Secondary ion yields of C, Si, and Ge in InP, and Cs surface density and concentration, in SIMS

Abstract

In secondary ion mass spectrometry (SIMS), the relationship between negative secondary ion yield ( γ −) of C, Si, and Ge in an InP substrate and sputtering yield ( Y), and that between the secondary ion yield and the surface density of reactive primary ion Cs ( n Cs(0); zero refers to substrate surface) were examined by changing the incident angle of the primary ion to the surface. The γ − value was derived by dividing the corresponding useful yield by the instrumental transmission factor, which was obtained by measuring the useful yield of an element with high electron affinity, such as fluorine. Monte Carlo simulations using TRIM code and theoretical analysis were performed to obtain the Cs density (atoms/unit volume) depth distribution, n Cs( x), in the sputtered substrate. The density depth distribution was measured directly using SIMS, under conditions where the energy of the O 2 + primary ion was sufficiently reduced. n Cs(0) was obtained by setting x of n Cs( x) to zero. The Cs ions used for sputtering were distributed mostly according to a Gaussian function. The n Cs( x) profile estimated on the basis of the Gaussian distribution showed good accordance with the measured one. At the sputtered surface, n Cs(0) was closely proportional to 1/ Y. The values of γ − for C, Si and Ge changed proportionally to the power of n Cs(0), or 1/ Y. For C, Si, and Ge, the lower the γ −, the more sensitively γ − changed in response to n Cs(0). Furthermore, the composition at the Cs-sputtered surface was examined using Auger electron and wavelength dispersive X-ray spectrometry. These measurements showed that n In(0), n P(0) and n Cs(0) changed almost linearly with 1/ Y InP, suggesting that the Cs surface concentration (at.%), C Cs(0), is proportional not to 1/ Y itself, but to a function of 1/ Y. However, under certain special conditions, C Cs(0) is also proportional to 1/ Y itself, as is n Cs(0).