David L. Willenborg

Detection of thermal waves through optical reflectance

We show that thermal wave detection and analysis can be performed, in a noncontact and highly sensitive manner, through the dependence of sample optical reflectance on temperature. Applications to the study of microelectronic materials are illustrated by an example of measuring the thickness of thin metal films.

Thermal-wave detection and thin-film thickness measurements with laser beam deflection

A new technique has been developed that employs highly focused laser beams for both generating and detecting thermal waves in the megahertz frequency regime. This technique includes a comprehensive 3-D depth-profiling theoretical model; it has been used to measure the thickness of both transparent and opaque thin films with high spatial resolution. Thickness sensitivities of ±2% over the 500–25,000-A range have been obtained for Al and SiO2 films on Si substrates.

Ion implant monitoring with thermal wave technology

A new method, based on thermal wave technology, is used to monitor the ion implantation process in silicon. It is a noncontact, nondestructive technique that requires no special sample preparation or processing, has high sensitivity even at low dose, and provides a one‐micron spatial resolution capability. This method allows, for the first time, the ability to monitor the critical ion implantation process directly on the patterned product integrated circuit wafers as well as on the usual test wafers.

Thin‐film thickness measurements with thermal waves

We have developed a method for measuring the thickness of thin films that is nondestructive, noncontact and that can make measurements with 2‐μm spatial resolution (i.e., 2‐μm spot size) on both optically opaque as well as optically transparent films. With this method, which is based on the use of high‐frequency thermal waves, thicknesses of Al and SiO2 films on Si substrates have been measured in the 500–25 000‐A range.A method and apparatus for thin film thickness measurements with thermal waves in which heating and detection laser beams are focused onto the film, normal to the surface of the film, with the two beams parallel and non-coaxial.

Beam profile reflectometry: A new technique for dielectric film measurements

We describe a new technique for measuring the thickness and optical constants of dielectric, semiconducting, and thin metal films. Beam profile reflectometry provides excellent precision for films as thin as 30 A and as thick as 20 000 A. The technique is also capable of simultaneous 2 and 3 parameter measurements and it performs all measurements with a submicron spot size.

Multiparameter measurements of thin films using beam-profile reflectometry

Beam‐profile reflectometry is a new technique for measuring the thickness and optical constants of dielectric, semiconducting, and thin metal films. The technique consists of measuring the intensity profile of a highly focused beam reflected from the sample. By using a linearly polarized light source and a tightly focussed beam, the S‐ and P‐polarization reflectivities of the film are simultaneously obtained over a wide range of angles. The focusing also provides a submicrometer spot, thus allowing these measurements to be performed in very small geometries. How the information present in the reflectivity profiles can reveal information about as many as three unknown film parameters simultaneously is described, and results from film samples for which the multiparameter fitting capability was essential to the success of the measurement are also presented.

Detection of thermal waves through modulated optical transmittance and modulated optical scattering

We show that variations in local optical constants induced by the presence of thermal waves can be used for thermal wave detection and analysis through measurements of thermal wave‐induced modulated transmittance and scattering.

Nondestructive technique for the detection of dislocations and stacking faults on silicon wafers

We demonstrate the imaging of dislocations and stacking faults in silicon wafers in a noncontact, nondestructive fashion using laser based modulated optical reflectance. By comparison with conventional wet decoration etching, we show that the sensitivity of the modulated optical reflectance method can resolve the difference between two types of dislocations.

Effects of feature edges on thickness readings of thin oxides

Measurements of ultra-thin films (<100A) and small geometries (< 1/mm) of IC product wafers require more than simply a smaller measurement spot size. An optical artifact has been discovered when using spectrophotometers to measure ultra-thin films near feature edges. A model of this effect will be presented. This artifact is a subtle effect that produces measurable reflectivity errors tens of microns from a feature edge. While this error is small, it is not negligible for film thickness measurements below 400A. Experiments have been performed on typical spectrophotometers and data from these experiments will be presented. These data will be compared to a newly developed laser-based dielectric film thickness measurement system that significantly reduces this edge effect.

Thermal Wave Characterization of Semiconductors and Superconductors

Thermal wave technology has proven to be a very effective means for investigating the near surface region of several different materials. Although there are many methods for generating and detecting thermal waves the most desirable for quantitative NDE are the noncontact and nondamaging laser methods. When a material is excited with an intensity-modulated laser pump beam a thermal wave is generated within the near surface of the sample. Since the complex refractive index of most materials depends on temperature, the laser pump induced modulations in the local temperature of the sample will induce a corresponding modulation in the local refractive index. This variation in refractive index can in turn be detected through the modulation in the reflectance of a laser probe beam from the surface of the material [1,2]. This method is not only a highly effective method for generating and detecting thermal waves, but also permits thermal wave measurements to be performed with micron scale spatial resolution by utilizing highly focused pump and probe laser beams.