BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to isolation devices used to separate two different atmospheres in continuous semiconductor processing apparatus, and more particularly to the structure of an entry-exit seal for continuous vapor deposition apparatus of the types disclosed in copending applications: Ser. No. 345 filed on Jan. 2, 1970, and entitled "Method and Apparatus for Diffusion Limited Mass Transport" or Ser. No. 825,827 filed May 19, 1969, and entitled "Continuous Systems for Fabricating Semiconductor Substrates to Contain a Diffused Conductivity Type Determining Impurity Therein," both assigned to the assignee of the instant invention.
2. Description of the Prior Art
Atmospheric isolation devices of the prior art are found to consist of three basic types: (1) restricted aperture types, (2) positive pressure types and (3) negative pressure types. These devices find application in various processing apparatus where it is desirable to prevent gaseous phase material, or other contaminants, from passing through an opening through which workpieces must pass. These devices are most appropriately used between ambient atmosphere and reaction vessel atmosphere, such as found in vapor deposition or sputtering apparatus.
The restricted aperture prior art device consists of a relatively long, low ceilinged, or restricted, passage which utilizes gas viscosity in combination with a gas pressure head developed over an extented distance to maintain positive controlled flow in a direction either toward or away from a particular part of the apparatus. These devices have their primary application where a relatively large pressure differential exists between the two atmospheres, for example, ambient atmosphere and a pressure vessel.
The positive pressure devices utilize a multiple stream concept wherein a single external gas stream, usually an inert gas, is divided such that part of the gas flows into the ambient and part flows into the processing vessel, thereby preventing the mixing of gases of the two atmospheres to be isolated. This type device has its primary application in apparatus where the mixing of two atmospheres on both sides of the seal is not permissible, for example, where the process gas is explosive in air.
Negative pressure devices are constructed similarly to the positive pressure types but the gas flow is reversed. Gases are drawn from both the process vessel and the atmosphere to prevent leaking of the process gases into the atmosphere.
The above-described prior art devices, although effective in certain applications, prove to be undesirable in the semiconductor processing apparatus disclosed in the above-referenced applications. The prior art devices require, depending on the type used, either high-gas flow rates or high-back pressure. Furthermore, when reasonable stream velocities needed to provide a proper margin of safety are used, sufficient lift is developed by the Bernouilli effect to cause semiconductor substrates to be lifted bodily from their carriers, thereby causing irreparable damage to the substrates.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to prevent damage to semiconductor wafers while passing from one atmosphere to another in a semiconductor processing system.
It is another object to provide an improved structure for a gaseous phase isolation device having improved efficiency without increasing costs for materials.
It is a further object of this invention to provide an effective gaseous phase isolation device which uses relatively low-back pressures more efficiently than prior art devices.
The invention herein disclosed is constructed to realize the aforementioned objects, goals and advantages, and comprises in its preferred form a positive pressure isolation device having a gas inlet means and two atmospheric communicating passages. The passages are formed by a series of alternating restricted apertures and expansion chambers. The restricted apertures provide means to increase back pressure and limit gas flow rates, while the expansion chambers provide relief from the Bernouilli effect caused by the restrictions and also produce a drop in enthalpy, or internal energy, of the gas stream by allowing controlled expansion, thereby allowing higher pressures to be used than possible with the low ceiling, or extended restricted aperture, type devices.
The foregoing and other objects, features and advantages of the invention will be apparent from the more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a partial isometric view showing the instant invention mounted on the process apparatus of copending application Ser. No. 345.
FIG. 2 is a partial cross-sectional view of the entry-exit isolation device taken through line 2--2 of FIG. 1 and shows the internal structure of the invention.
FIG. 3 is a sectional view of FIG. 2 taken along the line 3--3.
DETAILED DESCRIPTION OF THE INVENTION
Although the structure of the instant invention may be utilized as either an entry, exit or intermediate isolation device, only the application as an exit seal need be discussed in detail, as the structure and operation of the device is independent of its location in the processing apparatus.
Referring now to FIG. 1 there is shown the isolation device 10 of the instant invention mounted on the exit end of the process apparatus 12 of the aforementioned copending application, Ser. No. 345, the description and operation of which is herein expressly incorporated by reference. The isolation device is mounted on plate 14 which may be bolted, or otherwise attached, to the process apparatus. Mounted in a recess on carriers 16 shown leaving the isolation device 10, are semiconductor substrates 18. In operation, carriers 16 are continuously passed through the apparatus. Inlet gas tube 20 mounted on the top of isolation device 10 is provided to supply inert gas, for example argon, or other suitable gas, to effectuate operation of the device. Tube 20, normally is connected to a metered source of inert gas, not shown. It should be understood that the operation of the device is independent of the particular gas used and that any gas compatible with the two atmospheres to be isolated may be used.
Referring to FIG. 2, gas is delivered in a downward direction, as indicated by arrow 21, through tube 20 into chamber 22 from which it exits through gas restricting means, apertures 24 and 26. The restricted apertures are formed partially by the upper surface of carrier 16 and substrates 18 and partially by projections 28 of a top plate 30. Projections 28 may be formed by transverse grooves milled into top plate 30 or may be separately constructed and mounted into position. The shape of the restricted apertures is a gap of uniform width formed by the substantially parallel spaced relation between the upper surfaces of carriers 16 and substrates 18 and the lower edges or surface of projections 28.
Located adjacent to apertures 24 and 26 are expansion chambers 32 into which the gas then passes. Thereafter, there are provided a series of alternating restricted apertures 24 and 16 and expansion chambers 32, the particular number and size of which is optional depending upon the desired design characteristics of the isolation device as will be discussed below.
Carriers 16, normally consisting of machined high purity graphite, supporting semiconductor substrates 18 are continuously passed through the apparatus, for example, from left to right.
FIG. 3 shows a sectional view of the isolation device and more clearly illustrates the shape of restricted apertures 26 and expansion chambers 32. The carriers 16 are shown supported by bottom plate 34 and guided through recess 35. Both bottom plate 34 and top plate 30 may be constructed of stainless steel or other suitable material compatible with the atmospheres found in the process apparatus.
In summary, it will be seen that there is provided a gas inlet tube 20 and a plurality of first gas restricting means, apertures 24, leading to the first atmosphere, or the process apparatus, and a series of second gas restricting means, apertures 26, leading to the second or ambient, atmosphere, and adjacent to each gas restricting aperture is an expansion chamber 32.
The particular number, size and location of the various elements of the isolation device are best determined depending on the particular inert gas used, as well as the processing system requirements.
A typical example of an application of the instant invention would be in a vapor deposition process such as silicon deposition. In such a system it is desirable to utilize a certain net gas flow rate into the process apparatus to assure proper operation and safety of the system. For example, 1 liter per minute through a 4 inch wide restriction would be typical. Utilizing this flow rate and providing a typical gas restricting aperture size of 0.010 inch, the linear gas flow rate at the process side of the device would be about 5 centimeters per second, depending upon the width of carrier 16.
In order to determine the appropriate thickness of each projection 28 and thereby set the effective lift produced by the flowing gas, Bernouilli's equation must be used.
In its simplest form Bernouilli's equation may be written as:
P 1 - P2 = 1/2ρ(V22 -V12).
When applied to the lifting of a substrate from its carrier, P1 and V1 refer to the underside of the substrate where the velocity is zero and P2 and V2 refer to the top side of the substrate. It is obvious that at any streaming velocity the pressure gradient will be such as to provide a lifting force. Applying a minimum safe streaming velocity of 5 centimeters per second with, for example, argon gas having a density (ρ) of 1.78 grams per liter and utilizing a substrate area of 2.57×10-3 square meters the lift obtained is approximately 5.7 grams. Since the typical wafer weighs approximately 3 grams it must lift off the carrier. The solution as described herein is to provide a thickness for projection 28 which will reduce the effective lift area of reduced pressure to prevent the lifting of substrates. With a weight of 3 grams and a lift factor of 5.7 grams over the area of the wafer, it is obvious that a reduction in surface area of 50 percent is required to prevent lifting of the wafer. This can be accomplished by a combination of equal width expansion chambers and gas restricting apertures. The spacing between the substrate and the aperture will determine the pressure required to produce the 5 centimeter per second streaming velocity through one set of gas restricting apertures and expansion chambers. The number of apertures and expansion chambers will determine the overall system pressure. Apparatus constructed in accordance with the above description having 10 expansion chambers produces four times the back pressure of a smooth restricted aperture at the same streaming velocity. For example, a back pressure of approximately 4 ounces may be expected to provide a flow rate of 1 liter per minute through the device into the process apparatus, having an internal pressure of 0.1 ounce.
A similar determination may be made for the second, or ambient, atmosphere side of the isolation device. Typically a flow rate of twice that flowing into the process apparatus is desirable for the ambient side of the device, that is, a series of five sets of expansion chambers and apertures.
It should be understood that the structure of the above described device is not limited to positive pressure operation and may in some applications be utilized with a negative pressure. It should also be understood that, although the preferred embodiment utilizes equal width apertures, apertures of different size and shape may be desirable in some applications.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in the form and details may be made therein without departing from the spirit and scope of the invention.