Device to determine the thickness of a conductive layer

The device comprises at least a measuring head (7) with a transmitter (7A) and a receiver (7B). The head comprises means to measure the thickness through surface resistivity and optical means to measure said thickness by measuring the transparency of the substrate and of the relative layer applied to it.

TECHNICAL FIELD

The present invention relates to a device for contact free measurement of the thickness of a layer of material, particularly conductive material, such as a metal or the like, deposited with a vacuum deposition procedure on a substrate or medium, such as a sheet of paper, a plastic film or the like.

STATE OF THE ART

One of the most urgent requirements in the plastic film and paper metallization industry is the need to determine the quality of the treatment in terms of uniformity of the coating, that is the layer deposited. In the case of films which are transparent and have uniform coloring, the most commonly used method to determine this coating is by measuring the transparency. In fact, the thicker the coating, the more opaque the treated material will be. The sizes of interest are therefore “transmittance” and “optical density”. The equipment normally used is composed of a system of photometer heads. The photometers can be used in the case of treatment with metallic coatings on transparent and uniformly colored films as they work in the visible or in the infrared.

With regard to metallization on paper or on film that does not have uniform coloring, such as a preprinted film, however, it is not possible to use an optical system to detect the thickness of the coating. In this case the trend of the surface electric resistance may be used as an indicative parameter of the quality of metallization, as the thicker the coating, the lower the surface resistance is. The parameter of reference is also the surface resistivity of the film in some specific applications, such as films for the capacitor industry.

The method for contact free measuring of surface resistivity is to make use of the attenuation which a radiofrequency electromagnetic field, emitted by an emitter, undergoes while passing through the film or other substrate or medium. The lower the surface resistance and, consequently, the thicker the coating, the greater the attenuation is. This attenuation depends on the intensity of the current induced in the metallized layer, said current causing a dispersion in power. The surface resistance may be determined in two different ways:by measuring with a receiver, positioned in front of the emitter on the opposite side in relation to the substrate, the field attenuated by the metallized medium. Examples of devices which operate with this method are described in U.S. Pat. No. 4,220,915;by measuring the power dispersed due to eddy currents in the metallized layer. Examples of devices based on this technology are described in IT-B-1229313, GB-B-1452417; GB-B-1108084.

OBJECTS AND SUMMARY OF THE INVENTION

The object of the present invention is to produce a measuring device which allows the thickness of the coating to be measured on a plurality of substrates or media.

In substance, according to the invention a device is provided, for contact free measurement of the thickness of a thin layer coated on a substrate, comprising at least a measuring head with a transmitter and a receiver, in which said head comprises means to measure said thickness through the surface resistivity and means to measure said thickness through optical measuring of the transparency of the medium and of the relative layer applied to it.

It is thus possible, with the same head or with a plurality of heads substantially equivalent to and side by side with one another, to perform measurements both by transparency and based on the surface resistivity of the metallized substrate, according to the type of material produced. According to a possible embodiment, the measurements according to the two techniques are alternate, in the sense that a central control unit will, on the basis of programming by the operator, activate either one or the other of said two measurement systems, according to requirements and in particular to the nature of the metallized substrate. However, the possibility of using the two measurement methods simultaneously is not excluded. According to yet another embodiment, the two measurement systems may in any case always be active and the control unit may visualize or in any case alternately use the measurements obtained with one or other of the two techniques.

According to a practical and preferred embodiment of the invention, the transmitter comprises a transmission coil, to produce an electromagnetic field, and an optical emitter positioned coaxially to said transmission coil, and the receiver comprises a receiver coil, to detect the electromagnetic field emitted by the transmission coil and an optical receiver positioned coaxially to the receiver coil, the substrate passing between said transmission coil and said receiver coil.

In this manner a particularly compact device and a head of modest size are obtained, in which the space inside the coils, which constitute the inductances forming the transmitting and receiving antennae to produce and receive the electromagnetic field, is exploited to house optical emission and receiving means.

In a practical embodiment the transmission coil and the receiving coil are each wound on a respective bobbin, made of plastic or in any case non-ferromagnetic material, inside which the optical emitter and the optical receiver are respectively positioned.

Further advantageous features and embodiments of the invention are indicated in the attached dependent claims.

FIG. 1generically indicates a system for vacuum metallization of a continuous substrate, for example a plastic film or a paper strip, indicated with F. The numeral1generically indicates the vacuum chamber or bell jar, inside which vaporization sources3are positioned. The numeral5indicates a process roller, on which the film F to be metallized is driven, delivered from a bobbin B1and which, after having been metallized, is rewound on a bobbin B2.

Positioned along the path of the film F are various driving rollers and the heads to measure the thickness of metallization, indicated as a whole with7, are arranged in an appropriate position.FIG. 1shows a single head, it being understood that the others are aligned orthogonally to the plane of the figure, so as to cover the entire transverse extension of the film. The heads may also be arranged staggered in relation to one another in the direction of advance of the film F, to provide a greater number of measuring heads and hence obtain a more accurate measurement.

As can be seen inFIG. 2, each head7has two parts indicated with7A and7B which respectively house the optical and electromagnetic emission elements and the optical and electromagnetic receiving elements. In the diagram inFIG. 2the numerals9,10and11,12schematically indicate the electronic boards which house the circuit part of the head. Connected to the electronic board10is an inductance formed of a coil or solenoid13wound in an annular seat of a bobbin17mounted on the electronic board10. The inductance formed by the coil13is inserted in an electronic circuit which shall be described below with reference toFIG. 6.

The bobbin17has an axial through hole17F which ends with a conical flared part17C, facing the part7B of the head. Housed inside the axial hole17F is a piano-convex focusing lens18and positioned behind this is a light source19(not shown inFIG. 2), carried by the board10and constituted by a solid state laser with integral photodiode. This is fitted in a circuit which shall be described below with reference toFIG. 4.

Furthermore, the bobbin17has two longitudinal grooves diametrically opposite to clamp the electronic board9in position.

Electrically connected to the electronic board11of the part7B of the head7is an inductance constituted by a second coil or solenoid23, housed in an annular seat formed in a bobbin27fixed to the electronic board12and substantially equivalent to the bobbin17. In the same manner as this, the bobbin27is also provided with a through hole27F ending with a conical flare27C facing the part7A of the head. The inductance formed of the coil23is fitted in an electronic circuit which shall be described with reference toFIG. 7.

Housed inside the hole27F is a plano-convex convergent lens28positioned in front of a photosensitive element29, for example a photodiode, fixed on the board12and which receives radiation emitted from the source19which manages to pass through the film F with the relative metallization coating. The photosensitive element29is inserted in a circuit which shall be described in detail with reference toFIG. 5.

Analogously to the bobbin17, the bobbin27also has two diametrically opposite longitudinal grooves in which the board11is inserted.

With reference toFIG. 4, the solid state laser19forming the source is controlled by a circuit, indicated as a whole with31, which comprises an oscillator33and a programmable gain amplifier controlled by a microprocessor37. The signal of the oscillator33is amplified by the amplifier35and then transferred to a control buffer with automatic gain control, indicated with39, of the laser19. The circuit31also comprises a monitor photodiode41which continuatively reads the laser19emission and the signal of which is amplified by an amplifier43. The output signal of the amplifier43is filtered through a band pass filter45with a band centered on the frequency of the emission pulses of the laser19. This laser, in fact, is controlled in pulsed mode, for example at a frequency around 1 kHz, so as to obtain high peak powers respecting the dissipation limits of the device.

The use of the pulsed laser also makes it possible to reduce the effects of ambient luminosity and any offsets. In fact, if the photodiode29is not saturated the light pulses emitted by the laser19can be easily discriminated by the background and, moreover, the offsets and dark current of the photodiode29(FIG. 5) can be compensated.

The output signal from the band pass filter is sent to a peak detector47and the output signal from this is compared in a comparator49with a reference signal51. The output signal from the comparator49, appropriately amplified by an amplifier53, is used to regulate the gain of the automatic gain amplifier39of the laser19. The emission power is varied using the amplifier35and the change in emission is implemented when the signal received is excessively low or excessively high. In the first case emission is increased and in the second case it is decreased.

The circuit which contains the photodiode or other photosensitive element29, generically indicated with55and represented schematically inFIG. 5, comprises a programmable gain amplifier57, a band pass filter59, centered on the frequency of the pulses of the laser19, a peak detector61and a microprocessor63. The microprocessor63may vary the gain of the amplifier57if necessary in the event of the signal received being too low or too high.

The microprocessors37of the circuit31of the various measuring heads arranged in sequence along the transverse direction of the film F are connected to one another by a serial line, as are the processors63of the circuit55. The serial line connects the processors of various measuring heads to a central control unit, not shown. This is used to control the heads and to acquire the results of the reading. In particular, it is used to set the type of reading (optical or by detecting the surface resistance) to be implemented as a function of the type of substrate to which the coating is applied. Alternatively, the heads may constantly operate according to both types of reading and the control unit may be programmed to acquire and process information coming from only one of the operating modes, possibly displaying it or storing it in a suitable manner. Again alternatively, the central unit may be programmed to process both types of measurement simultaneously.

The photodiode29receives an optical signal inversely proportional to the thickness of the coating on the film F and this signal is used as a parameter to measure the thickness of the coating. Variations in the signal during transit of the film F in front of the measuring head are an index of an oscillation in the thickness of the coating.

Operation of the optical measuring means described above in the entire interval of optical density required means that the dynamic range of the signal is very wide. For example, an optical density variable from 0 to 4 implies a dynamic range of 1 to 10,000. This is very difficult to produce. The method used to overcome this difficulty consists in dividing the operating range into several intervals with a scale change system. The signal received by the photodiode29is compared with two thresholds preset by the microprocessor63. If the resulting value is above the high level, to avoid saturation of the receiver, amplification of the system must be reduced, while if it is below the lower threshold amplification must be increased to obtain a significant signal. Variation of total amplification is obtained both by acting on the amplifier57on the receiving side and on the power emitted, through the amplifier35. Indeed, if the signal received is very low, comparable with the background noise, amplification of the signal received through the amplifier57on its own has no advantages. In this case it is necessary to increase the signal-to-noise ratio. This is obtained by increasing the signal inciding on the photodiode29, that is by increasing the power emitted.

The control circuit of the electromagnetic field emission inductance13is schematically represented inFIG. 6and indicated as a whole with71. It comprises a radiofrequency generator73, a band pass filter with high selectivity75, ceramic or SAW, which receives the input signal produced by the generator73and the output of which is amplified by an automatic gain amplifier77. The energizing output signal from the latter is applied to the coil or inductance13. An emission control circuit is also provided to maintain electromagnetic emission of the inductance13constant.

The control circuit comprises a measuring circuit79of energization of the transmitter which produces a voltage signal proportional to the signal emitted by the inductance13. This is compared in a comparator81with a signal of reference83. The output of the comparator81, suitably amplified by an amplifier85, controls the gain of the amplifier77.

Moreover, the energizing signal is also processed by the microprocessor37, described with reference toFIG. 4.

Positioned on the part7B of the head7is a receiving circuit generically indicated with91, represented schematically inFIG. 7, to receive the electromagnetic signal which, emitted by the inductance13, passes through the metallized film or substrate F and reaches the inductance23. The circuit91comprises a differential amplifier93, which receives the incoming signal picked up by the inductance23and the output of which is connected to a band pass filter95with high selectivity, ceramic or SAW, the pass band of which, just as that of the band pass filter75of the circuit71, is centered on the emission frequency of the radiofrequency generator73. The pass signal passes through a diode97and a mean square value detector, indicated with99. The signal thus obtained is sent to the microprocessor63, already described with reference toFIG. 5.

It is understood that the drawing merely shows a practical example of the invention, which may vary in form and layout without however departing from the scope of the underlying concept of the invention.