Patent Application
20250129468
This disclosure relates generally to devices, systems, and method for sputtering of high-quality superconducting films.
In a physical deposition process (PVD) ions of an inert gas (e.g., argon) are electrically accelerated toward a “target” of source material. These ions essentially eject or “sputter” the target material, atom by atom. The ejected atoms are deposited onto a substrate (e.g., wafer) to form a thin film on the substrate. One example of a target material may include niobium (Nb). During the process, however, large amounts of argon becomes incorporated into the growing thin film, which results in a high resistivity film and poor material quality.
The following presents a simplified summary to provide a basic understanding of the subject disclosure. This summary is not an extensive overview of the subject disclosure. It is not intended to identify key/critical elements or to delineate the scope of the subject disclosure. Its sole purpose is to present some concepts of the subject disclosure in a simplified form as a prelude to the more detailed description that is presented later.
One example of the subject disclosure is a method of forming a thin film that includes providing a physical vapor deposition apparatus having a vacuum chamber. A target material is disposed on a back plate near a top portion of the vacuum chamber. A chuck to support a substrate is located near a bottom portion of the vacuum chamber. The chuck has coolant flowing continuously through it during processing and is in thermal contact with the substrate to provide cooling. A working gas comprising a heavy noble gas of either krypton or xenon is injected into the vacuum chamber. Power settings are applied to the physical vapor deposition apparatus to produce an electromagnetic field to induce electric currents inside the vacuum chamber. The heavy noble gas is ionized into an inductively coupled plasma by the electric currents. Positive ions from the inductively coupled plasma are accelerated toward the target material to displace atoms from the target material to deposit a thin film of the target material of a desired thickness on the substrate. An electric field is applied to the substrate (e.g. a substrate electrical bias) to accelerate ions to the surface of the substrate.
Another example of the subject disclosure is a method of forming a superconductive thin film that includes providing a physical vapor deposition apparatus having a vacuum chamber. A target material comprising a superconductor material selected from a group consisting of niobium, indium, aluminum, titanium, molybdenum, and the like is bonded on a back plate near a top portion of the vacuum chamber. A chuck to support a substrate is located near a bottom portion of the vacuum chamber, where a temperature of the chuck is in a range from 15° C. to 150° C. A working gas comprising a heavy noble gas of either krypton or xenon is injected into the vacuum chamber. Power settings are applied to produce an electromagnetic field to induce electric currents inside the vacuum chamber. The noble gas is ionized into an inductively coupled plasma by the electric currents. Positive ions from the inductively coupled plasma are accelerated toward the target material to displace atoms from the target material to deposit a thin film of the target material on the substrate to a desired thickness.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate various systems, methods, and other examples of the disclosure. Illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. In some examples one element may be designed as multiple elements or multiple elements may be designed as one element. In some examples, an element shown as an internal component of another element may be implemented as an external component and vice versa.
The disclosure is now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the subject disclosure. It may be evident, however, that the subject disclosure can be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing the subject disclosure.
While specific characteristics are described herein (e.g., thickness, orientation, configuration, etc.), it is to be understood that the features, functions and benefits of the subject disclosure can employ characteristics that vary from those described herein. These alternatives are to be included within the scope of the disclosure and claims appended hereto.
Argon gas is typically used as a sputtering gas in a physical vapor deposition (PVD) process to create a thin film for use in superconducting applications. During the PVD process, however, argon atoms, due to their low atomic mass, can become incorporated into a thin film deposited on a substrate resulting in a high resistivity thin film and poor material quality. Specifically, during the PVD process, when the argon gas impacts a target material to displace atoms from the target material, atoms from the argon gas are reflected back (mass reflection effect) toward the thin film. These argon atoms become interstitially incorporated into the thin film. Thus, there is a large concentration of argon atoms in the resulting thin film, which as mentioned above, results in a high resistivity thin film and poor material quality. In addition, the incorporated argon atoms lead to material defects and can also act as flux pinning sites which trap flux loops in undesirable locations within circuits in the thin film. The flux trapping is a major source of repeatability and yield issues within superconducting circuits.
Still further, long process times are required for each lot (e.g., 8 wafers). The reason for the long processing time is that the process requires, 1) a high cathode power (e.g., 20 KW) applied between a target material and a chuck, 2) a high wafer bias (e.g., 300 W), and 3) a high-temperature chuck (wafer pedestal). The cathode power and the high wafer bias are major sources of creating heat within chamber. In addition, the high-temperature chuck has limited active cooling capabilities. Thus, the deposition process must be slowed in order to not to exceed a 150° C. thermal budget requirement of the wafers.
Disclosed herein is a physical vapor deposition (PVD) method of depositing a niobium (Nb) thin film onto a substrate using krypton (Kr) as the sputtering gas that overcomes the aforementioned disadvantages. When used as a sputtering gas, krypton has several beneficial properties over argon. For example, krypton has a larger atomic mass than argon and thus has a mass reflection effect that is approximately eight times less than that of argon. In other words, due to the larger atomic mass of krypton, krypton vaporizes the target material (e.g., niobium) more efficiently than argon and thus has less chance to be reflected at the surface of the sputtering target. Therefore, krypton atoms are less likely to become incorporated into the resulting thin film during the PVD process as opposed to argon atoms. In addition, krypton has a reduced kinetic reflection of atoms from the sputter target and thus a lower kinetic velocity at the wafer surface which also reduces the amount of krypton atoms incorporated into the resulting thin film. As a result, the concentration of the sputtering gas atoms (i.e., krypton atoms) in the thin film is significantly less than the concentration of argon atoms. Alternatively, xenon can be used as the working gas since xenon has similar properties and advantages as krypton.
In addition to the use of krypton, the method uses an inductive plasma coupling to provide a high degree of krypton ionization. The method also includes modifying parameters (e.g., cathode power, wafer bias, etc.) of a sputtering chamber associated with the sputtering process and providing an actively cooling, low-temperature, biasable electrostatic chuck in the sputtering chamber to ensure that the wafer remains below the 150° C. thermal budget. More specifically, the process includes applying a high voltage across two electrodes with where a wafer is disposed between the two electrodes. The high voltage ionizes the krypton gas and the positively charged ions impact the electrode containing the target material and thus vaporizes the target material. As mentioned above, the larger krypton atomic mass vaporizes the target material more efficiently than argon, which mitigates the reflection at the surface of the sputtering target. The target material vapor then diffuses through the krypton gas while a radio frequency field is applied via an inductive coil. This causes a higher degree of ionization of the plasma and reduces the kinetic energy of the target atoms as they drift towards the wafer. Finally, a second RF field is capacitively applied to the substrate through the biasable electrostatic chuck which collimates the incoming material and provides energy at the wafer surface to aid in surface diffusion of the incoming target material. This process allows controlled growth of high purity conformal thin films (e.g., niobium) with large grains.
Results from the process described above have demonstrated that the krypton deposition process provides an approximate 20% improvement in electrical resistivity and an approximate 300% improvement in residual resistivity ratio (RRR) compared to the argon deposition process. As a result, the amount of current that can be conducted through the circuits increases when using krypton as the working gas as opposed to argon. Thus, the process reduces resistivity and improves yield. The benefits of the low resistivity film can be incorporated into superconducting electronics to improve functional performance, allow scaling to smaller line sizes, and improve ground plane performances. Still further, krypton produces a higher surface roughness due to significantly larger grains produced in the thin film, which provides further evidence of improved material quality.
The PVD apparatus 100 further includes a low-temperature, biasable chuck 116 located near a bottom of the vacuum chamber 102. The chuck 116 also serves as a pedestal for a substrate (e.g., wafer) 120. The pedestal is conductive and thus functions as an electrode. The thin film is formed on the substrate 120 during the deposition process. A cooling line 118 is incorporated into the chuck 116. The cooling line 118 provides a coolant (e.g., water) to the chuck 116 to maintain the chuck 116 at a low temperature in a range from approximately 15° C. to 150° C. Specifically, the cooling line 118 continuously provides the coolant to the chuck 116 such that the coolant is in thermal contact with the substrate during the PVD process so that the substrate 120 does not exceed 150° C.
The PVD apparatus 100 further includes a gas inlet port 122 and a gas outlet port 124. A working gas (e.g., krypton, xenon) 126 is introduced into the vacuum chamber 102 via the gas inlet port 122 and exits the vacuum chamber 102 via the gas outlet port 124.
At 204, the following power settings are applied to the PVD apparatus 100. A first DC power source 130 supplies DC power to supply a voltage between the back plate 104 and the chuck 116 in a range of approximately 1 kW-20 kW. An AC power source 132 provides an AC bias to the substrate 120 in a range of approximately 100 W-300 W. An RF coil 134 provides power inside the vacuum chamber 102 in a range of approximately 800 W-3000 W to increase ionization of the target material 106 to thereby overcome a deposition rate of the target material 106. Specifically, the RF coil 134 provides an electromagnetic field 136 inside the vacuum chamber 102. The electromagnetic field 136 induces electric currents in the vacuum chamber 102. The electric currents in turn ionize the working gas into an inductively coupled plasma where positive ions from the inductively coupled plasma are accelerated toward the target material to displace atoms from the target material. A second RF field is capacitively applied to the substrate 120 via an electrostatic force generator 138 through the biasable electrostatic chuck 116, which collimates the incoming material and provides energy at the wafer surface to aid in surface diffusion of the incoming target material 106. A second DC power source 140 provides a DC offset power 142 inside the vacuum chamber 102 in a range of approximately 1000 W-3000 W.
At 206, after waiting a predetermined amount of time sputtered atoms from the target material 106 are deposited on the substrate 120 to form the thin film of a desired thickness. The predetermined time may vary based on the desired thickness of the thin film. The AC power source 132 that provides a bias to the substrate 120 in the presence of the plasma within the vacuum chamber 102 causes a bias to develop on the chuck (conductive pedestal) 116 so that the sputtered target material 106 ions accelerate toward the substrate 120.
The descriptions above constitute examples of the disclosure. It is, of course, not possible to describe every conceivable combination of components or method for purposes of describing the disclosure, but one of ordinary skill in the art will recognize that many further combinations and permutations of the disclosure are possible. Accordingly, the disclosure is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims.
The invention was made under U.S. Government Contract. Therefore, the U.S. Government has rights to the invention as specified in that contract.
