Patent Description:
The present invention relates generally to superconductors, and more particularly to a methodology for forming superconductor structure.

Superconducting circuits are one of the leading technologies proposed for quantum computing and cryptography applications that are expected to provide significant enhancements to national security applications where communication signal integrity or computing power are needed. They are operated at temperatures <<NUM> Kelvin. Efforts on fabrication of superconducting devices have mostly been confined to university or government research labs, with little published on the mass producing of superconducting devices. Therefore, many of the methods used to fabricate superconducting devices in these laboratories utilize processes or equipment incapable of rapid, consistent fabrication. Recently there has been a movement to mass producing superconducting circuits utilizing similar techniques as those utilized in conventional semiconductor processes.

One well-known semiconductor process is the formation of contacts and conductive lines in a multi-level interconnect stack to couple devices to one another over different layers of an integrated circuit. One such fabrication process for formation of conductive contacts and lines is known as a (single or dual) damascene process. This technique has recently been attempted in the formation of superconducting circuits. During the fabrication of (single or dual) damascene superconducting circuits, as known from <CIT> or <CIT>, for example, via/trench structures are patterned, etched, filled with metal (e.g., niobium, tantalum, aluminum), then polished back using a chemical mechanical polishing (CMP) process. The next level dielectric is then deposited, and the sequence begins again, building up a multi-level interconnect stack. The CMP process and any exposure to oxygen prior to deposition of the next dielectric layer can result in oxidization of the conductive contacts and lines, and dielectric layers which degrades performance.

<CIT> also discloses several techniques to fabricate superconductive circuits where a niobium/aluminum oxide/niobium trilayer and individual Josephson Junctions (JJs) may be formed. A protective cap may be provided to protect a JJ during fabrication. Depositing of superconducting metal layer may be stopped or paused to allow cooling before completion. In addition, multiple layers may be aligned by patterning an alignment marker in a superconducting metal layer. Furthermore, a method for forming a superconducting via is taught which comprises forming a superconducting protective capping layer, particularly of TiN or NbTiN, over a first superconducting metal layer, particularly of Nb, depositing a dielectric layer thereon, etching a hole through the dielectric layer, and filling the hole with a second superconducting metal layer.

<CIT> also generally discloses an integrated circuit of electrically conductive interconnects formed of a superconducting material. In this manner, the electrically conductive interconnects can be made very small, and yet still have adequate conductivity. In various embodiments, all of the electrically conductive interconnects are formed of the superconducting material. In some embodiments, the electrically conductive interconnects are formed of a variety of different superconducting materials. In one embodiment, only the backend electrically conductive interconnects are formed of the superconducting material. In some embodiments no vias are formed of the superconducting material.

<CIT> discloses that by performing a wet chemical process after etching a via, contaminations may be removed and a thin passivation layer may be formed that may then be readily removed in a subsequent sputter etch process for forming a barrier/adhesion layer. In a particular embodiment, the wet chemical process may be performed on the basis of fluoric acid and triazole or a compound thereof.

<CIT> discloses a method of stabilizing a fluorosilicate glass (FSG) used as an inter-level dielectric layer that includes via or holes formed there through for contacting an aluminum or copper metallization. The method includes subjecting the FSG layer to a plasma of nitrogen and hydrogen and then to an argon plasma. The nitrogen plasma creates a nitrogen-rich surface, which reacts with the after deposited titanium to form a thin stable TiN layer. The hydrogen plasma removes free fluorine in the FSG which can create problems with peeling. The hydrogen/nitrogen may be supplied in the form of forming gas that contains less than <NUM> % hydrogen. The subsequent argon plasma cleans the surface, including removing a nitrided aluminum surface formed by the nitrogen plasma from an aluminum metallization. D4 discloses that this process may also be applied to a copper dual-damascene metallization. After the two plasma treatments, a liner/barrier layer of Ti/TiN for Al and Ta/TaN for Cu, is coated on the walls of the hole over the top of the FSG layer. Finally a metal (e.g., copper) is deposited in the hole to fill the hole.

In one aspect of the present invention, a method of forming a superconductor structure according to independent claim <NUM> is provided. The method comprises forming a superconducting element in a first dielectric layer, the superconducting element having a top surface aligned with a top surface of the first dielectric layer, forming a second dielectric layer over the first dielectric layer and the superconducting element, and forming an opening in the second dielectric layer to the top surface of the superconducting element. The method also comprises performing a cleaning process on the top surface of the superconducting element to remove oxides formed on the top surface of the superconducting element and on a surface of the second dielectric layer at a first processing stage, forming a niobium nitride protective barrier solely over the cleaned top surface of the superconducting element at the first processing stage, and moving the superconductor structure to a second processing stage for further processing. The method further comprises performing an argon sputter clean to remove the niobium nitride protective barrier.

In a second aspect of the present invention, another method of forming a superconductor structure according claim <NUM> is provided. The method comprises forming a first opening in a first dielectric layer overlying a substrate, performing an etch to remove oxides from the first dielectric layer caused by the forming of the first opening, forming a first superconductive line in the first opening, the first superconductive line having a top surface aligned with a top surface of the first dielectric layer, and performing a first cleaning process on the top surface of the first superconductive line and the top surface of the first dielectric layer to remove oxides from the top surface of the first superconductive line and the top surface of the first dielectric layer. The method also comprises forming a second dielectric layer over the first dielectric layer and the first superconductive line, forming a via opening in the second dielectric layer to the top surface of the superconductive line, and a trench opening surrounding the via opening and partially extending into the second dielectric layer, and performing a second cleaning process on the top surface of the first superconductive line and on the second dielectric layer including a surface of the opening to remove oxides formed on the top surface of the first superconductive line and the remaining portions of the superconductor structure at a first processing stage. The method further comprises forming a niobium nitride protective barrier solely over the cleaned top surface of the first superconductive line at the first processing stage to mitigate oxide formation over the top surface of the superconductive line, moving the superconductor structure to a second processing stage for further processing, performing an argon sputter clean to remove the niobium nitride protective barrier, and forming a contact in the via opening and a second superconductive line in the trench opening. Further advantageous features are set out in the dependent claims.

The present invention is directed to a method for forming superconducting elements (e.g., conductive lines, contacts, microstrips, coplanar waveguides, stripline transmission lines, filter designs) in superconductor structures. The method incorporates a preclean process to remove oxide layers from superconducting metal elements followed by formation of a protective barrier over the superconducting metal elements when moved to subsequent processing stages to protect the elements from oxides. The oxides can be as a result of a chemical mechanical process (CMP), and/ or as a result of the exposure of the superconductor interconnect structure to oxygen outside of a vacuum environment. In one example, the method integrates the preclean process and protective barrier formation into a dual damascene process for scaling into a high density multilevel interconnect submicron technology. The method can employ a tetrafluoromethane (CF<NUM>) (fluorine) based plasma clean etch process and a nitridation formation process prior to dielectric deposition of a next layer in the dual damascene process to assure a smooth clean surface of the metal interconnect elements on the underlying layer when moving between process locations.

The process enhanced method reduces the RF losses associated with the interfaces surrounding the signal line. RF losses cause signal degradation due to the dissipation factor of the surrounding materials. The materials can deplete the energy of the signal line due to the bulk dielectric or interfaces between the signal line and dielectric material. One of the major sources for signal loss is the unintended formation of dielectric oxides, as well as metal oxides (e.g., niobium oxide) created during chemical mechanical polishing (CMP) processes and photoresist strip. In one example employing niobium as a superconducting metal for forming the superconducting elements, the methodology removes these unintended oxides and create niobium nitride layers which inhibit oxidation of niobium.

<FIG> illustrates a cross-sectional view of a superconductor interconnect structure <NUM>. The superconductor interconnect structure <NUM> includes a substrate <NUM> that can be formed of silicon, glass or other substrate material. A first dielectric layer <NUM> overlies the substrate <NUM>, and a second dielectric layer <NUM> overlies the first dielectric layer <NUM>. Both the first and the second dielectric layers <NUM> and <NUM> can be formed of a low temperature dielectric material that can be formed at low temperatures (e.g., less than or equal to <NUM> degrees Celsius) typically utilized in the formation of superconducting devices.

A first superconductive line <NUM> and a second superconductive line <NUM> are embedded in the first dielectric layer <NUM>. A superconductive contact <NUM> extends from the first superconductive line <NUM> at a first end to a third superconductive line <NUM> in the second dielectric layer <NUM>. A fourth superconductive line <NUM> is disposed in the second dielectric layer <NUM> above and isolated from the second conductive line <NUM> in the first dielectric layer <NUM>. Each of the superconductive contact and superconductive lines are formed of a superconducting material, such as niobium. A first protective barrier <NUM> overlies a top surface of the third superconductive line <NUM> and a second protective barrier <NUM> overlies a top surface of the fourth superconductive line <NUM>. The first and second protective barriers <NUM> and <NUM> can be formed from, for example, niobium nitride. The first and second protective barriers <NUM> and <NUM> protect the top surfaces of the third and fourth superconductive lines <NUM> and <NUM> from oxidation, which effects superconducting performance, when being transported between process stages and/or chambers for further processing.

Turning now to <FIG>, fabrication is discussed in connection with formation of interconnects in the superconducting device of <FIG>. The present example will be illustrated with respect to two single damascene conductive lines etched into a dielectric thin film to form bottom conductive lines followed by a dual damascene process to form top conductive lines. The methodology demonstrates the techniques used to reduce the RF and microwave losses of the dielectric material inherent to oxides of a superconductor interconnect such as niobium as applied to the design of microstrip, coplanar waveguide and stripline transmission lines and filter designs using resonant frequency of a dielectric material. The process flow example described in <FIG> will form a microstrip transmission line using a dual damascene method to create the superconducting wires within the dielectric material. Niobium oxides are removed and replaced by superconducting niobium nitride using either in-situ and/or ex-situ to the dielectric used for the resonating material.

<FIG> illustrates a cross-sectional view of a superconductor structure in its early stages of fabrication. The superconductor structure resides in an etch chamber for forming vias and trenches in one or more dielectric layers. The superconductor structure <NUM> includes a first dielectric layer <NUM> deposited over an underlying substrate <NUM>. The underlying substrate <NUM> can be, for example, a silicon or glass wafer that provides mechanical support for subsequent overlying layers. Any suitable technique for forming the first dielectric layer <NUM> may be employed such as Low Pressure Chemical Vapor Deposition (LPCVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), High Density Plasma Chemical Vapor Deposition (HDPCVD), sputtering or spin-on techniques to a thickness suitable for providing an interconnect layer. A typical dielectric material to use for the first dielectric layer <NUM> would be a silicon rich film with low levels of oxygen and hydrogen.

As illustrated in <FIG>, a photoresist material layer <NUM> has been applied to cover the structure and patterned and developed to expose trench openings <NUM> in the photoresist material layer <NUM> in accordance with a trench pattern. The photoresist material layer <NUM> can have a thickness that varies in correspondence with the wavelength of radiation used to pattern the photoresist material layer <NUM>. The photoresist material layer <NUM> may be formed over the first dielectric layer <NUM> via spin-coating or spin casting deposition techniques, selectively irradiated (e.g., via deep ultraviolet (DUV) irradiation) and developed to form the trench openings <NUM>.

<FIG> also illustrates performing of an etch <NUM> (e.g., anisotropic reactive ion etching (RIE)) on the first dielectric layer <NUM> to form extended trench openings <NUM> (<FIG>) in the first dielectric layer <NUM> based on the trench pattern in the photoresist material layer <NUM>. The etch step <NUM> can be a dry etch and employ an etchant which selectively etches the underlying first dielectric layer <NUM> at a faster rate than the overlying photoresist material layer <NUM>. For example, the first dielectric layer <NUM> may be anisotropically etched with a plasma gas(es), herein carbon tetrafloride (CF<NUM>) containing fluorine ions, in a commercially available etcher, such as a parallel plate RIE apparatus or, alternatively, an electron cyclotron resonance (ECR) plasma reactor to replicate the mask pattern of the patterned of the photoresist material layer <NUM> to thereby create the extended trench openings <NUM>. The photoresist material layer <NUM> is thereafter removed using an oxygen based plasma and wet cleaned to remove organic residues. As a result of the oxygen exposure, the etched dielectric surface will have layers of silicon dioxide <NUM> as illustrated in <FIG>.

Silicon dioxide is known to have high losses at RF and microwave frequencies. The oxide layer <NUM> is removed using an hydrofluoric acid wet etch <NUM> in a wet etch chamber, as illustated in <FIG>. Next, the structure is placed into a material deposition chamber. The structure undergoes an argon pre-clean in-situ to remove any monolayers of native oxide grown during movement of the structure from the wet clean to the deposition chamber. The structure then undergoes a contact material fill to deposit a superconducting material <NUM>, such as niobium, into the trench openings <NUM> to form the resultant structure shown in <FIG>. The contact material fill can be deposited employing a standard contact material deposition. Following deposition of the contact material fill, the superconducting material <NUM> is moved into a polish chamber and is polished via chemical mechanical polishing (CMP) down to the surface level of the dielectric layer <NUM> to planarize the surface of the conductor level. The CMP utilizes a slurry which is selective to the dielectric layer <NUM> to form superconductive lines <NUM> and provide the resultant structure of <FIG>.

The slurry used to polish the metal niobium contains a peroxide component and results in a surface layer of about <NUM>Å niobium oxide <NUM> overlying the first and second superconductive lines and a dielectric oxide <NUM> of about 50A overlying the first dielectric layer <NUM>. The presence of this niobium oxide will degrade the performance of the superconducting circuits (losses in the metal lines), so it needs be removed prior to the deposition of the next dielectric layer. Since these surface thin films can cause high losses, a reactive clean is used to remove the niobium oxide <NUM> and the dielectric oxide <NUM> using a fluorine based plasma etch chemistry. This reactive clean can be done either ex-situ to the deposition chamber, for example, plasma etch chamber clustered to a PECVD chamber on a vacuum mainframe with low background level oxygen concentration, or in-situ to the deposition i.e., reactive clean process as part of a recipe sequence prior to the deposition process.

As illustated in <FIG>, the superconductor structure with the cleaned top surface is then moved to a deposition chamber to undergo a subsequent dielectric deposition process for forming the next interconnect layer in the superconductor interconnect structure. The resultant structure is illustrated in <FIG> with a second dielectric layer <NUM> overlying the structure and encapsulating the first and second superconductive lines <NUM>.

A photoresist material layer <NUM> is applied to cover the structure and is then patterned (e.g., DUV imaged) and developed to expose an open region <NUM> in the photoresist material layer <NUM> in accordance with a via pattern. <FIG> also illustrates performing of an etch <NUM> on the second dielectric layer <NUM> to form extended via opening <NUM> (<FIG>) in the second dielectric layer <NUM> based on the via pattern in the photoresist material layer <NUM>. The extended via opening <NUM> extends to one of the first superconductive lines <NUM>. The etch <NUM> utilizes the same plasma chemistry described before for the first dielectric layer <NUM>. The photoresist material layer <NUM> is thereafter stripped so as to result in the structure shown in <FIG>. After the photoresist strip using oxygen plasma, another dielectric oxide layer <NUM> and a layer of niobium oxide <NUM> are formed.

As represented in <FIG>, a photoresist material layer <NUM> is applied to cover the structure and is then patterned and developed to expose open trench regions <NUM> in the photoresist material layer <NUM> in accordance with a trench pattern. <FIG> also illustrates performing of an etch <NUM> (e.g., anisotropic reactive ion etching (RIE)) on the second dielectric layer <NUM> to form extended openings <NUM> and <NUM> (<FIG>) that partially extend into the second dielectric layer <NUM> based on the trench pattern in the photoresist material layer <NUM>. The etch <NUM> also removes the layer of niobium oxide and portions of the dielectric oxide not covered by the photoresist material layer <NUM>. The photoresist material layer <NUM> is thereafter stripped so as to result in the structure shown in <FIG>. After the photoresist strip, another dielectric oxide layer <NUM> and a layer of niobium oxide <NUM> are formed.

<FIG> also illustrates simultaneous removal of the resulting dielectric oxide <NUM> and niobium oxide <NUM> using a fluorine based plasma etch <NUM> and selective growing of a subsequent niobium nitride layer using a nitrogen plasma to form a niobium nitride barrier <NUM> (<FIG>). A typical plasma etch chamber can be used to perform these functions. The two processes oxide clean and nitridation are done ex-situ to the PVD chamber so that non-line of sight areas of the structure are cleaned using a plasma etch chamber. The niobium nitride barrier <NUM> is grown to passivate the surface and therefore inhibit further oxidation of the conductor level. The resultant structure is illustrated in <FIG>.

Next, the niobium nitride barrier <NUM> is removed using an argon sputter pre-clean in-situ to the PVD niobium chamber which is line of sight. The structure undergoes a contact material fill to deposit superconducting material <NUM>, such as niobium, into the via <NUM> and trenches <NUM> and <NUM> employing a standard contact material deposition to provide the resultant structure in <FIG>. Following deposition of the contact material fill, the contact material is polished via chemical mechanical polishing (CMP) down to the surface level of the second dielectric layer <NUM> to provide the resultant structure in <FIG>. The resultant structure of <FIG> includes a third superconductive line <NUM> and a fourth superconductive line <NUM>, another dielectric oxide layer <NUM> overlying the second dielectric layer <NUM> and another niobium oxide layer <NUM> overlying the top surface of the third superconductive line <NUM> and the fourth superconductive line <NUM>. Another fluorine based plasma etch is performed on the structure of <FIG> to remove the dielectric oxide layer <NUM> and the niobium oxide layer <NUM>. Finally, the structure is completed with a niobium nitridation process to provide a first niobium nitride barrier <NUM> overlying the third superconductive line <NUM> and a second niobium nitride barrier <NUM> overlying the fourth superconductive line <NUM> to prevent oxidation of the niobium during transfer of the structure to the next process and/or chamber. The final resultant structure is illustrated in <FIG>.

Since the electric fields between the conductor and ground plane occur across the dielectric, the niobium nitride layer does not contribute to losses of the microstrip. The process flow concept described here can be extended to the formation of stripline as well as multilevel transmission line formation in the vertical plane.

Claim 1:
A method of forming a superconductor structure (<NUM>, <NUM>), the method comprising:
forming a superconducting element (<NUM>, <NUM>, <NUM>) in a first dielectric layer (<NUM>, <NUM>), the superconducting element (<NUM>, <NUM>, <NUM>) having a top surface aligned with a top surface of the first dielectric layer (<NUM>, <NUM>);
forming a second dielectric layer (<NUM>, <NUM>) over the first dielectric layer (<NUM>, <NUM>) and the superconducting element (<NUM>, <NUM>, <NUM>);
forming an opening (<NUM>) in the second dielectric layer (<NUM>, <NUM>) to the top surface of the superconducting element (<NUM>, <NUM>, <NUM>);
performing a cleaning process (<NUM>) on the top surface of the superconducting element (<NUM>, <NUM>, <NUM>) and on a surface of the second dielectric layer (<NUM>, <NUM>) to remove oxides formed on the top surface of the superconducting element (<NUM>, <NUM>, <NUM>) at a first processing stage;
forming a niobium nitride protective barrier (<NUM>) solely over the cleaned top surface of the superconducting element (<NUM>, <NUM>, <NUM>);
moving the superconductor structure (<NUM>, <NUM>) to a second processing stage for further processing; and
performing an argon sputter clean to remove the niobium nitride protective barrier (<NUM>).