Multipoint X-Ray Reflectometry Faster, Cost-Effective
Staff -- Semiconductor International, 2/1/1998
Although a powerful tool, ellipsometry is inaccurate unless the optical property's absorbance and refraction index of the film it is used to measure are known. Also, since it relies on light reflected from a subsurface thin film substrate interface, it is useless for opaque film measurements.
Because of the radiation's penetrability for all materials, grazing-incidence X-ray reflectometry is useful in measuring opaque and transparent thin films. Thin film structural properties' thickness, density and microroughness can be determined by analyzing interference fringes formed by X-rays re-flected over a range of angles (see figure). The angular distance between fringes gives an accurate measurement of film thickness and density relative to the substrate. The amplitude of the reflection provides information on the microroughness of the air/thin film and thin film/substrate interfaces.
However, as a metrology tool, X-ray reflectometry has not been practical because it required time-consuming step-and-repeat measurements. In the traditional method, a fine beam of monochromatic X-rays is aimed at a sample at a particular value of angle and the reflected signal is measured until a statistically meaningful amount of data is accumulated. The process then moves to the next value of angle and repeats the measurement. At least several minutes were required to analyze a thin film structure by this method.
An X-ray reflectometer measurement technology called the Metal Thin-Film Metrology System (MTMS), developed by AXIC (Santa Clara, Calif.), promises a fast means to measure multiple thin film and substrate parameters simultaneously, non-inferentially and non-destructively.
The system is based on a proprietary X-ray reflectometry method that measures thickness, density and surface and interface microroughness of single films and multilayer stacks, without prior knowledge or inference of any material constants (sound propagation speed, optical qualities, density) and without reference to standards. The only parameter needed is the X-ray wavelength. The technique is insensitive to the material's phase, whether crystalline, amorphous or liquid.
The MTMS uses a cylindrical monochromator positioned between an X-ray and a sample to focus a converging fan of X-rays to the sample. A pixelated detector then senses the X-rays reflected from the sample over a range of angles (0° to 2°, for example) all at once. Processing time is decreased by as much as 97%. The technique's disadvantage is a 50 Å rms upper limit on roughness. There is also an upper limit on thin film thickness, because the fringes become too dense to resolve above about 2000 Å unless extreme measures are taken.
Because of its speed, the MTMS makes X-ray reflectometry a practical multipoint mapping metrology tool for the first time. It is useful for monitoring and controlling wafer-scale nonuniformities in CVD and CMP processes and for monitoring substrate microroughness. It is the only rapid, non-destructive method for measuring thin film or substrate density. Particularly with the coming of 300 mm, being able to map these processes quickly and accurately over the whole wafer is crucial.
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Three tantalum pentoxide thin films were examined. Experimental data (solid lines) were fitted with theoretical calculations (dotted lines) modeling the film's X-ray specular reflectivity.
