DE102020102959B4 - Surface measuring system and method for measuring a surface of a test piece - Google Patents

Abstract

Surface measuring system for measuring a surface (38) of a test piece (40), with (a) a topography sensor (32) for measuring a change in distance (Δdj) from the topography sensor (32) to the surface (38) of the test piece (40 ), (b) a guide (34) for guided movement of the topography sensor (32), and (c) an autocollimator (10) having (i) a mirror (14) attached to the topography sensor (32). (ii) arranged to emit an output light beam (20) and (iii) arranged to measure a tilt angle (α) of the topography sensor (32), (d) the mirror (14) being arranged to reflect of at least two discrete partial light beams (30.1, 30.2) at different reflection angles (ρ1, ρ2) when irradiated with the light beam (20).

Description

The invention relates to a surface measuring system for measuring a surface of a test piece, with (a) a topography sensor, in particular an interferometer unit, for measuring a change in distance from the topography sensor to the surface of the test piece, (b) a guide for moving the topography - Sensors. According to a second aspect, the invention relates to a method for measuring a surface of a test object and (c) an autocollimator which (i) has a mirror which is attached to the topography sensor, (ii) is designed to emit an output light beam and (iii) arranged to measure a tilt angle of the topography sensor,.

It is known from JP 2000 - 88 551 A It is known to measure the topography of the surface of a smooth specimen by moving a topography sensor along the surface of the specimen by means of a guide. When an interferometer unit is used, the surface acts as a mirror reflecting the light from the interferometer unit. The distance between the surface of the test object and the topography sensor can be determined for each position. The topography of the surface is then reconstructed from a large number of such measurements. The disadvantage of this system is the comparatively small measuring range.

The article "CCD-area-based autocollimator for precision small-angle measurement" by Jie Yuan et al describes the use of a CCD sensor in the autocollimator to increase the accuracy of the angle measurement. The disadvantage of this system is the comparatively small measuring range.

In order to increase the measurement range, the article "Cube-corner autocollimator with expanded measurement range" by Renpu Li et al suggests using a retroreflector in the shape of a cat's eye. The disadvantage of such a system is that very small measurement uncertainties are difficult to achieve.

In "Error compensation for long-distance measurements with a photoelectric autocollimator" by Renpu Li et al, an algorithm is proposed to compensate for the error that results from the finiteness of the expansion of the light source over long measurement distances. The measuring range cannot be extended in this way.

If the interferometer unit is mentioned below, a topography sensor is always meant in general, provided that properties specific to interferometer units are not involved.

If the surface is macroscopically curved, the topography sensor must be tilted, otherwise the measuring range of the topography sensor will be left and/or a measurement error will occur. If the topography sensor is an interferometer unit, for example, the outgoing light rays are no longer reflected back in such a way that they can be brought to interference. The tilt angle must be measured with a low measurement uncertainty, otherwise the topography can only be recorded with a high measurement uncertainty. It has turned out to be disadvantageous that such surface measuring systems are difficult to measure such specimens in which the normal vectors on the surface lie in an angular interval that is too large, for example more than 5°.

The object of the invention is to reduce the disadvantages of the prior art.

The invention solves the problem by means of a generic surface measuring system, in which the mirror is designed to reflect at least two discrete partial light beams at different reflection angles when irradiated with the output light beam. In other words, the mirror is multi-reflective.

The invention also solves the problem with a method for measuring a surface of a specimen with the steps (a) measuring a change in distance from a topography sensor to the surface of the specimen using the topography sensor, (b) moving the topography sensor using a guide and (c) measuring a tilt angle of the topography sensor using an autocollimator having a mirror fixed relative to the topography sensor. The mirror is designed to reflect at least two discrete partial light beams at different reflection angles when irradiated with exactly one light beam.

The invention is based on the consideration that an autocollimator only has a limited angle measurement range. All those angles measured relative to the output light beam are in the angle measurement range, under which an input light beam can enter the autocollimator in order to measure the tilt angle relative to an output light beam.

The output light beam is emitted from the autocollimator and hits the mirror. The input light beam is the light beam that is reflected by the mirror and falls back into the autocollimator. If the tilt angle is too large, the input light beam leaves the angle measurement range and the tilt angle can no longer be measured.

The mirror, which can be described as multi-reflecting and reflects at least two discrete partial light beams at different reflection angles when it is precisely irradiated with a light beam, ensures that at least one partial light beam always enters the autocollimator in the angle measuring range. In this way, the tilt angle measurement range, in which the tilt angle must lie in order to be measured by the autocollimator, is significantly increased without the angle measurement range having to be increased.

The second partial light beam (and further partial light beams) has an angular offset compared to the first, which can either be determined with the autocollimator if both partial light beams arrive at its detector camera or can be known from prior knowledge. If you swivel the mirror, you can count how many partial light beams are currently visible on the detector. The angular offset is measured by the offset angle.

In the context of the present description, the topography sensor is understood to mean in particular a distance measuring device that can be used to record distance measurement data that indicate the respective distance of the topography sensor or a point on the topography sensor from the surface to be measured.

The topography sensor can be an interferometer unit, a chromatic-confocal sensor or a capacitive sensor, for example.

The interferometer unit is understood to mean, in particular, a device that is designed for interferometrically measuring a distance between the interferometer unit and the surface of the test object. In particular, the interferometer unit is preferably constructed in such a way that the surface of the specimen reflects an interferometer measuring beam emitted by the interferometer unit.

If a capacitive sensor is used, it is advantageous if the test object has an electrically conductive surface.

The guide for moving the topography sensor is understood to mean, in particular, a linear guide. It is possible, but not necessary, for the guide to be a two-dimensional guide, which means that the topography sensor can be positioned in one plane, ie in two spatial degrees of freedom. In particular, it is also possible for the topography sensor to be a 1D linear guide. In principle it is possible that the guidance is also not a straight guidance, but this usually leads to an increased measurement uncertainty.

The mirror is preferably designed to reflect at least three, in particular four, particularly preferably at least five, partial light beams. The mirror is preferably designed to reflect at most one hundred partial light beams. The angular gradation can also be formed in two mutually perpendicular directions. The mirror can be referred to as a multi-mirror because of its property of reflecting back several light beams from an incident laser beam.

Each partial light beam is reflected at a reflection angle. The angle of reflection is measured relative to the light beam falling on the mirror, i.e. usually the exit light beam.

Two partial light beams form an offset angle with each other. It should be pointed out that the offset angle for each partial light beam is determined in relation to the partial light beam whose reflection angle is smaller and for which the smallest offset angle results. In this sense, the offset angle between adjacent partial light beams is measured.

It should be noted that it is useful, but not necessary, for offset angles to be measured, defined, or reported in this way. All that matters is that the relationships specified in this description are fulfilled when the specified definition is used.

The feature that the partial light beams are discrete means in particular that the autocollimator can identify them as different light beams.

According to a preferred embodiment, the autocollimator has an angle measurement range that has an angle measurement range width. The mirror is preferably designed in such a way that an offset angle between two partial light beams corresponds at most to the width of the angle measurement range. If the tilt angle is so large that an angle at which a partial light beam enters the autocollimator comes to the edge of the angle measurement range, another partial light beam enters the autocollimator. The tilt angle can then be measured using this second partial light beam.

It is pointed out that it is possible, but not necessary, for always exactly two partial light beams to be detected by a detector of the autocollimator.

In particular, it is possible that—at least for some tilt angles—three or even more partial light beams are detected by the detector of the autocollimator.

According to a preferred embodiment, the mirror comprises a first planar mirror element and at least one second planar mirror element which is tilted with respect to the first mirror element. If the tilt angle changes, the mirror element that reflects the incident light beam changes.

Alternatively or additionally, the mirror comprises at least two wedge-shaped reflector elements, each having a first reflector element surface and a second reflector element surface, which influence a wedge angle. This wedge angle is preferably smaller than the angle measuring range width. Alternatively or additionally, it is favorable if the wedge angle is between 0.5° and 2.5°.

According to a preferred embodiment, the multi-mirror is implemented by a computer-generated hologram. This optically reproduces the function of the classic wedge arrangement. Such holograms are introduced into a transparent element, in particular a glass element, for example by means of a laser. The mirror preferably has a transparent element, in particular a glass element, into which a diffraction mask is introduced, so that the hologram is produced.

So that the measurement uncertainty depends as little as possible on the temperature of the mirror, it is favorable if the transparent element consists of a material whose coefficient of thermal expansion has a zero crossing. It is particularly favorable if this zero crossing is at a temperature between 0° and 40°.

The surface measuring system preferably comprises an evaluation unit which is set up to automatically carry out a method with the steps (i) detecting the angle of reflection of the at least one partial light beam which is incident on the autocollimator, (ii) detecting a measured tilt angle value using the autocollimator and ( iii) Calculating the tilt angle from the reflection angle and the tilt angle measurement value.

For example, the rough tilt angle can be determined from the previously measured tilt angles. If the tilt angle is known at the beginning of the measurement, for example zero, then it is also known which of the partial light beams is used to determine the tilt angle. Since the offset angles of the partial light beams are known to each other, it can be determined from the last measured tilt angle which partial light beam or which partial light beams are detected by the autocollimator.

The offset angles are, for example, between 0.05°, in particular 0.1° and 3°, in particular 1°.

The measured tilt angle value is recorded as is usual with autocollimators, for example by determining the position at which the at least one partial light beam falls on a detector of the autocollimator, for example a CCD chip or another position detector. The tilt angle can be calculated from the known offset angle and the tilt angle measured value. The evaluation unit does this automatically.

The evaluation unit is preferably set up to automatically carry out a method with the steps (i) detecting a topography sensor position of the topography sensor along the guide, (ii) detecting changes in the distance of the topography sensor as a function of the topography sensor position, (iii ) detecting the tilt angle as a function of the topography sensor position and (iv) calculating a topography of the surface from the topography sensor position, the changes in distance and the tilt angle. The topography is understood to be that point cloud or a description of this point cloud that describes the course of the surface.

According to the invention is also an autocollimator with a light source for emitting at least one output light beam, if necessary a beam splitter, a mirror for attachment to a test object whose tilt angle is to be measured, for reflecting the output light beam so that an input light beam is created, and a detector. The mirror is designed to reflect at least two discrete partial light beams at different reflection angles when irradiated with exactly one light beam. The preferred embodiments given above also apply to this autocollimator.

• 1 eine schematische Ansicht eines erfindungsgemäßen Autokollimators und eines erfindungsgemäßen Oberflächen-Messsystems zum Durchführen eines erfindungsgemäßen Verfahrens,
• 2a eine schematische Detailansicht einer ersten Ausführungsform des Spiegels des erfindungsgemäßen Autokollimators aus 1 und
• 2b eine schematische Detailansicht einer zweiten Ausführungsform des Spiegels des erfindungsgemäßen Autokollimators aus 1.

The invention is explained in more detail below with reference to the accompanying drawings. while showing

• 1 a schematic view of an autocollimator according to the invention and a surface measuring system according to the invention for carrying out a method according to the invention,
• 2a a schematic detailed view of a first embodiment of the mirror of the autocollimator according to the invention 1 and
• 2 B a schematic detailed view of a second embodiment of the mirror of the autocollimator according to the invention 1 .

1 10 shows an autocollimator 10 according to the invention, which has a light source 12, a mirror 14 and a detector 16. FIG. In the present case, the autocollimator also has a beam splitter 18. The light source 12 has an aperture for generating an output light beam 20. This is parallelized by a lens 22 and impinges on the mirror 14. This creates an input light beam 24, which from the beam splitter 18 is diverted to the detector 16. A change in a tilt angle α of the mirror 14 is measured in this way.

The mirror 14 consists, as in 2a is shown, in the present case of several reflector elements 26.i, which are connected to each other. Each reflector element has a first reflector element surface 28a.i and a second reflector element surface 28b.i, which form _{a wedge angle κ i with one another.}

At each of the boundary layers between a first reflector element surface 28a.i and a second reflector element surface 28b.i+1, the output light beam 20 is reflected, so that several partial light beams 30.i (i = 1, 2, ...N) develop. Preferably N < 50.

Two partial light beams each enclose an offset angle βi. Thus, the first partial light beam 30.1 and the second partial light beam 30.2 enclose the first offset angle β1. The second partial light beam 30.2 and the third partial light beam 30.3 enclose the offset angle β2. It is possible, but not necessary, for all offset angles β to be of the same size. As a rule, the offset angles βi differ from one another. It is favorable if the individual offset angles βi are known with a low measurement uncertainty. The uncertainty is preferably at most 10 ^-6 °.

1 shows that the mirror 14 is attached to a topography sensor 32 which is attached to a guide 34 for linear displacement. In the present case, the topography sensor 32 is an interferometer unit.

The interferometer unit 32 emits at least one measuring laser beam 36.j (j=1, 2, ...) onto a surface 38 of a specimen 40. The reflected laser beam is brought to interference with the measuring laser beam 36.j, so that a change in distance Δd _{j of} the distance d _{j is} measured.

The test specimen 40 can be accommodated on a receptacle 42 . In this case, the guide 34 is preferably rigidly connected to the receptacle 42 .

The interferometer unit 32 is moved on the guide 34 in order to measure the surface 38 . The respective changes in distance Δd _j from all measuring laser beams 36 are recorded and sent to an evaluation unit 44 . In addition, the respective tilt angle α is measured by the autocollimator 10 and also transmitted to the evaluation unit 44 . A topography sensor position P _{32 is} measured along the guide 34 by means of a distance meter (not shown) and is also transmitted to the evaluation unit 44 . From this, evaluation unit 44 determines a topography Δd _j of surface 38.

The interferometer unit 32 has an actuator 46, by means of which the tilt angle α of the interferometer unit 32 relative to the test object 40 can be changed.

If the tilt angle α becomes greater than the first offset angle β1, the second partial light beam 30.2 enters the autocollimator 10 in addition to the first partial light beam 30.1. Two light spots then appear on the detector 16 . The tilt angle α is then calculated, for example, by using the light spot on the detector 16, which belongs to the first partial light beam 30.1, to calculate the tilt angle α. The offset angle β1 is also calculated from the second light spot, which belongs to the second partial light beam 30.2. If the tilt angle α increases further, the corresponding light spot leaves the area of the detector 16. The tilt angle α is then calculated from the light spot that the second partial light beam 30.2 forms on the detector 16.

Reference List

10

autocollimator

12

light source

14

mirror

16

detector

18

beam splitter

20

output light beam

22

lens

24

input light beam

26

reflector element

28a

first reflector element surface

28b

second reflector element surface

30

partial light beam

32

Topography sensor, interferometer unit

34

guide

36

measuring laser beam

38

surface

40

examinee

42

admission

44

evaluation unit

46

actuator

48

transparent element

a

tilt angle

αmess

tilt angle

i

running index

j

running index

β

offset angle

Δdj

distance change

P32

Topographic sensor position

W

angle measuring range

B

angle measuring range

k

wedge angle

ρ1, ρ2

angle of reflection

Claims (9)

Surface measuring system for measuring a surface (38) of a test piece (40), with (a) a topography sensor (32) for measuring a change in distance (Δd _j ) from the topography sensor (32) to the surface (38) of the test piece ( 40), (b) a guide (34) for guided movement of the topography sensor (32) and (c) an autocollimator (10) which (i) has a mirror (14) which is mounted on the topography sensor (32) is attached, (ii) is designed to emit an output light beam (20) and (iii) is arranged to measure a tilt angle (α) of the topography sensor (32), wherein (d) the mirror (14) is designed for Reflecting at least two discrete partial light beams (30.1, 30.2) at different reflection angles (ρ ₁ , ρ ₂ ) when irradiated with the light beam (20). surface measuring system claim 1 , characterized in that the topography sensor (32) is an interferometer unit. Surface measuring system according to one of the preceding claims, characterized in that (a) the autocollimator (10) has an angular measuring range (W) which has an angular measuring range width (B) and (b) the mirror (14) is designed such that a Offset angle (β) between at least two partial light beams (30) corresponds at most to the angle measuring range width (B). Surface measuring system according to one of the preceding claims, characterized in that the mirror (14) has (a) a first planar mirror element and at least one second planar mirror element which is tilted in relation to the first mirror element, or (b) at least two wedge-shaped reflector elements (26.1 , 26.2), each having a first reflector element surface (28a.1, 28a.2) and a second reflector element surface (28b.1, 28b.2) which enclose a wedge angle (κ), or (c) a computer-generated hologram and/or (d) has a transparent element (48), in particular a glass element, which is structured in such a way that it reflects light at different discrete angles. Surface measuring system according to one of the preceding claims, characterized by an evaluation unit (44) which is set up to automatically carry out a method with the steps: (i) detecting the reflection angle (ρ ₁ ) of the at least one partial light beam (30.1, 30.2) , which is incident on the autocollimator (10), (ii) detecting a tilt angle _{measured value (α mess} ) by means of the autocollimator (10) and (iii) calculating the tilt angle (α) from the reflection angle (ρ ₁ ) and the tilt angle measured value (α _mess ) . surface measuring system claim 5 , characterized by an evaluation unit (44) which is set up to automatically carry out a method with the steps: (i) detecting a topography _{sensor position (P 32} ) of the topography sensor (32) along the guide (34), (ii) Detection of changes in distance (Δd _j ) from the topography sensor (32) as a function of the topography sensor position (P ₃₂ ), (iii) detection of the tilt angle (α) as a function of the topography sensor position (P32) and (iv) Calculating a topography of the surface (38) from the topography sensor position (P ₃₂ ), the distance changes (Δd _j ) and the tilt angles (α). Autocollimator (10) with (a) a light source (12) for emitting at least one output light beam (20), (b) a beam splitter (18), (c) a mirror (14) for attachment to a test object (40), whose tilt angle (α) is to be measured, for reflecting the output light beam (20) so that an input light beam (24) is formed, and (d) a detector (16), (e) the mirror (14) being designed to reflect at least two discrete partial light beams (30) at different reflection angles (ρ ₁ , ρ ₂ ) when irradiated with the output light beam ( 20). Method for measuring a surface (38) of a specimen (40), with the steps: (a) measuring a change in distance (Δd _j ) from a topography sensor (32) to the surface (38) of the specimen (40) by means of the topography sensor (32), (b) moving the topography sensor (32) by means of a guide (34), (c) measuring a tilt angle (α) of the topography sensor (32) by means of an autocollimator (10), which (i) has a mirror (14) which is attached to the topography sensor (32), (ii) the mirror (14) being designed to reflect at least two discrete partial light beams (30.1, 30.2) at different reflection angles (ρ ₁ , ρ ₂ ) when irradiated with a light beam. procedure after claim 8 , characterized by the steps: (a) detecting which partial light beam (30) the autocollimator (10) uses to measure the tilt angle (α), (b) detecting a measured tilt angle value (α _mess ) using the autocollimator (10) and (c ) Calculating the tilt angle (α) from the offset angle (β) of the respective partial light beam (30) and the tilt angle measured value (α _mess ).

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