Photoresists Look to Friendlier Chemicals

When it comes to operating manufacturing facilities, the semiconductor industry has a relatively good safety record (see "EHS Fab Hazards: A Review "), despite all the toxic gases and chemicals in use. According to the SIA, the U.S. semiconductor industry has been making significant reductions in the use of ozone-depleting substances, reducing emissions by ~75% since 1987.
 
But it surely wouldn't be hard to make a case that some of the waste produced by the chip manufacturing process is not as environmentally friendly as it could be. In fact, Silicon Valley has the largest concentration of Superfund toxic waste sites in the United States, as the area struggles to clean up semiconductor manufacturing solvents that have seeped into groundwater.
 
Of course, which substances are the least friendly is not always known, with improved test capabilities and statistics bringing environmental dangers to light after years of a chemical's use. For example, the chip industry of the 1960s and 1970s used trichloroethylene (TCE) to clean silicon wafers, with disposal tanks later found to be leaking the suspected carcinogen into groundwater.¹ Another example is ethylene glycol ethers, solvents used up until the early 1990s in novolak positive-tone photoresists that have been linked to miscarriages and congenital defects.

Reflected in a decision from the U.S. Environmental Protection Agency (EPA) a year ago, the SIA and SEMI successfully retained the use of another group of substances used in photoresists. Originally aiming to completely phase out the use of perfluorooctyl sulfonates (PFOS) because of toxicity and environmental concerns, the EPA conceded to an exemption for photolithography, agreeing that the application used such small quantities and the industry manages the substances responsibly. Nonetheless, resist manufacturers are phasing out their use of PFOS because of the environmental concerns, regardless of the exemption. For some of its uses, they have managed to find suitable replacements, while for others there is still work to be done, with no clear alternatives in sight.

The EPA began its investigation of PFOS in 1999. After 3M (St. Paul, Minn.) — then the only manufacturer of PFOS-based substances in the United States and the principal supplier worldwide — announced in May 2000 that it would phase out all PFOS-based production, the EPA began to pursue regulatory action to limit any other manufacture or import of PFOS.

The EPA proposed a significant new use rule (SNUR) for PFOS later in 2000, proposing severe limitations for the use of 90 PFOS chemicals and derivatives. The agency enacted the SNUR late last year, now requiring manufacturers and importers to notify EPA at least 90 days before beginning to manufacture or import PFOS-based substances for certain significant new uses. Actually, the ruling refers to a broader category of chemicals known as perfluoroalkyl sulfonates (PFAS). This category encompasses PFOS, but reserves the term PFOS to identify primarily those chemicals with an eight-carbon chain length.

PFOS is highly persistent in the environment (it effectively degrades only through incineration), and has a strong tendency to bioaccumulate. Studies have shown small amounts of PFOS in humans and animals. Although it is not fully known what toxicity implications PFOS holds, the EPA is concerned that it could have considerable adverse effects in people and wildlife if it continues to be produced, released and built up in the environment.

However, after the EPA's initial proposal, the semiconductor industry fought hard to make a case for itself, and ultimately won a few exemptions, as noted in the final ruling. Key to the semiconductor industry is exemption of PFOS as a photoresist component (as a surfactant or photo acid generator), or as part of an antireflective coating (ARC) used in photolithography to produce semiconductors or similar components.

Shortly after the publishing of the initial proposed SNUR, SEMI and the SIA jumped to action to form the PFOS Working Group, a coalition of semiconductor and resist manufacturers that was created to work with the EPA to resolve the chip industry's contentions with the proposed SNUR. Micron's Rob Sterling and JSR Micro's Basil Falcone co-chaired the Technical Group, while Intel's Steve Harper and Shipley's Greg Dripps co-chaired the Legal and Communications Group.

The group successfully persuaded the EPA that the semiconductor industry's use of PFOS is in such small amounts and is so well controlled that it should be exempted from the EPA's ban. "The PFOS Working Group worked with the EPA to demonstrate the extremely low impact, volume, usage, exposure and negligible environmental release produced by the semiconductor industry," noted Mark Slezak, technical manager at resist manufacturer JSR Micro (Sunnyvale, Calif.).

Following the EPA's lead, the British government's Department of the Environment, Food and Rural Affairs (DEFRA) announced recently that it was coordinating a risk reduction strategy for PFOS and related substances. SEMI is one group now working with DEFRA as it did with the EPA.

When 3M let its customers know in mid-2000 that it was phasing out the use of PFOS in its surfactants, it was a voluntary move based on testing that showed PFOS persists in the environment, and accumulates in humans and animals. 3M committed to discontinue production of all PFOS-based chemicals by the end of last year (Table 1 ). About a month after 3M's initial announcement, the EPA met with various fluorochemical factions to announce preliminary concerns regarding PFOS.

At SEMICON West 2001, 3M Electronic Materials introduced a family of non-PFOS photoacid generators (PAGs) for use in advanced 248 and 193 nm photoresists. According to 3M, there are a limited number of PAGs that have proven effective for 193 nm photoresists, and most are PFOS-based. 3M's alternative formulations offer high acid strength, excellent solubility in resins and solvents, high purity, high thermal stability and low volatility, according to 3M.

3M is also developing a new generation of electronic-grade fluorochemical surfactants, intended for a variety of uses within the semiconductor industry, including photoresists, resist strippers and antireflective coatings. The first was introduced late last year for use in photoresist strippers. The new surfactants are based on perfluorobutane sulfonate (PFBS), which has a much better Environmental, Health, Safety and Regulatory (EHS&R) profile than does PFOS. The PFBS-based surfactants are expected to be sustainable replacements for PFOS-based surfactants.

Although the new surfactants fall into the broader PFAS category, they have very different environmental and toxicity properties than PFOS-based surfactants. The key difference between PFBS and PFOS is the carbon chain length — PFBS having four perfluorinated carbon atoms as opposed to PFOS's eight. The carbon chain length causes considerable differences in bioconcentration factors (BCFs) among PFAS formulations and perfluorinated carboxylic acids (the longer the chain length, the higher the BCF).

According to 3M, PFBS-based surfactants are practically non-toxic, with low mammalian toxicity and low ecotoxity. Unlike PFOS, PFBS does not fit within the EPA's persistent, bioaccumulative and toxic (PBT) chemical policy. It is classified as an insignificant hazard by the U.S. National Institute of Occupational Safety and Health (NIOSH), and requires no label warning by the European Union.

However, though PFBS does not bioconcentrate, it is still persistent in the environment. So to minimize exposure to the environment, 3M is limiting sales of PFBS-based surfactants to non-dispersive applications or applications with low emissons to the environment.

3M is already supplying PFBS-based products for some industries, but is still developing alternative chemistries for others, including the semiconductor industry, according to Ron Wenaas, who handles public relations for the company.

With the exemptions in place, resist suppliers are unaffected by the EPA's ruling. And, although 3M was formerly a major supplier of PFOS to the semiconductor industry, the chemical developer's decision to pull out of PFOS production has not been a significant consideration for resist developers. "While JSR Micro does import an extremely small amount of PFOS for production, it does not purchase any raw materials from 3M, and was therefore not directly influenced by its decision," Slezak said.

Ralph Dammel, director of technology, 193 and 157 nm lithography, at Clariant Corp., AZ Electronic Materials (Somerville, N.J.), agreed that 3M was not a factor. "In our application segment, we place a premium on the metal content of these materials. The materials that we would purchase would be in the low-ppb levels for metal," he said. "3M didn't meet these levels. Most actually come out of Japan."

In a letter to its customers in March 2001, Shipley Co. LLC (Marlborough, Mass.) conceded that its products were affected by 3M's decision. At the time, Shipley used PFOS-based surfactants and PFOS in some of its PAGs. To offset this effect, Shipley secured a multi-year supply of the PFOS-based materials to ensure product continuity.

In an October 2001 letter to the EPA from the PFOS Working Group, Harper and Dripps wrote, "The electronics industry recognizes that it is sometimes prudent to respond to new conditions by substituting process materials. Generally, the industry has been adept at doing so. Depending on the significance of a material and the viability of alternatives, however, the industry's resourcefulness cannot fully bridge its technology imperatives with a material substitution strategy."

Despite these concerns — and despite the EPA's ultimate ruling of exemption for lithography applications — resist manufacturers have been moving away from the use of PFOS in continued resist development. They import the necessary PFOS (primarily from Japan) to continue existing product lines for less advanced lithography techniques, but are using alternative chemistries for their new resists.

"PFOS surfactants are generally part of older-generation resists (g- and i-line), and most PAGs that contain PFOS have EPA permission to import under LVEs (low-volume exemptions)," JSR's Slezak said. "We are not using any PFOS surfactants in our new resists."

PFOS serves several important functions in photoresists (see "The Role of PFOS in Resist Chemistries "), and alternative chemistries are not always forthcoming.

JSR, for one, has been working on R&D for non-PFOS chemistries, such as alternatives to PFOS PAGs, Slezak said. The acid generated by PAGs is used to create a deblocking reaction, which in turn liberates more acid to continue the reaction. Typically, one acid molecule can deblock thousands of sites, Slezak explained, noting PFOS's importance in this reaction: "Control of the diffusion length of the acid generated is critical to many aspects in the lithography process, including key components such as resolution, isolated to dense biasing, and line edge roughness (LER). PFOS-based PAGs allow for the creation of an acid that is relatively large and bulky. The large size of this acid allows us to control diffusion and therefore modulate these key aspects of lithography."

Clariant has never used PFOS in its PAGs, opting instead for a PFBS acid. "By the time 193 nm lithography had matured to the point where we were looking at two or three or four PAGs, PFOS was already on the horizon (as an environmental concern)," Dammel said. "We actually jumped over the PFOS formulation stage."

However, Clariant does use PFOS as a surfactant in its older-generation photoresists and top ARC products. Surfactants are critical to photoresists, Dammel explained, and fluorinated surfactants such as PFOS surfactants lower surface tension without causing a negative impact on resists. Nonetheless, Clariant discontinued the use of PFOS for new development, instead opting for PFBS-based surfactants. "In my view, there's a clear difference in performance, but it's workable," Dammel said.

"For resists, we can get the same performance," said Kathy Durham, marketing manager for Clariant. "But not yet for top ARCs." Although non-PFOS top ARCs are workable for i-line and 248 nm materials, they have not yet found a good solution for 193 nm ARCs, Dammel added. "Top ARCs need a very low refractive index for the material to work," he said. "The top ARC refractive index should be as close as possible to the square root of the refractive index of the resist. ... That can only be achieved with fluorinated polymers."

Some chipmakers were relieved when the EPA conceded to the photolithography exemption for its PFOS ruling, concerned that they would not get the same performance from alternatives or that they would have to requalify a new line of resists. Other customers, meanwhile, wanted nothing to do with PFOS-based formulations. "There are some customers that accept the fact that, with incineration of the majority of the material, the amount is very small," Clariant's Dammel said. "Other customers are very cautious for reasons of their own, and they prefer not have any PFOS-containing materials enter their facilities."

Certain customers are very sensitive to PFOS issues, Durham agreed. "Some customers are telling us that they won't accept any formulations that have anything to do with PFOS." However, she added, requalifying i-line or other older resists in order to eliminate the PFOS factor is prohibitively time-consuming and expensive in most cases. This is particularly true for IC production related to military or automotive applications, where extremely tight specifications call for a lengthy requalification process.

Until the use of PFOS goes away altogether, chipmakers are reducing the already small amount of PFOS used by reducing their own resist and ARC consumption. The more efficient use of materials helps to reduce manufacturing costs. According to statements from the PFOS Working Group, the volume of photoresist used per chip layer has dropped from ~8 mL in the early 1990s to 2 mL in 2001, and is expected to ultimately approach 1 mL. Increasing wafer sizes also result in a net decrease in resist used per device (Fig. 1 ).

1. With continually reduced volumes of resist used per wafer, combined with increasing wafer sizes, photoresist consumption per wafer area is falling. (Source: Dataquest, Prismark, SEMI)

The amount of PFOS PAGs consumed by the semiconductor industry in the United States is actually quite small, JSR's Slezak said. "Typically, 0.0157 g of resist film is deposited per wafer. PFOS PAG makes up less than 1% of that resist," he said. "PFOS PAG and its reactants do not remain on the final product (wafer), and the small amount that is generated is destroyed in standard waste containment, such as thermal destruction."

Shipley's 2001 customer letter, signed by Dripps, Shipley's global manager, product integrity and toxicology, also emphasized the low levels of PFOS use: "The Shipley products impacted by 3M's decision contain non-volatile perfluorooctanyl surfactants and PAGs at concentrations ranging from 0.002% to 2% by weight. Their dilute, non-volatile nature results in little opportunity for inhalation exposure."

The method of spin-coating resists and ARCs onto the wafers means that only a small amount of the materials is left on the wafer. Some 90-99% of the material is drained to the waste line, then collected for incineration, according to information supplied to the EPA by Clariant. About 1-3% of the materials get washed away by developers, the PFOS component presumably going through water treatment and into the drain. Clariant estimated in 2001 that U.S. chipmakers release ~12 kg/yr of PFOS into the environment through top ARCs, ~3 kg/yr through resist surfactants, and ~15 kg/yr through resist PAGs.

The trend toward thinner resists influences the amount of resist that goes to wastewater vs. incineration. The thinner the films, the less of the resist that's left on the wafer, thereby sending more of the resist to safe incineration. Table 2 shows the effect of thinner films, at a constant resist volume of 2 mL.

For any new developments, resist manufacturers are studying the environmental compatibility of chemicals, trying to ensure that their products are safe throughout the supply chain — from exposure to workers at various plant sites on through to the environment. But, as Clariant's Dammel noted, there's always the possibility that new information will bring today's developments into question. There is still much to learn about the effects of PFOS itself, although reduced use will certainly obviate some of that need.