Molecular depth profiling

To start, it is appropriate to reflect on the wonderful venue and the hospitality of our hosts for the SIMS XVIII conference here in Riva del Garda. The scenery has been spectacular, the food and wine of culinary delight, and the science inspirational indeed. All the attendees owe a warm thanks to the terrific job of the local organizing committees in providing a great treat for everyone. There can be no better occasion than to celebrate the 101st anniversary (1) of the discovery of secondary ion mass spectrometry with this group. The subject of this paper is molecular depth profiling. It is a fascinating topic for our lab since it is a subject that is quite new in the scheme of the 100 year history of SIMS. As is well known, atomic bombardment of molecular solids rapidly leads to chemical damage with the concomitant loss of chemical information as the fluence exceeds the static limit. With cluster bombardment, however, this damage is mitigated in some cases, and even eliminated completely in others, allowing erosion of the material without loss of chemical information. In this lecture, some of the critical issues will be reviewed, along with a discussion of various protocols that are becoming available to characterize the effectiveness of a depth profile. The goal will be to decide what makes it, what breaks it, and what fixes it. There are many valuable resources available to supplement this discussion, in addition to these proceedings. Of particular note is the SIMS society website, www.simssociety.org, which is being developed by the community for use by the community. The site has links to other SIMS activities, job opportunities and PowerPoint presentations from a number of review-type lectures, including this one. The SIMS International Committee encourages contributions to this site, which can be arranged through any committee member. It is fortunate that many of the issues associated with molecular depth profiling have already been addressed quite rigorously by the dynamic SIMS community over the last 30 years or so. These experiments have been focused on determining elemental dopant distributions very near the surface of semiconductors such as Si, and on characterizing structures ranging from epitaxial layers to quantum wells. Extending these approaches to molecular depth profiling opens more opportunities for applications of SIMS since the study of buried interfaces in molecular solids with nanometer depth resolution is virtually impossible with other techniques. Key applications so far appear most promising in organic electronics through the study of organic light emitting diodes (OLED's), optoelectronic materials (2), photovoltaics and multilayer polymer structures with a number of examples found in these proceedings. There are also opportunities in the pharmaceutical industry, where formulation of drugs into complex layered structures is now standard practice. Beyond these applications, there are also important implications for chemical imaging. In a 1 μm2 pixel, there are ~106 molecules per pixel, depending upon molecule size, of course. For static SIMS, the damage threshold is ~1%, the instrument transmission is ~10%, and the ionization efficiency for typical organic molecules is ~0.01%. Hence, the maximum signal in imaging mode is on the order of 0.1 counts per pixel, not too impressive. If the static limit no longer applies, and the pixel can be integrated to a voxel, the count rate increases by a factor of 105, opening many new possibilities. In fact, the combination of molecular depth profiling with 2-dimensional imaging to create a 3-dimensional rendering of the chemistry is clearly a future growth area for our field. Incidentally, if somebody could figure out a way to improve ionization of the target molecules, the count rate could be increased by another 4 orders of magnitude, changing the game yet again. These simple calculations have another important implication. As is clear from papers presented at this conference, and from the discussion to follow, virtually all molecular depth profiling experiments are performed using 2 different modes – one where the beam is on continuously to erode the material and one where the beam is pulsed to acquire a ToF SIMS spectrum. This latter approach is clearly inefficient since no mass spectral information is gained during the erosion cycles. It will be critical to design instrumentation going forward that removes this inefficiency if the advantages noted above are to be fully realized.