Studies of Sodium in SiO[sub 2] Films by Neutron Activation and Radiotracer Techniques

Abstract

Electrical instabilities in SiO2 films under high fields at elevated temperatures have been widely attributed to mobile ionic impurities, with sodium suspected as the most important ion. Phosphate glass treatments on SiO2 are known to have a stabilizing effect. This paper reports neutron activation experiments to determine sodium contamination on and in steam‐grown SiO2 films and also diffusion and drift experiments with tracer sodium. Phosphate glass‐treated SiO2 films were included in the samples. SiO2 films grown and handled under various conditions had sodium densities of 7×1011 normalto 3×1014 normalatoms/cm2 , which could be easily removed or exchanged by washing and light etching. Densities remaining inside the 6000Aå films after washing and light etching were ⋜4×1011–4×1012/cm2 .Tracer diffusions into SiO2 at 600°C produced profiles which were high near the surface and lower through the interior of the film, similar to those observed by neutron activation for unintentional sodium contamination. Phosphate glass layers on SiO2 absorbed and stopped the sodium under similar conditions. With an applied field at elevated temperature (+4v, 1 min, 400°C) tracer sodium drifted through an SiO2 film under gold dot electrodes, piled up at the normalSi‐SiO2 interface, and carried a substantial part of the charge transported. There was no detectable diffusion in uncharged areas. A layer of phosphate glass applied over an SiO2 film absorbed sodium and prevented it from drifting into the SiO2 film under the gold dots.

Preparation and Properties of Pyrolytic Zirconium Dioxide Films

“…Hot phosphoric acid (220~ was used to remove ZrO2 at a rate of 50 A/min. The Na penetration was determined using the technique developed by Buck et al (16). Figure 6a and b are plots of Na22/cm3 vs. depth in angstroms for ZrO2 on silicon, for diffusion times of 22 and 200 hr, respectively.…”

Phosphosilicate glass stabilization of FET devices

“…However, Na+ contamination incorporated in the SiO, near its surface only (as obtained by oxidation of a NaCl contaminated bare silicon surface) will be reduced by more than an order of magnitude during subsequent stabilization [l I]. When NaCl is evaporated onto the surface after stabilization, a brief heating period at relatively low temperature suffices to lodge the contaminant firmly in the outermost layer of the glass [5]. Even when entrapped in a PSG film, Na+ ions will affect the surface potential, since they will be partially compensated [2] by a space charge layer in the silicon (see Fig.…”

“…In the absence of such a film, cation impurities (notably Na') will rapidly drift [2]- [6] through the SiO, layer under the influence of an electric field and change the threshold voltage for conduction. The PSG film acts as a Na' ion getter and a barrier against Na' ion drift [3], [5], thereby stabilizing the device characteristics.…”

Determination of Sodium in Furnace Atmospheres by Atomic Absorption Spectroscopy

“…Manuscript submitted Jan. 2, 1969; revised manuscript received May 7, 1969. This was Paper 522 presented at the Montreal Meeting, Oct. [6][7][8][9][10][11]1968.…”

“…The distribution of this contaminant in these films and on cleaned silicon surfaces has been studied by flame emission, radiotracer, and neutron activation analytical methods (7)(8)(9)(10)(11). The main source of this impurity has been suggested to be the furnace tube and liner in which oxidation and diffusion reactions occur (4).…”