S. Radelaar

Technology for high‐performance n‐channel SiGe modulation‐doped field‐effect transistors

A fabrication process for n‐channel SiGe modulation‐doped field‐effect transistors has been developed in which the strain in the heterostructure is completely preserved by taking into account an appropriate temperature range for the formation of ohmic contacts to source drain by implantation. It yields a low specific contact resistance of 7.1×10−5 Ω cm2 to be compared to 5.7×10−2 Ω cm2 for alloyed Au/Sb contacts. With the same thermal upper limit the application of different gate oxides has been studied. All oxides show negligible gate leakage enabling a superior gate voltage swing compared to devices with a Schottky gate. Application of thermal oxide grown at 650 °C yields a superior field‐effector transistor performance compared to devices with sputter and plasma‐enhanced chemical‐vapor deposition deposited oxides due to its lower and negative charge level. Gate recessing of 0.5 μm Schottky gates yields a transconductance increase up to 310 mS/mm.

Elastic constants and thermal expansion coefficient of metastable C49 TiSi2

The elastic constants and thermal expansion coefficient of C49 TiSi2 thin films have been investigated by in situ curvature measurement during heat treatment and ex situ x‐ray diffraction measurements. The C49 TiSi2 compound was formed from Ti‐Si multilayers deposited on monocrystalline silicon and sapphire substrates. The films were polycrystalline without any evident texture. Young’s modulus (142 GPa), Poisson’s ratio (0.27), and the thermal expansion coefficient (10.9×10−6 K−1) have been determined. Note that these values are averages over random crystal orientations. Directly after formation C49 TiSi2 films exhibit tensile stress. This stress relaxes considerably above 375 °C. Below this temperature the thermal expansion is found to be independent of the state of relaxation of the C49 TiSi2 film.

In situ sensitive measurement of stress in thin films

A method for the in situ measurement of mechanical stress in thin films deposited in a vacuum system is presented. The bending of the substrate, a measure for mechanical stress in the deposited layer, is detected by reflecting two parallel laser beams off the surface of the substrate and measuring the angle between the two reflected beams. A hollow mirror in the path of the reflected beams acts as an ‘‘optical cantilever’’ and increases the sensitivity of this method. In the present setup it is possible to detect the difference between a flat substrate and a substrate with a radius of curvature of 6.5 km.

Growth kinetics and inhibition of growth of chemical vapor deposited thin tungsten films on silicon from tungsten hexafluoride

The growth kinetics and inhibition of growth of chemical vapor deposited thin W films on Si(100) from WF6 was studied with in situ growth stress and reflectivity measurements and ex situ weight gain measurements. A systematic series of experiments at varying WF6 flow, total pressure, and temperature show that the thickening kinetics and inhibition of the growth are controlled by two processes: WF6 diffusion through the gas phase and Si diffusion through the thickening columnar film. The steady state growth kinetics are controlled by WF6 diffusion in the gas phase whereas inhibition of the growth occurs at the transition from WF6 gas diffusion limited to Si solid state diffusion limited growth. A simple model based on WF6 gas phase diffusion and Si solid state diffusion is presented which gives a quantitative description of the experimental results.

Phase transformations in Al–Cu thin films: precipitation and copper redistribution

Abstract The (micro-)structural changes occurring in thin Al–Cu films of about 500 nm thickness and containing up to 1 at% Cu have been investigated. In particular, the copper distribution and the precipitation behaviour have been studied in situ during thermal cycling between 323 and 773 K. After slow cooling, large second-phase particles are observed which are mostly located at grain boundary triple points; the shape and size depend on the copper concentration. The average distance between these particles is about 16 μm, which is larger than the average grain size of approximately 1 μm. No second-phase particles have been observed within the aluminium grains. After cooling, a relatively large amount of copper (about 0.2 at%) as compared with bulk Al–Cu (0.001 at%) is not contained in second-phase particles. Most likely, this copper has segregated at grain boundaries, interfaces and dislocations. The temperature at which Al 2 Cu starts to form in the thin films is well below the copper solvus for bulk material.

The evolution of growth stresses in chemical vapor deposited tungsten films studied by in situ wafer curvature measurements

An in situ study of the evolution of the biaxial state of intrinsic stress during nucleation and growth of polycrystalline tungsten chemical vapor deposition films deposited by the hydrogen reduction of tungsten hexafluoride is presented. The evolution of biaxial stress was determined from in situ wafer curvature measurements. It is shown that the intrinsic stress is a growth stress, i.e., a stress developing in close vicinity to the advancing surface of the film due to metastable film growth processes. The stress developing depends strongly on the thickness of the film. High tensile stress (≊4 GPa) is observed during the initial stage of growth, compressive stress (up to −1 GPa) is observed in an intermediate thickness regime after film closure and tensile stress (0.1–1 GPa) is observed in the thick film regime. The associated stress gradients in the film are preserved during and after growth. The development of growth stress is determined by deposition temperature and growth rate. The tensile stress in ...

Transient creep in free-standing thin polycrystalline aluminum films

We studied room-temperature transient creep in polycrystalline, free-standing Al films with a thickness between 220 and 550 nm using a high-resolution bulge test technique. A transient logarithmic creep strain is observed. The time and stress dependence of the creep strongly support the idea that dislocation glide, limited by forest dislocation cutting, is the prevailing rate limiting mechanism. This is in contradiction with the misfit dislocation model for thin-film strengthening but in agreement with recent work on plasticity in thin Ag and Cu films on a substrate. A comparison is made with data on bulk Al. Both the transient creep strain and the initial fast strain are at least three orders of magnitude smaller for the thin-film samples. We argue that the strain hardening coefficient is the key parameter distinguishing thin film from bulk creep.

Micromagnetics and magnetoresistance of a Permalloy point contact

We performed micromagnetic calculations for a Permalloy point contact. The magnetization configurations show the formation of a Neel wall in the constriction. The occurrence of a Neel wall instead of a Bloch wall results from the dipole–dipole energy in this region. The evolution of the magnetization with field differs strongly for fields parallel and perpendicular to the electrodes. In the former case, the pattern evolves abruptly and in a narrow range, while in the latter case it evolves smoothly and in a rather wide range. From the magnetization patterns, we estimate the domain-wall magnetoresistance and the anisotropic magnetoresistance, which we compare with experimental data.We performed micromagnetic calculations for a Permalloy point contact. The magnetization configurations show the formation of a Neel wall in the constriction. The occurrence of a Neel wall instead of a Bloch wall results from the dipole–dipole energy in this region. The evolution of the magnetization with field differs strongly for fields parallel and perpendicular to the electrodes. In the former case, the pattern evolves abruptly and in a narrow range, while in the latter case it evolves smoothly and in a rather wide range. From the magnetization patterns, we estimate the domain-wall magnetoresistance and the anisotropic magnetoresistance, which we compare with experimental data.

A new fabrication process for metallic point contacts

Abstract We present a new process to fabricate metallic point contacts. The key feature of this process is the use of a Si membrane. Advantages of the process are i) the possibility to fabricate very small holes (down to 10 nm diameter), while the lithographic resolution requirements are modest, ii) the possibility to fine-tune the size of the hole by thermal oxidation and iii) probably a better controlled filling of the hole due to its tapered shape and its sharp edge. From electrical transport measurements it follows that the process yields highly ballistic point contacts.

Ultrahigh depth resolution secondary ion mass spectrometry with sub-keV grazing O2+ beams

Compositional analysis of interfaces in semiconductor materials grown with the most modern equipment requires a substantial improvement of the depth resolution of secondary ion mass spectrometry (SIMS). The lowering of the impact energy to improve depth resolution is limited on most magnetic-sector instruments to ∼1.5 keV. In this work it is shown that in the VG IX70S magnetic-sector instrument a reduction of the impact energy to 600 eV is possible. The reduction is achieved by use of a deceleration electrode in the primary beam line, allowing for independent variation of the energy and the incidence angle θ (50°<θ<80°). The best depth resolution obtained—for a shallow Ge δ layer with a 600 eV θ=80° O2+ beam—was 1.6 nm full width at half-maximum and an exponential decay length of 0.65 nm; about three times better than achievable on any other magnetic-sector SIMS instrument. In addition, a very shallow (∼3–4 nm) Ge δ layer has been analyzed by high-resolution Rutherford backscattering and SIMS. There is go...