CN105737759A - Long trace profile measurement device - Google Patents

Abstract

The invention provides a long-distance surface shape measurement device, which is used for surface shape detection on the surface of an optical device to be tested, which includes a moving optical head, and the moving optical head includes a surface light source, a single-hole screen, a beam splitter, and a Fourier transform A lens and an area array detector, the beam splitter is closely attached to the upper surface of the single-hole screen, the single-hole screen is arranged on the side of the surface light source and forms an oblique angle with the surface light source, so that the surface The light beam in the normal direction of the light source can be vertically reflected to the surface of the optical device to be tested through the beam splitter of the single-hole screen hole, the Fourier transform lens is arranged above the beam splitter horizontally, and the area array detector is arranged horizontally above the Fourier transform lens. The invention reduces the systematic error introduced by the lateral movement of the measuring beam when measuring different angles, thereby improving the measuring accuracy.

Description

A long-range surface shape measuring device

technical field

The invention relates to the field of high-precision mirror surface shape detection, in particular to a long-distance surface shape measurement device.

Background technique

In the fields of synchrotron radiation, large astronomical telescopes, and extreme ultraviolet lithography, it is necessary to use slender, high-precision mirrors with a length of about 1m and a surface error of less than 0.1 microradians to focus X-rays into nanoscale spots . The shape of this kind of mirror used for focusing directly determines the quality of the x-ray spot, so it needs to be accurately measured. Long Trace Profile (LTP for short) is one of the main instruments used to detect the surface profile of this large-scale, high-precision mirror.

The long-range surface profiler itself is an optical system composed of precision optical components. Its working principle is to inject a reference beam of a specific incident direction onto the optical device to be tested, and measure the angle of the reflected beam at different points on the optical device to be tested. value, so as to realize the surface shape detection of the optical device to be tested. Since the long-range surface profiler adopts non-contact measurement mode, it will not cause damage to the reflective surface of the optical device to be tested during the measurement process, and its measurement accuracy is high, which can realize the accurate detection of large-scale mirror surface shape. Therefore, in the past 20 years, the long-range surface profiler has made great progress, and there have been LTP-I, LTP-II, LTP-V, PP-LTP (pentaprism long-stroke profiler), online LTP, multi-function LTP, NOM (Nano Optical Detector) and other long-stroke surface profilers based on the principle of thin beam scanning detection. Among them, NOM is one of the most accurate surface shape testing instruments in the world.

With the continuous development of science and technology, various application fields have put forward higher requirements for the detection of mirror surface shape. In order to improve the detection capability of the long-range surface profiler, it is necessary to correct or eliminate various system errors. Among these system errors, the most important type is caused by the slight difference between the optical elements used in the optical path system of the long-range surface profiler and the ideal optical elements, which is mainly manifested in two aspects:

1) The small difference between the surface shape of the reflective optical element in the optical system and the surface shape of the ideal reflective optical element and the uneven refractive index of the transmissive optical element will introduce system errors, because when the measuring beam is incident on the non-ideal optical element, the non-ideal optical The component will cause a slight deviation between the direction of the outgoing beam and the ideal outgoing direction, thereby introducing an angle measurement error;

2) The light beam reflected back by the optical device under test will produce lateral shifts on each optical element in the system as the measurement angle changes, thus introducing errors at different points on the same optical element in the measurement system.

Figure 1 shows a schematic diagram of the optical structure of the existing pp-LTP, which includes a laser light source 1', a fixed optical head, a moving optical head and an f-θ angle detection system, and the fixed optical head includes a phase plate 2' and a beam splitter 3 'and plane mirror 4', the moving optical head includes a pentaprism 5', and the f-θ angle detection system includes an FT (Fourier transform) lens 7' and an area array detector 8'. When the light beam is vertically incident on the mirror surface 6' from the pentaprism 5', if the measuring point on the mirror surface 6' is not horizontal, the reflected light will be reflected at a certain angle with the incident light. Let this angle be θ angle, then five The distance s on the prism 5' represents the lateral displacement of the reflected light beam on the reflective surface of the pentaprism 5' when θ is equal to 0° and θ is not equal to 0°. It can be seen from Fig. 1 that the measuring beam starts to deviate from the measurement point on the mirror surface 6' to be measured, so the measurement point on the mirror surface 6' to be measured is the reference point for calculating the lateral displacement of each optical element in the pp-LTP, Therefore, for the same deflection angle, the farther the optical device in the system is from the geometric optical path of the measuring point on the mirror surface 6' to be measured, the greater the lateral shift of the measuring beam on the optical device is, and it is this lateral shift that makes each of the systems in the system Optics introduce errors at different points. The more transmission and reflection optical devices used in the measurement system, the greater the amount of lateral movement of the measurement beam, and the greater the system error introduced.

It is impossible to obtain ideal optical components. In order to reduce the systematic error caused by lateral movement in the detector, there are two main approaches from the above analysis, one is to reduce the number of optical components used in the detection system, and the other One is to reduce the distance between the reference point for lateral displacement calculation and each optical element in the detection system, so it is urgent to provide such a measuring device.

Contents of the invention

The purpose of the present invention is to provide a high-precision long-distance surface profile measurement device, so as to reduce the system error by reducing the lateral movement caused by the measurement beam when the measurement angle is different.

To achieve the above object, the present invention adopts the following technical solutions:

A long-distance surface shape measurement device is used for surface shape detection on the surface of an optical device to be tested, which includes a moving optical head,

The moving optical head includes a surface light source, a single-hole screen, a beam splitter, a Fourier transform lens and an area array detector, the surface light source is vertically arranged, and the beam splitter is closely attached to On the upper surface, the single-hole screen is arranged on one side of the surface light source and forms an oblique angle with the surface light source. The size of the angle makes the beam in the normal direction of the surface light source reflect to the On the surface of the optical device to be tested, the Fourier transform lens is horizontally arranged above the beam splitter, and the area array detector is horizontally arranged above the Fourier transform lens.

Further, the mobile optical head further includes a casing, and the surface light source, single-hole screen, beam splitter, Fourier transform lens, and area array detector are all arranged in the casing.

Further, the measurement device also includes a fixed optical head and a plane mirror, the plane mirror is fixed on the moving optical head, the fixed optical head is configured to project a reference beam to the plane mirror, and detect the The light beam reflected by the flat mirror.

Preferably, the fixed optical head is an autocollimator or an f-θ angle detection system.

Preferably, the surface light source is an incoherent surface light source.

Further, the measurement device also includes an optical platform and a linear translation platform, the linear translation platform is located on the optical platform, and the moving optical head is installed on the linear translation platform.

Preferably, when the surface light source is arranged vertically, the angle of inclination between the single-hole screen and the surface light source is 45°.

When measuring, the present invention passes the measuring beams of different angles reflected from the surface of the optical device to be measured through the screen hole of the single-hole screen, so that the center point of the screen hole of the single-hole screen is used as the value of the lateral displacement of each optical element in the measuring device. Calculate the reference point, and then it can be considered that the measuring beam is offset from the screen hole of the single-hole screen. Compared with the scheme in the prior art that uses the measurement point of the optical device to be tested as the reference point for the lateral displacement calculation, the present invention makes the distance between each optical element and the reference point by transferring the reference point to the screen hole center point of the single-hole screen The distance is greatly shortened, thereby reducing the amount of lateral movement of the measuring beam on each optical element, thereby reducing the systematic error caused by the lateral movement. In addition, the refractive and reflective optical devices used in the present invention only have beam splitters and Fourier transform lenses, but since the beam splitters are placed close to the single-hole screen, only the beam-splitter area at the screen hole of the single-hole screen will be used, so the beams reflected by different measurement points on the optical device to be tested will pass through the same area of the beam splitter, although this area will introduce errors, but the error is the same for each measurement point, so it can be considered that the beam splitter The mirror introduces the same error to the measurement values at different angles, so the systematic error introduced by the beam splitter has no effect on the relative variation of the measurement results, only the relative variation of the measurement results is meaningful for characterizing the surface shape of the optical device to be tested. That is to say, the beam splitter does not contribute to systematic errors, and only the Fourier transform lens actually introduces errors in the present invention, thereby reducing the number of optical elements in the measuring device that introduces systematic errors.

Description of drawings

Fig. 1 is the optical structure schematic diagram of pp-LTP in the prior art;

Figures 2a and 2b are schematic optical schematics of surface light source specular reflection, wherein Figure 2a shows that the plane mirror is in a horizontal position, and Figure 2b shows that the plane mirror is in an inclined position;

Fig. 3 is a schematic view of the optical structure of a long-distance profile measuring device of the present invention;

Figures 4a and 4b are schematic diagrams of light path propagation in the present invention, wherein Figure 4a is a light path diagram incident to the optical device to be tested, and Figure 4b is a light path diagram after reflection from the optical device to be tested.

detailed description

Below in conjunction with the drawings, preferred embodiments of the present invention are given and described in detail.

As we all know, as shown in Figure 2a, if the surface light source 100 is placed horizontally behind the hole 200, the light beam emitted by the surface light source 100 passes through the hole 200 and is reflected by the plane mirror 300, which can be regarded as formed by the plane mirror 300 facing the light source 100 A beam of light emitted by the image 100A and transmitted through the aperture image 200A. It can be seen from the principle of mirror reflection that the light beam passing through the center of the hole 200 and the hole image 200A after mirror reflection must propagate along the normal direction of the plane mirror 300, so the light beam passing through the hole 200 after mirror reflection is a beam that propagates along the normal direction of the mirror surface and has a small The divergence angle of the thin light beam is determined by the diameter of the hole 200 and the distance from the hole 200 to the mirror surface of the plane mirror 300 . If the angle of the plane mirror 300 changes, as shown in Figure 2b, the position of the image 100A of the surface light source 100 and the position of the hole image 200A will also change accordingly. The light beam that is emitted by the image 100A formed by the plane mirror 300 against the light source 100 and passes through the hole image 200A, so the light beam that is mirrored back to the hole 200 is still a small beam that propagates along the normal direction of the mirror surface and has a small divergence angle. .

Based on the above principles, the present invention provides a high-precision long-distance surface shape measuring device. In the embodiment shown in FIG. 3 , the measuring device includes a moving optical head 1 , an optical device to be tested 2 , an optical platform 3 , a linear translation stage 4 , a plane mirror 11 and a fixed optical head 12 .

As shown in Figure 3, the optical table 3 of the present invention is realized by using a common optical table in the existing LTP, wherein the linear translation stage 4 is horizontally arranged above the optical table 3, and the moving optical head 1 is fixed on the linear translation stage 4 and moves along with it. The linear translation stage 4 moves horizontally to carry out horizontal scanning measurement (the scanning direction is shown by the arrow in Fig. 3 ) for the optical device 2 to be tested; the fixed optical head 12 is fixed on the side wall of the optical platform 3, and the plane mirror 11 is fixed on the On the outer wall of the housing 10 of the optical head 1, wherein the fixed optical head 12 is arranged opposite to the plane reflector 11, for projecting a reference beam to the plane reflector 11 and detecting the light beam reflected by the plane reflector 11 for the reference beam, and then The influence of the movement error caused by the vibration of the moving optical head 1 during the measurement process is corrected. In the field, the solution of correcting the motion error of the moving optical head 1 by using the fixed optical head 12 and the plane mirror 11 belongs to the known technology, and its working principle will not be repeated here. In addition, the fixed optical head 12 in the present invention can be realized by using an existing autocollimator or f-θ angle detection system, and the fixed optical head 12 shown in FIG. 3 is an autocollimator.

Referring to Fig. 3 again, the mobile optical head 1 of the present invention comprises a housing 10 and a surface light source 5, a single hole screen 6, a beam splitter 7, a Fourier transform lens 8 and an area detector installed in the housing 10 9. Wherein, the surface light source 5 is vertically arranged, the single-hole screen 6 is arranged on one side of the surface light source 5 and forms an angle of 45° with the surface light source, the beam splitter 7 is closely attached to the upper surface of the single-hole screen 6, and the Fourier transform lens 8 It is arranged horizontally above the beam splitter 7, and the area array detector 9 is arranged horizontally above the Fourier transform lens 8. According to the simple geometrical optics theory in this field, it can be known that the light beam emitted by the surface light source 5 can be regarded as the light beam emitted by the image 5A formed by the beam splitter opposite the surface light source after being reflected by the beam splitter 7 . It should be noted here that the single-hole screen and surface light source can also be set at other angles. Because the beam splitter is placed close to the single-hole screen, it is necessary to use the beam splitter to reflect the beam from the normal direction of the surface light source to the optical mirror to be tested. If the surface light source is placed vertically, the best angle of the single-hole screen is 45° °, but if the surface light source has a certain inclination, the single-hole screen should also have a certain inclination, as long as the angle of the single-hole screen is adjusted to keep the light beam in the normal direction of the surface light source to reflect to the surface of the optical device to be tested.

When the surface shape detection of the optical device 2 to be tested is carried out, as shown in Figure 4a, part of the light beam emitted by the surface light source 5 first passes through the screen hole 13 of the single-hole screen 6 and is incident on the beam splitter 7 behind the single-hole screen 6. The light beam 14 is then reflected by the beam splitter 7 and incident on the surface of the optical device 2 to be tested; then, as shown in FIG. 4 b , the light beam 15 is reflected back from the optical device 2 to be tested. According to the specular reflection principle shown in Figures 2a and 2b, the beam 14 reflected back by the surface of the optical device 2 to be tested and the beam 15 passing through the single-hole screen 6 must be a beam along the normal direction of the measuring point on the surface of the optical device 2 to be tested. Propagated thin beam, the thin beam 15 is the measurement beam of the system, the measurement beam passes through the beam splitter 7 and converges on the area array detector 9 through the Fourier transform lens 8 to form a measurement spot, according to the measurement spot The change value of the inclination angle of the surface of the optical device 2 to be tested can be measured from the position change data of the center of gravity.

Compared with the prior art, the present invention has the advantages of:

1. The optical path system adopted in the present invention is beneficial to reduce the lateral displacement of the measuring beam on each optical element. Specifically, the starting point for calculating the lateral movement of traditional long-range surface profilers is the measurement point on the optical device to be tested, so it is difficult to reduce the lateral movement by reducing the geometrical optical path between the calculation reference point of the lateral movement and the optical components of the system. Purpose; and in the present invention, because the measuring light beam of different angles is all reflected to the area array detector 9 by the screen hole 13 of single-hole screen 6 and forms the measurement spot, thus the central point O of the screen hole 13 of single-hole screen 6 is The reference point for the calculation of the lateral movement of each optical device in the system, compared with the solution in the prior art that uses the measurement point of the optical device to be measured as the reference point for the calculation of the lateral movement, the present invention transfers the reference point to the single-hole screen 6 The center point O of the screen hole makes the distance between each optical element, such as the Fourier transform lens 8 compactly arranged with the single-hole screen 10, and the reference point is greatly shortened, thereby reducing the lateral shift of the measuring beam on the optical element , thereby reducing the systematic error introduced by the traverse.

2. The optical path system adopted in the present invention reduces the optical elements that produce errors. Specifically, there are multiple optical devices in the optical path of the traditional long-range surface profiler, such as the pentaprism 5' and the beam splitter 3' in Figure 1, which have multiple optical surfaces, and they themselves have non-uniform refractive index Transmissive body, these all can cause to introduce systematic error because of measuring beam traverse; And in the present invention, the optical element that causes measuring beam to deviate from ideal direction only has beam splitter 7 and Fourier transformation lens 9, but because single-hole screen 6 It is arranged close to the beam splitter 7, and only the area of the beam splitter 7 at the screen hole 13 of the single-hole screen 6 will be used, so the light beams reflected by different measurement points on the optical device 2 to be tested are all in the whole measurement process. Will pass through the same area of the beam splitter 7, although this area will introduce errors, but the error is the same for each measurement point, so it can be considered that the beam splitter 7 introduces the same error for the measured values of different angles, so the beam splitter The systematic error introduced by the mirror 7 has no influence on the relative variation of the measurement result, but it should be understood that only the relative variation of the measurement result is meaningful for characterizing the surface shape of the optical device to be measured, that is to say, only the error is really introduced in the present invention Fourier transform lens 9, thereby reducing the number of optical elements that introduce systematic errors.

3. Traditional long-range surface profilers, such as pp-LTP, require the light source 1' to have good directionality, and lasers are often used as light sources; however, this system does not require light source directionality, and the surface light source 5 can use incoherent surface light sources. This is beneficial to reduce the interference of diffracted light caused by impurities in the air or screen hole diffraction during laser propagation.

4. In the traditional long-distance surface profiler based on laser light source, the directional error will be introduced due to the direction drift of the laser beam; and in the present invention, the measuring beam passing through the single-hole screen 6 is a beam equal to the optical device 2 to be tested. The thin beam 15 with a small divergence angle propagating in the normal direction at the upper measurement point always points to the normal direction at the measurement point on the optical device 2 to be tested, so there is no directivity error problem in the present invention.

What is described above is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Various changes can also be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made according to the claims and description of the application of the present invention fall within the protection scope of the claims of the present invention. What is not described in detail in the present invention is conventional technical contents.

Claims (7)

1. a long-range profile measurement apparatus, for the surface of optical device under test is carried out surface testing, it includes flying optical head, it is characterised in that

Described flying optical head includes area source, single hole screen, beam splitter, Fourier transform lens and planar array detector, described beam splitter is close to the upper surface of described single hole screen, described single hole screen is arranged on described area source side and at an angle with described area source, this angle is configured such that area source normal direction light beam can pass through single hole screen shield aperture part beam splitter vertical reflection to optical device under test surface, described Fourier transform lens is horizontally set on above described beam splitter, and described planar array detector is horizontally set on above described Fourier transform lens.

2. long-range profile measurement apparatus according to claim 1, it is characterised in that described flying optical head also includes housing, and described area source, single hole screen, beam splitter, Fourier transform lens and planar array detector are arranged in described housing.

3. long-range profile measurement apparatus according to claim 1, it is characterized in that, this measurement apparatus also includes fixing optical head and plane mirror, described plane mirror is fixed on described flying optical head, described fixing optical head is set to project reference beam to described plane mirror, and detects the light beam reflected through described plane mirror.

4. long-range profile measurement apparatus according to claim 3, it is characterised in that described fixing optical head is autocollimator or f-θ angle detection system.

5. long-range profile measurement apparatus according to claim 1, it is characterised in that described area source is incoherent area source.

6. long-range profile measurement apparatus according to claim 1, it is characterised in that this measurement apparatus also includes optical table and linear translation platform, and described linear translation platform is positioned on described optical table, and described flying optical head is arranged on described linear translation platform.

7. the angle of inclination of long-range profile measurement apparatus according to claim 1, it is characterised in that when described area source is vertically arranged, described single hole screen and described area source is 45 °.