Environmentally friendly wet cleans protect yields, bottom line

11/01/2004

New global guidelines and semiconductor industry targets for protecting the environment are compelling chipmakers to consider options for waste/effluent treatment and reduction. Historically, IC manufacturing has required large volumes of highly concentrated chemicals with limited lifetimes to fabricate devices. Once these chemicals surpass their useful lifetimes, typically on the order of hours, they must be either collected for disposal or sufficiently treated and neutralized before being released into the surrounding environment.

While improvements in temperature control, filtration, and purity levels have extended the useful life of concentrated chemicals, the growth of fabrication lines worldwide and the concentration of wafer fabs in new locations have increased pressures on chipmakers to consider a variety of available dilute chemistries and alternative wet-processing techniques for advanced front-end-of-line (AFEOL) cleaning steps. Many of these new wet processes are more cost-effective than their predecessors, but the industry's movement toward these alternative methods of cleaning and surface-preparation steps has been relatively slow. A major hurdle slowing the adoption of environmentally benign processes is the need to requalify fabrication lines for new processes of record. In many cases, fab operators prefer to "recenter" existing qualified processes rather than adopt new ones, but this practice can become costly as environmental protection guidelines grow more stringent.

Early IC processing employed a variety of "buckets" or containers for the various cleaning solutions. These solutions were used for as few as one process cassette prior to being dumped, requiring a new bath to be mixed for the next batch of wafers. Single-use chemistries included an FEOL cleaning mixture of sulfuric acid, hydrogen peroxide, and DI water (often called a "piranha" clean). These H₂SO₄/H₂O₂-based piranha cleans were often used in ratios of 2:1 with water to strip photoresists and other organic compounds, which are detrimental to the operation of devices.

New chemistries and processes

In response to emerging environmental guidelines and targets, wet-processing tool suppliers have developed new cleaning technologies designed to protect the earth while helping device manufacturers improve yields and reduce costs. One example is the use of ozonated water for resist-strip processes instead of commonly used H₂SO₄/H₂O₂ piranha chemicals. Sulfuric acid is a viscous, corrosive formulation that requires copious amounts of neutralizer for disposal purposes, making it environmentally unfriendly.

Data showing the effect of process temperature and dissolved ozone concentration on the average stripping rate of the DIO3 process.Click here to enlarge image

In contrast, Akrion's patented DIO3 process uses pure water, which is super-saturated with ozone to accomplish the same process results as sulfuric acid. Ozonated water has proven to be an effective alternative for stripping many types of photoresist in Akrion's Allentown Class 1 applications laboratory for both wafer and mask manufacturing (see figure). The process has been introduced into several wafer fabs worldwide. Moreover, this process has also been shown to be more cost-effective than sulfuric acid.

Here's how it works. Ozonated water oxidizes organics from the wafer surface. The chemical reaction that takes place in the process [1] is:

What remains are all by and large benign to the environment: oxygen, carbon dioxide, and water.

In contrast, the reactions that take place in a piranha process are:

The ozonated water process accomplishes the organic removal operation without the treatment or disposal concerns associated with piranha processes, which leave behind spent sulfuric acid to be neutralized for disposal. An added benefit of the ozonated water process is that it also eliminates the post-rinse operation that normally follows the sulfuric acid process. This decreases the amount of water required for the process by ≥50 gallons
un. The ozonated water process uses small volumes of ozone, and the ozone molecules have relatively short lifetimes, making the process a cost-effective and environmentally sound solution for the IC industry. Other benefits include a longer life expectancy for the wet-processing tool (sulfuric acid is corrosive) and enhanced safety.

Other cleaning applications

Ozonated deionized water is also a component of several other processes, including AFEOL cleaning, where, as with resist-strip processes, ozonated water replaces sulfuric acid for removal of organic contaminants from wafer surfaces. The advantages in the stripping process also are inherent in AFEOL cleaning processes using ozonated water.

For particle removal, many fabs continue to use a highly concentrated (1:1:5) solution of ammonium hydroxide, hydrogen peroxide, and water in traditional RCA cleaning processes, known as APM or SC1. Due to elevated temperatures and high vapor pressures, however, these solutions must either be continuously spiked with additional chemicals or be discarded and replaced with a new bath. Similarly, a mixture of hydrogen peroxide, hydrochloric acid, and water is also used in a high concentration (1:1:6) for HPM or SC2 cleaning steps to remove metallic impurities from wafer surfaces.

Compared to these widely used process steps, new AFEOL systems require fewer chemicals overall. Hydrofluoric acid for oxide removal is run at very dilute concentrations compared to most existing processes. These new formulations are largely composed of water, resulting in lower costs for process materials and disposal. In the standard process, RCA cleaning chemistries are used in high concentrations. With advanced cleaning systems, SC1 processes are run with very dilute concentrations and SC2 is eliminated. For controlling metallic impurities, a small amount of hydrochloric acid is added to the rinse water prior to the dry operation, thus eliminating the need for a dedicated SC2 tank and corresponding rinse.

This type of AFEOL process also offers a performance benefit. The use of HF eliminates chemically grown oxide, reducing impurities in the chemicals (e.g., hydrogen peroxide). Consequentially, the metallic signature associated with these impurities is also eliminated. Independent analysis by Balazs Analytical Services confirmed that metallic contamination levels in Akrion's AFEOL processes are <7E9 atoms/cm² for all elements. This is an order of magnitude better than the standard cleans, resulting in better charge-to-breakdown performance and increased yield of good devices/wafer.

With all these environmental advantages available, the biggest remaining challenge is to convince chipmakers to adopt environmentally benign processes as standard processes of record. Typically, new generations of products are introduced every 6–12 months, creating a continuous flow of device qualifications for high-volume wafer fabrication lines. Qualifications can be costly and time-consuming and allow little room for change. While higher device yields are desirable, new processes and changes in chemistries are often avoided because production lines must be requalified. While device manufacturers often aim to recenter qualified processes of record, eventually environmental regulations and potential cost savings from new wet-processing steps will win out and require changes in the FEOL.

Acknowledgments

DIO3 and ICE-1 are registered trademarks of Akrion Inc.

Reference

1. I. Kashkoush, et al., "Using an Ozonated-DI-water Technology for Photoresist Removal," Micro, January 2001.

For more information, contact Glenn Marshall at Akrion Inc., 6330 Hedgewood Dr., Allentown, PA 18106; ph 610/530-3559, fax 610/391-1982, e-mail [email protected].