Chemical mechanical polishing slurry for tungsten

Details Chemical mechanical polishing slurry for tungsten Chemical mechanical polishing slurry for tungsten

1997-12-23

2001-09-04

Utech, Benjamin L. (Department: 1765)

Compositions

Etching or brightening compositions

C252S079200, C252S079400

Reexamination Certificate

active

06284151

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the planarization of semiconductor devices by Chemical-Mechanical-Polishing (CMP) techniques and, more particularly, to the formulation of polishing slurries for metals including tungsten wherein the slurry has a prolonged stability against settling of abrasive material and which is less corrosive to equipment.
2. Description of the Prior Art
Chemical-Mechanical-Polishing techniques are used in the semiconductor processing industry to enable interconnect metallurgy of the highest density to be fabricated. Based on the teachings in U.S. Pat. No. 4,789,648 to Chow et al. and U.S. Pat. No. 4,944,836 to Beyer et al., CMP has enabled the continued shrinking of semiconductor device dimensions by the practical and manufacturable introduction of planarization of both metals and interlevel dielectrics to semiconductor processing. By maintaining the surface of a semiconductor device as flat as possible, the rendering of optical images of ever smaller size continues to be possible.
CMP generally is based on the principles that most hard materials are reactive with oxidants to form oxides and other softer compounds which, in turn, can be polished away. If the process of reacting and polishing can be controlled sufficiently, semiconductor metallurgy will continue to lead the path to miniaturization.
It is known to planarize tungsten metallurgy by polishing with an aqueous slurry comprising alumina in ferric nitrate. See “PROCESS OPTIMIZATION OF TUNGSTEN CMP,” V. Blaschke and K. Holland, Chemical Mechanical Polishing—Metals Seminar (CMP), SEMICON/Southwest '95; Austin, Tex. Oct. 23, 1995. This slurry, when used in an IPEC-Westech Systems wafer polisher at a feedrate of about 125 ml per minute, has the capability of polishing tungsten at 3,000 Angstroms per minute. The slurry, however, has the following disadvantages:
1. The alumina colloidal suspension is unstable and the ferric nitrate solution must be agitated constantly to keep the suspension dispersed in the solution. If left alone, the suspension collapses within a few hours rendering it impossible to formulate batches of slurry in advance. The settling of the alumina particles indicates that the particles are not well dispersed at all, a condition which could easily lead to scratching and cause other polishing irregularities since the slurry actually contains particulates and aggomerates large enough to settle out of suspension. This also can cause Foreign Material (FM) problems and loss of product yield.
2. The polishing slurry leaves rust residue on items throughout the CMP tool area. Whenever some slurry is spilled or sprayed, orange rust is left behind when the slurry dries. The stains are an indication that the ferric ion is prone to polymerizing to oxo-bridged species, leading to ferric-bearing oxide residues. These residues are a source of FM. The polymerization can also account for agglomeration of the alumina, as the polymerized ferric species likely promotes adhesion of alumina particles to each other. This condition often causes tools to be shut down for cleaning and removal of slurry cakes by the use of a hammer.
3. The heterogenity of the slurry also prohibits the bulk feeding of the slurry to the tools. Because the particulates in the slurry have a tendency to settle out, the particulates will precipitate out within any delivery system clogging tubing and valves.
4. The tendency of the ferric components to form rust residue causes corrosion wherever the slurry lands and dries on stainless steel parts of the polishing tools. This is a cause of FM problems and eventually destroys polisher components.
Other ferric-based salts have been proposed and include ferricyanides as oxidizers, combined with an acetate buffer and acetic acid using a silica abrasive as described in U.S. Pat. No. 5,407,526 to Danielson et al. and U.S. Pat. No. 5,516,346 to Cadien etal.
U.S. Pat. No. 5,527,423 to Neville et al. describes a polishing slurry for polishing metals. The slurry includes ferric nitrate nonahydrate and deionized water in which a special fumed alumina or silica is used to provide stability of particles in suspension. The disclosure also suggests other additives and stabilizers which may be added immediately prior to use. The differences in polish rates obtained are attributed to the high surface area fumed silica or alumina of the invention and the resulting slurry suitable for most metals.
SUMMARY OF THE INVENTION
Thus, an improved slurry must avoid all of the above problems and also be capable of polishing at a rate of about 3,000 Angstroms per minute and have a selectivity to interlevel dielectrics, particularly silicon oxides of at least 10:1.
It is therefore an object of the invention to provide an effective slurry for polishing tungsten that maintains a high polish rate while reducing the quantity of chemicals used.
It is another object of the invention to provide a polishing slurry in which the abrasive particulates are truly distributed in the solution and do not separate out of solution.
It is yet another object of the invention to provide a polishing slurry in which metal contaminates are maintained in solution and do not precipitate out of solution.
Another object of the invention is to provide more efficient polishing slurries in which scratching is reduced because particles do not agglomerate.
These and other objects of the invention are achieved by providing a slurry for CMP polishing comprising an alumina abrasive, an oxidizer having an anion which forms soluble salts with metals having atomic numbers less than 83, water and an acid having the same anion as the oxidizer. More specifically, a CMP polishing slurry comprising ferric nonahydrate, an alumina slurry, nitric acid and water. More specifically the slurry comprises 175 to 225 grams of ferric nonahydrate added to about 340 to 410 ml 9% by weight alumina slurry diluted to about 4.1 to 4.8 liters with deionized water and pH balanced to a range of 1.0 to 1.5.
These and other objects of the invention will become more apparent when viewed in the light of the following description of the preferred embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENT
CMP technology has been practiced for many years and is based on the principle that relatively hard materials can be polished, or more properly planarized, by a combination of a chemical conversion of the material to a softer compound and the physical removal of that softer material with an abrasive process. The basis of CMP of many metals is the surface oxidation of the metal followed by the abrasive removal of the oxide. In the case of tungsten the preferred oxidant is the ferric ion and the following reaction is believed to occur.
W+6Fe
3+
+3H
2
O→WO
3
+6Fe
2+
+6H
+
E
0
=0.86 v  (1)
The ferrous ion is oxidized back to ferric by the nitrate ion by the following reaction.
3Fe
2+
+4H
+
+NO
3
−
→3Fe
3+
+2H
2
O+NO E
0
=0.19 v  (2)
thus sustaining the reaction rate.
The oxide, WO
3
, is then removed by the alumina (Al
2
O
3
) abrasive exposing additional unoxidized tungsten which undergoes more oxidation, etc. until the process is terminated.
The problem of providing a true dispersion of a solid oxide in aqueous suspension is best considered by knowing the electrical charge on the oxide particles. All oxides suspended in water exhibit a characteristic pH. Oxide particulates will cause a solution to exhibit a pH at which their surfaces will exhibit no net charge, the iso-electric point (IEP). In order for particles of an oxide to repel each other in suspension, the pH must be set either several unit more basic than the IEP (imparting negative charge) or several units more acidic (imparting positive charge). When the pH is thus set, the like-charged particles mutually repel and the suspension remains dispersed. If the ionic strength is raised too high, the surface charge around the particles is disrupted and the suspension

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