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 XPS  Ion sputtering speed - page 1.

 Les abaques sur quelques matériaux.

ESCA (Electron Spectroscopy for Chemical Analysis) or XPS (X-ray Photoelectron Spectroscopy) is a semi-quantitative technique for surface chemical analysis allowing obtaining an atomic percentage of the various elements present in the analysed specimens as well as information about atomic environment of each element. This technique coupled to ion sputtering allows analysing superficial layers and interfaces as well as establishing depth profiles of atomic concentrations within a specimen.

The main objective of these pages is to establish a database summarising ion sputtering speeds of some materials typically used by Altis Semiconducteur. It is actually very difficult to predict the sputtering speed from a material's physical properties (such as roughness, density, conductivity, etc). Such a database can be seen as a comparison tool for XPS users who would like to:

  • On one hand, adjust the ion sputtering parameters, knowing the thickness of the various layers that must be removed and the corresponding sputtering speed for each material. This is especially important when interfaces must be analysed.

  • On the other hand, find out from the ion sputtering speeds, approximate thicknesses of the various deposited layers.

  •  Experimental methods

    Experiments were conducted on an ESCALAB 220I XL from Thermo Electron. Such a system includes typically three main instruments: An x-ray source (exciting the target material which in turn emits electrons), a hemispherical analyser (analysing, sorting and counting photoelectrons) and an ion gun (used for material sputtering in order to obtain the depth concentration profile of the analysed material).

    This study is aimed at:

  • Obtaining from different Altis Semiconducteur departments that are specialised in thin layer deposition different specimen representative from extremely pure material deposited with a known thickness e on a silicon substrate (a 20 cm diameter wafer).

  • Establish a depth profile from these specimens in order to obtain the sputtering time t necessary for a complete thin layer removal.

  • Deduct from e and from t the ion sputtering speed under given experimental sputtering conditions (especially for a given sputtering measured current)

  • Finally gather and plot all these data on a graphic showing sputtering speeds versus ion sputtering measured current.

  • A depth concentration profile consists in completing a series of alternating sputtering and analysis phases. Each of the two phases follows a well-defined protocol

    The surface analysis method can be decomposed in several steps. Specimen preparation consists in obtaining six specimens, 1 to 2 cm2, coming from a single radius of a silicon wafer, thus allowing characterisation of wafer deposition thickness radial symmetry. The specimens are bonded on the specimen holder by the mean of a carbonated adhesive (non insulating). This assembly is mounted on a probe housed inside the transfer chamber. A primary pumping followed by a turbomolecular pumping is applied for about 20 minutes, after which the specimen holder is inserted inside the reactor. The system stays in standby until the pressure goes down to a value comprised between 10-9 and 10-10 mbar (depends on introduced material degasing). The analysis time depends on the number of elements to be quantified and on the number of scans used for each element.

    For ion sputtering, a neutral gas is introduced, a part of its atoms will be ionised and accelerated toward the material causing superficial sputtering. Argon was used during these experiments. The analysis chamber internal pressure increases when this gas penetrates the reactor. This pressure should not rise beyond 1 to 2.10-7 mbar. For the study the work pressure was 1.3.10-7. Before any experiment, the ion gun was started but stayed inactive during the argon pressure rise. Software parameters are identical to those used for surface analysis. Only the sputtering time is added under the form of a number of cycles and an ion sputtering time for each cycle. A complete depth profile can take up to 8 hours for a layer of approximately 100 nm thickness. Ar+ ion energy is 5 keV for most of the following experiments.

    The transition phases between surface analysis and ion sputtering are fully automated from the control software delivered with the instrument by its manufacturer, Thermo Electron.

    Some of the parameters are explained below:

  • Analysis step size: This is in the smallest energy division shown on the spectrum X-axis. This parameter is purely software driven. The energy resolution actually directly depends on the 6 analyser channeltrons (default value: 0.1eV for high-resolution spectra or zooms and 1 eV for standard spectra or surveys).

  • Dwell time: This is the duration of each analysis step. The higher the dwell time, the higher the number of detected electrons (default value: 20 ms for surveys and 100 ms for zooms).

  • PE or Pass Energy, the potential difference between both analyser hemispheres. When the pass energy is increased, we allow more electrons to pass through the analyser. And we actually obtain higher peaks that are also wider i.e. showing less precision (default value: 20eV).

  • Scan number: When increasing the scan number, the peak intensity increases without any increase of the peak full width at half maximum (FWHM) hence their precision (default value: 10).

  • Spot size (size of the analysed region): The risk of analysing high heterogeneity region increases when the spot size decreases is (default value: 500 µm).

  •  Sputtering parameters:

    - Sputtering spot tuned to Mag 5.
    - Ion energy: 5 keV.
    - 2 µA current intensity measured on the specimen holder by measurement beginning.
    - about 1.7.10-7 mbar pressure inside reactor during cycle sputtering steps
    - Ion gun emission current: 13.5 mA (90% of 15 mA)
    - Scan amplitude fixed by Thermo Electron
    - Ion incidence angle: 62°
    - Sputtering time per cycle: 90 seconds

     Analysis parameters:

    - About 1.3.10-7 mbar pressure inside reactor during cycle analysis steps (non sputtering)
    - Analysing source: Al, mono
    - Analysis spot size: 500 µm
    - Identical software parameters spectra processing
    - Cathode electron bombardment current intensity: 6.5 mA
    - Emission current: 12 mA (for a 500 µm spot size)
    - Emission voltage: 10 kV

    The first work to carry out was making specimens containing as many as possible of the thin layer materials used at Altis Semiconducteur's site. All these thin layers were deposited on a new or reconditioned silicon wafer. Copper was deposited on a tantalum layer itself deposited on a Si substrate[1]. 8 materials were collected and etched in order to find out their sputtering speed.
    [1] Copper/silicon binding is weak.