Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2000-09-08
2003-11-25
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S055000
Reexamination Certificate
active
06654096
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
This invention relates to a projection exposure apparatus and a device manufacturing method, for photolithographically transferring a fine pattern formed on an original onto a substrate such as a wafer, for example.
The fine pattern forming procedure for a semiconductor device or a liquid crystal display, for example, uses a projection transfer technology, called photolithography. The projection transfer is performed in the following manner. An original pattern formed on a quartz glass substrate, called a reticle or a mask, is illuminated. In response, through a projection optical system, a latent image pattern is photolithographically transferred onto a substrate such as a semiconductor wafer or a liquid crystal forming glass substrate, for example. The latent image pattern is then developed into a resist pattern. Thereafter, an etching process of a high processing selection ratio between the pattern and the base material surface underlying the resist is performed, whereby the substrate is microprocessed.
In the projection exposure process included in such a fine pattern forming procedure, particularly for production of a semiconductor device as represented by current MPU or DRAM wherein extraordinary minuteness and very high processing precision are required, a reduction projection exposure apparatus called a stepper is mainly used. The stepper is a step-and-repeat type exposure apparatus wherein equivalently divided exposure regions (exposure shots) on a wafer are sequentially moved into an exposure picture angle below a projection optical system by means of a wafer carrying stage, whereby pattern exposures are repeatedly performed.
There is a step-and-scan type exposure apparatus, called a scanner. In this type of exposure apparatus, a wafer and a reticle are scanned and exposed while being scanningly moved relative to a projection optical system having a rectangular illumination region. As compared with a stepper, it has a wider exposure picture angle, and the pattern uniformness is higher.
In any of the stepper and the scanner, in order to meet requirements, of miniaturization of a semiconductor, improvements of the resolving power of a projection optical system have been desired. Many attempts have therefore been made in the development and products.
Examples of conventional measures for improving the resolution of a projection optical system are enlarging the numerical aperture (NA) of a projection optical system while holding the wavelength fixed, and shortening the exposure wavelength such as from g-line to i-line or to the emission wavelength of a KrF or ArF excimer laser, for example. Also, there is a shape changing illumination method in which the shape of an illumination light source is changed to enhance oblique incidence illumination light, or a phase shift mask method wherein a phase difference is produced in transmission light between adjacent reticle patterns. These are attempts to extend the process limit in the optical exposure.
With the improvements in the resolving power, the semiconductor process requires a more strict control precision, while the process margin such as, for example, the depth of focus of a projection optical system and a total overlay tolerance, is being reduced. On the other hand, separately from the improvements of resolution, improvements of the overlay precision itself have been required. This is for the reason that improving the overlay precision leads to narrowing the layout margin which enables reduction of the device size. This leads to an increase of device yield rate per a unit substrate, and thus to a decrease of the cost.
In projection exposure apparatuses, in order to meet these requirements, improvements are being made in regard to an exposure focus system and an alignment system which is directly influential to the overlay precision. Now, a conventional example of an exposure focus system will be described and, after that, an alignment system influential to the overlay precision will be described.
A focus detecting unit in am exposure focus system generally comprises an off-axis type wherein a probe light is obliquely projected onto a surface to be detected and wherein the focus detection is made on the basis of the position where the reflected light is collected. Usually, the detecting unit is fixed at the peripheral portion of an image plane of the projection optical system. In a stepper, after an exposure shot is positioned within the exposure picture angle, a wafer stage is moved upwardly/downwardly and tilted (for focusing) on the basis of the tilt amount and the level of the wafer surface as measured by the detecting unit, whereby the imaging plans of the projection lens and the plane of the transfer region are brought into registration. The exposure is then performed. In a scanner, the measurement and the focusing process described above are performed simultaneously with the scan exposure.
The wafer alignment is definitely a dominant factor of the overlay precision. The alignment system comprises an alignment detecting unit for measuring alignment marks formed on a wafer, and aligning means for positioning each shot at the exposure position on the basis of the results of processing the measured values of the positions in accordance with a predetermined method. The former alignment detecting unit measures the position of an exposure shot on the basis of the positions of alignment marks formed adjacent to that exposure shot. As regards the detection method, there is a TTL method wherein the position measurement is made through a projection optical system, and an off-axis method wherein the measurement is made without intervention of a projection optical system. In both of these methods, for the detection, a focusing operation for bringing the wafer alignment mark into registration with the detection plane is necessary. The measurement of the detection plane level is made also by using the focus detecting unit of the projection optical system or, alternatively, the alignment system itself has a focus detecting device.
As regards the aligning means, there is a die-by-die method wherein, for every exposure shot, the exposure position is measured and the alignment operation is made. However, at present, an alignment method called a global alignment method is used in many cases. In this alignment method, position measurement is made to sample shots of an appropriate number designated beforehand, and all the shot positions are estimated by preparing a linear correction formula to the positions on the basis of the results of measurements. By the position correction formula based on the global alignment method, not only the wafer shift component but also the magnification, the orthogonality and the rotation of the wafer as a whole related to the shot layout can be corrected. Further, depending on the measurement point, the magnification and the rotation of the shot itself can be corrected. As described, the global alignment method has superior advantages such as higher throughput and precise alignment, for example. Additionally, because the alignment operation is made to the whole wafer surface region in accordance with the same correction formula, the state of alignment can be detected once measurements are made to a few points on the substrate. Thus, this method is superior also in respect to the easiness in use.
On the other hand, when the positional deviation between exposure shots has no linearity to the position, namely, when it has a non-linearity, the non-linear deviation swerving from the linear correction amount directly leads to an alignment error, which causes degradation of the overlay precision. Further, when a non-linear deviation is produced at a sample shot position, it causes an error in the linear correction formula to be determined on the basis of the sample shot position. Therefore, with respect to the improvement of the overlay precision, reducing the non-linear deviation between exposure shots is an important matter.
As described above, the overlay precision is
Fujita Itaru
Nogawa Hideki
Takabayashi Yukio
Tsukamoto Izumi
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Fuller Rodney
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