The formation of a sodium depletion layer during the electrostatic bonding of silicon to Pyrex is studied using transmission electron microscopy. In the cross section of a bond a sodium depletion layer is visible in the Pyrex near the bonded interface. Within this layer a black line is present, which is due to the pileup of less mobile potassium ions. The distance between the sodium depletion edge and the potassium pileup line is relatively insensitive to process conditions. The growth of the sodium depletion layer is studied as function of the three main bonding parameters: bonding time, voltage, and temperature. For long bonding times and higher bonding temperatures the sodium depletion layer reaches a maximum width, which is a function of the bonding voltage. For short bonding times, the width of the sodium depletion layer can be estimated from the external current measured during bonding. (C) 2003 The Electrochemical Society.