Mechanical redesign for lens focus module and design for thermal actuator

Published on by Development & Engineering

Paul van den Hoogenhof (25) finished his study Mechanical Engineering at Eindhoven University of Technology at NTS in the summer of 2018. His challenge was to optimise an existing module of NTS used for defining 3D-topology of the surface structure of silicium wafers. His assignment concerned making the existing module more accurate. During this process he was also asked to investigate if thermomechanical actuation would have an advantage compared to piezoelectric actuation. Both his redesign for the module as his design for the actuator are currently being further developed by NTS. A graduation with a lot of impact.

“NTS among others develops optic metrology systems to identify whether wafers change shape because the structure of the surface diverges too much,” Paul explains. “This is extremely important for the quality of the wafers. Minimal deviation is the aim. At the moment that I wanted to graduate, my professor had a couple of research assignments. This one stood out. The other assignments were not that interesting so I had soon made my choice."

Design of a more accurate module

“NTS’ assignment was defined quite clearly namely: ‘our current module for 3D topology definition of wafer surfaces is insufficiently accurate. Design an improved module’. Besides this, NTS had the idea that the piezoelectric controlled actuator that the module contained, could be of a higher quality by using a thermal actuator. A fairly well-defined assignment, but I had a lot of flexibility for a further definition and the execution of the research.”

Right balance between stiffness and degrees of freedom

“For defining 3D topology, NTS uses interferometry based on phased movement. This is executed by means of a special objective that needs to be moved back and forth very precisely. In this process it is essential that the controlled movement of the objective takes place in a precise and repeatable manner with steps of 50- 80 nm. This requires a right balance between stiffness and straight guide.”

Good match with study

“I developed a mechanic redesign for the module. During the redesign I looked at the performance, frequency, straight guide and stiffness of the module. A subject that matches my study perfectly. At TU/e I chose for the control systems group and the subgroup constructions and mechanisms. Eventually I produced a redesign that is much more specifically aimed at the exact application of the lens focus module.”

Other way of actuation

“After this, I looked at another form of actuation. Then you have a sound image of the way in which you want to dimension your actuator. What is done in a piezoelectric way in the current module they perhaps want to do in a thermal manner in the future. So, based on expansion by heath. The actuator is then roughly said just a little stick that pushes the objective away. This approach of course has some risks, you do not want to have heath in the rest of your module or machine.”

Impact of the research

Paul’s study proved that the thermal actuator is a very promising concept. That is why NTS’ R&D department is currently working on further developing it into an actuator that is able to elongate in both directions. This is done by applying the so-called Peltier element, among others known from the holiday coolers people use in their cars. With this NTS wants to provide its clients with a driver that offers a clear performance improvement for a number of specialised applications compared to the current piezo actuators.

Clear gain in performance

“The advantage of a thermomechanical actuator is that it can be less sensitive to noise. I have performed a lot of simulations and experiments. During this we have explored the boundaries of speed. Normally it takes seconds for the actuator to reach its position in a thermal way, in our case we have managed to achieve 20 milliseconds for the step size needed for our application. That is a clear gain in performance.”

Final solution seems to be very promising

“In the end I have made a proposal that combines the advantages of a thermal actuator namely its considerable stability, but that can also move in both directions by means of an actively cooled design. That seems to be a very promising concept for specialistic modules or systems.”

I like seeing results

“It was a nice, fairly multidisciplinary assignment. I like variation and get little energy out of doing extremely specialised work and I like it when all things come together and I can see results. Without this, to me the feeling of being useful kind of disappears. What I like most is being assigned with a problem and developing a solution for it. At NTS I have made a good first move towards something that eventually can be an alternative to existing technologies.”

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