NUMERICAL SIMULATION OF RADIANT THERMAL PROCESSING OF BILAYER MICROCANTILEVERS

Multilayered thin-film structures in microelectro-mechanical systems (MEMS) and other microscale devices affect the radiative properties of the structures due to microscale heat transfer effects, which include thin-film interference, diffraction, and scattering. The difference between the radiative properties of the wafer and those of the structures creates a non-uniform wafer temperature during radiant thermal processing. This temperature non-uniformity may lead to defects and non-uniform processing. This paper presents the results of a numerical investigation of the radiative properties of bilayer microcantilever beams consisting of SiO2 and Si3N4 layers, separated from a Si substrate by an air gap created by etching. The study focuses on the effects of changes in the absorptivity (α) and emissivity (ε) of the beams due to varying etching depth (η). The radiative Properties calculated with numerical models are used in a simulation of the radiant heating of patterned wafers, studying the effects of Pattern size and separation on the temperature uniformity of the wafer. For a beam with Si3N4 thickness of 2000 Å and SiO2 thickness of 5000 Å, the results show that the total absorptivity and emissivity are highly dependent on the etch depth (η) for values less than approximately 1.5μm. For larger η, the radiative properties remain constant, approximately equal to those of the Si substrate. The temperature distributions for a patterned wafer become more uniform as the pattern size and spacing decrease. When the patterning size decrease for a given separation, the result is a better thermal interaction between the die and the wafer, resulting in a smaller temperature gradient. This, however, creates a large temperature difference between the wafer patterning area and the outer edge region, which results in an increased temperature gradient along the wafer edge. These heat transfer effects should be taken into consideration when creating fabrication mask sets radiant for thermal processing to design a pattern layout that would best avoid the problems caused by thermal gradients