BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional elevation view of the cooling apparatus of the present invention.
FIG. 2 is a plot of temperature versus time illustrating the measured cooling rates of a soda lime glass substrate for various gas pressures in a sealed gas volume utilizing the cooling apparatus of the present invention and also illustrating a plot of an estimated cooling rate utilizing prior art radiation cooling.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a schematic cross-sectional elevation view of the preferred embodiment of the substrate cooling apparatus of the present invention. The cooling apparatus is enclosed in a surrounding conventional vacuum chamber (not shown). A planar soda lime glass substrate 1 is supported on two metal substrate transport belts 2. These metal transport belts 2 may be of the type described in U.S. Pat. No. 6,423,565 and do not form part of the present invention. Rather, they are shown only for purposes of clarity. The metal transfer belts 2 extend out of the plane of FIG. 1. Upper and lower metal cooling plates 3, 8 are provided above and below substrate 1 and in close proximity to the flat upper and lower surfaces thereof. Upper and lower sealed gas volumes 5, 9 are contained and sealed by o-ring seals 6, 10. Sealed gas volumes 5, 9 are formed between upper and lower cooling plates 3, 8 and above and below substrate 1, respectively. Two gas supply tubes 4, 11 are provided to admit gas under controlled pressure from an external source to each of the sealed gas volumes 5, 9. Two cooling medium supply tubes 7, 12 are connected between an external source of coolant and cavities within each of cooling plates 3, 8 to facilitate controlled cooling of the cooling plates 3, 8. The cooling supply tube 7 passes through the top cooling plate 3 and the cooling supply tube 11 passes through the lower cooling plate 8 so that there are two ends to the cooling supply tubes 7, 11. The cooling plates 3, 8 are adapted to be moved vertically to allow close contact with substrate 1 and to also allow them to be moved away from substrate 1 to facilitate unimpeded horizontal movement of the substrate 1 through the vacuum chamber or other enclosure. A control valve (not shown) is provided in connection with each of the gas supply tubes 4, 11 so that the gas flow may be shut off when the cooling plates 3, 8 are moved away from the substrate 1. Multiple substrates 1 may be passed between the cooling plates 3, 8 to facilitate continuous processing of substrates. When the cooling plates 3, 8 are brought together in close proximity to the upper and lower surfaces of substrate 1, the o-ring seals 6, 10 are compressed to provide the seal necessary to admit gas through the gas supply tubes 4, 11 in order to bring the sealed gas volumes 5, 9 to a higher pressure than that of the surrounding ambient vacuum.
The lower cooling plate 8 must not be in direct contact with the lower surface of substrate 1 since this would harm the thin semiconductor films on that surface. Since the lower cooling plate 8 must be spaced further away from the substrate 1, the cooling rate due to the lower cooling plate 8 will not be as great as the cooling rate from the top cooling plate 3. However, the pressure in the lower sealed gas volume 9 is critical since the pressure in the lower gas volume 9 eliminates any deflection of large area substrates 1 away from the surface of the top cooling plate 3. If there is any deflection of large area substrates 1 away from the top cooling plate 3 the cooling rate will drop significantly. Pressures in the sealed gas volumes 5, 9 are equalized to eliminate the possibility of breaking the glass substrate 1 due to a pressure differential between the upper and lower surfaces thereof.
Any of a number of known cooling mediums can be passed through supply tubes 7, 12 including water. These cooling mediums may be conventionally cooled using chillers or brine baths, for example. The cooling rate of substrate 1 and the end point temperature of the cooling curve are determined by the temperatures of cooling plates 3, 8. These temperatures can be controlled by utilizing conventional PID control methods with a thermocouple control point. The gas pressure in the sealed gas volumes 5, 9 can also be measured and controlled using methods well understood in the art.
It has been found possible to improve the cooling rate of substrate 1 in a vacuum environment by enhancing the conduction cooling mechanism. Thermal conduction may be accomplished through solid contact or through gas phase conduction. This may be shown by placing the cooling plates 3, 8 in close proximity to the substrate 1 and increasing the ambient gas pressure in the sealed gas volumes 5, 9 between the cooling plates 3, 8 and the substrate 1. The heat transfer away from substrate 1 may be optimized in any of several ways. First, the cooling plates 3, 8 are preferably constructed of a material having high thermal conductivity, such as copper, so that excess heat is transferred away quickly. The surfaces of the cooling plates 3, 8 that face substrate 1 are preferably polished so that the surface roughness of the cooling plates 3, 8 is low. The spacing between the cooling plates 3, 8 and substrate 1 should be as small as possible. Preferably, the upper cooling plate 3 is in direct contact with the upper surface of substrate 1, with the average distance being approximately 0.005 inches or less. In this way, for any microscopically rough surface, the high points will be in contact and increase the probability of contact conduction. The lower cooling plate 8 of FIG. 1 must not be in direct contact with the lower surface of substrate 1 since this would harm the thin semiconductor films on that surface. As the ambient gas pressure in the sealed gas volumes 5, 9 is increased, the distance between the gas atoms decreases, and the probability of a collision between the gas atoms and the surrounding walls increases. These collisions of the gas atoms or molecules with the hot substrate 1 and the cold cooling plates 3, 8 increase the heat transfer from the substrate 1. The heat transfer due to the gas collisions are a form of conduction heat transfer through the gas phase in a confined space. In this case, there will be no convection currents or fluid flow in the gas, only random collisions of the gas atoms or molecules.
In order to illustrate the thermal efficiency of the present invention, a prototype test fixture was used to demonstrate that a soda lime glass substrate can be cooled from 170° C. to 37° C. in a two-minute time period and to 29° C. in a four-minute time period. The measured cooling rates of a soda lime glass substrate are shown in FIG. 2. The topmost curve in FIG. 2 is a predicted cooling rate for thermal radiation cooling of a glass substrate calculated by a finite difference analysis. The other three curves in FIG. 2 show the effect of increasing the gas pressure intervening space between the single top cooling plate 3 and a substrate 1. In this test prototype no o-ring seals 6 were used. In this prototype no deflection or stress was introduced so the lower cooling plate 8 was not required. As the gas pressure was increased from 40 mTorr to 1 Torr, the cooling rate increased significantly. The following parameters produced the test results shown in FIG. 2: 1) the distance between the cooling plate 3 and the substrate 1 was at most 0.005 in., with the surfaces thereof being in direct contact at some points; and 2) the temperature of the cooling plate 3 was approximately 20° C. The results shown in FIG. 2 demonstrate the efficiency of the current invention compared to thermal radiation cooling alone. In the working system the cooling rate can be increased by decreasing the temperature of the cooling plates 3, 8 below 20° C. and by using a combination of both cooling plates 3, 8. Another way to increase the cooling rate is to increase the pressure in the sealed gas volumes 5, 9 to pressures on the order of 10 Torr.
Patent Application
20080075853
