Chamber cleaning method using F2 and a process for manufacture of F2 for this method

Abstract

Elemental fluorine is often manufactured electrochemically from a solution of KF in hydrogen fluoride and contains varying amounts of HF as impurity. The present invention provides a method for chamber cleaning using F.sub.2 which contains more than 0.1% by weight and equal to or less than 10% by weight of HF. Surprisingly, such an F.sub.2 is very well suited for the purpose of chamber cleaning. In a preferred embodiment, the F.sub.2 which contains more than 0.1% by weight and less than 2.5% by weight of HF is electrolytically produced, cleaned, delivered and used on site, without any pressurizing treatment. Omitting cleaning steps and process and using process conditions leaving a relatively high HF content in the F.sub.2 allows at the same time to omit pressurizing steps. The advantage is that less cleaning steps.

Claims

1. A method for the cleaning of chambers, the method comprising using F.sub.2 which contains more than 0.01% by weight and less than 0.5% by weight of HF; wherein the applied F.sub.2 is obtained in a process comprising the following steps (I) and (II): A step (I) comprising: A) at least one step of electrolytically producing raw F.sub.2 from HF in the presence of KF B) at least one step of removing particles entrained in the raw F.sub.2 obtained in step A) C) at least one step of filtering any remaining particles out of the F.sub.2 leaving step B) by passing the F.sub.2 through a filter and D) a step of providing the F.sub.2 to a buffer tank or a storage tank; and A step (II) wherein F.sub.2 is withdrawn from the buffer tank or storage tank and delivered to the chamber to perform at least one step of chamber cleaning; wherein the steps A) to D) are performed on the site of chamber cleaning; wherein step A) is performed in an electrolyzer, step B) is performed in a static scrubber, step C) is performed using a metallic filter, step D) is performed in a buffer tank or storage tank, and the F.sub.2 leaving step D) is delivered to at least one chamber and used in the chamber in a chamber cleaning step (II), wherein the electrolyzer, the static scrubber, the metallic filter, the buffer tank or storage tank, and the at least one chamber are operably connected; and wherein the pressure of the F.sub.2 in step B) is lower than the pressure of F.sub.2 in step A), the pressure of the F.sub.2 in step C) is lower than the pressure of F.sub.2 in step B), the pressure of the F.sub.2 in step D) is lower than the pressure of F.sub.2 in step C), and the pressure of the F.sub.2 in step (II) is lower than the pressure of F.sub.2 in step D).

2. The method of claim 1 wherein the chamber is a CVD chamber.

3. The method of claim 1 wherein the chamber is used during the manufacture of a semiconductor, a micro-electromechanical system, TFT (flat panel display) or a solar cell for depositing at least one layer.

4. The method of claim 1 wherein the filter of step C) comprises pores having a diameter between 0.01 and 20 μm.

5. The method of claim 1 wherein the F.sub.2 pressure in step A) is equal to or lower than 1.1 bar (abs).

6. The method of claim 1 wherein the F.sub.2 is delivered to the chamber in step (II) at a pressure from equal to or greater than 0.2 bar (abs) to equal to or lower than 0.55 bar (abs).

7. The method of claim 1 wherein the F.sub.2 is not subjected to a pressurizing treatment.

8. The method of claim 1 wherein the filter used in step C) is cleaned with liquid HF.

9. The method according to claim 8 wherein the liquid HF is recovered and fed to a step of electrolytically producing F.sub.2.

Description

(1) In the following, this preferred embodiment is explained in further detail.

PREFERRED EMBODIMENTS OF STEP A)

(2) In step A), F.sub.2 is produced electrolytically. Often, salt compositions of the approximate formula KF.Math.(1.8−2.3)HF are applied. Such compositions have a melting point, depending on the composition, around 80° C. or above. Usually, an electrolyzer contains several electrolytic cells. Each cell often contains a multitude of anodes. Often, the cell vessel containing the molten salt serves as cathode, or separate electrodes. Fresh HF is supplied continuously or batch wise to the cell to replenish electrolyzed HF.

(3) HF is electrolyzed to form F.sub.2 and H.sub.2 which are collected in separate cell compartments. H.sub.2 may be dumped. The raw F.sub.2 formed contains up to 10% by weight of HF, and possibly even more; the raw F.sub.2 further contains entrained particles which are essentially constituted of solidified electrolyte salt.

(4) In step A), the raw F.sub.2 is withdrawn from the respective cell compartment having a pressure of equal to or greater than 1 bar (abs). Usually, the pressure of the raw F.sub.2 in the F.sub.2 compartments is approximately 1.05±0.05 bar (abs). Preferred embodiments of step B):

(5) In step B), the raw F.sub.2 is subjected to a purification treatment to separate all or a major part of solid impurities from the raw F.sub.2. Preferably, the solid-removing treatment comprises at least one step of contacting the raw F.sub.2 with cooled liquid HF in a static scrubber.

(6) The liquid HF with which the raw F.sub.2 is contacted has a low temperature and consequently a low vapor pressure. The temperature of the liquid HF during its purifying contact with the fluorine is equal to or higher than the melting point of the HF at the respective pressure in the static scrubber. Preferably, it is equal to or higher than −83° C., more preferably, it is equal to or higher than −82° C. It is preferably equal to or lower than −60° C. The temperature of the liquid HF is preferably in the range between −60° C. and −82° C. The HF can be cooled by cooling machines which provide a suitably cooled liquid to heat exchangers to keep the HF at the desired low temperature. In a preferred embodiment, the liquid HF in the scrubber is indirectly cooled by means of liquid N.sub.2 which, when passing from the liquid to the gaseous state, provides the desired cooling effect.

(7) The F.sub.2 can be contacted with technical grade liquid HF. If desired, purified liquid HF can be applied.

(8) Methods to purify HF, mainly to remove phosphorous compounds, sulfur compounds, arsenic compounds, metals, hydrocarbons and water are well known. See for example U.S. Pat. No. 5,362,469 and U.S. Pat. No. 5,585,085. According to U.S. Pat. No. 5,362,469, water, arsenic compounds, boron compounds, phosphorous compounds and sulfur compounds as well as carbon compounds and metal compounds are removed from HF by contacting liquefied hydrogen fluoride with lithium fluoride and elemental fluorine. The HF is then distilled to provide pure HF. According to U.S. Pat. No. 5,585,085, water and hydrocarbons are removed from HF by contacting it with elemental fluorine and subsequent distillation.

(9) The raw F.sub.2 from step A) may preferably be contacted with a purified liquid HF which is essentially free of at least phosphorous compounds, sulfur compounds and arsenic compounds. HF can be purified according to the process of U.S. Pat. No. 5,362,469 and then has a content of water of less than 1 ppm.

(10) Often, it is sufficient to perform step B) only once to achieve a desired degree of solids removal. But if desired, the contact between F.sub.2 and liquid HF can be repeated once, or even more often.

(11) The raw F.sub.2 is supplied to the static scrubber to be contacted with liquid HF under the pressure it has in the electrolytic cell compartment. No pump or compressor is used to raise the pressure of the raw F.sub.2 withdrawn from the electrolytic cell compartment. Due to the marginal pressure loss in the line from the electrolytic cell compartment to the scrubber, the raw F.sub.2 enters the scrubber with a pressure corresponding to the pressure in the electrolytic cell, i.e. 1.05±0.02 bar (abs).

(12) Due to the low temperature of the liquid HF during its contact with fluorine, the vapor pressure of HF is very low. Thus, not only the content of entrained solids is reduced, but also entrained HF is removed. The F.sub.2 after the contact with liquid HF contains more than 0.1% by weight and less than 2.5% by weight.

(13) The liquid HF which is used in the purification step is preferably circulated or reused, optionally after regeneration, e.g. by a step of distillation, to remove any solids washed out from the raw fluorine as described above. The advantage is that any water initially present in the HF reacts with F.sub.2 to form HF and OF.sub.2; after some time, the water is consumed, and the fluorine to be purified will not be consumed by water in a side reaction, and the fluorine will not take up any OF.sub.2 anymore because it is no longer formed.

(14) The F.sub.2 which leaves the static scrubber has a pressure which is, as a rule of thumb, approximately 40 to 80 mbar lower than upon entering the static scrubber. The HF content depends from several conditions, e.g. from the initial HF concentration, from the temperature of the liquid HF and from the contact time. The conditions of contact between the liquid HF and the F.sub.2 are selected such that the HF content is in the range given above, and notably, in the preferred ranges.

PREFERRED EMBODIMENTS OF STEP C)

(15) The F.sub.2 withdrawn from step B) is then treated in step C). Once again, no pump or compressor is used to raise the pressure of the F.sub.2 withdrawn from step B) and forwarded to step C). For solids removal, the F.sub.2 is passed in step C) through one or more particle filters with small pores to remove any residual solids content. The filter or filters may comprise pores in the range of 0.01 to 20 μm. Pore size denotes the pore diameter. Particle filters with a pore size greater than 20 μm could be applied but may not be effective enough. The particle filters serve to remove any solid particles before the contact with liquid HF, or to remove solids still entrained after the purifying treatment of the present invention; the particle filters may be constructed from materials resistant to F.sub.2, especially from steel or Monel metal.

(16) The temperature in the filter should advantageously be lower than the melting point of the solids (which is usually somewhat above 80° C.). Preferably, the temperature in the filter is equal to or lower than 50° C. Especially preferably, step C) is performed at ambient pressure.

(17) The F.sub.2 leaving the filter in step C) has a pressure which is approximately 10 to 150 mbar lower than upon entering the filter.

(18) F.sub.2 having passed steps B) and C) is suitably pure to be used as cleaning agent for chambers used for CVD-enhanced deposition of matter on items. It is especially suitable for cleaning of chambers which have been used for deposition and/or etching of layers, like CVD chambers, used in the manufacture of semiconductors, micro-electromechanical devices, solar cells, TFTs (thin film transistors).

PREFERRED EMBODIMENTS OF STEP D)

(19) The F.sub.2 leaving step C) is subjected to step D). In step D), it is delivered to the point of use, i.e. a chamber or a plurality of chambers to be cleaned. No pressurization is performed, thus it is not passed through a pump or a compressor.

(20) Preferably, in step D), the F.sub.2 leaving step C) is stored in a storage tank. In the storage tank, it is preferably stored at the pressure with which it is withdrawn from step C). If desired, a control valve may be applied to reduce the F.sub.2 pressure in the buffer tank. In the buffer tank, the F.sub.2 pressure preferably is in a range from equal to or greater than 0.2 bar (abs) to 0.8 bar (abs). In view of this relatively low pressure, the buffer tank or tanks may require a large internal volume. For example, one or more buffer tanks having an internal volume from 10 to 50 m.sup.3 are preferred. Using two or more buffer tanks, e.g. 4 buffer tanks, having an internal volume of 25 m.sup.3 provides sufficient capacity to provide a reliable F.sub.2 delivery.

(21) Preferably, the chamber cleaning method of the invention is performed such that step A) is performed in an electrolyzer, step B) is performed in a static scrubber, step C) is performed using a metallic filter, step D) is performed in a buffer tank or storage tank, and the F.sub.2 leaving step D) is delivered to at least one chamber and used in the chamber in a chamber cleaning step (II), wherein the electrolyzer, the static scrubber, the metallic filter, the buffer tank or storage tank and the at least one chamber are operably connected.

(22) Preferably, the pressure of the F.sub.2 in each of the steps B) to D) and in step (II) is lower than the pressure of F.sub.2 in the preceding step. Preferably, the F.sub.2 pressure in step A) is equal to or lower than 1.1 bar (abs), preferably equal to or lower than 1.05±0.02 bar (abs), and preferably, the F.sub.2 is delivered to the chamber in step (II) at a pressure from equal to or greater than 0.2 bar (abs) to equal to or lower than 0.55 bar (abs).

(23) Another aspect, as mentioned above, concerns process for the manufacture of F.sub.2 which contains more than 0.1% by weight and less than 2.5% by weight of HF, comprising a step (I) comprising the following steps: A) at least one step of electrolytically producing raw F.sub.2 from HF in the presence of KF B) at least one step of removing particles entrained in the raw F.sub.2 obtained in step A) C) at least one step of filtering any remaining particles out of the F.sub.2 leaving step B) and D) a step of delivering the F.sub.2 to a buffer tank or a storage tank.

(24) From this buffer tank or storage tank, it can be delivered according step (II) to the chamber to be cleaned.

(25) A preferred process for the manufacture of F.sub.2 which contains more than 0.1% by weight and less than 2.5% by weight of HF, comprising step (I) comprising the following steps: A) at least one step of electrolytically producing raw F.sub.2 from HF in the presence of KF B) at least one step of removing particles entrained in the raw F.sub.2 obtained in step A) C) at least one step of filtering any remaining particles out of the F.sub.2 leaving step B) and D) a step of delivering the F.sub.2 to a buffer tank or a storage tank, wherein the F.sub.2 pressure in step A) is equal to or lower than 1.1 bar (abs), preferably equal to or lower than 1.05±0.02 bar (abs), the pressure of the F.sub.2 provided to the chamber to be cleaned is from equal to or greater than 0.2 bar (abs) to equal to or lower than 0.55 bar (abs), and wherein the pressure of the F.sub.2 in each of the steps B) to D) is lower than the pressure of F.sub.2 in the preceding step.

(26) Preferably, the F.sub.2 is not subjected to a pressurizing treatment throughout the process and method.

(27) The F.sub.2 provided by the invention, containing HF from 0.1 to 2.5% by weight is very well suited as chamber cleaning agent. An F.sub.2 containing 1 to 2% by weight of HF can be manufactured technically easy (only a short contact time in the static scrubber is necessary, for example), and nevertheless very suitable as chamber cleaning agent. If it is desired to provide F.sub.2 with a very low content of HF, e.g. with an HF content from equal to or greater than 0.1% by weight to equal to or lower than 0.5% by weight, it is an option to contact the F.sub.2 for further HF removal between step B) and step C), especially with an adsorbent, e.g. with NaF. This complicates the process, though, without improving the suitability of the treated F.sub.2 as chamber cleaning agent.

(28) The F.sub.2 which is provided according to the invention is used as chamber cleaning gas for chambers used in the manufacture of the a semiconductor, a TFT or a solar panel.

(29) In a preferred embodiment, step (I), i.e., the electrolytic manufacture of F.sub.2, purification and delivery, optionally including storage, as described above are performed on site in combination with step (II). The electrolyzer apparatus, scrubber, filter or filters and storage tank are connected through lines in this embodiment.

(30) The method of chamber cleaning and the process of providing F.sub.2 according to the invention have many advantages over the prior art. Contrary to the common belief that highly pure F.sub.2, notably F.sub.2 essentially free of HF, is needed for chamber cleaning, it was found that F.sub.2 containing HF with a content of up to 2.5% by weight of HF is very suitable. Consequently, it is not necessary to subject F.sub.2 to thorough and careful purifying operations to remove even minute amounts of HF. An added advantage is the finding that the purifying steps according to the process of the invention can be performed such that no pump or compressor is needed to subject purified F.sub.2 to pressurization. The combination of both observations—simple purification with low pressure drop, and consequently, no need for pressurization—provides an economic advantage because of less energy consumption and less equipment; the latter also has technical advantages because less equipment and less treatment steps mean less incidents due to failure of apparatus, less maintenance costs and less downtime. Less apparatus, less treatment steps and a lower F.sub.2 pressure also provide a higher degree of safety.

(31) Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

(32) The following example is intended to explain the invention further without limiting it. It is performed according to the embodiment wherein the pressure in the electrolytic cell is 1.05 bar (abs).

EXAMPLE 1

Manufacture of F2, its Purification and its Delivery for Chamber Cleaning

(33) Step A). Manufacture of F.sub.2

(34) An electrolyte salt with a composition of about KF.Math.2HF is filled into an electrolysis cell, heated to about 80-120° C. and molten therein. A voltage between 8 to 10 V is applied, and current is passed through the composition of electrolyte salt dissolved in the hydrogen fluoride. HF is introduced into the electrolytic cell in an amount corresponding to consumed HF. Raw F.sub.2 under a pressure of about 1.05 bar (abs) and H.sub.2 form in the respective electrode compartments. H.sub.2 is passed through a water scrubber to removed entrained HF and is then released to the atmosphere.

(35) Step B). Removal of HF and Solids

(36) The raw F.sub.2 in the anode compartment of the electrolytic cells contains HF and entrained solids (mainly electrolyte salt). The raw F.sub.2 is withdrawn from the electrolytic cells at a pressure of about 1.05 bar (abs) and contacted in a static scrubber with liquid HF which has a temperature of about −80° C. Most of the solids and the major part of HF entrained in the F.sub.2 are removed in the static scrubber. The F.sub.2 leaving the static scrubber has an HF content of about 1% by weight, and its pressure is about 0.95 bar (abs.).

(37) Step C): Additional Solids Removal in a Filter

(38) The F.sub.2 leaving step B) is passed through a Monel metal frit with pores having a diameter of approximately 1 μm to further reduce the solids content. The F.sub.2 leaving the metal frit has a pressure of approximately 0.8 bar (abs.) and a very low content of entrained solids. The fits are plugged from time to time by filtered solids. They can be cleaned by a treatment with liquids to dissolve the solids.

(39) Step D): Delivery to the Buffer Tank

(40) The F.sub.2 leaving the Monel filter frit in step C) still has a content of about 1% by weight of HF, but a very low solids content. It is passed into a buffer tank having an internal volume of 25 m.sup.3; four such buffer tanks are assembled. In the buffer tank, the F.sub.2 is stored under a pressure of approximately 0.8 bar (abs.). If desired, a control valve can be foreseen between the Monel filter of step C) and the buffer tank; by means of the control valve, the pressure of the F.sub.2 in the buffer tank can be reduced to a still lower level (e.g., it can be stored at a pressure ranging from 0.2 to 0.55 bar).

(41) Step (II): As soon as F.sub.2 is needed as chamber cleaning agent, it is provided from the buffer tank through a delivery line. It is passed through a control valve which further reduces the pressure, e.g. to 0.5 bar (abs) if the F.sub.2 pressure in the buffer tank is greater than 0.5 bar (abs). The F.sub.2 which finally is delivered to a chamber (used for the manufacture of photovoltaic cells) as cleaning agent has an HF content of 1% by weight and a pressure of approximately 0.5 bar (abs).

(42) In the chamber, F.sub.2 containing HF is introduced as the cleaning agent, an in-situ plasma is ignited and the cleaning agent, having a pressure of about 0.5 bar (abs), removes deposits, e.g. deposits of Si and SiO.sub.2, from the walls and parts inside the chamber.

EXAMPLE 2

Manufacture of F2, its Purification and its Delivery for Chamber Cleaning

(43) Step A). Manufacture of F.sub.2

(44) An electrolyte salt with a composition of about KF.Math.2HF is filled into an electrolysis cell, heated to about 80-120° C. and molten therein. A voltage between 8 to 10 V is applied, and current is passed through the composition of electrolyte salt dissolved in the hydrogen fluoride. HF is introduced into the electrolytic cell in an amount corresponding to consumed HF. Raw F.sub.2 under a pressure of about 1.05 bar (abs) and H.sub.2 form in the respective electrode compartments. H.sub.2 is passed through a water scrubber to removed entrained HF and is then released to the atmosphere.

(45) Step B). Removal of Solids

(46) The raw F.sub.2 in the anode compartment of the electrolytic cells contains HF and entrained solids (mainly electrolyte salt). The raw F.sub.2 is withdrawn from the electrolytic cells at a pressure of about 1.05 bar (abs) and introduced into a settling box where most of the solids entrained in the F.sub.2 are removed by gravity. The F.sub.2 leaving the settling box has an HF content of about 5% by weight, and its pressure is about 0.95 bar (abs.).

(47) Step C). Final Solids Removal in a Filter

(48) The F.sub.2 leaving step B) is passed through a into a filtering device composed of a first Monel metal frit with pores having a diameter of approximately 30 μm and a second Monel metal frit with pores having a diameter of approximately 0.003 μm. The F.sub.2 leaving the metal frit has a pressure of approximately 0.8 bar (abs.) and a very low content of entrained solids. The frits are plugged from time to time by filtered solids. They are cleaned by a treatment with liquid HF to dissolve the solids. The HF recovered from the washing of the fits is recovered and fed in the liquid phase to step A).

(49) Step D): Delivery to the Buffer Tank

(50) The F.sub.2 leaving the Monel filter frit in step C) still has a content of about 5% by weight of HF, but a very low solids content. It is passed into a buffer tank having an internal volume of 25 m.sup.3; four such buffer tanks are assembled. In the buffer tank, the F.sub.2 is stored under a pressure of approximately 0.8 bar (abs.). If desired, a control valve can be foreseen between the Monel filter of step C) and the buffer tank; by means of the control valve, the pressure of the F.sub.2 in the buffer tank can be reduced to a still lower level (e.g., it can be stored at a pressure ranging from 0.2 to 0.55 bar).

(51) Step (II): As soon as F.sub.2 is needed as chamber cleaning agent, it is provided from the buffer tank through a delivery line. It is passed through a control valve which further reduces the pressure, e.g. to 0.5 bar (abs) if the F.sub.2 pressure in the buffer tank is greater than 0.5 bar (abs). The F.sub.2 which finally is delivered to a chamber (used for the manufacture of photovoltaic cells) as cleaning agent has an HF content of 5% by weight and a pressure of approximately 0.5 bar (abs).

(52) In the chamber, F.sub.2 containing HF is introduced as the cleaning agent, an in-situ plasma is ignited and the cleaning agent, having a pressure of about 0.5 bar (abs), removes deposits, e.g. deposits of Si and SiO.sub.2, from the walls and parts inside the chamber.