Dopant Distribution during Low Energy Implant Anneal Due to Photoresist Breakdown and Anneal Ambient Effects

Abstract

Photoresist outgassing during low energy ion implantation can affect the dosimetry of the process as well as the distribution of implanted dopants in the silicon substrate. The gaseous by-products of photoresist breakdown can become entrapped in the substrate, thereby altering the diffusion and clustering characteristics of implanted dopants during high-temperature anneal. In addition, they can lead to charge exchange reactions with incoming ions, thus affecting the dose control of the process. The latter effect is well understood and corrected for in modern ion implantation equipment. This work examines the effects of photoresist outgassing on the distribution of boron and arsenic in silicon during anneal, the diffusion and clustering of the dopants, and effects on substrate sheet resistance. Single-crystal and preamorphized silicon substrates are used as well as oxide-covered wafers in low energy boron and arsenic implants to study the dopant behavior during anneal. High carbon concentration on the wafer surface from photoresist implants has opposite effects on boron-and arsenic-implanted silicon. The transient enhanced diffusion of boron is supressed, whereas diffusion of arsenic is enhanced, with corresponding effects on sheet resistance. These effects can be minimized by the use of a thin oxide layer on the implanted substrates and careful selection of ambient during anneal.

Integrated Precision Polysilicon Resistors

“…The sequence of interactions is as follows; photoresist breaks down under ion bombardment releasing large quantities of hydrogen gas and creating a hardened carbonized crust at the top of the photoresist [5]. The released gas neutralizes incoming positive ions.…”