Michael J. Bozack

HIGH-TEMPERATURE OHMIC CONTACT TO N-TYPE 6H-SIC USING NICKEL

Specific contact resistances measured at elevated temperatures for Ni ohmic contacts to 6H‐SiC were reported. The specific contact resistances were measured with the linear transmission line method at both room temperature and at 500 °C and yielded values <5×10−6 Ω cm2 at both temperatures. The trend shows a decreasing contact resistance at higher temperatures. The annealed metal film is a nickel silicide with substantial mixing of C throughout the silicide layer.

Reliability of the aging lead free solder joint

Solder materials demonstrate evolving microstructure and mechanical behavior that changes significantly with environmental exposures such as isothermal aging and thermal cycling. These aging effects are greatly exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In the current study, mechanical measurements of thermal aging effects and material behavior evolution of lead free solders have been performed. Extreme care has been taken so that the fabricated solder uniaxial test specimens accurately reflect the solder materials present in actual lead free solder joints. A novel specimen preparation procedure has been developed where the solder uniaxial test specimens are formed in high precision rectangular cross-section glass tubes using a vacuum suction process. The tubes are then sent through a SMT reflow to re-melt the solder in the tubes and subject them to any desired temperature profile (i.e. same as actual solder joints). Using specimens fabricated with the developed procedure, isothermal aging effects and viscoplastic material behavior evolution have been characterized for 95.5Sn-4.0Ag-0.5Cu (SAC405) and 96.5Sn-3.0Ag-0.5Cu (SAC305) lead free solders, which are commonly used as the solder ball alloy in lead free BGAs and other components. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. In our total experimental program, samples have been solidified with both reflowed and water quenching temperature profiles, and isothermal aging has been performed at room temperature (25 degC) and elevated temperatures (100 degC, 125 degC and 150 degC). In this paper, we have concentrated on reporting the results of the room temperature aging experiments. Variations of the temperature dependent mechanical properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of room temperature aging time. Microstructural changes during room temperature aging have also been recorded for the solder alloys and correlated with the observed mechanical behavior changes

Lasing in whispering gallery mode in ZnO nanonails

We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be 17MWcm−2. Very sharp emission peaks (full width at half maximum of 0.08nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.

The effects of aging temperature on SAC solder joint material behavior and reliability

The effects of aging on mechanical behavior of lead free solders have been examined by performing creep tests on four different SAC alloys (SAC105, SAC205, SAC305, SAC405) that were aged for various durations (0-4 months) at room temperature (25degC), and several elevated temperatures (75, 100, and 125 degC). Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes. Variations of the creep properties were observed and modeled as a function of aging time and aging temperature. In addition, the chosen selection of SAC alloys has allowed us to explore the effects of silver content on aging behavior.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.

Vibration thresholds for fretting corrosion in electrical connectors

Single frequency vibration tests were used to induce fretting corrosion in tin alloy plated contacts. The samples used in this study were connectors consisting of 25 pairs of mated pin and socket contacts. Experimental results for a variety of vibration levels, frequencies, and wiring tie-off lengths are presented. The experiments consisted of running a series of vibration tests at each frequency where the excitation level was stepped through a range of g-levels. During each test run contact resistance was monitored as a performance characteristic. The results exhibit threshold behavior at each frequency for the onset of fretting degradation. Typically a plateau region was observed where similar g-levels produced similar fretting rates. It was also found that outside the plateau region the g-levels varied according to the dynamic behavior of the mechanical system. In addition, a transfer matrix model was used to analyze these results. An empirical fit of the data correlated well with the model when damping was used. This analysis revealed the importance of the bending moment induced at the contact interface as a result of excitation levels and tie-off configurations. Consequently, it is concluded that dynamic response of the mechanical system under various g-levels and tie off configurations can greatly impact the performance of a connector system subjected to vibration stresses.

The effects of SAC alloy composition on aging resistance and reliability

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that the observed material behavior variations of Sn-Ag-Cu (SAC) lead free solders during room temperature aging (25 °C) and elevated temperature aging (125 °C) were unexpectedly large and universally detrimental to reliability. Such effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. However, there has been little work in the literature, and the work that has been done has concentrated on the degradation of solder ball shear strength (e.g. Dage Shear Tester). Current finite element models for solder joint reliability during thermal cycling accelerated life testing are based on traditional solder constitutive and failure models that do not evolve with material aging. Thus, there will be significant errors in the calculations with the new lead free SAC alloys that illustrate dramatic aging phenomena.

Reduction of lead free solder aging effects using doped SAC alloys

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging have been shown to be relatively small in conventional Sn-Pb solders. Aging effects for lead free solder materials are especially important for the harsh applications environments present in high performance computing and in automotive, aerospace, and defense applications. In the current investigation, we have extended our previous studies to include a full test matrix of aging temperatures and SAC lead free solder alloys. In an attempt to reduce the aging induced degradation of the material behavior of SAC solders, we are also exploring various doped SAC-X alloys. These materials are SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at room temperature (25 °C), and several elevated temperatures (50, 75, 100, and 125 °C). Four “standard” SAC alloys have been examined in this work including SAC105, SAC205, SAC305, and SAC405. This selection has allowed us to explore the effects of silver content on aging behavior (we have examined SACN05 with N= 1%, 2%, 3%, and 4% silver; with all alloys containing 0.5% copper). The doped SAC solder materials being considered in our ongoing studies include SAC0307-X, SAC105-X, and SAC305-X. In this work, we will concentrate on presenting the results for SAC0307-X (SAC-X), where X is 0.1%Bi. This alloy has been proposed as a lower cost SAC variation suitable for enhancing drop reliability. For all of the solders, variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC-X alloy illustrates reduced degradations with aging for all of the aging temperatures considered. The stress-strain and creep mechanical properties of SAC-X are better than those of SAC105 after short durations of aging, and approach those of SAC205 with longer aging times. After long term aging, the SAC-X alloy was found to have more stable behavior than all of the standard SACN05 alloys. Analogous tests were performed with 63Sn-37Pb eutectic solder samples for comparison purposes.

Etching of 6H‐SiC and 4H‐SiC using NF 3 in a Reactive Ion Etching System

The use of pure NF 3 source gas in reactive ion etching of bulk and epitaxy Si-face, 6H-SiC, and 4H-SiC is reported. The effects of RF power and chamber pressure on etch rate and surface morphology are discussed. A process developed for a smooth, residue-free etch, with a relatively high etch rate of ∼1500 A/min is examined using scanning electron microscopy and Auger electron spectroscopy surface analysis. The process developed had a self-induced dc bias ranging from 25 to 50 V, a forward RF power of 275 W (1.7 W/cm 2 ), chamber pressure of 225 mT, and a NF 3 flow rate between 95 and 110 sccm. No chemical residue or aluminum micromasking was observed on any of the samples etched with the above process.

Improved predictions of lead free solder joint reliability that include aging effects

It has been demonstrated that isothermal aging leads to large reductions (up to 50%) in several key material properties for lead free solders including stiffness (modulus), yield stress, ultimate strength, and strain to failure. In addition, even more dramatic evolution has been observed in the creep response of aged solders, where up to 10,000X increases have been observed in the steady state (secondary) creep strain rate (creep compliance). Such degradations in the stiffness, strength, and creep compliance of the solder material are expected to be universally detrimental to reliability of solder joints in electronic assemblies. Traditional finite element based predictions for solder joint reliability during thermal cycling accelerated life testing are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle models) that do not evolve with material aging. Thus, there will be significant errors in the calculations with lead free SAC alloys that illustrate dramatic aging phenomena. In our current research, we are developing new reliability prediction procedures that utilize constitutive relations and failure criteria that incorporate aging effects, and then validating the new approaches through correlation with thermal cycling accelerated life testing experimental data. In this paper, we report on the first step of that development, namely the establishment of a revised set of Anand viscoplastic stress-strain relations for solder that include material parameters that evolve with the thermal history of the solder material. The effects of aging on the nine Anand model parameters have been examined by performing stress strain tests on SAC305 samples that were aged for various durations (0-6 months) at a temperature of 100 C. For each aging time, stress-strain data were measured at three strain rates (0.001, 0.0001, and 0.00001 1/sec) and five temperatures (25, 50, 75, 100, and 125 C). Using the measured stress-strain data, the Anand model material parameters have been determined for various aging conditions. Mathematical expressions were then developed to model the evolution of the Anand model parameter with aging time. Our results show that 2 of the 9 constants remain essentially constant during aging, while the other 6 show large changes (30-70%) with up to 6 months of aging at 100 C. Preliminary finite element simulations have also shown that the use of the modified Anand model leads to a strong dependence of the calculated plastic work dissipated per cycle on the aging conditions prior to thermal cycling.