Steven Brems

Reversing the Training Effect in Exchange Biased CoO/Co Bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.

Enhancement of cavitation activity and particle removal with pulsed high frequency ultrasound and supersaturation

Megasonic cleaning as applied in leading edge semiconductor device manufacturing strongly relies on the phenomenon of acoustic cavitation. As the occurrence of acoustic cavitation is incorporating a multitude of interdependent effects, the amount of cavitation activity in the cleaning liquid strongly depends on the sonication conditions. It is shown that cavitation activity as measured by means of ultraharmonic cavitation noise can be significantly enhanced when pulsed sonication is applied to a gas supersaturated liquid under traveling wave conditions. It is demonstrated that this enhancement coincides with a dramatic increase in particle removal and is therefore of great interest for megasonic cleaning applications. It is demonstrated that the optimal pulse parameters are determined by the dissolution time of the active bubbles, whereas the amount of cavitation activity depends on the ratio between pulse-off and pulse-on time as well as the applied acoustic power. The optimal pulse-off time is independent of the corresponding pulse-on time but increases significantly with increasing gas concentration. We show that on the other hand, supersaturation is needed to enable acoustic cavitation at aforementioned conditions, but has to be kept below values, for which active bubbles cannot dissolve anymore and are therefore lost during subsequent pulses. For the applicable range of gas contents between 100% and 130% saturation, the optimal pulse-off time reaches values between 150 and 340 ms, respectively. Full particle removal of 78 nm-diameter silica particles at a power density of 0.67 W/cm(2) is obtained for the optimal pulse-off times. The optimal pulse-off time values are derived from the dissolution time of bubbles with a radius of 3.3 μm and verified experimentally. The bubble radius used in the calculations corresponds to the linear resonance size in a 928 kHz sound field, which demonstrates that the recycling of active bubbles is the main enhancement mechanism. The optimal choice of the pulsing conditions however is constrained by the trade-off between the effective sonication time and the desire to have a sufficient amount of active bubbles at lower powers, which might be necessary if very delicate structures have to be cleaned.

Evaluation and interpretation of bubble size distributions in pulsed megasonic fields

The occurrence of acoustic cavitation is incorporating a multitude of interdependent effects that strongly depend on the bubble size. Therefore, bubble size control would be beneficial for biological and industrial processes that rely on acoustic cavitation. A pulsed acoustic field can result in bubble size control and the repeated dissolution and reactivation (“recycling”) of potentially active bubbles. As a consequence, a pulsed field can strongly enhance cavitation activity. In this paper, we present a modified methodology for the evaluation of the active bubble size distribution by means of a combination of cavitation noise measurements and ultrasonic pulsing. The key component of this modified methodology is the definition of an upper size limit, below which bubbles—in between subsequent pulses—have to dissolve, in order to be sustainably recycled. This upper limit makes it possible to explain and link the enhancement of cavitation activity to a bubble size distribution. The experimentally determined...

Time-resolved monitoring of cavitation activity in megasonic cleaning systems

The occurrence of acoustic cavitation in the cleaning liquid is a crucial precondition for the performance of megasonic cleaning systems. Hence, a fundamental understanding of the impact of different parameters of the megasonic process on cavitation activity is necessary. A setup capable of synchronously measuring sonoluminescence and acoustic emission originating from acoustically active bubbles is presented. The system also includes a high-speed-stroboscopic Schlieren imaging system to directly visualize the influence of cavitation activity on the Schlieren contrast and resolvable bubbles. This allows a thorough characterization of the mutual interaction of cavitation bubbles with the sound field and with each other. Results obtained during continuous sonication of argon-saturated water at various nominal power densities indicate that acoustic cavitation occurs in a cyclic manner, during which periods of stable and inertial cavitation activity alternate. The occurrence of higher and ultraharmonics in the acoustic emission spectra is characteristic for the stable cavitation state. The inertial cavitation state is characterized by a strong attenuation of the sound field, the explosive growth of bubbles and the occurrence of broadband components in the acoustic spectra. Both states can only be sustained at sufficiently high intensities of the sound field. At lower intensities, their occurrences are limited to short, random bursts. Cleaning activity can be linked to the cavitation activity through the measurement of particle removal on standard 200 mm silicon wafers. It is found that the particle removal efficiency is reduced, when a continuous state of cavitation activity ceases to exist.

The influence of interface roughness on the magnetic properties of exchange biased CoO/Fe thin films

We have investigated the correlation between magnetic and structural properties in exchange coupled polycrystalline CoO/Fe thin films. It has been found that an increase in interface roughness increases the exchange bias field as well as the coercivity. The magnetization reversal mechanism is also influenced by the interfacial morphology. Smooth interfaces are characterized by an asymmetric hysteresis loop, which is associated with domain wall motion for the first magnetization reversal after field cooling and spin rotation in all subsequent reversals. This asymmetry diminishes as the interface roughness increases, i.e., all magnetization reversals are dominated by spin rotation. Moreover, we have observed that the blocking temperature decreases with increasing interface roughness. We also report on a logarithmic time dependence of the magnetization which is different for both branches of the hysteresis loop of smooth CoO/Fe bilayers.

Broadband 10Gb/s graphene electro-absorption modulator on silicon for chip-level optical interconnects

We report the first silicon integrated graphene optical electro-absorption modulator capable of 10Gb/s modulation speed. We demonstrate low insertion loss and low drive voltage combined with broadband and athermal operation in a compact hybrid graphene-Si device, outperforming Si(Ge) optical modulators for future chip-level optical interconnect application.

Effects of Interfacial Strength and Dimension of Structures on Physical Cleaning Window

Four different types of FINs; amorphous Si (a-Si), annealed a-Si, polycrystalline Si (poly-Si) and crystalline Si (c-Si) were used to investigate the effect of interfacial strength and the length of structures on the physical cleaning window by measuring their collapse forces by atomic force microscope (AFM). A transmission electron microscope (TEM) and a nanoneedle with a nanomanipulator in a scanning electron microscope (SEM) were employed in order to explain the different collapse behavior and their forces. Different fracture shapes and collapse forces of FINs could explain the influence of the interfacial strength on the pattern strength. Furthermore, the different lengths of a-Si FINs were prepared and their collapse forces were measured and the shorter length reduced their pattern strength. Strong adhesion at the interface resulted in a wider process window while smaller dimensions made the process window narrower.

Exchange bias by implantation of O ions into Co thin films

An original approach for the formation of an exchange bias system is presented. Alternative to surface oxidation or deposition for the formation of Co/CoO bilayer exchange bias systems, implantation of oxygen ions into Co films is applied. The implantation results in the formation of CoxOy embedded in a Co matrix. Comparison with noble gas implantation unambiguously demonstrates that the observed exchange bias effect is induced by the implanted oxygen. Opposed to bilayers formed by surface oxidation, the implantation results in a different morphology of the interface between Co and CoxOy and also gives rise to a radically different magnetization reversal mechanism.

Study of ultrasound-assisted radio-frequency plasma discharges in n-dodecane

This paper investigates the generation of a stable plasma phase in a liquid hydrocarbon (n-dodecane) by means of ultrasound (US) and radio-frequency (RF) or electromagnetic radiation. It is demonstrated for the first time that ultrasonic aided RF plasma discharges can be generated in a liquid. Plasma discharges are obtained for different gas mixtures at a pressure of 12?kPa and at low ignition powers (100?W for RF and 2.4?W?cm?2 for US). Direct carbon deposition from the liquid precursor on Cu, Ni, SiO2 and Si substrates has been obtained and no apparent compositional or structural difference among the substrate materials was observed. Characterization of the deposited solid phase revealed an amorphous structure. In addition, structural changes in the liquid precursor after plasma treatment have been analysed. Optical emission spectroscopy (OES) allowed the estimation of several plasma characteristic temperatures. The plasma excitation temperature was estimated to be about 2.3?2.4?eV. The rotational and vibrational temperatures of the discharge in n-dodecane with Ar as a feed gas were 1400?K and 6500?K, respectively. In Ar/O2 plasma, an increased rotational (1630?K) and vibrational temperature (7200?K) were obtained.

Piezoresistive detection-based ferromagnetic resonance force microscopy of microfabricated exchange bias systems

Ferromagnetic resonance measurements were performed on CoO∕Co exchange biased microstructures with a low-temperature magnetic resonance force microscope (MRFM). The MRFM instrument relies on piezoresistive force detection, and the magnetic tip attached to the cantilever acts as a field gradient source. These features extend the applicability of the MRFM that can also be used as a conventional magnetic force microscope. Spatial variations of the MRFM signal, which are induced by a spatially modulated exchange bias, are monitored with a lateral resolution of about 5μm.