C. J. Doherty

Hydrogen passivation of point defects in silicon

Laser melting of crystalline silicon introduces electrically active defects which are observed by capacitance transient spectroscopy. The electrical activity of these point defects is neutralized by reaction with atomic hydrogen at 200 °C.

Study of Ni as a barrier metal in AuSn soldering application for laser chip/submount assembly

The possibility of replacing Pt in the Ti/Pt/Au base and traditionally used metallurgical structure by Ni, while bonding InP laser chip to a submount with AuSn (80% Au) solder, has been investigated. Various Ni‐based metal alloys have been prepared by evaporation. Reflow experiments were conducted in a chamber under forming gas‐controlled ambient. The Ti/Ni/AuSn system provided much longer surface local freezing duration compared to the Ti/Pt/AuSn system. Scanning electron microscopy analysis revealed a smoother surface morphology for the Ti/Ni/AuSn system after the metal refroze. Auger electron spectroscopy depth profiles indicated the formation of a Ni‐Sn‐Au interacted layer. The interaction took place in two steps: the first stage was the dissolution of Ni into the Au‐Sn liquid followed by precipitation of a Ni‐Sn‐Au intermetallic compound; the second stage was a solid‐state interdiffusion of Sn, Au, and Ni which occured in the interacted layer and in the original Ni layer. The latter step was a diffus...

Interfacial order in epitaxial NiSi2

High‐quality epitaxial NiSi2 films have been fabricated on 〈111〉 Si substrates and examined by grazing‐angle Rutherford backscattering and channeling techniques. The channeled backscattering yields are close to single‐crystal values and permit examination of the silicide‐silicon interface. The yields indicate that the number of disordered Ni atoms at the interface must be less than 1.5×1015 Ni/cm2.

Bonding of InP laser diodes by Au-Sn solder and tungsten-based barrier metallization schemes

Ti/W/Au-Sn and Ti/WxMy/Au-Sn schemes were studied as alternative metallization schemes to the traditionally used Ti/Pt/Au-Sn system for the bonding of InP laser diodes to heatsinks, and in particular to CVD diamond parts. The study comprised the Ti/W, Ti/W1(Au-Sn) and Ti/W1(Ni-Sn) barrier metal schemes, co-deposited in between the Au-Sn solder and the submount. In particular, reactivity and thermodynamic stability of the systems, acid the integrity of the barrier metal to the AuSn solder interface through the thermal bonding refreezing and reflow cycles were tracked. Premature freezing of the solder through the bonding cycles was attributed to the intermixing of the underlying barrier metal and the solder, suggesting an insufficient thermodynamic stability. Dewetting of the solder from the barrier metals through the reflowing cycle, subsequent to the completion of the bonding cycle, occurs due to the excellent inert nature of the solder to the barrier system, but exhibited the deficiency of poor solder to barrier metal adhesion. The TIM, system performed as an absolute inert barrier under the Au-Sn solder, in which no premature freezing phenomena were observed through the bonding cycle, resulting, however, in a delamination of the solder from the Ti/W, while reflown both under flux and forming-gas. In order to maintain the stable nature of this system, but to improve the barrier-solder interfacial integrity, W-Au, W-AuSn and W-NiSn co-deposited intermediate adhesion layers were introduced in between the W layer and the Au-Sn solder. As a result, the adhesion of the solder to the barrier metal improved, while the most stable performance was observed while applying the W/W(NiSn) barrier system under the Au-Sn solder. The first local freezing phenomenon of the bonding solder, while using this system, was observed only after heating the sample to 320 degrees C for more than 3 min and more than 1 h was needed to completely freeze the entire solder. in addition, an excellent solder to metal interfacial integrity was observed through the gas and flux reflow cycle. Thus, the W/W(NiSn) barrier metallization is recommended as a superior scheme to replace the traditionally used Ti/Pi system for bonding laser diodes to any type of submount using Au-Sn solder.

Vacuum‐evaporated films of chromium with the A‐15 structure

Continuous thin films of Cr 200–400 A thick and predominantly consisting of the A‐15 phase were grown by evaporation in high vacuum onto KCl and NaCl substrates held at 300 °C. The films have a chemical purity of better than 98%, determined by 4He Rutherford scattering. The A‐15 phase is not stabilized by impurities. The lattice parameter determined by electron diffraction is 4.60 A. The grain size is about 500 A. The films did not show superconductivity down to 0.5 °K.

Grain‐boundary diffusion of Ag through Cu films

The interdiffusion of bimetallic couples consisting of 1850 A Cu/1550 A Ag on sapphire substrates has been studied using Rutherford backscattering for anneal temperatures ranging from 225 to 500 °C. The Ag‐Cu couple was chosen because it represents a system in which only grain‐boundary diffusion as opposed to lattice diffusion should occur. No Ag‐Cu intermetallic compounds have been reported and the equilibrium solid solubility of Ag in Cu and Cu in Ag is under 3 at.% for the temperature range of interest. A series of in situ isothermal vacuum anneals established the following mechanism for interdiffusion in these couples. Initially, the Cu grain boundaries saturate with Ag. Then Ag atoms in the Cu grain boundaries diffuse to the Cu surface and spread out. Finally, the diffusion process ceases when about one monolayer of Ag covers the Cu surface. Also, at temperatures exceeding 350 °C, grain growth at the Ag‐Cu interface roughens that interface and makes Rutherford‐backscattering analysis more difficult. ...

Formation of p‐n junctions and Ohmic contacts at laser processed Pt‐Si surface layers

Pt‐Si alloy layers are formed during 130‐nsec pulsed irradiation of Pt coated Si with light from a Nd : YAG laser. The alloys form via surface melting and resolidifaction. Laser processing of arsenic of ion implanted, p‐type silicon results in formation of a p‐n+/Pt‐Si structure that exhibits rectifying electrical behavior. This structure arises because the As and Pt impurities are zone refined to different extents during resolidification of the molten surface layer.

CELLULAR SOLIDIFICATION OF LASER PROCESSED Ge-Si HETEROJUNCTIONS

125 nsec pulses of λ = 1.06 μm light from a Nd:YAG laser have been used to convert amorphous, vapor deposited layers of Ge and Ge + Si on (100) Si substrates to epitaxial heterostructures. Pulse energy densities in the range from 4 to 12 J/cm 2 melt the deposited layers, which regrow epitaxially. A cellular defect structure is formed in the resolidified material via a cellular crystal growth mechanism.

POST ILLUMINATION ANNEALING OF DEFECTS IN LASER-PROCESSED SILICON

Laser induced melting and resolidification of single crystal silicon introduces electrically active point defects. The electrical properties of laser processed silicon were monitored by capacitance transient spectroscopy (DLTS) and thermally stimulated capacitance (TSCAP) measurements. Removal of residual defects by thermal annealing occurs between 500°C and 700°C. More importantly, the defects are passivated by reaction with atomic hydrogen in the temperature range of 100–300°C.

Laser formation of Pt-Si Schottky barriers on silicon

Abstract135 nsec pulses ofλ = 1.06μm light from a Nd: YAG laser have been used to form Schottky barriers by irradiation of a 500Å thick metal film on n-type silicon. Large area barriers were fabricated by over-lapping individual 30μ diameter laser pulses of from 4 to 12 J/cm2. The barrier height was 0.73 ± 0.03 V, independent of the laser power. The barrier quality, as assessed by measurement of the forward current characteristic, decreased with laser power to a value of n = 1.5 at 12 J/cm2.