Near-Surface and Interfacial Profiling by Neutron Depth Profiling (NDP) and Secondary Ion Mass Spectrometry (SIMS)
Authors: Downing R. G. , Fleming R. F. , Maki J. T. , Simons D. S. , Stallard B. R. 
Published: March 2011

Information relating the spatial arrangement and concentration of intentional and intrinsic dopants is commonly required to fully understand the properties of a material, whether the application is chemical, electrical, or physical. We have synergistically coupled the near-surface techniques of thermal neutron depth profiling (NDP) and secondary ion mass spectrometry (SIMS) for the purpose of better determining the distribution of a few key elements in a number of matrices and thin-film interfacial applications.

The NDP facility, unique in the U.S., allows virtually non-destructive measurements of the absolute concentration of specific elements (e.g,, He, Be, Li, B, Na, Bi . . .) to be made versus their depth distribution in a specific matrix [1]. The quantitative information is derived from the number and the residual energy of emitted charged particles that are produced in situ by uniformly illuminating a sample volume with thermalized neutrons. Sensitivity, depth of view, and resolution are dependent upon the reaction cross-section for the element of interest and the characteristic energy loss for the elemental components of the matrix. However, experimental parameters, such as the sample angle relative to the detector, can be adjusted to extract the maximum depth or the best resolution information from the measurement [2]. Since the technique is non-destructive, samples can be subjected to a series of treatments and profiled after each step [3].

The more mature SIMS technique is able to detect most of the elements listed above with greater relative sensitivity but without an absolute concentration calibration. Therefore, by utilizing the abundance information obtained by NDP, a concentration scale can be established for the SIMS profile. SIMS is also useful in probing smaller surface areas, a few tens of micrometers square as opposed to a few millimeters square for NDP. The advantage in coupling the two techniques lies principally with the role NDP plays in distinguishing experimental artifacts from real concentration variations [4]. While some matrices and interfacial areas of a sample give rise to variable sensitivities in SIMS measurements. NDP, however, counts every event that emitted a charged particle within the solid angle subtended by the detector, thereby, making it more reliable for reporting the concentration information.

Shown in Figure 1 is a comparison of NDP and SIMS profiles determined for a boron-10 implant in a single-crystal silicon, a common processing step for semiconductor materials. The agreement between techniques is good. Possible sources of discrepancies between the two methods are briefly discussed by Ehrstein et al. [3].

The combined effort of SIMS-NDP is currently being utilized to study diffusion and boundary segregation in thin-film semiconductor applications. Accurate depth profiles have been difficult to obtain by other analytical approaches for such material systems. The ability of SIMS-NDP to profile across interfacial regions and thin films will allow many other electrical devices and material problems to be addressed more reliably.