Ultra high vacuum properties of some engineering polymers

Engineering Polymers are very good candidates for applications requiring mechanical properties comparable with metals, chemical inertia, high insulation capability, high temperature operation and ultra high vacuum (UHV) compatibility. The results of a systematic test series, aimed at qualifying the engineering resins VESPEL/spl reg/ SP1, PEEK and CELAZOLE/spl reg/ PBI as UHV seals, are reported. The study of the materials behavior has been carried out over a wide temperature interval, ranging from 20 to 400/spl deg/C. In addition to the tightness and permeation tests, thermal desorption and gas chromatographic-mass spectrometer (GC/MS) analysis have also been performed. The results obtained indicate that CELAZOLE/spl reg/PBI provides the best performance, since it can be operated safely up to 375/spl deg/C, without giving any sign of leak or other drawbacks. PEEK, on the contrary, does not stand temperatures higher than 275/spl deg/C but, below this limit, it remains a very cost effective and reliable alternative. VESPEL/spl reg/ SP1, in its turn, can be operated safely up to 325/spl deg/C but above this temperature its properties start to degrade even if not in an abrupt manner as is the case for the other two resins. The possible applications of some of these polymers in the field of nuclear fusion research are also briefly described.     

Effects of manufacturing technology on electrical breakdown and morphology of thin films of low density polyethylene blended with polypropylene copolymer

Extruded films prepared from blends of low-density polyethylene (LDPE) and random copolymer of ethylene and propylene (EP) with the T-die method were studied with respect to electrical properties and morphology. Comparisons with data on blown films are made. These blends are of interest as improved LDPE for making XLPE for insulated power cable. In the high temperature region (90/spl deg/C), a specimen with a slightly higher EP content had higher impulse breakdown strength than that with a lower EP content, but no improvement of DC breakdown strength by blending could be found. The improvement of impulse breakdown strength (90/spl deg/C) is explained in terms of morphological changes by blending such as the orientation of chains in a film and the size of spherulites on the assumption of the thermal breakdown. In comparison, a T-die film had higher impulse breakdown strength than that of a blown film for the same composition. The impulse breakdown strength also increased with the use of the higher density LDPE. In the current versus electric field characteristics at 30/spl deg/C, the blend polymer with EP content of 5-10% showed a transition from LDPE behavior at low field region to EP behavior at high field region. However, no appreciable difference in current behavior among the specimens was observed at 90/spl deg/C, which suggests an incompatibility between the two materials that exists at 30/spl deg/C but not at 90/spl deg/C.     

Fluorescent lamp model with power and temperature dependence for high-frequency electronic ballasts

The electrical behavior of a fluorescent lamp operating at high frequency can be characterized by a resistance. The lamp equivalent resistance varies with the operating power and ambient temperature. On this basis, an electrical circuit model of the fluorescent lamp for high-frequency steady-state operation is developed. A test system is set up to emulate the dimming operation of fluorescent lamps in a temperature range from -15/spl deg/C to 55/spl deg/C. For a given lamp, the voltage equation can be obtained from few test data. Then, the equations for the arc current and the equivalent resistance can be derived. These lamp equations can be used for predicting the operating characteristics of the dimmable ballast-lamp circuits with considerations on the temperature effects. The verification of the proposed lamp model is illustrated by implementation examples.     

Recovery of shifted MOS parameters induced by focused ion beam exposure

The effects of focused ion beam (FIB) exposure on MOS transistors within a circuit were examined. It was found that FIB exposure does not cause parameter shifts as long as the gate is connected to the drain of other MOS transistors. However, the threshold voltage (V/sub t/) does shift during isolating the gate using a FIB. Further FIB exposure on MOS transistors with a floating gate is shown to cause larger shifts. Thermal annealing was studied to recover shifted V/sub t/. We demonstrated that a 400/spl deg/C-450/spl deg/C anneal could recover shifted V/sub t/ almost completely. Ninety percent recovery can be reached by annealing at 400/spl deg/C-450/spl deg/C for 1-2 hours, and V/sub t/ shifts can be reduced to about 10 mV.     

Electric charge transport and trapping in dielectrics deduced from isothermal and nonisothermal measurements

By sequential use of the isothermal charging, the isothermal discharging, the final thermally stimulated discharge current and the final isothermal discharging current techniques, the charge dynamics in highly insulating materials may be investigated. The method is demonstrated for polyethylene terephthalate. The injected charge for a field of 20 MV m/sup -/1 and polarization temperatures up to 110/spl deg/C is almost totally trapped in the material and is released during the heating of the sample at 180/spl deg/C for a sufficiently long time. A significant current at high temperatures, about 90/spl deg/C above the poling temperature, was observed proving that it originates from charge detrapping. The final thermally stimulated discharge current peaks shift to higher temperature when the polarization temperature increases, and are characterized by activation energies in the range from 1.03 to 1.56 eV. They allowed the identification of the glass transition around 114/spl deg/C. The relaxation time of the trapped charge, at 180/spl deg/C, was determined to be about 3780 s, explaining the very good stability of trapped charge.     

Highly accelerated electromigration lifetime test (HALT) of copper

The reliability of copper interconnects is an important aspect in ULSI technology. The test time of the standard electromigration test is rising with improving interconnect systems. At moderate current densities, lifetime test could last more than 500 h. In this paper, lifetime tests on via-line test structures in a copper dual-damascene technology at extremely high temperatures have been investigated. This method is an alternative solution to the well-known SWEAT method where high current densities are used to accelerate the lifetime test. The used test system was a modified Suss probe station with a self-made reactor. The results have been compared with standard tests performed in commercial oven test equipment. Bimodal behavior was observed above 425/spl deg/C. Only one of the two observed failure types shows the expected thermal dependency and can be extrapolated to the standard test temperatures with Black's equation. The estimated activation energy E/sub A/=0.81 eV is comparable to the activation energy determined by standard tests below 350/spl deg/C. The benefit of this method is a reduction in test time of more than a decade at 425/spl deg/C in comparison to the standard test at 300/spl deg/C and a moderate current density.     

Long-wavelength quantum-dot lasers on GaAs substrates: from media to device concepts

Recent progress in semiconductor quantum-dot (QD) lasers approaches qualitatively new levels, when dramatic progress in the development of the active medium already motivates search for new concepts in device and system designs. QDs, which represent coherent inclusions of narrower bandgap semiconductor in a wider gap semiconductor matrix, offer a possibility to extend the wavelength range of heterostructure lasers on GaAs substrates to 1.3 /spl mu/m and beyond and create devices with dramatically improved performance, as compared to commercial lasers on InP substrates. Low-threshold current density (100 A/cm/sup 2/), very high characteristic temperature (170 K up to 65/spl deg/C), and high differential efficiency (85%) are realized in the same device. The possibility to stack QDs (e.g., tenfold) without an increase in the threshold current density and any degradation of the other device parameters allow realization of high modal gain devices suitable for applications in 1.3-/spl mu/m short-cavity transmitters and vertical-cavity surface-emitting lasers (VCSELs). The 1.3-/spl mu/m QD GaAs VCSELs operating at 1.2-mW continuous-wave output power at 25/spl deg/C are realized, and long operation lifetime is manifested. Evolution of GaAs-based 1.3-/spl mu/m lasers offers a unique opportunity for telecom devices and systems. Single-epitaxy VCSEL vertical integration with intracavity electrooptic modulators for lasing wavelength adjustment and/or ultrahigh-frequency wavelength modulation is possible. Arrays of wavelength-tunable VCSELs and wavelength-tunable resonant-cavity photodetectors may result in a new generation of "intelligent" cost-efficient systems for ultrafast data links in telecom.     

Single-mode distributed feedback and microlasers based on quantum-dot gain material

Quantum-dot gain material fabricated by self-organized epitaxial growth on GaAs substrates is used for the realization of 980-nm and 1.3-/spl mu/m single-mode distributed feedback (DFB) lasers and edge-emitting microlasers. Quantum-dot specific properties such as low-threshold current, broad gain spectrum, and low-temperature sensitivity could be demonstrated on ridge waveguide and DFB lasers in comparison to quantum-well-based devices. 980-nm DFB lasers exhibit stable single-mode behavior from 20/spl deg/C up to 214/spl deg/C with threshold currents < 15 mA (1-mm cavity length). Utilizing the low-bandgap absorption of quantum-dot material miniaturized monolithically integrable edge-emitting lasers could be realized by deeply etched Bragg mirrors with cavity lengths down to 12 /spl mu/m. A minimum threshold current of 1.2 mA and a continuous-wave (CW) output power of >1 mW was obtained for 30-/spl mu/m cavity length. Low-threshold currents of 4.4 mA could be obtained for 1.3-/spl mu/m emitting 400-/spl mu/m-long high-reflection coated ridge waveguide lasers. DFB lasers made from this material by laterally complex coupled feedback gratings show stable CW single-mode emission up to 80/spl deg/C with sidemode suppression ratios exceeding 40 dB.     

Single-mode 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP quantum well vertical cavity surface emitting lasers

The 1.27-/spl mu/m InGaAs:Sb-GaAs-GaAsP vertical cavity surface emitting lasers (VCSELs) were grown by metalorganic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current varies from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% from 0.17 to 0.11 mW/mA as the temperature is raised from room temperature to 75/spl deg/C. The VCSELs continuously operate up to 105/spl deg/C with a slope efficiency of 0.023 mW/mA. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is estimated to be 10.7 GHz with modulation current efficiency factor of /spl sim/5.25GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C. We also accumulated life test data up to 1000 h at 70/spl deg/C/10 mA.     

1.3-/spl mu/m AlGaInAs strain compensated MQW-buried-heterostructure lasers for uncooled 10-gb/s operation

We have successfully fabricated 1.3-/spl mu/m AlGaInAs strain-compensated multiple-quantum-well (MQW) buried-heterostructure (BH) lasers by narrow-stripe selective metalorganic vapor-phase epitaxy. Based on the optimization of AlGaInAs strain compensated MQW and the Al-oxidation-free BH process, we obtained a low-threshold current of 12.5 mA and a relaxation frequency of more than 10 GHz at 85/spl deg/C for Fabry-Perot lasers. For distributed feedback lasers, we demonstrated a 10-Gb/s operation and transmission of over 16 Km for a single mode fiber at 100/spl deg/C. Furthermore, a record-low 25.8-mA/sub p-p/ modulation current for a 10-Gb/s modulation at 100/spl deg/C was demonstrated with shorter cavity and high grating-coupling coefficient. A median life of more than 1/spl times/10/sup 5/ h at 85/spl deg/C was estimated after an aging test of over 5000 h for these lasers. These superior characteristics at high temperatures were achieved by the combination of the high differential gain of AlGaInAs strain compensated MQW and the BH structure.     

Development of a silicone modified unsaturated polyester varnish for rated electrical insulation application

We report on the development of a silicone modified unsaturated polyester (SUP) using a novel combination of well known principles and rigorous calculation and control of mole fraction to produce what we believe the first SUP varnish which can be used in all insulation systems including those requiring application in vacuum (solvent free, single component). Silicone modified polyesters are available but not in solvent free and unsaturated varnish form and their chemistry is different. We believe that the nature of the chemical structure generated by the manufacturing protocol is critical to the properties of the cured SUP varnish. The properties of SUP are consistently better than those of unsaturated polyesters (UP) with respect to chemical resistance as defined by ASTM tests to moisture, acids, bases, toluene, transformer oil and silicone oil. IEC comparative tracking resistance was 300 drops vs 160-240 for UPs. Results of electrical properties vs temperature tests and heat deflection temperature suggested application to high voltage electric motors and other high temperature uses. Heat aging stability of bond strength of SUP treated helical coils was tested for tensile strength (ts) (using Stecker) and for flexural strength (fs) to determine the temperature index (TI) of the varnish alone. The end point was taken to be 14 kgf (50% loss for both ts and fs). Also, motorettes insulated with materials of high temperature class were treated with SUP for TI evaluation. The TI on a motorette was sufficient (208/spl deg/C) to suggest application at 200/spl deg/C rating level (20,000 h level). The TIs of helical and Stecker coils were 194 and 193/spl deg/C, respectively. The literature values of TIs of UPs on motorettes with similar insulating materials never exceeded 180/spl deg/C. These properties are consistently better than the properties of conventional unsaturated polyester.     

InP-GaInP quantum-dot lasers emitting between 690-750 nm

We describe the growth, material characterization, and device characterization of InP-GaInP quantum-dot lasers for operation in the wavelength range 690-750 nm. We show that the growth conditions have a major influence on the form of the gain spectrum. Relatively flat gain can be achieved over a spectral width of 90 nm at 300 K using samples containing a bimodal distribution of dot sizes, or narrower gain spectra at shorter wavelength can be achieved by suppressing the bimodal distribution by using (211)B substrates. Optimization of samples grown on substrates with the growth surface of (100) misorientated by 10/spl deg/ toward [111] results in laser operation between 729 and 741 nm and with a room temperature threshold current density as low as 190 A/spl middot/cm/sup -2/ for a 2000-/spl mu/m-long device with uncoated facets.     

A 5-V operated MEMS variable optical attenuator by SOI bulk micromachining

We report the design, fabrication, and successful demonstration of microelectromechanical variable optical attenuator (VOA) using an electrostatic microtorsion mirror (0.6 mm in diameter) combined with a fiber-optic collimator. The VOA operates at low voltages (dc 5 V or less) for large optical attenuation (40 dB, corresponding to mirror angle of 0.3/spl deg/) and a fast response time (5 ms or faster). The mirror made of a bulk-micromachined silicon-on-insulator wafer has been designed to be shock resistant up to 500 G without any mechanical failure. We also have suppressed temperature dependence of optical performance to be less than /spl plusmn/0.5 dB at 10-dB attenuation in the range of -5/spl deg/C-70/spl deg/C by mechanically decoupling the parasitic bimorph effect from the electrostatic operation.     

Gimbal-less monolithic silicon actuators for tip-tilt-piston micromirror applications

In this paper, fully monolithic silicon optical scanners are demonstrated with large static optical beam deflection. The main advantage of the scanners is their high speed of operation for both axes: namely, the actuators allow static two-axis rotation in addition to pistoning of a micromirror without the need for gimbals or specialized isolation technologies. The basic device is actuated by four orthogonally arranged vertical comb-drive rotators etched in the device layer of an silicon-on-insulator wafer, which are coupled by mechanical linkages and mechanical rotation transformers to a central micromirror. The transformers allow larger static rotations of the micromirror from the comb-drive stroke limited rotation of the actuators, with a magnification of up to 3/spl times/ angle demonstrated. A variety of one-axis and two-axis devices have been successfully fabricated and tested, in all cases with 600-/spl mu/m-diameter micromirrors. One-axis micromirrors achieve static optical beam deflections of >20/spl deg/ and peak-to-peak resonant scanning of >50/spl deg/ in one example at a resonant frequency of 4447 Hz. Many two-axis devices utilizing four rotators were tested, and exhibit >18/spl deg/ of static optical deflection at <150 V, while their lowest resonant frequencies are above 4.5 kHz for both axes. A device which utilizes only three bidirectional rotators for tip-tilt-piston actuation achieves -10/spl deg/ to 10/spl deg/ of optical deflection in all axes, and exhibits minimum resonant frequencies of 4096 and 1890 Hz for rotation and pistoning, respectively. Finally, we discuss the preliminary results in scaling tip-tilt-piston devices down to 0.4 /spl times/ 0.4 mm on a side for high fill-factor optical phased arrays. These array elements include bonded low-inertia micromirrors which fully cover the actuators to achieve high fill-factor.     

Space charge studies in LDPE using combined isothermal and nonisothermal current measurements

Using a recently developed procedure combining isothermal and nonisothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the nonisothermal measurements appeared a broad peak (40/spl deg/C to 50/spl deg/C) that was possible to decompose into two or three peaks (35, 45 and 65/spl deg/C). At even higher temperature another peak was sometimes present (85/spl deg/C) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of /spl ap/15 /spl mu/m). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the nonisothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.     

Charge storage and its dynamics in porous polytetrafluoroethylene (PTFE) film electrets

The outstanding space charge storage stability of porous polytetrafluoroethylene (PTFE) film electrets is studied by isothermal surface potential decay measurements and open-circuit thermally stimulated discharge (TSD) experiments after corona charging at room and elevated temperatures, or corona charging at RT and then aging at different temperatures. Charge storage properties of porous PTFE, nonporous PTFE (Teflon/spl reg/ PTFE) and nonporous FEP (Teflon/spl reg/ FEP) electrets are compared. The results show that porous PTFE has the best charge storage stability of organic materials for both negative and positive charges, especially at high temperatures. The structure of porous PTFE, investigated by a scanning electron microscope (SEM), is important for understanding the electret properties of this material. Charge dynamics, including the influence of environmental humidity and temperature on charge stability and shift of mean charge depth, and the kinetics of detrapped charges for the porous PTFE film electrets were also investigated by means of isothermal surface potential decay measurements and analysis of the TSD current spectra in combination with the heat pulse technique. It is found that from about RT to 200/spl deg/C slow retrapping plays a dominant role; from about 200/spl deg/C to 300/spl deg/C fast retrapping controls the transport.     

Off-state breakdown of GaAs PHEMTs: review and new data

This paper reviews the literature dealing with off-state gate-drain breakdown in MESFET and HEMT structures, with particular emphasis on GaAs PHEMTs, in terms of: 1) the physics of the breakdown phenomenon; 2) the breakdown walkout effect; 3) the impact of design and process choices on the breakdown behavior; and 4) the experimental techniques used for breakdown characterization. A thorough temperature-dependent breakdown characterization of commercial PHEMTs is also shown and discussed. It is found that different physical mechanisms may dominate the gate-drain leakage depending on the reverse bias and temperature range considered, and the particular PHEMT technology. The main results shown here tell us the following. 1) The breakdown voltages are decreasing functions of temperature between room temperature and 160/spl deg/C. 2) Between room temperature and 90-100/spl deg/C, thermionic-field emission seems be dominant, with low activation energies below 0.15 eV; as a consequence, the temperature dependence of the breakdown voltage is weak. 3) Between 110/spl deg/C and 160/spl deg/C, higher activation energy mechanisms (possibly trap-assisted tunneling and thermionic emission over a field-dependent barrier) tend to dominate, and the temperature dependence of the breakdown voltages is stronger.     

Charge storage in corona-charged polypropylene films analyzed by LIPP and TSC methods

Negatively corona-charged 50-/spl mu/m-thick polypropylene (PP) film is measured using laser-induced pressure pulse (LIPP) and thermally stimulated current (TSC) in order to study the charge storage mechanism in the PP film. The LIPP can reveal the space-charge distribution in the depth direction of the PP films and the TSC can be used to measure the energetic depth of the charge trap. The LIPP shows that negative charge is deposited on the charged surface of the sample. Almost all surface charges are removed by soaking the sample in ethanol. However, about 5% of the surface charge is injected into the sample up to a depth of about 7 /spl mu/m from the surface. The injected charge is not removed by the dip-in-ethanol method because the ethanol does not penetrate into the sample. The injection of the surface charge increases with corona-charging temperature. Besides the negative charge injection, the injection of positive charge from the opposite surface is also observed when the sample is charged at higher than 60/spl deg/C. In addition, negative bulk charge is formed when the sample is charged at higher than 80/spl deg/C. The LIPP profile is compared with the TSC spectrum. It is shown that the space charge observed using LIPP disappears when the temperature of the sample exceeds 80/spl deg/C. However, TSC is observed at even higher than 80/spl deg/C. This indicates that the TSC is observed even after the disappearance of the space charge measured using LIPP.     

Investigation and minimization of underfill delamination in flip chip packages

A generalized plane strain condition is assumed for an edge interfacial crack between die passivation and underfill on an organic substrate flip chip package. C4 solder bumps are explicitly modeled. Temperature excursions are treated as loading conditions. The design factors studied include underfill elastic modulus, underfill coefficient of thermal expansion (CTE), fillet height, and die overhang. Varying underfill modulus and CTE produces a different stress field at underfill/die passivation interface, different stress intensity factor (SIF), and phase angle (/spl psi/) even under the same loading condition. The baseline case uses underfill with elastic modulus of 6 GPa, CTE of 36 ppm//spl deg/C and fillet height equal to half die thickness. Four more cases involving underfill material properties are investigated by varying elastic modulus between 3 and 9 GPa, and by varying CTE between 26 and 46 ppm//spl deg/C. The effect of fillet height is also studied by assuming no fillet and full fillet, i.e., fillet height equal to die thickness. Finally, two cases concerning the influence of die overhang, defined as the nominal distance between outermost solder joint and die edge, are investigated. Fracture parameters, including energy release rate (G) and phase angle (/spl psi/), are evaluated as a function of dimensions. Underfill material properties (elastic modulus and CTE), fillet configuration, and die overhang can be optimized to reduce the risk of underfill delamination in flip chip or direct chip attach (DCA) applications.     

Long-wavelength vertical-cavity surface-emitting lasers on InP with lattice matched AlGaInAs-InP DBR grown by MOCVD

1.3- and 1.55-/spl mu/m vertical-cavity surface-emitting lasers (VCSELs) on InP have been realized. High-reflectivity AlGaInAs-InP lattice matched distributed Bragg reflectors (DBRs) were grown on InP substrates. 1.7 (for 1.3 /spl mu/m) and 2.0 (for 1.55 /spl mu/m) mW single mode power at 25/spl deg/C, 0.6 mW single mode power at 85/spl deg/C and lasing operation at >100/spl deg/C have been achieved. 10 Gbit/s error free transmissions through 10 km standard single mode fiber for 1.3-/spl mu/m VCSELs, and through 15 km nonzero dispersion shift fiber for 1.55-/spl mu/m VCSELs, have been demonstrated. With the addition of an SOA, 100 km error free transmission at 10 Gbit/s also has been demonstrated through a negative dispersion fiber. No degradation has been observed after over 2500-h aging test.