RBS Thin Film Thickness Measurement: Principles, Methods, and Applications

Rutherford Backscattering Spectrometry (RBS) is a powerful ion beam analysis technique for accurately determining thin film thickness, composition, and elemental depth profiles without requiring reference standards. This non-destructive method offers quantitative results with nanometer-scale depth resolution, making it indispensable for semiconductor, optical, and coating industries. In this article, we explore the principles, sample types, testing procedures, and practical applications of RBS thin film thickness measurement.

Technical Overview

Rutherford Backscattering Spectrometry (RBS) is a quantitative surface and thin film analysis technique in which a beam of energetic ions, typically helium ions (He+ or He2+) accelerated to energies between 1.0 and 3.0 MeV, is directed at a sample. A small fraction of these ions undergo elastic collisions with the atomic nuclei in the specimen and scatter backward. The energy spectrum of these backscattered ions is collected by a silicon surface barrier detector positioned typically at a scattering angle of 160 to 170 degrees.

The fundamental physics governing RBS relies on two key principles. First, the kinematic factor, determined by classical two-body elastic scattering mechanics, relates the energy of the backscattered ion to the mass of the target atom. Heavier atoms cause smaller energy losses upon backscattering, allowing elemental identification. Second, the continuous energy loss of ions traveling through the material, described by stopping power coefficients, provides depth information. Ions scattered from atoms located deeper in the film lose more energy both before and after the collision event.

For thin film thickness measurement, the width of the elemental peak in the RBS spectrum is directly proportional to the areal density of atoms in the film, expressed in atoms per square centimeter. By dividing the measured areal density by the known or assumed film density, the physical thickness in nanometers can be calculated. A major advantage of RBS is that the scattering cross-sections are well known from Rutherford's formula, enabling standardless quantitative analysis with a typical accuracy of 3 to 5 percent.

RBS provides a depth resolution of approximately 10 to 20 nanometers near the surface and can probe films ranging from a few nanometers up to several micrometers in thickness. It is particularly sensitive to heavy elements in light matrices, achieving detection limits in the parts-per-million range for high-Z elements, while light elements such as oxygen, nitrogen, and carbon are more challenging to quantify in heavy-element matrices.

Testing Samples

Semiconductor Thin Films

Silicon-based dielectric films such as SiO2, Si3N4, and SiON are routinely analyzed by RBS to verify thickness uniformity and stoichiometry. High-k dielectric materials including HfO2, ZrO2, Al2O3, and their silicates are measured for both thickness and elemental ratios, which are critical parameters in gate stack engineering for advanced CMOS devices.

Metal and Barrier Layers

Metallic films used in interconnect and metallization schemes, including Ti, TiN, Ta, TaN, W, Al, and Cu, are characterized for thickness and impurity content. Diffusion barrier layers in the nanometer thickness range can be accurately profiled, and interfacial reactions or intermixing between layers can be detected and quantified.

Compound Semiconductor Films

Epitaxial and polycrystalline compound semiconductor layers such as GaAs, AlGaAs, InGaN, and SiGe are analyzed for composition gradient and layer thickness. RBS combined with ion channeling can additionally evaluate crystalline quality and lattice site occupancy of dopant atoms.

Optical and Protective Coatings

Multilayer optical coatings composed of alternating high and low refractive index materials, such as TiO2/SiO2 stacks, are measured for individual layer thicknesses. Hard and protective coatings including TiN, CrN, DLC, and various nitrides and carbides are characterized for thickness, composition, and interface sharpness.

Energy Material Films

Photovoltaic absorber layers such as CIGS (Cu(In,Ga)Se2) and CdTe, battery electrode coatings, and solid-state electrolyte films are analyzed for thickness and compositional uniformity, which directly influence device performance and efficiency.

Testing Items

RBS thin film thickness measurement encompasses the following analytical deliverables:

• Film thickness determination expressed as areal density (atoms/cm²) and converted physical thickness (nm)
• Quantitative elemental composition and stoichiometric ratios of single and multilayer films
• Elemental depth profiles revealing layer structure and interface intermixing
• Heavy element impurity detection and quantification within films and substrates
• Areal density measurement of ultrathin layers below 10 nm
• Interfacial reaction layer analysis and diffusion depth evaluation
• Density determination when RBS areal density results are combined with thickness data from X-ray reflectometry or profilometry
• Crystal quality assessment through RBS channeling measurements when applicable

Testing Methods

Standard RBS Measurement

The sample is mounted on a goniometer in a high-vacuum chamber, typically maintained at 10-6 Torr or better. A collimated beam of 2.0 MeV helium ions is directed at the sample surface at normal incidence. Backscattered ions are detected by a silicon surface barrier detector or passivated implanted planar silicon (PIPS) detector with an energy resolution of approximately 12 to 20 keV. The acquired energy spectrum is then analyzed to extract film thickness and composition.

Glancing Angle Geometry

To improve depth resolution for ultrathin films, the sample can be tilted so that the ion beam strikes the surface at a glancing angle, typically 75 to 85 degrees from the surface normal. This geometry increases the effective path length of ions within the film, enhancing depth resolution to a few nanometers at the cost of a reduced probing depth.

High-Energy RBS

For improved mass resolution between heavy elements or for analyzing thicker films, beams of higher-energy helium ions or heavier projectiles such as lithium ions may be employed. High-energy RBS also improves the sensitivity for detecting light elements within heavier matrices.

Spectrum Simulation and Data Fitting

Quantitative analysis is performed using dedicated simulation software such as SIMNRA or RUMP. The analyst constructs a model of the layered structure with assumed thicknesses and compositions, simulates the theoretical RBS spectrum using evaluated stopping powers and scattering cross-sections, and iteratively refines the model parameters until optimal agreement between simulated and experimental spectra is achieved. The final output includes layer thicknesses, compositions, and interface widths with statistical uncertainty estimates.

Sample Preparation Requirements

Samples should be flat, smooth, and compatible with high vacuum. A typical analysis area of 1 to 5 mm² is required, and the surface must be free of volatile contaminants. The maximum lateral dimension is generally limited to approximately 15 to 25 mm to fit standard sample holders. No conductive coating is required, although strongly insulating samples may require electron flood gun compensation to mitigate charging effects.

Applications

RBS thin film thickness measurement serves a wide range of industries and research fields:

• Semiconductor process development and qualification, including gate dielectric thickness verification and metal stack characterization in advanced logic and memory devices
• Calibration and cross-validation of in-line metrology tools such as ellipsometry and X-ray fluorescence, establishing certified reference film stacks
• Thin film materials research in universities and national laboratories, providing absolute quantitative thickness and composition data
• Magnetic multilayer development for spintronics and data storage, where individual layer thicknesses at the angstrom scale control magnetic coupling behavior
• Optical coating production control, ensuring multilayer stacks meet precise thickness and composition specifications
• Photovoltaic film optimization, correlating absorber layer thickness and composition gradients with device efficiency
• Failure analysis of coated components, identifying interfacial reactions, interdiffusion, and thickness non-uniformity as root causes of degradation
• Verification of deposition tool capability and process transfer between fabrication facilities

FAQ

What is the measurable thickness range of RBS?

RBS can accurately measure films from approximately 1 nanometer up to several micrometers in thickness. For very thin layers, the result is reported as areal density in atoms per square centimeter, which remains accurate even when the peak width approaches the detector energy resolution. For films thicker than the accessible depth, typically beyond 2 to 10 micrometers depending on composition, the backscattered signal from the deepest region becomes too low in energy to resolve.

How accurate is RBS thickness measurement?

Because RBS relies on well-established physical cross-sections, it is an absolute technique requiring no calibration standards. Typical accuracy for thickness and composition is 3 to 5 percent, with precision better than 1 percent for repeated measurements. The primary uncertainty sources include stopping power values, detector solid angle calibration, and charge collection accuracy.

What is the depth resolution of RBS?

At normal incidence with a standard silicon detector, the near-surface depth resolution is approximately 10 to 20 nanometers for typical materials. Using glancing incidence geometry, the resolution can be improved to approximately 2 to 5 nanometers for the topmost layers.

Can RBS detect all elements?

RBS detects all elements heavier than the projectile ion. With helium beams, this includes elements from boron upward. Sensitivity scales approximately with the square of the atomic number, so heavy elements in light matrices can be detected at parts-per-million levels, whereas light elements in heavy matrices may have detection limits of several atomic percent.

Is RBS destructive?

RBS is generally considered non-destructive. The ion beam deposits minimal energy, and most materials including semiconductors, metals, and ceramics are unaffected. However, certain radiation-sensitive materials such as some organic films and photoresists may exhibit beam-induced modification, in which case reduced beam currents and short acquisition times are employed.

How does RBS compare with other thickness measurement techniques?

Compared with ellipsometry, RBS requires no optical model assumptions and provides absolute composition data. Compared with TEM cross-sectioning, RBS is faster and non-destructive but offers lower depth resolution. Compared with X-ray fluorescence, RBS provides superior depth profiling capability. A common best practice is to combine RBS with X-ray reflectometry, which yields absolute density and thickness, to derive highly reliable film density values.

What information should be provided when submitting samples?

Clients should provide the expected film structure including layer sequence, approximate thicknesses, nominal compositions, and substrate material. This information significantly accelerates spectrum simulation and improves the accuracy of the final fitting results. Sample dimensions, surface finish, and any prior processing history are also helpful.