Method to Form Hemispherical Grain (HSG) Silicon by Implant Seeding Followed by Vacuum Anneal

This invention relates to hemispherical grain silicon and more particularly to a process for forming hemispherical grain silicon for capacitor structures used in semiconductor devices.

In dynamic semiconductor memory storage devices it is essential that storage node capacitor cell plates be large enough to retain an adequate charge (or capacitance) in spite of parasitic capacitance and noise that may be present during circuit operation. As is the case for most semiconductor integrated circuitry, circuit density is continuing to increase at a fairly constant rate. The issue of maintaining storage node capacitance is particularly important as the density of DRAM arrays continues to increase for future generations of memory devices.

The ability to densely pack storage cells while maintaining required capacitance levels is a crucial requirement of semiconductor manufacturing technologies if future generations of expanded memory array devices are to be successfully manufactured.

Storage cells formed with hemispherical grain polysilicon gains unique advantages as the hemispherical grain polysilicon enhances the surface area of a storage cell electrode without any additional complex processing. However, conventional methods of forming HSG polysilicon by deposition, develop roughness of the polysilicon which starts at the interface of the polysilicon and forms towards its surface. It is desirable to have the roughness of the polysilicon develop by starting at the polysilicon surface and continue to form towards the polysilicon's interface. The present invention develops such a technique.

U.S. Pat. No. 5,162,248, and U.S. Pat. No. 5,340,765, having the same assignee as does the present invention, are hereby incorporated by reference.

The present invention develops a process for forming hemispherical grain (HSG) silicon for fabrication of a semiconductor device, by the steps of:

introducing a crystallization nucleus (or nuclei) into a silicon material; and

converting the silicon material into the HSG silicon by promoting growth of the crystallization nucleus (or nuclei).

FIG. 1 is a flow chart depicting process steps of the present invention;

FIGS. 2-4 are cross-sectional views of an in-process wafer portion demonstrating general results of the process steps outlined in FIG. 1; and

FIGS. 5-8 are cross-sectional views of in-process wafer portions demonstrating the results of the process outlined in FIG. 1 for specific applications.

An embodiment of present invention is directed to maximizing storage cell surface area by providing uniform, storage cell structures across a given supporting substrate, in a high density/high volume DRAM fabrication process. The following process steps, as described below, serve as an example as well as a preferred method of implementing the present invention. However, the example is not intended to limit a broad scope of the invention which is depicted in FIG. 1.

A general process step flow chart of the present invention is depicted in FIG. 1. As represented by step 10, a crystallization nucleus (also crystallization seed) is introduced into an existing silicon material. Next, in step 11, the silicon material is converted into hemispherical grain (HSG) silicon by subjecting the silicon material to a high vacuum anneal. These general steps are discussed in further detail in the following discussion concerning FIGS. 2-4.

As shown in FIG. 2, a starting substrate 20 is overlaid by silicon material 21 (preferably by chemical vapor deposition). It is preferred that silicon material 21 be amorphous silicon, and it is further preferred that silicon material 21 be conductively doped amorphous silicon. Other silicon layers, such as crystalline silicon, polycrystalline silicon, or conductively doped polycrystalline silicon, may be substituted for amorphous silicon, though the structure of amorphous silicon (devoid of long -range periodic structure or there is no re-occurring crystal periodicity), is particularly effective when used in the present invention.

As shown in FIG. 3, a crystallization seed or nucleus is introduced into material 21. It is preferred that the crystallization nucleus (or nuclei) be implanted as the depth of the implant is controllable. It is also preferred that the depth of the implant of the crystallization nucleus reside at the upper surface boundary of material 21, as discussed in the process step depicted in FIG. 4. Even though implanting is the preferred method, it is conceivable that the crystallization nucleus could be introduced in situ during the formation of layer 21. The crystallization nucleus is an element selected from the group consisting of elements from groups IIIA, IVA or VA listed in the periodic table and any combination thereof. It is preferred that any one of the elements silicon (Si), boron (B), arsenic (As) or phosphorous (P) be used as the crystallization nucleus or any combination thereof. Group IIIA elements include boron (B), aluminum (Al), gallium (Ca), indium (In), and thallium (Tl). Group WA elements include carbon (C), silicon (Si), germanium (Ge), tin (Sn), and lead (Pb). Group VA elements include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).

Referring now to FIG. 4, silicon material 41, impregnated with at least one crystallization nucleus element, is converted to HSG silicon by subjecting the crystallization nucleus impregnated silicon material to an ultrahigh vacuum anneal. This high vacuum anneal promotes crystal growth of the crystallization nucleus and thereby transforms the impregnated silicon into HSG silicon. The anneal temperature must be at least 350.degree. C. and may range to 1050.degree. C., and a preferred anneal temperature range is from 550.degree. C.-750.degree. C. This ultrahigh vacuum anneal is performed at a pressure less than 100 mTorr. A preferred method of converting the crystallization nucleus impregnated silicon material to HSG silicon by subjecting the material to rapid thermal processing (at an anneal temperature of at least 350.degree. C. ranging to 1050.degree. C., and a preferred anneal temperature range from 550.degree. C.-750.degree. C.). The stated temperature range is intended to include all temperatures and temperature ranges assumed therein.

FIGS. 5-8 depict various capacitor structures that can be obtained by using the process steps discussed above during capacitor fabrication for a memory device (i.e., dynamic random access memories, floating gate devices and the like).

Referring now to FIG. 5, the storage capacitor plate 55 of each memory cell will make contact to the underlying diffusion region 51. Each underlying diffusion region 51 will have two storage node connections isolated from a single digit line contact by access transistors formed by poly word lines 53 overlying the active area of substrate 50. Normally each diffusion region 51 within the array is isolated from one another by a thick field oxide 56. Diffusion regions 51 and the poly wordlines 53 (control gates) form active field effect transistors (FETs serving as access transistors to each individual capacitor) that can be doped as NMOS or PMOS type FETs depending on the desired use. An etch, performed on planarized insulation layer 54, provides access to diffusion region 51 for placement of capacitor plate 55. Once capacitor plate 55 is formed, the silicon material making up the plate is processed in the manner outlined for FIGS. 2-4 to form the HSG silicon on the exposed surfaces of plate 55. FIG. 6 shows a completed capacitor with the inclusion of cell dielectric 61 and the second capacitor plate 62. From this point on the wafer is completed using conventional fabrication process steps.

FIGS. 7 and 8, show a second formation of a capacitor with HSG silicon which follows the discussion of FIGS. 5 and 6. Referring now to FIG. 7, the storage capacitor plate 76 of each memory cell will make contact to the underlying diffusion region 71. Each underlying diffusion region 71 will have two storage node connections isolated from a single digit line contact by access transistors formed by poly wordlines 72 overlying the active area of substrate 70. Diffusion regions 71 and the poly wordlines 72 (control gates) form active field effect transistors (FETs), serving as access transistors to each individual capacitor, that can be doped as NMOS or PMOS type FETs depending on the desired use. An etch, into planarized insulation layer 74, provides access to diffusion region 71. FIG. 8 shows a completed capacitor with the inclusion of cell dielectric 81 and the second capacitor plate 82. Conductive plug 73 has been formed prior to capacitor plate 76 formation and is used to connect plate 76 to underlying diffusion region 71. From this point on the wafer is completed using conventional fabrication process steps.

It is to be understood that although the present invention has been described with reference to a preferred embodiment, various modifications, known to those skilled in the art, may be made to the structures and process steps presented herein without departing from the invention as recited in the several claims appended hereto.