Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-11-29
2001-10-09
Gorgos, Kathryn (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S230200
Reexamination Certificate
active
06299741
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to planarization of metal substrates and more particularly to advanced electrolytic polishing of metal films on semiconductor wafers.
2. Description of the Related Art
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After a layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar outer surface presents a problem for the integrated circuit manufacturer. Therefore, there is a need to periodically planarize the substrate surface to provide a relatively flat surface. In some fabrication processes, planarization of the outer layer should not expose underlying layers.
Chemical mechanical polishing (CMP) is a current method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a “standard” pad or a fixed-abrasive pad. A fixed-abrasive pad has abrasive particles held in a containment media, whereas a standard pad has a durable surface, without embedded abrasive particles. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
An effective CMP process not only provides a high polishing rate, but also provides a substrate surface which is finished and flat. The polishing rate, finish and flatness are determined by the pad and slurry combination, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In applying conventional planarization techniques, such as CMP, it is extremely difficult to achieve a high degree of surface uniformity, particularly across a surface extending from a dense array of features, for example copper lines, bordered by an open field. A dense array of metal features is typically formed in an interlayer dielectric, such as silicon oxide layer, by a damascene technique wherein trenches are initially formed. A barrier layer, such as a Ta-containing layer e.g. Ta, TaN, is then deposited lining the trenches and on the upper surface of the silicon oxide interlayer dielectric. Copper or a copper alloy is then deposited, as by electroplating, electroless plating, physical vapor deposition (PVD) at a temperature of about 50° C. to about 150° C. or chemical vapor deposition (CVD) at a temperature under about 200° C., typically at a thickness of about 8000 Å to about 18,000 Å. In planarizing the wafer surface after copper metallization using CMP, undesirable erosion and dishing typically occur, decreasing the degree of surface uniformity or planarity and challenging the depth of focus limitations of conventional photolithographic techniques, particular with respect to achieving submicron dimensions, such as about 0.25 micron. Erosion is defined as the height differential between the oxide in the open field and the height of the oxide within the dense array. Dishing is defined as a difference in height between the oxide and Cu within the dense array.
Dishing and erosion formation are the most important parameters in evaluating metal CMP processes. Current processes using CMP generate at best 600-800 Å dishing and 400-1500 Å erosion, depending on the pattern density on the substrate. There are generally two causes for dishing formation: a) insufficient planarization and b) over-polish. CMP accomplishes planarization, but the efficiency of the planarization decreases significantly as the feature size increases on the substrate. Over-polish is performed to remove metal residue from a wafer's surface after CMP. Over-polish contributes significantly to dishing and erosion formation, especially when over-polish is done at a relatively high polish rate in order to have high throughput. Past efforts to improve dishing and erosion included modifications to the slurry, polishing pad and the process.
It remains desirable to have a process of planarization where dishing and erosion are decreased.
It is an advantage of the present invention to provide a method and apparatus for substrate planarization producing a good quality substrate surface with high throughput.
SUMMARY OF THE INVENTION
The problems of reducing dishing and erosion while achieving planarized processed substrates are solved by the present invention of an advanced electrolytic polish (AEP) assisted metal wafer planarization method and apparatus.
The advanced electrolytic polish (AEP) method polishes the metal surface of a layered substrate in a controlled way so that higher spots are removed more quickly than lower spots in the surface topography. AEP uses a reverse electroplating method along with surface modifying methods, either alone or in combination, to remove material from a substrate in order to planarize the substrate. A first surface modifying method is to include surface modifying additives in the electrolyte solution used for AEP. A second surface modifying method is pulsed electrolysis with application of alternating positive and negative potentials, in which the potential across the electrodes in AEP is periodically reversed causing high points on the substrate surface to be removed before low points on the substrate surface. Additives and pulsed electrolysis can be combined in an AEP process.
The polish in the present embodiment of the invention stops when a barrier layer (e.g.tantalum, tantalum nitride, titanium or titanium nitride) is substantially removed. Alteration of the electrolyte chemistry of the present invention, however, would alter the depth and the layers removed in the polish. Additives to the electrolyte solution, and the application of pulsed voltage during electrolytic polish further improve the planarity of the substrate polished surface.
In the AEP method herein described, substrates act as anodic electrodes and another metal plate is used as a cathodic electrode. The substrate and the cathode are configured in either an electroplating tank or in an adapted chemical mechanical polishing device. A voltage differential is applied to the anode and cathode under a predetermined anodic dissolution current density. This causes a reaction that provides a planarized surface on the metal wafers. Current electrolytic polishing methods generally leave at least 500 Å difference between high and low spots in the wafer surface topography. With the present advanced electrolytic polish, additives are included in the electrolyte solution which adsorb onto the wafer surface urging a higher removal rate at higher spots and a lower removal rate at lower spots. Also, another embodiment of the present invention is a pulsed-electrolytic process in which positive and negative potentials are applied to the anodic and cathodic electrodes alternately, further encouraging surface planarization. A further embodiment of the invention involves using the additive method and the pulsed method in combination.
AEP can be used either as a first step followed by a short mechanical polishing step (buffing) to remove the thin barrier layer (250 Å); or as a second step between an initial CMP polish with about 2000 Å copper layer remaining and a third step mechanical polish. The present invention may also be added as a last step of copper electroplating process and so may be used in the manufacture of all kinds of patterned metal wafers.
With the AEP technique, the “insufficient planarization” problem that occurs in normal CMP processes is substantially eliminated, the first CMP step is much easier, and dishing
Redeker Fritz
Sun Lizhong
Tsai Stan
Applied Materials Inc.
Gorgos Kathryn
Parsons Thomas H
Perkins Smith & Cohen
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