Application of Thin Crystalline Si.sub.3 N.sub.4 Liners in Shallow Trench Isolation (STI) Structures

The present invention generally relates to the manufacture of submicron integrated circuits and, more particularly, to the application of thin silicon nitride (Si.sub.3 N.sub.4) films in shallow trench (STI) structures as O.sub.2 -barrier films.

One of the most critical parameters affecting silicon integrated circuit functionality and yield is the amount of stress that is developed within the silicon substrate during device processing. Stresses, exceeding the mechanical yield strength of silicon (Si), will form Si crystal defects (e.g., dislocations and stacking faults), adversely affecting device performance and yield. A prominent source of stress occurs during the isolation process of electrical devices. Shallow trench isolation (STI) is used for features designed a 0.5 .mu.m or less. This process includes

etching a "shallow" trench (i.e., 0.5 .mu.m or less),

growing a thin oxide immediately after etch,

filling the trench with a dielectric (i.e., deposited oxide),

"densifying" the dielectric, either by oxidizing the dielectric or using a high-temperature anneal in an inert ambient, and

planarization of the dielectric.

As device geometries continue to shrink in size, and since stress is inversely proportional to area, it is of paramount importance to minimize stress during isolation processes as much as possible.

A 256 megabyte (MB) dynamic random access memory (DRAM) chip uses a "deep" trench array (as the capacitors) and STI to isolate the various transistors within the capacitor array. It has been determined that limiting the amount of oxide grown in the substrate is necessary to eliminate Si crystal defects in the trench-capacitor array. The method currently employed to "block-out" oxygen (O.sub.2) in the Si substrate is the use of a thin (<5 nm) silicon nitride (Si.sub.3 N.sub.4) film deposited by low-pressure chemical-vapor deposition (LPCVD) immediately after a thin oxide is grown in the STI. The thickness of the Si.sub.3 N.sub.4 film is specified as being 5 nm or less. The reason for this thickness limitation is that this film has been found to be etch-resistant in hot phosphoric acid baths (e.g., to remove pad Si.sub.3 N.sub.4) as well as in hydrofluoric acid baths (to remove thermally grown oxides). Thicker LPCVD-Si.sub.3 N.sub.4 films used as Si.sub.3 N.sub.4 liners in STI trenches have shown to etch readily in hot phosphoric acids.

One problem associated with the thin Si.sub.3 N.sub.4 liner has been its propensity to trap electrical charge. The charge-trapping behavior (interface and bulk) of the Si.sub.3 N.sub.4 liner has been observed to enhance STI-bounding leakage (N well to N well) in the sense amplifiers (i.e., "support circuitry") of the DRAMs, thus causing high standby currents. Recent data clearly shows that the Si3N4 liner lowers the threshold voltages (Vt) and enhances junction leakage by several orders of magnitude.

To ascertain the amount of charge that is trapped by the Si.sub.3 N.sub.4 liner, blanket wafer experimental lots were processed consisting of (1) thermally-grown oxide (SiO.sub.2 -10 nm), (2) LPCVD-Si.sub.3 N.sub.4 (4 nm), and (3) 10 nm SiO.sub.2 /4 nm LPCVD-Si.sub.3 N.sub.4. Simple metal-insulator-semiconductor (MIS) structures were fabricated by depositing aluminum through a dot mask onto the insulating film(s). C--V measurements (low- and high-frequency) were conducted and result indicated that (1) the thin Si3N4 film alone contains approximately two orders of magnitude more charge-trapping states than the oxide (i.e., 10.sup.12 versus 10.sup.10), and (2) the combination of the oxide and Si.sub.3 N.sub.4 decreases the density of charge-trapping states only marginally (e.g., .about.5.times.10.sup.11 versus 10.sup.10). Ideally, it would be best to develop or deposit a thin Si.sub.3 N.sub.4 film that would not trap charge and still be resistant to hot phosphoric acid and hydrofluoric acid.

It is therefore an object of the present invention to provide a thin Si.sub.3 N.sub.4 film which has a lower density of trapping centers than as-deposited LPCVD-Si.sub.3 N.sub.4 and is an extremely effective as an O.sub.2 -barrier film yet is resistant to hot phosphoric acid and hydrofluoric acid.

According to the invention, thin crystalline Si.sub.3 N.sub.4 films are applied in STI structures as O.sub.2 -barrier films. The crystalline Si.sub.3 N.sub.4 contains a lower density of trapping centers, is more resistant to hot phosphoric acid etches and hydrofluoric acid etches than as-deposited LPCVD-Si.sub.3 N.sub.4, and is extremely effective as an O.sub.2 -barrier film. The fact that crystalline Si.sub.3 N.sub.4 has a lower density of trapping centers lowers, and in some cases prevents altogether, the parasitic transistor caused by the as-deposited LPCVD-Si.sub.3 N.sub.4 film used in current STI technology. Furthermore, unlike the restrictive thicknesses that are specified in the current technology, this invention permits a larger range of crystalline Si.sub.3 N.sub.4 thicknesses (e.g., 5 nm to 10 nm) to be used as O.sub.2 -barrier films in STI trenches. This allows a larger processing window for thickness controllability in manufacturing.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIGS. 1A, 1B and 1C illustrate the process of forming thin crystalline Si.sub.3 N.sub.4 films in STI structures; and

FIGS. 2A and 2B are, respectively, a bright-field transmission electron micrograph (TEM) of crystalline Si.sub.3 N.sub.4 and an electron diffraction pattern of the TEM.

Referring now to the drawings, and more particularly to FIGS. 1A, 1B and 1C, there is shown the process of forming crystallized Si.sub.3 N.sub.4 films in STI trenches according to the invention. In FIG 1A, after a shallow trench has been etched, a thin thermal oxide (typically 10 nm thick) is grown to remove etch damage. Next, in FIG. 1B, a thin Si.sub.3 N.sub.4 film (5 to 10 nm) is deposited on top of the thermally-grown oxide layer in the STI trench. Then, in FIG. 1C, LPCVD-Si.sub.3 N.sub.4 is typically deposited at temperatures of 720.degree. C. to 780.degree. C., and as such, is generally deposited in an amorphous state. Immediately after deposition, a high temperature, rapid thermal anneal in pure nitrogen (RTN) or ammonia (NH.sub.4) is conducted. Crystallization of LPCVD-Si.sub.3 N.sub.4 begins at 1050.degree. C. and for anneal times on the order of 60 seconds. Temperatures greater than 1050.degree. C. and/or anneal times greater than 60 seconds will induce further crystallization and grain growth of the LPCVD-Si.sub.3 N.sub.4 film. In addition, thicker LPCVD-Si.sub.3 N.sub.4 films (up to 10 nm) can be crystallized at temperatures less than 1100.degree. C. This can be critically important in terms of limiting wafer warpage and slip line formation along wafer edges during the RTN process.

The typical morphology of crystalline Si.sub.3 N.sub.4 is shown in FIGS. 2A and 2B. FIGS. 2A and 2B are bright-field transmission electron micrograph (TEM) and corresponding electron diffraction pattern, respectively, of a 10 nm SiO.sub.2 /4 nm Si.sub.3 N.sub.4 insulator-couple that has been annealed at 1100.degree. C. At 1100.degree. C., the Si.sub.3 N.sub.4 film is found to be largely crystalline. At 1050.degree. C., the majority of the film is found to be amorphous; however, small crystallites of Si.sub.3 N.sub.4 are observed. Electron diffraction analysis of samples annealed at 1050.degree. C. and 1050.degree. C. shows that the crystalline S.sub.3 N.sub.4 film is the low-temperature, hexagonal (.alpha.) Si.sub.3 N.sub.4 phase.

Crystallized films of LPCVD-Si3N4 have much lower trap densities as compared to as-deposited LPCVD-Si.sub.3 N.sub.4 films. Results of C--V measurements of 10 nm SiO.sub.2 /4 nm Si.sub.3 N.sub.4 insulator-couples which have been crystallized by high temperature RTN anneals (1050.degree. C. to 1150.degree. C.) show that the trap density, at a minimum, has decreased by an order of magnitude. In addition, crystallized LPCVD-Si.sub.3 N.sub.4 films show a heartier etch resistance to hot phosphoric acid and hydrofluoric acids. Etch rate studies show that crystallized LPCVD-Si.sub.3 N.sub.4 films have, at a minimum, a 45% lower etch rate in hot phosphoric acid than as-deposited Si.sub.3 N.sub.4 and as much as a 30% lower etch rate in pure hydrofluoric acid.

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.