High-precision thickness and TTV measurement for wafer production

Sawing, grinding, lapping, polishing… These are the typical processing steps that lead from the initial product “Ingot” to high-quality wafers. Manufacturers in the fields of microelectronics, microsystems technology and photovoltaics have high demands on the production tolerances of the pre-product “wafer”, since even small deviations can have a negative impact on the quality in the downstream, cost-intensive process steps. This results in lower yields and reduced efficiency and reliability of the end products. High-quality, fully automatic multi-sensor measuring technology from FRT contributes to the control of process tolerances in wafer manufacturing and helps to maintain the required quality standards of the producers.

After sawing, the wafers are further processed using mechanical procedures such as grinding, lapping and polishing. The thickness must be observed over the entire wafer in the range of a few micrometers. Since this is usually provided with a doping with a defined depth of penetration, the grinding process must be adjusted in such a way that it does not come below a certain thickness. Especially for thin wafers with lower tolerances, the demands on accuracy and reproducibility are constantly increasing. In this case, optical non-contact thickness measurement has established itself. In addition to thin wafers, bonded or taped wafers can also be measured. FRT relies on a diametrically arranged, calibrated sensor configuration for wafer thickness and surface measurement, consisting of two non-contact chromatic point sensors. They measure the distance from the wafer at its top and bottom. In this way, the local wafer thickness as well as the thickness variation over the entire wafer surface is reliably determined in accordance with the standard of the SEMI industry association.

With a maximum wafer diameter of 300 mm, the system achieves a resolution of less than 10 nm for thickness measurement. At the same time, roughness and structure on both sides of the sample can be measured. A special specimen holder picks up the specimen in such a way that it only touches the outer edge. The sensors are arranged above and below the sample. The lower sensor remains fixed, once aligned, and is automatically at the correct working distance due to the sample holder.  The upper sensor is approached via the z-axis. The variation of the absolute thickness can be measured over the entire sample range.  The bow of the sample and the height deviation of the scanning table are irrelevant, since a difference measurement is carried out between the fixed sensors. Especially the thickness variation, defined by the so-called TTV value, is a quality indicator for wafer grinding. It allows conclusions whether the ablation has been carried out consistently. Only wafers with a very low TTV value can continue to be used for demanding applications.

For the determination of film thickness in the sub-micrometer range, from a few ten nanometers to several ten micrometers, as well as for the analysis of complex multilayer structures with high resolution, the FTR sensor was developed in-house. Depending on the requirements, the thin-film sensor is used in variants with different wavelength ranges, so that optimum measuring conditions are offered for different materials and film thickness.

For materials that are opaque for visible light but transparent for infrared radiation, FRT makes thickness measurement possible with the CWL IR. The adhesive layer of stacked wafers, such as voids and defects can also be determined. The sensor is characterized by a wide measuring range and an x, y-resolution of 6.5 µm. With the proven devices of the MicroProf® series, the CWL IR can be used for 3D layer thickness mappings and 2D layer thickness profiles.

The degree of automation ranges from manually operated measuring systems such as the MicroProf® 300, which automatically execute predefined routines, to fully automated wafer handling including automatic pre- and fine alignment in the MicroProf® MHU. FRT systems can be configured to handle SEMI-compliant and non-compliant wafers, often used in the MEMS industry.

For semiconductor applications, FRT offers its own device class for cleanroom production: the MFE series, with the MicroProf® FE and MicroProf® FS. MFE measuring systems can process 200 mm and 300 mm wafers in one system (FOUP, SMIF and open wafer cassettes possible). The included SEMI-compliant software package enables interactive or automated use, simple creation of measurement and evaluation recipes as well as integration into existing production control systems via the SECS/GEM interface.

Whether in laboratory, development, quality assurance or production – FRT offers the right measuring technology for your application. Do not hesitate to contact us if you have any questions. Our experts will be glad to help you solve your measurement tasks by creating the best possible system configuration for you.

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