Gas phase decomposition

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The gas phase decomposition ( English vapor phase decomposition , VPD) is a sample preparation method in analytical chemistry for the concentration of mainly metallic impurities on ( silicon ) samples. It is used, among other things, in the semiconductor industry to improve the sensitivity of total reflection X-ray fluorescence analysis (TXRF) or other analytical methods. The process was developed in 1984 by employees at Toshiba .

functionality

The preparation and analysis of impurities on and on the surface of a silicon wafer by means of gas phase decomposition is divided into three steps. In the first step, the silicon surface is converted into silicon dioxide through a chemical reaction with oxygen . Typical process parameters for this thermal oxidation of silicon are a process temperature of 1000 ° C and a process time of approx. 10 min, with an oxide layer approx. 15 nm thick. Metallic impurities are built into the oxide layer and collect in an area close to the surface on the silicon, cf. Segregation , or diffuse into the silicon substrate. In the example of iron , 50–90% of the iron atoms in the oxide are bound by oxidation. Alternatively, a clean silicon wafer with a previously applied oxide layer can also be used for some applications.

In the second sub-step, the thin oxide layer is dissolved and the metal ions are dissolved in a liquid. For this purpose, the surface of is the wafer to a hydrofluoric acid vapor exposure, which condenses on the wafer. Due to the hydrophilic nature of the silicon dioxide surface, a more or less thin film of hydrofluoric acid solution forms, which dissolves the surface silicon dioxide together with the metallic impurities. Since the silicon substrate itself has a hydrophobic character, this solution contracts to a drop after the silicon dioxide layer has dissolved (ideal case) in which an increased concentration (compared to the wafer surface or the thin oxide layer) of what was previously on and in the oxide layer Contaminants are included. In this process, however, particles or other influences can lead to the formation of not just one but several droplets on the substrate. In order to nevertheless obtain a mean value of the impurities normalized to the entire substrate surface, these droplets must first be brought together to form a drop, for example using the VPD-DC method (from English vapor phase decomposition-droplet collection , dt. 'Gas phase decomposition and droplet collection ') .

The third sub-step is the analysis of the droplets. For this purpose, the collected drop of liquid (approx. 100 µl to 1000 µl) is first dried, which results in a further concentration of the impurities collected over the entire wafer. The resulting granular residue can now be examined and characterized using various analytical methods. Typical measurement methods with which the VPD are combined are total reflection X-ray fluorescence analysis (TXRF), flame atomic absorption spectroscopy (F-AAS) or mass spectrometry with inductively coupled plasma (ICP-MS).

Analysis methods and detection limits

Since VPD essentially represents a sample preparation method, the detection limit of the method depends on the analytical method with which it is combined, for example total reflection X-ray fluorescence analysis , 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 with these methods is in the range of 10 8 -10 10 atoms per square centimeter.

The combination of VPD and total reflection X-ray fluorescence analysis (TXRF) increases the sensitivity for transition metal impurities ( copper , nickel , zinc , etc.) compared to TXRF by approx. Two orders of magnitude. However, the combination is only suitable for the analysis of elements with larger atomic numbers than silicon. Elements such as sodium or aluminum, which play an important role in semiconductor technology as metallic impurities, cannot therefore be detected.

This disadvantage can be circumvented by an element analysis using flame atomic absorption spectroscopy (F-AAS) or mass spectrometry with inductively coupled plasma (ICP-MS). In addition to the larger number of measurable chemical elements, combinations of VPD with this method achieve a detection limit that is approximately one order of magnitude lower for the analysis of an entire wafer. However, the analytical effort is correspondingly higher.

scope of application

The combination of VPD and an analysis method is used in semiconductor technology to determine possible metallic contamination of wafers. The method is used, among other things, to qualify production systems. For this purpose, a cleaned silicon wafer, on which there is already a thin oxide layer, is run through the system several times and then examined for metallic contamination.

literature

  • Karen A. Reinhardt, Werner Kern: Handbook of Silicon Wafer Cleaning Technology . 2nd Edition. Elsevier, 2007, ISBN 0-8155-1554-5 , pp. 623–633 (Detailed description of the method. Subsequently with further descriptions for the combination with various analysis methods).

Individual evidence

  1. ↑ A. Shimazaki, H. Hiratsuka, Y. Matsushita, S. Yoshii: Chemical Analysis of Ultrace Impurities in SiO 2 Films . In: Extended Abstracts of the 16th (1984 International) Conference on Solid State Devices and Materials, Kobe . 1984, p. 281-284 (Referenced in: Takeshi Hattori: Ultraclean surface processing of silicon wafers: secrets of VLSI manufacturing. Springer, 1998, ISBN 978-3-540-61672-6 , pp. 170-171.).
  2. ↑ Takeshi Hattori: Ultraclean surface processing of silicon wafers: secrets of VLSI manufacturing . Springer, 1998, ISBN 978-3-540-61672-6 , pp. 200 .
  3. ↑ Karl-Heinz Koch: Process analytical chemistry: control, optimization, quality, economy . Springer, 1999, ISBN 978-3-540-65337-0 , pp. 173 .
  4. ↑ C. Neumann, P. Eichinger: Ultra-trace analysis of metallic contaminations on silicon wafer surfaces by vapor phase decomposition / total reflection X-ray fluorescence (VPD / TXRF) . In: Spectrochimica Acta Part B: Atomic Spectroscopy . tape 46 , no. 10 , 1991, pp. 1369-1377 , doi : 10.1016 / 0584-8547 (91) 80186-7 .
  5. ^ Karen A. Reinhardt, Werner Kern: Handbook of Silicon Wafer Cleaning Technology . 2nd Edition. Elsevier, 2007, ISBN 0-8155-1554-5 , pp. 623-633 .