Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
2001-07-31
2003-11-11
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S022000, C356S399000
Reexamination Certificate
active
06645683
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention lies in the semiconductor technology and photolithography processing field. More specifically, the invention relates to a control system for photolithographic processes on semiconductor wafers coated with photoresist, and to a method of controlling photolithographic processes of this type by using a control system of this type. A control system according to this type includes a control loop with which, in an automatic process, specific process parameters are modified by measuring and evaluating the resist structures from already processed semiconductor wafers and used for the process management of the semiconductor wafers subsequently to be processed.
In the production of integrated microelectronic semiconductor circuits, as a rule, in at least one process step, a layer of a photoresist is applied to the surface of a semiconductor wafer and structured by means of photolithographic processes. The structuring is carried out by optical projection of a mask structure onto the resist layer and subsequent development of the resist layer. For the optical imaging, use is made of a wafer stepper or photostepper, as it is known, which uses a UV light source, a mask reticule, an optical imaging device and a support table for a semiconductor wafer. In the optical imaging method, at most a 5:1 reduction in the size of the mask onto the resist layer is performed. In X-ray lithography, on the other hand, an optical shadow of the mask located at a short distance above the resist layer is produced on the resist layer in the proximity method, as it is known.
As early as during the 1980s, the requirements on the accuracy of the exposure processes represented a problem, which could no longer be managed with tolerances on the systems. In order to optimize the exposure process, the photosteppers used as the exposure machines offer the possibility of changing the exposure intensity or dose, the focusing and the positioning of the semiconductor wafer on the support table continuously, manually or under computer control. In the case of a computer-controlled change, correction values (offset values) for the exposure intensity, the focusing or the xy positioning of the semiconductor wafer on the support table can be input on an input unit by an operator. When a new exposure process is started, this is carried out by the wafer stepper with the new correction values.
These setting possibilities then open up two variants with which, in spite of relatively high requirements, it is still possible to operate with the existing machines.
In the first variant, referred to as the precursor principle, first of all a wafer is removed from a batch (group of or predefined number of semiconductor wafers to be processed), provided with a photoresist layer, exposed in a photostepper, using a mask with defined structures with standard parameters, and then developed. The structured photoresist layer is then measured for positional errors (OV, overlay) and line width errors (CD, critical dimension) of the photoresist structures produced, by means of a scanning electron microscope or an optical microscope, and the measured deviations are documented. By using these measured results, the optimum setting values for the exposure intensity, the focusing and the xy-positioning of the semiconductor wafers of the entire batch are determined. The photoresist on the precursor wafer is removed and the entire batch is then exposed with the correction values (offset values) determined. This first variant offers the highest possible accuracy, but also has a critical disadvantage. The processing of an entire batch lasts about 45 to 60 minutes, since separate units for the individual process steps are available in the systems and therefore a number of semiconductor wafers can be processed simultaneously. However, the single operations of processing and measuring an individual semiconductor wafer last about 15 to 25 minutes plus measurement time, so that overall the throughput of the machines decreases by more than 30%.
For mass production, therefore, a second variant, namely the statistical monitoring of the measured values for determining optimum setting or correction values, has gained acceptance. At the start, the measured results are assessed once each day by an engineer and used to define the correction values in the form of exposure intensity and xy-positioning tables for the next 24 hours, the focusing mostly being kept to a constant value. For this type of evaluation, the “Western Electric Rules” had proved to be a reliable instrument. However, this evaluation must be carried out separately for each photostepper, each product and each exposure level. With the increasing number of exposure levels in various products, this optimization assumes dimensions which can only be managed by automating the evaluation.
The publication “A practical automatic feedback system for poly photo CD control” by J. H. Chen and C. Y. Wang in SPIE, Volume 2876, pages 225-31, describes a process control system for photolithographic processes in which, immediately before the start of the processing of a batch, from the measured line width errors and the exposure intensities used in the preceding five (or three) batches, and the desired line widths, a corrected exposure intensity for the batch to be processed is calculated by means of a known linear relationship between the line width (of a web or of a trench) and the exposure intensity used, and is set on the photostepper. Known control systems of this type may become unstable under certain condition if an incoming oscillation does not vanish after a short time but, instead, builds up.
This can occur, for example, if a batch is measured with a systematic measurement error, the measured results from this batch therefore wrongly differ significantly in a specific direction from those of another batch, and/or if a poor or incorrect mathematical computational model is used. If, for example, the average line width measured for a batch is significantly falsified, its use in the control system can lead to an incorrect value for the corrected exposure intensity, so that the error builds up and the oscillatory deviation is amplified.
From a statistical point of view, disturbances of this type could be averaged out by the calculation of the correction values being carried out from the measured data from a relatively large number of batches processed earlier. As a result, however, the reaction time of the control system increases in an unacceptable way, so that the real-time character of the control system is lost.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a control system for photolithographic processes which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides for the development of a generic control system for photolithographic processes in such a way that instabilities of the type described can largely be avoided.
With the above and other objects in view there is provided, in accordance with the invention, a method of controlling photolithographic processes, which comprises the following method steps:
a) photolithographically processing a batch of a predefined number of semiconductor wafers with predefined values for one of an exposure intensity and an xy-positioning;
b) measuring one of positional errors and line width errors on the processed semiconductor wafers;
c) calculating batch-related correction values for one of the exposure intensity and the xy position with reference to the processed batch based on parameters selected from the group consisting of the measured positional errors, line width errors, predefined desired values for the position, the line width, and predetermined algorithms for the calculation, and storing the batch-related correction values;
d) calculating optimized correction values for processing a next batch based on the correction values from previously processed batches lying within a predetermined value rang
Luhn Gerhard
Sarlette Daniel
Greenberg Laurence A.
Infineon - Technologies AG
Locher Ralph E.
Stemer Werner H.
Young Christopher G.
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