Patent Application
20100308923
The present disclosure is generally related to oscillators. Further, the present disclosure is related to voltage controlled oscillators.
In the following detailed description of the embodiments, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration of specific embodiments. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present disclosure.
Gigahertz frequency oscillator circuits can be key components of wireless transceiver technology, such as cellular telephone handsets, wireless networks, and local proximity wireless devices such as Bluetooth enabled devices. As these devices continue to be designed smaller and with more features, an oscillator having a relatively small footprint can be useful. Thus, presented herein are designs of oscillators comprising mutually phase-locked oscillating elements with each having a relatively small footprint, such as 50×50 nm2.
The oscillator may comprise magnetic thin film structures. Specifically, current-biased magnetic thin film structures can exhibit microwave oscillations that can be tunable with current as well as magnetic field. Oscillations in these structures can be due to a spin momentum transfer (SMT) effect between spin-polarized current and magnetic moments of the thin films. These structures may compose a single component oscillator with a footprint of a few hundred nanometers (nm) that are produced using thin film processes.
Referring to
In addition, the magnetic thin film structure 101 may have an oscillator array layer 110 disposed in a gap between the magnetic top pole layer 106 and the magnetic bottom pole layer 107. The oscillator array layer 110 may include nanocontact pillars 109 on spin valve layers 111. The nanocontact pillar array may be further composed of oscillator pillars and detector pillars that are electrically insulated or separated from each other. In a particular embodiment, the spin valves films are patterned much larger than corresponding nanocontact pillars to produce narrow oscillation linewidths, in the range of 2-20 MHz, at room temperature when subject to an out-of-plane approximately 0.7 T magnetic field.
There may be connections through or around the magnetic bottom pole layer 107 from the nanocontact pillars to the semiconductor device layers 104. The semiconductor 104 may include devices to regulate current to the oscillator pillars as well as other semiconductor processing components, such as an amplifier 114. The magnetic thin film structure 101 may be composed of thin film layers grown on the semiconductor 104.
During operation, current through the coil 112 can produce a magnetic field or flux in the gap between the top pole layer 106 and the bottom pole layer 107 where the oscillator array layer 110 is located. Current can then be applied to the nanocontact pillars in the oscillator array 110 to generate an oscillating giant magnetoresistive (GMR) signal between the top lead 113 and the bottom lead 115 of the oscillator array layer 110.
Referring to
The oscillator device 200 may include a metal bottom electrode layer 202 deposited on the insulating layer. The metal electrode layer 202 may connect to a current source through the insulating layer. In addition, a fixed layer 204 may be deposited on the metal electrode layer 202. The fixed layer 204 may comprise a relatively thick soft high moment material, such as, but not limited to, Co or Co90Fe10 having a thickness ranging from 20 nm to 50 nm. Alternately, the fixed layer may be grown on top of an antiferromagnetic film such as PtMn or IrMn or some other antiferromagnetic material. A metal spacer layer 206 may be deposited on the fixed layer 204. The metal spacer layer 206 may be Cu with a thickness ranging from 3 nm to 5 nm.
A free layer 208 comprising a soft magnetic material with a lower moment may be deposited on the metal spacer layer 206. The free layer 208 may comprise Ni80Fe20 with a thickness of 1 nm to 2 nm. An array of metal pillars 210 may be grown on the free layer 208 with a specified pitch, such as 100 nm to 500 nm. The metal pillars 210 may have a diameter or width of 50 nm. The spaces between the metal pillars 210 may be filled with an insulating material 212.
The oscillator array 200 may consist of a pillar array on a spin valve film structure that is patterned to 5 μm to 10 μm on a side depending on how many oscillator elements are desired and what there spacing is. The oscillator array 200 may be rectangular, circular, or some other planar configuration.
During operation, each of the metal pillars 210, when activated by sufficient current and a magnetic field, excites sustained large angle magnetization dynamics in a portion of the free layer 208 beneath each of the metal pillars 210. Natural mutual synchronization of the dynamics beneath multiple pillars leads to a stable mono-frequency spinwave excitation throughout the free layer 208. Described herein are examples of embodiments of how the magnetic oscillations can be captured as a useful electrical oscillation signal.
Phase locking of the magnetic excitations beneath each metal pillar 210 can be achieved through a combination of nonlinear oscillator response and available mutual coupling mechanisms. Phase locking stabilizes a common frequency and phase relationship for all the oscillating metal pillars 210. The phase locking may be insensitive to details of the actual coupling mechanism and actual nonlinear response of the oscillators. The overall advantage of phase locking is a combined power that increases as N2, where N is the number of oscillators, and a frequency linewidth that decreases in a range between N−1 to N−2.
Further, the oscillating pillars 210 may be slightly dissimilar and still produce a mutually phase locked system. Phase locking has been shown to occur in a system having two slightly dissimilar oscillators where the coupling is due to spinwaves traveling between the oscillators. The phase locked state may produce narrower oscillation linewidths and an enhanced power output. Yet further, coupling may also occur via an alternating current through a series connected set of SMT devices.
Referring to
In such an embodiment, all the metal pillars 306 may be electrically connected in parallel and current from the current supply is split to each metal pillar 306. Once activated by the current and a magnetic field, each metal pillar 306 becomes an oscillator that generates a local high amplitude spinwave disturbance in the free layer 302. Due to interaction between the spinwaves, a phase locked high amplitude spinwave mode is generated underneath the entire array of oscillators. The phase locked array of oscillators serves to stabilize the frequency of the oscillation (i.e. reduces the phase noise). An oscillating giant magnetoresistive (GMR) signal between the top metal electrode 304 and a bottom metal electrode (not shown), such as metal electrode 202 shown in
The electrodes of the oscillating device 300 may be capacitively coupled to a path that carries time-varying (AC) voltage due to the GMR signal to a semiconductor, such as semiconductor 104. The semiconductor 104 may include a transistor amplifier 104 connected to the path to receive and amplify the AC voltage for delivery to other circuit components.
The oscillating device 300 can serve as a voltage controlled oscillator whereby changes in the magnetic field, such as a change to current of the coil 112, or changes in the current to the oscillator array can be used to switch frequency.
In another particular embodiment, an array of parallel connected metal pillars may be utilized but is separated and electrically insulated from other detector pillars (sensors) on the same layer.
Referring to
The system 400 may also include an MTJ detector 414 adjacent to the free layer 408. In a particular embodiment, the MTJ detector 414 is located on an area of the free layer 408 that is separate from an area of the free layer 408 that is adjacent the metal pillars 410. In addition, the MTJ detector 414 may be electrically isolated from the top electrode 412 and the metal pillars 410. In a particular embodiment, more than one MTJ detector may be implemented to provide additional output power.
In a particular embodiment, the MTJ detector 414 may include a tunnel barrier 416 that can be grown directly on the free layer 408. The tunnel barrier 416 may comprise AlOx, TiOx, or MgO or other appropriate barrier materials. A pinned ferromagnetic layer 418, such as a Co/Ru/Co synthetic antiferromagnetic (SAF) structure, may be deposited on the tunnel barrier 416. An antiferromagnetic film 424, such as IrMn or PtMn, may be placed on the pinned ferromagnetic layer 418. Also, there may be a top electrode 426 on the antiferromagnetic layer 424.
The MTJ detector 414 can operate as a current perpendicular to the plane (CPP) structure and can be patterned as a pillar. In a particular embodiment the pillar diameter can be in the range of 300-400 nm; however, the pillar diameter can be decreased below 100 nm. A relatively large detector device can provide increased stability of the pinned layers as well as ease of fabrication. The area surrounding the MTJ detector 414 may be filled with an insulating material.
In a particular embodiment, an expected output of the MTJ detector 414 may be determined by considering the resistance area (RA) product and the magnetoresistance ratio (MR %). In a conservative analysis, a RA=50 Ωμm2 may be achieved with an MR % of approximately 50%. If the MTJ detector 414 has a 400 nm diameter, a 100 mV bias between bottom electrode 402 and top electrode 426, and a resistance change from 300Ω to 500Ω, assuming an excitation of the free layer 408 that produces half of the full magnetoresistive change, the MTJ detector 414 may produce 8.3 nW into a 50Ω load. By a coherent combining of ten MTJ detectors within an oscillating device, the overall output can be 0.83 μW. In addition, the output power can be increased when the RA is decreased and/or the MR % is increased.
However, additional embodiments may include variations in the location of detectors, such as shown in
The system 500 may also include detector pillars 510 on the spin valve mesa structure 502 to sense the spinwave disturbance. Electrodes 512 can provide an oscillating output signal from the detector pillars 510. The detector pillars 510 may be MTJ detectors as described with respect to
Referring to
As shown in
Referring to
As shown in
Optimum locations for detectors may correspond to points of maximum magnetization excursion for the particular spin wave excited. This may depend on numerous factors such as the shape of the spin valve structure, the quality of an edge of the spin valve structure, and the configuration of the oscillator pillars, as well as other possible factors. Determination of the optimum location for placing the detectors may be determined by detailed micromagnetic modeling.
The designs discussed herein contemplate a stand-alone oscillator device that may also be integrated with other components on a chip, such as for system-on-a-chip applications. Such application may include wireless transceivers. Transceivers utilizing these designs may benefit from extra space on the chip for other applications, such as on-board memory, due to the small footprint of the described magnetic VCOs. In a particular embodiment, the VCO designs discussed herein can utilize a 20 μm2 footprint.
In another embodiment, a magnetic VCO as described herein could be used as a microwave current source for a magnetic write head to enable microwave-assisted magnetic recording. The magnetic VCO could be bonded to a slider or could be fabricated within the thin film process of the read-write head itself. Microwave current from the write head could apply a microwave magnetic field to the recording media. When the microwave current is sufficiently tuned to the resonance frequency of magnetic grains in the recording media, then the switching field for the magnetic grains may be reduced.
The illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The illustrations are not intended to serve as a complete description of all of the elements and features of apparatus and systems that utilize the structures or methods described herein. Many other embodiments may be apparent to those of skill in the art upon reviewing the disclosure. Other embodiments may be utilized and derived from the disclosure, such that structural and logical substitutions and changes may be made without departing from the scope of the disclosure. Moreover, although specific embodiments have been illustrated and described herein, it should be appreciated that any subsequent arrangement designed to achieve the same or similar purpose may be substituted for the specific embodiments shown.
This disclosure is intended to cover any and all subsequent adaptations or variations of various embodiments. Combinations of the above embodiments, and other embodiments not specifically described herein, will be apparent to those of skill in the art upon reviewing the description. Additionally, the illustrations are merely representational and may not be drawn to scale. Certain proportions within the illustrations may be exaggerated, while other proportions may be reduced. Accordingly, the disclosure and the figures are to be regarded as illustrative and not restrictive.
