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New measurement station for wafer quality control

In the PTB laboratory at the Berlin synchrotron radiation facility BESSY II, a new measuring station has been put into operation. The set-up can determine smallest impurities on silicon wafers by means of X-ray fluorescence analysis. The station can analyse the composition of nanometre-thick layers as well.

The new measuring station at PTB’s plane grating monochromator beam line at BESSY II. A robot system (EFEM) transports the wafers from their standard transport containers (SMIF or, as in this case, FOUP) into a load lock from where a vacuum robot takes them to the (T)XRF analysis chamber.

Energy-dispersive X-ray fluorescence analysis (XRF) is a physical method for non-destructive elemental analysis. Several elements can be detected simultaneously. For investigations, the sample is irradiated with X-rays and the resulting element-specific fluorescence radiation is recorded with an energy-dispersive semiconductor detector. Besides other applications this method is also used for chemical ultra-trace analysis in the quality control of drinking water, foodstuffs and semiconductor surfaces.

Total reflection XRF (TXRF) is applied as a routine procedure in the semiconductor industry to control the quality of wafer surfaces. In this procedure, X-rays strike the sample under a grazing incident angle so small that they are totally reflected at the surface. By using monochromatic undulator radiation as generated at the synchrotron radiation source BESSY II in Berlin, even elements of low atomic numbers (C to Al) can be verified down to a limit of one picogram. This corresponds to a detection limit of approximately 108 foreign atoms per cm2 in the case of 200 mm diameter silicon wafers treated by a VPD (vapor phase decomposition) procedure.

Clean room conditions are necessary for this ultra-trace analysis to ensure that the wafer samples are not additionally contaminated when introduced in the analysis chamber. The new measuring arrangement for (T)XRF operates without reference samples. It was taken into operation at the PTB laboratory at BESSY II in late 2002. The station can handle and position wafers of up to 300 mm in size automatically at a low additional contamination risk. The method has been developed in cooperation with the semiconductor industry. It is to be implemented for quality control of new wafer cleaning procedures.

In addition, the measuring station will serve the analysis of multi-elemental multi-layered systems with layer thicknesses in the nanometre region. The fine structure of the X-ray absorption edge can be investigated by variation of the excitation energy. That allows speciating compounds of light elements on wafer surfaces found only in small amounts in the nanogram region.

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