Rapid thermal anneal activates light induced degradation due to copper redistribution

Abstract

While it is well known that copper impurities can be relatively easily gettered from the silicon bulk to the phosphorus or boron–doped surface layers, it has remained unclear how thermally stable the gettering actually is. In this work, we show experimentally that a typical rapid thermal anneal (RTA, a few seconds at 800 °C) used commonly in the semiconductor and photovoltaic industries is sufficient to release a significant amount of Cu species from the phosphorus-doped layer to the wafer bulk. This is enough to activate the so-called copper-related light-induced degradation (Cu-LID) which results in significant minority carrier lifetime degradation. We also show that the occurrence of Cu-LID in the wafer bulk can be eliminated both by reducing the RTA peak temperature from 800 °C to 550 °C and by slowing the following cooling rate from 40–60 °C/s to 4 °C/min. The behavior is similar to what is reported for Light and Elevated Temperature degradation, indicating that the role of Cu cannot be ignored when studying other LID phenomena. Numeric simulations describing the phosphorus diffusion and the gettering process reproduce the experimental trends and elucidate the underlying physical mechanisms.

Can hydrogenation mitigate Cu-induced bulk degradation in silicon?

“…It is worth to point out that in heavily Cu contaminated sample, the degradation is really significant as the bulk lifetime decreases from nearly a millisecond to less than 10 s. This was expected behavior based on literature although Cu-LID has not been studied earlier with such accelerated conditions [14]. The dominating degradation mechanism in the Cu-contaminated samples is likely Cu precipitation in the bulk, while the decrease in bulk lifetime in the No Cu area can be explained by conventional BO-LID.…”

“…Based on the results by Nampalli et al [14], Cu-LID should be more pronounced with the higher firing temperature. This is because firing was shown to dissolve possible Cu precipitates and drive mobile Cu atoms into the bulk making them more harmful for the bulk lifetime during subsequent illumination.…”

Low-T anneal as cure for LeTID in Mc-Si PERC cells

“…There are currently many speculations about the root cause of the LeTID defect and how to explain the similar DA recombination activity. The majority of both theories and experiments support the presence of hydrogen as one crucial component and rules out the role of metal contamination [9], [16], while other studies support the involvement of fast diffusing metals [14], [17]. In this work we study the latter approach, i.e.…”

“…The maximum temperature is limited to 300 °C in order to have a minimal impact on the junction or contact properties as the anneal times of interest are relatively long. Finally we will study if the degradation and regeneration during DA resemble metal precipitation kinetics, since earlier studies indicate that some metals may still be present in the bulk after the last high temperature step [4], [12], [13], [14], [15] although a strong evidence has been reported for hydrogen playing a crucial role as well [9], [16].…”

Vertically integrated modeling of light-induced defects: Process modeling, degradation kinetics and device impact

“…The physical mechanism behind LID-response of the different firing profiles is out-diffusion of gettered copper from the emitter back to the bulk during firing [33]. An experimental verification of this mechanism has been completed on reference lifetime samples and will be published in a separate study [34].…”