TimeDomain CVD, Inc.

High Density Plasma Deposition: Overview

HDP: What and Why?

Silicon dioxide films deposited by plasma-enhanced chemical vapor deposition (PECVD) under normal conditions in a showerhead reactor incorporates quite a bit of hydrogen, typically present mostly as silanol, Si-OH [see discussion of SiO2 deposition under FILMS]. However, if enough ion bombardment can be provided, most of the hydrogen can be driven off, as shown by the data below, courtesy of Karen Seaward of Hewlett-Packard:

 

For reasonably high deposition rates, we need a very high plasma density to achieve significant effects this way. Let's define for convenience High Density Plasma Deposition as occurring when the ion flux to the surface is larger than the net deposition flux. (In this case, every deposited atom gets whacked by an ion at least once on the average.) The ion flux is estimated from the Bohm velocity and the plasma density. The deposition flux can be calculated from the deposition rate and the molecular density. Let's take an example: for deposition of silicon dioxide at 1000 A/minute, the molecular flux to the surface is 3.8E15 molecules/cm2 second; this is equivalent to an electric current (of singly charged ions) of about 0.6 mA/cm2. By reference to our previous calculations we find that the plasma density required at the sheath edge to produce such a flux is at least 1E10 ions/cm3.

If we can combine such high densities with a reasonably low pressure and some bias voltage on the sheath, we can get an additional benefit: significant sputtering of the deposited film. This backhanded method of deposition (put it on and then take it off??) can be useful because the sputtering rate is dependent on the angle.

 

Let us define an angle of incidence for incoming species:

As the angle varies (which happens when the film is deposited over features such as trenches or holes), the deposition rate and the sputtering (etch) rate vary:

The film will grow most rapidly on the surfaces which are nearly horizontal, with surfaces inclined around 45-50 degrees growing slowly due to the high sputter etch rate. As the surface propagates upward the slowest-growing planes will dominate the curved parts of the surface...

...forming "facets" inclined to the surface, which eventually meet and partially annihilate. The net result is that holes and trenches will tend to be filled up and planarized (if their initial aspect ratio isn't too high!).

For more details...

"Profile Simulation of Plasma Enhanced and ECR Oxide Deposition with Sputtering"; C. Chang, J. McVittie, K. Saraswat, S. Lassig, J. Dong; 1993 Int'l Electron Devices Meeting, Washington DC, technical digest p. 853

So if we combine high plasma density and low pressure, we can get some neat benefits: high film quality at low deposition temperatures, and nice gap-filling and planarization properties at the same time. However, conventional showerhead reactors cannot achieve the high plasma densities cited above at low operating pressures. New reactor designs and operating principles are needed (and as we'll see, new problems arise).

 

 

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