X-ray Reflectivity（XRR）

Film thickness, film density, and surface/interfacial roughness of films deposited on a wafer can be determined by analyzing changes in the intensity of X-rays reflected from the sample surface under grazing-incidence conditions.
As shown below, the intensity of X-rays reflected from the film surface and the film-substrate interface varies with film thickness. Furthermore, by determining the critical angle for total external reflection, both the film density and film thickness can be obtained simultaneously.

XRR Measurement Principle

Below are the results obtained from an actual XRR measurement. A sample consisting of a DLC (diamond-like carbon) film deposited on a Si wafer was used for the measurement. The film density, thickness, and roughness were determined by varying the model parameters until the simulated profile (red line) matched the measured profile (blue line). The simulated profile was calculated using the model shown below.

               Thin-film model used for calculation                            Measured XRR Profile and Simulation

・Examples of applications of thin-film thickness and density evaluation are as follows:

1) Semiconductor materials and thin films: TaN, HfO₂, Low-k film, various ALD films, etc.

2) Optical devices: Multilayer mirrors, filters, etc.

3) Material science and thin-film materials: two-dimensional (2D) materials such as graphene and MoS₂

＜Features＞

1. Nondestructive analysis is possible.

2. Density and film thickness can be measured simultaneously. 

3. Both crystalline and amorphous materials can be measured.

4. Characterization of multilayer films consisting of a few layers.

5. Film thickness: a few nm to approximately 300 nm (depending on the material); no thickness limitation for density measurements.

＜Limitations＞

1. Measurements cannot be performed on curved samples.

2. A surface with nanometer-scale roughness is required.

3. A measurement area of several millimeters or larger is required.

4. Films with similar densities are difficult to distinguish.

5. Approximate compositional and structural information is required.