Method for Anodizing a Polysilicon Layer Lower Capacitor Plate of a DRAM to Increase Capacitance

This invention relates to semiconductor technology and, more specifically, to the design of capacitors used in dynamic random access memory (DRAM) cells. The focus of the invention is the formation of micro structures for increasing the capacitance of a DRAM cell having a polysilicon storage node capacitor plate, without increasing cell area and without utilizing any additional masking steps.

The invention uses various materials which are electrically either conductive, insulative or semiconductive, although the completed semiconductor circuit device itself is usually referred to as a "semiconductor."

The memory cells of dynamic random access memories (DRAMs) are comprised of two main components: a field-effect transistor and a capacitor. In DRAM cells utilizing a conventional planar capacitor (such as the one depicted in FIG. 1), far more chip surface area is dedicated to planar capacitor 11 than to field-effect transistor (FET) 12. The gate 13 of FET 12 and the word line 14 are formed from an etched polycrystalline silicon-1 layer. Bit line 15 connects with access-node junction 16. Capacitor 11 has a lower plate formed from the n+silicon substrate extension 17 of storage node junction 18 of FET 12. Upper capacitor plate (or field plate) 19 is formed from a layer of conductively-doped polycrystalline silicon. Substrate extension 17 is electrically insulated from upper plate 19 by a dielectric layer 20. Planar capacitors have generally proven adequate for use in DRAM chips up to the one-megabit level. However, planar capacitors constructed with conventional dielectric materials appear to be unusable beyond the one-megabit DRAM level. As component density in memory chips has increased, the shrinkage of cell capacitor size has resulted in a number of problems. Firstly, the alpha-particle component of normal background radiation will generate hole-electron pairs in the n+ silicon substrate plate of a cell capacitor. This phenomena will cause the charge within the affected cell capacitor to rapidly dissipate, resulting in a "soft" error. Secondly, as cell capacitance is reduced, the sense-amp differential signal is reduced. This aggravates noise sensitivity and makes it more difficult to design a sense-amp having appropriate signal selectivity. Thirdly, as cell capacitance is decreased, the cell refresh time must generally be shortened, thus requiring more frequent interruptions for refresh overhead. The difficult goal of a DRAM designer is therefore to increase or, at least, maintain cell capacitance as cell size shrinks, without resorting to processes that reduce product yield or that markedly increase the number of masking and deposition steps in the production process.

Several methods for providing adequate cell capacitance in the face of shrinking cell size are either in use or under investigation. Basically, the efforts fall into two categories. Efforts within the first category are aimed at creating complex three-dimensional capacitors; those within the second are aimed at improving the dielectric of the planar capacitor.

The three-dimensional technique currently receiving the most attention involves the creation of "trench" capacitors in the cell substrate. FIG. 2 depicts a DRAM cell having a typical trench capacitor 21. Similar in concept to planar capacitor 11 of FIG. 1, the trench is employed to provide greater plate area, and hence, greater capacitance. The lower plate 22 may be formed from the n+doped silicon substrate or it may be formed from a polysilicon layer which lines a trenoh cut in the n+ doped silicon substrate. The upper plate 23 is formed from a layer of conductively-doped polycrystalline silicon (poly). Lower plate 22 and upper plate 23 are electrically insulated with a dielectric layer 24. DRAM chips employing trench capacitors have been built by a number of European, Japanese and U.S. companies, including IBM Corporation, Texas Instruments, Inc., Nippon Electric Company, Toshiba, Matsuchita and Mitsubishi Electric Corporation. There are several problems inherent in the trench design, not the least of which is trench-to-trench capacitive charge leakage which is the result of a parasitic transistor effect between trenches. Another problem is the difficulty of completely cleaning the capacitor trenches during the fabrication process; failure to completely clean a trench will generally result in a defective cell.

Another three-dimensional technique, which is being used by Mitsubishi Electric Corporation, Hitachi, and Fujitsu, Ltd., is the stacking of capacitor plates between dielectric layers on the DRAM cell surface. FIG. 3 is a graphic representation of a typical DRAM cell having a stacked capacitor 31. The lower plate 32 is formed from an n-type polycrystalline silicon layer which is in contact with the silicon substrate 33 in the region of the FET storage node junction, while the upper plate 34 is formed from a conductively-doped polycrystalline silicon layer. The two layers are separated by a dielectric layer 35. Lower plate 32 and upper plate 34 are both stacked on top of FET 12 and word line 36, resulting in a high-profile cell which requires more stringent process control for the connection of bit line 37 to access-node junction 38.

Alternately, other schemes involve the use of ferroelectric materials for DRAM cell capacitor dielectrics. Since ferroelectric materials have a dielectric constant more than 100 times that of silicon oxides, the use of such materials has the potential for allowing the size of DRAM cell capacitor to be shrunk to one of the smaller cell elements without resorting to three-dimensional structures. Critics of ferroelectric materials point out that such materials suffer from a "wearout" mechanism. In addition, they warn that there are many chemical incompatibilities with the other materials used in integrated circuit fabrication and that the layering of ferroelectric films within integrated circuit structures has not yet been done successfully.

The present invention utilizes a porous semiconductor as a storage node capacitor plate. Micro structures are formed by anodizing the semiconductor in hydrofluoric acid to produce the porous semiconductor. An article entitled "Microstructure and Formation Mechanism of Porous Silicon" by M. I. J. Beale, N. G. Chew, M. J. Uren, A. G. Culis, and J. D. Benjamin presents a systematic study of the formation and properties of micro structures produced in a semiconductor subjected to anodization in hydrofluoric acid. Elongated pores are formed in the semiconductor where an anodizing current flows. Inhomogeneities at the semiconductor-electrolyte interface result in localization of the current flow and the initiation of pores. Since the semiconductor between the inhomogeneities is highly resistive in comparison to the electrolyte concentrated at the inhomogeneities, the current flow is concentrated down the electrolyte at the inhomogeneities consuming the silicon and forming elongated pores and resulting in a microstructure comprising elongated pores.

The present invention is applicable to DRAM cell designs, such as the stacked capacitor design heretofore described or a poly-lined trench design, that utilize a conductively-doped polycrystalline layer for the storage node, or lower, capacitor plate. The invention is a method for anodizing the storage node plate in hydrofluoric acid employed to create micro structures on the upper surface of the storage node plate. The micro structures are typified as elongated pores. The micro structures increase the capacitance of the stacked capacitor without increasing cell size by providing a larger storage node plate area in the same cell area. The porous film density after anodization is one measure of the effectiveness of the anodization in creating the micro structures. The porous film density is equal to the percent of weight loss (for a given volume of material anodized) experienced during anodizing.

In addition to the formation of micro structures, a texturization process may be employed to create three-dimensional asperities on the upper surface of the storage node plate prior to the anodizing. The texturization process provides more inhomogeneities for the localization of current and the initiation of pores. A final texturization process may also be employed subsequent to the anodization to provide three-dimensional asperities on the sides of the micro structures formed during anodization. The final texturization further increases the surface area of the storage node plate without increasing cell size.

Both the texturization prior to and subsequent to the anodization are accomplished by subjecting the storage node plate layer to a wet oxidation step. Since oxidation at the crystal grain boundaries on the surface of the poly layer proceeds more rapidly than elsewhere, the surface develops three-dimensional asperities. When maximum texturization has been achieved, the overlying oxide is removed during a wet etch step.

With texturization complete, a thin conformal (typically less than 100 Angstroms) dielectric layer of silicon nitride is deposited on top of the texturized poly layer. The nitride deposition is followed by the deposition of a second poly layer, which functions as the capacitor field plate. Since the nitride layer is thin in comparison to the texturized micro structures of the storage node plate layer, capacitive area is substantially augmented. Cell capacitance can be increased by more than 1,000 percent using a storage node plate so texturized.

It should be noted that the texturization process was developed by Xicor, Inc. as a means to increase electron injection during write and erase operations in electrically-erasable programmable read-only memory (EEPROM) cells. During EEPROM construction, texturization is followed by thermal oxidation of the texturized surface. The three-dimensional asperities on the surface of the poly create regions of increased electric field strength, which both lowers breakdown voltage of the thermally-grown oxide and enhances current leakage by several orders of magnitude. The thermal oxide thus functions as a tunneling layer. Because conduction in silicon dioxide is electrode limited, generally considered to be in accordance with what is known as the Fowler-Nordheim mechanism, the use of thin layers of silicon dioxide as a DRAM cell dielectric between two roughened surfaces is precluded. However, because conduction in silicon nitride is bulk limited, most likely in accordance with what is known as the Frankel-Poole mechanism, silicon nitride is a much more suitable thin layer dielectric for use in DRAMs where capacitive layers are not smooth.

This texturizing process was also described in U.S. Pat. application 7/460,453, filed Jan. 3, 1990, entitled "DRAM Cell Having a Texturized Polysilicon Lower Capacitor Plate for Increased Capacitance," by inventors Pierre C. Fazan and Ruojia R. Lee.

FIGS. 1 through 3 depict existing technologies, while FIGS. 4 through 11 depict the fabrication of a DRAM array having a capacitor design conforming to the present invention.

FIG. 1 is a cross-sectional view of a DRAM cell having a conventional planar capacitor.

FIG. 2 is a cross-sectional view of a DRAM cell having a typical trench capacitor.

FIG. 3 is a cross-sectional view of a DRAM cell having a typical stacked capacitor.

FIG. 4 is a cross-sectional view of a DRAM cell of the stacked capacitor design type during the fabrication process and subsequent to the deposition of the storage node poly layer.

FIG. 5 is a cross-sectional view of the DRAM cell of FIG. 4 following a wet oxidation of the storage node poly layer.

FIG. 6 is a cross-sectional view of the DRAM cell of FIG. 5 following a wet oxide etch which removes oxide that accumulated on the storage node poly layer during the wet oxidation step.

FIG. 7 is a cross-sectional view of the DRAM cell of FIG. 6 following anodizing of the storage node poly layer.

FIG. 8a is an exaggerated enlarged cross-sectional view of a portion of the microstructure formed during the anodizing step shown in FIG. 7.

FIG. 8b is a cross-sectional view of FIG. 8a following a wet oxidation of the storage node poly layer. following a wet oxide etch which removes oxide that accumulated on the storage node poly layer during the wet oxidation step.

FIG. 9 is a cross-sectional view of the DRAM cell of FIG. 7 following the oxidation and etch depicted in FIGS. 8b and 8c.

FIG. 10 is a cross-sectional view of the DRAM cell of FIG. 9 following the blanket deposition of a dielectric layer, the blanket deposition of a polysilicon cell plate layer, and the masking of the cell plate layer.

FIG. 11 is an exaggerated enlarged cross-sectional view of a portion of the DRAM cell of FIG. 10 following the blanket deposition of a dielectric layer and the blanket deposition of a polysilicon cell plate layer.

FIG. 12 is a cross-sectional view of the DRAM cell of FIG. 10 following the etching of the polysilicon cell plate layer.

Referring now to FIG. 4, a DRAM cell 50 of stacked capacitor design is shown during the early stages of the fabrication process using a portion of a wafer of intrinsic polycrystalline silicon (poly). The cell's field-effect transistor (FET) 51 and word line 52 have already been formed. The FET gate 53, formed from a poly-1 layer, is covered on the top and sides by a silicon oxide spacer layer 55. A poly-2 layer 57, from which will be formed the cell's storage node capacitor plate, has been deposited over the FET 51, the word line 52, the storage node junction 58, and the access node junction 59. Poly-2 layer 57 is then doped with phosphorus to produce an extrinsic conductive poly-2 layer.

Referring now to FIG. 5, poly-2 layer 57 has been subjected to a wet oxidation step, which has created a multitude of three-dimensional asperities (inhomogeneities) on the surface of poly-2 layer 57. Conductive doping of poly-2 layer 57 was done prior to the wet oxidation step because the oxidation proceeds more rapidly at polysilicon crystal boundaries when the material is doped. In addition, the wet oxidation process has created an intermediate oxide layer 60 which covers the now texturized poly-2 layer 57.

Referring now to FIG. 6, a wet oxide etch has removed intermediate oxide layer 60 which covered poly-2 layer 57, thus exposing the now texturized surface 61 of poly-2 layer 57.

Referring now to FIG. 7, the poly-2 layer 57 has been anodized by submerging the wafer in a container of electrolytic solution of hydrofluoric acid and subjecting the wafer to an electrolytic current. The electrolytic current is generated between two electrodes, an anode and a cathode. The wafer, or poly-2 layer 57, comprises the anode. The container, or a platinum electrode immersed in the electrolytic solution parallel to the poly-2 layer 57, comprises the cathode.

Typically anodizing creates a porous surface on the material employed as the anode. In this case, silioon 62, having a microstructure 63 resembling elongated pores, is formed by the anodic conversion of the poly-2 layer 57 silicon in the hydrofluoric acid. An enlarged exaggerated depiction of the porous substrate 62 formed by anodic conversion is shown in cross section in FIGS. 8a, 8b and 8c. FIG. 8a is an enlargement of portion 64 of FIG. 7 depicting the microstructure 63 of the porous layer 62. The anodic conversion of the poly-2 layer 57 increases the porous film density of the poly-2 layer 57. Since inhomogeneities at the start of anodization result in localization of the current flow to initiate pores, the wet oxidation prior to anodization will further increase the anodization of the poly-2 layer 57 typically has a charge within the range of 10 mA/cm.sup.2 and 100 mA/cm.sup.2 and is performed in a hydrofluoric acid solution having a concentration between 10 to 60 percent by weight. The anodization charge and concentration of hydrofluoric acid solution are typically chosen to produce a porous film density of substantially 45 percent.

Referring now to FIG. 8b, the porous silicon 62 has been subjected to a second wet oxidation step filling the pores with an intermediate oxide layer 65 which covers the porous silicon 62. The oxidation step creates a texturized surface 66 comprising a multitude of three-dimensional asperities on the sides of the filled pores 63.

Referring now to FIG. 8c, a wet oxide etch has removed intermediate oxide layer 65 which covered the porous silioon 62, thus exposing the texturized surface 66 of the porous silicon 62. The texturized surface 66 is present on the porous silicon 62 shown in FIGS. 9 through 11. However, because of its minute size it is not discernable in the Figures.

Referring now to FIG. 9, porous silicon 62 has been masked with first photomask 67 and etched (patterned) to form the cell's storage node capacitor plate 68. The etch also exposed the access node junction 59.

Referring now to FIG. 10, photomask 67 has been removed and the conformal blanket deposition of a dielectric layer of silicon nitride 69 has taken place. The dielectric layer has a thickness of less than 100 Angstroms. It is necessary for the dielectric layer to provide good step coverage and its deposition is followed by the blanket deposition of a poly-3 layer 70, conductive doping of poly-3 layer 70 with phosphorus, and the masking of poly-3 layer with second photomask 71.

FIG. 11 is an exaggerated enlargement of portion 72 of FIG. 10 depicting the conformal deposition of silicon nitride 69 followed by the blanket deposition of the poly-3 layer 70.

Referring now to FIG. 12, poly-3 layer 70 and silicon nitride layer 69 have been etched, forming a cell plate 73, which functions as the cell capacitor's field plate, being dielectrically insulated from storage node capacitor plate 68 by silicon nitride layer 69. The process is completed with a conformal deposition of oxide 74 etched to expose access node junction 59. A bit line 76 is then deposited and patterned to contact the access node junction 59.

By anodizing the poly-2 layer 57 no additional masking steps are needed to increase the surface area of the cell's storage node capacitor plate while retaining the lateral dimension of the cell's storage node capacitor plate. By increasing the surface area, the capacitance of the cell's storage node capacitor is greatly increased.

In addition to negatively doping of the poly with phosphorus other pentavalent impurity atoms comprising arsenic, and antimony, may be used. Alternately, the poly may be positively doped by using trivalent impurity atoms comprising aluminum, boron and gallium.

The invention may also be implemented wherein the stacked capacitor is fabricated overlaying the bit lines. Monocrystalline silicon may also be used in place of polycrystalline.

Although only a single embodiment of the invention has been described herein, it will be apparent to one skilled in the art that changes and modifications may be made thereto without departing from the spirit and the scope of the invention as claimed.