C. C. Williams

Atomic force microscope-force mapping and profiling on a sub 100-Å scale

A modified version of the atomic force microscope is introduced that enables a precise measurement of the force between a tip and a sample over a tip‐sample distance range of 30–150 A. As an application, the force signal is used to maintain the tip‐sample spacing constant, so that profiling can be achieved with a spatial resolution of 50 A. A second scheme allows the simultaneous measurement of force and surface profile; this scheme has been used to obtain material‐dependent information from surfaces of electronic materials.

Apertureless near field optical microscope

We demonstrate a new method whereby near-field optical microscope resolution can be extended to the nanometer regime. The technique is based on measuring the modulation of the scattered electric field from the end of a sharp silicon tip as it is stabilized and scanned in close proximity to a sample surface. Our initial results demonstrate resolution in the 3 nm range--comparable to what can be achieved with typical attractive mode atomic force microscopes. Theoretical considerations predict that the ultimate resolution achievable with this approach could be close to the atomic level.

Scanning capacitance microscopy on a 25 nm scale

A near‐field capacitance microscope has been demonstrated on a 25 nm scale. A resonant circuit provides the means for sensing the capacitance variations between a sub‐100‐nm tip and surface with a sensitivity of 1×10−19 F in a 1 kHz bandwidth. Feedback control is used to scan the tip at constant gap across a sample, providing a means of noncontact surface profiling. Images of conducting and nonconducting structures are presented.

Lateral dopant profiling with 200 nm resolution by scanning capacitance microscopy

Measurement of dopant density in silicon with lateral resolution on the 200 nm scale has been demonstrated with a near‐field capacitance technique. The technique is based upon the measurement of local capacitance between a 100 nm tip and a semiconducting surface. Lateral dopant imaging is achieved by the measurement of the voltage‐dependent capacitance between tip and sample due to the depletion of carriers in the semiconductor, as the tip is scanned laterally over the surface. Measurements of dopant density have been demonstrated over a dopant range of 1015–1020 cm−3. Capacitance‐voltage measurements have been made on a submicrometer scale.

Scanning thermal profiler

Apparatus is provided for investigating surface structures irrespective of the materials involved. A fine scanning tip is heated to a steady state temperature at a location remote from the structure to be investigated. Thereupon, the scanning tip is moved to a position proximate to, but spaced from the structure. At the proximate position, the temperature variation from the steady state temperature is detected. The scanning tip is scanned across the surface sturcture with the aforesaid temperature variation maintained constant. Piezo electric drivers move the scanning tip both transversely of, and parallel to, the surface structure. Feedback control assures the proper transverse positioning of the scanning tip and voltages thereby generated replicate the surface structure to be investigated.

Advances in experimental technique for quantitative two-dimensional dopant profiling by scanning capacitance microscopy

Several advances have been made toward the achievement of quantitative two-dimensional dopant and carrier profiling. To improve the dielectric and charge properties of the oxide–silicon interface, a method of low temperature heat treatment has been developed which produces an insulating layer with consistent quality and reproducibility. After a standard polishing procedure is applied to cross-sectional samples, the samples are heated to 300 °C for 30 min under ultraviolet illumination. This additional surface treatment dramatically improves dielectric layer uniformity, scanning capacitance microscopy (SCM) signal to noise ratio, and C–V curve flat band offset. Examples of the improvement in the surface quality and comparisons of converted SCM data with secondary ion mass spectrometry (SIMS) data are shown. A SCM tip study has also been performed that indicates significant tip depletion problems can occur. It is shown that doped silicon tips are often depleted by the applied SCM bias voltage causing errors...

Quantitative two-dimensional dopant profile measurement and inverse modeling by scanning capacitance microscopy

Quantitative dopant profile measurements are performed on a nanometer scale by using a scanning capacitance microsope. A nanometer scale tip of the microscope is positioned at a semiconductor surface, and local capacitance change is measured as a function of sample bias. The method incorporates a feedback system and procedure in which the magnitude of the AC bias voltage applied to the sample is adjusted to maintain a constant capacitance change as the tip is scanned across the sample surface. A one dimensional model is used to extract dopant density profiles from the measurements made by the scanning capacitance microscope.

Lateral dopant profiling in semiconductors by force microscopy using capacitive detection

Recently, high‐resolution mapping of dopant concentration has been demonstrated with the scanning capacitance microscope (SCM). Here, we demonstrate that a similar measurement can be made with the atomic force microscope using the previously demonstrated capacitive force sensing mode. By applying appropriate bias to the force tip, depletion‐induced capacitive variation is mapped over regions of varying dopant density. This method has a predicted sensitivity comparable to the SCM, and in addition allows imaging of trapped charge, as well as an independent measurement of the surface topography. Results of first‐order model calculations are presented which give estimates as to the limits in sensitivity and resolution of this method

Optical ranging by wavelength multiplexed interferometry

A new optical technique is described for measurement of absolute distance. The approach is based upon a wavelength multiplexed heterodyne interferometer with FM demodulation. By temporally multiplexing discrete wavelengths in a heterodyne interferometer, a complete elimination of interferometric range ambiguity can be achieved while maintaining the high range sensitivity and resolution of interferometry. The basic theory is presented and an algorithm is described for measurement of range over meter distances with submicrometer resolution. The experimental implementation of the wavelength multiplexed interferometer is described and ranging results with 2 μm resolution from 20 cm are presented. A scanned three‐dimensional map of a surface contour with 3‐mm topography is also presented.

Scanning capacitance microscope methodology for quantitative analysis of p-n junctions

Quantification of dopant profiles in two dimensions (2D) for p-n junctions has proven to be a challenging problem. The scanning capacitance microscope (SCM) capability for p-n junction imaging has only been qualitatively demonstrated. No well-established physical model exists yet for the SCM data interpretation near the p-n junction. In this work, the experimental technique and conversion algorithm developed for nonjunction samples are applied to p-n junction quantification. To understand the SCM response in the active p-n junction region, an electrical model of the junction is proposed. Using one-dimensional secondary ion mass spectrometry (SIMS) data, the carrier distribution in the vertical dimension is calculated. The SIMS profile and carrier distribution is then compared with the SCM data converted using a first-order model. It is shown that for a certain class of profiles, the SCM converted dopant profile fits well to the SIMS data in one dimension. Under this condition, it is possible to identify t...