Polishing method

Details Polishing method Polishing method

2000-07-18

2003-07-22

Utech, Benjamin L. (Department: 1765)

Semiconductor device manufacturing: process

Chemical etching

Combined with the removal of material by nonchemical means

C438S691000, C438S692000, C438S693000, C438S700000

Reexamination Certificate

active

06596638

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to the polishing of a metal film, and, in particular, the invention relates to a method of polishing a metal film in a semiconductor device interconnection process.
In recent years, as semiconductor integrated circuits (referred to hereafter as LSI) have become more complex, new microtechniques have been developed. One of these is chemical mechanical polishing (referred to hereafter as CMP), which is often used in LSI manufacture, in particular for flattening of interlayer insulating films, for forming metal plugs and for inlay of interconnections in multi-layer interconnection processes.
This technology is disclosed, for example, in U.S. Pat. No. 4,944,836.
To achieve higher speeds in a LSI, attempts are being made to use a low resistance copper alloy, instead of the conventional aluminum alloys, as an interconnection material, however, with copper alloy, microprocessing by dry etching, which was used with aluminum alloy, is difficult. Therefore, the “damascene” method is mainly employed, wherein an inlaid interconnection is formed by depositing a copper alloy thin film on an insulating film on which a groove is then formed by dry etching, and the copper alloy thin film is removed by CMP, except for the part inlaid in the groove. This technique is disclosed, for example, in Japanese Published Unexamined Patent Application No. 2-278822.
In general, slurries used for CMP of a copper alloy interconnection comprise a solid abrasive and an oxidizing substance as main components. The basic mechanism of CMP is to mechanically remove the oxide using a solid abrasive while oxidizing the surface of the metal by the oxidizing action of an oxidizing substance. This is disclosed on p. 299 of “The Science of CMP”, edited by Masahiro Kashiwagi and published by Science Forum on Aug. 20, 1997 (in Japanese).
As solid abrasives, an alumina abrasive and silica abrasive with a particle diameter of several 10-several 100 nm have been used but most solid abrasives for metal polishing on the market are of the alumina type.
Generally, as oxidizing substances, hydrogen peroxide (H
2
O
2
), ferric nitrate (Fe(NO
3
)
3
) and potassium iodate (KIO
3
) are used, and these are described on p.299-p.300 of the aforementioned “Science of CMP”.
However, when interconnections and plugs were formed by CMP using a conventional slurry containing a solid abrasive for metal film polishing as a main component, the following problems (1)-(8) occurred.
(1) Denting (referred to hereafter as dishing) occurs wherein the surface of the central part of the metal interconnection inlaid in the groove formed in the insulating film is polished excessively compared to the periphery thereof, or a phenomenon (referred to hereafter as erosion) occurs wherein the insulating film surface around the interconnection is polished (
FIGS. 5A
,
5
B).
The metal/insulating film selective ratio of a slurry intended for metal film polishing is as high as ten or more. This value is obtained by performing CMP on a wafer with only a flat metal film, and a wafer with only a flat insulating film, and comparing the polishing rates in the two cases.
However, it is known that when CMP is applied to a wafer where a metal film is deposited on an insulating film having a groove which is an interconnection pattern, excessive polishing occurs locally. This is due to the fact that there is unevenness on the surface of the metal film before CMP is performed, reflecting the groove which is the interconnection pattern. When CMP is applied, high pressure occurs locally according to the pattern density, and the polishing rate at these points is faster.
Therefore, dishing and erosion become conspicuous problems in pads of large area (are of about 0.1 mm side) or with crowded interconnection patterns. These problems are mentioned in J. Electrochem. Soc., p. 2842-2848. Vol. 141, No. 10, October 1994.
(2) Scratches (polishing marks) occur due to the solid abrasive used for polishing. In particular, alumina, which is the main material used as a metal polishing abrasive, has a greater hardness than silicon dioxide, which is the main material of the insulating film. Therefore, scratches occur on the surface of an insulating film exposed by CMP in addition to the surface of the metal film used for the interconnection. A slurry remains behind in the scratches on the insulating film surface, and this causes a malfunction of the semiconductor device due to heavy metal ion contamination. It also affects the shape of the upper layer interconnection, and causes short circuits. The scratches on the metal film surface cause poor continuity and deterioration of electromigration resistance.
In order to prevent scratches, the down force and the platen rotation speed are reduced when CMP is employed. However, it is difficult for even this method to prevent scratches in a soft metal, such as copper.
The scratches can be reduced by using a soft polishing pad, but dishing and erosion become more serious, and the flatness after CMP deteriorates. It was therefore suggested to perform CMP with a hard polishing pad in a first stage, and then to finish with a soft polishing pad, i.e. to perform a two-stage CMP. A new problem, however, arises in this case in that the throughput falls.
(3) Due to the high frictional force between the polishing abrasive and the metal film surface when CMP is performed, peeling occurs between the metal film and the lower insulation layer, or between the spin-on-glass (referred to hereafter as SOG) in the lower insulating layer and the chemical vapor deposition (referred to hereafter as CVD) oxide film. To prevent peeling, the down force and the platen rotation speed may be reduced, but if an attempt is made to completely prevent peeling, the CMP rate falls and the polishing time becomes longer, which is not practical. This can be resolved by using a soft polishing pad, but dishing and erosion become serious, and the flatness after CMP deteriorates.
(4) Since a large amount of polishing abrasive remains behind on the wafer surface after CMP, cleaning must be performed before applying the next step, and foreign matter must be removed until the amount thereof is below a specified level (e.g., there must be no more than 100 particles of foreign matter greater than 0.2 &mgr;m in one wafer). A cleaning machine which employs mechanical cleaning together with chemical cleaning is needed for this purpose.
The cleaning technique is very complicated, as shown in FIG.
11
. Brush-cleaning and megasonic cleaning that use a reagent fluid together are mainly used. The brush materials must be special materials which do not damage the metal film surface, and, for example, ammonium hydroxide or an aqueous solution of hydrofluoric acid are used as a reagent fluid.
Megasonic cleaning is a cleaning method using a high frequency of 800 kHz applied to the cleaning fluid so as to remove abrasive material from the substrate. This cleaning is more powerful than conventional cleaning by ultrasonic waves (40 kHz). In this technique, sufficient energy or force must be supplied to remove the abrasive material from the substrate. On the other hand, the output must be set in a range that does not damage the metal film and the insulating film. An example of post-CMP cleaning is disclosed on p. 172 of the May 1995 edition of Semiconductor World (in Japanese).
(5) Consumable items used for CMP are costly. This is because the production cost of abrasives used in the slurry is high, and great care must be taken to adjust the particle size. In particular, alumina abrasive is several times higher in price compared with silica abrasive.
In general, foaming polyurethane is used as a polishing pad. When CMP is performed, the polishing abrasive adheres to this polishing pad, clogging occurs, and the CMP rate drops.
To prevent this, the polishing pad surface needs to be sharpened with a whetstone (referred to hereafter as a conditioner) to which diamond particles were made to adhere. Therefore, the life of the polishing pad is short, and it represents

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