Method for the manufacture of electronic devices with purified fluorine
This application claims priority to European patent application
no. 10159285.5 filed April 8th, 2010, the whole content of this application being
incorporated herein by reference for all purposes.
5 The invention concerns a method for the manufacture of electronic devices
with purified elemental fluorine.
Elemental fluorine (F2) is often produced electrolytically from hydrogen
fluoride (HF) in the presence of an electrolyte salt, usually in the presence of a
molten HF adduct ofKF having a formula of about KF·(1.8 to 2.3)HF at a
10 voltage of at least 2.9 V (in practice, between 8 and 10 or 11 V) according to the
equation (1):
2HF -7Hz+ Fz (1)
F2 is useful as etchant for the manufacture of electronic devices, especially
semiconductors, photovoltaic cells, micro-electromechanical systems
15 ("MEMS"), TFTs (thin film transistors for flat panel displays or liquid crystal
displays), and the like, and as cleaning agent for treatment chambers used in the
manufacture of electronic devices. For the manufacture of semiconductors,
photovoltaic cells, MEMS and TFTs, several consecutive steps of deposition of
layers and etching a part of them are performed in treatment chambers ; fluorine
20 can be used for etching of layers constituted of very different constitution, for
example, for etching silicon containing layers. Usually, during deposition
processes performed in the treatment chambers - often CVD chambers
(chambers wherein layers are deposited on items via chemical vapor deposition,
e.g. plasma-enhanced CVD, metal organic CVD or low pressure CVD)-
25 undesired deposits form on the walls and on inner constructive parts of the
chamber and must be regularly removed. This is achieved by treating the
deposits thermally or plasma-enhanced with elemental fluorine as chamber
cleaning agent.
Fluorine is a very reactive molecule, and thus, for plants with a
30 respectively great demand, elemental fluorine is preferably produced on-site.
This means, that one or more fluorine generators are located on the site of the
tools, and preferably, in proximity of the tools (CVD chambers, etching
chambers) for manufacturing the electronic devices. It is especially preferred
wo 2011/124642 PCT /EP2011/055425
- 2-
that the fluorine generator or generators be in fluid contact with one or more of
the tools such that it is not necessary to fill the fluorine into a storage tank or into
pressure bottles and transport it on or to the site of use.
Especially for the use of elemental fluorine as an etchant, but also when
5 used as chamber cleaning agent, it is desirable that the elemental fluorine be very
pure. Entrained solids and hydrogen fluoride (HF) are considered as especially
undesired impurities.
WO 2006/043125 discloses a fluorine gas generator disposed in the gas
feed system of a semiconductor plant. The pressure in the electrolytic cells is set
10 to ambient pressure up to 820 torr and preferably, to ambient pressure up to
770 torr, and the pressure in a buffer tank is set, for example, to 0.18 MPa above
atmospheric pressure (i.e. to about 2.8 Bara).
US patent 5,589,148 discloses a method for purifying fluorine wherein
impure fluorine is contacted with an alkaline earth metal hydroxide and an iron
15 oxide.
US patent 2,960,835 and WO 02/50334 (corresponding to US patent
application publication 2002/0074013) disclose the removal ofHF from F2 by a
low temperature treatment.
Object of the present invention is to provide an improved process for
20 delivery of purified fluorine as agent to a process for the manufacture of
electronic devices wherein it is desirable to use pure reagents.
The invention concerns a method for the manufacture of electronic devices,
especially semiconductors, photovoltaic cells, MEMS, or TFTs, comprising at
least one step selected from the group consisting of etching of items in a chamber
25 using elemental fluorine as etching agent, and cleaning a chamber using
elemental fluorine as chamber cleaning agent, comprising at least a step of
manufacture of the elemental fluorine by electrolysis wherein, after its
manufacture and before its application as etching agent or chamber cleaning
agent, the fluorine is subjected to a low temperature treatment at a pressure
30 above ambient pressure to remove at least a part of entrained hydrogen fluoride.
The term "low temperature treatment" denotes a contact of the fluorine to
be purified with cooled surfaces of a cooling apparatus, e.g. a cooled trap or a
cooled heat exchanger wherein the cooled surfaces are cooled to a temperature of
equal to or lower than -50°C. Preferably, they are cooled to a temperature of
35 equal to or lower than -60°C. The temperature is preferably equal to or higher
than -185°C because, at ambient pressure, the boiling point of fluorine is
wo 2011/124642 PCT /EP2011/055425
- 3-
about -188°C. Cooling liquids for this temperature range are generally known,
e.g. from R. E. Rondeau, J. Chern. Eng. Data, II, 124 (1966). US patent
application publication 2009-0026410 provides heat transfer fluids comprising
an ether and an alkylbenzene which are suitable in heat exchangers operating at
5 temperatures as low as -115°C. The melting point ofHF under standard pressure
(100 kPa) is -83.6°C. Thus, under the conditions of the process of the invention,
and depending from the pressure, the HF in the trap will solidify if the
temperature is respectively low ; the higher the pressure, the lower the
temperature at which the HF solidifies.
10 The pressure of the fluorine during the low temperature treatment is
preferably equal to or greater than 1.5 Bar absolute (150 kPa abs.); the term
"Bar abs." is equivalent to "Bara". More preferably, the pressure is equal to or
greater than 2 bar ( abs ). The pressure is preferably equal to or lower than
20 Bara (2.000 kPa abs.), and more preferably, it is equal to or lower than
15 15 Bara (1.500 kPa abs).
Performing the process at a pressure in the upper region, e.g. between 6
and 20 bar ( abs. ), at a given temperature improves the separation of the entrained
HF because the partial pressure of HF is lower at a higher pressure, and thus the
residual content of HF in the treated fluorine is lower. On the other hand, for
20 practical reasons it may be preferred to perform the process at a lower pressure,
e.g. in the range from 2 to 4 bar (abs.) because F2 is a very aggressive compound.
While performing the process at a temperature as low as possible improves
the separation factor (because the partial pressure ofHF is lower), it is preferred
to select pressure and temperature such that the separated HF does not solidify
25 because the separated HF can be removed from the trap more easily in liquid
state than HF being in a solid state. The removal of HF at conditions at which
the condensed HF is liquid is preferred. As mentioned above, at ambient
pressure, HF solidifies at about -83.6°C. Under the elevated pressure of the
process of the invention, the solidification temperature is lower, thus, the process
30 can be performed at temperatures even lower than -83.6°C. A preferred range
for the treatment at low temperature is from equal to or greater than -70°C to
equal or higher than -82°C.
The fluorine can be manufactured, if desired, on site. This is a preferred
embodiment of the invention. It can be produced in one or more satellite plants,
35 e.g. in a fluorine generating cassette as described in WO 2004/009873. If
desired, each cassette can be allocated to one or more process chambers wherein
wo 2011/124642 PCT /EP2011/055425
- 4-
etching is performed ; or a plurality of fluorine generating cassettes is connected
to a fluorine gas distribution system which is connected to the chambers. The
means, e.g. a cooled trap, for performing the inventive method for the low
temperature purification can be integrated into the cassette.
5 The process can also be performed in the frame of a plant according to the
skid concept as described in unpublished US provisional patent applications
no. 61/383204, filed Sep. 15, 2010, and 61/383533, filed Sep. 16, 2010, both
filed in the name of Solvay SA.
If produced on site, one embodiment provides for the manufacture of
10 fluorine for the manufacture of photovoltaic devices, for example, photovoltaic
cells. The manufacture of photovoltaic cells and etching agents and etching
agent compositions comprising F2 useful therein are described in
WO 2009/092453 and EP-A 2159829. In this embodiment, the pressure of
delivery of the fluorine to the tool is preferably equal to or higher 1.5 Bar abs. It
15 is preferably equal to or lower than 5 Bara. A preferred range for the pressure in
this embodiment is 1.5 to 4.5 Bara. The process of the invention can be
performed at the same pressure. If desired, the process of the invention is
performed at a higher pressure, preferably from 8 to 12 Bara, because, as
explained above, the separation of HF is better at higher pressure. The fluorine
20 is then depressurized to a pressure in the preferred range of 1.5 to 5 Bara.
In another embodiment of on site production with greater demand of
fluorine, especially in the manufacture of TFTs for LCDs, the pressure of
delivery of the fluorine to the tool is preferably equal to or greater than 3 Bara
(300 kPa abs.), and especially preferably, equal to or greater than 4.5 Bara
25 ( 450 kPa abs ). In this embodiment, the pressure is preferably equal to or lower
than 12 Bara (1.200 kPa abs), and more preferably, it is equal to or lower than
11 Bara (1.100 kPa abs). An especially preferred range in this embodiment is
4.5 to 11 Bara ( 450 to 1.100 kPa abs ). In this embodiment, the process of the
present invention and the delivery to the tool are preferably performed at a
30 pressure in the range of 8 to 12 Bara (800 to 1200 hPa). The advantage is that no
additional pressurization is needed. If the process of the invention is performed
at a lower pressure than the step of the fluorine delivery to the tool, the fluorine
must be pressurized in a compressor or other means. The manufacture ofTFTs
and etching agents comprising F2 useful therein is described in unpublished EP
35 patent application 09174034.0 filed on 26 Oct 2009 (corresponding to
wo 2011/124642 PCT /EP2011/055425
- 5-
unpublished international patent application no. PCT /EP20 10/066109 filed
on Oct. 26, 2010).
By means of the low temperature treatment, HF which usually is entrained
in electrolytically produced F2 can be removed effectively. If desired, a further
5 treatment with an adsorbent or absorbent, e.g. NaF, can be performed after the
low temperature treatment. The fluorine can be contacted with the adsorbent or
absorbent under the same pressure as applied for removing HF from it at low
temperature. Alternatively, the fluorine can be pressurized to a higher pressure
or depressurized to a lower pressure for contacting it with the absorbent.
10 Alternatively or additionally, a step of pressure distillation can be performed.
Entrained solids which are mainly composed of solidified electrolyte salt can be
removed in a frit made from Fz-resistant material, especially stainless steel,
nickel or Monel metal.
According to one embodiment, the removal of solids from the
15 electrolytically produced F 2 can be performed by passing the F 2 through a jet
scrubber operated with liquid HF at a low temperature, e.g. with liquid HF at a
temperature of -70°C or lower, down to -82°C. Such a treatment is described in
European patent application no. 10172034.0 filed Aug 05, 2010 in the name of
Solvay SA.
20 In such a combination of low temperature treatment and absorption, the
advantage is that less absorbent must be regenerated. The removed HF can be
recycled to the electrolysis reactor.
The "chamber" is a chamber wherein electronic devices are etched
thermally or assisted by a remote and/or internal plasma, or it is a chamber used
25 for deposition oflayers, e.g. by chemical vapor deposition (CVD), especially a
plasma-enhanced CVD chamber (PECVD) which is to be cleaned from time to
time or according to a schedule to remove undesired deposits which accumulate
on the walls and internal parts.
The process of the present invention and the delivery of the purified
30 fluorine can be performed on the site of use or on a site apart from its point of
use. If it is performed apart from the site of use, the fluorine will be stored in a
storage tank for transport purposes. If the fluorine is produced on site, it may be
stored in a storage tank ; the storage tank can also be applied as buffer tank. If
the fluorine generator is sufficiently close to the fluorine applying tool, the
35 fluorine must not be collected in a storage tank.
wo 2011/124642 PCT /EP2011/055425
- 6 -
The step or steps of purification can be performed before or after storage or
buffering (if a step of storage or buffering is foreseen). If desired, the fluorine
can be purified before and after storage.
After its manufacture and purification, the fluorine is delivered to the point
5 of use. This is preferably performed under a pressure which is greater than
ambient pressure.
In a preferred embodiment, the fluorine is pressurized by means of
compressors. If acceptable for the intended purpose, pressurizing inert gases can
be applied, for example, nitrogen, oxygen, helium or argon. Preferably, no
10 pressurizing gas is applied, except for elemental fluorine as pressurizing gas.
The step of storage, if foreseen, preferably denotes the storage of the
elemental fluorine in suitable tanks, e.g. stainless steel bottles.
The fluorine is preferably generated on site of its point of use via
electrolysis in an apparatus which is in fluid communication with the process
15 chamber or process chambers. This means that the generated elemental fluorine
is not filled into a storage tank or into pressurized bottles, which are then
disconnected from the delivery line. If desired, the fluorine is stored in storage
tanks, buffer tanks or bottles only which remain connected to the delivery line.
Often, the fluorine generator is located on the same plant as the tools wherein it
20 is used, i.e. in a distance of less than 500 m from the manufacturing tools ; the
generator often will be located near the tools, e.g. in a distance of 100 m or less
from the tools. They can even be located in close proximity to the process
chamber as the point of use, e.g. the distance can be 10m or less.
The step of delivery preferably denotes passing the fluorine from the
25 manufacturing apparatus to the point of use through pipes, especially through
pipes which remain permanently connected to prevent intrusion of air into the
fluorine, and to prevent fluorine to leak out.
In a preferred embodiment, the step of low temperature treatment to
remove HF, the step of delivery of the treated F2 to the point of use, any step of
30 storage and the optional, but preferably performed steps of removing entrained
solids and removal of any residual HF by means of an adsorption or absorption,
preferably by contacting the F2 with NaF, are performed at a pressure greater
than ambient pressure; preferably, the steps are performed at a pressure equal to
or greater than 1.5 bar (abs), and more preferably, at a pressure equal to or
35 greater than 2 bar (abs). Preferably, said steps are performed at a pressure equal
to or lower than 20 bar (abs).
wo 2011/124642 PCT /EP2011/055425
- 7 -
According to one embodiment, said steps are performed at a pressure in the
lower range, especially in the range of 1.5 to 4.5 Bara (150 to 450 hPa) if
photovoltaic devices, for example, photovoltaic cells, are to be produced.
In another embodiment of on site production with greater demand of
5 fluorine, especially in the manufacture of TFTs for LCDs, the pressure of the
fluorine in said steps to the tool is preferably equal to or greater than 3 Bara.
The method of the present invention allows for the delivery of elemental
fluorine to the point of use with a low content ofHF, or with no HF at all.
Should the disclosure of any patents, patent applications, and publications
10 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.
The following example is intended to explain the invention in detail
without limiting it.
15 Example 1 :Provision of elemental fluorine with low HF content on-site to the
point of use for chamber cleaning
1. Manufacture of elemental fluorine
An electrolyte salt with a composition of about KF·2HF is filled into an
electrolysis cell, heated to about 80- 120°C and molten therein. HF is
20 introduced into the electrolytic cell. A voltage of between 8 to 10 V is applied,
and current is passed through the composition of the molten electrolyte salt and
hydrogen fluoride ; the content of the cell is kept in a range of about 80
to 1 00°C. Elemental fluorine and elemental hydrogen form in the respective
electrode compartments. The generated elemental fluorine is passed through a
25 Monel metal frit to remove solids and pressurized by means of a compressor to
about 10 Bar abs. and then passed through a trap cooled to -80°C ; in this trap,
entrained HF condenses. The gaseous F2 leaving the trap is passed through a bed
ofNaF to remove any residual HF.
2. Delivery of the purified F2 to the chamber
30 The compressed fluorine is then passed, if desired after depressurization to
about 2 to 3 Bar abs., in a pipe directly to a semiconductor-manufacturing tool
located on-site. In this tool, the elemental fluorine is applied for the cleaning of
the inner walls of a plasma chamber to remove silicon-containing residues
deposited during the PECVD (plasma-enhanced vapor deposition) layers in a
35 process for the manufacture of semiconductors.
wo 2011/124642 PCT /EP2011/055425
- 8 -
Example 2 : Provision of elemental fluorine with low HF content on-site to the
point of use for etching
1. Manufacture of elemental fluorine
The manufacture of the elemental fluorine including the low temperature
5 treatment is performed as described in example 1.1.
2. Delivery of the purified F2 to the tool
The compressed fluorine obtained in example 2.1 is then passed under a
pressure of 2 to 3 Bar abs. through a pipe directly to a semiconductormanufacturing
tool located on-site. In this tool, the elemental fluorine is applied
10 in admixture with N2 and Ar in a volume ratio of Ar:N2:F2 of 10:70:20 for the
etching of SixNy layers (xis about 3 and vis about 4) on a Si wafer in a plasma
chamber in a process for the manufacture of semiconductors.
Example 3 :Provision ofF2 with very low HF content for TFT manufacture
1. Manufacture of elemental fluorine
15 An electrolyte salt with a composition of about KF·2HF is filled into an
electrolysis cell, heated to about 80- 120°C and molten therein. HF is
introduced into the electrolytic cell. A voltage of between 8 to 10 V is applied,
and current is passed through the composition of the molten electrolyte salt and
hydrogen fluoride. Elemental fluorine and elemental hydrogen form in the
20 respective electrode compartments.
2. Delivery of the purified F2 to the tool
The generated elemental fluorine is passed through a Monel metal frit to
remove solids and pressurized by means of a compressor to about 10 Bar abs.
and then passed through a trap cooled to -1 00°C ; in this trap, entrained HF
25 condenses.
The gaseous F2 leaving the trap is passed through a bed ofNaF to remove
any residual HF and then delivered, under a pressure of about 10 Bara, to a tool
for the manufacture of thin film transistors for liquid crystal displays
(TFT-LCD).
30 Example 4 :Provision ofF2 with very low HF content for photovoltaic cell
manufacture
1. Manufacture of elemental fluorine
An electrolyte salt with a composition of about KF·2HF is filled into an
electrolysis cell, heated to about 80- 120°C and molten therein. HF is
35 introduced into the electrolytic cell. A voltage ofbetween 8 to 10 Vis applied,
and current is passed through the composition of the molten electrolyte salt and
wo 2011/124642 PCT /EP2011/055425
- 9 -
hydrogen fluoride. Elemental fluorine and elemental hydrogen form in the
respective electrode compartments.
2. Delivery of the purified F2 to the tool
The generated elemental fluorine is passed through a Monel metal frit to
5 remove solids and pressurized by means of a compressor to about 4 Bar abs. and
then passed through a trap cooled to -1 00°C ; in this trap, entrained HF
condenses.
The gaseous F2 leaving the trap is passed through a bed ofNaF to remove
any residual HF and then delivered, under a pressure of about 4 Bara, to a tool
10 for the manufacture of photovoltaic cells.
Example 5 :Provision ofF2 with very low HF content for photovoltaic cell
manufacture
1. Manufacture of elemental fluorine
An electrolyte salt with a composition of about KF·2HF is filled into an
15 electrolysis cell, heated to about 80- 120°C and molten therein. HF is
introduced into the electrolytic cell. A voltage of between 8 to 10 V is applied,
and current is passed through the composition of the molten electrolyte salt and
hydrogen fluoride. Elemental fluorine and elemental hydrogen form in the
respective electrode compartments.
20 2. Delivery of the purified F2 to the tool
The generated elemental fluorine is passed through a Monel metal frit to
remove solids and pressurized by means of a compressor to about 4 bar ( abs.)
and then passed through a trap cooled to -80°C, by means of a heat exchanger
through which a cooling liquid is passed ; in this trap, entrained HF condenses.
25 The gaseous F2 leaving the trap is passed at the pressure of 4 bar (abs)
through a bed ofNaF to remove any residual HF, passed again through a Monel
metal frit to remove any solids still contained, and is then delivered, under a
pressure of about 4 Bara, to a tool for the manufacture of photovoltaic cells.

1. A method for the manufacture of electronic devices comprising at least
a step of the manufacture of the elemental fluorine on site electrolytically and at
least one step selected from the group of etching of items in a chamber using
5 elemental fluorine as etching agent and cleaning a chamber using elemental
fluorine as chamber cleaning agent wherein, after its manufacture, the elemental
fluorine and before its application for etching or chamber cleaning is subjected to
a low temperature treatment at a pressure above ambient pressure to remove at
least a part of entrained hydrogen fluoride.
10 2. The method of claim 1 where in the pressure during the treatment is
equal to or greater than 1.5 Bar abs.
3. The method of claim 1 or 2 wherein the pressure during the treatment
is equal to or lower than 20 Bar abs.
4. The method of anyone of claims 1 to 3 wherein the electronic devices
15 are selected from the group consisting of semiconductors, photovoltaic cells,
MEMS, and TFTs.
5. The method of claim 4 for the manufacture of photovoltaic cells
wherein the pressure is in the range of 1.5 to 4.5 Bara.
6. The method of claim 4 for the manufacture ofTFTs wherein the
20 pressure is in the range of 4.5 to 11 Bara.
7. The method of anyone of claims 1 to 6 wherein the elemental fluorine
is cooled in the low temperature treatment to a temperature equal to or lower
than -50°C.
8. The method of claim 8 wherein the elemental fluorine is cooled in the
25 low temperature treatment to a temperature equal to or lower than -60°C.
9. The method of anyone of claims 1 to 8 wherein the elemental fluorine
is additionally subjected to at least one adsorption step, at least one absorption
step and/or at least one distillation step.
wo 2011/124642 PCT /EP2011/055425
- 11 -
10. The method of anyone of claims 1 to 9 wherein the elemental fluorine
is produced by electrolysis ofHF in the presence of a molten HF adduct ofKF as
electrolyte salt.
11. The method of anyone of claims 1 to 10 wherein elemental fluorine is
5 produced in a generator, and the generator for elemental fluorine is in fluid
communication with the chamber.
12. The method of anyone of claims 1 to 11 wherein the etching chamber
and/or the chamber to be cleaned is a thermal or plasma-assisted chamber.
13. The method of claim 1 wherein the cleaning is performed thermally or
10 plasma-assisted.
15
14. The method of anyone of claims 1 to 13 wherein the fluorine is
manufactured in a cassette.
15. The method of anyone of claims 1 to 13 wherein the fluorine is
manufactured in a skid.