Efficient phase defect detection system and method

Details Efficient phase defect detection system and method Efficient phase defect detection system and method

2001-05-04

2004-01-06

Rosenberger, Richard A. (Department: 2877)

Optics: measuring and testing

Inspection of flaws or impurities

C356S237500, C356S239300

Reexamination Certificate

active

06674522

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to optical inspection systems, and more particularly to an automated photomask inspection apparatus for detecting or classifying defects that might occur on optical masks, reticles, and the like.
BACKGROUND OF THE INVENTION
Integrated circuits are comprised of three-dimensional structures of conductors, semiconductors and insulators formed on a semiconductor substrate or wafer. The integrated circuit manufacturing process typically involves multiple processing steps, including repeated transferring of patterns to the wafer surface using photolithography. Precise and defect free execution of each step is essential for the production of functional semiconductor devices at a yield that is economical for the manufacturer.
Photolithography is used to transfer a pattern to a wafer by exposing a photoresist-covered wafer to a light intensity pattern and developing the photoresist. The light intensity pattern is produced from the modification of an incident light beam having an optical wave front, with a photomask or reticle. The photomask includes features having variations in optical properties that modify the amplitude and/or phase of the incident beam through reflection, transmission, absorption or any combination thereof. The light is then projected onto the wafer to produce a required intensity pattern. One type of photomask feature is an amplitude object that modifies the amplitude, or intensity, of an illumination beam. For a transmission photomask, an opaque layer of metal film is one example of an amplitude object.
A second type of feature is a phase object that modifies the phase of an illumination beam or phase distribution of the optical wave front thus producing desirable intensity distributions in the image projected on the wafer. A transparent layer of varying thickness on a transmission photomask is one example of a phase shift object (or “phase shifter”). A phase-shift mask (PSM) has a pattern of phase shifters on the mask that results in an illumination beam comprised of phase variations. In practice, a PSM may contain either phase objects or a combination of amplitude and phase objects, as well as attenuated phase objects that simultaneously induce a phase and amplitude variation at select wavelengths. For a transmitted-light PSM, phase objects can be variations in the thickness of the photomask, usually a fused quartz plate. As the light travels from the photomask to the wafer, interference occurs between the differently phase-shifted portions of the illumination beam. The system optics, including the illumination source and photomask, produce an interference pattern of light and dark areas on the wafer.
Semiconductor industry roadmaps call for decreasing the critical dimension (CD) in semiconductor wafers. Phase shift mask technology can enable smaller CD's and is increasingly being used along with other approaches such as reducing the wavelength of the illumination beam or increasing the numerical aperture of the objective lens of the wafer stepper. Since most defects on a PSM will print, i.e. replicate itself, on every wafer in the particular semiconductor device, inspection for phase defects on PSMs is critical for achieving high yields. The industry is currently developing technologies to take advantage of the lower CDs of PSMs. These technologies include the ability to produce photomasks and the ancillary equipment and techniques for using and testing photomasks. In particular, most commercially available photomask inspection apparatus were designed before the widespread use of PSMs, as described for example, in U.S. Pat. Nos. 5,572,598 and 6,052,478, both to Wihl et al.
Of particular interest here is the inspection of masks that contain phase objects. Inspection of phase masks should include the detection and classification of the phase error, amplitude error of the phase shifter, amplitude error of adjoining features, and context. Since photomask inspection equipment usually performs an optical inspection at wavelengths and optical configurations different from that of the intended stepper, these differences must be taken into account.
The optical inspection of phase objects is technologically challenging. Pure phase objects modify the electromagnetic field of the incident light through changes in phase, and not of the intensity. As most detectors respond to variations in intensity, phase objects can be difficult to measure directly. Even more challenging is the detection of phase objects in the presence of amplitude objects. Due to the large amount of information contained on a mask, i.e. greater than 10
15
pixels to write and greater than 10
12
pixels to inspect a 6 inch mask, automated mask inspection systems are used.
A defect is defined here as any unintended modification to the intended photolithographic pattern caused during the manufacture of the photomask, or as a result of the use of the photomask. Defects can be due to, but not limited to, a portion of the opaque layer being absent from an area of the photolithographic pattern where it is intended to be present, a portion of the opaque layer being present in an area of the photolithographic pattern where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, cleaning residue, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or opaque layer.
Phase-shift photomask inspection systems should meet several requirements to be commercially successful. These requirements include the ability to: determine and report defects among about 10
12
pixels in a reasonable time; inspect a single large die using gray-scale algorithms by rendering, from a database image of the PSM, both the transmission and phase for each pixel; handle multiple resolutions, including having a sensitivity to large (missing shifters) and small (sub-resolution) phase defects; process possible context information; account for off-wavelength inspection of phase errors; and distinguish between amplitude and phase objects. In addition to semiconductor applications, phase imaging systems are important in other fields, such as in the biological and medical areas.
Several imaging and defect detection systems for PSMs have been described in the prior art that address some of the above listed requirements. The following discussion provides information about several of the known techniques for obtaining phase information about objects useful in mask inspection systems.
Differential interference contrast techniques determine phase objects by interfering orthogonally polarized, laterally offset spots. Phase defects produce changes in the polarization state of the modified light that is detected as an intensity difference in the image, e.g. in a Nomarski microscope. Interference techniques have very high accuracy and can measure absolute phase. In practice however, interference techniques typically have very high alignment, optical wave-front quality, and system vibration requirements that are prohibitive or extremely costly to implement. Other problems include throughput and the need for a reference beam that could, in the case of the Nomarski technique, be blocked by chrome on the mask.
Differential phase contrast techniques, detailed in D. K. Hamilton and C. J. R. Sheppard,
J. Microscopy
133, 27 (1984), determine the phase gradients of an object by the resulting intensity differences in the pupil plane of the imaging system measurable using a split or quad detector in the case of a scanner. Sensitivity of this technique depends on orientation of the phase object relative to the split detector that could result in missed defects.
The Zernike phase contrast technique uses a 90 degree shifted annular-pupil-plane filter to shi

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