DD293891A5 - ARRANGEMENT FOR DETERMINING THE ABSORPTION AND LASER STRENGTH OF OPTICAL LAYERS - Google Patents

Abstract

The invention relates to an arrangement for determining the absorption and the laser resistance of optical layers, preferably the electrical layers. It can be used for testing of special optical layers and layer systems z. B. anti-reflection and mirror coating systems, layer polarizers and interference filters. The arrangement according to the invention comprises an oscillating quartz sensor element consisting of crystalline quartz and metal layer electrodes and a layer to be tested arranged in a specific manner thereon. Upon irradiation of the at least partially transparent arrangement with intense laser light, the induced thereby thermal and / or mechanical changes of the layers can be detected very accurately with immediate acquisition of digital signals. {Absorption; Laser strength; optical layers; dielectric layers; Quartz crystal sensor element, partially transparent; irradiation; thermal / mechanical changes}

Description

Title of the. ..I f i η you η g_

Arrangement for determining the absorption and laser resistance of optical layers

OQweD.dJrAn.tl- ⁵ Qe? Hlet the dun g OF INVENTION

The invention relates to an arrangement for determining the absorption and / or the laser resistance of optical, preferably dielectric layers. The field of application extends in the product areas of optics, optoelectronics, device construction to products in which low-absorption and / or laser-resistant layers with interference-optical properties are required, especially anti-reflection and mirror-coating systems, layer polarizers and interferometric filters. The arrangement according to the invention is particularly suitable for determining the absorption and / or the laser resistance of low-loss SchichL optical substances at all wavelengths of light for which powerful lasers or other intense Btrahlungsguellen are available, but especially for oxides and fluorides at the laser wavelength of 1.06 microns , In addition, the invention can be used in all fields of technology involving the determination of absorption and / or absorption-related properties of substances.

Characteristic of the known prior art de

The utility properties of optical layers and the optical layer devices fabricated therefrom are significantly determined by absorption and laser resistance. These properties have increasingly become utility-limiting factors, particularly in the design, manufacture and application of compact lasers with higher efficiency and performance. Low absorption optical

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In addition, layers are of general interest for high-performance layered construction elements.

For the determination of the absorption of optical media, the change of radiation fields or the change of properties of the media after interaction with radiation fields is generally analyzed. In a broader sense, these radiation fields include the gessainte spectrum of electromagnetic and particle radiation, 5D that the absorption analysis in the general case both optical measuring methods including the laser desorption and surface analysis methods 2.B. comprising electrons and ions.

With the use of light, absorption analysis in the narrower sense encompasses the optical and photothermal absorption measurement methods, of which the photothermal detection techniques are of particular interest for determining extremely small layer absorptions [JM Bennett, Optics News 7 (1985) 17, JM Etennett, Thin Solid Films, 123 (1985) ¹ Z '/ "], which are used in routine operation for absorption in the egg range 10 ~". , , 10 at a

i have 1 proven. In contrast to the optical measuring methods, which indicate the extinction and further properties of the medium interacting with the radiation field, in the photothermal method only the energy fraction of the incident beam field dissipated via absorption processes is measured. At the present state of technology of optical layers, sample heating is determined primarily by linear absorption. The photothermal absorption measuring techniques are classified according to the way of proving the sample heating (temperature, deformation, etc.), which is currently measured. can also be local, deep and time resolved

 A long-known photothermal measuring method is laser calorimetry, in which an intense laser beam is incident on the sample to be measured, the absorption-related heating is then measured with suitable thermoelectric transducers or as optical detuning of an interferometer. The evaluation can be done in different ways. Frequently, the absorption from the rate of change in temperature at constant irradiation into a thermally isolated sample (rate method) ΓΜ. Braunstein, J.E. Rudisill, J.A. Harrington, Appl.

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Opt. 16 (1977) 2843; E. Welsch, G. Lieder, HG Walther, E. Hacker, Thin Solid Films 91 (1982) 32, or determined by Hesjunq the temperature of the sample environment, which is adjusted to the sample temperature, so that no heat is released to the environment ( adiabatic method) [R. Atkinson, Appl. Opt. 24 (1985) 46411. Another method for determining the absorption is based on the IQQO of AG Bell first described photoacoustic effect, in which the sample is irradiated with time-modulated light. The absorption-based sample heating modulates the pressure in a connected gas volume having an amplitude which is Ma (3 for sample absorption [EL Kerr, Appl. Opt. 12 (1973) 2520; i. The necessity of connecting a gas volume leads to considerable Restrictions on the applicability of this method

 Another known method of determining the absorption of layers is photothermal radiometry, which measures the change in the radiant flux of a sample as a result of an absorption-induced temperature change, with a linear relationship in the region of low absorption. However, the disadvantage here is the layer material dependency of the emissivity CV.N. Lopatkin, Kvant. Elektron., 12 (1985) 339; SO. Kanstad, P.E. Nordal, Can. J. Phys. 64 (1986) 1159]

 Another known method for determining the absorption of surfaces and layers is the photothermal beam deflection (Mirage effect), which was first developed by A.C. Doccara, D. Fournier and J. E <adoz in the journal Appl. Phys. Lett. 36 (1930) 130. The method is based on the probing of time-varying refractive index fluctuations in a gas space adjacent to an absorbing sample. By a temporally modulated irradiation (heating jet), a local surface temperature change is induced on an absorbing sample, which also generates a time-variable temperature and thus refractive index fluctuation in the adjacent gas space. The probing is done with a jet stream in which the refractive index fluctuations cause phase front deformations that lead to time-varying beam deflections or divergence changes. A disadvantage of this method is the required high adjustment and focusing accuracy of heating and Me (3

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beam as well as the complicated signal interpretation

 Similarly, in the photothermal surface deformation (PTD) process, the thermoplastic sample deformation produced by the local preamble heating is used for signal recovery, which is accomplished by deflecting a reflected measuring beam CM.A. Ulmstead, N.M. Amer, S. Kohn, Appl. Phys. A 32 (1983) 14iJ or by adapted piezoelectric transducers. In addition to the already in the photothermal. Distortion, the signal amplitude in the PTD-I'lessung of layers is determined by the thermoe.Lastischen properties of the substrate, which is a serious shortcoming of this method

 In order to determine the laser strength, the erosion coefficients under laser irradiation are generally determined, which lead to irreversible changes in optical media. The laser resistance is highly dependent on the irradiation conditions and the detection techniques, so that the previously known methods differ greatly in terms of sensitivity and resolution. Universally applicable or generally accepted methods do not exist.

Irrespective of the conditions of the irradiation, a light or electron microscopic inspection is frequently used to detect changes in crystal size [A.A. Poplawski, GJ.P. Tichomiruv, T.S. Turovskaya, Sov. J. Techn. Fiz. 17 (1972) 1462] or catastrophic destructions and their morphology TW.H. Lowdermilk, D. Milam, F. Rainer, Thin Solid Films 73 (1980) 155; W.U. Lowdermilk, D. Mil., Appl. Phys. Lett., 36 (1980) 891; H.E. Dennett, A.T. Glass, A.H. Guenther, B.E. Newnam, Appl. Opt., 19 (1980) 2373] made. Other known detection methods relate to the registration of such efferent phenomena of laser-induced destruction, such as e.g. Plasnafunken [B.E. Newnam, L.6. De Shazer, NBS Spec. Publ. , 372 (1972) 123], diffusion CCW. Draper, L. Buene, J.M. Poate, D.C. Jacobson Appl. Opt. 20 (1931) 1730], change of absorbance CH.Fi. Bennett, A.T. Glass, A.H. Guenther, B.E. Newnam, Appl. Opt., 19 (1980) 2373; P.A. Temple, NBS Spec. Publ. 568 (1980) 333], light scattering [A.T. Glass, A.H. Guenther, Λρρί. Opt. 15 (1976) 1514] or transmission [A. Yes. Kuznetsov, I.S. Warna-

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Sheva, A.A. Poplagski, G.P. Tikhomirov, Sov. Üpt. Mech. Prom. 3 (1972) 39].

It is also tau fqt? solved the procedure known to you; on changes in the reflection behavior with respect to the laser beam or a guide beam, the scattered light b: -: w. the 'sparks are based CR.H. Picard, D. Milam, R.A. Bradbury NDS Spec. Publ., 435 (.1.97S) 272; T.W. Walker, A.N. Guenther, P.E. Nielsen, NBG Spec. Publ., 541 (1978) 226; N. Alyassini, J.H. Parks, NBS Spec. Publ., 43Li (1975) 284]. Also, the change in the photoacoustic signal produced in the irradiation source upon reaching the destruction intensity Ig has already been proposed for the purpose of determining the laser strength ΓΑ. Ra :: ancwaig, J.B. Willis, Appl. Phys. Lett., 36 (1980) 66 /; A. Rosencwaiq, L.S. Bacigalupi, J.B. Willis, Appi. Opt., 19 (1980) 4133]. In general, the methods mentioned are technically complex and grain interpretation with respect to the signal interpretation.

L UaL ^c Jl? E

The aim of the invention is to determine the absorption and laser stability of optical, preferably dielectric, layers with a uniform, sensitive detection method.

jd £? 5_Wesens the Erf_indunq

The invention has for its object to develop a uniform arrangement for determining the absorption and the laser resistance of optical, preferably dielectric layers.

The arrangement should be simple, avoid high adjustment effort and allow a simple Siqnalinterpretation

 This object is achieved with the arrangement for determining the absorption and the laser strength of optical

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Z 93

Layers or layer systems by means of laser light and sensors are achieved in that the sensor learns a scanning crystal sensor consisting of crystalline silicon and metal oxide layers, that the layer to be tested or the layer system on the quartz crystal is Sensor element is arranged and both are at least tt.il transparent with respect to the laser light. In this case, the Schwingquars Sensoreleroent depending on the specific Hesssituation a temperature-dependent frequency (AT-section) or a temperature-independent frequency (HT section.) Have.

By arranging the layers to be examined on the quartz crystal sensor element, it is possible to detect the induced during irradiation with intense laser light thermal and / or mechanical changes of the layers very accurately with immediate acquisition of digital signals. If the applied layers show absorption, a temperature increase occurs, which is also transmitted to the quartz crystal, which then changes its oscillation frequency based on the piezoelectric effect. Since laser destruction processes involve complex changes of mechanical properties, such as e.g. The elastic properties and crack and crater formation in unit are associated with additional Schallereiqnissen, these may also be selectively detected if necessary.

It is of subordinate importance whether the vibrating quartz sensor elements are of the Thick-Shear type (thickness oscillator 300 kHz, ... JLOO Hz) or strain and surface shear oscillator (bending oscillator 1 ... 50 kHz ). Experimentally and methodically essential, however, is the angle of intersection of the quartz, which determines the temperature dependence of the oscillation frequency.

The temperature response of the linear temperature coefficient a has a zero in the so-called AT-section, so that the frequency of the quartz f (T) = f ₍ -, (i · η α T) is practically no longer dependent on the temperature Therefore, in order to determine the thermal changes produced by absorption, the HT cut is advantageous because it has a maximum linear temperature

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1.93 * 94

koef f is it? n tone α of \ ~ \ round ·) -90 · 10ό-6α Kö-iö ii.efert, which is a frequency change of 0.0Oi V. per Kelvin in the interval of the meantime. 1 equals .10 ... 77O K · Although this is t? .In small Mt :? However, due to the high stability and precision of the oscillating crystals, it can be easily evaluated with purely digital methods. <br /><br/> For calibrated sensors, a fence of 0.002 K can be used, and relative temperature measurement blocks can be executed even more precisely. It is also possible to use the temperature-dependent NT cut for the detection of laser destruction (Yes, laser damage generally does not cause any systematic changes in the slice.

By an at least tei1 transparent arrangement of quartz crystal sensor element and layer or layer system, it is possible, even an intense laser beam on this arrangement su ste when the layer or the layer system is partially or hochtransmi tti erend. It is. to choose a wavelength of irradiation light wavelength that lies within the transparency range of quartz. The absorption of dec quartz can be eliminated by appropriate methodical design of the method. Their influence is in the case of reflective dielectric layer systems. As a result of the interaction between the laser beam and the layer as a result of absorption, heating and / or laser destruction, which, as already explained, can be detected by frequency changes of the quartz crystal, occur as a result of the transmittance of the layer to be examined.

The realization of an at least partially transparent arrangement takes place in a simple manner and by the metal layer electrodes arranged on opposite sides of the crystalline quartz being positioned relative to one another such that congruent, electrode-free regions exist with respect to the laser light on the opposite sides of the crystalline quartz at least tei1 are transparent.

Expediently, the metal layer electrodes arranged on opposite sides of the crystalline quartz have hole structures which are arranged approximately congruently and form a window for the laser light. These hole structures can be structured laterally using known methods for producing

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finished layers.

To realize the arrangement, it is favorable if the layer or the layer system is applied to at least one side of the crystalline quartz on the electrode-free region. However, an arrangement of the layer or of the layer system within the hole structure of the metal 1-layer electrodes is expedient. In principle, arrangements may also be advantageous in which the layer or layer systems to be investigated are respectively applied on opposite sides to electrode-free regions of the Schwinqquarz sensor element, so that an additive effect is produced, which is very poor for detecting absorption loss or high laser degradation. Fault thresholds is particularly suitable if the resulting in this arrangement interference effects are also exploited.

For highly sensitive determinations of the absorption of very low-loss, transmissive optical layers with laser wavelengths which lie in the transparency range of quartz, it is advantageous if the crystalline quartz has an optical quality with respect to the volume and the surface. In this case, the optical quality relates in particular to the optical losses of absorption and light scattering, which are to be minimized by a suitable choice of the photoconductivity and the surface treatment methods in a known manner.

With the arrangement according to the invention, it is possible, if necessary also selectively to determine the absorption and / or the laser strength of optical, preferably dielectric layers with a uniform, sensitive and stable detection method in an experimentally and methodically simple way, especially by the use of quartz crystal Sensor elements for detecting absorption and / or irreversible change itvfolgr? Intensive laser irradiation makes it possible to realize simply constructed arrangements that reduce high adjustment effort and provide a simple signal interpretation.

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For f u hr ^ injq's be is g y le

The invention will be explained in more detail with reference to two exemplary embodiments. It shows:

1 is a schematic illustration of the front view (a) and side view (b) of the invention for determining the absorption and laser resistance of optical, preferably dielectric layers by means of oscillating quartz sensor elements in the form of a biaxial bending oscillator

2 shows a schematic representation of the front view (a) and side view (b) of the arrangement according to the invention for determining the absorption and the laser stability of optical, preferably dielectric layers by means of a quartz oscillator element in the form of a thick-angle heater.

In Fig. I, a first embodiment of erfindunqsqemäßen arrangement with a biaxial bending oscillator is shown schematically as a quartz oscillating sensor element. The arrangement comprises an oscillatory quartz crystal rod i with two oscillation nodes 2 and two electrodes 3, electrically insulated from each other and arranged on opposite sides of the quartz crystal rod 1. According to one embodiment, on one of the electrode-free sides of the flexurally stable fluorescent crystal 1, the absorption and / or or laser resistance to be examined layer 4, which is a layer of titanium dioxide in the spc'ziel len embodiment.The direction of incidence of the laser beam is indicated in Fig. 1 by the direction F. In a specific embodiment, the radiation of a Nd: The use of bending vibrators as oscillating quartz sensor element is particularly favorable for the arrangement according to the invention, since this type of oscillator, due to its function-related electrode arrangement, can be used without additional measures (particularly those for transmissive electrodes) necessary layers are required

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IL

lend transparency with respect to the laser light used.

The uuarzkristal lstab i will with a suitable Schnittwinke? made of preferably synthetic ec ~ Ouarz and held in the Schwinqunqsknoten 2 with not shown elements, which may be simultaneously electrical connection elements. The realization of the vibrating quartz sensor cell is, in general, subject to no restrictions and, in principle, can also be carried out, for example , with multi-pole bending vibrators. However, in order to set the absorption of the layer 4, it is essential that the linear temperature coefficient of the resonant frequency of the quartz resonator be maximized. If only the laser strength of the layer 4 is to be determined, a quartz crystal having a temperature-independent frequency must be used. Both? Measuring situations can be achieved by a suitable choice of cutting angle and geometry. As electrodes 3 are vapor-deposited and / or baked metal coverings of silver or gold. These electrodes 3 are connected via not shown electrical Verbindunqselemente with also not shown electronic units. The characteristic of two-pole bending vibrator large Paral le.l capacity can be reduced in a known manner by a ground electrode connected to each separation electrode on each side of the quartz crystal rod i between the two electrodes 3 is disposed, the representation in Fig. 1, however, was omitted , This coating prevents the strong coupling of the two poles over the quartz dielectric. The layer 4 to be examined for absorption and / or laser strength can be produced by means of known coating methods, such as vapor deposition or sputtering, while maintaining the deposition conditions required for the respective layer-forming substance or for the layer properties.

By means of the electrode arrangement shown in FIG. 1, when oscillating voltages of several volts are reached, it is achieved that the vibratable * = quartz crystal rod 1 oscillates at the desired frequency. For excitation of bending vibrations two extending in the X direction, but oppositely directed fields are necessary, for example, in the in FIG.

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i are constructed for a half-period of oscillation specified polarity of the voltage. The quartz crystal rod 1 sees itself under the influence of the electric field on one side and expands on the other side, so that the center and the end faces of the quartz crystal rod 1 rotate in opposite directions at the points of contact 2, respectively Z direction to be deflected. The oscillations of the quartz crystal bar 1 are composed of strain and surface oscillations. As the width-length ratio of the bendable quartz crystal rod I increases, the influence of the surface shear oscillation increases, which increases the temperature coefficient α . For example, values in the order of magnitude -10 'K can be achieved for X.,. ^ Bending

If, due to the irradiation of the layer 4 by intense laser radiation (cw or pulse) as a result of absorption processes, a temperature change 6T 'in the layer 4 occurs, then a temperature change 6T occurs in the quartz crystal rod 1 due to heat transfer. which can be detected with high precision and stability by electronic units as a frequency change £ f = f ₍ -, (l + aGT) The absorption of the layer 4 can be determined by means of known methods, for example from the temperature or frequency change rate. With appropriate calibration of the quartz crystal sensor element, for example, with officially calibrated platinum resistors, the corresponding temperatures can be determined with high accuracy.The intrinsic absorption of the quartz crystal is generally much lower than the layer absorption can be determined by appropriate preliminary tests their egg count and taken into account accordingly.

In the case of very intensive laser irradiation, laser destructions can be used, which usually lead to abrupt mechanical changes (cracks, craters), the formation of such defects being associated with sound events. Thus, these destruction processes can be determined as abrupt changes in the signal behavior and also selectively to the absorption-induced signal. The use of temperature-independent oscillating quartz is also an exclusive observation of the mechanical behavior

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Layer 4 with intensive Laserbestrahlunqunq including the registry of Zerstörunqsereignissen possible

In a second embodiment, the arrangement according to the invention with a thickness oscillator as Schwinqquarz sensor element is schematically darqestelit in Fig. 2. The arrangement according to FIG. 2 comprises a thin-walled quartz crystal plate 5 with specially designed electrocan auf located opposite opposite sides. These specially dimensioned electrodes 6 have hole structures which are approximately congruent so that they form a window for incident laser light. According to the invention, on one side of the quartz crystal strip 5 within the hole structure, the layer 4 to be examined for absorption and / or laser resistance is arranged, which in a specific embodiment is a layer of titanium dioxide. The five directions of the laser beam are indicated in FIG. 2 by the arrow 1 direction. In a special embodiment, the radiation of a Nd: YAG laser with the wavelength 1.06 μτη is used.

The quartz crystal 1 plate 5 is made with a suitable cutting angle of preferably synthetic α-quartz and with elements not shown, the? can be electrical supplies at the same time, held. The realization of the P> c: hwinqquelz sensor element is generally subject to no restrictions. However, to determine the absorption of the layer 4, it is essential that the linear temperature coefficient of the resonant frequency of the quartz resonator be maximized. what e.g. can be realized by the so-called HT-cut. If only the laser resistance of the layer 4 is intended to be determined, a quartz crystal having a temperature-independent frequency must be used, which may be e.g. can be realized by the so-called AT-section. As specially designed electrodes 6, vapor-deposited and / or baked Met, 11-beläqe of silver or Bold serve. These specially designed electrodes 6 are connected via electrical connecting elements (not shown) to likewise non-illustrated electronic units, which serve to supply with an excitation voltage, to generate Hess signals and to compensate for systematic errors. In terms of absorption and / or

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Laser strength layer 4 to be examined can be prepared by means of known coating methods, such as e.g. Vapor deposition or sputtering while maintaining the deposition conditions required for the respective coating-forming substance or for the coating properties.

By the electrode arrangement shown in Fig. 2 is achieved when applying oscillating Spannunqen with guideline values of a few volts that the quartz crystal 1 plate 5 is due to the piezoslek.trischen effect in a thickness-Scherschwingunq with the frequency determined by the geometry. The temperature Qanq of this frequency has a zero in the so-called AT-section, so that (3 the frequency of the quartz f (T) = f ₍ -, (1 + C (T) practically no longer depends on the temperature, which for pure LaserssstsstiqkeitsmeBS For the temperature dependence required for absorption measurements, the frequency, on the other hand, the HT cut is advantageous because it gives a maximum linear temperature coefficient α of around +90 ».100-60 Kö-lö, which results in a frequency change of 0.001 7. per Kelvin in the application interval of 10... 770 K, comes now that due to the irradiation of the layer 4 by intensive laser radiation (cw or pulse) due to Absarptionsprozessen to a temperature change ST 'in the layer 4, comes it also in the quartz crystal 1 platelets 5 by heat transfer to a temperature change ST, which with a high precision and stability by electronic units as a frequency change ßf = f ₍ -) (i + C (GT) proved With appropriate calibration of the Schwinqquarz sensor element, for example, with officially qeeichten platinum resistors, and the corresponding temperatures can be determined with high accuracy. The intrinsic absorption of the quartz crystal is generally much lower than the layer absorption. By appropriate preliminary tests their Beitraq can be determined and considered accordingly.

If laser destruction occurs in the case of very intensive laser irradiation, abrupt mechanical changes (cracks, craters) occur in layer 4, the formation of which is associated with gaping conditions. Thus, these destructive processes can be considered abrupt changes in signal behavior as well

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Selective sum absorptiansbedingten signal can be determined. When using temperature-independent quartz crystals is also an exclusive observation of the mechanical behavior of the layer 4 as an indicator of Zerstörungsprosesse possible hit the inventive arrangement succeeds in an experimentally and methodologically simple way, the absorption and / or the laser strength of optical, preferably dielectric If necessary, to determine layers and layer systems with a uniform, sensitive and stable detection method. Especially by the use of quartz crystal sensor elements for detecting absorption and / or irreversible change due to intensive laser irradiation, it becomes possible to realize simple arrangements that avoid high adjustment effort and allow a simple Siqnalinterpretation. The applicability of different types of Schwinqquari sensing elements enables investigations in very wide frequency ranges.

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Claims (4)

5 Π sj) Γ y r b e .1 .. Arrangement for determining the absorption and the laser resistance of optical layers or layer systems by means of laser light and sensors, characterized in that (3 is a sensor consisting of crystalline quartz and Metallschichtelek- electrodes quartz crystal sensor element that the to be tested Layer or the layer system is arranged on the quartz crystal sensor element and both are at least tei1 transparent with respect to the laser light. 2. Arrangement according to claim i characterized in that (3, the quartz crystal sensor element is a bending oscillator and the layer to be tested is arranged on an electrode-free side. 3. Arrangement according to claim 1, characterized in that a quartz oscillating sensor element is a thickness oscillator and the metal layer electrodes arranged on two opposite sides are positioned relative to one another such that congruent electrode-free regions are provided which form a window for the laser light and in those the layer to be tested is introduced. 4. Arrangement according to claim 3, characterized in that the
Electrodes have a hole structure. - (For this i dtlatt drawings- 6833