P. De Wolf

Status and review of two-dimensional carrier and dopant profiling using scanning probe microscopy

An overview of the existing two-dimensional carrier profiling tools using scanning probe microscopy includes several scanning tunneling microscopy modes, scanning capacitance microscopy, Kelvin probe microscopy, scanning spreading resistance microscopy, and dopant selective etching. The techniques are discussed and compared in terms of the sensitivity or concentration range which can be covered, the quantification possibility, and the final resolution, which is influenced by the intrinsic imaging resolution as well as by the response of the investigated property to concentration gradients and the sampling volume. From this comparison it is clear that, at present, none of the techniques fulfills all the requirements formulated by the 1997 Semiconductor Industry Association roadmap for semiconductors [National Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 1997)]. Most methods are limited to pn-junction delineation or provide a semiquantitative image of the differen...

Cross-sectional nano-spreading resistance profiling

The nano-spreading resistance profiling (nano-SRP) method has been developed and improved such that it can now be used as an accurate tool for quantitative two-dimensional carrier profiling. Instrumental improvements include the use of batch-fabricated, conducting diamond-coated silicon probes, and a low-noise logarithmic current amplifier. The spatial resolution (10 nm), the dynamic range (1014–1020 atoms/cm3), and the sensitivity (1014 atoms/cm3) of the nano-SRP technique are illustrated by profiling a wide range of state-of-the-art device structures. Two-dimensional measurements of the carrier distribution inside fully processed metal–oxide–semiconductor transistors with gate lengths varying from 2 μm down to 0.25 μm illustrate the strength of the technique to map present and future devices. The nano-SRP method currently has sufficient resolution to demonstrate the small asymmetry in the source/drain profiles from transistors in which the sample was not rotated during the 7° implant. The electrical tra...

Epitaxial staircase structure for the calibration of electrical characterization techniques

Frequently electrical characterization techniques [such as the spreading resistance probe (SRP)], rely on the availability of a set of well-calibrated, homogeneously doped Si samples to establish the calibration curves (and parameters) necessary for the conversion of resistance measurements into carrier profiles. Although ideally such a calibration should be verified daily, in practice, time considerations limit the daily verification to one (or a few) calibration samples. To remedy this situation a special multilayer Si structure has been grown consisting of a decreasing B-doped staircase containing seven flat 4–5 μm thick calibration layers doped from 1020/cm3 down to 1015/cm3 separated by slightly (factor 2–3) higher doped 1–2 μm thick interface layers. The latter are included to facilitate the SRP calibrations as the SRP correction factor within the calibration layers now becomes very close to one. Since presently, a calibration curve can be generated quickly from a single measurement, daily measureme...

Evaluating probes for “electrical” atomic force microscopy

The availability of very sharp, wear-proof, electrically conductive probes is one crucial issue for conductive atomic force microscopy (AFM) techniques such as scanning capacitance microscopy, scanning spreading resistance microscopy, and nanopotentiometry. The purpose of this systematic study is to give an overview of the existing probes and to evaluate their performance for the electrical techniques with emphasis on applications on Si at high contact forces. The suitability of the characterized probes has been demonstrated by applying conductive AFM techniques to test structures and state-of-the-art semiconductor devices. Two classes of probes were examined geometrically and electrically: Si sensors with a conductive coating and integrated pyramidal tips made of metal or diamond. Structural information about the conductive materials was obtained by electron microscopy and other analytical tools. Swift and nondestructive procedures to characterize the geometrical and electrical properties of the probes p...

One‐ and two‐dimensional carrier profiling in semiconductors by nanospreading resistance profiling

Measurement of the carrier concentrations in silicon with lateral and in‐depth resolution on the sub‐100 nm scale has been demonstrated with an atomic force microscope (AFM) using conducting tips. The technique determines the local spreading resistance and hence inherits the attractive features of the conventional spreading resistance profiling (SRP) technique and will be referred to as nano‐SRP. For instance, the calibration curve established by measuring homogeneously doped substrates indicates a dynamic range of concentrations between 1014 and 1019 cm−3 and a monotonic relation between resistance and resistivity, similar to a conventional SRP calibration curve. In the present study, W‐coated diamond tips are used at a precisely controlled force of 70 μN, leading to a contact radius of 50 nm as determined from AFM analysis of the resulting imprints. The drastic reduction of the contact size implies that one can measure directly on a vertical cross section of the structure and overcome some limitations c...

Recent insights into the physical modeling of the spreading resistance point contact

The generation of accurate electrically active dopant profiles from raw spreading resistance probe (SRP) data requires a realistic physical description of the high‐pressure point contact used. Comparisons between SRP and secondary ion mass spectrometry for junction isolated structures have previously indicated a need to introduce, into the one‐dimensional Poisson calculations, rather high permittivity and band gap narrowing values close to the contact. To clarify the situation, a systematic survey of the literature has been made regarding the mechanical and electrical aspects of pressure contacts, including finite element calculations of the internal stress distributions and the characteristics of the high‐pressure phases of silicon. Furthermore correlation has been sought with current–voltage curves of the SRP point contact, nano‐SRP low weight measurements, and atomic force microscopy and transmission electron microscopy analysis of SRP probe imprints. The emerging physical contact model is one dominate...

Nanopotentiometry: Local potential measurements in complementary metal–oxide–semiconductor transistors using atomic force microscopy

In nanopotentiometry a conductive atomic force microscope tip is used as a voltage probe in order to measure the distribution of the electrical potential on the cross section of an operating device. The information thus obtained is complementary to the carrier profiles and provides a method for calibration of device simulations. The experimental procedure is discussed in detail with emphasis on preparation techniques and contact force calibration. The present results are obtained on the cross section of a 0.25 μm complementary metal–oxide–semiconductor transistor cell which was designed such that even after sectioning, the device is still operational. I–V characteristics of the transistors before and after preparation indicate that the sectioned transistors show a higher leakage current, but are still functionally operational. Using nanopotentiometry, potential maps are measured under various bias conditions, showing the progressive creation of the channel. The present results demonstrate that nanopotenti...

Quantification of nanospreading resistance profiling data

In nanospreading resistance, the resistance measured at a particular position on the sample cross section is not exclusively determined by the carrier concentration at this position, but by the entire surrounding carrier profile. The correct evaluation of this spreading effect requires a detailed calculation, leading to a deconvolution algorithm, which recovers the charge-carrier profile from the measured resistance profile. In this work, a general scheme for transforming a wide range of profiles is proposed. The scheme is based upon finite-element calculations of the potential distribution and the current spreading of a circular flat contact current source on a semi-infinite semiconductor sample with known carrier distribution. A correction factor database is formed as a function of typical profile characteristics such as (i) the distance to perfectly isolating or conducting boundaries, (ii) carrier gradient, and (iii) carrier curvature. In routine operation the transformation of resistance data into the...

Low weight spreading resistance profiling of ultrashallow dopant profiles

The application of the conventional spreading resistance profiling (SRP) method on ultrashallow profiles is endangered by the phenomenon of pressure enhanced carrier spilling which results in false dopant and carrier profile differences, the need for large correction factors, and the presence of surfaces states on the bevel. Furthermore, the tedious preparation of the conventional SRP probes requires a lot of expertise. In this work, it is shown that some of these limitations can be resolved by the application of the nano-SRP technique on beveled samples. The use of a single conductive diamond-coated silicon tip mounted on an atomic force microscope (AFM) maintains the strong points of SRP while eliminating the need for probe conditioning. Contact sizes for nano-SRP are a factor of one hundred smaller than in conventional SRP. The small contact size combined with small probe movements allows for considerably larger bevel angles and provides a geometrical resolution as small as 0.5 nm. The smaller contact,...

Comparison of two-dimensional carrier profiles in metal-oxide-semiconductor field-effect transistor structures obtained with scanning spreading resistance microscopy and inverse modeling

Scanning spreading resistance microscopy (SSRM) is used to determine the complete two-dimensional carrier profile of fully processed 0.29 μm p- and n-type metal–oxide–semiconductor field-effect transistors with various source/drain implants. A comparison is made between the quantified profiles determined using SSRM and the profiles extracted from the electrical device characteristics using an inverse modeling technique. This comparison includes source/drain and well implants, epilayers, and field implants. The data are compared in terms of depth precision and carrier-concentration accuracy and show a good agreement. This article also addresses the limitations and possible artifacts of both methods.