Chuan Hu

Noise characteristics of thin multiplication region GaAs avalanche photodiodes

It is well known that the gain‐bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This letter presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite ‘‘size effect’’ for multiplication regions less than approximately 0.5 μm. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process.

Thermal conductivity study of porous low-k dielectric materials

An experimental method based on the 3ω technique has been developed to measure thermal conductivity of porous Xerogel films as a function of porosity. The results show that the thermal conductivity of these porous dielectric films can be an order of magnitude smaller than that of SiO2. To account for the porosity dependence of thermal conductivity, two porosity weighted semiempirical models are introduced. These models suggest the scaling rule expressing the thermal conductivity as a function of porosity. The decrease observed in thermal conductivity of porous films suggests that the tradeoff between thermal and electrical performance is an important consideration when implementing porous dielectric materials as interlevel dielectrics for on-chip interconnects.

HIGH-SPEED RESONANT-CAVITY SEPARATE ABSORPTION AND MULTIPLICATION AVALANCHE PHOTODIODES WITH 130 GHZ GAIN-BANDWIDTH PRODUCT

Previously it has been shown that resonant-cavity, separate absorption and multiplication (SAM) avalanche photodiodes (APDs) exhibit high peak external quantum efficiency (∼75%), low dark current, low bias voltage (<15 V), and low multiplication noise (0.2<k<0.3). We describe the frequency response of resonant-cavity AlGaAs/GaAs/InGaAs SAM APDs. A unity-gain bandwidth of 23 GHz and a high gain-bandwidth product of 130 GHz have been achieved.

Porosity effect on the dielectric constant and thermomechanical properties of organosilicate films

This letter reports a study of the porosity effect on material properties of methylsilsesquioxane films, including the dielectric constant, thermal conductivity, and thermal stress behavior. In a porosity range from 0% to 50%, both the dielectric constant and thermal conductivity decreased with increasing porosity and no significant change was observed at the percolation point where pores became interconnected. In comparison, the stress–temperature slope also decreased with porosity, but as the porosity approached the percolation point, the slope showed a large drop of 40%, indicating a significant degradation of the thermomechanical properties due to percolation of pores. Assuming the coefficients of thermal expansion remain at 17 ppm/°C within the porosity range, the change in the stress–temperature slope corresponds to a decrease of the biaxial modulus from 7 to 5 GPa around the percolation point.

Thermal stress and glass transition of ultrathin polystyrene films

The thermal stress of thin and ultrathin polystyrene (PS) films on Si substrate has been studied and the glass transition temperature (Tg) is determined from the thermal stress data. Tg of PS turned out to be thickness independent for thick films but decreases when the film thickness is comparable to the end-to-end distance of the polymer chains (<100 nm). The thermal stress level and the slope of the stress temperature curve of the film also decrease as the film thickness decreases. The slope reduction indicates that the product of the biaxial modulus E/(1−ν) and the coefficient of thermal expansion (CTE) of the film decreases with film thickness. Assuming that the CTE increases for ultrathin films, the modulus is found to decrease significantly with respect to the bulk value.

Thermal conductivity and interfacial thermal resistance of polymeric low k films

The effective thermal conductivity of four polymeric thin films with distinct molecular morphologies has been measured as a function of film thickness down to 70 A using a 3ω technique. Comparing to SiO2, the intrinsic thermal conductivity of polymers is about 5–10 times smaller while the interfacial thermal resistance is about 2–10 times larger. The interfacial thermal resistances are explained in terms of the mismatches of acoustic and mechanical properties between polymers and crystalline materials. Both elastic and inelastic scatterings at the interface are examined.

Characteristics of GaAs and AlGaAs homojunction avalanche photodiodes with thin multiplication regions

The gain and noise of thin GaAs and Al0.2Ga0.8As homojunction avalanche photodiodes were measured. The gain and the excess noise factor were found to be significantly lower than would be expected using ionization coefficients reported in the literature. The discrepancy is believed to be due to physical effects that become significant in thin multiplication layers. It is shown that the gain and excess noise under electron injection can be accurately fit using conventional models with width-dependent ionization coefficients.

THERMAL DIFFUSIVITY MEASUREMENT OF POLYMERIC THIN FILMS USING THE PHOTOTHERMAL DISPLACEMENT TECHNIQUE. I. FREE-STANDING FILM CASE

The photothermal displacement technique has been used to measure the out-of-plane thermal diffusivity in free-standing polymer thin films. The technique can be applied to a single sample as well as a collection of samples of different film thickness. Polymers are well suited for this method because they usually possess a large vertical coefficient of thermal expansion and readily absorb UV laser radiation. In particular, this method yields a value of the thermal diffusivity for polymer films with thickness ranging from 125 to about 10 μm. Different polymers have been studied with particular attention paid to UPILEX-S (biphenyl tetracarboxylic dianhydride-p-phenyl diamine) polyimide whose thermal diffusivity has been determined to be 0.0025±0.0005 cm2/s. Our study shows that the measurements are not strongly affected by uncertainties in the optical absorption coefficient of the polymer as well as photothermal displacement contributions from the backside of the free-standing film. Furthermore, neither the e...

Determining effective dielectric thicknesses of metal‐oxide‐semiconductor structures in accumulation mode

Metal‐oxide‐semiconductor (MOS) capacitance–voltage (C–V) characteristics in the accumulation mode have been measured and simulated for polycrystalline Si gate MOS capacitors with various oxide thicknesses (40–200 A) on p‐type (100) Si substrates. The discrepancy between experimental data and theoretical prediction by classical MOS theories is clarified by taking quantization effects into account. The experimentally determined ‘‘effective dielectric thicknesses’’ in the semiconductors are found to be in good agreement with the values calculated from quantization effects for MOS capacitors with thinner oxides (<80 A). The effective dielectric thicknesses at oxide electric fields of 2–6 MV/cm have been determined to be 2–3 A larger for the quantum mechanical case than for the classical case.

Thermal diffusivity measurement of polymeric thin films using the photothermal displacement technique. II. On-wafer measurement

A three-dimensional analytical solution as well as experimental verification of the thermoelastically induced deformation in a substrate-constrained thin polymeric film have been developed. In this model, the elastic deformations of the two layers are treated separately since the typical polymer has a much smaller Young’s modulus but a much larger out-of-plane coefficient of thermal expansion than a silicon wafer. Results from measurements of biphenyl tetracarboxylic dianhydride-p-phenyl diamine thin films on silicon wafers agree very well with calculations based on this analysis, especially by using the sensitive frequency modulation method as outlined in part I of this two-part series. Thus, this development of a two-layer thermoelastic deformation model and measurement has advanced this technique to a thinner thickness scale with improved precision. This method is also amenable to studies of the interfacial thermal resistance Rt, and numerical analysis shows that typical interfacial thermal resistances...