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Conductive atomic force microscopy
In microscopy, conductive atomic force microscopy (C-AFM) or current sensing atomic force microscopy (CS-AFM) is a mode in atomic force microscopy (AFM) that simultaneously measures the topography of a material and the electric current flow at the contact point of the tip with the surface of the sample. The topography is measured by detecting the deflection of the cantilever using an optical system (laser + photodiode), while the current is detected using a current-to-voltage preamplifier. The fact that the CAFM uses two different detection systems (optical for the topography and preamplifier for the current) is a strong advantage compared to scanning tunneling microscopy (STM). Basically, in STM the topography picture is constructed based on the current flowing between the tip and the sample (the distance can be calculated depending on the current). Therefore, when a portion of a sample is scanned with an STM, it is not possible to discern if the current fluctuations are related to a change in the topography (due to surface roughness) or to a change in the sample conductivity (due to intrinsic inhomogeneities).
The CAFM is usually operated in contact mode; the tip can be kept at one location while the voltage and current signals are applied/read, or it can be moved to scan a specific region of the sample under a constant voltage (and the current is collected). Recently, some manufacturers provide the option of measuring the current in semi-contact mode. The CAFM was first developed by Sean O'Shea and co-workers at the University of Cambridge in 1993, and it is referred to in the literature by several names, including C-AFM, local-conductivity AFM (LC-AFM), conductive probe AFM (CP-AFM), conductive scanning probe microscopy (C-SPM) or conductive scanning force microscopy (C-SFM), although CAFM is the most widespread.
In order to transform an AFM into a CAFM, three elements are required: i) the probe tip must be conductive, ii) a voltage source is needed to apply a potential difference between the tip and the sample holder, and iii) a preamplifier is used to convert the (analogical) current signal into (digital) voltages that can be read by the computer. In CAFM experiments, the sample is usually fixed on the sample holder using a conductive tape or paste, being silver paints the most widespread. A Faraday cage is also convenient to isolate the sample from any external electrical interference. Using this setup, when a potential difference is imposed between tip and sample an electrical field is generated, which results in a net current flowing from tip-to-sample or vice versa. The currents collected by the CAFM obey the relationship:
where I is the total current flowing through the tip/sample nanojunction, J is the current density and Aeff is the effective emission area through which electrons can flow (from now on we will refer to it just as effective area). The most common mistake in CAFM research is to assume that the effective emission area (Aeff) equals the physical contact area (Ac). Strictly, this assumption is erroneous because in many different tip/sample systems the electrical field applied may propagate laterally. For example, when the CAFM tip is placed on a metal the lateral conductivity of the sample is very high, making (in principle) the whole sample surface area electrically connected (Aeff equals the area covered by the metallic film/electrode). Aeff has been defined as:"the sum of all those infinitesimal spatial locations on the surface of the sample that are electrically connected to the CAFM tip (the potential difference is negligible). As such, Aeff is a virtual entity that summarizes all electrically relevant effects within the tip/sample contact system into a single value, over which the current density is assumed to be constant." Therefore, when the CAFM tip is placed in contact with a metal (a metallic sample or just a metallic pad on an insulator), the lateral conductivity of the metal is very high, and the CAFM tip can be understood as a current collector (nanosized probe station); on the contrary, if the CAFM tip is placed directly on an insulator, it acts as a nanosized electrode and provides a very high lateral resolution. The value of Aeff when a Pt-Ir coated tip (with a typical radius of 20 nm) is placed on a SiO2 insulating film has been calculated to be typically 50 nm2. The value of Aeff can fluctuate depending on the environmental conditions, and it can range from 1 nm2 in ultra high vacuum (UHV) to 300 nm2 in very humid environments. On well-defined single crystal surfaces under UHV conditions it has even been demonstrated that measurements of the local conductivity with atomic resolution are possible.
Common problems in conventional CAFM include difficulty in managing high and low currents and avoiding unwanted side-effects such as the Joule, Bimetallic and local oxidation effects when using high currents. In order to produce accurate and reproducible measurements, frequent tip replacements and repeated experimental setup can be required. A book edited by Mario Lanza discusses these issues (Chapter 12, Pacheco and Martinez. 2017). This chapter, written by employees of AFM manufacturer Concept Scientific Instruments, claims that their company's module named ResiScope overcomes these issues, and provide supporting data, without any supporting peer-reviewed publication.
In order to overcome the narrow dynamic range limitations of conventional preamplifiers, advanced modules such as the ResiScope have been developed. The ResiScope allows for resistance measurements across a wide dynamic range — spanning over ten decades — from approximately 10² Ω to 10¹² Ω, while maintaining high spatial resolution. This capability enables the detailed study of materials with both low and high conductivity on the nanoscale.
CAFM was initially used in the field of nanoelectronics to monitor the electrical properties of thin dielectrics with very high lateral resolution. The first CAFM development in 1993 had the goal of studying the local tunneling currents through 12 nm thick SiO2 films. In 1995 and 1996, O'Shea and Ruskell further improved the lateral resolution of the CAFM technique, achieving values of 10 nm and 8 nm, respectively. This enhanced resolution allowed to observe the first topographic-current correlations, and the inhomogeneity observed in the current maps was associated to the presence of local native defects in the oxide. Following works by Olbrich and Ebersberger reported that, in SiO2 films thinner than 5 nm, the tunneling current increases exponentially with thickness reductions. Consequently, thickness fluctuations of tenths of nanometer in the SiO2 film could create electrically weak spots that reduce the reliability of the whole dielectric film, as the dielectric breakdown (BD) is a stochastic process. The capability of the CAFM for determining the thickness of thin oxides was further demonstrated by Frammelsberger and co-workers who statistically analyzed more than 7200 I-V curves, and reported SiO2 thicknesses with a sensitivity of ±0.3 nm. Other local phenomena like charge trapping, trap assisted tunneling and stress induced leakage current (SILC) can be also easily monitored with CAFM. In general, the CAFM can monitor the effect of any process that introduces local changes in the structure of the dielectric, including thermal annealing, dopping and irradiation, among others.
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Conductive atomic force microscopy
In microscopy, conductive atomic force microscopy (C-AFM) or current sensing atomic force microscopy (CS-AFM) is a mode in atomic force microscopy (AFM) that simultaneously measures the topography of a material and the electric current flow at the contact point of the tip with the surface of the sample. The topography is measured by detecting the deflection of the cantilever using an optical system (laser + photodiode), while the current is detected using a current-to-voltage preamplifier. The fact that the CAFM uses two different detection systems (optical for the topography and preamplifier for the current) is a strong advantage compared to scanning tunneling microscopy (STM). Basically, in STM the topography picture is constructed based on the current flowing between the tip and the sample (the distance can be calculated depending on the current). Therefore, when a portion of a sample is scanned with an STM, it is not possible to discern if the current fluctuations are related to a change in the topography (due to surface roughness) or to a change in the sample conductivity (due to intrinsic inhomogeneities).
The CAFM is usually operated in contact mode; the tip can be kept at one location while the voltage and current signals are applied/read, or it can be moved to scan a specific region of the sample under a constant voltage (and the current is collected). Recently, some manufacturers provide the option of measuring the current in semi-contact mode. The CAFM was first developed by Sean O'Shea and co-workers at the University of Cambridge in 1993, and it is referred to in the literature by several names, including C-AFM, local-conductivity AFM (LC-AFM), conductive probe AFM (CP-AFM), conductive scanning probe microscopy (C-SPM) or conductive scanning force microscopy (C-SFM), although CAFM is the most widespread.
In order to transform an AFM into a CAFM, three elements are required: i) the probe tip must be conductive, ii) a voltage source is needed to apply a potential difference between the tip and the sample holder, and iii) a preamplifier is used to convert the (analogical) current signal into (digital) voltages that can be read by the computer. In CAFM experiments, the sample is usually fixed on the sample holder using a conductive tape or paste, being silver paints the most widespread. A Faraday cage is also convenient to isolate the sample from any external electrical interference. Using this setup, when a potential difference is imposed between tip and sample an electrical field is generated, which results in a net current flowing from tip-to-sample or vice versa. The currents collected by the CAFM obey the relationship:
where I is the total current flowing through the tip/sample nanojunction, J is the current density and Aeff is the effective emission area through which electrons can flow (from now on we will refer to it just as effective area). The most common mistake in CAFM research is to assume that the effective emission area (Aeff) equals the physical contact area (Ac). Strictly, this assumption is erroneous because in many different tip/sample systems the electrical field applied may propagate laterally. For example, when the CAFM tip is placed on a metal the lateral conductivity of the sample is very high, making (in principle) the whole sample surface area electrically connected (Aeff equals the area covered by the metallic film/electrode). Aeff has been defined as:"the sum of all those infinitesimal spatial locations on the surface of the sample that are electrically connected to the CAFM tip (the potential difference is negligible). As such, Aeff is a virtual entity that summarizes all electrically relevant effects within the tip/sample contact system into a single value, over which the current density is assumed to be constant." Therefore, when the CAFM tip is placed in contact with a metal (a metallic sample or just a metallic pad on an insulator), the lateral conductivity of the metal is very high, and the CAFM tip can be understood as a current collector (nanosized probe station); on the contrary, if the CAFM tip is placed directly on an insulator, it acts as a nanosized electrode and provides a very high lateral resolution. The value of Aeff when a Pt-Ir coated tip (with a typical radius of 20 nm) is placed on a SiO2 insulating film has been calculated to be typically 50 nm2. The value of Aeff can fluctuate depending on the environmental conditions, and it can range from 1 nm2 in ultra high vacuum (UHV) to 300 nm2 in very humid environments. On well-defined single crystal surfaces under UHV conditions it has even been demonstrated that measurements of the local conductivity with atomic resolution are possible.
Common problems in conventional CAFM include difficulty in managing high and low currents and avoiding unwanted side-effects such as the Joule, Bimetallic and local oxidation effects when using high currents. In order to produce accurate and reproducible measurements, frequent tip replacements and repeated experimental setup can be required. A book edited by Mario Lanza discusses these issues (Chapter 12, Pacheco and Martinez. 2017). This chapter, written by employees of AFM manufacturer Concept Scientific Instruments, claims that their company's module named ResiScope overcomes these issues, and provide supporting data, without any supporting peer-reviewed publication.
In order to overcome the narrow dynamic range limitations of conventional preamplifiers, advanced modules such as the ResiScope have been developed. The ResiScope allows for resistance measurements across a wide dynamic range — spanning over ten decades — from approximately 10² Ω to 10¹² Ω, while maintaining high spatial resolution. This capability enables the detailed study of materials with both low and high conductivity on the nanoscale.
CAFM was initially used in the field of nanoelectronics to monitor the electrical properties of thin dielectrics with very high lateral resolution. The first CAFM development in 1993 had the goal of studying the local tunneling currents through 12 nm thick SiO2 films. In 1995 and 1996, O'Shea and Ruskell further improved the lateral resolution of the CAFM technique, achieving values of 10 nm and 8 nm, respectively. This enhanced resolution allowed to observe the first topographic-current correlations, and the inhomogeneity observed in the current maps was associated to the presence of local native defects in the oxide. Following works by Olbrich and Ebersberger reported that, in SiO2 films thinner than 5 nm, the tunneling current increases exponentially with thickness reductions. Consequently, thickness fluctuations of tenths of nanometer in the SiO2 film could create electrically weak spots that reduce the reliability of the whole dielectric film, as the dielectric breakdown (BD) is a stochastic process. The capability of the CAFM for determining the thickness of thin oxides was further demonstrated by Frammelsberger and co-workers who statistically analyzed more than 7200 I-V curves, and reported SiO2 thicknesses with a sensitivity of ±0.3 nm. Other local phenomena like charge trapping, trap assisted tunneling and stress induced leakage current (SILC) can be also easily monitored with CAFM. In general, the CAFM can monitor the effect of any process that introduces local changes in the structure of the dielectric, including thermal annealing, dopping and irradiation, among others.