Compositions – Etching or brightening compositions
Reexamination Certificate
2000-03-15
2002-12-10
Goudreau, George (Department: 1763)
Compositions
Etching or brightening compositions
C252S079200, C252S079300, C252S079400
Reexamination Certificate
active
06491843
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an aqueous slurry for use in chemical-mechanical polishing processes. More particularly, the present invention relates to an aqueous slurry that is particularly useful for selectively removing silicon dioxide in preference to silicone nitride from a surface of an article by chemical-mechanical polishing, and a method of removing silicon dioxide in preference to silicon nitride from a surface of an article by chemical-mechanical polishing.
BACKGROUND OF THE INVENTION
Chemical-mechanical polishing (“CMP”) is a technology which has its roots in the pre-industrial era. In recent years, CMP has become the technology of choice among semiconductor chip fabricators to planarize the surface of semiconductor chips as circuit pattern layers are laid down. CMP technology is well-known, and is typically accomplished using a polishing pad and a slurry which contains a chemical reagent and abrasive particles. The chemical reagent functions to chemically react with the surface of the layer being polished whereas the abrasive particles perform a mechanical grinding function.
One of the uses of CMP technology is in the manufacture of shallow trench isolation (STI) structures in integrated circuits formed on semiconductor chips or wafers such as silicon. The purpose of an STI structure is to isolate discrete device elements (e.g., transistors) in a given pattern layer to prevent current leakage from occurring between them. Recent technological advancements permitting the fabrication of very small, high density circuit patterns on integrated circuits have placed higher demands on isolation structures.
An STI structure is usually formed by thermally growing an oxide layer on a silicon substrate and then depositing a silicon nitride layer on the thermally grown oxide layer. After deposition of the silicon nitride layer, a shallow trench is formed through the silicon nitride layer and the thermally grown oxide layer and partially through the silicon substrate using, for example, any of the well-known photolithography mask and etching processes. A layer of a dielectric material such as silicon dioxide is then typically deposited using a chemical vapor deposition process to completely fill the trench and cover the silicon nitride layer. Next, a CMP process is used to remove that portion of the silicon dioxide layer covering the silicon nitride layer and to planarize the entire surface of the article. The silicon nitride layer is intended to function as a polishing stop that protects the underlying thermally grown oxide layer and silicon substrate from being exposed during CMP processing. In some applications, the silicon nitride layer is later removed by, for example, dipping the article in an HF acid solution, leaving only the silicon dioxide filled trench to serve as an STI structure. Additional processing is usually then performed to form polysilicon gate structures.
It should be readily apparent that during the CMP step of manufacturing an STI structure on a silicon semiconductor substrate, it would be highly advantageous to use a polishing agent that is capable of selectively removing silicon dioxide in preference to silicone nitride, which is used as the stop layer. Ideally, the rate for removing silicon nitride by CMP would be close to 0 whereas the rate for removing silicon dioxide by CMP would be as high as possible.
Throughout the specification and in the appended claims, the term “selectivity” is used to describe the ratio of the rate at which silicon dioxide is removed to the rate at which silicon nitride is removed by the same polishing agent during a CMP process. Selectivity is determined by dividing the rate at which the silicon dioxide film is removed (usually expressed in terms of mu/min) by the rate at which the silicon nitride film is removed. Conventional CMP slurries occasionally exhibit a silicon dioxide to silicon nitride selectivity of up to about 10, and typically of about 4 or 5.
Keeping the silicon dioxide to silicon nitride selectivity high for a CMP slurry is important. However, suppressing the rate of removal of silicon nitride is probably more important. It is known that the removal rate of the silicon dioxide trench fill material can be made to be quite high by varying polishing conditions such as increasing pad pressure and using various abrasive particles in the slurry. However, these polishing conditions also tend to increase the silicon nitride removal rate, which can affect the uniformity of the final silicon nitride layer thickness and can cause other defects, such as scratching, in the final product. Thus, it is important for a CMP slurry to promote a reasonable silicon dioxide removal rate while, at the same time, inhibiting or suppressing the rate of silicon nitride removal. This too, however, must be done in moderation for some applications. When the selectivity of a CMP slurry is too high coupled with a very low silicon nitride removal rate, other problems, such as dishing of the trench silicon dioxide, can occur, which can result in severe topography variations once the silicon nitride stop layer is removed. Thus, an aqueous slurry should be able to balance these factors in order to be useful in STI processing.
One of the factors that can affect the polishing rate during a CMP process is the pH of the slurry. For some CMP slurries, a slight change in pH can result in a substantial difference in polishing rate and selectivity. It is preferably for a CMP slurry to exhibit a relatively high polishing rate and high selectivity over a broad range of pH.
Most conventional CMP slurries used for polishing oxides typically comprise abrasive particles dispersed in an aqueous alkaline medium (i.e., high pH). Such slurries tend to polish silicon dioxide and silicone nitride at a substantial rate, with a selectivity for silicon dioxide of about 10 or below, and typically at about 4. A few CMP slurries are known which do provide a fairly high silicon dioxide to silicon nitride removal rate selectivity. However, none of the heretofore known CMP slurries exhibit high silicon dioxide to silicon nitride selectivity over a broad range of pH.
Hosali et al., U.S. Pat. No. 5,738,800, discloses a composition for polishing a composite comprised of silicon dioxide and silicon nitride. The CMP slurry according to Hosali et al. comprises an aqueous medium, abrasive particles, a surfactant, and a complexing agent having two or more functional groups each having a dissociable proton that complexes with the silicon dioxide and silicon nitride. The surfactant used in conjunction with the complexing agent in the CMP slurry according to Hosali et al. does not perform the usual function of surfactants (i.e., the stabilization of the particulate dispersion), but rather it is believed by the inventors to affect the rate of removal of silicon nitride from the composite surface. The chemistry of the interaction between the surfactant and the complexing agent is not explained. The composition according to Hosali et al. exhibits selectivity better than conventional CMP slurries, but only within a narrow range of pH (from about 6 to about 7).
Grover et al., U.S. Pat. No. 5,759,917, discloses a CMP slurry for selectively polishing a silicon dioxide overfill in preference to a silicon nitride film stop layer during the manufacture of integrated circuit's and semiconductors. The CMP slurry according to Grover et al. comprises a carboxylic acid, a salt, and a soluble cerium compound at a pH within the range of from about 3 to about 11. The specification of Grover et al. states that a silicon dioxide to silicon nitride removal selectivity of from about 5 to about 100 is obtainable, but the highest reported selectivity in any of the examples in Grover is 34.89, and the substantial majority of the examples yield a selectivity of less than 20.
Kodama et al, EP 0 786 504 A2, discloses a CMP polishing composition comprising silicon nitride particles, water, and an acid. The CMP polishing composition according to Kodama et al. is said to exhibit hi
America William G.
Babu Suryadevara V.
Her Yie-Shein
Srinivasan Ramanathan
Eastman Kodak Company
Goudreau George
Watkins Peyton C.
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