Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/54—Controlling or regulating the coating process
  • C23C14/542—Controlling the film thickness or evaporation rate
  • C23C14/545—Controlling the film thickness or evaporation rate using measurement on deposited material
  • C23C14/546—Controlling the film thickness or evaporation rate using measurement on deposited material using crystal oscillators
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/54—Controlling or regulating the coating process
  • C23C14/542—Controlling the film thickness or evaporation rate
  • C23C14/545—Controlling the film thickness or evaporation rate using measurement on deposited material
  • C23C14/547—Controlling the film thickness or evaporation rate using measurement on deposited material using optical methods
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B11/00—Measuring arrangements characterised by the use of optical techniques
  • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
  • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
  • G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
  • G01B11/0683—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating measurement during deposition or removal of the layer
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B21/00—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant
  • G01B21/02—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness
  • G01B21/08—Measuring arrangements or details thereof, where the measuring technique is not covered by the other groups of this subclass, unspecified or not relevant for measuring length, width, or thickness for measuring thickness
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
  • G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
  • G01B7/06—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
  • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
  • G01B7/02—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness
  • G01B7/06—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness
  • G01B7/063—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness using piezoelectric resonators
  • G01B7/066—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring length, width or thickness for measuring thickness using piezoelectric resonators for measuring thickness of coating
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D5/00—Control of dimensions of material
  • G05D5/02—Control of dimensions of material of thickness, e.g. of rolled material
  • G05D5/03—Control of dimensions of material of thickness, e.g. of rolled material characterised by the use of electric means

Definitions

• THIN FILMS CONTROL OF DEPOSITION OF THIN FILMS
• This invention relates to a method and apparatus for determining and controlling the properties of a thin film during deposition of that film.
• the term "thin film” means a film less than about 20 micrometres thick, and usually about 0.1 micrometres thick, and the words 'optical' and 'light* mean electromagnetic radiation at infra-red, visible and ultra-violet wavelengths.
• a film of this thickness may be referred to as an optical interference film, because its optical thickness, that is the product of its geometrical thickness and its refractive index, is usually a fraction of the wavelength of light, but may also be up to about three times the magnitude of the wavelength.
• Optical interference effects may occur due to a phase difference introduced by the film between two light waves reflected respectively at the two film boundaries.
• Films of this thickness are often applied to a substrate to alter the reflection properties of the substrate, for example to glass to give an anti-reflecting surface at visible wavelengths, or to a semi-conducting material to give an anti-reflecting surface at Lnfra-red wavelengths; the films may also be applied to provide beam splitters and broad and narrow band-pass filters at ultra-violet, visible, and infra-red wavelengths, or to give mechanical protection to a surface or to impart electrical conductivity to it.
• Single layer films may be required, or multilayer films comprising films of different optical properties.
• the films may be applied by any physical vapour deposition (PVD) or chemical vapour deposition (CVD) process.
• a material is sputtered or evaporated; examples are d.c. sputtering, radio frequency sputtering, magnetron sputtering; electron beam evaporation; ion plating and glow discharge.
• a material is decomposed by heat, optionally into a plasma, and optionally an active gas may be added.
• an ionised atmosphere is generated in the vicinity of the substrate.
• Materials used vary over a very wide range, and may be elements or compounds, metals, semiconductors, or insulators.
• the present invention is applicable to the majority of such films and is largely independent of the material of the film or the method of application.
• apparatus for sensing and controlling the deposition of a thin film on to a substrate from a gas or vapour phase comprises sensing means arranged to sense the optical reflectance or optical transmittance or electrical resistivity of the deposited film and to provide a first signal having a value representative of the value of the.sensed property; an oscillatable crystal, usually a quartz crystal, arranged adjacent to the substrate with at least one surface of the crystal exposed to the gas or vapour phase; and timing and calculating means connected to the sensing means and to the oscillatable crystal and arranged to* determine the absolute resonance frequency of the oscillatable crystal, to provide a second signal representative of that frequency, to determine the quotient of the change in the first signal and the change in the second signal over a first predetermined time interval, and to provide an output signal dependent on said quotient.
• sensing means arranged to sense the optical reflectance or optical transmittance or electrical resistivity of the deposited film and to provide a first signal having a value representative of the value of the.sensed property
• the timing and calculating means may be means such as a computer or microprocessor having a built-in clock.
• the change in resonance frequency of the crystal may be determined by a measurement either of the absolute frequency or of the resonance period.
• the first predetermined time interval is conveniently
• the period of the oscillatable crystal which is typically 10 seconds.
• This interval may be a predetermined time interval chosen so that the averaged quotient is unaffected by temporal fluctuations in the deposition rate.
• it may be a variable time interval, corresponding to a predetermined increase in film thickness.
• the first signal is arranged to be in the form of a frequency signal, the value of frequency representing the value of optical reflectance or transmittance or electrical resistivity, and the second signal is the output of the
• the two frequency signals can then be digitised and the numbers used to calculate quotient or averaged quotient.
• a method of sensing and controlling the deposition of a thin film on to a substrate from a gas or vapour phase comprises: sensing the optical reflectance or optical transmittance or electrical resistivity of the deposited film and providing a first signal having a value representative of the value of the sensed property; arranging an oscillatable crystal adjacent the substrate with at least one surface of the crystal exposed to the gas or vapour phase; determining by timing and calculating means the absolute resonance frequency of the oscillatable crystal and deriving a second signal having a value representative of said resonance frequency; determining over a first predetermined interval of time the quotient of the change in the first signal and the change in the second signal; providing an output signal dependent on said quotient; and controlling the variable of the deposition process in accordance with the output signal.
• Figure 5 is a schematic sectional drawing of a reactive d.c. sputtering apparatus.
• Figure 6 which is a schematic sectional drawing of a magnetron sputtering apparatus.
• Figure 1 shows the theoretical variation of the reflectance R of a film deposited on a glass substrate of refractive index 1,5 as the film thickness increases.
• the abscissa could also be considered to indicate increase with time.
• a deposition run typically, a deposition run lasts between 2 and 10 or even 20 minutes.
• Figure 2 shows a typical curve obtained by monitoring film growth using a conventimnal beam photometer and plotting reflectance against time.
• the curve irregularities are due to several effects such as instabilities in the deposition rate; for example if a powder is evaporated, volatilization may occur as a series of discrete events as particles come into contact with the heat source; alternatively, during a sputtering , process, gas may be released from a target compound, which results in a change in ion density in the gas and a variation in deposition rate.
• instabilities cause changes in the mass growth rate, which affects the rate of increase of geometrical thickness, and may also have secondary effects on the optical thickness if the film density and refractive index are dependent on growth rate.
• a further contribution to curve irregularities may be made by instrument noise, and the height of the measured maxima may decrease with time due to changes in the refractive index of the deposited film and to absorption.
• FIG. 5 A first embodiment of apparatus which allows such a dif erentiation to be carried out with optical thickness measured to terms of film mass is shown in Figure 5»
• a conventional vacuum chamber 10 and its contents are shown in schematic form; a substrate 12 to be coated and a quartz crystal l4 are both exposed to a source of material to be deposited 16, which is connected to a d.c. power supply 18 to give reactive d.c. sputtering.
• the crystal l4 is driven by an oscillator 15 outside the vacuum chamber.
• the power supply l8 is controlled by a microprocessor 24.
• the chamber can be evacuated by a pump 20, and a gas can be supplied through an inlet pipe 22. If the source is a metal and an oxygen atmosphere is provided, then a metal oxide film will be deposited.
• the quartz crystal is connected directly to the micro ⁇ processor 24.
• a light source 26 illuminates the substrate 12 with a collimated beam through: a beam splitter 28; reflected light is received via the beam splitter by a reflectance monitor 0. which is connected to the microprocessor; the source 26 and monitor 30 are connected together and to the microprocessor so as to allow operation in synchronism in a pulse mode controlled by the microprocessor.
• the position of a transmittance monitor 32 is indicated by the dotted line.
• the monitors 30 and 3 each comprise a light detector such as a vacuum photocell or a solid state device together with the necessary power supplies.
• the sensor may provide a voltage or current of a magnitude representing the reflectance or trans ⁇ mittance of the coated substrate.
• the chamber 10 is evacuated and oxygen is supplied
• a pressure of between 10 and 10 torr, and a conventional reactive d.c. sputtering process is established in which material from the source 16 is deposited on both the substrate 12 and the quartz crystal l4, changing the resonant frequency of the crystal; the absolute resonant frequency of the crystal is sensed using the internal clock of the microprocessor 24.
• the reflectance of the film of material deposited on the substrate 12 is sensed by the jreflectance monitor, which converts it to a frequency signal, the first signal, and supplies it to the microprocessor.
• the microprocessor therefore has available two digitised signals, and is arranged to calculate the required quotient over the first time interval. Conveniently, this first predetermined time interval is the period of the oscillatable crystal, and the change in the frequency of the first signal over this time interval is easily determined from the digitised information.
• the average value of the quotient is supplied to a display unit 34 and may be used to supply information to an operator when the apparatus variables are controlled manually. Alternatively, in a fully automatic system, the output signal is used to control the d.c. power supply 18. Information relating to a required value or programme of values of the quotient may be stored in the microprocessor, and may include a value at which deposition is to be terminated.
• the output of reflectance monitor R is a dimensionless value equivalent to reflected signal/incident signal. If this value is expressed as a frequency, then any change in frequency corresponds to a change S in R.
• This second interval of time may be, for example, 0.1 seconds or may be as long as 5 minutes for unusually prolonged deposition processes.
• Information relating to a required value or programme of values of this averaged quotient may also be stored in the micro ⁇ processor, and the source controlled in accordance with the requirement.
• refractive index of a film during deposition is constant, but film materials are known for which refractive index increases or decreases and film density also changes with growth of the film. When such established changes result in a departure from a linear relationship between increase in mass and increase in optical thickness, this can be allowed for when determining the value of the quotient at which deposition is to terminate.
• a microprocessor If a film thickness corresponding to a known number of quarter wave ⁇ lengths is required, then the microprocessor can be programmed to count the maxima and minima in the reflectance curve, corresponding to ".the number of zero points in the differential, and to terminate deposition when the required thickness is reached.. Termination at a point between a maximum and a minimum is also possible.
• Another beneficial aspect of the present invention is illustrated when for some reason the source fails and deposition ceases; a differential with respect to time would then be zero, because there is no temporal change. But the differential with respect to optical thickness remains at a constant value.
• the microprocessor can be programmed to recognise this condition., to give an alarm signal if required, and, when growth begins again, to continue the calculation of differential from the held value.
• quartz crystals appropriately spaced in the vacuum chamber and each used in conjunction with an optical sensing instrument, can easily be incorporated. This would allow deposition of very uniform films, or films on curved surfaces.
• the several crystals need not all have the same basic resonance frequency.
• several sources of material far deposition can be controlled independently by the microprocessor, which can rapidly determine the actual film mass distribution from the monitor crystals during deposition, and. determine the independent source emission rates needed to provide a required mass distribution.
• the microprocessor can control other variables of the apparatus, such as electron beam current if an electron beam evaporation apparatus is used, or the target and substrate bias potentials in an r.f. sputtering system, or other variables as appropriate.
• Electrostatic and magnetic shields can provide some protection, but cannot exclude energetic neutrals which can charge the crystal surface on impact by releasing electrons.
• the microprocessor can be used to switch off the power supply to the exciting discharge for a very short period, such as microsecond or milli ⁇ second inrfcervals, and to make a measurement of crystal frequency during that period.
• the time can be short enough for gas absorption on the film surface to be negligible.
• pulsed measurements of thickness can be made at known time intervals so that mass growth rate of the film can be determined.
• OMPI - 1.6- invention deposition in the absence of the discharge can be for such a short period that the properties of the film are essentially not affected.
• the invention is particularly applicable to the deposition of materials in which refractive index changes with deposition because the structure changes.
• the true film thickness then differs from* that derived from nL assuming ri and film density are constant.
• examples of such materials are zirconium and aluminium; respective oxides of these metals are often used in multi-layer anti-reflection films.
• Use of the invention will show turning points as the film masses and the optical thickness changes combine to give maximum or minimum reflected energy.
• the invention is utilised in otherwise conventional apparatus for preparing multilayer interference • filters by magnetron sputtering.
• magnetron sputtering metals are sputtered in oxygen-containing atmospheres to deposit oxide films.
• a reduced pressure of between 10 -3 and 10-2 torr is used and a magnetic field is superimposed on the electrode arrangement so that the electrons are trapped in magnetic loops and follow cycloidal paths.
• Figure 6 shows a vacuum chamber 4 ⁇ having a central circular cathode support carried by a hollow stem 44
• the support 42 is water cooled and the stem has water inlet and outlet pipes 48, 50 *
• a circular arra of small magnets 52 is* arranged with their axes radial to the cathode support and their upper surfaces flush with the surface of the support; the loop-shaped magnetic fields are illustrated by the dotted lines.
• the support is partly shielded by a grounded metal shield 53•
• the chamber 40 can be evacuated by a pump 55-
• a silicon cathode 54 lies on the cathode support; the cathode has a central aperture.
• a similarly shaped titanium cathode 6 is held by a cathode-changing mechanism 8 operated from outside the vacuum_-chamber.
• the hollow stem.44 of the cathode support is closed at its lower end by a glass window 60 through which a monitoring light beam can pass from a source-62, through a collimating lens 64 up through the hollow stem 44 and through a glass substrate 66 supported (by means not shown) above the aluminium cathode 54.
• the light beam is then reflected by a plane mirror ⁇ through a wavelength-selecting filter 70 to a photocell 72.
• the electrical signal from the photocell passes through an amplifier 74 to a microprocessor 78, which controls a light chopper 80 which chops the beam from the source 62.
• Adjacent the substrate 66 is a high-frequency crystal 82 with a free surface aligned with the surface of the substrate.
• the crystal is connected through a crystal oscillator 84 to the micro- processor 78.
• the microprocessor also controls via a servo-motor controlled needle valve 86 the supply to the vacuum chamber 4 ⁇ of a mixture of argon and oxygen; is connected to the thyristor power control 88 placed between a power source 90 and a d.c. power supply 92 which is connected to the aluminium cathode 4 ⁇ and is connected to a visual display unit -94.
• a glow discharge is generated in the vacuum chamber between the cathode 5 and the earthed shield 53 or base ⁇ plate 46 and silicon is sputtered from the silicon cathode 4 to be deposited on the surfaces of the glass substrate 66 and the crystal 82 as a film 96 of silicon dioxide.
• the presence of the • magnetic field confines ion bombardment to a narrow annulus on the ' cathode. This allows a central, aperture through which the light beam sensing film transmittance can pass.
• the microprocessor 78 combines the quotient of the change in resonant frequency of the crystal 82 and the change in transmittance of the film 96, as in the Figure 5 embodiment, to terminate deposition at a required value.
• the cathode-changing mechanism 58 is then operated to remove the silicon cathode 54 and place titanium cathode 56 on the electrode support. Titanium oxide film is then deposited as required.
• the sputtering rate can vary for several reasons, even if the discharge current and applied potential are constant. This may occur because the surface condition of the cathode varies, because contaminants are formed on or removed from its surface, or the profile changes as etched tracks form.
• the derivative of the frequency signal from the crystal 82 can be used to adjust the glow discharge conditions so that the change of resonant frequency with time, and thus the change of film ma'ss with time, is constant. Either the needle valve 86 is adjusted to alter the gas pressure and therefore the glow discharge power input, or the thyristor 88 is adjusted to alter the electrical power input to the discharge.
• the thyristor which is a semiconductor rectifier which can be arranged to become conducting for different periods during an a.c. cycle, can conveniently be used to switch off the electrical power supply during periods when the crystal oscillator 84 is energised and frequency measurements are being made.
• the measuring crystal 82 is therefore not exposed o the intense electron bombardment characteristic of r.f. and d.c. sputtering systems operated in the non-magnetron mode.
• a crystal exposed to a magnetron discharge may reach a potential several volts negative with respect to earth unless it is kept earthed during the coating periods.
• the microprocessor 78 can be programmed to ground the crystal until a frequency measurement is required. Alternatively a separate switch 85 can be provided in the oscillator 84.
• microprocessor could also be averaged over a variable time interval, each interval corresponding to a predetermined change in the second signal, that is, a predetermined increase in film thickness.
• a computer or microcomputer can be used, or any other calculating device having an internal clock or timing device which can be used to measure the absolute oscillation frequency of the quartz crystal.

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• Engineering & Computer Science (AREA)
• Metallurgy (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Organic Chemistry (AREA)
• Crystallography & Structural Chemistry (AREA)
• Automation & Control Theory (AREA)
• Physical Vapour Deposition (AREA)
• Length Measuring Devices By Optical Means (AREA)

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• 1979
  • 1979-08-10 WO PCT/GB1979/000138 patent/WO1980000504A1/en unknown
  • 1979-08-10 CH CH303680A patent/CH634424A5/fr not_active IP Right Cessation
  • 1979-08-10 JP JP50122979A patent/JPS55500588A/ja active Pending
• 1980
  • 1980-04-18 US US06/192,524 patent/US4311725A/en not_active Expired - Lifetime

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