Developing a Pneumatically Operated Automatic Staircase
Motorola?s MOS 8 fab solved a nagging problem: How can operators access upper tubes in its horizontal furnaces? This article explains what Motorola did.
By Mark McGreevey and Stan Webb
For years, bulky and dangerous roll-around step ladders have been used in the diffusion area of Motorola`s MOS 8 Fab. MOS 8 is a mature fab and has very little unused floor space. Therefore, the unwieldy step ladders were constantly in the way of area operations and taking up floor space, which could be used more efficiently. Motorola engineers faced the challenge of eliminating the old step ladders while improving the efficiency of the furnace load/unload activities.
The problem
The furnace operator needs to load wafer boats into an upper furnace tube that is up to 7 feet off the ground. In order to reach the upper furnace tubes, the operator has to find a step ladder, roll it into position, then climb it repeatedly to load the furnace. The old-style step ladders are approximately 72 ¥ 48 ¥ 48-in. and were mounted on casters. Because the step ladder was not anchored to the floor, it was possible for it to move while the operator was using it. Obviously, this jeopardizes the safety of the operator and the product being carried.
During the loading process, operators complained of wrist and arm problems resulting from the strain produced when they tried to load wafer boats up over their heads into the upper furnace tubes. In addition, the step ladders were bulky and always in the way, and had to be moved from one furnace to the next within very tight and confined areas. Some operators aggravated wrist and arm problems by not using the step ladders at all, as they were not readily and conveniently available. Realizing that there was probably a better way to load the furnaces, Motorola`s Diffusion Equipment Engineering Group decided to build a much safer and more efficient stair.
The solution
The engineers designed the Pneumatically Operated Automatic Operator Stair to alleviate the safety, space and availability problems associated with old step ladders. The stair is a self-contained, Clean Dry Air (CDA) or nitrogen (N2) driven retractable three-step stair which extends and retracts at the touch of a switch with fully adjustable speed. By incorporating small size, automatic operation, ease of use, and availability into one unit, the engineers developed a unit that could be permanently mounted on each furnace four-stack in a convenient but unobtrusive location under the furnace boatloader system.
When the stair is retracted its size is 14 ¥ 30 ¥ 45-in. When the steps are open, a full-size, three-step stair is anchored to the floor and cannot move.
The design
The top step is 14-in. wide by 45-in. long and is constructed from Ametco Safety grip 304 stainless steel channel encased by 1 ¥ 2-in. rectangular 304 stainless steel tubing. This material was selected for its non-skid characteristics because users are fully garbed in cleanroom attire and usually are carrying a full lot of wafers in one hand. Considering the 30-in. height of the top step, a non-skid surface was added.
The framework supporting the top step provides attachment points for the slides. The stairs utilize four 590-lb. capacity stainless-steel slides which are double the estimated weight requirement and have double roller races for added strength. Although the heavy duty slides more than meet nominal weight requirements, casters were installed on the lower step to reduce stress on all of the anchor points when being used.
For extra stability, a small, 60-in. hand rail was added. The hand rail can be installed on either side of the stair according to the user`s preferences and the space requirements of the mounting location.
To make sure that the stair unit was compactable, special brackets were used to mount the cylinders. The brackets were manufactured to allow the middle step to collapse completely. These brackets are bolted to the back of the framework and to the middle step. The upper cylinders power the middle step, and the lower cylinders power the bottom step. When the toggle switch is activated, the cylinders extend and cause the stairs to open as a telescoping crane would. Proper adjustment of the flow controllers enhance the slow, purposeful extension of the stairs. The speed of the extension and the retraction was adjusted to be very slow (approximately 0.2 inches per second) to let the user move away from the extending steps.
Constructed of the same materials, the middle and the lower steps are powered by air cylinders with a 12-in. stroke. Cylinder speed is regulated by flow controllers on the input and output pneumatic ports of the cylinders. Activation of the cylinders is accomplished by “toggling” a two-position five-ported Pilot Valve that activates the control air to a spool valve. The spool valve “switches” the air to the proper port of the cylinders. If there should be a sudden loss in air pressure, the spool valve prevents the unit from changing state. The stair will not retract if it was opening or opened, nor will it open if it was closed or closing.
In order to comply with strict adherence to cleanroom protocol, the exhaust air is diverted to an exhaust duct. This prevents lubricants in the pneumatic hardware from getting into the process area. Diverting the air to an exhaust duct aids in silencing the operation.
Summary
With a pneumatic stair installed at each furnace stack, operators have better access to upper horizontal diffusion furnace tubes without strain on their arms and wrists, and without the need to locate and transport a ladder to their work area. They are able to move up to the level of their loading activities which allows them to load the furnaces with reduced wafer breakage, quartz boat breakage and particle generation. n
Mark McGreevey and Stan Webb are engineers with Motorola MOS 8`s equipment engineering team.
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The stair unit in a retracted position.
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The stair in an extended position.