GB2276720A - Measuring the density of flux of material in a near-vacuum - Google Patents

Abstract

The material is excited by injecting energy into an excitation region in which the material is present and causing quanta of injected energy to make plural transits of said excitation region. The injected energy may be electrons or photons. Plural transits of electrons can be obtained using magnetic or electric fields, and plural transits of photons can be achieved by the use of mirrors. The invention has application in vacuum deposition systems for coating a substrate with the material. <IMAGE>

Description

"Measurina" This invention relates to measuring, and relates more specifically but not exclusively to methods and apparatus for measuring the density or flux of material in a near-vacuum.

When forming thin coatings of a given material on a substrate by the known technique of vacuum deposition, it is desirable or essential for accurate control of the vacuum deposition process and of the quality of the resultant coated product that the rate of deposition of the given material on the substrate and/or the total quantity of the given material deposited on the substrate (as thickness, mass per unit area, or some other parameter) be accurately and rapidly measured. A proposed arrangement for undertaking such measurements involves disposing a sensor to be deposited with the given material simultaneously with deposition of the given material on the substrate, vibrating the sensor, and observing changes in the vibration characteristics of the sensor as deposits of the given material accumulate on the sensor.However, this proposed arrangement lacks adequate precision and reliability.

It is also desirable to be able to measure the concentration of material within a near-vacuum, for example to determine the chemical composition of a residual gas in a vacuum chamber after the vacuum chamber has been evacuated.

It is therefore an object of the invention to provide methods and apparatus for measuring the density or flux of a material in a near-vacuum.

Underlying the present invention is the principle that it is a basic characteristic of material that their constituent atoms or molecules can be excited by various means, and that when these excited materials relax back into lower energy states, they do so by emitting radiation of wavelengths that are specific to and characteristic of the materials. The excitation mechanism can comprise bombardment of a material by electrons. The electrons can be injected into the material and subjected to a potential difference or generated by the material itself following the application of a steady or alternating electric field (commonly called plasma discharge), or by subjecting the material to electromagnetic radiation.Measurement of the concentration of the material that is present in the excitation region can be achieved either by determining how much of the excitation energy is absorbed or by determining the level of subsequent radiation emission.

The present invention as defined below particularly concerns increasing the resolution of the measurement technique discussed above by maximising the efficiency of absorption by the material of energy injected into the measuring system.

As used in this specification, the term 11near-vacuum11 refers to an environment (for example, the interior of a vacuum chamber) from which substantially all gases and vapours have been evacuated, possibly to the extent that the near-vacuum environment contains substantially no gas or vapour or plasma other than of the material(s) whose density or flux is to be measured, and whose presence in that environment prevents the vacuum therein being total.

According to a first aspect of the present invention there is provided a measuring method for measuring the density or flux of a material in a near-vacuum, said method comprising the steps of exciting the material by injecting energy into an excitation region in which said material is present, and causing quanta of injected energy to make plural trans its of said excitation region.

Where said quanta of injected energy are electrons, said electrons are preferably caused to make said plural trans its of said excitation region by applying an electrostatic field to said excitation region, said electrostatic field being such as to tend to cause an electron injected thereinto to travel on a path which oscillates across said excitation region. Said electrostatic field is preferably non-uniform in manner tending to destabilise the position of an electron located therein. Said excitation region may additionally or alternatively be subjected to a magnetic field such as to cause electrons injected into said excitation region to make plural transits thereof, in an oscillatory path or in an orbital path, or in a combination of such paths.

Where said quanta of injected energy are photons, said photons are preferably caused to make said plural trans its of said excitation region by at least partially bounding said excitation region with mirrors or other suitable forms of photonic reflector. Said mirror or other reflector forms may have photon reflecting surfaces which are planar or curved.

Since the absorption of excitation energy by atoms or molecules of a given material is a statistical process, then in order to increase the absorption of energy it is necessary to increase the amount of time that a unit of injected energy spends in the vicinity of the energy-absorbing material. In theory this can be achieved by making the excitation region very large, but it may be impracticable and sometimes impossible arbitrarily to increase the size of the excitation region. The present invention takes an alternative approach to increasing energy absorption in an excitation region of given size by causing quanta of injected energy to make a plurality of transits of the excitation region rather than a single transit.The efficiency of energy absorption by the given material in said region is thereby multiplied accordingly, thus improving the procedure for measuring the density or flux of said given material.

According to a second aspect of the present invention there is provided measuring apparatus for measuring the density or flux of a material in a near-vacuum, said apparatus comprising an excitation chamber defining an excitation region in which said material will be present in use of said apparatus if said material exists in measurable form, an energy source disposed to inject energy into said excitation region in use of said apparatus, and plural transit means to cause quanta of injected energy to make plural trans its of said excitation region in use of said apparatus.

In respect of the foregoing specification of material being present in said excitation region conditional upon said material existing in measurable form, this was a reference to the possibility of said material being absent from said excitation region because the material does not exist in the vacuum system in measurable form. Such a situation might arise, for example in a vacuum deposition system, where the source of the material in vapour or plasma form is quiescent or exhausted, such that there is no material in the measurable vapour or plasma form, the material being then present in the system only in the unmeasurable solid form of the quiescent source or as a fully-deposited layer on the target substrate, respectively. However, when the source commences or re-commences operation, the material will enter its measurable vapour or plasma phase and then be present in the excitation region to be subject to the plural transits of the quanta of injected energy. The specific condition is also intended to cover the situation where some of the material is deposited or condenses in or on the excitation chamber and thus leaves its measurable state which depends upon the material existing as a free vapour (or plasma). Thus notwithstanding that technically speaking, the material may then be in the excitation region, it is so only in a form that makes it irrelevant to the measuring process and therefore outside the scope of the present invention.

In said measuring apparatus according to the second aspect of the present invention said excitation chamber preferably comprises a material stream entrance and a material stream exit disposed to allow a stream of the said material in measurable form to pass through said excitation chamber and, in so doing, to traverse said excitation region.

Said energy source is preferably an electron source which may comprise an electrically heated filament having a thoriated coating, said electron source being functionally associated with an anode to cause electrons from said electron source to be injected into said excitation region in use of said apparatus, said plural transit means preferably comprises a pair of grids respectively disposed on mutually opposite sides of said excitation region with each said grid being aligned transverse the paths of electrons injected into said excitation region, both said grids being charged in use of said apparatus to similar or identical positive voltages to act as respective anodes each capable of passing electrons therethrough.The one of said grids that is closer to said filament or other electron source preferably also constitutes said anode that causes electrons from said electron source to be injected into said excitation region, said filament or other electron source and said grids being maintained in use at requisite relative potentials.

Said plural transit means may additionally or alternatively comprise magnetic field applying means to apply a magnetic field to said excitation region in use of said apparatus whereby to cause injected electrons to make plural transits of said excitation region.

Where said energy source is a photon source, said plural transit means preferably comprises mirrors or other suitable photon reflectors, which may have planar or curved photon reflecting surfaces, said mirrors or other photon reflectors being disposed to cause photons injected into said excitation region in use of said apparatus to make plural trans its of said excitation region.

Said apparatus preferably comprises a light channel associated with said excitation chamber to channel light from said excitation region to an external photon-sensitive detector whereby photonic emissions from decay of excited material may be externally detected and measured. Said photon-sensitive detector may be part of or separate from said measuring apparatus. Said detector is preferably sensitive only to a narrow band of optical frequencies appropriate to the excitation decay transitions of the material whose density or flux is to be measured, or alternatively, a photon-sensitive detector having relatively broad-band sensitivities may be employed in conjunction with a narrow-pass-band optical filter. Said photon-sensitive detector may comprise a photo-multiplier.

According to a third aspect of the present invention there is provided a vacuum deposition system comprising a vacuum chamber containing a source of vapour or plasma of a material to be deposited and substrate mounting means to mount one or more substrates to receive said vapour or plasma to deposit said material on said substrate(s) in use of said system, and a measuring apparatus according to the second aspect of the present invention disposed in said vacuum chamber between said source and said substrate mounting means, said measuring apparatus being operated in use of said system in accordance with the measuring method according to the first aspect of the present invention to measure the density of said material in said vacuum chamber when in the form of a vapour or plasma and/or to measure the flux of said material as it passes in the form of a vapour or plasma from said source towards said substrate mounting means.

According to a fourth aspect of the present invention there is provided a material-coated substrate when produced by the system according to the third aspect of the present invention.

Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings wherein: - Fig. 1 is an external elevation of a first embodiment of measuring apparatus in accordance with the present invention, viewed from the upstream side thereof; Fig. 2 is a view of the measuring apparatus corresponding to Fig. 1, but with the upstream-side cover of the apparatus removed and with the excitation arrangement removed from the interior of the apparatus; Fig. 3 is a view of the measuring apparatus corresponding to Fig. 2, but with the excitation arrangement located in its working position in the interior of the apparatus; Fig. 4 is a horizontal section of the measuring apparatus (viewed from below), including its excitation arrangement, the section being taken on the line IV-IV in Fig. 3;; Figs. 5 and 6 are respectively front and side elevations of a shield forming part of the excitation arrangement of the measuring apparatus of Fig. 1; Figs. 7 and 8 are respectively front and side elevations of one of two identical grids forming part of the excitation arrangement of the measuring apparatus of Fig. 1; and Figs. 9 and 10 are respectively side and end elevations of an electron-emitting filament forming part of the excitation arrangement of the measuring apparatus of Fig. 1.

Referring first to Fig. 1, this is an external elevation of an excitation chamber 10 comprised in the first embodiment of measuring apparatus in accordance with the present invention. The excitation chamber 10 comprises a rectiform casing having a body 12 and an upstream-side cover 14, both of stainless steel sheet.

The cover 14 has a rectangular entrance 16 for entry of a stream of vapour or plasma of a material whose flux or density is to be measured by the apparatus.

Referring now to Fig. 2, this shows that the back of the chamber body 12, which forms the downstream-side face of the excitation chamber 10, has a rectangular exit 18 for exit of the stream of vapour or plasma of the material whose flux or density is to be measured by the apparatus. The exit 18 is aligned with the entrance 16 and is of a similar shape, but somewhat larger to allow for divergence of the stream of vapour or plasma and so minimise or obviate interception of material within the chamber 10 by the casing of the chamber.

Figs. 1 and 2 both show a straight tubular light channel 20 which is attached to the excitation chamber 10 in alignment with the excitation region (detailed below) within the chamber 10. The light channel 20 is conveniently formed of a rigid stainless steel tube welded to the chamber body 12.

Referring now to Figs. 3 and 4, these show details of the electrode assembly forming the excitation arrangement. Two identical grids 22 are secured by screws 24 to stainless steel spacers 26 to lie mutually parallel on either side of an excitation region defined therebetween, this excitation region including the material stream path through the excitation chamber 10 from the entrance 16 to the exit 18. The grid and spacer assembly 22/26 is supported by a shield 28 through the intermediary of ceramic spacers 30 which keep the grids 22 electrically insulated from the remainder of the excitation arrangement. The shield 28 is formed of stainless steel and has the form of a J-section bracket, as may be seen in Figs. 5 and 6 which show the shield 28 as an individual component separate from the remainder of the apparatus.

The shield 28 has an elongate rectangular aperture 32 adjacent which is mounted an electron-emitting filament 34 on the side of the shield 28 opposite to that of the grids 22. The filament 34 (which is separately detailed in Figs. 9 and 10) is a finely-wound elongate coil of iridium wire coated with thorium oxide which is electrically self-heated by the passage of an electric current through the wire, to cause thermionic emission of electrons from the thorium oxide coating thereon.

The lower end 36 of the filament 34 is electrically connected to an external current supply (shown as a ghost-outline cable connector 38 in Fig. 1) by way of an electrically insulated feed-through 40 (Fig. 3), while the upper end 42 of the filament 34 is connected to the shield 28 for a ground return of the filament-heating current through the metal of the shield 28 and the casing body 12.

The two grids 22 are shown assembled in Figs. 3 and 4, and one of the two identical grids 22 is shown separately as an individual component in Figs. 7 and 8.

Each of the two grids 22 is a rectangular sheet of thin molybdenum having two holes 44 (Fig. 7) to accept the mounting screws 24 (Figs. 3 and 4), and a rectangular active mesh area 46 (Fig. 7) which is photo-etched to form a perforate grid through which electrons can pass in use of the apparatus as will now be detailed.

In use of the measurement apparatus, the excitation arrangement is assembled and mounted as shown in Figs.

3 and 4, and the excitation chamber 10 is closed by securing the cover 14 on the chamber body 12 as shown in Fig. 1. The cable connector 38 is attached, and the excitation chamber 10 is disposed in the vacuum chamber (not shown) of a vacuum deposition system (not shown) between the source (not shown) and the substrate mountings (not shown) which (in use) hold the substrate(s) on which material from the source is to be deposited, with the entrance 16 towards the source, and the entrance and exit 16, 18 on a line between the source and the substrate mountings such that material vapour or plasma from the source passes through the excitation chamber 10 and between the grids 22 on its way to be deposited. The excitation chamber 10 is conveniently so mounted in the vacuum chamber by using the light channel 20 as a cantilevered suspension bracket.

When the vacuum chamber of the deposition system is evacuated and the source is activated, a stream of material in the form of a vapour or plasma will pass through the excitation chamber 10 by way of the entrance 16 and the exit 18. The filament 34 is activated by passing a suitable heating current therethrough to stimulate thermionic emission of electrons from the thoriated coating of the filament 34. The two grids 22 (which are mutually electrically connected through the metal spacers 26) are maintained at a suitable positive potential (for example, +180 volts) relative to the ground potential of the shield 28 and of the metallic body 12 and cover 14 of the chamber 10. The grids 22 are charged through the cable connector 38 and a suitable connection (not shown) within the chamber 10.The grid 22 which is nearer the electron-emitting filament 34 acts as an anode to attract electrons emitted by the filament 34 through the aperture 32 in the grounded shield 28, and accelerate these electrons through the photo-etched perforations of the active mesh area 46 thereby to inject electrons into the excitation region between the two grids 22. Some electrons from the filament 34 will strike the metal of the nearer grid 22 to be captured thereby, but a sufficient number of electrons will pass through the grid perforations for the purposes of the present invention.

Electrons thus injected through the nearer grid 22 and into the excitation region between the grids 22 will pass across the excitation region and through the perforations of the active mesh region 46 of the grid 22 which is further way from the filament 34. Beyond this further grid 22, the steeply negative electrostatic field produced by the grounded casing 12 relative to the highly positive grid 22 will reverse electrons passing through the further grid 22 and so re-inject electrons into the excitation region between the two grids 22. (Again, some electrons will strike the metal of the further grid 22 to be captured thereby, but a sufficient number of electrons will pass through the grid perforations for the purposes of the present invention).

In a similar manner, electrons returning through the grid 22 which is nearer the filament 34 will be reversed and re-injected by the steeply negative electrostatic field produced by the ground shield 28 relative to the highly positive grid 22.

In this manner, the electrons not captured on the grids 22 will be caused to oscillate between the grids 22 and hence make plural trans its of the excitation region between the grids 22.

Since this excitation region is also being traversed by the stream of material vapour or plasma passing through the excitation chamber 10 by way of the entrance 16 and the exit 18, the atoms or molecules of the material will be exposed to electrons, with a probability of being temporarily excited by interaction with an electron and subsequently decaying by characteristic photonic emission. As previously mentioned, this probability is statistical and a function of the period of time which each quantum of injected energy is in the vicinity of the material to be measured, i.e. the duration of injected electrons in the excitation region.Thus the efficiency of excitation in an excitation region of given dimensions is substantially improved by causing the injected electrons to make plural transits of the excitation region, in comparison to an excitation arrangement in which electrons make only a single transit of the excitation region.

When the temporarily electron-excited atoms or molecules of the vapour or plasma of the material whose density or flux is being measured subsequently decay by characteristic photonic emission, these photonic emissions (flashes of light) are conveyed by the light guide 20 to a suitable photon detector (not shown) such as a photo-multiplier tube mounted outside the vacuum chamber of the deposition system. The detector either has an inherent narrow-band sensitivity or is fed through a suitable narrow-pass-band optical filter (not shown). If the detector output is suitably processed (e.g. in an external electronic circuit, not shown) to give a measurement of the instantaneous rate of photonic decay (assuming steady-state excitation conditions), this measurement will be proportional to the flux (mass flow per unit time) of deposition material in flight from source to substrate(s).By suitable calibration, the proportional measurement can be converted to an absolute flux measurement. The time integral of this measurement from commencement of deposition operation can give the deposition thickness or deposition mass per unit of substrate area, or some other appropriate parameter. Similarly, operation of the measurement apparatus in accordance with the measurement method but with the source inoperative can be used to measure the density of residual vacuum in the vacuum chamber, and in conjunction with suitable tailoring of the optical pass band, can be adapted to analyse the composition of the residual material in the near-vacuum of the evacuated vacuum chamber.

By way of example only, suitable dimensions of the illustrated embodiment of measuring apparatus in accordance with the invention are numerically denoted in Figs. 1-10 in millimetric units. While certain preferred materials for the various components of the measuring apparatus of Figs. 1-10 have been mentioned above, other suitable materials can be substituted therefor.

Other modifications and variations of the above-exemplified measuring apparatus and measuring method can be made without departing from the scope of the invention.

Claims (20)

1. A method for measuring the density or flux of a material in a near-vacuum, said method comprising the steps of exciting the material by injecting energy into an excitation region in which said material is present, and causing quanta of injected energy to make plural transits of said excitation region.

2. A method as claimed in Claim 1 wherein said quanta of injected energy are electrons.

3. A method as claims in Claim 2 wherein said electrons are preferably caused to make said plural trans its of said excitation region by applying an electrostatic field to said excitation region, said electrostatic field being such as to tend to cause an electron injected thereinto to travel on a path which oscillates across said excitation region.

4. A method as claimed in Claim 3 wherein said electrostatic field is preferably non-uniform in manner tending to destabilise the position of an electron located therein.

5. A method as claimed in any one Claims 2 to 4 wherein said excitation region is additionally or alternatively subjected to a magnetic field such as to cause electrons injected into said excitation region to make plural transits thereof, in an oscillatory path or in an orbital path, or in a combination of such paths.

6. A method as claimed in Claim 1 wherein said quanta of injected energy are photons.

7. A method as claimed in Claim 6 wherein said photons are preferably caused to make said plural trans its of said excitation region by at least partially bounding said excitation region with mirrors or other suitable forms of photonic reflector.

8. A method as claimed in Claim 7 wherein said mirror or other reflector forms have photon reflecting surfaces which are planar or curved.

9. A method for measuring the density or flux of a material in a near-vacuum, substantially as hereinbefore described with reference to the accompanying drawings.

10. Apparatus for measuring the density or flux of a material in a near-vacuum, said apparatus comprising an excitation chamber defining an excitation region -in which said material will be present in use of said apparatus if said material exists in measurable form, an energy source disposed to inject energy into said excitation region in use of said apparatus, and plural transit means to cause quanta of injected energy to make plural trans its of said excitation region in use of said apparatus.

11. Apparatus as claimed in Claim 10 wherein said excitation chamber comprises a material stream entrance and a material stream exit disposed to allow a stream of the said material in measurable form to pass through said excitation chamber and, in so doing, to traverse said excitation region.

12. Apparatus as claimed in either one of Claims 10 or 11 wherein said energy source is an electron source which comprises an electrically heated filament having a thoriated coating, said electron source being functionally associated with an anode to cause electrons from said electron source to be injected into said excitation region in use of said apparatus, said plural transit means comprising a pair of grids respectively disposed on mutually opposite sides of said excitation region with each said grid being aligned transverse the paths of electrons injected into said excitation region, both said grids being charged in use of said apparatus to similar or identical positive voltages to act as respective anodes each capable of passing electrons therethrough.

13. Apparatus as claimed in Claim 12 wherein one of said grids is closer to said filament or other electron source and constitutes said anode that causes electrons from said electron source to be injected into said excitation region, said filament or other electron source and said grids being maintained in use at requisite relative potentials.

14. Apparatus as claimed in any one of Claims 10 to 13 wherein said plural transit means additionally or alternatively comprises magnetic field applying means to apply a magnetic field to said excitation region in use of said apparatus whereby to cause injected electrons to make plural trans its of said excitation region.

15. Apparatus as claimed in Claim 10 wherein said energy source is a photon source.

16. Apparatus as claimed in Claim 15 wherein said plural transit means comprises mirrors or other suitable photon reflectors, which have planar or curved photon reflecting surfaces, said mirrors or other photon reflectors being disposed to cause photons injected into said excitation region in use of said apparatus to make plural trans its of said excitation region.

17. Apparatus as claimed in either of Claims 15 or 16 wherein said apparatus comprises a light channel associated with said excitation chamber to channel light from said excitation region to an external photon-sensitive detector whereby photonic emissions from decay of excited material may be externally detected and measured.

18. Apparatus for measuring the density or flux of a material in a near-vacuum, substantially as hereinbefore described with reference to the accompanying drawings.

19. A vacuum deposition system comprising a vacuum chamber containing a source of vapour or plasma of a material to be deposited and substrate mounting means to mount one or more substrates to receive said vapour or plasma to deposit said material on said substrate(s) in use of said system, and a measuring apparatus as claimed in any one of Claims 10 to 18 disposed in said vacuum chamber between said source and said substrate mounting means, said measuring apparatus being operated in use of said system in accordance with the measuring method as claimed in any one of Claims 1 to 9 to measure the density of said material in said vacuum chamber when in the form of a vapour or plasma and/or to measure the flux of said material as it passes in the form of a vapour or plasma from said source towards said substrate mounting means.

20. A material-coated substrate when produced by the system as claimed in Claim 17.