Abstract:
Memory cell transistors with back-channel isolation are produced without using an SOI substrate. With the word line stack acting as a mask, the semiconductor material is etched on both sides of the world line, first anisotropically and then isotropically to widen the etch hole and form an undercut beneath the gate electrode and at a distance from the ONO storage layer forming the gate dielectric. The undercut is filled, whereby a buried oxide layer of at least 20 nm maximum thickness is formed underneath the channel region. The latter is p-doped at a density of at least 10 17  cm −3 .

Description:
BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention lies in the field of electrically writable and erasable non-volatile flash memories comprising NROM-type memory cells preferably arranged in a virtual-ground NOR array. 
     Extremely small non-volatile memory cells are required for very large-scale integration densities in multimedia applications. However, while the minimum feature size, which is determined by lithography, continues to decrease, other parameters can no longer be scaled accordingly. 
     NROM-type memory cells are described in B. Eitan et al., “NROM: A novel localized trapping, 2-Bit nonvolatile memory cell”, IEEE Electron Device Letters 21, 543-545 (2000). Currently, NROM cells are fabricated as planar-type MOS transistors using a triple layer of oxide-nitride-oxide both as gate dielectric and as memory or programming layer. The intermediate nitride layer is used as a storage layer for trapping charge carriers, preferably electrons. Due to the specific properties of the used materials, typical source/drain voltages of 4 V to 5 V are necessary during program and erase operations. 
     At these comparatively high voltages, a punch-through occurs that impedes further scaling down of transistor channel lengths to values below 200 nm. Punching is supposed to take place between the n + -junctions of the source/drain regions through the semiconductor material below the channel region. Recent research in the physics of semiconductor devices demonstrated a superior punching behavior as well as an improvement of short channel properties when the channel is confined below by a buried oxide layer at a certain distance from the gate electrode. 
     MOSFETs on SOI substrates are described in the book by Jean-Pierre Colinge, “Silicon-on-insulator Technology: Materials to VLSI”, 2 nd  ed., Kluwer Academic Publishers, Dordrecht 1997, Chapter 5: “The SOI MOSFET”. The vertical extension of the channel region of SOI MOSFETs is limited by the insulating layer of the SOI substrate. The channel is partially or fully depleted, depending on the thickness of the channel region, which determines, inter alia, whether the space-charge regions adjacent to the gate-oxide and adjacent to the insulating layer are separated or join each other. The SOI MOSFET is completely electrically insulated towards the bulk substrate, unless there is a via provided through the insulating layer so that the bulk substrate can be electrically contacted from the upper surface. 
     At the 3 rd  European Workshop on Ultimate Integration of Silicon (ULIS 2002), Munich 2002, Thomas Skotnicki presented a NANO CMOS with 16 nm gate length. This type of transistor structure provides a backside channel isolation that is essentially confined to the channel region and is formed by removing an about 15 nm thick epitaxially grown SiGe layer and substituting it with electrically insulating material. The vertical dimension of the undoped and fully depleted channel is comparable to state of the art SOI MOSFETs. The heavily doped source and drain regions extend below the insulating layer level and are provided with LDD (lightly doped drain) regions as channel junctions. The channel structure is called SON, Silicon On Nothing. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a non-volatile memory cell and a fabrication method which overcome the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provide for a SONOS-type transistor memory cell having a minimum cross-sectional area of the active channel, at the same time allowing for an at least partial depletion of the channel region, and a method of producing such a memory cell and memory cell array. 
     It is a further object of the invention to disclose a method of fabricating NROM memory cells without using SOI substrates. 
     It is a yet a further object to provide an isolation of the memory cell that is self-aligned to the word line and suitable for an array of memory cells. 
     It is still a further object of the invention to provide an isolated channel transistor memory cell that can be arranged in a virtual-ground array, and a method of producing such a memory cell and memory cell array. 
     With the foregoing and other objects in view there is provided, in accordance with the invention, a method of producing a non-volatile memory cell having a semiconductor structure (a semiconductor body or a semiconductor layer on a substrate); buried bit lines formed at the surface of the semiconductor structure and conductor strips on the surface applied to the bit lines; a source region and a drain region each connected by one of the bit lines; a gate dielectric formed on the surface at least between the source region and the drain region; a gate electrode disposed on the gate dielectric; and a word line electrically connected to the gate electrode, the word line crossing the bit lines and being electrically insulated from the bit lines. 
     The method comprises the following steps:
         a) providing a semiconductor structure;   b) depositing a gate dielectric comprising a storage layer for trapping charge carriers;   c) depositing a layer for forming the gate electrode;   d) forming openings in the layer and forming spacers at sidewalls in the openings;   e) implanting a dopant through the openings to form the buried bit lines;   f) applying the conductor strips onto the buried bit lines, and applying electrically insulating cover layers onto the conductor strips;   g) applying at least one word line layer electrically connecting the gate electrode, and applying a hard mask layer on top of the at least one word line layer, the hard mask layer being used to structure the gate electrode and the at least one word line layer to form a word line stack;   h) etching on both sides of the word line stack between the bit lines anisotropically down into the semiconductor structure to a level below the source region and the drain region to form etch holes that are self-aligned to the word line stack; and   i) filling the etch holes with an electrically insulating material.       

     The NROM cell according to this invention is arranged at the surface of a semiconductor body or semiconductor layer supplied with electrically insulating regions extending vertically with respect to this surface straight down into the semiconductor body or semiconductor layer between bit lines and on both sides of a corresponding word line provided for addressing the memory cell, the electrically insulating regions being arranged self-aligned to the word line and possibly also self-aligned to the bit lines and extending at least to a lower boundary level of the source region and the drain region. The electrically insulating regions preferably comprise an undercut region or buried layer underneath the channel region that is situated between the regions of source and drain. Punching of the transistor is avoided or at least impeded by means of the electrically insulating regions. 
     The structure is produced by anisotropically etching between the word lines and bit lines after word line delineation at least to the level of the lower junctions of the source/drain regions and preferably by isotropically underetching the channel region of the transistor. This etching process takes place from both sides of the word line and is performed self-aligned to the word line. The etch holes and eventually the undercut are filled with the electrically insulating material. The gate reoxidation step can be used to grow a thermal oxide around the transistor body and to passivate the semiconductor surface. Additionally, boron dopant or another p-dopant species can be implanted for improved electrical insulation beneath the electrically insulating material filling the undercut. 
     A preferred embodiment comprises a deposition of a CVD oxide in combination with a dielectric material of a small value of the relative dielectric constant into the space between neighboring word lines. 
     The method is applicable to future memory devices appertaining to lithography generations with a word line half pitch of about 90 nm and less. 
     The method according to the invention of producing a non-volatile memory cell comprises the following steps: A storage layer is applied on a surface of a semiconductor body or semiconductor layer, and a layer provided for a gate electrode is applied on this storage layer. In the regions provided for the bit lines, openings are etched in the gate electrode layer and used to implant a dopant to form buried bit lines comprising regions for source and drain. Bit line stacks are applied on the buried bit lines to reduce the electrical resistance of the bit lines and are covered with an electrically insulating material. A word line crossing the bit lines and electrically connected to the gate electrode is applied and structured together with the gate electrode to form a word line stack. A number of bit lines arranged in parallel and word lines crossing there bit lines can be applied in the described manner to form a whole cell array of a memory device. 
     The word line stack is used as a mask to etch the semiconductor material on both sides of the world line, first anisotropically and, in a preferred embodiment, subsequently isotropically to widen the etch hole and form an undercut beneath the gate electrode and at a distance from the storage layer. The undercut is filled with an electrically insulating material, particularly oxide, to form an insulating buried layer, having a maximum thickness, i.e., a maximum dimension in the direction orthogonal to the surface of the semiconductor body or semiconductor layer, of at least 20 nm, in some embodiments more than 100 nm, underneath the channel region. 
     The source/drain regions are preferably of n-type conductivity, whereas the channel is of p-type conductivity. The channel is preferably doped at a density of at least 10 17  cm −3 . The threshold voltage of the cell transistor can be set to values between 0.5 V and 2.0 V in the erased state, and can be adjusted to typically about 1.5 V by a suitable choice of the device parameters. 
     An accomplishment of the inventive method is to provide a non-volatile cell transistor with isolated channel without using an SOI substrate. It provides means to reduce the effective channel length of the cell device, especially with the aim of further scaling down the device dimensions. An electrical insulation between the source region and the drain region laterally or immediately beneath the channel impedes a punch-through in this area. 
     With the above and other objects in view there is also provided, in accordance with the invention, a non-volatile memory cell, comprising:
         a semiconductor structure selected from the group consisting of a semiconductor body and a semiconductor layer and having a surface;   buried bit lines formed at the surface of the semiconductor structure and conductor strips on the surface applied to the bit lines;   a source region and a drain region each connected by one of the bit lines;   a gate dielectric formed on the surface at least between the source region and the drain region, the gate dielectric including a storage layer for trapping charge carriers;   a gate electrode disposed on the gate dielectric;   a word line electrically connected to the gate electrode, the word line crossing the bit lines and being electrically insulated from the bit lines; and   electrically insulating regions extending into the semiconductor structure between the bit lines and on both sides of the word line, the electrically insulating regions being self-aligned to the word line and extending at least to a lower boundary level of the source region and the drain region.       

     In accordance with an added feature of the invention, the electrically insulating undercut region extends continuously across the word line. 
     In accordance with an additional feature of the invention, the electrically insulating undercut region has a maximum thickness in a direction orthogonal to the surface of the semiconductor structure of at least 20 nm or of at least 100 nm. 
     In accordance with a concomitant feature of the invention, there is provided a channel region between the source region and the drain region. The channel region has a preferred doping density of at least 10 17  cm −3 . 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a non-volatile memory cell and method of fabricating such a memory cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  show cross-sections through the bit lines, taken along the line I—I in  FIG. 2 , after the first process steps of two alternative embodiments of the The method according to the invention; 
         FIG. 2  shows the cross-section through the word lines taken along the lines II—II in  FIGS. 1A and 1B ; 
         FIG. 3A  is a cross-section through the bit lines, taken along the line IIIa—IIIa in  FIG. 4A , after the formation of the underetch structures for a first alternative embodiment of the method; 
         FIG. 3B  is a cross-section through the bit lines, taken along the line IIIb—IIIb in  FIG. 4B , after the formation of the underetch structures for a second alternative embodiment of the method; 
         FIGS. 4A and 4B  show cross-sections through the word lines as indicated in  FIGS. 3A and 3B , respectively; 
         FIGS. 5A and 5B  show cross-sections through the word lines according to  FIGS. 4A and 4B  after subsequent steps of two alternative embodiments of the inventive method; and 
         FIG. 6  shows a cross-section through the word lines according to  FIG. 5B  for a further embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a detailed description of the preferred embodiments of the invention will be given referring to the steps of preferred methods of manufacture. In any of the embodiments, the fabrication process can start with steps that are known in themselves from the fabrication process of memory cells according to the state of the art. These steps may comprise depositing layers of pad oxide and/or pad nitride grown on a surface of a semiconductor body or semiconductor layer, especially a p-doped semiconductor wafer. All the known steps to form oxide-filled shallow trench isolations can be added, including the application of lithography for trench definition, and planarization. Standard implants may be carried out to form wells in the periphery region provided for the CMOS control integrated circuits. 
     Next, the storage layer, preferably an ONO-layer (oxide-nitride-oxide layer), is grown on a surface of the semiconductor body or semiconductor layer. A lithography step can follow to remove the storage layer in the periphery and to replace it there by one or more different gate oxides. On the storage layer, an electrically conductive layer is deposited, which is provided for the gate electrode to be produced. 
     Referring now to the figures of the drawing in detail and first, particularly, to  FIGS. 1A and 1B  thereof, there are shown cross-sections through a semiconductor body  1  carrying the thin storage layer  2  and the gate electrode layer  3  that may be comprised of poly-crystalline silicon. A nitride layer is deposited as a hard mask layer. By a lithography step, bit line openings running parallel to each other at a given spacing distance are formed in the gate electrode layer  3 . Spacers  4  are formed at the sidewalls of the gate electrode layer  3  within the openings. The spacers  4  are formed in the way that is known in itself from general semiconductor technology, by isotropically depositing a layer of the spacer material and subsequently anisotropically etching back this layer until only the sidewall spacers are left. The spacers  4  may be formed of oxide, preferably, or of nitride, especially silicon nitride. Then, dopant is implanted to form buried bit lines  5 , of n + -conductivity type in the described embodiment. 
     Bit line conductor strips  6  to reduce the electrical resistance are deposited on the surface of the buried bit lines  5 . The bit line conductor strips  6  can be formed of CoSi and/or polysilicon. When using a silicide, the bit line conductor strips  6  can partly be merged into the semiconductor material of the semiconductor body  1  as shown in FIG.  1 B. Bit line cover layers  7  are applied on the bit line conductor strips  6 . These cover layers  7  can be formed by depositing TEOS (tetraethylorthosilicate) or by growing silicon oxide on top of the bit line conductor strips  6 , if they were made of polysilicon. The cover layers  7  are planarized, and the hard mask layer of nitride is removed. 
     Next, a layer sequence is deposited that includes at least one word line layer provided for the formation of word lines. In the exemplary embodiment of  FIGS. 1A and 1B , respectively, there are shown a first word line layer  8 , for example polysilicon, a second word line layer  9 , for example comprising a metal, and a hard mask layer  10  formed of an electrically insulating material. The cross-section indicated in  FIGS. 1A and 1B  in a position between two bit lines is shown in FIG.  2 . 
       FIG. 2  shows the cross-section indicated in  FIGS. 1A and 1B  through the word lines after formation of the word line stack. The first word line layer  8 , the second word line layer  9 , the hard mask layer  10 , and the gate electrode  3  have been structured to form strips of word line stacks. The position of the cross-sections shown in  FIGS. 1A and 1B  is indicated in  FIG. 2  by the broken line. The lithography step can be split into two steps to etch the word lines first while the periphery devices remain protected by the hard mask layer. 
     As shown in the cross-sections of  FIGS. 3A and 3B , the openings in the gate electrode layer are used to perform an etching step to form holes in a self-aligned fashion at least to the word lines, and in this example also self-aligned to the bit lines. In this step, RIE (reactive ion etching) can be used to etch anisotropically through the storage layer  2  and straight down into the semiconductor material. This is shown in  FIG. 3A  by the vertically oriented arrows pointing downwards. The holes are etched down at least to the level of the lower boundaries of the source/drain regions and the buried bit lines  5 , i.e. to the level of the lower pn-junction between the source/drain/buried bitline region and the main semiconductor body or layer. By this, the semiconductor body or semiconductor layer is electrically insulated in a region adjacent to the transistor channel and vertically extending down as far as the source/drain regions. 
     After this anisotropic etching, the sidewalls of the bit line stacks and the word line stacks can be covered by a thin nitride cover  11 . Alternatively, cover  11  can be a thin oxide layer resulting from a word line reoxidation step. The cover  11  is removed from the surface of the semiconductor body  1  at least at the bottom of the etched holes. In a preferred embodiment, the etching process continues isotropically by using either a dry etchant such as SF 6  or, in the embodiment of  FIG. 3B , a wet etchant. In the embodiment of  FIG. 3B , the sidewalls are protected by the cover  11 , so that the underetched parts of the etch holes  12  are confined to a comparatively smaller vertical extension. The underetching of etch holes  12  achieved by the isotropic etching process can be further continued until the etch holes advancing from both sides of the word line meet and form one continuous opening underneath the channel region of the cell transistor. 
       FIGS. 4A and 4B  show the cross-sections through the word lines that are indicated in  FIGS. 3A and 3B . In the example of  FIG. 4A , the etching process has been continued until continuous openings have been formed below the remaining semiconductor channel region  17 . In the example shown in  FIG. 4B , the etching process was stopped before the etch holes produced from both sides of the word lines occupy the whole region underneath the channel region  17 . In the embodiment according to  FIG. 4B , the channel region is not totally separated from the semiconductor body. The remaining connection between the semiconductor material of the channel region  17  and the semiconductor body  1 , for instance silicon, can provide a sufficient isolation between the channel region and the semiconductor body, since the semiconductor material is depleted of charge carriers. However, a preferred embodiment provides a full isolation underneath the channel region by completely etching through across the total width of the word lines. In this preferred embodiment, there are pn-junctions only between the channel region and the source/drain regions as well as between the source/drain/buried bit line regions and the semiconductor material beneath these regions and/or on lateral sides facing away from the channel, but there are no pn-junctions laterally adjacent to the source/drain regions in the direction of the channel below the channel region. 
     The maximum thickness  19 , i.e., the maximum dimension in the direction orthogonal to the surface of the semiconductor body or semiconductor layer, of the undercut is usually present in the elongation of the plane of the lateral flanks of the word line stack, as indicated in the FIGS. 
     The upper limit of the area of cover  11  in  FIG. 4B  shows the position of the upper surface of the bit line stack. The cover  11  can be etched away to expose the surface to thermal oxide growth for providing a passivation. 
     The  FIGS. 5A and 5B  show cross-sections according to the  FIGS. 4A and 4B , respectively, through the word lines after the growth of a thermal oxide cover  13 . An electrically insulating material is deposited as fillings  15  to fill up the etch holes and the regions between the word lines up to the level of the hard mask layer  10 . The material of the fillings  15  is preferably chosen to have a low value of its relative dielectric constant. 
     The insulating layer in the undercut beneath the channel region  17  is formed with a maximum thickness  19  of at least 100 nm in the embodiment shown in FIG.  5 A. In the embodiment shown in  FIG. 5B , the cover  11  ( FIG. 4B ) has rendered the underetched parts of the etch holes  12  confined to a comparatively smaller vertical extension with a maximum thickness  19  of at least 20 nm. 
     The  FIG. 5B  shows optional additional features: a well  14 , formed by implanting boron or another dopant to provide p-conductivity regions in order to improve the isolation beneath the channel region  17 ; a nitride passivation layer  18  applied to the upper surface of the shown structure; and, in this case, a complete isolation of the channel region  17  from the semiconductor body  1  by dielectric material that was formed by thermal oxidation (oxide cover  13 ). 
     The  FIG. 6  shows the cross-section according to  FIG. 5B  for a further embodiment in which the growth of the thermal oxide cover was confined to the surfaces of the etch holes, whereas the nitride layer  16  was applied all over the surfaces of the word line stacks. This nitride layer  16  corresponds to the upper nitride passivation layer  18  in FIG.  5 B. Such a nitride layer can be used to encapsulate the word line. Thus, a self-aligned contact scheme for the bit lines can be applied. 
     It will be understood that variations and deviations from the exemplary embodiments of the disclosed method due to special requirements of the manufactured memory devices lie within the scope of this invention.