Paper AS-ThM4
TOF-SIMS at the Edge

Thursday, October 24, 2019, 9:00 am, Room A211

TOF-SIMS instruments are designed to extract secondary ions in the direction normal to the surface by creating a potential difference between the sample and the extraction device. The optimum geometry for getting the highest transmission is for the sample to consist of a functionally infinite flat plane. When sample topography deviates from this ideal significantly, secondary ion transmission is reduced. The effect on the yield of each secondary ion is a function of the ion mass, but also the initial ion emission angular and momentum magnitude distributions. The yields of the lightest atomic ions are least affected by topography, and the yields of heavier molecular fragments and ions most significantly affected.

To quantitatively explore these effects, two samples presenting topographic challenges were used in this study.

1. The edge of a magnetic recording disk is tightly controlled and therefore reproducible. The disk is extremely flat. The surface produces homogeneous signals for atomic species and molecular fragments of the disk lubricant and adsorbed hydrocarbons and other organic species where ion yield is not affected by topography. There is a slight bevel at the edge, much smaller in size than the edge effect on secondary ion collection. The sample height drop off past the edge in this experiment was effectively infinite.

2. A Si coupon was etched to produce a deep enough crater that ion yields from the crater bottom were affected by the topography. A lighter etch was performed over a wider area sufficient to reach dynamic equilibrium (and thus produce a surface damaged identically to that at the crater bottom). The sample was then exposed to air and allowed to oxidize and be contaminated by adventitious organics before analysis. This sample represents the condition where at a distance from the sample, the surface is essentially flat, but near to the surface the ion extraction will see non-normal fields that affect secondary ion trajectories.

With these samples, it was possible to look at the effect of the topography on the yields of a variety of secondary ions, and at a variety of instrumental conditions, both in the IonTof TOF-SIMS 5 and in the Physical Electronics NanoTof II. The analysis results of the two types of topographic challenges are shown to be differently affected by instrumental conditions. No one condition gives the ideal remediation for all topographic challenges.

The work with the edged sample promises to provide a simple test for differences in the initial angular and momentum magnitude distributions. Such distributions may provide added clues to secondary ion formation mechanisms.