Patent Application
20240209491
This disclosure relates generally to a power limiter and, more particularly, to a power limiter including a waveguide having an LaCoO3 film formed thereto.
RF power limiters are often employed in receiver front end circuits and other devices to protect sensitive electrical components, such as low noise amplifiers (LNA), from high power RF signals. Traditionally these power limiters are solid-state devices that employ semiconductor components, such as p-i-n diodes or MESFET devices, that limit input power for a certain frequency range. However, with the advance of power and agility in RF components and systems additional challenges in the form of low power consumption, higher incident power control and high reliability are placed on traditional RF power limiters. These challenges have led to the investigation of new power limiting materials that often employ an insulator-to-metal phase transition (IMT) in response to a certain temperature change that improve with increasing power. In response to increasing current, and thus temperature, an IMT material will transition from an insulative state to a metallic state, which shunts power above a given threshold.
One IMT material that can demonstrate IMT behavior for power limiting purposes is vanadium dioxide (VO2). VO2 has been synthesized by a wide variety of methods, such as molecular beam epitaxy (MBE), pulsed laser deposition and direct current and radio frequency sputtering. VO2 provides a sharp drop in resistance as the material heats to around 340 degrees Kelvin. However, this temperature is generally too low for practical device operation for many systems.
Another IMT material that can demonstrate IMT behavior is lanthanum cobalt oxide (LaCoO3). LaCoO3 has an IMT at a higher temperature than other IMT materials such as VO2. More particularly, LaCoO3 exhibits a large reduction in resistivity at around 500 degrees Kelvin as a result of a non-structural phase transition, such as electronic spin state transition or orbital melting mechanism, that provides a rapid electrical resistivity drop as a function of increasing temperature. Typical methods of synthesis for LaCoO3 found in the literature focus on the production of porous volumes because the majority of research on the material is in its application for catalysis for gas sensors and humidity sensors. Various processes have been reported in the literature for depositing thin films of LaCoO3 that typically focus on catalytic applications, which include MBE, ion beam sputtering, chemical vapor deposition and pulsed laser deposition. MBE is known to produce high quality, smooth, crystalline films. However, it is a slow, difficult and an expensive process. The literature shows that films of LaCoO3 can be produced using DC co-sputtering of metal targets, but those processes required post-deposition calcination steps to properly oxygenate and crystallize the films.
The following discussion of the embodiments of the disclosure directed to a power limiter including a waveguide having an LaCoO3 film formed thereto is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses.
This disclosure proposes producing RF devices that employ an LaCoO3 film that provide a quick drop in resistance as temperature increases above a threshold. The challenge is to produce smooth, high quality crystalline thin LaCoO3 films having a high insulator-to-metal transition point on various substrates that is relatively low cost, amenable to large scale production. It should also consist of only one step for simplicity and to protect underlying device layers from damage in high temperature calcination. The process will be able to deposit the LaCoO3 films onto a variety of substrates with varying dielectric properties in order to optimize the film properties and device performance.
In order to provide an RF device as described above, this disclosure describes a single step DC sputtering process for synthesis of LaCoO3 using relatively cheap source materials that can quickly produce large wafer scale material. Rather than use a ceramic target of pre-sintered LaCoO3, the proposed process utilizes cheaper metal targets of lanthanum and cobalt. The process uses a heated substrate in conjunction with deposition of the metal targets in a reactive oxygen/argon atmosphere. These metals are vaporized by a direct current magnetron gun that ejects material toward the substrate. Under these conditions, the metals react and crystallize directly onto the substrate surface to form the LaCoO3 film. Thus, no secondary processing steps are required to fully crystallize or oxygenate the film. An additional advantage of the proposed process is that using independently controlled metal targets allows for tuning of the composition of the film, affording greater control over optimization of the film properties. The ability to vary the substrate temperature with this process also allows for balancing the benefits between highly crystalline films grown at 700° C. and the amorphous films grown at low temperatures. Furthermore, while still able to produce smooth, quality films, sputtering is considered a low cost and fast production tool used for large wafer scale synthesis.
A negative bias potential is applied to the targets 34 and 36 by DC sources 52 and 54, respectively. Magnets (not shown) are employed to increase the ionization frequency by trapping a cloud of electrons near the surface of the targets 34 and 36, which increases the likelihood of collision and ionization of the argon gas. Positive argon ions bombard the targets 34 and 36, which releases cobalt atoms 56 from the target 34 and lanthanum atoms 58 from the target 36. The atoms 56 and 58 are drawn to the substrate 38 where they react directly with the oxygen to create a crystalized LaCoO3 film 60 on the substrate 38. A rotation device 62 rotates the substrate 38 so that the film 60 has a uniform concentration of cobalt and lanthanum. The sputtering process is continued until the thickness of the film 60 reaches a desired thickness, for example, 40-200 nm. The percentage of oxygen in the chamber 32 is carefully tuned to avoid significant oxidization of the targets 34 and 36.
The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.
This application is a divisional application of co-pending U.S. application Ser. No. 17/318,519, titled, LaCoO3 Thin Film Deposition by DC Metal Co-Sputtering, filed May 12, 2021.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17318519 | May 2021 | US |
| Child | 18597354 | US |
