1. Field of the Invention
This present invention relates generally to the field of electronic circuitry and, more specifically, to techniques for fabricating integrated circuits implementing small wavelength photolithography technology.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Microprocessor-controlled circuits are used in a wide variety of applications throughout the world. Such applications include personal computers, control systems, telephone networks, and a host of other consumer products. A personal computer or control system is made up of various different components that handle different functions for the overall system. By combining these different components, various consumer products and systems are able to meet the specific needs of an end user. As is well known, microprocessors are essentially generic devices that perform specific functions under the control of software programs. These software programs are generally stored in one or more memory devices that are coupled to the microprocessor and/or other peripherals.
The memory devices generally include many different types of integrated circuits that are typically fabricated from one or more semiconductor materials. The integrated circuits work together to enable the memory device to carry out and control various functions within an electronic device. With the current trend in decreasing the “footprint” or overall size of the electronic devices, the different components of the electronic devices, such as the memory devices, may be reduced to accommodate these requirements. Accordingly, the integrated circuits that make up the memory device may be designed to consume less space. The reduction in size of these integrated circuits is a key component to the technological development of many devices containing electrical components. Accordingly, the fabrication processes that are used to form these integrated circuits have experienced dramatic changes.
Integrated circuits, such as memory devices, are typically fabricated on a wafer surface through any number of manufacturing processes, such as layering, doping, and patterning. Layering generally refers to adding material to the surface of the wafer by a growth process, such as oxidation, or through a deposition process, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD). Doping generally refers to the process of implanting dopants into the wafer surface or overlying layer and may be used to increase the current carrying capacity of a region of the wafer or overlying layer of material. The doping process may be implemented before a layer is formed, between layers, or even after the layer are formed. Generally, the doping process may be accomplished through an ion implantation process, using boron or other similar dopants, or through thermal diffusion, for example.
Patterning refers to a series of steps that result in the removal of selected portions of layers or underlying wafer material. After removal of the selected portions of the layer(s), via a wet or dry etch process, a pattern of the layer is left on the wafer surface. The removal of material allows the structure of the device to be formed by providing holes or windows between layers or by removing unwanted layers. Patterning sets the critical dimensions of the integrated circuit structures being fabricated. Disadvantageously, errors in the patterning and removal process may result in changes and failures in the electrical characteristics in the device.
One commonly used patterning technique is photolithography. In using photolithography, a pattern may be formed by using a photomask to expose certain regions of a radiation sensitive material, such as a photoresist or resist, to a certain wavelength of light. Typically, the radiation source provides UV light to pattern the resist. However, certain resists may also be implemented using other energy types, such as X-rays. Exposure to the radiation changes the structure of the resist. If the resist is a negative resist, then the resist become polymerized where it is exposed. If the resist is a positive resist, the exposed region of resist becomes divided or softened. After the exposure to the radiation, the unpolymerized regions may be dissolved by applying an appropriate solvent.
To fabricate an appropriately sized structure, a bottom anti-reflectant coating (BARC) layer may be implemented underneath the resist to enhance the photolithography process. The BARC layer is used to absorb the radiation generated by the source, thereby reducing development of the photoresist caused by reflections from underlying layers. By providing an underlying layer for absorbing the radiation, the patterned structure is typically more defined with fewer defects than the methods wherein a BARC layer is not included. Once the resist has been patterned, the resist layer may be removed to allow the underlying structure to be developed. While it may be desirable to retain the BARC layer, it is typically desirable to remove the BARC layer through an etching process.
After patterning of the photoresist layer, various etchants may be selected to implement the removal of selected portions of material from the surface of the structure. Selectivity relates to the preservation of the surface underlying the etched material layer. The selectivity is generally expressed as a ratio of the etch rate of the material layer to the etch rate of the underlying surface. Further, selectivity may be used to refer to the removal rate of the photoresist with respect to the underlying material layer. As can be appreciated, as the material layer is being etched through the openings patterned in the photoresist, some of the photoresist may also be removed. The selectivity should be high enough to ensure that the photoresist layer is not removed before the etched patterns in the material layer.
One of the objectives in photolithography is to transfer a well-defined pattern with minimal ambiguities or anomalies. One technique that may be used to develop the respective gates, devices, or desired structures is to implement a multi-layer resist scheme. The multi-layer resist scheme employs multiple layers of resist for each gate or structure. This process employs different photoresists and etchants to ensure that the gate or structure is formed correctly. However, with the multi-layer resist process, the resulting gate or structure is more expensive because the various layers of resist may increase the number of steps in the process, which increases the time required to fabricate the device. This increase in processing time has a further negative effect of decreasing the quantity of devices that may be produced over a period of time, such as a week or month. Furthermore, the additional fabrication process steps increase the likelihood of potential errors by complicating the method of making the device.
To meet the ever-increasing demand for smaller integrated circuit structures, smaller wavelength photolithography techniques have been developed. One such technique implements 193 nm technology. That is to say that the photoresist used to pattern the underlying materials is developed by exposure to a radiation source having a wavelength of 193 nm. While 193 nm technology allows the resist to be patterned with smaller structures, the resist does not generally retain a crisp pattern throughout the entire etch process. Instead, the 193 nm resist tends to wrinkle, shred, and bend during the etch process. To mitigate some of these effects, an etchant to which the 193 nm photoresist has a higher selectivity could be implemented. However, in the 193 nm process, increased resist selectivity produces large striations through the resist. Regardless of whether the defects include wrinkles, striations, or other anomalies, defects in the photoresist are undesirable, since the defects may be transferred to the underlying layers. Disadvantageously, defects in the underlying layers may result in failures of the structure or integrated circuit device being fabricated.