Semiconductor Technology from A to Z

Oxide layers are transparent films. If light is irradiated onto the wafer and reflected, various properties of the light wave are changed which can be detected with metering devices. If there are multiple layers stacked on each other they must differ in optical properties to allow a determination of the materials.

To monitor the film thickness across the entire wafer, several measuring points are measured (e.g. 5 points on 150 mm, 9 points on 200 mm, 13–21 points on 300 mm wafers). Not only must the values of the individual points be compared with the specified target value, but also the thickness of the points relative to one another, since homogeneity is also important for subsequent processes. If the deposited layer is too thick or too thin, material has to be removed (e.g. by chemical mechanical polishing) or additional material has to be deposited (by repeating the corresponding process).

If light waves interfere with each other, they can be amplified, weakened, or even canceled out. This phenomenon is used in semiconductor manufacturing for measuring transparent layers.

Light rays incident on the wafer are partially reflected at a transparent layer, while part of the rays also penetrates the material and is reflected at the layer underneath.

A spectrum of different wavelengths is irradiated onto the wafer. Depending on the thickness of the layer being penetrated, the reflected rays interfere differently, resulting in a characteristic interference pattern for each film thickness and material. Using a photometer, the film thickness can be determined from the reflected light.

Interference measurements are possible for layers whose thickness corresponds to roughly one quarter of the irradiated light wavelength or more.

Ellipsometry is the determination of optical properties through the change in polarization of light. Linearly polarized light (the light waves have a specific oscillation) is irradiated onto the wafer at a fixed angle.

Illustration of possible polarizations of light (left linear, right circular):

     
(Source: Optics Group, University of Glasgow)

Upon reflection at the wafer surface or at the interface between two layers, the light is repolarized. This change can then be measured with an analyzer. From the known optical properties of the layers (e.g. refractive angles and absorption coefficient), the irradiated wavelength, and the polarization, the film thickness can be determined.

In contrast to measurement by interference, ellipsometry is suitable for thin layers whose thickness is less than one quarter of the irradiated light wavelength.

With these optical measurement methods, layer thickness is determined only indirectly; the optical parameters of the layers being measured must be sufficiently well known beforehand. Using these values, a model of the layer stack on the wafer is created and a measurement is simulated. The result is then compared with the actual measurement, and the layer thicknesses (as well as other optical indices) of the materials defined in the model are varied until the simulation and the measurement match as closely as possible.

Comparison of simulation and measurement in an optical scatterometry measurement

(Source: n&k Technology)

The more parameters that are varied in the model, the easier it becomes to achieve a match with the measurement, but the less certain the result also becomes. Typically, a parameter known as goodness of fit (GOF) indicates how well the simulation matches the measurement (0–100 %).