Demystifying Semiconductor Chip Inspection: A Comprehensive Comparison of SSRM and SCM

Today, as the semiconductor process has entered the 14nm or even 7nm/5nm node, the emergence of advanced structures such as FinFET and GAA has posed unprecedented challenges to the accurate measurement of the doped area inside the chip. In the detection of two-dimensional carrier concentration distribution (2D Carrier Profiling), scanning capacitance microscopy (SCM) and scanning spread resistance microscopy (SSRM) are the two most mainstream technologies. How should we choose for various R&D and failure analysis needs? This article will give you an in-depth breakdown of the physical mechanism and actual implementation difficulties.[1], [2]

The core practical advantages of SSRM compared to SCM

Although both SCM and SSRM can characterize microscopic electrical properties, they have significant differences in spatial resolution and quantitative analysis capabilities.[2]

Differences in spatial resolution mechanisms

The resolution of SCM is mainly limited by the depletion layer width of the semiconductor. As the process node shrinks below 14nm, the depletion layers of adjacent doped regions are prone to overlap, leading to signal crosstalk, and it is difficult for resolution to break through the bottleneck of 10-20nm. In contrast, the resolution of SSRM only depends on the contact patch radius of the conductive probe with the sample. By applying a high contact force on the nanonewton (nN) level, SSRM can compress the contact area to within 10nm. As long as the surface is properly treated, it is currently the highest-resolution atomic force microscope (AFM) method for 7nm/5nm node doping distribution mapping.[1], [2], [3]

Quantitative capability versus dynamic range

There is a serious nonlinear relationship between the measurement signal of SCM (dC/dV) and the carrier concentration. The signal in extremely highly doped (greater than 10^20 cm^-3) or extremely low doped regions is very weak, which makes it difficult for SCM to directly infer the absolute doping concentration and can usually only be used for qualitative comparison between processes. The SSRM measures the extended resistance, and its logarithmic value and the logarithmic value of the carrier concentration (Log(R) and Log(N)) show a highly linear relationship within a very large range (10^15 to 10^21 cm^-3). This wide and linear dynamic range can directly output a quantitative carrier concentration distribution curve, which has extremely high practical value for calibrating TCAD simulation models.[3]

Different sensitivities to surface states

SCM is extremely sensitive to the oxide thickness and surface state density on the surface, and it is easy for the capacitance signal to drift directly due to surface contamination. Instead, the core of SSRM lies in forming ohmic contacts. Although it is also affected by the surface state, through ultra-hard conductive probes and high contact force, it can directly penetrate the extremely thin natural oxide layer, effectively avoiding the formation of Schottky barrier, and thereby obtaining real internal electrical signals.[1], [3], [4]

Pushing the Limits: Hidden Thresholds in SSRM Sample Preparation

Although SSRM comprehensively wins in terms of data quality and resolution, its sample preparation is an extremely complex system engineering, and the fault tolerance rate is much lower than that of the relatively tolerant SCM. Preparing high-quality SSRM cross-section samples requires overcoming the following five core difficulties:

• Surface quality requirements are extremely high: The sample must reach atomic level flatness (surface roughness RMS is less than 0.5nm), because any surface undulations will directly change the contact area, thereby seriously affecting the measurement of contact resistance.[4]

• The surface damage layer is extremely controlled: Traditional mechanical polishing will inevitably introduce amorphous layers and residual stress, and the parameter window of chemical mechanical polishing (CMP) is extremely narrow. In addition, ion beam processing may also accidentally introduce additional doping or lattice damage.[4], [5]

• Surface state and oxide layer interference: The natural oxide layer that grows when exposed to air can easily form a Schottky barrier, hindering the formation of ohmic contact. Therefore, the final processing and measurement of samples often need to be completed quickly in an inert atmosphere or high vacuum to avoid interference of carrier injection by surface state density.[3], [4]

• Precise construction of ohmic contacts: For different doping types and concentrations, the contact conditions between the probe and the sample need to be finely adjusted. Especially in lightly doped regions, maintaining a stable ohmic contact is extremely challenging.[1]

• Structural peculiarities of cross-sectional samples: Chip cross-sections usually contain a variety of different materials (such as silicon, oxide, metal), and their polishing rates are different, which can easily produce step effects. At the same time, shedding or contamination is prone to occur at heterogeneous interfaces, which places extremely high requirements on the collaborative processing of multi-material surfaces.

As a professional semiconductor testing service provider, we not only have deep accumulation of SSRM sample preparation technology, but also can stably overcome key problems in the flatness, damage layer control, surface oxide layer treatment and ohmic contact construction of advanced device cross-sections. At the same time, we have also conducted exclusive in-depth research and development and optimization based on commercially available modules, forming a unique SSRM test module.

Through continuous improvements in system noise suppression, signal chain optimization, probe contact stability, and measurement control strategies, our modules have achieved significant improvements in test sensitivity, signal-to-noise ratio, and spatial resolution capabilities, and can more effectively support high-precision carrier distribution measurements at advanced process nodes.

Relying on the integrated capabilities of "high-quality sample preparation + self-developed high-performance modules", we are able to provide customers with more stable, more precise, and more quantitatively valuable SSRM test results, demonstrating professionalism in advanced semiconductor process research and development, device analysis, and failure diagnosis scenarios.

Demystifying Semiconductor Chip Inspection: A Comprehensive Comparison of SSRM and SCM

As semiconductor manufacturing processes advance to 14nm and dive deeper into 7nm/5nm nodes with complex structures like FinFET and GAA, the precise measurement of internal doping regions has become an unprecedented challenge. For 2D carrier profiling, Scanning Capacitance Microscopy (SCM) and Scanning Spreading Resistance Microscopy (SSRM) stand as the two most prominent analytical techniques. To help our clients and engineers make informed decisions for R&D and failure analysis, this article breaks down their physical mechanisms and practical application hurdles. [1], [2]

Core Practical Advantages of SSRM Over SCM

While both SCM and SSRM are capable of characterizing microscopic electrical properties, they exhibit significant differences in spatial resolution and quantitative analysis capabilities. [2]

Differences in Spatial Resolution Mechanisms

The spatial resolution of SCM is fundamentally limited by the depletion width of the semiconductor. As process nodes shrink below 14nm, the depletion layers of adjacent doped regions tend to overlap, causing signal crosstalk that makes it incredibly difficult to break the 10-20nm resolution barrier. In contrast, SSRM's resolution is solely dependent on the contact radius between the conductive tip and the sample. By applying a high contact force in the nano-Newton (nN) range, the contact area can be compressed to under 10nm. Provided the sample surface is properly prepared without damage, SSRM currently offers the highest spatial resolution among AFM-based techniques for mapping doping profiles at the 7nm and 5nm nodes. [1], [2], [3]

Quantitative Capability and Dynamic Range

SCM suffers from a severe non-linear relationship between its measurement signal (dC/dV) and carrier concentration. Its signal peaks at moderate doping levels but becomes extremely weak at both exceptionally high (greater than 10^20 cm^-3) and very low doping concentrations. This non-linearity makes absolute quantitative calibration highly difficult, restricting SCM primarily to qualitative comparisons between processes. SSRM, however, measures spreading resistance. The relationship between the logarithm of resistance and the logarithm of carrier concentration (Log(R) vs. Log(N)) exhibits strict linearity over a massive dynamic range spanning from spanning from 10^15 to 10^21 cm^-3. This allows SSRM to directly output quantitative carrier concentration curves, which are invaluable for calibrating TCAD simulation models. [3]

Varying Sensitivity to Surface Conditions

SCM is acutely sensitive to surface oxide thickness and surface state density; even minor surface contamination can lead directly to capacitance signal drift. Conversely, the core operational principle of SSRM is the formation of an Ohmic contact. While still affected by surface states, SSRM utilizes an ultra-hard conductive probe combined with high contact force to physically penetrate thin native oxide layers. This effectively prevents the formation of a Schottky barrier and ensures the extraction of genuine internal electrical signals. [1], [3], [4]

Pushing the Limits: The Hidden Challenges of SSRM Sample Preparation

Despite SSRM's comprehensive supremacy in data quality and resolution, preparing samples for it is a highly complex systems engineering task. The process is significantly more demanding and less forgiving than SCM preparation. Achieving high-quality cross-sectional samples for SSRM requires overcoming five primary hurdles:

• Exceptional Surface Quality: The sample must achieve atomic-level flatness with a root mean square (RMS) roughness of less than 0.5nm. Any topographic variation directly alters the contact area, thereby severely skewing the contact resistance measurement. [4]

• Strict Surface Damage Control: Standard mechanical polishing inevitably introduces amorphous layers and mechanical stress, while the parameter window for Chemical Mechanical Polishing (CMP) is extremely narrow. Furthermore, ion beam milling risks introducing unintended doping or crystal lattice damage.static. [4], [5]

• Native Oxide and Surface States: The natural oxide layer that forms upon exposure to air creates a Schottky barrier instead of the required Ohmic contact. Consequently, final surface treatments and subsequent measurements must often be executed rapidly in an inert atmosphere or high vacuum to prevent surface state density from interfering with carrier injection. [3], [4]

• Precise Ohmic Contact Formation: Maintaining a stable Ohmic contact requires meticulous adjustments to the tip-sample contact force based on different doping types and concentrations. This is particularly challenging to stabilize in lightly doped regions. [1]

• Complexities of Cross-Sectional Structures: Semiconductor cross-sections consist of multiple materials (e.g., silicon, oxides, metals) with varying polishing rates, which frequently results in undesired topographic steps. Additionally, heterogeneous interfaces are prone to peeling or contamination, demanding extraordinary precision when processing multi-material surfaces simultaneously.

As a professional semiconductor testing service provider, we offer not only strong expertise in SSRM sample preparation—covering critical challenges such as surface flatness, damage-layer control, native oxide management, and reliable Ohmic contact formation—but also a uniquely optimized SSRM module developed in-house on top of existing commercial platforms.

Through continuous improvements in noise suppression, signal-chain optimization, tip-sample contact stability, and measurement control strategies, our module delivers enhanced sensitivity, higher signal-to-noise ratio, and superior spatial resolution for high-precision carrier profiling in advanced technology nodes.

With this integrated capability combining high-quality sample preparation and a high-performance self-developed module, we provide customers with more stable, more refined, and more quantitative SSRM results, demonstrating internationally leading performance in advanced semiconductor process development, device characterization, and failure analysis.

Reference

[1] Park Systems. Scanning Spreading Resistance Microscopy (SSRM). 2024-03-02. Available at: https://www.parksystems.com/kr/products/research-afm/AFM-modes/Electrical-Modes/scanning-spreading-resistance-microscopy--ssrm-

[2] AZoNano. SSRM and SCM for Carrier Profiling. 2025-04-15. Available at: https://www.azonano.com/article.aspx?ArticleID=5636

[3] Microscopy Today / Oxford Academic. Dual Lens Electron Holography, Scanning Capacitance Microscopy ... 2021-04-30. Available at: https://academic.oup.com/mt/article/29/3/36/6813671?login=false

[4] NanoScientific. Primer: The Advancements and Applications of Scanning ... Available at: https://nanoscientific.org/articles/view/423

[5] ASM International. Aug_EDFA_Digital. Available at: https://static.asminternational.org/EDFA/202308/55/