Borophosphosilicate glass
Borophosphosilicate glass is with 3 to 5% of boron and phosphorus doped silicate - glass , which in the semiconductor technology is used, among other things, to planarize the surface.
Application in semiconductor technology
During the manufacture of integrated circuits , the deposition and structuring of different materials create height differences and steep flanks. Due to the shallow depth of field of the photolithography systems, these negatively influence the subsequent photolithographic processes. It is therefore necessary to reduce these height differences and flank angles. Different processes have been developed for this in the course of the history of semiconductor technology.
The process of planarization by melting ( reflow technology ) of doped silicate glasses was mainly used in the first metallization level until around the mid-1990s . A doped silicon dioxide layer is deposited on the structured surface by means of chemical vapor deposition (for example LPCVD at 400 ° C.). After the deposition, the coated surface is melted at high temperatures in a diffusion furnace, the silicate layer dissolves and forms a planarized surface. In order to prevent undesired diffusion of the foreign atoms into the silicon, an undoped silicon oxide layer, which acts as a diffusion barrier, is deposited in front of the doped silicate glass layer.
Doped silicate glasses are used because the admixture of pure silicon dioxide (1200 ° C) can significantly lower the melting point and thus reduce the thermal load on the substrate. This is important because at temperatures around 1000 ° C the doping atoms in silicon diffuse significantly, shifting the necessary flat doping profiles and changing the electrical properties of the transistors. The lowering of the melting point is possible because the foreign atoms fit into the silicate network, but in contrast to silicon, they can only build up three instead of four bonds to oxygen atoms and thus form a less rigidly bound network.
To begin with, a phosphosilicate glass (PSG) with a proportion of up to 8% phosphorus was used. This means that the melting point can be reduced to as much as 950 ° C. The phosphorus content can be increased further, since PSG then becomes highly hygroscopic and the phosphorus can form corrosive phosphoric acid together with moisture . With an additional boron doping, the melting point of the borophosphosilicate glass produced can be reduced to below 900 ° C. However, the maximum boron content here is also approx. 4%, since otherwise boric acid can form.
With the introduction of chemical-mechanical planarization (CMP), planarization by means of silicate glass liquefaction has lost its importance and is no longer used in modern manufacturing processes.
Individual evidence
- ^ A b Dietrich Widmann, Hermann Mader, Hans Friedrich: Technology of highly integrated circuits . Springer, 1996, ISBN 3-642-61415-9 , pp. 71 .
- ↑ a b c Jan Albers: Basics of integrated circuits: components and microstructuring . Hanser, 2007, ISBN 978-3-446-40686-5 , pp. 190 .