Patrick J. McNally

A comparative study of Pd/Sn/Au, Au/Ge/Au/Ni/Au, Au-Ge/Ni and Ni/Au-Ge/Ni ohmic contacts to n-GaAs

Abstract Under the LIGA process for fabricating microstructures having high aspect ratios and great structural heights, synchrotron radiation lithography produces a primary template which is filled with a metal by electrodeposition. The metallic structure so produced is used as a mould insert for fabricating secondary plastic templates which, in mass production, replace the primary template. This is a report about the status of work performed by the Karlsruhe Nuclear Research Center with the cooperation of Siemens AG and the Fraunhofer Institute for Solid State Technology: By irradiation and development of polymethyl methacrylate (PMMA) plates, primary templates were produced which, for structural heights of several hundred μm, exhibit deviations in critical dimensions of less then some 0.1 μm. The best results were obtained with an X-ray mask consisting of a 25 μm thick beryllium foil and 18 μm thick absorber structures consisting of copper and gold. Practically perfect metallic replicas were obtained by electrodeposition of nickel in the PMMA microstructures and even details in structure of less than 0.1 μm size were reproduced. Moulding was done with a methacrylate-based casting resin with an internal mould release agent. By electrodeposition of nickel in the secondary templates, secondary metal structures were produced which practically do not differ from the primary structures. The LIGA process can be expected to be superior to other methods for fabricating microstructures with high aspect ratios if, in series production of microstructures with complex shapes, stringent requirements are imposed on the resolution, the aspect ratio, the structural height, and the parallelism of the structural walls.

Determination of SF 6 reactive ion etching end point of the SiO 2 /Si system by plasma impedance monitoring

Plasma impedance monitoring is successfully used to determine the end point of reactive ion etching of a SiO2 layer lying on a Si substrate in SF6 plasma. The usefulness of this technique is demonstrated using a commercial Plasma Impedance Monitoring (PIM) system. The end point conditions are tested by monitoring changes in the fundamental and the first four harmonic components of the RF current, RF voltage, phase between RF voltage and current, RF discharge power and RF impedance. The best monitoring parameter found in this work is modelled as a polynomial equation of RF input power, chamber pressure and gas flow rate, from which the end point can be predicted with good precision and easily detected by the PIM. The end point conditions are confirmed by both the Fourier Transform Infrared Spectroscopy (FTIR) measurements and via observation of the plasma colour.

Highly conductive Sb-doped layers in strained Si

The ability to create stable, highly conductive ultrashallow doped regions is a key requirement for future silicon-based devices. It is shown that biaxial tensile strain reduces the sheet resistance of highly doped n-type layers created by Sb or As implantation. The improvement is stronger with Sb, leading to a reversal in the relative doping efficiency of these n-type impurities. For Sb, the primary effect is a strong enhancement of activation as a function of tensile strain. At low processing temperatures, 0.7% strain more than doubles Sb activation, while enabling the formation of stable, ∼10-nm-deep junctions. This makes Sb an interesting alternative to As for ultrashallow junctions in strain-engineered complementary metal-oxide-semiconductor devices.

Constraints on micro-Raman strain metrology for highly doped strained Si materials

Ultraviolet (UV), low penetration depth, micro-Raman spectroscopy, and high-resolution x-ray diffraction (HRXRD) are utilized as complementary, independent stress characterization tools for a range of strained Si samples doped by low energy (2keV) Sb ion implantation. Following dopant implantation, good agreement is found between the magnitudes of strain measured by the two techniques. However, following dopant activation by annealing, strain relaxation is detected by HRXRD but not by micro-Raman. This discrepancy mainly arises from an anomalous redshift in the Si Raman peak position originating from the high levels of doping achieved in the samples. This has serious implications for the use of micro-Raman spectroscopy for strain characterization of highly doped strained Si complementary metal-oxide semiconductor devices and structures therein. We find a direct correlation between the Si Raman shift and peak carrier concentration measured by the differential Hall technique, which indicates that UV micro-R...

Evaluation of mechanical stresses in silicon substrates due to lead-tin solder bumps via synchrotron X-ray topography and finite element modeling

Solder-based flip-chip packaging has prompted interest in integrated circuit (IC) packaging applications due to its many advantages in terms of cost, package size, electrical performance, input/output density, etc. The ball grid array (BGA) is one of the most common flip-chip packaging techniques used for microprocessor applications. However, mechanical stresses induced by the flip-chip process can impact adversely on the reliability of products. Synchrotron X-ray topography (SXRT), a non-destructive technique, has been employed to investigate the spatial extent of strain fields imposed on the underlying silicon substrate for Intel® Pentium® III microprocessors due to the lead-tin solder bump process for BGA packaging. Large area and section back-reflection SXRT images were taken before and after a simulation of the reflow process at 350 °C in atmosphere. The presence of induced strain fields in the Si substrate due to the overlying bump structures has been observed via the extinction contrast effect in these X-ray topographs. In addition, orientational contrast effects have also been found after the reflow process due to the severe stresses in the underlying silicon beneath the lead bumps. The estimated magnitudes of stress, |σ|, imposed on the underlying silicon were calculated to be of the order of 100 MPa. The spatial strains in the underlying silicon were relieved dramatically after the lead bumps were removed from the wafer, which confirms that the bumps are indeed a major source of strain in the underlying Si. Finite element modeling (FEM) has also been performed in two-dimensional (2-D) plane strain mode. The magnitudes and spatial distribution of the stresses after the reflow process are in good agreement with the SXRT results.

Synchrotron X-ray topography of undoped VCz GaAs crystals

For the first time vapour pressure controlled Czochralski (VCz) monocrystals of semi-insulating (SI) GaAs, grown at IKZ Berlin, have been investigated by synchrotron X-ray topography. The X-ray topographs of a typical VCz sample, taken from the cylindrical part, show dislocation images resembling those of SI vertical gradient freeze-grown GaAs crystals. From the disappearance of the dislocation image in selected topographs it is concluded that the Burgers vector for most dislocations is parallel to . The main part proves to be of 60° type. The cellular structure, typical for liquid encapsulated Czochralski material, is not seen in the VCz samples. Large volumes up to 0.5 x 0.5 × 0.5 mm 3 are dislocation-free. The results are compared with etch pit density (EPD) measurements from the same crystals. The average EPD is (1-2) x 10 4 cm -2 . The minimum value along is 2 × 10 3 cm 2 .

SYNCHROTRON X-RAY TOPOGRAPHIC AND HIGH-RESOLUTION DIFFRACTION ANALYSIS OF MASK-INDUCED STRAIN IN EPITAXIAL LATERALLY OVERGROWN GAAS LAYERS

Synchrotron x-ray back reflection section topographs of epitaxial lateral overgrown (ELO) GaAs samples grown on (001) GaAs substrates show images of the GaAs layers bent due to the interaction between the layer and the SiO2 mask. The topographs are simulated under the assumption of orientational contrast. Using the same data the measured x-ray diffraction curve is simulated. The calculations, which are in good agreement with the measurements, are used to gain information on the tilted (001) lattice planes in each ELO layer. We show that the bending of ELO lattice planes reaches a maximum at the center of the ELO stripes, where misorientation is at a minimum, and decreases towards the edges of the stripes, where misorientation reaches a maximum.

Antimony for n-type metal oxide semiconductor ultrashallow junctions in strained Si: A superior dopant to arsenic?

The creation of stable, highly conductive ultrashallow junctions in strained Si is a key requirement for future Si based devices. It is shown that in the presence of tensile strain, Sb becomes a strong contender to replace As as the dopant of choice due to advantages in junction depth, junction steepness, and crucially, sheet resistance. While 0.7% strain reduces resistance for both As and Sb, a result of enhanced electron mobility, the reduction is significantly larger for Sb due to an increase in donor activation. Differential Hall and secondary-ion mass spectroscopy measurements suggest this to be a consequence of a strain-induced Sb solubility enhancement following epitaxial regrowth, increasing Sb solubility in Si to levels approaching 1021cm−3. Advantages in junction depth, junction steepness, and dopant activation make Sb an interesting alternative to As for ultrashallow doping in strain-engineered complementary metal-oxide semiconductor devices.

Crack propagation and fracture in silicon wafers under thermal stress

The microcrack propagation and cleavage behaviour in silicon wafers during thermal annealing has been studied by in situ X-ray diffraction imaging (topography).

Geometric linewidth and the impact of thermal processing on the stress regimes induced by electroless copper metallization for Si integrated circuit interconnect technology

Mechanical strains and stresses are a major concern in the development of copper-based on-chip metallization. Synchrotron x-ray topography (SXRT), micro-Raman spectroscopy, finite element modeling (FEM), and atomic force microscopy (AFM) have been used to examine the strain fields imposed by electroless Cu metallization on the underlying Si. As expected, we have observed enhanced strain regions close to the metal line edges. These strain fields tend to zero at annealing temperatures approaching 200 ° C, and thereafter the magnitudes of the strain fields at 300 ° C and 400 ° C are much higher, implying a return to a higher strain regime. Although the strain transition point is slightly different from the SXRT result, the FEM results confirm the existence of a zero-strain transition point as a function of thermal anneal. We have also examined the generated stress in Si as a function of Cu linewidth L. We have found that the stress sXX due to the electroless copper metallization is empirically related to the Cu linewidth in terms of an exponential distribution. For Cu linewidths less than 20 mm, the stress magnitudes increased with decreasing Cu linewidth due to the thermal stress in the absence of self-annealing, whereas the stress decreased with increasing linewidths in the range of 60‐ 100 mm due to a relief of the thermal stress possibly via the self-annealing effect. This self-annealing phenomenon was observed using AFM. It is observed that the stresses in the Si shifted to a compressive state after annealing at 400 ° C.