Semiconductor device manufacturing: process – Chemical etching – Combined with the removal of material by nonchemical means
Reexamination Certificate
2000-05-25
2001-04-17
Mills, Gregory (Department: 1763)
Semiconductor device manufacturing: process
Chemical etching
Combined with the removal of material by nonchemical means
C438S756000, C438S757000, C216S088000, C216S097000, C216S099000
Reexamination Certificate
active
06218305
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to compositions which are useful as slurries for the chemical-mechanical polishing of substrates, especially substrates comprised of silica and silicon nitride. More specifically the slurries of the present invention include an aqueous medium, abrasive particles, a viscosity modifier, a surfactant, and a compound which complexes with silica and silicon nitride.
2. Description of Related Art
In integrated circuit technology, various active and passive elements generally must be isolated from one another within the integrated circuit structure. This has often been accomplished by deep or shallow trench isolation techniques. These techniques typically use silicon dioxide (silica) as a dielectric material and silicon nitride as a stop layer, requiring chemical-mechanical polishing (planarization) of each circuit layer. To achieve efficient polishing and adequate planarization, a polishing slurry is generally useful and should provide a high selectivity involving the removal rate of silica relative to silicon nitride.
In Silvestri et al., U.S. Pat. No. 4,526,631, a slurry of 6 weight percent colloidal silica adjusted to a pH of about 12 with KOH provides a polishing ratio of about 10 SiO
2
to 1 Si
3
N
4
. Beyer et al., U.S. Pat. No. 4,671,851, states that the polishing ratios between SiO
2
and Si
3
N
4
preferably should be between a lower limit of 4 to 1 and a higher limit of 40 to 1. Beyer describes obtaining a ratio of 6.2 to 1 using a colloidal silica in water with small amounts of a sodium salt of dichloroisocyanuric acid and sodium carbonate.
Even more recent patents such as Murase, U.S. Pat. No. 5,502,007, also describe obtaining selectivities of about 10 SiO
2
to 1 Si
3
N
4
removal rates using a colloidal silica slurry as a polishing agent. Kodera et al., U.S. Pat. No. 5,445,996, use ceria as well as silica for the abrasive particles in slurries, but they also report selectivities for SiO
2
to Si
3
N
4
removal rates in the range of 2 to 3.
SUMMARY OF THE INVENTION
A composition is provided for polishing a composite comprised of silica and silicon nitride comprising: an aqueous medium, abrasive particles, a surfactant, a viscosity modifier, and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different.
A further aspect of the invention is the method for polishing a composite comprised of silica and silicon nitride comprising: applying a slurry at a polishing interface between a polishing pad and the composite comprised of silica and silicon nitride, the slurry comprising: an aqueous medium, abrasive particles, a surfactant, a viscosity modifier, and a compound which complexes with the silica and silicon nitride wherein the complexing agent has two or more functional groups each having a dissociable proton, the functional groups being the same or different.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
It has been found that the addition of a compound which complexes with silica and silicon nitride to polishing slurries used in the chemical-mechanical polishing of composites comprised of silica and silicon nitride can provide a very high selectivity of rate of removal of SiO
2
to the rate of removal of Si
3
N
4
when a surfactant is used in conjunction with the complexing agent and when the concentration of the complexing agent in the slurry is sufficient to block the removal of Si
3
N
4
while not greatly affecting the removal of SiO
2
at the pH of the polishing slurry.
Compounds which act as complexing agents or chelating agents for SiO
2
and Si
3
N
4
are described in great detail in U.S. Pat. Nos. 5,391,258 and 5,476,606 which are made part of this specification by reference. These compounds must have at least two acid groups present in the structure which can affect complexation to the silica and silicon nitride surface. Acid species are defined as those functional groups having a dissociable proton. These include, but are not limited to, carboxylate, hydroxyl, sulfonic and phosphonic groups. Carboxylate and hydroxyl groups are preferred as these are present in the widest variety of effective species. Particularly effective are structures which possess two or more carboxylate groups with hydroxyl groups in an alpha position, such as straight chain mono- and di-carboxylic acids and salts including, for example, malic acid and malates, tartaric acid and tartarates and gluconic acid and gluconates. Also effective are tri- and polycarboxylic acids and salts with secondary or tertiary hydroxyl groups in an alpha position relative to a carboxylic group such as citric acid and citrates. Also effective are compounds containing a benzene ring such as ortho di- and polyhydroxybenzoic acids and acid salts, phthalic acid and acid salts, pyrocatecol, pyrogallol, gallic acid and gallates and tannic acid and tannates. In the examples to follow, a salt of phthalic acid is used as the complexing agent and, therefore, such salts are preferred complexing agents for this invention. Potassium hydrogen phthalate, “KHP”, was the phthalate salt used in the experiments described below.
The surfactant used in conjunction with the complexing agent in this invention is not present to perform the usual function of surfactants in slurries of stabilizing the particulate dispersion. As shown in the examples which follow, the surfactant in combination with the complexing agent affects the rate of removal of Si
3
N
4
from the composite surface. It is believed that any surfactant, whether it be an anionic, cationic, non-ionic or zwitter-ionic surfactant, might be effective in the compositions of this invention. Particularly useful would be fluorocarbons or hydrocarbons with phosphate end groups. In the following examples several different surfactants were shown to be effective. “ZFSP”, ZONYL™ FSP Fluorosurfactant, manufactured by the DuPont Company, was shown to be a particularly effective surfactant additive to the slurries of this invention. It is a long straight chain hydrocarbon with phosphate groups at one end and a fluoride at the other end.
In these examples, ceria was used for the abrasive particles in the slurry because it is an effective polishing abrasive for chemical-mechanical polishing at all pH conditions and is stable against gelation. Any other polishing abrasive, such as alumina, zirconia, silica, titania and barium carbonate could also be used.
To adjust the slurries of this invention to the pH range in which the highest selectivities of SiO
2
removal to Si
3
N
4
removal are obtained any base or amine compound might be used. In the examples to follow KOH is used to adjust the pH of the slurry compositions. Potassium hydroxide, ammonium hydroxide, and all types of soluble amine compounds may be used to adjust the pH of chemical-mechanical polishing slurries.
To inhibit scratching, a viscosity modifier is preferably added to the slurry compositions of the present invention. The viscosity modifier is intended to increase the polishing fluid boundary layer between the polishing pad and substrate being polished. Generally speaking, the height of the boundary layer is proportional to the square root of the viscosity. Therefore, if the viscosity is increased four fold, the boundary layer should approximately double. As the boundary layer increases, the opportunity for pad-to-substrate contact decreases, thereby diminishing the opportunity for unwanted scratching. A larger boundary layer will generally provide improved lubrication and heat transfer at the polishing interface and will generally provide more efficient slurry transport along the polishing interface. By increasing the polishing fluid viscosity and thereby also increasing the boundary layer, slurry particles have less opportunity to drag across a substrate surface, but rather, the slurry particles will tend to be accelerated through contact with the pad and cause the particle to impinge upon p
Cook Lee Melbourne
Hosali Sharath D.
Oliver Michael R.
Sethuraman Anantha R.
Wang Jiun-Fang
Benson Kenneth A.
Goudreau George
Kaeding Konrad
Mills Gregory
Rodel Holdings Inc.
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