Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2000-09-20
2002-04-02
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S053000
Reexamination Certificate
active
06366339
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the high resolution imaging of an etch resistant layer, commonly referred to as a “resist”. A resist may be used to fabricate patterns by etching or deposition. In particular, the invention relates to apparatus useful for manufacturing products such as integrated circuits and flat panel displays. In preferred embodiments, the apparatus uses a series of masks in an optical stepper.
BACKGROUND OF THE INVENTION
Today, most integrated circuits are fabricated using an imaging device known as a “stepper”. A stepper is an imaging device used in the semiconductor industry to project an image of a mask onto a semiconductor wafer. The stepper images a pattern carried by a mask onto the image-receiving surface, allowing subsequent selective etching or deposition. The process of fabricating high resolution patterns on planar objects by selective etching and deposition is well known in the art.
In general, the image receiving surface is covered by a protective layer known as a “resist”. The desired image, carried by a mask, is recreated on the resist, via photoexposure. The exposed parts of the resist (or possibly the unexposed parts for negative working resists) are removed, using a liquid or plasma developer. The imagewise removal of the resist uncovers the underlying material, which can now be etched through the openings in the resist layer while the resist protects the material still covered. Etching may be done, for example with wet chemicals or dry plasma (a process widely adopted in the semiconductor industry). In some cases, resists are also used in additive processes or “depositions”, wherein a substance is deposited through the openings in the resist, adding to the underlying material. Techniques for deposition include wet processes (used in the production of printed circuit boards) or dry processes, such as vacuum deposition by evaporation or sputtering. Another technique for exploiting an imaged resist involves chemical reactions, such as oxidation, which occur only in bare areas not covered by resist. After the etching, deposition, or oxidation, the remaining resist is normally removed.
Generally, resists are imagewise masks, which selectively control a chemical or physical process, forcing the process to occur in accordance with the imaged pattern. In some processes the imagewise patterned layer is not a resist at all but a functional layer which is not removed. Such functional layers can result from selective direct imaging of insulators and conductors. While these are not “masks”, they will be referred to using the generic term “resist”. The term “resist” should be interpreted to mean any patterned layer imaged by radiation, throughout the disclosure and the claims of this application.
Historically, most resists were photoresists. Photoresists are activated and imaged by the photonic action of light. For this reason, most imaging systems operate in the UV part of the spectrum, where the photon energy is high. Some resists are exposed by other types of radiation, such as electron beams. All photoresists and electron beam resists share one fundamental property; they respond to the total exposure, not the momentary illumination. In optics, total exposure is defined as the linear integral of illumination over time. For example, a photoresist can be exposed by illumination of 100 mW/cm
2
for a time of 1 second to yield an exposure of 100 mJ/cm
2
, or equivalently, can be exposed by an illumination of 1000 mW/cm
2
for 0.1 seconds to yield the same total exposure. This phenomenon is known as the “reciprocity law” and governs the exposure of photo resists and electron beam resists.
With respect to the accumulation of total exposure, photo and electron beam resists behave according to a linear function of power and time. Accordingly, the principle of linear superposition also applies to photo and electron beam resists. This principle applies in cases where f(a+b)=f(a)+f(b). In other words, the exposure response of the system to an illumination made up of multiple parts is equal to the sum of the exposure responses of the system to each part, as if it was applied separately. In making this statement, secondary effects such as coherence of the light source are ignored. Clearly multiple exposures will have an effect on the coherence but this effect is secondary compared to linear superposition.
When a certain threshold total exposure is reached, a chemical change occurs in the resist. Commonly, the resist undergoes a chemical change which results in a change in the solubility of the resist in a particular developing solution.
Because of the superposition principle and the reciprocity law, photo and electron beam resists require that they be exposed with apparatus which provides high contrast ratios and low stray light. For example , an imaging system may have light leakage or stray light of 1% (i.e. when the illumination is “off”, the light level does not drop to zero, but only to 1% of that of the “on” state). If the photoresist is exposed to the stray light for a long period of time, the effect of this stray light can be as large (or larger) than that of the main exposure. An even larger problem is caused when trying to image high resolution features; the point spread function of the optical system causes a “spreading” of light from each feature which cause overlap with adjacent features, effectively reducing the resolution. The reduction in resolution, is a particular problem in the semiconductor industry, where steppers are used to image increasingly high resolution features onto semiconductor wafers. For further details on microlithography and the operation of steppers, any modern book on the subject can be consulted, such as: “Handbook of Microlithography, Micromachining, and Microfabrication”, Volume 1 and 2, edited by P. Rai-Choudhury, SPIE Press 1997, ISBN 0-8194-2378-5 (V.1) and 08-8194-2379-3 (V.2).
Recently, a different type of resist, known as a thermoresist, has been developed for use in the manufacturing of printing plates and printed circuit boards. A thermoresist (also known as a thermal resist or heat-mode resist) changes solubility when a certain temperature, rather than a certain accumulated exposure, has been reached. Such thermoresists are typically imaged using near infra-red light and have, therefore, also become known as “IR resists”. Thermoresists respond to temperature and do not obey the reciprocity law or the principle of superposition. In contrast to a photoresist, which will be exposed by prolonged exposure to ambient light, a thermoresist does not follow the reciprocity law and, is not exposed by prolonged exposure to ambient temperature. While a photoresist can be shielded from ambient light, it is both inconvenient and expensive to do so.
Advantageously, prolonged exposure of a thermoresist to ambient temperature below the threshold temperature has little effect. Obviously, the threshold temperature needs to be well above the temperatures encountered in shipping and storage. When the chemical reaction in a thermoresist does not have a sharp threshold temperature, the chemical composition has to be formulated to keep the reaction rate very low at room temperature. Such a chemical composition is not difficult to create, as a large varieties of chemical reaction proceed at rates which double for approximately every 10 degrees centigrade increase. Thus, the reaction rate in a thermoresist exposed at 350 degrees centigrade may be one billion times faster than the reaction rate at 25 degrees centigrade. Laser sources make it fairly easy to raise the temperature of the thermoresist to over 1000 degrees centigrade. Such a thermoresist will appear to have a distinct threshold temperature simply because the reaction rate increases exponentially as the temperature is raised. In contrast, a typical thermal material which obeyed the reciprocity law would have a reaction rate which varies linearly with temperature. However, the chemical compositions typically used in thermoresists do not possess suc
Adams Russell
Creo Srl
Nguyen Hung Henry
Oyen Wiggs Green & Mutala
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