Ilkka Suni

Reactive sputter deposition and properties of Ta x N thin films

The aim of this work was to evaluate tantalum nitride thin films fabricated using reactive sputtering with adjusted deposition parameters. Thin TaxN films were deposited reactively on Si wafers using reactive RF magnetron sputtering at various N2/Ar gas ratios. The films were investigated by four-point probe sheet resistance measurement, profilometry, X-ray diffraction, scanning electron microscope, 2 MeV 4He+ backscattering spectroscopy, and atomic force microscopy. As the amount of nitrogen was increased, the phases in the as-deposited films were identified as β-Ta, Ta2N (5% N2-flow), hexagonal TaN (10% N2-flow) and f.c.c.-TaN (15% N2-flow) with resistivities of 166 µΩ cm, 234 µΩ cm, 505 µΩ cm and 531 µΩ cm, respectively. Only the phase obtained at 5% N2-flow showed a reasonable uniformity over the wafer suggesting suitability as thin film resistors. The value of temperature coefficient of resistance (TCR) determined for the Ta2N thin film resistor was - 103 ppm/°C.

Mechanically induced Si layer transfer in hydrogen-implanted Si wafers

Mechanically induced layer transfer of single-crystal silicon by hydrogen ion implantation, low-temperature wafer bonding, and subsequent mechanical splitting of the implanted wafer has been investigated. The bond strength measurements using the crack opening method in room environment yield a surface energy of ⩾2000 mJ/m2 after exposure to oxygen plasma and subsequent hydrophilic silicon/silicon dioxide bonding at 200 °C. Mechanically induced layer transfer was carried out for silicon wafers implanted to a dose of 5×1016 H2/cm2 at 100 keV and annealed for 2 h at 200 °C. No feature was observed by atomic force microscopy (AFM) measurements on the unbonded free surface after this heat treatment. For lower doses of implantation, annealing at higher temperatures is required to enable the mechanical transfer. AFM measurements on the split silicon surface indicate that low-temperature wafer bonding and mechanical transfer yield a root mean square surface roughness of 4 nm which is less than in the standard Sma...

Failure mechanism of Ta diffusion barrier between Cu and Si

The reaction mechanisms in the Si/Ta/Cu metallization system and their relation to the microstructure of thin films are discussed on the basis of experimental results and the assessment of the ternary Si–Ta–Cu phase diagram at 700 °C. With the help of sheet resistance measurements, Rutherford backscattering spectroscopy, x-ray diffraction, a scanning electron microscope, and a transmission electron microscope, the Ta barrier layer was observed to fail at temperatures above 650 °C due to the formation of TaSi2, the diffusion of Cu through the silicide layer, and the resulting formation of Cu3Si precipitates. However, in order for the TaSi2 phase to form first, the Ta diffusion barrier layer must be thick enough (e.g., 50–100 nm) to prevent Cu diffusion into the Si substrate up to the temperature of TaSi2 formation (∼650 °C). Independent of the Ta layer thickness, Cu3Si was present as large nodules, whereas the TaSi2 existed as a uniform layer. The resulting reaction structure was found to be in local equil...

Cold ion-cutting of hydrogen implanted Si

Abstract The strength of the H-implanted layer has been measured in 〈1 0 0〉 , 〈1 1 1〉 and 〈1 1 0〉 oriented Si wafers using the crack opening method. The required annealing temperature for mechanical layer transfer increases in the order 〈1 0 0〉 , 〈1 1 1〉 and 〈1 1 0〉 . The damage induced by the implantation has been studied by Rutherford backscattering in the channeling mode (RBS/C). The same methods have been used to investigate the influence of boron and arsenic doping on the mechanical exfoliation. Boron doping reduces the strength of the H-implanted layer thereby enabling mechanical layer transfer at temperatures below 200 °C. We found that the exfoliation takes place closer to the wafer surface in highly boron doped Si as compared to the undoped Si. The RBS damage peak also appears to move closer to the surface when the boron concentration of the H-implanted layer is >1019 cm−3. No lowering of the exfoliation temperature was observed for compensated and arsenic doped Si layers. We suggest that the lowering of the exfoliation temperature with increasing boron doping is related to Si–H bonds associated with the neutralization of shallow acceptors by hydrogen.

Single crystal Si layers on glass formed by ion cutting

The process of ion cutting was used to integrate single crystalline Si layers on glass for potential active matrix flat panel display and other applications. It was found that p-Si wafers implanted at 100–150 ° C with H with a dose in the order of a few times 10 16 cm −2 could be readily bonded to glass substrates when both of the surfaces were properly treated and activated. The as-implanted Si wafer surface was converted from p type to n type. Upon bonding at room temperature, annealing (300 ° C ) and exfoliation (450 ° C ), the transferred Si layer on glass and the as-exfoliated surface of the implanted Si wafer remained n type. A highly defective region was observed near the top of the Si layer on glass, however the crystalline quality was nearly defect free in the deeper region of the layer. Annealing at sequentially higher temperatures led to the recovery of p type conductivity at ∼600–650 ° C . The type conversion and the subsequent annealing behavior observed on the samples were rationalized in terms of ion enhanced oxygen diffusion and the presence of H-related shallow donors in the Si.

Chemical stability of Ta diffusion barrier between Cu and Si

The reactions in the Si/Ta/Cu metallization system produced by a sputtering process were investigated by means of sheet resistance measurements, XRD, RBS, SEM and optical microscopy. In particular, the reaction sequence was emphasised. The reaction mechanisms and their relation to the microstructure and defect density of the thin films are discussed on the basis of the experimental results and the assessed ternary Si-Ta-Cu phase diagram at 700 °C. It was found out that the effectiveness of the Ta barrier is mainly governed by the defect density and their distribution in the Ta film. The failure was induced by Cu diffusion through the Ta film and almost simultaneous formation of Cu 3Si and TaSi2.

Amorphous layer formation at the TaC/Cu interface in the Si/TaC/Cu metallization system

An amorphous Ta(O,C)x layer was found to form at the TaC/Cu interface in the Si/TaC/Cu metallization system. The formation of the layer was induced by oxygen trapped in the as-deposited films, since on the basis of thermodynamic evaluation of the ternary Ta–C–O system, the dissociation of the TaC layer and the formation of the Ta2O5 and graphite can be expected to occur during subsequent annealings in this case. However, as observed experimentally, the formation of the amorphous Ta(O,C)x preceded the formation of the stable tantalum oxide.

SIS junctions with frequency dependent damping for a programmable Josephson voltage standard

Experimental and computational results on programmable Josephson junction array (JJA) chips based on superconductor-insulator-superconductor (SIS) junctions are presented. Implications of circuit design and fabrication process on the performance are discussed. We introduce a method to decrease the attenuation of the pump microwave. Different designs are compared, suggesting that 1 V chips operating at the third constant voltage step with 70 GHz pump frequency can be produced with our process.

A coupled DC SQUID with low 1/f noise

A low-noise coupled DC superconducting quantum interference device (SQUID) especially optimized for low frequency is discussed. Using large Josephson junctions and a low loop inductance the contribution of the critical current fluctuation to 1/f noise can be minimized. To minimize the inductance the Josephson junctions are placed in the center of the washer close to the SQUID loop. A scaled-up copper model of the SQUID and the signal coil is used to analyze the impedance of the SQUID loop affected by the signal coil. An equivalent circuit model describing the effective inductance of the SQUID loop as a function of the frequency is used to design appropriate damping of the resonances. The DC SQUID characteristics are smooth and the noise performance of the SQUID does not markedly suffer from resonances. The contribution of the 1/f noise at 1 Hz is found to be about 5*10/sup -7/ Phi /sub 0// square root Hz.<<ETX>>

Optimization of a Josephson voltage array based on frequency dependently damped superconductor–insulator–superconductor junctions

We have developed a programmable Josephson voltage standard based on an array of superconductor–insulator–superconductor junctions. The junctions are damped by an external frequency-dependent shunt circuit, which allows operation at Shapiro steps with n>1. We derive optimization criteria for the design parameters for a fast and stable array with low power consumption. An array able to generate 1.0 VRMS ac voltage signal with sub-ppm accuracy is experimentally demonstrated. Theoretically it is also shown that a fast programmable 10 V array can be realized.