Absolute micro-encoder using image obtained by ball lens assembled inside wafer

An absolute micro-encoder based on code imaging is demonstrated. As the lens for the code imaging, a ◇300νm ball lens is assembled inside the cavity between wafer-bonded Pyrex glass and suspensions inside a Si wafer. By designing the appropriate lens holder, the assembling accuracy of the ball lens can be of the same level as that of the alignment accuracy of the aligner used. The photodiode array is prepared in the device Si layer of a silicon-on-insulator wafer realizing the optoelectronic integration. The code image is incident on the backside of the photodiode array. The current signal shows sufficient contrast for resolving the maximum length code having the minimum line width of 13.6νm. The obtained results correspond to the realization of a12-bit absolute encoder with the device size of 2 × 2 × 2mm³ using a ◇18mm code disc.