Sims analysis of residual gas elements.

A.Villegas, Yu. Kudriavtsev, A.Godines, R. Asomoza

Dep. Ingeneria Electrica – SEES, CINVESTAV-IPN, Av. IPN # 2508,

Mexico D.F. 07300, Mexico

SIMS is the most sensitivity method of the elemental analysis of solid surface and thin films with the detection limit (DL) of 10¹⁴ atoms/cm³ for the most elements. But in the case of elements like H, C, N and O, which form atmospheric gases, the DL is essentially worse. The reason is presence of these gases in the residual atmosphere of an analytical instrument. The gases adsorb continuous on the analyzed sample surface and become re-sputtered by an ion beam and registered as the background signal. Therefore the SIMS signal for these elements ( negative ions in our study) consists of the two components: particles, presented in the bulk of the target and the same particles, adsorbed and re-sputtered than:

, (1)

here s is the analyzed surface area, j_o is the primary ion current density, Y_x is the partial yield of element x, b_x is the ionization probability of element x, R_x is the transmission coefficient of the instrument, N_ad is the flux of molecules on analyzed surface from residual atmosphere, a_x is the sticking coefficient and n is the number of x atoms in the adsorbed molecules.

Under the typical SIMS regime the adsorption rate is almost 4 order less than the sputtering rate. And we can conclude that a balance between adsorbed and target atoms depends on the sputtering regime, only. Moreover, all adsorbed atoms will be re-sputtered back (in time unit).

Eq.(1) gives a method of experimental determination of both adsorbed and target atoms. Indeed, eq.(1) can be considered as a simple linear function () of the primary ion current density j_o. So, both component of eq.(1) can be found from a linear extrapolation of experimental dependence of SIMS signal on j_o.

We have applied described before technique for analysis of H, C, O, N, and F in Si and GaAs targets. The DL for the elements was found almost one order more than for the “standard” SIMS measurements. We estimated the residual gas composition in the sample chamber, basing on experimentally found adsorbed component in eq.(1) - N_ad.