brings to the finished memory device.  
         [0051]     Also in this embodiment, as mentioned in the foregoing, the recessed windows  315 , being filled by conductive polysilicon layer, shielding the floating gates of adjacent memory cells and reduce the coupling capacitances there between. The floating gate potential of the generic selected memory cell is thus not affected by the charge present on the floating gates of the adjacent cells.  
         [0052]     By the method just described, thanks to the present invention, it is possible to make a floating gate non-volatile memory device of very reduced size, wherein nevertheless the coupling capacitances with, and thus the effects of the adjacent memory cells of the memory matrix are very reduced.  
         [0053]     Thanks to the present invention, the above results is achieved by means of relatively simple process steps and without adding masks so as to respect the reference process flow.  
         [0054]     Moreover, it is particularly useful to apply the solution of the invention to multi-level flash memories, wherein the reduced threshold voltage margins between the different programming states make the correct operation of the memory cells particularly critical in the presence of coupling capacitances between adjacent cells.  
         [0055]     Moreover, the method according to the invention is very advantageous in the case of NOR and NAND flash type or multilevel floating-gate non volatile semiconductor memory devices, but it can be applied to any semiconductor device in which is necessary to have a reduced coupling capacitance between adjacent memory cells.  
         [0056]     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply, to the solution described above, many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to preferred embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice.  
         [0057]     For example, although in the preceding description reference has been made to a P-type substrate, the conductivity type of the region may be reversed.  
         [0058]     In addition, it is not strictly necessary that the recessed windows  250  or  315  extend past the level of the tunnel oxide layer: a significant reduction of the coupling capacitances may also be obtained with more shallow recessed windows.  
         [0059]     Moreover, the recessed windows may have different shapes.  
         [0060]     The shape and depth of the trenches may greatly vary.  
         [0061]     In addition, it is possible to use other profiles of the dopant concentrations.  
         [0062]     In any case, the use of alternative processes for realizing the proposed floating gate MOS transistor is possible.  
         [0063]     For example, it is possible to grow a sacrificial oxide layer over the substrate before forming the tunnel oxide layer.  
         [0064]     Moreover, before filling the trenches with the insulating layer, a thin layer of silicon oxide may be formed to cover the walls of the trenches.  
         [0065]     In addition, although in the preceding description of the first invention embodiment a conforming nitride layer is deposited in order to form the spacers, a stack of relatively thin, conforming silicon dioxide layer and nitride layer may be formed.  
         [0066]     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.