An aperture plate (1) is attached at its rim (203) to a support washer (3) by adhesive. Anchor grooves (204) having a zig-zag pattern in plan are machined in the lower surface of the rim (203 of the aperture plate before application of the adhesive. The grooves (204) extend out to the edge of the aperture plate (1). The anchor grooves have a depth in the range of 10 μm to 40 μm, and a width in the range of 20 μm to 150 μm, and an angular pitch in the range of 2.5° to 12.5°. Excellent bonding strength is achieved for long term reliable attachment in an environment of high frequency vibration and moisture and chemical corrosion.