Radial lithographic source homogenizer

A method includes identifying a contamination region of a collector in a light source, positioning a subset of a plurality of movable light-blocking elements around a periphery of a circular aperture of the light source to compensate for the contamination region, and transmitting light from the light source through the circular aperture.

BACKGROUND

1. Field of the Invention

The present disclosure generally relates to lithography techniques for the fabrication of semiconductor devices, and, more particularly, to a radial lithographic source homogenizer.

2. DESCRIPTION OF THE RELATED ART

Integrated circuits are formed on the basis of a plurality of sequential process steps to form nanoscale regions of precisely controlled size in one or more material layers of an appropriate substrate. These regions of precisely controlled size are typically obtained by applying lithography, etch, implantation, and deposition techniques at various manufacturing stages in order to pattern a material layer or layers in accordance with the underlying device design. The patterning of material layers formed on the substrate material may be typically accomplished by forming a type of mask layer, which may consist of or may be formed by means of a layer of resist material that is patterned by a lithography process, typically a photolithography process. To this end, the resist may be spin-coated onto the substrate surface and may then be selectively exposed to radiation through a corresponding lithography mask, such as a reticle, in order to image the reticle pattern into the resist layer, thereby forming a latent image therein. After developing the resist material, other mask materials and, finally, the actual device patterns may be formed by further manufacturing processes, such as etching and the like. Since the dimensions of the patterns in sophisticated semiconductor devices are continuously reduced, the finally accomplished resolution of the optical patterning process may, therefore, depend significantly on the imaging capability of the equipment used, the photoresist materials for the specified exposure wavelength, and the target-critical dimensions of the device features to be formed in the device level under consideration.

Extreme ultraviolet lithography (EUV) employs a wavelength shorter than 124 nm, e.g., on the order of 13.5 nm, using a laser-driven tin (Sn) plasma light source. A collector optic focuses collected light from the Sn light source and provides the light to an illuminator. The collector is directly exposed to the plasma. Tin droplets deposit on the collector mirror causing contamination and degrading its performance over time. Collector contamination leads to poor pupil uniformity. One technique for correcting pupil non-uniformities employs a uniformity correction module (UNICOM) that is positioned on the output of the illuminator. The UNICOM module employs a rectangular slit with movable light absorbing elements that are positioned along edges of the slit to set the outer boundary of the slit. The UNICOM module distributes the illumination variation resulting from the collector contamination across the pupil. However, asymmetric contamination of the collector cannot be adequately corrected by the UNICOM module and performance degrades over time, leading to poor pupil uniformity and eventually asymmetry or dead pixels that affect the programmed pupil fill of the illuminator. Restoration of the tool performance requires a maintenance procedure to change the collector, resulting in tool unavailability.

SUMMARY

The present disclosure is directed to various methods and apparatus that may avoid, or at least reduce, the effects of one or more of the problems identified above.

Generally, the present disclosure is directed to a radial lithographic source homogenizer. An illustrative method includes, among other things, identifying a contamination region of a collector in a light source, positioning a subset of a plurality of movable light-blocking elements around a periphery of a circular aperture of the light source to compensate for the contamination region, and transmitting light from the light source through the circular aperture.

Another illustrative method includes, among other things, generating a tin plasma in a light source having a circular aperture. A contamination region of tin is identified on a collector in the light source. The collector is to direct light from the light source to the circular aperture. A subset of a plurality of movable light-blocking elements is positioned around a periphery of the circular aperture to compensate for the contamination region. Light from the light source is transmitted through the circular aperture.

An illustrative apparatus disclosed herein includes, among other things, a plasma source to generate light, a collector to direct the light to a circular aperture, a radial homogenizer including a plurality of movable light-blocking elements proximate the circular aperture, and a controller to identify a contamination region of the collector and position a subset of the plurality of movable light-blocking elements around the circular aperture to compensate for the contamination region.

DETAILED DESCRIPTION

The present disclosure generally relates to a radial lithographic source homogenizer. With reference to the attached figures, various illustrative embodiments of the methods and devices disclosed herein will now be described in more detail.

FIG. 1is a simplified block diagram of a lithography scanner100, in accordance with some embodiments disclosed herein. The primary functional blocks of the lithography scanner100include an extreme ultraviolet (EUV) source105, an illuminator110, a reticle stage115, a projection optics unit120, and a wafer stage125. The EUV source105includes a plasma source130(e.g., Sn plasma source) and a collector135. The plasma source130generates EUV light using a plasma formed from tin (Sn) droplets and a high power CO2laser, and the collector135focuses the collected EUV light for transmission to the illuminator110. The light output from the EUV source105is provided to a radial homogenizer140. As described in greater detail below, the radial homogenizer140attempts to account for degradation of the collector135arising from the collecting of tin droplets thereon.

Light from the EUV source105is conditioned by the illuminator110based on the desired illumination conditions (e.g., dipole, quadrapole, annular, etc.). A uniformity correction module (UNICOM)145employs a slit for projecting the light from the illuminator110to a reticle150mounted to the reticle stage115to attempt to provide uniform light intensity across the entire reticle field. The reticle150includes the lithographic pattern to be imaged multiple times onto a wafer155held on the wafer stage125using a scanning technique. The reticle stage115continually scans the illumination beam from the illuminator110across the field of the reticle150. The projection optics unit120focuses the light from the reticle150onto the wafer155.

In general, the uniformity of the light from the collector135is affected by tin contamination. The illuminator110attempts to randomize the light rays from the collector135using programmable field and pupil facet mirrors (not separately shown). Non-uniformity of the light from the collector135is thus randomly distributed across the field. However, as the degree of contamination increases, this randomization loses effectiveness, and dead pixels are evident in the field. Pupil uniformity correction at the UNICOM145cannot correct for single pixel fails, since the pupil light has already been homogenized. The radial homogenizer140allows source balancing to reduce asymmetry. Correction by the radial homogenizer140may be conducted gradually over the course of the degradation of the collector135as tin droplets build up.

FIG. 2is a simplified diagram of a far-field EUV beam profile200from the collector135. In general, the collector135is an ellipsoidal mirror with a center aperture that allows laser light to be directed onto tin droplets to generate a plasma for the EUV light source. The far-field EUV beam profile200represents a two-dimensional projection of the collector135. Over time, tin contaminates the collector135due to molten tin droplets, tin flakes, or tin films (e.g., by chemical vapor deposition (CVD)) that deposit thereon resulting in one or more contamination regions. An example contamination region causes a dead region205in the far-field EUV beam profile200. For ease of illustration, only a single dead region205is illustrated. In general, contamination that is relatively evenly distributed can be compensated for by the UNICOM145. However, an asymmetric contamination region generates a single pixel dead region205for which the UNICOM145is unable to correct. The far-field EUV beam profile200represents a contamination map of the collector135.

FIG. 3Ais a simplified diagram of the radial homogenizer140. The radial homogenizer140includes a plurality of light-blocking elements300arranged radially around the circumference of an aperture305of the EUV source105through which light from the collector135is transmitted to the illuminator110. In the illustrated embodiment, the light-blocking elements300have a generally triangular shape that allow them to be inserted into various positions in the field. However, other shapes may be employed. For ease of illustration, the positioning mechanics for the light-blocking elements300are not illustrated. In some embodiments, the light-blocking elements300may simply move along a radial axis310as may be implemented by a radial actuator. In other embodiments, the light-blocking elements300may move along the radial axis310and may also be rotated about an axis315proximate one end thereof.

In some embodiments, the EUV source105may include a camera160for generating an image of the collector135from which a contamination map may be derived. The radial homogenizer140includes a controller165. Based on the contamination map generated by the camera160, the controller165configures the radial homogenizer140as shown inFIG. 3Bto position a subset of the light-blocking elements300A in the aperture305to compensate for asymmetric contamination regions, such as a contamination region causing the dead region205inFIG. 2. In some embodiments, a contamination map specifying the contamination region may be provided to the controller165by an operator of the EUV scanner100.

FIG. 4is a diagram of a far-field EUV beam profile400balanced by the radial homogenizer140configured as inFIG. 3B. The light-blocking elements300in the subset300A cause a corresponding dead region405in the EUV beam profile400that balances the dead region205caused by tin contamination of the collector135. In general, the dead region405exhibits radial (e.g., symmetry about the central axis of the collector135) with respect to the original contamination region205. The dead zone405mimics contamination on the collector135that is radially symmetric to the original contamination region205such that the EUV beam profile400is balanced (i.e., not asymmetric). Hence, there is no longer a single pixel dead region, and the UNICOM145is able to compensate for the variations in the beam profile400.

The radial homogenizer140may be configured over time to adjust the positioning of the light-blocking elements300as contamination builds up on the collector135. For example, the contamination analysis and correction using the light-blocking elements300may be conducted monthly. In some embodiments, the contamination analysis and correction using the light-blocking elements300may be conducted after a tool event occurs that is expected to generate contamination, such as an extended processing run of substrates. After changing the positions of the light-blocking elements300, the randomizing field/pupil mirrors in the illuminator110and the UNICOM145may require re-calibration. Periodically compensating for the contamination build-up on the collector135extends the operating life (e.g., time between maintenance procedures to change the collector135) of the EUV source105, thereby increasing throughput.