Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2002-04-30
2004-02-24
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
Reexamination Certificate
active
06696208
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention lies in the semiconductor fabrication technology field and relates, more specifically, to a method for determining imaging errors in optical exposure units as used for the lithographic structuring of semiconductors, a method for optimizing optical exposure units 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 mask 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 imaging quality of the mask on the photoresist is having to meet increasingly high requirements. In order to be able to produce even very small structures in the region 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.
A further potential source of errors is the optical exposure unit used for reproducing the mask on the photoresist. Spherical lenses only approximately permit error-free reproduction of the structures of a microelectronic circuit which are defined by the mask on the photoresist. Imaging errors are caused by lens defects and aberrations. It is true that these can be substantially avoided by the use of lens systems and of aspherical lenses. However, in order to qualify such optical exposure systems, such as wafer steppers and step-and-scan exposure units, for semiconductor production, it is necessary first to test them for imaging errors. In this context, it is of particular interest to investigate the effect of imaging errors under conditions close to those in production, i.e. for example also with the use of the mask types used in production and of the corresponding mask layout.
At present, the analysis of imaging errors of optical apparatuses which are used for the production of microchips is possible only by means of special analytical apparatuses with which the lens is measured by means of interferometry, or by complicated theoretical estimations. At present, a type of pinhole camera and simulations are used for analyzing the light intensity distribution on the wafer surface or in the photoresist. In this method, however, the result of the measurements can be influenced by the photoresist. Furthermore, there is in this method no simple relationship between a simulation on the basis of the Zernike polynomials and the data from the experiment carried out in practice. Limitation to locally fixed points of the exposure field is also disadvantageous.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an experimental method for determining imaging errors of optical exposure units 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 that are close to those in production.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of determining imaging errors of optical exposure units for the lithographic structuring of semiconductors. The method comprises the following steps:
(a) placing a photomask in an object plane of the optical exposure unit to be tested, the photomask defining a test image to be reproduced;
(b) placing a photoactivatable layer in the image plane of the optical exposure unit to be tested, the photoactivatable layer containing a photoactivatable component and a compound permitting linkage of an amplification agent;
(c) exposing the photoactivatable layer, the test 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 test image is produced;
(d) applying an amplification agent to the exposed photoactivatable layer to react the amplification agent with the compound in the photoreactive layer, the 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 as a function of the incident light dose;
(e) removing excess amplification agent;
(f) determining a local distribution of an increase in layer thickness of the photoactivatable layer;
(g) comparing the distribution of the increase in layer thickness with the test image to be reproduced and determining the local increases in layer thickness outside the test image to be reproduced; and
(h) assigning the local increases in layer thickness outside the test image to be reproduced to imaging errors of the optical exposure unit to be tested.
There is also provided, in accordance with the invention, a method of optimizing an optical exposure unit for the lithographic structuring of semiconductors, which comprises:
performing the method as outlined above to create a layer thickness distribution in a photoactivatable layer with an optical exposure unit to be optimized and using a test image;
determining an experimental distribution of the exposure intensity from the layer thickness distribution;
comparing the experimental distribution of the exposure intensity with a theoretical distribution of the exposure intensity;
determining an imaging error of the optical exposure unit from a difference between the experimental and the theoretical distribution; and
modifying the optical exposure unit by alleviating or completely eliminating the imaging error.
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 a photoactivatable layer comprising a photo-activatable material on a substrate;
exposing the photoactivatable layer to exposure radiation to produce a latent image in the photoactivatable layer;
subsequently treating the photoactivatable layer with an amplification agent that reacts, in
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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