The temperature dependences of the spectral and total hemispherical emissivities of silicon have been experimentally determined, by using a technique which combines isothermal electron beam heating with in situ optical measurements. Emission spectra were used to deduce the absorption coefficient for phosphorus-doped silicon samples for wavelengths between 1.1 and 1.6 μm, in the temperature range from 330 to 800 °C. For lightly doped samples, the data show good agreement with a model which includes the effects of the various phonon-assisted processes involved in interband transitions in silicon, as well as the free-carrier absorption. For heavily doped samples the agreement was less satisfactory, possibly because of inadequacies in the model for free-carrier absorption. It was shown that reflection spectra can also be used to determine the absorption spectrum, for the range where the absorption coefficient lies between 1 and ∼70 cm−1. By fitting the theoretical model to the absorption coefficients derived from the reflection spectrum, it is possible to deduce the temperature of a sample, which is especially useful for temperatures less than 300 °C, where the thermal emission is very weak. The total hemispherical emissivity of the specimens was determined from the input electron-beam power densities and the measured temperatures. The total emissivity of a 390-μm-thick specimen of lightly doped silicon rises from 0.12 at 280 °C to a limiting value of 0.7 at 650 °C. This behavior is a consequence of the increase in the free-carrier concentration with the temperature. For heavily doped specimens the total emissivity remains approximately constant at ∼0.7 between 200 and 800 °C because the carrier concentration is high even at room temperature, and the additional thermal generation of carriers produces an insignificant change in the total emissivity.
Temperature nonuniformity is a critical problem in rapid thermal processing (RTP) of wafers because it leads to uneven diffusion of implanted dopants and introduces thermal stress. One cause of the problem is nonuniform absorption of thermal radiation, especially in patterned wafers, where the optical properties vary across the wafer surface. Recent developments in RTP have led to the use of millisecond-duration heating cycle, which is too short for thermal diffusion to even out the temperature distribution. The feature size is already below 100nm and is smaller than the wavelength (200-1000nm) of the flash-lamp radiation. Little is known to the spectral distribution of the absorbed energy for different patterning structures. This paper presents a parametric study of the radiative properties of patterned wafers with the smallest feature dimension down to 30nm, considering the effects of temperature, wavelength, polarization, and angle of incidence. The rigorous coupled wave analysis is employed to obtain numerical solutions of the Maxwell equations and to assess the applicability of the method of homogenization based on effective medium formulations.
Isothermal electron beam heating combined with in situ optical measurements has been used to measure the temperature dependence of the spectral emissivity of lightly doped silicon in the range 345–723 °C for wavelengths between 1 and 9 μm. The absorption coefficient was deduced from the spectral emissivity and compared with the predictions of a model including phonon-assisted processes involved in interband transitions, free-carrier absorption, and lattice absorption. The experimental data agree well with the model’s results.
The challenge of achieving maximal dopant activation with minimal diffusion has re-awakened interest in millisecond-duration annealing processes, almost two decades after the initial research in this field. Millisecond annealing with pulsed flash-lamps or scanned energy beams can create very shallow and abrupt junctions with high concentrations of electrically active carriers, but solutions for volume manufacturing must also meet formidable process control requirements and economic metrics. The repeatability and uniformity of the temperature cycle is the key for viable manufacturing technology, and the lessons from the development of commercial rapid thermal processing (RTP) tools are especially relevant. Advances in the process capability require a fuller understanding of the trade-off between dopant activation, defect annealing. diffusion and deactivation phenomena. There is a strong need for a significant expansion of materials science research into the fundamental physical processes that occur at the short time scales and high temperatures provided by millisecond annealing.
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