Etch process for recessing polysilicon in trench structures

#Fabricating semiconductor devices is a complex process. It involves many steps that require extreme precision and highly pure materials. A semiconductor wafer is provided at an early step in the fabrication process. The wafer serves as a base substrate. Layers of material are added onto the substrate. The shape and composition of the added layers and the substrate are modified to fabricate device components.

#A variety of semiconductor devices can be made with a few basic components, such as transistors and capacitors. One widely used device is dynamic random access memory (DRAM). The basic DRAM device is a memory cell having one capacitor and one transistor. The capacitor stores a charge representing data. The transistor allows the data to be written to or read from the capacitor. By reducing the size of its components, semiconductor manufacturers can fit more DRAM devices onto a chip. The increase in the amount of DRAM devices results in greater memory capacity for the chip.

#One method of minimizing the size of devices is to vertically construct the components (i.e., where a semiconductor device includes components such as capacitors formed at several or more layers thereof). One way to accomplish such vertical construction may involve forming a trench in a semiconductor substrate. In the process of forming a capacitor, polysilicon (“poly-Si”) may be deposited in the trench as a conducting material or as a sacrificial fill that will be removed during later process steps. The poly-Si may be recessed by removing a portion of the poly-Si through an etching process. Layers of conductive or insulating material may then be deposited in the recessed area of the poly-Si.

#FIG. 1 graphically illustrates the poly-Si etch rate whereby a quantity of poly-Si is removed in accordance with examples of the present invention.

#FIG. 2 illustrates the result of one step in the present method of fabricating a semiconductor device.

#FIG. 3 illustrates the result of a further step in the present method of fabricating a semiconductor device.

#In accordance with a preferred embodiment of the present invention, a solution comprising NH4OH in water is applied to poly-Si in a trench. The solution is used to etch the poly-Si in the trench so that the poly-Si is recessed to a desired depth. The etch rate can be varied by altering solution concentration, temperature, and exposure time.

#The trenches containing the poly-Si can be of various dimensions, depending upon their application. For example, a DRAM capacitor has a main trench that will be filled with a material to form a capacitor therein, which will store the charge of the memory cell. The main trench is typically 6000 nm deep with a top width of less than 200 nm. Other trenches having different dimensions may be formed during fabrication of the DRAM memory cell.

#A wide range of concentrations and temperatures are available for the solution of NH4OH in water. In one example, the concentration is between 1:5 and 1:80 (NH4OH:H2O). The temperature range in this example is between 25° C. and 65° C.