Patent Number: 046481060
Section: summary

CROSS-REFERENCES TO RELATED APPLICATIONS This invention relates to and may be utilized in the lithographic system disclosed in U.S. application Ser. No. 487,943 filed Apr. 25, 1983 entitled "Lithographic System" by W. Thomas Novak, Inventor; and U.S. application Ser. No. 475,427 filed Mar. 15, 1983 entitled "Mask Alignment Apparatus" by Anwar Husain, Inventor. The disclosure of such applications are herein incorporated by reference. BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a lithographic alignment system particularly one usable in a system employing X-ray or other beams for printing replicas of patterns or masks on photo sensitive materials contained on semiconductor substrates such as silicon wafers, wherein the inert gas between the X-ray source and mask and the wafer fabrication process gas between the mask and wafer are controlled in terms of purity attenuation, oxygen content and leakage losses, and in pressurization levels affecting the mask-to-wafer gap. 2. Description of the Prior Art U.S. Pat. No. 4,185,202 discloses as prior art an X-ray lithography system involving an X-ray source, an X-ray transmission chamber 14 and an imaging or exposing chamber 18 into which a prealigned mask-wafer fixture is moved into position under the source at the bottom of the exposing chamber for each exposure. The transmission chamber is maintained in a vacuum at a pressure less than about 10.sup.-6 torr while helium is present in the imaging or exposure chamber or a 10.sup.-2 torr vacuum provided which limits the throughout capabilities of the system. The improvement in the '202 patent is directed to having a movable (expandible) exposure chamber 40 with a side wall 46 which is lowered to engage the mask-wafer fixture. Helium is flowed into chamber 40 for each exposure at a rate of 10 liters/minute through passageway 42. A separate atmosphere of inert gas e.g. nitrogen at a rate of 2.9 liter/minute at a pressure of 5-50 psi is provided into the fixture and the zone between the mask underside and the wafer. Flushing of the zone is initially started while the fixture is still being loaded remote from the exposure chamber. The '202 patent also mentions that it may be advantageous to maintain the pressure in the exposure chamber greater than in the mask-to-wafer zone to prevent gaseous constituents in the latter being introduced to the former and contemplates addition of a small amount of inhibiting oxygen in the mask-to-wafer zone. In the prior art the respective gases are both allowed to escape to the atmosphere in a relatively uncontrolled manner, typically the helium leaking profusely near the mask edge and exiting from the junction of the chamber and a bottom frame, and the process gas being injected in the vicinity of the wafer edge, in an attempt to maintain the proper gas environment and allowed to exit between the chuck and bottom frame. SUMMARY In an X-ray alignment apparatus, proper control of gas environments is essential for overall performance in two critical areas. The first relates to the helium gas between the X-ray exposure source and the X-ray mask, and the second relates to wafer fabrication process gases between the mask and the wafer which is being exposed. The problems arise due to several competing factors. First, the purity of the helium in the exposure chamber must be kept high in order to reduce the X-ray exposure time to a minimum. A second factor is the need, in certain cases, to have a special wafer fabrication process gas in between the mask and the wafer. As an example, it may be desirable to have a gas mixture of about 0.3% oxygen in nitrogen present above the substrate resist for an optimum exposure. The oxygen in the gas reacts with the resist during exposure to obtain certain desirable printing characteristics. A third factor is that the helium gas required is somewhat costly, and steps should be taken to minimize its use. This in turn improves the economic viability of the X-ray lithography system. A fourth factor is that due to the membrane nature of the X-ray mask, any differential pressure across the mask must be kept to a minimum. A pressure differential will cause the mask to deflect upward or downward, and therefore cause the mask-to-wafer gap to vary. This gap would also not be uniform across the wafer since the membrane would deflect more in its middle than at its supported peripheral edges and would introduce additional printing distortion. The allowable pressure difference is so small as to be not economically measurable by available sensors. However, the first two factors which have been mentioned tend to cause there to be two different gases on either side of the mask membrane. The present invention is an improved X-ray lithographic system and substrate or wafer fabrication method in which the gases in the exposure chamber and in the abutting mask-to-wafer zone are controlled more exactingly and their usage significantly reduced. These advantages are realized by providing an inert gas vent tube adjacent to the bottom of the exposure chamber and of a design to prevent back diffusion from the surrounding exterior ambient and prevent mask membrane deflection. Further, the helium purity within the chamber can be continually monitored by providing an oxygen detector in the vent line. The present invention also provides an improved means for venting, sealing and controlling the pressure in the mask-to-wafer zone, including a gas flange interposed between a wafer-holding chuck and the mask holder, or in an alternative embodiment providing a vent tube for allowing a controlled purge of gas from the mask-to-wafer zone. This prevents contamination of that zone by ingress of ambient air without unduly pressurizing the underside of the mask membrane being acted upon on its top side by the helium or other inert gas in the X-ray exposure chamber. Providing an essential zero pressure differential across the mask membrane obviates any change in the gap distance between the mask membrane and the wafer.