Self-aligned selective hemispherical grain deposition process and structure for enhanced capacitance trench capacitor

The invention relates to a semiconductor processing method and structure, and more particularly to a integrated circuit fabrication process and structure for capacitance enhancement of a trench capacitor structure, such as useful in a dynamic random access memory (DRAM), among others.

In a dynamic random access memory (DRAM), a plurality of memory cells are formed in a matrix manner on a substrate. Each memory cell in a DRAM device is constructed by at least one transistor and at least one storage capacitor. Commonly, either trench capacitor or stacked cells are used. The trenched structure is advantageous to the stacked structure in situations where the surface flatness of the device is of great importance. While trench capacitors are mostly used as storage capacitors in DRAMS, such structures are also used in some integrated circuits as general use capacitors, for example for decoupling purposes.

In DRAMs, as well as in other integrated circuits, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. The continuing trend to increase circuit density requires that DRAM cells occupy smaller and smaller areas over the long term. However, the inability to shrink cell voltages and currents as rapidly as the cell area means that proportionally more capacitance is demanded from the storage capacitor per unit of area defined by the initial etch of the trench at critical dimension. Thus, a need exists for a method of forming a trench storage capacitor having enhanced capacitance.

The present invention provides a way of increasing the amount of surface area available between plates of a trench storage capacitor by forming an uneven plate surface by deposited hemispherical silicon grains, and then forming the capacitor dielectric and a conductor on the other side of the dielectric.

Accordingly, a method is provided, in an integrated circuit, for forming an enhanced capacitance trench capacitor. The method includes forming a trench in a semiconductor substrate and forming an isolation collar on a sidewall of the trench. The collar has at least an exposed layer of oxide and occupies only a “collar” portion of the sidewall, while a “capacitor” portion of the sidewall is free of the collar. A seeding layer is then selectively deposited on the capacitor portion of the sidewall. Then, hemispherical silicon grains are deposited on the seeding layer on the capacitor portion of the sidewall. A dielectric material is deposited, and then a conductor material, in that order, over the hemispherical silicon grains on the capacitor portion of the sidewall.

FIG. 1 is an illustration of the a thin nitride layer deposition as provided by one embodiment of the present invention; and

FIG. 2 is an illustration of a hemispherical grain polysilicon (HSG) seeding as provided by one embodiment of the present invention.

One way to increase the surface area available as the dielectric interface between plates of a capacitor is to provide a plate having an uneven surface, and then depositing a compatible dielectric material over it, such that the dielectric material generally assumes the unevenness of the plate surface below that.

Depositing hemispherical silicon grains (hereinafter, “HSG”) onto sidewalls of a deep trench may be a way to create an uneven surface, and thereby increase the surface area of a trench capacitor. However, at small groundrules of 110 nanometers and below, HSG deposition in the context of existing trench capacitor processing appears problematic. For example, depositing HSG into deep trenches after forming the isolation collar could fail due to HSG adhering to the collar and pinching off the trench, causing few grains to be deposited in the bottom of the trench. Therefore, it becomes imperative to use a selective HSG process such that grains are deposited in a lower portion of the trench, while relatively few, if any, grains are deposited on the collar. In accordance with the method described below, the selective deposition of HSG eliminates a hard requirement of removing grains from the collar prior to depositing a node conductor.

For example, in an embodiment, a deep trench 100 is formed in a substrate 102 . Thereafter, an isolation collar 120 is formed in an upper portion 122 of the deep trench 100 , leaving the lower portion 130 free of the collar. The collar comprises at least an outer layer of oxide, but can be formed entirely of an oxide. A seeding layer 110 is then preferentially deposited on the sidewall in the lower portion 130 of the deep trench 100 . The seeding layer preferably comprises a thin layer of silicon nitride, between about 5 and 12 521 in thickness. The seeding layer is preferentially deposited on the silicon sidewalls in the lower portion 130 of the deep trench. Very little seeding material adheres to the sidewall of isolation collar 120 .

Next, as shown in FIG. 2 , hemispherical silicon grains (HSG) 210 are selectively deposited, from a source of amorphous silicon, onto the seeding layer 110 which is present in the lower portion 130 of the deep trench 100 . Very little HSG, if any, is deposited onto the collar 120 because of the absence of a seeding layer 110 or an otherwise suitable surface on which they can adhere. One way of selectively depositing HSG onto a seeding layer of silicon nitride, resulting in little deposition onto an oxide surface, is by a low pressure chemical vapor deposition at a temperature below 650° C. Next, although not a strict process-enabling requirement, if there is residual HSG adhering to the collar 120 , it can then be removed, as for example, with an etch, such as a wet etch, or other similar methods, as known to those skilled in the art. If an isotropic wet etch is used, the HSG grain size in the lower portion 130 of the trench may be reduced slightly, but still be satisfactory within the context of this method. Such etch can be performed, or omitted, as necessary, depending upon the amount of for a particular process, to prevent unwanted conduction and/or parasitic effects of leaving some conductive material between the isolation collar and the subsequently deposited capacitor dielectric 220 . Note that the method described herein results in the HSG enhancement of the trench capacitor in a lower portion 130 being self-aligned to the bottom 124 of the isolation collar 120 . Next, a capacitor dielectric 220 is deposited over the HSG 210 and a material such as highly doped polysilicon, a metal or a metal silicide is deposited and recessed in the trench to form a conductor 230 , thus forming a trench capacitor in the lower portion 130 .

While the invention has been described in accordance with certain preferred embodiments thereof, those skilled in the art will understand the many modifications and enhancements which can be made thereto without departing from the true scope and spirit of the invention, which is limited only by the claims appended below.