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 th ree generations old, is still considered quality surface-analysis equipment . It was designed more than two decades ago, at a time when power transisto rs were the means of rastering high voltages at high frequencies. In our mo dification, we have replaced the high-frequency amplifiers with a simple op erational amplifier (op amp) circuit that greatly enhances the capabilities of this instrument. In a Cameca dynamic SIMS instrument, the primary ion b eam is focused into a small spot (similar to 5 - 100 mu m) and then is rast ered over an area generally between 25 mu m x 25 mu m and 500 mu m x 500 mu m. The raster signal is a sawtooth waveform with separate high and low-fre quency components for X and Y, respectively. The transistor design of the r aster amplifier does not produce a sharp waveform at high frequencies. Inst ead, the signal appears rounded at the top and bottom. This, in turn, allow s the ion beam to spend more time at the edges of the rastered area, etchin g the sides at a greater rate, producing the so-called "dog ears" at the bo ttom 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-mu m-diameter circular analysis area for a 250 mu m x 250 mu m 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 a rea, there is a loss of depth resolution. Another problem occurs when sputt ering 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 inf ormation. (C) 1999 American Vacuum Society. [S0734-2101(99)05201-X].