157 nm Photoresist Materials
157 Photoresist Materials
Materials designed for use in single-layer photoresists for optical lithography must meet several requirements. These include low optical density at the exposure wavelength and resistance to image transfer processes such as plasma etching. The continual decrease of critical feature sizes predicted by Moore's Law and detailed in the SIA roadmap necessitates the use of shorter and shorter exposure wavelengths. The most common wavelengths used in current semiconductor lithography are i-line (365 nm) and DUV (248 nm). As device geometeries shrink past 150 nm, the lithography for critical levels will be performed at 193 nm.
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These resists are based on the photochemical generation of acid, which, during a post-exposure bake step, causes an acid-catalyzed reaction that produces base-soluble groups and hence changes the solubility of the exposed areas in developer. Since the acid generated may diffuse into unexposed areas during post-exposure bake and cause contrast loss, monitoring and controlling photoacid generation and diffusion processes in chemically amplified resists has received much attention.
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Biochips are devices which can contain hundreds to millions of individual biosensors on an area the size of a microscope slide. They are becoming increasingly useful tools in the areas of genomics and proteomics research, as well as in medical diagnostics. We aim to develop new ways of fabricating biochips and biosensors using photolithography.
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Cationic Graft Polymerization
We propose to study a new top-surface microlithographic scheme that overcomes the need for a bulk property switch in the polymer film. The surface of a polymer film can be modified in a manner that mimics the desired exposure pattern regardless of the polymer film’s absorbance characteristics at a given wavelength. We can then introduce a gas-phase silicon-containing monomer that is susceptible to acid-catalyzed cationic polymerization. Under the appropriate conditions, this material polymerizes on the exposed, acidic surfaces.
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Everyday experience has led to universal familiarity with the three states of matter: solids, liquids, and gases. However, there are many organic materials that demonstrate more than a single transition in passing from a liquid to a solid, thereby neces sitating the description of one or more intermediate phases. The mechanical properties and intermolecular packing arrangements of these phases are in between those of a liquid and those of a crystal. They have partial ordering of molecules similar to so lids, while maintaining the ability to flow akin to liquids. Therefore, these materials are classified as liquid crystals and exhibit one or more liquid crystalline phases, or more properly, mesophases.
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The demand for smaller and more uniform features on photomasks is rapidly increasing. The complexity of these patterns is also increasing with the need for optical proximity correction and phase shifting structures. These complex mask features demand unprecedented accuracy in pattern placement and dimensional control. We have conducted research designed to model and optimize the process for laser pattern generation by improving resolution and process latitude for 365 nm optical pattern generators. Lithographic simulation and photomask manufacturing trials have demonstrated sub-0.30 micron spaces in I-line resists using the ALTA 3500.
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Step and Flash Lithography (SFIL)
Step and Flash Imprint Lithography is an alternative to photolithography for patterning resist with sub-micron features. Several imprint lithography techniques are being investigated as a low cost, high throughput alternative to conventional photolithography for high-resolution patterning. Imprint lithography faces several challenges: i) pressure greater than 10MPa are typically required to imprint relief, ii) temperatures must be greater than the Tg of the polymer film, iii) aspect ratios experimentally have been limited to ~3:1. SFIL solves each of these issues by utilizing a low-viscosity, UV-sensitive solution that exhibits high etch contrast to organic films in O2 reactive ion etching.
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The microelectronics industry owes much of its success to the miniaturization of the integrated circuit. The imaging properties of the photoresists used in the lithographic steps of IC production have enabled this progress to continue. The diazonaphthoquinone (DNQ)/novolac photoresist system claims the most widespread usage due to the 1000-fold change in solubility of DNQ/novolac upon exposure and the excellent etch resistance of novolac. Although the photoresist’s dissolution behavior in aqueous base greatly affects its lithographic performance, much of this process remains unexplained in molecular terms.
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Water Soluble Resist
Since the 1980's, when studies revealed the health effects of exposure to ethylene-based glycol ether solvents, the search for alternative solvent systems has been a consideration of growing significance for resists vendors and semiconductor manufacturers. Propylene-based glycol ethers, lactic acid esters, and a host of other safe solvents have nearly completely supplanted ethylene glycol ethers as photoresist solvents since 1992, when the industry underwent a voluntary phase-out of their use.
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Molecular simulation of chemical species and systems is a method that has enjoyed much popularity and success, partially as a result of the advances made in the microelectronics industry. Improvements in processor speed and memory have enabled scientists to routinely perform computations of significant complexity, such as ab initio quantum calculations[i], simulations of reactions in plasmas[ii], and the determining the dynamics of protein folding[iii]. While the ability to study dynamic processes on a molecular scale has advanced greatly, there remains a significant limit to the time and space domains that may be simulated. These simulations are typically limited to time scales from nanoseconds to microseconds, and often consist of only 10 to 1000 molecules.
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Mass Persistent Photoresists
Current chemically amplified photoresists being used today are designed to lose mass upon reaction. This loss of mass occurs during the deprotection of the polymer with acid to yield an aqueous base soluble functional group. The mass that is lost during the lithography process is problematic for several reasons. The most important problem is the products formed during the deprotection of the polymer are volatile and these gases can potentially ruin the optical properties of the lenses. While this is a concern at 248 nm and 193 nm lithography it has proven to be a significant problem at 157 nm lithography. To overcome this problem the design of a resist that will function without the loss of mass is being sought.
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Our research group has been using real time spectroscopic ellipsometry to study fundamentals of photoresist processing. The ellipsometer is a J.A.Woollam M-2000, with a maximum acquisition rate of 10 Hz. We have characterized film relaxation due to acid catalyzed deprotection during PEB, and both the thickness change and change in optical constants can be monitored. We have also been using the ellipsometer as a dissolution rate monitor, and it is useful for studying the dissolution rate of bulk and thin films. It is also useful for characterizing films that swell during dissolution. Please click the link for further information.
Also,refer to the Acid Diffusion and Dissolution sections for further information.
Top Surface Imaging
Traditional single layer photoresists require high transparency in order to obtain a solubility switch from the top of the resist to the bottom. As exposure wavelengths shrink in order to print smaller features, this transparency requirement is growing steadily more challenging. An alternative to the process of generating new single layer materials is to create a system in which the chemical change occurs only in the top of the film, and the change is transferred through the resist. This can be performed by incorporating silicon into the chemically changed regions, and then performing a reactive oxygen etch. This process, called top surface imaging, is being explored and developed.