Use of a capping layer to reduce particle evolution during sputter pre-clean procedures

[0002] (1) Field of the Invention

[0003] The present invention relates to methods used to fabricate semiconductor devices and more specifically to a method used to pre-clean a surface prior to metal deposition.

[0004] (2) Description of Prior Art

[0005] The use of micro-miniaturization, or the ability to fabricate semiconductor devices with sub-micron features, has allowed the performance of the sub-micron semiconductor device to be increased, while the fabrication cost for semiconductor chips comprised with sub-micron semiconductor devices, has decreased. The use of sub-micron features have allowed performance degrading, parasitic capacitances to be reduced, in addition to allowing a greater number of smaller semiconductor chips to be obtained from a specific size starting substrate, thus reducing the fabrication cost for a specific semiconductor chip. The smaller semiconductor chips, comprised with sub-micron features, still provide device densities equal to, or greater than, counterpart semiconductor chips, comprised with larger features.

[0006] One example of the use of sub-micron features is formation of sub-micron contact holes, or openings, in specific dielectric layers, to expose a portion of an underlying metal structure. The use of micro-miniaturization have allowed contact holes, or via openings, with diameters less then 0.25 um, to routinely be formed. However the small contact area now presented by the sub-micron opening demands a clean, or oxide free metal surface to be exposed at the bottom of the sub-micron via opening to minimize resistance at this interface when overlaid with a subsequent, overlying metal plug structure. However several processes performed prior to metal deposition, can result in unwanted material residing on the exposed metal surface. The removal of the photoresist shape used to define the sub-micron contact hole, or via opening, results in the formation of a native oxide layer on the metal structure, exposed at the bottom of the sub-micron opening. In addition the insulator layer in which the sub-micron opening was defined in, when subjected to the plasma pre-clean procedure, used to remove native oxide from the exposed metal surface, can flake or be disturbed, re-depositing on metal surface as well as interfering with the native oxide removal procedure.

[0007] This invention will teach a procedure in which a metal pre-clean can be performed via argon sputtering, resulting in removal of native oxide from the surface of the metal structure exposed at the bottom of the sub-micron opening in a composite insulator layer, in addition to avoiding flaking or particle generation of components of the composite insulator layer exposed during the pre-clean procedure. This is accomplished via use of a disposable capping insulator layer, used to protect the composite insulator layers in which the sub-micron opening is defined in, from thinning, as well as from flaking or contaminating the exposed metal surface, during the metal pre-clean procedure. Prior art, such as Lai et al, in U.S. Pat. No. 6,136,680, describe a procedure for preparing a metal surface for acceptance of an overlying metal layer, however that prior art does not describe the novel, disposable capping insulator layer, used in the present invention, which allows preparation of a metal surface, exposed in a sub-micron opening, to be successfully performed.

[0008] It is an object of this invention to define an opening with a sub-micron diameter, in a composite insulator layer, to expose a portion of a top surface of a metal structure.

[0009] It is another object of this invention to employ a capping insulator layer, as a top layer of the composite insulator layer, to protect underlying components of the composite insulator layer from a metal pre-clean procedure performed in a plasma.

[0010] It is still another object of this invention to consume the capping insulator layer during the plasma metal pre-clean procedure.

[0011] In accordance with the present invention a method of plasma cleaning a metal structure, located at the bottom of a sub-micron diameter opening in an composite insulator layer, and featuring overlying, disposable capping insulator layer as a component of the composite insulator layer, is described. After creation of an underlying metal structure a composite insulator layer comprised of: a thin, underlying, silicon rich oxide (SRO), layer; a fluorinated silica glass (FSG), layer; a plasma enhanced oxide (PE-OX), layer; a silicon oxynitride, bottom anti-reflective coating, (BARC); and a thin, overlying, disposable capping oxide layer; is deposited. A photoresist shape is used as a mask to allow a sub-micron diameter opening to be defined in the composite insulator layer, exposing a portion of the top surface of the metal structure. After removal of the defining photoresist shape, an argon sputtering procedure is performed in a metal pre-clean chamber, removing native oxide from the top surface of the exposed metal structure, in addition to removing the capping insulator layer, which protected the underlying SiON BARC component of the composite insulator layer from the argon sputtering procedure, preventing SiON flaking and particles from contaminating the exposed surface of the plasma cleaned, metal structure. In situ deposition of a metal layer, is then accomplished filling the sub-micron diameter opening, resulting in an oxide free, and particle free interface between the filling metal layer and the top surface of the underlying plasma cleaned, metal structure.

[0012] The object and other advantages of this invention are best described in the preferred embodiment with reference to the attached drawings that include:

[0013] FIGS. 1-4, which schematically, in cross-sectional style, describe key stages of fabrication used to plasma clean a metal structure located at the bottom of a sub-micron diameter, contact hole opening, wherein the sub-micron opening is defined in a composite insulator layer, which in turn is comprised with an overlying, disposable capping insulator layer, used to prevent flaking of, and to protect, other components of the composite insulator layer during the plasma, metal pre-clean procedure.

[0014] The method for forming a sub-micron diameter opening in a composite insulator layer, and for preparing a metal surface, located at the bottom of the sub-micron diameter opening, prior to filling with a metal structure, will now be described in detail. A metal structure 1, comprised of a metal such as aluminum, aluminum—copper, copper, tungsten, or a metal silicide, is defined and schematically shown in FIG. 1. Insulator layer 2, comprised of a silicon rich oxide (SRO), layer, at a thickness between about 200 to 400 Angstroms, is deposited via plasma enhanced chemical vapor deposition (PECVD), procedures. To improve semiconductor device performance, a low dielectric constant (low k), layer 3, such as a fluorinated silica glass (FSG), layer, is deposited via chemical vapor deposition (CVD) procedures, to a thickness between about 5000 to 6000 Angstroms. FSG layer 3, with a dielectric constant between about 3.0 to 4.0, will present lower capacitance, and thus increased performance, when compared to counterpart insulator layers comprised with higher dielectric constant values. Silicon oxide layer 4, is then deposited again via PECVD procedures, to a thickness between about 1500 to 2500 Angstroms, followed by deposition of silicon oxynitride (SiON), layer 5, at a thickness between about 500 to 700 Angstroms, via CVD procedures. Silicon oxynitride layer 5, will serve as a bottom anti-reflective coating (BARC), layer, allowing increased acuity of a sub-micron feature, to be exposed and developed in an overlying photoresist shape, to be realized. The result of these depositions are schematically shown in FIG. 1.

[0015] If a sub-micron diameter opening were now formed in the composite insulator comprised of silicon oxynitride layer 5, silicon oxide layer 4, FSG layer 3, and SRO layer 2, via photolithographic and anisotropic, reactive ion etching (RIE), procedures, the removal of the defining photoresist shape would be accomplished using plasma oxygen ashing. The ashing procedure however results in the growth of native oxide on the portion of metal structure 1, exposed in the sub-micron diameter opening. The presence of the native oxide becomes more critical as the diameter of the opening decreases, due to the potential of increased interface resistance resulting from a decreased contact area available for an overlying metal plug structure. Therefore after ashing of the photoresist shape, and prior to metal deposition used to fill the sub-micron diameter opening, an in situ pre-clean procedure, such as an argon sputtering procedure, is used. However in addition to the positive result of removing native oxide from the surface of metal structure 1, the argon sputtering procedure can attack the exposed surfaces of silicon oxynitride layer 5, resulting in flaking or creation of silicon oxynitride particles, landing on the prepared surface of metal structure 1, thus negatively influencing the ability to minimize interface resistance. In addition the generation of silicon oxynitride particles can result in contamination of the pre-clean chamber, perhaps resulting in unwanted contamination and yield loss for products next used in the contaminated pre-clean tool. Therefore to protect the needed BARC or silicon oxynitride layer 5, from the argon sputter, pre-clean procedure, capping layer 6, comprised of a PECVD silicon oxide layer, is deposited at a thickness between about 200 to 300 Angstroms. The thickness of capping layer 6, shown schematically in FIG. 1, is chosen so that consumption of this layer can result from exposure to the argon sputtering, pre-clean procedure. Photoresist shape 7, with opening 8a, comprised with a diameter between about 0.21 to 0.27 um, is next formed, exposing a portion of the top surface of capping layer 6. This is schematically shown in FIG. 1.

[0016] An anisotropic RIE procedure, using CHF₃or CF₄as an etchant for capping layer 6, for silicon oxynitride layer 5, for silicon oxide layer 4, for FSG layer 3, and for SRO layer 2, is used to transfer opening 8a, in photoresist shape 7, to the composite insulator layer, in terms of sub-micron diameter opening 8b, featuring a diameter between about 0.19 to 0.25 um, exposing a portion of the top surface of metal structure 1. Removal of photoresist shape 7, via plasma oxygen ashing procedure, however results in native oxide layer 20, now residing at surface 9a, of metal structure 1. This is schematically shown in FIG. 2.

[0017] A critical pre-clean procedure, accomplished via an argon sputtering procedure, is next performed in situ, with a subsequent metal deposition, either in the same chamber to be used for the metal deposition, or in another chamber, however in the same tool to be used for metal deposition, thus avoiding exposure to the environment after the pre-clean procedure. The metal pre-clean procedure is performed in an argon plasma, at a power between about 200 to 400 watts, at a pressure between about 0.5 to 1.5 mtorr, and with a negative DC bias applied to metal structure 1. The bias applied, between about −200 to −300 volts, allows the positively charged argon ions to be accelerated to the surface of the negatively biased metal structure 1, removing via collisions of accelerated argon ions, native oxide layer 20, resulting in oxide free, metal surface 9b, shown schematically in FIG. 3. In addition the capping layer 6, used to protect silicon oxynitride layer 5, during the argon sputtering, pre-clean procedure, is slowly consumed during this same argon sputtering, pre-clean procedure, resulting in an absence of capping layer 6, at the conclusion of the pre-clean procedure. Thus the thickness chosen for capping layer 6, allowed protection of underlying components of the composite insulator layer to be realized, while the ultimate consumption of this layer, experienced at the conclusion of the pre-clean procedure, allowed the capacitance of the composite insulator layer to be reduced.

[0018] The deposition of a metal layer is next performed either in situ, in the same chamber just used for the argon sputter, pre-clean procedure, or in another chamber, but still in the same tool, thus avoiding exposure oxide free metal surface 9b, to the environment. A metal layer chosen from a group that includes aluminum, aluminum—copper, copper, tungsten, or a metal silicide, is obtained via plasma vapor deposition, or via chemical vapor deposition procedures, at a thickness between about 3000 to 5000 Angstroms, completely filling sub-micron diameter opening 8b. Removal of unwanted portions of the metal layer from the top surface of silicon oxynitride layer 5, is accomplished via chemical mechanical polishing procedures, or via a selective RIE procedure, using Cl₂as an etchant for the metal layer. The result of these procedures, metal deposition and removal, result in the definition of metal plug structure 10, in sub-micron diameter opening 8b. This is schematically shown in FIG. 4. The interface resistance between metal plug structure 10, and metal structure 1, is minimized as a result of the pre-clean procedure used to remove native oxide from the surface of metal structure 1, exposed in sub-micron diameter opening 8b. If desired instead of forming metal plug structure 10, an upper level metal structure can be defined, comprised of an underlying metal plug component, located in sub-micron diameter opening 8b, and an overlying metal shape component. This configuration can be realized via the same metal deposition procedure, however using conventional photolithographic and RIE procedures to define the metal shape component of the upper level metal structure. In addition, if desired a titanium—titanium nitride composite layer can be deposited prior to the metal deposition, with the titanium component serving as an adhesion layer, while the titanium nitride layers provides barrier characteristics. This composite layer would be deposited via plasma vapor deposition procedures, in situ after the argon sputtering pre-clean procedure.

[0019] While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.