What advantages does transmission small-angle X-ray scattering provide for measuring deep semiconductor structures?

Transmission SAXS allows X-rays to pass through the silicon wafer and interrogate deep, high-aspect-ratio features that are difficult or impossible to characterize using grazing-incidence techniques. Unlike destructive methods such as SEM cross-sectioning or TEM, it preserves the sample while providing quantitative three-dimensional structural information across large wafer areas.

How does T-SAXS determine the critical dimension of deep holes?

The average critical dimension is derived from the periodicity of the scattering intensity in reciprocal space. As hole diameter changes, the spacing between scattering fringes changes in a predictable manner, allowing the average feature width to be calculated with high sensitivity through model-based analysis of the diffraction pattern.

How are hole depth, sidewall angle, and tilt distinguished from one another?

Each structural parameter affects the scattering pattern differently. Hole depth changes the spacing of fringes in the out-of-plane scattering direction, sidewall angle alters the phase of the scattering intensity, and hole tilt shifts the symmetry of the scattering pattern. Because these signatures are distinct, all three parameters can be extracted simultaneously during fitting.

Why is wafer-scale measurement important for semiconductor manufacturing?

Measuring only a few isolated locations can miss systematic process variations. Wafer-scale measurements reveal trends such as changes in feature size, taper, or tilt from the center to the edge of the wafer, enabling engineers to identify non-uniform etching behavior and optimize manufacturing processes more effectively.

Can T-SAXS detect subtle process variations that affect device performance?

Yes. The method is sensitive to relatively small changes in critical dimension, sidewall angle, and feature inclination. It can also identify profile characteristics such as tapering and bowing, making it useful for monitoring process drift and evaluating the effects of etch process adjustments.

What types of semiconductor structures can benefit from T-SAXS measurements?

The technique is applicable to a wide range of high-aspect-ratio structures, including channel holes and slit structures in 3D NAND flash memory, capacitor holes in DRAM, trenches in CMOS image sensors, and other advanced three-dimensional device architectures that require precise dimensional control.

How does model-based analysis improve T-SAXS measurement accuracy?

Instead of assuming a simple feature geometry, the analysis divides the structure into multiple layers along its depth. Parameters such as critical dimension, center position, layer thickness, and sidewall characteristics are optimized using least-squares fitting until the calculated scattering pattern matches the measured data. This layered approach enables accurate reconstruction of complex three-dimensional feature profiles.

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Small-angle X-ray scattering shape metrology for 3D semiconductor devices

Takumi Goto

Winter 2025 Volume 41, No. 1 , 01-07

The etching technique for high-aspect-ratio hole structures is one of the key technologies in modern semiconductor device manufacturing. Accurately evaluating hole shapes is crucial for developing and controlling the etching process. In order to create a precise evaluation system for deep hole shapes, Rigaku has developed a transmission small-angle X-ray scattering (T-SAXS) instrument. In this technical note, we describe the principles of a small-angle X-ray scattering (SAXS) technique for the determination of three-dimensional semiconductor device structures and its measurement sensitivity based on simulation results. We also demonstrate its performance for SAXS metrology by the measurement of deep holes on a 300 mm wafer. As a result of these measurements we were able to obtain the distribution of deep hole sizes and their tilt across the entire wafer.

Highlights

Transmission small-angle X-ray scattering (T-SAXS) enables nondestructive, wafer-scale measurement of deep, high-aspect-ratio semiconductor structures that are inaccessible to conventional grazing-incidence SAXS methods.
The technique is highly sensitive to critical dimension (CD), hole depth, sidewall angle, and tilt, allowing accurate three-dimensional reconstruction of etched features.
Full-wafer mapping reveals systematic variations in hole diameter and tilt, providing process engineers with actionable information for optimizing deep-hole etching.

Summary

As semiconductor devices increasingly rely on three-dimensional architectures such as 3D NAND and advanced memory structures, precise characterization of deep, high-aspect-ratio features has become essential for process control. Transmission small-angle X-ray scattering (T-SAXS) provides a nondestructive method for measuring these structures by transmitting short-wavelength X-rays through the wafer rather than relying on grazing-incidence geometry. This approach allows X-rays to probe holes several micrometers deep while maintaining the ability to measure across an entire 300 mm wafer.

The technique analyzes scattering intensity in reciprocal space to extract multiple structural parameters. Variations in the scattering pattern along the in-plane direction are primarily sensitive to average critical dimension, while changes in the out-of-plane direction provide information about hole depth, sidewall profile, and feature tilt. Simulation results demonstrate that each parameter produces a distinct and measurable change in the scattering signal, allowing simultaneous reconstruction of complex three-dimensional geometries through model-based fitting.

Measurements performed on etched deep-hole arrays confirmed the capability of T-SAXS to quantify detailed structural profiles, including tapered sidewalls, bowing, and feature inclination. Comparisons between locations near the wafer center and edge showed measurable differences in average hole diameter and tilt, illustrating the technique's ability to detect process-induced variations across the wafer. Full-wafer mapping further revealed systematic radial trends in both critical dimension and hole inclination.

Because the measurements are nondestructive and provide statistically meaningful information over large areas, T-SAXS is well suited for semiconductor process development and manufacturing. The approach offers a practical means of monitoring etching uniformity, evaluating complex three-dimensional structures, and improving process control for advanced semiconductor devices.

Frequently asked questions

: Transmission SAXS allows X-rays to pass through the silicon wafer and interrogate deep, high-aspect-ratio features that are difficult or impossible to characterize using grazing-incidence techniques. Unlike destructive methods such as SEM cross-sectioning or TEM, it preserves the sample while providing quantitative three-dimensional structural information across large wafer areas.
: The average critical dimension is derived from the periodicity of the scattering intensity in reciprocal space. As hole diameter changes, the spacing between scattering fringes changes in a predictable manner, allowing the average feature width to be calculated with high sensitivity through model-based analysis of the diffraction pattern.
: Each structural parameter affects the scattering pattern differently. Hole depth changes the spacing of fringes in the out-of-plane scattering direction, sidewall angle alters the phase of the scattering intensity, and hole tilt shifts the symmetry of the scattering pattern. Because these signatures are distinct, all three parameters can be extracted simultaneously during fitting.
: Measuring only a few isolated locations can miss systematic process variations. Wafer-scale measurements reveal trends such as changes in feature size, taper, or tilt from the center to the edge of the wafer, enabling engineers to identify non-uniform etching behavior and optimize manufacturing processes more effectively.
: Yes. The method is sensitive to relatively small changes in critical dimension, sidewall angle, and feature inclination. It can also identify profile characteristics such as tapering and bowing, making it useful for monitoring process drift and evaluating the effects of etch process adjustments.
: The technique is applicable to a wide range of high-aspect-ratio structures, including channel holes and slit structures in 3D NAND flash memory, capacitor holes in DRAM, trenches in CMOS image sensors, and other advanced three-dimensional device architectures that require precise dimensional control.
: Instead of assuming a simple feature geometry, the analysis divides the structure into multiple layers along its depth. Parameters such as critical dimension, center position, layer thickness, and sidewall characteristics are optimized using least-squares fitting until the calculated scattering pattern matches the measured data. This layered approach enables accurate reconstruction of complex three-dimensional feature profiles.

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