SYSTEM AND METHOD FOR PROVIDING A
NITRIDE CAP OVER A POLYSILICON
FILLED TRENCH TO PREVENT
FORMATION OF A VERTICAL BIRD'S BEAK
STRUCTURE IN THE MANUFACTURE OF A 5
RELATED PATENT APPLICATION
The present invention is related to the invention disclosed 10 and claimed in U.S. patent application Ser. No. 11/201,761 filed concurrently with the present patent application.
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the manufacture of semiconductor devices and, more particularly, to a system and method for preventing the formation of a vertical bird's beak structure during a manufacturing process.
BACKGROUND OF THE INVENTION
method that would prevent the formation of a bird's beak structure in the first place during the manufacture of a semiconductor device.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation; the term "or," is inclusive, meaning and/or; "each" means every one of at least a subset of the identified items; the phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior uses, as well as future uses, of such defined words and phrases.
Some types of semiconductor manufacturing processes utilize a polysilicon buffered LOCOS (Local Oxidation of Silicon) field oxidation process to provide device to device 25 isolation in conjunction with a deep polysilicon filled trench. A standard prior art technique comprises growing field oxide over a polysilicon filled trench to isolate the trench polysilicon from further processing. This type of manufacturing process creates a vertical bird's beak structure on both sides of 30 the trench where the liner oxide of the trench intersects with the surface.
It has been shown that the presence of a vertical bird's beak structure is a very significant contributor to creating additional stress in the silicon due to the expansion of the silicon 35 dioxide in the confined trench liner region near the surface. The expansion of the liner oxide region causes compressive stress in the silicon surrounding the trench. This stress can be transmitted downwardly into the silicon bulk.
There have been prior art attempts to minimize the amount of stress that is produced by a vertical bird's beak structure. For example, a paper by Yang et al. entitled "Characterization of Collector-Emitter Leakage in Self-Aligned Double-Poly Bipolar Junction Transistors", 140 J. Electrochem. Soc. 3033-3037 (October 1993), suggests that the stress produced by the volume expansion from oxidation can be reduced by minimizing the size of the vertical bird's beak structure.
This can be achieved by reducing the deep trench surface oxide thickness (as long as adequate isolation can still be 5Q provided). To improve the quality of isolation oxide, a sacrificial layer can be grown and stripped off before the final layer is grown. The corner of the trench can be rounded off to reduce the stress that is generated due to volume misfit of the growing oxide. 55
The oxidation conditions, on the other hand, can be varied to relieve the oxide stress through the viscous elastic flow of the oxide. Variations in process conditions, such as the mixture of oxidation gas or oxidation temperature will affect the properties of the grown oxide as well as the generated defects. 60
The paper by Yang et al. also states that at higher oxidation temperature, the grown oxide has the ability to redistribute itself (due to lower viscosity) and thereby reduce the amount of stress on the neighboring silicon region.
This approach and others similar to it are directed at mini- 65 mizing the stress that is due to the presence of the bird's beak structure. It would be advantageous to have a system and
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which:
FIGS. 1 through 4 illustrate exemplary stages of manufacture of a prior art polysilicon filled trench that creates a vertical bird's beak structure;
FIGS. 5 through 14 illustrate exemplary stages of construction of a polysilicon filled trench of the present invention that does not create a vertical bird's beak structure;
FIGS. 15 and 16 illustrate a flow chart showing the steps of an advantageous embodiment of a method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1 through 16, discussed below, and the various embodiments used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented for any type of suitably arranged semiconductor device.
FIG. 1 is a diagram 100 illustrating a cross sectional view of a polysilicon filled trench in a substrate of a semiconductor device. Using conventional methods an isolation trench is etched in substrate 110. Then a liner oxide layer 120 is formed on the walls of the trench. The trench is then filled with polysilicon 130. Additional layers of material, polysilicon 140 and silicon nitride 150, are then applied over the oxide layer 120 to cover the polysilicon filled trench. The layers 120,140 and 150 may be collectively referred to as composite layers.
FIG. 2 illustrates the result of applying a mask and etch process to etch the composite layers 120,140 and 150 over the polysilicon filled trench. The resulting structure 200 is then subjected to a field oxide process to grow a field oxide 310 over the polysilicon filled trench. The presence of the field oxide 310 over the polysilicon filled trench is to isolate the polysilicon 130 in the trench from further processing. This prior art method creates a vertical bird's beak structure where the liner oxide 120 intersects with the field oxide 310 at the