Plasma based photoresist removal system for cleaning post ash residue

1. Field of the Invention

The present invention relates to a cleaning process used in a semiconductor lithographic manufacturing system and, in particular, to a method for cleaning ash reside on low dielectric constant films after photoresist removal.

2. Description of Related Art

Integrated circuits (ICs) are fabricated on semiconductor wafer substrates by a photolithographic process. The lithographic process allows for a mask pattern of the desired circuit or portion thereof to be transferred via radiant energy of selected wavelengths to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component of the photoresist material, create a latent image in the resist. The latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent processing, the resulting resist film pattern is used as an etch mask to remove underlying substrates from the areas of the patterned openings in the resist layer, or to drive dopants into areas of the substrate not protected by the resist layer.

All the photoresist must be removed before the substrates are subsequently processed. Additionally, any etch-related residues must be thoroughly removed before subsequent processing to avoid embedding impurities in the device. Various methods have been disclosed for cleaning resist residue. For example, U.S. Pat. No. 6,848,455 discloses that resist contaminants are removed from a semiconductor wafer by the in-situ generation of oxidizing species, by the simultaneous application of ultra-violet radiation and chemicals containing oxidants such as hydrogen peroxide and dissolved ozone. Ultrasonic or megasonic agitation is also employed to facilitate resist removal.

Wet cleaning processes have often damaged underlying substrate layers. Plasma based chemically reactive cleaning processes have been used for traditional IC structures that employ SiO 2 as the inter-layer dielectric since they do not result in any damage to the dielectric material. Such processes for stripping photoresist have employed a plasma formed from a mixture of gases with the presence of oxygen in the plasma. The highly reactive oxygen based plasma reacts with and oxidizes the organic photoresist to form volatile components that are carried away from the wafer surface.

It has been found that highly oxidizing conditions are also generally unsuitable for use on low dielectric constant (low-k) materials, i.e., those having a dielectric constant generally below about 2.7 to 3.0. Low-k materials have been used as inter-metal and/or inter-layer dielectrics between conductive interconnects employed to reduce the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric material, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit. Typically, low-k dielectrics are silicon-oxide based materials with some amount of incorporated carbon, commonly referred to as carbon doped oxide (CDO). An example of a CDO is CORAL brand carbon-doped oxides, from Novellus Systems, Inc. of San Jose, Calif. It is believed, although not necessarily proven, that the oxygen scavenges or removes carbon from the low-k materials. In many of these materials such as CDOs, the presence of carbon is instrumental in providing a low dielectric constant. Hence, to the extent that the oxygen removes carbon from these materials, it effectively increases the dielectric constant. As processes used to fabricate integrated circuits move toward smaller and smaller dimensions and requires the use of dielectric materials having lower and lower dielectric constants, it has been found that the conventional strip plasma conditions are not suitable.

Plasma processes that employ NF 3 in combination with He have been used with some success for cleaning post etch residues. However, if the amount of fluorine is not controlled properly, excessive residue may remain (F ratio too low), or there is excessive loss of dielectric (F ratio too high). Excessive loss of dielectric leads to loss of critical dimension (CD), the smallest dimension of a shape, pattern or feature that can be produced by the lithographic manufacturing system.

Hydrogen plasmas or hydrogen-based plasmas with a weak oxidizing agent are effective at stripping photo-resist and removing residues from low-k dielectric layers without the problems associated with conventional strip plasmas. However, these methods require a high hydrogen flow to achieve an acceptable strip rate. Because high hydrogen flow requires costly abatement and pump systems, it is desirable to have hydrogen flow as low as possible while maintaining an acceptable strip rate. In addition, it is desirable to reduce hydrogen flow due to hydrogen's flammability and the dangers associated with handling and abating it.

Others have reported using hydrogen-based plasmas with inert gases such as hydrogen and helium introduced with hydrogen at the plasma source. Han et al. U.S. Pat. Nos. 6,281,135 and 6,638,875 describe using a mixture of hydrogen, helium and fluorine and Zhao et al. U.S. Pat. Nos. 5,660,682 and 6,204,192 describe using a mixture of hydrogen and argon. However, helium or argon ions in the plasma have harmful effects. Mixtures of hydrogen and helium result in high plasma damage on low-k materials due to the long life of ionized helium plasma. Ionized argon causes unwanted sputtering of the quartz material in the plasma tube (the portion of some reactors where the plasma is formed). Introduction of argon to hydrogen plasmas has also been shown to reduce strip rate.

U.S. application Ser. No. 11/011,273 owned by the assignee of the instant invention discloses an improved method and an apparatus for stripping photoresist and removing etch-related residues from dielectric materials. After generating a plasma from a hydrogen gas, optionally containing a weak oxidizing agent such as carbon dioxide, an inert gas is introduced to the plasma downstream of the plasma source and upstream of a showerhead that directs gas into the reaction chamber. The inert gas mixes with the plasma, reducing the required hydrogen flow rate and improving the low-k dielectric strip rate and strip rate uniformity.

Despite the improvements of the latter in the ashing or stripping of the low-k photoresist materials, it has been found that some high silicon content residue may still remain after such hydrogen plasma cleaning.

Consequently, a need exists in the art for the development of an alternative or additional stripping and cleaning process that effectively remove post-ashing reside and that does not remove excessive amounts of the low-k dielectric materials or otherwise affect or materially alter the properties of low-k dielectric materials.

Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved method of cleaning resist from wafer substrates in a lithographic process.

It is another object of the present invention to provide a method of removing resist residue remaining after stripping of a resist layer from a dielectric layer.

A further object of the invention is to provide a method of removing resist residue without damaging an underlying low-k dielectric layer.

It is yet another object of the present invention to provide a method of removing resist residue without affecting critical dimension features etched into an underlying low-k dielectric layer.

Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.

The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention which is directed to a method of removing resist reside from a workpiece comprising providing a substrate having thereover a residue to be removed, introducing over the substrate and residue an ionizable gas consisting essentially of at least one noble gas, applying energy to the noble gas to create a plasma of the noble gas, and contacting the residue with the noble gas plasma to remove the residue without substantially harming the underlying substrate.

The preferred noble gas comprises helium. The method is particularly useful where the residue is resist residue, comprising silicon, and the substrate is a dielectric film, such as a low-k dielectric film having a dielectric value less than about 3.0.

In another aspect, the present invention is directed to a method of cleaning a low dielectric constant film in a lithographic process comprising providing a dielectric film having thereover a resist composition, the dielectric film having a dielectric constant no greater than about 4.0, and stripping the resist composition to leave a substantially silicon-containing ash residue on the dielectric film. The method then includes contacting the ash residue with plasma comprising an ionized, essentially pure noble gas to remove the resist residue without substantially affecting the underlying dielectric film.

Yet another aspect of the present invention is directed to a method of cleaning a low dielectric constant film in a lithographic process comprising providing a dielectric film having thereover a resist composition, the dielectric film having a dielectric constant no greater than about 4.0 and hydrogen plasma stripping the resist composition to leave on the substrate residue of the resist composition. The method then includes removing the hydrogen and introducing over the substrate and residue an ionizable gas consisting essentially of at least one noble gas, applying energy to the noble gas to create a plasma of the noble gas, and contacting the residue with the noble gas plasma to remove the residue without substantially harming the underlying substrate.

The aforementioned methods of noble gas plasma removal of the residue may take place at a temperature between about 20 and 450° C.

The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:

FIG. 1 is a cross-sectional elevational view of layers deposited on a wafer substrate, with a patterned photoresist layer over a low-k dielectric layer to be etched.

FIG. 2 is a cross-sectional elevational view of the wafer substrate layers of FIG. 1 after the low-k dielectric layer has been etched and the photoresist layer has been stripped, showing the residue layer. The residue may consist of sparsely dispersed patches of string-like material.

FIG. 3 is a cross-sectional elevational view of the wafer substrate layers of FIG. 2 after the residue layer has been cleaned by the noble plasma process gas method of the present invention, without damage to the low-k dielectric layer.

In describing the preferred embodiment of the present invention, reference will be made herein to FIGS. 1-3 of the drawings in which like numerals refer to like features of the invention.

While the method of the present invention may be used to efficiently and effectively to remove materials from low-k dielectric films, it is not limited to low-k dielectric films, or even to dielectrics. The invention is also not limited to any particular category of low-k dielectrics. For instance, the present invention may be effectively used with dielectrics with k values less than 4.0 (also known as first generation low-k dielectrics), dielectrics with k values less than about 2.8 (second generation low-k dielectrics) and dielectrics with k values less than about 2.0 (ultra-low-k dielectrics). The low-k dielectric may be porous or non-porous (the latter sometimes referred to as a dense low-k dielectric). Generally, low-k dense dielectrics are those having k values no greater than 2.8 and low-k porous dielectrics are those having k values no greater than 2.2. Low-k dielectrics of any suitable composition may be used, including silicon oxide based dielectrics doped with fluorine and/or carbon. Non-silicon oxide based dielectrics, such as polymeric materials, may also be used. Any suitable process may be used to deposit the low-k dielectric, including as spin-on deposit and CVD deposit techniques. In the case of forming porous dielectrics, any suitable method may be used. A typical method involves co-depositing a silicon-based backbone and an organic porogen and subsequently removing the porogen component, leaving a porous dielectric film. Other methods include sol-gel techniques. Specific examples of suitable low-k films are carbon based spin-on type films sold under the trademark SiLK by Dow Chemicals, Inc. and CVD deposited porous films sold under the trademark CORAL by Novellus Systems, Inc.

The method of the present invention is most advantageously used to remove resist residue remaining after ashing by the method of U.S. application Ser. No. 11/011,273, the disclosure of which is hereby incorporated by reference.

In general, reactive plasma cleaning is performed in situ in a plasma reactor in which the processing chamber promotes excitation and/or disassociation of the reactant gases by the application of RF energy with capacitively coupled electrodes disposed in the processing chamber. The plasma typically creates a highly reactive species that reacts with and etches away the unwanted deposition material present in the processing chamber.

The plasma reactor apparatus that may be used in practicing the present invention includes a vacuum pump for creating a vacuum in the process chamber. The apparatus of the invention also includes a process gas inlet assembly such as a pressurized gas cylinder coupled to an inlet conduit connected to a gas distribution faceplate or showerhead in the process chamber. The semiconductor wafer substrate or other workpiece rests on a pedestal or platen, which may apply a bias to the substrate. An RF or other power supply applies electrical power between the gas distribution faceplate or showerhead and the pedestal to excite the process gas or mixture of gasses to form a plasma within the cylindrical reaction region between the faceplate and pedestal.

The ionizable process gas used in the present invention is preferably helium, although other ionizable noble gases or mixture of noble gases may be used. Such other gases include neon, argon, krypton, xenon or radon, or mixtures thereof. The noble gas is essentially pure, preferably with a purity of at least about 99.99% by weight, and contains essentially no oxidizers or other reactive agents. During operation, the plasma process gas moves from one side of the vacuum chamber to the other side due to the vacuum generated on the side of the vacuum chamber opposite the side from which the ionizable process gas flows into the chamber. The plasma process gas diffuses across the surface of the wafer substrate removing unwanted resist residue and carrying the material towards the vacuum pump assembly. It may be desirable to keep the wafer temperature within a desired temperature range, such as about 20 to 450° C., and a heating or cooling element may be supplied in the process chamber for such purpose.

As shown in FIG. 1 , a wafer 20 comprises substrate layers 26 , 28 , which may various types of polysilicon, aluminum, copper or other typical films, over which is deposited a low-k dielectric layer 24 . A resist layer 22 over the dielectric layer is exposed to a device pattern and developed to remove a volume of resist material 30 corresponding to the pattern. The remaining resist layer 22 is then used as a mask to etch a corresponding volume of material 24 ′ from the dielectric layer 24 , for example, by reactive ion etching (RIE). The wafer may then be subject to an ashing process to strip and remove the resist layer, for example by transfer to a plasma reactor and hydrogen plasma stripping of the low-k dielectric film as in the aforementioned U.S. application Ser. No. 11/011,273. FIG. 2 shows the via or trench 32 etched into dielectric layer 24 . After stripping, the wafer 20 may still contain post-ash reside comprising a layer of silicon 22 ′ remaining from the resist layer composition. The wafer is then subject to the plasma cleaning method of the present invention using an essentially pure noble process gas. The noble gas plasma processing may be conducted in the same reactor used for the hydrogen plasma ashing process. Subsequently, as shown in FIG. 3 , the surface 24 a of the resist layer is substantially free of the post-ash silicon residue, and the dimensions of the etched via or trench 32 in the dielectric layer are unaffected.

The following example is intended as further illustration of the invention but is not necessarily limitative except as set forth in the claims. All parts and percentages are by weight unless otherwise indicated.

A batch Gamma 2130™ plasma reactor made by Novellus Systems can be used to ash a low-k dielectric film and clean it using the method of the present invention. The plasma reactor had six stations within the process chamber. A semiconductor wafer having a photoresist film over a dielectric film having a dielectric constant of 2.24 to 2.26 (according to Hg probe measurement of blanket film) was placed on the platen and subjected to the hydrogen plasma cleaning process of U.S. application Ser. No. 11/011,273 in the first five stations to remove the photoresist layer.

After the fifth station, a thin photoresist reside layer substantially comprising silicon remained on the low-k dielectric layer. In the sixth station, essentially pure helium 99.99% purity) was introduced over the low-k dielectric layer in an amount of about 2000 sccm. An RF power supply set at 500 W applied high frequency voltage between the electrode and the wafer pedestal, with a bias of about 13.56 mHz applied to the wafer, to ionize the helium gas. The plasma sputtering continued at about 70° C. for about 120 seconds, after which the process was stopped and the wafer with the low-k film was removed from the reactor.

Subsequent scanning electron microscopy revealed that essentially all of the residue was removed from the low-k dielectric film surface, without incurring any substantial damage to the dielectric film properties. Additionally, the critical or other dimensions of the via, trench or other features etched into the dielectric layer are unaffected.

The noble gas flow rate, RF power setting, time of exposure and other parameters may be adjusted to achieve desired results for other cleaning tasks.

Thus, the present invention provides an improved method of cleaning resist from wafer substrates in a lithographic process, particularly when removing resist residue remaining after stripping of a resist layer from a dielectric layer. The present invention eliminates the need for fluorine compounds, such as NF 3 or CF 4 , in the plasma gas, with improved control over the amount of photoresist and residue removal and without damaging the underlying low-k dielectric substrate.

While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.