Low-Cost Modification for the High-Frequency Raster on the Cameca IMS-3F Secondary Ion Mass Spectrometer

The Cameca IMS-3F secondary ion mass spectrometry (SIMS) instrument, now three generations old, is still considered quality surface-analysis equipment. It was designed more than two decades ago, at a time when power transistors were the means of rastering high voltages at high frequencies. In our modification, we have replaced the high-frequency amplifiers with a simple operational amplifier (op amp) circuit that greatly enhances the capabilities of this instrument. In a Cameca dynamic SIMS instrument, the primary ion beam is focused into a small spot [formula omitted] and then is rastered over an area generally between [formula omitted] and [formula omitted] The raster signal is a sawtooth waveform with separate high and low-frequency components for [formula omitted] and [formula omitted] respectively. The transistor design of the raster amplifier does not produce a sharp waveform at high frequencies. Instead, the signal appears rounded at the top and bottom. This, in turn, allows the ion beam to spend more time at the edges of the rastered area, etching the sides at a greater rate, producing the so-called “dog ears” at the bottom of the SIMS crater. Under most analysis conditions, these dog ears are not a problem because the analysis area is much smaller than the rastered area (e.g., a 60-μm-diameter circular analysis area for a [formula omitted] rastered area) and is taken from the center. However, when the sputter depth is great, or the analysis area approaches the size of the rastered area, there is a loss of depth resolution. Another problem occurs when sputtering through thin films deposited on glass or other insulating materials. During depth profiling, as the material is sputtered away, the edges reach the insulator before the analysis area, which causes a charge buildup. The operator must then stop the profile prematurely, resulting in a loss of information.