Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
1999-04-29
2002-07-30
Cintins, Ivars (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S684000, C210S685000, C210S688000, C210S692000, C210S912000
Reexamination Certificate
active
06426007
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to waste water treatment to remove soluble metals for safe water discharge. This invention is particularly concerned with removal of dissolved molybdenum (Mo) and tungsten (W) metals from effluent generated in the cleaning of screening masks and semiconductor substrate devices with aqueous tetramethyl ammonium hydroxide (TMAH) to remove conductive metallic paste residue. This invention is also particularly concerned with removal of soluble Mo in aqueous TMAH washings from an oxide removal process in a Mo etching method based on acidic pH etchants.
2. Description of Related Art
In the fabrication of multilayer ceramic substrates for packaging semiconductor devices, conductive metal patterns are screened onto individual punched ceramic green sheets using a mask, such as a metal mask. After screening, the green sheets are assembled, aligned, laminated and sintered to obtain a multilayer ceramic substrate. Conductive pastes used in screening processes for delineation of wiring and via metallurgy patterns comprise metal particles dispersed in an organic binder and solvent carrier along with other additives, typically, wetting agents, dispersants/surfactants, plasticizers, etc.
Most commonly used conductive pastes in multilayer ceramic fabrication are based on molybdenum metal powder dispersed in an organic polymer binder such as ethyl cellulose, polymethyl methacrylate, polyhydrocarbon resins and the like, in a high boiling organic solvent vehicle. Conductive pastes other than molybdenum-based or polymer-metal composites contain copper, tungsten and nickel or binary metals as bismuth coated copper and tin plated copper as the metal filler.
Conductive paste screening on ceramic green sheets through metal masks, for example, typically causes retention of some residue on the mask surface and around the mask features. Therefore, in order to prevent any product defects in the subsequently screened pattern due to such residue, the masks are cleaned after one or more screening passes. The pastes referred to above are readily removed from metal surfaces with organic solvents and until recently, solvent-based cleaning has been employed for mask cleaning and other processes requiring removal of organic-metal/inorganic particulate from substrates. The organic solvents (particularly chlorinated solvents) which are effective in cleaning such residues, however, have the problem of environmental regulatory issues and human health concerns in large scale processing, which has resulted in an increasing focus of industry on aqueous cleaning so as to minimize, and preferably to eliminate the use of chlorinated solvent-based cleaning.
An aqueous cleaning method for polymer-metal composite paste, e.g., Mo-based screening paste from metal mask and semiconductor cleaning, employs a dilute solution of tetramethyl ammonium hydroxide (TMAH) to replace organic solvent based cleaning. The effluent generated in this method contains TMAH and soluble metal after all the separated and suspended solids are removed by micro filtration and ultra filtration.
In general, the effluent from cleaning and water rinse steps is first filtered to remove separated solids including solid metals from the removal of paste metals such as molybdenum, tungsten, copper and nickel metal when using molybdenum-based or other metal based screening pastes, and inorganics, such as glass, silica, which may be added in paste formulations as well as precipitated organics. The filtrate is then subjected to submicron-filtration, preferably using 0.2 &mgr;m or less filter cartridges and finally membrane filtration or ultrafiltration to remove any residual filterable solids. Metal analysis of the filtered effluent using Ion-Coupled Plasma (ICP) technique shows, however, a residual significant level of dissolved metals, particularly Mo, anywhere from 15-200 ppm (mg/liter) depending on the composition of the Mo paste used for screening, the concentration of TMAH cleaning solution, volume of solution used to clean a mask, and the amount of residue on the mask. In addition to dissolved Mo, relatively low levels of dissolved tungsten (W), less than 50 ppm, and dissolved copper (Cu), less than 1 ppm, are observed when tungsten-based pastes and Cu-based pastes are used respectively.
Considering that the environmental regulations on waste water discharge for pollution control requires that dissolved heavy metals be removed substantially completely, for example for Mo the limit being <1 ppm in discharge water, it is necessary to further treat the waste water effluents in industrial processes so that Mo as well as W, Cu and Ni are effectively removed to provide a “metals-free” effluent before being discharged as waste water.
U.S. Pat. No. 4,189,381 (Laferty et al.) describe treatment of industrial waste water containing Mo, and heavy metals Fe, Mn, Pb, Cd, and also containing cyanide ion in mineral recovery processes such as the flotation recovery of molybdenite. According to this method, Mo is first removed from the effluent by ion exchange using a weak-base amine type anion exchange resin. The eluent is then subjected to lime precipitation and followed by coagulation/flocculation to remove other metal contaminants.
U.S. Pat. No. 4,891,193 (Beutier et al.) describes a method for Mo separation from acid solutions generated in the uranium recovery from ores by using a resin with a stationary phase having oxime active groups. In this method, a sulfuric acid solution from the recovery process is passed through an oxime resin bed, then eluted with an alkali metal salt or ammonium hydroxide, and carbonate.
U.S. Pat. No. 5,082,568 (Holler et al.) describes a composite cartridge-type filter to reduce lead (Pb) concentration in drinking water where the filter comprises an activated carbon black and a powdered deionizing resin which is converted to an ion exchange resin in-situ and during use of the filter. The composite filter is also effective in removing dissolved metals that are leached out from the carbon black.
U.S. Pat. No. 4,032,457 describes a polymer bonded activated charcoal and hollow interior filled with ion exchange resin to remove lead (Pb) and other metals.
Other methods of removing soluble molybdenum are based on lime precipitation to form insoluble calcium molybdate and further treatment with ferric salts such as FeCl
3
or Fe
2
(SO
4
)
3
to flocculate Fe(OH)
3
wherein molybdate ion is removed by adsorption.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of removing dissolved metals such as Mo and W in aqueous effluents particularly alkaline effluents, generated from the removal of screening paste residues from masks and from other semiconductor cleaning processes based on the use of aqueous alkaline compositions.
Yet another purpose of this invention is to provide a method for removing dissolved Mo and W in TMAH-containing effluents from mask and semiconductor cleaning processes.
Another purpose is to provide a method of removing dissolved metals from waste water using ion-exchange and absorption techniques.
Still yet another purpose of this invention is to provide a method based on ion-exchange for removing dissolved metals in aqueous TMAH-containing effluents from cleaning of conductive paste residue from screening masks.
It is also the purpose of this invention to remove dissolved Mo in aqueous waste solutions generated from the aqueous TMAH treatment of semiconductor substrates to remove Mo oxide surface layers on substrates as part of a Mo etch process using low pH etchants.
Another purpose of this invention is to provide a method for effective removal of dissolved Mo and W in aqueous effluents from mask cleaning to remove screening paste residues and semiconductor cleaning with aqueous TMAH to generate clean water which meets or exceeds the waste water discharge limit requirements for dissolved Mo, W, Cu and Ni.
Another purpose of the invention is concerned wit
Ahmad Umar M.
Sachdev Krishna G.
Cintins Ivars
DeLio & Peterson LLC
Pepper Margaret A.
Tomaszewski John J.
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