Dimensional calibration standards are an important metrology tool for quality control, inspection, and fault analysis. Tools such as atomic force microscopes (AFM), scanning probe microscopes (SPM), or scanning electron microscopes (SEM) require regular calibration to meet the needs of current and projected production processes. Suitable calibration standards have been expensive, difficult to use, and of limited utility. These limitations were, to a large degree, a result of the fabrication process and the accompanying measurement calibration paradigm. Any approach to microscope calibration should make calibration easier, less expensive, and more useful. An improved calibration standard would also be amenable to automation of the calibration process for use in production line instruments. The necessary features include: (1) the ability to calibrate the entire viewing field instead of discrete points; (2) the ability to easily locate and use the calibrated region; (3) the ability to calibrate on the nanometer scale where the most demanding applications push the state of the art; (4) significantly reduced specimen costs. There is an alternative production method for calibration specimens which meets the above criteria. It is based on the concept of physically replicating a light interference pattern to provide the essence of an interferometer in a simple calibration specimen. Modern optics technology has reached the point where large area, very accurate nd regular interference patterns in 1 and 2 dimensions can be produced. The basic physics of the process enables the periodicity of these patterns to be specified and controlled to fractions of a nanometer over these very large areas. This large-area interference pattern can be captured in a physical record suitable for viewing under the microscope. The issues affecting the accuracy and utility of this physical record and its preparation for use as a magnification standard will be discussed. Experience in sue in AFM applications indicates that calibration samples produced by this method can deliver repeatable accuracy of 1.5 nm if properly employed and analyzed. This methodology can be extended to other imaging microscope technologies.
A general purpose SPM can function as a metrology SPM when used with a new type of calibration standard and new data analysis software. The calibration standard is a 288-nm pitch, 1D holographic grating. The holographic exposure process assures uniform feature spacing over the entire specimen area, with an expected accuracy of 0.1 percent. We developed new software for data analysis and used it to diagnose and correct the residual scan nonlinearity of a standard NanoScope SPM. We improved the differential non- linearity of a 10 micron scan from 6.7 percent to 1.1 percent and we improved the integral non-linearity from 0.5 percent to 0.04 percent. We then applied the improved instrument to gauge feature spacing son magnetic disks, integrated circuits, and optical disks.
MOXTEK and its collaborators have developed technology for the fabrication of multilayer soft x-ray diffraction gratings. The diffraction gratings we will discuss here are planar, or lamellar amplitude gratings, with a structure similar to that shown in Figure 1 . The gratings we have fabricated and measured consist of a silicon (100) wafer substrate onto which we spin photo-resist. The photo-resist is patterned holographically and the substrate is then etched using reactive plasma processing techniques. The period, or pitch of these gratings Is 0.293 jim, and the grating active area can be as large as 6 cm x 6 cm. The linespace relationship is approximately 50-50. The substrates are etched until the grooves are about 1 200 A deep. This Is done to place the bottom of the grooves deep Into the substrate where It will not be able to scatter x-ray radiation efficiently. A typical substrate is shown in Figure 2.
X-ray phase diffraction gratings can be designed to behave in a fashion similar to blazed gratings, directing the majority of the energy into certain desired orders. They should be easy to fabricate using conventional semiconductor production technology, and offer advantages in design flexibility and efficiency over conventional amplitude grating or blazed grating structures. As a multilayered structure, a phase grating has structure in depth as well as across the surface. Most theoretical analyses in the literature treat the embedded structure through simplifying approximations or assumptions. We will discuss a model which treats the embedded structure explicitly using the Fresnel-Kirchhoff integral in the Fraunhofer diffraction limit. This approach produces a set of equations which are identical to the result for an amplitude diffraction grating except for an additional factor which depends on the phase relationships of the various surfaces in the multilayer stack.
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