The manufacture of integrated circuit devices involves the processing of a semicondutor wafer through sequential layers to add or remove material. Optical projection lithography, or photolithography, is the most common process used around the world to pattern each layer onto the wafer. The wafer is covered with a material that is sensitive to light called a photoresist. The photoresist is then selectively exposed using a patterned photomask in an exposure tool: a stepper, a scanner, or a contact printer. The developed photoresist is the positive or negative image of the photomask, and acts as a physical stencil for transferring a pattern on the photomask to the wafer through the process of etching. State-of-the-art photolithography has enabled the semiconductor industry to produce sub-wavelength features and to keep pace with “Moore's law”. This capability is due in large part to the recent introduction of advanced phase-shift photomasks (PSM) and optical proximity correction (OPC) technology. The increase in complexity of these photomasks has been reflected by large increases in PSM costs, and has made repair of defective photomasks an attractive alternative to replacement.
Inspection of masks using phase-shifting and OPC technologies is a difficult task, but necessary to ensure the initial and subsequent fidelity of the photomasks throughout the manufacturing process. Patterns on the photomasks must meet stringent criteria for size, shape, spacing, orientation, overlap, and placement of features. Defects must be repaired to prevent replication of errors across the printed wafers. There are two basic types of PSMs: embedded-attenuator PSMs (EAPSM), and alternating-aperture PSMs (AAPSMs) or “Levenson masks”. EAPSMs deposit a layer of partially-opaque such as molybdenum silicide (MoSi) or some other compound to effect phase shifts; the transmission of a layer with thickness sufficient for a half-wave of phase shift displays approximately 5–10% transmission. AAPSM or Levenson masks remove controlled amounts of quartz photomask substrate to effect the phase changes, without depositing attenuating layers. In either case, features are designed to cause a variety of phase shifts (0 degrees, 60, 90, 180, etc.) in order to produce a given aerial image pattern. If the construction of the photomask is incorrect, so that, for example, a phase shift of a designed 180 degree shift feature is instead, say, 170 degrees, the desired feature may not print on the wafer, possibly leading to device failure. This is a typical photomask phase defect which must be identified and corrected before wafer production can continue.
What is needed is a tool for inspecting advanced photomasks to determine errors in phase and amplitude with high spatial resolution and accuracy. Currently, photomasks are usually inspected with a scanning optical microscope. However, phase information is lost since it is the light intensity, rather than the electric field amplitude, that is integrated at each location on a mask. Generally, interferometric techniques, which provide information about the phase of the optical beams, are required in order to obtain information on optical path difference (OPD).
One example of interferometric techniques applied to phasemask metrology may be found in U.S. Pat. No. 6,559,953. In '953, two phase-coherent beams with a controllable phase difference are combined on a CCD detector. One beam is first passed through a region on the photomask defined by a variable pupil, while the other beam serves as a uniform reference. The fringe pattern thus generated, in conjunction with several other ‘control’ interferograms, is used to determine the transmission and phase of the photomask region. No imaging components are employed in the setup (unity magnification), which emphasizes the need for collimated light beams. From a photomask metrology perspective, the primary drawback of '953 is that the spatial resolution of the phase and transmission measurements is limited by the pixel size of the CCD detector, which is on the order of several micrometers. This limitation precludes the use of '953 to perform metrology where it is most needed: on the sub-micron features found on advanced photomasks.
A second example, combining interferometric techniques with microscopy to perform phase metrology, is described in H. Kusunose et al., “Direct phase-shift measurement with transmitted deep-UV illumination”, Proceedings of the SPIE Vol. 2793 pg. 251–260 (1996). Kusunose uses a lateral-shearing interferometer to combine two phase-coherent images, each of which has passed through the photomask slightly displaced from the other. The measurement is obtained by a photosensitive detector, mounted behind a final turning mirror, in the center of which a small hole has been drilled. The technique relies upon the quality of the microscope images, and thus the measurement precision and repeatibility depends on the aberrations and NA of the optics within the microscope.