WO2005061758A1

WO2005061758A1 - Transfer system

Info

Publication number: WO2005061758A1
Application number: PCT/GB2004/005346
Authority: WO
Inventors: Stuart Charles Coles; David Alan Turrell; Kristian Laskey; Allister Watson; David Paul Manson
Original assignee: The Boc Group Plc
Priority date: 2003-12-24
Filing date: 2004-12-21
Publication date: 2005-07-07
Also published as: TW200528374A; GB0329933D0

Abstract

Two techniques are described for inhibiting the Wilson Cloud Effect formed during rapid reduction in pressure in a load lock chamber, which can cause particulate deposits to be formed on a substrate located within the chamber. In a first technique, the relative humidity of gas within the load lock chamber is reduced by replacing atmospheric air with a dry inert gas. This gas is retained within the system and recirculated between pumping cycles to reduce cost. In a second technique, during pump down the temperature and pressure of the venting gas within the load lock chamber are monitored to maximise the speed of evacuation whilst inhibiting the Wilson Cloud Effect.

Description

TRANSFER SYSTEM

The present invention relates to a transfer system, and to a method of operating the same.

Vacuum processing is commonly used in the manufacture of semiconductor devices to deposit thin films on to substrates. Typically, a processing chamber is evacuated using a vacuum pump to a very low pressure, which, depending on the type of process, may be as low as 10^"6 mbar, and feed gases are introduced to the evacuated chamber to cause the desired material to be deposited on one or more substrates located in the chamber. Upon completion of the deposition, the substrate is removed from the chamber and another substrate is inserted for repetition of the deposition process.

Significant vacuum pumping time is required to evacuate the processing chamber to the required pressure. Therefore, in order to maintain the pressure in the chamber at or around the required level when changing substrates, transfer chambers and load lock chambers are typically used. The capacity of the load lock chamber can range from just a few litres to several thousand litres for some of the larger flat panel display tools.

The load lock chamber typically has a first window, which can be selectively opened to allow substrates to be transferred between the load lock chamber and the transfer chamber, and a second window, which can be selectively opened to the atmosphere to allow substrates to be inserted into and removed from the load lock chamber. In use, the processing chamber is maintained at the desired vacuum by the processing chamber vacuum pump. With the first window closed, the second window is opened to the atmosphere to allow the substrate to be inserted into the load lock chamber. The second window is then closed, and, using a load lock vacuum pump, the load lock chamber is evacuated until the load lock chamber is at substantially the same pressure as the transfer chamber, typically around 0.1 mbar. The first window is then opened to allow the substrate to be transferred to the transfer chamber. The transfer chamber is then evacuated to a pressure at substantially the same pressure as the processing chamber, whereupon the substrate is transferred to the processing chamber.

When vacuum processing has been completed, the processed substrate is transferred back to the load lock chamber. With the first window closed to maintain the vacuum in the transfer chamber, the pressure in the load lock chamber is brought up to atmospheric pressure by allowing a non-reactive gas, such as air or nitrogen, to flow into the load lock chamber. When the pressure in the load lock chamber is at or near atmospheric pressure, the second window is opened to allow the processed substrate to be removed. Thus, for a load lock chamber, a repeating cycle of evacuation from atmosphere to a medium vacuum (around 0.1 mbar) is required.

In order to increase throughput and consequently output of the finished product, it is desirable to reduce the pressure in the load lock chamber as rapidly as possible. Any such rapid reduction in pressure leads to a corresponding rapid reduction in temperature. A typical load lock chamber pressure characteristic (solid line) and its corresponding temperature curve (dashed line) are illustrated in Figure 1. The consequence of such a reduction in both temperature and pressure is that any vapour, typically water vapour, within the load lock chamber is likely to condense on the substrate, causing any particulate matter in the air to be deposited on the surface of the substrate. This is known as the Wilson Cloud Effect. These particles are left behind as deposits when the condensation subsequently evaporates, and so may cause irregularities in the later processing steps performed in the processing chamber, leading to an increased level of faults in the end product.

Some substrates, such as glass, are particularly prone to the formation of such condensates, and hence such problems can arise when large sheets of glass are introduced into the load lock chamber, as is the case in manufacture of flat panel displays. ln conventional systems, a "soft start" may be implemented whereby a significantly reduced pressure reduction rate is used, as illustrated in Figure 2, such that the temperature (dashed line) does not dip to such low levels during evacuation of the load lock chamber, and hence condensation formation may be avoided. The safety factor involved in this reduced pressure reduction rate leads to increased pump down time and hence cycle time and can, therefore, be undesirable. Alternatively, heating elements may be provided to prevent the temperature from falling to a level below that at which condensation may be formed at the pressures involved. However, providing such heating elements can lead to an increased level of complexity of the load lock system, which could result in reduced reliability together with an increased power requirement.

A further alternative, known technique is to modify the humidity of the environment locally to the substrate to reduce the likelihood of condensation formation thereon. These techniques rely on provision of a curtain of dry gas over the substrate during evacuation of the load lock chamber. Provision of additional equipment within the load lock chamber increases the complexity of the device which, in turn, reduces the reliability of the overall system. Any failing in the curtain causes a corresponding section of the substrate to be exposed to the ambient atmosphere present in the remainder of the load lock chamber and in these circumstances condensation may still form, resulting in an increased risk of at least part of the product being faulty.

It is an object of the present invention to reduce the particulate build up on a substrate in a transfer chamber whilst overcoming some of the aforementioned problems.

According to a first aspect of the present invention there is provided a method of operating a transfer system, the transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, the method comprising controlling the humidity of gas within the transfer chamber to inhibit condensation of vapour, for example water vapour, on a substrate located within the transfer chamber during evacuation of the transfer chamber, and subsequently returning to the transfer chamber the gas evacuated from the transfer chamber to raise the pressure therein.

According to a second aspect of the present invention there is provided a transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, means for evacuating the transfer chamber, means for controlling the humidity of gas within the transfer chamber to inhibit condensation of vapour, for example water vapour, on a substrate located within the transfer chamber during evacuation of the transfer chamber, and means for returning the gas evacuated from the transfer chamber to the transfer chamber to raise the pressure therein.

In the preferred embodiment the transfer system is a load lock system, and so the present invention also provides a method of operating a load lock system, the load lock system comprising a load lock chamber for receiving a substrate to be transferred to a process chamber, the method comprising controlling the humidity of gas within the load lock chamber to inhibit condensation of vapour, for example water vapour, on a substrate located within the load lock chamber during evacuation of the load lock chamber, and subsequently returning to the load lock chamber the gas evacuated from the load lock chamber to raise the pressure therein. The present invention further provides a load lock system comprising a load lock chamber for receiving a substrate to be transferred to a process chamber, means for evacuating the load lock chamber, means for controlling the humidity of gas within the load lock chamber to inhibit condensation of vapour, for example water vapour, on a substrate located within the load lock chamber during evacuation of the load lock chamber, and means for returning the gas evacuated from the load lock chamber to the load lock chamber to raise the pressure therein.

By controlling the humidity in the entire transfer chamber, the aforementioned problems associated with failure of a locally controlled transfer chamber environment are avoided. By recirculating the majority of this environment from one process cycle to the next, heavy costs and wastage associated with completely replenishing the dry environment can be avoided. Furthermore, since additional equipment within the transfer chamber is not required, modification of the transfer system can be avoided when implementing this technique within an existing process tool.

According to a third aspect of the present invention there is provided a method of operating a transfer system, the transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, the method comprising monitoring at least one parameter of the group of humidity, pressure and temperature of gas within the transfer chamber and using the results of the monitoring to maximise the rate of evacuation of the transfer chamber whilst inhibiting condensation of vapour, for example water vapour, on a substrate located within the transfer chamber during evacuation of the transfer chamber, wherein the evacuation rate is controlled by varying the conductance of a variable flow control device located between the transfer chamber and means for evacuating the transfer chamber.

According to a fourth aspect of the present invention there is provided a transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, means for evacuating the transfer chamber, monitoring means for monitoring at least one parameter of the group of temperature, pressure and humidity of gas within the transfer chamber, and control means for receiving the signal from the monitoring means and using the received signals to maximise the evacuation rate of the transfer chamber whilst inhibiting condensation of vapour, for example water vapour, on a substrate located within the transfer chamber, wherein the control means comprises a variable flow control device positioned between the transfer chamber and the evacuation means, and means for varying the conductance of the variable flow control device to control the rate of evacuation of the transfer chamber.

Figure 3 illustrates the condensation temperatures of air with varying relative humidity. The present invention makes use of this relationship by causing the humidity of the venting gas to be reduced below that of the ambient air to allow lower temperatures to be experienced before condensation occurs on the substrate. The reduction of humidity thus permits a faster transfer chamber evacuation time whilst maintaining a reduced risk of forming condensates on the substrate. Where the reduction in humidity is coupled with monitoring of relevant parameters within the transfer chamber, the evacuation time may be further reduced as the evacuation rate may be maximised for any particular process cycle.

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

Figure 1 shows a typical pressure and corresponding temperature curves over time for fluid within a load lock chamber under evacuation;

Figure 2 shows similar curves to Figure 1 where a "soft start", using a significantly slower evacuation rate, is used;

Figure 3 shows a graph of condensation temperature of air with varying relative humidity;

Figure 4 shows a first embodiment of a transfer system according to the present invention; and

Figure 5 shows a second embodiment of a transfer system according to the present invention.

Figure 4 illustrates a transfer system 1 according to a first embodiment of the present invention. The technology is equally applicable to various types of transfer system for transferring a substrate from one region of a process tool at a first ambient pressure to another region having a second ambient pressure. In one example, a conventional transfer chamber is located between the load lock chamber and the process chamber. Such a chamber may typically be evacuated by turbo-molecular vacuum pumps. In the example discussed below, the transfer system is a load lock system 1. The load lock system 1 comprises a load lock chamber 10. The load lock chamber 10 is connected to a transfer chamber (not shown) to enable a substrate inserted into the load lock chamber 10 to be transferred to a process chamber (not shown) for processing and to enable the subsequently processed substrate to be returned to the load lock chamber 10 for removal and replacement with a fresh substrate.

The load lock chamber 10 is in fluid communication with a buffer chamber 11 via control valve 21 , and with vacuum pumps 12 via control valve 22. The buffer chamber 11 and vacuum pumps 12 are in fluid communication via a control valve 23 to enable the buffer chamber to be pre-evacuated by the pumps 12. A gas filter 13 is provided downstream from the pumps 12 to remove impurities from the gas stream evacuated by the pumps 12.

A low-pressure storage vessel 14 is provided downstream from the filter 13. A first outlet from the storage vessel 14 is directly connected to a compressor 16, which is, in turn, connected to a first inlet of a high-pressure storage vessel 17. A second outlet from the storage vessel 14 is connected to a gas dryer 15, so that gas output from the storage vessel 14 may be selectively routed to the dryer 15 before passing to the compressor 16.

A source 18 of clean, dry vent gas, such as nitrogen, is connected to a second inlet of the high-pressure storage vessel 17 to provide, as required, additional clean gas. Optionally, the gas source 18 may also provide a source of purge gas for cooling the pumps 12 during operation. The outlet from the high-pressure storage vessel 17 is connected to the load lock chamber 10 via control valve 26. A pressure relief valve 19 is also typically located in fluid communication with the outlet of the high-pressure storage vessel 17 to prevent over pressurisation of the load lock system 1.

In operation, in order to control the humidity of gas within the load lock chamber 10, valves 25 and 26 are initially opened to enable the load lock chamber 10 to be supplied with a stream of vent gas from the gas source 18. The supply of vent gas may be controlled so as to create a pressure in the load lock chamber 10 which is greater than atmospheric pressure, so that when a substrate is inserted into the load lock chamber 10, the positive pressure gradient prevents ambient air from being drawn into the load lock chamber, thereby retaining the controlled humidity within the load lock chamber 10.

Once the controlled atmosphere has been established in the load lock chamber 10, valves 25 and 26 are closed, and a substrate is inserted into the chamber 10. In order to reduce the pressure in the load lock chamber 10 to a level substantially the same as that of the transfer chamber, the load lock chamber 10 is evacuated by opening control valve 21 to allow gas to flow from the load lock chamber 10 into the pre-evacuated buffer chamber 11. The degree of opening of the valve 21 controls the rate of evacuation of the load lock chamber 10. During periods when valve 21 is fully open, the noise from the passage of the gas into the pre-evacuated chamber 11 can be quite loud, and so, as illustrated in Figure 4, a diffuser 32 may be provided upstream from the pre-evacuated chamber 11 to minimise the noise generated during the early stages of evacuation of the load lock chamber 10.

Once the buffer volume 11 and the load lock chamber 10 have equalised in pressure (typically 300 mbar), valve 21 is closed and valve 22 opened to allow the vacuum pumps 12 to continue the evacuation of the load lock chamber 10 until the required operating conditions are reached (typically at or around 0.1 mbar). Once the target pressure for the load lock chamber 10 has been achieved, valve 22 is closed and the substrate is moved into the transfer chamber. During this period the vacuum pumps continue to run and valve 23 is opened to return the buffer volume 11 to its original lower pressure, in anticipation of the next cycle of load lock system operation.

Rather than exhausting the vent gas from the system during each evacuation cycle, the gas is recirculated to avoid the associated high levels of consumption. Downstream from the vacuum pumps 12, the exhausted vent gas is conveyed through the gas filter 13 to remove any impurities therefrom prior to entering low- pressure storage tank 14. From here, the exhausted vent gas may optionally be diverted through dryer 15 to dehumidify the exhausted vent gas, so that formation of condensation can be avoided in the load lock chamber 10 in the subsequent cycle. As it is necessary to raise the pressure of the exhausted vent gas so that elevation of the pressure in the load lock chamber 10 can be achieved rapidly following the return of the processed substrate to the load lock chamber 10, the exhausted vent gas is, consequently, passed through a compressor 16 and then stored at a higher pressure in the high-pressure storage tank 17.

In this embodiment, during each cycle, approximately 10% of the vent gas is replenished from gas source 18, by opening valve 25, to maintain the quality of the vent gas being recirculated through the load lock system 1. As the addition of fresh vent gas can lead to an over pressurisation of the system 1, any excess gas is purged from the system through pressure relief valve 19.

At the end of each cycle the processed substrate is returned to the load lock chamber 10 from the transfer chamber, and the pressure in the load lock chamber 10 is increased in preparation for the removal of the processed substrate from the load lock chamber 10. In order to prevent damage to the processed substrate from the rapid elevation in pressure, by returning the exhausted vent gas to the load lock chamber 10 from the high-pressure storage tank 17, the return of the vent gas is controlled by the controlled opening of valve 26. An additional diffuser 33 may also be provided downstream from the valve 26 to suppress noise generated during the return of the vent gas to the load lock chamber 10. As discussed above, the pressure in the load lock chamber 10 may be raised slightly above atmospheric levels to achieve a positive pressure gradient to and thereby inhibit entrainment of air into the load lock chamber 10 during the replacement of the processed substrate with a fresh substrate.

In addition to, or as an alternative to, the diffusers 32 and 33, an active noise control enclosure, as depicted by the dashed line in Figure 4, may be provided with a suitable control device 31. If the load lock chamber 10 is evacuated at the highest possible rate, as illustrated in Figure 1 the temperature within the load lock chamber 10 can approach -30°C. In order to avoid condensation forming within the load lock chamber 10 during evacuation, as illustrated in Figure 3 the relative humidity of the gas within the load lock chamber 10 should be in the region of 3%. As discussed above, soft starts introduce a significant safety factor and a steady, slow evacuation rate is realised to avoid formation of condensation. In order to increase the evacuation rate, in a second embodiment of the present invention, temperature and/or pressure of the gas within the load lock chamber 10 are monitored and used to control the rate of evacuation of the load lock chamber 10.

Figure 5 illustrates the second embodiment of the present invention, the features of which can be readily incorporated into the first embodiment of the invention. Similar to the first embodiment, the load lock chamber 10 is connected to a pre-evacuated buffer chamber 11 and two vacuum pumps 12. In this second embodiment, a sensor 41 is provided within the load lock chamber 10 to monitor environmental parameters, such as temperature, pressure and relative humidity of the gas within the load lock chamber 10. Sensor 41 is configured to generate a signal 42 indicative of the monitored parameters and to supply signal 42 to a controller 43. Controller 43 utilises this signal 42 to generate control signals, which are supplied to valves 21' and 22' to vary the conductance of the valves to control the rate of evacuation of load lock chamber 10 so as to achieve an optimised pump down that is able to achieve rapid evacuation whilst avoiding the condensation limits of Figure 3. Each of the valves 21', 22' may be any flow control device having a conductance that can be varied depending on, or in proportion to, a received control signal.

Such monitoring can lead to sophisticated active control such that the controller 43 is constantly sending signals to the valves 21' or 22' based on real time variations in the monitored parameters of the gas within the load lock chamber 10. Alternatively, in a repeatable process, the evacuation rate can be determined as a complex function of time elapsed such that predetermined instructions can be sent to the valves 21 ', 22' by the controller 43. These instructions may be derived from the monitored parameters of the previous cycle of the process or alternatively may be standardised for each particular type of process.

In a third intermediate example, the instructions may be based on a predetermined complex function varying with elapsed time, but constant monitoring can be undertaken in order to act as a verification check to ensure that condensation formation is avoided in the event of some deviation from the standard repeated process. In this way the control system may be used to effect real-time, fine tuning of the evacuation cycle rather than needing to achieve real-time full control of the evacuation.

In some applications the control valve 21 may need to be particularly sensitive and to implement such a valve may be prohibitively expensive. In these circumstances, the pump down may be initiated by conventional "soft start" techniques using the vacuum pumps 12 to enable an increased level of control. The valve 21 between the load lock chamber 10 and the buffer chamber 11 may then be opened part way through the evacuation procedure to gain the benefit of the rapid reduction in pressure.

Claims

1. A method of operating a transfer system, the transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, the method comprising controlling the humidity of gas within the transfer chamber to inhibit condensation of vapour on a substrate located within the transfer chamber during evacuation of the transfer chamber, and subsequently returning to the transfer chamber the gas evacuated from the transfer chamber to raise the pressure therein.

2. A method according to Claim 1 , wherein gas is returned to the transfer chamber so as to raise the pressure therein to a level above atmospheric pressure.

3. A method according to Claim 1 or Claim 2, wherein a portion of the gas returned to the transfer chamber is supplied from a source thereof.

4. A method according to any preceding claim, wherein at least one parameter of the group of humidity, pressure and temperature of the gas within the transfer chamber is monitored, and the results of the monitoring are used to maximise the rate of evacuation of the transfer chamber whilst inhibiting condensation of vapour on a substrate located within the transfer chamber during evacuation of the transfer chamber.

5. A method according to any preceding claim, the method comprising the steps of:

(a) supplying a vent gas from a source thereof to the transfer chamber to control humidity within the transfer chamber;

(b) inserting a substrate into the transfer chamber; (c) evacuating the transfer chamber to enable the substrate to be transferred from the transfer chamber to the process chamber; and storing the gas exhausted from the transfer chamber remotely from the transfer chamber,

(d) following return of a processed substrate to the transfer chamber, returning the stored gas to the transfer chamber to elevate the pressure within the transfer chamber, (e) removing the processed substrate from the transfer chamber, and

(f) cyclically repeating steps (b) to (e).

6. A method according to any preceding claim, wherein the transfer chamber is evacuated using a vacuum pump.

7. A method according to any preceding claim, wherein the transfer chamber is evacuated by opening a passage extending between the transfer chamber and a pre-evacuated buffer chamber.

8. A method according to Claim 7, wherein the buffer chamber is re-evacuated once the transfer chamber has been evacuated to a target pressure.

9. A method according to any preceding claim, wherein the rate of evacuation of the transfer chamber is controlled.

10. A method according to any preceding claim, wherein the rate of return of the stored gas to the transfer chamber is controlled.

11. A method according to any preceding claim, wherein the gas evacuated from the transfer chamber is filtered prior to being stored.

12. A method according to any preceding claim, wherein the humidity of gas evacuated from the transfer chamber is reduced prior to being returned to the transfer chamber.

13. A method according to Claim 5 and any of Claims 6 to 12 when dependant from Claim 5, wherein before step (d) a clean dry gas is added to the stored gas and any excess gas is released from the transfer system through a pressure relief valve.

14. A method according to Claim 13, wherein the clean dry gas is supplied from the source.

15. A transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, means for evacuating the transfer chamber, means for controlling the humidity of gas within the transfer chamber to inhibit condensation of vapour on a substrate located within the transfer chamber during evacuation of the transfer chamber and means for returning the gas evacuated from the transfer chamber to the transfer chamber to raise the pressure therein.

16. A transfer system according to Claim 15, comprising means for supplying a vent gas from a source thereof to the transfer chamber to control the humidity within the transfer chamber and means for storing gas exhausted from the transfer chamber remotely from the transfer chamber, wherein the returning means is arranged to return the stored gas from the storage means to the transfer chamber to elevate the pressure within the transfer chamber.

17. A transfer system according to Claim 16, comprising a gas filter located between the evacuation means and the storage means for removing any impurities from a gas passing therebetween.

18. A transfer system according to Claim 16 or 17, comprising a gas dryer located between the evacuation means and the storage means for removing moisture from a gas passing therebetween.

19. A transfer system according to any of Claims 16 to 18, comprising a pressure relief valve located between the storage means and the transfer chamber for releasing any excess gas from the transfer system.

20. A transfer system according to any of Claims 15 to 19, comprising sensor means located within the transfer chamber, the sensor means being configured to output a signal indicative of at least one parameter of the group of temperature, pressure and relative humidity of a gas within the transfer chamber, and an evacuation controller for receiving the or each signal and for controlling the evacuation rate of the transfer chamber in dependence thereon.

21. A transfer system according to Claim 20, wherein the evacuation controller is arranged to control the evacuation rate of the transfer chamber according to a predetermined function established from an earlier process cycle.

22. A transfer system according to Claim 20 or Claim 21 , wherein the evacuation controller comprises a variable flow control device positioned between the transfer chamber and the evacuation means, and means for varying the conductance of the variable flow control device to control the rate of evacuation of the transfer chamber.

23. A method of operating a transfer system, the transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, the method comprising monitoring at least one parameter of the group of humidity, pressure and temperature of gas within the transfer chamber and using the results of the monitoring to maximise the rate of evacuation of the transfer chamber whilst inhibiting condensation of vapour on a substrate located within the transfer chamber during evacuation of the transfer chamber, wherein the evacuation rate is controlled by varying the conductance of a variable flow control device located between the transfer chamber and means for evacuating the transfer chamber.

24. A method according to Claim 23, the method comprising the steps of: supplying a vent gas from a source thereof to the transfer chamber to control humidity within the transfer chamber; inserting a substrate into the transfer chamber; and evacuating the transfer chamber to enable the substrate to be transferred from the transfer chamber to the process chamber; wherein at least one parameter of the group of pressure, temperature and relative humidity of gas retained within the transfer chamber is monitored, the or each monitored parameter being used to control the evacuation rate of the transfer chamber.

25. A transfer system comprising a transfer chamber for receiving a substrate to be transferred to a process chamber, means for evacuating the transfer chamber, monitoring means for monitoring at least one parameter of the group of temperature, pressure and humidity of gas within the transfer chamber, and control means for receiving the signal from the monitoring means and using the received signals to maximise the evacuation rate of the transfer chamber whilst inhibiting condensation of vapour on a substrate located within the transfer chamber, wherein the control means comprises a variable flow control device positioned between the transfer chamber and the evacuation means, and means for varying the conductance of the variable flow control device to control the rate of pvacuation of the transfer chamber.