JP2831428B2 - Aspherical shape measuring machine - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aspherical shape measuring instrument for determining a surface shape of a lens, a mirror or the like, in particular, a surface shape of an aspherical lens by wavefront measurement.

[Conventional technology]

When measuring the surface shape of the surface to be measured by measuring the wavefront from the surface to be measured by interference measurement, it is necessary to eliminate or reduce the influence of aberration (hereinafter referred to as “self-aberration”) due to a manufacturing error of the measuring optical system as much as possible. Is required.

As conventional methods for solving this, (1) a method using a master and (2) a method using ray tracing are known. As an example of (1), there is a method described in JP-A-63-48406. This is a method using a master having a known shape, and using the parameters of the wavefront measured at the exit pupil of the measurement optical system for the master, performs a polynomial expansion of the wavefront aberration at the entrance pupil, and uses the expansion coefficient to obtain the master. The value of the wavefront aberration at the entrance pupil is calculated from the measured value of the wavefront at the exit pupil of the test surface having a relatively small deviation from, and the self-aberration of the measurement optical system is removed and reduced. or,
As an example of (2), Proc. ICO 14 353-354 1987) “A
spheric Measurement using PhaseShifting Interferom
etry "by K. Creath & JCWyant, which captures interference fringes using a Tweenman-Green normal interferometer and a large number of photodetectors, and captures the ideal non- A wavefront observed when a spherical surface is placed is obtained by ray tracing, this is used as a reference wavefront, and a difference between the reference wavefront and the measured wavefront is used to apply a fringe analysis technique.

The method (1) is characterized in that it does not require time-consuming and complicated calculations, and the method (2) simplifies the self-aberration by simplifying the measurement optical system as much as possible, and uses ray tracing. Accuracy is improved by matching with the simulation. The latter is also called a digital null method because a null fringe state (a state without interference fringes) with respect to a reference wavefront is an ideal state. This method does not require the creation of a null lens (aberration-free lens) or a computer generated hologram, and has the advantage of being highly versatile.

[Problems to be solved by the invention]

The method (1) using a master is good when the deviation between the surface to be measured and the master is small, but the error increases when the deviation increases. Further, since only the deviation between the surface to be measured and the master is measured, there is a drawback that the relative evaluation can be performed but the surface shape cannot be absolutely evaluated.

In the digital null method of the above (2), the number of interference fringes is too large for a so-called tight aspheric surface having a large amount of aspheric surface, and the realization of an image sensor having the number of pixels capable of detecting the interference fringe is far away. There is also a drawback that it is difficult to remove and reduce the influence of manufacturing errors of the measurement optical system, that is, the self-aberration.

In view of the above problems, the present invention does not require a master, enables absolute evaluation of the shape, and provides a large aspherical surface with a tight aspherical surface, without using an image sensor having a particularly large number of pixels, and thus has a measurement optical system. It is an object of the present invention to provide an aspherical shape measuring instrument capable of measuring by removing or reducing the influence of manufacturing errors, that is, self-aberration.

[Means and actions for solving the problem]

The aspherical shape measuring instrument according to the present invention comprises a means for illuminating a surface to be measured, a means for measuring a wavefront reflected from the surface by fringe scanning shearing interferometry, and It is characterized by comprising means for obtaining a wavefront measured when a spherical surface is placed by ray tracing, and means for performing fringe analysis by taking the difference between the measured wavefront and the ideal wavefront.

Further, according to the present invention, a mechanism is provided for rotating the test surface around the measurement optical axis.

The fringe scanning shearing interferometry causes a wavefront to be measured to interfere with a wavefront obtained by laterally shifting (shearing) the wavefront, and has a large amount of aspherical surface by combining the amount of shear and the method of fringe scanning. Even a tight aspheric surface can be measured with the same number of interference fringes and with the same accuracy as the Twyman-Green type. Therefore, by combining the fringe scanning shearing interferometry and the digital null method, most of the drawbacks of the conventional method can be eliminated, and a mechanism for rotating the surface to be inspected is added, and a plurality of measurements before and after the rotation are performed. By utilizing the result, it is possible to separate the yield including the manufacturing error of the measurement optical system and the aberration due to the deviation from the design value of the test surface, and reduce the influence of the manufacturing error of the measurement optical system. be able to.

〔Example〕

Hereinafter, the present invention will be described in detail based on illustrated embodiments.

FIG. 1 shows the configuration of an aspherical shape measuring instrument according to the present invention. 1 is a light source, 2 is an objective lens, 3 is a pinhole,
Reference numeral 4 denotes a beam splitter, 5 denotes a mirror, 6 denotes a beam splitter, 7 denotes a mirror, 8 denotes a collimator, 9 denotes an objective lens, and 10 denotes an optical element to be tested which is detachably held on an alignment table 11. The emitted light beam is converged by the objective lens 2, passes through the pinhole 3, is reflected by the mirrors 5 and 7, is collimated by the collimator 8, is once converged by the objective lens 9, diverges, and diverges Illuminating the surface 22 to be inspected,
The light beam reflected by the test surface 22 passes through an objective lens 9, a collimator 8, and a mirror 7, and is split into a reflected component and a transmitted component by a beam splitter 6.

The alignment table 11 holding the test optical element 10 is
As shown in FIG. 2, a clamp unit 23 for detachably holding the test optical element 10, a tilt adjusting unit 24, an x-axis direction slider 25,
The y-axis direction slider 26, the rotation unit 27, and the base 28 are vertically stacked, and the tilt adjustment unit 24, the x-axis direction slider 25, and the y-axis direction slider 26 align the optical element 10 to be measured. I have. That is, the optical element to be inspected
It has the function to tilt, shift and rotate 10. 12
Are mirrors, 13 and 14 are semi-transparent mirrors forming a fringe scanning shearing interference unit, 15 is an imaging lens, 16 is an image sensor, 17 is an interference fringe analysis unit, and mirror 14 is a pulse motor and a piezo not shown. The action of both shearing and fringe scanning is exerted by coarsely and finely changing the distance between the semi-transmissive mirror 13 and the driving unit composed of elements. The pulse motor gives a shearing amount, and the piezo element gives a fringe scanning amount. The reflection component from the beam splitter 6 forms interference fringes at a fringe scanning shearing interference unit composed of the semi-transparent mirrors 13 and 14, and forms an image on an image sensor 16 by an imaging lens 15. The output signal is output from the interference fringe analyzer 17
Thus, signal processing described later is performed.

Reference numeral 18 denotes a beam splitter, 29 denotes a pinhole, 19 denotes a photodetector, and 20 denotes a position detection photodetector. A light beam transmitted through the beam splitter 6 is split into two by a beam splitter 18 via a mirror 5 and a beam splitter 4. , One is pinhole 29
And then enters the photodetector 19, while the other is a position detection photodetector 20.
, And both are used for alignment of the test surface 22.

Next, the operation of this embodiment will be described with reference to FIGS.

The interference fringe analyzer 17 performs signal processing shown in the flowchart of FIG. That is, ray tracing is performed by assuming an optical system from the test aspheric surface 22 to the image sensor 16 as shown in FIG. 4 in advance, and an ideal aspheric surface as designed is placed in place of the test aspheric surface 22. In this case, a shearing wavefront in the image sensor 16 is obtained, and this is used as a reference wavefront. Next, the shearing wavefronts of the test aspheric surface 22 in two directions (x, y-axis directions) are measured, and the wavefronts are combined to obtain a two-dimensional measurement wavefront. By taking the difference between this and the reference wavefront and integrating the difference, the amount of deviation from the design value of the test aspheric surface 22 is obtained.

Next, the concept of correction will be described. The deviation includes a processing / assembly error of the measuring optical system, and it is desirable to remove or reduce the error. The wavefront determined by the digital null method and fringe scanning shearing interference is represented by ΔW _m (r,
θ, β). Here, (r, θ) represents the coordinates of the entrance pupil when the aspheric surface to be measured is used as the entrance pupil of the measurement optical system, and β represents the inclination angle of the outgoing ray with respect to the principal ray at (r, θ). This ΔW _m (r, θ, β) can be written as: ΔW _m (r, θ, β) = ΔW _r (r, θ, β) + ΔT (r, θ, β). Here, ΔW _r is the true shift amount, and ΔT is the error of the measuring optical system. Then, since ΔT is unknown, to erase the ΔT using the measurement wavefront obtained by rotating a plurality of times around the measurement optical axis 21 to be aspheric surface 22, obtaining the [Delta] W _r.
Specifically, given a known amount of rotation φ _i (i = 0, 1, 2,...), ΔW _m (r, θ, β _i ) = ΔW _r (r, θ + φ _i , β _i ) + ΔT (R, θ, β _i ) where i = 0, 1, 2,..., Β ₀ = β, φ ₀ = 0, β = β (r,
θ) is measured. Then, for simplicity, a case where ΔT is approximated by the following equation will be described.

_{ΔT (r, θ, β i} ) = ΔT (r, θ, β) + (∂ΔT / ∂β) · Δβ i _{However, ΔB i = β i -β,} i = 0,1,2, ... Δβ i Is known from the measured values, and the unknowns are ΔT, ∂ΔT / ∂β, Δ
Since there are three values of W _r , ΔW _r can be obtained from three measured values of the measured value before rotation and the measured values after two rotations. If you write only the result, Which is expanded into a so-called Zernike polynomial. here, Is expressed as follows.

However, | | denotes the determinant, Δa _l ^k, Δa _l ^-k represents the coefficient of the angular portion when deployed two rotating phi _1, the difference between the obtained wavefront phi ₂ to the polynomial of Zernike, A, B, C, and D represent the following quantities.

A = (cos kφ ₁ −1) Δβ ₂ − (cos kφ ₂ −1) Δ
β ₁ B = Δβ ₂ · sin kφ ₁ −Δβ ₁ · sin kφ ₂ C = −Δβ ₂ · sin Kφ ₁ + Δβ ₁ · sin kφ ₂ D = (cos kφ ₁ −1) ΔB ₂ − (cos kφ ₂ −1) ) Δβ
_{1. In an} actual example, for an aspherical surface having an aspherical amount of about 100 μm, it is necessary to perform interference measurement on a wavefront having a wavefront aberration of the magnitude shown in FIG. The number is too large and it is impossible. However, by using the digital null test in combination with the fringe scanning shearing interference and through the above-described correction procedure, the sixth
As shown in the figure, it is possible to obtain a shape deviation from a design value in which the influence of the self-aberration of the measurement optical system is reduced.

In the above embodiment, the fringe scanning shearing interferometry using a parallel plate is taken as an example, but the shearing interferometry utilizing the polarization characteristic disclosed in JP-A-59-154309 and the known shearing interferometry are used. It can be applied to any of them.
In addition to the mechanical method using a piezo element, a method using a semiconductor laser as a light source and changing the wavelength can be used for the stripe scanning method.

〔The invention's effect〕

As described above, according to the aspherical shape measuring apparatus according to the present invention, even in the case of a so-called tight aspherical surface having a large aspherical amount, the self-aberration of the measuring optical system can be removed and reduced without using an image sensor having a particularly large number of pixels. It becomes possible to measure the surface shape.
Moreover, the absolute evaluation of the shape is possible without requiring a master.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an aspherical shape measuring instrument according to the present invention, FIG. 2 is a diagram showing a configuration of an alignment table of the above embodiment, and FIG. 3 is a diagram showing a signal processing of an interference fringe analyzer of the above embodiment. FIG. 4 is a flowchart showing an optical system assumed when a reference wavefront is obtained in the above embodiment, and FIG.
FIG. 6 is a diagram showing the magnitude of the wavefront aberration of a wavefront that requires interference measurement on an aspherical surface having an aspherical amount of about μm. FIG. 6 shows a setting in which the influence of self-aberration of the measuring optical system obtained by the present embodiment is reduced. FIG. 6 is a diagram illustrating a shape deviation from a value. 1 ... light source, 2 ... objective lens, 3,29 ... pinhole,
4, 6, 18 beam splitter, 5, 7, 12 mirror, 8 collimator, 9 objective lens, 10 optical element under test, 11
...... Alignment table, 13,14 ... Semi-transparent mirror, 15 ... Imaging lens, 16 ... Imaging element, 17 ... Interference fringe analyzer, 19 ... Photodetector, 20 ... Position detection photodetector, 21 ... optical axis, 22 ...
Surface to be inspected, 23… Clamp part, 24… Tilt adjustment part, 25 ……
x-axis slider, 26 ... y-axis slider, 27 ...
… Rotating part, 28 …… Base.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-69933 (JP, A) JP-A-63-48406 (JP, A) JP-A-60-55214 (JP, A) JP-A-59-1984 154309 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G01B 11/00-11/30 G01B 9/00-9/10 G01M 11/00-11/08

Claims (2)

(57) [Claims] 1. A means for illuminating a surface to be measured, a means for measuring a wavefront reflected from the surface by fringe scanning shearing interferometry, and a case where an ideal aspherical surface according to a design value is placed in place of the surface to be measured. An aspherical shape measuring device comprising: means for obtaining a wavefront to be measured by ray tracing; and means for performing fringe analysis by taking a difference between the measured wavefront and the wavefront in an ideal case. 2. The aspherical shape measuring instrument according to claim 1, further comprising a mechanism for rotating the surface to be measured around a measurement optical axis.