ASML – Small Bolt Pretensioner
Background
Whether you are reading this on a computer, on a smart phone, on a tablet, or on any other device, you are using ASML's lithography products right now. In lithography, an image on a reticle is projected onto a wafer to etch the design into the wafer. Figure 1 shows a simplified version of some of the key components in the process. The bottom section shows the round wafer in a square carrier, while the middle section shows the rectangular reticle. The top circular section is part of the optical system.

Figure 1 – Simplified Lithography Process



Improvement in the lithographic process is so important because as lithography improves, so does the speed and capability of the microchips that are created from the process. Following "Moore's Law", advances in lithography systems, such as the one shown in Figure 2, have allowed chip manufacturers to continue to shrink feature sizes, with the current state of the art possessing the ability to manufacture feature sizes in the range of 22 nanometers, or about the size of 100 silicon atoms.

Figure 2: Lithography System


With feature sizes in that range, tolerance budgets are pressed to the atomic level. All components and processes contributing to the precision of the equipment must be optimized.

Problem Statement
For non-precision applications, torque wrenches / torque drivers, as shown in figure 3, can be used to create somewhat repeatable performance in bolt tightening. However, this method has the following disadvantages:

  1. Poor accuracy due to dependence on friction coefficients of components. For non-lubricated vacuum components, this depends highly on surface quality and particulate contamination and is impractical to know accurately and reliably.
  2. Large displacements of the clamped part due to friction between screw head and clamped part.
  3. Introduces surface defects on the clamped part, making subsequent actions less accurate and reliable


Figure 3 – Bolt / nut torque application
For larger screws, the use of hydraulic tensioners, shown in Figure 4, offers greater precision. However, no commercial product of this type exists for the small screw sizes used in ASML's precision mechatronic assemblies. Also, hydraulic devices are incompatible with the clean room requirements needed for semiconductor manufacturing.

Figure 4 – Hydraulic Bolt Pretensioner
For this project, students are challenged to develop a method to assemble a bolted connection for small screws (M1.6-M3) for subassemblies such as Figure 5 that meets the following functional requirements:

  1. Applies an accurate and known bolt tension (+/- 5% from ~20% to ~80% yield of the bolt).
  2. Minimal (5-10um) transverse alignment distortion of the clamped part with respect to the base part.
  3. Applicable to screws spaced 4 - 5.5mm (M1.6) to 8 mm (M3) apart.
  4. Does not cross contaminate high vacuum compatible components, e.g. no lubricants allowed.
  5. Works with a variety of fasteners of various styles (pan head screw, socket head screws, hex nuts, etc.) and materials (stainless steel, titanium, etc.)


Figure 5 – Sample Subassembly
   
End Game
In the end, if all goes as planned, you will have a system unique to the marketplace. A system such as this would be of tremendous benefit to ASML and its endeavors to keep Moore's Law alive for the next generation, but should also be of general value to industry in general. Exceptional designs with patentable concepts may even be the basis of your own business. This is by no means an easy endeavor, but we believe your group is up to it and you have 9 months to show us what you got.
If you have any questions, please contact:
Andy Judge, Engineering Manager
Andy.judge@asml.com
+1 203 761 4328

Bill Dornfeld, Senior Architect
William.Dornfeld@asml.com
+1 203 761 4325
Dave Ramirez, Senior Architect
d.ramirez@asml.com
+1 203 761 4320
While they will help you, they will not do everything for you. Good luck (and skill)!

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