Scatterometer

The scatterometry (Eng. about stray radiation measurement) is a non-destructive method for the analysis of particles and periodic surface structures using the elastic scattering and diffraction of electromagnetic waves (often visible light). The sizes of structures can extend up to the nanometer range, ie well below the Abbe limit of resolution (see resolution) of the light used.

In addition to the evaluation of the diffraction of visible light, there are also similar methods that usually use electromagnetic radiation of shorter wavelength, such as x-rays or particle. These methods are referred to in German as a rule, not as scatterometry, but as diffractometry, such as the neutron and X-ray diffractometry.

Classification

Scatterometric methods can be with respect to the radiation source used and the type of detection divided into the following sub- processes:

In addition, scatterometry method, which can be viewed as a combination of different basic shapes or were supplemented by additional parameters, for example, a further rotation angle in the sample plane ( φ scatterometry ) exist.

Operation

Hereinafter, the operation of the method on the example of a depth measurements using the spectral scatterometry will be briefly described. Measuring a plurality of tens of microns large field with a periodic line structure is irradiated with visible light, and detects the intensity and / or the polarization state of light reflected on the structures. It usually only the zeroth diffraction order is considered. Depending on the method, the light detection can be carried out as a function of the angle, the wavelength, or both. Of the detected spectrum is dependent on the structural parameters such as the line width, the depth, the flank angles of the side walls or to the material, and in one or more areas characteristic of the diffractive structure. However, a direct determination of the inverse structure parameters from the measured spectrum is not possible, since the necessary equations can not be solved analytically. The inverse determination is therefore indirectly by using a compensation bill to a previously created model that beschriebt the essential desired shape of the structure. Therefore, only the parameters roughly captured in the model are more accurately determined by the compensation calculation.

Application

Scatterometric methods are based on elastic scattering, and diffraction of light by particles, and non-planar surfaces. They will therefore be used for the detection of particles in a gas, liquid or surface to another for the characterization of periodic or random surface structures.

The spectral scatterometry has become in recent years an important method in the production control in the semiconductor technology. There she serves, inter alia, to characterize periodic profiles, such as the depth or side angles of etched structures, and the critical dimension (CD ); the method is therefore known in English as optical critical dimension metrology ( OCD metrology). In addition, it is also suitable to determine the overlay offset ( engl. diffraction based overlay DBO) or for measuring buried structures, which opens up new possibilities for process control, especially in advanced technologies such as FinFETs.

As optical measurement method, it can be in other optical measuring devices, such devices integrate for the determination of the overlay offset or layer thickness (eg, reflectometer, ellipsometer ), and also in process plants. The latter enables a very fast feedback to the process control system (sometimes in real time) and thus a better process control.

A disadvantage of scatterometry measurements is the relatively high space requirements for the test areas, these are often 50 microns x 50 microns in size and can therefore only placed in the scribe line (English scribe line ) between the actual chips in the rule. In - The measurements are therefore very rare, as precious chip area would be assigned here. In certain cases, however, directly periodic structures of the circuits, for example, larger DRAM or SRAM blocks can be used for the measurement, on the chip. An important disadvantage that comes just for the CD measurement to bear is that the OCD measurement usually allows no measurement of isolated line structures. In addition to the CD value of dense line- space structures, such information is necessary for today's photolithographic processes for the assessment of exposure dose influence. Therefore, in the industrial environment, the measurement of critical dimension usually not scatterometric process, but by scanning electron microscopes.

Pros and Cons

Scatterometric measurements are non-destructive and non-contact methods. For use in process control in the manufacturing of semiconductor products, they are characterized by a high sensitivity even to small material or structure changes from plant and is often technically relatively easy to implement. The scatterometric determination of profile depths are compared to alternative methods such as atomic force microscopes significantly faster, high-throughput operation is less susceptible to interference and also offer the possibility to determine more profile parameters such as line width, sidewall angle and layer thicknesses. The disadvantage, however, is that the measurements of the desired material and geometry data is made indirectly, that is, they require a matching calculation of a previously defined model. A direct determination of material data of an unknown system is usually not possible. Even larger deviations can possibly only be determined with larger error values ​​. In addition, the determination of isolated line widths, as they are often required for process control in photolithography, not possible.