Controlling the solubility of ALD aluminium oxide in deionised water

Riikka L. Puurunen • Jyrki Kiihamäki • Hannu Kattelus

VTT TECHNICAL RESEARCH CENTRE OF FINLANDVTT Information TechnologyVisiting address: Micronova, Tietotie 3, P.O. Box 1208FIN-02044 VTT, Finland

Tel. +358 20 722 6621Fax +358 20 722 7012riikka.puurunen@vtt.fiwww.vtt.fi

Solubility of as-deposited ALD-aluminium oxide

• To study the solubility of ALD-Al2O3 in deionised water (DIW), ALD-Al2O3 samples (grown in 1000 AlMe3/H2O ALD cycles) were immersed in DIW at 5-61°C for 30-1200 min. The thicknesses before and after immersion were measured by spectroscopic reflectometer(NanoSpec) in 5-point measurements.

•Al2O3 in the as-deposited state dissolved slowly in DIW.

• Etch rate was calculated from the thickness lost during immersion. The etch rate increased with temperature.

Introduction

• Conformal ALD-films with uniform thickness in the ten-nanometerrange find applications for example in the passivation of microelectromechanical systems (MEMS). ALD-aluminium oxide is also promising as an etch-stop layer in the dry etching of silicon.

• As a part of implementing ALD aluminium oxide in VTT’s MEMS processing, we have studied the solubility of ALD aluminium oxide in deionised water.

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VTT data Univ. Colorado (Ott et al., 1997) Univ. Helsinki (Matero et al., 2001) Helsinki Univ. Tech. (Putkonen et al., 2004)

Conclusions

• As-deposited ALD aluminium oxide dissolves in deionised water. Although the etch rate is low, the etching nevertheless prevents the use of as-deposited ALD-Al2O3 as a passivation layer in MEMS components, where the Al2O3 would be exposed to aqueous environments.

• The DIW-solubility of ALD-Al2O3 can be decreased practically to zero by heating so that crystallization of Al2O3 occurs.

• Depending on the substrate surface treatment, an unwanted “bubbling” phenomenon may occur during heating ALD-Al2O3. At the moment the mechanism of the bubbling is unclear, but it seems to be possible to avoid the bubbling by suitable surface treatments.

ALD growth characteristics

• Al2O3 was grown in a prototype of the Picosun SUNALETM ALD reactor. Substrates were Si (100) with 100 mm diameter. Before ALD, the substrates were subjected to the SC-1 and SC-2 cleans.

• The temperature dependency of the growth per cycle was similar as reported by other groups, and the growth was linear (in the detection limit of the reflectometric and ellipsometric measurements). The figures report the ellipsometer data. Thickness non-uniformity on a wafer (49 pt measurements) was typically <1%.

Effect on annealing on the structure of Al2O3

• The changes in Al2O3 structure during heating were monitored by measuring the refractive index (RI) by ellipsometer at 633 nm.

• RI was constant up to heating at about 750°C, beyond which (by 800°C) it increased to a higher level.

• The increase in RI was accompanied with a 10% decrease in film thickness.

• The change in refractive index is known to originate from the crystalli-zation of the originally amorphous aluminium oxide.

Solubility of annealed ALD-aluminium oxide

• To improve the stability of ALD aluminium oxide in DIW, sampleswere heated after the ALD step (N2, 30 min, 500 to 1000°C).

• The heat-treatment decreased the water-solubility of ALD-Al2O3significantly.

• After heating the samples to 800°C and beyond, the etch rate decreased below the detection limit (0.0003-0.01 nm/min, depending on the experiment).

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“Bubbling” detected during annealing of Al2O3

• In some samples, “bubbling” of ALD-Al2O3 was observed to occur during annealing (examples for rapid thermal annealing in Ar at 700°C for 5 min).

Acknowledgements

• Help with the experiments of especially Kimmo Solehmainen, KirsiJärvi, Meeri Partanen and Tapani Vehmas (VTT) and Sven Lindfors(Picosun) is gratefully acknowledged.

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• The bubbling could be avoided by pre-heating the samples (e.g., N2850°C, 1 h) before the ALD step, at least for samples grown in 1000 ALD cycles and heated after the ALD step at 500-900°C.

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