Electron and negative ion density have been measured in a modified Applied Materials decoupled plasma source commercial metal etch chamber using gas mixtures of BCl3, Cl-2 and Ar. Measurements were performed for four different substrate types to examine the influence of surface material on the bulk plasma properties: aluminum, alumina, photoresist, and 50% patterned aluminum/photoresist. Electron densities in the Cl-2/BCl3 mixtures varied from 0.25 to 4 X 10(11) cm(-3). Photodetachment measurements of the negative ion density indicate that the negative ion density was smaller than the electron density and that the electron to negative ion density ratio varied between 1 and 6. The presence of photoresist had a dominant influence on the electron and negative ion density compared to alumina and aluminum surfaces. In most cases, the electron density above wafers covered with photoresist was a factor of 2 lower, while the negative ion density was a factor of 2 higher than the aluminum or alumina surfaces.