Steam Generator Purifies Oxidation
Oxide apertures feature in most of the VCSELs deployed in optical mouse sensors and data communication networks. The structures deliver optical and current confinement (see figure 1), and are added by oxidation in a furnace with a controlled water vapor source.
The water vapor is generated by a bubbler or vaporizer. However, Jeff Spiegelman, president of Californian start-up Rasirc, says that VCSEL production yield and throughput improvements can be made if manufacturers switch to his company s steam purification system. "We are able to deliver [water vapor] at a rate and composition that gives us an advantage over everybody." The water vapor source provides the furnace with a continuous stream of gaseous water at a well defined temperature and pressure. If this uniformity is achieved, all of the VCSELs on the various processed wafers are oxidized identically and yields are very high.
Spiegelman believes that bubbler-based water vapor sources compromise this uniformity. Water is absorbed during the process, but the carrier gas – typically nitrogen or argon – is not consumed, which leads to changes in the composition and pressure of the gas. These variations alter the oxidation rate, resulting in non-uniform oxidation across the wafer. "The more nitrogen you have in your bubbler, the slower the growth rate and the worse the uniformity," claims Spiegelman.
Bubblers also suffer from contaminants in the gas stream. Any metallic or ionic contaminant in the gas can be absorbed by the oxide layer, which reduces the film s insulating properties and impacts the VCSEL s optical and current confinement. According to Spiegelman, vaporizers, which are also known as clean steam systems, can also produce high levels of contamination. This is because metallic parts can corrode and release ions. In addition, their performance is hindered by incomplete vapor formation. "The water drops onto a heated surface and you get some vapor, but you also get a clump of water molecules that haven t gone into the vapor state," explains Spiegelman. "When they go into the furnace they vaporize. [You need] a lot of energy to go from a liquid to a gas, which can lead to non-uniformity."
Spiegelman believes that Rasirc s Intaeger ultrahigh purity (UHP) steam purification system, which was launched six months ago and is based around a non-porous hydrophilic membrane, can address the shortcomings of vaporizers and bubblers. The machine can be fed with de-ionized water, which is relatively cheap, and contamination is also restricted by the membrane s selectivity. The membrane allows water molecules to pass through a million times more quickly than nitrogen molecules, thereby providing a source that is free from a carrier gas while removing contaminants such as other gases, ions, organic material, particles, viruses, bacteria and metals. Tests have shown that the Intaeger system can reduce 67 different metal contaminants to levels below detectable limits, and "virtually eliminate" nitrogen and carbon dioxide.
Rasirc is yet to sell its first UHP Intaeger steam generator to a VCSEL manufacturer, but it has increased oxidation rates at three silicon fabs. All of them have seen an improvement in growth rate of at least 14%, says Spiegelman. One fab reported a hike of 18%, which is equivalent to a time saving of four days a month. "That translates to three months or less for payback on the tool." Oxidation uniformity has also improved: one customer went from ±5 % at best to ±1.8 %, based on data from three lots of 25 wafers, which included wafer-to-wafer, across the wafer and across the furnace variations.
Rasirc s steam generator is more expensive than a bubbler and closer to the price of a vaporizer. However, Spiegelman points out that once the capital expense has been met, running costs are as low as $20 a year for the water, aside from the utility bill for boiling it. "And in these high-volume applications, like VCSELs, that cost differential is going to be critical."