Downstream mass spectrometry is successfully used in the reactive ion-beam etching of dielectric diffraction gratings of deep grooves with vertical sidewalls to achieve in situ endpoint detection. Silica (SiO2) gratings with a Sc2O3 etch-stop layer are fabricated by reactive ion-beam etching with CHF3 as etchant, and the mass spectrometric signal of SiF3+ produced by the reactive etching of the SiO2 grating material is monitored. When the etch-stop layer is reached, a drop of this signal occurs. By comparing the monitoring curves and resulting gratings of different etching methods, we find that the decrease of the monitored signal is strongly influenced by the sidewall steepness of the etched grating grooves. All conditions being equal, the greater sidewall steepness renders the faster decrease of the signal. Consequently, the proposed approach of endpoint detection applies well to the gratings with steep sidewalls. With the help of two previously developed methods, the sidewall steepness of grating grooves is increased, and the optimal endpoint is detected. Employing the proposed technique, we have reproducibly fabricated dielectric gratings with proper groove depth and even groove bottom. (c) 2008 Elsevier B.V. All rights reserved.