Patent Document

FEDERAL RESEARCH STATEMENT 
     This invention was made with Government support under Contract No.: W911NF-10-1-0324 awarded by the U.S. Army. The Government has certain rights in this invention. 
    
    
     BACKGROUND 
     The present invention relates to waveguides such as radio frequency (RF) structures, and more specifically, to RF structures for superconducting quantum computing circuits and chip mode isolation and cross-talk reduction implementing buried metal layers and through-vias. 
     Quantum computing using superconducting elements uses resonant structures such as coplanar waveguides and capacitively shunted Josephson junctions to store and manipulate information. These structures are built using conventional semiconductor manufacturing methods onto substrates such as Si wafers. A chip is diced out of a full wafer and then cooled and tested. This chip can support various resonant electromagnetic modes determined by the size (dimensions) of the chip. For chips in the several millimeter size range, these modes can be in the several GHz range, which is similar to the frequencies of the structures built on the chip and can therefore cause problems. RF structures used in superconducting quantum computing circuits need to be isolated from spurious resonant systems in order to maintain coherence, and in addition have to be isolated from other RF structures on the same chip. RF modes in the substrate itself can cause issues in this regard, both as a source of loss in the qubit (due to mode coupling), and also as an unintended conduction path between nominally separate structures. 
     Implementation of superconducting circuitry is seen as one of the strongest candidates to realize to enable quantum computing. The presence of chip modes is responsible for degradation of performance through the damping of qubit resonances and by the increase of crosstalk between circuit elements on-chip. 
     SUMMARY 
     Exemplary embodiments include a method for fabricating a chip surface base, the method including preparing a first substrate, preparing a plurality of vias in the first substrate, depositing metal fillings into the plurality of vias, preparing a second substrate, bonding the first and second substrates and exposing the metal fillings. 
     Additional exemplary embodiments include a method for fabricating a chip surface base, the method including preparing a first and second substrate, depositing a metal on at least one of the first and second substrates, bonding the first and second substrates, preparing a plurality of vias in the first substrate, depositing metal fillings into the plurality of vias and exposing the metal fillings. 
     Further exemplary embodiments include a chip surface base device, including a first substrate, a second substrate, a metal layer disposed between the first and second substrates and a plurality vias disposed on the first substrate. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a flowchart of a method for fabricating a base onto which qubit circuits can be manufactured; 
         FIG. 2  illustrates a first substrate; 
         FIG. 3  illustrates the substrate with etched vias; 
         FIG. 4  illustrates vias filled with metal fillings; 
         FIG. 5  illustrates the first substrate of  FIG. 1  with metal fillings and a second substrate; 
         FIG. 6  illustrates the first and second substrates of  FIGS. 1-5 , bonded to one another; 
         FIG. 7  illustrates a flowchart of another method for fabricating a base onto which qubit circuits can be manufactured; 
         FIG. 8  illustrates a first and second substrate; 
         FIG. 9  illustrates the first and second substrates of  FIG. 8  bonded to one another; 
         FIG. 10  illustrates a substrate with etched vias; and 
         FIG. 11  illustrates vias filled with metal fillings. 
     
    
    
     DETAILED DESCRIPTION 
     In exemplary embodiments, the systems and methods described include wafer bonding to create buried metal layers and metallic through-vias. The systems and methods described herein are implemented to reduce or eliminate chip modes, and in addition can provide cross-talk isolation between RF structures. It has been determined that existing chip modes can be reduced or eliminated by either quenching the mode by building metallic structures into the chip in order to disrupt the mode, or by isolating the structures in a Faraday cage in order to shield the structures from the chip modes. Incorporation of dielectrics into the fabrication sequence can lead to reduction in the coherence time of the qubits and should be avoided, as further described herein. 
     As described herein, superconducting transmon quantum computing test structures often exhibit significant undesired cross-talk. For experiments with only a handful of qubits this cross-talk can be quantified and understood, and therefore corrected. As quantum computing circuits become more complex, and thereby contain increasing numbers of qubits and resonators, it becomes more vital that the inadvertent coupling between these elements is reduced or eliminated. The task of accurately controlling each single qubit to the level of precision required throughout the realization of a quantum algorithm is difficult by itself, but coupled with the need of nulling out leakage signals from neighboring qubits or resonators would quickly become impossible. For example in one realization of a superconducting qubit circuit used for quantum calculations, each qubit has four neighbors and operations between four different pairs. If there is poor isolation, then each qubit is potentially also talking to qubits one hop away, up to nine total qubits. In another example, each qubit is an oscillator with a resonating frequency, that ideally resonates indefinitely (i.e., has an ideal coherence). For the case in which crosstalk occurs via chip modes, this communication is not nearest neighbor, but rather is non-local so that qubits which are widely separated can communicate with each other. Correcting for this situation would quickly become intractable as the size of the quantum circuit increases. A second issue exists in that if the qubit communicates to chip modes, then each communication is an energy transfer that can de-phase or reduce amplitude of the resonant frequency (i.e., a reduction in T 1  and T 2 ). As such, there is a reduction in the coherence time of the qubit if it couples into a chip mode. In quantum computing, coherence times are preserved in order to perform proper calculations. 
     In exemplary embodiments, the systems and methods described herein address these issues by selectively placing vias having metal fillings that are coupled to a buried metal surface. The locations of the vias are selected based on where modes of a subsequently fabricated qubit circuit will reside on a surface of the substrate in which the vias are disposed. In exemplary embodiments, modeling of the qubit circuit enables selection of locations of the vias. In operation, when the qubit circuit couples to chip modes, the modes conduct into the metal fillings and are shorted into the buried metal surface. Thus the modes are killed. In exemplary embodiments, the modes have wavelengths longer than distances between the vias. In exemplary embodiments, the locations of the vias can also be selected to isolate individual devices in the qubit circuit, thereby placing vias around the devices so that modes are kept isolated between the vias and then shorted to the buried metal layer. 
     Several embodiments of implementation of metallic through-vias coupled with buried metal ground planes can be implemented to both quench chip modes and to isolate RF components from these modes. The systems and methods described herein provide improvement in qubit coherence as well as increased cross-talk immunity for RF devices on the chip. By implementing wafer bonding to create a buried metal layer, advantages of via methods are obtained with ground backplane to isolate key circuit components and in addition other advantages are opened up as well including the ability to pattern the back metal layer (BML) and also incorporate structures into the second wafer which can be used for addressing RF structures. Several embodiments are now described. 
       FIG. 1  illustrates a flowchart of a method  100  for fabricating a chip-surface base onto which qubit circuits can be manufactured. The base provides chip mode isolation and cross-talk reduction through buried metal layers and through-vias.  FIG. 2  illustrates a first substrate  200 . At block  105 , the first substrate  200  is prepared for processing. In exemplary embodiments, the first substrate  200  is selected to reduce dielectric loss tangent at low temperatures. The first substrate  200  is also selected to be a material which can be etched selectively to the superconducting and dielectric material to be used for subsequent qubit circuit fabrication. For example, high resistivity Si wafers may be implemented. 
       FIG. 3  illustrates the substrate  200  with etched vias  205 . At block  110 , the vias  205  are etched into the substrate. Any suitable photolithography techniques can be implemented to pattern the vias  205 . In addition, any suitable etching techniques can be implemented including, but not limited to, Plasma-Enhanced Chemical Vapor Deposition (PECVD) for insulator deposition. For example, a Reactive Ion Etching (RIE) for etching samples and spinners to coat silicon chips and wafers with lithographic resist may be used. In addition, any etch alignment features for latter processing can be performed at block  110 . 
       FIG. 4  illustrates the vias  205  filled with metal fillings  210  (e.g., a superconducting material such as but not limited to aluminum (Al)). At block  115 , the metal fillings  210  are deposited into the vias  205 . In exemplary embodiments, the lengths of the metal fillings  210  are in the range of 50-160 microns. The first substrate can be polished to ensure a flush surface and that the metal fillings  210  are exposed. 
       FIG. 5  illustrates the first substrate  200  with metal fillings  210  and a second substrate  220 . In exemplary embodiments, a metal layer  225  can be deposited on the second substrate  220 . A metal layer  201  can also be deposited on the first substrate  200  over the metal fillings  210 . At block  120 , the second substrate  220  is prepared. In exemplary embodiments, the second substrate  220  is selected to reduce dielectric loss tangent at low temperatures. For example, high resistivity Si wafers may be implemented. The second substrate  220  is also selected to be a material which can be etched selectively to the superconducting and dielectric material to be used for subsequent qubit circuit fabrication. As described herein metal layers  201 ,  225  can be deposited on one or both of the first and second substrates  200 ,  200  respectively. The metal layers  201 ,  225  are preferably the same material as the metal fillings  210 . 
       FIG. 6  illustrates the first and second substrates  200 ,  220  bonded to one another. At block  125  the first and second substrates  200 ,  220  are bonded together with a low-temperature anneal. At block  130 , the surface of the first substrate  200  is polished to fully expose the vias. Subsequent fabrication, shone at block  135 , includes fabricating a qubit circuit  290  atop the first substrate, along with other resonators and top metallization. As described herein, the metal fillings  210  are positioned with the subsequent qubit circuit  290  in mind, and placed where the modes of the qubit circuit will reside. In this way, the metal fillings  210  conduct the modes and short the modes to the now buried metal layers  201 ,  225 , thereby suppressing the modes from leaking to other qubit circuits. 
       FIG. 7  illustrates a flowchart of another method  700  for fabricating a chip surface base onto which qubit circuits can be manufactured. The base provides chip mode isolation and cross-talk reduction through buried metal layers and through-vias.  FIG. 8  illustrates a first and second substrate  800 ,  820 . At block  705 , the first and second substrates  800 ,  820  are prepared for processing. In exemplary embodiments, the first and second substrates  800 ,  820  are selected to reduce dielectric loss tangent at low temperatures. The first and second substrates  800 ,  820  are also selected to be a material which can be etched selectively to the superconducting and dielectric material to be used for subsequent qubit circuit fabrication. For example, high resistivity Si wafers may be implemented. In exemplary embodiments, metal layers  801 ,  825  can be deposited on the first and second substrates  800 ,  820  respectively. As described herein metal layers  801 ,  825  can be deposited on one or both of the first and second substrates  800 ,  820  respectively. 
       FIG. 9  illustrates the first and second substrates  800 ,  820  bonded to one another. At block  710  the first and second substrates  800 ,  820  are bonded together with a low-temperature anneal. 
       FIG. 10  illustrates the substrate  800  with etched vias  805 . At block  715 , the vias  805  are etched into the substrate. Any suitable photolithography techniques can be implemented to pattern the vias  805 . In addition, any suitable etching techniques can be implemented including, but not limited to, Plasma-Enhanced Chemical Vapor Deposition (PECVD) for insulator deposition. For example, a Reactive Ion Etching (RIE) for etching samples and spinners to coat silicon chips and wafers with lithographic resist may be used. In addition, any etch alignment features for latter processing can be performed at block  715 . 
       FIG. 11  illustrates the vias  805  filled with metal fillings  810  (e.g., a superconducting material such as but not limited to aluminum and preferably the same material as the metal layers  801 ,  825 ). At block  720 , the metal fillings  810  are deposited into the vias  805 . In exemplary embodiments, the lengths of the metal fillings  810  are in the range of 50-160 microns. 
     At block  725 , the surface of the first substrate  800  is polished to fully expose the vias. Subsequent fabrication includes fabricating a qubit circuit atop the first substrate, along with other resonators and top metallization. As described herein, the metal fillings  810  are positioned with the subsequent qubit circuit in mind, and placed where the modes of the qubit circuit will reside. In this way, the metal fillings  810  conduct the modes and short the modes to the now buried metal layers  801 ,  825 , thereby suppressing the odes from leaking to other qubit circuits. 
     It can be appreciated that an implementation of a combination of buried metal layers with connected vias form isolation cage for RF structures on chip surface. The systems and methods described herein have an absence of dielectrics in buried layers which prevent a reduction in coherence times. Additional layers can be implemented to allow bonding (i.e., adhesion layers). All metallization procedures described herein are patterned in order to isolate grounds and prevent mode coupling between devices. 
     The methods described herein can include RIE end-pointing on Al coatings. In addition, the methods can include incorporation of wiring and structures into the bonding substrate wafer. This can be used for wiring between structures and provide access to the RF structures on the primary wafer. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.

Technology Category: g