Limiting factors for secondary ion mass spectrometry profiling

Abstract

Understanding the limitations of depth profiling with ion sputtering is essential for accurate measurements of atomically abrupt interfaces and ultra-shallow doping profiles. The effects of cascade mixing, sputtering statistics, ion-induced roughness, the inhomogeneity of ion beams, and sample rotation on the depth resolution of Si δ-doped, AlAs, and InAs monolayers in GaAs and an AlGaAs(5 nm)/GaAs(5 nm) superlattice were investigated. Atomic force microscopy (AFM) investigation of the ion-induced surface ripple formation on a GaAs substrate sputtered with 3 keV O+2 at angle of incidence θ=40° showed that ripples form rapidly below 200 nm depth. AFM measured root mean square roughness of Si δ-doped GaAs sputtered with 2 keV O+2 was 0.8 and 2.6 nm with and without sample rotation showing that ripples play a dominant role in depth resolution degradation at shallow depth under these conditions of bombardment. Sample rotation yielded the lowest full width at half-maximum, 4.1 nm for a Si δ layer at 120 nm depth corresponding to a depth resolution ΔZ=3.5 nm. Use of AFM enabled determination of the atomic mixing ΔZm and sputtering statistics ΔZss components of depth resolution to be identified directly for the first time. These components were 3.1 and 1.5 nm, respectively.

Towards the ultimate limits of depth resolution in sputter profiling: Beam‐induced chemical changes and the importance of sample quality

“…Figure 17 shows a comparison of the roughening effect for two different samples: the GaAs(5 nm/A1,,,GaAso.,(5 nm) superlattice already discussed72 and an Si sample containing four boron deltas. 73 In both experiments the profile width remained constant with depth if sample rotation was applied. With a stationary sample, on the other hand, the width increased beyond some critical depth.…”

Temporal evolution of a silicon surface subject to low energy ion irradiation and concurrent sample rotation

“…Sample rotation does not always prevent surface roughening, however. In some instances, the sample roughens while it is 3 simultaneously eroded and rotated, albeit at a slower rate than it does when it is stationary [13,15].…”

Resolution in sputter depth profiling assessed by AlAs/GaAs superlattices

“…21 ͑4͒ In this round-robin test the depth resolution ⌬z scarcely deteriorated with the sputtered depth z using O 2 ϩ primary ions, in contrast to the works reported by Yoshihara et al 10 and Cirlin et al 23 One reason is that the resolution was measured at a depth ranged from 10 to 100 nm under optimum conditions where no ripples form. Linnarsson et al say that, in the case of O 2 ϩ ions, the deterioration of ⌬z l is attributed to surface roughening induced by formation of aluminumoxygen compounds during O 2 ϩ bombardment.…”

“…In the SIMS case, it is well known that the depth resolution depends on ion energy and incidence angle and ion species: [21][22][23] concerning ⌬z l1 , the resolution for O 2 ϩ is larger than that for Cs ϩ ; concerning ⌬z t5 , it has a slight dependence on ion energy and angle of incidence. However, the depth resolution ⌬z varies to some extent at each leading edge or trailing edge.…”