EP0526491A1 - Gaseous radical producing apparatus - Google Patents

Abstract

An apparatus for producing gaseous radicals is described, provided with a discharge tube, a magnetic field sufficient for the confinement of electrons inside the discharge region, and a narrowing downstream of the discharge region. The use of a narrowing downstream of the discharge region prolongs the residence time in the discharge region, and therefore increases the probability of production of gas radicals. A preferred embodiment of the invention has a magnetic field sufficient to cause the electron cyclonic resonance (ECR) and a narrowing giving a ratio of 5: 1 or more between the gas pressure in the discharge region and the gas pressure just in downstream of the narrowing. Other preferred features of the invention include the use of a metal cylinder head, a metal casing surrounding the exposed parts of the device, a discharge tube with a low coefficient of recombination and microwave frequencies. industrial standard.

Description

GASEOUS RADICAL PRODUCING APPARATUS.

This invention relates to apparatus for the production of gaseous radicals.

Radicals are defined as atoms or molecules which possess an odd number of electrons. They are often short lived and are highly reactive, often combining with other radicals or molecules.

Gaseous radicals are often required for process such as etching, cleaning and layer growth of material in a chamber. Gaseous radicals used in such processes include oxygen, nitrogen and hydrogen.

One of the many processes for which oxygen radicals are used is growth of epitaxial layers of high-temperature superconductor material. They are often produced by a microwave cavity discharge within a stream (or flow) of molecular oxygen through a discharge tube (eg. R J Spah et al Appl. Phys. Lett. 53. p 442 1988; J. Kwo et al Appl. Phys. Lett. 53. p 2683 1988; N Missert et al IEEE Trans. Magnetics .25. (2) p. 2418 1989 and J Juns et al Conf. Proc. of "Materials and Mechanisms of Superconductivity and High Temperature Superconductivity" July 23-281989, Stanford, CA, USA). There appears to be no agreed model as to the yield of oxygen radicals produced by such apparatus, with very few workers in the field quoting the yield achieved by the use of their described apparatus. One of the few quoted yields is that of N Missert et al (Supra) of 5 x 10¹⁵ atoms cm"², although they did not state the mass flow of oxygen used. Thus, the ratio of radicals to molecular oxygen delivered to the growth chamber cannot be deduced by other worker results. J Kwo et al (Supra 1988 and 1989) used a constriction thus situated impairs the flow of oxygen, containing the flow within the microwave cavity discharge region for longer and leading to a greater likelihood of oxygen radicals occurring. Proof of use of such a constriction leading to a greater yield of radicals can be derived from the equations given by A M Mearns and A J Morris (Chem. Eng. Prog. Symp. Ser. No. 112, .67_ p 37 1971). However, J Kwo et al did not disclose the amount, or yield, of radicals delivered to the growth chamber. Other radical producing apparatus using a microwave cavity discharge includes US4699689, where radical producing apparatus is used for high gas flow rates of 1000 SCC/M.

Two of the main advantages of using apparatus producing oxygen radicals by means of a microwave cavity discharge are that the apparatus is relatively small, thus easily portable and adaptable for retrofit to layer growth apparatus, and that the microwaves are delivered from a microwave power source to a microwave coupling device via coaxial cable, which is flexible and compact. However, this apparatus will not operate at low oxygen flow rates - a requirement which is highly preferable for epitaxial growth by evaporation. Epitaxial growth using medium to high oxygen flow rates is possible when techniques such a sputtering [X. X. Xi et al. Z. Phys. B (to be published) and EPO 328 076 A2] or laser ablation are used (T. Venkatesen et al. App. Phys. Lett..54. p 5611989). However, growth by use of the evaporation technique at low oxygen flow rates enables the use of lower growth temperatures resulting in reductions in inter diffusion of layers and also of interactions of elements comprising the superconductor material with substrate material.

In order to use the evaporation technique for epitaxial growth of high temperature superconductors at low oxygen flow rats, oxygen ions generated by electron cyclotron resonance (ECR) are used instead (eg K Moriwaki etap. J. Apl. Phys. 27. p L2075 1988). Although this apparatus provides a means of supplying oxygen suitable for use in epitaxial growth at low oxygen flow rates, it has a number of associated disadvantages. An ECR ion source requires magnets to provide a magnetic field within the discharge region, in order that the ECR condition is created. These magnets are very large and require significant cooling. Microwaves are delivered from a microwave power source to a microwave coupling device via a waveguide which is inflexible and bulky. The cost of the required components makes an ECR ion source extremely expensive compared to a microwave cavity discharge radical source. The kinetic energy of the ions produced has to be monitored and kept low, in order to avoid sputtering of the epitaxially grown material. In addition to the above disadvantages, it has bee shown that material growth using ions as a source of activated (ie not molecular) oxygen is not as effective as when radicals are used as a source of activated oxygen (Missert et al supra). Although ECR ion sources do produce some radicals, the yield is small. More recently miniaturised ECR ion sources do produce some radicals, the yield is small. More recently miniaturised ECR ion sources have become available, (eg Wavemat Inc), 44780 Helm Street, Plymouth, M148170, USA and Applied Science and Technology Inc, 35 Cabot Road, Woburn, MA 01801, USA). These sets of apparatus are easier to use than conventional ECR ion sources because the delivery of microwaves is by co-axial cable. The apparatus is much smaller due to the reduction in size of all the components, and thus the purchase and running costs are greatly reduced. However, these sets of apparatus still retain the disadvantages that the kinetic energy of the ions must be kept low and that, inevitably, it is mainly ions which are produced to the virtual exclusion of radicals. Other apparatus which produces ions for deposition includes US4683838, which generates a microwave induced plasma, which is confined by electromagnets, and EP 0328076. The latter apparatus directs gas from a delivery tube into a large diameter microwave cavity having a microwave choke giving a high percentage of gas throughput.

It is the object of this invention to provide an activated gaseous radical source which is capable of producing gaseous radicals, when operating at low oxygen flow rates, to the virtual exclusion of ion production.

According to this invention apparatus for the production of gaseous radicals comprises:

a discharge tube having in serial order an inlet, a discharge region, a constriction which significantly restricts gaseous flow from the discharge region and an outlet. a means of coupling microwaves to the discharge region of he discharge tube, and

a magnetic field sufficient for electron confinement within the discharge region.

The invention enables gas, suitable for producing radicals, to enter the discharge tube and flow through to the discharge region. On flowing through the discharge region the gas encounters the co-operating effects of the coupled microwaves (as a discharge) and magnetic field, resulting in the production of gaseous radicals. The constriction in the discharge tube inhibits free through flow of the gas to the outlet, thus increasing the likelihood of a given gaseous molecule in the discharge region breaking down to become radicals and increasing the yield of radicals flowing through the outlet. The apparatus is normally operated with a gas pressure below atmospheric pressure providing radicals to low pressure systems. The magnetic field must occur in the discharge region, although it may also occur elsewhere. The strength of the magnetic field is preferably about sufficient to induce the ECR condition. This strength can be calculated from equation (1):

B = m x w

(1)

where B = magnetic field strength m = electronic mass w = angular frequency e = electronic charge

Thus, for a microwave frequency of eg 2.54 GHz, a field of 0.0875 Tesla is required within the discharge region, in order that the ECR condition occurs.

The constriction in the discharge tube is arranged to significantly restrict gaseous flow from the discharge region in order to lengthen residence time of through-flowing gas within the discharge region, and therefore presence of a constriction lowers gas conductance. Gas conductance is defined as volume flow/pressure difference. Preferably the constriction produces a ratio of gaseous pressure within the discharge region to gaseous pressure just downstream of the construction is 2:1 or greater. More preferably the ratio is 5:1 or greater. The ratio is greatly dependent upon such characteristics length of discharge tube downstream of the constriction and capacity of vacuum pumping facilities associated with any apparatus retrofitted to the apparatus for production of gaseous radicals. The closer the retrofitted apparatus occurs downstream of the constriction, then the higher the ratio becomes.

The preferred shape of the magnetic field within the discharge region is that of a magnetic-bottle, which may be seen in figure 4. This shape of magnetic field is preferred due to it producing a higher yield of radicals than other magnetic field shapes. One other typical magnetic field shape is that such as produced by annular permanent magnets, and may be seen in figure 5.

A preferred embodiment of this invention contains a metal yoke. This yoke is used in such a way as to provide both a mechanical support for the apparatus and a convenient path for the completion of the magnetic field circuit between magnets.

It is preferable to contain all exposed parts of the discharge tube and microwave coupling within a jacket of metal. This jacket can be used to help protect the discharge tube from breakage and to minimise or prevent microwave radiation leakage.

Preferably the discharge tube is made of a material with a low recombination coefficient for the radical to be produced. Typical materials would be silica or pyrex for oxygen and hydrogen radicals, or pyrolitic boron nitride for nitrogen radicals. The discharge tube has to be able to withstand relatively high temperature (typically up to about 500°C), and thus pyrex tubing may need to be cooled. The discharge tube material is preferably transparent to microwaves. Thin coatings of passivating materials on the inner surface of the discharge tube, eg B₂0₃ or P 0₅ can help to improve the radical yield. These coatings allow the discharge tube to be characterised by a lower recombination coefficient than would be achieved otherwise. Suitable passivating materials generally have low melting points, and thus can usually only be used as thin coatings.

Downstream of the constriction, it is preferable to have a discharge tube diameter which is as large as conveniently possible, in order to deliver the radicals to the processing point as quickly as possible. Typically, the inner diameter of the discharge tube downstream of the constriction is of the order of tens of millimeters, with actual sizes very dependent upon retrofit conditions.

The region of the discharge tube upstream of the constriction and where radicals are produced, called the discharge region, has a typical maximum inner diameter of about 30 mm for microwave frequencies of about 2.54 GHz. Smaller diameters are, however, preferable in order to minimize the size of the discharge region, although there is a diameter below which a decrease in discharge tube diameter leads to higher recombination.

The preferred means of coupling microwaves to the discharge cavity is by the use of a microwave cavity, with the preferred geometry of microwave cavity being one which will accommodate the discharge tube in such a way that the electric field (E) is perpendicular to magnetic field (B).

Where microwave are delivered to the coupling means via coaxial cable, then this cable preferably has low loss at microwave frequencies.

The frequency of microwaves delivered to the coupling means is preferably one of the "spot" frequencies reserved for industrial use, eg. 2.45 GHz. although frequencies of between 900 MHz and 3 GHz are conveniently suitable. Where higher frequencies are used, then high magnetic fields are needed, and delivery of microwaves by coaxial cable becomes significantly less efficient. The power of the microwaves is typically between 20 and 350 Watts. However, care must be taken when using high power levels not to induce excessive sputtering of the discharge tube walls.

The size of constriction in comparison with diameter of the discharge tube is a compromise between retaining the molecules within the discharge region for a sufficient length of time to dissociation, and removing the created radicals from the discharge region quickly enough to minimise recombination. The dissociation and recombination characteristics of a gaseous molecule are a function of many circumstances eg. shape and size of coupling means, shape of the magnetic field and the power of the delivered microwaves. The flow constriction is preferably a reduction of aperture to the equivalent of between 1 and 10mm diameter. The aperture may be one, or many holes in the constriction.

The invention will now be further described, by example only, with reference to the accompanying figures, in which:

Figure 1 is a cross-section view of apparatus for producing gaseous radicals.

Figure 2 is a cross-sectional view of figure 1 taken along the line x-x.

Figure 3 is a graph of oxygen radical yield against oxygen flow

(seem) .

Figure 4 is a schematic representation of a magnetic bottle - magnetic field shape.

Figure 5 is a schematic representation of magnetic field shape induced by the use of annular permanent magnets.

Gaseous radical producing source 1 may be seen in figure i. A flow of oxygen passes through tube 2, in which there is a contriction 3. Microwaves are coupled to a microwave cavity 4, formed within a casing 5, causing a discharge region 6. Oxygen flows through the tube 2 into the discharge region 6, whereupon oxygen radicals are produced by the co- operating effects of the coupled microwaves and a magnetic field produced by annular magnets 7. Free through-flow of the oxygen is inhibited by the constriction 3, and so there is an increase in the likelihood of an oxygen molecule within the discharge region 6 breaking down to produce oxygen radicals. A metal yoke 8 provides mechanical support for the source 1, and also a convenient path for the completion of the magnetic field circuit between magnets.

Molecular oxygen (of purity 99.999Z) is provided by a gas delivery system 9, which has a flow control valve 10 and a monitoring pressure gauge 11. The system 9 may optionally have a window 12, in order to provide a view down the length of the tube 2. The gas delivery system 9 is attached to the source 1 by bolting a flange 13 and a clamping block 14 to cylinder 15. Where the source is used to produce a flow of radicals into a low pressure system 16, ten collett 17 and stops 18 prevent suction of the tube 2 downstream. 0-rings 19 and 20 are used within cylinder 14 and against cylinder 21 respectively when low pressure sealing is required.

The yoke 8 provides support for a protective jacket 22. The jacket has arms 23 which locate and support the annular magnets 7. Additionally the jacket 22 minimises or prevents leakage of microwave radiation. A bolt 24 extends through the jacket 22 and the casing 5 and into the microwave cavity 4, and is used to adjust the microwave characteristics of the microwave cavity.

Figure 2 gives a cross-section view through the axis marked x - x in figure 1. Microwaves are generated and sent along coaxial cable 30. The microwaves are delivered to a microwave coupling 31 via connecter 32. Tuning of the microwaves in cavity 11 is achieved by use of screws 33 and 34. Screw 33 can be secured by locking screw 34. In the source 1 shown in figures 1 and 2, microwaves are generated at 2.54 GHz. The power of the applied microwaves during operation can vary between 20 and 350 Watts, although typical running powers are about 100 Watts.

The tube 2 is made of silica, with the portion containing discharge region 6 of the tube having an inner diameter of 20mm. The constriction 3 has an aperture diameter of 3mm, whilst downstream of the constriction the tube has an inner diameter of 22mm. The stoppers 18 are bulges blown onto the tube at the time of manufacture.

The constriction 3 is made of silica. It may be an integral part of the tube or be an end to one part of the discharge tube which is then attached to the other part by a "push-fit" connection.

Optionally, jacket 22 may also house cooling, such as possibly flushing water through carrier pipes, in the volume between its jacket 22 and casing 5. Microwave cavities and associated microwave couplings 31 are commercially available, with Evanson type such as available from Electromedical Supplies (Greenham) Ltd, Wantage, Oxfordshire 0X12 7AD as catalogue number 216L being an example of a suitable means of coupling.

The magnets 7 are positioned by means of the arms of the jacket. These magnets are ferrite, and produce a field of 0.0875 Tesla within the discharge region 6. The yoke 8 is made of iron.

Oxygen flows of up to about 100 seem can be used within the apparatus 20, although typical operating flows are 5 to 15 seem. As may be seen from figure 3, the yields of oxygen radicals increases with increasing flow. The normalized percentage yield of oxygen radicals achieved with source 1 is between 5^". and 1QZ . These yields are a measurement of yield occurring at a processing position within systems downstream of the source.

The preferred type of magnetic field shape within the discharge region is seen in figure 4. The magnetic bottle shape 40 is outlined as a dotted line. This type of shape is difficult to achieve with permanent magnets such as the ferrite magnets 7, but can be achieved with the use of electromagnetic coils.

Magnets 7 induce a magnetic field shape of type 50, such a scan be seen in figure 5. The exact field shape is dependent upon the positioning of magnets 7 with respect to the discharge region 6.

In order to use the source 1, mass flow controller 10 is set to allow a constant flow of between 2 and 10 seem through the discharge tube 2 and into the system 16 operating at very low pressures. Microwave power of about 100 Watts is turned on and delivered via coaxial cable 30 and microwave coupling 31 to microwave cavity 4. Initial runs of the source will require tuning of the microwave cavity by screws 24 and 33. Once this tuning has been carried out, then the cavity should need little, if any, adjustment for subsequent useage.

Window 12 can be used to view as to whether a discharge is present in the tube 2. Should there be no discharge, then a Tesla coil can be applied to window 12. This produces high AC Voltage, induces ions in the discharge tube and initiates the discharge.

The oxygen encounters the discharge of microwaves whilst flowing through the discharge region. The co-operating effects of the discharge and the magnetic field at, or near, the ECR condition lead to the production of oxygen radicals at low oxygen flow rates. The yield of radicals produced within the discharge region at low oxygen flow rates is enhanced by the constriction 3.

Monitoring of the reflected microwave power (which is reflected back along the coaxial cable 30) is carried on throughout the operation of source 1. Where the reflected microwave power is high eg. over 30Z, then this is an indication that the discharge is not present or that the microwaves are not coupled correctly in microwave cavity 4. The yield of radicals reaching a processing position in the downstream low pressure system can be monitored using the method outlined by R G Humphreys et al (to be published in Superconductor Science and Technology). The percentage yield is obtained from the measured radical flux, the known mass flow and the estimated system conductances. Using source 1 the percentage yield is between 5Z and 10Z, with actual yield dependent upon oxygen flow as seen in figure 3.

Claims

1 Apparatus for the production of gaseous radicals comprising a discharge tube having in serial order an inlet, a discharge region, a constriction which significantly restricts gaseous flow from the discharge region and an outlet,

a means of coupling microwaves to the discharge region of the discharge tube, and

a magnetic field sufficient for electron confinement within the discharge region.

2 Apparatus according to claim 1 where the constriction restricts gaseous flow to give a ratio of gaseous pressure within the discharge region to gaseous pressure just downstream of the constriction of at least 2:1.

3 Apparatus according to claim 2 where the ratio is at least 5:1.

4 Apparatus as claimed in any preceding claims and further comprising a metal yoke.

5 Apparatus as claimed in any preceding claims and further comprising a protective jacket.

6 Apparatus as claimed in any of the preceding claims where the magnetic field has a magnetic bottle shape.

7 Apparatus as claimed in any of claims 1, 2, 3, 4 or 5 where the magnetic field is produced by annular permanent magnets.

8 Apparatus as claimed in any of claims 1, 2, 3, 4 or 5 where the discharge tube material is of a suitable low recombination coefficient. Apparatus as claimed in claim 8 where the discharge tube material in one of the group silica, pyrex and pyrolitic boron nitride.

Apparatus as claimed in any of the preceding claims where the discharge tube inner surface has a thin coating of a passivating material.

Apparatus as claimed in claim 11 where the passivating material is B₂0₃ or P₂0₅.

Apparatus as claimed in any of the preceding claims where the means of coupling microwaves to the discharge region of the discharge tube is a microwave cavity.

Apparatus as claimed in claim 12 where the microwave cavity has a geometry allowing electric field and magnetic field to be perpendicular to one another.

Apparatus as claimed in any of the preceding claims where the microwave coupling means receives microwaves from a coaxial cable.

Apparatus as claimed in claim 14 where the coaxial cable has low loss at microwave frequencies.

Apparatus as Claimed in any of the preceding claims and further comprising a gas supply.

Apparatus as claimed in claim 16 where the gas supplied is any one of the group oxygen, nitrogen and hydrogen.