Method of Making a Tooth Shaped Capacitor Using Ion Implantation

The present invention relates to semiconductor capacitors, and more specifically, to a method of making a tooth shaped capacitor for a DRAM cell.

In recent years there has been a dramatic increase in the packing density of DRAMs. Large DRAM devices are normally silicon based, and each cell typically embodies a single MOS field effect transistor with its source connected to a storage capacitor. This large integration of DRAMs has been accomplished by a reduction in individual cell size. However, the reduction in cell size results in a decrease in storage capacitance leading to reliability drawbacks, such as a lower signal to noise ratio and undesirable signal problems. The desired large scale integration in DRAM devices along with reliable operation can be achieved by using DRAM storage capacitors with a high storage capacitance relative to its cell area.

Efforts to maintain or increase the storage capacitance in memory cells with greater packing densities have included the use of a stacked capacitor design in which the capacitor cell uses the space over the MOSFET device area for the capacitor plates. In a recent prior art DRAM, one of the two electrodes of a storage capacitor is formed to have a three-dimensional structure. This makes the capacitance larger by 30% to 40% than that of a two-dimensional storage capacitor having the same size as the three-dimensional one.

For example, a three-dimensional stacked capacitor is disclosed in U.S. Pat. No. 5,053,351. The storage node plate of this capacitor has an E-shaped cross-section. In another example, a hemispherical-grain (HSG) polysilicon storage node has been proposed (see "A New Cylindrical Capacitor Using Hemispherical-Grain Si for 256 Mb DRAMs", H. Watanabe et al., Microelectronics Research Laboratories, NEC Corporation). This memory cell provides a large storage capacitance by increasing the effective area of a simple storage node. However, the complex capacitor shapes tend to be difficult to fabricate and the standard processes need at least two masks for the complex capacitor shapes, and more particularly, for the toothed-shape capacitor node. Therefore, there is a need for a capacitor node with a high surface area that is simple to manufacture.

A method for manufacturing a capacitor on a semiconductor substrate is disclosed. The method comprises the steps of: forming a first conductive layer over said substrate; forming a dot silicon layer on said first conductive layer; doping oxygen into said dot silicon layer using an oblique angle to form anti-oxidation regions; thermal annealing said semiconductor substrate in an inert gas ambient to convert said dot silicon layer into silicon oxide; removing a portion of said first conductive layer using said silicon oxide as a mask; patterning and etching said first conductive layer to form a first storage node of said capacitor; forming a dielectric layer on the surface of said first storage node of said capacitor; and forming a second conductive layer over said dielectric layer to act as a second storage node of said capacitor.

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a cross section view of a semiconductor wafer illustrating the step of forming a gate structure on a semiconductor substrate;

FIG. 2 is a cross section view of a semiconductor wafer illustrating the step of forming an oxide layer and a first dielectric layer on the semiconductor substrate;

FIG. 3 is a cross section view of a semiconductor wafer illustrating the step of forming a contact hole in the first dielectric layer and the oxide layer, and forming a first conductive layer on the first dielectric layer, in the contact hole, and forming a HSG-Si layer on the first conductive layer;

FIG. 4 is a cross section view of a semiconductor wafer illustrating the step of forming silicon oxide;

FIG. 5 a cross section view of a semiconductor wafer illustrating the step of etching the first conductive layer using the silicon oxide as an etching mask;

FIG. 6 is a cross section view of a semiconductor wafer illustrating the step of forming a bottom storage node of a capacitor;

FIG. 7 is a cross section view of a semiconductor wafer illustrating the step of depositing a second dielectric layer on said bottom storage node; and

FIG. 8 is a cross section view of a semiconductor wafer illustrating the step of forming a second conductive layer on said second dielectric layer.

The formation of the capacitor described herein includes many process steps that are well known in the art. For example, the processes of photolithography masking and etching are well known in the art and are used extensively herein without a related discussion of these well known technologies.

Referring to FIG. 1, a P-type single crystal silicon substrate 2 with a <100> crystallographic orientation is provided. An isolation region 4 is formed using any suitable technique such as thick field oxide (FOX) or trench isolation technology. In the present invention, a thick field oxide (FOX) region 4 is formed to provide isolation between devices on the substrate 2. The FOX region 4 is created in a conventional manner. For example, the FOX region 4 can be formed via photolithography and dry etching steps to etch a silicon nitride-silicon dioxide composition layer. After the photoresist is removed and wet cleaned, thermal oxidation in an oxygen-steam environment is used to grow the FOX region 4 to a thickness of about 3000-8000 angstroms.

Next, a silicon dioxide layer 6 is created on the top surface of the substrate 2 to serve as the gate oxide for subsequently formed Metal Oxide Silicon Field Effect Transistors (MOSFETs). In one embodiment, the silicon dioxide layer 6 is formed by using an oxygen ambient, at a temperature of about 800.degree. to 1100.degree. C. Alternatively, the oxide layer 6 may be formed using any suitable oxide chemical compositions and procedures. In the preferred embodiment, the thickness of the silicon dioxide layer 6 is approximately 30-200 angstroms.

A doped first polysilicon layer 8 is then formed over the FOX region 4 and the silicon dioxide layer 6 using a Low Pressure Chemical Vapor Deposition (LPCVD) process. In this embodiment, the first polysilicon layer 8 has a thickness of about 2000-4000 angstroms. A capped oxide layer 10 is formed on the first polysilicon layer 8. Next, standard photolithography and etching steps are used to form a gate structure 12 and a word line 14. Then a LDD (lightly doped drain) structure 16 is formed by light ion implantation. Sidewall spacers 18 are generated by using well known techniques, and, subsequently, active regions 20 (i.e. the source and the drain) are formed by using well known processes to implant appropriate impurities in those regions.

Turning next to FIG. 2, an undoped oxide layer 22 is deposited using a CVD process on the gate structure 12, the word line 14, and the substrate 2. A first dielectric layer 24 is then formed on the undoped oxide layer 22. The first dielectric layer 24 can be formed by using any suitable material such as borophosphosilicate glass (BPSG) or TEOS-oxide.

As shown in FIG. 3, a contact hole 26 is formed in the first dielectric layer 24 and the oxide layer 22 to the active regions 20 by using conventional patterning and etching. A first conductive layer 28 is then formed over and in the contact hole 26 and on the first dielectric layer 24. The first conductive layer 28 is preferably formed using conventional LPCVD processing. The thickness of the first conductive layer 28, as measured over the first dielectric layer 24, is optimally 2000-6000 angstroms. The first conductive layer 28 is preferably chosen from doped polysilicon or in-situ doped polysilicon.

Subsequently, a dot silicon layer 30 is formed on the first conductive layer 28. Preferably, the dot silicon layer 30 consists of a Hemispherical Grained Silicon (HSG-Si) layer 30 that is formed by the "initial phase" technique. The HSG-Si layer 30 serves as an etching mask for subsequent processes. Other techniques currently available or developed in the future may also be used to form the dot silicon layer 30. The advantage of using dot silicon layer 30 is that the dot silicon layer 30 can be deposited with a resolution that is beyond the limitation of current photolithography techniques. The HSG-Si layer 30 is formed with a thickness about 50-1000 angstroms.

Turning next to FIG. 4, oxygen is implanted at an oblique angle into the dot silicon layer 30. The angle of the implant is from 0 degrees to 45 degrees. The advantage of the oblique implant is that the HSG-Si 30 acts as a mask to prevent the first conductive layer 28 from being bombarded by the implant. Therefore, only the HSG-Si 30 is implanted by oxygen. Next, a thermal anneal process is carried out in inert gas ambient to convert the HSG-Si 30 into dot silicon oxide 30a.

Next, as seen in FIG. 5, the first conductive layer 28 is etched using the silicon oxide 30a as an etching mask. The present invention uses the high etching selectivity between the silicon oxide 30a and the polysilicon 28 to create cavities 32 in the first conductive layer 28. Any suitable etchant can be used for this etching, such as C.sub.2 F.sub.6, SF.sub.6, CF.sub.4 +O.sub.2, CF.sub.4 +Cl.sub.2, CF.sub.4 +HBr, HBr/Cl.sub.2 /O.sub.2, Cl.sub.2, HBr/O.sub.2, BCl.sub.3 /Cl.sub.2, SiCl.sub.4 /Cl.sub.2, SF.sub.6, SF.sub.6 /Br.sub.2, CCl.sub.4 /Cl.sub.2, or CH.sub.3 F/Cl.sub.2. The etching is preferably reactive ion etching (RIE) or chemical dry etching (CDE). The CDE is an isotropic etching that can provide a rounded bottom surface of the cavities 32.

Referring to FIG. 6, the silicon oxide 30a is removed by wet etching. In preferred embodiment, buffer oxide etching (BOE), vapor HF or diluted HF solution is used as an etchant. Then, a photoresist is patterned on the first conductive layer 28 to define a capacitor bottom storage node. Subsequently, an etching process is performed to etch the first conductive layer 28 using the photoresist as a mask. The photoresist is then removed after the capacitor bottom storage node is formed.

Turning now to FIG. 7, a second dielectric layer 34 is deposited along the surface of the first conductive layer 28. The second dielectric layer 34 is preferably formed of either a double-film of nitride/oxide film, a triple-film of oxide/nitride/oxide, or any other high dielectric film such as tantalum oxide (Ta.sub.2 O.sub.5), BST.

Finally, as is shown in FIG. 8, a second conductive layer 36 is deposited using a conventional LPCVD process over the second dielectric layer 34. The second conductive layer 36 provides a top storage electrode and is formed of doped polysilicon, in-situ doped polysilicon, aluminum, copper, tungsten or titanium. Thus, a semiconductor capacitor is formed which comprises a second conductive layer 36 as its top storage electrode, a dielectric 34, and a first conductive layer 28 as the bottom storage electrode.

As will be understood by persons skilled in the art, the foregoing preferred embodiment of the present invention is illustrative of the present invention rather than limiting the present invention. For example, the method of the present invention can also be used in a COB (capacitor over bit line) structure. Thus, the invention is not to be limited to this embodiment, but rather the invention is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.