Photolithography advances push purge-gas purification
11/01/2002
By Xiaosong Jiang, Daniel Alvarez, Jr., Allan Tram, Jeffrey J. Spiegelman Aeronex, San Diego, California
overviewProcess success with advanced lithography involves a wide range of factors, including a tool's ambient environment. Use of shorter wavelengths of light has increased the potential for photon-induced reactions creating byproducts that deposit on lenses and hamper performance, dictating the need for extremely clean lens purge gases. One solution uses a new inorganic catalyst purification method that reduces contaminants in nitrogen or compressed dry air to sub-ppt levels.
DUV photolithography tools use purge gases, such as compressed dry air (CDA) or nitrogen, to keep optical surfaces clean and to provide a consistent environment for the light beam. Contaminants in these gases, particularly hydrocarbons and refractory compounds, can deposit on lens surfaces, reducing the transmissivity of the optical beam path and ultimately damaging optical surfaces (see "Lithography optics contamination issues").
Several groups of contaminants are especially damaging in photolithography: polar protic molecules, such as H2O and ROH; simple hydrocarbons, such as butane and toluene; refractory organic compounds, such as HMDSO; acidic compounds, such as SO2 and NO2; and basic compounds, such as ammonia. These contaminants come from several sources. For example, in the presence of DUV, elastomeric materials used within a photolithography tool, such as polymer tubing and adhesives, release hydrocarbons into the gas stream. Cleaning solvents and chemicals outgassing from resists add to atmospheric contaminants. The industry has finally realized how critical it is to remove contaminants in purge gases to low ppt levels.
Purge-gas options
Different gas-optics interfaces throughout the lithography process require different purge gases and flow rates. Small and medium nitrogen flows are usually used to purge small and enclosed locations, such as an internal lens space. CDA is a better choice for purging areas in an open or semi-open space in the light beam path. High flows are needed in wafer chambers.
CDA is an ideal purge gas because its refractive index matches that of ambient air. Since the flow rate is usually higher and the area being purged is large, CDA is far more economical than nitrogen. The disadvantage of CDA is that its contamination level is usually higher than that of nitrogen derived from a liquid nitrogen source.
 Figure 1. Calibration curve for sub-ppt detection of toluene via the TDT method. |
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To meet the purity requirements of a tool, nitrogen, CDA, and synthetic air should be used only with gas purification. (Synthetic air is a blend of nitrogen and oxygen, but it is costly and the ratio needs to be tightly controlled to match the refractive index of ambient air.) CDA, since it can be created on-site, offers a good value when combined with purification.
Requirements for purification
Gas purification in photolithography poses two significant needs: Purification technologies capable of removing contaminants to low ppt levels and techniques for measuring low-ppt levels of contaminant species. In addition, purge-gas purification technologies must not release additional contaminants into the gas stream. A purge gas purifier should operate at ambient temperature, and CDA or nitrogen gas temperatures after purification must stay within ±1°C of the ambient temperature so that the transmission of light is not altered.
Also required is the ability to remove oxygen, or not, as an application dictates. Moreover, since CDA quality can vary from fab to fab and the consumption of CDA is relatively high, high-capacity purification technologies are required so that CDA use is cost-effective.
Current purification technologies
A few gas decontamination technologies have been used to purify lithography lens gases to acceptable levels, particularly chemical filters (carbon beds impregnated with chemicals that promote reaction with the contaminants) and particle filters. As lithography has advanced, however, decontamination processes that were once sufficient have become unacceptable.
With filters, all cleanroom air is recirculated back into the working environment. Filters are typically contaminant-specific and cannot remove everything on the increasingly long list of potential contaminants. Specifically, filters are capable of removing only individual groups of contaminants such as hydrocarbons, acids, or bases. Moisture cannot be removed by using filters. Over time, a chemical filter may release a quantity of its impregnated chemicals.
Some purification technologies require heat or electrical stimulation that can result in outgassing and byproducts. Some technologies release particles that become entrained in the gas stream, especially at high pressure. Furthermore, some purification technologies required periodic replacement or activation during the service lifetime of a lithography tool. This causes an interruption in production and may increase the likelihood of outgassing, byproducts, and particles.
Until recently, a single method capable of accomplishing sub-ppb levels for all contaminants did not exist.
Inorganic catalyst purification
Relatively new, inorganic catalyst-based purification technology is a single-composition decontamination process for point-of-use removal of a variety of purge gas contaminants used in lens gas streams. It can be precisely formulated for specific decontamination projects and specified to last the service lifetime of an application. Inorganic catalyst-based purification removes neutral polar molecules, neutral polar aprotic molecules, amines, acids, bases, and hydrocarbons. Also, the technology does not contribute any contaminants to the gas stream being purified, generate particles in any significant amount, or release reaction products of the adsorbed gases.
The technology uses three types of material that function together to allow decontamination of lens purge gases down to sub-ppt levels:
- An electropositive metal component provides affinity for water down to 1ppb or lower. It is also effective for removing hydrocarbons and carbon oxides.
- A high silica-zeolite component is effective at removing hydrocarbons down to 100ppt or less, in many cases down to 1ppt.
- A metal oxide component has an active high surface area for removal of various gaseous contaminants, notably nonatmospheric contaminants such as SOx and NOx.
Inorganic-catalyst purifier technology is ideally suited for purge gas purification in photolithography tools. Contaminants in the gas stream are removed by chemisorption, oxidative addition, simple oxidation, and simple adsorption. All of these reactions take place at ambient temperature without added heat or power. Because the constituent components are inorganic, the process does not release any hydrocarbons or other organic compounds.
 Figure 2. GC chromagram for a 2ppt toluene TDT-GC sample. |
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The five groups of critical photolithography purge-gas contaminants are removed by this purification technology to below specifications required by photolithography tool manufacturers or to the current available lower detection limit (LDL) of analytical techniques. Inorganic catalyst technology can be used to purify CDA as well as nitrogen with or without oxygen removal capability.
Once the inorganic purifying material becomes saturated with contaminants, it can be regenerated and returned to its original clean state. Regeneration is done at ~300°C with a specific regeneration purge gas such as CDA or inert gas. Regeneration can be done in situ at a user's site with an automatic self-regeneration gas purification system.
New detection capability
In our work to validate that inorganic catalyst purification technology can remove hydrocarbons to low ppt levels, we developed an indirect method that provides quantitative measurement for hydrocarbons at sub-ppt levels. This method uses a thermal desorption tube (TDT) containing a molecular sieve, and a gas chromatograph (GC). The sample gas travels through the TDT to trap hydrocarbon contaminants found in the gas stream. The sample is collected for a fixed time, flow rate, and pressure. The TDT is then isolated and heated to re-volatize the trapped hydrocarbons, and the volatized hydrocarbon sample is injected into a GC for hydrocarbon analysis.
Using certified gas standards and a double dilution methodology, gas streams containing low-ppt levels of butane, toluene, and HMDSO were created separately and used to calibrate the TDT-GC method. The LDLs for butane, toluene, and HMDSO were 10 ppt, 1 ppt, and 3 ppt. For example, we show here our calibration curve for toluene measurement between 1 and 100 ppt (Fig. 1) and a GC chromagram, with an SNR of five, for a 2 ppt toluene sample (Fig. 2).
Using our TDT-GC method to test inorganic-catalyst technology's ability to purify either nitrogen or CDA, we looked at 60-ppb challenges of butane, toluene, and HMDSO. In all cases, the outlet contamination levels were at or below the LDL of the TDT-GC method.
Xiaosong Jiang is director of marketing at Aeronex, 6975 Flanders Dr., San Diego, CA 92121; ph 858/452-0124, fax 858/452-0229, [email protected].
Daniel Alvarez Jr. is director of technology development at Aeronex.
Allan Tram is an application engineer at Aeronex.
Jeffrey J. Spiegelman is president of Aeronex.
Lithography optics contamination issues
Gas-phase hydrocarbons absorb light at 248nm wavelength and below, reducing optical transmittance [1]. At a gas-optics interface, simple hydrocarbons, refractive organic compounds, and acid compounds decompose and react with high-energy photons leaving carbon, SiO2, and inorganic salt deposits on optical surfaces [2], causing nonuniform projection of the transmitted light. A 0.1% loss of projection can alter fabricated device CDs.
Sub-ppb levels of hydrocarbons exposed to 157nm light undergo photodecomposition (i.e., sigma bond cleavage) to form carbonaceous coatings on optical surfaces [3]. One monolayer of carbon on an optical surface results in a 1% loss of transmission; several monolayers, a 4% loss [4].
Refractory organic and acid compounds can react with lens coatings, causing inorganic salt deposits with very strong adhesive properties, especially on MgF2 and CaF2 lenses. Lens cleaning techniques can remove these deposits with limited success [5]. Downtime required for cleaning, which may take up to three weeks, can cause major production loss. In situ cleaning processes using high-energy processes with oxygen have also been explored, but they remain inefficient and impractical for production fabs. Clearly, a simpler solution is to prevent contaminants from entering the beam path and contacting optical components.
References
1.Andrew J. Dallas, et al., SPIE Conference on Metrology, Inspection, and Process Control for Microlithography XIV, Vol. 3998 (2000), pp. 863–873.
.A.Grayfer, et al., Microlithography World, Feb. 2002, pp. 20–22.
3.T.M. Bloomstein, presented at 157nm Technical Data Review, San Diego, Nov. 2001.
4.T.M. Bloomstein, et al., SPIE Conference on Optical Microlithography XV, Vol 4961, 2002, pp. 709–723.
5.T.M. Bloomstein, et al., presented at 2nd International Symposium on 157nm Lithography, May 15, 2001.