Today`s optical tops and breadboards are faced with several challenging laboratory and production requirements. Not only must they be flat, lightweight, inexpensive, stiff, and structurally well damped, they must also meet the rigorous cleanroom standards required for precision optics research and semiconductor manufacturing.
Optical table construction has resulted historically in a relatively dirty structure, incompatible with cleanroom standards. This is hardly surprising considering the conventional manufacturing methods used to build a top, virtually making a sandwich of two steel plates bonded onto a honeycomb structure. Such tops are large, heavy, and usually built in machine-shop-type environments that are in stark contrast to modern cleanrooms. Beyond these obvious environmental drawbacks, however, the basic design was flawed. Drilling and tapping thousands of holes through a top sheet of stainless steel into a steel honeycomb core creates an inherently oily, dirty structure with countless small pieces of steel debris-a hopeless mess to clean. Furthermore, even if the top surface could be well cleaned, the residual oils from the manufacturing process could cause contamination by outgassing and creeping up out of the holes. As a result, engineers have avoided using optical top technology in cleanrooms.
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Attempts were made to seal the core of early tables to prevent outgassing and contamination, but this generally amounted to little more than plugging the tapped holes with wax or plastic. The result was to render the holes almost unusable. Worse, attempting to use the holes broke the seal, creating more contamination.
Reversing the order
The CleanTop, introduced by Technical Manufacturing Corp. (TMC; Peabody, MA) in 1984, used a process that reversed the conventional sequence of steps taken to manufacture a top. Rather than drilling and tapping a monolithic structure that had already been assembled and epoxy-bonded, the new process required that the hole pattern be machined prior to bonding the top to the honeybomb core. In addition, the process involved capacitance-discharge welding a 1-in.-long cylindrical, nonloadbearing plated steel cup beneath each hole on the stainless steel top skin prior to bonding. The cylinders provided a barrier between the holes and the core. Small parts dropped into the holes might be retrieved, and outgassing from any damping materials inside became impossible.
Unfortunately, the discharge welding caused a small amount of weld-splatter, and the tapping of the holes was, therefore, done after the structure was bonded. The residue of cutting oil and steel debris could be cleaned from these blind holes, but without a time-consuming and expensive individual detailing of the holes, some residue remained.
The recent CleanTop II design solves these limitations of the original system. Holes are tapped and counter-sunk before adding the cylinders to allow the machined top sheet to be thoroughly cleaned with a high-pressure, high-temperature solution forced through each hole. The cylinders, made from chemically resistant nylon-6 or stainless steel, are now epoxy-bonded, not welded, under each hole (see Fig. 1).
The other sides
The sides and bottom of the optical top are also potential sources of contami-nation. Conventional tops have wood or steel sides, covered with vinyl or a plastic laminate and caulked at the seams-hardly ideal from a micro-contamination point of view. Because the tops can be quite large (16 ft and longer), the bottom surface is generally steel with a roll-on paint treatment. TMC`s Class One optical top combines the CleanTop II features with stainless-steel sidewalls and a stainless-steel bottom skin. The seams are sealed with nonoutgassing epoxy caulk to form an unbroken, clean structure that is wiped with alcohol and wrapped in plastic prior to shipping.
Microscope on a table
Increasingly, optical and similar subassemblies are finding their way into microlithography tools and semiconductor inspection equipment. Generally such subassemblies have been mounted on granite or a machined, anodized aluminum plate in an attempt to meet standards of cleanliness. Because such structures have provided only marginal vibration damping and rigidity, system designers have begun to look to steel honeycomb structures to provide the same advantages in a tool that they provide in the laboratory, in the form of optical tables.
Digital Instruments (Santa Barbara, CA) faced such a challenge. A maker of scanning probe microscopes (SPMs), and in particular atomic force microscopes (AFMs), the company has installed more than 3000 AFMs since 1989. Though AFM technology had been successful in a variety of applications, developing a version of the tool for semiconductor inspection presented new engineering hurdles. Vibration control was a high priority in the design of the AFM, but because the systems would be installed in fab lines producing sub-200-nm-linewidth semiconductors, control of micro-contamination was equally important.
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The tool designed by Digital Instruments was the first large-sample AFM and is versatile enough to provide nanometer-scale three-dimensional measurements for everything from semiconductor wafers to optics, data storage devices, and other large samples (see Fig. 2). From a vibration control standpoint, it is difficult to imagine a more demanding application than an AFM. The challenge is compounded by the addition of an optical microscope and a laser tracking system, which are also sensitive to vibration. Paradoxically, the tool is capable of measuring surface roughness to extremely high precision (0.5 Å), yet is typically installed in environments with severe floor vibration-the raised floors of cleanrooms. Such floors are lightweight and easily excited by other process equipment, people walking by, and the powerful overhead cleanroom air-handling systems. The result is floor vibration with low-frequency displacements on the order of 10 ?m, a factor of 50,000 worse than the resolution of the tool.
Floor vibration limits tool performance by causing relative motion between the sample and the equilibrium position of the AFM probe tip. Not only must the tool itself be designed with a great deal of stiffness to remain relatively inert to vibration, but also the floor vibration must be aggressively isolated and damped. Damping ensures that vibrational energy reaching the isolated surface from any source is dissipated. Stiffness ensures that vibration on the surface excites only high-frequency resonances, at which the tool is less sensitive.
To achieve the required stiffness and structural damping, Digital designed a ClassOne CleanTop II breadboard into the system to function as the backbone of the structure. To minimize vibration reaching the breadboard and tool, the company chose a gimbal piston pneumatic vibration isolation system for vertical and horizontal floor vibration down to 2 Hz. The system incorporates a self-leveling air spring and converts horizontal and vertical floor motion into the vertical motion of a diaphragm-piston assembly.
The honeycomb breadboard improves overall system rigidity and provides the side benefits of reducing system weight and making it easier to handle. The breadboard measures roughly 28 in. square by 4 in. thick with a custom pattern of tapped holes and sleeved through-holes. The tapped holes of various sizes are back-sealed. The top, side, and bottom surfaces are stainless steel, as are the sleeves lining the holes in the top.
The top is damped using a system of broadband mass dampers that convert energy at the resonant frequencies of the top to heat. The dampers move out of phase with the top`s bending modes through a viscous chemical compound that dissipates the motion. The dampers are hermetically sealed from the exterior environment for cleanliness. The result is a tool with an advanced vibration control system and no compromise in cleanroom compatibility.
STEVEN T. RYAN is vice president of marketing at Technical Manufacturing Corp., 15 Centennial Drive, Peabody, MA 01960; e-mail: [email protected].