New actuator concepts based on piezoelectric/MEMS actuation

Hi, I’m James Marvellous Muganda and I’m working within the Microsystems research group at the Department of Mechanical Engineering where I am focusing on new actuator concepts based on piezoelectric or MEMS actuation or a combination of both.

My first task was to identify how these actuators are applied in the ASML lithography wafer steppers, particularly on the wafer stages. From there I made analytical models to describe and analyze the application of piezoelectric actuators to the active wafer table. Using Kelvin and Bessel – Fourier Orthogonal functions as solutions to the Biharmonic plate equation, I was able to accurately infer the actuator requirements in terms of stroke, stiffness and actuator force. I also validated my analytical model with Finite Element Analysis in COMSOL. The models were both in agreement. Now I am in the process of carrying out experimental validation. From there I want to incorporate the actuators into the system design of the whole active wafer table.

Setting rules

The challenges I am encountering so far are in the structural analysis of the wafer table. My background is partly in Physics and partly in Electrical Engineering, so for this research I need very good structural analysis skills. This is something I am working on at the moment. The other problem is that from the scope of the research topic, there are no concrete design rules and requirements, so I set my own rules on what are the capabilities of the actuators.

The actuator requirements stem from the influence functions (IF) used to generate the wafer shape. An in-depth investigation into the effect of the influence function on the overall active wafer table performance revealed that the system is extremely sensitive to variation in the IF. Therefore the IF was modelled analytically and compared to the finite element analysis method (Figure 1).

Using the principle of linear superposition, the weighted sum of IF (Figure 2) can be used to generate different surface shapes (Figure 3).

Imaging the stars

My research is interesting to companies involved in x-ray synchrotrons, laser interferometer gravitational-wave observatory, laser-beam shaping, high power lasers, adaptive optics (both telescopic and medical), lithography scanners and space applications. Companies such as ASML (which is my main sponsor), TNO, Boston-Micromachines, Carl Zeiss and Cymer will gain a lot from my research. All of these companies need to control/manipulate surface shapes in their machines. For example in adaptive optics, for successful imaging of the stars, there is need to cancel the atmospheric induced wavefront aberrations by manipulating mirrors, commonly known as deformable mirrors. The actuators I am working can be used for that. In ASML, the wafer needs to be manipulated in order to reduce the focus and overlay errors. The actuators can be used to deform the wafer surface both in-plane and out-of-plane.

Complementary research

Designing the active wafer table is a complicated task. The source of wafer deformations vary. The most difficult deformations to correct are dynamic deformations such as laser induced wafer heating disturbances. David van den Hurk is looking at such deformation and modeling them while I am designing to cancel out such deformations. So our research is well matched. David and I regularly sit to discuss our progress. With a significant chance of our research overlapping, we constantly try to avoid that by keeping constantly each other updated what the other is working on.