WO2014170503A1

WO2014170503A1 - Pulsed valve cracker effusion cell

Info

Publication number: WO2014170503A1
Application number: PCT/EP2014/058142
Authority: WO
Inventors: Fernando Briones; Iván FERNÁNDEZ; Ambiörn WENNBERG
Original assignee: NANO4ENERGY SLNE
Current assignee: NANO4ENERGY SLNE
Priority date: 2013-04-19
Filing date: 2014-04-22
Publication date: 2014-10-23
Anticipated expiration: 2015-10-19
Also published as: GB201307073D0

Abstract

A pulsed valve cracker effusion cell comprising an evaporation chamber (11), a delivery system (13) and a conduit (20) in fluid communication with the evaporation chamber (11) and the delivery system (13), a high temperature heating element located in the conduit and a flow controller (17) for controlling the flux of a gas into the delivery system (13), characterised by the flow controller (17) comprising an on/off valve and a controller for controlling the on/off state of the valve. This invention relates to a pulsed valve cracker effusion cell for the injection of vapours generated by solid or liquid sublimation into a process system, mainly a vacuum system. The present invention also relates to the regulation of the vapour flow by means of an intermittent valve (17). The invention has been designed for linear and uniform injection of gases from solid or liquid sources, e.g. sulphur, tellurium, selenium etc. as an elemental atomized vapour.

Description

Title: Pulsed valve cracker effusion cell

Description: This invention relates to a pulsed valve cracker effusion cell, and in particular, but without limitation, to a pulsed valve cracker effusion cell for the injection of vapours generated by solid or liquid sublimation into a process system, mainly a vacuum system.

This invention also relates to the regulation of the vapour flow by means of an intermittent valve. The invention has been designed for various types of injection of gases from solid or liquid sources, e.g. sulphur, tellurium, selenium etc. as an elemental atomized vapour. The injection can be linear, uniform, point source or circular etc.

Cracker valves are used in extensively in vacuum deposition systems to introduce gasses, in particular, reactive gasses, into the process chamber. The gasses so introduced typically condense on substrates to form deposited layers, or act as reagents or catalysts that interact with the substrate or with other layers thereon.

A cracker valve typically comprises an evaporation chamber containing a quantity of material to be evaporated, and a control valve to control the egress of the vapour into a delivery system, which is typically a perforated tube extending into the process chamber. A cracking heater element is interposed between the evaporation chamber and the delivery system to crack the vapour into a desired reagent for the process.

Existing cracker vales, such as those disclosed in patent application numbers ES2067381, US5431735 and GB1307073.5, employ needle valves to control the flow of vapour or cracked vapour into the delivery system and hence into the process chamber of the overall system.

Needle valves have proven to be a good choice in most circumstances because a finely- tapered needle valve affords relatively accurate control of the flux of gas, as well as being reliable. For these reasons, existing cracker valves use needle valves to control the flow of gas into the process chamber.

The disadvantages of using needle valves, however, are manifold. Firstly, in order to obtain highly accurate control of the needle valve setting, a finely-pitched screw thread needs to be used to obtain the necessary granularity in its setting. This necessarily requires many turns of the needle valve to move it from a fully-open to a fully-closed position. Even using motorised actuators for rotating the needle valve, the time it takes to actuate a needle valve can be unacceptably slow, especially where the gas flux is a rapidly-changing process parameter.

Further, because the needle valve is controlled by a screw thread to convert rotation into axial displacement of the needle relative to the aperture, the screw thread provides sufficient mechanical advantage that even gentle manipulation of the needle valve upon collision of the needle with the aperture (i.e. when in the fully-closed position), can lead to plastic deformation of the needle. Thus, a needle valve needs to be re-calibrated every time it is first used, to counteract the plastic deformation of the needle that occurred the previous time it was fully-closed. Having to re-calibrate the needle valve is time-consuming and inconvenient.

A need therefore exists for an improved and/or alternative type of cracker valve that overcomes or addresses one or more of the above problems and/or which functions differently to existing cracker valves.

A first aspect of the invention provides a pulsed valve cracker effusion cell.

A second aspect of the invention provides an effusion cell comprising an evaporation chamber, a delivery system and a conduit in fluid communication with the evaporation chamber and the delivery system, a high temperature heating element located in the conduit and a flow controller for controlling the flux of a gas into the delivery system, characterised in that the flow controller comprises an on/off valve and a controller for controlling the on/off state of the valve. A third aspect of the invention provides a method of controlling the flux of gas from an effusion cell cracker by pulsing an on/off valve for controlling the flux of a gas into a delivery system of the effusion cell.

By providing an on/off valve, as opposed to a variable-setting valve, such as the needle valve of known systems, some or all of the problems identified above may be overcome or ameliorated. Specifically, an on/off valve typically has a faster switching speed (from the fully-on to the fully-off position), thereby enabling the flow controller to react more quickly to changes in controller inputs. Further, because an on/off valve generally comprises a planar closing surface, as opposed to the tapered needle arrangement of a needle valve, plastic deformation of the contacting parts of the on/off valve is avoided upon closing thereof. This may reduce, or avoid, the need for re-calibration each time the flow controller is first used.

Further, in systems having variable control valves, such as needle valves, the flux of gas into the system is variable. At relatively low fluxes, especially of organic monomers, a plasma process may produce mainly cracking, but also a black carbonaceous coating, which is a result of the dynamics of the reaction, whereby relatively low concentrations are able to react fully and thus produce the carbonaceous coating. However, by pulsing the flux such that short bursts of relatively high concentrations of evaporate are delivered, the overall amount of evaporate is moderated (i.e. the total amount of evaporate per unit time), but because it is delivered in short, relatively high- concentration bursts, the dynamics of the reaction are not able to keep up, thus reducing the amount of black carbonaceous coating in the process.

The foregoing is, of course, just an example, and it will be appreciated that the dynamics of a range of chemical reactions in the process can be better controlled by pulsing. For example, water could be introduced into the system, and under needle valve control, the water might be cracked into H and O, whereas, by pulsing the on/off valve, it may be possible to avoid cracking the water to deliver H₂0 into the process, which reacts differently to H and O. Suitably, the effusion cell comprises an evaporation vessel closed by means of a hermetic valve, which regulates its flow by the sublimation of a solid or liquid source material. By using an on/off valve, as opposed to a variable, or needle, valve several beneficial effects arise. Specifically, in a known system with a variable valve (as opposed to the on/off valve of the invention), the sublimated source material is essentially allowed to "leak" from the evaporation vessel. This makes it difficult to determine, with precision, the flux of gas from the evaporation vessel because the internal pressure is a variable quantity. However, by closing the valve, it is possible to obtain a known pressure of sublimated source material, and then by opening it for a known period of time, a known volume of sublimated source material can be released. Thus, the invention enables the delivery of sublimated source material to be controlled volumetrically, and with greater precision, than existing systems that rely on estimating the flux of sublimated source material based on the internal pressure of the evaporation vessel (a dynamic value) and the valve setting (which is prone to inaccuracy).

Suitably, the effusion cell comprises three, individually thermostatically controlled parts, namely: a hot thermostatically controlled evaporation zone; a thermostatically controlled gas conduction zone; and a high temperature distribution cracker. Being able to independently thermostatically regulate the three main components of the effusion cell is beneficial, because it becomes possible to avoid the condensation of source material on the on/off valve and within the different components of the effusion cell. From a practical perspective, providing independent thermostatic control of the three main components of the effusion cell enables the evaporation chamber to be removed and re-filled, for example, without requiring a complete system shut-down.

All the components of the effusion cell are suitably manufactured from quartz to avoid chemical corrosion. This is particularly beneficial when handling selenium, arsenic etc.

Suitably, the interconnections between the various parts of the effusion cell provided with polished conical or spherical fittings. The hot thermostatically controlled evaporation zone is used for the generation of vapour from solid or liquid source, where: the solid or liquid evaporates (source materials) are thermally heated in the thermo stated evaporation zone to a sublimation temperature sufficient to produce a vapour stream. Suitably, the hot thermo stated evaporation zone is hermetically closed with a heated quartz valve to minimize material waste. Suitably, thermocouple-based PID temperature control is used to obtain ±0.1°C temperature accuracy of the reservoir zone. Suitably, the hot thermo stated evaporation zone is constructed of quartz to avoid chemical corrosion.

The invention is additionally comprises an on/off flux control valve to control the flow of the said vapour (sublimated source material) stream into the vacuum chamber. The on/off valve suitably comprises a hermetic valve allowing complete flux shut-off. The on/off valve is suitably a fast actuation pulsed flux control valve, and is suitably manufactured from quartz for corrosion resistance. A high Curie temperature magnet is suitably operatively connected to the valve, which interacts with an externally actuated electromagnetic pulse generator, which actuates the valve. The electromagnetic pulse generator suitably comprises an electric coil, such that the coil and magnet together operate as a solenoid for controlling the opening and closing of the on/off valve.

The frequency of operation of the on/off valve can be at any desired frequency, on time and off time, although the on/off valve can suitably be operated in a continuously on, or continuously off state, or with an opening/closing rate of substantially 20Hz.

Suitably, the on/off flux control valve is temperature-controlled, and is suitably maintained above the temperature of the hot thermo stated evaporation zone to avoid material condensation on the valve.

The invention also provides a method of controlling the said actuating valve by changing the opening time of each pulse.

The effusion cell suitably comprises a high temperature thermo stated gas conduction zone to channel the vapour stream. Suitably, the gas conduction zone is maintained at a temperature above the temperature of the evaporation zone to avoid material condensation. Suitably, the gas conduction zone is constructed of quartz to avoid chemical corrosion.

The effusion cell suitably comprises a high temperature thermo stated linear distribution cracker to achieve a uniform distribution of vapour stream in the vacuum chamber. Suitably, the linear distribution cracker comprises an array of nozzles, which suitably form an even and/or uniform gas distribution. Suitably, the linear distribution cracker is constructed of quartz to avoid chemical corrosion. In a preferred embodiment of the invention, the linear distribution cracker is electrically heated up to 850⁵C by means of a resistance of refractory material, e.g. tantalum to achieve thermal dissociation of the vapour stream molecules.

Advantageously, the linear distribution cracker of the invention delivers a uniform vapour distribution along the cracker length. Advantageously, the pulsed valve cracker effusion cell of the invention enables users to introduce an essentially atomized vapour from a solid or liquid source with a great degree of controllability, offering the possibility to achieve long deposition campaigns with controlled parameters.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic view of an effusion cell vessel in accordance with the invention;

Figure 2 is a schematic view of the effusion cell of Figure 1; and

Figure 3 is a plot of the experimental results achieved with the invention.

Referring now to the drawings:

Figure 1 shows a schematic view of the effusion cell vessel. Solid or liquid source material is loaded in the thermo stated evaporation zone 1 which is heated and the temperature is controlled by using the power/thermocouple feedthroughs 2. The flux control valve opens the access of the vapour into a high temperature thermo stated gas conduction zone 3 to channel the vapour stream. This zone is heated and the temperature is controlled by using the power/thermocouple feedthroughs 4. The valve is remotely actuated by an external solenoid 5, fed with a pulsed or continuous electrical current. A high temperature cracker zone 6 is used to crack the vapour stream prior to its release in to the process system, mainly a vacuum system. This zone is heated and the temperature is controlled by using the power/thermocouple feedthroughs 7.

Figure 2 shows a schematic view of the effusion cell. The effusion cell provides a cooled external stainless steel vessel 9, which is connected to the process system by a vacuum flange 10. The effusion cell also comprises a thermo stated evaporation zone 11, a high temperature thermo stated gas conduction zone 12 and a high temperature cracking zone 13. These three zones are assembled with polished conical or spherical fittings 14a and 14b. The thermo stated evaporation zone is heated by an energy source and temperature controlled 15. The solid or liquid source material 16 is loaded in this zone and sublimated. The effusion cell further comprises a fast actuating valve 17, which is heated by an energy source to avoid material re-condensation. The vapour flow is regulated by an intermittent opening of the valve. A high Curie temperature magnet 18 is attached to the valve for external remote actuation by an external solenoid 19, fed with a pulsed electrical current. The high temperature thermo stated gas conduction zone and the valve are heated by an energy source and temperature controlled 20. The vapour stream passes through the valve and the high temperature thermo stated gas conduction zone prior to the injection to the high temperature cracking zone 13. During the transport the high temperature thermo stated gas is heated by an energy source and temperature controlled 21. The cracked vapour stream is then delivered to the process system through a series of holes, mainly into a vacuum system.

Figure 3 shows a plot of the experimental results achieved with the invention. The evaporation rate of the invention for different aperture times is represented as a function of the valve opening repetition frequency.

Claims

Claims:

1. A pulsed valve cracker effusion cell.

2. The pulsed valve cracker effusion cell of claim 1, comprising an evaporation chamber, a delivery system and a conduit in fluid communication with the evaporation chamber and the delivery system, a high temperature heating element located in the conduit and a flow controller for controlling the flux of a gas into the delivery system, characterised by the flow controller comprising an on/off valve and a controller for controlling the on/off state of the valve.

3. The pulsed valve cracker effusion cell of claim 1 or claim 2, wherein the evaporation chamber comprises a thermostatically-controlled heater for sublimating a source material, and wherein an outlet of the evaporation chamber is closed by the on/off valve.

4. The pulsed valve cracker effusion cell of claim 2 or claim 3, comprising a thermocouple-based PID temperature controller for controlling the temperature of the evaporation chamber.

5. The pulsed valve cracker effusion cell of claims 2 to 4, wherein the evaporation chamber is at least partially manufactured of quartz.

6. The pulsed valve cracker effusion cell of claims 2 to 5, wherein the on/off valve comprises a hermetic valve having two nominal settings, namely a fully-open setting and a fully-closed setting.

7. The pulsed valve cracker effusion cell of any of claims 2 to 6, further comprising a heater for the on/off valve adapted to maintain it at a temperature greater than that of the evaporation chamber.

8. The pulsed valve cracker effusion cell of any of claims 2 to 7, wherein the on/off valve is at least partially manufactured of quartz.

9. The pulsed valve cracker effusion cell of any of claims 2 to 8, further comprising a thermostatically controlled gas conduction zone.

10. The pulsed valve cracker effusion cell of claims 2 to 9, comprising a high temperature distribution cracker.

11. The pulsed valve cracker effusion cell of claim 10, wherein the high temperature distribution cracker comprises a linear distribution cracker.

12. The pulsed valve cracker effusion cell of claim 11, wherein the linear distribution cracker comprises an array of nozzles for forming, in use, an even and/or uniform gas distribution.

13. The pulsed valve cracker effusion cell of claims 11 or 12, wherein the linear distribution cracker comprises an electric heater for heating the cracker, in use, up to 850⁵C.

14. The pulsed valve cracker effusion cell of claim 13, wherein the electric heater comprises a refractory material resistor, such as tantalum.

15. The pulsed valve cracker effusion cell of any preceding claim, wherein the interconnections between the various components of the effusion cell are provided with polished conical or spherical fittings.

16. The pulsed valve cracker effusion cell of any preceding claim, wherein pulsed valve comprises a fast-actuation pulsed flux control valve.

17. The pulsed valve cracker effusion cell of any preceding claim, wherein pulsed valve comprises a high Curie temperature magnet operatively connected to the valve.

18. The pulsed valve cracker effusion cell of claim 17, further comprising an externally actuated electromagnetic pulse generator, wherein the electromagnetic pulse generator comprises an electric coil, such that the coil and magnet together operate as a solenoid for controlling the opening and closing of the valve.

19. The pulsed valve cracker effusion cell of any preceding claim, wherein pulsed valve is operable, in use, continuously on; or continuously off; or with an opening/closing rate of substantially 20Hz.

20. The pulsed valve cracker effusion cell of any preceding claim, further comprising a controller for controlling the opening time and frequency of each valve pulse.

21. The pulsed valve cracker effusion cell of any preceding claim, comprising a cooled external vessel, which is connectable, in use, to a process system by a vacuum flange.

22. The pulsed valve cracker effusion cell of any preceding claim, further comprising a vacuum system operatively connected to the evaporation chamber.

23. The pulsed valve cracker effusion cell of any preceding claim, comprising a delivery system comprising a tube arranged, in use, to extend into a process chamber of a deposition apparatus.

24. The pulsed valve cracker effusion cell of claim 23, wherein the tube is perforated.

25. The pulsed valve cracker effusion cell of claim 24, wherein the perforations comprise any one or more of the group comprising: through apertures in a sidewall of the tube; one or more slotted apertures in a side wall of the tube; a porous element having an open-porous structure; a reticulated, mesh or grille-type structure of the tube.

26. The pulsed valve cracker effusion cell of any preceding claim, further comprising a controller operatively connected to the on/off valve for controlling its on/off state and comprises a pulsed controller whereby the on time and off time of the on/off valve can be selected automatically.

27. The pulsed valve cracker effusion cell of claim 26, wherein the controller comprises a computer for calculating the desired on and off times for the on/off valve.

28. The pulsed valve cracker effusion cell of claims 26 or 27, wherein the controller comprises a feedback control system that monitors sensor inputs from a system comprising the cracker valve, and which dynamically amends the on and off times of the on/off valve to maintain the system or process within desired operating parameters.