Transient phenomena and impurity relocation in SIMS depth profiling using oxygen bombardment: pursuing the physics to interpret the data

Abstract

High detection sensitivity in bulk analysis or depth profiling by secondary ion mass spectrom etry (SIMS) can only be achieved, for positively charged ions, if the near surface regions of the sputter eroded sample are fully oxidized. Using oxygen prim ary ions, a stationary oxidation state is established after some tim e of bom bardm ent during which period the sputtering yield decreases and the ionization probability increases. The physical and chemical processes occurring during the transient period are reviewed w ith emphasis on the results for im purity analysis in silicon, i.e. the m atrix m aterial th a t has been studied most thoroughly in the past. The transient de crease in sputtering yield gives rise to a depth scale offset and an associated apparent shift of im purity profiles towards the surface. The effect is largest at normal beam incidence, ca. 1 nm/keVXOf), in which case silicon is fully oxidized. The transition depth, i.e. the depth sputtered before achieving a stable ion yield is about twice as large and increases as the im pact angle is turned away from normal. The shift and the depth scale offset can be measured safely using thin (delta) layers of isotopically pure tracers, for example 30Si in 28Si. Boron delta layers can serve as secondary standards because this im purity behaves almost the same as the host silicon atoms. The profile shift observed w ith other common dopants may contain contributions due to unidirectional relocation, often driven by segregation away from the surface. At O j energies below about 0.7 keV the transition depths fall below the thickness of typical native oxides, so th a t the transient changes in sputtering yield and ionization probability can disappear. The transient phenomena may be described by a simple sputtering-oxidation model th a t connects the depth scale offset to other observable param eters like the initial and final sputtering yield, the transition fluence and the oxide thickness. I n tr o d u c tio nOver the past 20 years secondary ion mass spectrom etry (SIMS) has been used quite successfully for depth profiling im purity distributions in solids. Most of the previous work has been done on semiconductor samples which constitute the m ajor challenge to the SIMS technique because of the need for high sensitivity in combination with high precision (Wilson et al. 1989). The excellent sensitivity reported for many ele ments is based on two characteristic features of SIMS: first, the absence of an inherent

One- and two-dimensional dopant/carrier profiling for ULSI

“…The increased sputter yield at non-normal incidence leads to a reduced oxygen incorporation level, causing a smaller differential shift despite the fact that the transient width increases quite drastically. A detailed description and modelling of this effect is based on the balance between the oxygen incorporation and sputter yield and was analysed qualitatively in [14] and more recently in detail in [ 15,16]. Obviously a low primary beam energy is a requirement to arrive at the correct depth scales.…”

Concentration-depth calibration and bombardment-induced impurity relocation in SIMS depth profiling of shallow through-oxide implantation distributions: a procedure for eliminating the matrix effect

“…The results for the commonly used matrix reference signals due to Si`or SiO`are in accordance with previously reported data. 18 The yields of all matrix speciÐc ions stabilize quite rapidly at a characteristic "transition timeÏ min that corret tr D 2 sponds to a transition Ñuence of 7 ] 1016 oxygen atoms cm~2. The signal passes through a local Si 2m aximum, very similar to that observed previously in measurements of oxygen build-up during argon bombardment in combination with oxygen Ñooding of the sample chamber.…”

“…18 To obtain a reliable concentration calibration one has to distinguish between the oxygen-concentrationdependent sensitivity ratio and density changes R i,m‰M of the matrix. The latter also determines changes in erosion rate, which must be known for a reliable timeto-depth conversion.…”

In-Line Characterization of Heterojunction Bipolar Transistor Base Layers by High-Resolution X-Ray Diffraction

“…The average difference is about 6.0 nm. Such a discrepancy can not be attributed alone to surface transient effects which are known to alter secondary ion yields (18)(19)(20)(21)(22). Even if we take into account the full error corresponding to the accuracy of thickness determination from the concentration profiles and add it to the SIMS differential shift, the observed deviation is still too large.…”