Vapour phase decomposition

The gas phase decomposition (english vapor phase decomposition VPD ) is a sample preparation method of the analytical chemistry for concentration of metallic impurities on priority ( silicon ) samples. It is inter alia used in the semiconductor industry to increase the sensitivity of the total reflection X-ray fluorescence analysis ( TXRF ), or other analytical methods. Was developed the method in 1984 by employees of Toshiba.

Operation

The preparation and analysis of impurities in and on the surface of a silicon wafer by the gas phase decomposition is divided into three steps. In the first part step, the silicon surface is converted by a chemical reaction with oxygen in the silicon dioxide. Typical process parameters for this thermal oxidation of silicon is a process temperature of 1000 ° C and a process time of 10 min, with an approximately 15 nm thick oxide layer is formed. Here, metallic impurities are incorporated into the oxide layer, accumulate in a surface region near the silicon, see segregation, or diffuse into the silicon substrate. In the example of iron 50-90 % of the iron atoms are bound in the oxide by the oxidation. Alternatively can be used with a previously applied layer of oxide for some applications, a clean silicon wafer.

In the second step the resolution of the thin oxide and the resulting solution of the metal ions in a liquid., The surface of the wafer is exposed to a hydrofluoric acid vapor to condense on the wafer. Due to the hydrophilic nature of the Silizumdioxidoberfläche a more or less thin film of hydrofluoric acid solution, which dissolves the superficial silicon dioxide together with the metal impurities forms. Since the silicon substrate having a hydrophobic character itself, this solution entails the dissolution of the silicon dioxide layer to a drop together ( ideally), located in the one (compared to the wafer surface and the thin oxide layer ) increased concentration of the above, and in the oxide layer impurities are included. In this method, however, it may come by particles or other influences quite to the fact that not only form a rather more droplets on the substrate. In order, nevertheless an average of the impurities normalized to get on the entire substrate surface, this droplet must first be led to a drop together, for example by means of VPD DC method ( by English vapor phase decomposition droplet collection, dt, gas phase decomposition and droplet collection ' ).

The third sub-step is to analyze the droplets. For this purpose the collected liquid drops (approximately 100 ul and 1000 ul) is first dried, whereby a further concentration of the collected over the entire wafer contamination occurs. The resulting granular residue can now be studied and characterized by various analytical methods. Typical measuring method with which the VPD are combined, the total reflection X-ray fluorescence analysis ( TXRF ), flame atomic absorption spectroscopy ( F- AAS) or inductively coupled plasma mass spectrometry (ICP -MS).

Analytical methods and detection limits

As VPD is a sample preparation method, essentially, the detection limit of the method is dependent on the analytical method with which it is combined, for example, total reflection X-ray fluorescence analysis, the flame atomic absorption spectroscopy ( F- AAS ), or mass spectrometry with inductively coupled plasma (ICP -MS). In general, the detection limit for the combination of the VPD is using this method in the range of 108-1010 atoms per square centimeter.

The combination of VPD and total reflection X-ray fluorescence analysis ( TXRF ) allows an increase in sensitivity for transition metal impurities (copper, nickel, zinc, etc. ) compared with TXRF by about two orders of magnitude. However, the combination is only suitable for the analysis of elements with atomic numbers greater than silicon. Elements such as sodium or aluminum, which play an important role as a metallic impurity in the semiconductor industry, therefore can not be detected.

This drawback can be circumvented by elemental analysis by flame atomic absorption spectroscopy (F- AAS) or inductively coupled mass spectrometry (ICP -MS). In addition to the large number of chemical elements achieve measurable combinations of VPD with this method an even lower by about one order of magnitude limit of detection for the analysis of a whole wafer. However, the analytical effort is correspondingly higher.

Scope

The combination of VPD and an analysis method is used in the semiconductor industry to identify possible metal contamination of wafers. The method uses include the qualification of production equipment. For this purpose, a purified silicon wafer on which there is already a thin oxide layer, multiple driven by the system and subsequently analyzed for metallic impurities.