Ion exchange purification of dielectric condensate precursor fluids and silicate esters such as tetraethylorthosilicate (TEOS)

[0002] This application derives priority from U.S. Provisional Application Ser. No. 60/088,579, filed Jun. 9, 1998.

[0003] This invention relates to semiconductor device fabrication and processing, and, in particular, to a method and system for removing inorganic contamination from dielectric condensate precursor fluids, such as silicate esters, e.g. tetraethylorthosilicate (TEOS), by passing these fluids through columns of ion exchange resins to increase the purity of the fluids.

[0004] In the manufacture of modern integrated circuits, for example, contamination of the insulating interlevel dielectric layer material causes many problems and should be reduced as much as possible. Contamination is becoming an increasing problem, particularly as the complexity and density of integrated circuits increase and the feature sizes drop below about 0.25 μm. Contamination may include contaminants, such as particulates, metallic ions, organic molecules, and other ionic species, that may be suspended or dissolved in the chemicals used for the manufacture of integrated circuits. Chemical processing utilized in the production of the integrated circuits often involves particle and metal removal steps followed by a drying step, interspersed with several rinsing steps.

[0005] Chemicals, such as dielectric condensate precursor fluids, are also used to form layers of insulating material between features of integrated circuits. Contaminants contained in such layers deposited on the surface of an integrated circuit substrate are particularly destructive when they are permanently included as part of the integrated circuit. These included contaminants may subsequently migrate to adjoining electrical device features and cause great damage to, or malfunction of, the integrated circuit and significantly reduce the product yield.

[0006] Dielectric condensate precursor fluids are typically applied as a fluid (gas, vapor, mist, or liquid) to the surface of an integrated circuit under vacuum and/or heat, and the fluid undergoes a conversion to a solid dielectric condensate. Conventional dielectric condensate precursor fluids include, but are not limited to silicate esters such as tetraethylorthosilicate (TEOS). Commercially available industrial, technical, or other grades of TEOS and other silicate esters typically include individual inorganic contaminants in concentrations up to about 10,000 parts per billion (ppb). Thus, for semiconductor manufacturing and certain other processes, the commercial manufacturer is requested, therefore, to further purify the silicate esters or other dielectric condensate precursor fluids.

[0007] Current methods for the purification of TEOS and related organic silicate fluids employ sequential fractional distillation at atmospheric or reduced pressures and higher temperatures. These methods suffer from several drawbacks that include the inability to remove significant amounts of boron and chloride ions, the facilitation of undesirable side reactions at operational temperatures, and the costly safeguards needed for distillation of flammable and hazardous materials.

[0008] A safer, less expensive, and more effective method of purifying TEOS, silicate esters, and other dielectric condensate precursor fluids is, therefore, desirable.

[0009] An object of the present invention is, therefore, to provide a system or method for removing contaminants from silicate esters or other dielectric condensate precursor fluids.

[0010] The present invention presents a method that employs ion exchange resins to remove metal and other contaminants from dielectric condensate precursor fluids and silicate esters, such as TEOS and to avoid certain drawbacks of the current distillation techniques. Ion exchange resin is currently used for the removal of contaminants from water and a few other chemicals, such as citric acid. The ion exchange resin beads have a positive or negative charge which attract contaminants of the opposite charge. Macroporous ion exchange resins additionally have numerous crevices at the surface of each resin bead. These crevices function to physically trap contaminant molecules, creating a molecular sieve effect that can capture contaminants even when the ion exchange resin is surrounded by an organic fluid.

[0011] The present invention includes, therefore, selecting or modifying ion exchange resin to be appropriate for the process, such as by converting the cations to hydrogen and the anions to hydroxide; drying or eliminating water from the ion exchange resin, such as by rinsing the resin with an evaporative alcohol; flushing the ion exchange tanks and associated chemical lines with the fluid to be purified; recirculating the commercial fluid feedstock through tanks of ion exchange resin one or several times to achieve optimum removal of contaminants; and delivering the purified fluid into a closed container to prevent subsequent contamination. The process may be carried out at room temperature and pressure.

[0012] Skilled persons will also appreciate that the present invention can be employed in the manufacturing of extremely pure silicate powders or slurries that can, for example, be used to polish silicon wafers.

[0013] Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof which proceeds with reference to the accompanying drawings.

[0014] FIG. 1 is a schematic diagram showing a simplified purification system that can be employed to purify dielectric condensate precursor fluids in accordance with the present invention.

[0015] With reference to FIG. 1, a simplified purification system 10 that can be employed to purify dielectric condensate precursor fluids in accordance with the present invention includes a supply tank 12, one or more ion exchange tanks 14 (14aand 14b), and a product collection tank 16. Supply tank 12 is supplied with commercially available industrial, technical, or other grades of a dielectric condensate precursor fluid or silicate ester. These grades can be defined as low purity fluids and typically include individual inorganic contaminants in concentrations greater than one part per billion and up to about 10,000 parts per billion (ppb). High purity fluids, on the other hand, can be defined as having less than one part per billion of individual metallic contaminants. Preferred silicate esters include, but are not limited to, tetraethylorthosilicate (TEOS) and methyltriethoxyorthosilicate (MTEOS), and other preferred dielectric condensate precursor fluids include, but are not limited to, hydrogen silsesquioxane (HSQ), methyl silsesquioxane (MSQ), polyarylene ether, benzocyclobutene (BCB), and orthosilicate glass (OSG). These fluids all exhibit adverse interactions with water.

[0016] Ion exchange tanks 14 preferably include macroporous strong acid cation and strong base anion ion exchange resins. Each ion exchange tank 14 may include a single mixed bed column of both anion and cation resins, or ion exchange tank 14amay include only cation resins while ion exchange tank 14bmay include only anion resins. Skilled persons will appreciate that the order of the separately-charged resin ion exchange tanks 14aand 14bcan be reversed and that these separately-charged resin ion exchange tanks are preferably included as paired sets.

[0017] Ion exchange resins typically comprise inert materials such as cross-linked polystyrene/divinylbenzene or polymethacrylate polymers that are chemically bonded to charged ions. The ion exchange resins are preferably prepared or purchased with macropores of about 0.3 to 1.2 millimeters and a density of about 40 to 50 pounds per cubic foot, but skilled persons will appreciate that particular resins can be selected to accommodate the sizes and densities of specific contaminants based on the quality of the source fluid and the size of the contaminants.

[0018] In a preferred embodiment, an H⁺ cationic resin and an OH⁻ anionic resin is obtained from a commercial source. In another embodiment, the cation resin is chemically converted to a hydrogen form rather than the commercially available sodium form, and the anion resin is converted to a hydroxide form rather than the commercially available chloride form. The cation conversion can be accomplished by treating or flushing the ion exchange resin with a strong acid, such as sulfuric acid or hydrochloric acid. The anion conversion can be accomplished by treating or flushing the ion exchange resin with a strong base, such as sodium hydroxide. The ion conversion treatments can be performed prior to putting the ion exchange resin into ion exchange tanks 14 or can be performed after tanks 14 are filled with the ion exchange resin. The preferred ionic forms can be used regardless of whether the ion exchange resins are used in individual columns or as a mixed bed. It is preferable to prepare the anion and cation ion exchange resins separately and than combine them to form a mixed bed ion exchange tank 14. Preferably, the ion exchange resins are obtained from commercial suppliers in the highest purity form available to prevent secondary contamination from the resin itself. Examples of commercial resins suitable for use in the present invention include Sybron Ionac products A-641, CFP-110, and NM-201SG.

[0019] Because water reacts with many of the dielectric condensate precursor fluids to form undesirable contaminants, it is desirable to dry or otherwise eliminate water from the ion exchange resin in addition to the ion conversion. For example, water reacts with or hydrolyzes TEOS to form silicon dioxide and other particulates, which adversely affect the uniformity and performance of the dielectric layer in an integrated circuit. A preferred drying technique involves rinsing the ion exchange resin with a drying agent, such as isopropyl alcohol, n-propyl alcohol, methyl alcohol, or other evaporative alcohols. In a preferred embodiment, the ion exchange resin is flushed with a volume of drying agent that is about 3-6 times the volume of the ion exchange resin, and the drying agent can be recycled. The drying process can be performed at room temperature or at elevated temperatures if desirable, and can be performed before or after placing the ion exchange resin into ion exchange tanks 14. System 10 and, particularly, ion exchange tanks 14 are typically flushed with the supply fluid before a purification run is initiated.

[0020] A tank supply valve 18 is opened to allow fluid to flow from supply tank 12 along a supply line 20 to a pressure regulating valve 22 that precludes backward flow of fluid along supply line 20. Fluid continues along supply line 20, passes a pump valve 24, and enters pump 26. Valve 24 allows fluid to reach pump 26 that is preferably adapted to pump industrial volumes of fluid at a rate of at least 20 gallons per minute. A pressure gauge 28 displays the pressure of fluid flowing through pump line 30 between pump 26 and ion exchange tank 14a, guarded by tank entry valve 31.

[0021] After passing through ion exchange tank 14a, fluid flows through tank line 32, between tank exit valve 34 and tank entry valve 36, and into ion exchange tank 14b. After passing through ion exchange tank 14b, fluid flows through tank exit valve 38, along filter line 40, through filter valve 42, and through filter 44. Filter 44 is preferably a conventional TEFLON™ filter, such as PTFE or PFA, and removes any remaining particulates from otherwise purified fluid flowing through filter line 40. Such remaining particulates may be generally 0.05 μm or larger and can include resin particles or hydrolytic or reaction products with any unremoved water, for example.

[0022] The resin-treated fluid can be recirculated along recirculation line 46 and through valves 48 and 24 to re-enter the ion exchange tanks 14aand 14b. Alternatively or simultaneously, the resin-treated or purified fluid can be directed through collection line 50, collection valve 52, flow meter 54, and tank valve 56 into purified product collection tank 16. In one embodiment, valves 24 and 48 are fully opened and valves 22 and 52 are partly opened so that supply line 12 and collection line 50 have similar flow rates that are substantially lower than the flow rates through recirculation line 46 and pump line 30, which has an even greater flow rate. This embodiment permits a continuous supply of source fluid and a continuous collection of resin-treated fluid. The supply and collection flow rates can be adjusted to optimize the purity of the collected fluid. The process can be carried out at room temperatures, although the process may be accelerated at higher temperatures.

[0023] The energy required to effect purification in accordance with this invention is significantly lower than the energy required to purify the fluids by distillation. For example, it is expected that resin treating the fluids will require less than $0.02 per gallon while distillation requires greater than $0.35 per gallon. Even though the resin-treating process is by itself less expensive and more effective than the conventional distillation process for purifying dielectric condensate fluid precursors, skilled persons will recognize that there may be advantages to distilling the fluids prior to passing them through the ion exchange resins.

[0024] Skilled persons will appreciate that numerous alternative designs for system 10 are possible, including altered placements, varieties, and number of valves, pumps, filters, and tanks. For example, numerous product collection tanks 16 can be added in parallel with appropriate lines and valves to allow sequential filling, removal, and replacement of collection tanks 16. Alternatively, product collection tanks 16 can be supplied with exit valves and lines that lead to process chambers where the fluid can be employed, and/or supply tank 12 can receive unused process fluid from such a chamber.

[0025] Skilled persons will also appreciate that system 10 can include only one ion exchange tank 14, a large number of mixed bed ion exchange tanks 14, or paired or uneven sets of ion exchange tanks 14aand 14bin series or in parallel so that the fluid can be highly purified in a single cycle or a small number of cycles through system 10. Such multiple ion exchange tanks 14, 14a, or 14bcan contain slightly different ion exchange resins having, for example, crevices of decreasing sizes and increasing numbers of crevices.

[0026] Skilled persons will also appreciate that the purity of the purified product fluid may be increased with an additional specified number of passes through ion exchange tanks 14 or until no further increase in purity can be detected. Typically 1 to 10 cycles through the ion exchange tanks 14 are performed to achieve the highest possible purity, containing less than 1 part per billion (ppb) of individual metallic contaminants.

[0027] The high purity dielectric condensate precursor fluids, silicate esters, or TEOS can be used for applications where the higher purity will benefit the quality or improve the yield of an unrelated process. Example applications include, but are not limited to, semiconductor manufacturing where an insulating dielectric layer is required on the surface of the semiconductor wafer, and in the manufacturing of precipitated amorphous silicate slurries where an organic silicate ester is a reactant or ingredient.

[0028] Major advantages of this process include significantly lower costs of purification, increased safety due to the ambient process temperatures, and significantly lower levels of boron and chloride contamination compared to existing distillation methods.

[0029] In this example, low purity TEOS containing greater than 1 ppb but less than 10,000 ppb of individual metallic contaminants was cycled through a system similar to system 10. Analysis of the purified fluids exhibited the general detectable levels of contaminants presented in Table 1. Skilled persons will appreciate that ppb in the table represents nanograms of contaminant per gram of fluid instead of the typical nanogram per milliliter since the fluids have a different density than water. The “less than” values are generally less than the detectable level for the particular contaminant.

[0030] Table 1 shows a reduction in boron contamination to less than 50 ppb, even to less than or equal to 20 ppb, preferably to less than or equal to 10 ppb, and more significantly to less than or equal to 1 ppb; a reduction in calcium contamination to less than 0.50 ppb and more significantly to less than or equal to 0.1 ppb; a reduction in copper contamination to less than 0.40 ppb and more significantly to less than or equal to 0.2 ppb; a reduction in iron contamination to less than 3 ppb and more significantly to less than or equal to 0.1 ppb; a reduction in magnesium contamination to less than 1.3 ppb and more significantly to less than or equal to 0.05 ppb; a reduction in potassium contamination to less than 0.23 ppb and more significantly to less than or equal to 0.1 ppb; a reduction in sodium contamination to less than 1.3 ppb and more significantly to less than or equal to 0.2 ppb; a reduction in zinc contamination to less than 50 ppb, preferably to less than or equal to 1 ppb and more significantly to less than or equal to 0.7 ppb; and a reduction in chloride contamination to less than 100 ppb, to less than 10 ppb, and more significantly to less than or equal to 1 ppb. Skilled persons will appreciate that these contaminant concentrations would be even lower when the fluids are resin-treated on a very large commercial scale, particularly the levels of boron would be even smaller where boron-containing silicate tanks or vessels are not employed.

[0031] It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiment of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.