Abstract:
Disclosed are methods for manufacturing floating gate memory devices and the floating gate memory devices thus manufactured. In one embodiment, the method comprises providing a monocrystalline semiconductor substrate, forming a tunnel oxide layer on the substrate, and depositing a protective layer on the tunnel oxide layer to form a stack of the tunnel oxide layer and the protective layer. The method further includes forming at least one opening in the stack, thereby exposing at least one portion of the substrate, and cleaning the at least one exposed portion with a cleaning liquid. The method still further includes loading the substrate comprising the stack into a reactor and, thereafter, performing an in-situ etch to remove the protective layer, using the at least one exposed portion as a source to epitaxially grow a layer comprising the monocrystalline semiconductor material, and forming the layer into at least one columnar floating gate structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a non-provisional of U.S. Provisional Patent Application Ser. No. 61/413,609 filed Nov. 15, 2010, the contents of which are hereby incorporated by reference. Further, this application claims priority to European Patent Application Serial No. 10190373 filed Nov. 8, 2010, the contents of which are hereby incorporated by reference. 
       BACKGROUND 
       [0002]    Though various types of floating gate non-volatile memory devices exist, many require obtaining a thin tunnel oxide layer on a semiconductor substrate, mostly commonly a silicon substrate. For example, Hurley (U.S. Patent Publication No. 2003/0011018) describes a method in which openings are made in a tunnel oxide layer and monocrystalline silicon is epitaxially grown over the tunnel oxide with the openings serving as seed areas for the epi-growth. This is an example of so-called selective epitaxial growth of silicon during which the nucleation of polycrystalline silicon on top of, for example, silicon nitride/oxide layers is prevented. The monocrystalline material will form the actual floating gate in the finished device. 
         [0003]    In selective epitaxial growth, it is important to clean the seed areas and to remove the native oxide before the epi-growth begins. This conventional cleaning process usually comprises a wet chemical clean having a last step treatment with aqueous solutions containing hydrogen fluoride, a so-called ‘HF dip’. However, this step may cause uncontrolled thinning of the tunnel oxide, which may lead to reproducibility issues, and possibly the appearance of areas of non-uniform tunnel oxide thickness. 
         [0004]    In some flash memory devices, arrays of columnar floating gate structures are produced from polysilicon to have a certain thickness between parallel sidewalls. An oxide-nitride-oxide (ONO) inter-poly dielectric runs along the sidewalls of the floating gate in order to provide a large capacitance between floating gate and control gate and therefore a large coupling ratio. 
         [0005]    In order to scale the cell, it is necessary to reduce one or more of the length of the floating gate, the length of the control gate, and the thickness of the inter-poly dielectric. For scaled devices, the length of the structures are becoming smaller and the patterns denser. The limit of scaling would be given by a half-pitch filled by the inter-poly dielectric (on both sides). This implies that the pitch of floating gates (defined as the sum of the floating gate length and the space between two adjacent floating gates) cannot be scaled below a value which corresponds to the sum of the floating gate length (scalable to about 5 nm), the control gate length (scalable to about 5 nm), and two times the ONO inter-poly thickness (scalable to about 15 nm). This gives a total sum of about 40 nm and therefore a limit of scaling of 20 nm half-pitch, meaning a half-pitch below 20 nm is difficult to achieve. While this value could be reduced by making a thinner vertical inter-poly dielectric, but this is not trivial for the ONO stacks typically in use. 
         [0006]    Floating gate memory devices may be operated array in a manner similar to NAND memories, as described, for example, in “Nonvolatile Memory Technologies with Emphasis on Flash: a Comprehensive Guide to Understanding and Using NVM Devices,” IEEE Press Series on Microelectronic Systems, pp. 227-237 (J. Brewer and M. Gill, eds., 2007). 
       SUMMARY 
       [0007]    Disclosed are methods for manufacturing floating gate memory devices. Also disclosed are the floating gate memory devices manufactured using the disclosed methods. 
         [0008]    In one embodiment, a method is disclosed that comprises providing a monocrystalline semiconductor substrate and forming a tunnel oxide layer on the substrate. The method further comprises depositing a protective layer over the tunnel oxide layer, thereby forming a stack comprising the tunnel oxide layer and the protective layer. The method still further comprises forming at least one opening in the stack, thereby exposing at least one exposed portion of the substrate, and cleaning the at least one exposed portion using a cleaning liquid, where the protective layer is substantially unaffected by the cleaning liquid. The method further includes loading the substrate comprising the stack into a reactor and, thereafter, removing the protective layer by an in-situ etching process that is selective with respect to the tunnel oxide layer, epitaxially growing a monocrystalline semiconductor layer using the at least one exposed portion as a source, and forming the semiconductor layer into at least one floating gate structure. 
         [0009]    In some embodiments, the method may further include, after loading the substrate into the reactor, annealing the substrate to remove any oxide from the at least one exposed portion. 
         [0010]    In some embodiments, the substrate may comprise silicon. Further, the tunnel oxide layer may comprise silicon dioxide (SiO 2 ), such as thermally-grown silicon dioxide. The protective layer may comprise amorphous or polycrystalline silicon. 
         [0011]    In some embodiments, the cleaning liquid may comprise hydrogen fluoride (HF). 
         [0012]    In some embodiments, the in-situ etching process may comprise exposing the stack to a halogen containing precursor, such as hydrogen chloride (HCl) or chlorine (Cl 2 ). 
         [0013]    In some embodiments, the in-situ etching process being selective with respect to the tunnel oxide layer may comprise the in-situ etching process having a selectivity for the protective layer over the tunnel oxide layer of about 100:1. 
         [0014]    In some embodiments, forming the semiconductor layer into at least one floating gate structure may comprise planarizing the semiconductor layer, depositing on the planarized semiconductor layer a hard mask layer comprising a dielectric material, depositing a resist layer on the hard mask layer, patterning the resist layer, etching the hard mask layer and the planarized semiconductor layer, thereby forming at least one floating gate structure. In some embodiments, the tunnel oxide layer substantially unaffected by the etch of the hard mask layer and the planarized semiconductor layer. 
         [0015]    In some embodiments, the at least one floating gate structure may comprise at least two columnar floating gate structure. In these embodiments, the method may further comprise etching trenches between the at least two columnar floating gate structures and filling the trenches with a dielectric material, thermally growing an oxide layer on sides of the at least two columnar floating gate structures, and depositing a control gate layer between and on top of the at least two columnar floating gate structures. 
         [0016]    The thermally-grown oxide layer may have a thickness between about 5 nm and about 15 nm, or between about 5 nm and about 10 nm, or even between about 6 nm and about 8 nm. Further, a half-pitch of the at least two columnar floating structures may be between about 10 nm and about 20 nm, or between about 10 nm and about 15 nm, or even between about 11 nm and about 13 nm. 
         [0017]    In another aspect, a method is disclosed comprising providing a monocrystalline semiconductor substrate and forming a tunnel oxide layer on the substrate. The method further comprises forming at least one opening in the tunnel oxide layer, thereby forming at least one exposed portion of the substrate, and cleaning the at least one exposed portion using a cleaning liquid. The method further includes loading the substrate comprising the tunnel oxide layer into an epitaxial reactor and, thereafter, epitaxially growing a monocrystalline semiconductor layer using the at least one exposed portion as a source, planarizing the semiconductor layer, depositing on the planarized semiconductor layer a hard mask layer comprising a dielectric material etching the planarized semiconductor layer, thereby forming at least two floating gate structures. The method further comprises etching trenches between the at least two columnar floating gate structures and filling the trenches with a dielectric material, thermally growing an oxide layer on sides of the at least two columnar floating gate structures, and depositing a control gate layer between and on top of the at least two columnar floating gate structures. 
         [0018]    In some embodiments, the method may further include, after loading the substrate into the epitaxial reactor, annealing the substrate to remove any oxide from the at least one exposed portion. 
         [0019]    In some embodiments, the tunnel oxide layer substantially unaffected by the etch of the hard mask layer and the planarized semiconductor layer. 
         [0020]    In some embodiments, the method may further include, after forming the tunnel oxide layer, depositing a protective layer over the tunnel oxide layer, thereby forming a stack comprising the tunnel oxide layer and the protective layer. In these embodiments, forming the at least one opening in the tunnel oxide layer may comprise forming the at least one opening in the stack. In these embodiments, the method may further comprise, after loading the substrate into the epitaxial reactor and before epitaxially growing the semiconductor layer, removing the protective layer by an in-situ etching process that is selective with respect to the tunnel oxide layer. In some embodiments, the in-situ etching process being selective with respect to the tunnel oxide layer may comprise the in-situ etching process having a selectivity for the protective layer over the tunnel oxide layer of about 100:1. 
         [0000]    In some embodiments, the substrate may comprise silicon. Further, the tunnel oxide layer may comprise silicon dioxide (SiO 2 ), such as thermally-grown silicon dioxide. The protective layer may comprise amorphous or polycrystalline silicon. 
         [0021]    In some embodiments, the cleaning liquid may comprise hydrogen fluoride (HF). 
         [0022]    In some embodiments, the in-situ etching process may comprise exposing the stack to a halogen containing precursor, such as hydrogen chloride (HCl) or chlorine (Cl 2 ). 
         [0023]    In some embodiments, the in-situ etching process being selective with respect to the tunnel oxide layer may comprise the in-situ etching process having a selectivity for the protective layer over the tunnel oxide layer of about 100:1. 
         [0024]    The thermally-grown oxide layer may have a thickness between about 5 nm and about 15 nm, or between about 5 nm and about 10 nm, or even between about 6 nm and about 8 nm. Further, a half-pitch of the at least two columnar floating structures may be between about 10 nm and about 20 nm, or between about 10 nm and about 15 nm, or even between about 11 nm and about 13 nm. 
         [0025]    In yet another aspect, a floating gate memory device is disclosed that comprises at least one array comprising a number of columnar floating gate structures comprising a monocrystalline semiconductor material. The floating gate memory device further comprises a number of control gate structures formed between each of the columnar floating gate structures. Each columnar floating gate structure comprises lateral sides on which oxide layers are formed, such that each floating gate structure is electrically isolated from one another. The oxide layers have a thickness between about 5 nm and about 15 nm, or between about 5 nm and about 10 nm, or even between about 6 nm and about 8 nm. Further, a half-pitch of the array of columnar floating structures is between about 10 nm and about 20 nm, or between about 10 nm and about 15 nm, or even between about 11 nm and about 13 nm. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]      FIGS. 1A-1M  illustrate the process flow of the fabrication of a floating gate memory device according to the method of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    With reference to  FIGS. 1A-1M , an example method of manufacturing a floating gate memory device is illustrated. While an example floating gate memory device is shown, it is to be understood that the invention is not so limited, and that the disclosed floating gate memory device could be other floating gate memory devices are possible as well. 
         [0028]    As shown in  FIG. 1A , a monocrystalline semiconductor substrate  1  is provided. The substrate  1  may be, for example, a monocrystalline silicon substrate. A tunnel oxide layer  2  is formed on the substrate  1 . The tunnel oxide layer  2  may comprise, for example, silicon dioxide and may be formed by, for example, thermal oxidation of the substrate. The tunnel oxide layer may have a thickness between about 5 nm and about 10 nm. 
         [0029]    A protective layer  3  is then formed on the tunnel oxide layer  2 , as shown. The protective layer  3  may comprise, for example, amorphous or polycrystalline silicon and may be formed by, for example, chemical vapor deposition on the tunnel oxide layer  2 . The protective layer  3  may have a thickness of, about, a few nm. The protective layer  3  and the tunnel oxide layer  2  may form a stack. In some embodiments, no such protective layer  3  may be formed. 
         [0030]    In  FIG. 1B , a stop structure  4  is formed on the protective layer  3  (or, in embodiments in which no protective layer  3  is formed, on the tunnel oxide layer  2 ). While only one stop structure  4  is shown, multiple stop structures may be formed. The stop structure  4  may serve to stop a chemical mechanical polish. In some embodiments, the stop structure  4  may comprise, for example, silicon nitride (Si 3 N 4 ) and may be formed by, for example, depositing a layer of silicon nitride and then patterning the silicon nitride to define one or more protrusions to act as the stop structure  4 . In embodiments where the stop structure  4  comprises stoichiometric silicon nitride, silicon does not nucleate on the stop structure  4 . 
         [0031]    In some embodiments, rather than being formed on the protective layer  3 , the stop structure  4  may be formed directly on the substrate  1  prior to the tunnel oxide layer  2  and the protective layer  3 . The tunnel oxide layer  2  and the protective layer  3  may then be formed on the substrate  1  comprising the stop structure  4 . 
         [0032]    As shown in  FIG. 1C , at least one opening is formed in the stack (i.e., in the tunnel oxide layer  2  and the protective layer  3  or, in embodiments in which no protective layer  3  is formed, in the tunnel oxide layer  2 ). As a result of forming the opening, an portion  5  of the substrate may be exposed. While only one exposed portion  5  is shown, in other embodiments more portions of the substrate may be exposed. 
         [0033]    The at least one opening may be formed by, for example, a lithography and dry etch process followed by a post-etch wet clean (e.g., strip) process. Following the post-etch wet clean, the exposed portion  5  may be covered by a native oxide layer, in which case the native oxide layer may be removed by a wet chemical clean, such as a hydrogen fluoride dip. The protective layer  3  may protect the tunnel oxide layer  2  during the hydrogen fluoride dip, such that the tunnel oxide layer  2  remains substantially unaffected by the hydrogen fluoride. In some cases, at the lateral sides of the exposed portions  5 , the tunnel oxide layer  2  may be slightly etched by the hydrogen fluoride dip; however, at the portions of the tunnel oxide layer  2  where the floating gate structures will be formed, as described below, the thickness of the tunnel oxide layer  2  will be unchanged. 
         [0034]    The substrate  1  comprising the stack  2 ,  3  may then be loaded into a reactor, such as an epitaxial reactor. In some embodiments, the substrate  1  comprising the stack  2 ,  3  may be annealed at a temperature between, for example, about 600° C. and about 1050° C., such as about 850° C. Such an anneal may serve to remove from the exposed portions  5  of the substrate  1  any oxides that may have formed between the hydrogen fluoride dip and the loading of the substrate  1  comprising the stack  2 ,  3  into the reactor. In embodiments where the protective layer  3  comprises an amorphous silicon layer, the anneal may also serve to change the amorphous silicon to polysilicon. Alternately, in some embodiments, no anneal may take place. 
         [0035]    As shown in  FIG. 1D , the protective layer  3  is removed through in-situ etching. The etching is selective with respect to the tunnel oxide layer  2 , such that the tunnel oxide layer  2  remains substantially unaffected by the etching. In some embodiments, the etching may include, for example, precursors containing hydrogen chloride (HCl) (or other halogen-based precursors) and may be carried out at a temperature between, for example, about 600° C. and about 1000° C., such as about 850° C. 
         [0036]    In some embodiments, the in-situ etching process being selective with respect to the tunnel oxide layer  2  may comprise the in-situ etching process having a selectivity for the protective layer  3  over the tunnel oxide layer  2  of about 100:1. Further, the in-situ etching process may be selective with respect to the substrate  1 , though may have a much lower selectivity; the in-situ etching process may have a selectivity for the protective layer  3  over the substrate  1  of about 10:1. Alternately, the in-situ etching may not be selective with respect to the substrate  1 , in which case the exposed portions  5  may be cleaned to remove oxides, and additional monocrystalline semiconductor material may be grown (e.g., by selective epitaxial growth) to replace any monocrystalline semiconductor material that is etched. In embodiments where no protective layer  3  is present, the in-situ etching may not be performed. 
         [0037]      FIG. 1E  illustrates selective epitaxial growth of a monocrystalline semiconductor layer  6  using the exposed portion  5  as a source. As shown, the semiconductor layer  6  extends over the tunnel oxide layer  2  to a thickness above the height of the stop structure  4 . In embodiments where multiple stop structures  4  are used, more than one portion  5  of the substrate may be exposed, such that a stop structure  4  may be formed between two adjacent exposed portions  5 . In these embodiments, the semiconductor layer  6  may be grown from each of the exposed portions  5 , and may connect to form a border above the stop structure  4 . The border may contain a number of irregularities. However, due to the stop structure  4 , the border may be formed far enough from the substrate  1  to avoid any negative effects. 
         [0038]    As shown in  FIG. 1F , the semiconductor layer  6  may be planarized. As shown in  FIG. 1G , the stop structure  4  may be removed (e.g., by a selective wet etch). 
         [0039]    As shown in  FIG. 1H , a hard mask layer  7  may be deposited on top of the planarized semiconductor layer  6 . The hard mask layer  7  may comprise, for example, a dielectric material, such as silicon nitride (SiN) or silicon dioxide. Further, as shown in  FIG. 1H , a resist layer  8  may be deposited on the hard mask layer  7  and patterned. 
         [0040]    In  FIG. 1I , the hard mask layer  7  and the semiconductor layer  6  are etched according to the patterned resist layer  8 , such that columnar floating gate structures  9  are formed. As shown, the etch stops at the tunnel oxide layer  2 . The floating gate structures  9  may have a half-pitch between about 10 nm and about 20 nm, or even between about 11 nm and about 13 nm. Further, the floating gate structures  9  may have a thickness of about 5 nm. 
         [0041]    As shown in  FIG. 1J , the tunnel oxide layer  2  and the substrate  1  are etched to form trenches  10 . The trenches  10  may have a depth between, for example, about 300 nm and about 350 nm. In  FIG. 1K , shallow trench isolation (STI) regions  11  are formed by filling the trenches  10  with an insulating material, such as silicon oxide. As shown, the STI regions  11  fill the trenches  10  to a height just above the tunnel oxide layer  2 , such that a large part of the floating gate structures  9  are still exposed. 
         [0042]    In  FIG. 1L , a silicon dioxide layer  12  is formed on the sidewalls of the floating gate structures  9 . The silicon dioxide layer  12  may be formed by, for example, a thermal oxidation of the semiconductor layer  6 . The silicon dioxide layer  12  may have a thickness between, for example, about 5 nm and about 15 nm, or between about 5 nm and about 10 nm, or even between about 6 nm and about 8 nm. 
         [0043]    As shown in  FIG. 1M , a control gate layer  13  is formed over the floating gate structures  9 . The control gate layer  13  may be formed by, for example, depositing (e.g., by chemical vapor deposition) a polysilicon material that surrounds the floating gate structures  9 . 
         [0044]    As shown, the hard mask layer  7  remains on top of the floating gate structures  9 . The hard mask layer  7  may prevent unwanted leakage between the top surface or top corners of the floating gate structures  9  and the control gate layer  13 . For example, the hard mask layer  7  may act as an inter-poly dielectric in between the floating gate structures  9  and the control gate layer  13 . In some embodiments, a thickness of the hard mask layer  7  may be greater than a thickness of the silicon dioxide layer  12 . For example, the hard mask layer  7  may be about 5 nm to about 15 nm thicker than the silicon dioxide layers  12 . 
         [0045]    The method illustrated in  FIGS. 1A-1M  may be used to produce a dense pattern of columnar floating gate structures  9 , each of which has a silicon oxide layer  12  on each side. 
         [0046]    The floating gate structures  9  may, in some embodiments, be operated in a manner similar to NAND memory arrays. For example, the control gate  12  and the floating gate structures  9  may be formed in a direction perpendicular to the STI regions  11 , such that every floating gate structure  9  is at the intersection of a portion of the substrate  1  where the STI regions  11  are not formed. The floating gate structures  9  may act as a NAND string, while the control gate  12  acts as a wordline.