Stack alignment techniques

A method for determining the alignment of substrates in a substrate stack that comprises a plurality of substrates, the method comprising:

• determining positions of a plurality of light beams that have passed through a respective plurality of alignment openings defined in each substrate in the substrate stack;
• and
• determining relative x, y, and Rz alignments of at least two substrates in the substrate stack in dependence on the determined positions;
• wherein:
• for each light beam path through the substrate stack, the alignment opening of one of the substrates on the light beam path has a smaller diameter than all of one or more other alignment openings of respective one or more other substrates on the light beam path;
• and
• for each one of at least two of the plurality of light beam paths, a different one of the substrates on the light beam path has an alignment opening with a smaller diameter than all of one or more other alignment openings of respective one or more other substrates on the light beam path such that, for each one of at least two substrates in the substrate stack, there are one or more light beams paths with positions that are indicative of the position of only said one substrate.

A substrate stack of substrates comprising beam manipulators, the substrate stack having at least two substrates, wherein in each substrate there are a plurality of alignment openings that align with corresponding alignment openings in the other substrates of the substrate stack such that there is a through passage through the substrate stack associated with each alignment opening in each substrate, wherein each of the plurality of through passages is for the passage of a light beam and the light beams are suitable for determining relative x, y, and Rz alignments of the substrates in the substrate stack;

• wherein:
• the alignment opening of one of the substrates, that defines the through passage for a corresponding light path through the through passage, has a smaller diameter than the other alignment openings that define the through passage;
• and
• a different substrate in the substrate stack has an alignment opening with a smaller diameter than the other alignment openings that define the corresponding through passage in the substrate stack.