Measurement of Boron and Phosphorus in BPTEOS Utilizing Quadrupole Secondary Ion Mass Spectrometry
Introduction
Boron and phosphorus concentrations in BPTEOS and BPSG films are typically in the 3-6 wt% range. The bulk concentrations may be varied to optimize the physical and electrical properties of the film. Measuring the concentration of boron and phosphorus as a function of depth provides a better understanding of the deposition process dynamics and the effects that result from changes made to tool operating parameters. Quadrupole SIMS offers several advantages over other analytical methods, including its ability to apply charge compensation, to utilize Cs+ cluster ion analysis and to use traceable standards to quantify results.
Cs+ Cluster Analysis
The recombination model is specifically valid for cluster ions of matrix species with one or two Cs atoms originating from the sputter beam. Basically, the model assumes the emission of Cs+ ions together with sputtered neutrals M that (re)combine above the surface to form MCs+ or MCs2+ cluster ions. The clusters may also be formed directly in the sputter process since Cs+ ions are also present at the surface. A surface covered with a sub-monolayer of Cs emits Cs+ ions with a very high degree of ionization. This high degree of ionization is virtually independent of the matrix since the work function is mainly defined by the Cs overlayer itself. If the Cs concentration becomes very high, the degree of ionization is reduced and becomes dependent on the surface concentration. This matrix independence minimizes ion-yield variations that often plague other types of SIMS analysis.
Figure 1
Typical Analytical Conditions
Atomika 4500 Quadrupole SIMS
Cs+, 5 keV primary
Secondary:Positive ions (CsB+, CsP+, CsO+, & CsSi+)
Raster size: 200 um x 200 um
Beam current: 40 nA
Tilt: 60 deg from Normal
Standardization
A BPTEOS film characterized by wet chemical analysis using NIST traceable standards is used to standardize the Quadrupole SIMS data. The characterized film is measured using the same Quadrupole SIMS analytical conditions as the unknown samples, allowing a direct comparison or quantification.
Figure 2
Example Results
Quadrupole SIMS can monitor specific isotopes of boron (10B and 11B) for the purpose of determining isotopic ratios or enhancement of one isotope over another. Figure 1 shows a BPTEOS film that has a normal isotopic abundance.
Figure 2 shows a spike of boron at the surface of the film. This high concentration spike of boron allows for improved reflow characteristics and better step coverage of the film. Figure 2 also shows minor differences in boron and phosphorus depth profiles between the center and edge of a 200mm wafer.
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