D. K. Reinhard

Temperature dependence and post-stress recovery of hot electron degradation effects in bipolar transistors

The authors present the results of a study of the BJT (bipolar junction transistor) degradation process due to hot electrons with the goal of better understanding the degradation rate of current gain and noise characteristics under various temperature and bias conditions. Degradation was produced by reverse biasing (-4 V) the base emitter junction of bipolar transistors at various temperatures (-75 to 240 C), with stress periods ranging from 1/60th of a second to over 1000 h. Post-stress recovery of the degradation was studied using both high-temperature annealing and base-emitter forward biases. Two mechanisms which decrease the rate of degradation at higher temperatures are the reduction in the number of hot electrons at higher temperatures and the simultaneous annealing of the states produced by hot electrons at higher temperatures. Experimental data are used to develop a model description of the hot-electron-induced gain degradation process which includes both the temperature dependence of the number of hot electrons and the temperature dependence of a simultaneous repassivation process which is observed at high ambient temperatures.<<ETX>>

Charged particle densities and energy distributions in a multipolar electron cyclotron resonant plasma etching source

The discharge downstream from a 9 cm i.d. multipolar electron cyclotron resonant (ECR) plasma was characterized in terms of positive ion density, electron energy distribution functions (EEDFs), ion energies incident on a biased conducting substrate, and silicon etching performance. Ion density in the process chamber was found to decrease exponentially away from the plasma source with a peak density of 6×1011 cm−3 in 1 mTorr argon with 250 W of microwave power. While EEDFs are non‐Maxwellian, no evidence of high‐energy ECR electrons was observed in the process chamber. The ion flux to a conducting substrate diverges by approximately 15° from normal but the ion incidence energy is easily controllable with a dc bias to the substrate. Despite the divergent ion flux from the ECR source, anisotropic etching of silicon was possible with etch rates of 300 nm/min. Uniform etching of silicon over 7.6 cm (3 in.) diam wafers was attained 8 cm below the plasma source.

Low temperature oxidation of silicon using a microwave plasma disk source

A new reactor concept, referred to as a microwave plasma disk source, is implemented and is used as a source of negative oxygen ions for low temperature oxidation of silicon. Oxide growth rates of 1000 A per hour, with only 100 W of absorbed power in the plasma, have been achieved on unheated silicon wafers in two configurations. In the first case, positively biased wafers are in actual contact with the plasma and in the second, the positively biased wafers are 15 cm downstream from the plasma ion source. In both cases the bias current density due to electrons and negative ions is on the order of 100 mA/cm2 and the plasma source electron density is on the order of 2×1012 cm−3. Oxide growth rates are correlated with plasma absorbed power, bias current, pressure, and plasma density. The reactor utilizes a single‐mode, internally tuned resonant cavity that can be continuously operated over a wide range of gas flow and pressure (>10 to <10−3 Torr). The plasma disk source is scalable with frequency. In the pre...

Electric field-dependent conductivity of polycrystalline diamond thin films

The dc electrical conductivity of polycrystalline diamond films with submicron grain sizes has been studied as a function of the applied electric field up to the point of electrical breakdown. For electric fields below approximately 105 V/cm, the films exhibited predominantly ohmic behavior. For higher electric fields, however, the conductivity was field activated according to Poole’s Law. The data is consistent with a Poole–Frenkel reduction of the ionization energy associated with Coulombic potentials surrounding ionizable centers, where the Coulombic potentials overlap. Consequently, the dielectric strength of the films in this study are lower than that reported for single‐crystalline diamond.

Plasma etching with a microwave cavity plasma disk source

A tunable resonant‐cavity microwave plasma disk source is applied to etching very large scale integrated circuit compatible profiles in crystalline silicon. Anisotropic etching downstream from the plasma is demonstrated without the use of large, potentially damaging wafer biases. Etch rate and degree of anisotropy are determined as a function of process chamber pressure, microwave power, and wafer bias. Etch rates from 200 to 490 A/min and degrees of anisotropy approaching 0.9 are obtained for pressures from 0.3 to 2 mTorr. Microwave power is varied from 150 to 200 W, and the wafer bias is varied from −10 to −50 V with a flowing mixture of 16 standard cm3/min (sccm)of CF4 and 4 sccm O2. Results are discussed in terms of simple microwave plasma etching theory and previously reported plasma characteristics.

An investigation of electromagnetic field patterns during microwave plasma diamond thin film deposition

The electromagnetic field patterns and intensities of a microwave‐sustained, diamond thin film deposition discharge were measured in a microwave plasma disk reactor (MPDR). The MPDR is a cylindrical, symmetrical cavity applicator which can be excited in single electromagnetic modes and utilizes internal matching and thus lends itself to experimental electromagnetic field diagnosis. The measured electric field intensities indicate that the plasma‐loaded TM011 mode provides optimum discharge excitation. During the thin film deposition conditions of 30–100 Torr and 100–300 sccm with mixtures of H2 /CH4 , the electric field intensities in the applicator are 100–200 V/cm and the tangential component of the electric field is the major electric field exciting the discharge. Measured discharge‐loaded cavity quality factors are 70–100 and microwave coupling efficiencies into the discharge are in excess of 98%; i.e., the transfer of microwave energy to the discharge is a very efficient process. Using estimated disc...

Hot-electron-induced degradation and post-stress recovery of bipolar transistor gain and noise characteristics

Hot-carrier-induced degradation and post-stress recovery of bipolar transistor gain and low-frequency noise are investigated. Forward-bias recovery allows a partial reversal of degradation, and is believed to be due primarily to a reduction of the number of electrons trapped in the oxide. Thermal annealing, which is capable of removing interface states as well, produces a larger recovery of both gain and noise performance measures. >

Fabrication and properties of ultranano, nano, and microcrystalline diamond membranes and sheets

Thin diamond membranes and free-standing sheets are of interest for a variety of potential applications. This article describes the film nucleation, microwave plasma-assisted chemical-vapor-deposition synthesis, and subsequent processing steps required to make free-standing strong and flexible diamond foils of several cm2. Films are initially deposited on silicon wafers as ultrananocyrstalline, nanocrystalline, or microcrystalline diamond by varying selected deposition parameters including gas composition, nucleation, power, substrate temperature, and pressure. Subsequently the diamond is separated from the original substrate and applied either to new substrates or to frames. Diamond membranes and sheets with thickness between 1 and 3μm have been fabricated from each of these film types. The sheets are drapable and can be applied to curved surfaces and wrapped around cylinders. Properties of the films including optical transmission, Young’s modulus and fracture strength are described. Several examples of ...

Adhesion of polycrystalline diamond thin films on single-crystal silicon substrates

Polycrystalline diamond films were deposited on (100) oriented silicon substrates using a microwave plasma disk reactor. Circular delaminations between the diamond thin film and the silicon substrate were produced by Vickers microindentation. While an overall relationship between mean delamination diameter and film thickness was not observed, an unexpectedly strong correlation was observed between delamination diameter and the square root of the coating grain size. A disadvantage of the Vickers microindentation technique is the damage sustained by the Vickers indenter tip after repeated loadings.

Temperature dependence of hot-electron degradation in bipolar transistors

Degradation of the base current and current gain observed in bipolar transistors that were electrically stressed at-75, 175, and 240 degrees C for 1000 h with a constant reverse-bias voltage applied to the emitter-base junctions is discussed. The rate of degradation was found to be temperature-dependent with a larger degradation occurring at the lower temperature. This temperature dependency is studied using an electron energy simulation technique and experimental data on degradation and postdegradation annealing. From the electron energy simulations, the number of hot electrons above a damage threshold energy was seen to increase with increasing ambient temperature at a constant reverse-bias voltage. This increase with temperature occurred because higher stress currents dominated over a reduction in the electron mean free path between collisions at higher temperatures. However, an actual degradation rate reduction at higher temperatures occurs because of simultaneous annealing of the states produced by hot electrons. A model that describes the temperature dependence of degradation and postdegradation annealing is described. >