WATER ACTUATED CYLINDER IN SEMI-CONDUCTOR WAFER ELEVATOR APPLICATION
An important high technology manufacturer builds processing equipment used in the fabricating of semiconductor integrated circuits. One operation in their chemical / mechanical polisher (CMP) involves lowering a cassette of silicon wafers into a rinse tank, then raising it. This sounds simple enough but a cassette can contain as many as 25 wafers etched with patterning for advanced- memory or microprocessor components. A full cassette of wafers can be worth more than $1 million, so smooth, precise and reliable elevator motion is important – for both operating and aesthetic reasons.
Older machines used a servomotor and precision ball-screw to raise and lower the cassette in incremental steps. When a new design eliminated the need for steps and called only for top- to- bottom travel, the lead senior design engineer, decided to investigate the use of air cylinders for this movement to reduce the cost.
The cylinders had to stroke 12 in. to immerse the cassette fully in the rinse tank and then to raise it to interface with a handling robot. Because this machine operates in a clean-room environment, no lubricants or particle-generating mechanisms can be present. In addition, the cylinder is exposed to harsh chemicals- for example; polishing slurries are present that can have ph’s that range from a very basic 1.5 to highly acidic 13.
From experience, the engineer knew that their inherent stiction would prevent most air cylinders from starting and stroking with the high degree of smoothness required in this application. He also knew that typical elastomeric seals could not meet the cleanliness specifications, nor could other materials used in typical air cylinders survive in the chemical environment. These limitations led him to Airpel pneumatic cylinders, built by Airpot Corp.
Airpel cylinders have no elastomeric seals between the piston and cylinder walls or between the rod and cylinder head. Instead, the Airpel design places a precision carbon/graphite piston inside a Pyrex glass cylinder-with a 0.0005 in. air gap surrounding the piston. The piston floats inside the barrel using air bearings principles. Instead of a rigid connection between piston and piston rod, a biaxially pivoting arrangement allows the piston to maintain its orientation despite lateral loading. These techniques dramatically reduce both static and dynamic friction, without lubrication. As little as 0.1 PSI can initiate movement in unloaded Airpels.
Rated for 100-PSI pressures, with an operating temperature range from -65 degrees to 150 degrees C, Airpels are well suited for applications that call for very smooth motion, low speeds, or short strokes, or where only low pressure is available. But they also will produce the same range of motions as conventional small air cylinders. To make the cylinder more rugged, Airpot adds a stainless-steel, impact- protection sleeve over the glass barrel, with a gap between the two. This protection lets the cylinder withstand impact testing with a 10 lb. load and continue to function.
While Airpel cylinders performed well during the initial tests, the elevator assembly did not quite meet the stringent motion requirements. Friction in the linear-guide mechanism and the variation in loads between full and empty cassettes resulted in motion that still was too irregular. The designer decided to see if he could actuate the CMP cylinders with water and capitalize on its incompressibility to achieve the smooth motion he wanted. This decision presented another problem. To meet the cleanliness requirements of the CMP machine, pressurized de-ionized water had to be used to drive the cylinder. And de-ionized water tends to corrode many materials. In particular it eats aluminum pretty quickly. To circumvent the corrosive effects of the de-ionized water, Airpot made some design modifications on the standard Airpel, These amounted to replacing the nickel plated aluminum piston couplings and cylinder heads with stainless steel components.
To test the modified cylinder, the designer used a 5 lb load and canted it at a 4” angle adding side loading to exaggerate the wear potential. With the raise-lower speed set at 6 in/sec. the modified water-powered Airpel was cycled more than a million times with no degradation in travel rate. (A small amount of demonized water leaked past the gap around the piston and was collected in a drain pan for disposal.) Testing on the actual elevator assembly followed, and the water actuated cylinder met all design and performance specifications.
Airpot Corporation manufactures instrument quality, precision Air Dashpots, Snubbers, Pneumatic Actuators, Airpel Anti-Stiction Air Cylinders and -AB Air Bearing Actuators. Airpot welcomes custom requirements and works with customers to adapt products to suit their individual application needs. Value added sub-assembly services are also a specialty. Airpot Corporation is a trusted supplier to many of the world’s most prestigious original equipment manufacturers. Visit www.airpot.com for detailed specifications. Call or email our technical staff for application assistance and ordering information. firstname.lastname@example.org 203-846-2021, 800-848-7681.
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