1. Field of the Invention
This invention relates to semiconductor processing equipment, and more specifically, to reticle pods and cassettes used in photolithography systems.
2. Description of the Relevant Art
Lithography is an integral part of integrated circuit fabrication. Optical lithography may be used to pattern various structures, such as circuit interconnect lines and trench isolation structures. To form these structures, a photosensitive material, referred to as a photoresist, is applied to a wafer of semiconductor substrate. Light is then projected onto the photoresist through a mask, which defines the pattern to be created on the photoresist. This is repeated for each die on the wafer, although the mask structure may include multiple instances of the same pattern, known as a mask reticle or xe2x80x9creticlexe2x80x9d, allowing the pattern to be projected to multiple die. Those areas of the photoresist that are exposed to the light source are rendered either soluble or insoluble to a particular solvent. Following the exposure of the photoresist to the light source, the solvent (often referred to as a developer) is used to remove soluble portions of the photoresist. The remaining areas of the photoresist may then be used to protect the covered portions of the semiconductor substrate, while etching those areas which are exposed.
FIG. 1 is a plan view of an exemplary reticle, with a pattern similar to one which may be applied to a semiconductor substrate. In this example, the chrome/opaque areas (within the lines) shield the substrate from the light source, while the quartz/clear areas allow light to pass to the photoresist on the semiconductor substrate. A typical reticle is configured to pattern only one level of the circuit. Thus, a different reticle is used to form the pattern for each level of the circuit. In some cases, a reticle may contain multiple instances of the same pattern, allowing the pattern to be projected onto multiple die simultaneously.
A type of lithography system that employs multiple reticles is referred to as a step-and-repeat system, or stepper system. FIG. 2 is an elevational view of a basic stepper system. Stepper system 10 includes a light source 23, one or more lenses 24 for focusing the light, several reticles 25, and a cassette 22 for storing the reticles. The reticle library cassette is housed within a SMIF (standard mechanical interface) pod 20, which includes a base 21. In the example shown, a reticle 25A may be drawn from cassette 22 and placed between two lenses 24. Light from a light source 23 may then be selectively projected through reticle 25A and onto a portion of wafer 30. After the pattern has been projected onto the entire wafer 30 by stepping and repeating this exposure process, reticle 25A is placed back into the cassette. Wafer 30 then undergoes processing to form the next level patterned by another reticle. After the each level is formed, another layer of photoresist is applied in order to form the next layer of the circuit. Wafer 30 is then placed back into the stepper system 10, and a different pattern is projected onto it using another reticle drawn from cassette 22. The reticle library cassette 22 is enclosed within SMIF pod 20 in order to prevent particulate contamination ingress upon the reticles.
Despite the ability of the SMIF pod to prevent particulate contamination of the reticles, other defects may occur to individual reticles. Many reticles are constructed of materials that are not electrically conductive, such as quartz or glass, and are therefore susceptible to static charge. Sufficient charge accumulation may lead to electro-static discharge (ESD). If ESD occurs on a reticle, its pattern may be damaged. ESD damage, if left unchecked, may result in erroneous patterns being projected onto the photoresist, thereby causing the manufacture of defective integrated circuits, which usually must be scrapped. Furthermore, most reticles, once damaged, must be replaced. This expense is not insignificant, as some reticles may cost on the order of $10,000 to $15,000 to replace. Furthermore, many SMIF pods and reticle library cassettes are constructed of materials that are not electrically conductive, making them susceptible to static charge buildup. ESD problems can be even further compounded by the geometry of a given SMIF pod and reticle library cassette. In particular, sharp edges and corners may increase ESD hazards, as static charge tends to gravitate to and/or accumulate in the sharply defined edges and corners of non-conductive materials.
Given the high cost of reticles, as well as the potential for the manufacture of defective integrated circuits caused by reticle rendered defective by ESD, not to mention the lost production time replacing damaged reticles, it would be desirable to have a lithography system that effectively prevents ESD problems within a reticle storage vessel (e.g., SMIF pod). Such a desirous system should be one that would retain those features which prevent particulate contamination to the wafer and/or reticles. In so doing, the improved reticle storage vessel or system may implement certain novel ESD protection features.
The problems outlined above may in large part be solved by a lithography system designed for ESD prevention. In one embodiment, the lithography system includes a light source, one or more lenses, a number of reticles having one or more masks (patterns), and a reticle library cassette. The reticle library cassette containing the reticles is enclosed within a reticle carrier such as a SMIF (Standard Mechanical Interface) pod in order to prevent particulate contamination. A spring-loaded reticle retainer is attached to the inside walls of the reticle carrier to keep the reticles securely in place within the reticle library cassette. The system is also designed to prevent ESD damage to the reticles and wafer. In particular, the SMIF pod and reticle library cassettes are designed with ESD prevention in mind.
In various embodiments, a SMIF pod (as well as the reticle retainer attached to the inside wall) is constructed of an electrically conductive material. One such material is a polycarbonate plastic, which is then impregnated with carbon fibers. A SMIF pod constructed of polycarbonate plastic, impregnated with carbon fibers, may have a maximum electrical resistance on the order of 104 ohms when measuring between the two most distant points on the pod. By impregnating the polycarbonate plastic with carbon fibers, conductivity of the material may be enhanced. However, the material selected may have enough resistivity to cause any static charges to drain slowly to a ground or reference point. Furthermore, to enhance the effectiveness for preventing static charge accumulation in areas susceptible to charge buildup (e.g. corners or edges of the SMIF pod), are rounded. As discussed above, static charges tend to accumulate near sharp edges and corners of objects. By eliminating sharp edges and corners, in conjunction with the use of a conductive material, static charge buildup may be minimized or even eliminated. The SMIF pod of such a design may effectively form an enclosed electrostatic shield, commonly known as a Faraday cage.
Various embodiments of the reticle library cassette may incorporate features similar to that of the reticle carrier. In particular, the ESD-safe reticle library cassette may be constructed of an electrically conductive material. For the cassette, these materials may include polycarbonate plastic (impregnated with carbon fibers), electrically conductive stainless steel, or other conductive material. As with the SMIF pod, the reticle library cassette may be designed to have a maximum measured resistance of 104 ohms when measuring from the furthest two points of the cassette. In addition, as with the SMIF pod, the material used in fabrication of the reticle library cassette may have enough resistivity to prevent a rapid draining of static electrical charges to a ground or reference point. Incorporating the use of an electrically conductive material into the fabrication of the reticle library cassette may offer additional ESD protection for instances when the SMIF pod must be removed for changing reticles, repairs, or maintenance.
Thus, in various embodiments, the lithography system including a reticle carrier (e.g., SMIF pod) and reticle library cassette designed for ESD protection may prevent damage to reticles from static electricity. Constructing the reticle carrier from an electrically conductive material, as well as eliminating sharp corners and edges from the pod may allow static electricity charges near the pod to be drained to ground. Incorporating similar ESD protection features into a reticle library cassette may afford additional ESD protection when it is necessary to open or remove the carrier.