N2O plasma etching of polyimides

Vacuum/volume

45humber

IO/l l/pages 1101 to 1102/1994 Elsevier Science Ltd Printed in Great Britain 0042-207x/94 $7.00+.00

Pergamon 0042-207X193)E0027-1

N,O plasma etching of polyimides J Muiioz and C Dominguez, Centro National

de Microelectr6nica-CSIC,

Campus UAB, 08193 Bellaterra, Spain

The N,O-based dry etching of a commercial polyimide has been investigated as a function of the process parameters. Surface modifications were found on the polyimide films depending on the pressure of the etching process. At pressures higher than I torr, oxygen-rich compounds were the main reactant species, producing high etch rates due to oxidation and scission of carbon chains. In contrast, at lower pressures nitrogen-rich species reacted with the polyimide surfaces, changing the etching mechanism to nitrogen addition into the polymeric material, as shown by FTIR spectra.

1. Introduction Polyimide materials are widely accepted and used in the electronic industry as interlayer dielectrics, as passivation layers and for other applications. Polyimide films provide excellent step coverage and can be used as a planarizing material in lithographic processes. The applications of polyimides involve patterning of the film by different etching techniques. Dry techniques are the most widely used for etching of polyimide films. The etch rate can be enhanced by mixing oxygen with other fluorine containing gaseous compounds such as CF, ‘, SF, ’ and NF, ‘. Structural changes on the polymeric surface during plasma etching processes have been studied by infrared spectroscopy techniques4. These analyses pointed out that oxygen-containing plasma treatments can generate an additional carbonyl function, thus modifying the surface properties. As an alternative to oxygen plasma, and in order to improve the directionality of the etching mechanism, oxygen mixtures with non-reactive gases have been used. Although the etch of polymeric films by N20 as a reactant gas is not very well understood’, in a N20 glow discharge NO, N02, N, and O2 are the main reactant specie@.

In this work, the dry etching of commercial polyimides by N20 plasma is reported. A change in the etching mechanism takes place as a function of the pressure of the reaction chamber, due to variations in the gas phase composition. As a consequence, the chemical modification of the polymer was characterized by FTIR spectroscopy.

2. Experimental In all experiments 2-in. silicon (n-type, 12-14 L2 cm, and (100) orientation) wafers were used as substrates. Onto these substrates a thermal silicon oxide layer of 600 8, was grown. The polyimide was supplied from OCG (HTR 3-200). Before applying the polymer a silylation process was carried out by employing a solution at 0.1% of 3-aminopropyltriethoxysilane (APS, Merck) in a methanol/water (19 : 1) solvent mixture. It was applied onto the oxide surface using the spinning technique, and then the substrate was baked at 90°C for 10 min. The polyimide was applied in two stages, and softbaked on a hot plate to obtain a thickness of approximately 10 pm. Measurements of etch depth were performed using a Tencor Alphastep 200 profilemeter.

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