New technology is ready for the removal of low-level metal contamination from UPW, organic solvent and other chemicals used in microelectronics manufacturing
By Bipin Parekh, Kunio Fujiwara, Makoto Komatsu, Yukio Hashimoto, Mutsuhiro Amari
During the manufacturing process for microelectronics devices, silicon wafers get exposed to high-purity water more frequently than other liquid chemicals—an eight-inch wafer requires more than 1,100 gallons of ultrapure rinse water during processing.
The purity of the DI water produced at the central water system in a fab is extremely high, with each critical metal concentration typically below 1 part per trillion (ppt). But it is difficult to maintain such high purity out of the central system during the distribution to points of use for wafer surface cleaning.
The process of distributing high-purity DI water often introduces particles and trace ionic contamination that can leach out from the plumbing components and process equipment. The International Technology Roadmap for Semiconductors 2001 (ITRS2001) guidelines indicate that the total metal contamination level on wafer surface be below 7 x 109 [atoms/cm2] for the line width 130-nanometer (nm) devices.1 (See Tables 1 and 2).
Some metal ions—especially copper, or silver—with higher electrochemical potential than silicon wafer can plate out on the wafer surface even under extremely low contamination levels.2 In his paper, Park experimentally demonstrated the Minority Carrier Lifetime (MCLT) reduction by changing the contact time of silicon wafers submerged in ultrapure water.
MCLT indicates the decay time of excited state electrons on the wafer surface; it provides useful information related to the wafer cleanliness and the effects of the metal contamination.3 Figure 1 shows the reduction of MCLT of over 20 percent upon soaking a silicon wafer in ultrapure water for 15 minutes. Such metal contamination on silicon wafer surface during the ultrapure water rinse process can be avoided by employing point-of-use ion removal purifiers.
Dissolved ions purifier/ion exchange membrane
New point-of-use dissolved ion purifier/filter technology has been developed for the removal of low-level metal contamination from ultrapure water, organic solvents and other chemicals used in the microelectronics industry.
The purifier/filter devices, assembled as 10-inch cartridges and disposable capsules, use an "ion exchange membrane" as an essential part in a pleated form. The ion exchange membrane is prepared using the "Radiation Induced Grafting Polymerization Method," which introduces ion exchange groups directly and covalently onto the surface of a microporous membrane.
The key characteristics of the membrane are:
- The active groups are attached homogeneously over the surface of the membrane and inside the micropores;
- The membrane maintains the structural characteristics of micropores in dry state, providing a large active area within thin film structure;
- The active group is sulfonic acid, which is a strongly acidic ion exchange group effective in removing positively charged metal ions from solutions.
The membrane purifier's ion removal reaction is identical to that of a strong acidic ion exchange resin; for example, the replacement of H+ on the sulfonic acid group by the positive ion in solution.
But there are key differences. The purifier has ion exchange groups on the surface of microporous membrane through which the fluids flow. On the other hand, the conventional ion exchange resin consists of spherical beads containing ion exchange groups inside the micropores, resulting in a diffusional resistance to ion exchange reactions.
The purifier's ion exchange membrane doesn't have such a rate-limiting factor for ion exchange reactions, enabling it to achieve high metal-removing conversion under high fluid flow rate with small filter size (Figure 2). Secondly, unlike ion exchange resin, there is no polymer swelling effect in the case of the ion exchange purifier membrane.
DI purification/In-house testing
In-house challenge tests of DI and IPA purification were conducted to determine ion removal efficiency (single and multiple) and capacity. The results include:
- High removal rate at single pass (Figure 3 for Cu and Figure 4 for several critical metals)
The ion removal experiments on membrane coupons show up to 99 percent metal removal. The results also show the ion removal capability is retained fairly well at high flow rates.
- High capacity (Figure 5, Cu breakthrough results)
The dynamic breakthrough tests show that the purifier has high Cu removal compared to a competitive purifier.
- Capable of multi-element removal from a given solution (Figure 6 for Cu and Na)
The tests show the ion exchange membrane shows selectivity to the higher valent Cu over Na.
Some metallic impurities in water exist in various forms—ionic or colloidal—depending on concentration, pH, temperature, and co-existing other impurities.
For example, ferric ion forms hydroxyl complexes with different coordination numbers under different concentration on hydroxyl ion. At neutral or higher pH, ferric ion forms amorphous Fe2O3, Fe(OH)3 and Fe(OH)2+ complexes with various ratios of hydroxide and oxide content.
These species form complexes and coagulate, forming dispersed colloidal particles in water with a wide range of particle size distribution and surface-charge density. The electrostatic interaction between such charged particles and ion exchange groups, and the particle diffusion, influence particle capture by the charged ion exchange membrane.
Smaller colloidal particles of higher charge density overcome inertial forces and are captured on the surface of ion exchange membrane by electrostatic forces. Larger and heavier colloidal particles of less charge density are more likely to follow streamlines and flow through the ion exchange membrane before being captured by the ion exchange surface.
To capture both ionic and colloidal impurities, the new devices have been designed using a laminated structure of ion exchange membrane and microporous ultra-high molecular weight polyethylene (UPE) membrane of sub-micron pore size rating.
Tests showed metal removal performance as a function of flow rate for the laminated membrane and the ion exchange membrane in pH 4.5 DI solution of 200 parts per billion (ppb) of Fe. The ion exchange membrane removed 80 to 90 percent of metals (at 10 to 30 liters per minute equivalent flow for a 10-inch cartridge) while the laminated device removed almost 100 percent of Fe (ionic and colloidal) from the solution.
Molecular Recognition Technology
The ion removal from low pH chemicals requires active groups with much higher binding constants than the new technology described above. One such chemical purifier, designed to remove ions from the POU recirculation HF baths, uses molecular recognition technology (MRT).
MRT describes the process of designing ligands having predetermined ion selectivities, immobilizing these macrocycle ligands to supports, such as membrane, and using the resulting immobilized membranes to effect desired chemical separations.4, 5 These materials allow separation of the desired ions in the parts-per-trillion range.6, 7, 8, 9, 10
In MRT, organic macrocycles/ligands are synthesized to selectively bind targeted molecules in a solution. Macrocycles are cyclic organic molecules containing an open cavity (Figure 7).
Within this cavity are free oxygen, nitrogen or sulfur atoms that can bind to metal ions small enough to enter the cavity. By properly designing the cavity size of the macrocycle and by introducing the right binding atoms within the cavity, these molecules can be customized to bind to individual metal ions specifically and tightly.
Macrocycles can be engineered by manipulating three factors:
- Cavity size, so that only the ions of interest will fit;
- The type of binding atoms on the interior cavity, which determines the complexing ability of the ligand to the ion of interest;
- The number of binding atoms in the interior, which dictates the bond's strength.
Macrocycles can be designed to be specific to an individual ion over all others, or specific to a class of ions. In both cases, the equilibrium constants for binding to macrocycles are much higher than for the conventional ion exchange resins or chelating resin.
Purification and filtration technologies have been developed to meet the challenges of stringent purity levels required of liquid chemicals in semiconductor manufacturing.
Other processes-related needs that will provide impetus for purification are:
- Use of dilute chemistry, requiring more stringent ionic specs to prevent metal deposition on wafers;
- More focus on waste DI reclaim/recycle applications for lithography resource conservation in the future;
- Decrease Fab UPW use from 6-8 gal/in2 in 2001 to 4-6 gal/in2 in 2005, and 3-5 gal/in2 beyond 2008.
BIPIN PAREKH is senior consulting engineer and group leader of the wet etch and cleans in the Liquid Applications Development at Mykrolis Corporation. He can be reached at email@example.com KUNIO FUJIWARA is senior chief research engineer of Radiation Graft Polymer Project at Ebara Research Co., LTD. He can be reached at firstname.lastname@example.org MAKOTO KOMATSU is senior research engineer of Radiation Graft Polymer Project at Ebara Research Co., LTD. He can be reached at komatsuL0062@erc.ebara.co.jp YUKIO HASHIMOTO is a senior engineer of application development at Mykrolis Corporation. He can be reached at email@example.com MUTSUHIRO AMARI is manager in the application technology development at Nihon Mykrolis, Tokyo. He can be reached atmutsuhiro_amari @mykrolis.com
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This paper was presented at the 22nd Annual Semiconductor Pure Water and Chemicals Conference, February 17-20, 2003.