motion systems

Motion-system considerations for the Medjed system

This section expands on the discussion on motion-systems for lithography machines ➡

The discussions above do not make any assumptions about the optical design of the system. The nature of the exposure unit is also a factor to consider. For example, the Raith system (which uses a X+YZ arrangement) makes a good bit of sense for the type of optical modulator used in that system: a Grated Light Valve (GLV). But it could use a XY+Z arrangement without problems too.

The Medjed system opts for a XY+Z arrangement. A X+YZ arrangement could also work, but for the project budget I think it would introduce alignment stability issues for the optics. The downside is that the stability of the XY axes is reduced, and particularly deflection of the upper linear axis holding the chuck are a problem. Either of these two arrangements are compatible with the 'low precision stage, high precision encoder' DMD paradigm.

One key advantage that the XY+Z has over X+YZ is in enabling backside alignment. If you want to be able to align based on features on the backside of the wafer, you will need to put some basic optics on the 'underside' of the stage. If you have an XY+Z system, you can get away with (in principle) a single stationary camera. No matter how poor the initial placement of the substrate, you can always move the substrate until you get the backside feature of interest in the center of the camera. In an X+YZ arrangement, there is no way to move the substrate in Y, so a static backside camera cannot be used as flexibly. In Raith's Picomaster systems (X+YZ), I recall that you have to put alignment markers in a specific region of the substrate so the alignment camera's get the marker somewhere in their FOV. This is suboptimal as it puts constraints on substrate layout and initial positioning accuracy when loading substrate into machine. And each marker would need its own camera; so 5 marker positions is not realistic.

Since the Medjed system features basic backside alignment, an XY+Z is the best choice.