Translation to the SSS Data Format Instrument Geometry Fields from VAMAS and ISO 14976 Data Transfer Format Geometrical Orientations

Paul Engle

Laboratory Support Mgr., Geller MicroÅnalytical Laboratory,

426e Boston Street, Topsfield MA. 01983, USA

email:pe@gellermicro.com

The American Vacuum Society Surface Science Spectra Database (SSS), the National Physical Laboratory Surface Chemical Analysis Standard Data Transfer Format (VAMAS) and the ISO 14976 Data Transfer Format (ISO) all employ comparable descriptions of instrument geometry and sample orientation. The systems all require specification of angular relationships that may not be commonly available in the instrument manufacturer’s literature. In order to facilitate translation to the SSS data format, and promote its use, a practical guide is presented for calculating the necessary values.

Format Requirements: The SSS format requires seven angular values to specify the relative orientation between sources, sample and analyzer. The coordinate system is referenced to the analyzer focal plane, and the excitation source axis (fig.1). The ISO and VAMAS formats each requires nine angular values, and are referenced to the sample stage and its X-Y translation axes (figs.2,3).

Nomenclature: For each of the three systems labels and diagrams have been preserved from the original format definitions. As a convenience symbols consistent with the SSS description have been added to the ISO / VAMAS labels, and figs.2,3 serve for both the ISO and VAMAS systems as they are almost identical. For clarity each of the symbols used is subscripted with s,i or v to identify its format. (e.g.: F sp_s denotes the SSS format Specimen Azimuthal Angle, F sp_i and F sp_v denote the ISO and VAMAS Sample normal tilt azimuth, each of which represents a different quantity.) The three diagrams (figs. 1-3) give an accurate description of how instrument configuration is represented in each format, however it is recognized that visualization of spatial relationships is sometimes difficult; every effort is made to give a clear narrative description. Reference the actual format definition documents for complete field descriptions.

Coordinate Systems: Sit in the operator’s chair of the instrument to be documented. Face the sample chamber directly. In the heart of the chamber is the sample stage, and the point on the sample being analyzed is the coordinate system origin for all three formats. The direction of stage motion toward you defines the Y axis for VAMAS, and the opposite direction (away from you) defines the Y axis for ISO. In both systems the X axis is defined by stage motion to your right, and Z is straight up through the stage (figs.2,3) The excitation source, the analyzer and possibly an ion gun are all pointed at the origin. Position the sample directly toward the analyzer. The plane of the sample is normal to the analyzer axis, and this orientation defines the X-Y plane for SSS. The analyzer axis and the source axis form a plane that cuts through the sample (analyzer focal plane) in a line. This line defines the X axis for SSS (fig.1). If the analyzer and source axes are coincident, then use the plane formed with the analyzer and ion gun axes to define X. Alternatively you may pick a different direction for the X axis and document it in Section C, Field 20 of the SSS format.

Measurement Angles: The SSS format measurements fall into three groups; those measured from the analyzer axis (Q e_s, Q s_s, Q ig_s), from the specimen normal axis (Y i_s,Y ig_s), and in the analyzer focal plane (F sp_s,F ig_s). These are referred to in the format as polar, incident, and azimuthal angles respectively (fig.1). The VAMAS and ISO format measurements also fall into three groups; those measured from the Z axis (Q s_v,i, Q a_v,i, Q ig_v,i, Q sp_v,i), in the stage (X-Y) plane (F s_v,i, F a_v,i, F ig_v,i, F sp_v,i), and in the plane of the sample (W r_v,i). These are referred to in both formats as polar, azimuth, and rotation angles respectively (figs.2,3). All polar and incident angles (Q ,Y ) are measurements made between two parts or axes pointing at the sample from above (e.g.: Q e_s the SSS emission angle is the angle between the analyzer axis and the specimen normal axis). All azimuth, and rotation angles (F ,W ) are measurements made within a plane, about the origin. (e.g.: F ig_v the VAMAS sputtering source azimuth is the angle in the stage plane (X-Y) from the Y axis clockwise to the orthogonal shadow of the ion gun axis on the plane). All F _i,v,W _i,v are measured in a clockwise direction, all F _s are measured in an anti-clockwise direction.

Practical Considerations: To obtain the polar and incident (Q ,Y ) measurements your best tools are the instrument manual, and a dose of trigonometry. If the angles can not be derived from printed specifications then empirical methods may be employed. Accurate results may be obtained with some common articles found in the laboratory.

A meter-stick, two long pencils and some adhesive tape can yield good results. First identify the two axes physically on the instrument. Have an assistant hold the pencils along the axes to be measured (e.g.: against the column and ion gun). Now secure the meter-stick to both pencils with tape. Take care to maintain the relative orientation of the pencils. Transfer to a flat surface and measure the two internal angles, b and g with a protractor (fig.4). The relationship to the desired angle a is given in (1.0).

(1.0) a = 180 – (b + g )

Instrument configuration may complicate the situation such that the meter-stick can not reach both pencils without interference from other components. Two meter-sticks can be used (fig.5), with the relationship given in (1.1). Clamp the two together tightly at a convenient angle d to ensure the apparatus remains entirely coplanar with the axes being measured.

(1.1) a = 360 – (b + g + d )

Azimuth measurements (F ) also should be derived from instrument documentation before resorting to empirical methods. If you must measure directly, cut a cardboard circle with the approximate diameter of your protractor. Poke a pencil through the center, and affix a length of string to one end of the pencil (fig.6). Align the pencil along the analyzer axis (fig.1) for SSS, or the Z axis (fig.2) for ISO and VAMAS. Have an assistant pull the string taught to each of the axes to be measured. Mark the disk’s edge at the point that the string touches or comes closest to. Label each mark. Take care that the disk does not rotate between measurements, and the pencil remains properly aligned. Remove the pencil and measure the marks with the protractor. SSS values are taken in an anti-clockwise direction from the analyzer source axis mark. ISO and VAMAS values are taken in a clockwise direction from the corresponding stage Y axis mark.

ISO and VAMAS formats both require a measurement of sample rotation angle (W ). This is clockwise rotation about the sample normal. If this is referenced to a particular direction on the sample this direction should be specified in a block comment line at item number 8 of the format. A real number equal to 1E37 may be used to indicate the value is not of consequence.

Translation from ISO and VAMAS to SSS:

Equation (2.0) may be used to obtain any of the SSS format values given the required ISO or VAMAS values. Take note that as of this writing, the SSS format is only defined for AES and XPS instrumentation.

Where P₁(x, y, z) and P₂(x, y, z) are coordinate sets given below for each of the SSS fields. If the analyzer and source axes are coincident (Q a = Q s and F a = F s) substitute coordinate set (2.9) for (2.6). For all F values, consult the paragraph below to determine the sign (± ) of the angle.

Use these coordinates (2.9) in place of (2.6) only if analyzer and source axes are coincident.

Finding the Sign ± of Azimuth Angles:

Using the values obtained for P₁ and P₂ if x₂ – x ₁ < 0 then the angle is negative and should be made positive by subtracting it from 360.

Translation from SSS to ISO and VAMAS:

The ISO and VAMAS coordinate systems are based on the orientation of the sample stage. The SSS fields do not contain information about the stage. Without This information translation from SSS to ISO and VAMAS is not possible.

Translation Between ISO and VAMAS:

The ISO 14976 Data Transfer Format is based on the original VAMAS format, and is nearly identical where instrument geometrical orientation fields are concerned. The only difference being that the y-axes are reversed.

References:

1. American Vacuum Society. Surface Science Spectra – An international journal devoted to archiving surface science spectra of technological and scientific interest. Ed: S. W. Gaarenstroom, M.P. Hecht. ISSN:1055-5269 Vol3, #4, 1994-1995 AES/XPS Contributors Form (Rev. 1/95)
2. DENCH, W.A. and SEAH, M.P. VAMAS Surface Chemical Analysis Standard Data Transfer Format with Skeleton Decoding Programs. NPL Report DMA(A)164, July 1988
3. DENCH, W.A. and SEAH, M.P. Specimen Files in VAMAS Standard Data Transfer Format (EXAMPLE1.TXT to EXAMPLE4.TXT) NPL Report DMM(A)90, March 1993
4. ENGLE, P.D. SSSTOOL.EXE (Ver 1.0) SSS Geometric Orientation Format Conversion Tool. MS Windows 95 and web page compatible 32 bit application program utility.
5. International Organization for Standardization. Surface chemical analysis – Data transfer format ISO/DIS 14976 Draft International Standard, 1996

Appendix A

Example 1:

Translation of instrument geometry for a Riber MAC 2 Auger micoprobe from VAMAS format to SSS format.

Start with the following 9 VAMAS field values.

Substitute these values into equation (2.0) to obtain

Repeat this procedure for (2.2) through (2.5) to obtain

Note that for each of the above calculations two of the terms are not VAMAS field values, but previously found SSS values.

From (2.7) ± F sp_s find the other coordinate set values in the same way.

P_2(v) (x₂) = 0

P_2(v) (y₂) = -0.9659

P_2(v) (y₂) = 0.2588

Substitute these values into equation (2.0) to obtain

± F sp_s = 0

Using the x coordinate values to find the sign, X₂ – X₁ = 0

Using the same procedure with (2.6) and (2.8) find ± F ig_s