Usually, F and Cl negative secondary ion currents forming background signals determine detection limits for these elements in secondary ion mass spectrometry (SIMS) measurements. Technique involving changing the primary-ion current was used to determine that the "memory effect" is the main source of the background signal for these elements. It is shown that energy distributions of F and Cl ions forming background signals are distinctly narrower than energy distributions of these ions reflecting element bulk concentration in a Si wafer. Using this fact, the F and Cl detection limits in Si were improved by energy filtering of secondary ions. For experimental conditions used, the filtering of the secondary ion energy in a region around 60-70 eV improves the F and Cl detection limits by a factor of more than 6. The effect of an electron beam used for charge neutralization of insulator samples in a negative SIMS mode on the F and Cl detection limits was also investigated. It is shown that the simultaneous use of the primary ion and electron beams causes a synergistic increase of the background signals. It was determined that both F and Cl detection limits for the SiO2 sample are almost the same as for the Si sample when the electron and ion beams are applied simultaneously. The possible mechanism of this effect is discussed. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3474983]