1. Theory

Definition: A scanning spreading resistance microscope is a contact-mode AFM in which a hard conductive tip is pressed on a biased semiconductor and the current through the nanocontact gives a local resistance, R = V/I, at each point (Park; Nxumalo).
Spreading resistance: For a flat circular contact of radius a, the resistance concentrated near the contact is $R_{s} = ρ / (4 a)$ , so the map reflects local resistivity and hence carrier concentration (Park; Vandervorst; background knowledge, Schroder).
Nanocontact: Diamond tips at micronewton forces reach contact pressures that can transform silicon to the metallic beta-tin phase, giving a near-ohmic contact (Nxumalo; Mylvaganam et al.).
Quantification: Resistance is converted to concentration with calibration staircases measured with the same tip and force (Park; Nxumalo; background knowledge).

2. Working Principle

Probe: Boron-doped diamond tips, coated or solid, on stiff cantilevers resist wear at high force (Park; Bruker; Nxumalo).
Force: Micronewton loads break through native oxide and contamination (Park; Brinciotti et al.; Mylvaganam et al.).
Bias and detection: A DC bias, for example 1.5 V, is applied to the sample and a logarithmic current amplifier covers ohm to gigaohm resistances (Park).
Scanning: Topography and resistance are recorded together on polished cross-sections, in air, nitrogen or vacuum (Park; Mylvaganam et al.; background knowledge).

3. Applications

2-D carrier profiling: Junctions, source and drain extensions and wells in cross-sectioned transistors (Park; Vandervorst).
FinFETs and nanowires: Sidewall doping conformality and out-diffusion gradients in three-dimensional devices (Vandervorst).
Phase-transformation studies: In-situ resistance during indentation of silicon (Mylvaganam et al.).
Battery and composite materials: Mapping conductive and insulating phases in cathodes under vacuum (Park).

Spreading Resistance and Contact

Name	Formula	Related Formulae	Variables/Constants
Spreading resistance of a point contact	$R_{s} = \frac{ρ}{4 a}$	$R_{m e a s} = R_{t i p} + R_{c} + R_{s} + R_{ba c k}$	$R_{s} : s p r e a d in g r es i s t an ce$ $ρ : l oc a l r es i s t i v i t y$ $a : co n t a c t r a d i u s$ $R_{t i p} : p r o b e r es i s t an ce$ $R_{c} : ba r r i er or co n t a c t r es i s t an ce$ $R_{ba c k} : ba c k - co n t a c t an d ser i es r es i s t an ce; P a r k, V an d er v or s t an d N xu ma l o : R = ρ /4 a f or a f l a t c i r c u l a r co n t a c t; N xu ma l o : j u n c t i o n m o d e l l e d a s a d e pl e t i o n c a p a c i t or in p a r a l l e l w i t h a ba r r i er r es i s t or, in ser i es w i t h t h e s p r e a d in g r es i s t an ce; ser i es t er m s (ba c k g r o u n d k n o w l e d g e, S c h r o d er; S p r e a d in g R es i s t an ce P r o f i l er p a g e)$ $(a ss u m e d : a = 5 nm; co m p u t e d : ρ = 0.01 Ω cm g i v es 5 kΩ an d ρ = 10 Ω cm g i v es 5 MΩ)$
Resistivity and carrier concentration	$ρ = \frac{1}{q ( n μ _{n} + p μ _{p} )}$	$lo g R_{s} \approx - lo g N + const (mobility roughly constant)$	$n, p : c a r r i er co n ce n t r a t i o n s$ $μ_{n}, μ_{p} : m o bi l i t i es$ $N : a c t i v e d o p in g; d r i f t r es i s t i v i t y (ba c k g r o u n d k n o w l e d g e, S c h r o d er); P a r k : 2 - D c a r r i er p r o f i l in g f r o m l oc a l r es i s t an ce$ $(a ss u m e d : n = 1 \times 1 0^{18} c m^{- 3}, μ_{n} = 270 c m^{2} / (V s); co m p u t e d : ρ = 0.023 Ω cm an d R_{s} = 11.6 kΩ a t a = 5 nm)$
Contact pressure and phase transformation	$P = \frac{F}{π a ^{2}}$	$a = \frac{F}{π P}, P_{β - S n} \approx 8 to 11 GPa$	$P : m e an co n t a c t p r ess u r e$ $F : t i p f or ce; N xu ma l o : f or ces b e l o w ab o u t 20 μ N, p r ess u r es a r o u n d 3.5 GPa or l ess, ab o u t 1.5 GPa t o s t a y co n t a c t - d o mina t e d, ab o v e ab o u t 8 GPa w i t h s h e a r t r an s f or min g S i t o m e t a l l i c b e t a - t in an d g i v in g a n e a r - o hmi c co n t a c t; M y l v a g anam e t a l . : b e t a - t in m e t a l l i c, 20 p er ce n t v o l u m e r e d u c t i o n, am or p h o u s S i o n u n l o a d in g, S S R M l o a d ab o u t 3.3 μ N w i t h a 5 N/m d iam o n d p r o b e o f 5 t o 10 nm b o tt o m r a d i u s; h y d r os t a t i c t h r es h o l d ab o u t 11 GPa (ba c k g r o u n d k n o w l e d g e)$ $(co m p u t e d : 3.3 μ N a t 11 GPa n ee d s a = 9.8 nm, a t 8 GPa a = 11.5 nm; c an t i l e v er d e f l ec t i o n 660 nm)$

Current Detection and Calibration

Name	Formula	Related Formulae	Variables/Constants
Logarithmic current amplifier	$V_{o u t} = V_{d} lo g_{10} \frac{I}{I _{0}}$	$I = \frac{V _{b}}{R _{m e a s}}$	$V_{o u t} : am pl i f i er o u tp u t$ $V_{d} : v o l t s p er d ec a d e$ $I_{0} : r e f er e n ce c u r r e n t$ $V_{b} : s am pl e bia s; P a r k : l o g a r i t hmi c am pl i f i er r e q u i r e d t o co v er o hm t o g i g a o hm, 1.5 V bia s o n a c a l ib r a t i o n s am pl e; l o g - am pl i f i er f or m (ba c k g r o u n d k n o w l e d g e)$ $(a ss u m e d : V_{d} = 0.5 V p er d ec a d e; co m p u t e d : nin e d ec a d es s p an 4.5 V; a t 1.5 V, 10 kΩ g i v es 150 μ A an d 1 GΩ g i v es 1.5 nA)$
Calibration curve	$N (x, y) = f_{c a l}^{- 1} [R_{m e a s} (x, y)]$	$R_{m e a s} (N_{i}) from staircase layers N_{1} to N_{k}$	$f_{c a l} : m e a s u r e d r es i s t an ce a g ain s t k n o w n co n ce n t r a t i o n; P a r k : I F X N P 20 s am pl e w i t h 20 p h os p h or u s an d b or o n s t e p s a t 1.5 V; N xu ma l o : d iam o n d - t i p c a l ib r a t i o n c u r v es o f v o l t a g e f or f i x e d 1, 5 an d 10 nA c u r r e n t s a g ain s t s u b s t r a t e d o p in g, co n s t an t - c u r r e n t m o d e w i t h f o u r or d er s o f ma g ni t u d e a d v an t a g e o v er co n s t an t v o l t a g e; s am e - t i p, s am e - f or ce c a l ib r a t i o n (ba c k g r o u n d k n o w l e d g e)$ $(co m p u t e d : 20 s t e p s o v er ab o u t f i v e d ec a d es i s f o u r s t e p s p er d ec a d e)$
Tip resistance limit at high doping	$\frac{Δ R _{m e a s}}{R _{m e a s}} = \frac{Δ R _{s}}{R _{t i p} + R _{s}}$	$R_{t i p} \approx 10 to 1000 kΩ (diamond on Pt)$	$R_{t i p} : p r o b e r es i s t an ce; B r u k er : so l i d b or o n - d o p e d p o l y cr y s t a l l in e d iam o n d p y r ami d s m e a s u r in g 10 t o 1000 kΩ o n pl a t in u m d e p e n d in g o n t i p r a d i u s; ser i es - se n s i t i v i t y r e l a t i o n (ba c k g r o u n d k n o w l e d g e)$ $(a ss u m e d : R_{t i p} = 10 kΩ, R_{s} = 5 kΩ; co m p u t e d : d o u b l in g ρ r ai ses R_{m} e a s b y a f a c t or o f 1.33 in s t e a d o f 2)$

Resolution and Applications

Name	Formula	Related Formulae	Variables/Constants
Lateral resolution	$d_{S S R M} \approx 2 a$	$a \approx 1 to 10 nm (diamond nanocontact)$	$d_{S S R M} : l a t er a l r eso l u t i o n; P a r k : r eso l u t i o n se t b y a p e x r a d i u s, co a t e d t i p s a f e w t e n s o f nm, S R P p r o b es ab o u t 1 μ m; N xu ma l o : m e t a l t i p s 30 nm co n t a c t an d 35 nm r eso l u t i o n, 20 nm w i t h b or o n - d o p e d d iam o n d; B r u k er : e l ec t r i c a l r eso l u t i o n b e l o w 1 nm, 0.5 nm o x i d e q u a l i f i c a t i o n s t r u c t u r e; H an t sc h e l e t a l . : s u b - nan o m e t r e e l ec t r i c a l r eso l u t i o n (t i tl e); nan oco n t a c t p r o t r u s i o n s (m y r e a d in g)$ $(co m p u t e d : t h e r a t i o f r o m a 1 μ m S R P p r o b e t o a 1 nm S S R M co n t a c t i s 1000)$
Force regime against other modes	$F_{S S R M} \sim 1 μ N, F_{S M M} \sim 1 nN$	$\frac{F _{S S R M}}{F _{S M M}} \sim 1 0^{3}$	$F : co n t a c t f or ce; B r in c i o tt i e t a l . : S M M u ses n N f or ces, t h r ee or d er s o f ma g ni t u d e b e l o w t h e u N f or ces o f S S R M, an d m e t a l in s t e a d o f d o p e d - d iam o n d t i p s; P a r k : hi g h f or ce t o b r e ak o x i d e an d co n t amina t i o n, s t i f f l e v er an d r i g i d t i p; M y l v a g anam : S S R M in d e n t a t i o n l o a d s 0.7 t o 4.3 μ N in a ni t r o g e n g l o v e b o x$ $(co m p u t e d : 4.3 o v er 0.7 μ N i s a f a c t or o f 6.1)$
Dopant gradient in nanostructures	$λ = \frac{Δ x}{Δ l o g _{10} N}$	$active fraction = \frac{N _{S S R M}}{N _{c h e m}}$	$λ : p r o f i l e s t ee p n ess$ $N_{c h e m} : c h e mi c a l co n ce n t r a t i o n f r o m S I M S or A P T; V an d er v or s t : nan o w i r e o u t - d i f f u s i o n g r a d i e n t s o f ab o u t 40 t o 100 nm/decade f or 100 t o 400 nm d iam e t er s, c a r r i er s f r o m 1 \times 1 0^{15} t o 1 \times 1 0^{21} c m^{- 3}, F in F E T w i t h 40 nm g a t e an d 16 nm f in co m p a r e d w i t h A P T an d S I M S a t 45 an d 10 d e g r ee t i l t$ $(co m p u t e d : a t 40 nm/decade a 16 nm f in s p an s 0.4 d ec a d es)$

Important Questions

Ques: What does SSRM measure and how does it relate to doping?

It measures the resistance of a nanoscale point contact, dominated by spreading resistance in the semiconductor. A biased diamond tip contacts the surface and the current gives $R = V / I$ (Park). For a flat circular contact of radius a, $R_{s} = ρ / (4 a)$ (Park; Vandervorst; Nxumalo), and resistivity is $ρ = 1/ (q n μ_{n})$ for n-type material. With a 5 $nm$ contact, 0.01 $Ω cm$ gives 5 $kΩ$ and 10 $Ω cm$ gives 5 $MΩ$ (assumed radius; computed). At $1 \times 1 0^{18}$ $c m^{- 3}$ and 270 $c m^{2} / (V s)$ , $ρ$ is 0.023 $Ω cm$ and $R_{s}$ about 11.6 $kΩ$ (assumed mobility; computed). Because mobility changes more slowly than concentration, log resistance falls almost linearly with log doping, the basis of the calibration curve (background knowledge, Schroder). Reference: Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary); W. Vandervorst, ‘Metrology at Imec’, IMEC slides 2010, hosted by NIST (fetch-tool summary; partly garbled); J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places); D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed., Wiley 2006 (background knowledge; not opened).

Ques: Why does SSRM need such high tip force?

To break through oxide and to form a stable, low-barrier nanocontact. Park notes that SSRM usually needs a high contact force to penetrate oxide and contamination, with a stiff lever and rigid tip. Mylvaganam et al. used a 5 $N/m$ diamond probe at about 3.3 $μ N$ , which deflects the lever by 660 $nm$ (computed), and loads of 0.7 to 4.3 $μ N$ in nitrogen. Nxumalo reports forces below about 20 $μ N$ and pressures around 3.5 $GPa$ or less; at about 1.5 $GPa$ the barrier contact resistance dominates rather than spreading resistance. Higher pressure transforms silicon under the tip to metallic beta-tin, so the barrier collapses and the measurement tracks spreading resistance (Nxumalo; Mylvaganam et al.). Reference: Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary); K. Mylvaganam, L. C. Zhang, P. Eyben, J. Mody and W. Vandervorst, ‘Evolution of metastable phases in silicon during nanoindentation’, Nanotechnology 20, 305705 (2009) (fetch-tool summary); J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places).

Ques: At what pressure does silicon transform to beta-tin under the tip?

Nxumalo gives above about 8 $GPa$ in the presence of shear stress, whereas the hydrostatic diamond-anvil value is about 11 $GPa$ (background knowledge); Mylvaganam et al. cite the anvil work only for its 20 per cent volume reduction. The values are not strictly in conflict: shear under a sliding tip lowers the transformation threshold, so the 8 $GPa$ figure is more likely for SSRM and the 11 $GPa$ figure for hydrostatic loading (my reading). For 3.3 $μ N$ , a mean pressure of 11 $GPa$ implies a contact radius of 9.8 $nm$ and 8 $GPa$ implies 11.5 $nm$ (computed), close to the 5 to 10 $nm$ tip radius Mylvaganam measured. On unloading, beta-tin becomes amorphous, which can leave a modified surface track (Mylvaganam et al.). Reference: J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places); K. Mylvaganam, L. C. Zhang, P. Eyben, J. Mody and W. Vandervorst, ‘Evolution of metastable phases in silicon during nanoindentation’, Nanotechnology 20, 305705 (2009) (fetch-tool summary).

Ques: Why does SSRM use a logarithmic current amplifier?

Because local resistance spans many decades. Park states that a logarithmic amplifier is needed to cover resistances from ohm to gigaohm in device materials, about nine decades (computed). At 1.5 V bias, a 10 $kΩ$ path gives 150 $μ A$ and a 1 $GΩ$ path gives 1.5 $nA$ (computed), a range that a linear amplifier cannot digitize with useful resolution at both ends. A log amplifier gives $V_{o u t} = V_{d} lo g_{10} (I / I_{0})$ , so equal resistance ratios occupy equal voltage steps; at an assumed 0.5 V per decade, nine decades span 4.5 V (computed). Nxumalo describes a logarithmic converter in constant-voltage mode, and notes constant-current imaging offers a four-order-of-magnitude advantage for dopant measurements over constant voltage. Reference: Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary); J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places).

Ques: What lateral resolution can SSRM reach?

From tens of nanometres in early work to about 1 $nm$ today. Nxumalo reports contact radii as small as about 30 $nm$ and resolution of about 35 $nm$ with metal tips, and 20 $nm$ after changing to boron-doped diamond to solve tip wear. Bruker states that electrical resolution of its solid diamond probes can be below 1 $nm$ on a buried oxide sample in an optimized environment, with a 0.5 $nm$ oxide qualification structure, and Hantschel et al. report sub-nanometre electrical resolution in their title. Park attributes resolution to the apex radius. A 1 $μ m$ SRP probe compared with a 1 $nm$ SSRM contact is a factor of 1000 (computed). Sub-nanometre values likely come from small protrusions on the diamond apex carrying the current (my reading). Reference: J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places); Bruker AFM Probes, ‘SSRM-HR’ diamond probe product page (fetch-tool summary); T. Hantschel et al., ‘Conductive diamond tips with sub-nanometer electrical resolution for characterization of nanoelectronics device structures’, Phys. Status Solidi A 206, 2077 (2009) (imec record metadata only); Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary).

Ques: How does tip resistance limit sensitivity at high doping?

It adds a series term that dilutes changes in spreading resistance. Bruker gives 10 to 1000 $kΩ$ for its diamond tips on platinum, depending on radius. The measured value is $R_{m e a s} = R_{t i p} + R_{c} + R_{s} + R_{ba c k}$ (background knowledge, Schroder), so when $R_{s}$ is small the tip dominates. With an assumed 10 $kΩ$ tip and 5 $kΩ$ spreading resistance, doubling resistivity raises the measured value by only a factor of 1.33 rather than 2 (computed). Highly doped regions near $1 \times 1 0^{20}$ $c m^{- 3}$ therefore compress in the map unless the tip is calibrated on the same staircase (background knowledge). The resistance range in a Vandervorst slide comparing metal and diamond tips is garbled in the extracted text and is not used here (my reading). Reference: Bruker AFM Probes, ‘SSRM-HR’ diamond probe product page (fetch-tool summary); D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed., Wiley 2006 (background knowledge; not opened); W. Vandervorst, ‘Metrology at Imec’, IMEC slides 2010, hosted by NIST (fetch-tool summary; partly garbled).

Ques: How is SSRM calibrated to carrier concentration?

With staircase samples of known doping measured under the same conditions. Park describes the IFX NP20 sample, with 20 phosphorus and boron steps, measured at 1.5 V bias; 20 steps over about five decades gives roughly four per decade (computed). Nxumalo shows diamond-tip calibration curves of voltage needed for fixed 1, 5 and 10 $nA$ currents against substrate doping for p- and n-type silicon, so the curve differs by carrier type. Because $R_{s} = ρ / (4 a)$ depends on contact radius, tip wear or force changes move the curve, so calibration is repeated with the same tip and load and checked before and after the device scan (background knowledge). The inverse curve, $N = f_{c a l}^{- 1} (R)$ , then converts each pixel to concentration. Reference: Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary); J. N. Nxumalo, ‘Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts’, PhD thesis, University of Manitoba (1998) (fetch-tool summary; OCR-garbled in places); D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed., Wiley 2006 (background knowledge; not opened).

Ques: How does SSRM compare with SCM and scanning microwave microscopy?

SSRM gives monotonic resistance with larger dynamic range but needs far more force. Brinciotti et al. state that quantitative scanning microwave microscopy works at nanonewton forces, three orders of magnitude below the micronewton forces typical of SSRM, and can use metal instead of doped-diamond tips. SCM measures dC/dV through a thin oxide, needs no high pressure but has a non-monotonic response and only about a factor of 2 signal change over four decades (SCM page), whereas SSRM resistance spans about nine decades (Park; computed). SSRM resolves about 1 $nm$ with diamond (Bruker), better than the roughly 20 $nm$ apex limit of SCM (SCM page). SSRM damages the surface and is unsuitable for soft materials (my reading). Reference: E. Brinciotti et al., ‘Probing resistivity and doping concentration of semiconductors at the nanoscale using scanning microwave microscopy’, Nanoscale 7, 14715 (2015) (fetch-tool summary); Park Systems, ‘Scanning Spreading Resistance Microscopy (SSRM)’, ModeNote (fetch-tool summary); Bruker AFM Probes, ‘SSRM-HR’ diamond probe product page (fetch-tool summary).

Ques: What can SSRM reveal in FinFETs and nanowires?

Active dopant distribution in three-dimensional structures. Vandervorst shows SSRM of FinFETs with a 40 $nm$ gate and 16 $nm$ fins, comparing sidewall and top doping with APT, SIMS and a model for 45 and 10 degree implant tilts; the extracted table lists SSRM values of 78 and 29 per cent beside APT values of 39 and 12.5, but its column meaning is ambiguous in the extracted text, so the numbers are given only as reported (my reading). For nanowire tunnel FETs of 100 to 400 $nm$ diameter, SSRM gave out-diffusion gradients of about 40 to 100 $nm/decade$ with carriers from $1 \times 1 0^{15}$ to $1 \times 1 0^{21}$ $c m^{- 3}$ (Vandervorst). At 40 $nm/decade$ , a 16 $nm$ fin spans 0.4 decades (computed), so nanometre resolution is essential. Reference: W. Vandervorst, ‘Metrology at Imec’, IMEC slides 2010, hosted by NIST (fetch-tool summary; partly garbled).

References

Park Systems, “Scanning Spreading Resistance Microscopy (SSRM),” ModeNote. Available: https://www.parksystems.com/content/dam/parksystems/product/research-afm/afmmodes/ssrm/ModeNote_SSRM_Park_Systems.pdf
W. Vandervorst, “Metrology at Imec: a Centre of Excellence Enabling Fundamental Understanding of Process and Materials development,” IMEC presentation, 2010. Available: https://www.nist.gov/document/vandervorst2011pdf
J. N. Nxumalo, “Cross-sectional Imaging of Semiconductor Devices using Nanometer Scale Point Contacts,” PhD thesis, University of Manitoba, 1998. Available: https://mspace.lib.umanitoba.ca/bitstreams/ee380cd1-3dd9-4499-b176-cfccced64036/download
D. K. Schroder, Semiconductor Material and Device Characterization, 3rd ed. Hoboken, NJ: Wiley, 2006 (cited from background knowledge).
K. Mylvaganam, L. C. Zhang, P. Eyben, J. Mody and W. Vandervorst, “Evolution of metastable phases in silicon during nanoindentation: mechanism analysis and experimental verification,” Nanotechnology, vol. 20, 305705, 2009. Available: https://www.unsw.edu.au/content/dam/pdfs/engineering/precision-and-nano/20250-05-evolution-of-the-metastable-phases-in-silicon-during-nanoindentation.pdf
Bruker AFM Probes, “SSRM-HR,” product page. Available: https://www.brukerafmprobes.com/t-NEW-SSRM-HR.aspx
T. Hantschel, C. Demeulemeester, P. Eyben, V. Schulz, O. Richard, H. Bender and W. Vandervorst, “Conductive diamond tips with sub-nanometer electrical resolution for characterization of nanoelectronics device structures,” physica status solidi (a), vol. 206, no. 9, pp. 2077-2081, 2009 (imec record metadata only). Available: https://imec-publications.be/entities/publication/e4958b7c-037c-411b-a352-f2e5044cc66f/full
E. Brinciotti, G. Gramse, S. Hommel, T. Schweinboeck, A. Altes, M. A. Fenner, J. Smoliner, M. Kasper, G. Badino, S.-S. Tuca and F. Kienberger, “Probing resistivity and doping concentration of semiconductors at the nanoscale using scanning microwave microscopy,” Nanoscale, vol. 7, pp. 14715-14722, 2015. Available: https://pubs.rsc.org/en/content/articlepdf/2015/nr/c5nr04264j

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Scanning Spreading Resistance Microscope (SSRM)