Radiation imagery chemistry: process – composition – or product th – Luminescent imaging
Reexamination Certificate
1999-05-26
2002-04-23
Baxter, Janet (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Luminescent imaging
C430S281100, C430S270100, C250S458100
Reexamination Certificate
active
06376149
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of lithography and more specifically relates to the use of chemically amplified photoresists in lithography.
BACKGROUND OF THE INVENTION
Semiconductors are widely used in integrated circuits for electronics applications, including information systems. Such integrated circuits typically employ transistors and multiple levels of device interconnects fabricated in silicon. Various device layers may be sequentially formed on a semiconductor wafer using a combination of microlithography and etch processes.
Microlithography is a commonly practiced process of creating a patterned mask on the surface of a semiconductor wafer so that subsequent patterned processes may be performed. Typically these subsequent patterned processes involve the addition or subtraction of a material by deposition, implant doping, or plasma etching. Frequently, the pattern is transferred from an exposure mask to the wafer using a photoresist layer and optical lithography exposure tools.
Many modern semiconductor fabrication processes involve the deposition of a photosensitive resist material upon a substrate such as a wafer that may have various material layers formed upon it. The resist material is then exposed to radiation of a particular frequency. The radiation interacts with the resist material and produces a pattern within the resist, termed a “latent image.”
There is a desire in the industry for higher circuit density in microelectronic devices that are made using lithographic techniques. One method of increasing the number of components per chip is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. The use of shorter wavelength radiation (e.g., “deep” or “extreme” ultraviolet (UV), in the range of from about 190 to about 315 nm) offers the potential for higher resolution. However, with deep UV radiation, higher exposure doses may be required to achieve the desired photochemical response.
As the exposure wavelength of modern microlithographic tools continues to decrease, chemically amplified photoresists are becoming increasingly important. Several acid catalyzed chemically amplified resist compositions have been developed. Chemically amplified resist compositions generally comprise a photosensitive acid (“photoacid”) generator (PAG) and an acid sensitive polymer. Upon exposure to radiation (e.g., x-ray radiation, ultraviolet radiation), the photoacid generator, by producing a proton, creates a photogenerated catalyst (usually a strong acid) during the exposure to radiation. During a post-exposure bake (PEB), the acid may act as a catalyst for further reactions. For example, the acid generated may facilitate the cross-linking in the photoresist. The generation of acid from the PAG does not necessarily require heat. However, many known chemically amplified resists require a post-exposure bake (PEB) of one to two minutes in length to complete the reaction between the acid moiety and the acid labile component. During this time, acid diffusion in the film can cause an undesirable effect if acid moieties migrate into unexposed regions.
Issues such as acid diffusion during bake steps can affect critical dimension and linewidth variation in a semiconductor. Accordingly, knowledge of the location, amount, and extent of diffusion of the photogenerated acid in the photoresist is crucial for understanding resist behavior. Since it can be extremely difficult to make optical contact with the acids directly, their location is generally inferred from scanning electron microscopy (SEM) images of developed patterns. These exposed images, however, are convolved with subsequent processes such as resist chemistry, baking, and chemical development. For this reason, it is desirable to have a method of detecting latent images in exposed photoresists which allows direct determination of acid location (i.e., without requiring additional baking or developing processes).
Previous studies of latent images have been undertaken with a variety of methods, including atomic force microscopy (see Ocola et al.,
Appl. Phys. Lett.
68, 717 (1996)), and photon tunneling microscopy (see Marchman and Novembre,
Appl. Phys. Lett.
66, 3269 (1995); Liddle et al.,
J Vac. Sci. Technol.
B 15, 2162 (1997)). These techniques rely on contrast mechanisms resulting from topographic and/or refractive index variations in exposed resist. It would be desirable, however, to have an imaging technique that is sensitive directly to the presence of the photogenerated acid molecules.
There is also a need to be able to evaluate whether or not a photoacid generator present in a chemically amplified photoresist is efficient (i.e., produces sufficient acid to catalyze the desired reaction). Similarly, there is a desire to be able to compare the efficiency of one photoacid generator to another. It is also desirable to be able to quantify the number of acid molecules generated by the photoacid generator upon exposure to radiation. Presently, these kinds of determination may be made by spectrophotometric titration (i.e., comparative onwafer absorbance measurements). However, this kind of measurement is generally very time consuming, and cannot be made with certain incident radiation (e.g., at 193 nm). A method that may be used with a broad range of radiation wavelengths, and which significantly reduces the amount of time required to make these determinations, is desirable.
SUMMARY OF THE INVENTION
It is an object of the invention to provide methods for imaging acids in a chemically amplified photoresist composition prior to post-exposure bake.
It is also an object of the invention to provide methods for imaging an acid in a chemically amplified photoresist by directly determining the location of the acid within the resist composition.
It is additionally an object of the present invention to provide methods for comparing the efficiency of photoacid generators.
It is yet another object of the invention to provide chemically amplified photoresist compositions that may be used to image the location of the acid generated in the photoresist composition during exposure to radiation.
It is yet another aspect of the invention to provide methods for measuring acid diffusion within a photoresist composition.
Accordingly, the present invention provides a novel method of imaging acid in a chemically amplified photoresist, by exposing to radiation a chemically amplified photoresist that generates acid when exposed to the radiation. The chemical amplified photoresist comprises at least one pH-dependent fluorophore that fluoresces in the presence of acid and when exposed to the radiation. The chemically amplified photoresist comprising a pH-dependent fluorophore may be made by adding an amount of pH-dependent fluorophore to a casting solution of a known chemically amplified photoresist composition. An image of the acid in the photoresist is then generated, preferably by fluorescent imaging. The chemically amplified photoresist may be applied to as substrate such as a silicon photoresist prior to exposure to radiation. The radiation may be ultraviolet (UV) radiation (including deep UV radiation), x-ray, or any other known means of radiation.. The pH-dependent fluorophore is preferably a rhodol derivative, although any pH-dependent fluorophore is useful in the practice of the present invention. The imaging of the acid in the chemically amplified photoresist provides a map of the location of acid generated in the photoresist.
The present invention also provides a method of making a chemically amplified photoresist, comprising admixing a polymeric resin, a photoacid generator, and a pH-dependent fluorophore. The polymeric resin may be, and preferably is, a novolak or novolak-based resin. The pH-dependent fluorophore is derivative of rhodol, and is more preferably one of the pH-dependent fluorophores Cl-NERF or DM-NERF.
The present invention further provides novel chemically amplified photoresist compositions that may be used for the detection and imaging of acids in the che
Bukofsky Scott Josef
Dentinger Paul Michael
Grober Robert David
Taylor James Welch
Baxter Janet
Clarke Yvette M.
Myers Bigel Sibley & Sajovec P.A.
Yale University
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