Gamma ray ellipsometry is an established thickness measurement technique and is widely used in the semiconductor industry to measure the thickness of ultra-thin layers. It is one of the few techniques that can measure thicknesses between 0.1nm and 200μm with good accuracy. The technique uses gamma ray to measure the thickness of a sample layer. Gamma rays are narrow-band, penetrating radiation detected by either scintillation crystals or semiconductor gamma-ray detectors.
When gamma rays pass through a sample, their intensity is reduced by the sample. The amount of attenuation is dependent on the thickness, material and geometry of the sample layer. The attenuation can be written as
A = e-(μ×t)
where A is the gamma ray intensity after passing through the sample, μg-1 is the linear attenuation coefficient for the sample material and t is the sample thickness.
In gamma ray ellipsometry, a sample is illuminated with gamma-ray radiation and the reflected beam is measured. The sample is placed in the beam path such that the gamma ray beams are incident and reflected at an angle, θ. The beam intensity is modulated as a function of angle.
The intensity of the gamma ray reflected from the sample can be related to the sample thickness by the following equation:
Iref = Iinc × cos2(θ) × e-(μ×t)
where Iref is the reflected gamma ray intensity, Iinc is the incident gamma ray intensity and θ is the angle of incidence.
By measuring the reflected beam as a function of angle of incidence, the thickness of the sample can be determined. This method is known as gamma ray ellipsometry and is used to measure the thickness of ultra-thin layers.
Gamma ray ellipsometry is a robust and easy to use thickness measurement technique. It is non-contact and does not require direct access to the surface of the sample. It is also highly accurate and can provide a detailed description of the layer structure by measuring the thickness of each layer in a stack.
The technique is well-suited for measuring thicknesses between 0.1nm and 200μm with sufficient accuracy (typically within 5%). It has been used successfully to measure the thickness of semiconductor layers, dielectric layers, corrosion layers, metal layers and other layers.
Gamma ray ellipsometry is also cost-effective compared to other techniques such as X-ray thickness measurements. The cost of the equipment is relatively low and the technique does not require special conditions or sophisticated infrastructure.
The main limitation of gamma ray ellipsometry is the source of gamma rays. Gamma rays have to be generated externally and can be hazardous if not handled with care. This limit the use of gamma ray ellipsometry in certain applications.
In conclusion, gamma ray ellipsometry is a well-established technique used to measure the thickness of ultra-thin layers. It is non-contact, accurate and cost-effective and can measure thickness between 0.1nm and 200μm. However, the use of gamma ray sources limits the application of this technique in certain cases.
