Selective oxidation technologies for high density MOS

Selective oxidation technologies using various thicknesses of silicon nitride formed by low-pressure chemical vapor deposition (LPCVD), plasma assisted nitridation in ammonia, and by nitrogen ion implantation were investigated. The transition region ("bird's beak") profiles were found to be related to the rigidity of the nitride film and also the oxidation underneath the nitride film via the buffer oxide or even a native oxide. With complete elimination of any oxide between Si3N4and Si achieved by implanting with nitrogen ions or nitriding in an ammonia plasma, a very abrupt transition region was achieved. This new sealed interface localized oxidation (SILO) technology appears to have low crystal defect density suitable for VLSI MOS technology.     

Variations of temperature coefficient and noise in thin Al and Al/Si resistors subjected to high current density

Electromigration (EM) in thin Al and Al/Si resistors was investigated by measuring the effect of the stress on the resistance, the temperature coefficientα, the low-frequency noise spectrum (20 mHz-1 Hz) and, for some Al samples, on the residual resistance atT = 11 K. It was found that the temperature coefficient variations versus the stress time have a different behavior for Al and Al/Si, that is an oscillation in the first part of the life of the sample and decreases in the last one, in the first case, whereas, in the second, only the oscillation was detected. This behavior, observed for the first time, has been modeled by means of the Mayadas theory, by supposing a Gaussian distribution of the reflection coefficient for electrons at the grain boundaries. The measurement of the residual resistance also supports the hypothesis that the behavior ofa andR _o (the resistance value at 273 K) is mainly due to the modification of the grain-boundary structure and cannot be explained by taking in account only the variation of the residual resistivity. Besides, the noise measurements give a further proof that the origin of the 1/fg spectra in thin films subjected to high current density must be ascribed to phenomena occurring at the grain-boundaries which are correlated to the phenomena causing temperature coefficient variations. This work was supported by the Progetto Finalizzato MADESS, CNR, Italy.     

Measurement of high electrical current density effects in solder joints

Measuring mechanical implications of high current densities in microelectronic packaging interconnects has always been a challenging goal. Due to small interconnect size this task has typically been accomplished by measuring the change in electrical resistance of the joint. This measurement parameter is global and does not give local mechanical state information. Also, understanding strain evolution in the solder over time is an important step toward developing a damage mechanics model.The real-time, full-field, strain displacement in a eutectic Sn/Pb solder joint during electrical current stressing was measured with Moiré interferometry (Post et al., High sensitivity Moire, Springer, New York, 1994) under in situ conditions. A finite element model simulation for thermal stressing was performed and compared with measured strain. The initial results show that the measured strain was largely due to thermal stressing versus the current density of 1.8 × 10² A/cm². A second Moiré interferometry experiment with thermal control distinguishes deformation of solder joint due to pure current stressing above 5000 A/cm².     

高功率密度系统需要大电流转换器

本文介绍了降压型稳压器设计中需要注意的问题及面临的新挑战,并介绍了LTC7150S和LTC7130两款满足特定需求的降压型变压器.     

InGaP/InGaAs HFET with high current density and high cut-offfrequencies

Doped channel pseudomorphic In_0.49Ga_0.51P/In _0.20Ga_0.80As/GaAs heterostructure field effect transistors have been fabricated on GaAs substrate with 0.25 μm T-gates and self-aligned ohmic contact enhancement. By introducing the channel doping and reducing the series resistances, a high current density of 500 mA/mm is obtained in combination with cut off frequencies of f_T=68 GHz and f_max=160 GHz. The channel doping did not affect the RF-performance of the device essentially, which is additionally reflected in noise figures below 1.0 dB with an associated gain of 14.5 dB at 12 GHz     

Degradation of MOS tunnel structures at high current density

The stability of tunneling-thin (2–3 nm) SiO₂ films during prolonged flow of high-density currents (10²–10³ A/cm²) was investigated. A sharp increase in the charge which a tunneling MOS structure is capable of transmitting without degradation on switching from Fowler-Nordheim injection to direct tunneling (10³ C/cm² and 10⁷ C/cm², respectively) was observed. The degradation of SiO₂ films was investigated using Al/SiO₂/n-Si/p ⁺-Si thyristor structures with a positive bias on the semiconductor, i.e., with reverse bias of the MOS structure. The use of these devices accounted for the uniformity of the current distribution over the area and made it possible to monitor the state of the insulator layer by measuring the device gain in the phototransistor mode. Fiz. Tekh. Poluprovodn. 32, 743–747 (June 1998)     

Selective anodic oxidation of silicon in oxygen plasma

Plasma-enhanced oxidation process at low temperature was studied for the selective oxidation of silicon. Silicon dioxide films as thick as 1 µm were routinely obtained at the substrate temperature of 600 °C. The oxidation was found to proceed through the motion of both oxygen and silicon ions and/or their vacancies under the electric field applied across the oxide. Aluminum oxide film was used as a mask for selective oxidation and very small lateral oxidation under the mask was found. Furthermore, no generation of dislocations was found at mask edges. The properties of the oxide were found comparable to those of thermally grown oxide. Electron-spin resonance measurements revealed one kind of defects in the oxide, which was attributed to the interface states by comparing with results obtained by measuring the capacitance-voltage characteristics. The morphology of the Si-SiO2interface, studied by transmission electron microscopy was not significantly different from that of thermally grown SiO2-Si systems. Impurity redistribution in the substrate was not observed.     

Improved method of anodic oxidation of GaAs

A new and better electrolyte for anodic oxidation of GaAs is described. It is an aqueous solution of tartaric or citric acid mixed with glycol and can produce, in an extremely stable manner, native oxide films with remarkably improved dielectric properties.     

High current density Si field emission devices with plasmapassivation and HfC coating

A novel self-aligned process was developed to fabricate gated Si field emission devices. At a gate voltage of 100 V, the emission current from an array of 100 tips increased from 283 to 460 μA and the turn-on voltage decreased from 31 to 21 V after H₂ plasma passivation using an inductively coupled plasma (ICP) source for 2 min. The improvements correspond to a 1.28-eV reduction in the effective work function of the emitters and the instability of the emission current decreased from ±1,25 to ±0.25% after H₂ plasma passivation. Emitter tips were also coated with Mo silicide and HfC. The emission current increased from 230 μA for uncoated emitters to 268 μA for emitters coated with Mo silicide and 389 μA for emitters coated with HfC. The turn-on voltage decreased from 50 to 41 and 25 V while the breakdown voltage increased from 126 to 129 and 143 V when Mo silicide and HfC were used for coating, respectively, which correspond to reductions of 0.95 and 2.23 eV, respectively, in the effective work function of the emitters. Single emitter tips have similar emission characteristics as high-density field emitter arrays, indicating excellent emission uniformity from the arrays     

Reliability of InGaAsP light emitting diodes at high current density

InGaAsP LED's emitting at 1.3 µm are attractive sources for long distance transmission systems. These devices have been shown to have excellent reliability at 8 kA/cm². In order to launch more power into the optical fiber it is desirable to operate at the highest possible current densities consistent with system reliability requirements. In this work, the high temperature aging behavior of these LED's has been studied at current densities from 20 to 40 kA/². Both the standard LED structure, where the small diameter p-contact is isolated with a dielectric layer, and a structure in which a Schottky barrier is used for isolation are examined. Dark spot defect (DSD) formation is greatly enhanced at these high current densities in devices with dielectric isolation, limiting MTTF atT_{J} = 70degC to 2 times 10^{5}h. In contrast, devices with Schottky-barrier isolation remain essentially free of DSD's and haveMTTF > 10^{7}h at 70°C. These results suggest that stress from the dielectric layer promotes the growth of DSD's. Schottky-barrier devices in which the dielectric layer is eliminated are, thus, better suited for high current-density operation.     

Degradation mechanisms induced by high current density in Al-gate GaAs MESFET's

High gate-current density, which can arise due to RF overdrive, induces two degradation mechanisms in Al-gate power GaAs MESFET's: 1) high current density through the Al-gate finger section causes gate interruption at the beginning of the metal stripe due to electromigration and makes it impossible to reach the pinchoff condition; 2) high current density through the Schottky junction promotes a fast Al/GaAs interaction, causing an increase in the Schottky-barrier height from 0.80 to 0.96 eV owing to the formation of an AlxGa1-xAs interfacial layer. Al/GaAs interaction appears to be enhanced by the electron current at a given temperature.     

Mapping of electrical leakage in transistors by anodic oxidation

Recently developed electrochemical methods permit the optical mapping of large and small bipolar transistors in an integrated circuit for electrical leakage before metallization. In one methods the emitters of the leaky transistors are decorated by a deposit of amorphous silicon produced as a result of anodic dissolution of the n⁺silicon in that region. Other methods depend on the colors of the anodic oxide grown on the emitter and base regions of the leaky transistors. A close relationship has been established between the oxide growth on the emitter and the base regions and the defective electrical characteristics of the transistor.     

Folded-cavity surface-emitting InGaAs-GaAs lasers withlow-threshold current density and high efficiency

Folded-cavity surface-emitting InGaAs-GaAs lasers (FCSELs) that employ high-quality internal 45° deflectors are demonstrated with low-threshold current density and high efficiency. A simplified process involving a stop etch to position the surface emitting output mirror close to the waveguide and ion-beam-etching (IBE) to form the 45° deflecting mirror is presented. FCSELs (cavity length 800 μm) with two 45° deflectors, are obtained with threshold current density as low as 112.5 A/cm² and surface-emission external quantum efficiency as high as 65% (0.82 W/A). The additional loss contributed by the folded-cavity design is estimated as 4.2 cm^-1     

Reliability of aluminum-bearing ohmic contacts to SiC under high current density

The degradation produced by high current density stressing of a contact to p-SiC consisting of an Al-bearing ohmic contact, a TiW diffusion barrier, and a thick Au overlayer was studied. The test structure allowed for vertical current stressing and the measurement of the specific contact resistance before and after stressing. A threshold current for contact failure was established for the Ti/Al and W/Al contacts, at which a large increase in specific contact resistance was measured and extensive voiding occurred in the ohmic contact region. The high current stressing generated a flux of Al from the ohmic contact layer, through the TiW barrier, to the surface to be oxidized, along with a flux of Au into the ohmic contact layer. The voiding in the ohmic contact layer, caused by the unequal fluxes of Al and Au, decreased the active area of the contact, consequently increasing the current density and the associated effects from electromigration and Joule heating, initiating a runaway event.     

Solid-state anion migration in the anodic oxidation of lutetium diphthalocyanine

The solid-state anion-migration mechanism proposed recently to account for the results of moving-boundary measurements on lutetium diphthalocyanine films was confirmed by quantitative radiotracer experiments using chloride and sulfate electrolytes. The same mechanism was independently demonstrated for these anions and for bromide by semiquantitative energy-dispersive x-ray spectroscopy. The red oxidation product containing chloride ion decomposed gradually in ambient air, while that containing sulfate or bisulfate appeared to be stable. This paper was presented in part at the 21st Electronic Materials Conference in Boulder, Colorado, June 27, 1979.     

An aqueous developable photoimageable silver conductor composition for high density electronic packaging

In our preliminary work, we have formulated aqueous developable photoimageable thick film conductor pastes, consisting of ‘inhouse processed’ submicron sized silver powders (functional material), micron sized lead borosilicate glass frits (permanent binder), epoxy acrylate resin with pendant –COOH group (base photoimageable polymer/temporary binder/organic vehicle) and 2,2′-dimethoxy-2-phenylacetophenone (photoinitiator). The conductor pastes thus formulated were manually screen printed on the alumina substrates, dried, exposed to the ultra-violet light through the desired test patterns, developed in a 1% aqueous sodium carbonate solution, and subjected to a standard one-hour thick film firing cycle. Solid content of the polymer appears to influence the paste performance. Prima facie observations indicate that the paste with the organic:inorganic ratio of 28:72 (corresponding to solid content of 89.9% of the polymer system) exhibits better electrical conductivity, relatively smooth surface finish and line/space resolution of 100 μm with ±5 μm accuracy. Investigations related to the effect of glass content on the properties of photoimageable conductor paste are also furnished in this communication.     

4H-SiC normally-off vertical junction field-effect transistor with high current density

4H-silicon carbide (SiC) normally-off vertical junction field-effect transistor (JFET) is developed in a purely vertical configuration without internal lateral JFET gates. The 2.1-/spl mu/m vertical p/sup +/n junction gates are created on the side walls of deep trenches by tilted aluminum (Al) implantation. Normally-off operation with blocking voltage V/sub bl/ of 1 726 V is demonstrated with an on-state current density of 300 A/cm/sup 2/ at a drain voltage of 3 V. The low specific on-resistance R/sub on-sp/ of 3.6 m/spl Omega/cm/sup 2/ gives the V/sub bl//sup 2//R/sub on-sp/ value of 830 MW/cm/sup 2/, surpassing the past records of both unipolar and bipolar 4H-SiC power switches.     

Flexible air-stable three-dimensional polymer field-effect transistors with high output current density

Flexible air-stable short-channel polymer organic field-effect transistor (OFET) arrays with high saturated output current density are demonstrated by utilizing a novel solution-processed naphthobisthiadiazole (NTz) based donor–acceptor semiconducting polymer (PNTz4T) and designing a three-dimensional vertical channel structure with an extremely large ratio of channel width to channel length. The saturated mean field-effect mobility of 0.16 cm²/V s of the short-channel polymer devices remains over one month resulting in air-stable OFET arrays with high on/off ratio over 10⁶ and powerful current–density exceeding 0.3 A/cm² under low operation voltage, both of which meet the requirements for such applications as driving organic light-emitting diodes in active-matrix displays.     

Low temperature passivation of Si1−xGex alloys by dry high pressure oxidation

Thermal passivation of Si_1−xGe_x using high pressure (70 MPa) oxidation was studied for potential use in MOS-device applications. Alloys of CVD-grown Si_1−xGe_x (x = 10 and 15 at.%, 200 and 150 nm thick, respectively), were oxidized at 500 and 550°C using high purity dry oxygen at a pressure of 70 MPa. For comparative purposes, a second set of alloys were oxidized using conventional wet atmospheric pressure oxidation at 800°C. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, transmission electron microscopy (TEM), and metal-oxide semiconductor capacitance-voltage (C-V) measurements were used to characterize the as-grown oxides. Chemical analysis by XPS confirmed that under high pressure conditions compositionally congruent oxides are grown from these alloys. High resolution TEM and Raman spectroscopy show that the as-grown oxide/semiconductor interface is planar and free of Ge enrichment on a scale of 1-2 monolayers. A midgap interface state density for both the 10 and 15 at.% samples of 1 × 10¹² cm⁻² eV⁻¹ was estimated based on 1 MHz C-V measurement.