INDUSTRY GaAs MANUFACTURING
through modifications to the showerhead, a part of the upper electrode assembly that ideally distributes an even gas flow over the entire diameter of the process chamber. Initially, a lack of high uniformity in the SiN films on the wafers appeared to follow no pattern, but rather, was quite random. Further investigations at Plasma-Therm identified an issue with the showerhead. This led to a change in the manufacturing process for this part, plus the establishment of a final quality control procedure that drew on the findings of the process testing of the showerheads.
This effort has driven down the level of non-uniformity that is to be expected in SiN films deposited by the Plasma Therm tools beyond what was originally specified by RFMD. Typical thickness non-uniformities on the order of ± 3-4 percent have been the norm but the lower the uniformity the better and the ‘magic’ ± 1.5 percent target is the goal for the future.
The improvements made by Plasma-Therm have driven down the non-uniformities, and today, it is not uncommon for the non-uniformity of the SiN process to be less than ±2 percent; and the uniformity is often significantly better than this. The showerhead, along with the chamber walls and internal chamber fixturing, is a major source of particles in any PECVD deposition system. To minimise these particulates, it is important to heat as many surfaces within the chamber. This leads to better adhesion of deposited material and less flaking or de-bonding of the deposited SiNx
from the chamber
fixturing. Parts that are heated include the upper electrode. After every in-situ clean, the showerhead is returned to its original manufactured operating condition.
Initially, the benefits of in-situ cleaning went unnoticed, due to the relatively high frequency of placement faults, which had to be addressed by opening the chamber. According to the initial preventative maintenance schedule, an open chamber clean was to take place every six months, but it made sense to also perform these cleans every time the chamber had been opened to address an indexer fault. The frequency of these faults varied from tool to tool, and in some cases took place every two- to-three months.
It is hard to know how long an in-situ chamber clean should be done, and this inevitably leads to ‘overkill’ in the cleaning process. This is very inefficient, because it reduces tool availability. One way to address this, which has
Figure 1. A robot is used to load the wafers into the tool. Each position on the indexer is ‘taught’ and the positions stored in memory for consistent loading and unloading
been pursued through the Plasma Therm-RFMD partnership, is to turn to Optical Emission Spectroscopy (OES). This provides clear, precise detection of when all the material has been removed (see Figures 3 and 4).
A common approach to removing SiN is to etch with a fluorine- based gas. This type of approach is used at RFMD, with in-situ cleaning involving a mixture of SF6
and N2 O – the latter acts as
a source of oxygen that bonds with the free sulphur to increase the etch rate. Cleaning is typically carried out after 1.5 µm of SiN deposition, using a process that takes about an hour to both clean (end pointed by OES) and carry out a subsequent post clean ‘conditioning’ deposition.
Thanks to improvements to the in-situ cleaning process, the intervals between open chamber cleans have been increased. In 2010, RFMD’s engineers decided that it was acceptable to carry out the open chamber cleans just once every year, and in late 2011, this interval was extended to two years. These intervals have increased due to the improvements and cooperation by Plasma-Therm and RFMD to resolve the issues
Figure 2. A small gain in uptime can translate into a throughput increase of thousands of wafers per year
June 2013
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