1. Field of the invention
The present invention relates to a non-contact conveying device for lifting up articles without physical contact with the articles by utilizing a gaseous medium such as air. Specifically, this invention relates to a device utilizing ejector effect and Bernoulli effect to lift up articles, as well as hovercraft effect and gas flow cushion effect to prevent injurious contact to the surface of the device.
2. Description of the Prior Art
The use of a gaseous medium such as air to support and/or lift articles is well known in the art and includes, for example, such devices as pickup heads and air truck systems.
In certain devices, suction, such as a vacuum or a negative pressure source, is employed to lift and/or support the articles. However, in many applications, for example processing and handling of semiconductor wafers, the use of a vacuum device is undesirable because physical contact between articles and a pickup device makes the articles susceptible to contamination and/or damage.
Other devices which use a positive pressure source to lift or support an article are of the so-called Bernoulli type.
Examples of Bernoulli type pickup head are disclosed in U.S. Pat. Nos. 3,438,668; 3,539,216; 4,002,254; 4,009,785; 4,029,351; 4,566,726; 3,523,706; 3,993,301; and a publication entitled "Wafer pickup with air barrier" (IBM Technical Disclosure Bulletin, Vol. 11, No. 2, July 1968).
All of these aforementioned devices are contructed to utilize the pressure-velocity relationship expressed in the Bernoulli principle to achieve a desired Pickup action.
The devices are constructed with a gas-flow boundary surface arranged adjacent the article that is to be picked up, such as a semiconductor wafer. A nozzle coupled to a positive pressure gas supply is formed in the middle of the flow boundary surface.
The velocity of the air flow is increased as it leaves the nozzle and consequently the pressure adjacent the flow boundary surface will be decreased. So, therefore, if the flow boundary surface is placed near a complimentary surface of an article to be picked up, the pressure between them will be sufficient to cause the article to be moved toward the flow boundary surface and thereby be picked up. At the same time, the flowing air provides a cushion to prevent damaging contact of the article with the surface of the device.
The aforementioned patents and publications disclose devices that utilize the Bernoulli effect to pickup articles and the gas flow cushion effect.
However, the non-contact conveying device of this invention utilizes not only ejector effects and Bernoulli effects to pick up articles, but also hovercraft effects and gas flow cushion effects to prevent the damaging contact.
U.S. Pat. No. 3,233,433 and a publication entitled "Self-Centering Non-Contact Pick-up", (IBM Technical Disclosure Bulletin, Vol. 2, Jan. 1980) have a chamber connected to a vacuum supply to lift articles as well as a chamber connected to a pressure supply to prevent contact with articles. However, in the pickup head of the present invention, the motion to lift up articles is achieved only by the gaseous flow of positive pressure.
U.S. Pat. No. 4,474,397 utilizes a suction force by an aspirator jet caused by exhaust gas. But, it has none of the Bernoulli effect, hovercraft and gas flow cushion effect.
A pickup head of the IBM Technical Disclosure Bulletin of September, 1976 (Vol. 19, No. 4) has a downwardly opening chamber having a downwardly inclined inner surface wherein the article exists at the bottom thereof, and a vertically extending inlet nozzle as shown in FIG. 26(b). FIG. 26(a) shows the pickup head of the present invention. FIG. 26(c) shows a graph of the relationship between the pressure caused on the surface of an article and the distance from the center of the pickup head, wherein P is the pressure caused on the surface of an article and Po is the pressure of the positive gas supply. It can be seen that the maximum negative pressure ratio P/Po of the present invention is about -1.5. The maximum negative pressure P/Po of the pick up head of the IBM device is -0.6 and is substantially smaller than that of the invention. The ratio P/Po of the IBM device is similiar to that of the prior pickup head of the Bernoulli type utilizing only the Bernoulli effect and gas flow cushion shown in FIG. 5. It is recognized that, in the IBM pickup head having a downwardly inclined inner slope, the only way of lifting the article is by means of the Bernoulli effect which is caused by the reduced static pressure produced by gaseous flow in the narrow space formed between the pickup head and the upper surface of the article, the same as the ordinary prior art shown in FIG. 5. Thus, the narrow space formed between the article and the pickup head of the IBM device does not work as a cushion-vacuum room.
In order to enable the opening chamber to function as a cushion-vacuum room, a diameter of the article should be larger than that of the Bernoulli Part which is the lowest part of the operating surface. However, in the pickup head of IBM, the outer surface of the article is positioned near the inclined inner slope of the pickup head, that is, the opening chamber of IBM pickup head is not the cushion-vacuum room. Furthermore, to keep the diameter of the article larger than that of the Bernoulli part, the gas flow will flow smoothly in the clearance between the article and the Bernoulli part, and accordingly, Bernoulli effect is effective, and the ejector effect in the cushion-vacuum room becomes larger because of the effective Bernoulli effect.
The pickup heads of British Patent No. 1,054,966 produce only a Bernoulli action which is caused by a reduced static Pressure by conducting a current of gas through a gap between the device and a surface of the article. A deflecting element is disposed to attain the highest possible jet velocity in the passage to decrease the resistance of flow from the conduit into the passage Russian Patent No. 1,151,497 lifts an article by a reduced static pressure due to Bernoulli effect.
As described above, the aforementioned patents and publications utilize only Bernoulli effect to lift articles and gas flow cushion to prevent injurious contact of articles with the surface of the device.
The non-contact conveying device of this invention utilizes not only ejector effect and Bernoulli effect to lift articles, but also hovercraft effect and gas flow cushion effect to prevent injurious contact of articles with the surface of the device.