Abstract:
A method of making a ferroelectric memory transistor includes preparing a silicon substrate including forming plural active areas thereon; depositing a layer of gate insulator on the substrate, and depositing a layer of polysilicon over the gate insulator layer; forming a source region, a drain region and a gate electrode; depositing a layer of bottom electrode material and finishing the bottom electrode without damaging the underlying gate insulator and silicon substrate; depositing a layer of ferroelectric material on the bottom electrode; depositing a layer of top electrode material on the ferroelectric material; and finishing the transistor, including passivation oxide deposition, contact hole etching and metalization.

Description:
FIELD OF THE INVENTION 
     This invention relates to high density integrated circuit non-volatile memory, and specifically to a method of making a ferroelectric device wherein etching of a bottom electrode is accomplished without damaging the underlying substrate. 
     BACKGROUND OF THE INVENTION 
     One of the most difficult steps in metal/ferro metal oxide semiconductor (MFMOS) ferroelectric memory transistor fabrication is that of etching the bottom electrode. In known NFMOS ferroelectric memory transistor fabrication, the bottom electrode must be selectively etched, without etching through the thin oxide located beneath the bottom electrode, and thereby penetrating the silicon substrate. The oxide located below the bottom electrode may be silicon dioxide, or any other suitable high-k insulator. If the silicon substrate is inadvertently etched, it will be impossible to form source/drain junctions which have adequate connections to the conductive channel of the transistor. 
     SUMMARY OF THE INVENTION 
     A method of making a ferroelectric memory transistor includes preparing a silicon substrate including forming plural active areas thereon; depositing a layer of gate insulator on the substrate, and depositing a layer of polysilicon over the gate insulator layer; forming a source region, a drain region and a gate electrode; depositing a layer of bottom electrode material and finishing the bottom electrode without damaging the underlying gate insulator and silicon substrate; depositing a layer of ferroelectric material on the bottom electrode, depositing a layer of top electrode material on the ferroelectric material; and finishing the transistor, including passivation oxide deposition, contact hole etching and metalization. 
     An object of the invention is to provide a production method for single transistor ferroelectric memory device fabrication. 
     Another object of the invention is to provide a method of forming a bottom electrode in a ferroelectric stack without penetrating the underlying silicon substrate. 
     This summary and objectives of the invention are provided to enable quick comprehension of the nature of the invention. A more thorough understanding of the invention may be obtained by reference to the following detailed description of the preferred embodiment of the invention in connection with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1-10 depict successive steps in a first embodiment of the method of the invention. 
     FIGS. 11-12 depict successive steps in a second embodiment of the method of the invention. 
     FIGS. 13-16 depict successive steps in an alternate embodiment of the method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The fabrication process for a ferroelectric memory transistor constructed according to the method of the invention does not require highly selective etching for the electrode etching process. Referring initially to FIG. 1, a silicon substrate  20  is a p-type silicon wafer. Boron is implanted into what will become the p-well regions of the wafer. The implanted wafer is heated to diffuse the implanted ions to form a p-well. A thin layer of gate insulator  22 , such as a layer of gate oxide, is grown and a layer of undoped polysilicon  24  is deposited. Alternately, the gate oxide may be replaced with a layer of high-k gate dielectric. A layer of photoresist  26  is applied prior to a trench isolation process, as shown in FIG.  1 . The figures depict the construction of two transistors wherein the left side of the drawing figure depicts one transistor and the right side of the drawing figure depicts a second transistor, turned 90° from the view of the left side of the drawing figure. 
     Turning to FIG. 2, shallow trenches  28 ,  30 ,  32  are etched through the polysilicon, the gate insulator and through about 500 nm of silicon substrate. The photoresist is then removed. Any plasma etching damage is removed, the wafer is cleaned, and a layer of oxide  34  is deposited onto the wafer. The thickness of the oxide is at least 1.5 times, and may be more than 2 times thicker than the depth of the shallow trenches. CMP is used, stopping at the level of polysilicon, to planarize the wafer. All of the polysilicon, except that on the active areas, is removed, as shown in FIG.  2 . 
     Photoresist is applied and the polysilicon selectively etched to form a source area  36  and a drain area  38 , and to form a gate electrode  40 . The source and drain of the device is implanted with arsenic or phosphorus ions, as shown in FIG.  3 . Exemplar implantation specification are by implantation of arsenic ions, at a dose of about 1·10 15  cm −2  to 5·10 15  cm −2 , and at an energy level of 30 keV to 60 keV, or implantation of phosphorus ions, at a dose of about 1·10 15  cm −2  to 5·10 15  cm − 2, and at an energy level of 10 keV to 30 keV, to form N+, heavily doped polysilicon. A thin layer of oxide  44  is deposited onto the wafer and the wafer is CMP plainarized, as shown in FIG.  4 . 
     Bottom electrodes  46  are deposited onto the wafer, and are finished by etching or CMP, without damaging underlying gate insulator layer  22  or silicon substrate  20 . If the bottom electrode cannot be removed by a CNIP process, such as when pure iridium is used as the bottom electrode, photoresist is applied prior to etching of the bottom electrode. With respect to conventional etching techniques, the bottom electrode has very similar characteristics to the remaining polysilicon gate, which assists in stopping the downward penetration of the etching process. In this embodiment of the method of the invention, bottom electrode  46  and polysilicon  40  are not perfectly aligned, as shown in FIGS. 5 to  12 . A thin layer of oxide  50  is deposited and is plainarized by CMP, resulting in the structure shown in FIG.  6 . 
     If the bottom electrode is formed of a material which may be polished, such as Pt, TiN, Ta, TaN, TiTaN, IrTa alloy and IrPt alloy, the steps of the preceding paragraph are replaced by selective etching of portions of N+ polysilicon, deposition of bottom electrode material, and CMP of the bottom electrode. In this case, the bottom electrode and the n+ polysilicon are self-aligned. The underlying oxide layer  22  and silicon substrate  20  are protected from penetration during polishing of the bottom electrode. 
     The wafer is now ready for deposition of the ferroelectric material. After a layer of ferroelectric thin film material  52  is deposited, the top electrode material  54  is deposited, as shown in FIG.  7 . 
     Photoresist is applied to mask the top electrode prior to etching. Top electrode  54  functions as a control gate, therefore, it extends beyond the horizontal boundaries of the active area. The ferroelectric thin film may also be etched during this step, with the resulting structure depicted in FIG.  8 . However, because etching a ferroelectric thin film usually degrades the ferroelectric property of the thin film, etching the ferroelectric thin film may be done in a separate step, using techniques less likely to degrade the ferroelectric properties of the thin film. 
     A thin layer of dielectric  56 , such as titanium oxide or aluminum oxide, is deposited to protect the ferroelectric thin film from hydrogen damage, as shown in FIG.  9 . 
     The remaining steps of passivation oxide deposition  58 , contact hole etching and metallization  60 ,  62 ,  64  and  66 , may be accomplished using any state-of-the-art process, resulting in the finished structure depicted in FIG.  10 . 
     Optionally, the structure of FIG. 9 may be masked an a plasma etch process used to remove the horizontally disposed portions of the dielectric thin film, except on the sidewall of the top electrode and ferroelectric stack, as is shown in FIG.  11 . The finished structure for this embodiment of the method of the invention is depicted in FIG.  12 . 
     Several alternative steps may be performed during fabrication of a ferroelectric memory transistor according to the method of the invention. One alternative step take place after those steps described in connection with FIG. 6, wherein a thin layer oxide  70 , having a thickness of between about 100 nm to 400 nm, is deposited as is shown in FIG. 13, over the already-deposited oxide and the bottom electrode. Photoresist  72  is applied, and the oxide is etched to open holes  74 ,  76  where ferroelectric memory material is to be deposited for the memory transistor, as shown in FIG.  14 . The resist is then removed . 
     A thin layer of barrier dielectric  78 , such as titanium oxide or aluminum oxide, is deposited and is plasma etched to form a protective layer at the sidewall of the holes previously opened, as is shown in FIG.  15 . 
     Ferroelectric material  52  is then deposited onto the wafer. Although both MOCVD and spin-on coating may be applied, the spin-on coating is preferred. A low viscosity precursor spin-on coating will fill the holes more economically. The ferroelectric material on the top surface is etched. This may be achieved with or without an etch mask, or by a CMP process. If the etching is done without a mask, the etching also etches a portion of the ferroelectric material at the transistor area, that is, in the hole area. 
     The remaining ferroelectric material thickness is the required ferroelectric material thickness for the memory transistor. Top electrode  54  is deposited and etched to form the control gate of the memory transistor, as shown in FIG.  16 . The remaining process steps of CVD of oxide, application of photoresist prior to opening contact holes, and final metallization may be accomplished by any of the state-of-the-art techniques. 
     Thus, a method of making a ferroelectric memory transistor has been disclosed. It will be appreciated that further variations and modifications thereof may be made within the scope of the invention as defined in the appended claims.