Sodium contamination of SiO₂caused by anodic bonding

Abstract

In this paper we present an investigation of sodium contamination of SiO2 (oxide) during anodic bonding. Sodium contamination can be deleterious to the electrical properties of silicon structures. Silicon wafers with metal–oxide semiconductor (MOS) capacitors were bonded to Corning 7740 (Pyrex) glass wafers. The concentration of mobile ions was measured on capacitors outside and within glass cavities using the triangular voltage sweep method. Using secondary ion mass spectrometry analysis, it was confirmed that the ions were sodium. We found an increase in sodium concentration Nm between 1010 and 1013 cm−2, depending on the oxide location and the geometry of the glass cavity. The gate aluminium of the MOS capacitor was found to partly shield the oxide from contamination, causing a two to five times smaller increase in Nm. Reducing the bonding voltage from 800 to 500 V did not affect the increase in Nm significantly. In contrast, changing the ambient in the bonding chamber from vacuum to 1020 mbar air, reduced the contamination of capacitors situated outside the glass. A plasma-enhanced chemical vapour deposited Si3N4 film was found to be very beneficial in protecting the capacitors. The Si3N4 prevented sodium contamination of the capacitors situated within the glass cavities, and radically reduced the contamination of the capacitors situated outside the glass. The results suggest that the contaminating sodium originated from the bulk glass.

Suspended microchannel resonators with piezoresistive sensors

“…31 The PECVD oxide was patterned and removed from the cantilever surface before the anodic bonding to minimize bending due to the mismatch of thermal expansion. Subsequent fabrication steps are identical to those for SMRs without piezoresistive readout.…”

Anodic bonding

“…72 All of these new lithiumcontaining glasses have been reported to have successfully bonded to silicon at y200uC in the presence of a suitable electric field. [69][70][71][72] The effects of sodium contamination during anodic bonding have recently been addressed by Schjølberg-Henriksen et al 73,74 in the context of monolithically integrated MEMS in which the relevant electronics for the device to operate sit together with the device on a single chip. Using metal-oxide semiconductor (MOS) capacitors as test structures, they have demonstrated that sodium contamination can occur during anodic bonding, but that its effect can be minimised to an acceptable level by the application of a 100 nm thick protective plasma-enhanced chemical-vapour-deposited silicon nitride layer protecting the electronics.…”

“…Using metal-oxide semiconductor (MOS) capacitors as test structures, they have demonstrated that sodium contamination can occur during anodic bonding, but that its effect can be minimised to an acceptable level by the application of a 100 nm thick protective plasma-enhanced chemical-vapour-deposited silicon nitride layer protecting the electronics. If such precautions are taken during anodic bonding, together with suitable precautions which avoid high electrical fields across sensitive electrical components, 73 the perceived incompatibility of anodic bonding with microelectronic components can be overcome. As Schjølberg-Henriksen et al 73 note, this will enable further device miniaturisation and may also lower production costs.…”

Wafer-level MEMS package and its reliability issues

“…1 is also effective to protect CMOS [11]. Na ion in a borosilicate glass is harmful for CMOS [12], but pn junctions are normally protected with a back-end-of-line (BEOL) layer. Therefore, this is more problematic for piezoresistors for strain sensing.…”