WO2002103089A2

WO2002103089A2 - Heating of an effusion cell for molecular beam epitaxy

Info

Publication number: WO2002103089A2
Application number: PCT/IB2002/003300
Authority: WO
Inventors: Esa Supponen
Original assignee: Dca Instruments Oy
Priority date: 2002-08-16
Filing date: 2002-08-16
Publication date: 2002-12-27
Also published as: WO2002103089A3

Abstract

An effusion cell for creating a molecular beam of source material to be deposited epitaxially on a substrate. The effusion cell comprises a crucible, which comprises a container for the source material of a first cross-sectional dimension, the container having a closed first end and an open second end, a substantially conical nozzle, which nozzle has its cross-section opening gradually along the direction of the molecular beam from a second cross-sectional dimension at its open first end to a third cross-sectional dimension at its open second end, and a neck part connecting the second end of the container to the first end of the conical nozzle, for conducting the molecular beam from the container to the nozzle, where said first and third cross-sectional dimensions are greater than said second cross-sectional dimension. The effusion cell further comprises at least one first resistive heating filament for heating the container, for vaporizing the source material in the container, and at least one second resistive heating filament for heating the substantially conical nozzle to a substantially uniform temperature along the direction of the molecular beam, said second heating filament being arranged immediate and in conformity with said substantially conical nozzle and to cover substantially the whole length of the conical nozzle from its first end to its second end.

Description

HEATING OF AN EFFUSION CELL FOR MOLECULAR BEAM EPITAXY

FIELD OF THE INVENTION This invention relates to a device and a method for generating an ultra-pure and uniform molecular beam of atoms or molecules, utilizing a molecular beam epitaxy (MBE) effusion cell comprising a crucible comprising a container for source material and a substantially conical nozzle, the cross-section of the nozzle opening gradually along the direction of the molecular beam. The invention relates specifically to a new and inventive resistive filament construction able to heat the nozzle to a substantially uniform temperature along the direction of the molecular beam.

BACKGROUND OF THE INVENTION Molecular beam epitaxy is a growth process where a thin film of material is deposited onto a substrate surface by directing molecular or atomic beams onto the substrate. MBE is widely used e.g. in semiconductor device producing industry and in other industries where thin-film deposition of elemental semiconductors, metals and insulating layers is needed.

A thermal effusion cell is utilized in MBE. Thermal effusion cells comprise a crucible containing the source material, for example aluminum, gallium or arsenic or indium. The crucible is heated by a resistive filament so much, that the source material is vaporized and effused out of an orifice as a molecular beam of atoms or molecules. The beam is directed on the substrate surface on which the film of the source material is wanted. MBE is carried out in an ultra high vacuum growth chamber. For optimal results, i.e. a uniform layer of the source material deposited on the substrate surface, the molecular beam effused from the crucible should be as homogenous and steady as possible. When designing a thermal effusion cell important factors concerning the steadiness of the flux from the crucible are the material and shape of the crucible and the construction of the resistive filament heating the crucible.

The material of the crucible is a subject to high demands. It has to withstand the high temperatures needed in MBE and have a high mechanical strength. The material also has to be very pure to avoid contamination of the source material. A material having the most versatility is pyrolytic boron nitride (PBN). The material naturally has to be selected depending on the source material to be melted in the crucible.

The heating of the crucible is critical for the results of the MBE process. A certain exact heating effect is needed to vaporize the source material in such a way that the vapor pressure in the container and the flux of atoms or molecules out of the crucible is exactly the wanted. If the beam of source material atoms or molecules confronts a cooler area on its way out from the crucible, the danger for condensation and formation of droplets arises. That would endanger the steady film formation on the substrate surface. To minimize this risk various systems have been proposed where the exit orifice part of the crucible is heated to a higher temperature than the part where the source material is being vaporized. One primary element of the effusion cell assembly is, of course, the resistive filament which is electrically heated and radiantly and by conduction heats the part of the crucible in which the material to be evaporated is contained and the part of the crucible through which the beam of atoms or molecules flows. A crucible for a thermal effusion cell has traditionally been formed as a substantially conical structure, having a closed end and an open end, the radius of the cross section of the cone gradually increasing from the closed end to the open end. The source material is kept, heated, melted and vaporized at the closed end, i.e. the bottom of the cone shaped crucible, from where the beam of source material atoms or molecules is directed via the gradually opening conical structure and through the open end, out of the crucible. Various systems of this kind have been developed. US-patent 5,034,604 by Streetman et al. introduces a solution for heating of a crucible of this kind. Another system is introduced in US-patent 5,041,719 by Harris et al.

Another kind of a known crucible comprises a substantially cylindrical container for the source material of a first cross-sectional dimension, and attached to the container a substantially conical nozzle, which nozzle has its cross-section opening gradually along the direction of the molecular beam from a second cross-sectional dimension at its first end to a third cross-sectional dimension at its second end. The said first and third dimensions are greater than the said second dimension. The cylindrical container and the first end of the conical nozzle are connected via a neck part, which is formed in a way to reduce the cross-sectional dimension of the substantially cylindrical container to the cross-sectional dimension of the first end of the conical nozzle. Crucibles of this kind are presented for example in US- patents 5,820,681 by Colombo et al. and 4,812,326 by Tsukazaki et al. Heating of the crucible has been arranged with resistive wires or filaments which have been arranged in a cylindrical manner around the crucible. The container part and the conical nozzle have usually had distinct heating filaments in order to apply a different heating power to these parts.

A conventional heating filament arrangement for a thermal effusion cell with a crucible comprising a cylindrical container, a neck part and a conical nozzle, as described above, encircles the whole crucible substantially in a shape of a cylinder. The heating filament is arranged adjacent the cylindrical container part, relatively close it. At the neck part the radius of the cross-section of the crucible reduces, but the heating filaments continue substantially straight, creating a substantial distance between the crucible and the heating filaments. Along the conical nozzle, the distance between the crucible and the heating filaments reduces, so that the crucible and the heating filaments are normally near each other at the exit end of the conical nozzle. The substantial distance between the crucible and the heating filaments at the neck part and at the first end of the conical nozzle cause a need for extra heating around these parts. If the heating filaments were closer to the crucible wall, a smaller heating effect would be needed to achieve the temperature needed on the wall. Excess heat reduces the life-time of the apparatus and causes an increased risk for releasing material from the apparatus and hence contamination of the molecular beam.

The method, described in the above mentioned US-patent 5,034,604, of arranging the heating of the conical crucible with one or more heating wires with different pitches or different spatial frequencies around different parts is a complicated way to produce the wanted temperatures on the crucible walls.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide an improved effusion cell for producing an ultra-pure and uniform molecular beam of atoms or molecules with molecular beam epitaxy (MBE), the effusion cell comprising a crucible which comprises a container for the source material, a substantially conical nozzle and a neck part connecting the container and the nozzle. Another object is to provide an improved heating arrangement for a MBE effusion cell crucible which comprises a container for the source material, a substantially conical nozzle and a neck part connecting the container and the nozzle.

It is another object to provide an improved resistive filament for a substantially conical nozzle of a MBE effusion cell crucible.

The effusion cell of the present invention is meant for creating a molecular beam of source material to be deposited epitaxially on a substrate. The effusion cell comprises a crucible, comprising a container for the source material having a closed first end and an open second end, a substantially conical nozzle having its cross-section opening gradually from its open first end to its open second end, and a neck part connecting the second end of the container to the first end of the conical nozzle. In this crucible according to the present invention the container is of a first cross-sectional dimension, the first end of the conical nozzle is of a second cross- sectional dimension and the second end of the conical nozzle is of a iMrd cross- sectional dimension, where said first and third cross-sectional dimensions are greater than the said second cross-sectional dimension. This means, that the substantially conical nozzle has its cross-section opening gradually along the direction of the molecular beam. The effusion cell of the present invention further comprises an at least one first resistive heating filament for heating the container, for vaporizing the source material in the container, and an at least one second resistive heating filament for heating the substantially conical nozzle to a substantially uniform temperature along the direction of the molecular beam. The second heating filament is arranged immediate said substantially conical nozzle and in conformity with the nozzle. The second heating filament is also arranged to cover substantially the whole length of the conical nozzle from the first end of the nozzle to the second end of it. Typically, the temperature of the conical nozzle is kept substantially higher than the temperature in the source material container. In an embodiment of the invention the container for the source material is formed substantially cylindrical, wherein first and second ends of the substantial cylinder establish the first and second ends of the container. This construction is simple and found effective and functional in practice.

In other embodiments of the present invention the second resistive heating filament or a separate third resistive heating filament is arranged to cover the neck part and arranged immediate said neck part for heating the neck part. The adequate heating of the neck part is important for a successful MBE process.

In other embodiments of the present invention the effusion cell further comprises an at least one support shield arranged immediate outside the second and/or third resistive heating filament. This kind of a support shield is preferably made of pyrolytic boron nitride (PBN). Preferably one or more support shields are arranged to cover and support both the heating filament or filaments positioned immediate outside the substantially conical nozzle and the heating filament or filaments positioned immediate outside the neck portion. Preferable the at least one support shield is positioned in conformity to the outside structure of the crucible and the heating filaments it is arranged to cover.

In another preferred embodiment of the present invention the crucible is made of pyrolytic boron nitride. In an very favorable embodiment of the present invention both the crucible and the support shield are made of PBN. PBN is an insulating material which is very suitable for a preferred construction where the resistive heating filament or filaments are positioned between the crucible and the support shield. In preferred embodiment of the present invention the substantially conical nozzle and the neck part are made of one single piece, and preferably of PBN. This way the construction of the effusion cell becomes more simple. To further optimize the construction of the invention the opening at the second end of the substantially cylindrical container is of the first cross-sectional dimension. In this case the neck part is preferably arranged to be connected tightly to said opening, that is, the against the container arranged part of the neck part is also of the first cross- sectional dimension.

In an preferred embodiment of the invention the second and/or third resistive heating filament is formed of a tantalum, molybdenum or tungsten foil. Preferably, the second heating filament foil is positioned in a serpentine path lengthwise about said conical nozzle, that is in the direction of the molecular flow. By this is meant that the foil is arranged to follow a distance in a first direction, that is the direction of the molecular flow, to turn about 90° to follow a distance in a second direction transverse to the direction of the molecular flow, to turn about 90° to again follow a distance in a first direction, to turn about 90° to follow a distance in a third direction which is the opposite direction compared to the second direction, turn about 90° to again follow a distance in a first direction, and so on. Preferably, the distances in the second and third direction get longer in the direction towards the second end of the conical nozzle. Even more preferably lengthwise gaps in the serpentine pattern of the foil, that is gaps formed between the parts of the foil arranged in the second and third direction, remain substantially constant along the length of the conical nozzle. The most favorable second resistive heating filament is formed of a foil of tantalum, molybdenum or tungsten, and the foil is formed to be at its broadest near the first end of the conical nozzle and narrowing in the direction of the second end of the conical nozzle, the foil being formed at its narrowest near the second end of the conical nozzle. This kind of a filament structure creates a more effective heat radiation to the area of the second end of the conical nozzle, creating a favorable environment for the molecular beam in the nozzle.

In a method of producing a resistive heating filament for a substantially conical nozzle of an effusion cell crucible according to the present invention, a sheet of tantalum, molybdenum or tungsten is cut, e.g. with a saw or a like, in a serpentine pattern. Preferably the sheet is cut into a serpentine-like foil which covers a substantial sector of a circle. According to the most favorable method the foil covering the substantial sector of a circle is formed to be at its broadest near the center point of the sector and narrowing in the direction of the periphery of the sector, and the foil being at its narrowest near the periphery of the sector. A filament produced in this way can easily be bent to form a substantial cone or a part of a cone to be positioned on a conical nozzle according to the invention.

The resistive heating filament according to the present invention can also be formed using a well known method of chemical vapor deposition (CVD). According to this method a substantially even layer of a suitable conductive material, such as pyrolytic graphite, is deposited on the surface of a substantially conical nozzle made of PBN. Then part of the graphite deposited is machined of in such a way that the graphite remained on the crucible forms the serpentine pattern of the inventive filament. After that, using CVD, a layer of PBN can be deposited on the crucible and the filament to function as a shield.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly cut side view of an effusion cell in accordance with the invention; FIG. 2 is a cross-section of a substantially conical nozzle, a neck part, and heating filament and support structures around them in accordance with the invention; FIG. 3 is a perspective view of a substantially conical nozzle, a neck part, and a heating filament structure around them in accordance with the invention; FIG. 4 is a view of a heating filament in accordance with the invention; FIG. 5 is a perspective view of a conical nozzle, a neck part, a heating filament structure and two support shields around them in accordance with the invention; FIG. 6 is a perspective view of a conical nozzle and a neck part, with a heating filament and cylindrical support structure around them in accordance with the invention;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates an effusion cell 1 according to the invention for creating a molecular beam of source material to be deposited epitaxially on a substrate. The effusion cell 1 comprises a crucible 2 made mainly of pyrolytic boron nitride (PBN). The crucible 2 includes a source material container 3, a substantially conical nozzle 4 and a neck part 5. The neck part 5 and the nozzle 4 are made of one single piece and attached to the container 3 in joint 6 by bolts not shown in the figures. The container 3 and the source material in it is heated with a first resistive heating filament 7, the conical nozzle 4 is heated with a second resistive heating filament 8 and the neck part 5 is heated with a third resistive heating filament 9. The effusion cell 1 further comprises an electrical feedthrough part 10 of the effusion cell 1 connected to the first end 11 of the container 3. The structure of the electrical feedthrough part 10 is well known and not presented here in detail. A typical electrical feedthrough part can comprise e.g. a thermocouple, power feedthrough and means for fastening the effusion cell 1 to an apparatus it is used with. The container 3 is substantially cylindrical and the first resistive heating filament 7 is arranged around it covering substantially the whole length of the container 3 from its closed first end 11 to its second end 12. The possible structures of the filament 7 heating the source material container 3 are well known from the prior art and not discussed here in detail. An opening 13 is arranged in the second end 12 of the container 3 for leading the vaporized source material out of the container 3. The cross-sectional dimension B of the opening 13 is substantially smaller than the cross-sectional dimension C of the second end of the cylindrical container 3. The neck part 5 is attached to the second end 12 of the container 3. The neck part 5 comprises a substantially round plate 14 with an opening 15 in the middle. The openings 13 and 15 are substantially of the same size B, and arranged together. The edge 16 of the opening 15 is bent away from the source material container 3, and connected to the first end 17 of the substantially conical nozzle 4. The neck part 5 and the conical nozzle 4 are formed as one piece and made of PBN. The diameter of the nozzle 4 increases rectilinearly from its first end 17 towards its second end 18, the diameter C at the second end 18 being approximately the same with the cross-sectional dimension A of the cylindrical source material container 3.

The second resistive heating filament 8 is arranged immediate the substantially conical nozzle 4 in conformity with the outer face 19 of the nozzle 4. The heating filament 8 is arranged to cover substantially the whole outer face 19 of the nozzle 4, substantially over the whole length of the conical nozzle 4 from its first end 17 to its second end 18. The round plate 14 of the neck part 5 is covered with the third resistive heating filament 9. Heating filaments 8 and 9 are made of tantalum foil. From FIG. 1 can be seen, that the second heating filament 8 is formed into a serpentine pattern.

FIG. 2 illustrates a cross-section of the substantially conical nozzle 4 and the neck part 5, with heating filaments and support structures around them according to a preferred embodiment of the invention. The second heating filament 8 is arranged in direct contact with the nozzle 4. In the same way the third heating filament 9 is arranged in direct contact with the neck part 5. A support shield 20, made of PBN, is arranged on the heating filaments 8 and 9, in direct contact with them. The whole conical structure is covered with a cylindrical cover 21, which is preferably made of molybdenum. The cover 21 can consist of one or more parts forming the cylinder. A ring 30 is arranged at the periphery of the round plate 14 of the neck part. The ring 30 is also well seen on FIGS 5 and 6. A step 31 is formed on the ring 30 to receive one of the ends of the cylindrical cover 21. The step 31 and the way the cylindrical cover 21 is arranged on it is best seen by looking at FIGS 5 and 6.

FIGS 3, 5 and 6 illustrate a perspective view of a substantially conical nozzle 4, a neck part 5, and heating filaments and support structures around them in accordance with a preferred embodiment of the present invention. FIG. 3 shows that the conical nozzle 4 and the neck part 5 are covered by resistive heating filaments 8 and 9 made of tantalum foil. The two parts 8 and 9 of the filament are connected with an electrical connection 22, so that they are in the same electrical circuit, and in that way their heating effect can be adjusted simultaneously. The electrical current to the heating filaments 8 and 9 is connected to wire terminals 23. The filaments 8 and 9 could also be arranged in different electrical circuits, for separate adjustment of the heating effects. The heating of the neck part 5 keeps the joint 6 between the container 3 and the neck part 5 hot.

In the preferred embodiment of FIG. 3 both the filaments 8 and 9 have a serpentine-like electrical path. The filament 8 is wrapped in the form of a cone or a part of a cone around the nozzle 4. The filament 9 forms a circle on the neck part 5. Such a serpentine pattern is favorable in order to provide the filaments 8 and 9 with a practical ohmic resistance and to uniformly distribute the radiated heat about the nozzle. The pattern and dimensions of the filaments 8 and 9 are selected so as to optimize the temperature gradient over the neck part 5 and the nozzle 4.

FIG. 5 illustrates how two support shields 20a and 20b are attached on the heating filament 8. Each shield 20 covers substantially one half of the nozzle 4 and the neck part 5. Support shields 20a and 20b support the thin heating filament 8 and decrease the heat loss to the outside. A structure of this kind with the resistive filament 8 between the insulating layers of PBN, that is the crucible 2 and support shields 20a and 20b, is very advantageous, e.g. because this way a substantial part of heating of the nozzle 4 and neck part 5 is conductive. When, as known before, the heating filament is situated at a distance from the nozzle, the heating of the nozzle is arranged mostly through radiative heating.

FIG. 6 shows how a cylindrical cover 21 is positioned to cover the whole conical structure. The cylindrical cover 21 is formed of one part, with its ends connected together by fastening means 28. An opening is formed in the cover 21 to have the wire terminals 23 available from the outside. The cylindrical cover 21 protects the apparatus inside it and decreases the heat loss to the outside. The cylindrical cover 21 could naturally comprise more than one part.

FIG. 4 shows a resistive heating filament 8 of a highly preferred embodiment of the invention. The heating filament 8 is made with a method in accordance with the present invention. The filament 8 has been cut of a sheet of tantalum into a serpentine-like formed foil which covers a substantial sector 24 of a circle. The foil 8 is formed to be at its broadest near the center point 25 of the sector 24, that is near the first end 17 of the conical nozzle 4. The foil 8 gets narrower in the direction of the periphery 26 of the sector 24, that is the second end 18 of the conical nozzle 4, and the foil 8 is at its narrowest near the periphery 26 of the sector 24. At the same time, the distance L between the parallel strips of the serpentine, that is the length of the gaps in the filaments 8 and 9, is kept substantially constant.

When using the apparatus of FIG. 1, source material, e.g. gallium, is positioned in the cylindrical container 3 of the crucible 2. By leading an electrical current through the resistive heating filament 7 positioned around the container 3, the source material is heated and vaporized. By adjusting the heating effect of the filament 7 the vapor pressure of the source material in the container is kept at the desired level. Through the opening 13 at the second end of the container and the opening 15 of the neck part, a beam of source material atoms or molecules is directed into the conical nozzle 4. The beam flows towards the second end 18 of the nozzle 4, through which the molecular beam of source material leaves the effusion cell 1.

Having the heating filaments 8 and 9 arranged immediate and in conformity with the neck part 5 and the conical nozzle 4, the heating effect needed from the filaments 8 and 9 can be reduced from the effect needed in a traditional solution, where the heating filament for the conical nozzle 4 is arranged in a cylindrical form around the nozzle 4. That traditional heating filament is positioned at the level of the heating filament arrangement of the source material container 3, approximately where the cylindrical cover 21 is situated in FIGS. 1 and 2. With the inventive construction of the present invention, heat from the filament 8 reaches the nozzle 4 almost directly both by conduction and radiation, avoiding the empty space 27 between the cylindrical cover 21 and the nozzle 4. By this inventive way a smaller heating effect is needed to create the same temperature on the nozzle 4. This means less heat loss and less impurities released from the structures.

With the inventive construction of the resistive heating filament 8, best shown in FIG. 4, surprising advantages are achieved. The foil 8 is arranged to get narrower in the direction of the periphery 26 of the sector 24, that is the second end 18 of the conical nozzle 4. The narrower the foil 8, the greater the resistivity of it, i.e. more heating energy is produced with the same electric current per length unit of the foil 8. When, preferably, at the same time the distance L between the parallel strips of the serpentine, that is the length of the gaps in the filaments 8 and 9, is kept substantially constant, more heat radiation is released per area of the conical nozzle 4 at the second end 18 of the nozzle 4. This is most advantageous, because more heat loss to the environment is encountered at the more widely open second end 18 of the conical nozzle 4. With the present invention, a uniform temperature is achieved at the inner wall 29 of the conical nozzle 4.

The method of the present invention of producing the resistive heating filament 8 for the substantially conical nozzle 4 of an effusion cell crucible 2 gives an easy and economical way to produce the heating filament 8 of this invention. Precise cutting of a sheet of tantalum, molybdenum or tungsten, e.g. with a saw or a like, preferably in a way showed in the FIG.4, has become possible and effective with available modern technology.

The descriptions above and the accompanying drawings should be interpreted in an illustrative and not a limited sense. While the invention has been disclosed in connection with the preferred embodiment or embodiments thereof, it should be understood that there may be other embodiments which fall within the spirit and scope of the invention as defined by the following claims.

Claims

CLAIMS l. An effusion cell for creating a molecular beam of source material to be deposited epitaxially on a substrate, comprising

- a crucible, comprising a) a container for the source material of a first cross-sectional dimension, the container having a closed first end and an open second end, b) a substantially conical nozzle, which nozzle has its cross-section opening gradually along the direction of the molecular beam from a second cross- sectional dimension at its open first end to a third cross-sectional dimension at its open second end, and c) a neck part connecting the second end of the container to the first end of the conical nozzle, for conducting the molecular beam from the container to the nozzle, where said first and third cross-sectional dimensions are greater than said second cross-sectional dimension,

- at least one first resistive heating filament for heating the container, for vaporizing the source material in the container, and

- at least one second resistive heating filament for heating the substantially conical nozzle to a substantially uniform temperature along the direction of the molecular beam, said second heating filament being arranged immediate and in conformity with said substantially conical nozzle and to cover substantially the whole length of the conical nozzle from its first end to its second end.

2. The effusion cell of claim 1 wherein the container for the source material is formed substantially cylindrical, wherein first and second ends of the substantial cylinder establish the first and second ends of the container.

3. The effusion cell of claim 1 wherein the second resistive heating filament is arranged to cover the neck part and arranged immediate said neck part for heating the neck part.

4. The effusion cell of claim 1 further comprising an at least one third resistive heating filament for heating the neck part, said third heating filament being arranged immediate said neck part.

5. The effusion cell of claim 1 further comprising a support shield arranged immediate outside said second resistive heating filament.

6. The effusion cell of claim 5 wherein the support shield is arranged to cover a heating filament arranged immediate outside the neck part.

7. The effusion cell of claim 1 wherein the second resistive heating filament is formed of a tantalum, molybdenum or tungsten foil.

8. The effusion cell of claim 1 wherein the crucible is made of pyrolytic boron nitride.

9. The effusion cell of claim 5 wherein the support shield is made of pyrolytic boron nitride.

10. The effusion cell of claim 1 wherein the substantially conical nozzle and the neck part are of one single piece.

11. The effusion cell of claim 2 wherein the opening at the second end of the substantially cylindrical container is of the first cross-sectional dimension and the neck part is arranged to be connected tightly to said opening.

12. The effusion cell of claim 1 wherein the second resistive heating filament is formed of a foil of tantalum, molybdenum or tungsten, the foil being positioned in a serpentine path lengthwise about said conical nozzle.

13. The effusion cell of claim 12 wherein lengthwise gaps in the serpentine pattern of the foil remain substantially constant along the length of the conical nozzle.

14. The effusion cell of claim 1 wherein the second resistive heating filament is formed of a foil of tantalum, molybdenum or tungsten, the foil being formed at its broadest near the first end of the conical nozzle and narrowing in the direction of the second end of the conical nozzle, the foil being formed at its narrowest near the second end of the conical nozzle.

15. A method of producing a resistive heating filament for a substantially conical nozzle of an effusion cell crucible by cutting a sheet of tantalum, molybdenum or tungsten, e.g. with a saw or a like, in a serpentine pattern.

16. The method of claim 15 wherein the sheet is cut into a serpentine-like formed foil which covers a substantial sector of a circle.

17. The method of claim 16 wherein the foil is formed to be at its broadest near the center point of the sector and narrowing in the direction of the periphery of the sector, and the foil being at its narrowest near the periphery of the sector.