GB1568530A - Spectrophotometer - Google Patents

Description

(54) SPECTROPHOTOMETER (71) We, LE?BOLD-HERAEus- VERWALTUNG, GMBH (sole personally rewble partner of LEYBOLD-HERA- EUS GMBH & CO. KOMMANDIT GESELLSCHAFT), a German Company, of Bonner Strasse, 504, 5, Koln-Bayental, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be par ticularly described in and by the following statement::- The invention relates to spectrophotometer apparatus for measuring the optical properties of transparent and reflecting thin layers on objects as a function of the light wavelength by spectrophotometry, and in particular to such apparatus for measuring the optical properties during the formation of such thin the on the objects in vacuum installations, as well as for quality control.

A measuring arrangement which enables the absorption and transmission behaviour of transparent media to be measured at quite specific wavelength ranges is described in specific wavelengths or wavelength ranges is described in German Offenlegungsschrift No.

2521 934. For this purpose, a radiator that does not emit a continuous spectrum but has instead intensity peaks within several narrow wavelength ranges, such as for example a mer- cury lamp, is used as light source. In order to be able to operate the known measuring apparatus in only one wavelength region, different filters are placed in succession in the beam paths With this prior art arrangement it is not possible to investigate the broadband spectral behavior of optical bodies, and to carry oat for example quality control or production checks. Moreover, the prior art device has only one receiver, which generates the different measurement signals in time sequence and feeds them in succession to an analogue store.Quite apart from the fact that the spectral resolution is extremely coarse, the time sequencing of the measurement signals in a mechanical way, namely by means of a filter wheel, involves a time delay which is several orders of magnitude greater than in the case of a purely electronic evaluation. The known apparatus is unsuitable for regulating rapid processes, such as for example vacuum evaporation processes, since the measurement result lags considerably behind the process.

On the other hand, on account of the use of an analogue store neither can it be used to regulate extremely slow processes, such as for example the vapour deposition of thin plastics foils, which extends over several days, or for slowly occurring cathode evaporation processes.

U.S.A. Patent Specfflcation No. 3 973 118 discloses a diode arrangement and an analysis circuit for the spectrotnetry of various objects, but the analysis circuit can, however, only detect the absolute values of the individual diode outputs. Filters with different spectral permeabilities are connected to the individual diodes, a sensitivity adjustment at each diode being possible either by means adjusting the amplification of the outputs or the crosssection of the diode window (aperture). Also, the differing sensitivities of the individual diodes must be adjusted separately for each diode. Furthermore, in order to use this known apparatus it is necessary for the substrate to have dimensions corresponding at least to the principal dimensions of the diode arrangement.A remote measurement over a fairly long distance, such as for example by means of a collimated measurement light beam, is not possible with the given apparatus; its use is likewise prohibited in a vacuum evaporation apparatus since each condensation of absorbing media on the diode arrangement would falsify the measurement result.

In the production and/or quality control of optical products such as filters, mirrors and lenses, the optical properties often have to be measured and graphically represented as a function of the light wavelength, or processed by computer techniques. An example of this is blooming coatings, in particular for wide band blooming, which should have as small a reflection as possible within the visible light spectrum. Such coatings generally consist of many individual layers with different refractive indices (so-called interference layer systems). In manufacture, the time build-up of each individual layer must be monitored, and the maintenance of tolerance limits must be checked in the end product. A further example are filter layers, e.g. infra-red filters, which retain thermal radiation but should let visible light through in as unhindered a manner as possible.In this case too the spectral dependency of the layer properties must be meausred during the formation of the layers and in the final check.

A light source is required in order to carry out the measurement. The light from the light source has a characteristic spectrum which, if the intensity is plotted as a function of the wavelength, exhibits a maximum in the middle of the visible range, and falls away relatively strongly on both sides. Moreover, the curve alters with the operating time of the light source. It is not possible without taking additional measures to measure the optical properties of objects as a function of the light wavelength using such light sources, since the measurement distorts the curve, which is therefore no longer valid for the observer. In addition, reproducible values cannot be obtained.

Also known is spectrophotometric apparatus consisting of a light source emitting a continuous spectrum for producing a collimated measurement light beam, an object holder, a spectral light splitting device, a photoreceiver device and an electronic evaluation arrangement that forms the intensity signals of the photo-receiver device as a function of the light wavelength and contains an analogue digital converter for the signal conversion and a pulse generator for synchronising wavelength signals and intensity signals of the type in which the photo-receiver device con sists of a photo-diode that is subjected to a succession of individual sections of the spectrum by means of a mechanical, synchronised scanning device in the form of a slit rotating cylinder.In order to compensate for errors, a reference light beam must be used which does not strike the object to be measured. The differing intensity of the light source at various wavelengths must be compensated by a plurality of individually adjustable potentiometers. Quite apart from the relatively slow snanning of the spectrum, the potentio meter compensation is extremely difficult and timeconsuming. Several dozen potentiometers must be provided and adjusted to provide the possibility of a high resolution over the wave length range to be measured. This expen diture is unacceptable, and accordingly only a relatively coarse resolution of the spectrum is normally carried out. However, even in this connection there are difficulties since the potentiometer compensation has to be repeated from time to time on account of changes in the characteristics of the light source.

The object of the invention is to provide a spectrophotometry apparatus that will make a manual potentiometer compensation super lfluous and will allow a practically instantaneous reading of the measurement results.

In accordance with the invention there is provided spectrophotometry apparatus for measuring the optical properties of transparent and reflecting thin layers on objects as a function of light wavelength during the formation of such thin layers on the objects in vacuum installations or for quality control, comprising a light source for emitting a continuous spectrum for generating a collimated measurement light beam, an object holder, a spectral dispersion device a photo-receiver device consisting of a plurality of individual measuring cells arranged to receive light from the dispersion device in respective different spectral bands, a frequency generator, an analogue-digital converter for processing the outputs from the measuring cells in a sequence determined by the frequency generator to provide signals proportional to the intensity of the light received by the measuring cells, a direct access store, a digital divider device, means for supplying the output signals from the converter as desired to the direct access store for storing a set of intensity values therein and to an input of the digital divider device, means connecting the direct access store to a second input of the digital divider device for the digital divider device to form the quotient of stored intensity values from the direct access store and the output signals from the converter from each spectral band, the frequency generator serving also to gate the output of the digital divider device to an indicator circuit or to a computing circuit.

Such an apparatus has the following advantages: By virtue of the specified arrangement of the individual measuring cells, which are connected via a multiple cable to the analogue-digital converter, it is possible to scan or read the measurement values of the individual cells with an extremely high frequency of for example 16 kHz and repeat the display or response in a correspondingly frequent manner so that the momentary value of the measurement is available at any time.

A high spectral resolution from a manometer is made possible by the series arrangement of suitably small measuring cells, so that for example the two yellow mercury spectral lines can still be separated well. The measure ment can be evaluated by a simple visible check on a cathode ray oscillograph screen or in a connected computing machine, and enables complicated optical multi-layer systems to be obtained. In the digital divider device the quotient of the instantaneous spectral measurement result and the measurement values previously fed into the direct access store is constantly being formed, whereby it is possible to eliminate completely the effect of the spectra characteristics of the light source and thereby simulate a so-called ideal light source with constant intensity over the whole wavelength range.The resulting curve is in this case a true picture of the optical properties of the object over the desired spectral range. Such a method of operation is particularly suitable for process monitoring during the formation of thin layers on substrates in vacuo. On the other hand, it is also possible to input a specific spectral dependence of for example a special filter into the direct access store and compare further objects with the measured values. In this case only the deviations or fluctuations appear in the display on account of the quotient formation in the digital divider device. In the case of complete agreement with the store input, which for example can be created by a standard filter, a horizontal line is produced in the display. Such a method of operation is particularly suitable for quality control of end products.

The accanpanying drawing illustrates dia grarmnatically a preferred embodiment of the invention and will now be described in detail.

A light source 1 comprising an incandescent lamp is connected to a voltagestabilised current source, not shown, and emits a measurement light beam which passes in the direction of the arrows passes through an optical image formation system 2, a window 3, an object holder 4 with an object (not defined in more detail), and through a further window 5. The windows 3 and 5 and also the object holder 4 'are shown in conjunction with a vacuum evaporation device 6, which has a shutter 7 to interrupt or end the evaporation process, and an evaporation source 8.

The items 3 and 5 to 8 are not part of the invention but are shown to illustrate a particularly preferred application of the apparatus of the invention. For example, for quality control (as described hereinafter) it is possible to arrange the object holder 4 and object alone in the previously described part of the beam path. It is also not essential to detect that part of the measurement light beam passing through the object, but instead to detect that part of the measurement light beam reflected from the object, for which purpose the previously described part of the arrangement should be altered in accordance with known models.

After its passage through the windcw 5 the measurement light beam enters a light guide 9 and is passed thereby to a spectrograph 10 comprising a mirror 11, a spectral light splitting device 12 in the form of a diffraction grating and mirror, an inlet aperture 13 and a photreceiver device 14, which latter consists of a known series arrangement of measuring cells in the form of a diode array, particularly a cPo-called self-scanning, linear diode array with 256 diodes. The photo receiver device 14 is connected via a suitable multiple cable to an analogue-digital converter 16, in which the measurement values for the individual spectral regions, which are still analogue values, are converted into digital signals.The output from the analogue-digital converter 16 is connected in the position of a switch 17 shown by the full line to a direct access store 18 having a capacity of 256 times 16 bits, which for its part is connected to a digital divider device 19. The switch 17 can, however, be brought into the position 17a shown by the dotted line, so that the output from the analogue-digital converter 16 is directly connected to the digital divider device 19. The output from this digital divider device carries a socalled intensity signal and is fed either via a line 20 to a digital evaluation device, for example a computing circuit 29, or via a digital-analogue converter 22 and a line 23 to a video display unit, for example an oscilloscope.

The scanning frequency for the photoreceiver device 14 is preset by a frequency generator 15, whose output is also connected to a digital counter 24 for address control. The output from the digital counter 24 is connected to a permanent store 25 in which information for the individual wavelengths is stored.

This permanent store is connected either via a line 26 to a digital evaluation device, for example, to the computing circuit to which the line 20 leads, or via a digital-analogue converter 27 and a line 28 to the video display unit to which the line 23 leads. The lines so/26 and 23/28 in principle provide the coordinates for a visual display or computational processing of the signals.

The method of operation of the apparatus as illustrated is as follows: Before beginning a measurement the switch 17 is in the full line position as illustrated. The object holder 4 initially contains no object, with the result that the spectral characteristics of the measurement light beam emitted by the light source 1, which are altered only by the optical properties of the optical system 2, the win dows 3 and 5 and the light guide 9, are fed into the direct access store 18. The switch 17 is then brought into the position 17s, whereupon a horizontal straight line is initially produced in the display since the measured values agree with the values in the direct access store, and thus the quotient 1 is formed over the whole wavelength range.If an object having specific absorption characteristics is now inserted in the object holder 4, the quotient at the output of the digital divider device 19 alters since the intensity I differs from the intensity Io (in the absence of an object) as a function of the wavelength. A graphical rep resentation can be produced by means of an oscilloscope using the outputs of the digital analogue converters 22 and 27 and the signals on the lines 23 and 28, which reproduces the exact spectral characteristics of the object.

The measured value is completely free of the spectral characteristics of the light source 1, optical system 2, windows 3 and 5, light guide 9 and spectrograph 10 together with the photoreceiver device 14, and is thus error-free.

Even a change in the transmission factor of the windows 3 and 5, which is unavoidable in vacuum evaporation installations, can be com- pensated in a simple manner by moving the switch 17 into the position shown by the full line, whereupon information that takes account of the change intransmissibility of the windows 3 and 5 is stored in the direct access store 18.

On account of the quotient formation, the change in the transmissibility of the windows is again eliminated.

With the quality control described above, the vacuum evaporation device 6 with the associated equipment is not present. A reference object serving for calibration purposes is first inserted in the object holder 4 and with the switch 17 in the full line position the spectral characteristics of the reference object (together with the disturbance magnitudes) are now supplied to the direct access store 18.

The switch 17 is then moved into the position 17a. When now an object to be checked is placed in the object holder 4 instead of the reference object, there is no fluctuation at the output of the digital divider device 19 as long as the spectral characteristics of the object agree with those of the reference object. If, however, fluctuations are present, these fluctuations exclusively can be made visible at the output of the digital divider device 19, for example by being shown on an oscilloscope screen.

WHAT WE CLAIM IS: 1. Spectrophototetry apparatus for measuring the optical properties of transparent and reflecting thin layers on objects as a function of light wavelength during the formation of such thin layers on the objects in vacuum installations or for quality control, comprising a light source for emitting a continuous spectrum for generating a collimated measurement light beam, an object holder, a spectral dispersion device, a photo-receiver device consisting of a plurality of individual measuring cells arranged to receive light from the dispersion device in respective different spectral bands, a frequency generator, an analoguedigital converter for processing the outputs from the measuring cells in a sequence determined by the frequency generator to provide signals proportional to the intensity of the light received by the measuring cells, a direct access store, a digital divider device, means for supplying the output signals from the converter as desired to the direct access store for storing a set of intensity values therein and to an input of the digital divider device, means connecting the direct access store to a second input of the digital divider device for the digital divider device to form the quotient of stored intensity values from the direct access store and the output signals from the converter from each spectral band, the frequency generator serving also to gate the output of the digital divider device to an indicator circuit or to a computing circuit.

2. Spectrophotometry apparatus according to Claim 1, wherein the photo-receiver device is a diode array.

3. Spectrophotometry apparatus substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. on the lines 23 and 28, which reproduces the exact spectral characteristics of the object. The measured value is completely free of the spectral characteristics of the light source 1, optical system 2, windows 3 and 5, light guide 9 and spectrograph 10 together with the photoreceiver device 14, and is thus error-free. Even a change in the transmission factor of the windows 3 and 5, which is unavoidable in vacuum evaporation installations, can be com- pensated in a simple manner by moving the switch 17 into the position shown by the full line, whereupon information that takes account of the change intransmissibility of the windows 3 and 5 is stored in the direct access store 18. On account of the quotient formation, the change in the transmissibility of the windows is again eliminated. With the quality control described above, the vacuum evaporation device 6 with the associated equipment is not present. A reference object serving for calibration purposes is first inserted in the object holder 4 and with the switch 17 in the full line position the spectral characteristics of the reference object (together with the disturbance magnitudes) are now supplied to the direct access store 18. The switch 17 is then moved into the position 17a. When now an object to be checked is placed in the object holder 4 instead of the reference object, there is no fluctuation at the output of the digital divider device 19 as long as the spectral characteristics of the object agree with those of the reference object. If, however, fluctuations are present, these fluctuations exclusively can be made visible at the output of the digital divider device 19, for example by being shown on an oscilloscope screen. WHAT WE CLAIM IS:

1. Spectrophototetry apparatus for measuring the optical properties of transparent and reflecting thin layers on objects as a function of light wavelength during the formation of such thin layers on the objects in vacuum installations or for quality control, comprising a light source for emitting a continuous spectrum for generating a collimated measurement light beam, an object holder, a spectral dispersion device, a photo-receiver device consisting of a plurality of individual measuring cells arranged to receive light from the dispersion device in respective different spectral bands, a frequency generator, an analoguedigital converter for processing the outputs from the measuring cells in a sequence determined by the frequency generator to provide signals proportional to the intensity of the light received by the measuring cells, a direct access store, a digital divider device, means for supplying the output signals from the converter as desired to the direct access store for storing a set of intensity values therein and to an input of the digital divider device, means connecting the direct access store to a second input of the digital divider device for the digital divider device to form the quotient of stored intensity values from the direct access store and the output signals from the converter from each spectral band, the frequency generator serving also to gate the output of the digital divider device to an indicator circuit or to a computing circuit.

2. Spectrophotometry apparatus according to Claim 1, wherein the photo-receiver device is a diode array.

3. Spectrophotometry apparatus substantially as hereinbefore described with reference to the accompanying drawings.