Quality control at wafer level is characterized by extremely high requirements with regard to many different parameters. The smallest deviation from production tolerances can have a negative impact on the quality of downstream, cost intensive process steps and eventually product quality. Therefore, tolerances during the wafer production process need to be closely monitored.
The starting-product of wafer production is the so called ingot, a block of silicon or a compound semiconductor material like Sapphire or Gallium Arsenide (GaAs). In a sawing process, single bare wafers are cut from the ingot.
Wafers as flat as possible – Process control during wafer thinning
In the next processing steps the bare wafers are ground, lapped and polished, which again requires metrological measurements in order to determine and control the optimal settings of all involved processing machines. Beside the assurance of a certain total thickness variation (TTV) across the entire wafer in the range of a few microns, the non-contact characterization of surface roughness is another parameter that needs to be determined. Especially ambitious specifications apply for processing of so called thin wafers which are used in the pioneering 3D-IC manufacturing technology. Production tolerances are in the area of a few micro- or nanometers.
For the measurement of total thickness variation (TTV), roughness, contour and topography, leading manufacturers trust in the wafer metrology tools of the proven MicroProf® series. It combines multiple measuring tasks in one universal system. The optical sensors that come to use offer distinct advantages over conventional capacitive methods with regard to measuring resolution. More importantly, the measurements are conducted according to standards defined by SEMI, the global industry association of semiconductor manufacturers. The MicroProf® with TTV setup achieves standard compliant TTV measurements through a diametrically positioned sensor configuration consisting of two optical chromatic point sensors. Each sensor determines the distance to the wafer surface on its front and back side respectively. The wafer’s thickness at a given position is reliably calculated from these data. The TTV values are determined with the same method, however based on multiple measuring points on the wafer and subsequent statistical evaluation. The total thickness variation provides information about the uniformity of substrate removal during grinding. Grinding uniformity, wafer flatness, TTV, Bow, Warp and other parameters are important for MEMS and microelectronics manufacturers to ensure process stability.
The sensors are based on the method of optical chromatic measurement. White light is focused on the surface by a measuring head with a strongly wavelength-dependent focal length. The spectrum of the light reflected on the surface generates a peak in the spectrometer. The wavelength of this peak is used to determine the distance to the sample surface. Profiles and 3D topography measurements are generated by moving the wafer with a precision x,y positioning stage. The resulting data is manually or automatically evaluated with specialized software.
These include areas of application in microelectronics and microsystems technology, such as 3D-IC manufacturing technology. It is an increasingly popular method in which individual chips are stacked and connected vertically to form packets. This makes it possible to create even more powerful, compact and efficient assemblies with short signal paths. FRT has developed a special solution for 3D-IC wafer metrology. This includes thickness measurements, surface and roughness analyses, the determination of single and multi-layer systems as well as the measurement of Through Silicon Vias (TSV) with a high aspect ratio.