Matthias Boehringer

Porous Silicon in a Semiconductor Manufacturing Environment

In the Bosch-proprietary “advanced porous silicon membrane process”, porous silicon (PSi) is used for the first time in high-volume industrial production of microelectromechanical devices. Nanoporous silicon acts as an auxiliary layer during manufacturing of monolithically integrated pressure sensors in a mixed-signal IC process supplemented by microelectromechanical-systems-specific steps in the front end of line. In this paper, the technical design and performance of a fully automated production tool capable of high-volume fabrication of PSi under the specific constraints of a semiconductor manufacturing environment are discussed. The process requires stringent control on the PSi layer thickness, uniformity, porosity, and morphology. The impact of chamber and electrode geometry, the electrolyte flow, and the mode of current coupling into the wafer back side on the uniformity of the PSi layer is addressed. The need for a well-defined PSi morphology demands high reproducibility and stability of the electrolyte composition, particularly with respect to the hydrofluoric acid concentration.

Improved copper detection in hydrofluoric acid by recombination lifetime measurements on dedicated silicon substrates

Copper (Cu) adsorption from diluted hydrofluoric acid (DHF) onto bare silicon surfaces strongly depends on the substrate doping. On highly phosphorus-doped silicon, the adsorption rate is up to three orders of magnitude larger than on moderately doped silicon. This may open a gap between Cu-induced semiconductor device degradation and the detection of Cu contaminations in DHF by minority carrier lifetime measurements. Using dedicated copper monitor wafers where a highly phosphorus-doped backsurface ensures strong Cu adsorption while a moderately doped frontsurface enables minority carrier lifetime measurements, we are able to improve the limit of detection for Cu in DHF by two orders of magnitude.

In-Line Copper Contamination Monitoring Using Noncontact Q ­ VSPV Techniques

A novel approach for the sensitive detection and unequivocal identification of trace amounts of copper introduced into p-type silicon and its oxide during high-temperature processing is discussed. Noncontact surface voltage and surface photovoltage (SPV) measurements are employed to determine the impact of copper impurities on distinct bulk silicon, interface, and oxide properties. For oxidized p-type Si, the characteristic copper signature comprises an increased oxide charge and a pronounced decrease in the minority carrier recombination lifetime upon optically induced formation of copper precipitates in the silicon bulk. During illumination, out-diffusion of copper to the silicon-oxide interface occurs simultaneously with precipitation and results in an increased interface trap density. For a complete overview on the distribution of the copper impurity on the various defect states before and after optical activation, the recombination lifetime, the interface trap density, and the total oxide charge have to be monitored.

Field Passivation of the Silicon Wafer Rear Surface for Reliable Bulk Recombination Lifetime Measurement

In noncontact charge-carrier lifetime techniques based on surface photovoltage measurements, uncontrolled surface recombination at the wafer back side may strongly interfere with carrier recombination in the silicon bulk for certain applications. An enhanced surface recombination rate decreases the sensitivity to metallic impurities in the bulk and may furthermore result in misleading data. To circumvent this problem, we use dedicated monitor wafers with a high/low p + /p 0 -junction close to the rear surface, where an electrostatic potential barrier hinders excess minority carriers to recombine at the back side. The benefit of this concept in lifetime measurements employing open-circuit photovoltage decay is demonstrated for the routine supervision of oxidation furnaces and implantation beam lines.