Oxide thickness determination by XPS, AES, SIMS, RBS and TEM

As the R_p of ion implants steadily decreases an ever-increasing percentage of the implant species lies in the oxide layer and is, therefore, not electrically active. For this reason it is important to have analytical techniques capable of accurately measuring the thickness of ultra-thin oxide layers. A round-robin study was performed on a series of SiO₂ films ranging from 0.3 to 20 nm in order to evaluate the advantages and disadvantages of five commonly used analytical techniques. High resolution cross-section transmission electron microscopy (TEM) offers the only true measurement of oxide thickness because no density assumptions are made. In this study TEM is used as the standard for all the other techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) offer precise measurements for ultra-thin (<3 nm) films, but are limited for thicker films (>15 nm) due to the exponential decay functions that describe the sampling depth in both techniques. Secondary ion mass spectrometry (SIMS) has historically been used for characterizing relatively thick films (>10 nm) but not for thinner films because of atomic mixing effects. Encapsulating oxides with amorphous silicon prior to performing a SIMS experiment can negate these effects. A comparison of SIMS on encapsulated and as received films is made. Rutherford backscattering (RBS) is an excellent technique for determining oxide thickness over a wide thickness range by channeling the Si signal from the crystalline substrate and analyzing oxygen from the amorphous oxide.