01/22/2019

Sawing, grinding and polishing - Tolerances and standards in wafer processing

Wafers are the substrate for the production of integrated circuits, light-emitting diodes, micromechanical components and solar cells. The particularly fine thin wafers are required primarily for the manufacture of 3D IC components. The micrometer-sized circuit packages are created by stacking and connecting in a vertical direction. Chips mounted on thin wafers are assembled. This miniaturization allows even more efficient circuits to be realized, for example for larger solid-state disks, more compact CMOS image sensors and more powerful and energy-efficient logic components. The requirements for the specifications of the output product wafers are immense. Sawing, grinding and polishing require maximum precision.

Sawing the starting product

The ingot, a block of silicon or a compound material such as sapphire or GaAs, is the starting material for wafer production. With a high-precision wafer saw, individual raw wafers are cut from it. This creates grooves whose width and depth must be controlled. Optical measuring systems with multi-sensor configuration are ideal for this task. The saw contour can not only be visualized three-dimensionally, but also quantitatively and metrologically characterized - a task in which classical optical microscopes reach their limits. The quantitative measurement allows the sawing process to be better controlled, so that the deviations in this work step are significantly lower. In addition, the tool wear of the processing machines can be monitored and the behavior of different materials during the sawing process can be evaluated.

Grinding evenly

After sawing, the wafers are thinned using mechanical processes such as grinding and lapping. During the grinding of a raw wafer or structured wafer, certain quality parameters must be maintained. For raw wafers, for example, the TTV value (a statistical value based on a metrological wafer thickness measurement indicating the absolute thickness variation of the entire wafer) indicates uniform removal during grinding. This is the only way to ensure that the wafer can continue to be used for the following process steps. Only wafers with a very low TTV value can continue to be used for process steps such as those occurring in microelectronics or microsystems technology. In addition to the thickness variation, it is also important for raw wafers to maintain flatness. Differences in height may lead to contacting problems during subsequent stacking to a 3D IC wafer package.

It has to be flat

In addition, the wafers must be ground to an exact thickness and the doping introduced into the substrate should not be removed too far. The wafer edge can also be checked at random with a 2D profile measurement. From this, the wafer manufacturer can determine the wear of the grinding tools and optimize the process parameters. Waste at the edge of the wafer also plays an important role. The more efficiently this is done, the more chips can be produced on the wafer later and the so-called fill factor increases. Processed wafers that are equipped with bump structures or solder balls, for example, can be measured in terms of height, width and coplanarity.

Meet the measurement requirements

Whether for measurements with several layers, for example with bonded or taped wafers, edge evaluation of the wafers or the evaluation of the coplanarity of bump structures, with modern optical measuring systems you can easily measure your wafers with high accuracy and keep pace with the increasing demands on accuracy and reproducibility.

Smooth polishing

In the thin wafer polishing process, a polishing pad applies pressure to make contact with the wafer surface and polish it. A polishing paste (slurry) containing chemically effective substances and abrasive materials are used. The removal on the wafer surface occurs by friction. In a dry-polish process, the chemical and abrasive material is bound in a pad. In this process, a lower contact pressure is sufficient. The optical measuring system MicroProf® is also used for the monitoring of the polishing process, because the determination of the roughness allows conclusions to be made about the surface quality.

One measuring instrument for all cases

The system can be equipped with a diametrically arranged sensor configuration (TTV setup) consisting of two non-contact chromatic point sensors. Thanks to modular multi-sensor technology, the system can be retrofitted at any time, for example with an infrared layer thickness sensor. With the MicroProf®, thickness variation, flatness, bump coplanarity and surface topography on raw wafers and structured or multi-layer wafers can be measured with high resolution and characterized quantitatively. The parameters are determined according to the standard of the semiconductor industry association SEMI. All wafer sizes up to 300 mm e.g. of silicon, gallium arsenide or sapphire can be measured fully automatically. These results can be used to adjust production plants and increase efficiency.

 

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