Hal Edwards

Fast, high-resolution atomic force microscopy using a quartz tuning fork as actuator and sensor

We report a new method of achieving tip–sample distance regulation in an atomic force microscope (AFM). A piezoelectric quartz tuning fork serves as both actuator and sensor of tip–sample interactions, allowing tip–sample distance regulation without the use of a diode laser or dither piezo. Such a tuning fork has a high spring constant so a dither amplitude of only 0.1 nm may be used to perform AFM measurements. Tuning-fork feedback is shown to operate at a noise level as low as that of a cantilever-based AFM. Using phase-locked-loop control to track excursions in the resonant frequency of a 32 kHz tuning fork, images are acquired at scan rates which are fast enough for routine AFM measurements. Magnetic force microscopy using tuning-fork feedback is demonstrated.

Analyzing atomic force microscopy images using spectral methods

Various statistical quantities (such as average, peak-to-valley, and root-mean-square roughness) have been applied to characterize surface topography. However, they provide only vertical information. Because spectral analysis provides both lateral and longitudinal information, it is a more informative measurement than all these commonly used statistical quantities. Unfortunately, a standard method to calculate power spectral density (PSD) is not available. For example, the dimensions of PSD are often denoted as either (length)3 or (length)4. This may lead to confusion when utilizing spectral analysis to study surface morphology. In this paper, we will first compare the definitions of PSD commonly used by various authors. Using silicon surface roughness measurements as examples, we will demonstrate the advantages of spectral methods on atomic force microscopic (AFM) image analysis. In this context, we study the effects of typical AFM imaging distortions such as image bow, drift, tip-shape effects, and acou...

SCANNING CAPACITANCE SPECTROSCOPY : AN ANALYTICAL TECHNIQUE FOR PN-JUNCTION DELINEATION IN SI DEVICES

Scanning capacitance spectroscopy (SCS), a variant of scanning capacitance microscopy (SCM), is presented. By cycling the applied dc bias voltage between the tip and sample on successive scan lines, several points of the high-frequency capacitance–voltage characteristic C(V) of the metal–oxide–semiconductor capacitor formed by the tip and oxidized Si surface are sampled throughout an entire image. By numerically integrating dC/dV, spatially resolved C(V) curves are obtained. Physical interpretation of the C(V) curves is simpler than for a dC/dV image as in a single-voltage SCM image, so that the pn junction may be unambiguously localized inside a narrow and well-defined region. We show SCS data of a transistor in which the pn junction is delineated with a spatial resolution of ±30 nm. This observation is consistent with the conclusion that SCS can delineate the pn junction to a precision comparable to the Si depletion width, in other words, the actual size of the electrical pn junction. A physical model t...

pn-junction delineation in Si devices using scanning capacitance spectroscopy

The scanning capacitance microscope (SCM) is a carrier-sensitive imaging tool based upon the well-known scanning-probe microscope (SPM). As reported in Edwards et al. [Appl. Phys. Lett. 72, 698 (1998)], scanning capacitance spectroscopy (SCS) is a new data-taking method employing an SCM. SCS produces a two-dimensional map of the electrical pn junctions in a Si device and also provides an estimate of the depletion width. In this article, we report a series of microelectronics applications of SCS in which we image submicron transistors, Si bipolar transistors, and shallow-trench isolation structures. We describe two failure-analysis applications involving submicron transistors and shallow-trench isolation. We show a process-development application in which SCS provides microscopic evidence of the physical origins of the narrow-emitter effect in Si bipolar transistors. We image the depletion width in a Si bipolar transistor to explain an electric field-induced hot-carrier reliability failure. We show two sam...

A terraced scanning super conducting quantum interference device susceptometer with submicron pickup loops

Superconducting quantum interference devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pickup loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pickup loops 300nm from the sample. The 600nm–2μm pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of ∼70 electron spins per root hertz at 4K.

In situ Si flux cleaning technique for producing atomically flat Si(100) surfaces at low temperature

We have developed a method for removing oxides and producing atomically flat Si(100) surfaces with single atomic height steps using a Si flux cleaning technique. By introducing a Si flux in the range 1.0–1.5 A/s at the onset of an SiO2 thermal desorption step as low as 780 °C, scanning tunneling microscopy (STM) and atomic force microscopy images reveal smooth surfaces with atomically flat terraces with an rms roughness of 0.5 A and single-step heights of 1.4 A. STM reveals that A- and B-type steps are present across the entire area of the scanned surface. Desorption of the surface oxide layer with Si fluxes below this range results in rougher surfaces with pits ∼50 A deep and 1000 A across. For Si fluxes above this range, no pits are seen but atomic steps are not visible on the surface.

Quantitative measurement of sheet resistance by evanescent microwave probe

Quantitative measurement of microwave sheet resistance by a novel type of near-field microwave microscope—Evanescent Microwave Probe (EMP)—has been demonstrated. The data cover a wide range of sheet resistance from the metal limit to the insulator limit. Both finite element analysis (FEA) and a simple coaxial ring model have been shown to fit the data well. The demonstration of sheet resistance measurement with high spatial resolution in the GHz range shows the potential of EMP for semiconductor metrology applications. The data also reveal issues related to the large penetration depth, allowing substrate properties to affect the signal.

Modification of parylene AF-4 surfaces using activated water vapor

In this study, we investigate the addition of oxygen functionalities to parylene AF-4 using X-ray photoelectron spectroscopy (XPS). Subsequent reactions with trimethylaluminum (TMA) were also studied by XPS. Samples were exposed to water vapor that was passed over a heated W filament, and comparison is made with similarly exposed polystyrene. Oxygen incorporation occurred in both polymers and was substantially greater in polystyrene. Defluorination occurred in the parylene sample, and X-ray photoelectron spectra indicate that the oxygen is chemically bound to the polymer surface. TMA was dosed at room temperature onto the parylene samples, and prior modification of the AF-4 surface resulted in enhanced reactivity toward TMA. Atomic force microscopy (AFM) is used to characterize the surfaces and demonstrates that no significant surface roughness occurred due to the modification process.

New method to estimate step heights in scanning-probe microscope images

A new algorithm, the polynomial step-function fit (PSFF), is presented. The PSFF algorithm extracts step heights from noisy and distorted scanning-probe microscope (SPM) images. A one-dimensional, line-by-line implementation as well as a two-dimensional, full-image version are presented. The PSFF algorithm allows the correction of image distortions due to nonlinearities in the piezoelectric scanner and Abbe offset errors, but piezoelectric creep and hysteresis must be corrected separately, and may set the ultimate physical limitations on the accuracy of the PSFF algorithm. The PSFF algorithm is demonstrated with a real sample.

Negative differential transconductance in silicon quantum well metal-oxide-semiconductor field effect/bipolar hybrid transistors

Introducing explicit quantum transport into Si transistors in a manner amenable to industrial fabrication has proven challenging. Hybrid field-effect/bipolar Si transistors fabricated on an industrial 45 nm process line are shown to demonstrate explicit quantum transport signatures. These transistors incorporate a lateral ion implantation-defined quantum well (QW) whose potential depth is controlled by a gate voltage (VG). Quantum transport in the form of negative differential transconductance (NDTC) is observed to temperatures >200 K. The NDTC is tied to a non-monotonic dependence of bipolar current gain on VG that reduces drain-source current through the QW. These devices establish the feasibility of exploiting quantum transport to transform the performance horizons of Si devices fabricated in an industrially scalable manner.