Abstract:
A saddled and wrapped stack capacitor DRAM and a method thereof are provided. The DRAM of the invention includes three factors in increasing the effective area for a capacitor. One is a storage poly layer comprising a first poly layer and a second poly layer, which is formed thick in a region over a field oxide layer through two steps; another is a spacer which is formed through an etchback technique for an oxide layer coated on another oxide layer being patterened to selectively remove the storage poly layer, and the spacer maximizes the size of the storage poly; another is an undercut which is formed in boundary regions on an upper oxide layer, on which a plat poly material is coated and wrapped.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to DRAM(Dynamic Random Access Memory) cells and a method for manufacturing them, and more particularly to a stacked capacitor DRAM cell which can obtain a high-capacitance without the increment of capacitor area in a semiconductor device. Recently, in the field of DRAM technology, the effort of minimizing the chip size has been completed, while the capacitance value needed for each cell has been maintained. According to such trend, the trench capacitor DRAM cell and stacked capacitor DRAM cell came on. These techniques contributed largely to the high-density DRAM technology. 
     To explain the method for manufacturing a stacked capacitor DRAM cell, referring to FIG. 1 which shows the vertical cross-sectional view of a stacked capacitor DRAM cell, the fabricating processes of the stacked capacitor DRAM cell are as follows. 
     At first, a p-well 32 is formed on a p-type substrate 31. A field oxide 34 is grown on an active region over the p-well, and a p +  layer is formed by injecting impurities. After above processes a gate poly 35 and a source-drain region for transistors are formed, and a contact is formed to form a storage poly 41, and said storage poly is etched selectively. 
     Thereafter, an insulating layer 46 for a capacitor is formed by oxidizing said storage poly, and a plate poly 48 is deposited. The layers of an oxide 49, a polycide 50, a B-PSG 51 and a metal 52 are formed in sequence, now the series of processes for manufacturing a DRAM cell are completed. 
     The effective area of a capacitor 41 of a DRAM cell manufactured by the above described way is determined by the top and side area of the storage poly. To increase the effective area of the stacked capacitor, the thickness of the storage poly 41 is increased up to now. 
     However, the magnitude of the capacitance manufactured by way of increasing the side area is not sufficient for high-density memories because the cell area of a 4M DRAM is 10 μm 2 , but that of a 16M DRAM reduces to 5 μm 2 . The increment of thickness of the storage poly in the entire area of the cell causes the deterioration of cell topology so that the patterning of the storage poly and the bit-line and the metal is difficult. The manufacturing of the high-density DRAMS beyond 4M DRAM is difficult by the conventional technology. The reason is that the capacitance per unit cell reduces remarkably when the 16M DRAM is manufactured by the prior technology. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to provide a stacked capacitor DRAM cell and a method thereof which makes the manufacturing of a 16M DRAM possible by maximizing the effective area of the stacked capacitor in a confined area. 
     The invention, in a first aspect, is a stacked capacitor DRAM in which the effective area of the capacitor is maximized with a storage poly layer for capacitor. There are three factors in increasing the effective area according to the present invention. 
     One factor is that the storage poly layer which is formed thick in a region over the field oxide layer. That is, the region in the storage poly layer over the field oxide layer is twice coated by the polysilicon so that the capacitance increases as much as that. 
     When a first poly layer is removed except the portion over the field oxide layer, it is preferable to use a saddle mask. That is because the etched surface of the storage poly layer is very rough, and this conditions makes the effective area for the capacitor increase in a large scale. 
     The formation of the storage poly layer through two steps leads a topology to be improved as the thickness in the region of a bit line contact is shallow. 
     Second, a spacer controls the magnitude of a capacitance. While the etching process of the storage poly layer employing the oxide layer on the storage poly as an etching mask, the etching pattern of the oxide layer is restricted by the resolution capability of a photoresist. Therefore, since the size of the etching pattern is determined by the resolution capability, the quantity of the storage poly layer to be removed is also effected by the resolution capability. However, the spacers cause the remaining storage poly layer to be large as the portion thereof to be removed is small. 
     An undercut is included as one of three factors. The undercut is carried out in the region just below the boundary region of the storage poly layer. Accordingly the effective area is increased once more. 
     As depicted in FIG. 4, the stacked capacitor according to the invention is increased in a curved surface with the saddle mask, and the oxide layer under the storage poly layer is wrapped by a capacitor dielectric layer. Now we call this structure the saddled and wrapped stacked capacitor. 
     The invention, in another aspect, is a method of manufacturing the saddled and wrapped stacked capacitor DRAM. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical cross-sectional view of a prior DRAM cell. 
     FIG. 2 is a plane layout of a stacked capacitor DRAM cell of this invention. 
     FIG. 3 is a vertical cross-sectional view of FIG. 2 taken along line A-A&#39; thereof. 
     FIG. 4 is a vertical cross-sectional view of FIG. 2 taken along line B-B&#39; thereof. 
     FIGS. 5A to 5L are vertical cross-sectional views of each process showing in sequential order the steps of an embodiment for manufacturing the DRAM cell of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described in details with reference to the accompanying drawings. 
     In FIG. 2, the reference numeral 60 is a gate poly and the numeral 62 is a saddle mask. The reference numerals 64, 66 are a buried contact and a storage poly respectively. The numeral 68 is a plate poly and a bit line contact is indicated by the numeral 70. The characters ML and BL shows a metal line and a bit line, respectively. 
     FIG. 5 is a vertical cross-sectional view showing in sequential order the steps of an embodiment for manufacturing a stacked capacitor DRAM. Referring now to FIG. 5A, a p-type well 2 and a n-type well 3 are formed on a p-type substrate 1. 
     Hereinafter, the preferred embodiment of this invention will now be described by setting the p-well on the basis of describing the embodiment. For the n-well, only the type of impurities is changed. 
     Impurities of p-type are injected into a region of the p-type well 2 which will be a field region in the well 2, and then a field oxide layer 4 is grown. From this resultant, a channel stop region of p +  -type is formed (FIG. 5B). 
     As shown in FIG. 5C, a gate oxide layer 4&#39; is grown on regions to be active regions by way of a conventional way, and impurities are injected in order to control a threshold voltage of a transistor. A poly layer 5 is then deposited over the substrate, and a gate poly oxide layer 6 is grown on the poly layer. The layers 4&#39;, 5 and 6 are selectively etched out employing a conventional photo-lithographic technique, and the layer 5 results in a gate poly layer. 
     FIG. 5D shows an oxide spacer 7 which is formed in the sidewall of the poly-silicon layer 5. Ion impurities of n-type are then injected into the p-well so as to form the regions which come to be a source S and a drain D of a transistor. 
     Thereafter, as shown in FIG. 5E, an oxide layer 8 (the lower oxide layer), a nitride layer 9 and an oxide layer 10 are formed in sequence. The layers 8,9,10 are formed by the method of Chemical Vapor Deposition, and the thickness of each layer is about 1000 Å. A first poly-silicon layer 11 is then coated with the thickness of about 3000 Å. 
     The layers 8,9 and 10 play the role of the inter-poly (laminated) insulating layer between the gate poly layer 5 and the first poly layer 11. The first poly layer may be formed with the poly doped initially with the n +  -type impurities, or n +  -type impurities may be doped after depositing the non-doped poly-silicon. 
     Referring now to FIG. 5F, the first poly layer 11 is selectively etched out, employing a saddle mask, except the poly-silicon coated over the upper surface side of the field oxide layer 4. During this process, the saddle mask causes the etched surface of the storage poly layer to be very rough so that this condition makes the effective area for the capacitor increase in a large scale. That is, the increment of the surface area with the curved surface states increases the quantity of the storage poly with respect to the another deposition of a poly-silicon layer. 
     Buried contacts 12 are then formed and the buried contact leads the source to be connected with a second poly which will be an electrode of a capacitor. The order of forming the first poly layer and the contact can be carried out reversely. 
     After that, as shown in FIG. 5G, the second poly layer 11&#39; is deposited with about 2500 Å thickness over the entire surface of the p-well, and an oxide layer 14 is coated by the thickness of about 1500-3000 Å, by way of CVD technique, on the second poly layer. As above indicated, owing to the rough, etched surface of the poly layer 11, the upper surface of the portion of the second poly layer 11&#39; overlying the first poly layer 11 is also rough. 
     FIG. 5H shows the next step. The reference numeral 13 is a poly storage layer consisting of the first and second poly layer 11 and 11&#39;, and hereinafter the first and second poly-silicon layer (laminate) 11, 11&#39; will be represented as the poly storage layer and a single hatching. 
     Employing a photoresist mask, the oxide layer 14 is selectively etched out per unit of a cell. In this etching process, the etching pattern of the oxide layer 14 is restricted by the resolution capability of the photoresist. Accordingly, the size of the etching pattern must be at least equal or large than the resolution capability. According to the present invention an oxide spacer is employed in order to maximize the capacitance. After forming the etching pattern of the storage poly, an oxide layer is coated on the oxide layer 14. The oxide spacer 15 is then formed through the etch back technique. The size of the spacer is determined with the magnitude of the storage layer to be remained, that is, with the capacitance of a capacitor according to a device characteristic. 
     In FIG. 5I, the exposed storage poly layer 13 is removed by employing the remaining oxide layer 14 and oxide spacer 15 as a mask. This etching process exposes the portions of the oxide layer 10 below the removed storage layer. Subsequently, using the isotropic technique the oxide layers 10, 14 are removed. The isotropic etching makes the oxide layer 10 (the uppermost lamina of the laminated insulating layer 8-9-10) have undercut surfaces because the boundary portions of the oxide layer 10 is etched out into apertures by the isotropic properties. Therefore, the exposed portions of the storage poly are increased, and the storage capability for charges is also increased. 
     The degrees of the undercut depend on the magnitude of capacitance to be obtained, and these are controlled by the difference of the thickness between the oxide layer 10 and 14. If the layer 14 is thicker than the layer 10, the degrees of the etching in the layer 10 may increase. The removal of the layer 10, 14 may be carried out separately. 
     The protection by the nitride layer 9 (the lamina exposed by the undercutting of the layer 10) for the oxide layer 8 prevents the short between the gate poly layer 5 and a plate poly layer 17 represented in FIG. 5J from occurring. 
     Referring now to FIG. 5J, a dielectric layer 16 for a capacitor is formed along the entire exposed surface of the storage poly layer 13. The plate poly layer 17 is then deposited over the p-well by about 1500 Å, and then the plate poly layer 17 is selectively removed as shown in FIG. 5J. 
     Then a plate poly oxide layer 18 is grown by oxidizing the plate layer 17 so as to have the thickness of about 1000 Å. 
     As shown in FIG. 5K, the BPSG(Boron Phosphorus Silicate Glass) 19 is deposited on the plate poly oxide layer 18 by the low temperature oxidation technique. A BPSG flow is then carried out for the planarization of the surface. Employing a mask, a bit line contact is formed and a policide layer 20 is filled therein. 
     After the above processes, in FIG. 5L, the BPSG 21 is coated and the glass 21 is flowed. A contact for metal line is then formed using a mask and a metal layer is deposited. The coated metal layer is selectively removed employing a mask. 
     The DRAM cell fabricated through the above-mentioned processes has a wide effective area for the capacitor as shown be in FIG. 2 and FIG. 3, because the storage poly-silicon for capacitor is formed with an enough thickness over the field oxide layer 4, as well as the exposed area of the storage poly layer 13 is increased by causing the undercut just below the boundary region of the storage layer to bring about; so that the effective area of the capacitor results in wide area. Furthermore, with the employment of the oxide spacer, the magnitude of capacitance is increased and controlled with the dimensions of the oxide spacer. 
     The effects of the DRAM cell manufactured according to the present invention are as follows. 
     Since the storage poly layer to be the capacitor electrode is formed not only thick in the region over the field oxide layer but also shallowly in the contact region of the bit line, the effective area of the capacitor increases and the topology of the bit line contact area is improved. 
     When the storage poly is removed, the gap between the remaining storage poly layer is narrow as much as possible by using the oxide spacer, so that the remaining area of the storage poly layer is wide and the effective area of the capacitor is increased once more. 
     The third factor which increases the capacitor area is the undercut phenomenon which breaks down in the region just below the boundary region of the storage poly layer. 
     Accordingly, since the effective area for the capacitor is enormously increased through the above-mentioned three factors according to the invention, the area of the DRAM decreases, and this result makes the manufacturing of the VLSI possible. 
     This invention is in no way limited to the example described hereinabove. Various modifications of the disclosed embodiment, as well as other embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.