WO1996025645A1

WO1996025645A1 - Device for determining the depths of coloured layers on transparent substrates

Info

Publication number: WO1996025645A1
Application number: PCT/DE1996/000279
Authority: WO
Inventors: Peter Kohns; Jörg KIECKHÄFER
Original assignee: Wissenschaftlich-Technisches Optikzentrum Nordrhein-Westfalen (Optikzentrum Nrw) E.V.
Priority date: 1995-02-16
Filing date: 1996-02-16
Publication date: 1996-08-22
Also published as: DE29502560U1

Abstract

The invention concerns a device for determining layer depths of coloured layers on transparent substrates by measuring the attenuation of a light beam as it passes through a given layer: monochromatic light sources of known wavelengths, the number (n + 1) of which is one greater than the number (n) of the layer depths to be determined, can be directed alternately at an optical sensor element in such a way that at any given time, only light of one wavelength (μi) falls on the sensor. By comparing the transmitted power of light of different wavelengths (μi) passing through the substrate and layer(s) with the power of light falling directly on the optical sensor element without undergoing such absorption, it is possible to determine the depths of the individual layers (di).

Description

Device for determining the layer thickness of colored layers on transparent substrates

The invention relates to a device according to the preamble of claim 1. In particular, the application of thin colored layers of paint is to be monitored. When measuring the applied layer thickness, a rapid acquisition of the measured value is important in order to also be able to act quickly on the production process.

In addition to the ellipsometric measurement of thin layers, photothermal determination is also used for such measurement tasks.

However, these methods require a great deal of effort and do not achieve the measuring speeds which are often required in order to keep the reject fraction low in mass production. In addition to these methods, it is also possible to utilize the absorbing effect of the layers and to determine the degree of absorption of light that passes through the layers and to use this as a measure of the thickness of the corresponding layer.

Sources of error that have a very negative influence on the measurement result are inhomogeneities in the substrate and contamination, such as Dust.

It is therefore an object of the invention to provide a device with a simple structure with which the thickness of colored layers can be determined in a short time with sufficient accuracy.

According to the invention, this object is achieved by the features mentioned in the characterizing part of claim 1. Advantageous embodiments and further developments result from the features contained in the subordinate claims.

When measuring the thicknesses of several applied layers, it is necessary to use a monochromatic light source more than the number n of layers. The wavelengths λ ,, ..., λ _{n + 1 must have} a certain sufficient distance from one another which depends on the absorption properties of the layers.

Using suitable optics, the light beams should be aligned so that they strike the same area of the test specimen at the same angle in order to rule out other sources of error. The individual light sources are switched so that one after the other which only impinges light of one wavelength on an optical receiving element and the transmitted light power is determined in each case. A distinction must be made here whether the light passes through the substrate and the applied layer (s) or not. If the light is radiated directly onto the optical receiving element, these measured values can be used as reference values, so that they are stored in an evaluation and control unit and are available as calibration values for determining the layer thickness or until further notice.

Accordingly, two monochromatic light sources that emit light with the wavelengths \ _x and λ _{2 are} required to determine the thickness of an applied layer. The light is absorbed differently by the layer. The light emitted from, for example, LEDs or laser diodes is focused using suitable optics and directed onto the surface to be measured using suitable means, for example mirrors. In this case, a highly reflecting and a dichroic mirror can be used to combine and align the two measuring beams.

The performance of both light sources is influenced by an evaluation and control unit such that their light is directed alternately in the direction of the optical receiving element and the transmitted light output is determined. The evaluation and control unit monitors which of the two wavelengths λ or λ ₂ strikes the optical receiving element and whether or not a test specimen was in the beam path. Thus, four different additives are used for the subsequent determination of the layer thickness. Stand sizes made available that can be processed computationally.

The respective measurement signal can be amplified with an amplifier and, after an analog-to-digital conversion in the evaluation and control unit, can be subjected to a calculation process or a comparison with calibration values stored there. The thickness of the layer in this case can be recalculated or the measured values can be calculated with values in

Calibration tables are compared so as to enable an assignment to a specific layer thickness.

To avoid errors caused by fluctuations in the light outputs of the individual light sources, these are monitored with reference detectors. For this purpose, the monitor photodiodes integrated in them can advantageously be used when using laser diodes.

The light emitted by the light sources is passed through optics to focus the beam so that it is parallel to the sample or focused on the substrate.

In order to reduce the influences that scattered light can exert, the optical receiving element is surrounded by black shielding surfaces. If this is not sufficient, this influence, which reduces the measuring accuracy, can be further restricted by emitting the light in a frequency-modulated manner and a frequency-adapted filter frequency-selecting the measuring signal. The invention is to be described in more detail below using exemplary embodiments.

Show:

Figure 1 shows the block diagram of an inventive device for determining the thickness of an applied layer;

FIG. 2 shows a further exemplary embodiment with a possibility of compensating for digitization defects and

Figure 3 shows an embodiment with a frequency-modulated light output.

The device shown in FIG. 1 for determining the thickness of a layer applied to a substrate has a sensor head 1 in which the elements for generating and detecting the measurement signals are accommodated. There are two light sources 2, 3 for generating monochromatic light which is absorbed differently by the layer to be measured. The light sources 2 and 3 can be switched on and off via driver circuits 10 and 11, which are again influenced by the control and evaluation unit, in such a way that either light with the wavelength λ or the wavelength λ _{2 is} emitted becomes. The control signals pass through the lines 19 and 20 to the driver circuits 10 and 11. The light emerging from the light sources 2 and 3 is focused with the focusing optics 4 and 5 and with the mirrors 15 and 16 onto the optical receiving element 8 directed. The mirror 16 is highly reflective and the mirror 15 is dichroic. The mirror 15 enables both beams to be aligned identically and both light beams to be same surface area of the test specimen 13, which can be inserted into the beam path, and the optical receiving element 8. This ensures comparable measurement conditions.

The signal detected by the optical receiving element 8 is amplified in an amplifier 9 and fed to the evaluation and control unit 7 via line 18, after which the measurement signals have been digitized in an A / D converter 6.

The evaluation and control unit 7 is also provided with a signal from a sensor 12, which indicates whether the light beam hits the optical receiving element (photodiode) 8 unhindered or through an absorbing test specimen 13, and thus a division into four different states is possible makes;

- Measurement without substrate, λ, on and λ ₂ off;

Measurement without substrate, λ _j off and λ _{2 on} ;

Measurement with substrate, λ, on and λ ₂ off;

Measurement with substrate, λ _j off and λ _{2 on} .

This sequence cannot be strictly adhered to, it is sufficient to carry out the measurements without a substrate at larger intervals and to keep the measured values available for this purpose in the evaluation and control unit 7. Exceptions to this are only necessary if there are fluctuations in the light outputs of the light sources 2, 3 or when the device is switched on.

The evaluation and control unit 7 can assign the measured values to one of these four states at any time. According to the Lambert-Beer's law, the voltages U _j , U ₂ , U ₃ and U ₄ , which correspond to the power of the light on the detector 8, can be determined according to the following equations:

U, = K * I _j = K * P ₁ * E, * (1-A) (I)

U ₂ = K * I ₂ = K * P ₂ * E ₂ * (1-A) (II)

U ₃ = K * I ₃ = K * P ₁ * E ₁ * (lA) * (lB) * exp (-d / μ (λ ₁ )) (III)

U ₄ = K * I ₄ = K * P ₂ * E ₂ * (lA) * (lB) * exp (-d / μ (λ ₂ )) (IV)

Then in the evaluation and control unit 7

W = (U ₂ * U ₃ ) / (U, * U ₄ ) = exp [d * (l / μ (λ ₂ ) -l / μ (λ,)] (V)

and from this the thickness of the applied layer with d = In W / (l / μ (λ ₂ ) - 1 / μ (λ,)) (VI)

certainly.

U_, U ₂ , U ₃ and U _{4 are} the output voltages of the amplifier for the four states; I and I ₂ are the currents which occur at the optical receiving element 8 under direct irradiation, ie without test specimen 13; I ₃ and I ₄ are the currents that occur at the optical receiving element 8 when illuminated by a test specimen 13, each for the light source 2 or 3. P _[ or P ₂ stand for the light powers emitted by the light sources 2 and 3 (in watts ), measured at the outlet from the sensor head 1. The sensitivity of the optical receiving element 8 is designated E] and E ₂ (ampere / watt) for the two wavelengths. K is the amplification factor of the amplifier 9 in volts / amperes. A takes into account the absorption of light by dust or dirt in the light path outside the substrate and is assumed to be constant in the spectral range under consideration. The losses due to reflection and scatter within the substrate, which have no influence on the layer thickness signal to be determined, are taken into account with the factor B. Both factors are dimensionless. The wavelength-dependent 1 / e absorption length of the colored layer is μ (λ) and is given in meters. When selecting the light sources, make sure that μ (λ,) is not equal to μ (λ ₂ ).

The greater the difference, the more precisely it can be measured.

The determined layer thickness value d can be reproduced on the display 21 for manual monitoring and / or can be used via an interface 14 for automatic process control. In this case, the control signal can already be generated in the evaluation and control unit 7 and transmitted to the coating device via the interface 14 in order to influence it in the required form as quickly as possible.

When determining layers with greater thickness and also with very different values for the wavelength-dependent 1 / e absorption length of the colored layer μ (λ _t ) and μ (λ ₂ ), the absorption of one of the two light beams with the wavelengths λ, or λ _{2 are} so large that an evaluation is no longer possible and one of the two values U ₃ or U ₄ can no longer be processed. The limit for this can already be set by the resolution of the A / D converter 6. In such a case, it is possible to work with a different gain factor for the measurement signals detected by the optical receiving element 8. This can be done by using a switchable amplifier, which is controlled by the evaluation and control unit 7, and by changing the amplification factor K, depending on the light source 2 or 3 that is switched on.

Another possibility for solving this problem is shown in FIG. Here, two different amplifiers 9a, 9b with the amplification factors K, (V / A) and K ₂ (dimensionless) are used. An analog switch 22 controlled by the evaluation and control unit 7 determines whether the

Measuring signal is amplified only by amplifier 9a or by both amplifiers 9a and 9b by establishing the connection between amplifier 9a or 9b to the A / D converter 6.

For example, the amplifier 9a in the states 1, 2 and 4 can be switched through to the converter 6. The amplifier 9b with an amplification factor K ₂ , which is set such that its voltage supplied to the A / D converter 6 can also be easily digitized, can also be effective in state 3.

It is expedient if the gain factor K _{2 is} kept constant over a period of time. However, it is also possible to carry out reference measurements in order to monitor the factor K ₂ . A defined voltage is applied to the amplifier input of 9b and then the output voltage of this amplifier 9b is determined.

The measured values and equations that can be used with a device according to FIG. 2 are reproduced below:

U ₂ = K, * I ₂ = K, * P ₂ * E ₂ * (1-A) (II ')

U ₃ = K, * K ₂ * I ₃ = K, * K ₂ * P, * E, * (lA) * (lB) * exp (-d / μ (λ ₁ )) (III ')

U ₄ = K, * I ₄ = K, * P ₂ * E ₂ * (lA) * (lB) * exp (-d / μ (λ ₂ )) (IV)

W = (U, * U ₃ ) / {(U, * U ₄ ) * K ₂ } = exp {d * (l / μ (λ ₂ ) -l / μ (λ,)} (V)

and d = In W / (l / μ (λ ₂ ) - 1 / μ (λ,)) (VI ')

With the device shown as a block diagram in FIG. 3, the influence of stray light or disturbing background light can be suppressed. The light emitted by the light sources 2 and 3 is clocked at a frequency in the kHz or MHz range. The clock frequency specifies an oscillator 24 connected to the driver circuits 10 and 11. If one of the two light sources 2 or 3 is switched on by the driver circuits 10 or 11, light which is modulated with the oscillator frequency is emitted. Suitable light sources are to be used for this. In this case, for example with the aid of reference detectors, it is not the average power of the light sources, but the amplitude of the modulation that must be kept constant. The optical receiving element 8 is followed by a narrow-band electronic filter 23, in which the transmission is greatest at the oscillator frequency. This prevents the low-frequency light caused by daylight or room light

Has an influence on the measurement signal, since the voltage values caused thereby cannot pass the filter 23.

The filtered measurement signal can then be evaluated in the form already described.

To understand the solution according to the invention, the description of the examples only applied to determining the thickness of only one

Shift turned off. However, the invention is not limited to only one layer, but can also be used for several layers. For this, at least one monochromatic light source must be used more than the number of layers n to be measured. Otherwise, the procedure is as described above. It only complicates the control of the light sources and the evaluation, since several light sources are switched on in a controlled manner and the corresponding measurement signals have to be assigned for the evaluation.

Here too, the measurements are carried out once with and once without a test specimen 13 for each emitted wavelength λ _| , where i reaches at least n + 1 values.

The equations apply once for the state without substrate and once for the state with substrate:

U _iι0hne = K * Pi * Ei * (lA) (VII) Ui.mi _t = K * P _i * E _i * (lA) * (lB) * exp [-d ₁ / μ ₁ (λ _i ) -d ₂ / μ ₂ (λ _i )

...- d _n / μ _n (λ _i )] (VIII)

P _; the powers of the individual light sources, E _j the detector sensitivities with respect to (i = 1, ..., n + 1), d: the thickness of the jth layer and μ: (λ ^ the 1 / e absorption length of the layer j at the wavelength λj (j = 1, ..., n).

The evaluation and control unit 7 carries out the assignment of the individual measurement signals, and values Q _k for k = 1,..., N are formed.

Q _k = in ((U _{n + lιθhne} * U _kmit ) / (U _{n + l with} * U _{k> without} ))

= d, [l / μ, (λ _{Il +} ,) - l / μ ₁ (λ _k )] + d ₂ [l / μ ₂ (λ _{n + 1} ) -l / μ, (λ _k )] + .. . + d _n [l / μ _n (λ _{n + I} ) -l / μ _n (λ _k )] (IX)

The coefficients in the square brackets are layer constants independent of the layer thickness and with the n equations the layer thicknesses d ,, ..., d _n for n layers can be calculated.

Claims

claims

1. Device for determining the layer thickness of colored layers on transparent substrates by determining the attenuation of a light beam as it passes through the layer, so that an increase in the number of layers (s) to be determined increases by one compared to the number of thicknesses to be determined

Number (n + 1) of monochromatic light sources (2, 3) of known wavelengths, so that an optical receiving element (8) can be alternately directed so that only light of one wavelength (λi) is incident at any time and by comparing the transmitted light outputs of the light with different wavelengths () when passing through the substrate (13) and the layer (s) and without such absorption with direct impact on the optical receiving element (8)

Thickness of the individual layers (d _j ) can be determined.

2. Device according to claim 1, characterized in that the monochromatic light sources

(2, 3) can be switched on and off alternately via a control device.

3. Apparatus according to claim 1 and 2, characterized in that the light beams of the monochromatic light sources (2, 3) impinge on the test object (13) with the same angle of incidence at the same point.

4. Device according to one of claims 1 to 3, characterized in that the signals determined by the optical receiving element (8) are amplified, digitized and can be fed to an evaluation and control unit (7) for carrying out the comparison.

5. Device according to one of claims 1 to 4, characterized in that the light sources (2, 3) are laser diodes.

6. Device according to one of claims 1 to 4, characterized in that the light sources (2, 3) are LEDs.

Device according to one of Claims 1 to 6, characterized in that the light sources (2, 3) can be switched via driver circuits (10, 11) which can be controlled by a control unit (7).

8. Device according to one of claims 1 to 7, characterized in that the focused beams of the light sources (2, 3) by means of mirrors (15, 16) are directed to the optical receiving element (8).

9. Device according to one of claims 1 to 8, characterized in that at least one dichroic mirror (15) for combining and aligning two different light beams is present.

10. Device according to one of claims 1 to 9, characterized in that at least one sensor (12) for detecting whether a test specimen (13) is in the beam path or not is present.

11. Device according to one of claims 1 to 10, characterized in that at least one amplifier (9) amplifies the measurement signal of the optical receiving element (8).

12. Device according to one of claims l to 11, characterized in that the amplifier (9) has switchable amplification stages.

13. Device according to one of claims 1 to 11, characterized in that the evaluation and control unit (7) switches two amplifiers (9a, 9b), which selectively amplify the measurement signals of the optical receiving element.

14. Device according to one of claims 1 to 13, characterized in that an oscillator (24) modulates the power of the emitted light from the light sources (2, 3) with a frequency and the optical receiving element (8) an electronic filter (23rd ) is connected downstream, the transmission of which is maximum at the oscillator frequency.

15. The device according to one of claims 1 to 14, characterized in that the light emissions of the light sources (2, 3) can be monitored by means of reference detectors.