Abstract:
A method of fabricating an a non-volatile memory includes forming trench isolation regions in an inactive region of a semiconductor substrate, adjacent trench isolation regions defining respective protrusions having rounded edges therebetween, wherein upper surfaces of the trench isolation regions are lower than an upper surface of the semiconductor substrate and wherein the protrusions define an active region, forming a tunnel insulating layer covering the protrusion of the semiconductor substrate, and forming, sequentially, a storage layer, a blocking insulating layer, and a gate layer covering the tunnel insulating layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation application of U.S. application Ser. No. 11/149,396 filed on Jun. 9, 2005, the disclosure of which is herein incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a non-volatile memory device in which stored data is maintained without a power supply and a method of fabricating the same, and more particularly, to an electrically erasable programmable read-only memory (hereinafter, referred to as ‘EEPROM’) and a method of fabricating the same. 
   2. Description of Related Art 
   Electrically erasable programmable non-volatile memory comes in different types, for example, EEPROM of a floating gate type, a metal-nitride-oxide-silicon (MNOS) type, a metal-oxide-nitride-oxide-silicon (MONOS) type, and a silicon-oxide-nitride-oxide-silicon (SONOS) type. 
   An example of a SONOS-type EEPROM is illustrated in  FIGS. 1 and 2 .  FIGS. 1 and 2  are cross-sectional views of the SONOS-type EEPROM, shown along a bit line and a gate, respectively. 
   Referring to  FIGS. 1 and 2 , the SONOS-type EEPROM has a stacked structure comprising a lower oxide layer  20 , a nitride layer  30 , an upper oxide layer  40 , and a polysilicon layer  50 . The lower oxide layer  20  is a tunnel oxide layer, the nitride layer  30  is a memory (storage) layer, and the upper oxide layer  40  is a blocking layer for preventing the loss of a stored charge. The polysilicon layer  50  is a gate. The lower oxide layer  20 , the nitride layer  30 , the upper oxide layer  40 , and the polysilicon layer  50  are sequentially formed on a substrate  10  in which isolation regions  15  are formed. Source/drain regions  60  are formed at both sides of the stacked structure in the substrate  10 . 
   The SONOS-type EEPROM can be used in a compact semiconductor memory cell because it needs less voltage to program and erase than an EEPROM of the floating gate type. To achieve a more highly integrated SONOS-type EEPROM, the size of a memory cell needs to be reduced. As the size of the SONOS-type EEPROM is reduced, isolation of each memory cell becomes more important. Thus, an area for isolation of each cell needs to be secured. The size of the area of isolation limits the size of an active area in this case. 
   Therefore, a need exists for a memory cell having an enhanced active channel region without increasing the size of a planar unit cell, and a method for fabricating the same. 
   SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, a method of fabricating an a non-volatile memory includes defining an active region and an inactive region in a semiconductor substrate, forming trench isolation regions in the inactive region for increasing an area of the active region from an upper surface of a protrusion of the semiconductor substrate between the trench isolation regions to a side of the protrusion, wherein upper surfaces of the trench isolation regions are lower than an upper surface of the semiconductor substrate, forming a tunnel insulating layer on the protrusion of the semiconductor substrate, including the side of the protrusion, forming, sequentially, a storage layer, a blocking insulating layer, and a gate layer on the tunnel insulating layer, and rounding angular edges of the protrusion after increasing the area of the active region, wherein the edges formed between the trench isolation regions and the protrusions remain angular. 
   According to an embodiment of the present invention, a method of fabricating an a non-volatile memory includes forming trench isolation regions in an inactive region of a semiconductor substrate, adjacent trench isolation regions defining respective protrusions having rounded edges therebetween, wherein upper surfaces of the trench isolation regions are lower than an upper surface of the semiconductor substrate and wherein the protrusions define an active region, forming a tunnel insulating layer covering the protrusion of the semiconductor substrate, and forming, sequentially, a storage layer, a blocking insulating layer, and a gate layer covering the tunnel insulating layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
       FIGS. 1 and 2  are cross-sectional views of a conventional SONOS-type EEPROM; 
       FIGS. 3 through 6  are cross-sectional views illustrating a method of fabricating a SONOS-type EEPROM according to an embodiment of the present invention; 
       FIG. 7  is a magnified view of a portion B shown in  FIG. 6 ; and 
       FIGS. 8 and 9  are cross-sectional views illustrating a method of fabricating a SONOS-type EEPROM according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The present invention will now be described more fully with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may be embodied in different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the concept of the invention to those skilled in the art. Characteristics, such as shape and thickness, of elements shown in the drawings may be exaggerated for clarity. The same reference numerals in different drawings represent the same elements, and thus their descriptions will not be repeated. 
     FIGS. 3 through 6  are cross-sectional views illustrating a method of fabricating a SONOS-type EEPROM according to an embodiment of the present invention. The cross-sectional views of the EEPROM are shown along the direction of a gate. 
   Referring to  FIG. 3 , a semiconductor substrate  110  is provided, in which a well (not shown) may be formed using ion implantation. Shallow trench isolation (STI) regions  115  are formed in the semiconductor substrate  110  to define active regions and inactive regions. The STI regions  115  are formed in the inactive regions of the semiconductor substrate  110  and are level with the upper surface of the semiconductor substrate  110 . 
   To fabricate the STI regions  115 , oxide and nitride layers are formed on the semiconductor substrate  110 . The oxide and nitride layers are patterned to form openings where trenches are to be formed. The trenches are formed to a depth of about 3000-6000 Å in the semiconductor substrate  110  with the patterned oxide and nitride layers being used as masks. Insulating layers are formed to fill the trenches. For example, the trenches may be filled with insulating layers in the manner where (i) an oxide liner and/or a nitride liner is formed along the inner walls of the trenches; (ii) a middle-temperature oxide (MTO) is formed on the liner; and (iii) a remaining portion of the trenches is filled with a material having excellent gap filling characteristics, e.g., undoped silicate glass (USG) or an oxide layer that is obtained using high-density plasma chemical vapor deposition (HDP CVD). After the trenches are filled, the upper surfaces of the insulating layers are planarized using chemical mechanical polishing (CMP) or an etch back method. During planarization, the patterned nitride layer acts as a stop for the CMP or the etch back method. The patterned oxide and nitride layers left on the semiconductor substrate  110  are removed to obtain the STI regions  115  whose upper surfaces are at the same height as, or slightly higher than, that of the semiconductor substrate  110 . 
   Referring to  FIG. 4 , the STI regions  115  are etched to a predetermined thickness forming recessed trench isolation regions  115   a , whose upper surfaces are lower than the upper surface of the semiconductor substrate  110 . A protrusion of the semiconductor substrate  110  between the trench isolation regions  115   a  forms an active region. The active area comprises the top and the sidewalls of the protrusion. Thus, according to the present invention, it is possible to increase the size of the active region without changing the two-dimensional layout of the active region. 
   The STI region  115  may be etched with a hydrofluoric acid (HF) solution using wet etch back. The HF solution is obtained by mixing HF with H 2 O at a ratio of between about 1:10 and 1:1000 at room temperature. The STI regions  115  are dipped in the HF solution or the HF solution may be sprayed onto the STI regions  115  so as to etch the STI regions  115  to the predetermined thickness. Alternatively, a buffered oxide etchant (BOE), which is a mixture of HF and NH 4 F, may be used in stead of the HF solution. The isolation regions  115   a  of a desired thickness can be obtained by appropriately increasing or reducing the time spent dipping the STI regions  115  in the HF solution or BOE or spraying the HF solution or BOE onto the STI regions  115 . 
   Accordingly, the active region protrudes above the semiconductor substrate  110  and allows effective isolation of devices even if the isolation regions  115   a  are thinner than a conventional field oxide layer. The thickness of the isolation region  115   a  can be increased if needed, given a recess of a certain depth, by increasing the depth of the trenches. 
   Referring to  FIG. 5 , a tunnel oxide layer  120  is formed to completely cover the isolation region  115   a  and the protrusion of the semiconductor substrate  110 . The tunnel oxide layer  120  is formed by thermally oxidizing the resultant structure of  FIG. 4 . Alternatively, the tunnel oxide layer  120  may be obtained by depositing a MTO on the resultant structure of  FIG. 4  using low-pressure CVD (LPCVD) and annealing the MTO. The annealing process is performed in a gas atmosphere comprising N 2 O and/or NO. However, for the as-deposited MTO, there is a higher probability of surface defects such as silicon dangling bonds. Performing the annealing process under an atmosphere of N 2 O and/or NO removes such defects, thus improving the leakage current characteristics and the reliability of the MTO. 
   After the formation of the tunnel oxide layer  120 , a nitride layer  130 , which acts as a memory (storage) layer, is formed on the tunnel oxide layer  120 . The nitride layer  130  may be formed by nitrifying the tunnel oxide layer  120  or using LPCVD. A blocking oxide layer  140  is formed on the nitride layer  130  to prevent loss of stored charges. The blocking oxide layer  140  may be formed by thermally oxidizing the nitride layer  130 . 
   Referring to  FIG. 6 , a polyslicon gate conductive layer  150  is formed on the blocking oxide layer  140 . The interface between the polysilicon gate conductive layer  150  and the blocking oxide layer  140 , above each of the isolation regions  115   a , is lower than the upper surface of the semiconductor substrate  110 . Deposition of the polysilicon gate conductive layer  150  may be performed at temperature of about 500-700° C. using LPCVD. The polysilicon gate conductive layer  150  may be formed by first depositing a polysilicon layer, which is not doped with impurities, on the resultant structure of  FIG. 5  and ion-implanting arsenic (Ar) or phosphorous (P) into the polysilicon layer so as to make the polysilicon layer conductive. Alternatively, the polysilicon gate conductive layer  150  may be made by doping polysilicon with impurities and depositing the impurity-doped polysilicon on the blocking oxide layer  140  by using an in-situ process. 
   As shown in  FIG. 6 , an EEPROM according to an embodiment of the present invention comprises the isolation regions  115   a  that are formed lower than the upper surface of the semiconductor substrate  110 , the tunnel oxide layer  120  covering the upper surfaces of the isolation regions  115   a  and the semiconductor substrate  110  that protrudes between the upper surfaces of the isolation regions  115   a , the nitride layer  130  formed on the tunnel oxide layer  120 , the blocking oxide layer  140  formed on the nitride layer  130 , and the polysilicon gate conductive layer  150  formed on the blocking oxide layer  140 . The interface between the polysilicon gate conductive layer  150  and the blocking oxide layer  140 , above the isolation regions  115   a , is lower than the upper surface of the semiconductor substrate  110 . 
   A SONOS-type EEPROM according to an embodiment of the present invention comprises an active region, which has a 2D layout substantially similar to a conventional SONOS-type EEPROM and a 3D structure, which increases an effective channel width of the active area. The active region comprises both the upper surface and the sidewalls of a protrusion of the semiconductor substrate  110  between the isolation regions  115   a.    
     FIG. 7  is a magnified view of the region B of  FIG. 6 . Referring to  FIG. 7 , additional storage capacity D is realized along the sidewalls of a protrusion of the semiconductor substrate  110 , thereby increasing the effective width of the active region without changing the cell layout. The formation of the additional storage facilities D results in the formation of an additional channel C along a portion of the semiconductor substrate  110 . As a result, it is possible to enhance the programming and erasing efficiencies, and reduce the amount of cell current needed when reading information stored in the memory cell. Due to an increase in the performance of the memory cell, the memory cell can be made smaller, while achieving at least the same level of performance as compared to a larger conventional memory cell, thereby increasing the integration of the memory. An active region of an EEPROM according to the present invention is a three-dimensional (3D) region having more area than a planar active region of a conventional EEPROM having a substantially similar 2D layout. Accordingly, a high-density and highly integrated EEPROM can be fabricated in which devices are effectively separated from one another. 
     FIGS. 8 and 9  are cross-sectional views illustrating a method of fabricating a SONOS-type EEPROM according to an embodiment of the present invention. 
   Referring to  FIG. 8 , from the structure shown in  FIG. 4 , edge portions of a semiconductor substrate  110 , which protrude above isolation regions  115   a , are rounded to form round edges E. 
   The round edges E may be formed by etching the edge portions using a mixture of NH 4 OH, H 2 O 2 , and H 2 O. Angular edges are etched at a higher rate by this mixture than round portions, and thus the angular edges become rounded without substantially affecting other structures. NH 4 OH, H 2 O 2 , and H 2 O may be mixed at a ratio between about 1:1:5 and 1:4:100 at a temperature of about 50-75° C. to form the mixture. 
   Alternatively, the round edges E may be made by oxidizing the angular edges and removing the oxidized portions of the edges. The structure of  FIG. 4  is exposed to an atmosphere comprising oxygen to oxidize the exposed portions of the semiconductor substrate  110 . In particular, the angular edges are easily oxidized as compared to other structures. An oxide layer covering the angular edges forms a round interface with the semiconductor substrate  110 . The oxide layer on the edges is wet-etched using an HF solution and removed to expose round edges E. 
   Referring to  FIG. 9 , a tunnel oxide layer  120  is formed to cover the isolation regions  115   a  and the protrusion of the semiconductor substrate  110  between the isolation regions  115   a . A nitride layer  130 , a blocking oxide layer  140 , and a polysilicon gate conductive layer  150  are sequentially formed on the tunnel oxide layer  120 . 
   The round edges E prevent the concentration of an electric field around the edges of the protrusion, thereby avoiding degradation of the tunnel oxide layer  120 . 
   A silicide layer  155  is formed over the polysilicon gate conductive layer  150 . The silicide layer  155  may be a cobalt silicide layer, a tungsten silicide layer, or a titanium silicide layer. The silicide layer  155  is formed by depositing a metal layer of cobalt, tungsten, or titanium on the polysilicon gate conductive layer  150  and performing a thermal treatment, such as rapid thermal annealing (RTA), on the metal layer. During RTA, the deposited metal layer reacts with silicon in the polysilicon gate conductive layer  150  to form the silicide layer  155 . Portions of the metal layer that do not react with the silicon are cleaned and removed. The silicide layer  155  has a lower resistance than the polysilicon gate conductive layer  150  and thus increases an operating speed of devices. 
   As described above, an EEPROM according to the present invention comprises trench isolation regions that are recessed compared to an upper surface of a semiconductor substrate, thus increasing an area of an active region to range from the upper surface of the semiconductor substrate to the side of a protrusion between the isolation regions. Thus, an active channel region can be broadened without needing to change the size of a planar unit cell, thus increasing the efficiency of programming the memory and erasing information from the memory. 
   According to the present invention, the performance of a cell can be increased and the size of the memory cell can be reduced. Therefore, it is possible to manufacture a highly-integrated EEPROM having a higher packing density.