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blue:-100 nm,  Red:+60 nm

Grinded Wafer: Same colorscale for AFM and StraDex a3 blue:-100 nm, Red:+60 nm

blue:-100 nm,  Red:+60 nm
Grinded Wafer: Same colorscale for AFM and StraDex a3 blue:-100 nm, Red:+60 nm
blue:-100 nm,  Red:+60 nm
Left: AFM image size 90x90µm2. Right: StraDex a3 image size 420x360µm2
Polished Wafer: blue:-10 nm, Red:+6 nm
Left: AFM image size 90x90µm2. Right: StraDex a3 image size 420x360µm2
Nominal Ra roughness value is 410 nm
Nominal Ra roughness value is 1.51 nm

Measurement of Roughness Parameters on grinded and polished Wafers

A wafer is comprised of semiconductor material which is used for electronic components and integrated circuits. Silicon is a very frequently used material because of its material properties and costs.

Grinding Issues

Grinding with corresponding wheels is a very complex process. The most important issue is that the wheel itself changes its cutting and grinding ability over time. The results of the grinding process can be subdivided into a) geometry and b) surface integrity. The geometry involves dimension shape and waviness/ flatness while the surface integrity mainly involves the micro-/nano- geometry as roughness. It is the result of residual stress, hardness and structure of the material.

Measurement Objective

Solid surfaces contain irregularities down to the order of inter-atomic distances. No machining method like fine-grinding or polishing, however precise, can produce a molecularly flat surface on conventional materials. Even more important changes in the grinding wheel will inevitable cause changes in the surface, beyond the specified values. Wafers not properly grinded or polished may have to be downgraded or even discarded for further processing. For technological applications, micro-/nano- topographic surfaces are of great importance. There is a need of measurements down to sub-nm roughness values.

Metrology Sensor and System

The multi-sensor capable wafer metrology system SemDex 301 comprises sensors for several metrology tasks:

  • Thickness (TTV)
  • Bow/ Warp and Flatness
  • Roughness
  • TSV depth/ CD

The system comprises an x/y stage for precise lateral movement to measurement points or sites. The user-friendly operation WaferSpect software can edit and run measurement recipes as well as evaluate and display the results instantly.

The roughness sensor StraDex a3 works as area sensor covering about (0.4 mm)2 without any lateral movement. This is a great advantage over other sensor (confocal, chromatic or mechanical tip) concepts which work on a point-by-point basis and require lateral scanning. No matter how, precise the mechanical stage, such a procedure is always prone to errors.

The measurement principle is based on white light interferometry. Low coherent light from an area source is deflected from a beam splitter and focused by a Mirau objective onto a sample covering a small area. Within the special objective light is hitting a reference mirror positioned at the same distance from the internal semi-reflective plate as the sample is located. After the sample and reference light is combined and leaving the objective, it is transmitted through the external beamsplitter and relayed to a 2D detector. Complete 3D information is obtained by moving the Mirau objective in height direction to satisfy the interference condition for each planar layer within the height range.

Great advantage of this principle is the extreme high accuracy of the 3D information due to the obviation of lateral scanning. The mechanical height scanning is performed with piezo-mechanics which has accuracies in the nm region. Further advantage is the 2D planar coverage of roughness which is yielding much more robust data than 1D planar roughness data (from confocal, chromatic or mechanical tip sensors). This is due to the higher tolerance against particles or dust or other imperfections.  

Comparison with AFM

In order to compare the two methods, various silicon substrates from a third party with differently treated surfaces were selected and measured with the StraDex a3 – 50 (sentronics metrology optical white light interferometer with a 50x objective) and with an AFM (“Atomic Force Microscopy”, Park Systems, model XE-150). The surfaces of the samples were grinded, fine grinded, polished and fine polished and only roughness data between 0 – 20 nm were evaluated in comparison with AFM. For example the measurements of the grinded wafer and of the polished wafer are shown.

Since the resolution of the AFM method is certainly higher, the results of both methods were analyzed by spectral decomposition (PSD =”Power Spectral Density”). The original roughness values for AFM will be slightly higher due to the extend of the spectral components into the high spatial frequency region, when its full bandwidth is used for the calculation. To compare both methods, the AFM-Results had to be truncated to the bandwidth of the white light interferometer.

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