A phase metrology pattern for attenuating phase masks. The phase error of this pattern can be determined to high accuracy by aerial image measurements. This pattern can be used to create an optical phase standard for calibrating phase metrology equipment for attenuated phase masks, or as a witness pattern on a product mask to verify the phase accuracy of that mask. The pattern includes an effective line to space ratio and can be tested using a microscope or stepper system or can be measured directly using a detector for the 0 order diffraction measurement.