DE102008001291B3 - Material stress determining method for e.g. high-homogeneous optical material, involves guiding light through object, and determining deviation of polarization angle of light withdrawing from object against light irradiated into object - Google Patents

Abstract

The method involves positioning an object consisting of a high-homogeneous optical material in a measuring device. A linearly polarized light is generated by a light source. The polarized light is guided through the object. A deviation of a polarization angle of the light withdrawing from the object against the light irradiated into the object is determined, where the object is thermally adjusted to ambient temperature of the measuring device before the measurement, till the temperature difference between the object and the measuring environment amounts to a specific value.

Description

The The invention relates to a method for determining material tensions in highly homogeneous optical materials by measuring stress birefringence with the features of the preamble of claim 1.

Such methods, with which one measures, for example, voltages in glass and in which one makes use of the influence of stress birefringence, are known. Devices with which this method can be carried out for example those marketed by the company Ilis GmbH under the designation "StrainMatic ^®". This reflects a polarimeter for the measurement of stress birefringence according to Sénarmont used. Linear polarized light (by an object to be measured sample The elliptically polarized light emerging from the sample is converted back into linearly polarized light by means of a λ / 4 plate, which then regularly has a different polarization direction than the irradiated linearly polarized light The difference or polarization angle α is then determined by means of an analyzer and from this the optical path difference R, of the partial waves caused by the voltage, is calculated according to the equation R = α ·λ / 180 ° λ denotes herein the wavelength of the light so determined optical path difference R is normalized to the thickness of the object in cm and given in the unit nm / cm as a measure of the birefringence.

While this system for testing simple glass products such as bottles, jars, ampoules, flat glasses or optical elements without higher claims suitable and in particular with regard to the optimization of the manufacturing process proven has, it pushes the above-mentioned highly homogeneous optical materials to his Limits. Such materials come, for example, in astronomy, Microscopy, microlithography, laser optics and lithography for the Manufacture of flat screens used and require one considerable production costs with long production times, in particular Temper times to avoid any form of stress in the glass.

The JP 05-306 133 A describes, for example, an annealing process in which the internal object stress of a glass body is optically monitored during the annealing time and the subsequent cooling time. A measured phase difference between a reference signal and a signal passing through the body is used as a controlled variable for controlling the temperature such that the measured stress birefringence occurs within a predetermined range.

in the same dimensions, As the production costs increase, so do the requirements to the quality control of Glass objects. For example, the detection limit is required and measurement accuracy for to further reduce stress birefringence. This is with the previously described method without further action not possible.

The measurement principle described at the outset for the determination of stress birefringence also makes use of that in the published patent application DE 10 227 345 A1 described method, according to which the optical material is scanned pointwise with a laser beam on a predetermined raster and also the polarization of the exiting light is determined. After all, stress birefringence can still be detected up to a value of 0.3 nm / cm.

Of the Invention is based on the object, a method of the preamble provided by claim 1, which is the determination of stresses in highly homogeneous optical materials with a improved resolution allows.

The Task is achieved by a method having the features of claim 1 solved.

According to the invention object to be measured thermally to the ambient temperature before the measurement the measuring device adjusted until the temperature difference between the object and the measuring environment is ± 0,1 K at most.

The inventors have found that, in addition to the intrinsic birefringence inherent in the material, there is a contribution to local stress birefringence, which is dependent on temperature fluctuations. Both contributions are superimposed in the conventional measurement of stress birefringence. Therefore, no clear conclusions can be drawn on the material-inherent stress birefringence if the influence of temperature predominates. Therefore, it is necessary to have a temperature stabilization in the to achieve the object to be measured. By means of the measure according to the invention, this is ensured to the extent that the temperature-induced fluctuations in the measured stress birefringence are not greater than the stress field to be detected and ± 0.2 nm / cm, preferably ± 0.1 nm / cm, as far as possible.

Prefers the measurement of stress birefringence is performed using a spatially resolving detector area. This saves measurement time by not, as in the prior art known to point the object by means of a laser point by point must be scanned, but the object over an area of predetermined size simultaneously is measured.

The Measurement of stress birefringence is particularly preferred using a switched between the object and the detector telecentric optics. The variation of the optics allows an adaptation the measuring apparatus to either a desired spatial resolution reach or measure a certain area simultaneously.

The Measurement of stress birefringence is preferably carried out with a spatial resolution of ≤ 1 mm.

Of Furthermore, it has to avoid temperature fluctuations as Advantageously, the light source by means of a thermal isolating window from the object to decouple. For this purpose are preferred Window made of an optically transparent material with as possible greater Thermal conductivity λ used. Preference is given to glasses with a λ ≥ 1.3 W / mK for use.

₂ -Quarzglas (z. B. Schott Lithosil ^®), Zerodur ^®, FK 51 or SF are particularly preferred window of high-purity synthetic SiO used 57th Apart from the high thermal conductivity, these materials are further characterized by the fact that they themselves can be produced as stress-neutral as possible and optically streak-free. Some properties of these glasses are summarized in Table 1 below: TABLE 1 Cover plates material coefficient unit SF57 N-FK51 Lithosil ^® Zerodur ^® Thermal conductivity λ W / (m · K) 0.62 0.911 1.31 1.46 Therm. Voltage factor φ MPa / K 0.626 1,507 0.04 0,002 Density ρ g / cm ³ 5.51 3.73 2.2 2.53 specific heat capacity c _p J / (g · K) 0.36 0.636 0.79 820 Thermal conductivity κ = λ / (ρ · c _p ) 10 ^-3 cm ² / s 0.31 0.38 0.75 0.001

In particular, Schott Zerodur ^® has proved to be a suitable material for a thermally insulating window in the measuring apparatus because it combines the properties of a very small stress factor φ and a high heat capacity c _p . A reference measurement of a Zerodur window (ie without a sample) revealed a background noise which was less than 0.2 nm / cm over the entire measurement area. This represents a significant improvement over the known window materials with a determined noise of at most 0.5 nm / cm. But also Lithosil is a suitable material because of the very small stress factor φ.

In a preferred embodiment of the method, thermal adaptation (thermalizing) of the object takes place at least over a predetermined, minimum adaptation period, which is determined as a function of the product of the thermal conductivity λ and the specific heat capacity c _{p of} the optical material. It has been found that the stress birefringence of optical materials in which the product of the thermal conductivity λ (in W / (m · K)) and the specific heat capacity c _p (in J / (g · K)) is less than 1 , more sensitive to temperature changes than materials with this product greater than one. An arrangement in the form of critical and uncritical materials is shown in Table 2. Table 2 material coefficient unit FK5HT LF5HT LLF1HT synthet. Quartz glass (Lithosil ^® ) CaF2 Crystal Thermal conductivity λ W / (m · K) 0.925 0.866 0,951 1.31 9.71 specific heat capacity c _p J / (g · K) 0.808 0.657 0.65 0.79 0.854 λ · c _p 0.75 0.57 0.62 1.03 8.29 sensitivity Critical uncritically

The Adjustment duration depends continue from the mass and the surface of the object to be measured. It is preferably 2 to 24 hours. longer Times are for some materials helpful but uneconomical for the production process.

Especially will have a thermal adjustment period of 6 to 12 hours in the first Step and another 6 to 12 hours in the second step. in special cases, in which a particularly high accuracy is sought, is an even further fine air conditioning sought

Though it basically requires no pretreatment in a separate room, if the examinee sufficient long stored in the measuring environment. However, this is to be borne the use of space and thus the utilization of the Messaparatur. Therefore the thermal adaptation of the object preferably takes place in a first Step (pretreatment) in a thermal of the measuring environment separate room or room area and in a second step (ready-to-heat) in the measurement environment, d. H. in the same room or room area, in which is also the measuring device.

Of the separate room or room area preferably has a temperature stability of ≤ ± 0.5K. This ensures that the measurement environment to be measured with the pre-painted object introduced energy regardless of external conditions (External temperature) is sufficiently known, so that the object to be measured no unexpected temperature fluctuations when relocating to the measuring environment is exposed and also the temperature of the measurement environment is not in unforeseen manner.

Further The thermal adjustment in the first step preferably follows one Temperature difference of at most ± 2.0 K to the measurement environment. This tolerance range ensures that the duration of the thermal adjustment in the second step, d. H. in the measuring environment, can be done in a sufficiently short time.

The Measurement environment itself preferably has a temperature stability of ≤ ± 0.1K and particularly preferably ≦ ± 0.05 K. on. This ensures that the temperature difference between the object and the measuring environment of ± 0.1 K at the most can.

According to one Another aspect of the invention is the positioning of the object preferably while avoiding heat input or decoupling. Because the positioning of the object in the measuring device for reasons the efficient use of the device immediately before the measurement it is necessary to take this step, on a thermal decoupling of the object from all heat sources to pay attention. The positioning of the object is therefore preferred using a gripping tool, so no hand heat in the Object is registered. Particularly preferably, the positioning takes place the object is automated, d. H. by means of a positioning unit (Handling robots) so that any heat input by persons in the measuring environment can be excluded.

Farther Arrangements are advantageous, by which computers, monitors, controls, or other electronics, power supplies, motors and other energy consumers as well as the waste heat management of Measuring device and the positioning unit relative to the object to be measured be shielded. Corresponding shields are preferred at least between the energy consumer or the waste heat management and the measuring station on the one hand and the deposit on which the prefabricated thermal treatment takes place the object happens to provide, on the other hand.

A advantageous development of the method provides that before each Measurement of stress birefringence on the object a calibration measurement performed without object becomes. The timely calibration ensures that any (temperature-related) temporal changes the zero measurement are taken into account, when the measurement of the object is corrected.

A further advantageous measure provides the measurement of stress birefringence on the object repeat at least once after 15 to 20 minutes to any eventuality Subtract the temperature-related residual fluctuations of the measurement result to be able to.

Further Objects, features and advantages of the invention will become apparent below based on an embodiment using the drawings closer explained.

It demonstrate:

1 a diagram with the course of stress birefringence after a sudden increase in temperature;

2 a schematic representation of the measuring device;

3 a diagram of an embodiment of the thermalization process according to the invention; and

4 a possible embodiment of the spatial arrangement for carrying out the method according to the invention.

In the diagram according to 1 the value of stress birefringence δ in nm / cm is plotted over the relaxation time t in hours for 4 different sudden temperature changes. Shown is the result of a simulation calculation assuming a cylindrical sample with a diameter of 270 mm and a thickness of 49.2 mm and a constant heat transfer coefficient of 5 W / m ² K on all cylindrical surfaces (top, bottom, lateral surface). Stress birefringence is determined at the outermost edge, ie at a radius of 135 mm. There, the influence of temperature fluctuations on the measurement result turns out to be the strongest with a sudden temperature difference of the ambient temperature.

As the simulation shows, exists between the stress birefringence and the temperature difference is a linear relationship. The maximum value The stress birefringence increases approximately every 0.1 ° K increase in the temperature jump around 0.15 nm / cm.

critical for the Thermalization duration is the sometimes considerably long relaxation time. So needed the value of stress birefringence 6 hours, until it is one tenth of the maximum value has dropped. This means that in the case of a sudden temperature increase of 1 K after about 6 hours the value of 0.15 nm / cm is reached and thus the desired accuracy of ± 0.2 nm / cm of the measurement can be achieved.

The Simulation also shows that in the limit of infinitely long relaxation time even after a temperature rise on the stress birefringence returns the value 0. That is, the absolute temperature in the measurement is not important plays, but only said temperature changes. The result is so Note that a temperature stabilization of the measurement environment to ± 0.1 K. and more preferably from ± 0.05 K should be sought.

The measurements made on a sample confirmed the in 1 shown context.

In 2 is the measuring device 200. represented schematically in an advantageous embodiment, as used in the inventive method. The measuring device 200. has a light source 202 on which unpolarized light radiates. The light becomes a polarizer 204 fed, which, from the unpolarized light, light with linear polarization happen. The linearly polarized light then passes through a thermally insulating window 206 , The window 206 is used for thermal insulation of the light source 202 and thus prevents the heat from the light source 202 on one above the window 206 positioned object 208 is transmitted.

As window material are typically glasses with one with the greatest possible thermal conductivity, in particular with a λ ≥ 1.3 W / mK. High demands are also placed on the optical properties of this window material. Thus, the material of the window should have a high homogeneity, ie a largely constant refractive index, streak-free and, in turn, be as low in tension as possible. Although material-inherent optical artifacts of the window are determined by a calibration measurement and subtracted from the result of the sample measurement. However, if possible, only residual errors of the apparatus should be eliminated in this way. Particularly suitable is a synthetic quartz glass (z. B. Schott Lithosil ^®), Zerodur ^®, FK 51 or SF found 57th

Above the window 206 is the sample or the object to be measured 208 positioned within the measuring device. It usually rests on a slide (not shown). Furthermore, it may be advantageous if the object is in communication with a holder, with which together it has already undergone the thermal adaptation. In this case, the object holder rests on the slide, and contributes to the isolation of the object 208 towards the slide on the device. The slide and / or the object holder 210 should therefore consist of a material of low thermal conductivity.

After the linearly polarized light has passed through the object to be measured, this has an elliptical polarization depending on the stress birefringence which has taken place in the object. The elliptically polarized light is hereafter by means of a λ / 4 plate arranged above the object 212 converted back into linearly polarized light. To the extent that a stress birefringence is the object to be measured 208 The polarization direction of linearly polarized light has turned. The difference angle to the original polarization direction, ie the position of the polarizer 204 , is determined by means of an analyzer 214 determined. The thus determined polarization angle indicates a measure of the birefringence and thus of the stress in the sample material.

Not necessary but advantageous if in the beam path a telecentric optics 216 , here simplified by a converging lens and a scattering lens, is switched on, which converts the parallel input beam into a parallel output beam with a different cross-section. Through such a telecentric optics 216 It is possible to change the image size regardless of the size of the measuring range to the cross-sectional area of a spatially resolving detector 218 adapt. In the case of the spatially resolving detector 218 it is preferably a CCD camera. The spatial resolution of the measuring device 200. is in this case by the pixel resolution of the CCD camera and the imaging ratio of the telecentric optics 216 certainly. The spatial resolution is preferably ≦ 1 mm.

In 3 the integration of the thermalization process according to the invention is shown schematically in the measurement process. Thereafter, the step of thermally adjusting the object to the ambient temperature of the measuring device consists of two thermalization steps.

In a first thermalization step T1, the object to be measured is exposed to certain thermal conditions over a predetermined adaptation period. The thermal conditions are characterized by the absolute temperature T ₁ and the thermal fluctuation range ΔT ₁ . The duration of thermalisation is fixed to a minimum time T ₁ ^min , the minimum duration of thermalisation or adaptation. It is preferably determined as a function of the product of the thermal conductivity and the specific heat capacity of the optical material of which the object consists. It is preferably between 2 and 24 hours and typically 12 hours for an object having a diameter of about 250 mm and a thickness of about 45 mm. Such objects (also referred to as "blanks") are for example starting material for the manufacture of high precision optical elements such as plates, filters, lenses, prisms or the like. The Thermalisierungsdauer also varies with the mass and the surface area of the object. The Thermalisierungstemperatur T ₁ is preferably in a temperature interval of ± 2.0 K, based on the temperature of the measurement environment used in the in 3 illustrated example at the same time corresponds to the thermalization temperature T ₂ .

The temperature stability during the first thermalization process is preferably in an interval ΔT ₁ of ± 0.5 K. This ensures that the object is supplied to the second thermalization step at a defined temperature. If the temperature T _{1 is} below the measurement ambient temperature T ₂ , this ensures that the sample always absorbs heat in the second thermalization step and does not give off. This ensures calculable temperature stability in the fine climate of the measurement environment.

After the thermalization step T1, a test step Q1 follows in which it is checked whether the temperature of the object T _{0 is} in the interval [T ₁ ± ΔT ₁ ]. If this is not the case, the sample is for a further duration to be determined, which may also be below the minimum thermalization time t ₁ ^min , Ther malisierungsschritt T1 supplied. If the condition is fulfilled, it is then fed to the second thermalization step T2.

The second thermalization step T2 is characterized by an absolute temperature T ₂ , a temperature fluctuation width ΔT ₂ and a thermalization time t ₂ ^min . The thermalization temperature T ₂ at the same time corresponds to the temperature of the measurement environment, the absolute value of which, as stated above, does not arrive. The fluctuation range ΔT _{2 of} the fine climate in the thermalization step T2 and the measurement environment is preferably in a range of ± 0.1 K and more preferably of ± 0.05 K. In order to ensure a sufficient adaptation of the object to the prevailing temperature, the minimum Thermalization time t ₂ ^min in an interval of 2 to 24 hours, preferably 12 hours.

After the thermalization step T2, the sample is subjected to a second temperature test in step Q2. If it is determined that the temperature of the sample T _{0 is} in the interval [T ₂ ± ΔT ₂ ], then the sample is fed to the measurement M1. Otherwise, the sample is again subjected to the thermalization step T2 for a duration to be determined, which may also be below the minimum thermalization time t ₂ ^min . If the thermalization time is sufficiently long, the test step Q2 as well as the test step Q1 can be omitted.

Unlike in 3 illustrated, the thermalization can also be done in a single thermalization step. The target parameters for the thermalization are the absolute temperature T ₂ and the fluctuation range ΔT _{2 of} the fine climate of the measurement environment. However, a disadvantage of the single-stage thermalization process is that the thermalization step immediately preceding the measurement should, if possible, take place without spatial separation in the area of the measurement environment in order to prevent another source of temperature fluctuations. This in turn means that the under-thermalized samples, depending on the ambient conditions (ambient temperature), bring different energy inputs into the measuring environment and thus it is difficult to keep the fine climate within the narrow temperature interval of ± 0.05 K.

Further it may also be provided instead of two thermalization steps to perform three or more thermalization steps, one of Thermalization step to thermalization step to the thermal Conditions of the measurement environment approximates.

In 4 a schematic arrangement in which the inventive method can be performed. The object to be measured 408 is first individually or in a batch of a certain size through a lock 412 in a closed first thermalization room 414 brought in. In the thermalization room 414 prevail the environmental conditions described above, which are described by the sizes T ₁ , ΔT ₁ . For this purpose, the first thermalization room 414 air-conditioned. The exhaust air is by means of a first air conditioning device (not shown) as efficiently as possible from the first thermalization space 414 led out to comply with the required temperature stability .DELTA.T ₁ even with larger energy inputs, by a large number of introduced objects with large total mass.

After elapse of a certain thermalization time t ₁ , which at least corresponds to the minimum thermalization time t ₁ ^min , the object or objects are in a second, thermally from the first thermalization space 414 separate room area (hereinafter second thermalisation room) 416 spent. In the second thermalization room 416 prevail the thermal conditions described above, characterized by the parameters T ₂ and ΔT ₂ . The same conditions prevail in the immediate measuring environment 418 in which the measuring device 400 is set up. The second thermalisation room 416 and the measurement environment 418 are therefore not thermally but only functionally separated (shown a dashed dividing line). The second thermalisation room 416 is set up to store one or more objects to be measured in stock, namely over a thermalization time t ₂ , which corresponds at least to the minimum thermalization time t ₂ ^min . The objects to be measured are preferably kept for this purpose in air-permeable shelves or cupboards (also in the immediate vicinity) to the measuring station. The thermal conditions in the second thermalisation room 416 or the measurement environment 418 are ensured by a second air conditioning device (not shown), the exhaust air is also passed to the outside.

Also in 4 implied that the lighting unit 402 thermally from the measurement environment 418 is decoupled. A separate room area 420 is provided for this, which inter alia by the window described above 406 from the measurement environment 418 is disconnected. The lighting room 420 is further provided with its own air conditioning, the exhaust air is led to the outside. The lead out of the exhaust air is done as far as possible isolated, ie far-flung past the object to be tested and the storage bins and / or with appropriate insulation measures.

The same applies to waste heat generated by any machine (not shown), for example a transfer system for the transfer of the object or objects from one room / room area to the next or a positioning unit for positioning the object in the measuring device 400 ,

to Compliance with the set by the air conditioning device After all, it is advantageous if the thermal conditions prevail Number of persons in particular in the measuring environment in is committed to the process flow. If possible, should not more than one person in the measuring environment or the second thermalisation room stop and at best should while no person can be found in the room or room area be.

200

measuring device

202

Light source / lighting unit

204

polarizer

206

thermal insulating window

208

object

210

object holder

212

λ / 4 plate

214

Polarizing filter, analyzer

216

telecentric optics

218

detector

400

measuring device

402

lighting unit

406

window

408

object

412

lock

414

first Thermalisierungsraum

416

second Thermalisierungsraum

418

measurement environment

420

lighting space

Claims (15)

Method for determining material tensions in highly homogeneous optical materials by measuring stress birefringence, comprising the steps of: - positioning an object to be measured ( 208 . 408 ) consisting of the highly homogeneous optical material in a measuring device ( 200. . 400 ) - generating linearly polarized light by means of a light source ( 202 ) - passing the linearly polarized light through the object ( 268 . 408 ) - determining a by a material tension in the object ( 208 . 408 ) caused deviation of the polarization (polarization angle) of the object ( 208 . 408 ) exiting light in relation to the object ( 208 . 408 ) radiated light characterized in that the object ( 208 . 408 ) thermally to the ambient temperature of the measuring device before the measurement ( 200. . 400 ) is adjusted until the temperature difference between the object ( 208 . 408 ) and the measurement environment ( 418 ) is at most ± 0,1 K. A method according to claim 1, characterized in that the deviation of the polarization by means of a λ / 4-plate ( 212 ) and a linear polarizing filter (analyzer) ( 214 ) is determined. Method according to one of the preceding claims, characterized in that the measurement of stress birefringence using a spatially resolving detector ( 218 ) is flat. A method according to claim 3, characterized in that the measurement of stress birefringence using a between the object ( 208 . 408 ) and the detector ( 218 ) switched on telecentri optical appearance ( 216 ) he follows. Method according to one of the preceding claims, characterized characterized in that the measurement of stress birefringence with a sensitivity of at least ± 0.1 nm / cm takes place. Method according to one of the preceding claims, characterized in that the light source ( 202 ) by means of a thermally insulating window ( 206 ) thermally from the object ( 208 . 408 ) is decoupled. Method according to one of the preceding claims, characterized in that the thermal adaptation of the object ( 208 . 408 ) takes place at least over a predetermined, minimum adaptation period, which is determined as a function of the product of the thermal conductivity λ and the specific heat capacity c _{p of} the optical material. Method according to one of the preceding claims, characterized in that the thermal adaptation of the object ( 208 . 408 ) in a first step in a thermally of the measuring environment ( 418 ) separate room or room area ( 420 ) and in a second step in the measurement environment ( 418 ) he follows. Method according to claim 8, characterized in that the separate room or room area ( 420 ) has a temperature stability of ≤ ± 0.5 K. A method according to claim 8 or 9, characterized in that the thermal adjustment in the first step to a temperature difference of at most ± 2.0 K to the measurement environment ( 418 ) he follows. Method according to one of the preceding claims, characterized in that the measuring environment ( 418 ) has a temperature stability of ≤ ± 0.05 ° K. Method according to one of the preceding claims, characterized in that the positioning of the object ( 208 . 408 ) while avoiding heat coupling or extraction. Method according to one of the preceding claims, characterized in that the positioning of the object ( 208 . 408 ) using a gripping tool. Method according to claim 13, characterized in that the positioning of the object ( 208 . 408 ) is automated. Method according to one of the preceding claims, characterized in that before each measurement of stress birefringence on an object ( 208 . 408 ) a calibration measurement is performed without an object.