and acquisition speed. This is enabled by ... Strong confinement as well as an about 25-fold ..... [1] Alvis, R. and Kelly, T. F., Microscopy Today, September. 2008 ...
ISTFA 2010 Proceedings from the 36th International Symposium for Testing and Failure Analysis November 14–18, 2010, Addison, Texas, USA
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Quantitative, nanoscale free-carrier concentration mapping using terahertz nearfield nanoscopy J. Wittborn and R. Weiland Infineon Technologies AG, Munich, Germany A. J. Huber Neaspec GmbH, Martinsried, Germany F. Keilmann Max Planck Institut of Quantum Optics and Center for NanoScience, Garching, Germany R. Hillenbrand CIC nanoGUNE Consolider, Donostia – San Sebastian, Spain Ikerbasque, Basque Foundation for Science, Bilbao, Spain
Abstract
Spatial resolution and detectable doping range varies between the methods.
We use ultra-resolving terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips to analyze 65-nm technology node transistors. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm. Images of semiconductor transistors provide evidence of 40 nm (λ/3000) spatial resolution at 2.54 THz (wavelength λ = 118µm) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. The mobile carrier contrast can be clearly related to near-field excitation of THz-plasmons in the semiconductor regions. The extraordinary high sensitivity of our microscope provides THz near-field contrasts from less then 100 mobile electrons in the probed volume.
Here we demonstrate the benefits of near-field nanoscopy. We use laser light at 2.54 THz (wavelength λ = 118µm) scattering at metallised atomic force microscope tips to achieve 2dimensional, quantitative mapping of carrier concentration in the range 1016 to 1019 carriers/cm3 at a spatial resolution of 40 nm (λ/3000). Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm. Images of 65-nm technology node CMOS transistors demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. Electromagnetic radiation at THz frequencies addresses a rich variety of light-matter interactions because photons in this low energy range can excite molecular vibrations and phonons, as well as plasmons and electrons of non-metallic conductors [10, 11, 12, 13]. Consequently, THz radiation offers intriguing possibilities for material characterization currently motivating major efforts in the development of THz imaging systems [14, 15]. Diffraction unfortunately limits the spatial resolution to about half the wavelength which is in the order of 100 µm. For this reason, THz mapping of micro- or nanoelectronic devices could not be attained. A promising route to break the diffraction barrier and to enable sub-wavelength scale imaging is based on fine-focusing of THz radiation by millimeter-long tapered metal wires [16, 17, 18, 19]. Acting as antennas, the wires capture incident THz waves and convert them into strongly confined near fields at the wire tip apex [20]. When this confined field becomes modified by a close-by scanned sample, the scattered radiation carries information on the local dielectric properties of the sample [18, 21, 22]. THz images can be obtained by recording the scattered radiation by a distant THz receiver. Attempts of realizing such THzscattering near-field optical microscopy (THz-SNOM), however, suffered from extremely weak signals and faint material contrasts owing to strong background scattering.
Introduction Background Measurement of carrier- or doping-concentration of nanostructured devices still remains a challenge for the semiconductor industry. Secondary ion mass spectroscopy (SIMS) and spreading resistance profiling (SRP) are useful methods for measuring dopant concentration and carrier concentration, respectively, but are limited to 1-dimensional depth profiles. In addition, both these methods require relatively large, laterally homogenous sample areas. Atom probe microscopy faces the opposite problem; it yields 3dimensional measurements of the nanoscale dopant concentration [1] but the maximum sample size limits its use for many failure analysis applications. Scanning probe microscopy (SPM) based methods [2] such as scanning capacitance microscopy (SCM) [3, 4, 5, 6, 7], scanning spreading resistance microscopy (SSRM) [8, 9] and scanning microwave microscopy (SMM) utilizes different tip-sample interaction mechanisms to yield 2-dimensional doping maps. 20
scattering the AFM is operated in dynamic mode where the cantilever oscillates at its mechanical resonance frequency Ω, here at 35 kHz, with amplitude of about 100 nmpp. The bolometer signal is subsequently demodulated at harmonic frequencies nΩ (with n = 2 or 3) yielding a background-free THz signal amplitude sn [21, 31].
Novel probes for THz focusing are thus a subject of current interest [23, 24]. In 2008, Huber et al. [25, 26] introduced THz near-field microscopy achieving unprecedented resolution of about 40 nm, paired with extraordinarily high image contrast and acquisition speed. This is enabled by interferometric detection of THz radiation scattered from cantilevered atomicforce-microscope (AFM) tips. Building on this pioneering work we demonstrate THz mapping of mobile carriers within a single nano-device and its relevance for semiconductor technology. Theory The optical near-field scattering, depending on refractive index and absorption of the sample, can be enhanced by phonon polaritons in polar dielectrics or plasmon polaritons in metals and doped semiconductors [27]. Using a wavelength near the polariton resonance of the material of interest thus causes sharp contrast between this and other materials. A dipole model for the field scattered by the tip that has proven to agree well with experimental results has been developed and is thoroughly described in a paper by Keilmann and Hillenbrand [21].
Figure 1: Scheme of our experimental setup based on an AFM: A laser emitting a monochromatic beam at 2.54 THz is used for illuminating a cantilevered AFM tip and interferometric detection is used for recording the backscattered THz radiation simultaneously with the AFM topography.
The essential ingredient to nanoscale resolved THz near-field microscopy is a strongly confined THz near field for generating highly localized scattering. Nanoscale near-field confinement can be achieved by plane-wave illumination of a conical metal tip, similar to visible [28] and infrared [29] frequencies. Strong confinement as well as an about 25-fold field enhancement (compared to the incident field) are essentially caused by the lightning-rod effect [30]. We note that because of the short tip length (L
