This invention relates to a method of etching hardmask stacks for deep trench openings in silicon. More particularly, this invention relates to a method of etching high aspect ratio deep trenches in silicon through a multilayer hard mask stack in a single chamber.
Multilayer hard mask stacks used for patterning silicon prior to etching deep, straight walled trenches in the silicon comprise a plurality of layers. One typical hard mask stack, as shown in FIG. 1, comprises in sequence a silicon substrate 10, a thin layer of thermally grown silicon oxide 12, called pad oxide, over the silicon substrate 10; a layer of silicon nitride 14 over the pad silicon oxide layer 12; a layer of silicon oxide hardmask 16, which can be doped (PSG, BPSG) or undoped silicon oxide over the silicon nitride layer 14; a polysilicon hardmask layer 18 over the silicon oxide layer 16; an antireflective coating 20 over the polysilicon layer 18; and a patterned layer of photoresist 22 thereover.
Patterning these different layers requires different etchants and different etch conditions, and thus the substrate and its various layers are presently transferred between up to five different processing chambers. Since processing is carried out in high vacuum plasma chambers, and since changing processing conditions in the chambers is a lengthy process, a tool has been developed that connects several reaction chambers together by means of a central vacuum chamber that connects to each of the processing chambers. A suitable device, such as a robot, picks up a substrate, such as a silicon wafer having the layers thereover as in FIG. 1, and inserts it into a first silicon etch chamber to open the antireflection layer 20 and the polysilicon hard mask layer 18, as shown in FIG. 2.
After processing, the substrate 10 is transferred to the central vacuum chamber and then into a second reaction chamber, known as an ASP or ashing chamber, to remove the remaining photoresist 22 using oxygen. The resultant substrate is shown in FIG. 3 where the polysilicon is a patterned hard mask layer. The substrate 10 is then transferred to a third, cleaning chamber where any remaining photoresist is removed.
The silicon oxide hard mask layer 16, the silicon nitride layer 14 and the thin pad oxide layer 12 are pattern etched in a fourth, dielectric etch chamber, as shown in FIG. 4. The etch stops when the silicon substrate 10 is reached. The polysilicon hard mask layer 18 is then removed, as shown in FIG. 5. A deep trench etch is carried out next using the silicon oxide hard mask 16 as the patterning layer in a fifth etch chamber. The resultant substrate is shown in FIG. 6.
The substrate is never exposed to the atmosphere or to non-vacuum conditions using the above tool, until all of the sequence of steps has been carried out. However, this method requires five chambers and multiple transfers of the silicon substrate by the robot, which can cause damage to the substrate and adds to the time and costs of processing.
The multiple chambers and the multiple steps carried out in the chambers is expensive both in terms of equipment costs and in terms of the time required for processing a single substrate. It would be highly desirable to reduce the amount of equipment required, the time required to process a single substrate, and to eliminate multiple transfers of the substrate.
We have found that once the photoresist and antireflective layers are patterned, the remaining layers can be etched down to the silicon substrate, and a deep trench etched therein, in a single, high aspect ratio trench etch chamber. This method can be carried out simply by changing the reactant gases and reaction conditions in the chamber. The method not only saves transfer time, but reduces damage and defects that can occur during transfers of the substrate between one chamber and another. Another advantage of this process is that it is self-cleaning. Ths use of fluorine-containing etch gases also serves to remove contaminants from the walls and fixtures of the single etch chamber.