Semiconductor device manufacturing: process – Chemical etching – Combined with the removal of material by nonchemical means
Reexamination Certificate
2000-08-01
2001-12-25
Le, Hoa Van (Department: 1752)
Semiconductor device manufacturing: process
Chemical etching
Combined with the removal of material by nonchemical means
C438S704000, C438S715000, C438S746000, C216S063000, C216S073000
Reexamination Certificate
active
06333268
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to methods and apparatus for removing adherent layers from substrates, and more particularly to methods and apparatus for removing residues from semiconductor wafers, including post-etch, post-ash, and other post-process residues.
The fabrication of integrated circuits and other devices on semiconductor wafers depends on the photolithographic patterning of successive layers of materials applied on and into the wafer. In each photolithographic step, a layer of photoresist is applied to the wafer, soft baked, and patterned by exposure to radiation through a precisely aligned mask. Such exposure alters the solubility of the resist material in a particular solvent, thus allowing selective removal of the resist in accordance with the pattern defined by the mask. After the subsequent fabrication steps are completed, it becomes necessary to remove the remaining insoluble photoresist to permit further fabrication. In some cases, the photoresist may be rendered insoluble to common solvents by the fabrication step which has been performed. For example, ion implantation, radiation accompanying plasma etching, and any other process raising the wafer temperature above 150° C. to 200° C. for a significant period of time, will frequently cause the photoresist material to become heavily cross-linked, making it particularly difficult to remove.
During photolithography, a photoresist mask is formed on a semiconductor substrate. A hard, cross-linked polymeric crust is typically formed at the top of the photoresist and residues are formed along the walls of any trenches or vias during subsequent processing. For instance, impurities, such as P, B, and As, may be bonded chemically with the photoresist polymer during ion implantation, and the surface layer of the photoresist mask will change in quality and become a very hard layer known as a carbonized layer. The photoresist polymer inside the photoresist mask beyond the reach of impurities will remain as an unchanged layer.
In addition to cross-linking, contamination of a photoresist layer during a wafer fabrication step can also reduce its solubility. For example, when photoresist is used for patterning a silicon dioxide or aluminum layer during plasma etching, the organic photoresist material may become contaminated by silicon, aluminum, or other inorganic materials. Such contaminated photoresists are frequently refractory to normal solvent removal.
Some techniques of removing refractory photoresists involve thermal and photochemical oxidation of the photoresists. Such oxidation typically requires elevated temperatures, which can cause undesired diffusion within the wafer, and can cause sputtering of the metallic components made of, e.g., aluminum and titanium nitride. Oxidation of such metallic components will form metal oxides such as aluminum or titanium oxides, which are ceramic residues that are extremely difficult to remove. Others employ low temperature “ashing” in an oxygen plasma, which has the disadvantage that the plasma discharge required can result in damage to the wafer substrate. Wet oxidative stripping of insoluble photoresists has also been used. Such wet stripping techniques, however, often require temperatures above 150° C. to be effective.
In U.S. Pat. No. 5,201,960 to Starov, which is incorporated herein by reference in its entirety, a densified fluid cleaning method for removing adherent matrices involves exposing the matrix to a vapor phase solvent to allow the solvent to penetrate the matrix and then condensing the vapor to physically disrupt the adherent matrix to promote fragmentation of the matrix and facilitate removal. The solvent typically used is ammonia. While this dry cleaning method has the advantages of lower cost, improved safety, and reduced environmental impact, it is sometimes not effective in completely removing post-etch or other post-process residues. For instance, the interaction between ammonia and the photoresist can be quite slow, making the removal of thick photoresist layers difficult. In cases where there is a stubborn polymeric crust on top of the photoresist, the stresses produced during interaction with ammonia may cause curling or rolling of the polymeric crust and the underlying photoresist, resulting in macroscopic flakes over the substrate surface. Thus, there is a need for more effective methods and apparatus for removing adherent matrix layers such as post-etch or other post-process residues.
SUMMARY OF THE INVENTION
The present invention is useful for removing a wide variety of adherent matrix layers which have been deposited over substrates, including both organic and inorganic matrix layers, and is particularly useful for removing cross-linked organic polymers which have limited solubility in, but are penetrable by, certain chemical and/or physical solvents. The present invention will find its greatest use in removing highly cross-linked photoresist present over semiconductor wafers, where removal has heretofore generally required either high temperature oxidation, plasma ashing, or combinations of both. The present invention can achieve removal of even highly refractory cross-linked photoresists, such as photoresists that have been hardened by exposure to certain wafer fabrication processes, at relatively low temperatures without exposure of the wafer substrate to potentially damaging radiation.
The present invention is particularly useful as a part of otherwise conventional photolithographic processes for transferring patterns from a mask containing circuit-design information to the surface of a silicon wafer. In such processes, the wafer is first coated with a substantially continuous layer of photoresist which is then soft-baked to remove residual solvents, promote adhesion, and harden the resist. The resist is then exposed using a radiation source, usually a light source or electron beam source, through the preformed mask or by projection printing to selectively alter the characteristics of well-defined portions of the photoresist. In the case of negative photoresists, an activator within the organic polymer matrix absorbs the radiation which in turn promotes cross-linking of the polymer. The cross-linked areas inhibit solubilization in the subsequent development step. In the case of positive photoresists, an inhibitor is present that prevents wetting and attack by the developer. Exposure to the radiation degrades the inhibitor, thus allowing the exposed regions to be removed upon subsequent exposure to the developer. After development, i.e., removal of the more soluble regions of the photoresist layer to expose the wafer thereunder, the photoresist is usually further baked to improve surface adhesion, increase the strength of the matrix, and drive off volatiles that have been retained from the developer. The patterned photoresist is then ready for use in a subsequent wafer fabrication operation, such as plasma etching, wet etching, ion implantation, sputtering, electroplating, and the like. Many of these processes will result in further cross-linking or otherwise hardening of the remaining photoresist layer, rendering the layer refractory to many removal techniques. The present invention is advantageously used as a stripping process for removing the residual photoresist material after the desired wafer fabrication step has been completed.
Specific embodiments of the present invention provide more effective methods and apparatus for removing adherent matrix layers such as post-etch residues. Some embodiments of the invention employ a plasma pretreatment to remove the bulk photoresist as well as much of any polymeric crust that is present from the substrate surface, followed by a densified fluid cleaning step to substantially remove completely the remaining residues. The plasma pretreatment modifies the photoresist and residues in a way that facilitates their complete removal by the subsequent densified fluid cleaning step. The plasma pretreatment does so by chemically reacting the plasma species with the photoresi
Basha Syed S.
Erez Shmuel
Kabansky Aleksandr
Reinhardt Karen A.
Shimanovich Arkadiy I.
Le Hoa Van
Novellus Systems Inc.
Tso Roland
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