Search

The critical current density J c 0 required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (MRAM) cell is proportional to the product of the damping parameter, saturation magnetization, and thickness of the free layer, α M S t F. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of a sub-monolayer thickness, we have reduced t F while minimizing α and maximizing M S , ultimately leading to lower J c 0 while maintaining high thermal stability. Bottom-pinned MRAM cells with a device diameter in the range of 55–130 nm were fabricated, and J c 0 is the lowest for the thinnest (1.2 nm) FLs, down to 4 MA / cm 2 for 65 nm devices, ∼ 30 % lower than 1.7 nm FLs. The thermal stability factor Δ dw , as high as 150 for the smallest device size, was determined using a domain wall reversal model from field-switching probability measurements. With high Δ dw and the lowest J c 0 , the thinnest FLs have the highest spin-transfer torque efficiency. [ABSTRACT FROM AUTHOR]

With the fast growth of the number of electronic devices on the internet of things (IoT), hardware‐based security primitives such as physically unclonable functions (PUFs) have emerged to overcome the shortcomings of conventional software‐based cryptographic technology. Existing PUFs exploit manufacturing process variations in a semiconductor foundry technology. This results in a static challenge–response behavior, which can present a long‐term security risk. This study shows a reconfigurable PUF based on nanoscale magnetic tunnel junction (MTJ) arrays that uses stochastic dynamics induced by voltage‐controlled magnetic anisotropy (VCMA) for true random bit generation. A total of 100 PUF instances are implemented using 10 ns voltage pulses on a single chip with a 10 × 10 MTJ array. The unipolar nature of the VCMA mechanism is exploited to stabilize the MTJ state and eliminate bit errors during readout. All PUF instances show entropy close to one, inter‐Hamming distance close to 50%, and no bit errors in 104 repeated readout measurements. [ABSTRACT FROM AUTHOR]

As the areal density of magnetic recording increases well beyond 100 Gb/[in.sup.2], the critical dimensions of recording heads continue to shrink at a rate of 15%-30% per year. Read sensors with 100 nm physical read widths are being routinely fabricated using undercut resist images and solvent-based liftoff processes. However, because standard liftoff processes using undercut photoresist images have reached their limits and cannot be performed reliably below 100 nm, sensor stabilization with a hard magnet contiguous junction is compromised because of induced variations in junction profile and hard magnet geometry. Furthermore, poor liftoff can induce fencing and excessive read gap topography leading to side reading and poor sensor performance. An alternative sensor patterning approach is proposed based on chemical mechanical polishing (CMP) and no-undercut resist images to define the read sensors critical dimensions. Ultra-narrow giant magnetoresistive (GMR) read heads have been successfully fabricated with physical read widths in the 20-80 nm range and using various sensor designs: Current in-plane (CIP) GMR, current perpendicular to the plane (CPP) GMR, and CPP tunnel magnetoresistive (CPP-TMR) sensors. Index Terms--Chemical mechanical polishing (CMP), magnetic recording, narrow track read heads.

Author(s): Shehzaad Kaka (corresponding author) [1]; Matthew R. Pufall [1]; William H. Rippard [1]; Thomas J. Silva [1]; Stephen E. Russek [1]; Jordan A. Katine [2] The spin torque [1, [...]

The in-stack stabilization of unshielded and shielded magnetic tunnel junction (MTJ) sensors have been studied experimentally by quasi-static and recording tests as well as theoretically by micromagnetic modeling. Results showed the viability of in-stack stabilization over a range of design and material conditions. Performance tradeoff studies and design optimization results led to the fabrication of MTJ read heads with good magnetic stability and high readback sensitivity. Index Terms--Longitudinal recording, magnetic materials for heads, read and write theory and modeling, test and characterization.

Spin-transfer torque (STT) magnetic random access memory (MRAM) is gaining commercial traction in low-density embedded and standalone memories, but the available market opportunities would grow exponentially if STT-MRAM could approach dynamic random access memory (DRAM) bit density. Of course, achieving DRAM density will require, among other things, demonstrating the ability to pattern MRAM bits at an extremely tight pitch. Here, an experimental demonstration of fabrication and electrical testing of a STT-MRAM bit array are reported. By optimizing the hard mask and ion beam etching (IBE) processes, MRAM bit arrays with a full pitch down to 50 nm are fabricated, in which the bits are individually tested using a novel bottom-point-contact method. [ABSTRACT FROM AUTHOR]

Hardware implementation of artificial neural networks (ANNs) using conventional binary arithmetic units requires large area and energy, due to the massive multiplication and addition operations in the inference process, limiting their use in edge computing and emerging Internet of Things (IoT) systems. Stochastic computing (SC), where the probability of 1s and 0s in a randomly generated bit-stream is used to represent a decimal number, has been devised as an alternative for compact and low-energy arithmetic hardware, due to its ability to implement basic arithmetic operations using far fewer logic gates than binary operations. To realize SC in hardware, however, tunable true random-number generators (TRNGs) are needed, which cannot be efficiently realized using existing complementary metal-oxide-semiconductor complementary metal-oxide-semiconductor (CMOS) technology. In this letter, we address this challenge by using magnetic tunnel junctions (MTJs) as TRNGs, the stochasticity of which can be tuned by an electric current via spin-transfer torque. We demonstrate the implementation of ANNs with SC units, using stochastic bit-streams experimentally generated by a series of 50 nm perpendicular MTJs. The numerical value (1 to 0 ratio) of the bit-streams is tuned by the current through the MTJs via spin-transfer torque with an ultralow current of <5 µA (= 0.25 MA⋅cm-2). The MTJ-based SC-ANN achieves 95% accuracy for handwritten digit recognition on the MNIST database. MRAM-based SC-ANNs provide a promising solution for ultra-low-power machine learning in edge, mobile, and IoT devices. [ABSTRACT FROM AUTHOR]

A single resonance-type spin-torque microwave diode (STMD) can be used as a quadratic detector to determine the frequency $f$ of an external microwave signal when the power $P_{\rm rf}$ of this signal is known. However, unless the signal's frequency $f$ exactly coincides with the detector's resonance frequency $f_{\rm res}$ , there exists a finite frequency determination error $\Delta f$. In this letter, we show theoretically and experimentally that if a pair of independent STMDs with different resonance frequencies connected in parallel is used for the signal frequency determination, knowledge of the signal power is not necessary, and the frequency determination error can be substantially reduced to $\Delta f \approx $ 30 MHz at the frequency of $f \approx $ 5.0 GHz. [ABSTRACT FROM AUTHOR]

The ferromagnetic resonance approach is employed to explain the spin wav spectra observed in the exchange-coupling modified Permalloy nanodot chain arrays. The interdot exchange coupling is shown to lead to the generation of satellite peaks in the system.

The effect of interdot exchange coupling on the magnetization reversal processes in nanodots is examined in an analysis on permalloy dot arrays. The studies have shown that along the perpendicular-to-the-coupling direction, the exchange interaction has little effect and magnetic reversal is identical to arrays of isolated dots of the same geometry.

The magnetization dynamics of a ferromagnetic system in the proximity of the unstable perpendicular equilibrium is investigated. The analysis of Permalloy thin films and nanoscale dots have shown that multiple resonances might occur in a certain frequency range because of the existence of local minimum in the frequency-field dispersion curves.

A study is done where the permalloy dot arrays are fabricated using e-beam lithography. Variable frequency ferromagnetic resonance is used to probe the dynamics of both vortex and single domain states and the uniform resonance mode is present at fields above the vortex annihilation when field sweeping up and vortex nucleation field when field sweeping down.

Spin wave spectra in exchange-coupled nanoscale dot chain arrays were studied using ferromagnetic resonance. The dot chain arrays, with dot diameters of 300 nm and thicknesses of 40 nm, coupled via Permalloy bridges of width ranging from 0 (no bridge) to 60 nm, were fabricated using e-beam lithography. In the perpendicularly magnetized isolated dots, multiple sharp ferromagnetic resonant peaks were observed, which are associated with the quantized in-plane wave vector due to the finite dot radius. These spectrum lines shift to higher fields for samples with wider bridges due to the increasing effective demagnetizing factor. For coupled dots, additional satellite peaks were observed at both sides of higher order spin wave modes and at the lower field side of the uniform mode. We show that these satellite peaks can be attributed to the excitation in each bridged dot and the interdot exchange coupling, respectively. [ABSTRACT FROM AUTHOR]

We are investigating the magnetization dynamics of a ferromagnetic system in the proximity of the unstable perpendicular equilibrium. The test system utilized is Permalloy in thin film and dot arrays with the applied field along a small angle off the film normal. Frequency/field dispersion curves (up to 15 GHz) obtained at different small angles (θH) reveal significant deviations from the perpendicular linear dispersion at low frequencies, and they form almost frequency independent flat plateau with a local minimum near the demagnetization field. The existence of the local minimum results in three resonances in a constant frequency/swept field scans. Such behavior is modeled successfully, in particular, in the film case, by using an approach for oblique angle resonance [P. E. Tannenwald, Phys. Rev. 105, 377 (1957)]. For the dots, additional influence from either induced in-plane anisotropy or vortex formation is discussed. [ABSTRACT FROM AUTHOR]

Time-resolved scanning Kerr microscopy (TRSKM) is a powerful technique for the investigation of picosecond magnetization dynamics at sub-micron length scales by means of the magneto-opticalKerr effect (MOKE). The spatial resolution of conventional (focused) Kerr microscopy using a microscope objective lens is determined by the optical diffraction limit so that the nanoscale character of the magnetization dynamics is lost. Here we present a platform to overcome this limitation by means of a near-field TRSKM that incorporates an atomic force microscope (AFM) with optical access to a metallic AFM probe with a nanoscale aperture at its tip. We demonstrate the near-field capability of the instrument through the comparison of time-resolved polar Kerr images of magnetization dynamics within a microscale NiFe rectangle acquired using both near-field and focused TRSKM techniques at a wavelength of 800 nm. The flux-closure domain state of the in-plane equilibrium magnetization provided the maximum possible dynamic polar Kerr contrast across the central domain wall and enabled an assessment of the magneto-optical spatial resolution of each technique. Line profiles extracted from the Kerr images demonstrate that the near-field spatial resolution was enhanced with respect to that of the focused Kerr images. Furthermore, the near-field polar Kerr signal (~1 mdeg) was more than half that of the focused Kerr signal, despite the potential loss of probe light due to internal reflections within the AFM tip. We have confirmed the near-field operation by exploring the influence of the tip-sample separation and have determined the spatial resolution to be ~550 nm for an aperture with a sub-wavelength diameter of 400 nm. The spatial resolution of the near-field TRSKM was in good agreement with finite element modeling of the aperture. Large amplitude electric field along regions of the modeled aperture that lie perpendicular to the incident polarization indicate that the aperture can support plasmonic excitations. The comparable near-field and focused polar Kerr signals suggest that such plasmonic excitations may lead to an enhanced near-field MOKE. This work demonstrates that near-fieldTRSKMcan be performed without significant diminution of the polarKerr signal in relatively large, sub-wavelength diameter apertures, while development of a near-field AFM probe utilizing plasmonic antennas specifically designed for measurements deeper into the nanoscale is discussed. [ABSTRACT FROM AUTHOR]

We report experimental results on write error rate and read disturbance as a function of read/write pulse width and amplitude in electric-field-controlled magnetic tunnel junctions (MTJs). Results are shown for 50 nm perpendicular MTJs. We also design and simulate the performance of a 256 kilobit (Kbit) magneto-electric random-access memory (MeRAM) macro in a 28 nm complementary metal-oxide semiconductor (CMOS) process, based on the measured MTJ device data. The results show that existing electric-field-controlled MTJs are capable of delivering write error rates below 10-9 for 10 ns total write and verify time and read disturbance below 10-16 for 2 ns read time in a 256 Kbit MeRAM array. [ABSTRACT FROM PUBLISHER]

Resistance drift of the amorphous states of multilevel phase change memory (PCM) cells is currently a great challenge for the commercial implementation of a reliable multiple-bit-per-cell memory technology. This paper reports observation of a stable intermediate state for a multilevel PCM cell that is achieved through nonuniform heating with a square current injection top electrode. Drift coefficient of the intermediate state is an order of magnitude lower than reset and has weaker temperature dependence. Using finite-element simulations and an analytical model for the subthreshold current–voltage characteristics, based on thermally activated hopping of charge carriers across Coulombic donor-like traps, we conclude that the defect density is two orders of magnitude larger in the intermediate state. We attribute the low drift coefficient of the intermediate state to a large number of stable interfacial defects which dominate the electron transport. Current findings give way to a more stable ultrahigh-density PCM device. [ABSTRACT FROM PUBLISHER]

We review the recent progress in the development of magnetoelectric RAM (MeRAM) based on electric-field-controlled writing in magnetic tunnel junctions (MTJs). MeRAM uses the tunneling magnetoresistance effect for readout in a two-terminal memory element, similar to other types of magnetic RAM. However, the writing of information is performed by voltage control of magnetic anisotropy (VCMA) at the interface of an MgO tunnel barrier and the CoFeB-based free layer, as opposed to current-controlled (e.g., spin-transfer torque or spin-orbit torque) mechanisms. We present results on voltage-induced switching of MTJs in both resonant (precessional) and thermally activated regimes, which demonstrate fast (<1 ns) and ultralow-power (<40 fJ/bit) write operations at voltages \sim 1.5-2~V . We also discuss the implications of the VCMA-based write mechanism on memory array design, highlighting the possibility of crossbar implementation for high bit density. Results are presented from a 1 kbit MeRAM test array. Endurance and voltage scaling data are presented. The scaling behavior is analyzed, and material-level requirements are discussed for the translation of MeRAM into mainstream memory applications. [ABSTRACT FROM AUTHOR]

We present a comparative study of high frequency dynamics and low frequency noise in elliptical magnetic tunnel junctions with lateral dimensions under 100 nm presenting current-switching phenomena. The analysis of the high frequency oscillation modes with respect to the current reveals the onset of a steady-state precession regime for negative bias currents above J = 107A/cm², when the magnetic field is applied along the easy axis of magnetization. By the study of low frequency noise for the same samples, we demonstrate the direct link between changes in the oscillation modes with the applied current and the normalised low frequency (1/f) noise as a function of the bias current. These findings prove that low frequency noise studies could be a simple and powerful technique to investigate spin-torque based magnetization dynamics. [ABSTRACT FROM AUTHOR]

In this work, we experimentally study the temperature dependence of the perpendicular magnetic anisotropy (PMA) and of the voltage-controlled magnetic anisotropy (VCMA) in nanoscale MgO|CoFeB|Ta-based magnetic tunnel junctions. We demonstrate that the temperature dependences of both the PMA and the VCMA coefficient follow power laws of the saturation magnetization, but with different exponents. We also find that the linear dependence of the PMA on electric field is maintained over a wide temperature range, although the VCMA strength decreases faster as a function of temperature as compared to the PMA. Possible mechanisms leading to the different exponents are discussed. [ABSTRACT FROM AUTHOR]

Stochastic dynamics of spin torque oscillators can be described in terms of magnetization drift and diffusion over a current-dependent effective energy surface given by the Fokker-Planck equation. Here we present a method that directly probes this effective energy surface via time-resolved measurements of the microwave voltage generated by a spin torque oscillator. We show that the effective energy approach provides a simple recipe for predicting spectral linewidths and line shapes near the generation threshold. Our time domain technique also accurately measures the fieldlike component of spin torque in a wide range of the voltage bias values. [ABSTRACT FROM AUTHOR]

We report the temperature dependences of tunneling magnetoresistance ratio, coercivity, thermal stability, and switching current distribution of magnetic tunnel junctions (MTJs) in the temperature range 25–80 °C, the most probable working environment for spin-transfer torque random access memory (STT-RAM). Two distinct temperature dependence of the switching current density are apparent due to two switching mechanisms: a switching current density decrease with increasing temperature in the long-pulse ( > 1 μs) regime, a result of thermally activated switching, but no decrease in the short-pulse (> 10 ns) regime, as a result of precessional switching. In the temperature range studied, the switching current density variation is less sensitive to environmental temperature than it is to switching time. Thus, switching time is the more important factor to consider in STT-RAM design. [ABSTRACT FROM PUBLISHER]

The bias dependence of the torque that a spin-polarized current exerts on ferromagnetic elements is important for understanding fundamental spin physics in magnetic devices and for applications. Several experimental techniques have been introduced in recent years in attempts to measure spin-transfer torque in magnetic tunnel junctions. However, these techniques have provided only indirect measures of the torque and their results regarding bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in magnetic tunnel junctions can be measured directly by using time-domain techniques to detect resonant magnetic precession in response to an oscillating spin torque. The technique is accurate in the high-bias regime relevant for applications, and because it detects directly small-angle linear-response magnetic dynamics caused by spin torque it is relatively immune to artefacts affecting competing techniques. At high bias we find that the spin-torque vector differs markedly from the simple lowest-order Taylor series approximations commonly assumed. [ABSTRACT FROM AUTHOR]

Plasmonic devices are capable of efficiently confining and enhancing optical fields, serving as a bridge between the realm of diffraction-limited optics and the nanoscale. Specifically, a plasmonic device can be used to locally heat a recording medium for data storage. Ideally, the recording medium would consist of individually addressable and non-interacting entities, a configuration that has been regarded as the ultimate future hard-drive technology. Here, we describe a plasmonic nano-antenna that is fully integrated into a magnetic recording head and its use for thermally assisted magnetic recording on both continuous and fully-ordered patterned media using nanosecond pulses in a static tester configuration. In the case of patterned media at 1.5 Pb m−2 (∼1 Tb inch−2) with 24-nm track pitch, we show ideally written bits without disturbing neighbouring tracks. We find a dramatic improvement in track width and optical efficiency compared to continuous media and show that this is largely due to advantageous near-field optical effects. [ABSTRACT FROM AUTHOR]

A nano-sized oscillator for RF applications is presented which is based on two spintronic effects, the tunneling magnetoresistance (TMR) and the spin momentum transfer torque. The oscillation frequency is several GHz and can be tuned by both a DC bias current and an external DC magnetic field. High compactness, high tunability and full compatibility with standard CMOS process make this spin torque nano-oscillator (STNO) a promising candidate for future RF transceivers. The main issues to be addressed are spectral purity and output power. First measurements on a hybrid built connecting the STNO to a dedicated wideband amplifier show that today's performance in terms of power is close to but not yet compatible with telecommunication standard requirements. Using time domain analysis we show that frequency fluctuations are an issue for spectral purity. Frequency synthesis concepts based on STNOs are also discussed. [ABSTRACT FROM AUTHOR]

We studied the spin transfer torque (STT) driven switching voltage distribution systematically by characterizing the switching probability density function (PDF) with large statistics (10^5 trials) across a wide time scale from 5 ns to 1 \mus. The skew normal distribution function is found to be a good one to fit the measured switching PDF down to low values, which would be used as a guideline to extrapolate read disturb rate (RDR) and write error rate (WER) in STT-RAM design. Moreover, the asymmetry of switching probability density function is observed to flip when the pulse width decreases. It is related to the fluctuation mechanism transition from the thermal agitation to the initial magnetization trajectory dispersion. [ABSTRACT FROM AUTHOR]

We report the spin-torque switching of the magnetization of the reference layer in a perpendicular magnetic anisotropy nanopillar with a spin-valve structure made of a combination of [Co/Pd] and [Co/Ni] multilayers. Around critical current, the switching is random, but becomes deterministic for larger currents. The threshold current is different for the two current directions, which shows that heating is not the main driving force but the spin torque interactions. A possible memory application is to store the magnetic bit in the magnetization of the hard magnetic reference layer and use the soft free layer to read and write it. [ABSTRACT FROM AUTHOR]

Extraordinary Magnetoresistance devices with critical dimensions below 50 nm have been developed in InAs 2-D electron gas (2DEG) quantum well heterostructures, and show the same magnetic sensitivity as Giant Magnetoresistance magnetic recording sensors. A new device design, in which fingers of metal extend into the semiconductor mesa to form leads, yields higher signal and operating currents than the more conventional design with semiconductor tab contacts. Devices built with the 2DEG very close to the surface yield high-signal and high-current carrying capacity, as required for scanning probe and magnetic recording applications. [ABSTRACT FROM AUTHOR]

Spintronic devices that utilize spin transfer torque are promising for integrated memory applications. However, these devices face substantial energy consumption challenges due to the high current densities required for switching. Conversely, voltage‐driven spintronic devices, using capacitive displacement charge, can realize switching operations that are energy‐efficient (≈1–10 fJ bit−1). This work investigates switching based on voltage control of the interlayer exchange coupling in perpendicular magnetic anisotropy (PMA) multilayered heterostructures. Unlike previous works that utilized gating techniques that employ ionic transport mechanisms to control interlayer exchange coupling, this study employs electrostatic gating by using MgO, which is more compatible with modern spintronic‐based memories. These results suggest that the magnetization anisotropy, and the magnitude, and phase of the Ruderman‐Kittel‐Kasuya‐Yosida (RKKY) coupling function with spacer layer thickness can be controlled through electric gating, providing a promising avenue for the development of energy‐efficient magnetic data storage devices. [ABSTRACT FROM AUTHOR]

Time-resolved scanning transmission x-ray microscopy measurements were performed to study the current-induced magnetization switching mechanism in nanopillars exhibiting strong perpendicular magnetic anisotropy. This technique provides both short-time (70 ps) and high-spatial (25 nm) resolutions. Direct imaging of the magnetization demonstrates that, after an incubation time of ∼1.3 ns, a 100 × 300 nm2 ellipsoidal device switches in ∼1 ns via a central domain nucleation and opposite propagation of two domain walls toward the edges. High domain-wall velocities on the order of 100 m/s are measured. Micromagnetic simulations are shown to be in good agreement with experimental results and provide insight into magnetization dynamics during the incubation and reversal periods. [ABSTRACT FROM AUTHOR]

Probabilistic (p-) computing is a physics-based approach to addressing computational problems which are difficult to solve by conventional von Neumann computers. A key requirement for p-computing is the realization of fast, compact, and energy-efficient probabilistic bits. Stochastic magnetic tunnel junctions (MTJs) with low energy barriers, where the relative dwell time in each state is controlled by current, have been proposed as a candidate to implement p-bits. This approach presents challenges due to the need for precise control of a small energy barrier across large numbers of MTJs, and due to the need for an analog control signal. Here we demonstrate an alternative p-bit design based on perpendicular MTJs that uses the voltage-controlled magnetic anisotropy (VCMA) effect to create the random state of a p-bit on demand. The MTJs are stable (i.e. have large energy barriers) in the absence of voltage, and VCMA-induced dynamics are used to generate random numbers in less than 10 ns/bit. We then show a compact method of implementing p-bits by using VC-MTJs without a bias current. As a demonstration of the feasibility of the proposed p-bits and high quality of the generated random numbers, we solve up to 40 bit integer factorization problems using experimental bit-streams generated by VC-MTJs. Our proposal can impact the development of p-computers, both by supporting a fully spintronic implementation of a p-bit, and alternatively, by enabling true random number generation at low cost for ultralow-power and compact p-computers implemented in complementary metal-oxide semiconductor chips., (Creative Commons Attribution license.)

The spin-transfer nano-oscillator (STNO) offers the possibility of using the transfer of spin angular momentum via spin-polarized currents to generate microwave signals. However, at present STNO microwave emission mainly relies on both large drive currents and external magnetic fields. These issues hinder the implementation of STNOs for practical applications in terms of power dissipation and size. Here, we report microwave measurements on STNOs built with MgO-based magnetic tunnel junctions having a planar polarizer and a perpendicular free layer, where microwave emission with large output power, excited at ultralow current densities, and in the absence of any bias magnetic fields is observed. The measured critical current density is over one order of magnitude smaller than previously reported. These results suggest the possibility of improved integration of STNOs with complementary metal-oxide-semiconductor technology, and could represent a new route for the development of the next-generation of on-chip oscillators.

The excitation of the steady-state precessions of magnetization opens a new way for nanoscale microwave oscillators by exploiting the transfer of spin angular momentum from a spin-polarized current to a ferromagnet, referred to as spin-transfer nano-oscillators (STNOs). For STNOs to be practical, however, their relatively low output power and their relatively large line width must be improved. Here we demonstrate that microwave signals with maximum measured power of 0.28 μW and simultaneously narrow line width of 25 MHz can be generated from CoFeB-MgO-based magnetic tunnel junctions having an in-plane magnetized reference layer and a free layer with strong perpendicular anisotropy. Moreover, the generation efficiency is substantially higher than previously reported STNOs. The results will be of importance for the design of nanoscale alternatives to traditional silicon oscillators used in radio frequency integrated circuits.