High-precision binary optical element fabricated by novel self-aligned process

In the conventional procedure for fabricating binary optical elements (BOEs ), we ascertained, by optical simulation, that the alignment error is a pri ncipal factor in diffraction efficiency. To deal with this problem we have developed a novel self-aligned procedure, and we have fabricated BOEs with high diffraction efficiency. The key point of the novel procedure is that e very step of the element is determined by the first Cr hard mask. To achiev e this, we used a negative photoresist with back exposure. We fabricated fo ur-level BOEs, and the diffraction efficiency of the BOEs is only 3% below the theoretical value. Furthermore, we investigated the influence of the re maining fabrication error in the novel method. The remaining error consists of the etching depth error and critical dimension (CD) error. According to the optical simulation, the CD error negligibly affects the diffraction ef ficiency. The only factor we should consider is the etching depth error.