DEVICE FABRICATION BY X-RAY LITHOGRAPHY UTILIZING STABLE BORON NITRIDE MASK

Abstract: A hydrogen-free boron-containing membrane (28) is a state of tension exhibits advantageous properties for use in a mask for X-ray lithography.

Description: Background of the Invention This invention relates to the fabrication of microminiature devices and, more particularly, to the manufacture of such devices utilizing the technology known as X-ray lithography. X-ray lithography is a technology that is being explored for the manufacture of microminiature devices, such as very-large-scale integrated-circuit devices with submicron feature sizes. In this technology, a resist-coated workpiece is exposed to X-rays via a mask that is interposed between a source of X-rays and the workpiece. The mask is a key component of X-ray technology and is thus the subject of extensive development. Such a mask comprises a X-ray-transparent membrane (or layer) having an X-ray-opaque device pattern defined thereon, to form a mask having some portions that are opaque to X-rays and other portions that are transparent thereto, in accordance with the pattern of the device to be manufactured. It is critical that such a membrane exhibit a high degree of dimensional stability. Further, to facilitate mask-to-workpiece alignment, it is thought to be advantageous that the membrane be optically transparent. Various materials from which to make the membrane have been suggested. One such promising material is an amorphous boron compound, such as boron nitride or boron carbide, that is made to be tensile by virtue of being boron rich or being doped with silicon, as described in U.S. Patent No. 4,171,489. Another is amorphous hydrogenated boron nitride which exhibits excellent X-ray transmissivity relative to competing materials such as silicon or silicon carbide. It has been shown that boron nitride in a state of tension can be fabricated into large-area membranes with high X-ray and optical transparencies, low defect densities, and excellent mechanical properties. For example, U.S. Patent No. 4,522,842 describes one such hydrogenated boron nitride membrane useful for X-ray lithography. Recently, workers have investigated the effects on hydrogenated boron nitride of radiation damage caused by long-term X-ray exposure, high-flux X-ray exposure, or both. The results of these investigations are reported in an article entitled "Radiation Damage Effects in Boron Nitride Mask Membranes Subjected to X-ray Exposures" by W.A.Johnson et al., published in Journal Vacuum Science Technology B , Vol. 5, No. 1, January/February 1987, pages 257-261. As reported in the cited article, X-ray radiation exposure can degrade both the optical and mechanical properties, and thus reduce on the useful lifetimes, of membranes made of amorphous hydrogenated boron nitride. Specifically, it is reported in the article that undesirable growths will form on exposed boron nitride surfaces irradiated in oxygen-containing ambients; optical transmission of X-ray irradiated membranes will degrade with cumulative absorbed dose; and, most significantly, the tensile stress of a hydrogenated boron nitride membrane will decrease under irradiation and thereby introduce distortions in the device pattern thereon. In practice, the technology of X-ray lithography is progressing towards higher power X-ray sources and step-and-repeat modes of operation. As this progress occurs, X-ray membranes must tolerate significantly greater radiation doses. But, as indicated, comulative radiation-induced changes appear to constitute a serious and fundamental wear-out mechanism for membranes made of hydrogenated boron nitride. In turn, this mechanism threatens to curtail the use of this otherwise attractive material in masks for X-ray lithography. Accordingly, considerable efforts by workers in the art are being directed at trying to understand the aforestated radiation-induced phenomena and to devise a membrane for X-ray lithography masks that do not exhibit the above-described problems. According to the present invention, there is provided a structure as defined in claim 1. Such a membrane for X-ray lithography masks can be made in accordance with the invention, applicant has discovered, with a membrane of material that comprises a hydrogen-free boron-containing compound, in particular, boron nitride. An advantageous such material has a boron-to-nitrogen atomic ratio of 1-to-1 and is designated herein as BN. A membrane of BN, however, inherently exhibits an undesirable compressive stress condition. Hence, further in accordance with applicant's invention, a hydrogen-free BN membrane is put into a condition (or state) of suitable tension. One particular way of doing so is to combine with the BN membrane a layer of an inherently tensile material, such as hydrogen-free B ₃ N . By suitably adjusting the thickness of the BN membrane, a membrane in the form of a composite layer is achieved, with the BN membrane in the condition of suitable tension. The drawing, not drawn to scale, is a cross-sectional schematic diagram of a portion of a specific illustrative X-ray lithography system in accordance with a specific embodiment of the invention. Detailed Description Hydrogen-containing (or hydrogenated) boron nitride of the type priorly used to form mask membranes for X-ray lithography exists in practice in different compositional forms. The particular form of hydrogenated nitride that is actually deposited is a function of the specific process parameters (e.g., temperature, pressure, B₂H₆/NH₃ ratio, B₂H₆ partial pressure, etc.) selected to deposit the material. One particular form of known hydrogenated boron nitride may be represented as B₃NH. This is the specific composition of the amorphous boron nitride investigated and reported upon in the above-mentioned article by Johnson et al. As indicated earlier above, there are three radiation-induced effects in an X-ray mask membrane made of a known material such as B₃NH which can cause both the surface and bulk properties of the membrane to be deleteriously modified. First , the aforementioned growths on the surface of the membrane are believed to result from the availability in the material of excess free boron. Under irradiation, surface dangling bonds of boron are created. In turn, these bonds can react with an oxygen environment to form growths of boric oxide crystallites. These growths can degrade optical alignment signals. Second , dangling boron bonds interior to the surface of the membrane can create midband gap traps (or color centers) which decrease the optical transparency of the membrane. And third , when a B₃NH membrane is irradiated at sufficiently high doses and dose rates, the bonding configurations of the hydrogen therein are modified such that more of the hydrogen becomes loosely bonded. This loose bonding in turn causes the membrane to become less tensile and to relax. Such variable stress changes in the membrane can cause large and variable distortions in the device pattern of the mask. These variable changes cannot be tolerated if accurate overlay alignments are to be achieved in a multi-mask fabrication process. In accordance with the principles of applicant's invention, a membrane for X-ray lithography is made of a material comprising hydrogen-free boron-containing membrane. By insuring that no hydrogen exists in the material, the basis for stress variations therein is removed. Moreover, in accordance with one specific illustrative embodiment of applicant's invention, the hydrogen-free membrane is advantageously made of BN. Significantly, no excess free boron exists in such a material. Consequently, the aforedescribed problems of radiation-induced crystallite growth and degradation of optical transparency are obviated. The aforementioned BN material inherently exhibits a compressive stress which generally would make it unsuitable by itself for use as a membrane for X-ray lithography. Therefore, in accordance with applicant's invention, various approaches are utilized to impart to the BN membrane a tensile stress in a specified range, i.e., sufficient to put the BN membrane into a suitable condition of tension and keep it there. The Figure shows a portion of a specific illustrative X-ray lithography system in accordance with a specific embodiment of the invention. In the depicted system, a conventional source 10 emits X-rays that are represented by dash lines 12. The emitted X-rays are directed, as indicated by arrowheads, toward a mask 14. Spaced apart from the mask 14 is a workpiece 16 (e.g., a semiconductor wafer) that includes thereon a layer 18 constituting a standard X-ray-sensitive resist. Illustratively, the workpiece 16 comprises a cylindrical disc or wafer having a diameter d of about ten centimeters (cm). The mask 14 comprises a dimensionally stable ring 20 made, for example, of glass. A ring 22 made of silicon is affixed to the ring 20 by any suitable means such as a layer 21 of epoxy cement. In turn, a multi-layer structure comprising layers 24, 26, 28 and 30 is supported by the ring 22. The mask 14 also includes an X-ray-opaque pattern comprising elements 32. These elements are definitive of a pattern to be transferred into the resist layer 18 on the workpiece 16. The layer 28 comprises the above-described inherently compressive membrane made of BN. In accordance with a feature of applicant's invention, the layer 28 is in effect tuned or adjusted to achieve a tensile stress condition in a specified range by combining therewith a layer in a state of tension. The layer 26 constitutes this latter layer. By way of example, the layer 26 comprised hydrogen-free boron nitride of the compositional form B₃N. This material is inherently highly tensile; and, because it contains no hydrogen, its stress condition remains stable even when the layer 26 is intensely irradiated with X-rays for a prolonged period. Illustratively, the layer 26 made of B₃N exhibits a tensile stress of approximately 1 x 10⁹ dynes per square centimeter. This stress condition is highly stable and is essentially independent of the thickness of this layer 26. In one embodiment, it was convenient to make the layer 26 about three micrometers (µm) thick. The resulting overall composite stress of the bilayer structure comprising the inherently compressive BN layer 28 and the inherently tensile B₃N layer 26 is determined by the thickness of the layer 28 deposited on the layer 26. As the thickness of the layer 28 increases, the composite stress of the structure becomes less tensile. In practice, a composite structure consisting of a BN layer about 0.5 µm thick combined with a 3-µm-thick B₃N layer exhibits an overall tensile stress of approximately 0.2 x 10⁹ dynes per square centimeter. This is within the limits of 0.1-to-1 x 10⁹ dynes per square centimeter, which defines the range of tensile stresses usually desired for such a structure. For a reason that will be apparent later below, it is advantageous to establish a barrier to prevent excess free boron from the B₃N layer 26 from diffusing into the silicon wafer from which the ring 22 is formed. A 30 nanometer-thick layer 24 made, for example, of BN (or other barrier material such as SiO₂) serves as an effective such barrier. Additionally, the layer 24 is effective to passivate the top surface of the layer 26 thereby to prevent the growth of crystallites thereon. Illustratively, the pattern elements 32 formed on the bottom surface of the BN layer 28 are made of a suitable X-ray-opaque material such as gold. Advantageously, these elements may be covered by the layer 30 which is made of an X-ray-transparent material such as beryllium or a standard monomer or polymer plastic material. The layer 30 protects the elements 32 from damage during handling but may be removed during actual use of the mask in an X-ray lithographic system. The layer 26 made of B₃N is typically not sufficiently optically transparent after annealing (as specified below) to permit optical alignment between corresponding marks (not shown) formed on the mask and workpiece. To facilitate such alignment by means of an optical microscope (not shown) positioned above the top of the depicted mask, it is advantageous to form plural so-called viewing ports in the layer 26. Two such illustrative ports 34 and 36, each about two millimeters in diameter, are shown in the drawing. As will be evident from the mask fabrication sequence described below, these ports also extend through the BN layer 24. A specific illustrative fabrication sequence for making the mask 14 will now be described. In the initial step, the layer 24 is formed on one main planar surface of a silicon wafer that is, for example, about ten cm in diameter and approximately 500 µm thick. (The ring 22 shown in the drawing will subsequently be made by etching this silicon wafer.) Illustratively, the layer 24 is formed in a low-pressure chemical-vapor-deposition (LPCVD) process utilizing diborane (B₂H₆) and ammonia (NH₃) as constituent gases. Satisfactory deposition occurs in the temperature range of about 450-to-500 degrees Celsius (°C) and in the pressure range of approximately 30 to 130 Pascals. In one particular process sequence, 475°C and 30 Pascals were selected, together with a flow rate for NH₃ of about 3000 standard cubic centimeters per minute (sccm) and a flow rate for B₂H₆ (actually 15 percent B₂H₆ in nitrogen) of approximately 40 sccm. As a result, the 30 nm-thick hydrogen-containing BN layer 24 was deposited on the surface of the silicon wafer at a rate of about 0.5 to 1.0 nm per minute. Alternatively, the layer 24 can be formed by utilizing a borane triethylamine complex (C₂H₅)₃ N·BH₃ and NH₃, or a borane diethylamine complete (C₂H₅)₂ NH·BH₃N and NH₃, as constituents. Illustrative LPCVD process parameters in this case include: 375-to-850°C (e.g., 800°C), 30 to 130 Pascals (e.g., 65 Pa), a flow rate for NH₃ of about 250 sccm and a flow rate of approximately 500 sccm for nitrogen bubbled through either of the aforespecified complexes, with the bubbler heated to about 120°C. This process yields a deposition rate of about 1.0 to 2.0 nm per minute. For such depositions carried out at about 800°C, the resulting layer of BN is typically hydrogen-free. Subsequently, the layer 26 is deposited on the surface of the layer 24 in an LPCVD process. Illustratively, this is done at about 375°C and at about 30 Pascals, with a B₂H₆ flow rate of about 50 sccm and an NH₃ flow rate of approximately 50 sccm. A 3-thick layer of hydrogen-containing B₃N can be thereby deposited at a rate of about 1.0 to 2.0 nm per minute. Significantly, the above-specified procedure produces in practice a hillock-free layer 26 made of hydrogen-containing B₃N. Such a hillock-free layer 26 obviates the need to add thereto a planarizing layer made, for example, of polyimide which itself might introduce distortions during X-ray exposure. The hydrogen-containing B₃N layer 26 can also be deposited by utilizing the aforementioned borane triethylamine complex and NH₃, or the borane diethylamine complex and NH₃. Illustratively, this is done at about 800°C and 65 Pascals by stopping the flow of NH₃ or adjusting it to produce the desired stoichiometry. Next, the layer 28 is deposited on the surface of the layer 26 by following either of the process sequences set forth above for making the layer 24. A 500 nm-thick layer 28 is thereby formed. Advantageously, the layers 24, 26 and 28 are each formed by utilizing the aforementioned borane complex and NH₃ constituents. In practice, the use of such a common chemistry simplifies the deposition of these layers. At that point, the silicon wafer with the three aforedescribed layers 24, 26 and 28 thereon is annealed at a relatively high temperature to remove all hydrogen therefrom. By way of example, this is carried out at a temperature greater than 1075°C (e.g., 1100°C) for about one hour in a vacuum or in an inert ambient such as argon or nitrogen. High-temperature annealing of the aforementioned layers to remove hydrogen therefrom can also be achieved in other ways. Illustratively, rapid-thermal-annealing (RTA), laser annealing or charged-particle-beam annealing techniques can be utilized to remove hydrogen from the layer 24, 26 and 28. A layer of X-ray-opaque material is then deposited on the surface of the layer 28. Next, by utilizing standard etching techniques, the interior portion of the aforedescribed silicon wafer is totally removed, thereby leaving only the silicon ring 22 shown in the drawing. The glass ring 20 is then affixed to the silicon ring 22. The aforementioned etching of the silicon wafer to produce the ring 22 would be made more difficult if boron diffused from the B₃N layer 26 into the wafer during the aforespecified hydrogen-removal step at 1100°C. To prevent such high-temperature diffusion and thus to facilitate etching, the layer 24 has been interposed between the layer 26 and the wafer to serve as a diffusion barrier to boron, as mentioned earlier above. Subsequently, the viewing ports 34 and 36 are formed in the layers 24 and 26 by conventional dry or wet etching techniques. Then the X-ray-opaque material is patterned to form the elements 32; and lastly the X-ray transparent protective layer 30, such as beryllium, is deposited. As a result of the above-described high-temperature annealing step, the B₃N layer 26, which is initially transparent to light, darkens. This darkening is believed to be due to phase separation of the annealed B₃N structure into a boron-rich phase and a boron nitride phase. Due to its bandgap of approximately 1.5 eV, the boron-rich phase absorbs light and causes the noted darkening of the annealed layer 26. The aforementioned phase separation provides a basis for selectively etching the layer 26 with respect to overlying or underlying layers made of BN. Illustratively, this etching is carried out by utilizing hot nitric acid. Such selective wet etching can be used in conjunction with standard lithographic techniques to form, for example, the above-specified viewing ports 34 and 36. In accordance with the principles of applicant's invention, techniques other than the particular illustrative one described above and depicted in the drawing are feasible for adjusting the inherently compressive stress of the BN layer 28 to be in a condition of tension within a suitable range. Thus, for example, a stress-inducing constituent such as silicon can be selectively added to the BN layer 28 to adjust its stress condition to a specified value. In that case, the layer 28 rests directly on the silicon ring 22 and layers 24 and 26 shown in the drawing are not needed. Further, a membrane for X-ray lithography masks utilizing the BN layer 28 without the layers 24 and 26 can be adjusted in other ways to exhibit a suitable tensile stress. For example, the layer 28 can be mechanically stretched to impart tension thereto. Illustratively, this can be done by positioning a weighted ring within the cavity defined by the rings 20 and 22. Such a weighted ring in contact with peripheral portions of the top surface of the layer 28 is effective to establish the interior portion of the layer 28 in a condition of tension. Additionally, the stress of a membrane utilizing the BN layer 28 without the layers 24 and 26 can be adjusted in still other ways. For example, the supporting ring 22 can be made of a material such as quartz that exhibits a lower linear coefficient of expansion than does the material of the layer 28 adhered directly thereto. As a result of cooling during fabrication of such a structure, the desired tensile stress condition can thereby be imparted to the BN layer 28. Applicant's discovery that a hydrogen-free boron nitride membrane exhibits a highly stable stress condition under irradiation provides a basis for constructing yet other membranes suitable for X-ray lithography masks. By way of example, a hydrogen-free layer of B₃N with a thin diffusion barrier made, for example, of BN between the B₃N layer and the supporting silicon ring 22 can serve as such a membrane. (As mentioned earlier above, B₃N by itself exhibits a tensile stress of about 1 x 10⁹ dynes per square centimeter.) To facilitate optical alignment with such a membrane, viewing ports are advantageously included in the B₃N and BN layers and alignment marks are formed on the bottom surface of the B₃N layer to bridge the respective openings defined by these ports. These marked are then visible through the viewing ports and can be respectively aligned with corresponding marks formed on an associated workpiece. By utilizing laser or charged-particle-beam techniques, it is also feasible to anneal only localized protions of a tensile hydrogen-containing B₃N layer. In that way, only portions of that layer to be subjected to X-rays would be annealed to remove hydrogen therefrom. Only these localized portions would be thereby darkened. Unannealed portions of the layer would remain transparent to light and hence usable for optical alignment purposes without the need for ports therethrough. Finally, it is to be understood that the above-described structures and processing techniques are only illustrative of the principles of the present invention. In accordance with these principles, numerous modifications and alternatives may be devised by those skilled in the art. Thus, for example, applicant has also determined that other hydrogen-free boron-containing materials are suitable for forming stable membranes for use in masks for X-ray lithography. Hydrogen-free boron carbide is one such illustrative other material. Additionally, hydrogen-free boron is attractive for use as a membrane; substantially pure boron is highly transparent to X-rays, inherently tensile, and extremely strong (exhibiting a Young's modulus that is higher by a factor of ten than that for B₃N). Moreover, in a mask comprising a hydrogen-free boron membrane, it is advantageous to include a diffusion barrier layer on one major surface of the boron membrane and a passivating layer on the other major surface thereof. BN, for example, is a suitable material for forming both the barrier and the passivating layers. Also, in such a mask comprising a hydrogen-free boron membrane, it is advantageous to include viewing ports, of the type described earlier above, to facilitate optical alignment.

Claims: A mask structure for use in fabricating devices by X-ray lithography, the structure comprising a X-ray transparent membrane (28), and a X-ray opaque device pattern (32) formed on the membrane, CHARACTERIZED IN THAT the membrane (28) comprises a boron-containing layer in a state of tension, the boron-containing layer containing hydrogen in a concentration that is sufficiently low so as to suppress decrease in optical transparency, and to suppress stress relaxation, of the layer in response to the X-rays. The structure of claim 1 in which the boron-containing layer comprises boron carbide. The structure of claim 1 in which the boron-containing layer comprises BN. The structure of claim 3 CHARACTERIZED FURTHER BY means for maintaining the boron-containing layer in the state of tension, said means comprising a stress-inducing constituent contained in said layer. The structure of claim 4 in which the constituent comprises silicon. The structure of claim 3 CHARACTERIZED FURTHER BY means for maintaining the boron-containing layer in the state of tension, said means comprising a tensile layer (26) comprising B₃N located on the boron-containing layer. The structure of claim 6 CHARACTERIZED FURTHER IN THAT a barrier layer against the diffusion of boron is located on the tensile layer (26). The structure of claim 1 in which the boron-containing layer is B₃N. The structure of claim 8 FURTHER CHARACTERIZED IN THAT a barrier layer against the diffusion of boron is located on the boron-containing layer. The structure of claim 1, 2, 3, 4, 5, 6, 7, 8 or 9 in which the membrane (28) is transparent to X-ray shaving prescribed wavelengths and the device pattern (32) is opaque to X-rays having the prescribed wavelengths.

Inventors: LEVY, ROLAND ALBERT
Applicants: AMERICAN TELEPHONE AND TELEGRAPH COMPANY
Application year: 1988
Main classification: G03F1
IPC codes: G03F1, C30B29, H01L21