Glass Flotation for PV and FPDs
by Drew Devitt
Founder, Chairman and Chief Technology Officer
New Way Air Bearings
Aston, Pennsylvania, USA
edited by Laura Peters
Editor-in-Chief, Semiconductor International Magazine
For large-generation flat-panel displays and PV modules, it has become prevalent to float the glass through inspection areas on an air conveyor. We demonstrate a new approach for Gen 10 panels that saves 75% of machine weight compared with moving gantries, while exploring several options for controlling these heavy panels in ambient to vacuum environments.
Large LCD TV screens (1 meter and larger) are forecast to be one of the fastest growing segments in the display market. Accordingly, manufacturers are turning to increasingly larger sheets of glass. Generation 8 was considered large at 2.0m2. One leading manufacturer is working now to commission a Generation 10 fab. Another is planning a Generation 11 facility.
Dimensionally, Gen 10 glass is 3.1 m long by 2.8 m wide, yet less than 0.7 mm thick. Such large, thin glass is difficult to handle. With surface areas like sails, these large sheets of glass are significantly influenced by the downdraft air currents typical of a cleanroom environment.
For larger generation sizes it has become prevalent, especially in automated optical inspection (AOI) applications, to float the glass through the inspection area on an air conveyor. Air conveying has also become attractive for laser scribing and cleaning of thicker glass substrates used in the manufacture of photovoltaic (PV) solar modules. This new approach to FPD and PV has required rethinking on the part of equipment manufacturers.
Traditionally, processing or inspection would occur by one of two means: a large, moving vacuum chuck holding the glass would pass beneath a stationary gantry, or a moving gantry would translate over the glass, held on a stationary vacuum chuck.
By moving only the glass, the architecture of the machine may be changed dramatically. Where, previously, huge granite bases with moving gantries spanning 3 meters were required, by this new means a substrate weighing only a few kilograms can be moved through stationary inspection zones. The structural loop of the machine is now only necessary in the inspection zone itself. So a Gen 10 machine may be a meter or less in length axially, and may weigh less than the Gen 6 equipment it has superseded.
This float-the-glass arrangement requires the same amount of cleanroom space as the moving chuck architecture mentioned previously, but the new design saves 75% of the machine weight. When compared to the moving gantry approach, a float-the-glass design would require some extra cleanroom space, but would still provide for a much lighter machine structure, and would avoid issues with accelerations and/or decelerations of the gantry on the top of the machine (Figure 1).
Floating the glass on air conveyors vs. moving gantry architecture.
The glass in these two examples is the same size.
Understanding the dynamics of glass flotation is critical to achieving precision and avoiding damage. The simplest of flotation systems involves large plate with many holes drilled in it (Figure 2).
A drilled plate or table.
In this example a simple blower issues air pressure of <7 kPa. Despite this seemingly low pressure, because 1 cm² of FPD glass weighs a small fraction of a gram, this pressure provides significant capacity. As shown, the highest pressure is generated under the center of the glass, because the air nearest the edge can more easily flow out, causing the glass to bulge in the center.
In a laminar-flow cleanroom, air flows from the ceiling to the floor. When this air flow encounters the horizontal glass, pressure gradients are caused on top of the glass as well. This creates a zero velocity area in the center of the glass and a high velocity area near the edge, which can cause the glass to flutter like a flag in the wind.
Ambient grooves in the plate, under the floating glass, are one method used to minimize the development of pressure differentials. Another method uses hollow aluminum bars as a platform, rather than a single large plate. These bars introduce air pressure beneath the floating glass through small holes drilled at regular intervals. The spaces between the bars avoid a pressure gradient build-up. But there is still high pressure above the glass created by the clean room down draft. This force can amount to a multiple of the weight of the glass, increasing sag between the conveyors. The air flow counteracting this force will have a high pressure point in the center of each aluminum bar. This point acts as a fulcrum for the glass, which will sag from this point to its lowest point, directly between the conveyors. Because FPD glass is not very stiff, this sag can be surprising – even disastrous – if the glass feeds onto a perpendicular surface.
Ironically, the atmosphere itself can be used to gain control over the glass; to eliminate variables and make glass behavior more deterministic. This can be accomplished simply by using low-pressure vacuum areas beneath the glass, causing the atmosphere to push down with a force far exceeding the weight of the glass, or the force from the down draft. With a differential of <1 kPa pressure, the down force distributed across a Generation 8 glass substrate can be 100 kg. When this down force of more than 10× the weight of the glass meets the opposing (10 kPa) force from the area of pressurized air conveyor surface, it puts the glass under control. No longer floating freely through the cleanroom, subject to the influence of every air current, the atmosphere presses it down evenly against a stiffer, higher-pressure air film.
The math and the physics are clear, but the effect may seem counterintuitive. It is easy to think that the further away the glass is from the conveyor the safer it is, but this is not the case. Glass floating hundreds of microns above a bearing surface is actually out of control, because the common forces in the cleanroom become comparable to the supporting forces. Stiffness, damping and flow all change approximately with the cube of the air gap, so the smaller gaps prevalent with the use of vacuum dramatically increase stiffness and damping, and reduce flow.
Vacuum holes are the most common way to introduce low-pressure into an air bearing film, but because FPD glass will bend easily over a distance of just ten millimeters, contact becomes likely with such configurations.
Another option is to use porous media, which will more evenly distribute the air pressure across the face of the bearing surface. With a porous media surface, pressure issues right up to the edge of a vacuum hole, so contact difficulties are reduced significantly. This also helps minimize problems associated with the glass bending between vacuum and pressure holes, and provides support across the entire pressure surface. Wherever you might press down on the glass, there is always a surface land issuing high pressure, pushing back, resisting contact.
A porous surface has other advantages over discrete pressure holes, as well. Because the porous media distributes a higher air supply pressure across the entire bearing surface through millions of submicron-sized holes, the land surface provides for 20× higher resistance to grounding. This is possible because the higher resistance to airflow though the passageways of the porous material allows for the use of higher air pressure (10 kPa), but at much lower flows. The energy consumption to compress the air is about the same, but the load capacity or resistance to contacting the glass is 20× higher (at a lower gap). Additionally, airflow from a porous land has, essentially, zero velocity a fraction of a millimeter from the surface, and air currents that could carry contamination above the glass are avoided.
Figure 3 demonstrates another reason to use vacuum in a conveyance-only area. Glass floating without vacuum (on air pressure only) is similar to the ocean’s surface, and subject to low-frequency waves. These waves have the effect of shortening the length of the glass and, because the glass is stiff in plane, position and image stability issues can occur due to the shifting of the glass axially from the inertial forces. The glass could be oscillating over the conveyance area, while in the precision zone it will be perfectly stable in the Z axis. It is more difficult to provide stability in X and Y, however, when not using vacuum in the conveyance areas.
Waves over a drilled extrusion can result in motion in the inspection area.
So exactly how stable can the glass be held in Z? According to measurements collected using its state-of-the-art 3D optical profiler, custom designed for the Display inspection market, Zygo Corp. has been able to collect repeatable interferometric photo height spacer measurements with vertical glass stability on the order of 1 nm while the glass is flying at 25 µm.
This is in large part due to the short structural-loop advantage noted earlier. The large vacuum chucks used previously to hold the glass by contact were kinematically mounted. This left a lot of unsupported chuck and, so, a diaphragm effect. By bolting vacuum-preloaded, porous carbon precision chuck segments directly to their metrology frame, Zygo was able to reduce background vibrations in the system.
Additionally, the need for a large substrate-sized vacuum chuck was eliminated in favor of a row of brick-size precision chucks. Further, because nothing is cantilevered, the result is a very stiff, dimensionally-stable reference (for the glass) that is bolted directly to Zygo’s metrology frame.
The precision chucks used employ a slightly higher vacuum pressure (-2.50 to -10.00 kPa as compared with -0.25 to -1.00 kPa used in similar conveyors). The vacuum is projected across a narrower/smaller area as the glass is stiff locally. The vacuum holes are engineered so that they match the flow through the porous media. In this way, the flow through a vacuum hole remains the same whether there is glass over it or not, so there is no change in restriction or flow, and so no change in pressure or fly height.
A groove is another way to distribute vacuum under the glass. Grooves used for this purpose still feature integral vacuum holes, but they are not directly closed when the glass passes over, so they damp the issue of sudden flow changes through the vacuum hole.
Grooves also provide low pressure continuously at the leading edge of the glass as it moves across the array. This enables safe, high-speed glass conveyance by preventing the leading edge from “taking off.” The continuous down-force of a groove is also more practical when glass warping or stress from coatings requires the flattening functionality of vacuum. When the glass is thicker and stiffer, or not of as high a quality, as is the case with most solar substrates, a conveyor with four or more continuous grooves may be necessary to flatten the glass. Yes, it is counterintuitive to press the glass down harder to get it to float better. But because there is so much down force, and the lifting lands are not very wide, such a conveyor floats the glass at a relatively low fly height. Achieving higher preload force would require high input pressures to achieve the same fly height.
In practice, solar glass quality is not as bad as may be assumed from reading the standard.
A single vacuum groove down the center of a porous-media conveyor will maintain continuous low-pressure at the leading edge of the glass and still prevent glass ‘fly-aways’ at speed. But because there is just a single vacuum groove, instead of four, the vacuum force is four times less. This allows for a higher fly height with lower air pressure for a given vacuum level. This, in turn, reduces the air input requirements, while still maintaining a stiff hold on the glass.
This function also provides several other important advantages. With the glass traveling parallel to the conveyors in an array, the center groove pulls the glass down in the center, pushing it up along either side. This has the fortunate effect of flattening the glass, and reducing sag between conveyors. This is where the single vacuum groove provides a beneficial natural effect (Figure 4).
The single, center vacuum groove pulls the glass down in the center,
pushing it up along either side, flattening the glass, and reducing sag between conveyors.
In summary, when floating PV or FPD glass there are basic aerodynamic issues resulting from both the environment and the air flotation system which, when recognized, allow for deterministically controlling the glass to nanometer stability.
Drew Devitt is the founder, chairman and CTO of New Way Air Bearings. He is also a past president of the American Society for Precision Engineering (ASPE) and a member of the Education Committee for the Bearing Specialists Association (BSA).
by Drew Devitt
Founder, Chairman and Chief Technology Officer New Way Air Bearings Aston, Pennsylvania, USA