Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2002-04-30
2004-10-05
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S005000
Reexamination Certificate
active
06800407
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the semiconductor technology and semiconductor manufacturing fields. More specifically, the invention relates to a method for determining imaging errors of photomasks for the lithographic structuring of semiconductors, a method for optimizing the layout of a photomask for the lithographic structuring of semiconductors, and a method for determining the local exposure dose.
In the production of miniaturized electronic circuits on microchips, the structuring of the semiconductor materials, for example silicon wafers, is carried out at present mainly by optical lithography methods. First, a thin layer of a photoresist is applied to the semiconductor. This layer is then exposed to laser light, a photomask which corresponds to a magnified image of the electronic circuit being arranged in the beam path. During the exposure, a miniaturized image of the photomask is produced in the photoresist layer. Depending on the photoresist used, the exposed parts in the case of a positive photoresist or the unexposed parts in the case of a negative photoresist can then be removed in further steps. The photoresist remaining on the semiconductor forms a mask corresponding to the electronic circuit so that, for example, the semiconductor can be etched or doped selectively in the bare parts or further layers can be deposited selectively on the bare surfaces of the semiconductor. In the course of the constantly increasing miniaturization of the semiconductor elements of electronic circuits, the quality of the imaging of the photomask on the photoresist is having to meet increasingly high requirements. In order to be able to produce even very small structures in the range of less than 1 &mgr;m without defects, the properties of all components of the imaging means, i.e. of the exposure apparatus, of the photomask and of the photoresist, are of decisive importance. Owing to their high image contrast, the halftone phase masks used in the production of integrated semiconductor elements permit the production of virtually perpendicular sidewalls in the structured photoresist, even in the case of very small dimensions of the structures, but they have the undesired effect of sidelobe printing. This means that, in addition to the maximum of the incident exposure dose, secondary maxima occur in parts of the photoresist outside the reproduced structure of the circuit and lead there to undesired structuring of the photoresist. This can in certain circumstances cause a defect in the integrated circuit. In the case of so-called alternating phase masks, such as chromium masks, phase conflicts play a dominant role. These effects, too, can lead to deviations from the required structure size of the elements on the semiconductor module and cause shorts or openings in the case of critical mask structures and hence lead to reduced yields during production of microchips.
Both during the development of the mask layout and later during the production of the semiconductor modules, therefore, all elements of the imaging equipment must continually be tested in order to prevent yield losses. The imaging equipment should as far as possible be tested under circumstances close to those in production, in other words using instruments and under conditions such as are also used in production. Only in this way is it possible unambiguously to analyze the interplay between photomask and optical system, including the optical imaging errors, and to optimize the layout and/or photomask production on the basis of the results.
In the case of halftone phase masks, appropriate design of the mask and a variation in the exposure equipment make it possible to alter the weighting between principal maxima and secondary maxima, so enabling side lobes to be minimized. In the design of the mask layout, however, it must be borne in mind that, for example, the side lobes of adjacent exposure maxima are imaged on the photoresist in such a way that they are not additive. The exposure dose, then stronger, could otherwise bring about structuring of the photoresist at the site of the side lobes at the development stage.
To date, qualitative and quantitative characterization of photomasks has been possible only on the basis of simulations and special areal image measurement techniques, such as AIMS, for example, offline, i.e. outside the production line.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method of verifying imaging errors in photomasks for the lithographic structuring of semiconductors, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which can be carried out rapidly and simply under conditions close to those in production.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining imaging errors of photomasks for lithographic structuring of semiconductors, the method which comprises:
(a) placing a photomask to be tested in an object plane of an optical exposure device, the photomask defining a mask image to be reproduced;
(b) placing a photoactivatable layer in the image plane of the optical exposure device, the photoactivatable layer containing a photoactivatable component and a compound permitting linkage of an amplification agent;
(c) exposing the photoactivatable layer, the mask image to be reproduced being reproduced in the photoactivatable layer and a chemical or physical change to the photoactivatable layer being effected in the photoactivatable layer in dependence on an incident light dose, for producing a latent image of the mask image;
(d) applying an amplification agent to the exposed photoactivatable layer to react the amplification agent with the compound in the photoreactive layer, a reaction between the compound and the amplification agent being dependent on a local exposure dose incident on the photoactivatable layer, to increase a layer thickness of the photoactivatable layer in dependence on the incident light dose;
(e) removing excess amplification agent;
(f) determining a local distribution of the increase in layer thickness of the photoactivatable layer;
(g) comparing the distribution of the increase in layer thickness with the mask image to be reproduced and determining local increases in layer thickness outside the mask image to be reproduced; and
(h) assigning the local increases in layer thickness outside the mask image to be reproduced to imaging errors of the photomask.
There is also provided, in accordance with the invention, a method for optimizing the layout of a photomask for lithographically structuring semiconductors, which comprises:
determining imaging errors in a first mask with the method outlined in the foregoing; and
taking into account the imaging errors in the first mask, producing a second photomask that is similar to the first photomask, and thereby eliminating at least some of the imaging errors found in the determining step.
With the above and other objects in view there is also provided, in accordance with the invention, a method for determining a local exposure dose, which comprises:
producing at least one photoactivatable layer comprising a photoactivatable material on a substrate;
producing a latent image in the photoactivatable layer by exposure to exposure radiation;
subsequent to the exposure, treating the photoactivatable layer with an amplification agent that reacts, in dependence on a locally incident exposure dose, with components of the photoactivatable material and thereby forms at least one chemical bond; and
subsequent to the treatment with the amplification agent, determining a local increase in the layer thickness of the photoactivatable layer and assigning the local increase in layer thickness to the local exposure dose.
The method for determining imaging errors of photomasks is based on an effect as described in U.S. Pat. Nos. 5,234,794 and 5,234,793 and in corresponding European patent EP 0 395 917 B1. In that method, after development of the r
Czech Günther
Richter Ernst-Christian
Scheler Ulrich
Sebald Michael
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
Young Christopher G.
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