Issue Date: August 6, 2007
Cleaning Up In Electronics
TAKESHI HATTORI, a former Sony electronics executive who now consults on semiconductor cleaning, described a high-tech future for the practice of cleaning silicon wafers at last month's Semicon West trade show in San Francisco. He sees environmentally friendly cleaning based on solvents such as supercritical carbon dioxide and even nanotweezer tools that remove unwanted particles from wafer surfaces.
Hattori's vision may be realized someday, but for now the job of keeping wafers clean during the semiconductor fabrication process is decidedly more prosaic. Computer chip makers use a witch's brew of acids, solvents, chelating agents, surfactants, and corrosion inhibitors to remove photoresists, polishing slurries, metal residues, and other debris generated during the multistep process of creating transistors and circuitry on top of a silicon wafer.
These cleaning mixtures are supplied by the electronics divisions of a number of chemical companies. Leaders in the field include Air Products & Chemicals, DuPont, ATMI, and Mallinckrodt Baker, although numerous other firms vie for market share. Analysts at Semiconductor Equipment & Materials International, the trade association that hosts Semicon, estimate that the global market for cleaning and other wet chemicals used in semiconductor fabrication was worth about $905 million last year, up almost 7% over 2005.
Big or small, all industry players face a multipronged challenge. They have to develop formulas that gently but effectively clean the new materials that are being introduced in the most advanced chips, including copper circuitry, hafnium oxide transistor insulators, and carbon-doped silicon oxide dielectrics. At the same time, they must respond to customer demands for lower cost products that meet environmental regulations.
"It's not just about meeting technical needs," said Douglas J. Holmes, director of worldwide marketing and product management at DuPont EKC Technology, speaking to C&EN at Semicon. "It's also environmental, and many products don't meet customers' environmental standards anymore."
A case in point is a class of cleaning formulas called post-etch residue removers. A key step in the chip-making process is the creation of circuit lines by applying a thin layer of photoresist to a silicon wafer, exposing the resist to light shone through a photomask, dissolving the exposed resist, and then depositing metal into the newly created grooves. The leftover resist is typically burned away by a plasma-etching technique; the residue is washed off with post-etch residue removers.
IN 2001, chip makers started switching the circuit line metal in their most advanced chips from aluminum to copper. They soon found that they also needed new post-etch residue remover formulas because, among other things, the hydroxylamine solvent that cleans aluminum so well tends to corrode copper.
Chris Reid, an applications engineer at DuPont EKC, told a Semicon technical session that the initial formulas for removing copper residue were based on fluorinated compounds and organic solvents. But, he said, customers soon started clamoring for products that don't contain N-methyl pyrrolidone, dimethylacetamide, or other unwanted solvents.
DuPont EKC won a technology innovation award at Semicon for CuSolve EKC520, an aqueous cleaner designed to remove photoresist ash as well as trace copper oxide and organometallic copper compounds. And because it's water-based, EKC520 doesn't need to be followed by a solvent-removal rinse step. Reid said EKC520 was developed in cooperation with a leading contract chip maker in Asia.
As Holmes and Reid well know, winning an award is not the same as winning business, and DuPont faces competition from several directions. John Bubel, director of surface preparation technologies at ATMI, claims that his company is the market leader in post-etch residue removal products for copper-based chips, and it plans to stay that way.
Although ATMI executives are aware that regulatory authorities in Europe and elsewhere are cracking down on the use of certain solvents, Bubel said his company doesn't plan to go totally solvent-free. "We don't limit ourselves to aqueous or solvent technologies," he said. "Our goal is to solve customers' problems, and two customers at the same technology node could have two completely different sets of needs."
DuPont also faces newcomer General Chemical, which is entering the post-etch residue removal market on a cost-cutting platform. Best known as a supplier of simple high-purity acids and solvents, General started selling formulated cleaning products in 2000 following the acquisition of Moore Enviro-Chemical. Since then, the company has come out with photoresist removers and other cleaning and stripping formulas.
At Semicon, the company was promoting a new series of low-cost aqueous residue removers marketed under the PMD name. Robert Rovito, a former Air Products scientist who joined General about a year ago as product development director, said his company has streamlined product formulas by using multifunctional ingredients that combine, for example, buffering as well as chelating. As a result, he claimed, General can sell its residue removers for up to 50% less than the $60-$70 per gal that other companies charge.
According to Rovito, relentless customer demand for lower prices is turning formulated electronic chemicals into commodities, albeit extremely high-purity commodities, just like the acids and solvents that General has long sold to the semiconductor industry. Competitors, though, aren't responding to this market demand, he said. "The products currently on the market have a lot of components and are very complex. They are expensive to make and expensive to support."
COST CONCESSIONS are already apparent in older cleaning chemistries, such as residue removers for aluminum circuitry. At Semicon, ATMI was promoting Ideal Clean 815, a fluoride-based formula for aluminum post-etch residue removal developed jointly with Anji Microelectronics, a Shanghai-based electronic materials company. ATMI product literature promises "substantially lower cost of ownership."
Mallinckrodt Baker played a variation of the cost card with the announcement of plans to expand production of its ALEG-380 photoresist stripper and residue remover. The company isn't making overt low-price claims, but Gary Dailey, global marketing manager for integrated circuit materials, asserted that ALEG-380 saves money by delivering a yield improvement of more than 3% to electronics customers. The company adds that the product's lack of fluorinated compounds and hydroxylamine eliminates many health, safety, and waste disposal concerns.
In fact, Mallinckrodt and ATMI both emphasize that their aluminum cleaners don't contain hydroxylamine, a semiconductor cleaning chemical that is patented by DuPont EKC and licensed only to Air Products. Dailey acknowledged that hydroxylamine-based products lead the aluminum market, but he said customers are looking for alternatives. The chemical is expensive—BASF is the world's only producer—and must be followed by an intermediate rinse step.
In addition to post-etch residue removers, chemical makers at Semicon were pushing cleaners used after chemical mechanical planarization (CMP), an increasingly popular semiconductor processing step in which silicon wafers covered with unfinished chips are immersed in an abrasive-containing slurry and pressed against a spinning polymeric pad. The pad/slurry system smoothes and levels the wafer, but it leaves a watery film of metal and chemical particles, just as traditional sanding leaves a layer of dust.
Ashutosh Misra, director of materials development at Air Liquide Electronics U.S., said his firm has been developing post-CMP cleaners for copper-based chips for the past four years and doing business with a major semiconductor firm for the past two. Air Liquide chose Semicon to step up marketing of the products, which it sells under the CuClean name.
Planarizing and then cleaning aluminum circuitry was relatively simple, Misra pointed out, because aluminum is self-passivating, meaning it forms a thin oxide layer that prevents further corrosion by water. The oxide layer that forms on copper, in contrast, offers less robust protection and must be complemented with corrosion inhibitors such as benzotriazole (BTA).
The catch is that BTA forms a multilayer film that threatens to cause copper delamination during subsequent high-temperature wafer processing steps. According to Misra, Air Liquide's CuClean products can be formulated to leave a minimal amount of BTA or even completely remove it by replacing the inhibitor with a non-BTA alternative. Other ingredients help remove abrasive particles and organics, he added.
Earlier this year, Air Liquide opened a CMP applications and development laboratory at its Dallas wet chemicals facility to accelerate post-CMP cleaner development. The lab's cleanroom houses metrology equipment and a Mirra Mesa CMP machine from semiconductor equipment supplier Applied Materials. "Air Liquide has been investing heavily in this area," said Scott Zentack, the company's CMP business manager. "Other companies, I'm not so sure."
Patrick X. Murphy, business manager for electronic formulated products at Air Products, countered that his company's investments have catapulted it into a wide variety of semiconductor materials markets-more so than any other firm. One advantage of this position, Murphy added, is an unequaled view of how various semiconductor materials and fabrication steps interact.
For example, Murphy noted, Air Products supplies dimethoxymethylsilane and a companion pore-generating material, which some chip makers are using to create a porous insulator around their copper circuit lines.
After customers use CMP to polish the copper, Air Products can follow up with post-CMP cleaners such as its CP72B, which contains wetting agents to improve contact with the hydrophobic insulator. The company also sells novel ancillary products like its OptiPattern surface conditioner. This product uses surfactants to keep capillary force from causing ultrathin circuit lines to collapse during cleaning.
OTHER FIRMS are working with partners to improve their purview of the semiconductor landscape. A week before Semicon, DuPont announced that it has joined the Belgian electronics research center IMEC as an industrial affiliate. During an initial two-year program, DuPont EKC will provide resident technical staff to IMEC and work with the center in developing new post-CMP cleaning chemistries and post-etch residue removers.
DuPont's Holmes explained that IMEC will provide the tools to electrically characterize the fragile insulating material both before and after post-CMP cleaning. "Semiconductor makers are typically very guarded with the information they give out," he said. "With IMEC, we can assess our products over the next few generations of chips."
Zentack would argue that Air Liquide's new labs and equipment can carry out the same kind of analysis. But, like General's Rovito, he cautions against getting too carried away by sophisticated technologies and cleaning formulas. "If a simpler chemistry works, why have a more complicated one?" he asked. "Because no matter how complex this industry is, it is cost-driven at the wafer level."
- Chemical & Engineering News
- ISSN 0009-2347
- Copyright © American Chemical Society