Patent Number: 061335770
Section: summary

FIELD OF THE INVENTION The present invention relates generally to photolithographic techniques and apparatus for use in the fabrication of semi-conductor components and more particularly to a method for producing extreme ultra-violet light for use in a photolithography system for facilitating the production of semi-conductor components having geometries of 10 nm and smaller. BACKGROUND OF THE INVENTION The use of photolithographic techniques in the fabrication of semi-conductor components such as dynamic RAM chips (DRAM), is well known. In the practice of such photolithographic techniques, light is utilized to cure or harden a photomask which prevents the chemical etching of various semi-conductor, conductor, and insulator portions of the device, as desired. As those skilled in the art will appreciate, the trend is toward semi-conductor components having greater and greater densities. This is particularly true in the area of memory, wherein it is extremely desirable to provide as much memory as possible in a given package. As those skilled in the art will appreciate, it is necessary to decrease the line size or geometry of the various semi-conductor, conductor, and insulator lines formed upon the component substrate in order to facilitate such increased density. That is, by making the individual devices, i.e., transistors, diodes, etc., formed upon the integrated circuit chip smaller, a larger number of such devices may be formed thereon. This, of course, facilitates fabrication of DRAM chips having greater capacity, for example. However, when utilizing photolithographic techniques, the lower limit on the line size is defined by the wavelength of the light utilized in the photolithographic process. Thus, extreme ultra-violet light (EUV) is capable of forming smaller line sizes (resulting in greater packaging densities) than is ultra-violet or visible light. Because of this, it is highly desirable to utilize extreme ultra-violet light in the photolithographic processes associated with the fabrication of integrated circuit components. According to contemporary methodology, two important goals associated with the use of extreme ultra-violet light in such photolithographic processes tend to be mutually exclusive. As those skilled in the art will appreciate, it is desirable to provide an intense source of extreme ultra-violet light and it is also desirable to minimize the generation of debris during the generation of such light. The curing time is directly proportional to the intensity of the light source. Thus, it is desirable to have an intense light source such that mask curing time may be reduced and the production rate correspondingly increased. It is desirable to minimize the generation of debris since such debris undesirably absorbs the extreme ultra-violet radiation prior to its being utilized in the curing process. Such debris also undesirably contaminates and degrades the performance of the optics which are utilized to collect and focus the extreme ultra-violet light. It also increases the vacuum pumping and filtering load on the system. The generation of such debris is inherent to contemporary methodologies for producing extreme ultra-violet light and tends to increase as an attempt is made to increase the intensity of the extreme ultra-violet light. According to one exemplary contemporary methodology for generating extreme ultra-violet light, a radiated energy beam such as the output of a high energy laser, electron beam, or arc discharge is directed onto a ceramic, thin-film, or solid target. Various different solid targets have been utilized. For example, it is known to form such targets of tungsten, tin, copper, and gold, as well as sold xenon and ice. The low reflectivity of mirrors which are suitable for use at the desired extreme ultra-violet light wavelength inherently reduces the transmission of extreme ultra-violet light through the optical system and thus further necessitates the use of a high intensity extreme ultra-violet light source. Degradation of the mirrors and other optical components by contamination due to debris formed during the extreme ultra-violet light generation is thus highly undesirable. Of course, as the intensity of the extreme ultra-violet light generation process is increased (by increasing the intensity of the radiated energy beam directed onto the target), more debris are formed. Thus, when utilizing such solid target configurations, the goals of debris reduction and intensity enhancement tend to be mutually exclusive. Consequently, the use of lasers and/or electron beams to ionize a gas flow so as to emit the desired intensity of extreme ultra-violet light while mitigating the production of undesirable debris is presently being investigated. Thus, it is known to utilize gas jets for the targets of lasers and electron beams in the production of extreme ultra-violet light. It is also known to cryogenically cool noble gases such as xenon and argon, so as to cause the gas to assume a super cooled state, wherein the individual atoms are drawn together into large clusters of several thousand atoms or more. While the use of such gas jets and/or cryogenic cooling methodologies have proven generally suitable for laboratory demonstrations, the vacuum pumping requirements necessary for such steady-state operation at high extreme ultra-violet light production rates is economically prohibitive. As such, it is desirable to provide means for producing high intensity extreme ultra-violet light while minimizing the undesirable production of debris. It is further desirable to accomplish such extreme ultra-violet light production utilizing methodology which substantially reduces the vacuum pumping requirements, thereby correspondingly reducing the size, cost, and power requirements of the system. SUMMARY OF THE INVENTION The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, the present invention comprises a method and apparatus for producing extreme ultra-violet light. The method comprises the steps of flowing a gas at a supersonic velocity, directing a radiated energy beam into the flowing gas to stimulate emission of extreme ultra-violet light therefrom, and capturing a substantial portion of the gas so as to mitigate contamination caused thereby. By capturing a substantial portion of the gas, the amount of debris available to contaminate the system's optical components is mitigated. As used herein, the term debris is defined to include any atoms, molecules, electrons, ions, or other material which is a component of the flowing gas or which results from the interaction of the flowing gas in the radiated energy beam. As those skilled in the art will appreciate, a substantial portion of such debris is trapped within the gas flow jets, which itself is then captured so as to prevent contamination. The step of flowing a gas at a supersonic velocity preferably comprises flowing a pressurized gas through a converging-diverging nozzle, so as to increase the velocity thereof. The converging-diverging nozzle preferably has a generally rectangular cross-section. The converging-diverging nozzle also preferably has a length substantially greater than the width thereof (a high aspect ratio). According to the preferred embodiment of the present invention, both the nozzle from which the supersonic gas flows and the opening in the diffuser into which the supersonic gas is received are approximately 9 mm long and approximately 0.9 mm wide, thus giving both an aspect ratio of approximately 10. The diffuser preferably comprises a converging portion proximate the opening thereof having walls angled at approximately 6.degree. to the gas flow axis thereof, so as to generate a stable system of shocks. The shocks decrease the velocity of the gas within the diffuser and increase the pressure thereof, as discussed in detail below. Those skilled in the art will appreciate that the dimensions of the nozzle and the diffuser may be varied substantially, as desired. Moreover, the throat area, inlet to throat area ratio, throat length, and exit divergence angle of the diffuser are preferably optimized according to well known principles for a given jet in order to obtain desirable pressure recovery and minimize gas bypass (gas not received within the diffuser) of the diffuser. The step of flowing a gas at a supersonic velocity preferably comprises expanding the gas so as to substantially decrease the temperature thereof. As those skilled in the art will appreciate, decreasing the temperature of the gas substantially increases a density thereof, by causing the atoms or molecules of the gas to tend to clump together, preferably in large clusters thereof. As those skilled in the art will further appreciate, the density increase due to such clumping substantially enhances the emission of extreme ultra-violet light therefrom. According to the preferred embodiment of the present invention, the gas comprises a noble gas, preferably argon, helium, and/or xenon. The gas is preferably flowed at a velocity of at least Mach 2, preferably Mach 3. The gas is preferably flowed at a supersonic velocity through a vacuum chamber, so as to facilitate photolithography, such as in the fabrication of integrated circuit components. The radiated energy beam preferably comprises either an electron beam, a laser beam, or a microwave beam. Those skilled in the art will appreciate that various other forms of radiated energy may likewise be suitable. According to the preferred embodiment of the present invention, the radiated energy beam is directed proximate the converging-diverging nozzle from which the gas flows, such that the radiated energy beam passes through the flowing gas in a manner which mitigates absorption of the extreme ultra-violet light stimulated thereby back into the flowing gas. Thus, according to the preferred embodiment of the present invention, reabsorption of the stimulated extreme ultra-violet light, particularly by the flowing gas, is minimized. In order to accomplish this, the radiated energy beam is directed through the flowing gas proximate a surface thereof, so as to reduce the distance that the extreme ultra-violet light stimulated thereby must travel through the flowing gas. It will be appreciated that the amount of gas through which stimulated extreme ultra-violet light must travel is proportional to the distance between the point of emission, i.e., the point of interaction between the radiated energy beam and the flowing gas, and the outer edge or surface of the flowing gas, beyond which the extreme ultra-violet light passes substantially only through vacuum. Thus, by positioning the radiated energy beam such that it passes through the flowing gas proximate a surface thereof, extreme ultra-violet light stimulated by the radiated energy beam within the flowing gas travels through less of the flowing gas than would be the case if the radiated energy beam were positioned deeper inside the flowing gas. According to the present invention, a substantial portion of the gas is received within a diffuser which is configured to reduce the velocity of the gas and also to increase the pressure thereof. Thus, the diffuser mitigates the contamination of system optical component by reducing the amount of gas flowing within the vacuum chamber. The use of the diffuser also reduces the load upon the vacuum pump by reducing the pumping requirements thereof. Further, according to the methodology of the present invention, the gas captured by the diffuser is recycled such that it repeatedly flows from the nozzle and is repeatedly stimulated to provide extreme ultraviolet light. Further, according to the preferred embodiment of the present invention, that gas removed from the vacuum chamber by the vacuum pump is also recycled. The aspect ratio of the cross-section of the diffuser, a the opening thereof, is preferably similar to and approximate that of the aspect ratio of the cross-section of the converging-diverging nozzle, at the exit thereof from which the gas flows. Alternatively, the aspect ratio of the cross-section of the diffuser, at the opening thereof, may be different from that of the aspect ratio of the cross-section of the converging-diverging nozzle at the exit thereof. For example, the opening of the diffuser may optionally be substantially larger in cross-sectional area than the exit of the converging-diverging nozzle, so as to enhance the capture of the flowing gas. As those skilled in the art will appreciate, when the cross-sectional area of the opening of the diffuser is substantially larger than the cross-sectional area of the exit of the converging-diverging nozzle, then the aspect ratio of the opening of the diffuser becomes less critical. Thus, according to the methodology of the present invention, a substantial portion of the kinetic energy of the gas is converted into pressure, so as to facilitate more efficient recycling thereof. As those skilled in the art will appreciate, the gas must be provided to the nozzle at a substantial pressure, so as to effect supersonic flow thereof. By converting a substantial portion of the kinetic energy of the gas into pressure, the pumping requirements of the system are substantially reduced, thereby reducing the costs of constructive and operating the system. The pumping requirements are substantially reduced since the difference between the input and output pressure of the pump is reduced when the input pressure is increased, as by converting a substantial portion of the kinetic energy of the gas flow into pressure. The gas captured by the diffuser, and optionally the gas removed by the vacuum pump as well, is compressed, so as to increase the pressure thereof to that pressure required for achieving the desired gas flow speed from the nozzle. Heat is removed from the gas prior to its being provided to the nozzle, so as to facilitate the desired cooling thereof upon expansion as the gas exits the nozzle. According to the preferred embodiment of the present invention, the diffuser comprises at least one knife edge which is configured so as to reduce the velocity of the gas captured thereby. As those skilled in the art will appreciate, various different configurations of such knife edges are suitable for reducing the velocity of the gas captured by the diffuser. For example, the knife edges may comprise concentric, generally parallel sets thereof, having generally rectangular, round, or oval shapes, for example. Alternatively, the knife edges may comprise a plurality of generally horizontal or vertical members. It is also contemplated that one or more point-type knife edges, configured generally as pointed needles may alternatively be utilized to generate shock waves. Thus, according to the present invention, the nozzle and diffuser inlet are configured to utilize gas dynamics properties of a supersonic jet of gas to direct debris formed during interaction of the radiated energy beam and the gas jet into the diffuser, and thus mitigate contamination of the system's optical components thereby. In this manner the collecting and focusing optics, for example, are maintained in a substantially contamination free manner, so as to enhance the integrated circuit fabrication process performed therewith. As those skilled in the art will appreciate, by reducing the contamination of such optical components, maintenance, i.e., cleaning, of the system's optical components, is substantially reduced and the production rate is increased, thereby providing a substantial economic advantage. Collecting and focusing optics collect the extreme ultra-violet light and focus the extreme ultra-violet light upon the desired target, e.g., a mask being cured upon the integrated circuit component(s) being fabricated. Thus, the methodology and apparatus of the present invention provides means for producing extreme ultra-violet light in an photolithography system for facilitating the production of semi-conductor components having geometries of 10 nm (nanometers) and smaller. According to the present invention, means for producing high-intensity extreme ultra-violet light while minimizing the undesirable production of debris are provided. Such extreme ultra-violet light production is further accomplished utilizing methodology which substantially reduces the vacuum pumping requirements, thereby correspondingly reducing the size, cost, and power requirements for the system.