Donald L. White

Technique for rapid at-wavelength inspection of extreme ultraviolet mask blanks

We have developed two new methods for at-wavelength inspection of mask blanks for extreme-ultraviolet (EUV) lithography. In one method an EUV photoresist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. In the second method, the photoresist is applied to an EUV transparent membrane that is placed in close proximity to the mask and then exposed and developed. Both reflectivity defects and phase defects alter the exposure of the resist, resulting in mounds of resist at defect sites that can then be located by visual inspection. In the direct application method, a higher contrast resist was shown to increase the height of the mounds, thereby improving the sensitivity of the technique. In the membrane method, a holographic technique was used to reconstruct an image of the mask, revealing the presence of very small defects, approximately 0.2 μm in size. The demonstrated clean transfer of phase and amplitude defects to resist features on a membrane will be ...

Rapid at-wavelength inspection of EUV mask blanks by photoresist transfer

We have developed a new technique for at-wavelength inspection of EUV mask blanks. In this technique a thin layer of EUV photoresist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. We have demonstrated using atomic force microscopy (AFM) that a change in reflectance of only 12% can cause an approximately 50 Angstrom mound in ZEP photoresist. We propose that such mounds could be detected by existing optical inspection tools, and have demonstrated the detection of similar features in ZEP (created using electron beam lithography) by an automated darkfield optical scanning instrument. The greatest advantage of the technique is speed, since both steps (EUV flood exposure and optical inspection) can be done rapidly. Difficulties, such as contamination and resist roughness, are discussed along with possible solutions.

Development of a Technique for Rapid At-Wavelength Inspection of EUV Mask Blanks

We have dramatically increased the sensitivity of a technique for the rapid inspection of EUV multilayer-coated mask blanks. In this technique an EUV sensitive resist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. Reflectivity defects in the mask blank results in mounds in a partially developed positive resists that appear as high contrast objects in a standard Nomarski microscope. The use of a higher contrast resist is shown experimentally to result in the creation of dramatically taller mounds. A simple model for the exposure and development of the resists has been developed and the predictions of the model compare well with the experimental results.

Lithographic projectors with dark-field illumination

A new resolution enhancement technique for photolithography that allows the illumination of each and every feature on a reticle to be customized is described. Customization is important because different feature types, e.g., contact holes, isolated lines, dense gratings, etc., must be illuminated differently if the best possible image at the largest depth-of-focus is to be produced. Examples of the imaging performance of a conventional lithographic projector retrofitted with dark-field illumination for three feature types commonly encountered in semiconductor lithography are presented.

Printing high-density patterns with dark-field 193-nm lithography

In dark-field (DF) lithography, light from the condenser illuminates the reticle at such a steep angle that non- diffracted light is lost from the system. A DF reticle contains a series of sub-resolution amplitude gratings to diffract light from the condenser into the projection lens and, thus, to precisely control the amplitude, phase and direction of light from every point on the reticle. In this paper we show how DF lithography can be used to print high- density patterns at higher contrast than is currently possible with conventional 193-nm lithography in a single exposure.

Phase-mask effects by dark-field lithography

A new resolution enhancement technique for photolithography that makes use of dark-field (DF) illumination was recently reported. In a DF projector, light from the condenser illuminates the reticle at such a steep angle that zero order light is lost from the system. The mask for a DF projector contains a series of sub-resolution gratings that diffract light into the lens and define features so be printed on the wafer. These sub-resolution gratings can be employed to precisely control the amplitude, phase and direction of light diffracted from each point on the mask. Given such precise control, many effect is can be produced with DF lithography that are not possible with enhanced optical lithography, e.g., terminating lines and resolving phase conflicts.