Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Radiation sensitive composition or product or process of making
Reexamination Certificate
2003-01-31
2004-12-28
Kelly, Cynthia H. (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Radiation sensitive composition or product or process of making
C430S905000, C430S907000, C430S910000, C430S909000, C430S325000, C430S326000, C430S914000
Reexamination Certificate
active
06835528
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a chemically amplified photoresist that has increased transparency at a wavelength of 157 nm.
Microchips are produced in a large number of operations in which modifications are made in a targeted manner within a small section of the surface of a substrate, generally a silicon wafer, in order, for example, to introduce trenches for deep-trench capacitors into the substrate or to deposit thin conductor tracks or electrodes on the substrate surface. To be able to produce such small structures, a mask is first produced on the substrate surface so that those parts that are to be processed are bare while the other parts are protected by the material of the mask. After the processing, the mask is removed from the substrate surface, for example by ashing. The mask is produced by first applying a thin layer of a photoresist which contains a film-forming polymer and a photosensitive compound. This film is then exposed, a mask which contains the information relating to the structure to be produced and through which a selective exposure of the photoresist film is effected being introduced, for example, into the beam path. Owing to diffraction effects, the smallest producible structure size is substantially determined by the wavelength of the radiation used for the exposure.
To be able to keep pace with increasing requirements with regard to the computational power of microprocessors and storage capacity of memory modules, there is a need to develop, in increasingly short periods, microchips having an increasingly high density of components and hence increasingly small structures. To be able to produce structure sizes in the range from 100 to 70 nm, below the currently obtainable dimensions, it is necessary to develop novel processes since the processes currently used industrially for the production of very fine structures and based on radiation having a wavelength of 193 nm are reaching the limits of the resolutions to be realized.
Extensive know-how has been acquired in the structuring of microchips using lithographic processes. To be able to continue using this knowledge, work has been carried out particularly intensively on a further development of the known lithographic processes for an exposure wavelength of 157 nm. The development of novel photoresists is required for this purpose, since the materials used to date for wavelengths of 248 nm and 193 nm are unsuitable for a wavelength of 157 nm.
The poor transparency of the materials used to date presents a particular difficulty. The transparency of the resist is influenced primarily by the polymer that is contained in the resist. The best polymers currently used in photoresists achieve an absorption coefficient of about &mgr;
10
=1/&mgr;m at a wavelength of 157 nm. At an exposure wavelength of 157 nm, these polymers thus still have an absorption that is about 50 times higher compared with polymers that are used for exposures with radiation having a wavelength of 193 or 248 nm. For this reason, it has been possible to date to realize only very thin resist layers having layer thicknesses of not more than 50 nm in the case of the 193 nm and 248 nm materials, giving rise to problems with the structuring of the material underneath. To be able to produce a mask structure without errors, complete chemical modification of the polymer must be initiated during the exposure, even in the deeper parts of the photoresist layer. For this purpose, a sufficiently high light intensity must be ensured even in deep layers of the resist. In spite of the thin layers, however, the absorption of these resists is so high that only about 40% of the original light intensity reaches the lowermost resist layer in a 50 nm thin layer. This increases the risk that the resist will exhibit resist feet after development. The use of such a small layer thickness gives rise to further problems. Thus, the defect density in the thin layers increases with decreasing layer thickness. In addition, the small volume of resist material makes the ultrathin resist layers susceptible to contamination. The structures produced with these ultrathin layers using an exposure wavelength of 157 nm therefore have very rough edges and a limited structure resolution. Therefore, a first requirement which a novel photoresist should meet is as high a polymer transparency as possible at a wavelength of 157 nm.
A further problem is sufficient stability of the photoresist for radiation having a wavelength of 157 nm. In the polymers contained in photoresists used to date, silicon-containing groups are present for increasing the etch stability to an oxygen plasma. As a result of the high energy of an exposure radiation having a wavelength of 157 nm, however, bonds may be broken in the polymer. This leads to expulsion of silicon-containing compounds in gaseous form, which form irreversible deposits on the optical systems of the exposure unit. A polymer suitable for a photoresist for exposure at 157 nm therefore must not include any silicon-containing side groups.
A photoresist suitable for the industrial production of microchips has to meet a large number of further requirements. For economic reasons, exposure times that are as short as possible desired in the transfer of the structure defined by a mask to the photoresist. To be able to achieve a comprehensive chemical modification of the photoresist even with low exposure intensities, most resists used at present operate with chemical amplification. Here, the exposure initiates a photochemical reaction that catalyzes a change in the chemical structure of the photoresist. In the case of a positive-working chemically amplified resist, for example, the exposure produces a strong acid which effects a catalytic conversion or cleavage of the resist in a subsequent heating step. As a result of this chemical reaction, the solubility of the polymer in a developer is dramatically changed so that a substantial differentiation between exposed and unexposed parts is possible. For this purpose, the polymer contained in the photoresist contains, for example, carboxylic acid tert-butyl ester groups, from which carboxyl groups can be liberated under acid catalysis. In chemically amplified resists, a large number of polar groups therefore can be liberated by an individual photon. In contrast to chemically unamplified photoresists, chemically amplified photoresists therefore have a quantum yield of more than 100%. A novel photoresist must therefore also fulfill the conditions of chemical amplification.
The structured photoresists serve as a rule as a mask for further processes, such as, for example, dry etching processes. There, the structure produced in the photoresist is transferred to a substrate disposed under the photoresist. For this purpose, it is necessary for the photoresist to have a higher stability to the plasma than the substrate, so that only the substrate is etched as selectively as possible. If it is intended to use the photoresist, for example, to structure an organic chemical medium underneath, such as, for example, in two-layer resists, high etch resistance of the structured, upper photoresist is required in comparison with this. The effect of an etch plasma can be divided roughly into a physical part and a chemical part. The physical part effects removal of material virtually independently of the material. The components of the plasma strike the substrate surface and knock out particles there. To achieve a differentiation between sections covered by the resist mask and uncovered sections, the resist mask must therefore have a certain layer thickness so that a sufficient layer thickness of the resist is still present on the covered sections at the end of the etching process in order to protect those sections of the substrate surface which are present underneath. The chemical part of the etching process is based on a different reactivity of the plasma with respect to different materials. Thus, organic materials are converted in an oxygen plasma into gaseous compounds s
Greenberg Laurence A.
Infineon - Technologies AG
Kelly Cynthia H.
Lee Sin J.
Locher Ralph E.
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