Investigating methods for improving the removal of inorganic and organic contaminants, perceived and real, from water, thus producing ultrapure water to minimize yield failure concerns, is an ongoing process. As line widths become tighter, it has become critical to address not only particulate and ionic contaminants, but specific organic issues. Evoqua Water Technologies has developed processes that specifically address the Total Organic Carbon (TOC) levels coming from elements such as Chloroform, Urea, IPA and others to reduce them to sub part-per-billion (ppb) levels. This has been shown to have significant benefits in integrated chip (IC) manufacturing and improved life cycle costs of critical process equipment components.
Moore’s Law continues to play a defining role in integrated circuit (IC) manufacturing; “the law,” simply said, is that the number of transistors in a single microprocessor will double every eighteen months in order to keep up with leading edge technology in the industry. In order to meet the technology requirements, circuits are to be packed into a microprocessor at 22 nm and below. The common microchip is also being packaged into smaller devices and applications, presenting a number of challenges for manufacturers, including providing critical ultrapure water.
The International Technology Roadmap for Semiconductor (ITRS) indicates a continuing need for the following to reduce the risk of yield problems, especially in the area of logic chip manufacturing:
- Further reduction in organic and inorganic contaminants
- Further reduction in the amount of water consumed in terms of litres/cm2 of wafer
- An increasing recovery of used water in terms of ultrapure water recycled (reuse after treatment or reclaimed including extracting useful components from waste)
The latest version of ITRS issued in 2009 with certain updates in 2010 shows a continuation of this trend.
One of the key elements in this direction is to constantly seek ways to reduce Total Organic Carbon (TOC) levels. This can be stated to be the quantity of carbon bound up in an organic compound. Most analyses measure the total carbon levels and deduct the inorganic levels to arrive at the organic value.
TOC typically comes from decaying natural organic matter such as humic acid, fulvic acid, amines, in addition to urea from fertilizers, animals and some chemical processes. The synthetic chemical sources may come from incompletely treated industrial and domestic waste streams such as detergents, pesticides, and herbicides. The impact of these organics on semiconductor facilities can be directly attributed to seasonal changes and agricultural activities in most regions. In many municipal water feed streams, the combination of chlorine and the organics produces trihalomethanes (THM’s), such as chloroform.
The impact on TOC of 1.0 part per billion (ppb) of chloroform is approximately 0.1 ppb. The impact on TOC of 1.0 ppb of bromoform is approximately 0.047ppb. Therefore, removing chloroform or preventing its introduction is very important in minimizing TOC levels.
The primary methods of removing TOC are through the use of activated carbon absorption and the use of hyper-filtration (typically reverse osmosis). Increasing the pH of the liquid takes advantage of the hydration impact on the organics to increase the molecular weight and therefore improve the rejection rate, as shown in Fig 1. Increasing the pH above 9 has an additional benefit of also improving the rejection of boron by the reverse osmosis (RO) membrane. THM’s are removed readily by activated carbon. However, the absorption capacity of the carbon is limited, and TOC spikes associated with THM’s are common where the carbon process is designed for chlorine removal or is infrequently changed.
The other main method of reducing the residual level of TOC is through applying heat, especially combustion and ultraviolet irradiation, with or without a chemical oxidizer such as persulphate or iron and hydrogen peroxide (Fenton’s reaction), or with the use of a catalyst such as platinum or titanium dioxide.
In a January 2003 article in Ultrapure Water magazine, entitled Removal of Trihalomethanes from RO Product Water Using UV 185-NM Technology, Gareth Thomas and Dr. Avijit Dey described a process using 185-nm wavelength UV to reduce THM’s from high purity water, especially chloroform . It showed that, subject to the energy input level, at least 40% could be removed.
Brominated hydrocarbons (e.g. Bromoform CHBr3) break down easily in the presence of 254-nm wavelength ultraviolet light into carbon dioxide and bromide ions. However, chlorinated hydrocarbons such as chloroform (CHCl3) do not break down easily. The use of heat changes the bond strength of the organic structure of chloroform. The change enables the gas transfer membrane to extract the now volatile organics, thus reducing the residual TOC levels.
In a November 2003 article in Ultrapure Water magazine, entitled Advanced Organics Oxidation – Removing Urea from High-Purity Water, the authors referred to the use of sodium bromide and ozone, together with UV, for oxidation and ozone destruction, producing a reduction in urea. This was developed to address what was believed to be the natural decomposition of urea into ammonia. The concern was that this decomposition would adversely affect the “acid catalyzed chemical amplified photoresists that are used with DUV (Deep Ultraviolet) lithography.”
For certain complex organics, especially non-degradable and volatile organics, there has been a move towards advanced oxidation commonly referred to as the “Advanced Oxidation Process” (AOP). There have been a number of different methods such as:
- Hydrogen Peroxide & Ultraviolet Irradiation
- Ozone & Ultraviolet Irradiation
- Ozone & Hydrogen Peroxide & Ultraviolet Irradiation
- Fenton’s reagent (Ferrous Sulphate and Hydrogen Peroxide) & Ultraviolet Irradiation
- Persulphate & Ultraviolet Irradiation
- Caro’s Acid (Peroxymonosulfuric acid) & Ultraviolet Irradiation
Where Isopropyl-Alcohol (C3H8O) (IPA) is present, the use of ultraviolet irradiation changes it into Acetone. The problem with acetone as an organic is that it is so close in molecular weight to water that it is virtually impossible to completely break it down; therefore, an additional process is needed to completely reduce IPA.
Oxidant and UV has been used in the semiconductor industry for some time but is not always a successful TOC reduction process. The use of persulphate previously had been limited in its scope due to the impact of contaminants such as sulphate, which is known to aid in the breeding of bacteria and is an ionic contaminant.
The persulphate/UV process has now been refined with critical improvements in the reactor design, allowing for reduction in power and capital costs along with reduction in chemical utilization.
Evoqua has recently introduced the Vanox™ system (patent pending) to effectively reduce and control THM’s, Urea and IPA in addition to controlling other critical process parameters. These are the primary organics that can require a more elaborate treatment and cause potential risk to an Integrated Circuit manufacturing facility.
The technology has been successfully applied at semiconductor Fabs in Point-Of-Use (POU) applications, in which the TOC level is normally low but the customer experiences TOC excursions in the ultrapure water polishing loop throughout the year.
The system is designed as a POU system and provides the best solution for removal of target trace organics with an expected positive effect on yield. It also provides a more effective destruction of IPA and other organic compounds than traditional ozone and peroxide/UV systems, in typical re-use and reclaim designs.
Figure 6 represents the impact of a TOC elevation in the feed water that worked itself through the complete ultrapure water system. The primary organic in the system during the TOC elevation shown here is Urea.
The ultrapure water system is fed to the Vanox POU system to maintain a TOC level below 1.0 ppb, typically 0.5 ppb, with up to 18 ppb in the feed water, (polish loop quality). Note: Both TOC analyzers were calibrated PPT analyzers.
In applications using medium pressure ultraviolet light (UV), the generation of hydroxyl ions and hydrogen peroxide is well understood. However, once the water enters the mixed bed, there is a catalytic effect whereby the dissolved oxygen is released. There is typically a trade off of TOC reduction vs. an oxygen increase in many applications. The Vanox™process includes the use of the membrane vacuum degassifier to remove the generated dissolved oxygen. Ammonia from the urea and carbon dioxide from the breakdown of the organics is removed in the mixed bed.
Evoqua has developed a process that purifies ion exchange resin to close to equilibrium values, so that the product quality for residual metals is reduced to sub-ppt levels. The NR-30 MEG Nano resins are part of the process with the Vanox™ system so that any trace metals and ionic contaminants are consistently removed. The residual metals levels are below 1.0 ppt.
The goal is to provide consistent process control to critical tools. As a result of sound process design, detailed engineering, disciplined construction and particle control, the quantity of residual particles is consistently less than 100 cts/l @ 0.05 micron.
The heat exchanger control is also carefully monitored so that the temperature variability is maintained within +/- 0.1 degrees centigrade. This helps ensure that there is minimal variability in the feed water condition, and maximizes the effectiveness of the ultrapure water when it is used by the tools.
Ultrapure water continues to play a critical role in the manufacture of integrated circuits, while at the same time, environmental conditions and the desire to reduce water use in today’s semiconductor facilities drive the need for new process technology. The Vanox™ system is designed to maintain consistent Point-of-Use (POU) product quality during excursions from the feed or reuse water sources for critical manufacturing processes.
