1. Field of the Invention
The present invention relates generally to imaging, and more particularly to measuring dielectric properties using a near-filed scanning microwave microscope.
2. Related Art
Dielectric thin film research has become increasingly important as the demand grows for smaller, faster, and more reliable electronics. For example, high permittivity thin films are under study in order to fabricate smaller capacitors while minimizing leakage. Low permittivity materials are being sought to allow smaller scale circuits while minimizing undesirable stray capacitance between wires. Nonlinear dielectrics, which have a dielectric permittivity which is a function of electric field, are being used in tunable devices, particularly at microwave frequencies. Finally, ferroelectric thin films are a solution for large-scale, non-volatile memories.
All of these dielectric thin film technologies demand high-quality, homogeneous films. However, this goal is often difficult to achieve because of the complicated fabrication process involved. Consequently, it is important to have a set of reliable techniques for evaluating thin film properties, such as permittivity and tunability. A number of different techniques are available. One standard low-frequency (xe2x89xa61 MHz) method uses thin film capacitors to measure the normal and in-plane components of the permittivity tensor. Another uses reflection measurements from a Corbino probe. Still another method provides microwave measurements of permittivity by using transmission measurements through a microstrip structure. However, these techniques average over large areas and involve depositing thin film electrodes which itself can alter the properties of the sample. Dielectric resonators have been used as well, but also have the problem of low spatial resolution. More recently, near-field microscopy techniques have allowed quantitative measurements with spatial resolutions much less than the wavelength. These techniques use a resonator which is coupled to a localized region of the sample through a small probe and have the advantage of being non-destructive. However, it is still difficult to arrive at quantitative results and maintain high spatial resolution.
Therefore, what is needed is a non-destructive, non-invasive, system and method for imaging quantitative permittivity and tunability at high spatial resolution.
The present invention meets the above-mentioned needs by providing a system, apparatus, and method for quantitatively imaging the dielectric properties of bulk and thin film dielectric samples. Permittivity and dielectric tunability are two examples of dielectric properties capable of measurement by the present invention.
The system uses a near-field scanning microwave microscope (NSMM). The NSMM is comprised of a coaxial transmission line resonator having one end coupled to a microwave signal source and the other end terminating with an open-ended coaxial probe. The probe, which has a sharp-tipped center conductor extending beyond the outer conductor, is held fixed while the sample is raster scanned beneath the probe tip. A spring-loaded cantilever sample holder gently presses the sample against the probe tip with a force of about 50 xcexcN (microNewtons). A feedback circuit keeps the microwave signal source locked onto a selected resonant frequency of the microscope resonator. Because the electric fields generated by the microwave signal are concentrated at the probe tip, the resonant frequency and quality factor of the resonator are a function of the sample properties near the probe tip. Once the microwave signal has been applied to the sample through the probe tip, it is reflected back through the system. The feedback circuit is then able to receive the reflected microwave signal from the coaxial transmission line resonator and calculate a resonant frequency shift. The resonant frequency shift value is then stored in a computer. The computer also controls the scanning of the sample beneath the probe. To obtain quantitative results, the system uses calibration curves to exhibit the relationship between the calculated resonant frequency shift data values and the dielectric properties of a sample.
The invention described herein has the advantage of being able to provide quantitative results for samples on a length scale of about 1 xcexcm or less. This allows for the measuring of sample sizes relative to the actual environment in which they will be used.
The invention also has the advantage of providing more accurate quantitative results because the sharp protruding center conductor is represented as a cone during modeling.