HK1018024A - On-site generation of ultra-high-purity buffered-hf for semiconductor processing - Google Patents

Description

On-site generation of ultra-high purity buffered HF for semiconductor processing

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The invention relates to the provision of ultra-pure buffered HF (buffered hydrofluoric acid) and/or ammonium fluoride (NH) for semiconductor manufacturing₄F) The method of (1).

Background: contamination in integrated circuit fabrication

Contamination is often the first significant problem in integrated circuit fabrication. In modern integrated circuit fabrication, a significant portion of the steps are such or such cleaning steps; these cleaning steps may require removal of organic contaminants, metal contaminants, photoresist (or inorganic residues thereof), etch by-products, native oxides, and the like.

As reported in 1995, the cost of new front-ends (integrated circuit wafer fabrication facilities) is typically in excess of one billion dollars ($1,000,000,000), a significant portion of which is spent on particle control, decontamination, and pollution control measures.

One important source of contamination is impurities in process chemicals. Contamination due to the chemical process of decontamination is highly undesirable because decontamination operations are very frequent and critical.

The extremely high purity levels required in semiconductor manufacturing are rare or unique in industrial processes. At such extremely high purity levels, chemical transport is inherently undesirable (and certainly not completely avoided). Exposure of ultrapure chemicals to air must be minimized, especially in environments where operators are also present. This exposure risks the introduction of particulate matter and, as a result, contamination. The transport of ultrapure chemicals in closed containers is still not ideal because of the higher risk of contamination at the site of the manufacturer or user. In addition, undetected contamination can damage a large number of expensive wafers.

Because a wide variety of corrosive and/or toxic chemicals are commonly used in semiconductor processing, the reagent supply is typically separate from the front-end worker. Piping and maintenance of ultra-high purity gases and liquids is well known in the semiconductor industry, and therefore most gases and liquids can be delivered to the wafer fabrication site from any part of the same building, even at the same location.

Ammonia purification

The present inventors have developed a method for producing ultra-high purity ammonia in an in-situ system at a semiconductor wafer production site, comprising: ammonia vapor is withdrawn from the liquid ammonia reservoir, passed through a microporous filter, and the filtered vapor is washed with high pH purified water, preferably deionized water that has been equilibrated with an ammonia stream. This finding enables commercial grade ammonia to be converted to ammonia of high enough purity for high precision manufacturing without the need for conventional column distillation. The drawing of the ammonia vapor from the supply reservoir itself serves as a single stage distillation, excluding non-volatile and high boiling impurities such as alkali and alkaline earth metal oxides, carbonates and hydrides, transition metal halides and hydrides, and high boiling hydrocarbons and halogenated hydrocarbons. Volatile active impurities that can be found in commercial grade ammonia, such as certain transition metal halides, group III metal hydrides and halides, certain group IV hydrides and halides, and halogens, previously thought to require removal by distillation, have been found to be removed by scrubbing to a degree suitable for high precision operation. This is a surprising discovery because scrubber technology is traditionally used to remove macroscopic quantities of impurities rather than trace quantities.

Wet and dry processing

One of the long-term technological changes in semiconductor processing is the change (and attempted change) between dry and wet processing. In dry processing, only gaseous or plasma phase reactants are in contact with the wafer. In wet processing, various liquid reagent formulations are used for various purposes, such as etching silicon dioxide or removing native oxide layers, removing organics or trace organic contaminants, removing metals or trace organic contaminants, etching silicon nitride, etching silicon.

Plasma etching has many attractive properties, but this is not suitable for cleaning. There is indeed no ready chemical method to remove some of the least desirable impurities, such as gold. Wet cleaning is therefore essential for modern semiconductor processing and is likely to be so in the foreseeable future.

Plasma etching is performed with the photoresist in place, followed by a high temperature step, which strips the photoresist, thus necessitating a cleaning step.

The substances that must be removed by the purification step may include: photoresist residue (organic polymer), sodium, alkaline earth metals (such as calcium or magnesium) and heavy metals (such as gold). Many of these species do not form volatile halides and therefore plasma etching cannot carry them away. It is necessary to purify it by wet-chemical methods.

As a result, the purity of the process chemistry in plasma etching is less critical because a cleaning step is always performed after these steps and before the high temperature step, which can remove dangerous contaminants from the surface before the high temperature step back-drills these contaminants. However, liquid chemistry purity is much more important because the intrusion rate on the semiconductor surface is typically one million times higher than plasma etching, and the liquid cleaning step is followed by a high temperature step.

However, wet processing has one major drawback, namely ionic contamination. Integrated circuit structures use only a few dopant species (boron, arsenic, phosphorus, and organic antimony) to form the desired p-type and n-type doped regions. Many other species are electrically active dopants and are highly undesirable contaminants. Many of these contaminants are well below 10¹³cm^-3The concentration of (a) can have deleterious effects, such as increased junction leakage current. In addition, some of these less desirable contaminants are concentrated in the silicon, i.e., where the silicon contacts the aqueous solution, the equilibrium concentration of contaminants in the silicon is higher than in the solution. Furthermore, some of these less desirable contaminants have a diffusion coefficient that is so high that incorporation of these dopants into any portion of the wafer can result in diffusion of these contaminants throughout, including at the junction sites where these contaminants can cause leakage.

Therefore, it is desirable that all liquid solutions used on semiconductor wafers have low concentrations of various metal ions. The total concentration of all metal ions is less than 300ppt (per 10)¹²Parts by weight) are suitable, and each metal is less than 10ppt, the smaller the better. In addition, theContamination by both anions and cations must be controlled (some anions may have adverse effects, e.g., the complexed metal ion may be reduced to a metal atom or ion that is mobile in the silicon lattice.)

The front-end facility typically includes an on-site purification system that produces high purity water (referred to as "DI" water, i.e., deionized water). However, it is more difficult to obtain production chemicals of the desired purity.

With in situ generation of buffered HF and/or NH₄Novel system and method for F manufacturing semiconductors

Systems and methods for preparing ultrapure chemicals on-site at a semiconductor manufacturing facility so that the chemicals can be piped directly to the point of use are disclosed. The disclosed system is a very compact unit that can be located in the same building (or in an adjacent building) as the head, thus avoiding transport.

It has now been found that a process and system similar to those used to prepare ultrapure aqueous ammonia can be used to prepare ultrapure hydrofluoric acid.

Anhydrous HF generally utilizes fluorite (CaF)₂) Adding sulfuric acid to prepare the catalyst. Unfortunately, many fluorites contain arsenic, which leads to contamination of the HF formed. Arsenic contamination is a major problem in HF purification. One source (from china) contains minimal As and is the best feedstock for ultra-pure HF. HF produced from this raw material is available from Allied Chemical company, USA. In conventional systems, other impurities come from the HF generation and transport system. These impurities result from the degenerative deterioration of these systems; these systems are designed for applications that are much less demanding than the semiconductor industry. These contaminants must be removed in order to achieve good semiconductor performance.

HF purification and Evaporation

The HF process includes a batch arsenic removal and vaporization section, a fractionating column to remove most of the other impurities, an ion purification column to reduce contaminants not removed by the fractionating column, and finally HF or NH₄F supplier (HFS or NH)₄FS)。

Arsenic by adding an oxidant (KMnO)₄Or (NH)₄)₂S₂O₈) And a cation source (e.g., KHF)₂) Formation of salt K₂AsF₇Converted to the +5 valence state and retained in the evaporator during distillation. This is a batch process because the reaction is slow and sufficient time must be allowed to complete before distillation can take place. This process requires a contact time of about 1 hour at ambient temperature. To achieve complete reaction in a continuous process, high temperatures and pressures (which are detrimental to safety) or large vessels and piping are required. In this process HF is introduced into a batch evaporator and treated with an oxidizing agent under agitation for a suitable reaction time.

The HF is then distilled under reflux heating in a fractionating column to remove most of the metal impurities. Elements that show significant reduction in this step include:

group 1(I) of Na,

group 2(II) Ca, Sr, Ba

Group 3-12(IIIA-IIA) Cr, W, Mo, Mn, Fe, Cu, Zn

Group 13(III) Ga

Group 14(IV) Sn, Pb,

group 15 (VII) Sb

The fractionating column acts as a series of simple distillations; this is accomplished by packing the column with a high specific surface material and reversing the liquid flow to ensure that the descending liquid is fully equilibrated with the ascending vapor. In this column only one partial condenser is installed to provide reflux and the purified gaseous HF is subsequently introduced into an HF ion purifier (HF IP). At this stage, the HF is pure to normal standards, with the only possibility of carrying out arsenic treatment chemicals or quenchers needed to remove these chemicals.

HF IP is used as an additional purity guarantee before introducing HF gas into the supplier system. These elements may be present in the treatment solution or introduced into the IP for absorption of sulfate carried over in the HF stream. IP testing has demonstrated a significant reduction in HF gas stream contamination of the following elements:

group 2(II) of Sr and Ba,

group 6-12 (VIA-IIA) Cr, W and Cu

Group 13(III) B of (I),

group 14(IV) Pb, Sn and

group 15 (V) Sb.

Many of these elements are useful in reducing As contamination. Any carry-over from the distillation column due to excess in As treatment can be purged in this step.

It should be noted that the batch As removal step can be avoided if HF with a sufficiently low arsenic content is obtained. This class of material was available in the United states from Allied Chemical as published 1995.

Ultra pure buffered HF and NH₄In situ preparation of F

As mentioned above, hydrofluoric acid (HF) is an extremely important production chemical in semiconductor manufacturing. It is often used in a buffered form to reduce the pH change after it has been loaded with etch byproducts in the acid solution (HF reacts with silicon to produce fluorosilicic acid, which will change the pH of the solution and thus the etch rate). Buffering of acids for this purpose is well known, but for ultra-pure chemicals the buffering requirements present additional problems, as the buffer itself is also a source of contamination and must be pure enough not to degrade the system.

In buffered hydrofluoric acid (buffered HF), the buffering of the acid solution is often provided by an ammonium component. According to disclosed embodiments of the invention, buffered hydrofluoric acid may be prepared by bubbling ammonia into an acid solution.

The process includes buffered HF and ammonium fluoride, the only process difference being NH₃Molar ratio to HF. NH (NH)₄The molar ratio of the F solution was 1.00, while the molar amount of HF buffered was excessive. The same equipment was used for both solutions except that the set points of the concentration measurement equipment were set at the desired molar ratios, respectively.

The disclosed invention will be described with reference to the accompanying drawings, which represent important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

FIG. 1 is a process flow diagram of an example of an apparatus for producing ultra-pure ammonia.

Fig. 2 is a block diagram of a semiconductor manufacturing line to which the generator of fig. 4 may be connected.

FIG. 3A shows an overview of a process flow for a generator arrangement in which ultra-pure ammonia is introduced into hydrofluoric acid to produce buffered HF; 3B1-3B3 show detailed piping and representative diagrams of the process flow sample set-up of FIG. 3A.

Figure 4 shows an in situ HF purifier according to a sample embodiment of the disclosed invention.

The numerous innovative aspects of the present application will be described with particular reference to the preferred embodiments of the invention (by way of example, and not of limitation), in which:

NH₃purification of (2)

According to the invention, ammonia vapor is first withdrawn from the vapor zone of the liquid ammonia supply reservoir. The extraction of the vapor in this manner serves as a single stage distillation, leaving some solids and high boiling impurities in the liquid phase. The supply reservoir may be any conventional supply tank or other reservoir suitable for containing ammonia, either in anhydrous form or in an aqueous solution. The vessel may be maintained at atmospheric pressure, or above atmospheric pressure when it is desired to increase the flow of ammonia through the system. The vessel is preferably thermally controlled so that the temperature is from about 10 to about 50 c, preferably from about 15 to about 35 c, and most preferably from about 20 to about 25 c.

Impurities that may be removed as a result of withdrawing ammonia from the vapor phase include metals from groups I and II of the periodic Table, and the aminated forms of these metals formed as a result of contact with ammonia. Also removed are oxides and carbonates of these metals, and hydrides such as beryllium hydride and magnesium hydride; group III elements and oxides thereof, and ammonium adducts of hydrides and halides of these elements; a transition metal hydride; and heavy and halogenated hydrocarbons, such as pump oils.

The ammonia drawn from the reservoir is passed through a filtration device to remove any solid material entrained with the vapor. Microfiltration and ultrafiltration devices and membranes are commercially available and can be used. The grade and type of filtering means are selected as desired. The preferred embodiment of the invention uses a coarse filtration device followed by a 0.1 micron filter followed by an ion purifier, after which no further filtration takes place.

The filtered vapor is then passed to a scrubber where the vapor is washed with a high pH purifying (preferably deionized) water. The high pH water is preferably an aqueous ammonia solution, which is brought to saturation by circulating through the scrubber. The scrubber may conveniently be operated as a countercurrent conventional scrubbing column. Although the operating temperature is not critical, the column is preferably operated at a temperature of about 10-50 deg.C, preferably about 15-35 deg.C. Likewise, the operating pressure is not critical, but preferably is from atmospheric to about 30psi above atmospheric. The column is typically packed with conventional column packing to provide sufficient contact between the liquid and the gas, and preferably with a demister section.

In a preferred embodiment of the invention, the column has packing height of about 3 feet (0.9 meters) and an internal diameter of about 7 inches (18cm) to provide a packing volume of 0.84 cubic feet (24 liters), operating at a pressure drop of about 0.3 inches of water (0.075 kilopascals) and an overflow of less than 10%, a circulation flow of about 2.5 gallons per minute (0.16 liters per second) nominal flow, or 5 gallons per minute (0.32 liters per second) at 20% overflow, a gas inlet below the packing, and a liquid inlet above the packing but below the demisting level. Preferred packing within the column in this illustration are those materials having a nominal size less than the diameter 1/8 of the column. The demisting portions of the columns are similarly or more densely packed and are otherwise of conventional construction. It should be clear that all descriptions and dimensions in this paragraph are only examples. Various system parameters may vary.

In a typical operation, the operation is started by first saturating deionized water with ammonia to form a solution for use as the starting scrubbing medium. During operation of the scrubber, small amounts of liquid in the sump in the column are periodically drained to remove accumulated impurities.

Examples of impurities that can be removed by scrubbers include reactive volatiles such as Silane (SiH)₄) And arsine (AsH)₃) (ii) a Halides and hydrides of phosphorus, arsenic and antimony; a general transition metal halide; and group III and VI metal halides and hydrides.

The apparatus described so far can be operated either batchwise, continuously or semi-continuously. Continuous or semi-continuous operation is preferred. The volumetric processing rate of the ammonia purification system is not critical and can vary widely. But in most operations contemplated for application of the invention, the flow rate of ammonia through the system is from 200 milliliters per hour to several thousand liters per hour.

The ammonia exiting the scrubber may optionally be further purified prior to use, depending on the particular type of manufacturing process in which the purified ammonia is to be used. For example, if ammonia is to be used for chemical vapor deposition, it may be advantageous to include a dehydration unit and a distillation unit within the system. The distillation column may also be operated in a batch mode, a continuous mode, or a semi-continuous mode. In batch operation, a typical operating pressure may be 300 pounds absolute per square inch (2068 kilopascals) with a batch size of 100 pounds (45.4 kilograms). In this example, the column was 8 inches (20 cm) in diameter and 72 inches (183 cm) in height, operated at 30% overflow, with a steam velocity of 0.00221 feet per second (0.00067 meters per second) and a height corresponding to 1.5 inches (3.8 cm) of theoretical plates and 48 equivalent plates. The size of the boiler in this example was about 18 inches (45.7 cm) in diameter, 27 inches (68.6 cm) long, a reflux ratio of 0.5, and circulating cooling water at 60 ° F (15.6 ℃) when entering and 90 ° F (32.2 ℃) when leaving. Again, this is just one example; distillation columns having wide variations in structure and operating parameters may be used.

Depending on the application, the purified ammonia (with or without a distillation step) may be used in the form of a purified gas or an aqueous solution, in which latter case the purified ammonia is dissolved in purified (preferably deionized) water.

FIG. 1 shows a flow diagram depicting an example of an ammonia purification apparatus of the present invention. Liquid ammonia is stored in reservoir 11. Ammonia vapor 12 is withdrawn from the vapor phase in the reservoir and then passed through a shut-off valve 13 and through a filter device 14. The flow rate of the filtered ammonia vapor 15 is controlled by a pressure regulator 16 and then passed into a washing column 17 which contains a packed fraction 18 and a demister pad 19. While the ammonia vapor flows upward, the saturated ammonia water 20 flows downward, the liquid is circulated by the circulation pump 21, and the liquid level is controlled by the liquid level sensor 22. The waste material 23 is periodically withdrawn from the liquid retained at the bottom of the scrubber. The scrubber 17 is supplied with deionized water 24 at high pressure maintained by means of a pump 25. The washed ammonia 26 is routed to one of three alternative routes. Namely:

(1) distillation column 27, where the ammonia is further purified. The distilled ammonia 28 is then passed to the point of use.

(2) A dissolving unit 29 in which ammonia is combined with deionized water 30 into an aqueous solution 31 which is passed to the point of use. For plant operations with multiple points of use, the aqueous solution may be collected in a holding tank from which ammonia is pumped into various lines for use by multiple points of use of the same plant.

(3) A transfer line 32 which delivers ammonia in gaseous form to the point of use.

The second and third of these alternative routes, which do not use distillation column 27, are suitable for producing any metal impurities in amounts less than every 10¹²100 parts of ammonia in the parts. However, for some applications, it is preferred to include distillation column 27. An example is the use of ammonia in furnace deposition or Chemical Vapor Deposition (CVD). For exampleIf ammonia is used for chemical vapor deposition, the distillation column will remove non-condensable species, such as oxygen and nitrogen, that may interfere with CVD. In addition, since the ammonia leaving the scrubber 17 is saturated with water, optionally a dehydration unit may be added to the system between the scrubber 17 and the distillation column 27, depending on the characteristics and efficiency of the distillation column.

For any of the above alternatives, the resulting stream (ammonia gas or aqueous solution) may be divided into two or more sub-streams, each directed to a different point of use, such that the purification unit simultaneously supplies purified ammonia to multiple points of use.

Purification of HF

Figure 4 shows an in situ HF purifier according to a sample embodiment of the disclosed invention.

HF is purified by first oxidizing arsenic to the +5 oxidation state and then fractionating to remove As⁺⁵And metal impurities. See U.S. patent 4,929,435, which is incorporated herein by reference. As shown in the literature, a wide variety of oxidants have been used for this purpose; see, for example, the following patents and patent applications, all of which are incorporated herein by reference: US #3,685,370; CA81-177347 s; EP #351,107; JP #61-151,002; CA 74-101216; CA 78-23343; US #5,047,226; USSR #379,533; CA81-177348 t; US #4,954,330; US #4,955,430; EP #276,542; US #4,083,441; and CA98-P200672 f.

Fluorine (F2) has been shown to be effective (work by others already published) and is considered to be the presently preferred embodiment. F₂Expensive plumbing systems and safety measures are required but have been shown to be feasible.

A second preferred alternative embodiment uses ammonium persulfate ((NH)₄)₂S₂O₈) It is conveniently obtained in ultra-high purity.

Generally, an oxidizing agent that does not introduce metal ions is preferred. Thus other candidates include H₂O₂And O₃。

A less desirable candidate is caronic acid (persulfuric acid, H)₂SO₅Which produces H in solution₂O₂). Another option is ClO₂But has the serious disadvantage of being explosive. Other options include HNO₃And Cl₂But they all introduce anions that must be separated out (the reduction of non-metallic anions is not as important as the reduction of metallic ions, but it is still desirable to achieve anion concentrations of 1ppb or less, so the initial introduction of anions adds to the burden of the ion purification step).

Allied Signal, in cooperation with the present inventors, utilized an initial As oxidation step to achieve successful production of ultrapure HF in their Geismar La facility. The inventors are not aware of all the steps of this method, but the success of Allied in this regard further confirms the practical feasibility of the disclosed invention.

KMnO₄Is the most commonly used oxidizing agent and is expected to be useful for ultra-purification if followed by the disclosed ion purifiers and HF removal processes. However, this reagent places a large cationic burden on the purifier and therefore metal-free oxidants are preferred.

In another embodiment, high purity 49% HF, substantially free of arsenic, may be used. This low arsenic material is expected to be available from Allied in the third quarter of 1995, and can be combined with an in situ ion purification process that does not include an arsenic oxidizing agent to produce ultra pure HF on site.

The HF process flow includes batch arsenic removal and evaporation steps, a fractionation column to remove most of the other impurities, an ion purifier column to reduce contaminants not removed by the fractionation column, and finally an HF Supply (HFs).

By adding an oxidizing agent (KMnO)₄Or (NH)₄)₂S₂O₈) And a cation source (e.g., KHF)₂) To form a salt K₂AsF₇Arsenic will be converted to the +5 valence state and remain in the evaporator during distillation. This is a batch process, since the reaction is carried out in the presence of a catalystIt should be slow and sufficient time must be allowed for the reaction to complete before the distillation can take place. This process requires a contact time of about 1 hour at the usual temperature. High temperatures and pressures (which are detrimental to safety) or large vessels and piping are required to achieve complete reaction in a continuous process. In this process HF is introduced into a batch vaporizer and treated with an oxidizing agent under agitation for a suitable reaction time.

The HF is then distilled in a fractionating column under reflux heating to remove most of the metal impurities. Elements significantly reduced in this step include:

in the group 1(I) of Na,

group 2(II) Ca, Sr, Ba,

groups 3-12(IIIA-IIA) Cr, W, Mo, Mn, Fe, Cu, Zn

In the group 13(III) Ga,

group 14(IV) Sn, Pb and

in group 15(VIII) of Sb,

the fractionating column acts as a series of simple distillations; this is accomplished by packing the column with a high specific surface material and reversing the liquid flow to ensure complete equilibrium between the descending liquid and the ascending vapor. In this column only one partial condenser is installed to provide reflux and the purified gaseous HF is subsequently introduced into an HF ion purifier (HF IP). At this stage, the HF is pure to normal standards, with the only possibility of carrying out arsenic treatment chemicals or quenchers needed to remove these chemicals.

HF IP is used as an additional purity guarantee before introducing HF gas into the supplier system. These elements may be present in the treatment solution or introduced into the IP to absorb sulfate carried over in the HF stream. IP testing has demonstrated a significant reduction in HF gas stream contamination of the following elements:

group 2(II) of Sr and Ba,

group 6-12 (VIA-IIA) Cr, W and Cu,

group 13(III) B of (I),

group 14(IV) Pb, Sn and

group 15 (V) Sb

Many of these elements are useful in reducing As contamination. Any carry-over from the distillation column due to excess in As treatment can be purged in this step.

Various modifications can be made to the concentration control loop (replacing the sound speed with conductivity, etc.) if desired.

In another embodiment of the disclosed invention, the on-site purifier can use high purity hydrofluoric acid with reduced arsenic content as a starting material for the batch. In this embodiment no oxidation step is required.

Buffered HF generation

Fig. 3A represents an overview of a process flow for a generation plant in which ultra-pure ammonia is introduced into hydrofluoric acid to generate buffered HF, and fig. 3B1-3B3 show detailed piping and representative diagrams for the process flow sample set-up of fig. 3A.

In a preferred embodiment of the invention, the liquid volume of the ammonia purifier is 10 liters and the maximum gas flow rate is about 10 standard liters per minute. The wash liquor is purged sufficiently (continuously or incrementally) to be refreshed at least once in 24 hours.

The product concentration (in both production steps) is determined with a sound velocity measuring device (Mesa Labs), but can also be determined with electrical conductivity, density, refractive index or infrared spectroscopy.

In another embodiment of the disclosed invention, the on-site purifier can use high purity hydrofluoric acid with reduced arsenic amounts as a batch starting material. No oxidation step is required in this embodiment.

For setting up the process, it must be ensured that the solution is dissolved in waterTotal HF and NH of₃And (4) concentration. For example, 1kg of a 40% by weight solution of amine fluoride contains 400g of NH₄F and 600g of ultrapure water. Since for pure NH₄F, HF and NH₃In a molar ratio of 1: 1, 400g NH₄F should comprise 216g of anhydrous HF and 184g of anhydrous NH₃(NH₄F molecular weight 237, HF molecular weight 20, NH₃Molecular weight 17).

At the completion of the HF generation cycle, 216g of HF was dissolved in 600g of water or at a weight concentration of 26.5%. The onboard instrumentation is dispatched to add HF to this concentration. Alternatively, 49% HF may be diluted to this concentration.

After 26.5% HF solution had formed, 189g NH were added₃To form 40% NH₄And F, solution.

Other concentrations and molar ratios may be set by adjusting the concentration testing equipment for different applications.

Wafer cleaning

Some of the clean benches in a conventional semiconductor manufacturing line are shown in fig. 2. The first device in the purge line is a photoresist stripping station 41 where an aqueous hydrogen peroxide solution 42 and sulfuric acid 43 are mixed and applied to the semiconductor surface to strip the photoresist. Followed by a rinse station 44 where deionized water is rinsed away from the stripping solution. Immediately downstream of the rinse station 44 is a purge station 45 where ammonia and aqueous hydrogen peroxide are applied. This solution is supplied in one of two ways. In the first mode, the aqueous ammonia 31 is mixed with the aqueous hydrogen peroxide 46, and the resulting mixture 47 is introduced into the purification station 45. In the second mode, pure ammonia gas 32 is bubbled into the aqueous hydrogen peroxide solution 48 to form a similar mixture 49, which is also introduced into the purification station 45. Once cleaned with the ammonia/hydrogen peroxide mixture, the semiconductor passes through a second rinse station 50 where deionized water is applied to remove the cleaning solution. The next station is another decontamination station 54 where an aqueous solution of hydrochloric acid 55 and hydrogen peroxide 56 is mixed and applied to the semiconductor surface for further decontamination. Followed by a final rinse station 57, where it is appliedDeionized water for HCl and H removal₂O₂. A dilute buffered HF (for removal of native or other oxide films) is applied to the wafer at the stripping station 59. The buffered dilute hydrofluoric acid is supplied directly from the generator 70 through a closed conduit. As described above, the reservoir 72 contains anhydrous HF from which a stream of HF is fed to the generator 70 via the ion purifier 71, ammonia, preferably in gaseous form, is also bubbled into the generator 70 to form a buffered solution, and ultra-pure deionized water is added to achieve the desired dilution. This is followed by rinsing in ultrapure deionized water (at station 60) and drying at station 58. The wafers and wafer lots 61 are held on wafer supports 52 and are transferred from one station to the next by a robot 63 or some other conventional method of performing sequential processing. The transfer means may be fully automatic, semi-automatic or not automatic at all.

The system shown in fig. 2 is but one example of a purge line for semiconductor manufacturing. In general, the purge line used for high precision manufacturing may vary widely from that shown in FIG. 2, either by eliminating one or more of the devices shown, or by adding or replacing devices not shown. However, the concept of the present invention for on-site production of high purity ammonia is applicable to all of these systems.

The use of ammonia and hydrogen peroxide as semiconductor purification media at a platen, such as the purification platen 45 shown in fig. 2, is well known throughout the industry. Although the ratio may vary, a nominal system is made up of deionized water, 29% ammonium hydroxide (by weight) and 30% hydrogen peroxide (by weight) mixed in a volume ratio of 6: 1. The scavenger is used for removing organic residues and, in combination with ultrasonic oscillations at a frequency of about 1MHz, for removing particles up to submicron level.

In one class of embodiments, the purification (or purification and generation) system is located in close proximity to the point of use of the ultrapure chemical in the production line, leaving only a short distance between the purification device and the production line. Alternatively, for plants with multiple points of use, the ultrapure chemicals from the purification (or purification and generation) unit may be passed through an intermediate storage tank before reaching the point of use. And then sent from the sump to each point of use via a separate outlet line. In each case, the ultrapure chemicals can be applied directly to semiconductor substrates without packaging or shipping, and without storage except for a small in-line reservoir, and thus without contact with potential contamination sources typically encountered when manufacturing and preparing for use at a site outside of a manufacturing facility. In such embodiments, the distance between the point at which the ultrapure chemical exits the purification system and its point of use in the production line is typically a few meters or less. When the purification system is a factory wide central system for pipeline to two or more user stations, the distance may be two thousand feet or more. The transfer can be carried out with ultra-clean transfer lines that do not introduce contaminating materials. In most applications, stainless steel or polymers (such as high density polyethylene or fluorinated polymers) can be used successfully.

Because the purification unit is located adjacent to the production line, the water used in the unit can be purified according to semiconductor manufacturing standards. These standards are commonly used in the semiconductor industry and are well known to those familiar with the process and experienced in industry practice and standards. Methods of purifying water that meet these criteria include ion exchange and reverse osmosis. Ion exchange processes typically involve most or all of the following: chemical treatments, such as chlorination to kill organisms; removing particles by sand filtration; filtering with activated carbon to remove chlorine and trace organic matter; filtering with diatomite; anion exchange to remove strongly ionized acids; mixed bed finishing, the bed containing cation and anion exchange resins to further remove ions; sterilization, including chlorination or ultraviolet light; filtered through a 0.45 micron or finer filter. Reverse osmosis processes will involve, instead of one or more devices in the ion exchange process, water under pressure flowing through a selectively permeable membrane through which many dissolved or suspended substances pass. Typical standards for the purity of water obtained by these processes are a resistivity of at least about 15 megaohm-cm at 25 deg.C (typically 18 megaohm-cm at 25 deg.C), an electrolyte content of less than about 25ppb, and a particle content of less than about 150/cm³Particle size less than 0.2 microns and microbial content less than about 10/cm³The total organic carbon is less than 10 ppb.

In the method and system of the present invention, a high degree of control over product concentration and hence flow rate is achieved by accurate monitoring and metering with known equipment and instrumentation. One convenient way to do this is to use acoustic velocity sensing. Other methods will be apparent to those skilled in the art. Various modifications can be made to the concentration control loop (replacing the speed of sound with conductivity, etc.) if desired.

Those skilled in the art will recognize that the innovative concepts described in the present application can be modified and varied over a wide range of applications and that the scope of the patented subject matter is therefore not limited by any of the specific exemplary illustrations given.

For example, the disclosed innovative techniques are not strictly limited to the fabrication of integrated circuits, but are also applicable to the fabrication of individual semiconductor elements, such as optoelectronic and power devices.

As another example, the disclosed inventive techniques are also applicable to the creation of other technologies that employ integrated circuit fabrication methods, such as the fabrication of thin film magnetic heads and active matrix liquid crystal displays; the primary application is integrated circuit fabrication, while the disclosed techniques are of secondary utility to other areas.

As another example, it is not strictly necessary to use a scrubber for liquid-vapor contact; a bubbler may be used instead, but it is much less desirable because of the inefficient gas/liquid contact.

Optionally, additional one or more filtration stages may be added to the disclosed purification apparatus.

It should also be noted that additives may be added to the purified water if desired, although this is not the case in the preferred embodiment of the invention.

As mentioned above, the basic embodiment is an on-site purification system. Alternatively, in a less preferred class of embodiments, the disclosed purification system can also be modified to operate as part of a manufacturing unit to produce ultra-high purity water for shipment; however, this alternative embodiment does not have the advantages of in situ purification described above. Such applications encounter the inherent risks of transporting ultra-high purity chemicals discussed above; the disclosed invention provides at least a means to achieve higher initial purity than other technologies can achieve for users who require packaged chemicals (plus attendant shipping). In addition, in this application, it is also possible to use a drying station after the ion purifier.

As mentioned above, the object of the basic embodiment is to provide ultra-pure aqueous chemicals which are most critical to semiconductor manufacturing. However, the disclosed system and method embodiments may also be used to supply a purified gas stream (in many cases, it may be useful to use a dryer downstream of the purifier for this purpose).

It should also be noted that the ultrapure chemicals routed in the semiconductor front end with piping may comprise an in-line reservoir or a pressure reservoir. Thus, references to "direct" piping in the claims do not preclude the use of such reservoirs, but preclude exposure to uncontrolled atmospheres.

Claims (5)

1. An on-site subsystem in a semiconductor device manufacturing facility for providing ultra-high purity buffered ammonium fluoride or hydrofluoric acid to a semiconductor manufacturing operation, the system comprising;

a first evaporation source connected to receive an HF source and to provide an HF vapor stream therefrom;

a second evaporation source connected to receive a source of liquid ammonia and to provide a flow of ammonia vapor therefrom;

said HF vapour stream is connected to a first ion purifier unit which provides a circulating volume of highly purified water in contact with said HF vapour stream, the water containing a high concentration of HF, the first purifier emitting purified HF gas;

said ammonia vapor stream is connected to a second ion purifier means which provides a circulating volume of highly purified water in contact with said ammonia vapor stream, the water containing a high concentration of ammonium hydroxide, the second purifier discharging purified ammonia gas;

a first generator means connected to receive the HF gas stream from the first purifier and to mix the HF gas with high purity acidic deionized water to produce ultra-pure hydrofluoric acid, the first generator means also emitting an impure HF gas top stream contaminated with small amounts of impurities;

said flow of ultra-pure hydrofluoric acid and ammonia vapor being connected to a second generator which mixes the ammonia vapor into the ultra-pure hydrofluoric acid to produce a controlled concentration of buffered ultra-pure hydrofluoric acid; and

a piping connection system that routes said aqueous solution to various points of use in a semiconductor device manufacturing facility.

2. The system of claim 1, wherein the HF source consists of anhydrous HF.

3. The system of claim 1, wherein none of the circulating volumes of high purity water contains any additive.

4. The system of claim 1, wherein the HF source is substantially free of arsenic.

5. The system of claim 1, wherein the HF source uses an ultra-pure arsenic-free aqueous HF solution.