We have, on combining holography with photolithography, improved the etching conditions of making binary elements and produced photolithographic holograms which can yield continuous phase modulation. This paper points out that it is most important to strictly control the ratio of etching rates, gives the curves of the etching rates in respect of the flow of etching gas in the case of constant response power, A proper working point has been found. As the applications of the curves, high-quality optical elements can be made.