Cluster secondary ion mass spectrometry and the temperature dependence of molecular depth profiles

Abstract

The quality of molecular depth profiles created by erosion of organic materials by cluster ion beams exhibits a strong dependence upon temperature. To elucidate the fundamental nature of this dependence, we employ the Irganox 3114/1010 organic delta-layer reference material as a model system. This delta-layer system is interrogated using a 40 keV C(60)(+) primary ion beam. Parameters associated with the depth profile such as depth resolution, uniformity of sputtering yield, and topography are evaluated between 90 and 300 K using a unique wedge-crater beveling strategy that allows these parameters to be determined as a function of erosion depth from atomic force microscope (AFM) measurements. The results show that the erosion rate calibration performed using the known Δ-layer depth in connection with the fluence needed to reach the peak of the corresponding secondary ion mass spectrometry (SIMS) signal response is misleading. Moreover, we show that the degradation of depth resolution is linked to a decrease of the average erosion rate and the buildup of surface topography in a thermally activated manner. This underlying process starts to influence the depth profile above a threshold temperature between 210 and 250 K for the system studied here. Below that threshold, the process is inhibited and steady-state conditions are reached with constant erosion rate, depth resolution, and molecular secondary ion signals from both the matrix and the Δ-layers. In particular, the results indicate that further reduction of the temperature below 90 K does not lead to further improvement of the depth profile. Above the threshold, the process becomes stronger at higher temperature, leading to an immediate decrease of the molecular secondary ion signals. This signal decay is most pronounced for the highest m/z ions but is less for the smaller m/z ions, indicating a shift toward small fragments by accumulation of chemical damage. The erosion rate decay and surface roughness buildup, on the other hand, exhibit a rather sudden delayed onset after erosion of about 150 nm, indicating that a certain damage level must be reached in order to influence the erosion dynamics. Only after that onset does the depth resolution become compromised, indicating that the temperature reduction does not significantly influence parameters like ion-beam mixing or the altered-layer thickness. In general, the wedge-crater beveling protocol is shown to provide a powerful basis for increased understanding of the fundamental factors that affect the important parameters associated with molecular depth profiling.

Figure 1

Conventional sputter depth profiles of Irganox 1010/3114 multilayer sample at 300 K, 250 K, 150 K and 90 K with C₆₀⁺ incident at 40° with respect to the surface normal. The Irganox 1010 matrix signal (C₂H₃O₂⁻ m/z 59) is normalized to surface intensity, and the Irganox 3114 Δ-layer signals (C₃₃H₄₆N₃O₅⁻ m/z 564, CNO⁻ m/z 42) are normalized to their maximum intensity during the entire depth profile.

Figure 2

Erosion rate calculated from conventional depth profiles by dividing the known depth of each delta layer by the ion fluence needed to reach the peak signal corresponding to this layer.

Figure 3

Depth profiles of different Irganox 1010 matrix signals at 300 K (top panel) and 90 K (lower panel) with 40-keV C₆₀⁺ at 40° incident angle. Each fragment signal is normalized to its surface intensity.

Figure 4

AFM topography scan along the wedge shaped craters eroded into a 380-nm thick Irganox 1010 film doped with four Irganox 3114 Δ-layers by a 40-keV C₆₀⁺ ion beam impinging at 40° with respect to the surface normal at 90 K, 130 K, 160 K, 210 K, 250 K and 300 K temperature. The blue curves represent the roughness fluctuations around the average crater profile calculated by subtracting the smoothed AFM scan data (black curves) from the original data.

Figure 5

RMS roughness (R_q value, see text) as a function of eroded depth during sputtering of the Irganox 1010/3114 Δ-layer film with a 40-keV C₆₀⁺ ion beam impinging under 40° with respect to the surface normal. The data were obtained at six different sample temperatures during ion bombardment at 90 K, 130 K, 160 K and 210 K (top panel) and 250 K, 300 K (lower panel).

Figure 6

Erosion rate as a function of eroded depth during bombardment of the Irganox 1010/3114 Δ-layer film at 90 K, 130 K, 160 K, 210 K (top panel) and 250 K, 300 K (lower panel). All six wedge craters were created by a 40-keV C₆₀⁺ ion beam impinging at 40° with respect to the surface normal.