Near‐field laser ablation inductively coupled plasma mass spectrometry: a novel elemental analytical technique at the nanometer scale

Abstract

An analytical technique utilizing a near-field effect (to enhance the incident light energy on the thin tip of an Ag needle) in a laser ablation inductively coupled plasma mass spectrometry (NF-LA-ICP-MS) procedure was developed. To produce the thin needles with a tip diameter in the hundreds of nm range a robust needle etching procedure was established. The 'sample-to-tip' distance was controlled via the measurement of a tunnel current between the needle and sample surface. The NF-LA-ICP-MS technique thus developed was applied for the analysis of copper isotopic standard reference material NIST SRM 976 and tungsten-molybdenum alloy NIST SRM 480 in the nm resolution range. The observed craters ranged from 200 nm to about 2 microm in diameter and were dependent on the needle used as well as on the 'sample-to-tip' distance. The mass spectrometric measurements of (63)Cu(+) ion intensity on NIST SRM 976 showed that using near-field enhancement in laser ablation allowed a roughly 6-fold increase in the ion intensity of the analyte when the needle was about 100 nm (and below) from the surface, in contrast to when it was far away (e.g. 10 microm) from the sample. The relative standard deviation (RSD) of the (65)Cu(+)/(63)Cu(+) isotopic ratio measurements by NF-LA-ICP-MS was 3.9% (n = 9). The detection efficiencies obtained for the compared LA-ICP-MS and NF-LA-ICP-MS methods were found to be 4.6 x 10(-3) counts per second (cps)/ablated atom and 2.7 x 10(-5) cps/ablated atom, respectively.

Metal imaging on surface of micro- and nanoelectronic devices by laser ablation inductively coupled plasma mass spectrometry and possibility to measure at nanometer range

“…To minimize the vibrations during the measurements the Iso-Plate vibration damping system from Thorlabs Inc. (Newton, NJ) was used. Further details about NF-LA-ICP-MS experimental setup can be found elsewhere [30,31].…”

“…Ion intensity of 197 Au ϩ was measured during "single shots" of the unfocused laser when the tip of the thin Ag needle far (d Ͼ10 mm), no near-field enhancement was observed; and at a distance of the needle about 10 nm (and lower) from the sample surface, near-field enhancement of incident laser energy took place. The controlling of tip-to-sample distance was performed via measurement of tunnel current by developed tuning system as described elsewhere [30,31]. First, the needle slowly approached the sample surface (no tunnel current was measured).…”

“…First, the needle slowly approached the sample surface (no tunnel current was measured). When the needle was about 1 nm from the sample surface (corresponding to a tunnel current of about 1 nA [31], specially developed software triggered the piezo to keep this position constant. It was then possible to move the piezo (with the sample) to a defined distance from the needle tip.…”

“…Under these experimental conditions, the needle approached the sample surface very slowly and the mass spectrometric signal of ablated Au was monitored with the running laser. Further details of the measurement procedure used can be found elsewhere [30,31].…”

“…The principle of the procedure is to apply an enhancement of incident laser energy (light) on the tip of a thin Ag needle (diameter ϳ 200 nm). "Focused" in this way, the laser beam will produce an ablation crater in the size proportional to the needle diameter used [30,31]. An experimental arrangement of NF-LA-ICP-MS (including controlling system for sample-to-tip distance via the measurements of tunnel current between the needle and sample surface) was successfully established in our laboratory and applied for nanolocal analysis of thin (20 nm) Au film deposited onto a Si substrate [30].…”