Buried channel technology for chemical microsystems

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School of Electrical Engineering | Master's thesis
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Optiikka ja molekyylimateriaalit
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xi + 68
Buried micro channels are made by deep reactive ion etching (DRIE) and conformal deposition methods. The processes rely on anisotropy/isotropy control in DRIE, and closing of the channels by polysilicon or alumina using low pressure chemical vapour deposition (LPCVD) or atomic layer deposition (ALD) alone or in combination. First DRIE depth of silicon, sidewall passivation material and methods, passivation removal etches, isotropic DRIE lateral etch rate and closing deposition conformality were characterized Based on the above characterization, the design rules of the buried channel technology were optimized. The use of ALD alumina resulted in a better sidewall passivation compared to thermal oxide and LPCVD silicon nitride due to its massive selectivity against silicon in DRIE. Extremely thin layers (less than 30nm) of ALD alumina were employed, which enabled extensive control over the size of the buried cavities during isotropic DRIE. The closing of the channels was carried out with ALD alumina and LPCVD polysilicon. The channel closure and surface profile were analyzed and it was observed that ALD alumina provides improved surface quality and channel closure compared to LPCVD polysilicon. ALD has a slow growth rate, which is a major limitation to deposit thicker layers (greater than 1µm) for the sealing of the micro channels with wider openings (greater than 2µm). LPCVD polysilicon is a better candidate for wider channel openings but improvement in the surface profile is needed. Reduction in the grain size serves the purpose and can be achieved with careful adjustment in the LPCVD process parameters. Buried channels enable concentric micro channels to be made, which is of considerable interest in many fluidic applications. The process flow is ideal foe making 3D micro fluidic networks for fluidic handling. New applications are foreseen in microfuel cells, micro reactors and mass spectrometry ships.
Franssila, Sami
Thesis advisor
Sainiemi, Lauri
buried channels, deep reactive ion etching, ALD alumina
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