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Browse Prior Art Database

Detection of Trace Amounts of Sodium in Materials

IP.com Disclosure Number: IPCOM000077780D
Original Publication Date: 1972-Sep-01
Included in the Prior Art Database: 2005-Feb-25
Document File: 2 page(s) / 33K

Publishing Venue

IBM

Related People

Ziegler, JF: AUTHOR

Abstract

Sodium contamination of field-effect transistors (FET's) is a major problem. The concentration levels necessary to seriously degrade performance is of the order to 5X10/10/ atoms/cm/2/ in the covering oxide. Typically, on the surface of a silicon wafer there is 5000 Angstroms of SiO(2). It is desired to detect sodium in that SiO(2), unambiguously, to the trace level of 5X10/10//cm/2/ or 0.05 parts per million. A method for such detection is set forth below.

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Detection of Trace Amounts of Sodium in Materials

Sodium contamination of field-effect transistors (FET's) is a major problem. The concentration levels necessary to seriously degrade performance is of the order to 5X10/10/ atoms/cm/2/ in the covering oxide. Typically, on the surface of a silicon wafer there is 5000 Angstroms of SiO(2). It is desired to detect sodium in that SiO(2), unambiguously, to the trace level of 5X10/10//cm/2/ or 0.05 parts per million. A method for such detection is set forth below.

An ion beam 2, consisting of argon ions, is incident on a wafer target 4, the latter containing a 5000 Angstroms thick layer 6 of SiO(2), the latter containing possible sodium contamination. The argon ion beam has a diameter of approx.
0.2 cm, is accelerated to 1300 KeV, and its current is 10 microamps. The wafer target is mounted on a copper heat sink 8, suitably cooled to maintain the temperature of the wafer below 100 degrees C.

When the ion beam impinges on the SiO(2), x-rays are generated by all the atoms of the film. A thin Al filter 10 (approx. 0.0015") is used to eliminate the K proportional to x-rays of the silicon atoms in both the oxide and the substrate. A crystal spectrometer 12 is used to focus the x-rays on the detector 14 and eliminate the x-rays from all but those emanating from the sodium atoms. The detector 14 is a flow-proportional counter with an ultrathin front window 16 to pass, with little attenuation, the soft sodium x-rays.

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